Reinforcing fillers precipitated silicas

Schaefer, D. W. Chen, C., Structure Optimization in Colloidal Reinforcing Fillers Precipitated Silica. Rubber Chem. Technol. 2002, 75(5), 773-793. 

Fillers. They are generally added to reinforce NBR adhesives. However, fillers can be added to promote tack, to increase the storage life, to improve heat resistance or to reduce cost. The most common fillers are carbon blacks. Precipitated silica can be used in applications where black colour is not acceptable, but excessive amounts tend to reduce adhesion. Titanium dioxide can be used to impart whiteness, improves tack and extend storage life. 

Reinforcing fillers (active) Fumed Silica (Si02) precipitated calcium carbonate (CaCOi) carbon black Thixotropic reinforcing agents (non-slump), adjustment of mechanical properties (cohesion) provide toughness to the elastomer as opposed to brittle materials. 

Zinc salt of maleated EPDM rubber in the presence of stearic acid and zinc stearate behaves as a thermoplastic elastomer, which can be reinforced by the incorporation of precipitated silica filler. It is believed that besides the dispersive type of forces operative in the interaction between the backbone chains and the filler particles, the ionic domains in the polymer interact strongly with the polar sites on the filler surface through formation of hydrogen bonded structures. 

The main chemical routes for vulcanisation of silicone elastomers are 1) Elevated temperatures cures and 2) Room temperature vulcanisation mechanisms. Organic peroxide cures are used in elevated temperature cures. Since the organic peroxides are inhibited by most carbon blacks, non black reinforcing fillers such as precipitated silicas, titanium dioxide and zinc oxide are used. Room temperature vulcanisation is normally used with low consistency silicone elastomers. 

The first reinforcing silica for rubber, which was in the form of a calcium silicate and known as Silene, was introduced to the U.S. market as early as 1939 (II). The first reinforcing fillers for rubber articles available in West Germany after the Second World War were also calcium and aluminum silicates. The production of Hi-Sil, a silicate with a high silica content, started in 1946 (12). The first pure precipitated silica was brought onto the European market in 1953 and was called Ultrasil VN 3 (13). 

Very finely divided colloidal silica powders can also be obtained by treating certain silicate minerals such as clay or calcium silicate with acid, followed by suitable heat treatment in an alkaline medium. Similarly, finely divided colloidal silicas can be produced by precipitating silica from a solution of sodium silicate with carbon dioxide. Such precipitated silicas are commonly used as reinforcing fillers, for elastomers because they are extremely finely divided, and the ultimate particles are easily broken apart. Finely divided aerogels of silicas may be employed, such as those described by Kistler in U.S. Pat. Nos. 2,093,454 and 2,249,767. 

Silicas used as reinforcing fillers are mainly obtained by precipitation (Legrand, 1998 EP0157703 et al., 1984 Chevallier andMorawski, 1986). The process basically consists in the preparation of a silica glass by alkaline fusion of pure sand and an alkaline salt. Then this glass is solubilized in water at high temperature and acid precipitated. The silica suspension obtained is then filtered, washed, and dried. 

Two major breakthroughs have transformed this facility product into a reinforcing filler that can achieve all carbon black mixes properties with, in addition, a decreased hysteresis of major interest for tire applications. The first step was made in the 1970s by Wolff, who proposed a specific silane coupling agent, the TESPT [11]. The second step arises in the 1990s with an R. Rauline s patent, which introduces the use of specific precipitation silica, elastomers, and mixing conditions to achieve reinforcement [12],... 

The precipitated silica and ZnO interact with each other and this decreases the reinforcement effect of the filler. 

In the rubber industry, silica is widely used as a non-black reinforcing filler to improve the mechanical properties of NR composites due to its high specific surface area, particularly tensile strength, tear resistance, abrasion resistance and hardness. Silica can be obtained by various methods, such as precipitated silica by precipitation of an aqueous sodium silicate solution, fumed silica by pyrogenic process, silica from natural resources e.g. rice husk ash and fly ash), and in situ silica or silica sol by the sol-gel process. 

---