JPS6090281A - Adhesive and bonding method - Google Patents

Abstract

PURPOSE:To provide a thermosetting adhesive for wood, paper, rubber, plastic, etc., having excellent adhesivity and adhesive strength, and composed of an active hydrogen-containing polyfunctional compound containing polymeric polyol as an essential component, a polyisocyanate, and a trimerization catalyst, etc. CONSTITUTION:The objective adhesive is composed of (A) an active hydrogen- containing compound having an average molecular weight of 1,000-10,000 and composed of (i) a polymeric polyol selected from polyester polyol and polyether ester polyol, and if necessary (ii) other polymeric polyol (preferably a polyether polyol having a molecular weight of 800-10,000) and/or a crosslinking agent (preferably polyol, polyamine or aminoalcohol having a molecular weight of 30- 300), (B) a large excess of a polyisocyanate compound based on the active hydrogen-containing group of the component A and free from isocyanurate ring (preferably having an NCO index of 1,100-2,000), (C) a trimerization catalyst, and if necessary (D) an organic solvent. The adhesive is inserted between a rigid plastic sheet and a flexible plastic sheet, and cured by heating at >=50 deg.C to effect the bonding of both sheets.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermosetting adhesive and an adhesion method, and more particularly to a thermosetting adhesive having excellent heat resistance.

Conventionally, various adhesives have been used in many laminated materials and composite materials. In addition, so-called polyisocyanate adhesives, such as mixtures of urethane polymers or polyisocyanates and active hydrogen atom-containing compounds, have strong adhesive strength;
Wood has many characteristics, including excellent water resistance and chemical resistance, as well as elasticity and flexibility when roughened, making it suitable for bonding parts that are subject to bending or deflection.

Textile materials, natural and synthetic rubber, plastics.

There is. In recent years, plastic has become an industrial and building material.

It has come to be widely used in packaging materials and the like, and there is a growing demand for adhesives that can bond plastics and have heat resistance, especially in automotive materials.

However, conventional polyisocyanate adhesives have poor heat resistance, and various adhesives such as other rubber adhesives and epoxy adhesives do not have sufficient heat resistance.

The present inventors have developed a polyisocyanate compound (13) which does not have a large excess of isocyanurate rings with respect to the active hydrogen-containing groups of the active hydrogen-containing polyfunctional compounds (2) and (5) which have a specific polymeric polyol as an essential component. ), the present invention was achieved by discovering that the above object could be achieved by using a thermosetting adhesive consisting of an allization catalyst (O).That is, the present invention relates to a polyester polyol.

A polymer polyol selected from polyether ester polyols and, if necessary, other polymer polyols and/or
Or an average molecular weight of 1000 to 10000 consisting of a crosslinking agent
An active hydrogen-containing polyfunctional compound (a), a polyisocyanate compound CB that does not have a large excess of isocyanurate rings with respect to the active hydrogen-containing group of (b), an octamerization catalyst (an isocyanurate consisting of Q and an organic solvent if necessary) An adhesion method characterized in that the adhesive is present between a ring-forming thermosetting adhesive (first invention) and a substrate and is heated and cured (
2nd invention).

The active hydrogen-containing polyfunctional compound (4) used in the present invention includes two or more active hydrogen-containing groups (hydroxyl group, carboxyl group) at the terminal or in the molecular chain.

(amino group, mercapto group, etc.), and at least a portion of the polymer polyol contains at least one type of polymer polyol consisting of polyester polyol and polyether ester polyol.

Examples of polyester polyols and polyether ester polyols include condensed polyester polyols obtained by reacting polyols with dicarboxylic acids (or dicarboxylic anhydrides and alkylene oxides) and polyester polyols obtained by ring-opening polymerization of lactones. can give. Examples of the polyol include low-molecular polyols such as ethylene glycol, propylene glycol, 1.6 hexanediol, diethylene glycol, and neopentyl glycol.

Diols such as bis(hydroxymethyl)cyclohexane and bis(hydroxyethyl)benzene; trimethylolpropane, glycerin, etc.; polyether polyols (polyalkylene glycols, etc.), and mixtures thereof. In addition, dicarboxylic acids include succinic acid,
Glutamic acid.

Adipic acid, sebacic acid, maleic acid, fumaric acid.

