Precipitated hydrated silica

Hydrated precipitated silica has been used in the rubber industry over 40 years. Not only is hydrated precipitated silica a major inorganic filler used in rubber compounding to impart reinforcement, it is also commonly used as part of the HRH in situ rubber-to-metal adhesion systems just discussed. 

Hydrated precipitated silica Precipitated hydrated silica Hydrated silica Precipitated silica 

SiC + Natural Silica 2NaOH Sodium Hydroxide A Na2SiC + Sodium Silicate 

Si02 + Natural Silica Na2C03 - Sodium Carbonate A Na2Si03 + Sodium Silicate 

The following ASTM standards can be used to classify different grades of hydrated precipitated silica used in rubber compounding and HRH systems. 

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The surface of hydrated (precipitated) silica is highly polar and hydrophilic due to its numerous silanol groups. The polar surface chemistry assists the development of better... 

Hydrated precipitated silica is derived from sodium silicate (water glass) shown in Figure 5.30. 

Figure 5.30 Derivation of hydrated precipitated silica from sodium silicate, i.e., water glass Synonyms...

Hydrated precipitated silica used with general-purpose elastomers in HRH adhesive systems comes from water glass (sodium silicate), which is made from natural silica (sand) as shown in Figure 5.31, where it is reacted either with sodium hydroxide or sodium carbonate at very high temperatures. 

Hydrated precipitated silica has many other nonrubber uses including coatings and plastics. 

While it is possible to get some adhesion using HRH without the hydrated silica, many times insufficient adhesion is imparted. Hydrated precipitated silica is a very important component of HRH, if this system is being used to achieve adequate rubber-to-... 

There is some history of shortages in hydrated precipitated silica because of increasing demand brought on by the need for improvements in tire rolling resistance and the fact that there are relatively few producers of this filler. 

The tire industry has grown somewhat dependent on using hydrated precipitated silica in tread compounds to achieve better rolling resistance without the loss of traction or wear resistance. The lower per-pound energy requirement to produce this silica vs. carbon black also makes this white filler more green than carbon black. 

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. 

Since fumed silica is generally used only in silicone rubber compounds, the rubber industry uses far less fumed silica than hydrated precipitated silica. On the other hand, fumed silica demand is growing significantly faster than the hydrated precipitated silica normally used. 

Fumed silica has an ultimate particle size as small as 5 nm, which is finer than hydrated precipitated silica. With a conventional Banbury mixer, a filler this fine would be very difficult to disperse in a general-purpose elastomer compound. Fumed silica can be dispersed using a dough mixer in a gum silicone to achieve the ultimate reinforcement improvement. Fumed silica costs more to produce than hydrated 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). 

Today, for the hydrated precipitated silica to be effective in reducing the rolling resistance in a tire tread, it must be used with an organosilane. The most common organosilane used today as a rubber compounding ingredient for use with silica is TESPT (bis-[3-(triethoxysilyl)propyl] tetrasulfide). 

Sodium carbonate reacts with natural silica at high temperatures to produce sodium silicate (water glass). An aqueous solution of sodium silicate, when treated with an acid, will precipitate silica with a fine particle size when conditions are controlled properly. This hydrated precipitated silica is commonly used in rubber compounding to achieve a more resilient (lower energy loss during flexing) cured rubber and to achieve better adhesion of rubber to steel tire cord. 

Sodium silicate (water glass) under acidic conditions can precipitate hydrated, precipitated silica for use as a filler for rubber. 

Anhydrous, fumed, silicas have a higher li than hydrated, precipitated silicas and, moreover, the lower the specific surface area, the lower the dispersive component, irrespective of the production process. According to Wang et al., the difference in the dispersive component between both types of synthetic silica might depend on the surface topology of their particles, and less on the concentration of hydroxyl groups. 

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