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Richardson, George Gary (1948) The properties of semi-ebonite. Doctoral thesis, Northern Polytechnic.
Semi-ebonite has been regarded as a definite compound, "polyprene monosulphide" (C10H16S)x, as a result of collected evidence of sudden changes in physical properties at that approximate composition. Various physical methods have been employed to follow the progressive vulcanisation of rubber mixes containing 20-25 parts of sulphur per 100 of rubber.
Swelling and recovery after flexing tests have helped to define the semi-ebonite stage with reasonable accuracy. Tensile tests and other common physical measurements, give little help in following the semi-ebonite reaction. Chemical tests involving the determination of free sulphur in the progressively cured vulcanisates, and the determination of unsaturation by a new bromine method, have not shown any sudden changes in the semi-ebonite region.
The effect of varying the amount of organic accelerator from 1 to 6, and sulphur from 15 to 30 parts per 100 of rubber, have been studied in a large series of cures ranging from 10 to 120 minutes.
It has been shown possible to make a reasonably good semi-ebonite from reclaim. Loading with gas black has not enhanced the physical, properties of semi-ebonite.
A survey of semi-ebonite formation from the physical and chemical aspects, has led to the development of a theory of vulcanisation involving a three- stage reaction of rubber with sulphur. The first reaction, visualised as the forma tion of strong monosulphide linkages at the terminal methylene groups of adjacent long chain rubber molecules, gives rise to soft vulcanised rubber. The second reaction, regarded as the formation of disulphide linkages joining adjacent rubber molecules at alternate double linkages, culminates logically in the formation of semi-ebonite, (C10H16S)x. Half the unsaturation is left. The third reaction saturates the remaining double bonds, giving rise to ebonite.
GR-S - sulphur vulcanisates analogous to natural rubber semi-ebonites, have excellent physical properties. The vulcanisation of Gr-S is regarded as three-dimensional cross linking.
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