JP7740021B2 - adhesive composition - Google Patents

Description

The present invention relates to an adhesive composition. More specifically, it relates to an adhesive composition that exhibits excellent flexibility of an uncured coating film and exhibits excellent humid solder heat resistance and peel strength at high temperatures as an adhesive for flexible printed wiring boards.

Flexible printed circuit boards (FPCs) are boards with electrical circuits formed on a base material made by bonding a thin, soft insulating film, such as polyimide, to a conductive metal, such as copper foil, with an adhesive. Unlike rigid boards, they are extremely thin and flexible, allowing them to be used in small gaps and bending moving parts of electronic devices, and are therefore used in many of the electronic devices we use every day, such as personal computers and smartphones. In recent years, more and more FPCs have been installed in automobiles, requiring adhesives that can adhere at high temperatures.

Polyesters are widely used as raw materials for resin compositions used in coatings, inks, adhesives, etc., and are generally composed of polycarboxylic acids and polyhydric alcohols. Because of their flexibility and the ability to freely control molecular weight through the selection and combination of polycarboxylic acids and polyhydric alcohols, they are widely used in a variety of applications, including coatings and adhesives.

Polyester has excellent adhesive properties (peel strength) with metals, including copper, and has been used as an adhesive for FPCs when combined with a curing agent (see, for example, Patent Document 1).

JP 9-125043

However, the polyester resin described in Patent Document 1 consists solely of polyester resin with a low glass transition temperature (Tg) of 80°C or less, and therefore has insufficient peel strength at high temperatures.

The present invention was made in response to these problems in the prior art. Specifically, the object of the present invention is to provide an adhesive composition that exhibits excellent flexibility in the uncured coating film (B-stage), and that exhibits excellent humid solder heat resistance and peel strength at high temperatures as an adhesive for flexible printed wiring boards.

As a result of extensive investigations, the present inventors have found that the above problems can be solved by the following means, and have arrived at the present invention.
That is, the present invention comprises the following configurations.

An adhesive composition containing a polyester (A1) having a glass transition temperature or melting point above 80°C and a polyisocyanate (B).

Furthermore, it is desirable to contain polyester (A2) having a glass transition temperature of 0°C or lower and a melting point of 80°C or lower or being amorphous.

It is desirable that the content of the polyester (A2) be 10 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the polyester (A1).

The content of polyisocyanate (B) is desirably 0.1 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the total of polyester (A1) and polyester (A2).

The adhesive composition can be suitably used for flexible printed wiring boards.

The adhesive composition of the present invention has excellent flexibility in the uncured coating film, and as an FPC adhesive it has excellent humid solder heat resistance and peel strength at high temperatures.

One embodiment of the present invention is described in detail below. However, the present invention is not limited to this embodiment, and can be implemented in various modified forms within the scope described above.

<Polyester (A1)>
The adhesive composition of the present invention contains a polyester (A1) having a glass transition temperature or melting point of greater than 80° C. By using a polyester (A1) having a glass transition temperature (Tg) or melting point (Tm) of greater than 80° C., it is possible to improve the humid solder heat resistance and the peel strength at high temperatures.
The polyester (A1) has a chemical structure that can be obtained by polycondensation of a polycarboxylic acid component or a polycarboxylic acid ester component with a polyhydric alcohol component, and the polycarboxylic acid component or the polycarboxylic acid ester component and the polyhydric alcohol component each consist of one or more selected components.

The polycarboxylic acid component constituting the polyester (A1) is not limited, but the following polycarboxylic acids or their esters, and polycarboxylic acid anhydrides can be used. Specific examples of polycarboxylic acids include terephthalic acid, isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acid, tetrahydrophthalic acid, methyltetrahydrophthalic acid, adipic acid, sebacic acid, dimer acid, 1,3-cyclohexanedicarboxylic acid, 1,4-cyclohexanedicarboxylic acid, fumaric acid, maleic acid, 5-sodiumsulfodimethylisophthalic acid, trimellitic acid, pyromellitic acid, and their esters. Examples of polycarboxylic acid anhydrides include phthalic anhydride, tetrahydrophthalic anhydride, succinic anhydride, trimellitic anhydride, pyromellitic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, and hydrogenated naphthalenedicarboxylic acid. Aromatic polycarboxylic acid components such as naphthalenedicarboxylic acid and terephthalic acid are particularly preferred. The use of aromatic polycarboxylic acids increases Tg and improves peel strength at high temperatures.

