JP7695451B2 - Head-mounted display - Google Patents

Description

The present invention relates to a resin lens for a head-mounted display.

In recent years, various electronic technologies known as VR (Virtual Reality) and AR (Augmented Reality) have rapidly developed, and head mounted display (HMD) products are becoming popular as image display devices for these technologies. Since head mounted displays are image display devices worn on the head, they are required to be small, lightweight, and cause minimal discomfort when worn.

As a means for miniaturizing head mounted displays, a method has been proposed in which a lens is combined with a quarter wavelength plate and a reflective polarizing plate, and the polarization state of light after passing through the lens is changed to switch between reflection and transmission, causing the image to make one and a half round trips through a single lens (Patent Documents 1 and 2).
This method works as follows: a quarter-wave plate and a reflective polarizer are placed on the back of a lens with a partially reflective coating on the front surface, and the light that enters the front surface of the lens as circularly polarized light is converted to linearly polarized light by the quarter-wave plate after passing through the lens; this linearly polarized light is reflected by the reflective polarizer, and is converted again by the quarter-wave plate into circularly polarized light opposite to the original, which enters the lens from the back surface and reaches the partially reflective coating on the front surface; the light that is reflected by the partially reflective coating and leaves the back surface of the lens is converted to linearly polarized light with a direction 90° different from the original by the quarter-wave plate, passes through the reflective polarizer, and enters the eye as an image. In this way, a high magnification and wide field of view can be obtained even in a thin optical module.

U.S. Pat. No. 6,563,638 JP 2017-21321 A

However, in the above-mentioned head-mounted displays, if the polarization state changes while passing through the lens, for example, some of the light will be transmitted through the reflective polarizing plate after passing through the lens for the first time, and low-magnification images and high-magnification images will overlap, making it difficult to obtain a clear image. Although clear images can be obtained by using glass lenses with low birefringence, this is undesirable as it increases the weight. Therefore, there is a strong demand for clear images to be obtained even with plastic lenses, which are lightweight but generally prone to birefringence.

The object of the present invention is therefore to provide a resin lens for head-mounted displays that can provide clear images even in an optical system that uses polarized light.

As a result of extensive research to solve the above problems, the inventors discovered that the phase difference within the effective diameter caused by the birefringence of the resin lens, and more surprisingly, the amount of fluorescent material contained in the resin lens, affect the clarity of images displayed on a head-mounted display, leading to the invention.

That is, the present invention is as follows.
[1]
A head mounted display including a resin lens,
the resin lens has an average absolute value of retardation within an effective diameter of 5 nm or less;
the resin lens has a fluorescent substance content of 0.1 to 4.0×10 −9 mol/L calculated from the fluorescence intensity at a wavelength of 530 nm when a 2.0% by mass solution obtained by dissolving the resin lens in chloroform is measured at an excitation wavelength of ⁴³⁶ nm, using a concentration-intensity conversion formula for a fluorescein ethanol solution ;
The head mounted display uses polarized light and transmits image light through the resin lens multiple times by repeating reflection.
A head mounted display comprising:
[2]
The head mounted display according to claim 1 , wherein the resin lens has a ratio (T450/T680) of a transmittance (T450) at a wavelength of 450 nm to a transmittance (T680) at a wavelength of 680 nm of 0.98 or more.
［ ３ ］
The head mounted display according to [1] or [2] , wherein the resin lens has a glass transition temperature (Tg) of more than 120° C. and not more than 160° C.
［ 4 ］
The head mounted display according to any one of [1] to [3] , wherein the resin lens has an absolute value of a photoelastic coefficient of 3.0×10 ⁻¹² Pa ⁻¹ or less.
［ 5 ］
The head mounted display according to any one of [1] to [ 4 ] , wherein the resin lens contains a methacrylic resin.
［ ６ ］
The head mounted display according to [ 5 ], wherein the resin lens contains a methacrylic resin having a structural unit (X) having a ring structure in a main chain.
［ ７ ］
The head mounted display according to [6], wherein the structural unit (X) of the resin lens includes at least one structural unit selected from the group consisting of a structural unit derived from an N -substituted maleimide monomer, a glutarimide structural unit, and a lactone ring structural unit.
［ 8 ］
The head mounted display according to [ 7 ] , wherein the structural unit (X) of the resin lens includes a structural unit derived from an N-substituted maleimide monomer.
［ ９ ］
The head mounted display according to [ 7 ] , wherein the structural unit (X) of the resin lens includes a glutarimide structural unit.

The present invention provides a resin lens for head-mounted displays that can provide clear images even in an optical system that uses polarized light.

1 is a schematic diagram of a simulator that reproduces the principle of a head-mounted display described in JP 2017-21321 A. In the examples, the simulator was used to evaluate the clarity of images.

Below, a form for carrying out the present invention (hereinafter referred to as the "present embodiment") will be described in detail, but the present invention is not limited to the following description and can be carried out in various modified forms within the scope of its gist.

[Resin lenses for head mounted displays]
The resin lens for head mounted displays of this embodiment is characterized in that the average absolute value of phase difference within the effective diameter is 5 nm or less, and the content of the fluorescent substance, calculated from the fluorescence intensity at a wavelength of 530 nm when a 2.0 mass % solution obtained by dissolving the lens in chloroform is measured at an excitation wavelength of 436 nm, using the concentration-intensity conversion formula for a fluorescein ethanol solution, is 0.1 to 4.0 × 10 ^-9 mol/L.

- Phase difference within the effective diameter -
In the resin lens for head mounted displays in this embodiment, the average absolute value of the phase difference within the effective diameter of the resin lens is 5 nm or less, preferably 4 nm or less, and more preferably 3 nm or less. By having the average absolute value of the phase difference of 5 nm or less, the image can be clearly viewed without being doubled.
Here, the effective diameter of the resin lens represents the range in which an image can be viewed when the lens is incorporated into the housing of a head mounted display, and is expressed as the diameter of a circle centered on the optical axis of the lens. If the range in which the image can be viewed is not a perfect circle, the minor axis is taken as the effective diameter. The phase difference within the effective diameter of the resin lens can be specifically measured by the method described in the Examples below.

- Content of fluorescent substances -
In the present embodiment, the resin lens for head mounted display has a content of a fluorescent substance calculated from the fluorescence intensity at a wavelength of 530 nm measured at an excitation wavelength of 436 nm for a 2.0 mass % solution obtained by dissolving the resin lens in chloroform using a concentration-intensity conversion formula for a fluorescein ethanol solution, which is 0.1 to 4.0×10 ⁻⁹ mol/L, preferably 0.2 to 3.5×10 ⁻⁹ mol/L, more preferably 0.3 to 3.0×10 ⁻⁹ mol/L, and even more preferably 0.3 to 2.5×10 ⁻⁹ mol/L. By having the content of the fluorescent substance be equal to or less than the upper limit, the image can be clearly viewed without blurring. Furthermore, by having the content of the fluorescent substance be equal to or more than the lower limit, it is possible to suppress the entry of low-wavelength light that is harmful to the eyes into the eyes more than necessary.
Although it is unclear why images appear blurred when the fluorescence intensity is high (the content of fluorescent substances is high), it is presumed that when fluorescence is generated by light passing through the lens, the image appears blurred because the light is directed in a direction other than the original traveling direction of the transmitted light. Furthermore, in lenses for head-mounted displays that use polarized light and are repeatedly reflected and pass through the lens multiple times, it is presumed that the blurring of images becomes more noticeable because the optical path length passing through the lens becomes longer and the deviation from the original traveling direction of the light becomes larger due to the reflective polarizing plate and partial reflection coating.
The content of the fluorescent substance can be specifically measured by the method described in the Examples below.

- Glass transition temperature -
The glass transition temperature (Tg) of the resin lens for head mounted displays in this embodiment is preferably higher than 120° C. and 160° C. or lower.
It is preferable that the glass transition temperature of the resin lens for a head mounted display exceeds 120° C. from the viewpoint of heat resistance against heat generated from electronic devices of the head mounted display and from the viewpoint of occurrence of photoelastic birefringence due to dimensional change. From the viewpoint of dimensional stability under the temperature of the usage environment, the glass transition temperature (Tg) is more preferably 125° C. or higher, and further preferably 130° C. or higher.
On the other hand, when the glass transition temperature (Tg) is 160° C. or lower, melt processing at extremely high temperatures can be avoided, thermal decomposition of the resin, etc. can be suppressed, and a good product can be obtained. From the viewpoint of further obtaining the above-mentioned effects, the glass transition temperature (Tg) is preferably 150° C. or lower, more preferably 140° C. or lower.
The glass transition temperature (Tg) can be determined by measurement in accordance with JIS-K7121. Specifically, it can be determined by the method described in the examples below.

- Photoelastic coefficient C _R -
The absolute value |C _R | of the photoelastic coefficient C _R of the resin lens for head mounted displays of this embodiment is preferably 3.0×10 ^-12 Pa ^-1 or less, more preferably 2.0×10 ^-12 Pa ^-1 or less, even more preferably 1.5×10 ^-12 Pa ^-1 or less, and even more preferably 1.0×10 ^-12 Pa ^-1 or less. The photoelastic coefficient is described in various documents (see, for example, Chemical Review, No. 39, 1998 (published by the Academic Society Publishing Center)) and is defined by the following formulas (i-a) and (i-b). It can be seen that the closer the value of the photoelastic coefficient C _R is to zero, the smaller the birefringence change due to external force.
|C _R |=|Δn|/σ _R ...(ia)
|Δn|=|nx−ny| ...(ib)
(In the formula, C _R is the photoelastic coefficient, σ _R is the tensile stress, |Δn| is the absolute value of birefringence, nx is the refractive index in the stretching direction, and ny is the refractive index in the in-plane direction perpendicular to the stretching direction.)
When the absolute value |C _R | of the photoelastic coefficient C _R of the resin lens of this embodiment is 3.0 × 10 ⁻¹² Pa ⁻¹ or less, the stress generated when fixing the lens and the photoelastic birefringence generated due to dimensional changes due to temperature are sufficiently small, and a resin lens that can provide a clear image can be obtained.
The photoelastic coefficient C _R is measured by cutting the resin lens into small pieces and pressing them into a film using a vacuum compression molding machine. Specifically, it can be determined by the method described in the Examples below.

-Light transmittance-
In the resin lens for head mounted displays of this embodiment, the ratio (T450/T680) of the light transmittance at the thickest part measured with a spectrophotometer under a D65 light source and a 10° visual field, of the transmittance at a wavelength of 450 nm (T450) to the transmittance at a wavelength of 680 nm (T680) is preferably 0.95 to 1.03, more preferably 0.97 to 1.01, and even more preferably 0.98 to 1.00. When the ratio (T450/T680) is within the above range, an image with good color tone can be obtained.
The light transmittance can be measured by the method described in the Examples below.

- Molecular weight and molecular weight distribution -
The resin lens for head mounted displays in this embodiment has a weight average molecular weight (Mw) in terms of polymethyl methacrylate measured by gel permeation chromatography (GPC) in the range of preferably 80,000 to 170,000, more preferably 100,000 to 170,000, even more preferably 100,000 to 150,000, and still more preferably 120,000 to 150,000. When the weight average molecular weight (Mw) is in the above range, the balance between mechanical strength and fluidity is excellent.

The weight average molecular weight (Mw), number average molecular weight (Mn), and Z average molecular weight (Mz) of the resin lens can be measured using the following device and conditions.
Measurement device: gel permeation chromatography (HLC-8320GPC) manufactured by Tosoh Corporation
Measurement conditions:
Column: One TSKguard column Super H-H, two TSKgel Super HM-M, and one TSKgel Super H2500 are connected in series in this order.
Column temperature: 40°C
Developing solvent: tetrahydrofuran, flow rate: 0.6 mL/min, 2,6-di-t-butyl-4-methylphenol (BHT) was added as an internal standard at 0.1 g/L.
Detector: RI (differential refraction) detector Detection sensitivity: 3.0 mV/min Sample: 20 mL of tetrahydrofuran solution of 0.02 g of resin lens Injection amount: 10 μL
Standard sample for calibration curve: The following ten types of polymethyl methacrylate (PMMA Calibration Kit MM-10, manufactured by Polymer Laboratories) having different molecular weights and known monodisperse weight peak molecular weights are used.
Weight peak molecular weight (Mp)
Standard sample 1 1,916,000
Standard sample 2 625,500
Standard sample 3 298,900
Standard sample 4 138,600
Standard sample 5 60,150
Standard sample 6 27,600
Standard sample 7 10,290
Standard sample 8 5,000
Standard sample 9 2,810
Standard sample 10 850
Under the above conditions, the RI detection intensity is measured versus the elution time of the resin lens.
The weight average molecular weight (Mw), number average molecular weight (Mn), and Z average molecular weight (Mz) of the resin lens are determined based on each calibration curve obtained by measuring the standard samples for calibration curves, and the molecular weight distributions (Mw/Mn) and (Mz/Mw) are determined using these values.

-Methanol insolubles-
The ratio of the amount of methanol insoluble matter in the resin lens for head mounted displays in this embodiment to the total amount 100% by mass of the amount of methanol insoluble matter and the amount of methanol soluble matter is preferably 95% by mass or more, more preferably 95.5% by mass or more, even more preferably 96% by mass or more, still more preferably 96.5% by mass or more, particularly preferably 97% by mass or more, and most preferably 97.5% by mass or more. By making the ratio of the amount of methanol insoluble matter 95% by mass or more, it is possible to suppress molding troubles such as the generation of silver streaks during injection molding.
The methanol-insoluble fraction and the methanol-soluble fraction refer to those obtained by dissolving a resin lens for a head mounted display in chloroform to prepare a chloroform solution, dropping the solution into a large excess of methanol to effect reprecipitation, separating the filtrate and the residue, and then drying each of them.

Specifically, it can be determined as follows. After dissolving 5 g of resin lens in 100 mL of chloroform, the solution is placed in a dropping funnel and dropped into 1 L of methanol stirred with a stirrer for about 1 hour to perform reprecipitation. After the entire amount is dropped, the solution is left to stand for 1 hour, and then suction filtration is performed using a membrane filter (Advantec Toyo Co., Ltd., T050A090C) as a filter. The filtered material is vacuum dried at 60°C for 16 hours to obtain methanol insoluble matter. In addition, the filtrate is evaporated in a rotary evaporator with a bath temperature of 40°C and a vacuum degree gradually reduced from the initial setting of 390 Torr to a final value of 30 Torr to remove the solvent, and the soluble matter remaining in the eggplant-shaped flask is collected to obtain methanol soluble matter. The mass of the methanol insoluble matter and the mass of the methanol soluble matter are each weighed, and the ratio (mass%) of the amount of the methanol soluble matter to the total amount (100 mass%) of the amount of the methanol soluble matter and the amount of the methanol insoluble matter is calculated (methanol soluble matter ratio).

[Resin composition]
The resin lens for a head mounted display of the present embodiment is preferably made of a resin composition containing a methacrylic resin.

[Methacrylic resin]
The methacrylic resin contained in the resin lens for head mounted display of this embodiment preferably contains a methacrylic resin having a structural unit (X) having a ring structure in the main chain and a structural unit derived from a methacrylic acid ester monomer. By containing a methacrylic resin, particularly a methacrylic resin containing a structural unit (X) having a ring structure in the main chain and a methacrylic acid ester monomer unit, it is possible to obtain a resin lens having a sufficiently small phase difference within the effective diameter of the lens and a sufficiently small photoelastic coefficient.

Each structural unit is explained below.

-Structural units derived from methacrylic acid ester monomers-
Examples of structural units derived from methacrylic acid ester monomers include structural units derived from monomers selected from the methacrylic acid esters shown below. Examples of methacrylic acid esters include methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, 2-ethylhexyl methacrylate, cyclopentyl methacrylate, cyclohexyl methacrylate, cyclooctyl methacrylate, tricyclodecyl methacrylate, dicyclooctyl methacrylate, tricyclododecyl methacrylate, isobornyl methacrylate, phenyl methacrylate, benzyl methacrylate, 1-phenylethyl methacrylate, 2-phenoxyethyl methacrylate, 3-phenylpropyl methacrylate, and 2,4,6-tribromophenyl methacrylate.
These monomers may be used alone or in combination of two or more kinds.
As the structural units derived from the methacrylic acid ester monomer, structural units derived from methyl methacrylate and benzyl methacrylate are preferred in that the resulting methacrylic resin has excellent transparency and weather resistance.
The structural unit derived from the methacrylic acid ester monomer may be contained in only one type, or in two or more types.

In the resin lens for head mounted displays of this embodiment, the ratio of the structural unit (X) having a ring structure in the main chain and the structural unit derived from the methacrylic acid ester monomer is appropriately adjusted to reduce birefringence caused by orientation during molding or residual stress, and a resin lens for head mounted displays having an average absolute value of retardation within the effective diameter of 5 nm or less can be obtained. In addition, by appropriately adjusting the above ratio, sufficient heat resistance can be imparted to the methacrylic resin. From these viewpoints, the content of the structural unit derived from the methacrylic acid ester monomer is preferably 50 to 97% by mass, more preferably 55 to 97% by mass, even more preferably 55 to 95% by mass, even more preferably 60 to 93% by mass, and particularly preferably 60 to 90% by mass, based on 100% by mass of the methacrylic resin.
The content of the structural unit derived from the methacrylic acid ester monomer can be determined by ¹ H-NMR measurement and ¹³ C-NMR measurement. ¹ H-NMR measurement and ¹³ C-NMR measurement can be performed, for example, using CDCl ₃ or DMSO-d ₆ as a measurement solvent at a measurement temperature of 40° C.

The structural unit (X) having a ring structure in the main chain will be described below.
The structural unit (X) having a ring structure in the main chain preferably contains at least one structural unit selected from the group consisting of structural units derived from N-substituted maleimide monomers, glutarimide structural units, and lactone ring structural units, and more preferably consists of at least one structural unit selected from the group consisting of structural units derived from N-substituted maleimide monomers, glutarimide structural units, and lactone ring structural units. The structural unit (X) having a ring structure in the main chain may be one type or a combination of multiple types.

--Structural unit derived from N-substituted maleimide monomer--
Next, the structural unit derived from the N-substituted maleimide monomer will be described.
The structural unit derived from the N-substituted maleimide monomer may be at least one structural unit selected from the group consisting of a structural unit represented by the following formula (1) and a structural unit represented by the following formula (2), and is preferably formed from both of the structural units represented by the following formula (1) and the following formula (2).

式（１）中、Ｒ^１は、炭素数７～１４のアリールアルキル基、炭素数６～１４のアリール基のいずれかを示し、Ｒ^２及びＲ^３は、それぞれ独立に、水素原子、酸素原子、硫黄原子、炭素数１～１２のアルキル基又は炭素数６～１４のアリール基のいずれかを示す。
また、Ｒ^２またはＲ^３がアリール基の場合には、Ｒ^２またはＲ^３は置換基としてハロゲン原子を含んでいてもよい。
また、Ｒ^１は、ハロゲン原子、炭素数１～６のアルキル基、炭素数１～６のアルコキシ基、ニトロ基、ベンジル基等の置換基で置換されていてもよい。

式（２）中、Ｒ^４は、水素原子、炭素数３～１２のシクロアルキル基、炭素数１～１２のアルキル基のいずれかを示し、Ｒ^５及びＲ^６は、それぞれ独立に、水素原子、酸素原子、硫黄原子、炭素数１～１２のアルキル基、又は炭素数６～１４のアリール基のいずれかを示す。

In formula (1), R ¹ represents either an arylalkyl group having 7 to 14 carbon atoms or an aryl group having 6 to 14 carbon atoms, and R ² and R ³ each independently represent either a hydrogen atom, an oxygen atom, a sulfur atom, an alkyl group having 1 to 12 carbon atoms, or an aryl group having 6 to 14 carbon atoms.
When ^R2 or ^R3 is an aryl group, ^R2 or ^R3 may contain a halogen atom as a substituent.
Furthermore, R ¹ may be substituted with a substituent such as a halogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, a nitro group, or a benzyl group.

In formula (2), R ⁴ represents any one of a hydrogen atom, a cycloalkyl group having 3 to 12 carbon atoms, and an alkyl group having 1 to 12 carbon atoms, and R ⁵ and R ⁶ each independently represent any one of a hydrogen atom, an oxygen atom, a sulfur atom, an alkyl group having 1 to 12 carbon atoms, and an aryl group having 6 to 14 carbon atoms.

