Two-Part Adhesive

1. Field of the Invention

The present invention relates to an adhesive. More particularly, it relates to a two-part adhesive having water incorporated.

2. Discussion of Background

There is an increasing demand for adhesives which can be cured in a short period of time at room temperature, from the viewpoint of energy saving, labor saving, saving of resources, etc.

Heretofore, as room temperature quick-curing adhesives, a two-part type quick-curing epoxy adhesive, an anaerobic adhesive, an .alpha.-cyanoacrylate adhesive and a second generation acrylic adhesive (SGA) have been known.

The two-part type quick-curing epoxy adhesive is designed to be used in such a manner that a main agent and a curing agent are measured, mixed and coated on an object to be bonded, whereupon the main agent and the main components of the curing agent will react for curing. If the measurement and mixing of the main agent and the curing agent are inadequate, the two-part type quick-curing epoxy adhesive tends to undergo a substantial deterioration in the strength. Further, even if the measurement and mixing are conducted adequately, it still has a drawback that it is poor in the peel strength and impact strength.

The above-mentioned anaerobic adhesive is cured by shutting off air by pressing the adhesive composition between the objects to be bonded. Therefore, at the time of bonding, the adhesive is likely to be pressed out from the objects, and the adhesive at the portion which is in contact with air, will not be cured. Further, it has a drawback that it will not be cured also in a case where the clearance between the objects to be bonded, is large.

The above-mentioned .alpha.-cyanoacrylate adhesive provides excellent operation efficiency, but it has low peel strength and impact strength and is poor in the moisture resistance and water resistance. Thus, it has a disadvantage that its useful range is very much limited.

The above-mentioned second generation acrylic adhesive (SGA) is a two-part type. However, it has excellent operation efficiency in that it is unnecessary to accurately measure the two liquids, and it is capable of being cured quickly in from a few minutes to a few tens minutes by extremely rough measurement and mixing (in some cases, merely by contacting the two liquids). Yet, it provides high peel strength and impact strength, and curing of the portion pressed out of the objects to be bonded, is excellent. Therefore, an adhesive of this type has been widely used.

Such SGA is disclosed in e.g. U.S. Pat. Nos. 3,890,407 and 4,096,201. However, it has a drawback that it does not provide adequate adhesiveness to a metal object.

With respect to improvement of the adhesiveness of SGA to a metal, U.S. Pat. Nos. 4,233,115 and 4,452,944 and Japanese Unexamined Patent Publication No. 84964/1984 disclose that the adhesiveness to a metal object can be improved by an addition of a monomer of a phosphoric acid ester type. However, even such improvement is not yet adequate with respect to the improvement of the adhesiveness to iron or stainless steel, and it has been found that the adhesiveness to such a metal object tends to be poor, and the bond strength tends to deteriorate.

The present invention has been made in view of such prior art, and it is an object of the present invention to provide an adhesive having particularly excellent adhesiveness to metals.

The present inventors have conducted extensive studies, and as a result, surprisingly have found that an adhesive having water incorporated among conventional non-aqueous adhesives where the presence of water has been avoided as far as possible, is capable of achieving the object. The present invention has been accomplished on the basis of this discovery.

The present invention provides a two-part adhesive consisting of a first liquid comprising 100 parts by weight of a polymerizable vinyl monomer and from 0.1 to 20 parts by weight of an organic peroxide and a second liquid comprising 100 parts by weight of a polymerizable vinyl monomer and from 0.05 to 15 parts by weight of a reducing agent, wherein either one or both of the first and second liquids contain from 0.4 to 5% by weight of water, and the content of water is more than 0.2% by weight based on the total amount of the first and second liquids.

Now, the present invention will be described in further detail.