The polyhydric alcohol constituting the polyester (A1) is not particularly limited, but examples thereof include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1-methyl-1,8-octanediol, 2-methyl-2-ethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 2-ethyl-2-n-propyl-1,3-propanediol, 2,2-di-n-propyl-1, Glycol components such as 3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-di-n-butyl-1,3-propanediol, 2,4-diethyl-1,5-pentanediol, 2-ethyl-1,3-hexanediol, 1,4-cyclohexanedimethanol, tricyclodecane dimethanol, polytetramethylene glycol, polypropylene glycol, and the like, as well as glycerin, trimethylolpropane, trimethylolethane, pentaerythritol, α-methylglucose, mannitol, sorbitol, and dimer diol can be used, and one or more of these can be used. In particular, polyhydric alcohols not having long-chain alkyl groups, such as ethylene glycol, 1,2-propanediol, and 1,3-propanediol, and alicyclic polyhydric alcohols such as 1,4-cyclohexanedimethanol and tricyclodecane dimethanol are preferred. The use of these polyhydric alcohols increases Tg and improves peel strength at high temperatures.

The polyester (A1) can also be copolymerized with a trivalent or higher polycarboxylic acid component and/or a trivalent or higher polyhydric alcohol component. Examples of trivalent or higher polycarboxylic acid components include aromatic carboxylic acids such as trimellitic acid, pyromellitic acid, benzophenonetetracarboxylic acid, trimesic acid, trimellitic anhydride (TMA), and pyromellitic anhydride (PMDA), and aliphatic carboxylic acids such as 1,2,3,4-butanetetracarboxylic acid. These can be used alone or in combination of two or more. Examples of trivalent or higher polyhydric alcohol components include glycerin, trimethylolpropane, trimethylolethane, pentaerythritol, α-methylglucose, mannitol, and sorbitol. These can be used alone or in combination of two or more.

The polyester (A1) can also be copolymerized with a lactone or lactam. For example, ε-caprolactone or ε-caprolactam can be used.

Examples of polymerization/condensation reaction methods for producing the polyester (A1) include: 1) a method in which a polycarboxylic acid and a polyhydric alcohol are heated in the presence of a known catalyst, followed by a dehydration esterification step and then a polyhydric alcohol removal/polycondensation reaction; 2) a method in which an alcohol ester of a polycarboxylic acid and a polyhydric alcohol are heated in the presence of a known catalyst, followed by a transesterification step and then a polyhydric alcohol removal/polycondensation reaction; and 3) a method in which depolymerization is performed. In methods 1 and 2, some or all of the acid components may be replaced with acid anhydrides.

When producing the polyester (A1), conventional polymerization catalysts can be used, such as titanium compounds such as tetra-n-butyl titanate, tetraisopropyl titanate, and titanium oxyacetylcetonate; antimony compounds such as antimony trioxide and tributoxyantimony; germanium compounds such as germanium oxide and tetra-n-butoxygermanium; and acetates of magnesium, iron, zinc, manganese, cobalt, and aluminum. These catalysts can be used alone or in combination of two or more.

Methods for increasing the acid value of the polyester (A1) include, for example, (1) adding a trivalent or higher polycarboxylic acid and/or a trivalent or higher polycarboxylic anhydride after the polycondensation reaction is complete and allowing the reaction to proceed (acid addition), and (2) intentionally modifying the resin during the polycondensation reaction by applying heat, oxygen, water, or the like. The polycarboxylic acid anhydride used in the acid addition method is not particularly limited, but examples include phthalic anhydride, tetrahydrophthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, pyromellitic anhydride, hexahydrophthalic anhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride, 3,3',4,4'-biphenyltetracarboxylic dianhydride, and ethylene glycol bisanhydrotrimellitate, and these can be used alone or in combination of two or more.

The acid value of the polyester (A1) is preferably 200 eq/ ¹⁰ g or less, more preferably 100 eq/ ¹⁰ g or less, even more preferably 50 eq/ ¹⁰ g or less, particularly preferably 30 eq/ ¹⁰ g or less, and most preferably 20 eq/ ¹⁰ g or less. By setting the resin acid value within the above range, good pot life, substrate adhesion, and improved crosslinkability can be expected. Furthermore, the lower the acid value, the more the moisture absorption can be suppressed, and the more the humid solder heat resistance is improved.

