Wood Plastic Composites

Home , Composite lumber

PB 1779 Wood Plastic Composites - A Primer - Funding was provided by the Wood Education and Resource Center and the Sustainable Wood Production Initiative of the Focused Science Delivery Program, U.S. Forest Service, Pacific Northwest Research Station, Portland, Oregon.

Adam Taylor1, Assistant Professor and Extension Specialist Vikram Yadama2, Assistant Professor and Extension Specialist Karl R. Englund2, Assistant Research Professor and Extension Specialist David Harper1,Assistant Professor, and Jae-Woo Kim1, Research Associate.

1 Tennessee Forest Products Center, University of Tennessee 2 Wood Materials and Engineering Laboratory, Washington State University

3 Introduction ood plastic composite is a relatively new material that has many The use of wood – a natural potential uses. The markets for WPC decking lumber have been and renewable resource – can reduce the “carbon footprint” of expanding recently and new applications are being pursued. W plastics, because less fossil energy Door and window components, deck handrails and fencing are other mar- and material are required to make kets for the WPC industry. The flexibility of manufacturing methods and the final product. WPCs are also potentially recyclable, because re- product performance attributes provide the potential for a variety of new covered material can be melted and markets. re-formed. WPCs may be identified This publication is intended as an introduction to wood plastic com- as sustainable materials, due to the posite technology for those people who are interested in WPC technology wood particles predominately being a byproduct of sawmill and other and their applications. wood-processing waste streams, and because much of the plastic is de- rived from consumer and industrial What are WPCs? Advantages recycling efforts. WPCs offer great flexibility in Wood plastic composites WPCs offer a number of po- the shapes and colors of the materi- (WPCs) are roughly 50:50 mixtures tential benefits. The presence of als produced. Materials usage can of thermoplastic polymers and wood in a plastic matrix can result be also be reduced through the en- small wood particles. The wood in a stiffer and lower-cost material gineering of special shapes – e.g., and thermoplastics are usually than if plastic alone was used. Also, hollow-core decking boards. compounded above the melting the compression properties (resis- temperature of the thermoplastic tance to crushing) for most WPCs polymers and then further pro- are superior to that of wood loaded Disadvantages cessed to make various WPC prod- perpendicular to the grain. The The wood component within ucts. WPC can be manufactured in plastic in the product is not subject WPCs does impart some positive a variety of colors, shapes and to water absorption or biological attributes compared to plastic; sizes, and with different surface attack, so the WPC can have lower however, the inherent problems textures. Depending on the pro- maintenance requirements than sol- with wood (moisture sorption and cessing method, WPCs can be id wood. WPC lumber will not susceptibility to mold and decay) formed into almost any shape and warp, splinter or check. remain. Water can penetrate into thus are used for a wide variety of applications, including windows, door frames, interior panels in cars, railings, fences, landscaping timbers, cladding and siding, park benches, molding and furniture. One commonly available example of WPCs is the decking lumber that is often better known by its various brand names – e.g., Trex™, ChoiceDek™, Eon™ or SmartDeck™ – or as the generic term “composite lumber.”

WPCs can be produced in almost any color and shape. Hollow decking boards can reduce material usage. 4 WPCs, albeit at a much lower rate Codes and standards • 12-7A-5 Fire-Resistive Standards and level compared to solid wood ASTM standards to assess per- for Decks and Other Horizontal or other wood composites. The re- formance of WPCs relative to Ancillary Structures sulting sorption of water can pro- building code requirements have mote the growth of mold and decay been developed. These include: fungi; however, aesthetics − not How WPCs are made structural issues − dominate con- • ASTM D 7031-04 Guide for WPCs are commercially pro- sumer callbacks. Color fade from Evaluating Mechanical and duced by a number of companies sunlight is also accelerated when Physical Properties of Wood- throughout the world. While there wood is added to thermoplastics, plastic Composite Products is considerable variation in the pro- causing a whitening or graying of cess employed and the products the surface of the composite. • ASTM D 7032-04 Specification produced, there are many common WPCs are also usually quite for Establishing Performance elements. This section will briefly heavy and not as stiff as solid Ratings for Wood-plastic describe the components of WPCs wood. This limits the potential use Composite Deck Boards and and how they are put together. of WPCs in many structural appli- Guardrail Systems Guards or cations and creates the potential Handrails creep or sagging problems, especially in a warm environment. On • ASTM D 6662-01 Specification the other hand, this flexibility can for Polyolefin-based Plastic be an advantage: WPC can be bent Lumber Decking Boards on-site to make attractive patterns. WPC is touted as having envi- Other standards exist that pertain ronmental benefits, because it is to specific applications. These made from residues (wood) or re- would apply to WPC products, if cycled materials (plastic). However, they were used in those applica- virgin plastics are commonly sup- tions. Some examples include: plemented in WPC operations to • Roofing Material WPCs are manufactured in facilities maintain tighter quality control and IBC standards for roofing-related across the United States. For more offset highly fluctuating recycled materials information, visit plastic inventories. WPC also re- http://www.wpcinfo.org/ quires large amounts of energy to • Playground Equipment produce. WPC is theoretically recy- F1487-01e1 Standard Consumer clable; it could be re-melted and re- Safety Performance Specification Wood formed into new decking lumber. for Playground Equipment for Particle geometry However, no recycling of this new Public Use Wood used in WPC manufac- product is currently underway, with turing is in the form of dry particles exception of recycling of off-specifi- • F1148-03 Standard Consumer with a powdery consistency, often cation material during manufacture. Safety Performance Specification called “wood flour.” In general, the The collection, cleaning and trans- for Home Playground Equipment wood waste ‘raw material’ is in the portation of old WPC to a recy- form of sawdust and/or planer • Products used in buildings in cling center for remanufacture are shavings. wildfire risk areas: likely to be prohibitively expensive. There are two steps to produce 12-7A-2 Fire-Resistive Standards wood flours: size reduction and size for Exterior Windows classification (screening). In case of 12-7A-1 Fire-Resistive Standards large pieces of wood, size may be for Exterior Wall Siding and reduced using equipment such as a Sheathing hammer mill, hog or chipper. Wood

