Recycled plastic lumber (RPL) is a material made of recycled plastic with a look similar to wood lumber that is considered for several applications primarily to replace conventional products made of wood, concrete, or metals. Conventional RPL is primarily used in open-air applications such as decking, benches or tables thanks to its waterproof properties and resistance to most weather conditions. RPL has the advantage to be resistant to insects, moisture, chemicals, graffiti, stains, and to UV; therefore, its long term costs may be considerably decreased with respect to wood. RPL may also have lower replacement costs as it does not rot, rust, warp, crack, splinter or absorb bacteria, being common issues with either woods or metals and may be factors which may significantly decrease the lifetime or aesthetics of the product. However, compared to conventional materials, RPL has a limited bending ability (especially if reinforced), which does not allow complex designs similar to those offered using metals. RPL can be divided into five main types:
Homogeneous recycled plastic (single plastic up to 95%)
This type of RPL is the most common and is made of only one plastic waste type, which is in general either polyethylene or polypropylen. This RPL type may not require reinforcements thanks to its homogeneous composition, which also allows a better product design. Generally the stiffness yet remains lower than wood which brings a significant limitation to the number of offered applications.
Heterogeneous recycled plastics (commingled)
The range of materials possible for RPL is wide but may vary according to the local availability of plastics waste. Manufacturers have also developed RPL by mixing recycled plastics, as well as blends of recycled plastics with virgin plastics. The structural performance may be poorer than wood, which may lead to larger profile dimensions and shapes.
Wood-filled RPL is made either with mixed or non-mixed recycled plastics and wood chips or sawdust. Thanks to the addition of wood, this RPL type may have enhanced properties with respect to other RPL types, such as better traction thanks to its greater surface roughness which may be similar to wood lumber. However, the addition of wood may lead to some drawbacks, such as the absorption of moisture, lower resistance in low temperature environments, not insect resistant, discolouring, and other minor issues.
This type of RPL is considered primarily for structural and higher-performance applications, in particular for applications where other RPL types usually fail. In order to withstand larger loads, RPL profiles are reinforced with glass-fibres (as in the PROWASTE project where pultruded rods are used), metals, or other materials which aim at enhancing the overall mechanical properties.
Other combinations of materials
Research is being conducted also to identify optimal blends between recycled plastics with other materials to further enhance, and possibly overcome, the main drawbacks of RPL for different applications. RPL has so far been mixed with rubber, peanut shells, concrete, and other materials.
PROWASTE project, Deliverable N°5.1 Market analysis and road-mapping – June 2011