Since the first intensive literature review on silicas and silicates was published, significant new technology for their use in rubber has developed. Usage in the rubber industry has broadened considerably, and increases in their consumption have occurred during the ensuing fifteen years. From the “soles and heels” fillers of the 1950's, the usage of precipitated silicas and silicates since has gained significantly in quality components of tires and mechanical goods. Potential applications in the immediate future are no longer limited by the “good, but second to carbon black” performance that has prevailed for the past twenty-five years. Besides the traditional unique properties of tear resistance, hardness, stiffness, flex resistance, heat resistance, low modulus, and neutral color imparted to rubber by silicas and silicates, it is now possible to add adhesion, low heat buildup, high resilience, high modulus, and high abrasion resistance. This is part of the new technology which has developed that provides an alternative type of filler to meet the demanding needs of tire components, conveyor belts, automotive mounts, gaskets and seals, and other quality rubber compounds previously inaccessible to silicas. The present review covers the period 1960–1975. Because of the volume of available literature, it became necessary to be quite selective, in a way perhaps that may seem arbitrary. Nevertheless, coverage will be confined largely to that relevant to more general rubber uses. Silicone rubber technology was not covered because of its highly specialized and generally proprietary nature. The definitions used in the previous review were expanded somewhat. Silicas and silicates (henceforth, “silicas”) germane to this review were generally restricted to those which are synthetic and amorphous. However, the size limitations were relaxed somewhat to include those having a specific surface area greater than 25 m2/g or an average particle diameter less than 100 nm. Natural products (e.g., clays, diatomaceous earth, sand, etc.), as well as their modified forms, were excluded.

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.

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