5 Higher aspect ratios can increase mechanical properties such as strength, elongation and impact energy, but high aspect ratio could de- crease processability. The equipment used to break down and clas- sify the wood particles will influence the aspect ratio.

Wood species: The commonly used wood species for commercial WPC production are pine, maple and oak. As with many wood-based products, regional availability and cost are key factors in species selection. WPCs have been manufactured in the lab using a wide variety of species, including osage orange, Any species can be incorporated into WPCs. Non-wood fibers can also be used. walnut, yellow poplar, hickory, from such processes is coarse and is Wood flours obtained from size Utah juniper, salt cedar and emer- usually ground further using an at- reduction processes or as byprod- ald ash borer-infested ash. Research trition mill (grinding between ucts from wood manufacturing con- indicates that species effects on the disks), rollers, hammer or knife tain various sizes of particles. These mechanical properties of WPCs are mills. Wood flour can also be ob- wood particles are classified using small; however, they can have a sub- tained from wood products opera- vibrating, rotating or oscillating stantial influence on the processing tions such as sawmills, mill work or screens. The size of wood particles attributes of the WPC. In practice, window and door manufacturers is often described by the mesh of wood species has not been consid- that produce sawdust as a the wire cloth sieves used to make ered to be an important variable; byproduct. A study conducted by them. For example, 20-mesh indi- any fiber source that is readily avail- WSU has shown that sawmill resi- cates wood flour that passes No.20 able and inexpensive is generally dues, including bark, can be suc- U.S. standard sieve (0.841mm per preferred. cessfully utilized to extrude WPCs. side square openings). Typically, Recent research indicates that It was observed that bark content wood particles between No. 10 to the inclusion of wood species with as high as 25 percent could be in- 80 U.S. standard sieve (2 and distinctive properties can positively cluded in the wood flour without 0.18mm openings, respectively) are influence the characteristics of the significantly compromising the me- used. Companies exist that special- resulting WPC. For example, WPCs chanical properties significantly; ize in supplying wood fibers for made with eastern red cedar and additionally, moisture resistance WPCs. Prices vary but currently are cherry are more resistant to water properties were shown to improve in the range of $0.05-$0.15/lb. absorption, swelling and rot than with inclusion of bark. It has also An important aspect of wood those made with other wood been shown that wood from insect- particles for inclusion in WPCs is species (Kim et al. 2008). infested trees, even with significant their aspect ratio – the length-to- In addition to various wood levels of deterioration, could poten- diameter ratio. Normally, this value species, various other natural fibers tially be used for wood-plastic com- will range from 1 to 5. Wood flour have been included into fiber-plastic posite production (Yadama, et al. with a low aspect ratio is easier to composites. Examples include bast 2008). process (readily metered and fed). fibers (flax, hemp, jute, kenaf, ra-