Specific examples are given below.
Examples of monomers (N-arylmaleimides, N-aromatic substituted maleimides, etc.) that form the structural unit represented by formula (1) include N-phenylmaleimide, N-benzylmaleimide, N-(2-chlorophenyl)maleimide, N-(4-chlorophenyl)maleimide, N-(4-bromophenyl)maleimide, N-(2-methylphenyl)maleimide, N-(2,6-dimethylphenyl)maleimide, N-(2-ethylphenyl)maleimide, N-(2-methoxyphenyl)maleimide, N-(2-nitrophenyl)maleimide, and the like. N-(2,4,6-trimethylphenyl)maleimide, N-(4-benzylphenyl)maleimide, N-(2,4,6-tribromophenyl)maleimide, N-naphthylmaleimide, N-anthracenylmaleimide, 3-methyl-1-phenyl-1H-pyrrole-2,5-dione, 3,4-dimethyl-1-phenyl-1H-pyrrole-2,5-dione, 1,3-diphenyl-1H-pyrrole-2,5-dione, 1,3,4-triphenyl-1H-pyrrole-2,5-dione, and the like.
Among these monomers, N-phenylmaleimide and N-benzylmaleimide are preferred because the resulting methacrylic resin has excellent heat resistance and optical properties such as birefringence.
These monomers may be used alone or in combination of two or more kinds.

Examples of monomers that form the structural unit represented by formula (2) include N-methylmaleimide, N-ethylmaleimide, N-n-propylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide, N-isobutylmaleimide, N-s-butylmaleimide, N-t-butylmaleimide, N-n-pentylmaleimide, N-n-hexylmaleimide, N-n-heptylmaleimide, and N-n-octylmaleimide. Examples of the cyclohexylmaleimide include cyclohexyl-3-methyl-1H-pyrrole-2,5-dione, 1-cyclohexyl-3,4-dimethyl-1H-pyrrole-2,5-dione, 1-cyclohexyl-3-phenyl-1H-pyrrole-2,5-dione, and 1-cyclohexyl-3,4-diphenyl-1H-pyrrole-2,5-dione.
Among these monomers, N-methylmaleimide, N-ethylmaleimide, N-isopropylmaleimide, and N-cyclohexylmaleimide are preferred because they provide methacrylic resins with excellent weather resistance, and N-cyclohexylmaleimide is particularly preferred because it has excellent low moisture absorption properties, which are required for optical materials in recent years.
These monomers may be used alone or in combination of two or more.

In the methacrylic resin of the present embodiment, it is particularly preferable to use a structural unit represented by formula (1) and a structural unit represented by formula (2) in combination, in order to develop highly controlled birefringence characteristics.
The molar ratio (X1/X2) of the content (X1) of the structural unit represented by formula (1) to the content (X2) of the structural unit represented by formula (2) is preferably more than 0 and 15 or less, more preferably more than 0 and 10 or less.
When the molar ratio (X1/X2) is within this range, the resin lens of the present embodiment maintains its transparency, is not yellowed, and does not lose its environmental resistance, and exhibits good heat resistance and good photoelastic properties.

The content of the structural unit derived from the N-substituted maleimide monomer is preferably in the range of 5 to 40% by mass, and more preferably in the range of 5 to 35% by mass, based on 100% by mass of the methacrylic resin.
Within this range, the methacrylic resin can obtain a more sufficient improvement effect in heat resistance, and can obtain more preferable improvement effects in weather resistance, low water absorption, and optical properties. Note that, keeping the content of structural units derived from N-substituted maleimide monomers to 40 mass% or less is effective in preventing a decrease in the physical properties of the methacrylic resin caused by a decrease in the reactivity of the monomer component during the polymerization reaction and an increase in the amount of unreacted remaining monomer.
Furthermore, by appropriately adjusting the content of the structural units derived from the N-substituted maleimide monomer within this range, it is possible to reduce birefringence caused by orientation during molding or residual stress, and obtain a resin lens for a head mounted display having an average absolute value of retardation within the effective diameter of 5 nm or less. The optimal content of the structural units derived from the N-substituted maleimide monomer varies depending on the type of N-substituted maleimide, but for example, when methyl methacrylate is used as the methacrylic acid ester monomer and N-phenylmaleimide and N-cyclohexylmaleimide are used as the N-substituted maleimide monomers, it is preferable to adjust the content within the ranges of 79 to 83 mass% of structural units derived from methyl methacrylate, 6 to 8 mass% of structural units derived from N-phenylmaleimide, and 11 to 13 mass% of structural units derived from N-cyclohexylmaleimide.

The methacrylic resin having a structural unit derived from an N-substituted maleimide monomer, which constitutes the resin lens for head mounted displays of the present embodiment, may contain a structural unit derived from another monomer copolymerizable with the methacrylic acid ester monomer and the N-substituted maleimide monomer, within a range not impairing the object of the present invention.
For example, the other copolymerizable monomer may include aromatic vinyls; unsaturated nitriles; acrylic esters having a cyclohexyl group, a benzyl group, or an alkyl group having 1 to 18 carbon atoms; glycidyl compounds; and unsaturated carboxylic acids.
Examples of the aromatic vinyl include styrene, α-methylstyrene, and divinylbenzene.
Examples of the unsaturated nitrile include acrylonitrile, methacrylonitrile, and ethacrylonitrile.
Examples of the acrylic ester include methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, and butyl acrylate.
The glycidyl compound includes glycidyl (meth)acrylate and the like.
The unsaturated carboxylic acids include acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, and half-esters or anhydrides thereof.
The structural unit derived from the other copolymerizable monomer may be of only one type, or may be of two or more types.

The content of the structural units derived from these other copolymerizable monomers is preferably 0 to 10 mass%, more preferably 0 to 9 mass%, and even more preferably 0 to 8 mass%, based on 100 mass% of the methacrylic resin.
It is preferable for the content of structural units derived from other monomers to be within this range, since this makes it possible to improve the molding processability and mechanical properties of the resin without impairing the inherent effect of introducing a ring structure into the main chain.

The content of structural units derived from the N-substituted maleimide monomer and the content of structural units derived from other copolymerizable monomers can be determined by ¹ H-NMR measurement and ¹³ C-NMR measurement. ¹ H-NMR measurement and ¹³ C-NMR measurement can be performed, for example, using CDCl ₃ or DMSO-d ₆ as a measurement solvent at a measurement temperature of 40° C.

-Glutarimide structural unit-
Examples of methacrylic resins having glutarimide structural units in the main chain include methacrylic resins having glutarimide structural units described in JP-A-2006-249202, JP-A-2007-009182, JP-A-2007-009191, JP-A-2011-186482, and Republished Japanese Patent Publication No. 2012/114718, and the like. The resins can be formed by the methods described in these publications.
The glutarimide structural unit constituting the methacrylic resin of the present embodiment may be formed after polymerization of the resin.
Specifically, the glutarimide structural unit may be represented by the following general formula (3).

上記一般式（３）において、好ましくはＲ^７及びＲ^８は、それぞれ独立して、水素原子又はメチル基であり、Ｒ^９は、水素原子、メチル基、ブチル基、シクロヘキシル基のいずれかであり、より好ましくは、Ｒ^７は、メチル基であり、Ｒ^８は、水素原子であり、Ｒ^９は、メチル基である。
グルタルイミド系構造単位は、単一の種類のみを含んでいてもよいし、複数の種類を含んでいてもよい。

In the above general formula (3), preferably, ^R7 and ^R8 are each independently a hydrogen atom or a methyl group, and ^R9 is any one of a hydrogen atom, a methyl group, a butyl group, and a cyclohexyl group, and more preferably, ^R7 is a methyl group, ^R8 is a hydrogen atom, and ^R9 is a methyl group.
The glutarimide-based structural unit may include only one type, or may include a plurality of types.

In the methacrylic resin having a glutarimide structural unit, the content of the glutarimide structural unit is preferably in the range of 3 to 70% by mass, and more preferably in the range of 3 to 60% by mass, based on 100% by mass of the methacrylic resin.
It is preferable that the content of the glutarimide structural unit is within the above range, since a resin having good moldability, heat resistance, and optical properties can be obtained.
Furthermore, by appropriately adjusting the content of the glutarimide structural unit within this range, it is possible to reduce birefringence caused by orientation or residual stress during molding, and obtain a resin lens for head mounted displays having an average absolute value of retardation within the effective diameter of 5 nm or less. The optimal content of the glutarimide structural unit varies depending on the type of the substituents R ⁷ to R ⁹ in general formula (3), but for example, when R ⁷ and R ⁸ are hydrogen atoms and R ⁹ is a methyl group, if the content of the glutarimide structural unit is in the range of 3 to 10 mass%, it is possible to reduce birefringence caused by orientation or residual stress during molding, and obtain a resin lens for head mounted displays having an average absolute value of retardation within the effective diameter of 5 nm or less.
The content of glutarimide structural units in the methacrylic resin can be determined by the method described in the aforementioned patent document.

The methacrylic resin having a glutarimide structural unit may further contain an aromatic vinyl monomer unit, if necessary.
The aromatic vinyl monomer is not particularly limited, but examples thereof include styrene and α-methylstyrene, with styrene being preferred.

The content of aromatic vinyl units in the methacrylic resin having glutarimide structural units is not particularly limited, but is preferably 0 to 20% by mass, with the methacrylic resin having glutarimide structural units being 100% by mass.
When the content of the aromatic vinyl unit is within the above range, it is possible to achieve both heat resistance and excellent photoelasticity, which is preferable.
For example, in the case of obtaining a resin by glutarimidating a methyl methacrylate-styrene copolymer obtained by copolymerizing methyl methacrylate as the methacrylic acid ester monomer and styrene as the aromatic vinyl monomer, by adjusting the ranges of 25 to 90 mass % of structural units derived from methyl methacrylate, 5 to 15 mass % of structural units derived from styrene, and 5 to 70 mass % of glutarimide-based structural units, it is possible to reduce birefringence caused by orientation and residual stress during molding and to obtain a resin lens for a head mounted display having an average absolute value of retardation within the effective diameter of 5 nm or less.

-Lactone ring structural unit-
Methacrylic resins having a lactone ring structural unit in the main chain can be formed by the methods described in, for example, JP-A-2001-151814, JP-A-2004-168882, JP-A-2005-146084, JP-A-2006-96960, JP-A-2006-171464, JP-A-2007-63541, JP-A-2007-297620, JP-A-2010-180305, etc.

The lactone ring structural unit constituting the methacrylic resin of the present embodiment may be formed after polymerization of the resin.
In the present embodiment, the lactone ring structural unit is preferably a six-membered ring because this provides excellent stability of the ring structure.
As the 6-membered lactone ring structural unit, for example, a structure represented by the following general formula (4) is particularly preferred.

上記一般式（４）において、Ｒ^１０、Ｒ^１１及びＲ^１２は、互いに独立して、水素原子、又は炭素数１～２０の有機残基である。
有機残基としては、例えば、メチル基、エチル基、プロピル基等の炭素数１～２０の飽和脂肪族炭化水素基（アルキル基等）；エテニル基、プロペニル基等の炭素数２～２０の不飽和脂肪族炭化水素基（アルケニル基等）；フェニル基、ナフチル基等の炭素数６～２０の芳香族炭化水素基（アリール基等）；これら飽和脂肪族炭化水素基、不飽和脂肪族炭化水素基、芳香族炭化水素基における水素原子の一つ以上が、ヒドロキシ基、カルボキシル基、エーテル基、エステル基からなる群から選ばれる少なくとも１種の基により置換された基；等が挙げられる。

In the above general formula (4), R ¹⁰ , R ¹¹ and R ¹² are each independently a hydrogen atom or an organic residue having 1 to 20 carbon atoms.
Examples of the organic residue include saturated aliphatic hydrocarbon groups (e.g., alkyl groups) having 1 to 20 carbon atoms, such as a methyl group, an ethyl group, a propyl group, etc.; unsaturated aliphatic hydrocarbon groups (e.g., alkenyl groups) having 2 to 20 carbon atoms, such as an ethenyl group, a propenyl group, etc.; aromatic hydrocarbon groups (e.g., aryl groups) having 6 to 20 carbon atoms, such as a phenyl group, a naphthyl group, etc.; and groups in which one or more hydrogen atoms in these saturated aliphatic hydrocarbon groups, unsaturated aliphatic hydrocarbon groups, and aromatic hydrocarbon groups have been substituted with at least one group selected from the group consisting of a hydroxy group, a carboxyl group, an ether group, and an ester group.

The lactone ring structural unit can be formed, for example, by copolymerizing an acrylic acid monomer having a hydroxy group with a methacrylic acid ester monomer such as methyl methacrylate to introduce a hydroxy group and an ester group or a carboxyl group into the molecular chain, and then causing dealcoholization (esterification) or dehydration condensation (hereinafter also referred to as "cyclization condensation reaction") between the hydroxy group and the ester group or the carboxyl group.

Examples of acrylic acid monomers having a hydroxy group used in the polymerization include 2-(hydroxymethyl)acrylic acid, 2-(hydroxyethyl)acrylic acid, 2-(hydroxymethyl)alkyl acrylates (e.g., methyl 2-(hydroxymethyl)acrylate, ethyl 2-(hydroxymethyl)acrylate, isopropyl 2-(hydroxymethyl)acrylate, n-butyl 2-(hydroxymethyl)acrylate, t-butyl 2-(hydroxymethyl)acrylate), and 2-(hydroxyethyl)alkyl acrylates. Preferred are 2-(hydroxymethyl)acrylic acid and 2-(hydroxymethyl)alkyl acrylates, which are monomers having a hydroxyalkyl moiety, and particularly preferred are methyl 2-(hydroxymethyl)acrylate and ethyl 2-(hydroxymethyl)acrylate.

The content of the lactone ring structural unit in the methacrylic resin having the lactone ring structural unit in the main chain is preferably 5 to 40% by mass, and more preferably 5 to 35% by mass, based on 100% by mass of the methacrylic resin.
When the content of the lactone ring structural unit is within this range, the effects of introducing a ring structure, such as improved solvent resistance and improved surface hardness, can be achieved while maintaining moldability. In addition, by appropriately adjusting the content of the lactone ring structural unit within this range, it is possible to reduce birefringence caused by orientation and residual stress during molding, and to obtain a resin lens for a head mounted display having an average absolute value of retardation within the effective diameter of 5 nm or less.
The content of the lactone ring structure in the methacrylic resin can be determined by the method described in the aforementioned patent document.

The methacrylic resin having a lactone ring structural unit in the main chain may have a structural unit derived from another monomer copolymerizable with the above-mentioned methacrylic acid ester monomer and the acrylic acid monomer having a hydroxy group.
Examples of such other copolymerizable monomers include monomers having a polymerizable double bond, such as styrene, vinyl toluene, α-methyl styrene, α-hydroxymethyl styrene, α-hydroxyethyl styrene, acrylonitrile, methacrylonitrile, methallyl alcohol, ethylene, propylene, 4-methyl-1-pentene, vinyl acetate, 2-hydroxymethyl-1-butene, methyl vinyl ketone, N-vinylpyrrolidone, and N-vinyl carbazole.
The copolymer may contain only one type of these other monomers (structural units), or may contain two or more types.

The content of the structural units derived from these other copolymerizable monomers is preferably 0 to 20% by mass, relative to 100% by mass of the methacrylic resin, and from the viewpoint of weather resistance, it is more preferably less than 10% by mass, and even more preferably less than 7% by mass.
The methacrylic resin in the present embodiment may have only one type of structural unit derived from the other copolymerizable monomer, or may have two or more types.

In this embodiment, the methacrylic resin preferably has at least one structural unit selected from the group consisting of structural units derived from N-substituted maleimide monomers, glutarimide structural units, and lactone ring structural units. Among these, it is particularly preferable that the methacrylic resin has structural units derived from N-substituted maleimide monomers, since it is easy to highly control optical properties such as the photoelastic coefficient without blending with other thermoplastic resins. Furthermore, it is particularly preferable that the methacrylic resin has glutarimide structural units, particularly from the viewpoint of obtaining a resin lens with high strength.

[Methacrylic resin manufacturing method]
Hereinafter, the method for producing the methacrylic resin of this embodiment will be described.

-Method for producing methacrylic resin containing structural units derived from N-substituted maleimide monomer-
Examples of a production method for a methacrylic resin having a structural unit derived from an N-substituted maleimide monomer in the main chain (hereinafter, may be referred to as a "maleimide copolymer") include any of bulk polymerization, solution polymerization, suspension polymerization, precipitation polymerization, and emulsion polymerization. From the viewpoint of reducing the amount of residual monomer and impurities contained in the resin lens, preferred are suspension polymerization, bulk polymerization, and solution polymerization, and more preferred is solution polymerization.

In the production method of the present embodiment, any of a batch polymerization method, a semi-batch polymerization method, and a continuous polymerization method can be used as the polymerization method.
In this embodiment, a method in which a part of the monomer is charged into a reactor before the start of polymerization, a polymerization initiator is added to start the polymerization, and then the remainder of the monomer is supplied, that is, a so-called semi-batch polymerization method, is used, from the viewpoint of reducing the amount of maleimide remaining at the end of polymerization and reducing the fluorescence intensity (reducing the content of the fluorescent substance).

In a methacrylic resin having structural units derived from an N-substituted maleimide monomer, methods for controlling the resulting resin lens to exhibit a predetermined fluorescence intensity (content of fluorescent substance within a predetermined range) include at least one selected from (1) using an N-substituted maleimide monomer with a controlled content of specific impurities as a raw material, (2) applying a polymerization method that reduces the amount of unreacted N-substituted maleimide remaining after completion of polymerization, and (3) applying a devolatilization method that reduces shear, and among these, it is preferable to select a manufacturing method that combines (1) and (3), or a manufacturing method that combines all three.

(1) Control of impurities in N-substituted maleimide One of the impurities in N-substituted maleimide that gives fluorescence to methacrylic resin is 2-amino-N-substituted succinimide produced by reaction of N-substituted maleimide with primary amine. Examples of 2-amino-N-substituted succinimide include 2-cyclohexylamino-N-cyclohexylsuccinimide and 2-anilino-N-phenylsuccinimide. Not only does 2-amino-N-substituted succinimide itself have fluorescence, but the inventors' studies have revealed that when 2-amino-N-substituted succinimide is heated to 300° C. or higher or subjected to a volatilizing device having shear, a thermally denatured product having fluorescence is produced, although the detailed modification mechanism is unknown. That is, in order to control the fluorescence intensity of the methacrylic resin and the resulting resin lens, it is preferable to reduce the amount of 2-amino-N-substituted succinimide contained in the N-substituted maleimide and to use a volatilizing device without a rotating part in the production of the methacrylic resin and perform volatilizing at 300° C. or lower.

As a method for controlling impurities in the N-substituted maleimide, a pretreatment step of washing the N-substituted maleimide with water (water washing step) and/or dehydrating the N-substituted maleimide with water (dehydration step) can be provided.
The pretreatment step may be only washing with water, or a combination of washing with water and dehydration. Moreover, washing with water and dehydration may each be performed once or multiple times. The pretreatment step may further include a concentration adjustment step of adjusting the concentration of the N-substituted maleimide solution obtained in the dehydration step.

In the present embodiment, for example, 2-amino-N-substituted succinimide in the N-substituted maleimide is removed by a water washing step described below, and then water is removed in a dehydration step, whereby an N-substituted maleimide solution suitable for preparing a methacrylic resin having controlled fluorescence intensity and good color tone can be obtained.
In the water-washing step, for example, a method can be employed in which the N-substituted maleimide is dissolved in a water-insoluble organic solvent, separated into an organic layer and an aqueous layer, the organic layer is mixed and washed with one or more liquids selected from an acidic aqueous solution, water and an alkaline aqueous solution by a batch system, a continuous system, or both of these systems, and then the organic layer and the aqueous layer are separated.

The amount of 2-amino-N-substituted succinimide in the organic layer after the water washing step is preferably 5 ppm by mass or less, more preferably 0.1 ppm by mass or more and 1 ppm by mass or less, when the amount of N-substituted maleimide in the organic layer is taken as 100% by mass. When the amount of 2-amino-N-substituted succinimide in the organic layer is within this range, the concentrations of the fluorescent substance in the methacrylic resin and the obtained resin lens can be controlled within the above-mentioned range, and a resin lens for a head mounted display that can provide a clear image can be obtained, which is preferable. It is also preferable in terms of the cost of washing. When the dehydration step is not provided, the organic layer containing the N-substituted maleimide obtained in the water washing step may be used in the polymerization step as an N-substituted maleimide solution.
The amount of 2-amino-N-substituted succinimide in the organic layer can be measured, for example, by gas chromatography, liquid chromatography, or the like using isopropyl benzoate as an internal standard substance. Specifically, it can be measured by the method described in the Examples below.

