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Ramakrishnan, Chathapuram Seshier (1948) Vulcanisation of rubber by sulphur under homogeneous conditions "the soft rubber reaction". Doctoral thesis, Northern Polytechnic.
The first systematic study of the mechanism of vulcanisation of natural rubber was undertaken by C.O. Weber. He studied the action of sulphur monochloride on a solution of natural rubber in benzene and confirmed the earlier observation of Henriques that sulphur monochloride added as such to the rubber molecules to form polyprenesulphochlorides. Although he postulated the existence of a series of such compounds, later work especially by Hinrichsen & Kindecher proved the existence of only one compound corresponding to the highest state of vulcanization.
By analogy, Weber postulated that the reaction between rubber and sulphur should also be chemical and he proceeded to investigate the nature of the reaction. Weber made a mixture of pure para rubber (100 parts) and neutral precipitated sulphur (10 parts) and vulcanised in the form of thin strips in an autoclave, kept at a constant temperature. At the end of 30 minutes or 60 minutes the autoclave was opened and one strip was removed, cut into bits and analysed for combined sulphur after acetone extraction. The curves plotted from these results showed several inflections which, according to Weber, were quite independent of experimental error. These inflections in the curves were attributed to the formation of definite chemical compounds and, since no hydrogen sulphide was evolved, the reaction was considered an addition process. Weber considered that the process of vulcanization consisted in the formation of a continuous series of addition products of rubber and sulphur with probably C100H160S as the lower and C100H160S20 as the upper limit. L.E. Weber in discussing these results pointed out that these inflections appeared at different places on the curves with different temperatures of vulcanisation and so no special significance could be attributed to them.
Thie research of Weber gave rise to a considerable amount of speculation, and many attempts were made to verify his results and to arrive at a definite conclusion regarding the nature of rubber-sulphur reaction product.
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