6 mie), rice hulls, leaf fibers (sisal, Plastics pineapple, abaca), seed fibers (cot- WPCs are made ton), fruit fibers (coconut husk or with thermoplastic coir) and stalk fibers (straw of vari- polymers. These materi- ous kinds). Use of some of these als melt and flow at fibers could add additional steps to high temperatures and processing due the presence of high harden when cooled. To silica content and cuticle wax prevent excessive dam- (Rowell et al. 1997). age to the wood component, thermoplastics Moisture content: for WPCs have process- Wood flour must be dry before ing temperatures of less it is used in WPC manufacturing. than 220 degrees C. Moisture evaporates and increases Common materials gas pressure during the high-tem- used include polypro- perature compounding and forming pylene, polystyrene, process. In addition, moisture in polyvinylchloride and wood can create voids in the final polyethylene (low and product and thus adversely affect high density). Recycled mechanical properties. Moisture polymers are often content levels of 2 to 8 percent (dry used, but they must be weight basis) are typical, but also relatively clean and ho- vary based upon the manufacturing mogeneous; polymers platform. Moisture is removed from of different types don’t the wood prior to composite forma- mix well. A twin-screw extruder tion via a variety of drying meth- • Lubricants help the molten WPC ods, including steam tubes and ro- Additives mixture move through the pro- tary drums driers. Moisture is also While the bulk of a WPC is cessing equipment removed during the composite pro- wood flour and thermoplastic poly- cessing step, minimizing its negative mer, a variety of materials are add- • Coupling agents improve the influence and allowing for higher ed in relatively small quantities. wood and polymer interaction. product output. Wood flour and These additives are included for a Wood is naturally hydrophilic (at- even pre-compounded material can variety of reasons. tracts water), while the thermo- absorb moisture under ambient plastic polymers are hydrophobic conditions. Therefore, the con- Chiller trol of moisture in Resin Silo wood flour and Compounder or Extruder Die Calibrator Spray Cooling compounded mate- Wood Flour Blender rial is very impor- Silo Extruding System tant in processing wood plastic composites. Additives • Lubricants Embossing

• Colorants Stacker Cut-off Saw Puller • UV Inhibitors • Biocides Brushing • Coupling Agents

The WPC manufacturing process, with extrusion forming

7 (repel water). This basic chemical Forming: in the die to aid in consolidation. incompatibility makes it very dif- Most WPCs are manufactured Tandem extruders have one compo- ficult to bond polymers to wood. using profile extrusion, which cre- nent for the compounding step and The use of coupling agents can ates long continuous elements, such one for the shaping process. help to overcome this incompat- as deck boards and window compo- While extrusion methods create ibility. nents. The wood-thermoplastic lineal elements, injection molding mixture (in pellet form) is conveyed produces three-dimensional parts • Fillers, such as talc, are used to into a hopper that feeds the extrud- and components. The unique reduce the cost of materials and er. As the material enters the first shapes and profiles that can be cre- to improve stiffness and zone of the extruder, the heated ated with injection molding provide durability. screws and barrel melt or soften the the potential for diversifying from • Biocides can be added to protect thermoplastic. The molten material the current WPC markets. The in- the wood component of WPCs is then forced through a die to make jection molding process involves from fungal and insect attack. a continuous profile of the desired two steps. The first is to melt-blend Zinc borate is the most commonly shape. Molten WPC material is or compound the wood-plastic mix- used wood preservative added to highly viscous, so the equipment ture, and the second is to force the WPCs. Fire-retardant chemicals needs to be powerful enough to molten WPC into a mold under reduce the tendency of the WPCs force the material though the ma- high pressure. The molten material to burn. UV stabilizers help pro- chinery and out of the die. As the fills the cavity in the mold and so- tect the plastics from degrading in material exits the extruder, it is lidifies as it is cooled. Injection the sun. cooled in a water spray chamber or molding is used to manufacture a • Pigments are added to provide a bath to rapidly harden the variety of parts, from small compo- desired color to the product. UV thermoplastic matrix, embossed nents to large objects. Injection stabilizers can help to protect the with a desired pattern, and cut to a molding is a common method of color, but some fading and whit- final length. Extruders can have sin- production and is especially useful ening will occur with most WPC’s gle screw or twin feed screws, which for making irregularly shaped exposed to sunlight. are counter- or co-rotating. These pieces. screws can be parallel, for mixing Other types of molding pro- Manufacturing only, or conical, to increase pressure cesses include compression, vacuum Technologies Compounding: Manufacturing of WPCs can be done using a variety of processes; however, the key to making any WPC is through efficient dispersion of the wood component into the thermoplastic matrix (‘compounding’). Generally, this can be accom- plished in twin-screw extruders or other melt-blending processes. Once the materials are sufficiently mixed, the composite can then be formed into the final shape using forming technologies such as extrusion or injection molding.

