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GCC is also widely used in bulk thermoset applications, especially in unsaturated polyester (UPR) formulations such as sheet and bulk molding compounds. In many of these applications, it is used in conjunction with glass fiber. The main requirements of the GCCs used in these applications are low cost, low moisture content, absence of troublesome impurities, and low oil absorption. The latter is particularly important as high filler loadings are usually required. UPR polymers contain high levels of polar groups which can interact strongly with the filler, and so surface treatments of the fatty acid type are not usually used, although the formulation may contain dispersants to reduce viscosity.
GCC is also frequently used in commodity elastomer applications, especially carpet backing based on lattices such as styrene butadiene rubber (SBR) and in general
Table 3 Effect of calcium carbonate particle size (as indicated by specific surface area) on the properties of a sulfur-cured SBR compound
rubber goods (GRG). It is little used in tire applications. Most elastomers are inherently weak unfilled and require small particle-size fillers to provide sufficient strength for most applications. Because of their size, only the finest GCCs are able to provide much reinforcement, and so they are commonly used in conjunction with more reinforcing fillers, such as carbon black or precipitated silica. Both uncoated and fatty acid-coated GCCs are used, and fatty acids are also frequently present in the formulation and able to coat the filler surface “in situ.” The effect of calcium carbonate particle size on reinforcement of an SBR rubber is illustrated in Table 3. It can be seen how reducing the particle size of the GCC brings its performance closer to that of PCC, although the magnitude of the effect varies with the property being measured.
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