Polychloroprene mercaptan modified

If tertiary chlorine atoms are indeed critical to heat resistance, then reactions that consume them should improve polymer stabiUty. This is indeed the case. Post-reaction of polychloroprene with dodecyl mercaptan (111), use of higher levels of ethylene thiourea for curing (112), and inclusion of reactive thiols such as mercaptobenzimidazole in cure systems (113) all improve heat resistance. This latter technique is especially effective in improving the heat resistance of mercaptan modified polychloroprene. 

Mercaptan-modified polychloroprenes only contain sulphur at the ends of the polymer chain. Polymerization is carried out in the presence of a mercaptan to act... 

Mercaptan-inodified polychloroprenes provide lighter colour to solutions and are more resistant to discolouration than sulphur-modified polychloroprenes. Also mercaptan-modified polychloroprenes exhibit better ageing, and improved thermal and solution stability. 

Neoprene WHV-A. It is a non-peptizable and mercaptan-modified polychloro-prene elastomer. It is a slow-crystallizing, high molecular weight type and contains only 85% trans-, 4 structure. It is generally used in blends with low molecular weight crystallizing polychloroprene types to increase solution viscosity. 

The cross-linking efficiency of the more branched polychloroprene latex (mercaptan modified) has been found to be higher than the less branched sulfur-modified one. The latex dispersion is found to display higher rate of cross-linking than the coagulated and subsequently dried mbber films [386] due to higher concentration of radical in the former. 

Mercaptan modified polychloroprene compounds vulcanised with a thiourea give superior heat ageing and compression set compared to... 

Mercaptan modified (W) types of polychloroprene are not as good as sulphur modified (G) types, the copolymer (GRT) being preferred. Gel-containing polymers are particularly poor. 

The mercaptan modified polychloroprenes are vastly superior to the sulphur modified (G) types in set resistance although Neoprene GW has partly closed the gap. For good set resistance at low temperatures, the crystallisation resistant copolymers must be used, with compounding appropriate to low temperature resistance. 

Compounds of either type of colour must contain a non-discolouring type antioxidant (particularly Antioxidant 2246), which will inhibit discolouration in sunlight, and must avoid the use of aromatic oils in favour of organic ester plasticisers. These compounds are normally made from mercaptan modified types of polychloroprene which do not darken during cure, provided the curing system is free from sulphur or lead oxide. 

Mercaptan modified polychloroprenes are preferred because they not only give better insulation values but are more extendable and have better extrusion properties such as speed and smoothness. However, sulphur modified (G) types are sometimes used if high tear strength is needed. 

The polymer, formed by this technique, consists of about 85% trans-1,4 units, 10% cw-1,4 units, 1.5% 1,2 units, and 1.0% 3,4 units. The polymer is essentially linear with a molecular weight equal to approximately 100,000. The sulfur-modified polychloroprenes are sold under the tradename Neoprene-G. An unmodified version prepared with mercaptan chain-transferring agents (Neoprene W) is a polymer with a molecular weight of about 200,000. ... 

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