**1. Introduction**

The increasing generation of plastic waste by industry and in urban areas in recent years has prompted concern in society and efforts to recycle discarded and unused plastic materials [1–3]. Among the alternatives to minimize plastic waste accumulation is using postconsumer plastics to produce plastic lumber as a substitute for natural wood [4, 5].

According to the American Society for Testing and Materials (ASTM) [4, 5], the term plastic lumber applies to products made primarily from plastic (with or without additives), with a rectangular cross-section and size typical of wood products used for building. However, plastic lumber products can also exhibit a circular cross-section, as well as other shapes, with applications such as furniture and farming, among others.

Most plastic lumber products on the market are made from polyethylene, particularly high-density polyethylene (HDPE), but can be obtained using polymers, such as polypropylene (PP), polystyrene (PS) and polyvinyl chloride (PVC), or mixtures of different plastic wastes [6]. Additionally, fillers and additives, such as natural fibers, sawdust [7–9], mineral fillers and glass fiber, can be added to plastic lumber formulations [10, 11].

Both composition and processing conditions are largely responsible for the final characteristics of plastic lumber products. Research and patents demonstrate that

**Figure 1.** *Examples of different plastic lumber profiles.*

different processes and recycling equipment are used to produce plastic lumber [12–15]. Factors such as the properties of the material to be processed, how plastic waste reaches the processing stage, the presence of additives and the moisture content of the material require extruders with specific technical characteristics when compared to processing virgin plastic [12]. These characteristics include shortening the residence time of the plastic inside the equipment, maintaining a constant feed rate inside the extruder and good degassing and homogenization of the material.

Due to their natural origin, wood-based products may exhibit a series of structural defects, such as knots, cracks, warping, wormholes and fungal damage, as well as low-dimensional stability and other imperfections resulting from varying moisture content and drying, which significantly influence the final strength of products and are difficult to control [16].

Plastic lumber has several advantages over natural wood in a number of applications and can be made from used plastics such as bottles, cups, packing and other products with a short useful life, thereby minimizing the accumulation of plastic material in the environment. It can be worked using conventional carpentry tools and planed, sawn, drilled and nailed in the same way as natural wood [6]. The advantages of plastic lumber over natural wood include being waterproof, resistant to weathering, mold and borers, and not requiring regular painting or maintenance, meaning it can be used in environments that natural wood would be unable to withstand for long periods. These include wet or underwater structures such as sea dikes in coastal areas [17, 18]. Plastic lumber can also be used to protect forests by preventing new trees from being felled to make furniture, decking, fencing and piers [6]. Different plastic lumber profiles are shown in **Figure 1**.