The water-insoluble organic solvent to be used is not particularly limited as long as it dissolves the N-substituted maleimide and the 2-amino-N-substituted succinimide, undergoes phase separation from water, and has an azeotropic point with water.Specific examples of the water-insoluble organic solvent that can be used include aromatic hydrocarbons such as toluene and xylene, aliphatic hydrocarbons such as normal hexane and cyclohexane, halogenated hydrocarbons such as chloroform and dichloroethane, and the like.
The water used may be sewage water, pure water, or clean water.
The acidity of the acidic aqueous solution or the alkaline aqueous solution is not particularly limited.

The concentration of the N-substituted maleimide in the organic layer before the water washing step is preferably from 0.5 to 30% by mass, more preferably from 10 to 25% by mass, and further preferably from 20 to 25% by mass.
The temperature at which the organic layer and the aqueous layer are mixed and washed may be 40° C. or higher, preferably 40° C. or higher and 80° C. or lower, and more preferably 50° C. or higher and 60° C. or lower.
The mass ratio of the aqueous layer to the organic layer is preferably 5% by mass or more and 300% by mass or less, more preferably 10% by mass or more and 200% by mass or less, and even more preferably 30% by mass or more and 100% by mass or less, when the mass of the organic layer is 100% by mass.
It is preferable that the concentration of the N-substituted maleimide in the organic layer, the liquid temperature during washing, and the mass ratio of the aqueous layer are within these ranges, since the reaction between the N-substituted maleimide and water does not easily proceed and the 2-amino-N-substituted succinimide is easily extracted into the aqueous layer.

When washing is performed in a batchwise manner, the reaction vessel used may be made of stainless steel or glass-lined, or other reaction vessels may be used. The shape of the stirring blade is not particularly limited. As specific stirring blades, for example, three-blade swept-back blades, four-blade paddle blades, four-blade inclined paddle blades, six-blade turbine blades, and anchor blades can be used, and Twin Star and Full Zone manufactured by Kobe Steel Eco-Solutions can be used.
The stirring time is preferably from 10 to 120 minutes, more preferably from 30 to 60 minutes. The stirring speed is appropriately selected so that the mixture is in a turbulent state without emulsifying. When the stirring time and stirring speed are within these ranges, the stirring efficiency and the extraction efficiency of 2-amino-N-substituted succinimide into the aqueous layer are improved, and the 2-amino-N-substituted succinimide in the N-substituted maleimide can be further reduced, which is preferable.

When washing is carried out in a continuous manner, an empty tower, a packed tower, or a tray tower can be used, and a stationary mixer such as a static mixer or a rotary mixer such as a dynamic mixer can also be used.
The contact time between the organic layer and the aqueous layer is preferably set to 1 second to 60 minutes, more preferably 30 seconds to 10 minutes, since a contact time in this range makes it difficult for the reaction between the N-substituted maleimide and water to proceed and facilitates extraction of the 2-amino-N-substituted succinimide into the aqueous layer.

In the dehydration step, the organic layer is placed in a reaction vessel and heated under reduced pressure to remove water. There are no particular limitations on the pressure and temperature as long as the solvent and water used form an azeotropic composition.
The water content in the organic layer after the dehydration step is preferably 100 ppm by mass or less, assuming that the mass of the organic layer is 100% by mass. If the water content after the dehydration step is in this range, deterioration of color tone due to water during polymerization of the methacrylic resin can be suppressed, and the amount of solvent distilled off together with water can be reduced, which is also preferable from the viewpoint of cost.
The organic layer containing the N-substituted maleimide obtained in the dehydration step may be used in the polymerization step as an N-substituted maleimide solution, or an N-substituted maleimide solution whose concentration has been adjusted in the concentration adjustment step may be used in the polymerization step.

In the concentration adjustment step, for example, the organic layer of the N-substituted maleimide obtained in the dehydration step or the like may be diluted with the water-insoluble organic solvent. It is preferable that the water-insoluble organic solvent used in the concentration adjustment step is the same as the water-insoluble organic solvent used in the water washing step.

The N-substituted maleimide solution obtained in the pretreatment step is preferably a solution in the non-water-soluble organic solvent (organic layer).
The N-substituted maleimide solution obtained in the pretreatment step preferably has a mass ratio of 2-amino-N-substituted succinimide contained in the N-substituted maleimide solution of 5 ppm by mass or less, more preferably 0.1 ppm by mass or more and 1 ppm by mass or less, relative to 100% by mass of N-substituted maleimide contained in the N-substituted maleimide solution.
The water content in the N-substituted maleimide solution obtained in the above pretreatment step is preferably 200 ppm by mass or less, and more preferably 100 to 200 ppm by mass, based on 100% by mass of the N-substituted maleimide solution.
The mass ratio of the N-substituted maleimide contained in the N-substituted maleimide solution obtained in the above pretreatment step is preferably 5 to 30 mass %, more preferably 5 to 25 mass %, relative to 100 mass % of the N-substituted maleimide solution. When it is within this range, the N-substituted maleimide is less likely to precipitate and can be transported as a homogeneous solution, which is preferable.

When a plurality of types of N-substituted maleimide solutions are used in the polymerization step, it is preferable that a mixture of the plurality of types of N-substituted maleimide solutions satisfies the mass ratio of the 2-amino-N-substituted succinimide, the water content, and/or the mass ratio of the N-substituted maleimide, and it is more preferable that each N-substituted maleimide solution satisfies the mass ratio of the 2-amino-N-substituted succinimide, the water content, and/or the mass ratio of the N-substituted maleimide.
By employing the above-mentioned N-substituted maleimide washing step and dehydration step, it is possible to reduce the amount of fluorescent 2-amino-N-substituted succinimide and to remove moisture that causes deterioration of the color tone of the methacrylic resin, and it is therefore preferable since it is possible to obtain a methacrylic resin or a composition of said resin having good color tone even in lenses with long optical paths.
In the polymerization step, the N-substituted maleimide solution obtained in the pretreatment step may be mixed with a methacrylic acid ester monomer, any other monomer, a polymerization initiator, a polymerization solvent, a chain transfer agent, etc. to prepare a monomer mixture, which may then be used in the polymerization.

(2) Reduction of Unreacted N-Substituted Maleimide at the End of Polymerization The present inventors have found that if unreacted N-substituted maleimide is present at the end of polymerization of a methacrylic resin, a low-molecular-weight fluorescent reaction by-product containing the N-substituted maleimide as a structural unit may be generated in a heated volatilizer or the like, although the detailed mechanism is unclear.

In order to control the content of the fluorescent substance in the methacrylic resin and the resulting resin lens within the above-mentioned range, the total mass of unreacted N-substituted maleimide remaining after completion of polymerization is preferably 1,000 ppm by mass or less, and more preferably 10 ppm by mass or more and 500 ppm by mass or less, relative to 100% by mass of the polymerization solution at the time of completion of polymerization.
Furthermore, when an N-arylmaleimide such as N-phenylmaleimide is used as the N-substituted maleimide, the total mass of unreacted N-arylmaleimides remaining after completion of polymerization is preferably 500 ppm by mass or less, more preferably 10 ppm by mass or more and 500 ppm by mass or less, and even more preferably 10 ppm by mass or more and 50 ppm by mass or less, relative to 100% by mass of the polymerization solution at the time of completion of polymerization.
When the content is within these ranges, the content of the fluorescent substance in the methacrylic resin and the resulting resin lens can be suppressed within the above-mentioned ranges, which is preferable. Furthermore, in order to make the amount of unreacted N-substituted maleimide less than 10 ppm by mass, it is necessary to increase the polymerization temperature or the amount of polymerization initiator, which is undesirable because it increases the amount of thermally denatured maleimide products and active radicals, causing deterioration in the color tone of the methacrylic resin.

As a means for controlling the amount of unreacted N-substituted maleimide after the polymerization is completed within the above range, a semi-batch polymerization method can be mentioned. In the semi-batch polymerization method, in the polymerization step, it is preferable to add 5 to 35% by mass of methacrylic acid ester monomer, assuming that the total mass of all monomers provided to the polymerization (e.g., methacrylic acid ester, N-substituted maleimide, and any other monomers) is 100% by mass, after 30 minutes from the start of addition of the polymerization initiator. In other words, it is preferable to charge 65 to 95% by mass of the total mass of all monomers provided to the polymerization (100% by mass) into the reactor before adding the polymerization initiator, and to add the remaining 5 to 35% by mass of the methacrylic acid ester monomer after 30 minutes from the start of addition of the polymerization initiator. The amount of methacrylic acid ester monomer to be added is more preferably 10 to 30% by mass, assuming that the total mass of all monomers provided to the polymerization is 100% by mass. If the amount of methacrylic acid ester monomer added is within the above range, the unreacted N-substituted maleimide reacts with the additionally added methacrylic acid ester monomer, and the amount of unreacted N-substituted maleimide after polymerization is completed can be controlled within the above range, which is preferable.

The start time of additional addition of the monomer, the speed of additional addition, etc. may be appropriately selected depending on the polymerization conversion rate. Furthermore, in addition to the methacrylic acid ester monomer, a monomer mixture containing an N-substituted maleimide monomer and other monomers may be additionally added within a range that does not impair the effects of the present invention or the reduction in the amount of unreacted N-substituted maleimide.
By employing the semi-batch polymerization method as described above, it is possible to reduce the amount of unreacted N-substituted maleimide monomer at the latter stage of polymerization and minimize the generation of fluorescent substances in the devolatilization step, and it is preferable because it is possible to obtain a methacrylic resin and a composition of said resin having good color tone even in lenses with long optical paths.

Below, we will specifically explain an example of a method for producing a methacrylic resin having structural units derived from an N-substituted maleimide monomer, using a solution polymerization method in a semi-batch radical polymerization system.

In the semi-batch polymerization method, it is preferable to additionally add 5 to 35% by mass of the methacrylic acid ester monomer, based on the total mass of all monomers (methacrylic acid ester, N-substituted maleimide, and any other monomers) to be supplied to the polymerization, 30 minutes or more after the start of the addition of the polymerization initiator. In other words, it is preferable to charge 65 to 95% by mass of the total mass of all monomers to be supplied to the polymerization, which is 100% by mass, into the reactor before the start of the polymerization, and to additionally add the remaining 5 to 35% by mass of the methacrylic acid ester monomer, 30 minutes or more after the start of the addition of the polymerization initiator.
The amount of the methacrylic acid ester monomer to be additionally added is more preferably 10 to 30% by mass, with the total mass of all the monomers to be supplied to the polymerization being 100% by mass.

The start time of additional addition of the monomer, the speed of additional addition, etc. may be appropriately selected depending on the polymerization conversion rate.
Furthermore, in addition to the methacrylic acid ester monomer, a monomer mixture containing the N-substituted maleimide monomer and other monomers may be additionally added within a range that does not impair the effects of the present invention or the increase in the conversion rate of the N-substituted maleimide monomer.

By adopting the semi-batch polymerization method described above, it is possible to increase the conversion rate of the N-substituted maleimide monomer at the latter stage of polymerization, reduce the content of fluorescent substances, provide excellent light transmittance for lenses with long optical path lengths, and make it easier to control the molecular weight distribution of the resulting polymer. This is particularly preferable because it makes it possible to obtain resins and compositions of said resins that have flow properties suitable for injection molding.

The polymerization solvent to be used is not particularly limited as long as it can increase the solubility of the maleimide copolymer obtained by polymerization and can appropriately maintain the viscosity of the reaction solution for the purpose of preventing gelation, etc.
Specific examples of the polymerization solvent that can be used include aromatic hydrocarbons such as toluene, xylene, ethylbenzene, and isopropylbenzene; ketones such as methyl isobutyl ketone, butyl cellosolve, methyl ethyl ketone, and cyclohexanone; and polar solvents such as dimethylformamide and 2-methylpyrrolidone.
These may be used alone or in combination of two or more.
Furthermore, alcohols such as methanol, ethanol, and isopropanol may be used in combination as a polymerization solvent within a range that does not inhibit dissolution of the polymerization product during polymerization.

The amount of the solvent during polymerization is not particularly limited as long as the polymerization proceeds and the amount can be easily removed without causing precipitation of the copolymer or monomers used during production, but for example, when the total amount of the monomers to be blended is taken as 100 mass%, the amount is preferably 10 to 200 mass%, more preferably 25 to 150 mass%, even more preferably 40 to 100 mass%, and even more preferably 50 to 100 mass%.
In the present embodiment, a method of polymerization in which the amount of solvent during polymerization is within a range of 100 mass % or less when the total amount of the monomers to be blended is taken as 100 mass % can also be preferably used, in which the solvent concentration is appropriately changed during polymerization.
More specifically, an example of the method is to blend 40 to 60% by mass in the initial stage of polymerization, blend the remaining 60 to 40% by mass during the polymerization, and finally blend the amount of the solvent in a range of 100% by mass or less when the total amount of the blended monomers is 100% by mass.
By employing this method, it is possible to increase the polymerization conversion rate, and further to control the molecular weight distribution, and it is possible to obtain a resin and a resin composition that have excellent injection moldability, a reduced content of fluorescent substances, and good color tone even when a lens with a long optical path length is prepared. This is therefore preferable.

In solution polymerization, it is important to reduce the dissolved oxygen concentration in the polymerization solution as much as possible; for example, the dissolved oxygen concentration is preferably 10 ppm or less. The dissolved oxygen concentration can be measured, for example, using a dissolved oxygen meter DO meter B-505 (manufactured by Iijima Electronics Co., Ltd.). Methods for reducing the dissolved oxygen concentration can be appropriately selected from a method of bubbling an inert gas into the polymerization solution, a method of repeatedly pressurizing a container containing the polymerization solution to about 0.2 MPa with an inert gas before polymerization and then releasing the pressure, a method of passing an inert gas through a container containing the polymerization solution, and the like.

There are no particular limitations on the polymerization temperature as long as it is a temperature at which the polymerization proceeds, but it is preferably 70 to 180°C, more preferably 80 to 160°C, even more preferably 90 to 150°C, and even more preferably 100 to 150°C. From the viewpoint of productivity, it is preferable to set the temperature at 70°C or higher, and it is preferable to set the temperature at 180°C or lower in order to suppress side reactions during polymerization and obtain a polymer with the desired molecular weight and quality.

The polymerization time is not particularly limited as long as it is a time that can obtain the required degree of polymerization at the required conversion rate, but from the viewpoint of productivity, etc., it is preferably 2 to 15 hours, more preferably 3 to 12 hours, and even more preferably 4 to 10 hours.

As the polymerization initiator, any initiator generally used in radical polymerization can be used. Examples of the polymerization initiator include organic peroxides such as cumene hydroperoxide, diisopropylbenzene hydroperoxide, di-t-butyl peroxide, lauroyl peroxide, benzoyl peroxide, t-butylperoxyisopropyl carbonate, t-amylperoxy-2-ethylhexanoate, t-amylperoxyisononanoate, and 1,1-di(t-butylperoxy)cyclohexane; and azo compounds such as 2,2'-azobis(isobutyronitrile), 1,1'-azobis(cyclohexanecarbonitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and dimethyl-2,2'-azobisisobutyrate.
These may be used alone or in combination of two or more.
The amount of the polymerization initiator added may be 0.01 to 1% by mass, and preferably 0.05 to 0.5% by mass, when the total amount of the monomers used in the polymerization is taken as 100% by mass.
The method of adding the polymerization initiator is not particularly limited as long as it is not a constant addition rate but a variable addition rate according to the monomer concentration remaining in the polymerization solution, and may be added continuously or intermittently. When the polymerization initiator is added intermittently, the amount added per unit time is not considered for the time when the polymerization initiator is not added.

In this embodiment, it is preferable to appropriately select the type and amount of the polymerization initiator, the polymerization temperature, and the like so that the ratio of the total amount of radicals generated from the polymerization initiator to the total amount of unreacted monomers remaining in the reaction system is always equal to or less than a certain value.
By employing these methods, it is possible to suppress the production of oligomers and low molecular weight materials in the later stages of polymerization, and to prevent overheating during polymerization, thereby ensuring the stability of the polymerization.

During the polymerization reaction, a chain transfer agent may be added as necessary.
As the chain transfer agent, any chain transfer agent used in general radical polymerization can be used, and examples thereof include mercaptan compounds such as n-butyl mercaptan, n-octyl mercaptan, n-decyl mercaptan, n-dodecyl mercaptan, and 2-ethylhexyl thioglycolate; halogen compounds such as carbon tetrachloride, methylene chloride, and bromoform; and unsaturated hydrocarbon compounds such as α-methylstyrene dimer, α-terpinene, dipentene, and terpinolene.
These may be used alone or in combination of two or more.
These chain transfer agents may be added at any stage as long as the polymerization reaction is in progress, and there are no particular limitations on the addition stage.
The amount of the chain transfer agent added may be 0.01 to 1% by mass, and preferably 0.05 to 0.5% by mass, based on 100% by mass of the total amount of monomers used in the polymerization.

As a method for recovering a polymer from a polymerization liquid obtained by solution polymerization, there can be mentioned a method in which the polymerization solvent and unreacted monomers are separated through a step called a devolatilization step, and the polymerization product is recovered. Here, the devolatilization step refers to a step in which volatile matters such as the polymerization solvent, residual monomers, and reaction by-products are removed under heating and reduced pressure conditions.
In this embodiment, it is preferable to control the content of unreacted N-substituted maleimide monomer contained in the polymerization solution containing the methacrylic resin to be subjected to the devolatilization step to a certain concentration or less. Details are as described above in "(2) Reduction of Unreacted N-Substituted Maleimide at the End of Polymerization". In addition to the above-mentioned methods, for example, in order to increase the monomer conversion rate, a method of carrying out polymerization by lengthening the polymerization time as much as possible or changing the rate of addition of the polymerization initiator in accordance with the concentration of the unreacted monomer in the polymerization solution; a method of carrying out polymerization while appropriately changing the solvent concentration during polymerization; a method of additionally adding another monomer that is highly reactive with the N-substituted maleimide monomer remaining in the latter half of polymerization; a method of adding a compound that is highly reactive with N-substituted maleimide, such as α-terpinene, at the end of polymerization, and the like can also be used.

--Remaining amount of N-substituted maleimide monomer--
The amount of N-substituted maleimide monomer remaining in a polymerization solution containing a methacrylic resin can be determined, for example, by collecting and weighing a portion of the polymerization solution, dissolving this sample in chloroform to prepare a 5% by mass solution, adding n-decane as an internal standard substance, and measuring the concentration of the N-substituted maleimide monomer remaining in the sample using gas chromatography (GC-2010 manufactured by Shimadzu Corporation). More specific measurement conditions can be those described in the Examples below.

(3) Devolatilization method with reduced shearing It is preferable to use a devolatilization device mainly composed of a heat exchanger and a reduced pressure vessel and having no rotating part as a structure for the devolatilization step. By not providing a rotating part, it is possible to reduce shearing during devolatilization, and a resin with even lower fluorescent emission can be obtained.
Specifically, a volatilizing apparatus can be used which is composed of a volatilizing tank having a reduced pressure vessel with a heat exchanger disposed on the top thereof and a size large enough for volatilization, to which a reduced pressure unit is attached, and a discharge device such as a gear pump for discharging the polymer after volatilization.
The volatilizer preheats the polymerization solution by feeding it to a heated heat exchanger, such as a multi-tube heat exchanger, a plate fin heat exchanger, or a flat plate heat exchanger having a flat plate flow path and a heater, which is disposed at the top of the reduced pressure vessel, and then supplies it to a volatilizer tank under heating and reduced pressure to separate and remove the polymerization solvent, unreacted raw material mixture, polymerization by-products, and the like, and the polymer. By using a volatilizer without a rotating section as described above, it is possible to suppress low molecular weight, fluorescent reaction by-products derived from unreacted N-substituted maleimide, and it is preferable because the fluorescence intensity (content of fluorescent substance) in the methacrylic resin and the obtained resin lens can be controlled within a predetermined range. It is also preferable because it is possible to obtain a methacrylic resin having a good color tone.

Examples of devices with a rotating part include thin-film evaporators such as Wiplen and Exeva manufactured by Kobelco Eco-Solutions Co., Ltd., and Contra and tilted blade Contra manufactured by Hitachi, Ltd.; vented extruders, etc.

