Stock material or miscellaneous articles – Layer or component removable to expose adhesive
Reexamination Certificate
2002-10-04
2004-03-16
Ahmad, Nasser (Department: 1772)
Stock material or miscellaneous articles
Layer or component removable to expose adhesive
C428S040200, C428S041300, C428S041500, C428S341000, C428S35500R, C523S200000, C523S331000, C524S460000, C524S356000, C524S558000, C524S560000, C525S242000, C525S301000, C526S264000, C526S317100, C526S319000
Reexamination Certificate
active
06706356
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to emulsion polymers, especially acrylic polymers useful as coatings and adhesives.
BACKGROUND OF THE INVENTION
Acrylic polymers are well known and are commonly used to produce adhesives, including PSA tapes, labels, and other constructions. They are also used as, or in, a variety of coatings, including paints, primers, barrier layers, scratch-resistant hard coatings, ink-receptive coatings, and chemical-resistant coatings. They can be prepared by a variety of polymerization processes, including bulk, solvent, and emulsion polymerization. In emulsion polymerization, a number of monomers are dispersed in a continuous aqueous phase with the aid of one or more emulsifiers (surfactants) and polymerization is catalyzed by, e.g., a free-radical initiator. The resulting product, a colloidal dispersion of polymer particles, is called a latex, an emulsion polymer, or simply, an emulsion. Particle size and molecular weight are typically distributed across a range of values, which can be expressed using statistical means.
In the production of PSA tapes and labels, it is desirable to employ relatively high solids content polymers. Because less water is present in high solids content emulsion polymers, several distinct cost-savings can be realized. More material can be formed in a given production cycle; transportation costs are reduced; and faster line speeds can be employed as less water needs to be removed when the polymer coating is dried. At the same time, however, the viscosity of an emulsion polymer typically increases as the polymer's solids content is increased. If the viscosity is too high, coatability and processing becomes more difficult. Paste-like emulsions, for example, cannot be coated with Meyer rod and similar coaters traditionally used in the production of PSA tapes, labels, and other constructions. In general, the solids content of emulsion polymers used in the PSA industry is rarely higher than about 67% by weight. A need exists for inherently tacky, emulsion polymers having both high solids content for economy and low viscosity for processability.
PSAs are used in a wide variety of applications, at different temperatures and environmental conditions. A given PSA may perform (adhere) well when applied to a given substrate, or at a particular temperature, yet be wholly unsuitable for other substrates and use temperatures. Particularly problematic are low temperature (<0° C.) applications. In order to achieve even a nominal degree of adhesion, it is usually necessary to lower the glass transition temperature of the PSA polymer well below the use temperature of the adhesive, the temperature at which the PSA tape, label or other construction is applied to a substrate or put to use. However, not all low-T
g
emulsion polymers are suitable for use as PSAs at low temperatures. It is not uncommon for low-T
g
polymers to have undesirably high creep values, making converting (slitting, die-cutting, matrix stripping, and so forth) problematic. A need exists for improved emulsion polymers that function as PSAs when applied to a variety of substrates even at low temperatures.
SUMMARY OF THE INVENTION
It has now been discovered that acrylic emulsion polymers characterized by both high solids content and low viscosity can be prepared using a blend of surfactants and a split monomer feed. The resulting polymers have a multimodal (or at least bimodal) particle-size distribution.