The number average molecular weight of the polyester (A1) is preferably 5,000 or more and 100,000 or less. It is more preferably 10,000 or more. Furthermore, it is even more preferably 50,000 or less. When the number average molecular weight is within the above range, the polyester is easy to handle when dissolved in a solvent, and has high cohesive strength after curing, thereby exhibiting excellent peel strength.

Increasing the Tg or Tm of a polyester to above 80°C can be achieved, for example, by introducing a rigid structure such as an aromatic compound or alicyclic compound, or by increasing the concentration of ester bonds.

<Polyester (A2)>
The adhesive composition of the present invention preferably further contains a polyester (A2) having a Tg of 0°C or lower and a melting point of 80°C or lower, or being amorphous. By containing a polyester (A2) having a Tg of 0°C or lower and a melting point of 80°C or lower, or being amorphous, flexibility can be imparted to the uncured coating film, and humid solder heat resistance can also be imparted. The Tg of the polyester (A2) is preferably -10°C or lower.
Furthermore, being amorphous means that the material does not show a melting peak in the measurement of the melting point described below.

The content of the polyester (A2) is preferably 10 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the polyester (A1) having a Tg or Tm of above 80°C. It is more preferably 25 parts by mass or more. It is also more preferably 70 parts by mass or less. When the content of polyester (A2) is within the above range, the flexibility of the uncured coating film, peel strength at high temperatures, and humid solder heat resistance are excellent.

The acid value of the polyester (A2) is preferably 200 eq/ ¹⁰ g or less, more preferably 100 eq/ ¹⁰ g or less, even more preferably 50 eq/ ¹⁰ g or less, particularly preferably 30 eq/ ¹⁰ g or less, and most preferably 20 eq/ ¹⁰ g or less. By setting the resin acid value within the above range, good pot life, substrate adhesion, and improved crosslinkability can be expected. Furthermore, the lower the acid value, the more the moisture absorption can be suppressed and the more improved the humid solder heat resistance.

The number average molecular weight of the polyester (A2) is preferably 5,000 or more and 100,000 or less. It is more preferably 10,000 or more, even more preferably 20,000 or more, and particularly preferably 25,000 or more. When the number average molecular weight is above the above value, the polar group concentration is relatively reduced, thereby suppressing moisture absorption and improving humid solder heat resistance. Furthermore, the number average molecular weight is more preferably 50,000 or less. When the number average molecular weight is below the above value, the polyester is easy to handle when dissolved in a solvent, retains cohesive strength even before curing, and exhibits flexibility in the uncured coating film.

To make a polyester have a glass transition temperature of 0°C or below and a melting point of 80°C or below, or to make it amorphous, this can be achieved, for example, by copolymerizing a monomer component having a long-chain alkyl group or a monomer component having an asymmetric structure to disrupt the crystallinity.

<Polyisocyanate (B)>
The adhesive composition of the present invention contains a polyisocyanate (B). The polyisocyanate (B) used in the present invention is not particularly limited as long as it is an isocyanate compound that reacts with a polyester resin and cures.

Examples of polyisocyanate (B) include aromatic or aliphatic diisocyanate compounds and trivalent or higher polyisocyanate compounds. These isocyanate compounds may be either low molecular weight compounds or high molecular weight compounds. Examples include aliphatic diisocyanates such as tetramethylene diisocyanate and hexamethylene diisocyanate; aromatic diisocyanates such as toluene diisocyanate, diphenylmethane diisocyanate, and xylylene diisocyanate; alicyclic diisocyanates such as hydrogenated diphenylmethane diisocyanate, hydrogenated xylylene diisocyanate, dimer acid diisocyanate, and isophorone diisocyanate; and trimers of these isocyanate compounds. Further examples include compounds containing terminal isocyanate groups obtained by reacting an excess amount of the isocyanate compound with a low-molecular-weight active hydrogen compound such as ethylene glycol, propylene glycol, trimethylolpropane, glycerin, sorbitol, ethylenediamine, monoethanolamine, diethanolamine, or triethanolamine. Further examples include compounds containing terminal isocyanate groups obtained by reacting an excess amount of the isocyanate compound with a high-molecular-weight active hydrogen compound such as various polyester polyols, polyether polyols, or polyamides. These isocyanate compounds can be used alone or in combination of two or more. Among these, a trimer of a hexamethylene diisocyanate compound is particularly preferred.

