Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-07-27
2004-04-06
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...
C526S210000, C526S318400, C526S348000, C524S503000, C524S459000, C525S243000
Reexamination Certificate
active
06716941
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a high solids ethylene-vinyl acetate dispersion and a process for producing such a dispersion by batch polymerization of a monomer mixture having vinyl acetate and ethylene, in the presence of a stabilizing system of polyvinyl alcohol without additional surfactants. The dispersion has a solids level of greater than 65 percent by weight, and a viscosity of less than 5000 mPa·s when measured at 65 percent solids at 25° C. The dispersion can be dried to form a redispersible powder. Both the dispersion and powder are useful in adhesive, coating and cementitious formulations.
BACKGROUND OF THE INVENTION
Ethylene-vinyl acetate dispersions, and powders produced by drying these dispersions, are widely used in adhesive, coating and cementitious formulations. High solids dispersions are of special interest due to favorable economics through increasing the reactor efficiency. High solid dispersions that will be spray dried benefit from having less water to remove, increasing the efficiency of the spray dryer in both throughput and energy savings. A high solids dispersion must have a low enough viscosity for practical use.
U.S. Pat. No. 4,921,898; 5,070,134; 5,629,370; 5,936.020; 5,939,505; and 6,001,916 all disclose ethylene-vinyl acetate dispersions having a solids level of greater than 65 percent. These dispersions are produced in a batch reactor, and using a stabilizer system of polyvinyl alcohol and a surfactant. The presence of a surfactant in the dispersion negatively affects water-resistance, spray-drying, and redispersibility of polymer powders.
EP 1067147 discloses a continuous process for the production of a high solids ethylene-vinyl acetate dispersion using low molecular weight polyvinyl alcohol as the emulsifying agent without the use of a surfactant, producing an dispersion having greater than 65 percent solids and a viscosity of 1000 to 3000 cps at 25° C. The application cites that conventional batch processes have not been adapted to make high solids vinyl acetate/ethylene dispersions without a surfactant.
Surprisingly it has been found that an ethylene-vinyl acetate dispersion having a solids level of greater than 65 percent by weight, and a viscosity at 65 percent by weight solids of less than 5000 mPa·s., can be produced in a batch process using polyvinyl alcohol as the stabilizer, without additional surfactants.
SUMMARY OF THE INVENTION
The present invention is directed to a polymer composition comprising an ethylene-vinyl acetate polymer dispersion stabilized with polyvinyl alcohol, wherein said dispersion has a solids level of greater than 65 percent by weight, and wherein said dispersion has a bimodal particle size distribution.
The invention is also directed to a process for forming an ethylene-vinyl acetate polymer dispersion comprising polymerizing a monomer mixture comprising vinyl acetate and ethylene in a batch process, in the presence of a stabilizing system consisting of polyvinyl alcohol, to form an ethylene-vinyl acetate polymer dispersion, wherein said copolymer dispersion has a solids level of greater than 65 percent by weight, and a viscosity of less than 5000 mPa·s when measured at 65 percent solids at 25° C.
The invention is further directed to process for producing a redispersible polymer powder comprising polymerizing a monomer mixture comprising vinyl acetate and ethylene in a batch process, in the presence of a stabilizing system consisting of polyvinyl alcohol, to form an ethylene-vinyl acetate polymer dispersion; and drying said polymer dispersion to form a redispersible polymer powder, wherein said copolymer dispersion has a solids level of greater than 65 percent by weight, and a viscosity of less than 5000 mPa·s.
The invention is also directed to the use of the high solids dispersion and redispersible powder in adhesives, coatings and cementitious formulations.
DETAILED DESCRIPTION OF THE INVENTION
The dispersion polymer of the present invention is formed in a batch free radical polymerization of vinyl acetate, ethylene, and optionally other comonomers including functional monomers, in the presence of a polyvinyl alcohol stabilizing system. Water forms the continuous phase, with polymer particles forming the dispersed phase.