In one embodiment of the invention, a high solids content, low-viscosity emulsion comprises an emulsified product of copolymerizing a plurality of monomers in the presence of a plurality of surfactants, wherein the plurality of monomers comprises, on a percent-by-weight basis, based on the total weight of monomers, about 90-98% alkyl acrylate(s) having an alkyl group containing from 4 to 12 carbon atoms; about 0.1 to 0.5% N-vinyl pyrrolidone; about 0.5 to 4.5% ethylenically unsaturated carboxylic acid(s); and 0 to about 0.5% (more preferably, about 0.1 to 0.5%) multifunctional crosslinking monomer(s); wherein the sum of all monomer weight percentages is 100%. The emulsified product has at least a bimodal particle-size distribution and a solids content of at least 68% by weight, yet, nevertheless, is pourable, with a viscosity of from about 300 to 15,000 centipoise (cps). In some embodiments, the plurality of monomers may also contain a minor amount of methyl or ethyl acrylate, e.g., up to about 6% by weight, based on the total weight of all monomers. Minor amounts of other monomers commonly used to prepare PSA polymers may also be employed.
Emulsion polymers according to the present invention are prepared in a straightforward manner. In one embodiment, the emulsified product is formed sequentially by polymerizing (a) a pre-emulsified first portion of the plurality of monomers, which is fed into a reactor in two distinct feeds, wherein a first feed is relatively slow and contains about 0.4% to 1% by weight of the pre-emulsified first portion of the plurality of monomers, and a second feed is relatively fast and contains about 99 to 99.6% by weight of the pre-emulsified first portion of the plurality of monomers; and (b) a second portion of the plurality of monomers. The second portion of the plurality monomers can be, but does not need to be, pre-emulsified before it is fed into the reactor and polymerized.
In an alternate embodiment, the emulsified product is formed by polymerizing a single pre-emulsion of the plurality of monomers, which is introduced into the reactor in two distinct feeds, wherein a first feed is relatively slow and a second feed is relatively fast, and the first and second feeds contain, preferably, the relative weight percentages stated above.
Preferably, the surfactant system consists of three anionic surfactants, each having a particular primary function, i.e., particle generation, emulsion stabilization, and wet-out/coating enhancement.
DETAILED DESCRIPTION OF THE INVENTION
According to a preferred embodiment of the invention, a high solids content, low-viscosity emulsion is provided and comprises an emulsified product of copolymerizing a plurality of monomers in the presence of a plurality of surfactants, wherein the plurality of monomers comprises, on a percent by weight basis, based on the total weight of monomers, about 90 to 98% alkyl acrylate(s) having an alkyl group containing from 4 to 12 carbon atoms; about 0.1 to 0.5% N-vinyl pyrrolidone; about 0.5 to 4.5% ethylenically unsaturated carboxylic acid(s); and 0 to about 0.5% (more preferably, 0.01 to 0.5%) multifunctional crosslinking monomer(s). Optionally, the plurality of monomers contains a positive amount up to about 6% by weight of methyl acrylate or ethyl acrylate. The emulsified product of polymerization has at least a bimodal, more preferably a multimodal, particle-size distribution.
Nonlimiting examples of alkyl acrylates having 4 to 12 carbon atoms in the alkyl group thereof include butyl, pentyl, hexyl, heptyl, octyl, iso-octyl, ethylhexyl, nonyl, decyl, and dodecyl acrylate. As between 2-ethylhexyl acrylate, butyl acrylate, and iso-octyl acrylate (three commodity acrylates commonly used to make PSA acrylic polymers), 2-ethylhexyl acrylate is preferred, because copolymers based on 2-ethylhexyl acrylate have a lower T
g
than copolymers whose main component is butyl acrylate. Iso-octyl acrylate is less preferred, because emulsion polymers based on iso-octyl acrylate appear to be less stable than polymers based on 2-ethylhexyl acrylate.
Nonlimiting examples of ethylenically unsaturated carboxylic acids include acrylic acid, methacrylic acid, beta-carboxyethyl acrylate, and itaconic acid. A mixture of acrylic acid and methacrylic acid is preferred.
As used herein, the term “multifunctional crosslinking monomer” means a monomer which is copolymerizable with acrylic monomers and which as at least one carbon-carbon double bond and at least one other functional group capable of entering into a polymerization or crosslinking reaction. Nonlimiting examples of suc
Ahmad Nasser
Avery Dennison Corporation
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