Wood-plastic composites: current trends in materials and processing The aesthetic and low maintenance properties of wood-plastic composites are continuing to drive growth in wood replacement applications. 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. Jennifer Markarian reports for Plastics Additives & Compounding.

September / October 2005

Wood-plastic composites are widely used in decking (Photo: Chemtura).

The global wood and natural fibre composites (WPC) market was 771 million kg (1.7 billion lbs) in 2003, says Jim Morton, partner at U.S.-based Principia Partners, a marketing and business consulting firm for the plastics and related industries that has organized a WPC conference for October 2005 in the U.S. Nearly 85% of the WPC market is in North America, primarily due to success in building product applications. WPC's aesthetic and 'low-maintenance' advantages over wood continue to drive growth in wood-replacement applications.

In North America, the WPC market will continue to experience double digit growth 2003 to 2010, says Principia. The decking market was the original driver for WPC growth, and continues to account for a nearly 65% share of the WPC market. WPC decking is estimated to have penetrated nearly 15% share of the North American decking market. North American WPC producers are seeking to translate WPC into other building product areas, such as fencing, trim board, windows, and WPC sheet for replacing plywood. WPC is also being considered for roofing and siding. The industry is increasing capacity to meet growing demand.

Wood plastic composite mouldings from Atlas Precision.

The European WPC market is small but growing, with mostly small manufacturing players, say industry experts. The European market, excluding natural fibres other than wood, was 65,000 tonnes in 2003, estimates Geoff Pritchard, consultant at the Hackwell Group, a UK-based plastics consultancy that publishes reports on the WPC market. Of this, 36,000 tonnes was in automotive applications, the largest sector for European WPC in 2003. Demand for WPC in automotive is expected to be steady, but under pressure of potential replacement by flax and other natural fibre. Non-automotive uses such as construction, furniture and infrastructure are increasing, adds Mr. Pritchard. WPC in Europe are still too expensive, since Europe lacks a large, established market to provide economies-of-scale, such as the decking market in the U.S., comments John Nash, consultant at UK-based Applied Market Information Ltd, which is organizing its annual WPC conference for December this year in Austria. While North America has an established supply chain, the channel to market is different for each European country. "Problems in the European WPC industry are largely commercial rather than technical. We see a lack of industry leadership to overcome these problems," says Mr. Nash. "We see trepidation in Europe about using wood flour because there is no large decking market now, but we think decking will take off in the UK and in Europe. The European market has a huge potential, and is just waiting for one or two large players to start producing in Europe," predicts Simon Lewis, sales development manager at WTL International Ltd., a UK-based wood flour company that sells wood flour into decking and other applications. WTL introduced Natraplast® wood flour-polypropylene compounds about three years ago, as a development tool for companies to test-market WPC parts. Natraplast is currently being used primarily in injection moulding applications, where processors are exploring new products and markets.

WPC decking is ideal for demanding outdoor applications. (Photo: Chemtura).

While the WPC market outside North America and Europe is very small, there is potential for growth. "We see interest in WPC not just in North America but Europe, South America and Asia. Interest seems to be in opportunities to use WPC products domestically, rather than in export to North America," comments Mike Fulmer, product manager for plastics additives at Struktol Company.

WPC's growth rate has been limited by a lack of consumer awareness, say industry experts in both North America and Europe. National ‘do-it-yourself’ home improvement retail stores in North America have begun marketing and advertising WPC decking, which should lead to expanded consumer awareness and increased market share in North America. "Consumer awareness has improved over the last couple of years, but the average person is still not aware of the features and benefits of WPC," says Mr. Crostic. Lack of consumer awareness is even more serious in Europe, with no advertising by national stores to help educate consumers, says Mr. Nash. A lack of established performance standards and lack of performance consistency are other obstacles to growth, say industry experts. Building code regulations and standards were established around existing materials and are not necessarily applicable to WPC, notes Mr. Nash. In North America, the newly formed Composite Fence, Deck and Railing Manufacturers Division of the American Fence Association (AFA) will be working on WPC standards. The European Committee for Standardization, known as Comite European de Normalistion (CEN), through its working group 13, is also looking at WPC test standards.

Top: Fruiting body (mushroom from decay fungus) that appeared on an untreated WPC sample that was exposed at a US Borax field site in Hawaii. These fruiting bodies only appear after the decay fungi has matured and consumed a significant quantity of wood. Middle: WPC product treated with Borogard ZB from Borax. Bottom: Untreated WPC product.

