Fine Particle Reinforcing Silicas and Silicates in Elastomers

Abstract

Reinforcement has been described as “the enhancement of one or more properties of an elastomer by the incorporation of some ingredient thus making it more suitable for a given application”. This definition gives an excellent qualitative description of the reinforcement of an elastomer, but lacks the quantitative aspects necessary to prescribe specifications for materials to be included in the present review on reinforcing siliceous fillers. The scope of the present review will therefore be defined in terms of the properties of the siliceous fillers rather than in terms of the degree of enhancement of one or many elastomer properties. Experience has shown that substantial improvement in the physical properties of vulcanizates generally cannot be expected from siliceous fillers greater than 50 millimicrons in number average particle size, or less than 50 square meters per gram in specific surface area. These specifications will be used to delineate the silicas and silicates included in the present review, a general outline of which is presented in the following paragraphs: Description of the silica and silicate fillers is made difficult by lack of established categories for these materials. To aid in their classification, a brief discussion of manufacturing methods is presented. Then, lists of commercially available products are introduced. Physical and chemical properties of commercially available reinforcing silicas and silicates are presented, based on information in current brochures obtained from manufacturers. Failure by the manufacturer to specify his methods has occasionally diminished the usefulness of published data. for example, the term average particle size is not specific as it does not state which statistical average value is intended. The lengthy treatment given testing methods constitutes a plea to define or specify test methods. The three properties of primary interest in reinforcing silicas and silicates are the particle size, the chemical nature of the surface, and the state of aggregation. A comprehensive discussion of the chemical nature of the surface is prohibitive because of the wide scope of the topic. Nevertheless, a quick survey was made of this subject with the objective of presenting adequate information to allow an interpretation of the behavior of the fillers in elastomers. The groups encountered are principally siloxane (Si—O—Si), silanol (SiOH), and reaction products of the latter with various hydrous oxides. Attention is focused on silanol groups. Their dehydration and rehydration, their chemical reactivity, and their propensity for hydrogen bonding are considered briefly to aid in understanding interactions of the fillers with elastomers and with curatives.

Effects of Surface Chemistry of Silica Particles on the Mechanical Properties of Silica Filled Styrene–Butadiene Rubber Systems

“…The yield phenomenon at a low strain has been thought to be the result of a breakdown of aggregates in the rubber matrix. 1,2,20,21 Payne studied the effect of strain on the storage modulus for black filled vulanizate and reported 22,23 that the storage modulus of the vulcanizate decreased with increasing the strain amplitude with this tendency more prominent in the higher concentration of filler in the vulcanizate. Further, at a larger strain, the effects of filler concentration on the storage modulus became small.…”

“…[2][3][4][5][6][7][8] The other one was a poor reinforcement, such as a low tensile strength, low abrasion resistance and so on, in comparison with carbon black filled rubber composites. [1][2][3][4][5][6][7][8] The chemical environment of silica particle is quite different from that of carbon black due to the existence of silanol groups in the particles. Thus, the primary discussion on the properties of silica filled rubber composites has been concerned with the interactions between silica particles and the interactions between silica particles and rubber molecules.…”

“…[1][2][3] However, the utilization of silica caused two serious problems. One was a bad processability, such as a lower dispersiblity of silica particles in rubber matrix and a high viscosity of silica filled unvulcanizates.…”

Rheometric and Dynamic Mechanical Analysis of Complex Natural Rubber Based Composites

“…This value of γc (the strain amplitude, where half of the clusters are broken) increases when silica is more present in the NR matrix. The largest γc-values of 12.91% and 13.6% (for 2 Hz and 10 Hz respectively) are found for the NR-h-CB0-S40 composite, suggesting the highest stability, which is due to poor compatibility of silica filler to hydrocarbon rubber, its polar character and the ability to form hydrogen bonds [34]. The empirical Kraus-parameter m, that considers the power law dependency of the number of surviving clusters on the applied strain, shows very small variation with the type of applied fillers and also negligible variations with frequency [32].…”

Silica-filled tire tread compounds: an investigation into the viscoelastic properties of the rubber compounds and their relation to tire performance

“…Silica as a filler shows a very strong tendency to networking due to poor compatibility with hydrocarbon rubbers, its polar character and the ability to form hydrogen bonds 57 . It has been recognized that silica possesses a low dispersive component of surface energy while that of carbon black is high.…”