Vinyl chloride polymers processing

Raw Materials. PVC is inherently a hard and brittle material and very sensitive to heat it thus must be modified with a variety of plasticizers, stabilizers, and other processing aids to form heat-stable flexible or semiflexible products or with lesser amounts of these processing aids for the manufacture of rigid products (see Vinyl polymers, vinyl chloride polymers). Plasticizer levels used to produce the desired softness and flexibihty in a finished product vary between 25 parts per hundred (pph) parts of PVC for flooring products to about 80—100 pph for apparel products (245). Numerous plasticizers (qv) are commercially available for PVC, although dioctyl phthalate (DOP) is by far the most widely used in industrial appHcations due to its excellent properties and low cost. For example, phosphates provide improved flame resistance, adipate esters enhance low temperature flexibihty, polymeric plasticizers such as glycol adipates and azelates improve the migration resistance, and phthalate esters provide compatibiUty and flexibihty (245). 

X-ray studies indicate that the vinyl chloride polymer as normally prepared in commercial processes is substantially amorphous although some small amount of crystallinity (about 5% as measured by X-ray diffraction methods) is present. It has been reported by Fuller d in 1940 and Natta and Carradini in 1956 that examination of the crystalline zones indicates a repeat distance of 5.1 A which is consistent with a syndiotactic (i.e. alternating) structure. Later studies using NMR techniques indicate that conventional PVC is about 55% syndiotactic and the rest largely atactic in structure. 

Consideration of the methods of processing vinyl chloride polymers is most conveniently made under the following divisions ... 

Originally, vinyl chloride polymers were based on acetylene. The switch to ethylene,chemistry came after the development of the oxychlorination process for vinyl chloride described in Chapter 9. Today very little acetylene-, based vinyl chloride monomer (VCM) processing remains. 

Carter, E.V. (1988) A new synthesis process for the manufacture of lamellar iron oxides for pigment use in anti corrosive coatings. J. Oil and Colour Chem. Assoc. 5 132-133 Carty, P. White, S. (1999) Flammability studies on plasticised chlorinated poly(vinyl) chloride. Polymer Degradation Stability 63 455-463... 

Raw Materials. PVC is inherently a hard and brittle material and very sensitive to heat it thus must be modified with a variety of plasticizers, stabilizers, and other processing aids to form heat-stable flexible or semillexible products or with lesser amounts of these processing aids fur the manufacture of rigid products. See also Vinyl Chloride Polymers. 

Alternately, it may be argued that the suppression of the 3 relaxation of PPO by PS as indicated by dynamic mechanical studies (3,4) raises the stress level required to activate significant strain softening. The importance of the 3 relaxation in controlling stress-activated processes has been revealed in the study of several poly(vinyl chloride) polymer blends (15,16). 

PE CE n. Post-chlorinated vinyl chloride polymer. The post-chlorination process increases chlorine content form 57 to 64%. The resulting polymer is soluble in... 

Poly(vinyl chloride). Poly(vinyl chloride) can be molded in liquid or powdered form. The liquid plastisols are fluid suspensions of fine particle-sized resins in a plasticizing liquid. PVC compounds are easily processed. They can be formulated to produce articles ranging from flexible to semirigid, with durometer hardness of 60 Shore A to 65 Shore D (see Vinyl Chloride Polymers). 

Use of seed polymers introduces a new degree of flexibility into the kind of product that may be produced. Within wide limitations, the seed polymer can be a latex based on any monomer—not only one based on vinyl chloride. For example, the seed may be a hexyl acrylate-based polymer or copolymer that may confer internal, permanent plasticization to the vinyl chloride polymer ultimately associated with it. Furthermore, as in many of these processes, the monomer added to the seed may consist of a mixture of several monomers to yield a large variety of copolymers that have significantly different properties from copolymers prepared without the use of a seed (co)polymer. Whether the products of such procedures are graft copolymers, intertwined chains within the latex particle, mixtures of latex particles of different chemical composition, or combinations of these probably varies with each system. Investigation of the fine structure of such latex systems is difficult. Therefore the technique itself is widely used. The physical properties of the system are related to the operations involved in the preparation rather than with the overall composition and conformation of the polymer chains. 

The principal effects of introducing vinyl acetate into a vinyl chloride polymer are to increase solubility and to improve moulding characteristics by lowering the temperature at which the material can be manipulated and by increasing flow. Thus vinyl chloride-vinyl acetate copolymers are used for non-flexible mouldings for which the processing characteristics of the homo-... 

Once the principal route to vinyl chloride, in all but a few percent of current U.S. capacity this has been replaced by dehydrochlorination of ethylene dichloride. A combined process in which hydrogen chloride cracked from ethylene dichloride was added to acetylene was advantageous but it is rarely used because processes to oxidize hydrogen chloride to chlorine with air or oxygen are cheaper (7) (see Vinyl polymers). 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

AUoys of ceUulose with up to 50% of synthetic polymers (polyethylene, poly(vinyl chloride), polystyrene, polytetrafluoroethylene) have also been made, but have never found commercial appUcations. In fact, any material that can survive the chemistry of the viscose process and can be obtained in particle sizes of less than 5 p.m can be aUoyed with viscose. 

Poly(vinyhdene chloride) (PVDC) film has exceUent barrier properties, among the best of the common films (see Barrier polymers). It is formulated and processed into a flexible film with cling and tacky properties that make it a useful wrap for leftovers and other household uses. As a component in coatings or laminates it provides barrier properties to other film stmctures. The vinyUdene chloride is copolymerized with vinyl chloride, alkyl acrylates, and acrylonitrile to get the optimum processibUity and end use properties (see Vinylidene chloride monomer and polymers). 

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