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
1997-03-28
2002-12-03
Reddick, Judy M. (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...
C524S417000, C524S423000, C524S745000, C524S747000, C524S748000, C524S757000, C524S832000, C524S833000, C526S317100, C526S318400
Reexamination Certificate
active
06489387
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to acrylic emulsion pressure-sensitive adhesives which when coated and dried as a film are clear and which resist whitening by the action of hot and ambient temperature water.
BACKGROUND OF THE INVENTION
A high degree of resistance to water whitening, especially hot water, by pressure-sensitive adhesives (PSAs) is required for some label applications. For example, bottles labeled with clear film PSA labels can be subjected to pasteurization conditions where the label is expected to remain clear through the process. Historically, solvent based adhesives have been used for such demanding applications. Environmental considerations, however, favor elimination of solvents. This and higher coating speeds attainable with emulsion PSAs, favor their use in place of solvent based PSAs.
Emulsion based PSA polymers are made in the presence of surfactants and other water soluble electrolytes such as initiators. In addition, comonomers such as unsaturated carboxylic acids are employed to impart or enhance adhesive properties and tend to be present at the latex-particle interface and remain there during film formation. Films cast from such emulsions are sensitive to water and become cloudy simply when a drop of water is placed on the film. Water whitening is almost always assured when the water is hot, e.g., the temperature of the water is 70° C. or higher.
The water resistance of acrylic emulsion polymer films has long been addressed in the art particularly with respect to paint films. The art has observed that the presence of water soluble electrolytes, surfactants and polyelectrolytes such as sodium polymethacrylate cause or enhance water sensitivity (“The Applications of Synthetic Resin Emulsions”, H. Warson, Ernest Benn Ltd., London, 1972, incorporated herein by reference). It is also well known that surfactants are migratory species that with time move towards and bloom to the surface of films also rendering them water sensitive.
The art has taught that the use of low levels of surfactant, a reduction or elimination in soluble electrolytes and crosslinking between the emulsion particles can all serve to improve or impart water resistance.
U.S. Pat. No. 5,202,375 to Biale, incorporated herein by reference, teaches for this purpose a polymer emulsion that is synthesized with less than about 1% by weight surfactant, has a high surface tension (i.e., greater than 45 dynes/cm) and is made in the presence of high levels of a crosslinking agent. Although the use of such emulsion polymers as PSAs is contemplated, no useful PSA properties are reported. Moreover, with such high levels of crosslinking and low levels of surfactant, poor PSA properties would be expected in addition to significant stability problems during polymerization. The contemplated use is as a water resistant edge sealer for composite boards and plywood. Low swelling of the films by water after long term drying is used to demonstrate water resistance.
European Patent Publication 554,832 to Lu et al., incorporated herein by reference, teaches high solids, moisture resistant PSA emulsion polymers. The emulsion polymer is synthesized in the presence of copolymerizable ionic surfactant and in the presence of a hydrophobic polymeric tackifier with a number average molecular weight ranging from 400 to about 50,000.
There is no mention of water whitening of the PSA film in the '375 and '832 patent specifications.
U.S. Pat. No. 4,994,538 to Lee, et al, incorporated herein by reference, teaches a silicone acrylate PSA copolymer having good water whitening resistance synthesized in the presence of an anionic surfactant. Methyl acrylate and vinyl acetate are optionally present.
U.S. Pat. No. 5,286,843 to Wood, incorporated herein by reference, teaches a process for improving the water whitening resistance of a PSA by use of ion exchange to remove water soluble ions. The deionized latex is claimed to provide superior resistance to water whitening.
SUMMARY OF THE INVENTION
It has now been found that water whitening can be eliminated or significantly reduced in clear or transparent acrylic emulsion PSA polymer laminate constructions where the PSA polymer is based on alkyl acrylates by including in the monomer charge at least one partially water soluble comonomer and at least one polar comonomer and forming the copolymers in the presence of a nonionic surfactant containing at least about 8 moles (mols) ethylene oxide per mole (mol) and an ionic surfactant containing less than about 10 mols ethylene oxide per mol preferably in which the weight ratio of nonionic to anionic surfactant is at least one-to-one (at least 1:1), and in which the formed emulsion is neutralized with a base to a pH above 7. Polymerization is preferably under conditions which form emulsions having mean “volume-average” particle diameters (“Psize”) of up to about 210 nm.
The adhesive laminate products formed preferably have an opacity less than about 5%, and more preferably less than about 2.5%, as measured using the boiling water resistance test as described herein. Opacity is measured on a laminate of a layer of the PSA exposed to the action of boiling water for 10 minutes (10 min.) or ambient temperature water for 24 hours (24 hr.) while on a biaxially oriented polypropylene film stock and then laminated to a 2 mil polyester film after exposure. Opacity is measured using a spectrocolorimeter.
Independent of the opacity characteristics of the adhesive film, opacity can be stabilized against change with time by the post-polymerization addition to the formed emulsion copolymer of an inorganic electrolyte.
The PSAs of the present invention may be employed as adhesives which as part of a label construction have a “no label look”. The “no label look” is a printed label which, on application to a container, substrate or the like, gives the appearance of direct printing on the container, substrate or the like. To this end, the adhesive constructions remain clear despite the action of moisture under pasteurizing condition.
It has been further found that the tolerance of the coated and dried adhesive film and/or PSA laminate constructions for water surprisingly enables their use under autoclave conditions in that these materials will either remain clear or quickly return to a clear state after autoclaving where solvent based adhesives do not. This makes the adhesives particularly useful for pharmaceutical and medical applications.
The adhesives of the present invention have been further established to have excellent mandrel performance which further enhances their use as adhesives for labels attached to curved surfaces such as bottles and vials used in pharmaceutical and medical applications as well as battery label applications where the adhesive is required to adhere a label to the cylindrical case of a battery. Such applications are also enhanced by established resistance and stability of the adhesives under conditions of high temperature, high humidity and thermal shock.
The ability to absorb moisture without turning white further makes the adhesives particularly attractive to labeling of bottles having wet surfaces, where the wet surface on the bottle is formed by condensation of water on the bottle's outer surface. Such applications include the labeling of bottles having a low surface energy fatty acid or wax coating to prevent scratching of bottles in route to a labeling station.
Another unique application for the adhesives of the present invention is as adhesives for fleet marking films where the coated and dried adhesive films not only provide resistance to water whitening during label application and in use, but also enable the constructions to conform to rivets and corrugated aluminum sidings and to retain that conformity over time.
DETAILED DESCRIPTION
There are provided, in accordance with an embodiment of the instant invention, acrylic emulsion PSA polymer compositions which, when coated and dried as a film, are substantially unaffected by the action of hot and ambient temperature water.
In addition to
Albrecht Kregg D.
Dilts George
Li Xing-Ya
Mallya Prakash
Petrasko Robert D.
Avery Dennison Corporation
Christie Parker & Hale LLP
Reddick Judy M.
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