Mention may be made of phthalic acid, terephthalic acid, dimer acid, and mixtures thereof. Furthermore, polyether ester polyols obtained by reacting polyoxyalkylene polyols with polycarboxylic acid anhydrides and cyclic ether compounds as described in Japanese Patent Publication No. 48-10078 can also be used. A combination of polyether polyol and polyester polyol or polyether ester polyol (for example, in a weight ratio of 5=95 to 95:5) can also be used.

At least a portion of the polymeric polyol (e.g. 5% by weight)
In addition to the above, a polymer polyol selected from polyether polyols and polyether ester polyols (preferably 10 to 90% by weight), and if necessary,
Other polymeric polyols (e.g. polymer polyols, OH
Contains vinyl polymer.

OH-terminated urethane prepolymers and polyether polyols) can be used in combination. Examples of polymer polyols include those made from polyether polyols and/or polyester polyols and/or polyether ester polyols and optionally crosslinking agents and ethylenically unsaturated monomers. Polyether polyols used in the production of polymer polyols include polyhydric alcohols, polyhydric phenols, and ammonia.

Examples include compounds having a structure in which alkylene oxide is added to a polyfunctional compound containing an active hydrogen atom, such as amines and polycarboxylic acids. The polyhydric alcohols include dihydric alcohols such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol, and neopentyl glycol, and glycerin, trimethylolpropane, pentaerythritol, sorbitol, Trivalent or higher polyhydric alcohols such as sucrose; polyhydric phenols include polyhydric phenols such as pyrogallol and hydroquinone, and bisphenols such as bisphenol A; amines include monoamines such as butylamine, ethylenediamine, etc. Aliphatic polyamines such as cyclohexylene diamine, alicyclic polyamines such as isophorone diamine, aromatic amines such as phenylene diamine, heterocyclic polyamines such as piperazine, and alkanolamines such as monoethanolamine; Examples include aliphatic polycarboxylic acids such as succinic acid and aromatic polycarboxylic acids such as phthalic acid. Two or more kinds of the above-mentioned active hydrogen atom-containing compounds can also be used.

Examples of the alkylene oxide to be added to the active hydrogen atom-containing compound include ethylene oxide (hereinafter abbreviated as EO), propylene oxide (hereinafter abbreviated as PO), butylene oxide, tetrahydrofuran, styrene oxide, epichlorohydrin, and the like.

The alkylene oxides may be used alone or in combination of two or more, and in the latter case, block addition, random addition, or a mixture of both may be used. Among these alkylene oxides, I) 0 and/or EO are preferred, but it is also preferable to use these together with a small amount of other alkylene oxides (butylene oxide, tetrahydrofuran, styrene oxide, etc.) depending on the required function.

Examples of the polymerizable unsaturated monomer used in the polymer polyol include the following.

(a) Acrylic acid, methacrylic acid, and their derivatives: acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid and their salts, methyl acrylate, methyl methacrylate, dimethylaminoethyl methacrylate, acrylamide, methacrylamide, etc.

(b) Aromatic vinyl monomer: styrene, α-methylstyrene, etc.

(C) Olefinic hydrocarbon monomer: ethylene.

propylene, butadiene, isobutylene, isoprene,
l,4-hentagenato.

(d) Hinyl ester monomer: vinyl acetate, etc.

(e) Vinyl halide monomer: vinyl chloride, vinylidene chloride, etc.

(f) Vinyl ether alone, such as vinyl methyl ether.

Among these, preferred is acrylonitrile.

These are methyl methacrylate, styrene, and butadiene. Particularly preferred is acrylonitrile.

Combination of acrylonitrile and styrene (weight ratio of styrene:acrylonitrile 0:100 to 60:40)
).

The proportions of polyol and monomer used in the polymer polyol can vary over a wide range.

Usually, the amount of the ethylenically unsaturated monomer (or polymer) is 2 to 70 parts by weight, preferably 5 to 40 parts by weight, per 100 parts by weight of the polyol.

As a polymer polyol, Japanese Patent Publication No. 41-8478.

Examples include those described in Japanese Patent Publication No. 89-24787, Japanese Patent Publication No. 54-19488, and Japanese Patent Publication No. 56-26924.

