JP7713305B2 - Methacrylic resin casting plate - Google Patents

Description

The present invention relates to methacrylic resin casting plates.

Methacrylic resins have excellent transparency, colorability, moldability, weather resistance, and surface hardness, and are widely used in various fields as signs, decorative materials, lighting covers, automotive parts, glazing materials, etc. In recent years, due to the above-mentioned properties such as transparency, this resin has come to be used for acrylic partitions to prevent droplet infection, which can be easily installed at reception windows and reception counters in facilities such as stores, companies, nurseries, kindergartens, schools, and hospitals. Methacrylic resin sheets used for this purpose are prone to adhesion of droplets, fingerprints, dust, etc., and therefore need to be regularly disinfected with a disinfectant such as ethanol.

Generally, methacrylic resins have relatively good chemical and oil resistance, but because they are thermoplastic resins, they tend to be relatively vulnerable to organic solvents such as ethanol. In particular, parts that have been cut, polished, hot bent, etc. tend to be more vulnerable to organic solvents than other parts because the stress applied during processing remains. Therefore, if the processed parts of a methacrylic resin plate are repeatedly exposed to organic solvents, or if a methacrylic resin plate including a processed part is immersed in an organic solvent, cracks or whitening may occur in the processed parts of the resin plate.

In general, solvent resistance is improved by using a methacrylic resin composition containing a crosslinked methyl methacrylate (MMA) polymer. A method for producing a resin plate containing a crosslinked MMA polymer includes a method of casting and polymerizing a polymerizable raw material containing multiple types of monomers including MMA and a crosslinking agent, or a syrup containing a non-crosslinked MMA polymer and one or more monomers including MMA to which a crosslinking agent and, if necessary, one or more monomers including MMA have been added.

For example, Patent Document 1 discloses a method for producing a methacrylic resin cast plate having excellent solvent resistance, in which a polymerizable raw material obtained by adding a crosslinking agent and a polymerization initiator to a prepolymerized syrup is cast and polymerized, and the degree of polymerization of the trunk polymer crosslinked by the crosslinking agent is set to an intrinsic viscosity [η] of 0.05 to 0.15 (l/g) (claim 1).
In the section entitled "Examples" of Patent Document 1, "solvent resistance" is measured by applying a stress of 140 kg/cm ² to a sample while dropping a styrene solvent onto the sample, and measuring the time to break the sample.

Patent Document 2 discloses a method for producing a methacrylic resin cast plate having excellent solvent resistance, which comprises casting and polymerizing 99.5 to 60% by weight of one or more monomers including MMA in the presence of 0.5 to 40% by weight of a crosslinking agent represented by a specific chemical formula (claim 1).
In the section entitled "Examples" of Patent Document 2, "solvent resistance" is evaluated in terms of light transmittance when immersed in an organic solvent such as benzene or acetone.

Special Publication No. 6-70098 Special Publication No. 44-020626 Japanese Unexamined Patent Publication No. 1986-108630

As described above, the use of a methacrylic resin composition containing a crosslinked MMA polymer improves solvent resistance. However, when a polymerizable raw material containing MMA and a crosslinking agent is polymerized, gelation tends to occur more quickly and polymerization shrinkage tends to progress quickly. In this case, poor transfer of the mold surface, called "sink marks," may occur at the peripheral portion of the plate where polymerization is likely to be delayed. Poor transfer of the mold surface leads to reduced productivity and is undesirable. This problem is more likely to occur when a large amount of crosslinking agent is added. For example, Patent Document 3 describes the problem that "if the amount of crosslinking agent used in the sieve exceeds 25% by weight, sink marks due to hardening will occur in the molded product." Therefore, it is not preferable to use a large amount of crosslinking agent, as in Patent Document 2, in order to improve solvent resistance.

Methacrylic resin casting panels used in applications such as acrylic partitions for preventing droplet infection should preferably have good ethanol resistance without using a large amount of crosslinking agent, and without causing defects in appearance such as cracks and whitening even when ethanol is used repeatedly over a long period of time.

The present invention was made in consideration of the above circumstances, and aims to provide a methacrylic resin casting plate that is free of appearance defects such as sink marks and has excellent ethanol resistance.

As a result of intensive research conducted by the inventors to solve the above problems, they discovered that by optimizing the weight-average molecular weight (Mw) and content of the non-crosslinked methyl methacrylate (MMA) polymer contained in the methacrylic resin cast plate and the dynamic viscoelasticity of the methacrylic resin cast plate, it is possible to provide a methacrylic resin cast plate that suppresses the occurrence of sink marks during cast polymerization and has good ethanol resistance, thus completing the present invention.

The present invention provides the following methacrylic resin cast plates [1] to [5].
[1] A methacrylic resin composition comprising a non-crosslinked methyl methacrylate polymer (P) and a crosslinked methyl methacrylate polymer (C),
The weight average molecular weight of the non-crosslinked methyl methacrylate polymer (P) is 500,000 to 1,500,000;
The content of the non-crosslinked methyl methacrylate polymer (P) is 6 to 15% by mass,
The content of the crosslinked methyl methacrylate polymer (C) is 94 to 85% by mass,
A methacrylic resin casting plate, in which, when dynamic viscoelasticity measurement is carried out, the storage modulus (E') in the rubber-like plateau region is 3 MPa or more at 200°C, the peak value of tan δ is 1.55 or less, and the peak temperature of tan δ is 130°C or more.