In the devolatilization step in this embodiment, the shear rate applied to the polymerization solution is preferably 20 s ^-1 or less, more preferably 10 s ^-1 or less, and even more preferably 0.1 s ^-1 or more and 10 s ^-1 or less. By setting the shear rate to 0.1 s ^-1 or more, the flow of the molten resin does not become too slow, and deterioration of color tone due to an increase in residence time can be suppressed. In addition, by setting the shear rate to 20 s ^-1 or less, the generation of reaction by-products having fluorescent properties due to shear can be suppressed.
Here, for example, the shear rate γ in the extruder is calculated by the following formula.
γ = (π × D × N) / H
(In the formula, D represents the screw diameter (m), N represents the screw rotation speed per second, and H represents the screw groove depth (m).)
In the case of a flat plate type flow channel, the shear rate γ is calculated by the following formula.
γ=(6×Q)/(w×h ² )
(In the formula, Q represents the volumetric flow rate (m ³ /s) passing through the flat channel, w represents the width (m) of the flat channel, and h represents the distance (m) between the plates.)

In this embodiment, it is preferable to use a flat-plate heat exchanger having a flat-plate flow path and a heater as the heat exchanger to be placed at the top of the reduced pressure vessel. More preferably, it is a flat-plate heat exchanger having a heater and a flat-plate flow path having a laminated structure with multiple slit-shaped flow paths with rectangular cross sections on the same plane.

The polymerization solution fed to the volatilizer is sent from the center of the heat exchanger to the slit-shaped flow passage and heated. The heated polymerization solution is fed from the slit-shaped flow passage into a reduced pressure vessel integrated with the heat exchanger under reduced pressure and flash evaporated.
Such a devolatilization method is also called flash devolatilization, and in the present invention, it will hereinafter also be referred to as flash devolatilization.

It is also possible to install two or more of the above-mentioned devolatilizers in series to perform devolatilization in two or more stages.

The temperature range for heating in the heat exchanger attached to the volatilization device may be 100°C or higher and 300°C or lower, preferably a temperature of the glass transition temperature (Tg) of the methacrylic resin + 100°C to Tg + 160°C, and more preferably a temperature of Tg + 110°C to Tg + 150°C. The temperature range for the heated and kept warm volatilization tank may be 100°C or higher and 300°C or lower, preferably a temperature of Tg + 100°C to Tg + 160°C, and more preferably a temperature of Tg + 110°C to Tg + 150°C. If the temperature of the heat exchanger and volatilization tank is within this range, it is preferable because it is possible to suppress the thermal denaturation of the remaining 2-amino-N-substituted succinimide and suppress the generation of fluorescent substances. It is also preferable because it is effective in preventing the remaining volatile content from increasing, and the thermal stability and product quality of the resulting methacrylic resin are improved.

The degree of vacuum in the devolatilizer tank may be in the range of 5 to 300 Torr, with a range of 10 to 200 Torr being preferred. If the degree of vacuum is 300 Torr or less, unreacted monomer or a mixture of unreacted monomer and polymerization solvent can be efficiently separated and removed, and the thermal stability and quality of the resulting thermoplastic copolymer will not decrease. If the degree of vacuum is 5 Torr or more, industrial implementation is easier.

The average residence time in the devolatilizer tank is 5 to 60 minutes, preferably 5 to 45 minutes. If the average residence time is within this range, devolatilization can be performed efficiently and coloration and decomposition due to thermal denaturation of the polymer can be suppressed, which is preferable.

The polymer recovered through the devolatilization step is processed into pellets in a step called a granulation step.
In the granulation process, the molten resin is extruded in a strand shape by at least one discharge granulation device selected from a gear pump having a multi-hole die as an auxiliary equipment, a single screw extruder, a twin screw extruder, etc., and processed into pellets by a cold cut method, an air hot cut method, an underwater strand cut method, or an underwater cut method. From the viewpoint of suppressing the generation of fluorescent reaction by-products due to shear, it is preferable to select a conveying device with a low shear rate without using an extruder.

In this embodiment, in order to obtain a highly controlled resin composition, it is preferable to adopt a granulation method that can quickly cool and solidify the resin composition in a molten state at high temperature while minimizing contact with air as much as possible.
In this case, it is more preferable to carry out granulation under conditions in which the molten resin temperature is as low as possible, the residence time from the multi-hole die outlet to the cooling water surface is as short as possible, and the cooling water temperature is as high as possible.
For example, the molten resin temperature is preferably 220 to 280°C, and more preferably 230 to 270°C, the residence time from the multi-hole die outlet to the cooling water surface is preferably 5 seconds or less, and more preferably 3 seconds or less, and the cooling water temperature is preferably 30 to 80°C, and more preferably in the range of 40 to 60°C.

By carrying out the process within these ranges of molten resin temperature and cooling water temperature, it is preferable to obtain methacrylic resins and compositions thereof that have less coloration and a lower moisture content.

The smaller the content of monomers remaining in the methacrylic resin after the devolatilization step, the better from the viewpoints of thermal stability and product quality. Specifically, the content of methacrylic acid ester monomers is preferably 3000 ppm by mass or less, more preferably 2000 ppm by mass or less. The total content of N-substituted maleimide monomers is preferably 200 ppm by mass or less, more preferably 100 ppm by mass or less.
The content of the residual polymerization solvent is preferably 500 ppm by mass or less, and more preferably 300 ppm by mass or less.

-Method for producing methacrylic resin containing glutarimide structural units-
Examples of a method for producing a methacrylic resin having a glutarimide structural unit in the main chain include a bulk polymerization method, a solution polymerization method, a suspension polymerization method, a precipitation polymerization method, and an emulsion polymerization method. Of these, the suspension polymerization method, the bulk polymerization method, and the solution polymerization method are preferred, and the solution polymerization method is more preferred.
In the production method of the present embodiment, any of a batch polymerization method, a semi-batch polymerization method, and a continuous polymerization method can be used as the polymerization method.
In the production method of the present embodiment, it is preferable to polymerize the monomers by radical polymerization.

The methacrylic resin having a glutarimide structural unit in the main chain is, for example, a methacrylic resin having a glutarimide structural unit, as described in JP 2006-249202 A, JP 2007-009182 A, JP 2007-009191 A, JP 2011-186482 A, WO 2012/114718 A, etc., and can be formed by the method described in the publication.
Hereinafter, as an example of a method for producing a methacrylic resin having a glutarimide structural unit, a batch-type radical polymerization method using a solution polymerization method will be specifically described.

First, a (meth)acrylic acid ester polymer is produced by polymerizing a (meth)acrylic acid ester such as methyl methacrylate. When an aromatic vinyl unit is to be included in a methacrylic resin having a glutarimide structural unit, a (meth)acrylic acid ester and an aromatic vinyl (e.g., styrene) are copolymerized to produce a (meth)acrylic acid ester-aromatic vinyl copolymer.

Examples of the solvent used in the polymerization include aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; and ketones such as methyl ethyl ketone and methyl isobutyl ketone.
These solvents may be used alone or in combination of two or more kinds.
The amount of the solvent during polymerization is not particularly limited as long as the polymerization proceeds and the amount can be easily removed without causing precipitation of the copolymer or monomers used during production, but for example, when the total amount of the monomers to be blended is taken as 100 mass%, the amount is preferably 10 to 200 mass%, more preferably 25 to 200 mass%, even more preferably 50 to 200 mass%, and even more preferably 50 to 150 mass%.

The polymerization temperature is not particularly limited as long as it is a temperature at which the polymerization proceeds, but is preferably 50 to 200° C., more preferably 80 to 200° C., even more preferably 90 to 150° C., even more preferably 100 to 140° C., and even more preferably 100 to 130° C. From the viewpoint of productivity, it is preferably 70° C. or higher, and it is preferably 180° C. or lower in order to suppress side reactions during polymerization and to obtain a polymer with the desired molecular weight and quality.
The polymerization time is not particularly limited as long as the target conversion rate is satisfied, but from the viewpoint of productivity, etc., it is preferably 0.5 to 15 hours, more preferably 2 to 12 hours, and even more preferably 4 to 10 hours.

During the polymerization reaction, a polymerization initiator or chain transfer agent may be added as necessary.

The polymerization initiator is not particularly limited, but for example, the polymerization initiators disclosed in the preparation method of the methacrylic resin having a structural unit derived from the N-substituted maleimide monomer can be used.
These polymerization initiators may be used alone or in combination of two or more kinds.
These polymerization initiators may be added at any stage as long as the polymerization reaction is in progress.
The amount of the polymerization initiator to be added may be appropriately set depending on the combination of monomers, reaction conditions, and the like, and is not particularly limited. When the total amount of the monomers to be used in the polymerization is taken as 100 mass%, the amount may be 0.01 to 1 mass%, and preferably 0.05 to 0.5 mass%.

As the chain transfer agent, a chain transfer agent used in general radical polymerization can be used, for example, the chain transfer agents disclosed in the above-mentioned method for preparing a methacrylic resin having a structural unit derived from an N-substituted maleimide monomer can be used.
These may be used alone or in combination of two or more.
These chain transfer agents may be added at any stage as long as the polymerization reaction is in progress, and there are no particular limitations on the addition stage.
The amount of the chain transfer agent to be added is not particularly limited as long as it is within a range in which a desired degree of polymerization can be obtained under the polymerization conditions used, but is preferably 0.01 to 1 mass %, and more preferably 0.05 to 0.5 mass %, when the total amount of the monomers to be used in the polymerization is taken as 100 mass %.

In the polymerization step, a suitable method for adding a polymerization initiator and a chain transfer agent may be, for example, the method described in the above-mentioned method for preparing a methacrylic resin having a structural unit derived from an N-substituted maleimide monomer.
The dissolved oxygen concentration in the polymerization solution may be, for example, the value disclosed in the above-mentioned method for preparing a methacrylic resin having a structural unit derived from an N-substituted maleimide monomer.

Next, an imidization reaction is carried out by reacting the (meth)acrylic acid ester polymer or the methacrylic acid ester-aromatic vinyl copolymer with an imidization agent (imidization step). This makes it possible to produce a methacrylic resin having glutarimide structural units.

The imidizing agent is not particularly limited as long as it can generate the glutarimide structural unit represented by the general formula (3) above.
Specifically, the imidizing agent may be ammonia or a primary amine. Examples of the primary amine include aliphatic hydrocarbon group-containing primary amines such as methylamine, ethylamine, n-propylamine, i-propylamine, n-butylamine, i-butylamine, tert-butylamine, and n-hexylamine; alicyclic hydrocarbon group-containing primary amines such as cyclohexylamine; and the like.
Of the above imidizing agents, ammonia, methylamine and cyclohexylamine are preferred from the standpoint of cost and physical properties, with methylamine being particularly preferred.

In this imidization process, the content of glutarimide structural units in the resulting methacrylic resin having glutarimide structural units can be adjusted by adjusting the ratio of the imidization agent added.

The method for carrying out the imidization reaction is not particularly limited, but a conventionally known method can be used. For example, the imidization reaction can be carried out using an extruder or a batch reaction tank.

The extruder is not particularly limited, but for example, a single screw extruder, a twin screw extruder, a multi-screw extruder, etc. can be used.
Among these, it is preferable to use a twin-screw extruder, which can promote mixing of the raw material polymer and the imidizing agent.
Examples of twin-screw extruders include non-intermeshing co-rotating, intermeshing co-rotating, non-intermeshing counter-rotating, and intermeshing counter-rotating.
The above-mentioned extruders may be used alone or in combination of a plurality of extruders connected in series.
It is particularly preferable to equip the extruder to be used with a vent port capable of reducing the pressure to below atmospheric pressure, since this makes it possible to remove by-products of the reaction, such as the imidizing agent, methanol, and the like, and monomers.

In producing a methacrylic resin having a glutarimide structural unit, in addition to the above-mentioned imidization step, an esterification step of treating the carboxyl group of the resin with an esterification agent such as dimethyl carbonate can be included. In this case, a catalyst such as trimethylamine, triethylamine, or tributylamine can also be used in combination.
The esterification step can be carried out, for example, by using an extruder or a batch reaction tank, in the same manner as in the imidization step.
For the purpose of removing excess esterifying agent, by-products such as methanol, or monomers, the apparatus used is preferably equipped with a vent port capable of reducing the pressure to atmospheric pressure or below.

The methacrylic resin that has been subjected to the imidization step and, if necessary, the esterification step is melted and extruded in the form of strands from an extruder equipped with a multi-hole die, and processed into pellets by a cold cut method, an in-air hot cut method, an underwater strand cut method, an underwater cut method, or the like.
In order to reduce the number of foreign matters in the resin, it is also preferable to use a method in which the methacrylic resin is dissolved in an organic solvent such as toluene, methyl ethyl ketone, or methylene chloride, the obtained methacrylic resin solution is filtered, and then the organic solvent is removed.

From the viewpoint of reducing the fluorescence intensity (content of the fluorescent substance), it is preferable to imidize the polymer solution after the completion of the polymerization in a batch reaction tank without using a twin-screw extruder which is subjected to a shearing force.
The imidization reaction is preferably carried out at 130 to 250° C., more preferably at 150 to 230° C., and even more preferably at 170 to 190° C. The reaction time is preferably 10 minutes to 5 hours, and more preferably 30 minutes to 2 hours.
After the imidization step, an esterification step may be carried out as necessary, and then the volatilization is carried out by the volatilization method with reduced shear (3) described in the above-mentioned method for preparing a methacrylic resin having a structural unit derived from an N-substituted maleimide monomer, followed by pelletization, which is preferable from the viewpoint of reducing fluorescence intensity.

-Method for producing methacrylic resin containing lactone ring structural unit-
As a method for producing a methacrylic resin having a lactone ring structural unit in the main chain, a method in which a lactone ring structure is formed by a cyclization reaction after polymerization is used. In order to promote the cyclization reaction, however, it is preferable to polymerize monomers by radical polymerization using a solution polymerization method using a solvent.
In the production method of the present embodiment, any of a batch polymerization method, a semi-batch polymerization method, and a continuous polymerization method can be used as the polymerization method.
Methacrylic resins having a lactone ring structural unit in the main chain can be formed by the methods described in, for example, JP-A-2001-151814, JP-A-2004-168882, JP-A-2005-146084, JP-A-2006-96960, JP-A-2006-171464, JP-A-2007-63541, JP-A-2007-297620, JP-A-2010-180305, etc.

As an example of a method for producing a methacrylic resin having a lactone ring structural unit, a batchwise radical polymerization method using a solution polymerization method will be specifically described below.
As a method for producing a methacrylic resin having a lactone ring structural unit, a method of forming a lactone ring structure by a cyclization reaction after polymerization is used. In order to promote the cyclization reaction, solution polymerization using a solvent is preferred.

Examples of the solvent used in the polymerization include aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; and ketones such as methyl ethyl ketone and methyl isobutyl ketone.
These solvents may be used alone or in combination of two or more kinds.

There are no particular restrictions on the amount of solvent used during polymerization, so long as the polymerization proceeds and gelation can be suppressed. For example, if the total amount of the monomers to be mixed is taken as 100% by mass, it is preferable to use 50 to 200% by mass, and more preferably 100 to 200% by mass.

In order to sufficiently suppress gelation of the polymerization liquid and promote the cyclization reaction after polymerization, it is preferable to carry out the polymerization so that the concentration of the polymer produced in the reaction mixture obtained after polymerization is 50% by mass or less. It is also preferable to appropriately add a polymerization solvent to the reaction mixture to control the above concentration to 50% by mass or less.

The method for appropriately adding the polymerization solvent to the reaction mixture is not particularly limited, and for example, the polymerization solvent may be added continuously or intermittently.
The polymerization solvent to be added may be a single solvent or a mixed solvent of two or more solvents.
The polymerization temperature is not particularly limited as long as the polymerization proceeds at any temperature. From the viewpoint of productivity, the polymerization temperature is preferably 50 to 200°C, and more preferably 80 to 180°C.
The polymerization time is not particularly limited as long as the target conversion rate is achieved, but from the viewpoint of productivity, etc., it is preferably 0.5 to 10 hours, more preferably 1 to 8 hours.

During the polymerization reaction, a polymerization initiator or a chain transfer agent may be added, if necessary.
The polymerization initiator is not particularly limited, but for example, the polymerization initiators disclosed in the above-mentioned method for preparing a methacrylic resin having a structural unit derived from an N-substituted maleimide monomer can be used.
These polymerization initiators may be used alone or in combination of two or more kinds.
These polymerization initiators may be added at any stage as long as the polymerization reaction is in progress.
The amount of the polymerization initiator to be added may be appropriately set depending on the combination of monomers, reaction conditions, etc., and is not particularly limited. When the total amount of the monomers used in the polymerization is taken as 100 mass%, the amount may be 0.05 to 1 mass%.

As the chain transfer agent, a chain transfer agent used in general radical polymerization can be used, for example, the chain transfer agents disclosed in the above-mentioned method for preparing a methacrylic resin having a structural unit derived from an N-substituted maleimide monomer can be used.
These may be used alone or in combination of two or more.
These chain transfer agents may be added at any stage as long as the polymerization reaction is in progress, and there are no particular limitations on the addition stage.
The amount of the chain transfer agent to be added is not particularly limited as long as it is within a range in which a desired degree of polymerization can be obtained under the polymerization conditions used, but it may be preferably 0.05 to 1% by mass, where the total amount of the monomers to be used in the polymerization is taken as 100% by mass.

A suitable method for adding the polymerization initiator and chain transfer agent in the polymerization step may be, for example, the method described in the preparation method of the methacrylic resin having structural units derived from the N-substituted maleimide monomer.

The dissolved oxygen concentration in the polymerization solution may be, for example, the value disclosed in the method for preparing a methacrylic resin having structural units derived from the N-substituted maleimide monomer.

The methacrylic resin having a lactone ring structural unit in this embodiment can be obtained by carrying out a cyclization reaction after the completion of the polymerization reaction. Therefore, it is preferable to subject the polymerization reaction liquid to the lactone cyclization reaction while still containing the solvent, without removing the polymerization solvent from the polymerization reaction liquid.
When the copolymer obtained by polymerization is subjected to a heat treatment, a cyclization condensation reaction occurs between the hydroxyl groups (hydroxyl groups) and the ester groups present in the molecular chain of the copolymer, forming a lactone ring structure.
In the heat treatment for forming the lactone ring structure, a reaction apparatus equipped with a vacuum device or a volatilizing device, an extruder equipped with a volatilizing device, or the like can be used to remove alcohol that may be by-produced by the cyclocondensation.

When forming the lactone ring structure, a heat treatment may be carried out using a cyclization condensation catalyst, if necessary, in order to promote the cyclization condensation reaction.
Specific examples of the cyclization condensation catalyst include monoalkyl phosphite, dialkyl ester, or triester phosphite, such as methyl phosphite, ethyl phosphite, phenyl phosphite, dimethyl phosphite, diethyl phosphite, diphenyl phosphite, trimethyl phosphite, and triethyl phosphite; monoalkyl phosphate, dialkyl ester, or trialkyl ester phosphate, such as methyl phosphate, ethyl phosphate, 2-ethylhexyl phosphate, octyl phosphate, isodecyl phosphate, lauryl phosphate, stearyl phosphate, isostearyl phosphate, dimethyl phosphate, diethyl phosphate, di-2-ethylhexyl phosphate, diisodecyl phosphate, dilauryl phosphate, distearyl phosphate, diisostearyl phosphate, trimethyl phosphate, triethyl phosphate, triisodecyl phosphate, trilauryl phosphate, tristearyl phosphate, and triisostearyl phosphate; and organic zinc compounds such as zinc acetate, zinc propionate, and octyl zinc.
These may be used alone or in combination of two or more.

The amount of the cyclization condensation catalyst used is not particularly limited, but is, for example, preferably 0.01 to 3 mass %, more preferably 0.05 to 1 mass %, based on 100 mass % of the methacrylic resin.
When the amount of the catalyst used is 0.01% by mass or more, it is effective in improving the reaction rate of the cyclization condensation reaction, and when the amount of the catalyst used is 3% by mass or less, it is effective in preventing the obtained polymer from being discolored or from being crosslinked, which makes it difficult to melt mold the polymer.

The timing of adding the cyclization condensation catalyst is not particularly limited, and for example, it may be added at the beginning of the cyclization condensation reaction, or during the reaction, or it may be added at both times.
When the cyclization condensation reaction is carried out in the presence of a solvent, devolatilization can also be carried out at the same time.

The apparatus used when the cyclocondensation reaction and the devolatilization step are carried out simultaneously is not particularly limited, but a devolatilization apparatus consisting of a heat exchanger and a devolatilization tank, an extruder equipped with a vent, or an apparatus in which a devolatilization apparatus and an extruder are arranged in series is preferable, and a twin-screw extruder equipped with a vent is more preferable.
The vented twin-screw extruder used is preferably one having a plurality of vent ports.

