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GCC is widely used in thermoplastics, especially PVC and polyolefins. The biggest use is in plasticized PVC applications, especially in cable insulation, carpet backing, and floor tiles. Cost is a major consideration together with low oil absorption. Plasticized PVC can accept high levels of filler and loadings can be as high as 400 parts per hundred of resin (phr) in some applications. In cable insulation, very low levels of conductive soluble impurities are required.

Rigid PVC (unplasticized or u-PVC) can only accept lower filler loadings, and GCC levels are typically 2-40 phr, although low-grade products containing up to 100 phr can be found. The main applications are in extrusions, such as pipes, cable management systems, vinyl sidings and window, and other building profiles.

Processing of u-PVC is a complex operation. Due to the thermal instability of PVC, the polymer granules need to be fused together rather than melted as is the case with other thermoplastics, and the speed and completeness of this process are critical. Fillers have a significant effect on this fusion process, and this has to be taken into account. The finer particle sizes usually give the most rapid and complete fusion. Fatty acid treatments also help. The finer particle-size grades also give the best physical and mechanical properties in u-PVC, especially when fatty acid is coated. Although u-PVC used to be a significant market for the very fine PCC, advances in milling technologies have led to this largely being displaced by GCC today.

GCCs are the main filler used in polypropylene (PP) compounds. Apart from cost, their main role is to increase stiffness without too much detriment to other properties, especially impact resistance. Higher aspect ratio fillers, such as talc and wollastonite,

Fig.3 An SEM image of a breathable film showing the polymer debonding from the GCC surface

are more effective at increasing stiffness, but have a greater detrimental effect on impact strength. Fatty acid coating aids processing and properties and is frequently used. The finer grades also give the best mechanical properties.

Microporous film is a special application for GCC in both PP and polyethylene (PE). The GCC particles are used to generate the micropores by debonding from the film when it is being stretched during production (see Fig. 3). In order to achieve the best results, a fatty acid-coated GCC with 1-2 ^m particle size and a narrow particle- size distribution are required, and special grades are offered for this application. One of the major uses for microporous film is in disposable diapers.

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