Rubber-sulphur Ratio and Cure Time

If we take a rubber-sulphur ratio of 68 32 in an ebonite compound and then cure it at 155°C, the vulcanization coefficient increases practically to a constant value of 47 after about five hours and the uncombined sulphur decreases during the first four hours. This state may be called a full cure in the chemical sense. There is also a reduction in volume of about 6%. It is known that after the combination of the first few percent of sulphur, the material passes through a leathery stage with low strength and poor resistance to oxidation and with time passing a true ebonite is formed with increased impact strength. 

If we increase the sulphur to a 65 35 ratio at full cure, the uncombined sulphur falls to about 4% while the vulcanization coefficient rises to about 49, but neither value is changed by further curing. At this stage tensile strength and cross braking strength are increased. Impact strength is reduced. 

The effects of reducing the rubber-sulphur ratio from 68 32 to 72 28 are very significant. The uncombined sulphur can be reduced, the vulcanization coefficient being about 40. Tensile and cross braking strength are a little lower. Impact strength is much higher. An ebonite 

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