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
1998-10-05
2003-04-01
Dawson, Robert (Department: 1712)
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...
C526S279000
Reexamination Certificate
active
06541566
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a pressure sensitive adhesive which is prepared by emulsion polymerizing an ethylenically unsaturated monomer and 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer. The alkoxy silane crosslinks during film formation to provide the adhesive with increased shear strength without the disadvantage of decreased loop tack.
BACKGROUND OF THE INVENTION
An important characteristic of pressure sensitive adhesives is the shear strength, also known as the cohesive strength or the internal strength of the adhesive, which measures the tendency of a pressure sensitive adhesive to flow or creep under an applied load. Especially in rolled paper applications where the pressure sensitive adhesives are used at temperatures considerably above their glass-transition temperatures, the shear strength must be greater than the adhesiveness to prevent the adhesive from creeping beyond the edges of the paper and bleeding through the paper which results in sheets of paper sticking together.
A crosslinked pressure sensitive adhesive sample or one with a high degree of entanglement (higher viscosity) would be expected to have a higher resistance to shear than a pressure sensitive adhesive sample with few entanglements (low viscosity). For these reasons, silane monomers have been used to provide crosslinking in polymer compositions. U.S. Pat. No. 3,729,438 describes an aqueous dispersion of a crosslinkable latex polymer prepared from vinyl acetate and a vinyl hydrolyzable silane. Upon removal of water from the latex polymer, the polymer further polymerizes or cures into a crosslinked polymer.
U.S. Pat. No. 3,706,697 describes an emulsion polymerization product of an acryloxyalkylsilane monomer with an alkyl acrylic ester wherein the silane is introduced to the polymerization after a portion of the other monomers are polymerized. International Patent Application WO 97/15624 describes curable aqueous compositions prepared with an emulsifiable polymer having pendant and/or terminal silyl ester groups and pendant terminal acidic groups neutralized with fugitive bases.
The disadvantages of using such alkoxy silane monomers, however, are that (1) the alkoxy silane monomers may crosslink during the emulsion polymerization and form grit; (2) the alkoxy silane monomers may hydrolyze during the emulsion polymerization so that even if the alkoxy silane monomer was polymerized the reactivity necessary for subsequent cross-linking would be destroyed; or (3) the alkoxy silane modified latex may hydrolyze during storage.
There continues to be a need for pressure sensitive adhesive compositions which provide the necessary shear strength at high temperature and high relative humidity environments, and provide increased shelf life without the disadvantages present in prior art pressure sensitive adhesives.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a pressure sensitive adhesive composition.
It is also an object of the invention to provide a pressure sensitive adhesive which bonds to a substrate and maintains sufficient shear strength.
It is another object of the invention to provide a pressure sensitive adhesive which may be applied to paper without bleeding through the paper or exhibiting excessive creep beyond the edges of the paper.
With regard to the foregoing and other objects, the present invention provides a pressure sensitive adhesive composition comprising the emulsion polymerization product of at least one ethylenically unsaturated monomer and from 0.01 to 2 parts per hundred monomer (pphm), preferably from about 0.05 to about 1 pphm of a sterically hindered alkoxylated silane monomer, wherein the sterically hindered alkoxylated silane monomer has the structure
R
1
—Si—(OR
2
)
n
(R
3
)
3−n
wherein R
1
is selected from the group consisting of an alkylene, arylene, and aralkylene group; R
2
is independently a sterically hindered alkyl group having 3 to 10 carbon atoms in a straight or branched chain configuration; R
3
is a monovalent alkyl group having 1 to 10 carbon atoms; and n is an integer of from 1 to 3.
According to another aspect the invention provides a method of making a pressure sensitive adhesive, said method comprising (I) mixing an acidic metal salt curing agent with an aqueous emulsion polymer to form an adhesive, said polymer comprising the reaction product of at least one ethylenically unsaturated monomer and from 0.01 to 2 pphm of a sterically hindered alkoxylated silane monomer as described above; (II) applying the adhesive formed in Step (I) to a substrate; and (III) curing the adhesive with air or radio frequency to form a film.
An additional aspect of the invention provides a tape comprising a backing coated with the pressure sensitive adhesive composition as described above.
During film formation of the pressure sensitive adhesive of the invention, the sterically hindered alkoxysilane functionality of the silanes crosslink resulting in pressure sensitive adhesives which exhibit a significant increase in shear strength without the expected decrease in loop tack as compared to pressure sensitive adhesives prepared without sterically hindered alkoxylated silanes. This increase in shear strength allows pressure sensitive adhesives of the invention to be applied to paper under high relative humidity conditions (80% to 90%) wherein the paper is rolled without the adhesive creeping out or bleeding though the paper. Moreover, the presence of the sterically hindered alkoxy groups in the polymers minimizes crosslinking during emulsion polymerization and storage of the pressure sensitive adhesive.
DESCRIPTION OF THE INVENTION
The pressure sensitive adhesive compositions of the present invention are prepared from an aqueous emulsion polymer which is curable to form a film. The polymer is the reaction product of at least one ethylenically unsaturated monomer and a sterically hindered alkoxylated silane monomer. The sterically hindered alkoxylated silane is incorporated in the backbone of the polymer. The steric hindrance of the alkoxylated silane minimizes hydrolysis of the alkoxylated group during polymerization and storage. The aqueous emulsion polymer has acidic groups which are neutralized with a base. The release of the base upon application of the polymer frees the acid of the ethylenically unsaturated monomer for catalyzing the cure of the polymer.
While not wishing to be bound by any particular theory, the present inventors believe that crosslinking occurs between the sterically hindered alkoxysilane functionality on the polymer by means of a hydrolysis reaction to give silanols with subsequent condensation reaction between silanols and/or carboxyl groups on the polymer. Such crosslinking occurs during film formation of the pressure sensitive adhesive. The advantage of preparing the pressure sensitive adhesive with sterically hindered alkoxylated silane monomers is that crosslinking during the emulsion polymerization of the polymer and storage of the pressure sensitive adhesive, especially in the presence of carboxyl groups, is minimized.
The sterically hindered alkoxylated silane monomer is present in a critical amount of from 0.01 to 2 pphm, preferably 0.05 to 1 pphm. The sterically hindered alkoxylated silane monomer has the structure:
R
1
—Si—(OR
2
)
n
(R
3
)
3−n
wherein R
1
is selected from the group consisting of an alkylene, arylene, and aralkylene group; R
2
is independently a sterically hindered alkyl group having 3 to 10 carbon atoms in a straight or branched chain configuration; R
3
is a monovalent alkyl group having 1 to 10 carbon atoms; and n is an integer of from 1 to 3.
Suitable sterically hindered alkoxylated silane monomers for use in the invention include vinyltriisopropoxy silane, vinylpropyltriisopropoxy silane, vinylpropyltriisobutoxy silane, vinyltriisobutoxy silane, vinylpentyltri-t-butoxy silane, vinylbutyldimethyloctyl silane, vinylpropylmethyldipentoxy silane, vinylpropyltri-sec-butoxysilane. The sterically hindered alkoxylated silane monomer is
Baird Arnold S.
Boutillier Douglas N.
Farwaha Rajeev
Hayes Patrick J.
Phan Lien
Dawson Robert
National Starch and Chemical Investment Holding Corporation
Robertson Jeffrey B.
Roland, Esq. Thomas F.
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