Terminology

• WPC: wood-plastic composite
• Wood flour: finely ground wood used as filler

History and demand

Invention

• 1960: Covema of Milan, founded by the Terragni brothers, invented the process and sold it as Plastic-Wood
• A few years later, Icma San Giorgio patented adding wood fiber or flour to thermoplastics

First uses in construction

• Early 1990s: WPCs entered the decking market
• Most widespread in North America as outdoor decking

Sourcing

• Composite decking is made mainly in North America and China, about 69 percent of output; North America is about 44 to 47 percent of demand (market research)
• Named producers are Trex, AZEK, and Fiberon in North America and Tianzhen Bamboo Flooring and Taizhou Huali in China; the top five hold about 56 percent

Composition and production

Raw materials

• Wood fiber or flour with PE, PP, PVC, or PLA; other lignocellulosic or inorganic fillers possible
• Wood content up to about 70 percent; 50/50 is more common
• Coupling agents and other additives improve bonding between polymer and wood

Manufacturing process

• Ground wood mixed with heated thermoplastic, then extruded; injection molding also used
• Some plants pelletize and remelt; others mix and extrude in one step
• Processing temperature about 50°F (28°C) lower than for unfilled plastic

Product forms

• Extruded solid and hollow profiles: deck boards, railing, fencing, cladding, trim, window and door frames
• Sandwich boards with WPC skins over a low-density core

Properties

Mechanical

• Lower strength and stiffness than wood; behavior depends on time and temperature

Thermal and moisture

• Thermal expansion: 1.9 × 10⁻⁵ in./in./°F (ASTM D1037, Trex cut sheets, Transcend and Enhance lines); gap boards side to side and end to end
• Water absorption: wood fibers absorb water, reducing stiffness and strength

Fire behavior

• Most formulations begin to burn near 400°F (204°C)
• Plastic content raises heat content and melts; fire performance is worse than wood alone

Durability

• Resists rot, decay, corrosion, and marine borers
• Polymer degrades under UV; some formulations stain easily
• Freeze-thaw effect on strength is not settled

Standards

Material specifications

• ASTM D7032: performance ratings for WPC and plastic lumber deck boards, stair treads, guards, and handrails; current edition D7032-21

Uses in buildings

Building elements

• Exterior Decking: deck boards, stair treads, fascia
• Guards and Railings: railing systems
• Wall Cladding: siding and cladding profiles

Pros and cons

Pros

• Resists rot, decay, and borers
• Large recycled-content share in some lines
• Extruded profiles in many colors and textures

Cons

• Lower strength and stiffness than wood
• Thermal expansion requires gapping
• Burns more readily than wood alone; hard to recycle after use

Sustainability

Recycled content and end of life

• ICC-ES SAVE listings for Trex: Transcend at least 95.4 percent recycled content by weight, Signature at least 92.6 percent
• Polymers and adhesives make post-use recycling difficult; scrap can be reused in new WPC

Manufacturing and cost

Main manufacturers and suppliers

• Trex
• Fiberon
• MoistureShield
• TimberTech

Sources

• Wikipedia: Wood-plastic composite, https://en.wikipedia.org/wiki/Wood-plastic_composite
• Trex: Transcend decking and fascia cut sheet, https://documents.trex.com/is/content/Trex/trex-transcend-decking-and-fascia-cut-sheetpdfpdf
• ASTM: D7032-21, https://store.astm.org/d7032-21.html
• Wikimedia Commons: Deck in wood plastic composite at Nagano airport.jpg, photo by Geolam (CC BY-SA 3.0), https://commons.wikimedia.org/wiki/File:Deck_in_wood_plastic_composite_at_Nagano_airport.jpg
• MarketPublishers: global composite decking market, https://marketpublishers.com/report/chemicals_petrochemicals/global-composite-decking-market-2019-by-manufacturers-regions-type-n-application-forecast-to-2024.html
• Technavio: decking market industry analysis, https://technavio.com/report/decking-market-industry-analysis