Vinylidene chloride copolymer latice

Vinylidene Chloride Copolymer Latex. Vinyhdene chloride polymers are often made in emulsion, but usuaUy are isolated, dried, and used as conventional resins. Stable latices have been prepared and can be used direcdy for coatings (171—176). The principal apphcations for these materials are as barrier coatings on paper products and, more recently, on plastic films. The heat-seal characteristics of VDC copolymer coatings are equaUy valuable in many apphcations. They are also used as binders for paints and nonwoven fabrics (177). The use of special VDC copolymer latices for barrier laminating adhesives is growing, and the use of vinyhdene chloride copolymers in flame-resistant carpet backing is weU known (178—181). VDC latices can also be used to coat poly(ethylene terephthalate) (PET) bottles to retain carbon dioxide (182). 

Vinylidene chloride copolymers are available as resins for extrusion, latices for coating, and resins for solvent coating. Comonomer levels range from 5 to 20 wt %. Common comonomers are vinyl chloride, acrylonitrile, and alkyl acrylates. The permeability of the polymer is a function of type and amount of comonomer. As the comonomer fraction of these semicrystalline copolymers is increased, the melting temperature decreases and the permeability increases. The permeability of vinylidene chloride homopolymer has not been measured... 

Many synthetic latices exist (16-18). They contain butadiene and styrene copolymers (elastomeric), styrene-butadiene copolymers (resinous), butadiene and acrylonitrile copolymers, butadiene with styrene and acrylonitrile, chloro-prene copolymers, methacrylate and acrylate ester copolymers, vinyl acetate copolymers, vinyl and vinylidene chloride copolymers, ethylene copolymers, fluori-nated copolymers, acrylamide copolymers, styrene-acrolein copolymers, and pyrrole and pyrrole copolymers. Many of these latices also have carboxylated versions. 

Vinylidene chloride copolymers are prepared by suspension and emulsion processes. Suspension processes are generally preferred for the production of melt-processable polymer whilst latices are used directly for barrier coatings. The procedures used are essentially as described for the preparation of poly(vinyl chloride) (section 4.2.3). The polymerization rate for vinylidene chloride/vinyl chloride mixtures is markedly less than for either monomer alone so somewhat higher temperatures and initiator levels are used together with longer reaction times. Molecular weights are generally relatively low (M 20000-50000). 

Poly(vinylidene chloride) latices can be easily prepared by the same methods but have few uses because they do not form films. Copolymers of high VDC content are film-forming when freshly prepared but soon crystallize and lose this desirable characteristic. Because crystallinity in the final product is very often desirable, eg, in barrier coatings, a significant developmental problem has been to prevent crystallization in the latex during storage and to induce rapid... 

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