Polychloroprene sulphur-modified

Peptizables grade polychloroprenes, sulphur-modified and stabilized with thiuram disulphide... 

During emulsion polymerization, a high conversion of monomer to polymer produces cross-linked rubber which is insoluble. To obtain a high conversion in the polymerization reaction and a processable polymer, suitable polymer modification should be made. The use of sulphur moieties allows this goal to be reached [2]. Sulphur-modified polychloroprenes contain di- and polysulphide sequences in the polymer chains. After the polymerization reaches the desired degree, reaction is stopped by adding thiuram disulphide ... 

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. 

Although there are several manufacturers of polychloroprene elastomers, Du Pont probably has the broader range of polychloroprene grades on the market. As an example in Table 3, the equivalencies between the peptizables sulphur-modified and stabilized with thiuram disulphide polychloroprenes are given... 

Neoprene GN (1942). It is a general-purpose sulphur-modified polychloroprene used in the shoe industry. This polychloroprene has two limitations the viscosity decreases on ageing and a black discolouration appears during storage. 

An important class of polychloroprenes are the so-called sulphur-modified polychloroprenes. In these cases the chloroprene is polymerized in the presence of small amounts of sulphur. It has been demonstrated, by the use of radioactive sulphur, that the sulphur is... 

Commercial polymerizations are typically carried out in jacketed glass-lined kettles at about 40 C using potassium persulphate as initiator with the emulsifier being formed in situ using rosin, aqueous caustic soda and the sodium salt of a naphthalene sulphonic acid-formaldehyde condensation product. Monomer conversion is followed by measuring the specific gravity of the emulsion. In the case of sulphur-modified polychloroprenes polymerization may be... 

The above theory does not explain the fact that sulphur-modified polychloroprenes do not require bis-alkylating agents for a satisfactory degree of cure. Nor does it account for the curing reaction with ethylene thiourea. 

With the sulphur-modified polymers cure may be brought about by zinc oxide and magnesium oxide in combination either alone or together with an accelerator such as ethylene thiourea. In the case of the homopolymers it has been common practice to support the zinc oxide/magnesium oxide/ethylene thiourea system with a further component. This component consists of a sulphide or a blend of sulphides of the type more commonly used as accelerators for the diene hydrocarbon polymers. These include mercaptobenzothiazole disulphide (MBTS), diorthotolyl guanidine (DOTG) and tetramethyl thiuram monosulphide (TMTM). In the polychloroprene homopolymers these materials appear to act as retarders of cure at processing temperatures but are accelerators at vulcanization temperatures. Their mechanism does not appear to have been fully elucidated. 

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. 

Discolouration during vulcanisation is a specific problem in coloured cable jackets or hose covers made by the lead press method. The cause is the reaction between the lead sheath and any available sulphur in the compound, forming black lead sulphide. For this reason, sulphur modified polychloroprenes should not be used or any ingredients containing or liberating sulphur during cure, such as TMTD or factice. ETU in combination with MBTS would be the preferred accelerator. 

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. 

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... 

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. 

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