When a vented extruder is used, the reaction temperature is preferably 150 to 350° C., more preferably 200 to 300° C. If the reaction temperature is less than 150° C., the cyclocondensation reaction may be insufficient, resulting in a large amount of residual volatile matter. Conversely, if the reaction temperature exceeds 350° C., the resulting polymer may become discolored or decomposed.
When using a vented extruder, the degree of vacuum is preferably 10 to 500 Torr, more preferably 10 to 300 Torr. If the degree of vacuum exceeds 500 Torr, volatile matter may easily remain. Conversely, if the degree of vacuum is less than 10 Torr, industrial implementation may be difficult.

When the above cyclization condensation reaction is carried out, it is also preferred to add an alkaline earth metal and/or amphoteric metal salt of an organic acid during granulation in order to deactivate the remaining cyclization condensation catalyst.
Examples of the alkaline earth metal and/or amphoteric metal salt of an organic acid that can be used include calcium acetylacetate, calcium stearate, zinc acetate, zinc octoate, and zinc 2-ethylhexylate.

After the cyclocondensation reaction step, the methacrylic resin is melted and extruded in the form of strands from an extruder equipped with a multi-hole die, and processed into pellets by a cold cut method, an in-air hot cut method, an underwater strand cut method, or an underwater cut method.
The lactonization for forming the lactone ring structural unit may be carried out after the production of the resin and before the production of the resin composition (described later), or may be carried out during the production of the resin composition, together with melt-kneading of the resin and components other than the resin.

From the viewpoint of reducing the fluorescence intensity (content of fluorescent substance), it is preferable to lactone-cyclize the polymerization solution after the completion of polymerization in a batch reaction tank and not to use a twin-screw extruder that is subjected to shearing force. After the lactone-cyclization step, it is preferable to devolatilize the solution using the devolatilization method with reduced shearing (3) described in the above-mentioned method for preparing a methacrylic resin having structural units derived from an N-substituted maleimide monomer, and then pelletize the solution, from the viewpoint of reducing the fluorescence intensity.

[Additives]
The resin composition constituting the resin lens for head mounted displays of the present embodiment may contain various additives within ranges that do not significantly impair the effects of the present invention described above.
The additives are not particularly limited, and examples thereof include antioxidants, light stabilizers such as hindered amine light stabilizers, ultraviolet absorbers, release agents, other thermoplastic resins, softeners/plasticizers such as paraffinic process oil, naphthenic process oil, aromatic process oil, paraffin, organic polysiloxane, and mineral oil, flame retardants, antistatic agents, inorganic fillers such as organic fibers and pigments such as iron oxide, reinforcing agents such as glass fibers, carbon fibers, and metal whiskers, colorants, organic phosphorus compounds such as phosphites, phosphonites, and phosphates, other additives, and mixtures thereof.

-Antioxidants-
The resin composition constituting the resin lens for head mounted displays of the present embodiment preferably contains an antioxidant that suppresses deterioration and coloring during molding or use.
The antioxidant includes, but is not limited to, hindered phenol-based antioxidants, phosphorus-based antioxidants, sulfur-based antioxidants, and the like.
These antioxidants may be used alone or in combination of two or more.
From the viewpoint of improving thermal stability and suppressing molding defects, it is preferable to use multiple types of heat stabilizers in combination. For example, it is preferable to use at least one selected from a phosphorus-based antioxidant and a sulfur-based antioxidant in combination with a hindered phenol-based antioxidant.

Examples of the hindered phenol-based antioxidant include, but are not limited to, pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], thiodiethylene bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, , 3,3',3'',5,5',5''-hexa-tert-butyl-a,a',a''-(mesitylene-2,4,6-triyl)tri-p-cresol, 4,6-bis(octylthiomethyl)-o-cresol, 4,6-bis(dodecylthiomethyl)-o-cresol, ethylene bis(oxyethylene)bis[3-(5-tert-butyl-4-hydroxy-m-tolyl)propionate], hexamethylene bis[ 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 1,3,5-tris[(4-tert-butyl-3-hydroxy-2,6-xylin)methyl]-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, 2 ,6-di-tert-butyl-4-(4,6-bis(octylthio)-1,3,5-triazin-2-ylamine)phenol, 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate, 2-tert-butyl-4-methyl-6-(2-hydroxy-3-tert-butyl-5-methylbenzyl)phenyl acrylate, and the like.
Particularly preferred are pentaerythritol terakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, and 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate.

In addition, as the hindered phenol-based antioxidant as the antioxidant, a commercially available phenol-based antioxidant may be used. Examples of such commercially available phenol-based antioxidants include, but are not limited to, Irganox 1010 (Irganox 1010: pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], manufactured by BASF), Irganox 1076 (Irganox 1076: octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, manufactured by BASF), Irganox 1330 (Irganox 1330: octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, manufactured by BASF), and Irganox 1330 (Irganox 1330: octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, manufactured by BASF). 1330: 3,3',3'',5,5',5''-hexa-t-butyl-a,a',a''-(mesitylene-2,4,6-triyl)tri-p-cresol, manufactured by BASF Corporation), Irganox 3114 (Irganox 3114: 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)-1,3,5-triazine-2,4,6(1H,3H,5H)-trione, manufactured by BASF Corporation), Irganox 3125 (Irganox 3125, manufactured by BASF), Adeka STAB AO-60 (pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], manufactured by ADEKA), Adeka STAB AO-80 (3,9-bis{2-[3-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyloxy]-1,1-dimethylethyl}-2,4,8,10-tetraoxaspiro[5.5]undecane, manufactured by ADEKA), Sumilizer BHT (Sumilizer BHT, manufactured by Sumitomo Chemical), Cyanox 1790 (Cyanox 1790, manufactured by Cytec), Sumilizer GA-80 (Sumilizer GA-80, manufactured by Sumitomo Chemical), Sumilizer GS (Sumilizer Examples of the acrylates include GS: 2-[1-(2-hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate, manufactured by Sumitomo Chemical Co., Ltd.), Sumilizer GM (2-tert-butyl-4-methyl-6-(2-hydroxy-3-tert-butyl-5-methylbenzyl)phenyl acrylate, manufactured by Sumitomo Chemical Co., Ltd.), and Vitamin E (manufactured by Eisai Co., Ltd.).
Among these commercially available phenolic antioxidants, Irganox 1010, Adeka STAB AO-60, Adeka STAB AO-80, Irganox 1076, Sumilizer GS, and the like are preferred from the viewpoint of the effect of imparting thermal stability to the resin.
These may be used alone or in combination of two or more.

In addition, examples of phosphorus-based antioxidants as the antioxidant include, but are not limited to, tris(2,4-di-t-butylphenyl)phosphite, bis(2,4-bis(1,1-dimethylethyl)-6-methylphenyl)ethyl ester phosphorous acid, tetrakis(2,4-di-t-butylphenyl)(1,1-biphenyl)-4,4'-diylbisphosphonite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite, bis(2,6-di-t-butyl-4-methyl phenyl) pentaerythritol diphosphite, bis(2,4-dicumylphenyl) pentaerythritol diphosphite, tetrakis(2,4-t-butylphenyl)(1,1-biphenyl)-4,4'-diylbisphosphonite, di-t-butyl-m-cresyl-phosphonite, 4-[3-[(2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepin)-6-yloxy]propyl]-2-methyl-6-tert-butylphenol, etc.

Furthermore, a commercially available phosphorus-based antioxidant may be used as the phosphorus-based antioxidant. Examples of such commercially available phosphorus-based antioxidants include, but are not limited to, Irgafos 168 (Irgafos 168: tris(2,4-di-t-butylphenyl)phosphite, manufactured by BASF), Irgafos 12 (Irgafos 12: tris[2-[[2,4,8,10-tetra-t-butyldibenzo[d,f][1,3,2]dioxaphosphen-6-yl]oxy]ethyl]amine, manufactured by BASF), Irgafos 38 (Irgafos 38: bis(2,4-bis(1,1-dimethylethyl)-6-methylphenyl)ethyl ester phosphorous acid, manufactured by BASF), and ADK STAB 329K (ADK STAB 329K). STAB-229K, manufactured by ADEKA), ADK STAB PEP-36 (ADK STAB PEP-36, manufactured by ADEKA), ADK STAB PEP-36A (ADK STAB PEP-36A, manufactured by ADEKA), ADK STAB PEP-8 (ADK STAB PEP-8, manufactured by ADEKA), ADK STAB HP-10 (ADK STAB HP-10, manufactured by ADEKA), ADK STAB 2112 (ADK STAB 2112, manufactured by ADEKA), ADK STAB 1178 (ADKA STAB 1178, manufactured by ADEKA), ADK STAB 1500 (ADK STAB 1500, manufactured by ADEKA) Sandstab Examples of such an agent include P-EPQ (manufactured by Clariant), Weston 618 (manufactured by GE), Weston 619G (manufactured by GE), Ultranox 626 (manufactured by GE), Sumilizer GP (4-[3-[(2,4,8,10-tetra-tert-butyldibenzo[d,f][1,3,2]dioxaphosphepin)-6-yloxy]propyl]-2-methyl-6-tert-butylphenol, manufactured by Sumitomo Chemical Co., Ltd.), and HCA (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, manufactured by Sanko Co., Ltd.).
Among these commercially available phosphorus-based antioxidants, from the viewpoint of the effect of imparting thermal stability to the resin and the effect of using them in combination with various types of antioxidants, Irgafos 168, ADK STAB PEP-36, ADK STAB PEP-36A, ADK STAB HP-10, and ADK STAB 1178 are preferred, and ADK STAB PEP-36A and ADK STAB PEP-36 are particularly preferred.
These phosphorus-based antioxidants may be used alone or in combination of two or more kinds.

In addition, examples of the sulfur-based antioxidant as the antioxidant include, but are not limited to, 2,4-bis(dodecylthiomethyl)-6-methylphenol (Irganox 1726, manufactured by BASF Corporation), 2,4-bis(octylthiomethyl)-6-methylphenol (Irganox 1520L, manufactured by BASF Corporation), 2,2-bis{[3-(dodecylthio)-1-oxopropoxy]methyl}propane-1 ,3-diylbis[3-dodecylthio]propionate] (ADEKA STAB AO-412S, manufactured by ADEKA Corporation), 2,2-bis{[3-(dodecylthio)-1-oxopropoxy]methyl}propane-1,3-diylbis[3-dodecylthio]propionate] (ChemiNox PLS, manufactured by Chemipro Chemical Co., Ltd.), and di(tridecyl) 3,3'-thiodipropionate (AO-503, manufactured by ADEKA Corporation).
Among these commercially available sulfur antioxidants, Adeka STAB AO-412S and Cheminox PLS are preferred from the viewpoints of the effect of imparting thermal stability to the resin, the effect of using them in combination with various types of antioxidants, and ease of handling.
These sulfur-based antioxidants may be used alone or in combination of two or more kinds.

The content of the antioxidant may be any amount that provides the effect of improving thermal stability. If the content is excessive, problems such as bleeding out during processing may occur, so the content is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, even more preferably 0.8% by mass or less, still more preferably 0.01 to 0.8% by mass, and particularly preferably 0.01 to 0.5% by mass, relative to 100% by mass of the methacrylic resin.

There are no particular limitations on the timing of adding the antioxidant, and examples of such methods include adding the antioxidant to the monomer solution before polymerization and then initiating polymerization, adding the antioxidant to the polymer solution after polymerization and mixing it, then subjecting it to the devolatilization step, adding the antioxidant to the molten polymer after devolatilization and then mixing it, then pelletizing it, and adding and mixing it when the pellets after devolatilization and pelletization are melt-extruded again. Of these, from the viewpoint of preventing thermal deterioration and coloration in the devolatilization step, it is preferable to add the antioxidant to the polymer solution after polymerization and mix it, then add it before the devolatilization step, and then subject it to the devolatilization step.

- Hindered amine light stabilizer -
The resin composition constituting the resin lens for head mounted displays of the present embodiment may contain a hindered amine-based light stabilizer.
The hindered amine light stabilizer is not particularly limited, but is preferably a compound containing three or more ring structures. Here, the ring structure is preferably at least one selected from the group consisting of an aromatic ring, an aliphatic ring, an aromatic heterocycle, and a non-aromatic heterocycle, and when one compound has two or more ring structures, they may be the same or different from each other.
Examples of the hindered amine light stabilizer include, but are not limited to, bis(1,2,2,6,6-pentamethyl-4-piperidyl)[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]butyl malonate, a mixture of bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate and methyl 1,2,2,6,6-pentamethyl-4-piperidylsebacate, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, N ,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)-N,N'-diformylhexamethylenediamine, polycondensate of dibutylamine/1,3,5-triazine/N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)-1,6-hexamethylenediamine and N-(2,2,6,6-tetramethyl-4-piperidyl)butylamine, poly[{6-(1,1,3,3-tetramethylbutyl)amino-1,3,5-triazine-2,4-diyl}{(2,2,6,6-tetramethyl tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate, tetrakis(2,2,6,6-tetramethyl-4-piperidyl)butane-1,2,3,4-tetracarboxylate, 1,2,2,6,6-pentamethyl-4-piperidiol and β,β,β',β'-tetramethyl-2,4,8,10-tetraoxaspiro[5 .5]undecane-3,9-diethanol, a reaction product of 2,2,6,6-tetramethyl-4-piperidiol and β,β,β',β'-tetramethyl-2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diethanol, bis(1-undecanoxy-2,2,6,6-tetramethylpiperidin-4-yl)carbonate, 1,2,2,6,6-pentamethyl-4-piperidyl methacrylate, 2,2,6,6-tetramethyl-4-piperidyl methacrylate, and the like.
Among these, bis(1,2,2,6,6-pentamethyl-4-piperidyl)[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl]methyl]butylmalonate, a polycondensate of dibutylamine, 1,3,5-triazine, N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)-1,6-hexamethylenediamine and N-(2,2,6,6-tetramethyl-4-piperidyl)butylamine, and poly[{6-(1,1,3,3-tetramethylbutyl)amino-1,3,5-triazine-2,4-diyl} {(2,2,6,6-tetramethyl-4-piperidyl)imino}hexamethylene{(2,2,6,6-tetramethyl-4-piperidyl)imino}], a reaction product of 1,2,2,6,6-pentamethyl-4-piperidiol and β,β,β',β'-tetramethyl-2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diethanol, and a reaction product of 2,2,6,6-tetramethyl-4-piperidiol and β,β,β',β'-tetramethyl-2,4,8,10-tetraoxaspiro[5.5]undecane-3,9-diethanol are preferred.
The content of the hindered amine light stabilizer may be any amount that provides an effect of improving light stability. If the content is excessive, problems such as bleeding out during processing may occur. Therefore, the content is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, still more preferably 0.8% by mass or less, still more preferably 0.01 to 0.8% by mass, and particularly preferably 0.01 to 0.5% by mass, relative to 100% by mass of the methacrylic resin.

--Ultraviolet absorbing agent--
The resin composition constituting the resin lens for a head mounted display of the present embodiment may contain an ultraviolet absorbing agent.
The ultraviolet absorbent is not particularly limited, but is preferably an ultraviolet absorbent having a maximum absorption wavelength of 280 to 380 nm, and examples thereof include benzotriazole-based compounds, benzotriazine-based compounds, benzophenone-based compounds, oxybenzophenone-based compounds, benzoate-based compounds, phenol-based compounds, oxazole-based compounds, cyanoacrylate-based compounds, and benzoxazinone-based compounds.
Benzotriazole compounds include 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol], 2-(3,5-di-tert-butyl-2-hydroxyphenyl)-5-chlorobenzotriazole, 2-(2H-benzotriazol-2-yl)-p-cresol, 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, 2-benzotriazol-2-yl-4,6-di-tert-butylphenol, 2-[5-chloro(2H)-benzotriazol-2-yl]-4-methyl-6-t-butylphenol, 2-(2H-benzotriazol-2-yl)-4,6-di-t-butylphenol, 2-(2H-benzotriazol-2-yl)-4-(1,1 ,3,3-tetramethylbutyl)phenol, 2-(2H-benzotriazol-2-yl)-4-methyl-6-(3,4,5,6-tetrahydrophthalimidylmethyl)phenol, methyl 3-(3-(2H-benzotriazol-2-yl)-5-t-butyl-4-hydroxyphenyl)propionate/polyethylene glycol 300 reaction products, 2-(2H-benzotriazol-2-yl)-6-(linear and side chain dodecyl)-4-methylphenol, 2-(5-methyl-2-hydroxyphenyl)benzotriazole, 2-[2-hydroxy-3,5-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxy-C7-9 side chain and straight chain alkyl esters.
Among these, benzotriazole-based compounds having a molecular weight of 400 or more are preferred. Examples of commercially available products include Kemisorb (registered trademark) 2792 (manufactured by Chemipro Chemicals), Adeka STAB (registered trademark) LA31 (manufactured by ADEKA Corporation), and Tinuvin (registered trademark) 234 (manufactured by BASF).
Benzotriazine compounds include 2-mono(hydroxyphenyl)-1,3,5-triazine compounds, 2,4-bis(hydroxyphenyl)-1,3,5-triazine compounds, and 2,4,6-tris(hydroxyphenyl)-1,3,5-triazine compounds. Specific examples of the benzotriazine compounds include 2,4-diphenyl-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-ethoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-(2-hydroxy-4-propoxyphenyl)-1,3,5-triazine, and 2,4-diphenyl-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazine. 1-(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-hexyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-octyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-dodecyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-benzyloxyphenyl)-1,3,5-triazine , 2,4-diphenyl-6-(2-hydroxy-4-butoxyethoxy)-1,3,5-triazine, 2,4-bis(2-hydroxy-4-butoxyphenyl)-6-(2,4-dibutoxyphenyl)-1,3-5-triazine, 2,4,6-tris(2-hydroxy-4-methoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-ethoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-propoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine , 2,4,6-tris(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-hexyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-octyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-dodecyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-benzyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-ethoxyethoxyphenyl)-1,3,5-triazine, 2 ,4,6-tris(2-hydroxy-4-butoxyethoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-propoxyethoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-methoxycarbonylpropyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-ethoxycarbonylethyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-4-(1-(2-ethoxyhexyloxy)-1-oxopropan-2-yloxy)phenyl)-1,3,5-triazine azine, 2,4,6-tris(2-hydroxy-3-methyl-4-methoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-ethoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-propoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-butoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-butoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-butoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-butoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-hexyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-octyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-dodecyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-benzyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-ethoxyethoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-butoxyethoxyphenyl)-1 ,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-propoxyethoxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-methoxycarbonylpropyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-ethoxycarbonylethyloxyphenyl)-1,3,5-triazine, 2,4,6-tris(2-hydroxy-3-methyl-4-(1-(2-ethoxyhexyloxy)-1-oxopropan-2-yloxy)phenyl)-1,3,5-triazine, and the like.
As the benzotriazine-based compound, commercially available products may be used, such as Kemisorb 102 (manufactured by Chemipro Chemicals), LA-F70 (manufactured by ADEKA Corporation), LA-46 (manufactured by ADEKA Corporation), TINUVIN 405 (manufactured by BASF), TINUVIN 460 (manufactured by BASF), TINUVIN 479 (manufactured by BASF), and TINUVIN 1577FF (manufactured by BASF).
Among these, ultraviolet absorbers having a 2,4-bis(2,4-dimethylphenyl)-6-[2-hydroxy-4-(3-alkyloxy-2-hydroxypropyloxy)-5-α-cumylphenyl]-s-triazine skeleton (wherein "alkyloxy" refers to a long-chain alkyloxy group such as octyloxy, nonyloxy, decyloxy, etc.) are more preferably used because of their high compatibility with acrylic resins and excellent ultraviolet absorbing properties.
As the ultraviolet absorber, from the viewpoints of compatibility with the resin and volatility upon heating, in particular, benzotriazole-based compounds and benzotriazine-based compounds having a molecular weight of 400 or more are preferred, and from the viewpoint of suppressing decomposition of the ultraviolet absorber itself due to heating during extrusion processing, benzotriazine-based compounds are particularly preferred.
The melting point (Tm) of the ultraviolet absorber is preferably 80° C. or higher, more preferably 100° C. or higher, even more preferably 130° C. or higher, and even more preferably 160° C. or higher.
The ultraviolet absorber preferably has a weight loss rate of 50% or less, more preferably 30% or less, even more preferably 15% or less, even more preferably 10% or less, and even more preferably 5% or less, when heated from 23°C to 260°C at a rate of 20°C/min.
These ultraviolet absorbents may be used alone or in combination of two or more. By using two types of ultraviolet absorbents having different structures in combination, ultraviolet rays in a wide wavelength range can be absorbed.
The content of the ultraviolet absorber is not particularly limited as long as it is an amount that does not impair heat resistance, moist heat resistance, thermal stability, and moldability and exhibits the effects of the present invention, but is preferably 0.1 to 5 mass %, more preferably 0.2 to 4 mass %, more preferably 0.25 to 3 mass %, and even more preferably 0.3 to 3 mass %, relative to 100 mass % of the methacrylic resin. When it is within this range, an excellent balance of ultraviolet absorbing performance, moldability, etc. is achieved.

