JP7851168B2 - Cationic curable epoxy resin composition - Google Patents

Description

This invention relates to an epoxy resin composition that cures by cationic polymerization.

For bonding optical components, UV-curing adhesives have traditionally been widely used due to the demands for transparency and rapid curing. UV-curing adhesives mainly consist of radical-curing acrylics and cationic-curing epoxys. However, the former suffers from long-term reliability issues due to polymerization inhibition by oxygen in the air and a decrease in adhesive strength due to hydrolysis of the ester group. Therefore, in fields where reliability is critical, epoxy-based UV-curing adhesives are widely used.

In these epoxy adhesives, inorganic fillers such as silica are sometimes incorporated to improve physical properties such as thixotropy and adhesive strength. For example, adhesives for fixing optical components include those comprising an epoxy resin with two glycidyl groups directly linked by aromatic rings, an alicyclic polyether-backed epoxy resin, an alicyclic epoxy compound, a photocationic polymerization catalyst, a silane coupling agent, and silica nanoparticles (Patent Document 1), and adhesives containing an alicyclic epoxy, an oxetane compound, a photocationic polymerization initiator, and silica (Patent Document 2).

However, increasing the amount of inorganic filler made the adhesive prone to sedimentation, and even with the addition of a dispersant to prevent this, it tended to thicken quickly. Furthermore, when inorganic fillers were heavily packed to reduce curing shrinkage and thermal expansion, the viscosity became significantly higher, sometimes leading to problems such as reduced workability and fluidity. Therefore, there was a need for an adhesive suitable for optical components that possessed good workability, a sufficiently low thermal expansion coefficient, and good storage stability.

Japanese Patent Publication No. 2016-94509 Patent No. 6709730

The problem that this invention aims to solve is to provide a cationic curable epoxy resin composition that maintains good viscosity for workability even when the silica content is high and the filler is dense, has good storage stability, and exhibits a low coefficient of linear expansion of the cured product.

To solve the above problems, the invention of claim 1 provides a cationic curable epoxy resin composition comprising an epoxy resin (A), silica (B), calcium carbonate (C), and a cationic polymerization initiator (D), wherein (A) comprises an epoxy resin (a1) having a cycloalkene oxide skeleton and a diglycidyl ether (a2) having an aromatic ring, (B) comprises spherical silica (b1) with an average primary particle diameter of 1.5 to 30 μm and spherical silica (b2) with an average primary particle diameter of 0.05 to 1.0 μm, and (D) comprises a photocationic polymerization initiator (d1) and a thermal cationic polymerization initiator (d2) .

The invention of claim 2 provides the cationic curable epoxy resin composition according to claim 1, characterized in that (a2) is resorcinol diglycidyl ether .

The invention of claim 3 provides a cationic curable epoxy resin composition according to claim 1 or 2, characterized in that the blending ratio of (a2) is 5 to 50% by weight relative to (A) .

The invention of claim 4 provides a cationic curable epoxy resin composition according to any one of claims 1 to 3, characterized in that it is an adhesive for optical components or optical devices .

The cationic epoxy resin composition of the present invention maintains a viscosity that is highly workable even with a high silica content and high filler content, and exhibits good storage stability and a low coefficient of thermal expansion of the cured product, making it useful as an adhesive for optical applications.

The present invention will be described in detail below.

The epoxy resin composition of the present invention comprises epoxy resin (A), silica (B), calcium carbonate (C), and a cationic polymerization initiator (D).

The epoxy resin (A) used in this invention is a highly transparent, heat-resistant, and flexible main constituent resin having epoxy groups that open ring-open and form a crosslinked structure upon activation by a cationic polymerization initiator. It includes an epoxy resin (a1) having a cycloalkene oxide skeleton and a diglycidyl ether (a2) having an aromatic ring.

