LATEX FOR PACKAGING APPLICATIONS

Abstract: A low surfactant content latex polymer suitable for use as a barrier layer is disclosed. The latex polymer comprises an aryl vinyl monomer, an aliphatic conjugated diene monomer, and an acrylamide-based monomer. The latex polymer has a moisture resistance of no greater than about 290 g/m<2>/day as measured by ASTM Test D-1653-93.

Description: LATEX FOR PACKAGING APPLICATIONS Field of the Invention The present invention relates to a latex polymer suitable for use as a paper coating. More particularly, the invention relates to a latex polymer having a low surfactant content and high moisture and oil barrier properties that are useful in coating applications. Background of the Invention Polymers formed from emulsion processes are employed in a variety of applications including films and coatings which are applied to cellulose substrates. In particular, the coatings are used in commercial products such as microwave oven boxes, popcorn bags, drinking cups, pet food bags, paper plates, and detergent boxes. These coatings are primarily designed to serve as a barrier to moisture and oil . The barrier facilitates the preservation of the contents of the paper package and is also required to prevent the package from leaking its contents to the outside. Also important to the design of these coatings is the rheological behavior of the emulsion latex. Typically, a coating will experience extremely high shear as it is applied to the cellulose substrate. This mandates that the coating be inherently stable. Typical emulsion polymerization processes employ dispersants, i.e., surfactants, to provide stability to the polymerization system. In particular, a primary factor governing the stability relates to the charge on the surface of dispersed particles in the emulsion. When conventional soaps are utilized as the dispersants, the surface charge occurs primarily from the absorbed soap molecules which are mobile and in dynamic equilibrium with other particles in the emulsion. The presence of the soaps, however, has been found to adversely effect latex polymer end properties such as moisture resistance. Accordingly, efforts have concentrated on lowering the surfactant content while maintaining the stability of the emulsion so as to obtain a more moisture resistant polymer. For example, U.S. Patent No. 3,882,070 to Ceska proposes a process for preparing latex which employs no emulsifier. In particular, the polymers formed are disclosed to have sulfate end-capped groups. In spite of these efforts, there remains a need in the art to provide a latex coating composition which possesses other end properties in addition to moisture resistance. In particular, such properties are desirable in that they facilitate the processing of the coated substrate. Some of the critical parameters are the surface tension, coating rheology, mechanical emulsion stability, tensile strength of the coating, and repulpability. As an example, the latex coating is often applied to plain paper using some coating application technique. Blade, rod, air knife and certain coaters are a few common application methods. Accordingly, it would be desirable that the coating possess appropriate rheology and emulsion stability to withstand tne forces which are exerted by processing equipment . Failure to achieve this results in coating surface defects {scratches, skips, whiskering) . Additionally, it would be desirable that the coating exhibit repulpability such that the paper product may be recycled subsequent to use . A typical repulping process will use mechanical action to destabilize the coating and the pulp. This mechanical agitation serves to separate cellulose fibers and coating. Many times this requirement is in direct opposition to the requirements for a stable, rheologically balanced coating. Therefore, it is an object of the present invention to provide a latex polymer which possesses other desirable end properties in addition to moisture resistance. Summary of the Invention To this end, the present invention provides a low surfactant content latex polymer which possesses desirable end properties, including barrier resistance to moisture and oil . The latex polymer comprises an aryl vinyl monomer, an aliphatic conjugated diene monomer, and an acrylamide-based monomer. The latex polymer has less than about 1.0 percent by weight surfactant and has a moisture resistance of no greater than 290 g/m 2 /day as measured by ASTM Test D-1653-93. In one embodiment, the acrylamide-based monomer may be selected from the group consisting of acrylamide, N-methyolacrylamide, N-methyolmeth- acrylamide, methacrylamide, N-isopropylacrylamide, N- tert-butylacrylamide, N-N' -methylene-bis-acrylamide, and blends and mixtures thereof. Preferably, the aryl vinyl monomer is styrene and the aliphatic conjugated