Extrusion, Wood

Interfacing Wood-plastic composites industries in the U.S.

Interfacing Wood-plastic composites industries in the U.S.

The term wood-plastic composites refers to any composites that contain wood (of any form) and thermosets or thermoplastics. Thermosets are plastics that, once cured, cannot be melted by repeating. These include resins such as epoxies and phenolics, plastics with which the forest products industry is most familiar. Thermoplastics are plastics that can be repeatedly melted, such as polyethylene and polyvinyl chloride (PVC). Thermoplastics are used to make many diverse commercial products such as milk jugs, grocery bags, and siding for homes.

Wood-thermoset composites date to the early 1900s. An early commercial composite marketed under the trade name Bakelite was composed of phenol-formaldehyde and wood flour. Its first commercial use was reportedly as a gearshift knob for Rolls Royce in 1916 (Gordon 1988). Wood-thermoplastic composites have been manufactured in the United States for several decades, and the industry has experienced tremendous growth in recent years. This article focuses on wood-thermoplastic composites, which are most often simply referred to as wood-plastic composites (WPCs) with the understanding that the plastic is a thermoplastic.

The birth of the WPC industry involved the interfacing of two industries that have historically known little about each other and have very different knowledge, expertise, and perspectives. The plastics industry has knowledge of plastics processing, and the forest products industry has more experience and resources in the building products market. Not surprisingly, some of the earliest companies to produce WPCs were window manufacturers that had experience with both wood and plastics.

The plastics industry has traditionally used talc, calcium carbonate, mica, and glass or carbon fibers to modify the performance of plastic; about 2.5 billion kg (5.5 billion lb.) of fillers and reinforcements are used annually (Eckert 2000). The industry was reluctant to use wood or other natural fibers, such as kenaf or flax, even though these fibers are from a renewable resource and are less expensive, lighter, and less abrasive to processing equipment than conventional fillers. Most plastics processors ignored wood fiber because of its low bulk density, low thermal stability, and tendency to absorb moisture. The majority of thermoplastics arrive at a manufacturer as free-flowing pellets or granules with a bulk density of about 500 kg/m^sup 3^ (31 pcf). The plastics processor is faced with the problem of how to consistently meter and force the low bulk density wood fiber into small feed openings typical of plastics processing equipment. In addition, the processing temperature for even low melting point plastics is often too high for incorporating wood fiber without thermal degradation. The high moisture content of wood and other natural fibers is also problematic to the plastics industry, which considers about 1 to 2 percent moisture content high. Even plastics processors with vented equipment capable of removing moisture during processing were averse to removing 5 to 7 percent moisture from wood fibers. Resin dryers, which are occasionally needed to dry plastics, are not appropriate for wood particles or fibers, and drying the fine wood particles poses a fire hazard. Plastics processors who tried to use wood or other natural fibers often lacked knowledge about wood, and their failed attempts made the industry generally skeptical of combining wood and plastic.

For the wood products industry, thermoplastics were a foreign world, albeit one that occasionally intruded on traditional markets (e.g., vinyl siding). Competing in different markets, forest products and plastics industries had few material and equipment suppliers in common and they processed materials very differently and on entirely different scales (Youngquist 1995).

The perspective of some plastics industries has changed dramatically in the last decade. Interest has been fueled by the success of several WPC products, greater awareness and understanding of wood, developments from equipment manufacturers and additive suppliers, and opportunities to enter new markets, particularly in the large-volume building applications sector. Forest products industries are changing their perspective as well. They view WPCs as a way to increase the durability of wood with little maintenance on the consumer's part (one of the greatest selling points). Some forest products companies are beginning to manufacture WPC lumber and others are distributing this product. These ventures into WPCs are being driven by customer demand and opportunities based on the industry's experience in building products). (Anonymous 2001).

Brief History

In the United States, WPCs have been produced for several decades, but they were produced even earlier in Europe. However, major growth in the United States did not occur until fairly recently. This section describes some historical developments in the U.S. WPC industry through the mid-1990s.

In 1983, American Woodstock, now part of Lear Corporation in Sheboygan, Wisconsin, began producing automotive interior substrates using Italian extrusion technology (Schut 1999). Polypropylene with approximately 50 percent wood flour was extruded into a flat sheet that was then formed into various shapes for interior automotive paneling This was one of the first major applications of WPC technology in the United States.