[2] The methacrylic resin cast plate of [1], in which when the cut end surface of the laser-cut methacrylic resin cast plate is wiped with ethanol at 20 to 25 ° C. for 1,200 times or more, no cracks having a length of 0.5 mm or more are generated.
[3] The methacrylic resin cast plate of [1], in which when the cut end surface of the cut methacrylic resin cast plate is flame polished and then wiped with ethanol at 20 to 25°C for 1,200 times or more, no cracks longer than 0.5 mm are generated.
[4] The methacrylic resin cast plate of [1], in which when the cut end surface of the cut methacrylic resin cast plate is buffed and then wiped with ethanol at 20 to 25°C for 1,200 times or more, no cracks longer than 0.5 mm are generated.
[5] The methacrylic resin cast plate of [1], in which when the laser-cut methacrylic resin cast plate is immersed in ethanol at 20 to 25 ° C. for 24 hours, no cracks with a length of 0.5 mm or more are generated.

The present invention provides a methacrylic resin casting plate that is free of appearance defects such as sink marks and has excellent ethanol resistance.

The present invention will be described in detail below.
In this specification, "methacrylic" and "acrylic" may be collectively referred to as "(meth)acrylic". The same applies to "(meth)acrylic acid" and (meth)acryloyl. In this specification, "crack" includes crazes and cracks.

[Methacrylic resin casting plate]
The methacrylic resin cast plate of the present invention comprises a methacrylic resin composition containing a non-crosslinked methyl methacrylate (MMA) polymer (P) and a crosslinked methyl methacrylate (MMA) polymer (C).
In the methacrylic resin casting plate of the present invention,
The weight average molecular weight (Mw) of the non-crosslinked MMA polymer (P) is 500,000 to 1,500,000;
The content of the non-crosslinked MMA polymer (P) is 6 to 15 mass %,
The content of the crosslinked MMA polymer (C) is 94 to 85% by mass.
When dynamic viscoelasticity measurement is carried out on the methacrylic resin cast plate of the present invention, the storage modulus (E') in the rubber-like plateau region is 3 MPa or more at 200°C, the peak value of tan δ is 1.55 or less, and the peak temperature of tan δ is 130°C or more.

In general, the use of a methacrylic resin composition containing a crosslinked MMA polymer improves solvent resistance. However, when a polymerizable raw material containing MMA and a crosslinking agent is polymerized, gelation tends to occur early and polymerization shrinkage tends to progress quickly. In this case, poor transfer of the mold surface, called "sink marks," may occur at the peripheral portion of the plate where polymerization is likely to be delayed. This problem is more likely to occur when a large amount of crosslinking agent is added.
In this specification, the term "sink" refers to a transfer defect of a mold surface caused by peeling of a resin from a mold surface such as a glass surface due to polymerization shrinkage during polymerization, etc. For example, if the mold surface is a mirror surface, a "sink" is a part that lacks specularity because the mirror surface was not transferred well.

The present inventors have found that, by optimizing the weight-average molecular weight (Mw) and content of the non-crosslinked MMA-based polymer (P) contained in the methacrylic resin cast plate, and the dynamic viscoelasticity of the methacrylic resin cast plate, it is possible to provide a methacrylic resin cast plate that has good ethanol resistance and does not suffer from poor appearance such as cracks or whitening even when ethanol is used repeatedly over a long period of time, without using a large amount of crosslinker.
In the present invention, since it is not necessary to use a large amount of a crosslinking agent, sink marks caused by polymerization shrinkage that occurs when a crosslinking agent is used can be effectively suppressed.
The present inventors have also found that by optimizing the weight-average molecular weight (Mw) of the non-crosslinked MMA polymer (P), it is possible to suppress sink marks caused by polymerization shrinkage when a crosslinking agent is used, and to improve the ethanol resistance of a methacrylic resin cast plate.
Due to the above-mentioned effects, the present invention can provide a methacrylic resin cast plate that is free from appearance defects such as sink marks and has excellent ethanol resistance.

According to the present invention, it is possible to provide a methacrylic resin cast plate that does not develop cracks having a length of 0.5 mm or more when the cut end surface of the laser-cut methacrylic resin cast plate is wiped with ethanol at 20 to 25°C 1,200 times or more.
According to the present invention, it is possible to provide a methacrylic resin cast plate that does not develop cracks having a length of 0.5 mm or more when the cut end surface of the cut methacrylic resin cast plate is flame polished and then wiped with ethanol at 20 to 25°C 1,200 times or more.
According to the present invention, it is possible to provide a methacrylic resin cast plate that does not develop cracks having a length of 0.5 mm or more when the cut end surface of the cut methacrylic resin cast plate is buffed and then wiped with ethanol at 20 to 25°C 1,200 times or more.
According to the present invention, it is possible to provide a laser-cut methacrylic resin cast plate that does not develop cracks having a length of 0.5 mm or more when immersed in ethanol at 20 to 25°C for 24 hours.
For specific methods for the above four types of ethanol resistance tests, please refer to the section below titled "Examples."

A DMTA curve showing the relationship between temperature and storage modulus can be obtained by dynamic thermomechanical analysis (DMTA). See FIG. 1 of WO 2015-122174. In the DMTA curve, the storage modulus drops significantly when the glass transition point (Tg) is exceeded, but then a rubbery plateau appears where the storage modulus does not change significantly even when the temperature is increased. The rubbery plateau is a region where the polymer molecular chains move but do not completely melt. If the temperature is then further increased to enter the flow region, the storage modulus drops significantly again.

