Wood-Plastic Composites Mineral Fillers

COMPOSITION OF WOOD-PLASTIC COMPOSITES MINERAL FILLERS 

Minerals, such as calcium carbonate, talc, silica, are quite common fillers in plastic industry. They, often at abont 6-15 cent/lb, replace a much more expensive plastic, increase stiffness of the filled product, and render the plastic more flame resistant. The world filler market for plastics is dominated by carbon black and calcium carbonate. Of abont 15 billion pounds of filler in America and Europe, about half the filler volume goes into elastomers, a third into thermoplastics, and the reminder into thermosets. About 15% of all manufactured plastics contain fillers. 

However, only few wood-plastic composites (WPCs) use mineral fillers along with cellulose fiber. Table 4.1 lists these composites. Some of them contain minerals only because they employ rice hulls as a principal filler, as rice hulls typically contain 19 1% of silicates. Hence, 50% of rice hulls (by weight) in WPG results in about 9.5% of minerals. In comparison, wood flour typically contains about 0.25% of minerals by weight, and 50% of wood filler would result in only 0.125% (w/w) of minerals in WPG. 

This chapter is not a comprehensive description of fillers in plastics and of the respective properties of the resulting materials. Books are written on this subject (see, for example. Ref. [1]). This chapter aims at a brief description of minerals which are either used as fillers in WPG or can be used readily, taking into account their cost and value-added properties. Besides, this chapter gives some examples of properties of WPGs after mineral fillers were added to the composition. 

Wood-Plastic Composites, by Anatole A. Klyosov Copyright 2007 John Wiley Sons, Inc. 

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COMPOSITION OF WOOD-PLASTIC COMPOSITES MINERAL FILLERS TABLE 4.3 Effect of inorganic fillers and wood flour on flexural strength and flexural modulus of polypropylene (homopolymer) [2] ... 

B. English, N. Stark, and C. Clemons. Weight reduction wood versus mineral fillers in polypropylene. In Fourth International Conference on Wood-Plastic Composites, Forest Product Society, Madison, WI, May 12-14, 1997, pp. 237-244. 

Mineral fillers, such as calcium carbonate, have linear coefficients of thermal expansion (CTE) about 20 times lower than those of plastics and about 10 times lower than those of wood-plastic composites (WPCs). CTE of minerals are close to CTE of wood (along grain), which in turn has CTE of 0.17 - 0.25 X 10 1/F (0.31 - 0.45 X 10 1/C). CTE of wood will be discussed below in more detail. 

Second generation wood plastics composites may also contain mineral fillers or nanoflUers. The latter could be used to overcome any doubts about mechanical strength and incidentally reduce the flammability. 

B. -J. Kim, F. Yao, G. Han, Q. Wang, and Q. Wu, Mechanical and physical properties of core-shell structured wood plastic composites Effect of shells with hybrid mineral and wood fillers. Composites Part B Engineering 45(1), 1040-8 (2013). 

Considering that plastic often takes 40-50% w/w of flame retardants, there is no room for wood filler in WPC, which will not be WPC anymore but rather a mineral-filled plastic. At any rate, replacement of wood fiber of 3-5 0/lb with mineral flame retardant of 20-30 0/lb would significantly increase the cost of the resulting material. All these questions pose a great challenge to WPC manufacturers aiming at fireproof composite deck boards. 

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