--Release agent--
The resin composition constituting the resin lens for head mounted displays of the present embodiment may contain a mold release agent. Examples of the mold release agent include, but are not limited to, fatty acid esters, fatty acid amides, fatty acid metal salts, hydrocarbon-based lubricants, alcohol-based lubricants, polyalkylene glycols, carboxylates, and hydrocarbon paraffin-based mineral oils.

The fatty acid ester that can be used as the release agent is not particularly limited, and any of the conventionally known fatty acid esters can be used.
Examples of fatty acid esters that can be used include ester compounds of fatty acids having 12 to 32 carbon atoms, such as lauric acid, palmitic acid, heptadecanoic acid, stearic acid, oleic acid, arachic acid, and behenic acid, with monohydric aliphatic alcohols, such as palmityl alcohol, stearyl alcohol, and behenyl alcohol, and polyhydric aliphatic alcohols, such as glycerin, pentaerythritol, dipentaerythritol, and sorbitan; and complex ester compounds of fatty acids, polybasic organic acids, and monohydric aliphatic alcohols or polyhydric aliphatic alcohols.

Examples of such fatty acid esters include cetyl palmitate, butyl stearate, stearyl stearate, stearyl citrate, glycerin monocaprylate, glycerin monocaprate, glycerin monolaurate, glycerin monopalmitate, glycerin dipalmitate, glycerin monostearate, glycerin distearate, glycerin tristearate, glycerin monooleate, glycerin dioleate, glycerin trioleate, and glycerin monolinoleate. , glycerin monobehenate, glycerin mono 12-hydroxystearate, glycerin di-12-hydroxystearate, glycerin tri-12-hydroxystearate, glycerin diacetomonostearate, glycerin citric acid fatty acid ester, pentaerythritol adipic acid stearate, partially saponified montanic acid ester, pentaerythritol tetrastearate, dipentaerythritol hexastearate, sorbitan tristearate, and the like.
These fatty acid esters may be used alone or in combination of two or more.
Examples of commercially available products include the Rikemal series, Poem series, Rikestar series, and Rikemaster series manufactured by Riken Vitamin Co., Ltd., and the Excel series, Leo D'or series, Exepar series, and Coconard series manufactured by Kao Corporation. More specific examples include Rikemal S-100, Rikemal H-100, Poem V-100, Rikemal B-100, Rikemal HC-100, Rikemal S-200, Poem B-200, Rikestar EW-200, Rikestar EW-400, Excel S-95, and Leo D'or MS-50.

The fatty acid amide is also not particularly limited, and any of the conventionally known fatty acid amides can be used.
Examples of fatty acid amides include saturated fatty acid amides such as lauric acid amide, palmitic acid amide, stearic acid amide, behenic acid amide, and hydroxystearic acid amide; unsaturated fatty acid amides such as oleic acid amide, erucic acid amide, and ricinoleic acid amide; substituted amides such as N-stearyl stearic acid amide, N-oleyl oleic acid amide, N-stearyl oleic acid amide, N-oleyl stearic acid amide, N-stearyl erucic acid amide, and N-oleyl palmitic acid amide; methylol amides such as methylol stearic acid amide and methylol behenic acid amide; methylene bisstearic acid amide, ethylene biscapric acid amide, ethylene bislauric acid amide, and ethylene bisstearic acid amide (ethylene bis saturated fatty acid bisamides such as ethylene bisstearylamide, ethylene bisisostearic acid amide, ethylene bishydroxystearic acid amide, ethylene bisbehenic acid amide, hexamethylene bisstearic acid amide, hexamethylene bisbehenic acid amide, hexamethylene bishydroxystearic acid amide, N,N'-distearyl adipic acid amide, and N,N'-distearyl sebacic acid amide; unsaturated fatty acid bisamides such as ethylene bisoleic acid amide, hexamethylene bisoleic acid amide, N,N'-dioleyl adipic acid amide, and N,N'-dioleyl sebacic acid amide; and aromatic bisamides such as m-xylylene bisstearic acid amide and N,N'-distearyl isophthalic acid amide.
These fatty acid amides may be used alone or in combination of two or more.
Examples of commercially available products include the Diamid series (manufactured by Nippon Kasei Chemical Industry Co., Ltd.), the Amide series (manufactured by Nippon Kasei Chemical Industry Co., Ltd.), the Nikka Amide series (manufactured by Nippon Kasei Chemical Industry Co., Ltd.), the Methylol Amide series, the Bis Amide series, the Slipax series (manufactured by Nippon Kasei Chemical Industry Co., Ltd.), the Kaowax series (manufactured by Kao Corporation), the Fatty Acid Amide series (manufactured by Kao Corporation), and ethylene bis stearic acid amides (manufactured by Dainichi Chemical Industry Co., Ltd.).

The fatty acid metal salt refers to a metal salt of a higher fatty acid, and examples thereof include lithium stearate, magnesium stearate, calcium stearate, calcium laurate, calcium ricinoleate, strontium stearate, barium stearate, barium laurate, barium ricinoleate, zinc stearate, zinc laurate, zinc ricinoleate, zinc 2-ethylhexoate, lead stearate, dibasic lead stearate, lead naphthenate, calcium 12-hydroxystearate, and lithium 12-hydroxystearate. Among these, calcium stearate, magnesium stearate, and zinc stearate are particularly preferred because the resulting transparent resin composition has excellent processability and extremely excellent transparency.
Examples of commercially available products include the SZ series, SC series, SM series, and SA series manufactured by Sakai Chemical Industry Co., Ltd.
When the fatty acid metal salt is used, its content is preferably 0.2% by mass or less relative to 100% by mass of the resin composition from the viewpoint of maintaining transparency.

The above release agents may be used alone or in combination of two or more.

The release agent to be used preferably has a decomposition onset temperature of 200°C or higher. Here, the decomposition onset temperature can be measured by the 1% mass loss temperature using TGA.

The amount of the release agent may be any amount that is effective as a release agent. If the amount is excessive, problems such as bleed-out during processing or poor extrusion due to screw slippage may occur, so the amount is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, even more preferably 0.8% by mass or less, still more preferably 0.01 to 0.8% by mass, and particularly preferably 0.01 to 0.5% by mass, relative to 100% by mass of the methacrylic resin. Adding the amount in the above range is preferable because it suppresses the decrease in transparency due to the addition of the release agent and also tends to suppress poor release during injection molding.

-Other thermoplastic resins-
The resin composition constituting the resin lens for head mounted displays of the present embodiment may contain thermoplastic resins other than methacrylic resins for the purpose of adjusting birefringence or improving flexibility without impairing the object of the present invention.

Other thermoplastic resins include, for example, polyacrylates such as polybutyl acrylate; styrene-based polymers such as polystyrene, styrene-methyl methacrylate copolymer, styrene-butyl acrylate copolymer, styrene-acrylonitrile copolymer, and acrylonitrile-butadiene-styrene block copolymer; and further, for example, acrylic rubber particles having a 3-4 layer structure described in JP-A-59-202213, JP-A-63-27516, JP-A-51-129449, JP-A-52-56150, etc.; rubbery polymers disclosed in JP-B-60-17406 and JP-A-8-245854; methacrylic rubber-containing graft copolymer particles obtained by multistage polymerization described in WO 2014-002491; and the like.
Among these, from the viewpoint of obtaining good optical properties and mechanical properties, preferred are rubber-containing graft copolymer particles having, on their surface layer, a graft portion made of a composition compatible with a styrene-acrylonitrile copolymer or a methacrylic resin containing a structural unit (X) having a ring structure in the main chain.

The average particle size of the aforementioned acrylic rubber particles, methacrylic rubber-containing graft copolymer particles, and rubber polymer is preferably 0.03 to 1 μm, and more preferably 0.05 to 0.5 μm, from the viewpoint of improving the impact strength and optical properties of the film obtained from the composition of this embodiment.

The content of other thermoplastic resins is preferably 0 to 50% by mass, and more preferably 0 to 25% by mass, when the methacrylic resin is taken as 100% by mass.

[Method of producing resin composition]
The method for producing the resin composition constituting the resin lens for head mounted displays of this embodiment is not particularly limited as long as a composition satisfying the requirements of the present invention can be obtained. For example, a method of kneading using a kneader such as an extruder, a heating roll, a kneader, a roller mixer, or a Banbury mixer can be mentioned. Among them, kneading using an extruder is preferred in terms of productivity. The kneading temperature may be in accordance with the preferred processing temperature of the polymer constituting the methacrylic resin or the other resins to be mixed, and is roughly in the range of 140 to 300°C, preferably in the range of 180 to 280°C. In addition, it is preferable to provide a vent port in the extruder for the purpose of reducing volatile content.

Here, in the resin composition constituting the resin lens for head mounted displays of the present embodiment, the amount of remaining solvent (residual solvent amount) is preferably less than 1000 ppm by mass, more preferably less than 800 ppm by mass, and even more preferably less than 700 ppm by mass.
Here, the remaining solvent refers to the polymerization solvent (excluding alcohols) used during polymerization, and the solvent used when redissolving and dissolving the resin obtained by polymerization. Specific examples of the polymerization solvent include aromatic hydrocarbons such as toluene, xylene, ethylbenzene, and isopropylbenzene; ketones such as methyl isobutyl ketone, butyl cellosolve, methyl ethyl ketone, and cyclohexanone; polar solvents such as dimethylformamide and 2-methylpyrrolidone; and the like. Examples of the solvent used for redissolution include toluene, methyl ethyl ketone, and methylene chloride.

The resin composition constituting the resin lens for head mounted displays of this embodiment preferably has a residual alcohol amount (residual alcohol content) of less than 500 ppm by mass, more preferably less than 400 ppm by mass, and even more preferably less than 350 ppm by mass.
The remaining alcohol herein refers to an alcohol by-produced in the cyclization condensation reaction, and specific examples thereof include aliphatic alcohols such as methanol, ethanol, and isopropanol.

The amount of remaining solvent and the amount of remaining alcohol can be measured by gas chromatography.

Whichever method is selected, it is preferable to prepare the composition after reducing oxygen and water as much as possible.
For example, the dissolved oxygen concentration in the polymerization solution in solution polymerization is preferably less than 300 ppm in the polymerization step, and in a preparation method using an extruder or the like, the oxygen concentration in the extruder is preferably less than 1% by volume, more preferably less than 0.8% by volume. The water content of the methacrylic resin is preferably adjusted to 1000 ppm by mass or less, more preferably 500 ppm by mass or less.
Within these ranges, it is advantageous since it becomes relatively easy to prepare a composition that satisfies the requirements of the present invention.

[Method of manufacturing resin lens for head mounted display]
The resin lens for head mounted displays of the present embodiment is formed by molding the above resin composition. As a method for producing the resin lens for head mounted displays of the present embodiment, a molding method such as injection molding, compression molding, extrusion molding, etc. can be used. Among these, injection molding is preferred from the viewpoint of productivity.

Injection molding typically consists of (1) an injection process in which resin is melted and the molten resin is filled into a temperature-controlled mold cavity; (2) a pressure holding process in which pressure is applied to the cavity until the gate is sealed, and an amount of resin is injected equivalent to the amount of molten resin filled in the injection process that contracts when it comes into contact with the mold and cools; (3) a cooling process in which the molded product is held until the resin cools after the pressure holding process is released; and (4) the mold is opened and the cooled molded product is removed.

In this case, the molding temperature is preferably in the range of Tg+100°C to Tg+160°C, preferably Tg+110°C to Tg+150°C, based on the glass transition temperature (Tg) of the resin composition. Here, the molding temperature refers to the controlled temperature of a band heater wrapped around the injection nozzle. The higher the molding temperature, the lower the retardation of the resin lens that can be obtained. However, at a constant temperature, coloring due to thermal degradation during retention in the molding machine is promoted, so that the molding temperature should be appropriately selected.
The mold temperature is preferably in the range of Tg-70°C to Tg, and more preferably in the range of Tg-50°C to Tg-20°C, based on the glass transition temperature (Tg) of the resin composition.

The injection speed can be appropriately selected depending on the thickness and dimensions of the resin lens to be obtained, and can be appropriately selected, for example, from the range of 5 to 1000 mm/sec.
The pressure for maintaining the pressure can be appropriately selected depending on the shape of the resin lens to be obtained, and can be appropriately selected within the range of, for example, 30 to 120 MPa.
The pressure for holding the pressure referred to here is the pressure maintained by a screw for further feeding the molten resin from the gate after the molten resin has been filled.

In addition, an annealing process may be performed to alleviate residual stress caused by injection molding and reduce the phase difference of the resin lens. The annealing temperature is preferably in the range of Tg-50°C to Tg, based on the glass transition temperature (Tg) of the resin composition, and more preferably in the range of Tg-30°C to Tg-10°C.

The surface of the resin lens for head mounted displays of this embodiment can be further subjected to surface functionalization treatments such as hard coat treatment, anti-reflection treatment, transparent conductive treatment, electromagnetic wave shielding treatment, gas barrier treatment, etc. The thickness of these functional layers is not particularly limited, but is usually in the range of 0.01 to 10 μm.

The hard coat layer to be applied to the surface can be formed, for example, by applying a coating liquid prepared by dissolving or dispersing an acrylate such as a silicone-based curable resin, an organic polymer composite inorganic fine particle-containing curable resin, a urethane acrylate, an epoxy acrylate, or a polyfunctional acrylate, and a photopolymerization initiator in an organic solvent, onto a film or sheet obtained from the resin composition of the present embodiment by a conventionally known coating method, drying the coating, and photocuring the coating.
In addition, before applying the hard coat layer, in order to improve adhesion, for example, a method of forming an easy-adhesion layer, a primer layer, an anchor layer, or the like containing inorganic fine particles in its composition and then forming the hard coat layer can be used.
The antiglare layer applied to the surface is formed by forming fine particles of silica, melamine resin, acrylic resin or the like into an ink, applying it onto the other functional layers by a conventionally known application method, and curing it with heat or light.
Examples of the anti-reflection layer applied to the surface include thin films of inorganic materials such as metal oxides, fluorides, silicides, borides, nitrides, and sulfides, and single or multiple layers of resins with different refractive indexes such as acrylic resins and fluororesins. Also usable are thin layers containing composite fine particles of inorganic compounds and organic compounds.

The present invention will be specifically described below with reference to examples and comparative examples. Note that the present invention is not limited to the following examples.

<1. Measurement of the amount of 2-cyclohexylamino-N-cyclohexylsuccinimide in N-cyclohexylmaleimide>
N-cyclohexylmaleimide or N-cyclohexylmaleimide/meta-xylene solution was collected and weighed to prepare a 25% by mass N-cyclohexylmaleimide meta-xylene solution, to which isopropyl benzoate was added as an internal standard substance. Measurement was performed under the following conditions using gas chromatography (Shimadzu Corporation GC-2014) to determine the content.
Detector: FID
Column used: HP-5ms
Measurement conditions: After holding at 80°C for 5 minutes, the temperature was increased to 300°C at a rate of 10°C/min, and then held for 5 minutes.

<2. Measurement of the amount of 2-anilino-N-phenylsuccinimide in N-phenylmaleimide>
N-phenylmaleimide or N-phenylmaleimide/meta-xylene solution was collected and weighed to prepare a 10% by mass N-phenylmaleimide meta-xylene solution, to which isopropyl benzoate was added as an internal standard substance. Measurement was performed under the following conditions using liquid chromatography (Waters Corporation, UPLC H-class) to determine the content.
Detector: PDA (detection wavelength: 210 nm to 300 nm)
Column used: ACQUITY UPLC HSS T3
Column temperature: 40°C
Mobile phase: 50% acetonitrile aqueous solution containing 0.1% formic acid Flow rate: 0.4 mL/min

3. Determination of remaining amount of N-substituted maleimide monomer
A part of the analysis target (polymerization solution after polymerization, or methacrylic resin pellets) was collected and weighed, and this sample was dissolved in chloroform to prepare a 5% by mass solution. n-Decane was added as an internal standard substance, and measurement was performed using gas chromatography (Shimadzu Corporation GC-2010) under the following conditions to determine the concentration.
Detector: FID
Column used: ZB-1
Measurement conditions: After holding at 45°C for 5 minutes, the temperature was increased to 300°C at a rate of 20°C/min, and then held for 15 minutes.

4. Analysis of structural units
Unless otherwise specified, each structural unit in the methacrylic resins produced in the Production Examples and Comparative Examples described below was identified for the methacrylic resins and methacrylic resin compositions by ¹ H-NMR measurement and ¹³ C-NMR measurement, and the amount of each structural unit was calculated. The measurement conditions for ¹ H-NMR measurement and ¹³ C-NMR measurement are as follows.
・Measuring equipment: JNM-ECZ400S manufactured by JEOL Ltd.
Measurement solvent: CDCl ₃ or d ₆ -DMSO
・Measurement temperature: 40℃

<5. Molecular weight and molecular weight distribution>
The weight average molecular weight (Mw), number average molecular weight (Mn), and Z average molecular weight (Mz) of the methacrylic resin compositions produced in the Production Examples and Production Comparative Examples described below were measured using the following apparatus and under the following conditions.
Measurement device: gel permeation chromatography (HLC-8320GPC) manufactured by Tosoh Corporation
Measurement conditions:
Columns: one TSKguard column Super H-H, two TSKgel Super HM-M, and one TSKgel Super H2500 were connected in series in this order and used.
Column temperature: 40°C
Developing solvent: tetrahydrofuran, flow rate: 0.6 mL/min, 2,6-di-t-butyl-4-methylphenol (BHT) was added as an internal standard at 0.1 g/L.
Detector: RI (differential refraction) detector Detection sensitivity: 3.0 mV/min Sample: 20 mL of tetrahydrofuran solution of 0.02 g of methacrylic resin composition Injection amount: 10 μL
Standard sample for calibration curve: The following ten types of polymethyl methacrylate (PMMA Calibration Kit MM-10, manufactured by Polymer Laboratories) having known monodisperse weight peak molecular weights and different molecular weights were used.
Weight peak molecular weight (Mp)
Standard sample 1 1,916,000
Standard sample 2 625,500
Standard sample 3 298,900
Standard sample 4 138,600
Standard sample 5 60,150
Standard sample 6 27,600
Standard sample 7 10,290
Standard sample 8 5,000
Standard sample 9 2,810
Standard sample 10 850
Under the above conditions, the RI detection intensity was measured versus the elution time of the methacrylic resin composition.
Based on each calibration curve obtained by measuring the standard samples for the calibration curve, the weight average molecular weight (Mw), number average molecular weight (Mn), and Z average molecular weight (Mz) of the methacrylic resin composition were determined, and the molecular weight distributions (Mw/Mn) and (Mz/Mw) were determined using these values.

<6. Phase difference within the effective diameter of the resin lens>
For the resin lenses obtained in the examples and comparative examples, a surface distribution of the phase difference of the lens was measured from the optical axis direction at a wavelength of 520 nm using a birefringence evaluation system PA-200-L manufactured by Photonic Lattice, Inc., and an area was designated within the effective diameter (Φ41 mm) of the lens to determine the average absolute value (nm) of the phase difference.

7. Content of fluorescent substance in resin lens
The resin lenses obtained in the examples and comparative examples were shredded, weighed into a glass sample bottle, and chloroform was added. The bottle was then shaken at a speed of 800 revolutions per minute for 30 minutes using a shaker to prepare a 2.0 mass % chloroform solution of the resin lens. The fluorescence intensity was measured using a fluorescence spectrophotometer (Fluorolog3-22, manufactured by Horiba Jobin Yvon).
The measurement conditions were as follows: a xenon lamp was used as the light source, a PMT was used as the detector, the excitation wavelength was 436 nm, the slit width was 2 nm on both the excitation side and the observation side, the time constant was 0.2 s, and the measurement mode was Sc/Rc, which normalizes the emission intensity by the excitation light intensity of each wavelength, and the measurements were performed with 90° observation using a quartz cell with an optical path length of 1 cm.
The obtained fluorescence intensity at a wavelength of 530 nm was normalized using the fluorescence intensity of a fluorescein/ethanol solution.
The fluorescence intensity at a wavelength of 530 nm was determined when ethanol, and 5×10 ^-8 mol/L and 1×10 ^-6 mol/L fluorescein/ethanol solutions were each subjected to spectroscopic analysis at an excitation wavelength of 436 nm, and after subtracting the background of ethanol, a concentration-intensity conversion formula was prepared. The fluorescence emission spectrum of chloroform was measured, and the emission intensity obtained by subtracting the emission intensity at a wavelength of 530 nm of chloroform from the emission intensity at a wavelength of 530 nm of the 2.0 mass % chloroform solution of the resin lens was converted into a concentration using the concentration-intensity conversion formula for the fluorescein/ethanol solution obtained above, to determine the content (mol/L) of the fluorescent substance.

