JP7785495B2 - Pressure-sensitive adhesive composition - Google Patents

Description

The present invention relates to an adhesive composition, and more specifically to an adhesive composition that has excellent adhesion to fluorine-treated glass.

Touch panels are electronic devices that combine display and position input functions, and are widely used in electronic devices such as smartphones, tablets, ATMs, and vending machines. When inputting data onto a touch panel, users touch it with their fingertips, which makes it easy for dirt such as fingerprints to adhere to the panel. When such dirt adheres to the panel, not only does it reduce visibility, but it can also cause malfunctions when inputting data.

As a result, fluorine-treated glass, which is less susceptible to adhesion of dirt components and can be easily removed if it does adhere, is now being used for touch panel surfaces. Meanwhile, in smartphones and other devices, a protective film is often attached to the touch panel surface, but fluorine-treated glass has low wettability, and conventional adhesives do not provide sufficient adhesive strength.

Patent Document 1 discloses an anti-slip adhesive film for lenses having an anti-fouling layer formed from a fluorine silane compound, but does not evaluate the adhesive strength required for a protective film for touch panels.
Japanese Patent Application Laid-Open No. 2006-299115

The objective of the present invention is to provide an adhesive composition that has excellent adhesion to fluorine-treated glass.

The present invention relates to a pressure-sensitive adhesive composition containing an acrylic copolymer obtained by polymerizing an acrylic monomer composition and an isocyanate compound. The pressure-sensitive adhesive composition is characterized in that when a film of the pressure-sensitive adhesive composition formed to a thickness of 20 μm on a 50 μm thick PET film is adhered to fluorine-treated glass and peeled off, the peel strength is 20 gf/25 mm or more.

The adhesive composition of the present invention has excellent adhesion to fluorine-treated glass, making it particularly useful as an adhesive for protective films for touch panels that use fluorine-treated glass as the surface material.

The pressure-sensitive adhesive composition of the present invention contains an acrylic copolymer obtained by polymerizing an acrylic monomer composition. The acrylic monomer includes a (meth)acrylic monomer and a monomer copolymerizable therewith. Note that, in the present invention, (meth)acrylic encompasses both acrylic and methacrylic, and a (meth)acrylic monomer refers to a monomer having a (meth)acrylic acid ester structure.

Examples of (meth)acrylic monomers having a substituent with 8 or less carbon atoms include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, sec-butyl (meth)acrylate, t-butyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate.

Examples of (meth)acrylic monomers having a substituent with 9 to 11 carbon atoms include isononyl (meth)acrylate, decyl (meth)acrylate, and undecyl (meth)acrylate.

Examples of (meth)acrylic monomers having a substituent with 12 or more carbon atoms include lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, pentadecyl (meth)acrylate, hexadecyl (meth)acrylate, heptadecyl (meth)acrylate, stearyl (meth)acrylate, behenyl (meth)acrylate, isomistyryl (meth)acrylate, isostearyl (meth)acrylate, isododecyl (meth)acrylate, isotridecyl (meth)acrylate, isopentadecyl (meth)acrylate, isohexadecyl (meth)acrylate, and isoheptadecyl (meth)acrylate.

Examples of hydroxyl group-containing (meth)acrylic monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxy-3-chloropropyl (meth)acrylate, and 2-hydroxy-3-phenoxypropyl (meth)acrylate.

Other monomers include amide group-containing monomers such as (meth)acrylamide, dimethyl(meth)acrylamide, diethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, (meth)acryloylmorpholine, N-vinylpyrrolidone, and diacetone(meth)acrylamide; carboxyl group-containing monomers such as (meth)acrylic acid, itaconic acid, and maleic acid; styrene, acrylonitrile, and vinyl acetate.

Among these, it is preferable that the monomer composition contains 55 to 80% by weight of a (meth)acrylic monomer having a substituent with 12 or more carbon atoms, 10 to 35% by weight of a (meth)acrylic monomer having a substituent with 8 or less carbon atoms, and 1 to 10% by weight of a hydroxyl group-containing (meth)acrylic monomer. By using these ranges, it is possible to increase adhesion to fluorine-treated glass and reduce the dielectric constant.

Furthermore, the (meth)acrylic monomer having a substituent with 12 or more carbon atoms is preferably isostearyl acrylate.
The acrylic copolymer can be prepared by polymerizing these monomers by known methods such as solution polymerization, emulsion polymerization, and bulk polymerization, but solution polymerization is preferred.

