Vulcanization conditions

It can be seen that every type of vulcanization system differs from every other type in the kind and extent of the various changes that together produce the vulcanized state. In the vulcanization processes, consideration must be made for the difference in the thickness of the products involved, the vulcanization temperature and thermal stability of the rubber compound. The word cure to denote vulcanization is believed to have been coined by Charles Goodyear and the same has been a recognized term in rubber industry circles [2]. The conditions of cure will vary over a wide range according to the type of vulcanizate required and the facilities available in a rubber factory. Many factors must be predetermined, including the desired hardness of the product, its overall dimensions, the production turnover required and the pretreatment of the rubber stock prior to vulcanization. Hardness will normally be determined by the composition of the stock but it can also be influenced by the state of cure. 

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. 

Each type of rubber has a definite range of temperatures which may be used for vulcanization. These temperatures may vary somewhat, but it is quite important not to exceed the maximum for each since some form of deterioration will occur. This effect is either shown by the appearance of the finished product or by its physical properties. 

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With rubbers a similar situation is met, but now with the aid of a rotation viscometer. The Mooney viscosity is measured as the torque needed to rotate two parallel plates, between which the rubber mass is present, with respect to each other. This provides a rough indication of the viscosity, and thus of the molar mass. This measurement can also be used to characterize the vulcanization behaviour under vulcanization conditions the increase of the Mooney viscosity indicates the onset of network formation. When the network develops further, a continuous rotation can, of course, no longer be applied because the viscosity increases unlimitedly therefore an oscillation method is mostly used with a cone-and-plate geometry. Initially, the viscosity is being measured, and later on the build-up of the E-modulus of the network. Another characterization of the viscosity of unvulcanized rubbers is the Hoekstra method. The rubber is present between two parallel plates, which are moved towards each other with a certain speed the force needed to do so is an indication of the viscosity. 

Special considerations zinc oxide is crosshnking agent carbon black is a more efficient UV stabilizer in sulfur-cured SBR than in radiation or peroxide cured " vulcanization conditions affects electric conductivity of carbon black filled SBR solvent diffusion and swelling rate decrease as concentration of carbon black increases ... 

For the past several deeades scientific research on the vulcanization mechanism was carried out but until now there has been no unified theory about it. Moreover, for some accelerators there are several different hypotheses regarding the meehanism of vulcanization of different rubbers and even different hypotheses for a particular accelerator with a particular rabber. It became known that the effeet of the aeeelerators on vulcanization is multifunctional and depends on the type of rabber, the type of aeeelerator, the nature of the filler and vulcanization conditions, but the meehanism of reaeting is complex and not sufficiently explained. It was diseovered that the aeeelerators show their full activity in the presenee of other substanees whieh are the so called activators. For the past several deeades seientifie researeh on the vulcanization mechanism was carried out but until now there has been no imified theory about it. Moreover, for some aeeelerators there are several different hypotheses regarding the meehanism of vuleanization of different rubbers and even different hypotheses for a partieular aeeelerator with a particular rabber. 

Djagarova, E, Gegova, E. Influence of the Formulation and Vulcanization Conditions on the Amount of the Mercaptans and Reducing Substances in an Extract of Butyl Rubber Vulcanizates. Polymers, Proceedings of the Institute for Rubber Industry Sofia, 1984, V0I.2, >fol6, 69 - 76 (in Bulgarian)... 

The heat resistance of NR is not good enough for many uses, and it is exceeded by many synthetic rubbers. It is affected by the choice of vulcanization system, vulcanization conditions, choice of protective agents, and even choice of filler. To obtain good aging resistance of NR vulcanizates, one must use protective agents in the compound and use relatively short curing cycles at relatively low temperatures. 

The characteristics of silicone rubber vary with both the kind and amount of additives used and the mixing and vulcanization conditions. Various thermal properties of silicone rubber can be inspected by means of thermal analysis. 

SAXS can also be used to investigate the expected outcome of preparation processes of rubber-based materials. One example is the study of the evolution of the crosslink density of natural and styrene-butadiene rubbers, reported by Salgueiro et al.P who observed that changes in the vulcanization conditions bring about a shift and a widening of the SAXS signal. 

If a study of vulcanization chemistry is to become part of a routine technological investigation it is necessary for a technique to be developed that will give very rapidly information on the distribution of rubber-sulphur reaction products for a specified set of vulcanization conditions (e.g. sulphur level, accelerator level, cure time, cure temperature). A most useful contribution in this direction has been made by Lautenschlaeger (1977). The model compound 2-methyl-2-pentene was heated with typical curing systems in Pyrex tubes from 10 to 100 minutes over a range of temperatures from 100 C to 150 C. The reaction products were then subjected to gas... 

Fig. 8.8. Gas chromatography spectrum of MCV products. Effective vulcanization conditions and high concentration of reagents. Si—monosulphides S2—disulphides S3— trisulphides. (After Lautenschlaeger, 1977.)...

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