The epoxy resin (a1) having a cycloalkene oxide skeleton used in the present invention has a structure in which epoxy groups are directly arranged on an alicyclic skeleton, exhibits good reactivity with cationic catalysts, good adhesion and curability, and excellent low-temperature curability. Because of its excellent curability, the amount of uncured material remaining in the cured product after curing can be reduced, and the increase in shrinkage stress can be suppressed even in high-temperature and high-humidity environments, thereby improving resistance to high temperatures and high humidity. It is preferable that it be liquid at room temperature, and in terms of reactivity, it is preferable that the epoxy groups be bifunctional or more. Examples of bifunctional or more (a1) include 3',4'-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, 3,4,3',4'-diepoxybicyclohexane, and ε-caprolactone-modified 3',4'-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate), which can be used alone or in combination of two or more. Among these, 3',4'-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, which yields a cured product with low viscosity and high crosslink density, is preferred.

The mixing ratio of (a1) is preferably 40 to 90% by weight, more preferably 45 to 85% by weight, and particularly preferably 50 to 75% by weight relative to (A). A ratio of 40% by weight or more ensures a sufficiently high glass transition temperature (hereinafter referred to as Tg), while a ratio of 90% by weight or less ensures sufficient photocurability. Furthermore, the mixing ratio of (a1) to the total of (a1) and (a2) is preferably 45 to 95% by weight, and more preferably 50 to 90% by weight.

The aromatic ring-containing diglycidyl ether (a2) used in this invention is a low-viscosity resin with high photocuring sensitivity. Examples include resorcinol diglycidyl ether, 4-ethyl resorcinol diglycidyl ether, catechol diglycidyl ether, hydroquinone diglycidyl ether, etc., which have one aromatic ring, and bisphenol-type and biphenyl-type glycidyl ethers, which have multiple aromatic rings. These can be used individually or in combination of two or more types. Among these, it is preferable to include resorcinol diglycidyl ether, which is readily available and a crystalline epoxy that allows for high silica filling.

The mixing ratio of (a2) is preferably 5 to 50% by weight, more preferably 8 to 45% by weight, and particularly preferably 20 to 40% by weight relative to (A). A ratio of 5% by weight or more ensures sufficient curability, while a ratio of 50% by weight or less ensures a sufficiently high Tg. Furthermore, the mixing ratio of (a2) to the total of (a1) and (a2) is preferably 5 to 55% by weight, and more preferably 10 to 40% by weight.

In addition to (a1) and (a2) above, a reactive diluent (a3) containing a monofunctional or bifunctional linear aliphatic or branched aliphatic epoxy resin may be added to adjust the viscosity of the composition. Among these, a linear aliphatic diglycidyl ether is preferred due to its very low viscosity, and 1,4-butanediol diglycidyl ether and 1,6-hexanediol diglycidyl ether, which have a viscosity of 30 mPa·s or less at 25°C, are even more preferred.

The overall blending ratio of (A), including (a1), (a2), and (a3), is preferably 15 to 45% by weight, more preferably 20 to 40% by weight, and particularly preferably 25 to 35% by weight, based on the total solid content. A ratio of 15% by weight or more ensures sufficient adhesive strength and film curing properties, while a ratio of 45% by weight or less allows for a sufficiently low coefficient of linear expansion of the cured product.

The silica (B) used in this invention is formulated to suppress curing shrinkage, reduce the coefficient of linear thermal expansion, and improve the fluidity of the composition. (B) comprises at least two types: spherical silica (b1) with an average primary particle diameter (hereinafter referred to as average particle size) of 1.5 to 30 μm, and spherical silica (b2) with an average particle size of 0.05 to 1.0 μm.

Spherical silica can be obtained by melting crushed silica in a flame at a high temperature of over 2000°C, and rapidly cooling the molten silica that has been sphericalized by surface tension. The resulting spherical silica is amorphous and has a very low coefficient of thermal expansion of 0.5 ppm/K. Therefore, even in resins with high expansion coefficients, incorporating it can suppress curing shrinkage and reduce the linear thermal expansion coefficient of the cured product.

By incorporating spherical silica having two different average particle sizes, (b1) and (b2), it becomes possible to position smaller particles in the gaps between larger particles, improving the packing efficiency, suppressing curing shrinkage, and significantly reducing the coefficient of linear thermal expansion. Furthermore, even with high packing, the distance between particles can be maintained, avoiding contact between particles and resulting in high fluidity. Especially when used as an adhesive for fixing optical components, this improves workability along with fluidity, and reduces displacement due to curing shrinkage.