In the early 1990s, Advanced Environmental Recycling Technologies (AERT, Junction, Texas) and a division of Mobil Chemical Company that later became Trex (Winchester, Virginia) began producing solid WPCs consisting of approximately 50 pent wood fiber in polyethylene. These composites were sold as deck boards, landscape timbers, picnic tables, and industrial flooring (Youngquist 1995). Similar composites were milled into window and door component profiles. Today, the decking market is the largest and fastest growing WPC market.

Also in the early 1990s, Strandex Corporation (Madison, Wisconsin) patented technology for extruding high wood fiber content composites directly to final shape without the need for milling or further forming. Strandex has continued to license its evolving technology.

Andersen Corporation (Bayport, Minnesota) began producing wood fiber-reinforced PVC subsills for French doors in 1993. Further development led to a wood-PVC composite window line (Schut 1999). These products allowed Andersen to recycle wastes from both wood and plastic processing operations. The market for WPC window and door profiles has continued to grow.

In 1996, several U.S. companies began producing a pelletized feedstock from wood (or other natural fibers) and plastic. These companies provide compounded pellets for many processors who do not want to blend their own material. Since the mid-1990s, activity in the WPC industry has increased dramatically. Technology is developing quickly and many manufacturers have begun to produce WPCs.

In 1991, the First International Conference on Woodfiber-Plastic Composites was convened in Madison, Wisconsin, with the intent of bringing together researchers and industrial representatives from both the plastics and forest products industries to share ideas and technology on WPCs. A similar conference (Progress in Woodfibre- Plastic Composites) began in Toronto, Ontario, the following year and is being held in alternating years. These conferences have grown steadily in the 1990s, and additional conferences have been held in North America and elsewhere as the market has grown. The current Progress in Woodfibre-Plastic Composites 2004 Conference will be held in Toronto, Ontario, Canada on May 10, 2004.
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Current Situation

Although the WPC industry is still only a fraction of a percent of the total wood products industry (Smith 2001), it has made significant inroads in certain markets. Current end product manufacturers are an interesting mix of large and small manufacturers from both the plastics and forest products industries. According to a recent market study, the WPC market was 320,000 metric tons (700 million lb.) in 2001, and the volume is expected to more than double by 2005 (Mapleston 2001b).

Materials

Because of the limited thermal stability of wood, only thermoplastics that melt or can be processed at temperatures below 200 deg C (392 deg F) are commonly used in WPCs. Currently, most WPCs are made with polyethylene, both recycled and virgin, for use in exterior building components. However, WPCs made with wood- polypropylene are typically used in automotive applications and consumer products, and these composites have recently been investigated for use in building profiles. Wood-PVC composites typically used in window manufacture are now being used in decking as well. Polystyrene and acrylonitrile-butadiene-styrene (ABS) are also being used. The plastic is often selected based on its inherent properties, product need, availability, cost, and the manufacturer's familiarity with the material. Small amounts of thermoset resins such as phenolformaldehyde or diphenyl methane diisocyanate are also sometimes used in composites with a high wood content (Wolcott and Adcock 2000).

The wood used in WPCs is most often in particulate form (e.g., wood flour) or very short fibers, rather than longer individual wood fibers. Products typically contain approxi\mately 50 percent wood, although some composites contain very little wood and others as much as 70 percent. The relatively high bulk density and free-flowing nature of wood flour compared with wood fibers or other longer natural fibers, as well as its low cost, familiarity, and availability, is attractive to WPC manufacturers and users. Common species used include pine, maple, and oak. Typical particle sizes are 10 to 80 mesh.

Wood and plastic are not the only components in WPCs. These composites also contain materials that are added in small amounts to affect processing and performance. Although formulations are highly proprietary, additives such as coupling agents, light stabilizers, pigments, lubricants, fungicides, and foaming agents are all used to some extent. Some additive suppliers are specifically targeting the WPC industry (Mapleston 2001a).

