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Introduction During the technological process of rubber compound vulcanization, the transformation of the material from plastic to elastic state occurs. This process is accompanied by the modification of the physico - mechanical properties of the material. Depending on the type of the final product, i.e., pneus components, conveyor belts, transmission belts, hoses, packings, vulcanization can be carried out in ovens, autoclaves, presses, injection molds, in chambers, on cylinders, etc. Autoclaves and presses are used to vulcanize tyres, tubes or rim strips. Vulcanization is an energy-consuming process, even though the chemical reaction is exothermic. The power consumption is necessary to convert rubber from plastic to elastic state, to form the tyre at the final shape and to initiate the chemical reaction. During the vulcanization stage, 30-35% of the total power and more than 50% of the utilities (steam, water, gas etc) involved in the production of a tyre are consumed. In order to reduce these consumptions, continuous improvements of vulcanizing machines and technologies are required. A few methods to reduce the power and material consumptions are shown below:
In the case of the classical system of vulcanizing in BoM presses, the time consumed for ancillary operations represents more than 65 % of the total time required. In order to reduce this percentage, the following can be done:
The time required for the vulcanization chemical process is determined by the vulcanization plateau, where the values of the most important physico-mechanical indicators (modulus, tensile strength, etc.) are optimal. Continuing the vulcanization beyond this plateau leads, especially in the case of natural rubber and cis-synthetic polyisoprene, to the reversion phenomenon. As a consequence, decrease of the previously mentioned indicators and increase of elongation at break is observed. In the case of tyres, an optimal vulcanization time, corresponding to 90% of the maximum values of the indicators, is recommended. The reason for this incomplete vulcanization is to avoid reversion and overcure that may occur due to the heat produced during the tyre use, which can lead to a continuation of the vulcanization process. The optimal vulcanization time can be determined in the laboratory for each rubber compound by means of the Mooney viscometer or the Monsanto rheometer. Many relationships were proposed for the calculation of the optimal vulcanization time (top) at a given temperature, by using data obtained with the Mooney viscometer:
where: t2, t5, t?10,13,20,50 represent times after which Mooney viscosity increases by 2, 5, 10, 13, 20, 30 and 50 units, respectively. Another important technological parameter is the vulcanization temperature, which is in the range 130oC - 190oC, depending on the crosslinking system. It is worth mentioning that, in the case of products with thick walls, temperature is not uniform inside the vulcanisates because of poor heat transfer. Therefore, it is expected that the vulcanization rate be variable within the product. The ratio between the vulcanization times t1 and t2, corresponding to temperatures T1 and T2, respectively, depends on the vulcanization rates and it is named thermal coefficient of vulcanization, C: where KT1 and KT2 are the vulcanization rate constants, which can be experimentally determined. An empirical equation to correlate t1 and t2 times has been also suggested: where DT = T2 - T1. The thermal coefficient has different values, depending on elastomer:
It is assumed that for a DT = 10oC rise of temperature, the vulcanization rate doubles on average, and the vulcanization time decreases accordingly by half. The vulcanization time is established practically by means of a nomograme built based on the calculated thermal coefficient and on some experimental results. Another important technological parameter is heat transfer, which depends on the parameters of the thermal agents and on mold and tyre thickness and composition. It was noticed that to an 1 mm increase of tyre thickness corresponds an 1 minute increase of vulcanization, but, of course, there is no law valid in all cases. The values of the coefficient of thermal conductivity, l, and of the calorific capacity, Cp, of the various materials involved are important for the heat transfer during the vulcanization process (Table 1). Table 1 - The coefficient of thermal conductivity and the calorific capacity of various materials involved in the vulcanization process
The values of these coefficients change as a function of the composition of rubber compounds. Thus, the addition of 60 parts carbon black to 100 parts synthetic 1,4 cis polyisoprene rubber can increase the coefficient of thermal conductivity of the compound up to 0.3 W/mK. The thermal effect has to be taken into account also during the design of vulcanization molds, when shrinkage (C) of the products after vulcanization is taken into consideration. Shrinkage may be express by the relation: where:
Below are given the coefficients of thermal expansion for several materials employed in the vulcanization process (K-1):
In the tyre vulcanization process, saturated steam, overheated water or hot gases (air, nitrogen, etc.) are used as thermal agents. The heat transfer coefficients, a (W/m2 K), are largely different, depending on the thermal agent:
The data displayed above show the low efficiency of hot air in the vulcanization process. The vulcanization system with water bag and the system with diaphragm differ each other not only by geometry and operating, but also by heat transfer. The equivalent thicknesses, displayed in table 2 for an identical heat transfer, show the superiority of diaphragm utilization. Table 2 - Equivalent thicknesses for water bags and diaphragms at the same heat transfer
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