The average particle size of (b1) is 1.5 to 30 μm, preferably 2 to 15 μm, and more preferably 3.0 to 10 μm. The average particle size of (b2) is 0.05 to 1.0 μm, preferably 0.2 to 0.9 μm, and more preferably 0.4 to 0.8 μm. This range allows for high packing, and a sufficient reduction in linear expansion coefficient and improvement in fluidity can be expected. The average particle size is the median diameter (d = 50) measured by laser diffraction/scattering method in accordance with JIS Z 8825-1.

The mixing ratio of (b1) to the total of (b1) and (b2) is preferably 60 to 95% by weight, more preferably 65 to 90% by weight, and particularly preferably 70 to 85% by weight. A ratio of 60% by weight or more allows for sufficient fluidity and reduction of the coefficient of linear expansion, while a ratio of 95% by weight or less allows for a viscosity with good workability.

In addition to (b1) and (b2) above, fumed silica (b3) may be further included to improve thixotropy and suppress the settling of filler components. The specific surface area of (b3) is preferably 50 to 300 ^m² /g, more preferably 80 to 200 ^m² /g, and particularly preferably 100 to 150 ^m² /g. By setting it within this range, a sufficient settling prevention effect can be expected due to the improvement of thixotropy. If the surface treatment is not performed, it will be hydrophilic and will easily absorb moisture, so it is preferable that the silica be surface treated, and dimethyldichlorosilane and the like can be used as surface treatment agents. The specific surface area can be measured by the BET method by gas adsorption.

The mixing ratio of (b3) is preferably 3.0% by weight or less relative to (B), more preferably 0.3 to 2.0% by weight, and particularly preferably 0.5 to 1.5%. This range ensures sufficient anti-settling effect.

The amount of (B), which includes (b1), (b2), and (b3), is preferably 130 to 350 parts by weight, more preferably 150 to 300 parts by weight, and particularly preferably 170 to 270 parts by weight, per 100 parts by weight of (A). An amount of 130 parts or more allows for sufficient reduction of the coefficient of linear expansion and improvement of fluidity, while an amount of 350 parts or less ensures viscosity and fluidity suitable for workability. Furthermore, the blending ratio of (B) to the total solid content is preferably 50 to 75% by weight, more preferably 55 to 70% by weight, and particularly preferably 57 to 68% by weight. Within this range, a composition with a balanced curability and fluidity can be obtained.

The calcium carbonate (C) used in this invention is added to suppress the increase in viscosity of the composition caused by the high concentration of silica. While the reason for the reduced viscosity is not entirely clear, one possible reason is that it neutralizes the composition's increased acidity. For the same reason, it is expected to suppress thickening over time and improve shelf life.

The average particle size of (C) is preferably 0.1 to 30 μm, more preferably 1.0 to 20 μm, and particularly preferably 3.0 to 10 μm. This range allows for a viscosity reduction effect on the composition, while simultaneously providing high density as a filler within the film together with silica, thus maintaining a sufficiently low coefficient of linear expansion.

The amount of (C) is preferably 1 to 15% by weight, more preferably 2 to 10% by weight, and particularly preferably 3 to 8% by weight, relative to the total solid content. Furthermore, the amount of (B) per 100 parts by weight is preferably 2 to 20 parts by weight, more preferably 3 to 15 parts by weight, and particularly preferably 5 to 12 parts by weight. By setting the viscosity within this range, it is possible to sufficiently suppress the increase in viscosity of the composition and achieve a viscosity range with excellent workability.

The cationic polymerization initiator (D) used in this invention preferably contains a photo-cationic polymerization initiator (d1) and a thermal cationic polymerization initiator (d2) as initiators of photo- and thermal polymerization reactions.

The aforementioned (d1) is an initiator that generates cationic ions upon irradiation with active energy rays such as visible light, ultraviolet light, ^and electron beams, and is an onium salt _composed of aromatic organic atom cations such as antimony, phosphorus, sulfur, nitrogen, and iodine, and anions such as ^FG- , ^FGa- , _BF4- , ^PF6- , _SbF6- , and _{( C2F5} ⁾ _3PF3- . Specifically ^, _these include aromatic diazonium salts, aromatic sulfonium salts, aromatic iodonium salts, etc., which can be used alone or in combination of two or more. Among these, aromatic triarylsulfonium salt systems are preferred because they have low toxicity, excellent solubility in monomers, and photosensitivity.

