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Shankar, Uma (1950) An investigation of the chemical cause of reversion of vulcanised rubber under heat. Doctoral thesis, Northern Polytechnic.
Physical and chemical evidence is presented to show that during reversion the breakdown of crosslinks (including polysulphides) predominates over any simultaneous formation of crosslinks. A result not previously reported in literature, but shown by the present data, is that during reversion the modulus falls exponentially with time. This finding has been confirmed by an examination of seventeen instances of reversion taken from literature. The molar free energy of activation has been calculated from the modulus-time data and has been foundd to be 33.3 ± l.7 Kcal., irrespective of the temperature or the atmosphere of cure, or the composition of the mix. This energy is sufficient to rupture an -S-S- bond. The degree or crosslinking, estimated from the equilibrium swelling measurements in benzene, has been found to decrease with a fall in the modulus, and to increase with its rise.
The exclusion of oxygen does not prevent reversion if the temperature is high enough to supply the energy necessary for the breakdown of crosslinks. During reversion in nitrogen, hydrogen sulphide is evolved and the C/H ratio increases above the value for C₅H₈. Elemental sulphur is split from the polysulphide links by heat, and thereafter recombines with rubber in new positions or appears as free sulphur extractable by cold isopentane. Combined sulphur decreases. During air cures, reversion generally brings about a decrease in free sulphur, but the combined sulphur has been found ultimately to decrease. Reactions of the vulcanisates with methyl iodide show that the mechanisms of formation and modification of the polysulphide bonds are different in the two atmospheres. These mechanisms are discussed. Recent data on the reaction of sulphur with polyisoprenes and 2-Me-2-butene are examined and found to support these mechanisms.
In the course of this investigation precautions were taken to avoid, at every step where possible, heat, light and oxygen, whereever these could interfere with the reactions being studied. Three rubber-sulphur mixes (one accelerated by hexamethylenetetramine) prepared from latex, have been examined.
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