An injection molder

8 bag, resin transfer (RTM), reaction Markets ing materials (WPCs can use waste injection (RIM) and matched die The wood-plastic composites wood and recycled plastics) have molding. These manufacturing market share has been growing rap- also contributed to greater accep- technologies each have the ability to idly, especially for applications such tance of WPCs by builders and ho- keep the full length of the fiber and as decking and railing. The main meowners. provide high strength composite, drivers for WPC acceptance are the Two-thirds of the WPCs pro- such as those used in automotive perceived improved performance duced are decking and railing prod- applications. The main disadvan- and appearance attributes (e.g., no ucts, accounting for almost $1 bil- tages to these techniques are that checking) over existing products lion annually. Other important they are batch processes, which re- such as treated wood decking. products are window and door quire longer processing times and Stricter regulations on the use of frames. Additional potential appli- are more costly. All of these meth- chemicals in building materials, cations for WPCs include siding, ods could have application to such as the phasing out of CCA- roofing, residential fencing, picnic WPCs; however, only limited re- treated lumber for residential deck- tables, benches, landscape timber, search has addressed their use. ing and the desire for ‘green’ build- patios, gazebos and walkways, and playground equipment.

WPC Manufacturing Furnish (dry tons) Green Wood (tons) Wood Number of Operating Production Production 55% wood 34% (80% MC) dry Roundwood Extruders Hours/Year (nominal MBF) (dry tons) (2% MC) HDPE basis no bark equivalents w/bark 1 2000 425 1250 701 425 1238 1386 4000 851 2500 1403 850 2475 2772 5 2000 2127 6250 3506 2125 6188 6930 4000 4255 12500 7013 4250 12375 13860 20 2000 8509 25000 14025 8500 24750 27720 4000 17018 50000 28050 17000 49500 55440 Raw material requirements for a typical WPC plant (Brackley and Wolcott, 2008)

$ millions per phase

Capital Estimate Phase 1 Phase 2 Phase 3 Total

Equipment 5.87 7.12 11.43 24.42

Installation 3.95 1.68 3.80 9.43

Site Development 1.34 0.06 1.20 2.60

Buildings 4.18 0.00 3.19 7.37

Indirect Costs 2.07 1.20 2.65 5.92

Contingency 0.87 0.50 1.11 2.48

Total 18.28 10.56 23.38 52.22

Capital investment estimates for WPC plant – starting with 2 extruders in Phase 1, and expanding to 10 extruders in Phase 2 and 20 extruders in Phase 3 (Evergreen Engineering, 2005)

9 Summary WPCs are new and rapidly evolving products. WPCs offer a number of advantages, including

Environmentally flexibility of production and the use of recycled material to create a recyclable product. Despite the wide array of possible finished forms, the manufacture of WPCs is relatively simple and uniform. WPCs have become well-established building materials, especially for residential decking, and are likely to be used for a wider array of applications in the future.

For more information http://www.wpcinfo.org/

Potential applications and desired attributes of WPCs for playground equipment as indicated by Parks & Recreation officials (Yadama and Shook, 2004)

10 References:

Brackley, A. and M. P. Wolcott. 2008. Economic Development – Market for wood composite decking material and impact of local economics. Alaska Wood Tides, Issue No. 7. Pacific Northwest Research Station, USDA FS, Sitka, Alaska.

Evergreen Engineering. 2005. Prospectus: The Opportunity to Manufacture Wood Plastic Composite Products in Louisiana. Project #1655.0, Eugene, OR.

Kim J-W, DP Harper and AM Taylor. 2008. Effect of wood species on water sorption and durability of wood-plastic composites. Wood and Fiber Science 40(4):519-531

Klyosov AA. 2007. Wood-Plastic Composites. Wiley-Interscience. 698 pages - ISBN 0470148918

Rowell RM, Sanadi AR, Caulfield DF, and Jacobson RE. Utilization of Natural Fibers in Plastic Composites: Problems and Opportunities. In Lignocellulosic Plastic Composites, 1997.

Yadama V, and Shook S. 2004. Market Evaluation of Wood-Plastic Playground and Building Components. Progress in Woodfibre-Plastic Composites 2004, Toronto, Canada.

Yadama V, Lowell EC, Petersen M. and Nicholls D. 2009. Wood- thermoplastic composites manufactured using beetle-killed spruce from Alaska. Polymer Engineering and Science 49(1):129-136 Visit the UT Extension Web site at http://www.utextension.utk.edu/

PB1779-500-1/09 E12-5115-00-013-08 09-0111 Programs in agriculture and natural resources, 4-H youth development, family and consumer sciences, and resource development. University of Tennessee Institute of Agriculture, U.S. Department of Agriculture and county governments cooperating. UT Extension provides equal opportunities in programs and employment.