Sulfur modification

Chloroprene Elastomers. Polychloroprene is a polymer of 2-chloro-l,3-butadiene. The elastomer is largely composed of the trans isomer. There are two basic polymer types the W-type and the G-type. G-types are made by using a sulfur-modified process W-types use no sulfur modification. As a result, G-types possess excellent processing and dynamic properties, and tend to be used in V-belts. However, they have poorer aging properties than W-types. The W-types tend to be used in appHcations requiring better aging, such as roUs and mechanical goods (see Elastomers, SYNTHETIC-POLYCm.OROPRENE). 

A very severe problem has arisen in South Russia and Kazakhstan as a result of stockpiling millions of tons of elemental sulfur over the past few decades. This sulfur has been extracted from high sulfuric oil and natural gas but has not been used because it is chemically unstable when it comes into contact with atmospheric gases and moisture and thus can become a serious ecological threat. The last problem might be resolved by transform ing the orthorhombic sulfur modification (equilibrium under ambient conditions) into its polymeric modification the latter is chemically more inert and can be used as a valuable component of large scale composite materials such as sulfur concrete, sulfur bitumen, and so on. 

D.A. Olsen, A.J. Osteraas, Sulfur modification of pofyethylene surfaces II modification of polyethylene surfaces with fuming sulfuric add. J. Polym. Sci. A-1 7, 1921-1926 (1969)... 

A number of papers have been published [5.5,6] about the complex equilibria resulting from the different sulfur modifications these papers show that complete reaction of the sulfur components to elemental sulfur cannot be expected until the temperature is reduced to less than 140° C. Figure 5.9 shows the sulfur yield versus temperamre at an overall pressure of 1 bar. It is obvious that the conversion rate drops in the area of purely thermal incineration (combustion chamber) and then rises steeply again as the temperatures decrease. As the reaction velocity is very small at temperatures of less than 350° C, catalysts have to be used for low temperatures. Reactions in this temperature range are accelerated by highly active AI2O3 catalysts which are normally doped with cobalt and molybdenum to improve CS2 and COS conversion. 

However, at about 159 C, the viscosity of sulfur increases rather abruptly by two orders of magnitude (Figure 33-1). In X-ray crystallography measurements at the same temperature the position of the intensity peak shifts quite sharply. From the distances between the intensity maxima, the number of nearest neighbors in this sulfur modification (S sulfur) is determined to be two. Therefore, a polymerization process must be involved. 

TOF-SIMS data for Armco iron point on the highest degree of sulfur modification being the result of friction against ebonite, SBR vulcanizate and cured polysulfide rubber. The spectra represent the highest amount of species containing sulfur. 

Tribological characteristics confirm the influence of sulfur modification of metal counter face on lowering fiiction for the metal-polymer couples studied. In the case of ebonite, the coefficient of friction reduced significantly for the whole experimental mn, whereas application of SBR vulcanizate or polysulfide rubber was effective only during the first period of experimental cycles. Any improvement of tribiological characteristics was not assigned for polysulphone. The polymer was observed to worn the surface of iron counterface, what resulted in increase of the coefficient of friction in this case. 

W-types use no sulfur modification. The W-type polymers tend to be used in applications requiring better aging, such as rolls and mechanical goods (see Chloroprene Polymers). W-type polychloroprenes require an organic accelerator. 

Sulfur modification improves the breakdown of the rubber during mastication, permitting the production of low-viscosity compounds with good building tack. Only zinc oxide and magnesium oxide are needed for vulcanization. In many cases... 

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