Examples of the OH-containing vinyl polymer include acrylic polyols, such as JP-A No. 49-98499 and Japanese Patent Application No. 55-657.
Examples include the hydroxyalkyl (meth)acrylate (co)polymer described in No. 67, vinyl acetate, and (co)polymer hydrolyzate.

The OH-terminated urethane prepolymer is obtained by reacting a polymeric polyol with an insufficient amount of an organic polyisocyanate.

As the polymer polyol used in the production of the above prepolymer, polyether polyol, polyester polyol, polyether ester polyol, and/or OH-containing vinyl polymer can be used. and those described in the section of OH-containing vinyl polymers). Examples of organic polyisocyanates include those described in ■) below (
In particular, MDI, TDI, etc.) can be used.

As the polyether polyol, those mentioned above (described in the section of polymer polyol production) can be used. The weight ratio of the polymer polyol to the other polymer polyol can be varied, for example, from 100:0 to 5:95, preferably from 95:5 to 10:90.

By using these together.

It may be possible to lower the viscosity and improve workability and permeability. The molecular weight of the high molecular weight polyol is 2,000 to 10,000. If it is less than 2,000, the reaction with isocyanate will be high and it will be disadvantageous to extend the pot life.

Furthermore, polymer polyols having a molecular weight of more than 10,000 tend to produce by-products during production, are not common, and have high viscosity, which is disadvantageous in terms of work. The OH value is usually 400-15, preferably 200-20, more preferably 130-25. The number of functional groups is usually 2 to 4, preferably 2 to 3.

As the crosslinking agent used as necessary in the present invention,
Low-molecular polyfunctional compounds containing active hydrogen and having a molecular weight of 30 to 300, such as polyols and polyamines.

and amino alcohols. Examples of low-molecular polyols include poly(+
lIi alcohol; and polyhydric alcohol.

Examples include low molar alkylene oxide adducts of polyfunctional compounds containing active hydrogen atoms, such as polyhydric phenols, ammonia, amines, and polycarboxylic acids.

Preferred low molecular weight polyols are ethylene glycol, 114-butanediol, trimethylolpropane, glycerin and alkylene oxide (PO and/or EO) adducts thereof. Examples of polyamines include aliphatic, alicyclic,
Examples include aromatic and heterocyclic polyamines. Examples of amino alcohols include ethanolamine and pronolamine. The amount of crosslinking agent used can be adjusted depending on the purpose and required performance, but it is generally less than 80% (highly concentrated polyols and crosslinking agents). (Based on the total weight of the agent, the same applies hereinafter.) Particularly preferably 10% or less. When the crosslinking agent force 5 exceeds the above range, the adhesive 11 becomes brittle.

The average molecular weight of the active hydrogen-containing polyfunctional compound (A) (polymer polyol and optionally a crosslinking agent) [1000 to 1
It is 0000. The average number of functional groups is usually 2 to 4, preferably 2 to 3.

The overall viscosity of (4) is usually 100 cps (25°C) ~ 1
000,000 cps (25°C) preferably 500 cps (25°C)
°C) to 100,000 cps (25 °C).

As the polyisocyanate compound (B) having no isocyanurate-1 ring used in the present invention (Y).

Those conventionally used in the production of polyurethane can be used, including urethane polymers obtained by reacting organic polyisocyanates and polyols with an excess of organic polyisocyanate. Examples of organic polyisocyanates include aliphatic polyisocyanates (hexamethylene diisocyanate, lysine diisocyanate, etc.)
, alicyclic polyisocyanates (hydrogenated diphenylmethane diisocyanate, isophorone diisocyanate, hydrogenated tolylene diisocyanate, etc.), aromatic polyisocyanates [tolylene diisocyanate (TDI).

diphenylmethane diisocyanate (MDI), naphthylene diisocyanate, xylylene diisocyanate, etc.] and mixtures thereof.

Among these, aromatic diisocyanates are preferred, and TDI and MDI are particularly preferred. These polyisocyanates are crude polyisocyanates, such as crude 1'DI, crude MDI [crude diaminodiphenylmethane (condensation product of formaldehyde and aromatic amines or mixtures thereof; Phosgenated products of (mixtures with the above polyamines) polyallylpolyisocyanates, such as liquefied MDI (carposiimide modified, trihydrocarbyl phosphate modified, etc.).

