Adhesive compositions containing graft copolymers

Details Adhesive compositions containing graft copolymers Adhesive compositions containing graft copolymers

2001-09-13

2003-07-08

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Adhesive outermost layer

C525S063000, C525S064000, C525S069000

Reexamination Certificate

active

06589651

ABSTRACT:

The present invention relates generally to improved adhesive compositions. More particularly, the present invention relates to improved pressure sensitive adhesive compositions containing graft copolymers.
Adhesives have been used since antiquity to hold substrates together via surface attachment. The term “adhesive”, as used herein, is a substance that is typically a liquid or tacky semisolid, or at least for an instant to contact and wet a surface, and be applied in a relatively thin layer to form a useful joint capable of transmitting stresses from one substrate to another. The term “pressure sensitive”, as used herein, refers to adhesives which typically do not undergo hardening after they have been applied to the surface of the substrate and the joint is formed. These adhesives are capable of holding substrates together when the surfaces are mated under briefly applied pressure at room temperature.
The properties of tack, peel strength and shear resistance, which are frequently mutually exclusive properties, may be highly important in tailoring an adhesive composition that is suitable for a particular application. Tack is a measure of viscous flow under conditions of fast strain rates and low stress magnitudes and generally relates to the initial attraction of an adhesive to a substrate. Peel strength is a measure of resistance to flow at intermediate strain rates and moderate to high stress magnitudes and generally relates to the measure of bond strength between an adhesive and a substrate. Shear resistance is a measure of resistance to flow at intermediate stress magnitudes and generally relates to the internal or cohesive strength of an adhesive.
Pressure sensitive adhesives (“PSAs”) may be generally comprised of rubber, acrylic or silicone based formulations and may be manufactured via such methods as solvent, emulsion, or hot melt processes. Pressure-sensitive adhesive compositions based upon aqueous emulsions and dispersions of acrylic are known and widely used. Exemplary of such pressure sensitive adhesives include, for example, graft copolymers. The term “graft copolymers”, as used herein, refers to macromolecules formed when polymer or copolymer chains are chemically attached as side chains to a polymeric backbone. Generally, the side chains are of a different polymeric composition than the backbone chain. Because graft copolymers often chemically combine unlike polymeric segments in one molecule, these copolymers have unique properties compared to the corresponding random analogues. These properties include, for example, mechanical film properties resulting from thermodynamically driven microphase separation of the polymer.
The term “comb copolymer”, as used herein, refers to a type of graft copolymer, where the polymeric backbone of the graft copolymer is linear, and each side chain of the graft copolymer is formed by a “macromonomer” that is grafted to the polymer backbone. The term “macromonomers”, as used herein, are low molecular weight polymers having at least one functional group at the end of the polymer chain that can further polymerize with others monomers to yield comb copolymers. See e.g., Kawakami in the “Encyclopedia of Polymer Science and Engineering”, Vol. 9, pp. 195-204, John Wiley & Sons, New York, 1987. The term “linear”, as used herein, is meant to include polymers where minor amounts of branching has occurred through hydrogen abstraction that is normally observed in free radical polymerizations. The comb copolymers are commonly prepared by the free radical copolymerization of macromonomer with conventional monomer (e.g., ethylenically unsaturated monomers).
Comb copolymers prepared with water-insoluble macromonomers have been predominantly prepared using bulk and solution polymerization techniques. However, such processes typically involve the use of solvent or monomer as the medium in which the polymerization is conducted. The use of such materials is undesirable, for example, due to toxicity concerns. Thus, efforts recently have focused on developing methods for preparing comb copolymers that may be suitable for use as PSAs via an aqueous emulsion process.
A parameter which is frequently relevant for selecting a grafted copolymer or comb copolymer that imparts the proper balance of properties for the adhesive composition, particularly for PSA compositions, is the respective glass transition values (T
g
) of the “hard phase” and the “soft phase”. As used herein, the term “hard phase” generally refers to the polymer or copolymer side chains or grafts, whereas the term “soft phase” generally refers to the polymeric backbone of the grafted copolymer. It is generally believed that the T
g
of the hard phase strongly influences the shear properties of the resulting adhesive compositions.
The degree of compatibility for the hard and soft phases also effects the shear and other properties of the adhesive compositions to a lesser degree. It is important that the copolymer side chains, or grafts, have low or no compatibility with the polymeric backbone so that separate phases are formed.
Compatibility, as used herein, refers to a measure of the mutual solubility of two materials, such as the hard and soft phases of the graft or comb copolymer. Compatible blends may be characterized by (1) the existence of a single homogeneous phase which contains no discrete domains of either component, and (2) a single glass transition temperature for the blend of components as discussed in P. B. Rim and E. B. Orler, “Dependence of T
g
on Composition for a Compatible Polymer/Oligomer Blend”, Macromolecules, Vol. 20, pp. 433-435 (1987).
In blends of polymers or blends of additives with polymers, a further aspect of compatibility relates to the differences in refractive indices between components. A lack of compatibility is generally evidenced by haziness in the dried adhesive film due to large domains of individual components of differing refractive index. Compatibility is typically favored between materials which are similar in chemical and/or physical characteristics. To effectively modify the performance of an adhesive containing graft or comb copolymers, the selected additives should preferably be at least partially compatible with the soft phase of the copolymer and have very limited, or no compatibility, with the hard phase.
U.S. Pat. No. 4,554,324 to Husman et al. (“Husman”) discloses PSA compositions that comprise a polymerized acrylic or methacrylic acid ester backbone having grafted pendant polymeric moieties. The grafted pendant polymeric moieties are comprised of macromonomers that may be prepared by anionic or free-radical polymerization processes using alkali metal hydrocarbons, alkoxide salts, or free-radical initiators, respectively. The reactive double bond of the macromonomer is an acrylate or methacrylate linkage to a desired polymeric repeat unit such as styrene or methyl methacrylate.
Husman teaches the use of macromonomers with acrylic compositions to reinforce the cohesive strength, or shear resistance, of polymers as seen in shear strength while maintaining a desirable balance of other PSA properties such as peel and tack. The adhesive compositions in Husman are made into films from solvent solutions or via melt related coating processes, such as extrusion or hot melt coating. Further, Husman does not teach the use of polymeric additives such as tackifiers to improve the properties of the PSA compositions.
U.S. Pat. No. 5,006,582 to Mancinelli (“Mancinelli”) discloses acrylic hot melt PSA compositions that contain acrylic comb copolymers. Mancinelli teaches that the acrylic comb copolymers, which generally consist of a methyl methacrylate macromonomer repeat unit that is linked to an acrylate or methacrylate terminal double bond, are made via group transfer polymerization. Mancinelli discloses the use of cobalt chain transfer agents to produce macromonomers with a nonacrylate type of terminal double bond that still reacts well with acrylates and methacrylates.
Mancinelli further teaches that the PSA properties and melt processability of the

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