Solvents used in solution polymerization include organic solvents such as methyl acetate, ethyl acetate, butyl acetate, benzene, toluene, xylene, acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, n-hexane, isopropyl alcohol, n-butanol, propylene glycol monomethyl ether, propylene glycol dimethyl ether, propylene glycol monomethyl ether acetate, and propylene carbonate. However, other solvents may also be used without any problems, and two or more solvents may be used in combination.

It is preferable to use a thermal decomposition type polymerization initiator as the polymerization initiator. 2,2'-azobisisobutyronitrile, 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), dimethyl-2,2'-azobis(2-methylpropionate), 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), 2,2'-azobis[N-(2-propionate)], azo compounds such as 2,2'-azobis(N-butyl-2-methylpropionamide), 2,2'-azobis(N-cyclohexyl-2-methylpropionamide), and 1,1'-azobis(1-acetoxy-1-phenylethane); lauroyl peroxide; octanoyl peroxide; Examples of organic peroxide compounds that can be used include peroxides, benzoyl peroxide, ethyl methyl ketone peroxide, cumene hydroperoxide, dicumyl peroxide, t-butyl hydroperoxide, cumyl peroxyneodecanoate, t-hexyl peroxyneodecanoate, t-butylperoxy-2-ethylhexanoate, dicumyl peroxide, isobutyl peroxide, di-t-butyl peroxide, t-butylcumyl peroxide, t-butyl peroxybenzoate, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, di(2-ethylhexyl)peroxydicarbonate, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, and 3,3,5-trimethylcyclohexanoyl peroxide. Redox polymerization of peroxide compounds can also be carried out by using a reducing agent such as N,N-dimethyltoluidine or N,N-diethyltoluidine in combination with the peroxide compound.

The polymerization initiator is used in an amount of 0.05 to 3 parts by weight per 100 parts by weight of monomer. Increasing the amount of polymerization initiator used tends to decrease the molecular weight of the resulting copolymer, while decreasing the amount of polymerization initiator used tends to increase the molecular weight of the resulting copolymer.

It is preferable to add a crosslinking agent to the pressure-sensitive adhesive composition of the present invention. The crosslinking agent reacts with the functional groups in the acrylic copolymer to improve various physical properties. Examples of crosslinking agents include isocyanate compounds and metal chelate compounds.
Examples of the isocyanate compound include 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, tolylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, and adducts, isocyanurates, biurets, and allophanates derived from the above isocyanates.
The amount of the crosslinking agent to be blended is preferably 0.001 to 20 parts by weight per 100 parts by weight of the acrylic copolymer.

The adhesive composition of the present invention preferably has a relative dielectric constant of 3.5 or less at a frequency of 100 kHz. A relative dielectric constant of 3.5 or less reduces the risk of malfunction when used in a touch panel. In order to achieve a relative dielectric constant of 3.5 or less, it is preferable to use the aforementioned acrylic monomer composition.

The adhesive composition of the present invention preferably has a peel strength of 20 gf/25 mm or more when a film made by applying the adhesive composition to a 20 μm thickness on a 50 μm thick PET film is attached to fluorine-treated glass. This reduces the risk of the protective film peeling off from the touch panel during use when used as an adhesive for the protective film.

The adhesive composition of the present invention may further contain various additives such as ultraviolet absorbers, near-infrared absorbers, antioxidants, preservatives, antifungal agents, tackifying resins, plasticizers, antifoaming agents, and wettability adjusters.

The present invention will be explained in more detail below with reference to examples and comparative examples, but these are intended to be specific examples and are not intended to be limiting. All parts are by weight.

Example 1
A monomer mixture was prepared by adding 10 parts of lauryl methacrylate (LMA), 60 parts of isostearyl acrylate (ISTA), 23.9 parts of 2-ethylhexyl acrylate (2EHA), 6 parts of 4-hydroxybutyl (meth)acrylate (4HBA), 0.1 parts of acrylic acid (AA), 0.1 parts of 2,2'-azobisisobutyronitrile as a polymerization initiator, and 20 parts of ethyl acetate as a solvent. 50 parts of ethyl acetate were added as a solvent to a separable flask equipped with a stirrer and reflux condenser, and the mixture was heated to 65°C. Nitrogen gas was introduced for 30 minutes or more to remove oxygen from the polymerization system. Next, the monomer mixture was added dropwise over 3 hours while maintaining the temperature at 75±1°C, and the mixture was allowed to react for another hour while maintaining the temperature at 75±1°C. The reaction temperature was then raised to 80°C and maintained at 80±1°C for 2 hours to complete the polymerization reaction. After the reaction was completed, the mixture was diluted with 30 parts of ethyl acetate to obtain a pale yellow, transparent, viscous acrylic copolymer solution. The viscosity was 800 mPa·s, the solid content was 50%, and the weight-average molecular weight was 200,000. Furthermore, 0.3 parts by weight of a trifunctional isocyanate compound, Coronate HX (manufactured by Tosoh Corporation, trade name), and 0.3 parts by weight of an aluminum chelate compound, Narcem Aluminum (manufactured by Nippon Chemical Industry Co., Ltd., trade name), were added as crosslinking agents per 100 parts by weight of the acrylic copolymer to prepare the pressure-sensitive adhesive composition of Example 1.

