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
2002-09-30
2004-06-08
Egwim, Kelechi C. (Department: 1713)
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...
C524S560000, C526S201000, C526S328000, C526S329700
Reexamination Certificate
active
06747084
ABSTRACT:
BACKGROUND OF THE INVENTION
Pressure-sensitive adhesives (sometimes referred to as PSA) which are permanently tacky in dry form at room temperature are widely used for making labels and tapes which can be applied to a variety of substrates and adhere on application of slight pressure. They are also used for laminating polymeric films such as poly(vinyl chloride) and polyester Mylar, silicone coated papers, and film release liners for forming decals and other related products.
Water based pressure-sensitive adhesives are of interest because of their low VOC emissions. Although the performance is not equivalent to solvent based pressure-sensitive adhesives, they satisfy emission standards and are easy to process. Common types of pressure-sensitive adhesives, both water based and solvent based, are derived from acrylic ester based copolymers, such as alkyl acrylate and alkyl methacrylate copolymers.
The following patents and articles are representative of acrylic based pressure-sensitive adhesives:
Hidalgo, et al. “Polystyrene(1)/poly(butyl acrylate-methacrylic acid)(2) core-shell emulsion polymers. Part II: Thermomechanical properties of latex films,”
Colloid and Polymer Science
, 1992, Vol. 270, pages 1208-1221, disclose the formation of polystyrene/poly(butyl acrylate-methacrylic acid) latexes by a two stage process. Initially, a polystyrene seed is prepared and then the butyl acrylate and methacrylic acid polymerized in the presence of the seed forming a core/shell polymer in a ratio of 2/3.
EP 0 593231 A1 discloses the formation of pressure-sensitive acrylic adhesives by the addition of low molecular weight (<7,000) ethylene oxide-block-propylene oxide copolymer surfactants to acrylic pressure-sensitive adhesives for the purpose of improving low temperature adhesion. These pressure-sensitive adhesives are based upon 2-ethylhexyl acrylate and acrylic and methacrylic esters of C
4-12
alkanols, such as butyl acrylate.
U.S. Pat. No. 6,225,401 discloses filterable aqueous dispersions of pressure-sensitive adhesive suited for labels formed by copolymerizing acrylic or methacrylic esters in the presence of an inhibitor. A wide variety of hardening comonomers can be included in the emulsion polymerization process and these include the styrenes, acrylonitrile, vinyl esters, and so forth.
U.S. Pat. No. 6,254,985 discloses aqueous emulsions of pressure-sensitive adhesives base upon esters of acrylic and methacrylic acid. The patentees disclose the use of an emulsifier consisting of at least 5% by weight of aromatic carbon atoms, typically including at least two sulfonate groups to improve adhesion and cohesion.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to an improvement in a process for preparing an aqueous emulsion of a pressure-sensitive adhesive based upon acrylic esters which have a good balance of adhesive and cohesive properties and to the resulting emulsion. In the basic process, a pressure-sensitive adhesive formulation comprised of at least one ester of acrylic or methacrylic acid is polymerized in the presence of water and an emulsifier thereby forming an emulsion polymerized pressure-sensitive adhesive polymer. The improvement resides in effecting the polymerization of said pressure-sensitive adhesive formulation comprised of an ester of acrylic or methacrylic acid and a styrene containing polymer containing at least 80 percent by weight styrene, said styrene containing polymer present in an amount of from 5 to 30 percent by weight of the pressure-sensitive adhesive formulation.
Significant advantages of the process and product can be realized and they include:
an ability to include a small proportion of a low cost filler polymer into an aqueous pressure-sensitive adhesive based upon acrylic and methacrylic esters without adversely affecting the adhesive properties;
an ability to overcome deficiencies in film formation from blends of polymers, e.g., blends of polystyrene and acrylic and methacrylic copolymers;
an ability to include a low cost “filler” into a pressure-sensitive adhesive by a simple method without the need for special equipment;
an ability to prepare a pressure-sensitive adhesive with high Tg polystyrene filler in one reaction, and in a single reactor, while maintaining the performance advantages of the pressure-sensitive adhesive including those formed by the blend method; and,
an ability to eliminate the need for the high Tg polystyrene latex to be of a specific particle size, expensive macromers, and minimum amounts of surfactants.
DETAILED DESCRIPTION OF THE INVENTION
Emulsion polymerization of a pressure-sensitive adhesive formulation comprised of esters of acrylic and methacrylic acid including ethylenically unsaturated monomers to produce aqueous based pressure-sensitive adhesive polymer emulsions is well known. A representative pressure-sensitive adhesive formulation is comprised of an aqueous polymer dispersion wherein the polymer is comprised of polymerized units, based on the total weight of units, of (a) from 60 to 95% by weight of at least one C
6-12
alkyl acrylate; (b) from 0 to 10% by weight of an ethylenically unsaturated compound having a glass transition temperature of above 0° C. and contain no functional groups other than ethylenically unsaturated group; (c) from 0 to 10% by weight of an ethylenically unsaturated compound having at least one acid or acid anhydride group; and (d) from 0 to 20% by weight of a further ethylenically unsaturated compound; the weight percentages based on the total weight of polymer. Typically, unsaturated compounds in group (c) and (d) are included at less than 5% each, when used. Thus, compounds in group (c) and (d) comprise a small proportion of the pressure-sensitive adhesive.
Particularly suitable alkyl acrylates in group (a) are 2-ethylhexyl acrylate, octyl acrylate, decyl acrylate or dodecyl acrylate.
Monomers in group (b) can include methyl methacrylate, methyl acrylate, n-butyl acrylate and tert-butyl acrylate; vinyl esters of C
1-20
carboxylic acids such as vinyl laurate, stearate, propionate, the vinyl ester of Versatic acid, and vinyl acetate; vinyl aromatics such as styrene, and so forth. Methyl methacrylate is preferred.
Examples of group (c) monomers can include acrylic and methacrylic acid, maleic acid, or maleic anhydride. Group (d) monomers can include C
1
to C
10
hydroxyalkyl (meth)acrylates.
There are two mechanisms in the process for forming the pressure-sensitive adhesive including the styrene containing polymer, e.g., polystyrene filler. In one mechanism, a styrene containing polymer is dissolved in a pressure-sensitive adhesive formulation comprised of a mixture of monomers. The resulting solution, then, is emulsified with surfactants and water and, with the aid of energy supplied by high shear mixing, converted to a stable emulsion of relatively small particle size particles. The resulting emulsion is polymerized by emulsion polymerization. In a second method, a seed latex of styrene polymer is prepared by emulsion polymerization and the pressure-sensitive adhesive formulation emulsion polymerized in the presence of the seed latex. The second method has the advantage of allowing for reduced emulsifier in the final product, elimination of high shear mixing and allowing the reaction to be carried out in a single reactor.
The styrene containing polymer is one containing at least 80% by weight styrene and typically one containing 100% styrene by weight. Optional monomers that may be included in producing styrene containing polymers are &agr;-methyl styrene, &rgr;-methylstyrene, acrylonitrile, methacrylonitrile, methacrylonitrile, methyl methacrylate and trace levels of other monomers leading to a high Tg polymer, at least 80° C.
The styrene copolymer is incorporated into the pressure-sensitive adhesive in an amount from 5 to 30% by weight, which includes the base pressure-sensitive adhesive polymer and the styrene containing polymer. Levels above about 30% by weight detract from the performance of the pressure-sensitive adhesive. Levels below about 5% by weight
DiStefano Frank Vito
Gruber Bruce Alan
Air Products Polymers, L.P.
Bongiorno Mary E.
Egwim Kelechi C.
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