The polymerizable vinyl monomer useful in the present invention may be any radically polymerizable vinyl monomer or a combination of two or more radically polymerizable vinyl monomers. Among them, a liquid or solid (meth)acrylate or (meth)acrylic acid is preferred. Specific examples are as follows:

1 A monomer of the formula Z--O--R.sub.1 wherein Z is a (meth)acryloyl group, CH.sub.2 .dbd.CHCOOCH.sub.2 --CH(OH)CH.sub.2 --or CH.sub.2 .dbd.C(CH.sub.3)COOCH.sub.2 --CH(OH)CH.sub.2 --, and R.sub.1 is H, a C.sub.1-20 alkyl group, a cycloalkyl group, a benzyl group, a phenyl group, a tetrahydrofurfuryl group, a glycidyl group, a dicyclopentyl group, a dicyclopentenyl group or a (meth)acryloyl group.

Such a monomer may, for example, be (meth)acrylic acid, methyl (meth)acrylate, cyclohexyl (meth)acrylate, benzyl (meth)acrylate, tetrahydrofuryl (meth)acrylate, dicyclopentyl (meth)acrylate, dicyclopentenyl (meth)acrylate, glycerol (meth)acrylate or glycerol di(meth)acrylate.

2 A monomer of the formula Z--O--(R.sub.2 O).sub.p --R.sub.1, wherein Z and R.sub.1 are as defined above, R.sub.2 is --C.sub.2 H.sub.4 --, --C.sub.3 H.sub.6 --, ##STR1## --C.sub.4 H.sub.8 --or --C.sub.6 H.sub.12 --, and p is an integer of from 1 to 25.

Such a monomer may, for example, be 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, ethoxyethyl (meth)acrylate, polyethylene glycol (meth)acrylate, phenoxyethyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, phenoxydiethylene glycol (meth)acrylate, tripropylene glycol di(meth)acrylate or 1,6-hexanediol di(meth)acrylate.

3 A monomer of the formula: ##STR2## wherein Z and R.sub.2 are as defined above, R.sub.3 is H or a C.sub.1-4 alkyl group, and q is an integer of from 0 to 8.

Such a monomer may, for example, be 2,2-bis(4-meth)acryloxyphenyl)propane, 2,2-bis(4-(meth)acryloxyethoxyphenyl)propane, 2,2-bis(4-(meth)acryloxydiethoxyphenyl)propane, 2,2-bis(4-(meth)acryloxypropoxyphenyl)propane and 2,2-bis(4-(meth)acryloxytetraethoxyphenyl)propane.

4 A (meth)acrylate of a polyhydric alcohol not included in the above monomers 1, 2 and 3.

Such a monomer may, for example, be trimethylolpropane tri(meth)acrylate, neopentyl glycol di(meth)acrylate, pentaerythritol tetra(meth)acrylate and dipentaerythritol hexa(meth)acrylate.

5 A urethane prepolymer having (meth)acryloyloxy groups.

Such a prepolymer is obtainable, for example, by reacting a (meth)acrylate having a hydroxyl group, an organic polyisocyanate and a polyol.

Here, the (meth)acrylate having a hydroxyl group may, for example, be hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate or hydroxybutyl (meth)acrylate.

The organic polyisocyanate may, for example, be toluene diisocyanate, 4,4-diphenylmethane diisocyanate, hexamethylene diisocyanate or isophorone diisocyanate.

The polyol may, for example, be polyethylene glycol, polypropylene glycol, polytetramethylene glycol or polyester polyol.

6 An acidic phosphoric acid compound of the following formula (I) or its amine salt: ##STR3## wherein R is CH.sub.2 .dbd.CR.sub.4 CO(OR.sub.5).sub.m -- (wherein R.sub.4 is H or CH.sub.3, R.sub.5 is --C.sub.2 H.sub.4 --, --C.sub.3 H.sub.6 --, ##STR4## --C.sub.4 H.sub.8 --, --C.sub.6 H.sub.12 -- or ##STR5## and m is an integer of from 1 to 10) and n is 1 or 2.

Specific examples of the acidic phosphoric acid of the formula (I) or its amine salt include acid phosphooxyethyl (meth)acrylate, acid phosphooxypropyl (meth)acrylate and bis(2(meth)acryloyloxyethyl)phosphate, and amine salts thereof.

The above monomers 1, 2, 3, 4, 5 and 6 may be used alone or in combination of two or more of them.

