Optimization of Silane-Silica OTR Compounds. Part 1: Variations of Mixing Temperature and Time during the Modification of Silica with Bis-(3-Triethoxisilylpropyl)-Tetrasulfide

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Abstract
This study of the TESPT modification reaction of silicas gives the compounder new and valuable information about rubber filler-silane interactions. It allows TESPT to be used more practically and economically. It also suggests several areas that would be interesting and worthwhile for further investigation. The conclusions based on this study are: (1) The modification reaction of silica with TESPT in situ cannot be considered as an equilibrium reaction. (2) The degree of modification increases with TESPT reaction time and temperature. Temperature has more effect than time. (3) In natural rubber, the highest and most practical modification reaction temperature is about 160°C. By performing the modification reaction between 150 and 160°C, it produces optimum cure rate and 300% modulus. Above 160°C, the thermal reaction of TESPT with natural rubber starts and reduces the sum of filler/rubber and rubber/rubber crosslinks available in the final cured rubber. (4) Up to 160°C, the number of filler/rubber and rubber/rubber crosslinks remains constant. But increasing temperature and reaction time lead to an increasing ratio of filler/rubber to rubber/rubber bonds. This improves the properties that are the most important for an OTR tread. (5) The higher the yield of filler/rubber bonds achieved by using the optimum reaction temperature and time, the less TESPT is necessary to get the desired properties. This produces better cost/performance. (6) To get the highest modification with TESPT, it should be added with the silica, to make best use of the reaction time dependency. (7) Most of the physical properties are dependent on the ratio of filler/rubber to rubber/rubber bonds and can simply be correlated to the 300% modulus of the compound. (8) The previously published standard mixing procedures' do not have to be changed with regard to the mixing sequence. However, by controlling mixing conditions, it may be possible to lower the level of TESPT and still obtain the desired improvements in compound physical and performance properties.

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