Specific Gravity of Wood-Plastic Composites and Its Effect on WPC Properties

DENSITY (SPECIFIC GRAVITY) OF WOOD-PLASTIC COMPOSITES AND ITS EFFECT ON WPC PROPERTIES 

Importance of density (specific gravity) of wood-plastic composite (WPC) materials cannot be overestimated. By density we here mean not an absolute density of different WPCs, but a density of the same WPC material that can be lower compared to the highest possible density of the same WPC, determined by specific gravities of its ingredients. 

Let us consider an example of, say, a Trex deck board. It consists of 50% w/w polyethylene (LDPE/LLDPE and/or HDPE) and 50% w/w of wood flour. Specific gravity of LDPE/LLDPE is 0.925 g/cm and that of HDPE is 0.96 g/cm Specific gravity of wood flour is 1.30 g/cm These two ingredients define the density of the composite material at their natural compaction, which would be 

08 g/cm for LDPE/LLDPE-based Trex and 1.10 g/cm for HDPE-based Trex (see the insert for calculations). Trex reported that the actual density is 0.91-0.95 g/cm (Trex data). Hence, 14-21% of all Trex composite volume is taken by voids, porosity. 

It is practically impossible to make industrial WPC boards without any porosity, hence, without any decrease in density compared to its theoretical value. Even traces of moisture in wood/cellulose fiber create steam at hot melt tempera-tnres, hence, porosity. Plastic decomposition during processing prodnces volatile organic compound (VOC), hence, porosity. Wood extractives decomposition prodnces VOC, hence, porosity. Wood fibers lignin decomposition at plastic hot 

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