When dynamic viscoelasticity measurement is performed, the storage modulus (E') in the rubber-like flat region being 3 MPa or more at 200°C means that a crosslinked structure having ethanol resistance that does not generate cracks or breaks of 0.5 mm or more in length in the above four types of ethanol resistance tests has been produced.
As shown in FIG. 1 of WO 2015-122174, in a methacrylic resin casting plate, 200 ° C. is the reference temperature on the high temperature side of the rubber-like flat region.
When dynamic viscoelasticity measurement is carried out, the peak value of tan δ is 1.55 or less and the peak temperature of tan δ is 130° C. or more, which means that the amount and molecular weight of the non-crosslinked MMA polymer (P) having low ethanol resistance are within the amount and molecular weight ranges that do not prevent the crosslinked MMA polymer (C) from forming and maintaining a crosslinked structure having ethanol resistance.

[Methacrylic resin casting plate manufacturing method]
The methacrylic resin casting plate of the present invention may, for example, be produced by a process comprising the steps of: (1) preparing a syrup (S) containing one or more monomers (M) including methyl methacrylate (MMA) and a non-crosslinked methyl methacrylate (MMA)-based polymer (P);
(2) a step of adding one or more monomers (M3) including methyl methacrylate (MMA) and one or more crosslinking agents (M4) to the syrup (S) to obtain a polymerizable raw material;
and (3) subjecting the obtained polymerizable raw material to cast polymerization.

The Mw of the non-crosslinked MMA polymer (P) can be adjusted to a desired range by appropriately selecting polymerization conditions and adjusting the degree of polymerization when polymerizing the raw material monomers for the non-crosslinked MMA polymer (P).
By appropriately selecting the composition of the polymerizable raw materials and the cast polymerization conditions, the storage modulus (E') of the rubber-like flat region of the methacrylic resin cast plate, and the peak value and peak temperature of tan δ can be adjusted to the desired range.

(Step (1))
The prepolymerization method is preferred as a method for preparing the syrup (S) because it is easy to produce a syrup (S) containing a non-crosslinked MMA polymer (P) having a weight-average molecular weight (Mw) of 500,000 to 1,500,000. The syrup (S) obtained by this method is a prepolymerized syrup (S1) containing a non-crosslinked linear MMA polymer (P1) and one or more unreacted monomers (M1) obtained by prepolymerizing one or more monomers (M1) including MMA in the presence of a polymerization initiator.
After the prepolymerization, one or more monomers (M1), such as MMA, may be further added for dilution, if necessary.

In addition to the above-mentioned preliminary polymerization method, the syrup (S) can be prepared by a polymer dissolution method. The syrup (S) obtained by this method is a dissolved syrup (S2) obtained by dissolving a non-crosslinked linear MMA polymer (P2) polymerized using one or more monomers including MMA in one or more monomers (M2) including MMA.
However, since it takes a lot of time and effort to dissolve the non-crosslinked MMA polymer (P) having a weight average molecular weight (Mw) of 500,000 to 1,500,000 in one or more monomers (M2) containing MMA, the prepolymerization method is preferable to the polymer dissolution method.

The non-crosslinked MMA polymer (P) may be a homopolymer of MMA, a copolymer of MMA and one or more other alkyl methacrylates, or a copolymer of one or more alkyl methacrylates including MMA and one or more other copolymerizable unsaturated monomers.

The number of carbon atoms in the alkyl group of the alkyl methacrylate used as the raw material for the non-crosslinked linear MMA polymer (P) is preferably 1 to 20, more preferably 1 to 12. Examples of such alkyl methacrylates include MMA, ethyl methacrylate, n-propyl methacrylate, i-propyl methacrylate, n-butyl methacrylate, i-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, and cyclohexyl methacrylate.

Other copolymerizable unsaturated monomers that can be used in combination with alkyl methacrylate as a raw material for the non-crosslinked linear MMA polymer (P) include alkyl acrylates such as methyl acrylate, ethyl acrylate, propyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and cyclohexyl acrylate; hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and 2-hydroxy-3-chloropropyl (meth)acrylate; (meth)acrylic acid; metal (meth)acrylates; vinyl monomers such as vinyl chloride, vinyl acetate, and vinyl toluene; acrylonitrile; acrylamide; styrene monomers such as styrene and α-methylstyrene; maleic anhydride, etc.

The concentration of the non-crosslinked MMA polymer (P) in the syrup (S) is not particularly limited, and is preferably 5% by mass or more. If the concentration of the polymer (P) in the syrup (S) is less than 5% by mass, polymerization shrinkage in step (3) becomes large, and sink marks may occur.
If the viscosity of the syrup (S) is too high, it becomes difficult to add and mix a diluting monomer such as MMA to the syrup (S), and it becomes difficult to degas the polymerizable raw material obtained in the next step and to inject it into a mold. From the viewpoint of optimizing the viscosity of the syrup (S), the concentration of the polymer (P) in the syrup (S) is preferably 40% by mass or less, more preferably 30% by mass or less, and particularly preferably 20% by mass or less.

The weight average molecular weight (Mw) of the non-crosslinked linear MMA polymer (P) is 500,000 to 1,500,000. By setting the Mw of the non-crosslinked MMA polymer (P) within this range, sink marks caused by polymerization shrinkage when a crosslinking agent is used can be effectively suppressed, and the ethanol resistance of the methacrylic resin cast plate can be improved.
If the Mw is less than 500,000, the ethanol resistance of the resulting methacrylic resin cast plate may be insufficient.If the Mw is 1,500,000 or more, the viscosity of the syrup (S) becomes high, making it difficult to add and mix a diluting monomer such as MMA to the syrup (S), and making it difficult to degas the polymerizable raw material obtained in the next step and to pour it into a mold.
The Mw of the non-crosslinked linear MMA polymer (P) is preferably from 700,000 to 1,200,000, and more preferably from 700,000 to 1,000,000.

