Coefficient of Vulcanization

The coefficient of vulcanization is usually defined as the number of units of weight of sulphur combined with 100 units by weight of unsaturated hydrocarbon. Ebonites prepared from natural rubber are 

Type of rubber Theoretical coefficient of vulcanization at ebonite stage Coefficient after correcting for rubber constituents 

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Table 3.2 Coefficient of vulcanization of natural and synthetic rubbers...

The vulcanization temperature must be chosen in order to produce a well cured product having uniform and optimum physical properties in the shortest possible time. The term temperature coefficient of vulcanization can be used to identify the relationship between different cure times at different temperatures. With this information optimum cure times at higher or lower temperature can be estimated for many rubber compounds with known coefficient of vulcanization. For approximately most rubber compounds the coefficient of vulcanization is 2. This indicates that the cure time must be reduced by a factor of 2 for each 10°C increase in cure temperature or if the temperature is reduced to 10°C, the cure time must be doubled. 

Ambelang J.C., K.C. Beach, and D.F. Sullivan. 1962. Temperature coefficient of vulcanization for present-day tire compounds. Rubber Chem. Technol. 35 92-104. 

The diffusion coefficient D is inversely related to the cross-link density of vulcanized rubbers. When D is extrapolated to zero concentration of the diffusing small molecules, it is related to the distance between the cross-links. Thus, as the cross-link density increases D becomes smaller, as expected. Further, the diffusion coefficient is less for crystalline polymers in comparison with the same polymer except in the amorphous state. In fact, this can be roughly stated as follows. 

The diffusion coefficient D is always related inversely to the crosslink density of vulcanized elastomers. When Z)is extrapolated to zero concentration of the diffusate, it is related to the weight of the principal section of the elastomer, i.e., the weight of the segments between crosslinks. 

Spence et al. (1911, 1912) found that the temperature coefficient of sulphur vulcanization of natural rubber was about 2-5 per 10 degrees Celsius which was in the range expected of a chemical rather than a physical reaction, a result supporting the earlier conclusions of Weber. They also reported that the maximum percentage of sulphur that could be combined was 32. This was consistent with the following empirical equation ... 

The advantages of block copolymer of butadiene and styrene are high coefficient of friction, good elasticity, broad hardness range, no need for vulcanization, good color and colorability. 

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. 

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... 

The macro-dispersion of the fillers can be determined by light-microscopic techniques with computer-assisted image processing on glazed cuttings of the vulcanized samples. At least five picture details have to be evaluated for each specimen. The dispersion coefficient D is calculated from the ratio of non-dispersed filler agglomerates and the volume fracture of the filler in the composites in accordance with ASTM D2663. 

Diffusion coefficients are proportional to 1/M, the molecular weight of linear chains [7]. They are not well known and therefore neither is the interdigitated thickness. Hence, it is not possible to say whether the observed behavior has to be related to the interdiffusion depth, or to the number of crosslinks formed in the interfacial region or, most hkely, to both effects. These results given for the elastomer joints crosshnked by a sulfur-based vulcanizing system show that it is very difficult to separate interdiffusion and crosshnking mechanisms because the temperature influences both the chain mobihty and the kinetics of network formation. 

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