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Currently, particulate mineral fillers, such as calcium carbonate and talc, dominate the filler market. This is because they are low cost, are fairly easy to extract and purify, have good thermal stability, and have little effect on color or water absorption of the final product. The main drawbacks of such products are lack of true sustainability and for some applications high specific gravity, with the lightest being amorphous silica (or oil-derived carbon blacks) at 1.8-2.0, while other common mineral fillers are in the range 2.5-3.0.

While plant-derived products have significant limitations, especially thermal stability, presence of deleterious impurities, color, and hydrophilicity, they are potentially more sustainable and also can often have specific gravities in the range 1.0—1.5. These two advantages are beginning to outweigh the limitations in some important applications, especially for automotives.

In order to offer good economics for volume applications, any filler has to be available in large quantities at any given location, which also needs to be near to the consumer. This limits the choice, with the main opportunities currently identified being wood flour, starch products, lignin, and rice hulls.

There are several different routes to particulate filler particles from plant sources. The first is through mechanical size reduction of plant materials such as wood. This is the simplest but generally results in complex mixtures, often with some significant limitations. Another method is the extraction and possibly size reduction of particulate species present in the plant. This is more complex but gives a simpler product with potentially superior properties. The final option is to extract a solution of a biopolymer and convert this into particulate form (e.g., by precipitation). Each of these approaches has received some degree of attention and is covered in the following sections.

 
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