diene monomer is 1,3- butadiene. The low surfactant content latex polymer may further comprise a non-aromatic unsaturated mono- or dicarboxylic ester monomer. The latex polymer may also further comprise an unsaturated mono- or dicarboxylic acid monomer. The present invention also provides a paper coating. Specifically, the paper coating comprises a barrier layer formed from a low surfactant latex polymer. The low surfactant latex polymer comprises an aryl vinyl monomer, an aliphatic conjugated diene monomer, and an acrylamide-based monomer. The latex polymer has less than 1.0 percent by weight surfactant and has a moisture resistance of no greater than 290 g/m 2 /day as measured by ASTM Test D-1653. Detailed Description of the Preferred Embodiment As summarized above, the present invention is directed to a low surfactant latex polymer which exhibits desirable end properties such as moisture resistance. Specifically, the polymer includes an aryl vinyl monomer, an aliphatic conjugated diene monomer, and an acrylamide-based monomer. Suitable aryl vinyl monomers which may be employed include, for example, styrene and styrene derivatives such as alpha-methyl styrene, p-methyl styrene, vinyl toluene, ethylstyrene, tert-butyl styrene, monochlorostyrene, dichlorostyrene, vinyl benzyl chloride, fluorostyren , alkoxystyrenes (e.g., paramethoxystyrene) , and the like. Blends and mixtures of the above may also be used. The aryl vinyl monomer may be used in an amount, based on the total weight of the starting monomers, from about 10 to 90 percent by weight, preferably from about 50 to 70 percent by weight, and most preferably from about 60 to 65 percent by weight. A particularly preferred aryl vinyl monomer is styrene. Suitable aliphatic conjugated dienes are C 4 to C 9 dienes and include, for example, butadiene monomers such as 1,3-butadiene, 2-methyl-1, 3-butadiene, 2 chloro-1, 3-butadiene, and the like. Blends or copolymers of the diene monomers can also be used. The aliphatic conjugated diene may be used in an amount, based on the total weight of the starting monomers, from about 10 to 90 percent by weight, preferably from about 20 to 50 percent by weight, and most preferably from about 30 to 35 percent by weight. A particularly preferred aliphatic conjugated diene is 1, 3-butadiene. The acrylamide-based monomer which may be employed may include, for example, acrylamide, N- methyolmet acrylamide, N-methyolacrylamide, methacrylamide, N-isopropylacrylamide, N-tert- butylacryla ide, N-N' -methylene-bis-acrylamide, alkylated N-methylolacrylamides such as N- methoxymethylacrylamide and N-butoxymethylacrylamide, and blends and mixtures thereof. The acrylamide-based monomer may be used in an amount, based on the total weight of the starting monomers, from about 0.1 to 5 percent by weight, preferably from about 0.2 to 3 percent by weight, and most preferably from about 0.5 to 2 percent by weight . Suitable non-aromatic unsaturated monocarboxylic ester monomers may be added to the low surfactant content latex polymer and include, for example, acrylates and methacrylates. The acrylates and methacrylates may include functional groups such as amino groups, hydroxy groups, epoxy groups and the like. Exemplary acrylates and methacrylates include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, 2-ethylhexyl acrylate, glycidyl acrylate, glycidyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, isobutyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2- hydroxybutyl methacrylate, n-propyl methacrylate and the like. Exemplary amino-functional methacrylates include t-butylamino ethyl methacrylate and dimethylamino ethyl methacrylate. Suitable non- aromatic dicarboxylic ester monomers are alkyl and dialkyl fumarates, itaconates and maleates, with the alkyl group having one to eight carbons, with or without functional groups. Specific monomers include diethyl and dimethyl fumarates, itaconates and maleates. Other suitable non-aromatic dicarboxylic ester monomers include di (ethylene glycol) maleate, di (ethylene glycol) itaconate, bis (2-hydroxyethyl) maleate, 2-hydroxyethyl methyl fumarate, and the like. The non-aromatic unsaturated mono- or dicarboxylic ester monomer may be used in any appropriate amount for the purposes of the invention. Preferably, the monomer is used from about 50 to 70 percent based on the total weight of the starting monomers, and most preferably from about 60 to 65 percent by weight. A particularly preferred non- aromatic unsaturated monocarboxylic ester monomer is methyl methacrylate. Suitable monomers based on the half ester of the unsaturated dicarboxylic acid monomer may also be added to the latex polymer and include mono esters of maleic acid or fumaric acid having the formula ROOC- CH=CH-COOH wherein R is a C 1 to C 12 alkyl group, for example monomethyl