|Wood Plastic Composites (WPCs), with upto 90% wood fiber or wood flour content is gaining popularity. Wood filler, usually added in ratios of 40-60% of a given WPC formulation, adds stiffness and decreases the tendency of plastic to creep. Design professionals continue to discover the advantages of plastic composite building products, including durability, light weight, corrosion resistance, high strength and low maintenance requirements. Because WPCs are used almost exclusively outdoors and exposed to sunlight, rain, and freeze/thaw cycles, their development, especially in regard to product service life is a cumbersome process. WPCs are a complex blend of a hygroscopic material (wood flour) with a hydrophobic material (plastic) and assorted pigments and additives. Generally the polymer matrix is PP, PE, PVC. Over 50% of all WPCs are made with polyethylene (PE). But a small but increasing number of manufacturers use polypropylene (PP) and polyvinyl chloride (PVC), polymers that are stronger and stiffer, but more difficult to process. The polymer matrix or plastic resin used in wood-fiber composites can consist of:
• polyethylene (common in external building components, being explored for building profiles)
• Polyvinyl chloride (PVC) (used in window, decking)
• polypropylene (PP)
• acrylonitrile-butadiene-styrene (ABS)
WPCs are more rigid and have a lower thermal expansion coefficient than polymers, with the attractive appearances of wood. WPCs also are more against splintering and equal wood in terms of shape formations. The composite can be shaped by way of extrusion, injection-molding or compression molding to almost all types of parts: from small, complicated, or hollow parts and profiles to large three-dimensional parts. WPC producers are seeking improved products, with focus on physical property retention, colour retention, mould and mildew resistance. Because the WPC industry is relatively young, the long-term- 10-20 year durability of outdoor products like decking is still being proven in the field. To achieve good WPCs, additives are very critical and indispensible even though they are used at just small percentages, as they give WPCs sufficient stiffness,rigidity and good stability against light and heat.
A crucial contributor to quality of WPCs is flame retardants- as without flame retardants, WPCs will have very limited applications. Flames spread rather slowly in WPCs because of their high wood content, but they are highly combustible once they catch fire. Safety standards in construction and transportation can be met with the addition of flame retardants. Most flame retardants do not have major adverse effect on mechanical properties, they provide ecological and environmental benefits compared to some other flame retardants. They are non halogenated and do not release corrosive gases in case of a fire, and they also emit very low levels of smoke.
Coupling agent is very essential in achieving polarity. Typically polyolefins grafted with maleic anhydride act as coupling agents, significantly improving mechanical qualities. The polar functional groups of the coupling agent react amongst others with the OH groups of the wood and form a genuine chemical bond while the non-polar areas of the coupling agent have strong affinity to the non-polar polyolefin chains. Coupling agents improve adhesion between resin and fibres, which reduces rate of moisture absorption and increases strength properties such as impact, tensile and flexural strength. Coupling agents also aid in fibre dispersion, which may allow increased wood content. Coupling agents also aid in foaming by aiding dispersion and by improving melt strength at the die exit.
As demand is on the rise for more colour variety, especially dark wood colours, a wider colour palette of pigments is being designed for WPCs. Light stabilizers are also needed to maintain the original colour as WPC products are used in outdoor applications. Light stabilizers are also essential to impart weather resistance; they arrest brittleness due to weathering. HALS is considered to be good light stabilizers for both PE and PP based WPCs.
Biocides are needed to control mould and mildew, driven both by consumer complaints of surface staining or discoloration and by concern for long-term durability. Addition of antimicrobials protects the wood in WPCs against decay fungi as well as inhibiting surface mould growth, and also slows down the effects of UV degradation. Additives are being developed to stabilize the wood portion to prevent it from lightening.
Lubricants are widely used in WPCs to improve extrusion rates, and several new products claim better compatibility with other additives and greater processing improvements. Foaming of WPCs is becoming increasingly important as they offer benefits such as weight and cost reduction, increases stiffness. Foaming makes the WPC more like wood for cutting and fastening, and maintains cost-competitiveness. Although foaming increases process complexity and may reduce throughput rate, the raw material savings outweigh other issues.
High-performance additives transform WPCs into a true high-tech material for many demanding applications. The aesthetic and low maintenance properties of wood-plastic composites are continuing to drive growth in wood replacement applications. They are being increasingly recommended for porch flooring, railings, or various profiles. The furniture industry and automotive suppliers are also increasingly using WPC for chairs and shelf elements as well as for interior components of passenger cars. Volumes are growing, both in North America where the market is well established, but now also in Europe where it is now starting to get a foothold. Demand for wood plastic composite is expected to rise in quest for minimal maintenance requirements, excellent weatherability and high resistance to wear and tear in construction applications.