The degree of polymerization, weight average molecular weight (Mw) and content of the non-crosslinked MMA polymer (P) capable of suppressing sink marks during cast polymerization while ensuring the ethanol resistance of the methacrylic resin cast plate can be indexed by the peak value and peak temperature of tan δ in the dynamic viscoelastic properties. In the methacrylic resin cast plate of the present invention, it is necessary that the peak value of tan δ is 1.55 or less and the peak temperature of tan δ is 130° C. or more.
If the peak value of tan δ exceeds 1.55 or the peak temperature of tan δ is less than 130°C, even if the type and amount of crosslinking agent are designed to be the preferred type and amount described in this specification, the ethanol resistance of the resulting methacrylic resin cast plate may be insufficient.
The peak value of tan δ is preferably 0.35 or more and 1.55 or less.
The peak temperature of tan δ is preferably 130° C. or higher and 200° C. or lower.

Instead of the syrup (S) containing MMA and the non-crosslinked MMA-based polymer (P), a syrup or gel containing a partially crosslinked MMA-based gel-like polymer can also be used. This syrup or gel is obtained by adding a crosslinking agent to the dissolved syrup (S2) and prepolymerizing it, and contains the non-crosslinked MMA-based polymer (P), the crosslinked MMA-based polymer (C), and one or more monomers including MMA.

(Step (2))
In step (2), one or more monomers (M3) including methyl methacrylate (MMA), one or more crosslinking agents (M4), and a polymerization initiator are added to the syrup (S) to obtain a polymerizable raw material.
As the monomer (M3), in addition to MMA, one or more other alkyl methacrylates and/or one or more other copolymerizable unsaturated monomers can be used as necessary. The addition of the monomer (M3) may be combined with the addition of the monomer (M1) as necessary after the preliminary polymerization. Examples of the other alkyl methacrylates and other copolymerizable unsaturated monomers that can be used as the monomer (M3) are the same as the examples of the raw material monomers for the non-crosslinked MMA polymer (P).

As the crosslinking agent (M4), a monomer having two or more (meth)acryloyl groups in the molecule is preferably used. The molecular weight of the crosslinking agent (M4) is not particularly limited, and is preferably 400 or less. For example, ethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, (1,3-butanediol di(meth)acrylate), 1,4-butylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, and neopentyl glycol di(meth)acrylate are preferred.

If a crosslinking agent having a molecular weight of more than 400 is used, the molecular weight between crosslinking points of the resulting crosslinked MMA polymer (C) will be large, and the ethanol resistance of the resulting cast methacrylic resin plate may become insufficient.
The molecular weight between crosslinking points of the crosslinked MMA polymer (C) is affected by the molecular weight and amount of the monomer (M3) and the crosslinking agent (M4), and the amount of the non-crosslinked MMA polymer (P).
The molecular weight between crosslinking points of the crosslinked MMA polymer (C) can be indexed by the storage modulus (E') in the rubber-like flat region of the methacrylic resin cast plate. In the present invention, the storage modulus (E') in the rubber-like flat region must be 3 MPa or more at 200°C. If the storage modulus (E') in the rubber-like flat region is 3 MPa or more at 200°C, it can be said that a crosslinked structure is formed that does not generate cracks of 0.5 mm or more in length in the above four types of ethanol resistance tests.
The storage modulus (E') in the rubber-like plateau region is preferably 3 MPa or more and 200 MPa or less at 200°C.

Polymerization initiators are not particularly limited, and examples include 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), acetylcyclohexylsulfonyl peroxide, isobutyryl peroxide, cumyl peroxyneodecanoate, diisopropyl peroxydicarbonate, di-n-propyl peroxydicarbonate, dimyristyl peroxycarbonate, di-(2-ethoxyethyl) peroxydicarbonate, di-(methoxyisopropyl) peroxydicarbonate, and di-(2-ethylhexyl) peroxydicarbonate.

In step (2), a chain transfer agent and/or an ultraviolet absorber can be added as necessary. Examples of chain transfer agents include styrene dimers such as α-methyl-styrene dimer; mercaptans such as n-octyl mercaptan, n-dodecyl mercaptan, and hyophenol; thioglycolic acid or its esters such as thioglycolic acid, ethyl thioglycolate, and butyl thioglycolate; β-mercaptopropionic acid or its esters such as β-mercaptopropionic acid, methyl β-mercaptopropionate, and octyl β-mercaptopropionate; and terpinolene. Examples of ultraviolet absorbers include 2-(2'-hydroxy-5'-methylphenyl)benzotriazole.

In step (2), one or more other additives can be added as necessary. Examples of other additives include different types of resins, antioxidants, dispersants, fillers, resin granules, pigments, dyes, pattern materials such as natural stone granules, and mold release agents.

The blending ratio of the raw materials is not particularly limited, and the total amount of the raw materials other than the polymerization initiator, the chain transfer agent, the UV absorber, and other additives is taken as 100 parts by mass.
The amount of the syrup (S) is designed so that the content of the non-crosslinked MMA polymer (P) in the polymerizable raw material becomes 6 to 15% by mass.
If the content of the polymer (P) in the polymerizable raw material is less than 6% by mass, the polymerization shrinkage during cast polymerization becomes large, and sink marks may occur in the resulting methacrylic resin cast plate. If the content exceeds 15% by mass, the amount of components soluble in the solvent increases, so that even if the type and amount of the crosslinking agent (M4) are designed to be the preferred type and amount described in this specification, the ethanol resistance of the resulting methacrylic resin cast plate may be insufficient.

