Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-08-20
2001-08-28
Hampton-Hightower, P. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S221000, C525S309000, C525S412000, C525S414000, C525S421000, C524S272000, C524S314000, C524S501000, C524S507000, C524S512000
Reexamination Certificate
active
06281298
ABSTRACT:
FIELD OF INVENTION
The present invention relates to water-based pressure sensitive adhesives comprising at least one water-based polymer and at least one aminoplast resin such as melamine formaldehyde resins, urea formaldehyde resins, alkylated melamine formaldehyde resins and combinations thereof. More particularly, the invention relates to water-based pressure sensitive adhesives comprising the reaction product of at least one water-based polymer and water dispersible melamine-formaldehyde resins. Specifically, it relates to water-based pressure sensitive adhesives having enhanced bonding characteristics including tack, peel strength and shear strength.
BACKGROUND OF THE INVENTION
Aminoplast resins have been used in a variety of applications such as in fire retardant compositions, glass fiber thermal insulation, plywood processing, molding composition and decorative lamination. Melamine-formaldehyde resins have been used as a crosslinking agent for the preparation of pressure sensitive adhesives and primers. For example, aqueous polymer dispersions are often used with said resins to enhance bonding characteristics. Several patents describing such adhesives and primers are described below.
Fitts et al., U.S. Pat. Nos. 5,075,360 and 5,187,210 disclose adhesives employed in manufacturing corrugated board. The adhesive composition comprises starch, boron containing compound, a basic agent and optionally, a water resistant additive comprising aminoplast resins such as urea formaldehyde, ketone formaldehyde and melamine formaldehyde.
Sheets et al., U.S. Pat. No. 5,059,264 discloses an aqueous dispersed primer-sealer composition applied to a surface layer of overlay paper or surface mat. The primer-sealer is based on a polymeric binder composition containing melamine resin, carboxyl functional acrylic emulsion copolymer, vinyl-acetate-vinyl versatate copolymer, emulsified fatty acid, and polyvinyl alcohol.
Pressure sensitive adhesives are used in a variety of applications due to their ability to form a bond at room temperature immediately upon contact, merely by the application of light pressure.
Tack, peel and shear are three properties which are useful in characterizing the nature of pressure sensitive adhesives. The tack measures the adhesive's ability to adhere quickly. Peel measures the ability of the adhesive to resist forced delamination. Shear measures the adhesive's ability to hold in position when shearing forces are exerted. Although tack and peel are directly related to each other, they are inversely related to shear.
A major disadvantage of pressure sensitive adhesives is that by chemical nature, they often have an inherently low cohesive strength. In addition, since they are thermoplastic and soften rapidly at a temperature in excess of about 140° F. (60° C.), they are limited to applications where the stress of the bond or assembled joint is low, i.e., generally less than 2 psi in shear.
Hoeschele et al., U.S. Pat. No. 4,234,500 and Traynor et al., U.S. Pat. No. 4,726,982 disclose improving loop tack and peel by formulating pressure sensitive adhesives with a tackifying resin. However, shear resistance could only be improved by sacrificing peel and loop tack in a single polymer composition or by formulating with a high shear product such as an elastomer or natural rubber latex. Thus, maintaining a requsite balance of poperties while improving one adhesive property, may detrimentally affect one or more of the desirable properties of pressure sensitive adhesives.
Sanderson et al., U.S. Pat. No. 4,077,926 disclose pressure sensitive adhesives containing polymerized alkoxyalkyl esters of unsaturated carboxylic acid. While the polymer of the invention is preferably an organic solution or less desirably an aqueous dispersion prepared in those media, it may also be used and/or prepared as a nonaqueous dispersion, a bulk polymerizable, an organic solvent solution of the polymer emulsified in water, and the like. In a preferred embodiment, an organic compound having two or more functional groups capable of reacting with the carboxylic and hydroxyl groups in the polymer is included to crosslink the said carboxylic acid groups. Among the polyfunctional compounds are phenol-formaldehyde, aminoplasts such as melamine-formaldehyde and alkylated melamine-formaldehyde. The amount of crosslinker is preferably from about 1% to about 5%. According to the invention, shear resistance is improved by crosslinking while retaining excellent tack.
A drawback with pressure sensitive adhesives described in the prior art references relates to shear strength. The pressure sensitive adhesives often have high tack, high peel strengh but lack adequate shear strength.
Therefore, there remains a need for water based pressure sensitive adhesives that exhibit excellent tack and peel strength with substantial improvements in shear strength when subjected to increased thermal and/or mechanical stress.
SUMMARY OF THE INVENTION
The present invention relates to a water-based pressure sensitive adhesive comprising:
a) at least one water-based polymer selected from the group consisting of polyacrylic, polyurethane, polyurethane-acrylic hybrids, polyamides, styrene-butadiene rubber, polychoroprenes, acrylonitrile-butadiene-styrene, polyisoprene, polyisobutylene, polyurea, natural latex, polysaccharides, gum resins and combinations thereof;
b) at least one aminoplast resin present in an amount ranging from about 0.2% by weight to about 8% by weight based on total solids.
Another aspect of the present invention relates to a water-based pressure sensitive adhesive comprising the reaction product of:
a) at least one aqueous polyacrylic dispersion present in an amount ranging from about 90% by weight to about 99.8% by weight based on total solids; and
b) at least one melamine formaldehyde resin present in an amount ranging from about 0.2% by weight to about 8% by weight based on total solids.
Surprisingly, the pressure sensitive adhesives of the present invention have a loop tack greater than about 3 newtons/cm, a peel strength greater than about 5 newtons/cm, and a shear strength greater than about 10 hours. It is surmised that this unique combination of properties can be attributed to the pre-crosslinked network and/or the inherent characteristic of an interpenetrating polymer network formed by the reaction product of at least one aqueous polymer dispersion and at least one water dispersible aminoplast resin.
A further aspect of the present invention relates to a method of enhancing the properties of water based pressure sensitive adhesives comprising the steps of:
a) providing a water-based pressure sensitive adhesive comprising at least one water-based polymer selected from the group consisting of polyacrylic, polyurethane, polyurethane-acrylic hybrids, polyamides, styrene-butadiene rubber, polychoroprenes, acrylonitrile-butadiene-styrene, polyisoprene, polyisobutylene, polyurea, natural latex, polysaccharides, gum resins and combinations thereof; and
b) reacting with at least one aminoplast resin present in an amount ranging from about 0.2% by weight to about 8% by weight based on total solids.
Another aspect of the present invention relates to a method for the preparation of dry-bond laminates, the method comprising the steps of:
a) coating a first flexible substrate with a water-based pressure sensitive adhesive comprising the reaction product of:
i) at least one water-based polymer selected from the group consisting of polyacrylic, polyurethane, polyurethane-acrylic hybrids, polyamides, styrene-butadiene rubber, polychoroprenes, acrylonitrile-butadiene-styrene, polyisoprene, polyisobutylene, polyurea, natural latex, polysaccharides, gum resins and combinations thereof; and
ii) at least one aminoplast resin present in an amount ranging from about 0.2% by weight to about 8% by weight based on total solids;
b) drying the coated substrate with forced air and heat, to form a dry-coated substrate; then
c) applying onto the dry-coated substrate a second substrate using pressure.
Still another aspect of t
H. B. Fuller Licensing & Financing Inc.
Hampton-Hightower P.
Quan Nancy
Su Bin
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