Wood-plastic composites have seen a tremendous growth in the last 2-3 years. Wood plastic composites are believed to be growing at an average rate of about 20% and consumption is expected to reach more than 1 million tons by the turn of this decade. Polyolefin based wood-composites have a major market share, followed by PVC. Styrenics also have some market share. Among Polyolefins, HDPE seems to have a major share. In fact, recycled HDPE finds an avenue in this sector.

Typically, these composites have more than 50% wood flour. Of late, talc has also found usage as a partial replacement for wood, as it apparently enhances mechanical properties.
Higher level of wood primarily need a coupling agent to achieve dispersion in the polymer matrix, particularly because wood is much polar compared to the polymer matrix. More coupling agent is required when the polymer in usage belongs to polyolefins, essentially non polar. Typically reactive polyolefins such as maliec anhydride modified polyolefins are used at about 1-3% levels. Crompton, Dupont, Eastman and Equistar offer several grades of coupling agents specially designed for wood-Polyolefin composites. Clariant also offers coupling agent masterbatches for improvement of the dispersion. Crompton also offers a compatibilizer for PVC matrix.

Lubricants increase throughput and improve surface appearance. Standard lubricants for polyolefins and PVC such as ethylene bis-stearamide (EBS), zinc stearate, paraffin waxes and oxidized PE can be used. Lubricants are being developed to eliminate the adverse reaction of metallic stearates on the coupling reaction in HDPE. Wood composites use about twice as much lubricant as standard plastics. For HDPE, with a typical 50% - 60% wood content, lubricant level can be 4% to 5%, while a similar wood-PP composite typically uses 1% - 2%. Total lubricant level in wood-PVC is 5 to 10 phr. Crompton, Lonza, Struktol have developed special lubricants for wood-Plastic composites. Clariant offers a fluroelastomer type lubricant.

There is keen interest in chemical foaming of WPCs to reduce weight and material cost and to improve surface appearance, processing speed, and ease of sawing, nailing, screwing as well as painting finished products. About 20% of all WPC products, mostly wood-PVC, are currently foamed.

Both endothermic and exothermic chemical foaming agents (CFAs) are used. Foaming is tougher for crystalline polymers like PE and PP as compared to amorphous polymers like PVC and PS, as the latter generally have better melt strength. When higher wood content is added to the mix, as in polyolefin decking products, the challenge is particularly daunting. Mold, mildew, and stains on some WPC decking are driving manufacturers to consider antifungal biocides that protect either the plastics component and maintain its surface appearance or that preserve the wood component from decay and also reduce moisture absorption. Many products can do both the jobs.

Ciba offers Irgaguard F3000, a thiazolyl benzimidazole said to be a broadly effective fungicide for wood-polyolefin and wood-PVC composites. Developmental EB 43-25 combines Irgaguard F3000 with a proprietary additive. It is also a broad-spectrum fungicide and can eliminate or reduce stains due to interactions of iron, tannin and moisture.
U.S. Borax offers Borogard ZB zinc borate as a preservative for wood-polyolefin and wood-PVC composites. It has broad-spectrum activity against wood-destroying organisms plus heat and UV stability and resistance to leaching and weathering.
Rohm and Haas sells Vinyzene biocides, which are based on dichloro-octyl-isothiazolone (DCOIT) for wood-PVC composites. Newly commercial Vinyzene SB27 is a concentrate of DCOIT in a vinyl carrier.
The company is also field testing new SB27-ELV, which is 10% DCOIT in a polyolefin terpolymer, aimed at wood-polyolefin composites. This leaching-resistant and uv-stable product is said to be effective at 0.04% - 0.1%.

New developments for treating natural lumber planks also have potential for treating the wood flour used in WPCs to prevent color change. Lonza introduced an alternative to chromium copper arsenic (CCA), a carcinogenic wood preservative banned by the EPA in Dec.2003. Lonza's Carboquat is a quaternary ammonium compound that could be used to treat wood flour for WPCs. Ciba also has a new proprietary wood treatment that is ready for sampling. Struktol is working on anti-staining wood lubricants that will neutralize tannins.