Examples of polyols used in the production of the urethane prepolymer include glycols such as ethylene glycol, propylene glycol, diethylene glycol, and dipropylene glycol; l-liols such as trimethylolpropane and glycerin; high-functional polyols such as pentaerythritol and sorbitol; These alkylene oxide (EO and/or ]'0) adducts are mentioned. The above polyol has an equivalent weight of 30 to 5
00, particularly preferably 30 to 200. Among these, those having 2 to 8 functional groups are preferred. The free isocyanate group content of the modified polyisocyanate and prepolymer is usually 10 to 85%, preferably 15 to 85%.
32%, particularly preferably 20 to 30%. The polyisocyanate compound (B) is a combination of two or more types (for example, a modified polyisocyanate and a prepolymer are used in combination)
can do. The viscosity (25°C) of (B) is usually 50
-20,000 cps, preferably 800-1000 cps.

In the present invention, it is necessary that the polyisocyanate compound does not have an isocyanurate ring.

Those having an isocyanurate ring have a high viscosity, are unstable, and tend to change in viscosity due to deterioration, making it impossible to obtain uniform adhesion.

In producing the thermosetting adhesive in the present invention, the isocyanate index is usually 200 to 5000, preferably 50.
It is 0-3000, more preferably 1100-2000. If the isocyanate index is lower than the above range, the heat resistance will be insufficient, and if it is higher, it will become brittle. The weight ratio of (5) to (5) is usually 100/20 to 1001500, preferably 100150 to 100/800.

As the a-quantification catalyst used in the present invention, known catalysts can be used. Examples of such catalysts include, for example, J.R. Salanders, "Polyurethanes, Chemistry and Technology", pp. 94-97 (Interscience Publishers 1964) or [Journal of Cellular Plastics]. J Vol, 1,
Catalysts such as, but not limited to, those described in 41.85-905r can be used. Some specific examples of octamerization catalysts include (a) alkali metal salts of various carboxylic acids (alkali metal salts of aliphatic monocarboxylic acids such as acetic acid, propionic acid, butyric acid, octanoic acid, oleic acid, and stearic acid; Alkali metal salts of aliphatic dicarboxylic acids such as malonic acid, maleic acid, fumaric acid,
Benzoic acid, cinnamic acid. , aromatic carboxylic acids such as phthalic acid, β-naphthalene carboxylic acid, etc., or alkali metal salts of cyclic carboxylic acids such as furanic acid). (b) Strong organic base [e.g. 2.4.6-tris(dimethylaminomethyl)phenol, N, N', N
'-tris(dimethylaminopropyl)sym-hexahydrotriazine, benzylmethylammonium hydroxide, benzyltrimethylammonium methoxide, sodium methoxide, 1,8-diazabicyclo(5,4,0)undecene-7, etc.), ( c) Metal salts of naphthenic acids, (ω merging catalysts (triethylenediamine and aldehydes, triethylenediamine and epoxy compounds, etc.). (e) Alkali metal salts of weakly acidic substances other than carboxylic acids (e.g. benzenesulfinic acid, P-diamine, etc.). potassium salt of trophenol).

(f) Organic acid salts of strong organic bases (eg fatty acid salts of 1,8-diazabicyclo(5,4,0)undecene-7, etc.). Moreover, two or more types may be used in combination, and other co-catalysts may be used as necessary. Preferred among these trimerization catalysts are 8th-class amines or salts thereof with a strong base, and catalysts having temperature-sensitive catalytic activity are particularly preferred. Specifically, organic acid salts (phenol salts, fatty acid salts, etc.) of 1,8-diazabicyclo(5,4,0 undecene-7), which are neutralized salts of strong bases, are preferable.

The amount of catalyst used is usually 0.1 to 1 per 100 parts of β).
0 parts, preferably 1 to 5 parts.

The organic solvent used as necessary in the present invention can be arbitrarily selected depending on the purpose. Solvents include ketones such as acetone and methyl ethyl ketone; hydrocarbons such as toluene; ethers such as tetrahydrofuran and dioxane; amides such as N,N-dimethylpormaldehyde; nitriles such as acetonitrile; esters such as ethyl acetate; Halogenated hydrocarbons such as methylene chloride;
and mixed solvents thereof can be used.

In terms of volatilization rate, compounds with relatively low boiling points, such as acetone and toluene, are preferred.

