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is scope to consider WPCs as a destination for reclaimed timber if of suitable quality and species. However, the softwood carcassing timbers which dominate construction and demolition wood would be less desirable. Polymer matrices for WPCs include polyethylene, polypropylene, PVC, or polystyrene or may increasingly include biopolymers such as polylactic acid (PLA) or thermoplastic starch. The exact formulations depend on the manufacturer and technology employed. Each manufacturer has their own range of different target markets and properties, relating to the polymer characteristics. The largest market is extruded decking planks and cladding sections, and this has become well established [80]. As for other applications, the presence of solid contaminants in some grades of recycled wood will present a challenge to the milling equipment [27]. If metal or inorganics pass through the milling stage, they present a further risk to the compounding equipment, extruders, and moulding equipment. Many of the contaminants can be removed using metal detection, and screening systems for removing non-ferrous metals and inorganic particles. Such technologies have evolved greatly in recent decades, as discussed above. The other contaminants present in recycled wood, such as paint, varnish, or treatment products and glues, may contribute additional considerations [27]. For example, if the organic contaminants degrade in the high temperature of the process, or give off gases. Also if the contaminant causes chemical interactions to occur, or lead to a reduction in compatibility between the particles and the polymer matrix. It is reported that the glues and other contaminants present gave poorer results for recycled wood than virgin wood in a study that considered a wide range of wood and paper wastes as feedstock for WPCs [81]. A study using relatively clean recycled wood, which was finely milled for WPC manufacture showed strength properties which were very similar to the WPC using virgin wood; however, the water absorption for the recycled fibre was higher [82]. An early example of recycled wood as a feedstock for WPC materials used the sawdust from MDF and particleboard manufacturing plants [83]. This fine material may be less desirable for other products where sawdust is typically used. The sawdust was added at different percentages and the properties compared with virgin MDF and particleboards (rather than with WPC made from virgin wood). Comparison with generic data for WPC materials [80] indicates that the modulus of elasticity (2.5–3.0 GPa), modulus of rupture (25–35 MPa), and unnotched impact strength (400–550 J/m) values obtained at 60% fibre loading were within the expected range for uncompatibilised WPCs [27]. The WPC made using MDF sawdust performed better than the WPC containing particleboard sawdust, which was attributed to the fibrous quality of the MDF sawdust [83]. Similar results might be expected if waste MDF was segregated and the fibre recycled into a WPC product. The method of separation appears significant, with slightly higher tensile and impact properties achieved for WPC made with fibre separated by the thermo-hydrolytic method compared to fibre from mechanical separation [84]. Similarly, recycled newsprint and other paper fibres have been used to form WPC materials with improved properties, relating to the fibrous rather than particulate character of the filler [85]. A more recent study considered Tanalith E-treated wood flour within a PLA matrix for WPCs. The presence of the preservative treatment led to enhanced resistance against decay fungi ( Trametes versicolor and Coniophora puteana ) in decay tests; however, the water resistance of samples containing both treated and untreated wood was poor [86]. The research on the use of waste wood in WPC manufacture is limited, and focus has been on WPCs using recycled plastics. There appears to have been relatively little activity to commercialise a product in this area, but it remains a potential option for the use of recycled wood. The recurring theme of cleaning the timber from contaminants must be taken into account.
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