Ethylene and Propylene Elastomers

The commercial ethylene-propylene rubbers typically range in propylene content from 30 to 60%, depending upon intended use. Such copolymers are prepared with Ziegler–Natta type catalysts. Soluble catalysts and true solution processes are preferred. The common catalyst systems are based on VCl₄, VOCl₃, V(Acac)₃, VO(OR)₃, VOCl (OR)2, VOCl₂(OR), etc. with various organoaluminium derivatives. The products are predominantly amorphous. Polymerization reactions are usually carried out at 40C in solvents like chlorobenzene or pentane. The resultant random copolymers are recovered by alcohol precipitation. Because these elastomers are almost completely saturated, cross-linking is difficult. A third monomer, a diene is, therefore, included in the preparation of these rubbers that carry the trade names EPTR or EPDM. Inclusion of third monomers presents some problems in copolymerization reactions. For instance, it is important to maintain constant feed mixtures of monomers to obtain constant compositions. Yet, two of the three monomers are gaseous and the third one is a liquid. Natta [86] developed a technique that depends upon maintaining violent agitation of the solvent while gaseous monomers were bubbled through the liquid phase. This was referred to as “semi-flow technique.” The process allows the compositions of gaseous and liquid phases to be in equilibrium with each other and to be more or less constant [87]. Other techniques evolved since. All are designed to maintain constant polymerization mixtures. The vanadium-based catalyst systems deteriorate with time and decrease in the number of catalytic centers as the polymerizations progress. The rate of decay is affected by conditions used for catalyst preparation, compositions of the catalysts, temperature, solvents, and Lewis bases. It is also affected by the type and concentration of the third monomer [88–90]. Additions of chlorinated compounds to the deactivated catalysts, however, help restore activity [91, 92]. Catalyst decay can also be overcome by continually feeding catalyst components into the polymerization medium [93]. While third monomer can be a common diene, like isoprene, more often it is a bridged ring structure with at least one double bond in the ring. In typical terpolymer rubbers with 60–40 ratios of ethylene to propylene the diene components usually comprise about 3% of the total. Some specialty rubbers, however, may contain 10% of the diene or even more. Reaction conditions are always chosen to obtain 1,2 placement of the diene. Dienes in common use are ethylidine norbornene, methylene norbornene, 1,4-hexadiene, dicyclopentadiene, and cyclooctadiene:

In addition to the above, the patent literature describes many other dienes. An idealized picture of a segment of an uncross-linked gum stock might be shown as having the following structure:

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متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

رائحة كريهة في الجسم.. "علامة خفية" أخطر مما تتصور

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مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

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المزيد

مواضيع ذات صلة

Synthetic Polyisoprenes

High Molecular Weight Polybutadiene

Liquid Polybutadiene

Polybutadiene

Ionomers

Copolymers of Ethylene with a-Olefins and Ethylene with Carbon Monoxide

Poly (4-methyl pentene-1)

Polyisobutylene

Syndiotactic Polypropylene

Manufacturing Techniques

Polypropylene

Materials Similar to Polyethylene