Crepe hardening

Uses Silicone chemical antistructuring (crepe hardening) additive for filled silicone... 

Unlike hydrated precipitated silica, fumed silica is used almost exclusively in silicone rubber compounds. Fumed silica is incorporated into silicone gum stock with the use of dough mixers. Fumed silica is unique to silicone rubber because it significantly improves the physical properties of the cured compound (better than hydrated precipitated silica). On the other hand, fumed silica will have a greater tendency than the hydrated precipitated silica to impart crepe hardening to the uncured silicone compound. Many times, the use of fumed silica in silicone rubber is extended with the less expensive precipitated silica. 

Fumed silica is a better reinforcing agent for silicone compounds than precipitated hydrated silica. While the hydrated precipitated silica does not stiffen the silicone compound as much (crepe hardening), it also does not impart as much true reinforcement (such as better tensile strength, tear strength, and abrasion resistance). 

P. Vondracek, M. Schatz. Bound rubber and crepe hardening in sulicone rubber. /. Appl. Polym. Sd., 21,3211-3222,1977. 

Natural rubber is a constituent of the latex of Hevea brasiliensis a tree grown in the tropics. The latex is present in laticiferous vessels and can be collected after incision of the bark. It contains about 30 % of rubber in form of small droplets and coagulates after the addition of acids. The coagulate is processed to bandlike sheets of raw" rubber (smoked sheets, crepes) which in the industry are heated with sulfur (vulcanization) to harden it. At present time about half of the rubber used in the industry is prepared from Hevea latex, the other half is of synthetic origin. 

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