Coating processes – Solid particles or fibers applied
Reexamination Certificate
2000-01-11
2001-05-22
Parker, Fred J. (Department: 1762)
Coating processes
Solid particles or fibers applied
C427S208400, C427S288000
Reexamination Certificate
active
06235344
ABSTRACT:
This invention relates to a method of improving an adhesive article containing at least one paper facestock layer and at least one adhesive layer. More particularly, this invention relates to a method of improving an adhesive article containing at least one paper facestock layer and at least one adhesive layer, wherein the adhesive layer contains a pressure sensitive adhesive and voided latex particles.
A variety of pressure sensitive adhesives, hereinafter referred to as “PSA,” are available for use, inter alia in tape and label applications where the transparency of the PSA is generally a desirable property, especially in those applications using a transparent facestock. If the PSA is transparent, then it does not interfere with viewing the facestock or substrate to which the adhesive article is attached nor does it otherwise detract aesthetically from the final product. In applications where a transparent PSA cannot be achieved or where the color of the PSA is not stable, formulators generally add fillers, such as colored clays and inexpensive pigments, to uniformly color the PSA.
Paper tapes (such as masking tapes), paper labels and repositionable paper are made using paper as the facestock layer with at least one PSA layer applied to at least a portion of the paper facestock layer. If the paper utilized is lightweight or inferior or if the PSA bleeds through the paper facestock layer, the clarity and appearance of the printing on the facestock of the paper article is diminished. In addition, if the substrate onto which the paper article is applied shows through the facestock the appearance of the paper article is damaged. Thus, it would be desirable to find a solution to this appearance problem that does not compromise the adhesive properties of the PSA, including tack, peel adhesion and shear strength, to an unacceptable level and does not cause wear to the processing equipment, such as die cutters and knives used to slit tape rolls or guillotine label sheet stock.
One possible solution is to add an opacifying agent, such as titanium dioxide, calcium carbonate, calcined clay, kaolin clay, zinc oxide, hollow glass beads, voided latex particles, to the paper facestock layer either in the paper making process or as a coating layer on the paper facestock. One possible disadvantage of this solution to the tape or label manufacturer is that the manufacturer may not be able to control how the opacifying agent is provided (type, level, cost, etc.) on the paper by the paper manufacturer or converter. Another possible disadvantage, especially in repositionable paper applications, is that adding the opacifying agent to the paper facestock layer would be wasteful and costly if the cause of the poor appearance were associated only with the bleed-through of the adhesive layer and such adhesive layer were only applied to a small portion of the paper facestock layer.
Applicant has found an advantage in adding a specific opacifying agent to the adhesive layer rather than to the paper facestock layer, especially for those applications where the manufacturer wants control of the aesthetic properties of the adhesive article or where the adhesive layer will be applied to only a portion of the paper facestock layer. Applicant has found a solution to the opacity problem that actually improves the shear strength of the adhesive layer without unacceptably compromising the tack and peel adhesion of the adhesive layer. Applicant's solution also reduces the level of wear, relative to conventional opacifying agents, such as titanium dioxide, calcium carbonate, calcined clay, kaolin clay and zinc oxide (all having Moh hardness of 2.0-6.5), to processing equipment typically used to make paper tape, paper labels and repositionable paper.
U.S. Pat. No. 5,045,569 discloses hollow, polymeric, acrylate, infusible, inherently tacky, solvent-insoluble, solvent-dispersible, elastomeric PSA microspheres having an average diameter of at least one 1 &mgr;m (1,000 nm) and preferably multiple voids. Because of the relatively low Tg of the hollow microspheres of U.S. Pat. No. 5,045,569 (less than −20° C.), the hollow microspheres themselves may be used as an adhesive. Consequently, these hollow microspheres are malleable and would collapse and thus would not contribute to the opacity of the adhesive layer.
STATEMENT OF THE INVENTION
The invention is a method of improving an adhesive article, including the steps of:
a. providing at least one paper facestock layer; and
b. applying at least one adhesive layer to at least a portion of the paper facestock layer, wherein the adhesive layer, contains at least one pressure sensitive adhesive.
The voided latex particles are incorporated at a level of 1% by weight solids to 80% by weight solids, based on the total weight of the adhesive layer prior to drying, of voided latex particles into the adhesive layer prior to application of the adhesive layer to the paper facestock layer. The voided latex particles useful in the method of the invention have a particle size diameter of 100 nm to 2,000 nm, a void fraction of 10% to 75%, and a glass transition temperature of at least 20° C.
As used herein, the term “facestock” shall mean the paper backing layer in an adhesive article. In paper labels, the facestock is the paper portion of the label and does not include any release liner. In paper tapes, such as masking tapes, the facestock is the paper portion of the tape.
As used herein, the term “repositionable paper” shall mean paper having applied thereto an adhesive that permits positional adjustments after application to a substrate, yet displays high peel adhesion and shear strength, and displays clean removal from most substrates.
The paper facestock layer of the invention may be any type of paper or other cellulosic material conventionally used in making adhesive articles (tape, label and repositionable paper). The paper layer may optionally be coated with an opacifying agent, including voided latex particles useful in the method of the invention, in addition to providing the voided latex particles in the adhesive layer. The thickness of the paper facestock layer is determined by the final application (tape, label, repositionable paper) and by manufacturing conditions. The paper facestock layer may be colored. By incorporation of the voided latex particles into the adhesive layer, it may actually be possible to use a lower weight or thinner layer of paper facestock.
The adhesive layer of the invention minimally contains at least one PSA and voided latex particles. Pressure sensitive adhesives useful in the invention include any conventional PSA, including without limitation, hot melt adhesives, solvent-based solution rubber adhesives, styrene-butadiene rubber emulsion adhesives and acrylic adhesives. In addition to the polymer component, the adhesive formulation may also contain conventional additives, including without limitation, tackifiers, fillers, wetting agents, surfactants, coalescents, rheology modifiers, thickeners, preservatives and defoamers.
The crux of the invention is the incorporation of voided latex particles into the adhesive layer. The voided latex particles may be incorporated directly into the adhesive formulation at a level of 1% by weight solids to 80% by weight solids, based on the weight solids of the PSA. Preferably, the voided latex particles may be incorporated at a level of 5% by weight solids to 50% by weight solids, based on the weight solids of the PSA. Most preferably, the voided latex particles may be incorporated at a level of 10% by weight solids to 25% by weight solids, based on the weight solids of the PSA. If the voided latex particles are incorporated at a level of less than 1% by weight solids, then the voided latex particles will not be able to provide the desired opacity to the adhesive article. If the voided latex particles are incorporated at a level of greater than 80% by weight solids, then the voided latex particles will unacceptably compromise the adhesive properties, especially tack and peel of the adhesive layer.
The
Falk Stephen T.
Parker Fred J.
Rohm and Haas Company
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