The amount of solvent used can be varied, but generally the solid content of the adhesive is usually 15 to 100%, preferably 30 to 70%.
The amount used is If the solid content is high, it is good for printing, but it is disadvantageous for spraying; if the solid content is low, it takes time for the solvent to evaporate, which is not desirable.

Various additives can be used in the adhesives of the present invention. For example, antioxidants (e.g. 2,6-di-ter-butyl-4) may be added to increase product stability if desired.
- methylphenol, 2,5-di-ter-butylhydroquinone, etc.), ultraviolet absorbers (such as Tinuvin P
, Irganox 1010, etc.), hydrolysis inhibitors (e.g., 8,8'-diisopropyl-4,4'-biphenylenecarboxylic acid), anti-shallowing agents (e.g., 8-hydroxyquinoline, etc.), etc. It may be added in an amount up to about 5% per minute. In addition, the special public interest public
171, tack imparting resins, softeners, flame retardants, natural resins or synthetic resins described in Japanese Patent Publication No. 84-2145,
It is also possible to mix in organic pigments, inorganic pigments, or fibrous materials. In addition, in order to improve adhesive strength to metals, for example, Japanese Patent Publication No. 51-18979, U.S. Pat.
Epoxy resins described in Nos. 809261 and 8891054.

A silane coupling agent can also be blended.

The adhesive according to the present invention is preferably applied in a one-component form by pre-mixing (5), (2), (C) and, if necessary, an organic solvent and other additives. B
).

(It is also possible to form an adhesive consisting of Q and, if necessary, an organic solvent and other additives.For example, a two-component type [(2)
and (Q component, or (2) and (Q
and (B) component) on the surface to be adhered at the same time, and applying one component of the two liquids to one side of the adhered TH and the other component to the other side, and applying pressure bonding. Here are some ways to do it.

When bonding using the adhesive of the present invention, all commonly known methods can be employed. For example, applying the adhesive to the adhesive surface by spraying or brushing; adhering the adhesive by gluing, dipping, etc., volatilizing the solvent if necessary, then aligning the adhesive surfaces and heating under pressure if necessary; A common method is to cure the adhesive. The curing temperature is usually 50°C or higher, preferably 60-looo°C. The curing time is usually 1 minute to 1 hour, preferably 5 minutes or less.

The adhesive of the present invention exhibits extremely excellent adhesion, especially when adhering plastics and sheets, whereas conventional isocyanate adhesives exhibit poor adhesion at high temperatures. It exhibits great adhesive strength not found in known adhesives. Moreover, as conventional isocyanate-based adhesives have a longer pot life, the curing time also increases, whereas polyisocyanurate cures rapidly, so the pot life can be extended at room temperature, and the heat curing time may be longer. It has the excellent feature of being able to shorten the length. In addition, isocyanate-based adhesives that already have isocyanurate rings are used in the handling of isocyanate compared to adhesives that form isocyanurate rings on the substrate according to the present invention.

Moreover, it is disadvantageous in terms of high cost and heat resistance.

The adhesive according to the present invention can be used in a wide variety of applications for bonding various materials. For example, coating, spraying.

By methods such as impregnation, wood9 paper, fiber materials, natural and synthetic rubber, hard or soft plastics (e.g. AS resin, BSI resin, modified polyphenylene oxide resin, nylon, vinyl chloride, polyester, etc.), urethane foam, plastic foam Sheets (e.g. foamed PVC sheets, etc.) leather.

Various laminated materials and composite materials can be provided by bonding metals, glass, ceramics, concrete, etc. of the same type or different types.

The adhesive according to the invention can optionally also be used as a primer. For example, after coating the iron plate with adhesive, use foamed urethane or sealant.

Paint etc. can be applied.

Applications of the adhesive according to the present invention include adhesion of plastics and sheets, glass laminates, pretreatment of reinforced plastic substrates, lamination of reinforced plastics, production of heat-resistant cloth, binder for abrasive materials, adhesion of plywood, and heat-resistant sealed packaging. etc.

The present invention will be explained below with reference to examples, but the present invention is not limited thereto (parts shown in the examples are parts by weight).

Example 1 100% of polyester polyol 1 in 160g of acetone
g and Catalyst I are combined in an amount of 8 gM. Then isocyanate l
An adhesive composition was prepared by mixing 150 g. The isocyanate index at this time was 1,300.