<8. Glass transition temperature of resin lens>
The glass transition temperature (Tg) (° C.) of the resin lens was measured in accordance with JIS-K7121.
First, test pieces of about 10 mg each were cut out from a resin lens that had been conditioned (left at 23° C. for one week) under standard conditions (23° C., 50% RH) at four points (four locations).
Next, a differential scanning calorimeter (Diamond DSC manufactured by PerkinElmer Japan Co., Ltd.) was used under the condition of a nitrogen gas flow rate of 25 mL/min, where the sample was heated from room temperature (23°C) to 200°C (first heating) at 10°C/min, held at 200°C for 5 minutes to completely melt the sample, then cooled from 200°C to 40°C at 10°C/min, held at 40°C for 5 minutes, and heated again under the above heating conditions (second heating). Among the DSC curves drawn during this period, the intersection point (midpoint glass transition temperature) between the step-like change partial curve during the second heating and a straight line equidistant in the vertical direction from each baseline extension line was measured as the glass transition temperature (Tg) (°C). Four measurements were performed per sample, and the arithmetic average (rounded off to the nearest whole number) of the four measurements was taken as the measured value.

<9. Photoelastic coefficient of resin lens>
The resin lenses obtained in the examples and comparative examples were each shredded and then pressed into films using a vacuum compression molding machine to prepare measurement samples.
Specific sample preparation conditions were as follows: using a vacuum compression molding machine (manufactured by Shinto Metal Industries, SFV-30 type), the resin lens was preheated at 260°C under reduced pressure (about 10 kPa) for 10 minutes, compressed at 260°C and about 10 MPa for 5 minutes, and after releasing the reduced pressure and press pressure, transferred to a cooling compression molding machine for cooling and solidification. The obtained pressed film was aged for 24 hours or more in a constant temperature and humidity room adjusted to 23°C and humidity 60%, and then a test specimen for measurement (thickness about 150 μm, width 6 mm) was cut out.
The photoelastic coefficient C _R (Pa ⁻¹ ) was measured using a birefringence measuring device described in detail in Polymer Engineering and Science 1999, 39, 2349-2357.
The film-like test piece was placed in a film tensioning device (manufactured by Imoto Manufacturing Co., Ltd.) similarly installed in a constant temperature and humidity chamber so that the distance between the chucks was 50 mm. Next, a birefringence measuring device (manufactured by Otsuka Electronics, RETS-100) was placed so that the laser light path of the device was located at the center of the film, and the birefringence of the test piece was measured while applying a tensile stress at a strain rate of 50%/min (distance between chucks: 50 mm, chuck movement speed: 5 mm/min).
From the relationship between the measured birefringence (Δn) and the tensile stress (σ _R ), the slope of the line was determined by least squares approximation, and the photoelastic coefficient (C _R ) (Pa ⁻¹ ) was calculated. Data for the tensile stress in the range of 2.5 MPa≦σ _R ≦10 MPa was used for the calculation.
C _R = Δn/σ _R
Here, the birefringence (Δn) has the value shown below.
Δn=nx−ny
(nx: refractive index in the stretching direction, ny: refractive index in the in-plane direction perpendicular to the stretching direction)

<10. Light transmittance of resin lens>
The resin lenses obtained in the Examples and Comparative Examples were measured for transmittance (%) at 5 nm intervals in the wavelength range of 380 to 780 nm with a D65 light source and a 10° field of view, with the light source passing through the thickest part of the lens in the thickness direction, using a spectrocolorimeter (SD-5000, manufactured by Nippon Denshoku Industries Co., Ltd.) The measured values were used to determine the ratio (T450/T680) of the transmittance at a wavelength of 450 nm (T450) to the transmittance at a wavelength of 680 nm (T680).

11. Image clarity
Using the resin lenses obtained in the examples and comparative examples, a simulation device reproducing the principle of the head mounted display described in JP 2017-21321 A as shown in FIG. 1 was produced in a dark room.
In the simulation device, a liquid crystal display 1, a polarizing plate 2, a quarter-wave plate 3, a half mirror 4, a resin lens 5 obtained in the examples and comparative examples, a quarter-wave plate 6, and a reflective polarizing plate 7 are arranged on the same axis in this order. When light along the optical axis from the liquid crystal display 1 passes through the polarizing plate 2, the quarter-wave plate 3, the half mirror 4, the resin lens 5, and the quarter-wave plate 6 to reach the reflective polarizing plate 7, it is reflected by the reflective polarizing plate 7 (first reflection), then passes through the quarter-wave plate 6 and the resin lens 5 to reach the half mirror 4, it is reflected by the half mirror 4 (second reflection), and again passes through the resin lens 5, the quarter-wave plate 6, and the reflective polarizing plate 7 to reach the human eye. The half mirror 4 allows a part of the light to be reflected and the remaining light to be transmitted. The light increases the phase delay of 90 degrees each time it passes through the quarter-wave plate. The image is enlarged by the above two reflections and reaches the human eye.
A still image displayed on the liquid crystal display 1 was observed through the simulator, and the clarity of the image was evaluated according to the following criteria.
[Evaluation Criteria]
A: The image is enlarged and there is no bleeding or blurring.
Δ: An enlarged image is overlapped with an image with a lower magnification, making the image somewhat unclear.
×: Images with low magnification are mainly seen and are unclear.

[Raw materials]
The raw materials used in the examples and comparative examples described later are shown below.
[[monomer]]
Methyl methacrylate (MMA): manufactured by Asahi Kasei Corporation N-cyclohexylmaleimide (chMI): manufactured by Nippon Shokubai Co., Ltd. (The mass ratio of 2-cyclohexylamino-N-cyclohexylsuccinimide (CCSI) to the mass of chMI is 80 ppm by mass)
N-Phenylmaleimide (phMI): manufactured by Nippon Shokubai Co., Ltd. (The mass ratio of 2-anilino-N-phenylsuccinimide (APSI) to the mass of phMI is 60 ppm by mass)

-Pretreatment of N-substituted maleimide (chMI, phMI) (water washing step and dehydration step)-
The above N-cyclohexylmaleimide (chMI) and N-phenylmaleimide (phMI) were subjected to the following water washing and dehydration steps to remove 2-amino-N-substituted succinimide (CCSI, APSI) and water contained therein.

--Washing and dehydration of N-cyclohexylmaleimide (chMI)--
---Reducing CCSI in N-cyclohexylmaleimide to 5 ppm by mass or less---
250.0 kg of chMI and 750.0 kg of meta-xylene (hereinafter referred to as "mXy") were weighed and added to a 2.0 ^m3 glass-lined reactor equipped with a jacket temperature control device and three receding blades as stirring blades, and steam was blown into the jacket to raise the solution temperature in the reactor to 56 ° C., and the organic layer was obtained. Next, 350.0 kg of 2% by mass sulfuric acid water was weighed and added to the reactor, the solution temperature was kept at 56 ° C., and the mixture was stirred at 100 rpm for 10 minutes. The stirring was stopped, the mixture was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum.
The same procedure was repeated two more times, and the organic layer was washed with aqueous sulfuric acid three times in total. The amount of CCSI in the organic layer was quantified by gas chromatography, and found to be 4.9 ppm by mass relative to chMI in the organic layer.
Thereafter, 350.0 kg of ion-exchanged water was added to the reactor, and the solution was stirred at 100 rpm for 10 minutes while maintaining the solution temperature at 55° C. The stirring was stopped, the solution was allowed to stand for 20 minutes, and the aqueous layer was extracted into a drum can.
The same procedure was repeated once more, and the organic layer was washed with ion-exchanged water twice in total. The amount of CCSI in the organic layer was quantified by gas chromatography, and found to be 4.6 ppm by mass relative to chMI in the organic layer.
Next, the solution temperature was kept at 50° C., and the pressure in the reactor was gradually reduced while stirring at 100 rpm, until the pressure in the reactor reached 5 kPa. Thereafter, the solution temperature was raised to 60° C., and an azeotropic dehydration operation was performed. 131.6 kg of the water/mXy mixture was distilled off, and the water concentration in the organic layer was quantified with a Karl Fischer water content meter, which was 102 ppm by mass relative to the mass of the organic layer.
Concentration adjusting agent mXy was added to the organic layer to obtain 1,034.3 kg of an organic layer having a chMI of 24.0 mass %, a water concentration of 191 mass ppm, and a CCSI of 4.6 mass ppm relative to the mass of chMI.

---Reducing CCSI in N-cyclohexylmaleimide to 1 ppm by mass or less---
80.0 kg of chMI and 240.0 kg of mXy were weighed and added to a 0.50 ^m3 glass-lined reactor equipped with a jacket temperature control device and a Kobe Steel Eco-Solutions Fullzone stirring blade, and steam was blown into the jacket to raise the solution temperature in the reactor to 55°C, followed by stirring to obtain an organic layer. Next, 112.0 kg of 2% by mass sulfuric acid water was weighed and added to the reactor, the solution temperature was kept at 55°C, and the mixture was stirred at 100 rpm for 30 minutes. The stirring was stopped, the mixture was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum.
The same procedure was repeated three more times, and the organic layer was washed with aqueous sulfuric acid four times in total. The amount of CCSI in the organic layer was quantified by gas chromatography, and found to be 0.57 ppm by mass relative to chMI in the organic layer.
Thereafter, 112.0 kg of ion-exchanged water was added to the reactor, and the solution was stirred at 100 rpm for 30 minutes while maintaining the solution temperature at 55° C. The stirring was stopped, the solution was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum can.
The solution temperature was kept at 50° C., and the pressure in the reactor was gradually reduced while stirring at 100 rpm, until the pressure in the reactor reached 5 kPa. The solution temperature was then raised to 60° C., and azeotropic dehydration was performed. 54 kg of the water/mXy mixture was distilled off, and the water concentration in the organic layer was quantified with a Karl Fischer water content meter, which was 46 ppm by mass relative to the mass of the organic layer.
Concentration adjusting agent mXy was added to the organic layer to obtain 384.0 kg of an organic layer having a chMI of 20.3 mass %, a water concentration of 125 mass ppm, and a CCSI of 0.60 mass ppm relative to the mass of chMI.

--Washing and dehydration of N-phenylmaleimide (phMI)--
---Reducing APSI in N-phenylmaleimide to 5 ppm by mass or less---
150.0 kg of phMI and 720.0 kg of mXy were weighed and added to a 2.0 ^m3 glass-lined reactor equipped with a jacketed temperature control device and three receding blades as stirring blades, and steam was blown into the jacket to raise the solution temperature in the reactor to 55°C, followed by stirring to obtain an organic layer. Next, 336.0 kg of 7% by mass sodium bicarbonate water was weighed and added to the reactor, the solution temperature was kept at 55°C, and the mixture was stirred at 100 rpm for 10 minutes. The stirring was stopped, the mixture was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum.
Next, 336.0 kg of ion-exchanged water was added to the reactor, and the solution was stirred at 100 rpm for 10 minutes while maintaining the solution temperature at 55° C. The stirring was stopped, the solution was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum can.
Thereafter, 336.0 kg of 2% by mass aqueous sulfuric acid was weighed and added to the reactor, and the solution temperature was kept at 55° C. and stirred at 100 rpm for 10 minutes. The stirring was stopped and the solution was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum can.
The same washing operation with ion-exchanged water was carried out two more times in the same manner as above.
The amount of APSI in the organic layer was quantified by liquid chromatography and found to be 3.6 ppm by mass relative to the phMI in the organic layer.
Next, the solution temperature was kept at 50° C., and the pressure in the reactor was gradually reduced while stirring at 100 rpm, until the pressure in the reactor reached 5 kPa. Thereafter, the solution temperature was raised to 55° C., and an azeotropic dehydration operation was performed. 190 kg of the water/mXy mixture was distilled off, and the water concentration in the organic layer was quantified with a Karl Fischer water content meter, which was 47 ppm by mass relative to the mass of the organic layer.
Concentration adjusting agent mXy was added to the organic layer to obtain 1,340.7 kg of an organic layer having a phMI of 10.8 mass %, a water concentration of 170 mass ppm, and an APSI of 3.4 mass ppm relative to the mass of phMI.

---Reducing APSI in N-phenylmaleimide to 1 ppm by mass or less---
50.0 kg of phMI and 240.0 kg of mXy were weighed and added to a 0.50 ^m3 glass-lined reactor equipped with a jacket temperature control device and a Kobe Steel Eco-Solutions Fullzone stirring blade, and steam was blown into the jacket to raise the solution temperature in the reactor to 55°C, followed by stirring to obtain an organic layer. Next, 112.0 kg of 7% by mass sodium bicarbonate water was weighed and added to the reactor, the solution temperature was kept at 55°C, and the mixture was stirred at 100 rpm for 15 minutes. The stirring was stopped, the mixture was left to stand for 10 minutes, and the aqueous layer was extracted into a drum. The same operation was repeated once more, and the organic layer was washed with sodium bicarbonate water twice in total.
Next, 112.0 kg of ion-exchanged water was added to the reactor, and the solution was stirred at 100 rpm for 30 minutes while maintaining the solution temperature at 55° C. The stirring was stopped, the solution was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum can.
Thereafter, 112.0 kg of 2% by mass sulfuric acid water was weighed and added to the reactor, and the solution temperature was kept at 55° C. and stirred at 100 rpm for 30 minutes. Stirring was stopped, the solution was allowed to stand for 10 minutes, and the aqueous layer was extracted into a drum. The same operation was repeated two more times, and the organic layer was washed with sulfuric acid water three times in total.
The same washing operation with ion-exchanged water was carried out two more times in the same manner as above.
The amount of APSI in the organic layer was quantitatively determined by liquid chromatography, and was found to be 0.37 ppm by mass relative to the phMI in the organic layer.
The solution temperature was kept at 50° C., and the pressure in the reactor was gradually reduced while stirring at 100 rpm, until the pressure in the reactor reached 5 kPa. The solution temperature was then raised to 55° C., and azeotropic dehydration was performed. 72 kg of the water/mXy mixture was distilled off, and the water concentration in the organic layer was quantified with a Karl Fischer water content meter, which was 60 ppm by mass relative to the mass of the organic layer.
Concentration adjusting agent mXy was added to the organic layer to obtain 432.8 kg of an organic layer having a phMI of 10.3 mass %, a water concentration of 180 mass ppm, and an APSI of 0.42 mass ppm relative to the mass of phMI.

[[Polymerization initiator]]
1,1-di(t-butylperoxy)cyclohexane: "Perhexa C" manufactured by NOF Corporation
[[Chain transfer agent]]
n-Octyl mercaptan: manufactured by Kao Corporation [hindered phenol antioxidant]
Irganox 1010: BASF [phosphorus-based antioxidant]
Irgafos 168 (melting point 180-190°C): manufactured by BASF

[Methacrylic resin composition]
[Production Example 1]
374.8 kg of a 10.3 mass% mXy solution of phMI (APSI was 0.42 mass ppm relative to the mass of phMI) that had been through the water washing process and the dehydration process, and 323.6 kg of a 20.3 mass% mXy solution of chMI (CCSI was 0.60 mass ppm relative to the mass of chMI) that had been through the water washing process and the dehydration process were weighed, and added to a 1.25 ^m3 reactor equipped with a jacketed temperature control device and a stirring blade, and 160.8 kg of mXy was distilled off under reduced pressure while stirring at a solution temperature of 60°C and a reactor pressure of 5 kPa. Next, the reaction vessel was returned to normal pressure, and 16.7 kg of mXy was added to prepare a mixed solution of 38.6 kg of phMI, 65.7 kg of chMI, and 450.0 kg of mXy. 445.7 kg of MMA and 0.413 kg of n-octyl mercaptan as a chain transfer agent were weighed and charged and stirred to obtain a monomer mixture solution.
The liquid in the reactor was bubbled with nitrogen at a rate of 30 L/min for 1 hour to remove dissolved oxygen.
Thereafter, steam was blown into the jacket to raise the solution temperature in the reactor to 125° C., and polymerization was initiated by adding a polymerization initiator solution prepared by dissolving 0.23 kg of 1,1-di(t-butylperoxy)cyclohexane in 2.77 kg of mXy at a rate of 0.5 kg/hour while stirring at 50 rpm, and the addition was stopped 6 hours after the initiation of polymerization.
During the polymerization, the solution temperature in the reactor was controlled at 125±2° C. by adjusting the temperature with a jacket.
After 8 hours had elapsed since the start of polymerization, a polymerization solution containing a methacrylic resin having a ring structure in the main chain was obtained.
The amount of N-substituted maleimide contained in the obtained polymerization solution was evaluated, and it was found to contain 1,340 ppm by mass of phMI and 4,390 ppm by mass of chMI.
To this polymerization solution, 0.1% by mass of Irganox 1010 and 0.05% by mass of Irgafos 168 were added with stirring, based on 100% by mass of the polymer contained in the solution.
This polymerization solution containing the antioxidant was filtered through a filter made of SUS316L metal fiber and having a filtration accuracy of 2 μm.
In order to recover the polymer from the polymerization solution, a devolatilization apparatus comprising a flat plate heat exchanger having a flat slit-type flow path and a heat medium flow path, and a SUS heat medium jacketed reduced pressure vessel having an internal volume of about 0.3 ^m3 (hereinafter referred to as a devolatilization tank) was used as the apparatus used in the devolatilization step.
The solution containing the polymer obtained by polymerization was supplied to a heat exchanger installed at the top of the reduced pressure vessel at a rate of 30 L/h and heated to 260°C, and then supplied to a devolatilization tank heated and reduced pressure to an internal temperature of 260°C and a vacuum degree of 30 Torr, where devolatilization treatment was performed. The shear rate in the devolatilization apparatus was calculated to be 5.3 s ^-1 from the apparatus shape and operating conditions.
The polymer after devolatilization was pressurized from the bottom of the devolatilization tank by a gear pump, extruded through a strand die, cooled with water, and pelletized to obtain a methacrylic resin composition A.
The composition of the methacrylic resin composition A was confirmed to be 81.2 mass%, 7.1 mass%, and 11.7 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively. The weight average molecular weight Mw was 148,000, Mw/Mn was 2.12, Mz/Mw was 1.63, and the glass transition temperature was 133°C.

