Modification Mechanism And Process Optimization Of Silica By ?-mercaptopropyltriethoxysilane

Under the background of energy saving and environment protection,the“green tire”industry is rapidly expanding,and silica is more and more widely used as an important renforcing filler in tire.In order to improve the dispersity of silica in rubber,the conventional solution is to modify the particle of silica with silane.But due to the limit of testing and research methods the silanization mechanism of silica has not been comprehensively and systematically investigated,and is still unclear.In this paper,experimental methods and molecular simulations were employed to revealing the silanization mechanism of silica.Firstly,in order to ascertain the effects of acid,alkali and water on the silanization mechanism of silica by 3-mercaptopropyltriethoxysilane(MPTS),the active energies of all the hydrolysis reactions,dimerization reactions and grafting reactions that related in the modification process of silica,were calculated.The interactions between silica and sliane,before and after grafting,were also simulated.Then by analyzing the simulation results of hydrolysis and dimerization reactions,the most favorable conditions for the hydrolysis of MPTS were determined.The hydrolysis experiments of MPTS were carried out in different conditions,the simulation results were verified and the hydrolysed products of MPTS with varying degrees were also prepared.By analyzing the simulation results of grafting reactions,the modification process in solutions was optimized to a“two-step”modification process of“adsorption at low temperature in acidic condition and grafting at high temperature in alkaline condition”and a“pre-hydrolysis of silane”procedure was added to the in-situ modification of silica.Finally,The modification experiments were carried out,the results of grafted contents of silane were used to evaluate the optimization effects of the“two-step”modification process,and the properties of the compound rubbers and vulcanizates were used to evaluate the optimization effects of the“pre-hydrolysis of silane”in the in-situ modification of silica.The main conclusions are as follows:The active energies for the three-steps hydrolysis reactions of MPTS and the dimerization reactions of the hydrolysed products in neutral condition are very high.Acid and alkali both can catalyze the hydrolysis and dimerization reactions.The hydrolysis and dimerization reactions are S_N2 nucleophilic substitution of back-attack in acidic condition.But in alkaline condition,the hydrolysis reactions are separated into two steps,five coordinated intermediates are firstly formed and subsequently ethanol is eliminated.The dimerization reactions in alkaline condition are S_N1 nucleophilic substitution.The results of hydrolysis experiment show that the hydrolysis and condensation reaction are very slow in neutral conditions,the hydrolysis and condensation reactions are accelerated in acidic and alkline conditions.The hydrolysis reactions in acidic condition are the fastest and the condensation products are the earliest to form in alkaline condition,indicting that the catalytic efficiency of acid is higher in hydrolysis reaction and the catalytic efficiency of alkali is higher in dimerization reaction.The experimental results are in good agreement with the simulation results.The results of DFT research on the interactions of silanes and silica shows that:the silanes with varying hydrolysis degrees in neutral,acidic and alkaline conditions are all tend to adsorbed flat on the silica surface before and after grafting reactions,via dispersion forces and hydrogen bonds.But the contributions of the dispersion force to the adsorption energies and the types of hydrogen bonds that dominate the adsorption behaviors,are apparently different in neutral,acidic and alkaline conditions.In neutral condition,the contribution of dispersion force to adsorption energy is between 32%and53%.Hydrogen bonds of H(Sur)-S(A-Me)?H(Sur)-O(A-E)?H(Sur)-O(A-S)and O(Sur)-H(A-S)play a leading role during the adsorption.Hydrolysis of silane can reduce the adsorption energy and stabilize the adsorption.In acidic condition,the adsorption energies are improved co MPared to those in neutral condition,the contributions of the dispersion force to the adsorption energy are reduced to 18%-32%,and the strong hydrogen bonds of O(Sur)-H(A-E)or O(Sur)-H(A-DS)play a leading role during the adsorption.The adsorption energies in alkaline condition is the lowest of all.The contribution of the dispersion force to the adsorption energy is reduced to about 10%,and the extremely strong hydrogen bond of H(Sur)-O(A)plays a leading role during the adsorption.For MPTS and its hydrolyzed products in neutral,acidic or alkaline condition,the O atoms are the nucleophilic sites and the Si atoms are the electrophilic sites.The reactivities of silanes can be improved by hydrolysis,or in acidic and alkaline conditions.The active energies are obviously reduced after hydrolysis in neutral condition,the grafting reactions also can be catalyzed by acid and alkali,but the catalytic efficiency of alkali is higher.The grafting reactions in neutral and acidic conditions are S_N2nucleophilic substitution,but S_N1 nucleophilic substitution in alkaline condition.The hydrolysis degrees of MPTS can be dominated by controlling the reaction time and the molar ratio of water in solution.In acidic condition at 25°C,it takes 40 minutes for silanes to reach the saturated adsorption on silica surface.After saturated adsorption in acidic condition,the slurry was adjusted to different p Hs,raised to 60°C and reacted for 8 h.In neutral condition,the grafting contents of silane were between 3.0%-3.9%,and increased after hydrolysis;in acidic condition,the grafting contents of silane increased to3.4%-4.3%;in alkaline condition,the grafting contents of silane were as high as 5.6%-6.2%.It is indicating that hydrolysis of silane promotes the grafting reaction.Acid and alkali can catalyze the grafting reaction,but the catalytic efficiency of alkali is higher than acid,which is consistent with the simulation results of grafting reactions.Co MPared with one-step modification in acidic condition,the contents of grafted silane of silica prepared the“two-step”modification process are increased and the“two-step”modification process are more efficiency.The activation energies of grafting reaction in gas phase can be reduced by hydrolysis of silane,which promote the grafting reactions in the in-situ modification of silica.Coared with properties of compound rubbers and vulcanizates prepared by the conventional in-situ modification process,the performance of compound rubbers and vulcanizates prepared by the optimized in-situ modification process with the procedure of“pre-hydrolysis of silane”is better.The procedure of“pre-hydrolysis of silane”can promote the in-situ modification efficiency of silica,improving the compatibility of silica and rubber to achieve high dispersity of silica in rubber,which improve the properties of the compound rubbers and vulcanizates.