Processing

The manufacture of thermoplastic composites is often a two-step process. The raw materials are first mixed together in a process called compounding, and the compounded material is then formed into a product. Compounding is the feeding and dispersing of fillers and additives in the molten polymer. Many options are available for compounding, using either batch or continuous mixers. The compounded material can be immediately pressed or shaped into an end product or formed into pellets for future processing. Some product manufacturing options for WPCs force molten material through a die (sheet or profile extrusion), into a cold mold (injection molding), between calenders (calendering), or between mold halves (thermoforming and compression molding) (Youngquist 1999). Combining the compounding and product manufacturing steps is called in-line processing.

The majority of WPCs are manufactured by profile extrusion, in which molten composite material is forced through a die to make a continuous profile of the desired shape (Fig. 1). Extrusion lends itself to processing the high viscosity of the molten WPC blends and to shaping the long, continuous profiles common to building materials. These profiles can be a simple solid shape, or highly engineered and hollow. Outputs up to 3 m/min. (10 ft./min.) are currently possible (Mapleston 2001b).

Although extrusion is by far the most common processing method for WPCs, the processors use a variety of extruder types and processing strategies (Maples ton 2001c). Some processors run compounded pellets through single-screw extruders to form the final shape. Others compound and extrude final shapes in one step using twin-screw extruders. Some processors use two extruders in tandem, one for compounding and the other for profiling (Mapleston 2001c). Moisture can be removed from the wood component before processing, during a separate compounding step (or in the first extruder in a tandem process), or by using the first part of an extruder as a dryer in some in-line processes. Equipment has been developed for many aspects of WPC processing, including materials handling, drying and feeding systems, extruder design, die design, and downstream equipment (i.e., equipment needed after extrusion, such as cooling tanks pullers, and cut-off saws). Equipment manufacturers have partnered to develop complete processing lines specifically for WPCs. Some manufacturers are licensing new extrusion technologies that are very different from conventional extrusion processing (Mapleston 2001c,d).

Compounders specializing in wood and other natural fibers mixed with thermoplastics have fueled growth in several markets. These compounders supply preblended, free-flowing pellets (Fig. 2) that can be reheated and formed into products by a variety of processing methods. The pellets are a boon to manufacturers who do not typically do their own compounding or do not wish to compound in- line (for example, most single-screw profilers or injection molding companies).

Other processing technologies such as injection molding and compression molding are also used to produce WPCs, but the total poundage is much less than what is produced with extrusion (English et al. 1996). These alternative processing methods have advantages when processing of a continuous piece is not desired or a more complicated shape is needed. Composite formulation must be adjusted to meet processing requirements (e.g., the low viscosity needed for injection molding can limit wood content).

Performance

The wide variety of WPCs makes it difficult to discuss the performance of these composites. Performance depends on the inherent properties of the constituent materials, interactions between these materials, processing, product design, and service environment. Moreover, new technologies are continuing to improve performance (Mapleston 2001d). General comments regarding performance can be made, but there are exceptions.

Adding wood to unfilled plastic can greatly stiffen the plastic but often makes it more brittle. Most commercial WPC products are considerably less stiff than solid wood. Adding fibers rather than flour increases mechanical properties such as strength, elongation, and unnotched Izod impact energy (Table 1). However, processing difficulties, such as feeding and metering low bulk density fibers, have limited the use of fibers in WPCs.

Because WPCs absorb less moisture and do so more slowly than solid wood (Fig. 3), they have better fungal resistance and dimensional stability when exposed to moisture. For composites with high wood contents, some manufacturers incorporate additives such as zinc borate to improve fungal resistance. Unfilled plastics absorb little, if any, moisture, are very resistant to fungal attack, and have good dimensional stability when exposed to moisture. However, most plastics expand when heated and adding wood decreases thermal expansion.

The fire performance of WPC materials and products is just beginning to be investigated (Malvar et al. 2001, Stark et al. 1997). These composites are different from many building materials in that they can melt as well as burn, making testing for fire resistance difficult. Light stability is also an area of considerable investigation (Lundin 2001). Most WPCs tend to lighten over time (Falk et al. 2001). Some manufacturers add pigments to slow this effect. Others add a gray pigment so that color change is less noticeable. Still others co-extrude a UV-stable plastic layer over the WPCs.

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