DETAILED DESCRIPTION The monomers forming the polymeric core of the heteropolymer are selected to provide a glass transition temperature (T.
sub.
g) in the core of -30.
degree.
C.
or lower.
It is believed that the resulting rubbery character of the core in cooperation with the crosslinking character of core and shell results in the cracking resistance and flexibility at room temperature and low temperature as well as the impact resistance provided by the finished polymer.
The core polymer is formed by emulsion polymerization of a first monomer composition comprising a major amount of a principal monomer sytem and a minor amount of a crosslinking monomer system.
The principal monomer system typically contains a C.
sub.
1 -C.
sub.
8 alkyl acrylate wherein the alkyl group is straight or branched chain, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, and mixtures thereof.
Part of the alkyl acrylate, up to a maximum of about 20% by weight, may be replaced with a non-crosslinking (with respect to the alkyl acrylate) monoethylenically unsaturated monomer having alpha, beta-unsaturation.
Examples of such other monomers are vinylidene chloride, vinyl chloride, acrylonitrile, vinyl esters such as vinyl formate, vinyl acetate and vinyl propionate, and mixtures of ethylene and such vinyl esters, alkyl methacrylic esters, acrylic and methacrylic acid esters of alcohol-ethers such as diethylene glycol monoethyl or monobutyl ether, styrene, ring-alkyl styrenes such as ortho-, meta and paramethyl styrenes, alpha-alkyl styrenes such as alpha-methyl styrene and the like, mixtures of ethylene with other alpha olefins such as propylene, butylene, pentene and the like, and combinations of ethylene with vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, vinyl 2-methoxylethyl ether, vinyl 2-chloroethyl ether and the like.
Of the foregoing monomers, butyl acrylate is particularly preferred along or in combination with minor amounts of methyl methacrylate, of the order of about 70-95% by weight of butyl acrylate and about 1-15% by weight of methyl methacrylate, based on the total weight of the monomers in the monomer composition forming the core polymer
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Liquid Purification 