The amount of component (d1) is preferably 0.1 to 5.0 parts by weight, more preferably 0.3 to 4.0 parts by weight, and particularly preferably 0.5 to 3.0 parts by weight, per 100 parts by weight of (A). Within this range, sufficient photocurability and storage stability can be ensured. Commercially available products include the CPI-100 series, 200 series, and 300 series (product names: all manufactured by Sunapro Co., Ltd.). In particular, the non-antimony CPI-310FG, which is highly sensitive to i-line (365 nm) light and does not produce discolored cured products, is preferred.

The above (d2) is _{an initiator} ^that generates cationic ions upon heating, ^and examples include _onium salts of ^nitrogen , _onium salts of sulfur ^, _onium salts of phosphorus, and _onium salts of iodine. Examples of anionic ^components of these onium salts include _SbF₁₆₁

If the solubility of (d2) in (A) is low, it is preferable to dissolve (d2) in a solvent first before blending. Examples of solvents include γ-butyrolactone, but the solvent is not particularly limited as long as it is a good solvent for solvent (d2) and has good compatibility with other components including (A). Other solvents besides γ-butyrolactone may also be used. Additives such as stabilizers may also be added.

The amount of (d2) is preferably 0.1 to 3.0 parts by weight, more preferably 0.3 to 2.5 parts by weight, and particularly preferably 0.5 to 2.0 parts by weight, per 100 parts by weight of (A). Within this range, sufficient thermosetting properties and storage stability can be ensured. Examples of commercially available products include TA-100 (trade name: manufactured by Sunapro, aromatic sulfonium salt compound), CXC-1612, and CXC-1821 (both trade names: manufactured by KING INDUSTRIES, quaternary ammonium salt type compounds).

The amount of (D), which is the sum of (d1) and (d2), is preferably 0.3 to 8.0 parts by weight, more preferably 1.0 to 6.0 parts by weight, and particularly preferably 2.0 to 5.0 parts by weight, per 100 parts by weight of (A). Within this range, sufficient low-temperature curing properties and storage stability can be ensured. Furthermore, the mixing ratio of (d1) to (d2) is preferably (d2)/(d1) = 0.30 to 0.95, more preferably 0.50 to 0.90, and particularly preferably 0.65 to 0.85. By reducing the amount of (d2) compared to (d1), the risk of (d2) cleaving due to photoacid generated from (d1) during light irradiation can be reduced, and as a result, the fixing accuracy of the optical components can be maintained.

The composition of the present invention (hereinafter referred to as "this composition") can be further improved by incorporating a silane coupling agent (E) to enhance adhesion to the adherend, suppress precipitation, and improve long-term storage. (E) has a structure that combines functional groups that bond with both organic and inorganic materials within its molecule. Examples of functional groups that react with organic materials include vinyl groups, epoxy groups, amino groups, methacrylic groups, and mercapto groups, while hydrolyzable silyl groups react with inorganic materials.

The aforementioned (E) is preferably a type having epoxy functional groups that have good compatibility with (A), and a type having an isocyanurate skeleton is also preferred as it is highly effective in improving adhesive strength and heat resistance. A commercially available example of the former is GLYMO (product name: Evonik Industries), and a commercially available example of the latter is KBM9659 (product name: Shin-Etsu Chemical Co., Ltd.). The amount of silane coupling agent is preferably 0.5 to 8 parts by weight, and more preferably 1 to 5 parts by weight, per 100 parts by weight of (A).

In addition to the above, antioxidants, photosensitizers, tackifiers, leveling agents, defoamers, curing accelerators, colorants, thickeners, flame retardants, organic microparticles, etc., may be added as needed, provided that they do not impair the performance of this composition.

