Chlorosulphonated Polymers

As an alternative to copolymerization as a means of introducing structural irregularities, it is also possible to modify polyethylene molecules already formed. Being alkanes such polymers are of very limited chemical reactivity and in practice modification is restricted to halogenation and certain related processes. Of the various halogens only chlorine is used commercially. 

The introduction of chlorine groups into the polyethylene molecule has two opposing effects  

It reduces chain regularity restricting and eventually eliminating the ability of the polymer to crystallize. In view of the low T, of polyethylene this tends to increase rubberiness. 

It increases interchain attractions causing the Tg to be raised, eventually to a point where the amorphous polymer is no longer rubbery. 

This is clearly illustrated in Table 12.2 (Oakes and Richards, 1946). 

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It may also be mentioned that a number of commercial polymers are produced by chemical modification of other polymers, either natural or synthetic. Examples are cellulose acetate from the naturally occurring polymer cellulose, poly(vinyl alcohol) from polyfvinyl acetate) and chlorosulphonated polyethylene (Hypalon) from polyethylene. 

DuPont have produced a modified chlorosulphonated polyethylene based polymer (trade name Acsium). In this modified polymer the chlorine content is reduced, but an additional pendant alkyl group is used to restrict the ability of the polymer to crystallise. The result is a polymer with a lower Tg than the conventional CSM polymer. 

As with chlorinated polyethylene rubber, chlorosulphonated polyethylene exhibits good resistance to oxygen, ozone and light. The polar nature of the polymer chain also confers oil resistance. 

Graft polymers were obtained by mastication of a 60/40 blend of natural rubber and chlorosulphonated polyethylene (/). From 10-55% natural rubber was obtained as side chains. Grafting presumably proceeds by transfer of chlorine atoms to the rubber radicals to give grafting sites for combination with rubber radicals. Soluble linear polymers were also obtained by mastication for 50-180 min under nitrogen for a blend 50/50 of natural rubber and a polyurethane rubber (Vulcaprene A) (/). 

The work reported so far has concerned fibres spun from solutions of either (a) 5-6% polymer in PPA or (b) 10 % polymer in a mixture of 97,5 % methanesulphonic acid (MSA)/2.5% chlorosulphonic acid. The processing of PBT fibres from PBT/PPA dopes has however two advantages over spinning from PBT/MSA dopes. The... 

At room temperature, PE is a semi-crystalline plastomer (a plastic which on stretching shows elongation like an elastomer), but on heating crystallites melt and the polymer passes through an elastomeric phase. Similarly, by hindering the crystallisation of PE (that is, by incorporating new chain elements), amorphous curable rubbery materials like ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), ethylene-vinyl acetate copolymer (EVA), chlorinated polyethylene (CM), and chlorosulphonated polyethylene (CSM) can be prepared. 

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... 

Relatively high solubility of PBLG in many solvents can be explained by the interaction of flexible side groups with solvents. On the other hand, for aromatic polyamides with p-structure, e.g. PPTA, the solubility is observed only for a very narrow range of solvents, mostly for concentrated acids (sulphuric, phosphoric, chlorosulphonic, hydrofluoric, and other acids). In this case the interaction of acid molecules with amide groups of a polymer reaches the energy of chemical reactions. 

A wide range of polymers which form thin, flexible films are used. They can be split into two categories, polymeric elastomers and rubbers. The polymers include polyurethanes, polyvinyl chloride, polyvinylidene chloride, polyethylene, polytetrafluoroethylene, silicone elastomers, polyacrylates, and chlorinated and chlorosulphonated polyethylenes. 

The staining technique for TEM uses a material that absorbs electrons and preferentially attaches itself to or reacts with certain regions of the polymer rather than other regions. Materials frequently used are uranyl acetate and osmium tetroxide. For polyethylene the technique of chloro-sulphonation can be used. In this method, which involves immersing the sample in chlorosulphonic acid, the electron-absorbing material becomes attached to lamellar surfaces, so that lamellae (see section 3.4.2) with their planes parallel to the direction of the electron beam become outlined in black in the micrographs. 

In general, the more polar the plasticizer the greater its impact on the polymer and compounds featuring phosphate, sulphonamide, chlorosulphonate, chloro, amino and ester groups are commonly effective. 

A copolymer of methyl methacrylate and vinyl chloride containing labelled chlorine ( "Cl) has been examined using thermovolatilization analysis and radiochemical assay. The yields of methyl chloride and hydrogen chloride agree with predictions made from sequence distribution calculations. The thermal degradation of a number of chlorine-containing polymers, poly(vinyl chloride), chlorinated polyethylene, chlorosulphonated polyethylene, polychloroprene, poly-epichlorhydrin, and co- and ter-polymers of epichlorhydrin has been compared and structural effects elucidated. ... 

Before vulcanization, chlorosulphonated polyethylene is a tacky, rubbery material of low tensile strength it is soluble in chlorinated hydrocarbons. The sulphonyl chloride groups are reactive and may be used for cross-Hnking the polymer. Vulcanization is generally carried out by heating with metal oxides such as litharge or magnesium oxide in the presence of a little water. Reaction... 

Another similar polymer, chlorosulphonated polyethylene (Hypalon), also requires special modifications to achieve adequate adhesion, but in this case it is the basic polymer formulation that is modified, by the addition of from 20 to 40% of NR this gives a good increase in achieved adhesion without too great a loss of the outstanding weathering and ageing properties of the polymer. 

Modification of already formed polymer, usually polyethylene, for example by chlorination or chlorination/chlorosulphonation as with the Du Pont product Hypalon. 

Figure 11.1 (a) Transmission electron micrograph of chlorosulphonated polyethylene, (b) Small-angle X-ray scattering pattern of the same polyethylene sample. Intensity is plotted on an arbitrary units scale. Unpublished data of M. Hedenqvist, Dept of Polymer Technology, Royal Institute of Technology, Stockholm, Sweden. 

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