Vinyl acetate and ethylene monomers are polymerized to form the dispersion polymer. In general, the final polymer contains from 75 to 99 percent by weight of vinyl acetate, and from 1 to 25 percent by weight of ethene. Preferably the level of vinyl acetate is from 85 to 95 percent by weight and the level of ethylene incorporated is from 5 to 15 percent by weight.
In addition to vinyl acetate and ethylene, one or more other ethylenically unsaturated monomers may also be present in the monomer mixture at up to 15 percent by weight, preferably from 5 to 10 percent by weight of the total polymer solids. Examples of said comonomers include, but are not limited to, comonomers conventionally used in compositions with ethylene and vinyl esters such as acrylates and maleates, e.g. butyl acrylate, and 2-ethylhexyl acrylate. Functional monomers may also be included at up to 10 percent by weight, and preferably from 1 to 5 percent by weight. Examples of suitable functional monomers are carboxylic acids, such as acrylic, methacrylic and maleic acid as well as hydroxyl and amide functional monomers, e.g. hydroxyethylacrylate, hydroxypropylacrylate, acrylamide, N-vinyl formamide, N-vinyl acetamide and the like. Crosslinking monomers can also be present, such as N-methylol acrylamide, and the n-alkyl esters thereof.
Additionally, certain copolymerizable monomers that assist in the stability of the copolymer dispersion, e.g., vinyl sulfonic acid and 2-acrylamido-2-methylpropane sulfonic acid or their salts may be used herein as latex stabilizers. If present, these stabilizers are added in amounts of from about 0.2 to 1 percent by weight of the monomer mixture.
The initiator is any free radical initiator, or initiator system known in the art. Suitable as polymerization initiators are the water-soluble free-radical-formers generally used in emulsion polymerization, such as hydrogen peroxide, sodium persulfate, potassium persulfate and ammonium persulfate, as well as t-butyl hydroperoxide, in amounts of between 0.01 and 3 percent by weight, preferably 0.1 and 1 percent by weight based on the total amount of the polymer dispersion. They can be used alone or together with reducing agents such as sodium formaldehyde-sulfoxylate, iron-II-salts, sodium dithionite, sodium hydrogen sulfite, sodium sulfite, sodium thiosulfate, ascorbic acid, erythorbic acid as redox catalysts in amounts of 0.01 to 3 percent by weight, preferably 0.1 to 1 percent by weight, based on the total amount of the polymer dispersion. The free-radical-formers can be charged in the aqueous emulsifier solution or be added during the polymerization in doses. Oil soluble initiators such as t-butyl hydrogen peroxide are preferred.
The stabilizing system of the present invention is low molecular weight polyvinyl alcohol. The polyvinyl alcohol is preferably partially hydrolyzed polyvinyl acetate and is used in amounts of 1 to 15 percent by weight, preferably 4 to 10 percent by weight, based on the weight of the polymer solids. Generally, the degree of hydrolysis will vary from 50 to 99 percent, preferably from 80 to 99 percent of the acetate groups. The polyvinyl alcohol should also exhibit a viscosity of about 2 to 45 mPa·s., preferably 3 to 30 mPa·s, and most preferably 3 to 10 mPa·s for a 4 weight percent aqueous solution at 20° C. as determined by the Hoeppler falling ball method. Exemplary of the polyvinyl alcohol component include AIRVOL A205, a low molecular weight, 87 to 89 percent hydrolyzed polyvinyl acetate; and AIRVOL A203, a low molecular weight, 87 to 89 percent hydrolyzed polyvinyl acetate, which are marketed by Air Products Corporation. Blends of various polyvinyl alcohols may also be used in order to increase the water resistance of the resultant powder and can include 92 to 99 percent hydrolyzed polyvinylalcohol, s
Kerr John C. H.
McLennan Alistair J.
Mitchell Robert N.
Pattenden Caroline S.
Almer Charles W.
National Starch and Chemical Investment Holding Corporation
Roland Thomas F.
Sastri Satya B
Wu David W.
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