Wood Plastic Composites

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  Wood Plastics Composites - A new opportunity

See also: Wood Plastics Composites - A Review Paper

The concept of wood-plastics composites (WPC) is starting to excite interest in Europe as methods of combining the two materials are developed and as the market continues to expand in the USA and other parts of the world. This new group of materials cover a wide range of polymer matrix types (including PP, PE and PVC) as well as a wide range of fillers and stiffeners (including wood flour, flax, jute and other cellulose based fibre fillers). The materials offer added strength, lower costs and a wide range of finishes but the high cellulose filler content of some products (up to 70%) may lead to some confusion in the market - is it wood or is it plastic? The plastics processing industry may change yet again as wood product companies transform themselves into composites companies.

The majority of the current applications are in the USA where significant advances are being made and finished products such as decking, cladding and window frames are already on the market. For the past 5 years the WPC market has grown at a rate of 100% per year and shows no signs of slowing down as new applications are developed for the materials. The majority of current applications are for the outdoor replacement of wood products but structural engineering applications are now being developed to use the improved physical properties of WPC.

The production of WPC typically uses a fine wood waste (sawdust in the 40 to 60 mesh range) mixed with various plastics. The powder is extruded to a dough-like consistency and the profile is then extruded through a single-step die (typically costing between 3,000 and 8,000) with no additional calibration and only a simple water bath for cooling.

Processing temperatures are less than 150 deg C, a temperature that allows high processing rates and low energy consumption. During production, the flow characteristics of cross-linked composite, through the extrusion system, permit the use of simple dies for even the most complex profiles. The simplicity of the die design means that lead times of six to eight weeks from design concept to production are usual.

With up to 70 percent of the WPC being cellulose material, the materials behave like wood and turn, drill, sand, saw, mitre, rout, tenon and plane like wood using conventional woodworking tools. WPC products can also be used with fasteners such as nails, screws and staples and can hold fasteners with up to two to four times better than wood. This permits further design freedom since smaller fasteners may be used to achieve equal hold. Special adhesives can be used to provide excellent adhesion on all types of joints. During installation, silicone or acrylic seals and wood fillers can be used successfully.

WPC products are extremely moisture-resistant (water absorption of 0.7 percent compared to 17.2 percent for Ponderosa pine) with less thickness swell (0.2 percent compared to 2.6 percent). Since there is little or no water present, fungal attack and decay are not issues. The coefficient of thermal expansion of WPC is very similar to that of aluminium and the mechanical properties can actually improve over time at higher temperatures. Low temperature performance is good and extrusions perform better than wood in temperature extremes. In Underwriters Laboratories tunnel tests; unmodified standard Strandex WPC has achieved flame-spread ratings of 46 (c.f. redwood = 100). This can be improved by the addition of flame and smoke retardants at the mixing stage to improve performance.

Pigments, UV stabilisers and/or fire retardants can be added to the compound prior to extrusion. A unique clear primer has been developed, exclusively for Strandex, which provides a surface that can be painted or stained with any desired shade or colour, or laminated with any type of foil or veneer. High speed off-line embossing can be employed to produce a wood-grained or deep decorative surface, which can be painted or stained if required.

Typically, exterior profile wall thicknesses range from 4mm to 6mm, interior walls range from 2.5mm to 3.5mm. Intricate details may be extruded in line at tolerances less than plus or minus 0.2mm.

Economic edge

With WPC, wood waste and recycled resins become enhanced products. Wood waste, which in many operations has had a negative value, becomes an asset. Virtually any waste stream can be recycled and used - hardwood, softwood, plywood, peanut hulls, bamboo, straw, corncobs, newsprint and numerous other materials.

In terms of cost, these wood composite profiles are usually very competitive with finger-jointed pine, complex MDF and particle- board components and PVC-U.

Benefits

Wood composite profiles are ideal in many applications traditionally using PVC-U extrusions such as window profile manufacture but many other products can benefit from the following attributes:

See also: Wood Plastics Composites - A Review Paper

 

Last edited: 11/03/10

Tangram Technology Ltd. 2001

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