The amount of the (meth)acrylate to be used, is preferably at least 70% by weight based on the total weight of the polymerizable vinyl monomers to be used, in view of the curing speed, etc.

As polymerizable vinyl monomers other than (meth)acrylate, styrene, .alpha.-alkylstyrene, divinylbenzene, vinyl ether, divinyl ether, N-vinylpyrrolidone, 2-vinylpyridine and a vinyl ester (such as vinyl acetate or vinyl propionate) may, for example, be mentioned.

Further, to the adhesive of the present invention, polymerizable substances other than polymerizable vinyl monomers may be incorporated. As such substances, polycarboxylic acids having polymerizable unsaturated bonds (such as maleic acid, maleic anhydride, fumaric acid, itaconic acid, itaconic anhydride and citraconic acid) and polymerizable olefinic hydrocarbons having at least 6 carbon atoms, may, for example, be mentioned.

When such polymerizable substances other than the polymerizable vinyl monomers, are to be used, their proportion is preferably less than 30% by weight based on the total amount of the polymerizable vinyl monomers. Such substances may be used alone or in combination as a mixture of two or more of them.

The Organic peroxide to be used for the first liquid of the present invention, may, for example, be cumene hydroperoxide, p-menthane hydroperoxide, tert-butyl hydroperoxide, diisopropylbenzene dihydroperoxide, methyl ethyl ketone peroxide, benzoyl peroxide or tert-butyl peroxybenzoate. However, the organic peroxide is not limited to such specific examples.

The amount of the organic peroxide is usually from 0.1 to 20 parts by weight per 100 parts by weight of the polymerizable vinyl monomer. If the amount is less than 0.1 part by weight, the curing speed tends to be slow, and if it exceeds 20 parts by weight, the storage stability tends to be poor.

The reducing agent to be used for the second liquid of the present invention may be any conventional reducing agent so long as it is capable of reacting with the above organic peroxide to generate radicals. As typical reducing agents, a tertiary amine, a thiourea derivative and a metal salt may, for example, be mentioned. The tertiary amine may, for example, be triethylamine, tripropylamine, tributylamine or N,N-dimethylparatoluidine. The thiourea derivative may, for example, be 2-mercaptobenzimidazole, methylthiourea, dibutylthiourea, tetramethylthiourea or ethylenethiourea. The metal salt may, for example, be cobalt naphthenate, copper naphthenate or vanadyl acetyl acetone.

The amount of the reducing agent is usually from 0.05 to 15 parts by weight per 100 parts by weight of the polymerizable vinyl monomer.

The adhesive of the present invention is characterized in that either one or both of the first and second liquids contain from 0.4 to 5% by weight of water, and the content of water is more than 0.2% by weight, preferably from 0.4 to 5% by weight, based on the total amount of the first and second liquids. If the water content is not more than 0.2% by weight, no adequate effect for improvement of the adhesiveness will be obtained. On the other hand, if the water content exceeds 5% by weight, water may sometimes be separated.

Substances such as polymerizable vinyl monomers used in the present invention usually contain water in an amount of 0.2% by weight or less as an impurity. In the present invention, water is added so that either one or both of the first and second liquids have a water content of from 0.4 to 5% by weight, and the water content in the liquid obtained by mixing the two liquids is more than 0.2% by weight, preferably from 0.4 to 5% by weight, to adjust the content of water.

There is no particular restriction as to the water to be used in the present invention. For example, the water may be commonly available water or purified water (such as distilled water, deionized water, deaerated water or water obtained by precise filtration). Further, it may be water containing a polymer substance (such as polyvinyl pyrrolidone, polyethylene oxide, an alkyl cellulose or polyvinyl alcohol), or water containing a surfactant. Furthermore, it may be water in the form of an aqueous synthetic resin emulsion or rubber latex.

Water may be incorporated by an optional method. It is common to employ a method of adding water during the preparation of the first liquid, the second liquid or a mixture thereof. However, water may be preliminarily incorporated to substances to be used, such as the polymerizable vinyl monomers, the organic peroxide and the reducing agent. To incorporate water uniformly, a conventional stirrer, line mixer or mixing-type coating machine may, for example, be employed.

