Chlorinated polyethylene , crosslinking

Within the family of polyolefins there are many individual families that include low density polyethylenes, linear low density polyethylenes, very low polyethylenes, ultra low polyethylenes, high molecular weight polyethylenes, ultra high molecular weight polyethylenes, polyethylene terephthalates, ethylene-vinyl acetate polyethylenes, chlorinated polyethylenes, crosslinked polyethylenes, polypropylenes, polybutylenes, polyisobutylene, ionomers, polymethylpentene, thermoplastic polyolefin elastomers (polyolefin elastomers, TP), and many others. 

The level of chlorination in these materials varies, and influences the properties of the material in exactly the same manner as the closely related chlorinated polyethylene. The introduction of the chlorosulphonyl group in small amounts, <1.5%, gives greater choice in the methods used to crosslink the polymer. However, in general, the properties exhibited by these materials are equivalent to those of chlorinated polyethylene. 

The moderate random chlorination of polyethylene suppresses crystallinity and yields chlorinated polyethylene elastomer (CPE), a rubber-like material that can be crosslinked with organic peroxides. The chlorine (Cl) content is in the range of 36 to 42%, compared to 56.8% for PVC. Such elastomer has good heat and oil resistance. It is also used as a plasticizer for PVC. They provide a very wide range of properties from soft/elastomeric too hard. They have inherent oxygen and ozone resistance, resist plasticizers, volatility, weathering, and compared to PEs have improved resistance to chemical extraction. Products do not fog at high temperatures as do PVCs and can be made flame retardant. 

A second family of ionic elastomers based on the sulfonation of chlorinated polyethylene was also introduced in the early 1950 s by E. I. du Pont de Nemours Co., Inc. Curing these materials with various metal oxides gives rise to a combination of ionic and covalent crosslinks, and these elastomers are commercially available under the trade name Hypalon. 

Esperfoam . [Witco/Argus] Peroxide derivs. initiator for polymerization of ethylene, styrene, acrylates, curing of unsat. polyester resins crosslinking agent for chlorinated polyethylene. 

Zhang and Xu [1993] blended EVAc and CPE (chlorinated-polyethylene) with PE (Table 11.9), because crosslinked PE has low flexibility for use as cable insulation. They mixed 30-70% HDPE with 70-30% EVAc or CPE, along with 0.5-1.0% of the antioxidant 1010 and 0.5-1.0% of zinc stearate in a SK-160B double roller mixer at 130-140°C. The blends so made were either hot-pressed into sheets or extruded into tubes, and irradiated with 1.5-MeV electrons to a dose of 100 kGy. Some properties of the irradiated blends and PE are compared in Table 11.21. As the data in this Table show, the flexibility and the elongation of the HDPE/EVAc and HDPE/CPE blends are better than those of the PE but their hardness and softening points are lower. The two blends also show the memory effect better than that shown by the PE. Moreover, the heat-shrunk tubes of the PE/EVAc and PE/CPE blends were more flexible than those of the heat-shmnk PE. 

Addition of ethylene copolymers to polyethylenes has been used to improve toughness, impact resistance, and chemical resistance in films and other forms [19]. Blends of polyethylene with ethylene-vinyl acetate formed two continuous phases, which could then be stabilized by crosslinking [20]. Addition of chlorinated polyethylene to polyethylene is helpful in reducing flammability [18]. 

Beside the compounds in which the characteristic constituents are polyolefins, other products also may be employed for insulation and jackets. These include crosslinked compounds and constituted from chloro-sulfonated polyethylene (CP insulation and jackets for wires and cables used for deep wall submersible pumps subjected to 60, 90, and 105°C) chlorinated polyethylene (CPE Insulation and jackets for wires subjected to 90°C) ethylene-propylene-diene monomer (EDPM)... 

Chlorinated Polyvinyl Chloride (CPVC) 9 Polyamide, Polyphthalamide (PPA) 8-9 Polyethylene, Crosslinked (XPE) 8... 

Polyethylene in solution is treated with chlorine and sulfur dioxide to introduce approximately 1.39k sulfur and 29% chlorine into the polymer. Most of the chlorine is attached directly to the carbon atoms in the backbone of the polymer, The remainder is in the form of sulfuryl chloride groups, SO CI, through which crosslinking occurs In the curing step with metal oxides. The material has good oxidation and ozone resistance and thus overall excellent weather resistance. Calendered stocks are used for lining ditches and ponds, for example. 

Lead stabilisers have been used in a variety of PVC as well as other polymers for many years. In some halogenated polymers, such as chlorinated PE (CPE), chlorosulphonated polyethylene (CSM), polychloroprene (CR) and epichlorohydrin (ECO), dibasic lead phthalate and dibasic lead phosphite are used to scavenge HC1 arising from crosslinking as well as from degradation. In some of these cases, the metal may participate in crosslink formation. With lead-based stabilisers, the result is typically a product with greater water and chemical resistance than if a light metal, with more soluble halide salts, were used instead. In other cases, lead stabilisers may be used solely for function in metal oxide... 

Polyolefins can be chemically modified with the purpose of changing some of their properties. For example, polyethylene can be chlorinated or chlorosulfonated. Chlorosulfonation can be done with a mixture of chlorine and sulfur dioxide, leading to a material partially chlorinated and partially containing sulfonyl chloride groups. This treatment imparts elastomer character and the capability to be crosslinked, for example, with metallic oxides. 

Although MEEP-metal salt complexes show high ionic conductivities, these materials have undesirable creep properties at ambient temperatures, thus precluding their use in practical devices. To overcome this problem, one approach has been to synthesize dimensionally stable poly(phosphazenes) by crosslinking poly-[(chloro)(methoxy ethoxy ethoxy) phosphazene] with polyethylene glycol). If some of the chlorines in poly(dichlorophosphazene) are left unsubstituted after the initial reaction with the sodium salt of methoxy ethoxy ethanol, these can be reacted further with the difunctional poly(ethylene glycol)to afford crosslinked materials (Eq. 13 and Scheme 14) [204]... 

This is a long essay question which can be answered using any of the major modification techniques listed in the chapter on polyethylene modification. All information needed to compare and contrast two modification techniques, the changes in polyethylene physical and chemical properties, and an example of real world applications for that modified polyethylene are to be found for each ofthe techniques Sulphochlorination, Chlorination, Plasma Modification, Crosslinking - Chemical and E-Beam, and Graft Modification with Maleic Anhydride. 

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