In the adhesive composition of the present invention, the content of polyisocyanate (B) is preferably 0.1 parts by mass or more and 10 parts by mass or less, per 100 parts by mass of the total of polyester (A1) and polyester (A2). It is more preferably 5 parts by mass or less. It is even more preferably 1 part by mass or more. By keeping the content of polyisocyanate (B) within the above range, excellent peel strength at high temperatures can be achieved.

In addition, in the present invention, inorganic flame retardants such as organic phosphorus compounds, zinc oxide, zinc sulfate, aluminum hydroxide, and barium hydroxide, leveling agents, and colorants such as dyes and pigments can be appropriately blended within a range that does not impair the properties of the adhesive composition of the present invention.

The present invention will be explained in more detail below with reference to examples. In these examples and comparative examples, parts simply refer to parts by mass.

(1) Measurement of polyester composition: Using a 400 MHz ^1H -nuclear magnetic resonance spectrometer (hereinafter sometimes abbreviated as NMR), the molar ratios of the polycarboxylic acid components and polyhydric alcohol components constituting the polyester were quantified. Deuterated chloroform was used as the solvent. When the acid value of the polyester was increased by acid post-addition, the molar ratio of each component was calculated by setting the total of the acid components other than the acid component used in the acid post-addition as 100 mol %.

(2) Measurement of Glass Transition Temperature and Melting Point Measurements were made using a differential scanning calorimeter (DSC-200, manufactured by SII). 5 mg of polyester was placed in an aluminum container with a lid, sealed, and cooled to -50°C using liquid nitrogen. The temperature was then increased to 150°C at a heating rate of 20°C/min. The endothermic curve obtained during the heating process was measured. The glass transition temperature (Tg, unit: °C) was determined as the temperature at the intersection of an extension of the baseline before the endothermic peak (below the glass transition temperature) and a tangent to the endothermic peak (a tangent showing the maximum slope between the rising part of the peak and the apex of the peak).
In addition, in the same endothermic curve, the temperature at the apex of the crystalline melting peak was taken as the melting point (Tm, unit: ° C.).

(3) Measurement of Number Average Molecular Weight A polyester sample was dissolved and/or diluted with tetrahydrofuran to a resin concentration of approximately 0.5 wt %, and filtered through a polytetrafluoroethylene membrane filter with a pore size of 0.5 μm to prepare a measurement sample. The molecular weight was measured by gel permeation chromatography (GPC) using tetrahydrofuran as the mobile phase and a differential refractometer as the detector. The flow rate was 1 mL/min, and the column temperature was 30°C. Showa Denko KF-802, 804L, and 806L columns were used. Monodisperse polystyrene was used as the molecular weight standard.

(4) Measurement of Acid Value 0.2 g of a polyester sample was dissolved in 40 ml of chloroform and titrated with a 0.01 N potassium hydroxide ethanol solution to determine the acid value equivalent per 10 ⁶ g of polyester (eq/10 ⁶ g). Phenolphthalein was used as an indicator.

Below is a production example of an adhesive composition containing the polyester of the present invention, a comparative polyester, and a polyisocyanate.

Production Example of Polyester (a1) A reaction vessel equipped with a stirrer, condenser, and thermometer was charged with 359 parts of terephthalic acid, 58 parts of dimethyl 2,6-naphthalenedicarboxylate, 67 parts of ethylene glycol, 189 parts of propanediol, 5 parts of trimethylolpropane, and 0.03 mol% of tetrabutyl orthotitanate as a catalyst relative to the total acid components. The temperature was raised from 160°C to 220°C over 4 hours, and an esterification reaction was carried out through a dehydration step. Next, in the polycondensation reaction step, the pressure in the system was reduced to 5 mmHg over 20 minutes, and the temperature was further increased to 250°C. Next, the pressure was reduced to 0.3 mmHg or less, and the polycondensation reaction was carried out for 60 minutes, after which the mixture was removed. As a result of composition analysis by NMR, the obtained polyester (a1) was found to have a molar ratio of terephthalic acid/2,6-naphthalenedicarboxylic acid/ethylene glycol/propanediol/trimethylolpropane=90/10/27/72/1. The glass transition temperature was 89°C. The results are shown in Table 1.