The viscosity of this lipid composition, as measured with an E-type viscometer at 25°C, rotor 3° × R7.7, and 1 rpm, is preferably 0.1 to 20 Pa·s, and more preferably 1 to 15 Pa·s. Furthermore, the increase in viscosity after standing at 23°C for 24 hours is preferably 1.6 times or less of the initial value, and more preferably 1.3 times or less. Keeping the increase rate below this level ensures sufficient shelf life.

The linear expansion coefficient of this composition, measured in a 1 mm thick sample at a heating rate of 10°C/min, is preferably α1 (linear expansion coefficient below Tg) of 30 ppm or less, and more preferably 20 ppm or less. Furthermore, α2 (linear expansion coefficient above Tg) is preferably 60 ppm or less, and more preferably 40 ppm or less. This range allows for sufficient use as an adhesive for joining optical components such as optical communication devices where high reliability is required.

Any known light source can be used to cure this composition, such as low-pressure mercury lamps, high-pressure mercury lamps, ultra-high-pressure mercury lamps, carbon arc lamps, xenon lamps, metal halide lamps, LED lamps, and electrodeless ultraviolet lamps. The curing conditions are exemplified by an irradiation intensity of 50 mW/ ^cm² to 3000 mW/ ^cm² , with an integrated light dose of 50 to 6000 mJ/ ^cm² . The irradiation atmosphere may be air or an inert gas such as nitrogen or argon.

The following examples and comparative examples will specifically describe the cation-curable epoxy resin composition of the present invention. These are merely examples and are not limiting to the present invention. Unless otherwise specified, measurements were taken at a room temperature of 25°C and a relative humidity of 65%.

Example 1
In a light-shielding bottle, (a1) Celoxide 2021P (product name: manufactured by Daicel Corporation, 3',4'-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate), (a2) EX-201-IM (product name: manufactured by Nagase ChemteX Corporation, resorcinol diglycidyl ether), (a3) YED216D (product name: manufactured by Mitsubishi Chemical Corporation, 1,6-hexanediol diglycidyl ether, viscosity 15 mPa·s), (b1) FB-5SDC (product name: manufactured by Denka Corporation, average particle size 5 μm, fused silica), (b2) FSP-130MC (product name: manufactured by Denka Corporation, average particle size 0.5 μm, fused silica), and (b3) TS610 (product name: manufactured by CABOT Corporation, fumed silica, treated with dimethyldichlorosilane, specific surface area 130 m²). ² /g) was added, along with (C) Cypron A (trade name: manufactured by Cipro Chemical Co., Ltd., average particle size 5 μm), (d1) CPI-210S (trade name: manufactured by Sunapro Co., Ltd., diphenyl[4-(phenylthio)phenyl]sulfonium tris(pentafluoroethyl)trifluorophosphate), (d2) CXC1612 (trade name: manufactured by KING INDUSTRIES, quaternary ammonium salt compound), (d2) γ-butyllactone as the solvent for (d2), and (E) KBM9659 (trade name: manufactured by Shin-Etsu Chemical Co., Ltd., tris-(triethoxysilylpropyl)isocyanurate) and GLYMO (trade name: manufactured by Evonik Japan, 3-glycidoxypropyltrimethoxysilane) in the amounts shown in Table 1. The mixture was stirred using a stirring and defoaming device until homogeneous to obtain the cationic curable epoxy resin composition of Example 1. The units in the formulation table are parts by weight.

Examples 2-10
In addition to the materials used in Example 1, YL980 (product name: Mitsubishi Chemical Corporation, bisphenol A epoxy, epoxy equivalent weight 180-190) as (a2), and CXC1821 (product name: KING INDUSTRIES Corporation, quaternary ammonium salt type compound) and TA-100 (product name: Sunapro Corporation, aromatic sulfonium salt type compound) as (d2) were added to a light-shielding bottle in the amounts shown in Table 1, and stirred using a stirring and degassing device until homogeneous to obtain the cationic curable epoxy resin compositions of Examples 2 to 10.

Comparative Examples 1-4
In addition to the materials used in the examples, YX8000 (product name: Mitsubishi Chemical Corporation, hydrogenated bisphenol A epoxy, epoxy equivalent 205) was added as (A) in the amounts shown in Table 2, and the mixture was stirred using a stirring and degassing device until homogeneous to obtain the cationic curable epoxy resin compositions of Comparative Examples 1 to 4.