The amount of the one or more monomers (M3) is preferably 6.2 to 12 parts by mass, more preferably 7 to 10 parts by mass, per 100 parts by mass of the total of the polymerizable monomer and the monomer unit.
The amount of one or more crosslinking agents (M4) is preferably from 2.5 to 25 parts by weight, more preferably from 4.5 to 20 parts by weight.
The amount of the chain transfer agent is preferably 0 to 0.5 parts by weight.
The amount of the polymerization initiator is preferably 0.05 to 3 g per kg of the total amount of raw materials other than the polymerization initiator, the chain transfer agent, the ultraviolet absorber, and other additives.
The amount of the ultraviolet absorber is preferably 0 to 2 g per kg of the total amount of the raw materials other than the polymerization initiator, the chain transfer agent, the ultraviolet absorber, and other additives.

(Step (3))
In step (3), the liquid polymerizable raw material obtained in step (2) is poured into a mold and subjected to cast polymerization.
Cast polymerization can be carried out by a known method. Examples of the mold include a mold consisting of a pair of plates such as reinforced glass, chrome-plated plate, or stainless steel plate and a soft polyvinyl chloride gasket, and a mold consisting of the opposing surfaces of a pair of endless belts running in the same direction at the same speed and gaskets running at the same speed as both endless belts on both side portions of the pair.

The polymerizable raw material is preferably polymerized and cured in two or more stages by changing the temperature. Two-stage polymerization can be performed as follows. First, the mold in which the polymerizable raw material is injected into the internal space is immersed in a water bath whose temperature is controlled at preferably about 55 to 85°C, and held for about 1 to 20 hours to perform the first stage of polymerization and curing. Next, the mold is placed in a hot air drying furnace, and heated preferably at a temperature of about 80°C for about 2 to 15 hours, and then heated preferably at a temperature of about 120 to 130°C for about 3 to 15 hours to drive the reaction, to perform the second stage of polymerization and curing.
In the manner described above, a cast methacrylic resin plate is produced from the methacrylic resin composition containing the non-crosslinked MMA polymer (P) and the crosslinked MMA polymer (C).
The flat resin plate thus obtained as the primary molded product may be secondarily molded into any three-dimensional shape by a known method such as vacuum molding or pressure molding.

As described above, according to the present invention, it is possible to provide a methacrylic resin cast plate which is free from appearance defects such as sink marks and has excellent ethanol resistance.
The methacrylic resin cast plate of the present invention is made of a methacrylic resin composition, and therefore has the inherent properties of methacrylic resins, such as transparency, colorability, moldability, weather resistance, and surface hardness.

[Application]
The methacrylic resin cast plate of the present invention can be suitably used for applications such as acrylic partitions for preventing droplet infection.
The thickness of the methacrylic resin cast plate of the present invention is not particularly limited, and is preferably 3 to 5 mm for applications such as acrylic partitions for preventing droplet infection.

Examples and comparative examples according to the present invention will be described.
[Evaluation items and evaluation methods]
(Weight average molecular weight (Mw) and molecular weight distribution ((Mw)/(Mn)) of non-crosslinked methyl methacrylate (MMA) polymer (P))
5 g of the syrup (S) was extracted with 200 ml of chloroform, filtered, and methanol was added to the collected filtrate to generate a precipitate. The precipitate was dried in a vacuum, and 0.12 g of the resulting dried product was dissolved in 20 ml of tetrahydrofuran to obtain a measurement sample. The molecular weight was measured by GPC (gel permeation chromatography) using Shimadzu Corporation's "LC-9A" as a molecular weight measurement device, Shimadzu Corporation's "GPC-802", "HSG-30", and "HSG-50" as columns, and Showa Denko K.K.'s "Shodex A-806". Note that Mw and Mn are polystyrene equivalent values.

(Content of non-crosslinked methyl methacrylate (MMA) polymer (P))
1 g of the syrup (S) was dried using an infrared heater ("IB-30" manufactured by CHYO), and the mass of the obtained dried product was measured to determine the solids concentration of the syrup (S). The content of the non-crosslinked MMA-based polymer (P) in the cast plate was calculated from the content of the syrup (S) in the polymerizable raw material and the solids concentration of the syrup (S).
For the cast plate, the mixture was extracted with chloroform, filtered, and methanol was added to the filtrate to generate a precipitate. The precipitate was dried using an infrared heater ("IB-30" manufactured by CHYO Corporation), and the mass of the dried product was measured to determine the content of the non-crosslinked MMA polymer (P) in the cast plate.

(exterior)
The appearance of the produced or prepared resin plate (a cast methacrylic resin plate or an extruded methacrylic resin plate) was visually observed and evaluated according to the following criteria.
Good (◯): No sink marks observed.
Poor (x): sink marks are observed.

(Dynamic Viscoelasticity)
The dynamic viscoelasticity of the produced or prepared resin plates (methacrylic resin cast plates or methacrylic resin extruded plates) was measured in accordance with JIS K7244-1 and JIS 7244-4.
First, the resin plate was cut to obtain a sample with a length of 20 mm, a width of 1.5 mm, and a thickness of 2 mm. Measurements were performed using a dynamic viscoelasticity measuring device (UBM's "Rheogel-E4000"). Sine wave vibrations with a set frequency and amplitude were applied to the sample at elevated temperature, and the stress response generated at that time was detected, and the phase difference between the dynamic stress waveform and the dynamic displacement waveform was obtained. Using an arithmetic formula based on linear viscoelasticity theory, data such as the storage modulus (E') and tan δ were plotted against the temperature of the measurement region to obtain a DMTA curve. The main measurement conditions were set as follows. From this, the storage modulus (E') at 200°C in the rubber-like flat region was obtained. In addition, the peak value and peak temperature of tan δ were obtained.
<Measurement conditions>
Measurement frequency: 1Hz,
Load: 1kg,
Measurement mode: Temperature dependence,
Measurement temperature: 30-250℃,
Temperature rise conditions: 3°C/min.