Production Examples of Polyesters (a2) to (a11) Polyesters (a2) to (a11) were synthesized according to the production example for polyester (a1), except for changing the types and blending ratios of raw materials. For polyester (a2), ε-caprolactone was added after the polymerization reaction was completed, and the mixture was reacted at 200°C for 30 minutes to carry out post-addition. For polyester (a3), trimellitic anhydride was added after the polymerization reaction was completed, and the mixture was reacted at 230°C for 30 minutes to carry out post-addition. The compositional analysis values, melting point (Tm), and glass transition temperature (Tg) of each polyester are listed in Table 1. PTMG1000 is polytetramethylene ether glycol (average molecular weight 1000). In Table 1, cases where a melting point (Tm) was not observed are indicated by "-".

Example 1
An adhesive composition was produced by blending 80 parts (solids) of polyester (a1) having a solids concentration of 50% dissolved in toluene, 20 parts (solids) of polyester (a2) having a solids concentration of 50% dissolved in toluene, and 2 parts of Sumidur N3300 (manufactured by Sumika Covestro Urethane Co., Ltd.) as a polyisocyanate. The resulting adhesive composition was evaluated for peel strength, humid solder heat resistance, and flexibility of the uncured coating film. The results are shown in Table 2.

Examples 2 to 14, Comparative Examples 1 to 4
Adhesive compositions were prepared in accordance with Example 1, but the types and blending ratios of the raw materials were changed as shown in Table 2, and various evaluations were carried out. The results are shown in Table 2.

Peel strength (adhesion)
The adhesive compositions prepared in each of the above examples were applied to a 12.5 μm thick polyimide film (Apical®, manufactured by Kaneka Corporation) to a dry thickness of 25 μm, and then dried at 140°C for 3 minutes. The adhesive film (B-stage product) thus obtained was then laminated to an 18 μm thick rolled copper foil (BHY series, manufactured by JX Nippon Mining & Metals Corporation). The laminate was bonded by pressing the shiny side of the rolled copper foil against the adhesive layer at 160°C under a pressure of 2 MPa for 30 seconds. The foil was then cured by heat treatment at 170°C for 3 hours to obtain a sample for peel strength evaluation. Peel strength was measured by a 180° peel test at 125°C, with the film pulled at a tensile speed of 50 mm/min. This test indicates the adhesive strength at 125°C.
<Evaluation criteria>
〇: 0.3N/mm or more △: 0.1N/mm or more but less than 0.3N/mm ×: 0.1N/mm or less

Humidified solder heat resistance: A sample was prepared in the same manner as for the peel strength evaluation. This sample was cut into a size of 2.0 cm x 2.0 cm, and allowed to absorb moisture for 2 days in a temperature and humidity environment of 40°C and 80%. The sample was then floated in a molten solder bath at 260°C, and the time until swelling occurred was measured.
<Evaluation criteria>
○: No swelling for 60 seconds or more △: Swelling occurs for 30 seconds or more but less than 60 seconds ×: Swelling occurs for less than 30 seconds

The adhesive compositions prepared in each of the above examples were applied to a 12.5 μm thick polyimide film (Apical (registered trademark), manufactured by Kaneka Corporation) so that the thickness after drying would be 25 μm, and then dried for 3 minutes at 140° C. The adhesive film thus obtained (B-stage product) was bent 90° to check whether cracks occurred in the coating film and evaluated.
<Evaluation criteria>
○: No cracks △: Bending marks ×: Cracks

The adhesive composition of the present invention has excellent flexibility of the uncured coating film, humid solder heat resistance, and peel strength at high temperatures. Therefore, it is suitable as an adhesive composition for FPCs exposed to high temperatures, such as automotive FPCs.

Claims (3)

An adhesive composition for flexible printed wiring boards, comprising a polyester (A1) having a glass transition temperature or melting point of more than 80°C and a polyisocyanate (B), and further comprising a polyester (A2) having a glass transition temperature of 0°C or lower and a melting point of 80°C or lower or being amorphous . 2. The adhesive composition for flexible printed wiring boards according to claim 1 , wherein the content of the polyester (A2) is 10 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the polyester (A1). 3. The adhesive composition for flexible printed wiring boards according to claim 1 , comprising 0.1 parts by mass or more and 10 parts by mass or less of the polyisocyanate (B) per 100 parts by mass of the total amount of the polyester (A1) and the polyester (A2 ).