Evaluation items and evaluation methods

Preparation of linear expansion coefficient test specimens <br/> The epoxy resin composition obtained above was cast to a size of 5 mm × 5 mm × 1 mm thickness after curing. It was irradiated with a Heraeus LH6/LC-6B ultraviolet irradiation device (electrodeless) under the conditions of a D bulb output of 100 mW/ ^cm² and an integrated light intensity of 6000 mJ/ ^cm² to photocur it, and then further cured by heat curing at 130°C for 2 hours.

Thermal expansion coefficient: Using a ThermoPlus2 TMA8310 thermomechanical analyzer manufactured by Rigaku Denki Co., Ltd., the thermal expansion coefficient was measured on the above-mentioned test specimen (5 mm x 5 mm x 1 mm thick) at a heating rate of 10°C/min. The temperature at which the slope of the temperature/expansion coefficient graph changed was defined as the thermal expansion coefficient. The temperature before the glass transition point was designated as α1, and the temperature after the glass transition point was designated as α2.

Viscosity: Measured using a Toki Sangyo E-type viscometer RE-215R at a cone angle of 3°R7.7°, a temperature of 25±1°C, and a rotation speed of 10 rpm. Values between 0.1 and 20 Pa·s were marked with ○, and values outside this range were marked with ×.

Storage life: The viscosity increase rate was measured after being left at 23°C for 24 hours. A value of 1.6 times or less compared to the initial value was marked with a circle (○).
A value greater than 1.6 times was marked as ×.

Glass Transition Temperature: A resin composition was poured into a 5 mm × 40 mm × t0.5 mm silicone mold, a release film was placed on top, and cured with Fusion D bulb at 100 mW/cm² and 6000 mJ/cm². The test specimen removed from the mold was then heat-treated in a 130°C constant temperature bath for 2 hours to obtain the sample. The obtained sample was subjected to a tensile test using a TA Instruments Q800 dynamic viscoelasticity analyzer from room temperature to 200°C at a heating rate of 3°C/min and a frequency of 1 Hz. The storage modulus and loss modulus were determined, and the temperature at the maximum point of the obtained TanΔ was defined as the glass transition temperature. Temperatures of 150°C or higher were marked with ○, and temperatures below 150°C were marked with ×.

Curability: Using a differential scanning calorimeter, the total heat generated by the resin before curing (A) and the total heat generated by the resin after curing (B) were measured when each epoxy resin composition was held at 100°C for 3 hours.
The response rate was calculated as (A-B)/A × 100, with a response rate of over 95% being marked with ◎, 90-95% with ○, and less than 90% with ×.

Evaluation Results The evaluation results are shown in Tables 3 and 4.

The epoxy resin compositions in the examples showed favorable results in all evaluations, including coefficient of linear expansion, viscosity, storage properties, glass transition temperature, and curability.

On the other hand, Comparative Example 1, which did not contain (B), had poor compatibility and separated; Comparative Example 2, which did not contain (b2), and Comparative Example 3, which did not contain (C), had high viscosity; and Comparative Example 4, which did not contain (a1), had a low Tg. In all cases, none of these were suitable for the present invention.

Claims (4)

A cationic curable epoxy resin composition comprising an epoxy resin (A), silica (B), calcium carbonate (C), and a cationic polymerization initiator (D), wherein (A) comprises an epoxy resin having a cycloalkene oxide skeleton (a1) and a diglycidyl ether having an aromatic ring (a2), (B) comprises spherical silica (b1) with an average primary particle diameter of 1.5 to 30 μm and spherical silica (b2) with an average primary particle diameter of 0.05 to 1.0 μm, and (D) comprises a photocationic polymerization initiator (d1) and a thermal cationic polymerization initiator (d2) . The cationic curable epoxy resin composition according to claim 1, characterized in that (a2) is resorcinol diglycidyl ether . The cationic curable epoxy resin composition according to claim 1 or 2, characterized in that the blending ratio of (a2) is 5 to 50% by weight relative to (A) . A cationic curable epoxy resin composition according to any one of claims 1 to 3 , characterized in that it is an adhesive for optical components or optical devices.