[Processing method]
(Laser cutting process)
The manufactured or prepared resin plate (methacrylic resin cast plate or methacrylic resin extrusion plate) was cut using a Comnet "Spirit GLS 100W" laser under conditions of 80% power control and a processing speed of 2 m/min, to obtain a laser-processed sample of 100 mm in length, 30 mm in width, and 3 mm in thickness.

(polishing)
The manufactured or prepared resin plate (a cast methacrylic resin plate or an extruded methacrylic resin plate) was cut with an electric saw to obtain two samples measuring 100 mm in length, 30 mm in width and 3 mm in thickness.
The cut end surface of one of the samples was flame polished to obtain a first mirror-finished sample.
Specifically, with the main surface of the sample lying on its side, a thin flame the size of a pencil lead was brought close to the cut end surface using a commercially available flame polisher, and when the color of the cut end surface began to change, the flame was slid horizontally at a speed of about 3 to 5 m/min while maintaining the distance between the cut end surface and the flame, and the cut end surface was melted by the heat of the flame. After the melted cut end surface cooled and solidified, it became a mirror surface. The distance between the cut end surface and the flame and the movement speed of the flame were designed within a range that would not cause deterioration such as burning or deformation of the sample.

The cut end surface of the other sample was polished to some extent with sandpaper, and then this cut end surface was buffed to obtain a second mirror-finished sample.
Specifically, buffing was performed using a commercially available buffing machine as follows. The saw marks on the cut end surface of the sample were removed using #600 sandpaper wetted with water. A cotton buff with a diameter of 100 to 300 mm and a thickness of 25 to 30 mm was used as the buff. With the buff with an abrasive attached applied to the cut end surface, the buff was rotated at a rotation speed of 1000 to 1500 rpm, and the surface condition of the cut end surface was confirmed while mirror finishing was performed. The force with which the buff was pressed against the cut end surface and the rotation speed of the buff were designed within a range in which buff burning due to overheating would not occur.

[Ethanol resistance test]
For each processed sample (specifically, the laser processed sample and the first and second mirror-finished samples), an ethanol resistance test was performed by the following two methods. The processed end surface was a cut end surface (100 mm x 3 mm) for the laser processed sample, and a mirror-finished end surface (100 mm x 3 mm) for the mirror-finished sample.
In the following two tests, anhydrous ethanol with a purity of 99.5% by mass or more was used as the ethanol.

(Ethanol wiping test)
A cellulose nonwoven fabric ("Bencotto M-3" manufactured by Asahi Kasei Corporation) was immersed in a container containing ethanol to soak the nonwoven fabric in ethanol.
The nonwoven fabric soaked in ethanol was held in the hand, one of the processed end faces (100 mm x 3 mm) of the processed sample was placed up, and the nonwoven fabric soaked in ethanol was brought into contact with this processed end face, and in this state, the nonwoven fabric was moved back and forth within a range of about 50 mm in length. One back and forth movement was counted as two wipes, and after every 50 back and forth movements (100 wipes), the nonwoven fabric used was immersed in a container containing ethanol to replenish the nonwoven fabric with ethanol. This test was repeated at room temperature of 20 to 25 ° C. until a crack of 0.5 mm or more in length was visually confirmed on the processed end face. The number of wipes up to the time when a crack was confirmed was determined. Evaluation was performed according to the following criteria.
Excellent (◎): Wipes over 1,500 times.
Good (◯): The number of wiping cycles was 1,200 or more and less than 1,500.
Acceptable (△): The number of wipes was 900 or more and less than 1200.
Poor (x): Wiping count is less than 900 times.
In Table 1, the numbers shown in the evaluation column for the ethanol wiping test indicate the number of times wiping was performed.

(Ethanol immersion test)
The processed sample was placed in a container containing ethanol with one processed end surface (100 mm x 3 mm) facing up, so that the processed end surface was completely immersed, and left at 23 ° C for 24 hours. The processed sample was then removed from the container, and the presence or absence and number of cracks on the large main surface (100 mm x 30 mm) and the processed end surface (100 mm x 3 mm) that are the non-processed surfaces were visually confirmed. In addition, the presence or absence of whitening on the large main surface (100 mm x 30 mm) and the processed end surface that are the non-processed surfaces was confirmed. Evaluation was performed according to the following criteria.

<Crack>
Excellent (○): No cracks observed.
Good (◯△): 1 to less than 6 cracks less than 0.5 mm in length were observed.
Fair (Δ): 6 to less than 10 cracks less than 0.5 mm in length were observed.
Fair (▲): 10 or more cracks less than 0.5 mm in length were observed.
Poor (x): Cracks having a length of 0.5 mm or more and less than 1.0 mm were observed.
Very poor (XX): Cracks having a length of 1.0 mm or more were observed.
In Tables 1 and 2, the numbers shown in the evaluation column for the ethanol immersion test indicate the number of cracks.

<Whitening>
Good (◯): No whitening observed.
Fair (△): Partial whitening was observed.
Poor (x): Whitening was observed over the entire surface.

[material]
The materials used are as follows:
<Crosslinking Agent>
NPG: neopentyl glycol dimethacrylate,
BG: 1,3-butanediol dimethacrylate.
<Polymerization initiator>
AIBN: 2,2'-azobis(isobutyronitrile),
V-65: 2,2'-azobis(2,4-dimethylvaleronitrile).
<Chain Transfer Agent>
TPL: terpinolene.

