COMPOUNDING PRECIPITATED SILICA IN NATURAL RUBBER
Commercial production of the first grades of precipitated silica took place in 1948. At this time, natural rubber had barely relinquished its position as the principal, if not only, elastomer in commercial use. In these circumstances, most of the early compound development work involved natural rubber as the base polymer. In some respects this was a fortunate situation for silica. In comparison to carbon black, abrasion resistance was only slightly reduced; high viscosity and slow cure rates, which appear in synthetic elastomers, were not significant problems. The major silica attributes of tear strength, heat resistance and adhesion to fabrics and metals were very much in evidence in natural rubber
compounds. It was not until the development of sophisticated dynamic testing procedures that the superiority of silica, frequently silane coupled, in the reduction of tire rolling resistance was discovered and put to commercial use.
2.2 SILICA AND CARBON BLACK
High surface area grades of precipitated silica provide an alternative to carbon black as a source of reinforcement for natural and synthetic rubber compounds. Among the criteria for reinforcement abrasion resistance, tear and tensile strengths unmodified silica is inferior only in respect to smooth surface abrasion resistance. This deficiency, together with higher prices, has confined silica production and consumption to roughly 10% of that of carbon black in the U.S. market. In view of this preponderance of carbon black compounding usage, it is appropriate to begin a compounding appraisal of silica in terms of silica’s relation to black.Valid comparisons in respect to both filler and compound properties are best made on a basis of comparable filler particle size or compound
hardness. In the following tables, 2.1a through 2.1c, a silica of 19 nanometers average particle size (180 m2 /g N2SA silica) and an ISAF carbon black of 22 nanometers (N220) are compared at 40 phr in compounds of equal hardness with appropriate changes in acceleration:
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