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Precipitated silicas are made up of small (10-30 nm) primary particles, aggregated into larger structures of complex shape. Because of their amorphous nature and high specific surface area, these products have a high absorbed water content (up to 7% w/w). In addition, the surface is highly hydroxylated, with the hydroxyls being equivalent to another 2-3% of water, although this is only released as such at much higher temperatures and does not contribute to the water content, as determined by conventional tests (e.g., drying at 105 °C).
This water plays an important role in polymer applications. It reduces the dustiness and appears to reduce particle/particle interaction, making dispersion easier. Both the absorbed water and surface hydroxyls are also important for reaction with the silane coupling agents frequently used in polymer compounds. On the other hand, the water often makes processing more difficult, as it has to be removed during compounding, if porosity is to be avoided during molding or extrusion. One little mentioned, but significant, advantage of all amorphous silicas is their relatively low specific gravity (about 2.0 for precipitated silica) compared with most other mineral fillers. Indeed, their specific gravity is not much higher than that of carbon blacks, and this is a distinct benefit in tire applications.
Because of their fine particle size and complex aggregate shapes, precipitated silicas in powder form have a low bulk density, usually about 0.2 kg per liter; as a result, they can also be very dusty. These problems are tackled by making densified and pelletized forms. While this sounds simple, it must be carefully carried out if the particles are to disperse readily in the final application. All the main producers offer such densified products, and they have been an important factor in the recent growth of the energy or green tire technology (see later).
For polymer applications, the very small size of the particles makes them most suited to elastomer use, where they can provide a similar reinforcing role to carbon blacks. Such applications require control over far more properties than is usual in the filler industry, and detailed measurements of size, surface characteristics, and aggregate structure are involved.
The main physical properties that are important are specific surface area and aggregate porosity. Specific surface area provides a guide to primary particle size. There are two measures of specific surface area in common use for fine fillers in the rubber industry: BET or nitrogen adsorption and cetyl trimethyl ammonium bromide (CTAB). The former uses a very small molecule (nitrogen) to access the surface, whereas the latter uses the much larger CTAB molecule. CTAB is regarded as being more representative of the surface available to the rubber molecules. As not all the surface detected by nitrogen is accessible to CTAB, the CTAB value is usually lower than the BET one.
Today, microporosity (a better term than the often used alternative of internal porosity, which wrongly implies closed pores) is also becoming an important parameter. This is usually determined by the difference in specific surface area when it is measured by the BET or nitrogen surface area method and the CTAB one. Microporosity is believed to be a good measure of ease of dispersion. Pore size distribution is also sometimes used as a guide to dispersability.
The primary particles of the precipitate are aggregated into secondary structures, and the porosity of these is of considerable significance for polymer applications. An old, but still useful, method for estimating the openness, or porosity, of the aggregates is the oil absorption test. In this test, a nonvolatile liquid which readily penetrates the pores is mixed with the precipitated silica until the mass loses its friable nature and can be molded into a putty. The oil absorption value is the amount of liquid just required to form this putty. Dioctyl phthalate (DOP) is often used as the liquid, and the results are usually expressed as milliliters of liquid per 100 g of powder. As mentioned above, precipitated silicas are subdivided into conventional, easy dispersing (ED), and highly dispersing (HD) grades. The more readily dispersing silicas are characterized by a more open structure, giving them a higher oil absorption. Thus, conventional precipitated silicas have dioctyl phthalate (DOP) oil absorption below 180 ml/100 g, ED silicas are in the range 180-200 ml/100 g, and HD silicas are above 220 ml/100 g.
The pH of the surface is also important, mainly because it may interfere with elastomer cure systems. Most precipitated silicas are slightly acidic, while the metal silicates are alkaline. Acidic surfaces have the most effect on elastomer cures, especially peroxide cures.
The hydroxylated nature of the surface of precipitated silicas also leads to poor wetting and dispersion in many polymers. Surface treatments are widely used to overcome this, the most common being organo-silanes (often referred to as coupling agents). These additives can be pre-reacted with the filler, or added during the composite manufacturing process, with the latter being by far more usual. The high specific surface area and high hydroxyl density means that very significant amounts of surface modifier (up to 10%) are needed to fully treat the surface. This level of treatment is expensive and proves to not be necessary or even useful in many applications. Tire treads are a notable exception and require high levels, approaching complete (monolayer) surface coverage.
The density of surface hydroxyls is of considerable significance and can be measured by titration with sodium hydroxide in strong salt solution. This gives what is known as the Sears number (Heinroth et al. 2008).
In addition, compacted forms are generally required by the tire companies for their ease of dosing. This must be achieved while still retaining good dispersability and adds further to the production complexity and costs.
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