This invention relates to a novel acrylic two-part type adhesive suitable for structural uses having excellent storage stabilities and adhesion performances.
Hitherto as acrylic adhesives have been known anaerobic adhesives, cyanoacrylate adhesives and the like. They are characterized by exhibiting adhesion in a short time and exhibiting a strong tensile strength although they are of one-pack type, while they are very weak against some strengths applied such as tearing and impact. Thus, their uses are restricted to the fixing of fitted portions or the like in the case of the anaerobic adhesives and to temporary fixing or the like in the case of the cyanoacrylate adhesives.
On the other hand, acrylic two-part adhesives wherein an elastomer such as acrylic rubber or epichlorohydrin rubber was dissolved in an acrylic monomer such as methyl methacrylate have been known (sometimes referred to as the first-generation acrylic adhesives). The adhesives of this type, however, do not undergo in a curing step a chemical reaction between the monomer and the elastomer, wherein the fragility caused by the acrylic monomer alone has been simply improved by the presence of the elastomer. Moreover, they exhibit good working performance but are considerably inferrior in adhesion performances, in comparison with the epoxy adhesives which are now used for structural uses in the greatest quantities.
Recently have been developed acrylic two-part adhesives called "the second-generation acrylic adhesives". In these adhesives, a chlorosulfonated polyethylene is often used as the elastomer component. The adhesives are characterized by such a mechanism that radicals are produced on the side chains of the elastomer in the course of curing and polymerizable monomers are graft-polymerized thereto. Thus, the adhesives have excellent adhesion properties in comprison with the first-generation adhesives which do not involve such a graft polymerization. More specifically, the adhesives have the tensile shear strength and impact strength equivalent or even superior to those of the epoxy adhesives, and also exhibit far better values with respect to the tear strength, fatigue strength and oily surface adhesion properties than the epoxy adhesives.
As to the working performances, the second-generation adhesives (as well as the first-generation adhesives) are advantageous in that they do not require troublesome operations such as weighing and mixing which are required in the case of the epoxy adhesives. In this respect, the acrylic adhesives of this type may be said to be an excellent adhesive. However, when they are used to bond metal articles, the adhesive containing chlorosulfonated polyethylene as the elastomer is dechlorinated after long term storage or upon heating at a high temperature and the resulting chlorine often corrodes the bonded metal surfaces to lower the adhesion strength. Thus, a small amount of an absorbent for the decomposed chlorine such as epoxy resins has been incorporated to stabilize the adhesive, but a satisfactory effect can not be exhibited. Moreover, a second-generation adhesive wherein a diene-elastomer was used has been developed, but this adhesive is also not satisfactory with respect to its adhesion performances and storage stability.
The present inventors have been engaged in the researches on curable compounds having ionic bonds, and invented one-pack anaerobic adhesives having excellent properties. Such adhesives include, for example, a rapidly curing anaerobic adhesive comprising an anaerobic mixture base of a polyvalent metal salt of an acid ##STR1## and an acrylate or methacrylate ester and a small amount of an organic acid adduct of an amine and organic peroxide (cf. Japanese Patent Publication No. 47492/1977); and a one-pack anaerobic adhesive, which is rapidly curable and can be strongly bonded to a metal other than iron and copper without a primer, comprising the above-mention anaerobic mixture base and o-benzsulfimide, tetrahydroquinoline and an organic peroxide (cf. Japanese Patent Publication No. 477766/1977).
After intensive researches, the present inventors have accomplished a novel adhesive of the second-generation acrylic type which is suitable for structural uses and excellent in storage stability and adhesion performances.