In the present invention, an elastomer component to improve the peel strength and the impact strength may be added to the adhesive. As such an elastomer component, various synthetic rubbers such as nitrile-butadiene rubber and chlorosulfonated polyethylene may be mentioned.

Further, for the purpose of adjusting the viscosity and fluidity, a thermoplastic polymer such as a methyl methacrylate-butadiene-styrene copolymer (MBS) or linear polyurethane, or fine silica powder, may also be employed.

In order to facilitate the curing of the portion which is in contact with air, various waxes may be added to the adhesive of the present invention.

Further, for the purpose of improving the storage stability, known substances such as various polymerization inhibitors or antioxidants may be incorporated. Further, a plasticizer, a filler, a coloring agent, a rust preventive agent or the like may also be added, as the case requires.

There is no particular restriction as to the type of the material of objects to be bonded by means of the adhesive of the present invention, and the adhesive of the present invention exhibits particularly excellent adhesive effects when used for bonding objects having a metal surface.

Now, the present invention will be described in further detail with reference to Examples and Comparative Examples. In the following Examples and Comparative Examples, the proportions of the respective materials are shown by parts by weight.

Various materials and water (deionized water) were mixed in the proportions as identified in Table 1 to obtain an adhesive consisting of the first and second liquids having the water contents as identified in Table 1. On one side of a test specimen (200.times.25.times.1.2 mm, non-treated SPCC-D), the first liquid was coated, and on one side of another test specimen (200.times.25.times.0.4 mm, non-treated SPCC-D), the second liquid was coated. Immediately, the coated surfaces were put together and bonded, followed by aging at room temperature for 24 hours to obtain a test sample for measuring the peel strength. The peel strength (unit: kg/25 mm) of the test sample was measured in accordance with ISO 4578-1979 (crosshead speed: 100 mm/min) at 23.degree. C. under a relative humidity of 50%. The results are shown also in Table 1.

The water contents of the first and second liquids were measured by means of a moisture measuring apparatus CA-03 Model (manufactured by Mitsubishi Kasei Corporation). To make the adhesive layer thickness uniform, glass beads having a particle size of 100 .mu.m were added in a very small amount to the first liquid.

Various materials and water (deionized water) were mixed in the proportions as identified in Table 2 to obtain an adhesive consisting of the first and second liquids having the water contents as identified in Table 2. As test specimens, a stainless steel sheet (non-treated SUS304) of 200.times.25.times.1.5 mm and a stainless steel sheet (non-treated SUS304) of 200.times.25.times.0.5 mm were used. The first and second liquids were mixed in equal amounts by weight, and the mixture was immediately coated on one side of each test specimen, and the coated surfaces were bonded to each other, followed by aging at room temperature for 24 hours to obtain a test sample for measuring the peel strength. The peel strength (unit: kg/25 mm) of the test sample was measured in the same manner as in Example 1. The results are shown also in Table 2.

Further, to make the adhesive layer thickness uniform, glass beads having a particle size of 100 .mu.m were added in a very small amount to each of the first and second liquids before mixing.

Various materials and water (deionized water) were mixed in the proportions as identified in Table 3 to obtain an adhesive consisting of the first and second liquids having the water contents as identified in Table 3. Test samples were prepared in the same manner as in Example 2, and the peel strength (unit: kg/25 mm) of a test sample was measured in the same manner as in Example 2 except that the obtained first and second liquids were used, and the following combinations (1) and (2) of test specimens were used. The results are also shown in Table 3.

Combination (1) of test specimens: Cold rolled steel sheets (non-treated SPCC-D) of 200.times.25.times.1.2 mm and 200 .times.25.times.0.4 mm.

Combination (2) of test specimens: Stainless steel sheets (non-treated SUS304) of 200.times.25.times.1.5 mm and 200 .times.25.times.0.5 mm.

The two-part adhesive of the present invention is excellent in the adhesiveness particularly to a metal material, and its industrial usefulness is significant.