maleate, monobutyl maleate and monooctyl maleate. Half esters of itaconic acid having C j to C 12 alkyl groups such as monomethyl itaconate can also be used. Blends or copolymers of the unsaturated mono- or dicarboxylic acid monomers and of the half ester of the unsaturated dicarboxylic acid can also be used. The unsaturated mono- or dicarboxylic acid or monomer based on the half ester of the unsaturated dicarboxylic acid is used in an amount, based on the total weight of the starting monomers, from about 0 to 5 percent by weight, and more preferably from about 1 to 2 percent by weight. Unsaturated mono- or dicarboxylic acid monomers and derivatives thereof may also be employed in the latex polymer and include components such as acrylic acid, methacrylic acid, itaconic acid, fumaric acid, and maleic acid, and the like. The unsaturated mono- or dicarboxylic acid monomers and derivatives thereof may be used in an amount, based on the total weight of the starting monomers, from about 0 to 5 percent by weight, and more preferably from about 1 to 2 percent by weight. Additional comonomers can be added to the latex polymer. Included among such additional comonomers are monoethylenically unsaturated substituted aliphatic hydrocarbons such as vinyl chloride, and vinylidene chloride; aliphatic vinyl esters such as vinyl formate, vinyl propionate and vinyl butyrate. Acrylonitrile may also be employed. In accordance with the invention, conventional surfactants can also be employed in an amount such that the resulting latex polymer has a low surfactant content. The term "low surfactant content" refers to the polymer having less than 1.0 weight percent, more preferably less than 0.5 weight percent, and most preferably less than 0.2 weight percent of surfactant. Any of the anionic or nonionic surfactants may be employed for the purposes of the invention. Polymerizable surfactants that can be incorporated into the polymer also can be used. Nonionic surfactants can include suitable alkyl esters, alkyl phenyl ethers, and alkyl ethers of polyethylene glycol . Exemplary nonionic surfactants are selected from the family of alkylphenoxypoly(ethyleneoxy) ethanols where the alkyl group typically varies from C 7 -C lβ and the ethylene oxide units vary from 4-100 moles. Various preferred surfactants in this class include the ethoxylated octyl and nonyl phenols, and in particular ethoxylated nonyl phenols with a hydrophobic/lipophilic balance (HLB) of 15-19. Anionic surfactants are preferred for the purposes of the invention and can be selected, for example, from the broad class of sulfonates, sulfates, ethersulfates, sulfosuccinates, diphenyloxide disulfonates, and the like, and are readily apparent to anyone skilled in the art. More particularly, the anionic surfactants can include a salt of an alcohol 477 PC17US96/18737 - 8 - sulfate (e.g., sodium lauryl sulfate) ; a salt of an alkylbenzenesulfonic acid (e.g., sodium dodecylbenzenesulfonate) ; and a sulfonic acid salt of an aliphatic carboxylic acid ester (e.g., sodium dioctylsulfosuccinate) . A preferred anionic surfactant is sodium dodecyl benzene sulfonate. The polymer can include additives to improve its various physical and mechanical properties, the selection of which is readily apparent to one skilled in the art. For example, crosslinking agents can be included such as vinylic compounds (e.g., divinyl benzene) ; allyllic compounds (e.g., allyl methacrylate, diallyl maleate) ; and multifunctional acrylates (e.g., di, tri and tetra (meth) acrylates) . The crosslinking agents can be included in amounts of up to about 7 percent by weight, and preferably about 0.05 to 5 percent by weight. Additional monomers can be included to improve specific properties such as solvent resistance (e.g., nitrile-containing monomers such as acrylonitrile and methacrylonitrile) and adhesion and strength (e.g., use of acrylamide or methacrylamide) . An initiator which facilitates polymerization can be employed and may include, for example, materials such as persulfates, organic peroxides, peresters, and azo compounds such as azobis (isobutyronitrile) (AIBN) . Commonly-used initiators include, for example, cumene hydroperoxide, dicumylperoxide, diisopropylbenzene hydroperoxide, and tert butyl hydroperoxide. Preferred initiators are pursulfate initiators such as ammonium persulfate, potassium persulfate, and the like. For the purposes of the invention, ammonium persulfate is a preferred initiator. Preferably, the amount of initiator ranges from about 0.1 percent to about 1 percent based on the weight of the total monomer. More preferably, the initiator ranges from 0.2 percent to about 0.4 percent. Reductants may be employed in the polymerization, and are typically employed in combination with the initiator as part of a redox system. Suitable reductants include sodium