Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
1998-02-09
2001-02-20
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C524S824000, C524S832000, C524S845000, C526S328000, C526S329200, C526S329700
Reexamination Certificate
active
06190767
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an aqueous emulsion for a pressure-sensitive adhesive and a process for preparing the same, and a pressure-sensitive adhesive prepared using the aqueous emulsion for a pressure-sensitive adhesive.
2. Description of Related Art
In recent years, in the field of pressure-sensitive adhesives, the conversion of solvent type pressure-sensitive adhesive compositions to aqueous emulsion type pressure-sensitive adhesive compositions has been desired from the viewpoint of the global environment, working environment, and effective utilization of resources. An emulsion type pressure-sensitive adhesive composition comprising a polymer containing a (meth)acrylic ester, with the alkyl having not more than 8 carbon atoms, as the main constituent monomer (hereinafter referred to as “acrylate emulsion type pressure-sensitive adhesive composition 1”) is known as one of such aqueous emulsion type pressure-sensitive adhesive compositions. This pressure-sensitive adhesive composition 1 is mainly prepared by emulsion polymerization and is extensively utilized in applications, such as labels. A coating formed of the acrylate emulsion type pressure-sensitive adhesive composition 1, however, is inferior in moisture resistance and water resistance to the solvent type pressure-sensitive adhesive composition. Therefore, the solvent type pressure-sensitive adhesive composition is generally used in applications where high performance is required, so that at the present time the conversion of the solvent type pressure-sensitive adhesive composition to the emulsion type pressure-sensitive adhesive composition is unsatisfactory.
The use of a radical-polymerizable emulsifying and dispersing agent in the emulsion polymerization in order to improve the water resistance of a coating formed from the polymer emulsion is known in the art. Specifically, the application of a product prepared by the emulsion polymerization of an acrylic ester using a radical-polymerizable emulsifier to a pressure-sensitive adhesive has been proposed in the art. The coating formed from the polymer emulsion has better water resistance than a coating formed from the conventional polymer emulsion. The water resistance, however, is not necessarily satisfactory.
The mechanism of the emulsion polymerization has been qualitatively elucidated by Harkins (see J. Polymer Sci., 5, 217 (1950)), and a quantitative theory on the mechanism has been developed by Smith and Ewart. According to Harkins, in the emulsion polymerization, the emulsifier first forms a micelle, and the monomer is solubilized in the micelle, followed by the attack of the monomer by a radical generated in an aqueous system to initiate polymerization. Subsequently, the monomer is gradually diffused from monomer oil droplets present in the aqueous system and fed into the micelle to permit the polymerization to proceed. Thus, micelle particles are grown into polymer particles.
When the water solubility of the monomer is excessively low, however, the monomer cannot be successfully fed from the monomer oil droplets into the micelle, inhibiting smooth progression of the polymerization reaction. This is the major reason why a polymer emulsion cannot be prepared from a sparingly water-soluble monomer by the emulsion polymerization. The (meth)acrylic ester is a sparingly water-soluble monomer. Therefore, when the (meth)acrylic ester is used as the monomer, no stable polymer emulsion can be prepared for the above reason.
In recent years, the number of kinds and quantity of plastic products used as industrial materials and consumer materials have been increased, and this tendency is particularly significant for the quantity of polyolefin plastics. Therefore, there is an ever-increasing demand for a pressure-sensitive adhesive having good tackiness properties against the polyolefin plastics.
The pressure-sensitive adhesive prepared from the acrylate emulsion type pressure-sensitive adhesive composition 1 is free from odor and possesses excellent weather resistance and heat resistance. It, however, is unsatisfactory in the adhesion to nonpolar adherends, such as polyolefin plastics. An attempt to add an aqueous emulsification product of a tackifier to the pressure-sensitive adhesive composition 1 has been made to reduce this drawback. This, however, results in remarkably lowered water resistance of the resultant pressure-sensitive adhesive due to the influence of an emulsifying and dispersing agent contained in the tackifier emulsification product. Further, the polymer constituting the acrylate emulsion type pressure-sensitive adhesive composition 1 has an unsatisfactory cohesive force and hence a pressure-sensitive adhesive prepared from the composition cannot satisfactorily retain the adhesive strength.
Further, JP-A 6-192307 (published on Jul. 12, 1994) discloses a process for-preparing an emulsion type pressure-sensitive adhesive composition comprising a polymer containing a (meth) acrylic ester, with the alkyl having 9 to 30 carbon atoms, as one of the monomers constituting the polymer (hereinafter referred to as “acrylate emulsion type pressure-sensitive adhesive composition 2”), and JP-A 6-192341 (published on Jul. 12, 1994) discloses a process for preparing an emulsion type pressure-sensitive adhesive composition comprising a polymer containing a (meth)acrylic ester, with the alkyl having 10 to 34 carbon atoms, as the major constituent monomer (hereinafter referred to as “acrylate emulsion type pressure-sensitive adhesive composition 3”). These methods are characterized by the size of monomer droplets in the emulsion during the polymerization. Further, the invention disclosed in JP-A 6-192307 is characterized by using a particular dispersion stabilizer and a radical polymerization initiator, and the invention disclosed in JP-A 6-192341 is characterized by the proportion of the (meth)acrylic ester in the monomer mixture. Neither JP-A 6-192307 nor JP-A 6-192341, however, refers to the weather resistance of a pressure-sensitive adhesive prepared from the composition 2 or 3 and the adhesion of the pressure-sensitive adhesive to nonpolar adherends, such as polyolefin plastics.
Accordingly, an object of the present invention is to provide an aqueous emulsion for a pressure-sensitive adhesive which can solve the above problems involved in the conventional emulsion type pressure-sensitive adhesive compositions and can be used to prepare a pressure-sensitive adhesive possessing excellent moisture resistance, water resistance, weather resistance, and adhesion to nonpolar adherends, such as polyolefin plastics, and at the same time well-balanced tackiness properties.
DISCLOSURE OF THE INVENTION
Summary of the Invention
The present inventors have made intensive studies with a view to overcoming the above problems. As a result, they have found that emulsification and dispersion of a monomer mixture having a particular composition in an aqueous system in the presence of an emulsifying and dispersing agent, preferably a radical-polymerizable emulsifying and dispersing agent, to bring the monomer mixture to fine particles followed by polymerization using the fine particles of the monomer mixture as a polymerization field can stably produce a highly stable aqueous emulsion for a pressure-sensitive adhesive which can provide a pressure-sensitive adhesive possessing excellent moisture resistance, water resistance, weather resistance, and adhesion to nonpolar adherends, such as polyolefin plastics, and, in addition, well-balanced tackiness properties. The above finding has led to the completion of the present invention.
Thus, according to the first aspect of the present invention, there is provided an aqueous emulsion for a pressure-sensitive adhesive comprising a polymer prepared by polymerizing a monomer mixture containing at least 50% by weight, based on the total weight of the monomer mixture, of a long-chain alkyl (meth)acrylate with the alkyl having 9 to 13 carbon atoms. In the present invention, the term “monome
Ishikawa Yoshinobu
Kitayama Hiroaki
Nomura Masaki
Birch & Stewart Kolasch & Birch, LLP
Copenheaver Blaine
Toyo Ink Manufacturing Co. Ltd.
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