Process for the production of general purpose PSA's

Details Process for the production of general purpose PSA's Process for the production of general purpose PSA's

1997-07-31

2001-10-23

Wu, David W. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S081000, C526S224000, C526S229000, C526S287000, C526S230000, C526S225000, C526S318430, C526S318440

Reexamination Certificate

active

06306982

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains to a process for reducing residual monomers in the formation of inherently tacky acrylic PSA emulsion copolymers containing a copolymerized amount of at least one vinyl ester and at least one diester of a dicarboxylic acid.
BACKGROUND OF THE INVENTION
Inherently tacky pressure sensitive adhesive (PSA) acrylic polymers formed by emulsion polymerization and which contains a polymerized amounts of at least one vinyl ester and at least one diester of a carboxylic acid, are described in U.S. Pat. Nos. 5,129,126, 5,164,444, 5,264,532 and 5,278,227 to Bernard each incorporated herein by reference. The adhesive products have enjoyed immense commercial success as a general purpose acrylic pressure sensitive adhesives.
The water borne pressure-sensitive adhesives exhibit excellent guillotinability and excellent adhesive performance at room temperature and after elevated temperature aging. Multifunctional monomers may be used to provide hydrogen bonding with functional groups such as carboxyl groups and enhance adhesion to polar substrates without affecting peel and tack. Shear performance may be enhanced by metal cross-linking. Metal cross-linking also creates a highly cohesive net work which enhances guillotine performance.
The inclusion of the vinyl esters, such as vinyl acetate and diesters of dicarboxylic acids, such as dioctyl maleate (DOM) create problems in production. Typical residual vinyl ester monomer content has been high, ranging from about 0.5 to about 1 percent by weight of total monomers fed to the reactor for a typical 3000 gallon stirred emulsion polymerization reactor. In addition, reaction rate is slow, requiring a slow feed lasting over a period of about 10 or more hours. It would be ecologically desirable to reduce residual monomer content of the emulsion polymerization reaction and, if possible, to economically reduce the amount of time required for monomer addition.
SUMMARY OF THE INVENTION
It is has now been found that the residual monomer content in commercial polymerization of inherently tacky acrylic emulsion polymers containing at least one vinyl ester and at least one diester of dicarboxylic acid, can be significantly reduced and rate of monomer addition significantly increased by conducting the reaction using a sequential polymerization process.
In the sequential polymerization process, there is fed a first charge of monomers constituting from 50 to about 95 percent, preferably 85 percent to about 90 percent, more preferably about 90 percent of the total monomers to form core polymer particles. Substantially all, and preferably all, of the vinyl esters and at least part, and preferably all, of the diester of dicarboxylic acids are included in the first charge of monomers to the reactor. This is followed by addition of the remaining monomers which are substantially free of vinyl esters and preferably free of diesters of dicarboxylic acids. By following this procedure, reaction time, for a 3000 gallon reactor, can be reduced to about 6 hours and residual monomer content reduced to about 0.1 percent or less and within 24 hours of adding a redox catalyst to levels of about 0.05 percent of total monomers fed to the reactor.
The inherently tacky, emulsion pressure-sensitive adhesive polymers of the instant invention comprise, on a polymerized basis and based on the total weight of the monomers forming the polymer, at least one soft monomer having a homopolymerization glass transition temperature (Tg) of less than 0° C. Preferably the soft monomer is an alkyl acrylate containing from about 4 to about 12 carbon atoms in the alkyl group, most preferably a mixture of 2-ethylhexyl acrylate and butyl acrylate. The total of the soft monomers are present in an amount of from about 35 to about 95 percent by weight of the monomers.
The polymer contains at least one hard monomer having a homopolymerization glass transition temperature (Tg) greater than about 0° C. and preferably further includes a soft diester monomer such as a maleate and/or fumarate.
More preferably, the inherently tacky emulsion pressure-sensitive adhesive polymers of the instant invention comprise, on a polymerized basis and based on the total weight of the polymer, at least one alkyl acrylate containing from about 4 to about 8 carbon atoms in the alkyl group, preferably a mixture 2-ethylhexyl acrylate and butylacrylate, said alkyl acrylates present in a total amount of from about 30 to about 65 percent by weight of monomers; at least one vinyl ester containing from 2 to about 16 carbon atoms in the alkyl chain of the acid, preferably vinyl acetate, said vinyl ester present in a total amount of from about 15 to about 35 percent by weight of the monomers; at least one diester of a dicarboxylic acid wherein each alkyl group of the diester independently contains from about 6 to about 12 carbon atoms, with dioctyl maleate or dioctyl fumarate being preferred, said diesters being present in a total amount of from about 20 to about 40 percent by weight; and at least one unsaturated carboxylic acid containing from 3 to 5 carbon atoms, preferably acrylic and/or methacrylic acid in an amount up to about 5 percent by weight, preferably about 1 to about 3 percent by weight of the monomers. The emulsion copolymer typically has a glass transition temperature of less than about −30° C. and a typical gel content of from about 50 to about 70 percent by weight of the copolymer particles.
The emulsion polymers may be formed in the presence about 0.1 to about 1 percent of a multifunctional monomer having an acrylate, methacrylate, cyanurate or phosphate functionality with tris(2-hydroxy ethyl)isocyanurate triacrylate and bis(methylacryloxyethyl)phosphate presently preferred. The use of the multifunctional monomers having cyanurate or phosphate moieties give a significant improvement in cohesive strength without much loss of peel and tack. The multifunctional monomers may be used with or without a chain transfer agent, and cross-linking agents, e.g. metal salts such as aluminum acetate.
The emulsion adhesive copolymers are optionally prepared using a reactive surfactant which polymerizes and becomes part of the emulsion polymer and which has been observed to enhance cohesive strength and aid in copolymerization of the monomers in forming the emulsion pressure-sensitive adhesive copolymers. The amount of reactive surfactant employed in the preparation of the emulsion pressure-sensitive adhesives of the present invention is in an amount of from 0 to about 0.4 percent by weight of the total monomers, typically from about 0.1 to about 0.25 percent by weight. The preferred reactive surfactants are anionic vinyl functional surfactants, such as sodium vinyl sulfonate, sodium styrene sulfonate and the like.
The aforementioned polymers are prepared in accordance with this invention by sequential polymerization. This amounts to emulsion polymerization of a first incremental charge of monomers and catalysts comprising the aforementioned monomers and typically all of the vinyl ester monomers to be copolymerized. This is followed by a second incremental monomer charge which is substantially free of vinyl esters and preferably substantially free of the diesters of dicarboxylic acid. The total amount of monomers contained in the second charge is sufficient to consume substantially all the vinyl ester and diesters of dicarboxylic acid monomers left after the first charge.
For a first charge of 90 percent followed by a second charge of 10 percent of total monomers, residual vinyl acetate monomer is expected to be about 0.1 to 0.3 percent of total monomers as compared to about 0.5 to 1 percent if all the monomers are fed incrementally over a continuous period of about 10 hours. The inclusion of the balance of the monomers in a second charge serves to reduce residual vinyl ester monomer content to below about 0.1 percent. It is preferred to add a redox catalyst system at conclusion of the polymerization reaction. This will consume additional residual monomers and reduce vinyl ester

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