[Production Example 2]
352.4 kg of a 10.3 mass% mXy solution of phMI (APSI was 0.42 mass ppm relative to the mass of phMI) that had been through the water washing process and the dehydration process, and 310.3 kg of a 20.3 mass% mXy solution of chMI (CCSI was 0.60 mass ppm relative to the mass of chMI) that had been through the water washing process and the dehydration process were weighed, and added to a 1.25 ^m3 reactor equipped with a jacketed temperature control device and a stirring blade, and 335.4 kg of mXy was distilled off under reduced pressure while stirring at a solution temperature of 60 ° C. and a reactor pressure of 5 kPa. Next, the reaction vessel was returned to normal pressure, and 8.9 kg of mXy was added to prepare a mixed solution of 36.3 kg of phMI, 63.0 kg of chMI, and 236.9 kg of mXy. 340.7 kg of MMA and 0.275 kg of n-octyl mercaptan as a chain transfer agent were weighed and charged, followed by stirring to obtain a monomer mixture solution.
Then, 123.1 kg of mXy was weighed and added to Tank 1.
Furthermore, 110.0 kg of MMA and 90.0 kg of mXy were weighed and stirred in tank 2 to prepare a monomer solution for additional addition.
The liquid in the reactor was bubbled with nitrogen at a rate of 30 L/min for 1 hour, and tank 1 and tank 2 were each bubbled with nitrogen at a rate of 10 L/min for 30 minutes to remove dissolved oxygen.
Thereafter, steam was blown into the jacket to raise the solution temperature in the reactor to 128°C, and polymerization was initiated by adding a polymerization initiator solution prepared by dissolving 0.37 kg of 1,1-di(t-butylperoxy)cyclohexane in 3.005 kg of mXy at a rate of 1 kg/hour while stirring at 50 rpm. 0.5 hour after the start of polymerization, the addition rate of the initiator solution was reduced to 0.25 kg/hour, and mXy was added from Tank 1 at 35.2 kg/hour for 3.5 hours.
During the polymerization, the solution temperature in the reactor was controlled at 128±2° C. by adjusting the temperature with a jacket.
Then, 4 hours after the initiation of polymerization, the addition rate of the initiator solution was changed to 0.75 kg/hour, and the monomer solution containing MMA was added from Tank 2 at a rate of 100.0 kg/hour for 2 hours.
Further, 6 hours after the start of polymerization, the addition rate of the initiator solution was reduced to 0.5 kg/hour, and 7 hours after the start of polymerization, the addition was stopped. The polymerization was continued for another 1 hour to obtain a polymerization solution containing a methacrylic resin having a ring structural unit in the main chain.
The amount of N-substituted maleimide contained in the obtained polymerization solution was evaluated, and it was found to contain 220 ppm by mass of phMI and 1,070 ppm by mass of chMI.
To this polymerization solution, 0.1% by mass of Irganox 1010 and 0.05% by mass of Irgafos 168 were added with stirring, based on 100% by mass of the polymer contained in the solution.
The polymerization solution containing the antioxidant was filtered through a filter made of SUS316L metal fiber and having a filtration accuracy of 2 μm.
In order to recover the polymer from the polymerization solution, a devolatilization apparatus having no rotating part and comprising a flat plate heat exchanger having a flat slit-type flow path and a heat medium flow path, and a SUS heat medium jacketed reduced pressure vessel having an internal volume of about 0.3 ^m3 (hereinafter referred to as a devolatilization tank) was used as the apparatus used in the devolatilization step.
The solution containing the polymer obtained by polymerization was supplied to a heat exchanger installed at the top of the reduced pressure vessel at a rate of 30 L/h and heated to 260°C, and then supplied to a devolatilization tank heated and reduced to an internal temperature of 260°C and a degree of vacuum of 30 Torr for devolatilization treatment. The shear rate in the devolatilization apparatus was calculated to be 5.3 s ^-1 from the apparatus shape and operating conditions.
The polymer after the devolatilization was pressurized from the bottom of the devolatilization tank by a gear pump, extruded through a strand die, cooled with water, and pelletized to obtain a methacrylic resin composition B.
The composition of the methacrylic resin composition B was confirmed to be 80.9 mass%, 7.0 mass%, and 12.1 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively. The weight average molecular weight Mw was 142,000, Mw/Mn was 2.32, Mz/Mw was 1.75, and the glass transition temperature was 134°C.

[Production Example 3]
A 10.8 mass% mXy solution (APSI was 3.4 mass ppm relative to the mass of phMI) was used as phMI that had been subjected to a water-washing step and a dehydration step, and a 24.0 mass% mXy solution (CCSI was 4.6 mass ppm relative to the mass of chMI) was used as chMI that had been subjected to a water-washing step and a dehydration step, and a methacrylic resin composition C was obtained in the same manner as in Production Example 2, except that the composition of the mixed solution prepared in the reactor was the same as in Production Example 2.
The amount of N-substituted maleimide contained in the obtained polymerization solution was evaluated, and it was found that the solution contained 210 ppm by mass of phMI and 1,090 ppm by mass of chMI.
The composition of the methacrylic resin composition C was confirmed to be 80.8 mass%, 7.1 mass%, and 12.1 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively. The weight average molecular weight Mw was 141,000, Mw/Mn was 2.31, Mz/Mw was 1.75, and the glass transition temperature was 134°C.

[Production Example 4]
In a reactor equipped with a jacketed temperature control device and an agitator, 40 parts by mass of MMA, 60 parts by mass of mXy, and 0.08 parts by mass of n-octyl mercaptan, a chain transfer agent, were charged, and nitrogen bubbling was performed for 1 hour to remove dissolved oxygen. Thereafter, steam was blown into the jacket to raise the solution temperature in the reactor to 120°C, and polymerization was performed by adding a polymerization initiator solution in which 0.02 parts by mass of 1,1-di(t-butylperoxy)cyclohexane was dissolved in 0.10 parts by mass of mXy at a constant speed for 5 hours while stirring at 50 rpm, and further aging was performed at 120°C for 3 hours, and the polymerization was terminated 8 hours after the start of polymerization to obtain a PMMA mXy solution. After the polymerization was completed, the liquid temperature was lowered to 50°C.
Next, a mixed liquid consisting of 12 parts by mass of monomethylamine and 12 parts by mass of methanol was dropped into the reactor at room temperature, and the liquid temperature was raised to 170° C. and stirred under pressure for 1 hour to allow the glutarimide cyclization reaction to proceed. The liquid temperature was lowered to 120° C., and the inside of the reactor was reduced in pressure to distill off unreacted monomethylamine and methanol, as well as a portion of mXy, to obtain a glutarimide-cyclized methacrylic polymer in an mXy solution of about 50% by mass.
To this polymerization solution, 0.1% by mass of Irganox 1010 and 0.05% by mass of Irgafos 168 were added with stirring, based on 100% by mass of the polymer contained in the solution.
The polymerization solution containing the antioxidant was filtered through a filter made of SUS316L metal fiber and having a filtration accuracy of 2 μm.
In order to recover the polymer from the polymerization solution, a devolatilization apparatus having no rotating part and comprising a flat plate heat exchanger having a flat slit-type flow path and a heat medium flow path, and a SUS heat medium jacketed reduced pressure vessel having an internal volume of about 0.3 ^m3 (hereinafter referred to as a devolatilization tank) was used as the apparatus used in the devolatilization step.
The solution containing the polymer obtained by polymerization was supplied to a heat exchanger installed at the top of the reduced pressure vessel at a rate of 30 L/h and heated to 260°C, and then supplied to a devolatilization tank heated and reduced to an internal temperature of 260°C and a degree of vacuum of 30 Torr for devolatilization treatment. The shear rate in the devolatilization apparatus was calculated to be 5.3 s ^-1 from the apparatus shape and operating conditions.
The polymer after the devolatilization was pressurized from the bottom of the devolatilization tank by a gear pump, extruded through a strand die, cooled with water, and pelletized to obtain a methacrylic resin composition D.
The composition of the obtained methacrylic resin composition D was confirmed to have an imidization rate of 3.2%, a glutarimide structural unit content of 5.1% by mass, a weight average molecular weight Mw of 93,000, Mw/Mn of 1.79, Mz/Mw of 1.51, and a glass transition temperature of 122°C.

[Production Example 5]
A methacrylic resin composition E was obtained in the same manner as in Production Example 2, except that the amounts of each monomer in the solution prepared in the reactor were 39.3 kg of phMI, 46.5 kg of chMI, and 354.2 kg of MMA.
The composition of the methacrylic resin composition E was confirmed to be 83.2 mass%, 7.6 mass%, and 9.2 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively, and the weight average molecular weight Mw was 143,000, Mw/Mn was 2.35, Mz/Mw was 1.81, and the glass transition temperature was 132°C.

[Comparative Production Example 1]
352.4 kg of a 10.3 mass% mXy solution of phMI (APSI was 0.42 mass ppm relative to the mass of phMI) that had been through the water washing process and the dehydration process, and 310.3 kg of a 20.3 mass% mXy solution of chMI (CCSI was 0.60 mass ppm relative to the mass of chMI) that had been through the water washing process and the dehydration process were weighed, and added to a 1.25 ^m3 reactor equipped with a jacketed temperature control device and a stirring blade, and 335.4 kg of mXy was distilled off under reduced pressure while stirring at a solution temperature of 60 ° C. and a reactor pressure of 5 kPa. Next, the reaction vessel was returned to normal pressure, and 8.9 kg of mXy was added to prepare a mixed solution of 36.3 kg of phMI, 63.0 kg of chMI, and 236.9 kg of mXy. 340.7 kg of MMA and 0.275 kg of n-octyl mercaptan as a chain transfer agent were weighed and charged, followed by stirring to obtain a monomer mixture solution.
Then, 123.1 kg of mXy was weighed and added to Tank 1.
Furthermore, 110.0 kg of MMA and 90.0 kg of mXy were weighed and stirred in tank 2 to prepare a monomer solution for additional addition.
The liquid in the reactor was bubbled with nitrogen at a rate of 30 L/min for 1 hour, and tank 1 and tank 2 were each bubbled with nitrogen at a rate of 10 L/min for 30 minutes to remove dissolved oxygen.
Thereafter, steam was blown into the jacket to raise the solution temperature in the reactor to 128°C, and polymerization was initiated by adding a polymerization initiator solution prepared by dissolving 0.37 kg of 1,1-di(t-butylperoxy)cyclohexane in 3.005 kg of mXy at a rate of 1 kg/hour while stirring at 50 rpm. 0.5 hour after the start of polymerization, the addition rate of the initiator solution was reduced to 0.25 kg/hour, and mXy was added from Tank 1 at 35.2 kg/hour for 3.5 hours.
During the polymerization, the solution temperature in the reactor was controlled at 128±2° C. by adjusting the temperature with a jacket.
Then, 4 hours after the initiation of polymerization, the addition rate of the initiator solution was changed to 0.75 kg/hour, and the monomer solution containing MMA was added from Tank 2 at a rate of 100.0 kg/hour for 2 hours.
Further, 6 hours after the start of polymerization, the addition rate of the initiator solution was reduced to 0.5 kg/hour, and 7 hours after the start of polymerization, the addition was stopped. The polymerization was continued for another 1 hour to obtain a polymerization solution containing a methacrylic resin having a ring structural unit in the main chain.
The amount of N-substituted maleimide contained in the obtained polymerization solution was evaluated, and it was found to contain 220 ppm by mass of phMI and 1,070 ppm by mass of chMI.
To this polymerization solution, 0.1% by mass of Irganox 1010 and 0.05% by mass of Irgafos 168 were added with stirring, based on 100% by mass of the polymer contained in the solution.
The polymerization solution containing the antioxidant was filtered through a filter made of SUS316L metal fiber and having a filtration accuracy of 2 μm.
Next, the obtained polymerization solution was introduced into a twin-screw extruder equipped with a plurality of vents for devolatilization, whereby devolatilization was carried out. The obtained polymerization solution was fed into the twin-screw extruder so as to be 10 kg/h in terms of resin, and the conditions were a barrel temperature of 260° C., a screw rotation speed of 150 rpm, and a vacuum degree of 10 to 40 Torr. The resin devolatilized by the twin-screw extruder was extruded through a strand die, cooled with water, and pelletized to obtain a methacrylic resin composition F. The shear rate in the devolatilizer was calculated to be 80 s ⁻¹ from the shape of the device and the operating conditions.
The composition of the methacrylic resin composition F was confirmed to be 80.9 mass%, 7.0 mass%, and 12.1 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively. The weight average molecular weight Mw was 136,000, Mw/Mn was 2.35, Mz/Mw was 1.81, and the glass transition temperature was 134°C.

[Comparative Production Example 2]
A methacrylic resin composition G was obtained in the same manner as in Production Example 2, except that commercially available unpurified phMI and chMI were used to prepare a mixed solution of 36.3 kg of phMI, 63.0 kg of chMI, and 236.9 kg of mXy. The shear rate in the devolatilizer was calculated to be 5.3 s ^-1 from the shape of the apparatus and the operating conditions.
The amount of N-substituted maleimide contained in the obtained polymerization solution was evaluated, and it was found to contain 220 ppm by mass of phMI and 1,070 ppm by mass of chMI.
The composition of the obtained pellet-like polymer was confirmed to be 80.9 mass%, 7.1 mass%, and 12.1 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively, and the weight-average molecular weight Mw was 142,000, Mw/Mn was 2.32, Mz/Mw was 1.75, and the glass transition temperature was 134° C.

[Comparative Production Example 3]
Commercially available unpurified phMI and chMI were used. 38.6 kg of phMI, 65.7 kg of chMI, 450.0 kg of mXy, 445.7 kg of MMA, and 0.413 kg of n-octyl mercaptan as a chain transfer agent were weighed out and charged into a 1.25 ^m3 reactor equipped with a jacketed temperature control device and an agitator, and stirred to obtain a monomer mixture solution.
The liquid in the reactor was bubbled with nitrogen at a rate of 30 L/min for 1 hour to remove dissolved oxygen.
Thereafter, steam was blown into the jacket to raise the solution temperature in the reactor to 125° C., and polymerization was initiated by adding a polymerization initiator solution prepared by dissolving 0.23 kg of 1,1-di(t-butylperoxy)cyclohexane in 2.77 kg of mXy at a rate of 0.5 kg/hour while stirring at 50 rpm, and the addition was stopped 6 hours after the initiation of polymerization.
During the polymerization, the solution temperature in the reactor was controlled at 125±2° C. by adjusting the temperature with a jacket.
After 8 hours had elapsed since the start of polymerization, a polymerization solution containing a methacrylic resin having a ring structure in the main chain was obtained.
The amount of N-substituted maleimide contained in the obtained polymerization solution was evaluated, and it was found to contain 1,340 ppm by mass of phMI and 4,390 ppm by mass of chMI.
To this polymerization solution, 0.1% by mass of Irganox 1010 and 0.05% by mass of Irgafos 168 were added with stirring, based on 100% by mass of the polymer contained in the solution.
This polymerization solution containing the antioxidant was filtered through a filter made of SUS316L metal fiber and having a filtration accuracy of 2 μm.
The polymerization solution containing the antioxidant was supplied to a concentration device consisting of a tubular heat exchanger and a vaporization tank previously heated to 170°C, and the concentration of the polymer contained in the solution was increased to 70 mass%, and then the obtained polymerization solution was supplied to a thin-film evaporator having a rotating part and a heat transfer area of 0.2 ^m2 , and devolatilization was performed.
The temperature inside the apparatus was 280° C., the supply amount was 30 L/hr, the rotation speed was 400 rpm, and the degree of vacuum was 30 Torr, and the polymer after devolatilization was pressurized by a gear pump, extruded from a strand die, cooled with water, and pelletized to obtain a methacrylic resin composition H. The shear rate in the thin-film evaporator having a rotating part was calculated to be 3200 ^s-1 from the shape of the apparatus and the operating conditions.
The composition of the obtained pellet-like polymer was confirmed to be 81.0 mass%, 7.2 mass%, and 11.8 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively, and the weight-average molecular weight Mw was 136,000, Mw/Mn was 2.21, Mz/Mw was 1.71, and the glass transition temperature was 134° C.

[Comparative Production Example 4]
A methacrylic resin composition I was obtained in the same manner as in Production Example 2, except that the amounts of each monomer in the solution prepared in the reactor were 41.3 kg of phMI, 41.3 kg of chMI, and 357.5 kg of MMA.
The composition of the methacrylic resin composition I was confirmed to be 84.1 mass%, 8.0 mass%, and 7.9 mass% of structural units derived from MMA, phMI, and chMI monomers, respectively, and the weight average molecular weight Mw was 145,000, Mw/Mn was 2.31, Mz/Mw was 1.77, and the glass transition temperature was 130°C.

[Comparative Production Example 5]
Polymethyl methacrylate was imidized with monomethylamine using a co-rotating twin-screw extruder to obtain a methacrylic resin composition J having a glutarimide structural unit.
Specifically, a 40 mm screw diameter co-rotating twin screw extruder was used, the extruder cylinder temperature was set to 270°C, the screw rotation speed was set to 150 rpm, polymethyl methacrylate with a weight average molecular weight Mw of 108,000 was supplied from the hopper at 20 kg/h, and nitrogen was flowed into the extruder at a flow rate of 200 mL/min. After the resin was melted and filled by the kneading block, 1.8 parts by mass of monomethylamine was injected from the nozzle per 100 parts by mass of the raw resin, and an imidization reaction was carried out. A reverse flight was inserted at the end of the reaction zone (before the vent port) to fill the resin. The by-products and excess methylamine after the reaction were removed by reducing the pressure of the vent port to 50 Torr. The resin that came out as strands from the die installed at the extruder outlet was cooled in a water tank and then pelletized with a pelletizer to obtain an imide resin.
Next, a 40 mm screw diameter co-rotating twin screw extruder was used, the extruder cylinder temperature was set to 250°C, and the screw rotation speed was set to 150 rpm. The obtained imide resin was fed at 20 kg/hr, and the resin was melted and filled by a kneading block. Then, a mixed solution of dimethyl carbonate and triethylamine was injected from a nozzle as an esterification agent to reduce the carboxylic acid group in the resin. Dimethyl carbonate was 3.2 parts by mass and triethylamine was 0.8 parts by mass per 100 parts by mass of imide resin. A reverse flight was placed at the end of the reaction zone to fill the resin. The by-products and excess dimethyl carbonate after the reaction were removed by reducing the pressure at the vent port to 50 Torr. The resin that came out as strands from the die installed at the extruder outlet was cooled in a water tank and then pelletized with a pelletizer to obtain a methacrylic resin composition J having a glutarimide structure.
The imidization rate of this resin composition was 3.3%, and the content of glutarimide-based structural units was 5.2% by mass. The shear rate in the twin-screw extruder was calculated to be about 80 s ^-1 from the device shape and operating conditions.
The weight average molecular weight Mw was 96,000, Mw/Mn was 1.82, Mz/Mw was 1.48, and the glass transition temperature was 122°C.

[Examples 1 to 5, Comparative Examples 1 to 5]
Using the methacrylic resin compositions A to J obtained in the Production Examples and the Production Comparative Examples, resin lenses were injection molded by the following method, and various properties were measured. The results are shown in Table 1.
<Molding of resin lenses>
Methacrylic resin compositions A to J were dried at 100° C. for 4 hours in a forced air circulation dryer, and then injection molding was performed using a mold for a spherical plano-convex lens having a diameter of 41 mm and a radius of 98 mm (3.2 mm at the thickest part, with a side gate (0.95 mm thick, 5.0 mm wide)) in an injection molding machine with a clamping force of 50 T, with a cylinder temperature of 270° C., a mold temperature of 116° C., an injection speed of 10 mm/s, and two-stage pressure dwelling, with the first stage being 50 MPa for 4 seconds and then the second stage being 30 MPa for 3 seconds, and a cooling time of 400 seconds.
Further, the composition was annealed for 3 hours in an oven set at a temperature 25° C. lower than the glass transition temperature of the methacrylic resin composition.

The resin lens for head-mounted displays of the present invention can provide clear images even in an optical system that uses polarized light, and can therefore contribute to making head-mounted displays smaller and lighter.

1: Liquid crystal display 2: Polarizing plate 3: 1/4 wavelength plate 4: Half mirror 5: Plastic lens 6: 1/4 wavelength plate 7: Reflective polarizing plate

Claims (9)

A head mounted display including a resin lens,
the resin lens has an average absolute value of retardation within an effective diameter of 5 nm or less;
the resin lens has a fluorescent substance content of 0.1 to 4.0×10 −9 mol/L calculated from the fluorescence intensity at a wavelength of 530 nm when a 2.0% by mass solution obtained by dissolving the resin lens in chloroform is measured at an excitation wavelength of ⁴³⁶ nm, using a concentration-intensity conversion formula for a fluorescein ethanol solution;
The head mounted display is characterized in that it utilizes polarized light, and transmits image light through the resin lens multiple times by repeating reflection. The head mounted display according to claim 1 , wherein the resin lens has a ratio (T450/T680) of a transmittance (T450) at a wavelength of 450 nm to a transmittance (T680) at a wavelength of 680 nm of 0.98 or more. The head mounted display according to claim 1 or 2, wherein the resin lens has a glass transition temperature (Tg) of more than 120° C. and not more than 160° C. The resin lens has an absolute value of a photoelastic coefficient of 3.0×10 ^-12 P ^-1 The head mounted display according to any one of claims 1 to 3, wherein: The head mounted display according to any one of claims 1 to 4, wherein the resin lens contains a methacrylic resin. The head mounted display according to claim 5 , wherein the resin lens contains a methacrylic resin having a structural unit (X) having a ring structure in a main chain. 7. The head mounted display according to claim 6, wherein the structural unit (X) of the resin lens includes at least one structural unit selected from the group consisting of a structural unit derived from an N-substituted maleimide monomer, a glutarimide structural unit, and a lactone ring structural unit. The head mounted display according to claim 7, wherein the structural unit (X) of the resin lens includes a structural unit derived from an N-substituted maleimide monomer. The head mounted display according to claim 7 , wherein the structural unit (X) of the resin lens includes a glutarimide-based structural unit.

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