Apply 30I adhesive to the adherend AS resin by spraying it.
Spray to a thickness of . After drying the solvent and pressurizing the foamed PVC sheet, it was heated at an ambient temperature of 90°C and a pressure of 1 kg/cm'.
As a comparative example, the urethane adhesive Bostic 7662 (manufactured by Bostic Japan Co., Ltd.) was used in the same manner.

Polyester polyol l is ethylene glycol, 1,
It is a polyester made from 4-butanediol and adipic acid and has an OH value of 45. Catalyst I is U-CA'l"
SA/161 (manufactured by Sun Abbott) Isocyanate 1 is a carposiimide-modified diphenylmethane isocyanate with an NC0% of 29%.

The results of measuring the adhesive strength (peeling at 180°C) at 20°C, 80°C, and 120°C are shown in Table 1.

Example 2 In the same manner as in Example 1, using polyester polyol 1, polyether ester polyol I, catalyst (1), and isocyanate (2), the adhesive peel strength of foamed PVC sheet and AS resin and the adhesive shear strength of aluminum plate were measured. Polyether ester polyol I is a glycerin PO adduct (
444 phthalic anhydride to 8000 g of OH-value = 56'
After reacting 264 g of EO, the OH value is 46.

1) Pot life is at 25℃ and 50% relative humidity.

2) Cohesive failure of foamed PVC layer.

Example 8 Using the same method as in Example 1, using polyester polyol I, polyether ester polyol 1, or a combination system of these and polyether polyol 2 or crosslinking agent 1, various isocyanate indexes, polymer polyols, and isocyanates were changed to prepare ASm resins. The adhesive peel strength of the foamed vinyl chloride sheet was measured.

Polyether polyol 2 is a polyether with a molecular weight of 3,000, which is obtained by adding PO to glycerin, and has an OH value of 56.

Crosslinker 1 is 1,4-butanediol.

Isocyanate 2 is a prepolymer of diphenylmethane diisocyanate and triethylene glycol with 0% NC.
is 24%.

Claims (1)

[Claims] 1. An active hydrogen-containing polyfunctional compound (with an average molecular weight of 1000 to 1oooo) consisting of a polymer polyol selected from polyester polyols and polyether ester polyols, and optionally other polymer polyols and/or crosslinking agents. 4), a polyisocyanate compound which does not have a large excess of isocyanurate rings with respect to the active hydrogen-containing group of (A), octamerization catalyst (Q, and, if necessary, an isocyanurate ring-forming thermosetting adhesive consisting of an organic solvent) 2. The adhesive according to claim 1, wherein a3) is used in an amount such that the isocyanate index is from 200 to 5,000. 8. Claim 1, wherein the weight ratio of the polymer polyol to another polymer polyol is 100:0 to 5:96.
The adhesive according to any one of Items 1 to 2. 4. Other polymeric polyols have a molecular weight of 800 to 10,000
Claims 1 to 8 are polyether polyols of
Adhesives described in any of the paragraphs. 5. The adhesive according to any one of claims 1 to 4, wherein the crosslinking agent is a polyol, polyamine, and/or amino alcohol having a molecular weight of 80 to 800. Claims 1 to 5, in which 6, (8) is an organic polyisocyanate or a urethane prepolymer obtained by reacting a polyol with an excess of an organic polyisocyanate.
Adhesives described in any of the paragraphs. 7. (The adhesive according to any one of claims 1 to 6, in which Q is an octamerization catalyst having temperature-sensitive catalytic activity. 8. Between the substrates, an adhesive selected from polyester polyol and polyether ester polyol An active hydrogen-containing polyfunctional compound (2) with an average molecular weight of 1,000 to 10,000, which is composed of a polymer polyol and, if necessary, another polymer polyol and/or a crosslinking agent; An adhesion method characterized by heating and curing in the presence of an isocyanurate ring-forming thermosetting adhesive consisting of a polyisocyanate compound (2) having no nurate ring, an octamerized M medium (Q, and, if necessary, an organic solvent). 9. The adhesive according to claim 8, wherein the thermosetting adhesive is cured by heating at a temperature of 50° C. or higher. 10. The thermosetting adhesive is applied between a hard plastic and a soft plastic sheet. The adhesion method according to claim 8 or 9, wherein the adhesive is cured by heating.