Performance
Performance depends on the inherent properties of the constituent materials, interactions between these materials, processing, and product design & service environment. New technologies continue to provide improved performance.

Adding wood to filled plastic can greatly stiffen the plastic but often make 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, impact strength. However, processing difficulties such as metering low bulk density fibers have limited their use in WPCs. Because WPCs absorb less moisture & do so more slowly than solid wood they have better fungal resistance& 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, & have good dimensional stability when exposed to moisture. However, most plastics expand when heated & adding wood decrease thermal expansion.

The fire performance of WPC materials is just beginning to be investigated. 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. Most WPCs tend to lighten over time. Some manufacturers add pigments to slow down 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.

Applications & Current Markets for WPCs
The greatest growth potential for WPCs is in building products that have limited structural requirements. Products include fencing, industrial flooring, and landscape timbers, railings & moldings. The figures given below list some wood plastic composite products currently available in North America. The decking, window & door profile & automotive markets represent the majority of the WPCs manufactured in the USA.

Decking
Pressure–treated lumber remains by far the most commonly used decking & railing materials (80 % of the approximately $ 3.2 billion market), but the market for WPC decking is growing rapidly. Market share grew from 2 % of the decking market in 1997 to 8 % in 2000, & is expected to be more than double by 2005.
Although WPC decking is more expensive than pressure–treated wood, manufacturers promote its lower maintenance, lack of cracking or splintering, & high durability. The actual lifetime of WPC lumber is currently being debated; most manufacturers offer a 10-year warranty. Compared with unfilled plastic lumber, the advantages of WPC lumber include increased stiffness & reduced thermal expansion. However, mechanical properties such as creep resistance, stiffness, & strength are lower than those of solid wood. Hence these composites are not currently being used in applications that require considerable structural performance. For example, WPCs are used for deck boards but not the substructure. Solid, rectangular profiles are manufactured as well as more complex hollow & ribbed profiles. Wood fiber, wood flour, & rice hulls are the most common organic fillers used in decking. About 50 % wood is typically used in decking & some products contain as much as 70% wood. A polyethylene matrix is used most often, but manufacturers of decking made with PVC & PP have recently entered the market. At least 20 manufacturers produce decking from WPCs.

Window and Door Profiles
Window & door profile manufacturers form another large industrial segment that use WPCs. Fiber contents vary considerably & can range anywhere from 30 to 70 %. PVC is most often used as the thermoplastic matrix in window applications, but other plastics & plastics blends are also used. Although more expensive than unfilled PVC, wood filled PVC is gaining favor because of its balance of thermal stability, moisture resistance & stiffness& strength. Patent activity is very high in this area.
Several industry leaders are offering wood – plastic composites in their product line, but their approaches vary. One leading manufacturer co extrudes a wood filled PVC with an unfilled PVC capstock for durability. Another manufacturer co extrudes a PVC core with a wood filled PVC surface that can be painted. A third manufacturer offers two different composites:
1. Highly wood filled PVC for stiffness and
2. A composite with a foamed interior for easy nailing & screwing.

Automotive Applications
Wood–filled PP sheets for interior substrate is still made in the USA by several manufacturers. However, manufacturers are beginning to investigate the use of other natural fibers such as kenaf, flax & hemp in air laid processes. Growth in the use of natural fiber filled thermoplastics, rather than unfilled plastics have been rather slower in the USA than in Europe, where environmental considerations are a stronger driving force. One market analyst cites he lack of delivery channels & high transportation costs as major factors that slow growth in the USA. One major U.S. Company has used German technology to produce automotive door quarter panels from natural fiber composites with PP & polyester. A number of other automotive components are being made with similar technology. Nonwoven mat technology is being used to make rear shelf trim panels with flax reinforced PP. Other products being tested include instrument panels; package shelves, load floors& cab back panels.