Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
1999-12-28
2002-04-23
Pezzuto, Helen L. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S089000, C526S236000
Reexamination Certificate
active
06376633
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an acrylic pressure-sensitive adhesive composition which is useful as adhesives in adhesive tapes or sheets and a process for producing the same.
BACKGROUND OF THE INVENTION
Acrylic pressure-sensitive adhesive compositions have been industrially produced mostly by solution polymerization or emulsion polymerization. In these methods, polymerization is usually initiated by radical initiators underheating or with radiation irradiation.
In the solution polymerization method, acrylic pressure-sensitive adhesive compositions are prepared by polymerizing an alkyl (meth)acrylate optionally together with modifier monomers (acrylic acid, styrene, vinyl acetate, etc.) in an organic solvent (toluene, etc.). In the solution polymerization using an organic solvent, the developed polymer chain undergoes chain transfer into the solvent, which generally makes it difficult to obtain a high-molecular weight polymer or to achieve a large cohesive force. Moreover, use of a large amount of an organic solvent is unfavorable from the viewpoints of resource saving and environmental health (i.e., protecting environment from air pollution).
In the emulsion polymerization method, on the other hand, acrylic pressure-sensitive adhesive compositions are prepared by polymerizing an alkyl (meth)acrylate optionally together with modifier monomers (acrylic acid, styrene, vinyl acetate, etc.) not in an organic solvent but in water which is employed as a medium with the use of an emulsifier, i.e., in a state having been emulsified in water. The acrylic pressure-sensitive adhesive compositions prepared by this method are advantageous from the viewpoints of resource saving and environmental health. In addition, these acrylic pressure-sensitive adhesive compositions obtained by this method have relatively high molecular weight and exhibit relatively large cohesive force as pressure-sensitive adhesives, compared with the polymers obtained by the solution polymerization method. However, the pressure-sensitive adhesive compositions obtained by this method suffer from some troubles such that they are poor in water resistance due to the emulsifiers contained therein and that the adhesive strength thereof is lowered since the emulsifiers are present on the surface thereof.
Accordingly, an object of the present invention is to provide an acrylic pressure-sensitive adhesive composition which contains a high-molecular weight polymer, is excellent in cohesive force and adhesive strength and can be prepared without using any organic solvent.
The present inventors have conducted intensive studies to achieve the above-described object. As a result, they have found out that an adhesive composition being excellent in cohesive force and adhesive strength can be obtained by radical-polymerizing monomer(s) containing an alkyl (meth)acrylate with the use of carbon dioxide as a diluent to thereby form a high-molecular weight polymer, thus completing the present invention.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a process for producing an acrylic pressure-sensitive adhesive composition, which comprises radical-polymerizing monomers containing an alkyl (meth)acrylate using carbon dioxide as a diluent to thereby prepare a pressure-sensitive adhesive composition containing an acrylic polymer having a weight-average molecular weight of 400,000 or more. The polymerization is performed, for example, under a pressure of 5.7 to 40 MPa at a temperature of 20 to 100° C.
The present invention further provides an acrylic pressure-sensitive adhesive composition which is obtained by polymerization using carbon dioxide as a diluent and contains, as the main component, an acrylic polymer containing an alkyl (meth)acrylate as a monomer unit and having a weight average molecular weight of 400,000 or more.
DETAILED DESCRIPTION OF THE INVENTION
The alkyl (meth)acrylate to be used as a monomer in the present invention includes alkyl acrylates and alkyl methacryaltes, each having 1 to 18 carbon atoms in the alkyl group. Example of the alkyl (meth)acrylate include methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, octyl acrylate, nonyl acrylate, isononyl acrylate, decyl acrylate, dodecyl acrylate, butyl methacrylate, 2-ethylhexyl methacrylate and octyl methacrylate. Those alkyl (meth)acrylates can be used alone or as mixtures of two or more thereof.
Only the above-described alkyl (meth)acrylate may be used as the monomer, but it is possible to use the above-described alkyl (meth)acrylate as the main component together with other monomers copolymerizable therewith. Examples of the copolymerizable monomers include carboxyl-containing ethylenically unsaturated monomers such as (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid and fumaric acid. Of those carboxyl-containing ethylenically unsaturated monomers, acrylic acid is particularly preferable. The carboxyl-containing ethylenically unsaturated monomers are an important component to form crosslinkages in the polymer. As other copolymerizable monomers, various monomers which are known in the art as modifier monomers used in acrylic pressure-sensitive adhesives can be used, for example, vinyl acetate, styrene, acrylonitrile, polar group-containing monomers other than the carboxyl-containing ethylenically unsaturated monomers as described above (hydroxyl-containing monomers, epoxy-containing monomers, amido-containing monomers, amino-containing monomers, etc.). By taking the adhesion characteristics into consideration, it is preferable that those copolymerizable monomers are used in an amount of 50% by weight or less based on weight of the total monomers containing the above-described alkyl (meth)acrylate.
The polymerization reaction can be carried out with the aid of a polymerization initiator capable of forming radicals upon decomposition, and the initiators which are generally used in radical polymerization can be used. For instance, when the polymerization is conducted at 40 to 100° C., dibenzoyl peroxide, tert-butyl permaleate, 2,2′-azobisisobutyronitrile, azobisisovaleronitrile, etc., are used as the polymerization initiator. When the polymerization is conducted at 20 to 40° C., a binary initiator (Redox initiator) such as a binary system composed of dibenzoyl peroxide and dimethylaniline is used.
The initiator is used in an amount usually used in polymerizing acrylic monomers, for example, about 0.005 to 10 parts by weight, preferably 0.1 to 5 parts by weight, per 100 parts by weight of the total monomers as described above.
The amount of carbon dioxide used in the present invention as a diluent is, for example, 5 to 2,000 parts by weight, preferably 20 to 900 parts by weight, per 100 parts by weight of the total monomers as described above. Although it is usually sufficient to use carbon dioxide alone as the diluent, the diluent may further contain a small amount of an organic solvent to improve the miscibility, etc., if necessary.
The polymerization is conducted in carbon dioxide under a pressure regulated to, for example, about 5.73 to 40 MPa in a temperature range of, for example, about 20 to 100° C. For example, the polymerization can be conducted in carbon dioxide in a supercritical state. The polymerization time is usually about 2 to 20 hours (for example, about 4 to 10 hours) The polymerization pressure and temperature may be varied stepwise, if necessary.
Since carbon dioxide is used as a diluent in the process of the present invention, chain transfer reaction of the polymer chain grown into the solvent does not occur. For this reason, relatively high molecular weight acrylic polymers as compared with those obtained by the solution polymerization, i.e., acrylic polymers having a weight-average molecular weight of 400,000 or more (for example, 400,000 to 2,000,000), are, obtained. Acrylic pressure-sensitive adhesives containing such polymers exhibit high cohesive force. The weight-average molecular weight can be regulated by co
Bamba Tomohide
Kanada Mitsuhiro
Minamizaki Yoshihiro
Taruno Tomohiro
Yamamoto Takayuki
Nitto Denko Corporation
Pezzuto Helen L.
LandOfFree
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