bisulfite, erythorbic acid, ascorbic acid, sodium thiosulfate, sodium formaldehyde sulfoxylate (SFS) , and the like. Other additives which may be used include other natural and synthetic binders, fixing agents, wetting agents, plasticizers (e.g., diisodecyl phthalate) , softeners, foam-inhibiting agents, froth aids, other crosslinking agents (e.g., melamine formaldehyde resin) , flame retardants, dispersing agents (e.g., tetrasodium pyrophosphate) , pH adjusting components (e.g., ammonium hydroxide) , sequestering or chelating agents (e.g., ethylene diaminetetraacetic acid (EDTA) ; wax additives; oxidizing agents; and other components. The selection of any of these additives is readily apparent to one skilled in the art. The low surfactant latex polymer of the present invention exhibits a high moisture resistance which makes it particularly suitable for use in a variety of applications described herein. Specifically, the latex polymer has a moisture resistance of at least 200 g/m 2 /day as measured by ASTM Test D-1653-93, the procedure of which is known to those skilled in the art . For the purposes of the invention, the publication describing the test is incorporated by reference in its entirety. (ASTM D-1653-93, "Standard Test Methods for Water Vapor Transmission of Organic Coating Films", Annual Book of ASTM Standards , (1994) , pp. 248-251.) Preferably, the latex polymer has a moisture resistance of no more than about 290 g/m 2 /day, more preferably of no more than about 100 g/m 2 /day, and most preferably, a moisture resistance of no more than about 6 g/m 2 /day. Additionally, the latex polymer may be repulpable, heat sealable, glueable, and contain a low odor level. The low surfactant latex polymer of the invention may be produced by any suitable emulsion processes, including batch and semi-continuous processes. The components which are utilized in the process are added according to known and accepted techniques. In accordance with the invention, the low surfactant content latex polymer is particularly suitable as a paper coating which may be employed, for example, in various packaging material applications. Packaging materials refers to all suitable materials which utilize polymeric coatings. Exemplary packaging materials include those utilized in food product applications such as microwave oven boxes, popcorn bags, drinking cups, and pet food bags; in paper products including, for example, copy paper ream wrap and paper plate coatings; and in secondary packages such as beer and soft drink cartons and detergent boxes. The methods for applying the latex polymer to the substrate may be carried out in accordance with any of the well known and suitable techniques. The present invention and its advantages over the prior art will be more fully appreciated from the illustrative examples which follow. It is to be understood that the examples are for the purpose of illustration and are not intended as being limiting upon the scope of the invention. A person skilled in the art will appreciate from these examples that this invention can be embodied in many different forms other than as specifically disclosed. Example Latex Composition A latex coating composition in accordance with the invention is prepared by first adding 1.56 lb of a 22% surfactant solution to a 148.5 lb water charge. 5.25 dry lb of seed latex is then added to the above solution. Subsequently, 1.75 lb of itaconic acid, 14 lb of water, and 6.16 oz . of ammonium persulfate is added. A mixture which contains 0.83 weight percent sulfole (chain transfer agent) , 34.6 weight percent butadiene, and 64.6 weight percent styrene is added to the above mixture at a rate of 37.5 lb/hr for one hour, and then 75.38 lb/hr for four additional hours. Separately and simultaneously, 59.36 lb of water, 7 lb of acrylic acid, 26.25 lb of acrylamide, and 2.63 lb of ammonium hydroxide are added during the first three hours of the above reaction. 2.45 lb of ammonium persulfate dissolved in 63 lb of water is added throughout the reaction at a rate of 11.8 lb/hr. The resulting latex composition exhibits high moisture resistance and is repulpable, heat sealable, glueable, and low odor containing. Comparative Example A latex coating according to U.S. Patent No. 3,882,070 to Ceska is prepared utilizing a N- methylolacrylamide monomer. Two latex coatings according to the present invention are prepared, one using an acrylamide monomer and another using a methacrylamide monomer. The water vapor transmission rate (WVTR) of the three latex coatings was measured in accordance with ASTM D-1653-93. The test results are described below: Monomer WVTR (g/m 2 /day) acrylamide 217.0 methacrylamide 170.5 N-methylolacrylamide 303.8 As seen, the latex coatings of the invention displayed superior moisture resistance to the coating disclosed by the Ceska '070 patent.

