Stamp adhesive

Details Stamp adhesive Stamp adhesive

1999-11-19

2001-06-05

Szekely, Peter (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

Reexamination Certificate

active

06242552

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a unique pressure-sensitive adhesive, self-adhesive postage stamps, and other constructions made with the adhesive.
BACKGROUND OF THE INVENTION
Since their introduction a few years ago, self-adhesive postage stamps have enjoyed tremendous popularity. In a typical construction, a sheetlet of self-adhesive postage stamps includes a laminate of an array of postage stamps made of a die-cut face stock, printed on one side with postage indicia, adhered to a release liner by a pressure-sensitive adhesive (PSA). U.S. Pat. No. 5,296,279 (Birnbaum et al.), assigned to Avery Dennison Corporation, is representative. ATM-dispensable, self-adhesive postage stamps are also known, and are disclosed in U.S. Pat. No. 5,503,436 (Alpaugh et al.), also assigned to Avery Dennison Corporation. Both patents are incorporated herein by reference.
The United States Postal Service (USPS) sets standards for United States-issued postage stamps. Government specification USPS-P-1238C (Jan. 24, 1996), which is incorporated by reference herein, identifies and describes certain “required,” “target,” and to-be-“monitored” properties of face stocks, adhesive layers (PSAs) and release liner backings (as well as linerless release coatings) to be used in self-adhesive postage stamps. An important property of self-adhesive postage stamps is the ability to form a permanent adhesive bond, after a short dwell time, to a variety of substrates, including woven paper, regular kraft, corrugated board, polyethylene based envelope stock (e.g., Tyvek®, sold by DuPont de Nemours, E.I., Co. of Wilmington Del.), and other materials commonly used in envelopes and parcels. To deter theft and re-use of stamps, it is desirable that the adhesive leave a telltale fiber tear mark on the paper envelope or package if attempts are made to remove the stamp.
Although progress has been made in the area of self-adhesive postage stamps, a need for improved adhesives and theft-deterring stamp constructions continues to exist. In addition, the USPS has expressed a desire for “environmentally benign” self-adhesive postage stamps that can be recycled during ordinary paper recycling processes, or at least easily removed from envelope substrates prior to repulping and other recycling processes.
It is well-known that many PSAs contribute to the formation of what are commonly referred to in the trade as “stickies.” Stickies tend to deposit on the felts, wires, drier cans, and other paper machine parts, and disrupt the smooth operation of paper machines. They also degrade the functional and cosmetic properties of paper products made with recycled paper filer. A need exists for self-adhesive stamps that either do not contribute to stickies formation or can be easily removed, preferably with water, before waste envelopes and parcels are repulped and recycled.
SUMMARY OF THE INVENTION
According to the present invention, there is provided an improved, general purpose permanent, environmentally benign, pressure-sensitive adhesive (PSA) that is ideal for use in self-adhesive postage stamps and other PSA constructions. The new PSA adheres well to most substrates used in postal application, including natural kraft, white woven, and recycled paper. Stamps and other constructions made with the new PSA are water-removable from a variety of substrates. In one embodiment, the PSA comprises an inherently tacky, acrylic copolymer prepared by emulsion or solvent polymerization of a monomer mixture comprising, on a percent by weight basis, based on the total weight of monomers, about 27 to 29% di-(2-ethylhexyl) maleate, 19 to 21% vinyl acetate, 40 to 43% ethylhexyl acrylate, 5 to 10% methyl methacrylate, 1% acrylic acid, and 1% methacrylic acid.
The present invention also provides self-adhesive postage stamps and other adhesive laminate constructions, including bulk mail stamps, address change labels, etc., made with the unique PSA. The constructions are conveniently prepared by coating or otherwise applying the PSA to a protective release liner, laminating the coated release liner to a suitable face stock having opposed first and second faces, and die-cutting the construction to form individual stamps or labels. Alternatively, the PSA is applied directly to a face stock, protected with a release liner, and die-cut. In some embodiments, either or both faces of the face stock are imprinted with, or imprintable with, postage indicia. When applied to an envelope substrate, such as a woven paper envelope, and then removed from the envelope, the self-adhesive postage stamps leave a fiber tear mark on the envelope, even after only very short dwell times (as little as five minutes or less).
The new adhesive has a low creep compliance and performs well in adhesive permanence and accelerated aging testing, over a range of temperatures and humidities. Constructions prepared with the adhesive also convert well, as a consequence of the adhesive's low creep compliance and excellent sheer characteristics. In addition, the adhesive is repulpable and is qualified as a screenable adhesive.
DETAILED DESCRIPTION
In one embodiment of the invention, an inherently tacky, acrylic PSA copolymer is made from a plurality of monomers comprising, on a percent by weight basis, based on the total weight of monomers, (a) about 27 to 29% di-ethylhexyl maleate, (b) about 19 to 21% vinyl acetate, (c) about 40 to 43% ethylhexyl acrylate, (d) about 5 to 10% methyl methacrylate, (e) about 1% acrylic acid, and (f) about 1% methacrylic acid. In a preferred embodiment, the copolymer is uncrosslinked, and the plurality of monomers contains substantially no multifunctional monomers having a cyanurate of phosphate moiety. That is, such monomers should be completely excluded or present in such a low amount that they cause no perceptible crosslinking.
The PSA copolymer is prepared by free radical emulsion polymerization, in the presence of a mixture of anionic and nonionic surfactants, a reactive (copolymerizable) surfactant such as sodium vinyl sulfonate, and at least one polymerization initiator. Preferred surfactants include both anionic and nonionic surfactants and stabilizers, including, without limitation, alkyl phenol ethoxylates and sodium alkyl phenol ethoxylate sulfates. Particularly preferred are POLYSTEP® surfactants from Stepan Company Inc. (Winnetka, Ill.), especially a mixture of POLYSTEP F-9 (a nonylphenol ethoxylate containing approximately thirty moles of ethylene oxide (EO) per mole of surfactant molecule) and POLYSTEP B-27, (a sodium nonylphenol ethoxylate sulfate containing approximately four moles EO per mole of surfactant molecule). The anionic and nonionic surfactants are included in the formulation in an amount sufficient to form a stable emulsion without causing phase separation.
A reactive surfactant, such as sodium vinyl sulfonate or a comparable alkali metal salt of an alpha-olefin sulfonic acid, is also preferably included in the polymerization reaction. A non-limiting example of such a surfactant is POLYSTEP A-18, a linear, sodium alpha-olefin sulfonate available from Stepan Company.
Nonlimiting examples of polymerization initiators include persulfates, such as sodium persulfate (Na
2
S
2
O
8
), potassium persulfate, and peroxy persulfates; peroxides, such as tert-butyl hydroperoxide (t-BHP); and azo compounds such as VAZO™ initiators; used alone or in combination with one or more reducing components or activators, such as bisulfites, metabisulfites, ascorbic acid, erythorbic acid, sodium formaldehyde sulfoxylate, ferrous sulfate, ferrous ammonium sulfate, etc. The activator is believed to form a redox couple with the initiator and promotes faster generation of free-radicals. Preferably, a small amount of base, e.g., ammonia, sodium hydroxide, sodium bicarbonate, etc. is added to the initiator and functions as a buffer. The base appears to stabilize the emulsion polymerization.
A defoamer, such as Drewplus L-191 (Ashland Chemical Co., Columbus, Ohio); a biocide, such as Kathon LX (available as a 1.5% solution from Rohm & Haas, Ph

Affiliated with

Also associated with

Rating

Stamp adhesive does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Stamp adhesive, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Stamp adhesive will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2446009