Foaming of WPC is becoming increasingly important. The primary benefits of foaming are weight and cost reduction. In addition, foaming increases stiffness and makes the WPC more like wood for cutting and fastening. "Escalating raw material prices, especially of resins, are compressing WPC margins. Foaming helps WPC maintain cost-competitiveness. Although foaming increases process complexity and may reduce throughput rate, the raw material savings outweigh other issues," explains Mr. Morton. Reduced weight makes WPC easier to use in both current applications and new applications such as fencing. Amorphous polymers such as polyvinyl chloride (PVC) and polystyrene (PS) foam well because they have good melt strength. Crystalline polymers such as polypropylene (PP) and linear polyethylene (PE) are more difficult to foam because they have lower melt strength and typically contain calcium stearate, which may act as a de-foamer, says Mr. Reedy. Choosing a compatible lubricant is also important, because some polar lubricants are active de-foamers, he adds. Water absorption in finished parts has been a problem for foamed WPC because the freeze-thaw cycle forces the composite apart. Reedy's patent-pending Saftec® AS reduces water absorption. For example, a PP formulation with 60% wood using Saftec AS absorbed 9% water, compared to more than 25% water in typical WPC and 200% water in real wood. Saftec AS acts as both a stabilizer that allows longer processing at higher temperatures and as a processing aid that improves flow characteristics, says the company.

Additives improve properties and processing
WPC producers are seeking improved products, with focus on physical property retention, colour retention, and 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. "The industry is more aware of long-term degradation issues and is taking steps to prevent problems by using additives," comments Bill Crostic, president of WC Consulting, Inc. As formulations become more engineered, users must understand the potential for negative additive interactions. This is leading to more tailor-made products and packaged blends coming from additive suppliers as well as masterbatch producers. "We expect to see more formal and informal agreements between additive suppliers as they cooperate to benefit WPC customers," says Mr. Fulmer at lubricant producer Struktol, which has begun providing lubricant-additive blends to address both process performance and end-product performance.

Biocide use to control mould and mildew in WPC decking is growing, driven both by consumer complaints of surface staining or discoloration and by concern for long-term durability. Several manufacturers have announced the addition of antimicrobials to their products, notes Ciba Specialty Chemicals, which provides Irgaguard® F antifungal additives for WPC. "Consumers expect mould and mildew protection as part of the WPC 'low-maintenance' requirement. Our consumer research shows that consumer satisfaction is linked to aesthetic and visual cues like discoloration or staining. WPC decking manufacturers are beginning to see biocides as a means of protecting their brand reputation," says Chris Springer, market manager for WPC biocides at Rohm and Haas Company. "In WPC decking, staining is caused by surface fungi. Surface fungi are treated with a broad-spectrum biocide that is effective in both plastics and wood applications," explains Mr. Springer. Rohm and Haas recently introduced two new Vinyzene DCOIT-based formulations that are designed to provide efficacy against surface fungi in WPCs. While surface fungi cause visible mould stains, decay fungi can eat away at any wood portion throughout the composite that is not fully encapsulated by plastic, causing the type of wood rot typically seen in unpreserved wood decking, says Mark Manning, manager of preservation technology at U.S. Borax. Laboratory tests show decay fungi can cause 10-20% weight loss, indicating a potential problem for long-term durability, he explains. Borax's Borogard® ZB zinc borate preservative protects the wood in WPC against decay fungi as well as inhibiting surface mould growth. Borogard ZB also slows down the effects of UV degradation, says the company.

WPC moulding (Photo: Courtesy of Atlas Precision).

Interest in coupling agents for WPC is growing as the industry becomes more sophisticated and aware of long-term product performance needs, say industry experts. 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. Coupling agents can improve the properties of composites with non-wood fibres, which typically are lower than properties of wood-fibre composites, giving users more fibre supply options notes Charlie Giaudrone, global industry leader for building and construction at Eastman Chemical Company. Several coupling agents are currently available for PE-WPC. Chemtura, formerly Crompton and Great Lakes, offers Polybond 3029 and Eastman offers Epolene G-2608. A new DuPont coupling agent for PE-WPC, DuPont™ Fusabond® W PC-576D, is an ethylene copolymer with anhydride functionality incorporated into the polymer backbone. The new coupling technology contains higher levels of anhydride than earlier grades. This creates more sites for chemical links, enabling significant performance improvements at low additive levels, explains Megan O'Brien, marketing programs manager for DuPont Industrial Polymers. Both Chemtura and DuPont are developing new coupling agents for polypropylene (PP)-WPC to meet a need brought about by the growing use of PP both in Europe and North America. There is also a market need for improved coupling agents for PVC-WPC. Further developments in coupling agents will be needed as WPC products aim for higher strength and durability to enter into applications such as siding or structural parts. "The current technology does not have the flex strength needed for structural parts. Companies are working on developments in both additives and fibres," comments Mr. Giaudrone.

Consumers and WPC producers are looking for more colour variety, especially dark wood colours like mahogany, notes Ciba, which is developing a wider colour palette of pigments designed for WPC. Light stabilizers are also needed to maintain the original colour, notes the company. "While modest colourfastness improvements have been made, there is still room for improvement. We have ongoing projects to develop integrated colorant and additive solutions to improve weather fastness, colourfastness, and physical prop-erty retention," says Ciba. There is a greater demand for WPC that is colourable, does not stain, and has a grained ‘natural wood’ look. Special effects additives that give a wood-grain appearance have been used for years in siding and are now being sought for WPC applications, notes John Hensler, business manager for NAFTA extrusions at Americhem, a U.S.-based masterbatch producer. Textured products provide marketing appeal in niche markets and are poised to grow, says Peter Bins, product manager at NorthWood Plastics, a North American WPC compound producer.