[Production Example 1] Preparation of prepolymerized syrup (S1) A prepolymerized syrup (S1) containing a non-crosslinked linear MMA polymer (P1) was obtained as follows.
2,2'-azobisisobutyronitrile (AIBN) as a polymerization initiator was added to MMA as a monomer (M), and the mixture was heated with stirring to polymerize until the weight average molecular weight (Mw) of the polymer reached about 800,000. Thereafter, the mixture was cooled to room temperature (20 to 25°C), and additional MMA was added to dilute the mixture, producing a prepolymerized syrup (S1) containing 8% by mass of a non-crosslinked MMA-based polymer (P1) (Mw = about 800,000).

[Production Example 2] Preparation of soluble syrup (S2) A soluble syrup (S2) containing a non-crosslinked linear MMA polymer (P2) was obtained as follows.
A raw material liquid was obtained by adding and dissolving 0.1 part by mass of a polymerization initiator (2,2′-azobis(2-methylpropionitrile) and 0.16 part by mass of a chain transfer agent (n-octyl mercaptan) in a monomer mixture consisting of 95 parts by mass of methyl methacrylate (MMA) and 5 parts by mass of methyl acrylate.
A mixed liquid was obtained by mixing 100 parts by mass of ion-exchanged water, 0.03 parts by mass of sodium sulfate, and 0.46 parts by mass of a suspension dispersant.
A pressure-resistant polymerization tank was charged with 210 parts by mass of the raw material liquid and 420 parts by mass of the mixed liquid, and the temperature was raised to 70° C. while stirring under a nitrogen atmosphere to initiate the polymerization reaction. After 3 hours had elapsed since the start of the polymerization reaction, the temperature was raised to 90° C., and stirring was continued for 1 hour to obtain a dispersion of a bead-shaped copolymer. The bead-shaped copolymer was taken out of the dispersion to obtain a non-crosslinked linear MMA polymer (P2). The obtained non-crosslinked linear MMA polymer (P2) had a weight average molecular weight (Mw) of about 130,000 and a molecular weight distribution (Mw/Mn) of 1.8.
While stirring MMA as the monomer (M), the non-crosslinked linear MMA polymer (P2) obtained above was added, and the mixture was heated to 60° C. and stirred for 90 minutes. Thereafter, the mixture was cooled to room temperature (20 to 25° C.) to obtain a dissolved syrup (S2) containing 30% by mass of the non-crosslinked MMA polymer (P2) (Mw=approximately 130,000).

Production Example 3 Preparation of Prepolymerized Syrup (S3) A prepolymerized syrup (S3) having a content of 15 mass% of non-crosslinked MMA-based polymer (P1) (Mw = about 800,000) was produced in the same manner as in Production Example 1, except that no additional MMA was added.

[Examples 1 to 3, Comparative Examples 1, 3 to 12]
In each of Examples 1 to 3 and Comparative Examples 1 and 3 to 12, a polymerizable raw material was prepared by mixing multiple materials according to the blending compositions shown in Tables 1 to 3. In these tables, the unit of blending amount of raw materials other than the polymerization initiator and the chain transfer agent is "mass %", and the total amount of raw materials other than the polymerization initiator and the chain transfer agent is 100 mass %. The blending amount of the polymerization initiator and the chain transfer agent is shown as the amount added [g] per 1 kg of the total amount of raw materials other than the polymerization initiator and the chain transfer agent.
The obtained polymerizable raw material was degassed and then poured into a mold consisting of a pair of tempered glass and a soft polyvinyl chloride gasket. Primary curing (prepolymerization) and secondary curing (postpolymerization) were carried out at the temperatures and times shown in Tables 1 to 3. After that, it was cooled to about the temperature of the primary curing, and a methacrylic resin casting plate having a length of 1250 mm, a width of 2500 mm, and a thickness of 3 mm was taken out from the mold.
The main production conditions and evaluation results are shown in Tables 1 to 3.

[Comparative Example 2]
The non-crosslinked MMA polymer (P2) was melt-kneaded using a 50 mmφ sheet extruder at a cylinder temperature of 250° C. to obtain an extruded methacrylic resin plate having a thickness of 3 mm.
The evaluation results are shown in Table 1.

[Summary of results]
(Examples 1 to 3)
The methacrylic resin cast plates obtained in Examples 1 to 3 had no sink marks and had good appearances. The cast plates obtained in these Examples contained 6 to 15 mass % of a non-crosslinked MMA polymer (P) having a Mw of 500,000 to 1,500,000 and 94 to 85 mass % of a crosslinked MMA polymer (C). The cast plates obtained in these Examples had a storage modulus (E') in the rubber-like flat region of 3 MPa or more at 200°C, a peak value of tan δ of 1.55 or less, and a peak temperature of tan δ of 130°C or more.

In the dynamic viscoelasticity measurement of Example 3, the storage modulus (E') at high temperatures was very high, and the safety device was activated when the temperature exceeded 150°C, making it impossible to measure. Therefore, in this example, the peak temperature of tan δ was considered to be 150°C or higher. In this example, the storage modulus (E') was considered to be a value higher than the storage modulus (E') of Example 2, in which the peak temperature of tan δ is 141.4°C, and was considered to be 5.00 MPa or higher. In this example, the peak value of tan δ was considered to be a value lower than the peak value of tan δ of Example 2, in which the peak temperature of tan δ is 141.4°C, and was considered to be 1.30 or less.
All of the cast plates obtained in Examples 1 to 3 had good ethanol resistance.

