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The Main Important Characteristics of Particulate Fillers
The cost of fillers varies enormously, with the spread from the crudest clays and carbonates to specialist synthetic types being over 100-fold. Large price variations are even experienced within one filler type, such as calcium carbonate, depending on factors such as color, purity, particle size and shape, and surface treatment. Transportation costs can also be very significant for the lowest-cost fillers and may well determine the choice of filler type and supplier. With increasing globalization, uniformity of filler grades across the world is becoming an important issue.
In considering potential cost savings from use of fillers, one must always bear in mind that they are generally considerably denser (two- to threefold) than the host polymer. Filler prices are generally quoted on a weight basis, but most applications are based on volume, and hence a significant correction has to be performed for a fair comparison to be made. A simplified view of the relative volume costs of fillers and polymers is presented in Table 1.
Factors other than the raw material cost also have to be taken into account in assessing whether cost savings will be obtained. One of the most important is any additional processing cost incurred. In many cases, the use of fillers increases processing costs due to the need for different equipment and higher energy costs and/or lower throughput. On the other hand, their use can give better heat transfer, speeding up cooling, and reducing cycle times. Any extra cost is much reduced with polymers that already have to be processed (e.g., to add curatives and stabilizers), and it is no accident that fillers are most widely used in such polymers (elastomers, thermosets, and PVC).
Filler effects on other ingredients, such as antioxidants and curatives, can also be significant. One would generally expect a reduction in the use of these per unit volume when filler is replacing some polymer. On the other hand, some fillers can negatively impact on polymer stability and also deactivate antioxidants and curatives, leading to the opposite effect. As with so much in this field, one has to consider each case on its own merits.
Table 1 Approximate relative volume costs of some common polymers and polymer grade particulate fillers (Note: commodity polymer and some filler prices are very much influenced by the price of oil so the above relationships must be treated with caution)
Despite the above, there are still significant areas where fillers are used principally because they give useful cost savings. These will particularly be where suitable compounding is already taking place to introduce other additives, as is the case with polyvinyl chloride (PVC), elastomers, and most thermosets, and where relatively expensive polymers, such as polyamides, are being used.
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