WPC decking (Photo: Chemtura).

The trend towards higher throughputs continues, as profile producers try to drive costs down to compete with pressure-treated wood. Lubricants are widely used in WPC to improve extrusion rates, and several new products claim better compatibility with other additives and greater processing improvements. While metallic stearate lubricants interfere with maleic anhydride (MA) coupling agents, alternative lubricant technologies are avail-able, notes Struktol, which has introduced new customer-specific blends of lubricants with other additives. Ferro's new SXT 2000 products are blends of metallic stearates and non-metallic lubricants, while the patent-pending SXT 3100 formulations are metallic stearate-free. Chemtura is also developing lubricants that do not interfere with coupling agents. Reedy's Saftec WLB Edge 100 allows increased extrusion rates and maximizes wetting to both polar and non-polar fillers, says the company. The product does not contain metal soaps and has a low acid value.

For the next generation of building and construction applications beyond decking, additives such as flame retardants and inorganic mineral fillers are being investigated. The WPC industry is rethinking the use of inorganic mineral modifiers such as talc and calcium carbonate in combination with wood fibres in WPC, says Mr. Morton. Minerals are not used solely as fillers, but can be used to increase mechanical properties. Mineral modifiers may also play a role in improving flame retardancy.

The FT80 twin screw extruder from Cincinnati Extrusion.

Processing
Methods of handling the high moisture content found in wood and natural fibres are a concern for processors. Because dry wood flour has an explosion risk, some WPC processors would like to feed undried flour to the extruder. While several equipment suppliers have developed extruders that can handle the high moisture contents found in wood and natural fibres, others caution that using extruders to dry fibre or wood flour can cause problems. "Because the moisture content of wood fibre, for example, varies from 4-10%, a loss-in-weight feeder metering a constant weight may not feed a consistent percentage of fibre," explains Paul Godwin, composites and compounding systems sales manager at American Maplan. Fibre should be dried to consistent moisture content prior to compounding, he says. For pre-pelletized compound users, NorthWood Plastics offers AriDry WPC compounds that contain less than 0.3% moisture, reducing drying costs and time for processors, says the company.

American Maplan continues to develop its two-stage planetary extruder for compounding WPC materials. The planetary extruder is designed to give high dispersive mixing using low shear, which is less damaging to fibres than the higher shear in co-rotating twin-screw extruders, says Mr. Godwin. The planetary extruder can process materials with up to 4% moisture, he notes. American Maplan also offers counter-rotating twin screw extruders designed for the direct extrusion of natural fibre composite profiles.

Cincinnati Extrusion's fourth-generation of Fiberex counter-rotating twin-screw extruders are designed for direct extrusion of WPC. Dosing devices, venting zone configuration, screw design, and corrosion-resistant materials enable processors to run high loadings of wood or natural fibres, says Hans Matthesius, president of Cincinnati Extrusion, Inc. Cincinnati's FiberexA 135/136
parallel, co-rotating twin-screw extruder designed for high-speed decking production should be commercially available by the end of 2005.

Contacts:
American Maplan Corporation
Tel: +1 620 241 6843
Website: www.maplan.com

Americhem
Tel: +1 800 228 3476
Website: www.americhem.com

Applied Market Information, Ltd.
Tel: +44 117 924 9442
Website: www.amiplastics.com

Borax
Tel: +1 661 287 5400
Website: www.borax.com

Chemtura
Tel: +1 203 573 2000
Website: www.chemtura.com

Ciba Specialty Chemicals
Tel: +1 914 785 2000
Website: www.cibasc.com/plasticadditives

Cincinnati Extrusion
Tel: +1 859 746 1530
Website: www.cet-austria.com

DuPont Industrial Polymers
Tel: +1 302 774 1000
Website: www.dupont.com/industrial-polymers/woodalternatives

Eastman Chemical Company
Tel: +1 800 327 8626 or +1 423 229 2000
Website: www.eastman.com/adhesives

Ferro Polymer Additives
Tel: +1 216 750 6647
Website: www.ferro.com

Hackwell Group
Tel: +44 1892 525576
Website: www.woodplasticcomposites.org

North Wood Plastics, Inc.
Tel: +1 800 571 0158
Website: www.northwoodplastics.com

Principia Partners
Tel: +1 484 459 2239
Website: www.principiaconsulting.com

Reedy International
Tel: +1 732 264 1777
Website: www.reedyintl.com

Rohm & Haas
Tel: +1 800 356 3402
Website: www.rohmhaas.com

Struktol Company
Tel: +1 800 FAST MIX
Website: www.struktol.com

WC Consulting, Inc.
Tel: +1 785 766 8575
E-mail: wcconsult@aol.com

WTL International Ltd.
Tel: +44 1260 223284
Website: www.wtl-int.com