Claims: THAT WHICH IS CLAIMED IS: 1. A low surfactant content latex polymer suitable for use as a barrier layer comprising an aryl vinyl monomer, an aliphatic conjugated diene monomer, and an acrylamide-based monomer, said latex polymer having less than about 1.0 percent by weight surfactant and having a moisture resistance of no greater than about 290 g/m 2 /day as measured by ASTM Test D-1653-93. 2. A low surfactant content latex polymer according to Claim 1 wherein the acrylamide-based monomer is selected from the group consisting of acrylamide, N-methyolmethacrylamide, methacrylamide, N- isopropylacrylamide, N-tert-butylacrylamide, N-N' - methylene-bis-acrylamide, and blends and mixtures thereof. 3. A low surfactant content latex polymer according to Claim 1 wherein the aryl vinyl monomer is styrene and the aliphatic conjugated diene monomer is 1, 3-butadiene. 4. A low surfactant content latex polymer according to Claim 1 comprising from 10 to 90 weight percent of the aryl vinyl monomer. 5. A low surfactant content latex polymer according to Claim 1 comprising from 10 to 90 weight percent of the aliphatic conjugated diene monomer. 6. A low surfactant content latex polymer according to Claim 1 comprising from 0.1 to 5 percent by weight of the acrylamide-based monomer. 7. A low surfactant content latex polymer according to Claim 1 further comprising a non-aromatic unsaturated mono- or dicarboxylic ester monomer. 8. A low surfactant content latex polymer according to Claim 1 further comprising an unsaturated mono- or dicarboxylic acid monomer. 9. A low surfactant content latex polymer suitable for use as a moisture barrier comprising 10 to 90 percent by weight of an aryl vinyl monomer, 10 to 90 percent by weight of an aliphatic conjugated diene monomer, and 0.1 to 5 percent by weight of an acrylamide-based monomer, said latex polymer having less than about 1.0 percent by weight surfactant and a moisture resistance of no greater than about 290 g/m 2 /day as measured by ASTM Test D-1653-93. 10. A low surfactant content latex polymer according to Claim 9 wherein the acrylamide-based monomer is selected from the group consisting of acrylamide, N-methyolmethacrylamide, methacrylamide, N- isopropylacrylamide, N-tert-butylacrylamide, N-N' - methylene-bis-acrylamide, and blends and mixtures thereof. 11. A low surfactant content latex polymer according to Claim 9 wherein the aryl vinyl monomer is styrene and the aliphatic conjugated diene monomer is 1, 3-butadiene. 12. A low surfactant content latex polymer according to Claim 9 further comprising a non-aromatic unsaturated mono- or dicarboxylic ester monomer. 13. A low surfactant content latex polymer according to Claim 9 further comprising an unsaturated mono- or dicarboxylic acid monomer. 14. A paper coating comprising a barrier layer formed from a low surfactant content latex polymer, said low surfactant latex polymer comprising an aryl vinyl monomer, an aliphatic conjugated diene monomer, and an acrylamide-based monomer, said latex polymer having less than about 1.0 percent by weight surfactant and a moisture resistance of no greater than about 290 g/m 2 /day as measured by ASTM Test D-1653-93. 15. A paper coating according to Claim 14 wherein the acrylamide-based monomer is selected from the group consisting of acrylamide, N- methyolmethacrylamide, methacrylamide, N- isopropylacrylamide, N-tert-butylacrylamide, N-N' - methylene-bis-acrylamide, and blends and mixtures thereof. 16. A paper coating according to Claim 14 wherein the aryl vinyl monomer is styrene and the aliphatic conjugated diene monomer is 1, 3-butadiene . 17. A paper coating according to Claim 14 comprising from 10 to 90 weight percent of the aryl vinyl monomer. 18. A paper coating according to Claim 14 comprising from 10 to 90 weight percent of the aliphatic conjugated diene monomer. 19. A paper coating according to Claim 14 comprising from 0.1 to 5 percent by weight of the acrylamide-based monomer. 20. A paper coating according to Claim 14 wherein the low surfactant latex polymer further comprises a non-aromatic unsaturated mono- or dicarboxylic ester monomer. 21. A paper coating according to Claim 14 wherein the low surfactant latex polymer further comprises an unsaturated mono- or dicarboxylic acid monomer.

Inventors: KINTZ KARL ANDREW, MERKEL MICHAEL, WILLETT ALBERT VANN JR, KINTZ, KARL, ANDREW, MERKEL, MICHAEL, WILLETT, ALBERT, VANN, JR.
Applicants: KINTZ KARL ANDREW, MERKEL MICHAEL, REICHHOLD CHEMICALS INC, WILLETT ALBERT VANN JR, KINTZ, KARL, ANDREW, MERKEL, MICHAEL, REICHHOLD CHEMICALS, INC., WILLETT, ALBERT, VANN, JR.
Application year: 1996
Main classification: D21H19
IPC codes: C08F12, C08F212, C08F220, C08L25, C08L33, C08L47, D21H19, D21H27