Examples 2 to 4, Comparative Examples 1 to 4
Each PSA composition was prepared in the same manner as in Example 1, except that lauryl acrylate (LA), behenyl methacrylate (VMA), n-butyl (meth)acrylate (BA), and 2-hydroxyethyl (meth)acrylate (2HEA) were used in addition to the monomers used in Example 1, and the monomer compositions and crosslinking agents were blended in the amounts shown in Table 1.

Preparation of adhesive sheets Each adhesive composition was applied to a release film (a silicone-treated PET film) so that the thickness after drying would be 20 μm, and after heating and drying at 100° C. for 2 minutes, a 50 μm-thick PET film was laminated onto the coated sheet. This sheet was then left to stand in an atmosphere of 40° C. and a relative humidity of 50% RH for 3 days to obtain an adhesive sheet.
Measurement of weight average molecular weight
The weight average molecular weight of the acrylic copolymer is a value measured by GPC (gel permeation chromatography) and calculated in terms of polystyrene. The sample was prepared by dissolving the sample in tetrahydrofuran to prepare a 0.1 wt % solution, leaving it in an ultrasonic cleaning device containing water for 10 minutes, and then filtering the solution through a 0.20 μm membrane filter.
Analytical equipment: Shimadzu LC20AD
Column: Shimadzu GPC-80M x 2
Eluent: tetrahydrofuran Flow rate: 1.0 ml/min
Inlet pressure: 10 kgf
Detector: differential refractometer (RI)
Column temperature: 40°C
Injection volume: 50 μl
Eluent: Tetrahydrofuran Detector: Differential refractometer Standard sample: Polystyrene

The adhesive sheet was cut into 25 mm wide strips, the release film was peeled off, and the strips were attached to fluorine-treated glass by rolling them twice with a hand roller to prepare test pieces. The test pieces were left standing in an atmosphere of 23°C and 50% RH for 24 hours, and then pulled in a 180° direction at a pulling rate of 300 mm/min, and the median value was recorded as the adhesive strength.

The pressure-sensitive adhesive layers of the pressure-sensitive adhesive sheets were laminated to form a laminated pressure-sensitive adhesive layer of about 200 μm, and the dielectric constant of the laminated pressure-sensitive adhesive layer was measured using the following device. The dielectric constant was determined by averaging the measured values of three samples.

Measurement method: LCR meter (device: Wayne Kerr 6440B impedance analyzer used)
Frequency: 1 MHz
Bias voltage: 2V
Measurement environment: 23±1°C, 50±1% RH

Each of the adhesive compositions in the Examples had high adhesive strength to fluorine-treated glass. On the other hand, each of the adhesive compositions in the Comparative Examples had poor adhesive strength to fluorine-treated glass.

Claims (2)

A pressure-sensitive adhesive composition comprising an acrylic copolymer obtained by polymerizing an acrylic monomer composition and an isocyanate compound, wherein a film of the pressure-sensitive adhesive composition formed to a thickness of 20 μm on a 50 μm thick PET film is attached to fluorine-treated glass, and the peel strength when peeled is 20 gf/25 mm or more;
the acrylic monomer composition contains 55 to 80% by weight of a (meth)acrylic monomer having a substituent with 12 or more carbon atoms, 10 to 35% by weight of a (meth)acrylic monomer having a substituent with 8 or less carbon atoms, and 1 to 10% by weight of a hydroxyl group-containing (meth)acrylic monomer;
A pressure-sensitive adhesive composition comprising isostearyl acrylate as the (meth)acrylic monomer having a substituent with 12 or more carbon atoms . 2. The pressure-sensitive adhesive composition according to claim 1 , wherein the acrylic copolymer has a molecular weight of 100,000 to 400,000.