(Comparative Example 1)
The methacrylic resin cast plate obtained in Comparative Example 1 had no sink marks and had a good appearance. The cast plate obtained in this Comparative Example was a non-crosslinked plate consisting of 100 mass % of a non-crosslinked MMA polymer (P) having an Mw of 500,000 to 1,500,000. The cast plate obtained in this Comparative Example had a storage modulus (E') in the rubber-like flat region of less than 3 MPa at 200°C and a peak value of tan α of more than 1.55. The cast plate obtained in this Comparative Example had poor ethanol resistance.

(Comparative Example 2)
The methacrylic resin extruded plate prepared in Comparative Example 2 had no sink marks and had a good appearance. The extruded plate prepared in this Comparative Example was a non-crosslinked plate consisting of 100 mass % of a non-crosslinked MMA polymer (P) having a Mw of 500,000 to 1.5 million. The extruded plate prepared in this Comparative Example had a storage modulus (E') in the rubber-like plateau region of less than 3 MPa at 200°C, a peak tan α value of more than 1.55, and a peak tan α temperature of less than 130°C.
The extruded plate prepared in this comparative example had extremely poor ethanol resistance at the processed end face, and the ethanol resistance of the main surface, which was the non-processed end face, was also poor.

(Comparative Examples 3, 5 to 9)
The methacrylic resin cast plates obtained in Comparative Examples 3 and 5 to 9 had no sink marks and had good appearances. The cast plates obtained in these Comparative Examples contained 6 to 15 mass % of the non-crosslinked MMA polymer (P) having a Mw of 500,000 to 1,500,000 and 94 to 85 mass % of the crosslinked MMA polymer (C). In these Comparative Examples, the amount of the crosslinking agent used was less than that in Examples 1 to 3.
The cast plates obtained in these comparative examples had a storage modulus (E') in the rubbery plateau region of less than 3 MPa at 200°C and a peak tan α value of more than 1.55. Comparative examples 6 to 9 had peak tan δ temperatures of less than 130°C.
The cast plates obtained in these Comparative Examples had poor ethanol resistance or were inferior to those of Examples 1-3.

(Comparative Example 4)
The methacrylic resin cast plate obtained in Comparative Example 4 had no sink marks and had a good appearance. The cast plate obtained in this Comparative Example had a non-crosslinked MMA polymer (P) with a Mw of less than 500,000 and a content of the non-crosslinked MMA polymer (P) of more than 15% by mass. In this Comparative Example, the amount of the crosslinking agent used was less than those in Examples 1 to 3.
The cast plate obtained in this comparative example had a storage modulus (E') in the rubbery plateau region of less than 3 MPa at 200°C, a peak tan α value of greater than 1.55, and a peak tan δ temperature of less than 130°C.
The cast plate obtained in this comparative example had extremely poor ethanol resistance at the processed end face, and the ethanol resistance of the main surface, which is the non-processed end face, was also poor.

(Comparative Examples 10 and 11)
The methacrylic resin cast plates obtained in Comparative Examples 10 and 11 had a content of the non-crosslinked MMA polymer (P) having a Mw of 500,000 to 1,500,000 of less than 6 mass % and a content of the crosslinked MMA polymer (C) of 94 mass % or more. The methacrylic resin cast plates obtained in these Comparative Examples had sink marks and poor appearance.

(Comparative Example 12)
In Comparative Example 12, since no additional MMA was added during preparation of the syrup (S), the obtained polymerizable raw material had a high viscosity and was difficult to inject into a mold, and thus a plate could not be produced.

Claims (5)

A methacrylic resin cast plate comprising a non-crosslinked methyl methacrylate polymer (P) and a crosslinked methyl methacrylate polymer (C),
The composition is made of a polymerized and cured product of a polymerizable raw material, which comprises a non-crosslinked methyl methacrylate polymer (P), one or more monomers including methyl methacrylate, one or more crosslinking agents, and a polymerization initiator, and the amount of the crosslinking agent is 2.5 to 25 parts by mass per 100 parts by mass of the total of the polymerizable monomers and monomer units;
The molecular weight of the crosslinking agent is 400 or less,
The weight average molecular weight of the non-crosslinked methyl methacrylate polymer (P) is 500,000 to 1,500,000;
The content of the non-crosslinked methyl methacrylate polymer (P) is 6 to 15% by mass,
The content of the crosslinked methyl methacrylate polymer (C) is 94 to 85% by mass,
A methacrylic resin casting plate, which is a laser cut plate, has a storage modulus (E') in the rubber-like plateau region of 3 MPa or more at 200°C when dynamic viscoelasticity is measured , a peak value of tan δ of 1.55 or less, and a peak temperature of tan δ of 130°C or more. The methacrylic resin casting plate according to claim 1, in which no cracks of 0.5 mm or more in length are generated when the cut end surface of the laser-cut methacrylic resin casting plate is wiped with ethanol at 20 to 25°C for 1,200 times or more. The methacrylic resin casting plate according to claim 1, in which no cracks of 0.5 mm or more in length are generated when the cut end surface of the cut methacrylic resin casting plate is flame polished and then wiped with ethanol at 20 to 25°C for 1,200 times or more. The methacrylic resin casting plate according to claim 1, in which no cracks of 0.5 mm or more in length are generated when the cut end surface of the cut methacrylic resin casting plate is buffed and then wiped with ethanol at 20 to 25°C for 1,200 times or more. The methacrylic resin cast plate according to claim 1, in which no cracks of 0.5 mm or more in length are generated when the laser-cut methacrylic resin cast plate is immersed in ethanol at 20 to 25°C for 24 hours.