Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-05-22
2002-05-14
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S849000, C524S853000, C156S327000, C156S329000
Reexamination Certificate
active
06388006
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a pressure sensitive adhesive composition. More particularly, it relates to a pressure sensitive adhesive composition which comprises an alkenyl group-containing vinyl polymer and a hydrosilyl group-containing compound and can be cured rapidly and can have a high solid content because the polymer has low viscosity.
BACKGROUND ART
Acrylic pressure sensitive adhesives show well-balanced adhesion characteristics without the addition of a tackifier resin and, therefore, are produced in large amounts side-by-side natural rubber-based pressure sensitive adhesives. Acrylic pressure sensitive adhesives have insufficient cohesive force because of their molecular weight and molecular weight distribution and, therefore, they are generally crosslinked for improving their cohesion. For the crosslinking, various techniques have already been developed. Thus, for example, techniques have been proposed which comprise adding a crosslinking agent, such as a polyisocyanate compound, epoxy compound, polycarboxylic acid, polyamine compound, phenolic resin or sulfur compound, or crosslinking an alkenyl-containing acrylic polymer in the presence of a condensation catalyst. In particular, pressure sensitive adhesives comprising an alkenyl-containing acrylic polymer as the main component, which are cured by crosslinking via siloxane bonding, has good weathering resistance.
For curing by siloxane crosslinking, however, moisture absorption is essential. In winter, in particular, when the humidity is low, undercure is a great problem. For solving such undercure problem without sacrificing the high weathering resistance resulting from siloxane crosslinking systems, an acrylic pressure sensitive adhesive utilizing the hydrosilylation reaction as the curing reaction has been proposed, as specifically disclosed, for example, in Japanese Kokai Publication Hei-03-95266, Japanese Kokai Publication Hei-04-93376and Japanese Kokai Publication Hei-04-145188. In curing systems in which the hydrosilylation reaction is used as the curing reaction, moisture absorption is not required but uniform cured products can be obtained by mere heating. In addition, the rate of curing is very rapid, so that such systems are suited for use in adhesive production lines.
In the pressure sensitive adhesive utilizing a hydrosilylation reaction, an alkenyl group-containing acrylic polymer is used as the main component and a hydrosilyl group-containing compound as the curing agent. The alkenyl-containing acrylic polymer can be prepared not only by copolymerization of an acrylic monomer(s) and an alkenyl-containing vinyl monomer but also by copolymerizing a vinyl monomer having a functional group such as a hydroxy or carboxyl group and then reacting the polymer with an alkenyl-containing compound having a further group capable of reacting with the functional group.
For a cured product to have elastic properties required of an pressure sensitive adhesive, it is necessary that the polymer have a high molecular weight and that the molecular weight of the portion between crosslinking sites of the polymer be increased by decreasing the amount of the comonomer having a an alkenyl group or a functional group derivable into an alkenyl group. However, when the molecular weight of the polymer is increased, the molecular weight distribution becomes broadened and the viscosity becomes high or a solid polymer is obtained, since the vinyl polymer is synthesized by free radical polymerization. For using such a polymer in a pressure sensitive adhesive, a problem is encountered, namely it is necessary that the viscosity be reduced by using a fairly large amount of a solvent. In addition, free radical polymerization results in almost random introduction of the comonomer containing an alkenyl group or a functional group derivable into an alkenyl group into the polymer. Therefore, to reduce the amount of the comonomer containing an alkenyl group or a functional group derivable into an alkenyl group means that the percentage of formation of polymers not containing alkenyl group increases. These polymers are not integrated into the crosslinked structure, so that the range which can control physical properties of cured products is remarkably narrowed. Furthermore, for obtaining a vinyl polymer showing a low viscosity and having a high percentage of alkenyl groups, it is necessary to use a monomer containing an alkenyl group or a functional group derivable into an alkenyl group in a considerably high proportion. This, however, results in shortened distances between crosslinking sites in cured products, and hence in failure to produce elastic properties required of pressure sensitive adhesives. Thus, the problem is that a system satisfactorily meeting the requirements concerning low viscosity of polymers, a high percentage of alkenyl groups in the polymers and elastic properties of cured products cannot be established.
In the case of solvent type pressure sensitive adhesives, a great amount of heat energy is consumed to evaporate the solvent after application to a substrate such as a film, or a fire may possibly be caused or the human body may be adversely affected. Solvent-free or high solid content pressure sensitive adhesives are thus demanded.
For solving the above problems and obtaining a high-performance pressure sensitive adhesive, a method is conceivable which uses, as the base polymer of the pressure sensitive adhesive, an acyclic copolymer having a relatively low molecular weight and a sufficiently low viscosity and having an alkenyl group capable of undergoing crosslinking or chain extension as introduced terminally to the polymer. In the prior art, however, it is difficult to produce alkenyl-terminated (meth)acrylic polymers.
Japanese Kokai Publication Hei-05-255415 discloses a method of producing latexes which comprises subjecting a vinyl monomer and a diene monomer to emulsion polymerization using a dialkenyl disulfide as a chain transfer agent. Japanese Kokai Publication Hei-01-247403 discloses a method of producing alkenyl-terminated acrylic polymers which comprises using an allyl-containing disulfide or thiuram disulfide as an initiator. By these methods, however, it is not easy to introduce the alkenyl group into a terminus with certainty. In Japanese Kokai Publication Hei-06-211922, the production of alkenyl-terminated vinyl polymers is described by obtaining a vinyl polymer having a hydroxy group at both termini by using a hydroxy-containing polysulfide chain transfer agent in large excess relative to the initiator, followed by hydroxy group conversion. This method has a problem from the production process viewpoint since the chain transfer agent, which is expensive, is used in large amounts, although terminal alkenyl group introduction can be effected with a relatively high rate.
Accordingly, the task imposed on the present invention is to provide a pressure sensitive adhesive composition in which a vinyl polymer having a narrow molecular weight distribution and a low viscosity and a high alkenyl group content is used as the main component and which can have a high solid content and is rapidly curable and by which the above problems can be solved.
SUMMARY OF THE INVENTION
The present invention provides a novel pressure sensitive adhesive composition comprising the constituents mentioned below, by which the above problems are solved.
Thus provided is:
1) a pressure sensitive adhesive composition which comprises the following two components:
(A) a vinyl polymer having at least one alkenyl group represented by the general formula (1):
CH
2
═C(R
1
)— (1)
wherein R
1
represents a hydrogen atom or a methyl group, and
(B) a hydrosilyl-containing compound; more specifically,
2) the pressure sensitive adhesive composition according to 1),
wherein the (A) component (meth) acrylic polymer has a weight average molecular weight-to-number average molecular weight ratio of less than 1.8 as determined by gel permeation chromatography; or
3) the pressure sensitive adhesive composition ac
Fujita Masayuki
Kusakabe Masato
Nakagawa Yoshiki
Connolly Bove & Lodge & Hutz LLP
Kaneka Corporation
Nutter Nathan M.
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