**1. Introduction**

Significant progress was achieved during the last years in wood-polymer composites (WPC) research and product development. In addition to the environmental advantages, the use of wood fiber in WPC manufacturing has several benefits, including low cost, renewability, biodegradability, low specific gravity, and high specific strength and stiffness [1, 2]. WPC use in construction and transport is in constant evolution due to these advantages [1–3]. The increase in environmental awareness increased the demand for environmentally friendly products [4]. WPC presents an excellent alternative to plastic products with lower environmental impact and cost [5]. Lately, many studies have tried to improve WPC properties and competitiveness by introducing new additives and processes [6–9]. However,

the polymers' prices are very volatile and depend on petroleum prices, affecting the competitiveness of such products. Thus, the economic aspect is also crucial to ensure the sustainability of WPC production. Using recycled materials appears to be an excellent way to lower the raw material cost and consequently the total production cost. The recycled thermoplastic polymer is commonly used as a matrix to produce WPC [10]. Moreover, the environmental advantages of using recycled polymers could decrease the WPC production costs.

In landfills, plastics are not biodegradable. The 2019 global production of plastics was 368 million tons. This important production leads to a large plastic waste stream, making recycling a great challenge [11, 12]. Recycling plastics in relatively long lifecycle products such as WPC could decrease the carbon footprint of the non-renewable polyolefin and avoid their burial in landfills [13]. Waste prevention comes to be an excellent way to avoid recycling energy uses and facilities expenses. In some cases, waste prevention could be less favorable by diminishing the downstream material recycling and preventing the low-impact of secondary production [14]. In recycling centers, sorting wastes is the most expensive part of the recycling process. Recycled plastics are always a mix of different kinds of polymers, such as polyethylene terephthalate (PET), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP). These polyolefins have different chemical compositions making them incompatible during processing which could significantly affect the properties of the resulting products.

Several reports [4, 15–21] investigated the effects of recycling on the properties of WPC, both components of these composites could be from recycled materials. Ashori and Sheshmani [15] showed that recycled newspapers in recycled PP composites showed maximum water absorption compared to non-recycled components formulation. Low et al. [4] used epoxy to produce recycled cellulose fiber epoxy composites. Results showed an improvement in mechanical properties. Tajvidi and Takemura [17] showed that recycled fibers increase the composites hydrophobicity due to better fiber-polymer adhesion. Nerenz et al. [16] showed that the addition of a sunflower hull diminished the tensile strength of composites compared to neat PP. Xiaolin et al. [18] studied the feasibility of composites made with recycled newspapers and magazines and recycled PP. The study showed that recycled fibers and PP could be a viable source for producing WPC.

Petroleum-based polymers such as PE, HDPE, PET, and PP serve for WPC production. These petroleum-based polymers are also large-scale products used for many applications such as packaging and constitute the main component of waste landfills.

Many studies investigated the potential of the polyolefin recycled matrix components for WPC [10, 21–36]. Chtourou et al. [21] showed that municipal waste plastics composed of 95% PE and 5% PP and pulp fibers produced WPC with average tensile properties compared to virgin polyolefin. Li et al. [22] used PET and PE to produce microfiber-reinforced composites. They reported that the mechanical properties of the PET composites greatly improved compared to the typical PET/PE blend at the same composition. Cui et al. [23] showed good compatibility of the treated wood post-consumer fiber with recycled HDPE and additives, giving WPC good mechanical properties. Beg and Pickering [24] studied the effect of eight times reprocessing on wood PP composites. The study showed a decrease in mechanical properties with the increase in reprocessing, and the thermal stability increased with the repeated process. In another study, Beg and Pickering [25] found that the equilibrium of moisture content of WPC decreases after eight times reprocessing. Ares et al. [26] showed that with more than 10% wood flour content, the reprocessed PP composites showed mechanical and rheological properties similar to those of virgin polymer composites. González-Sánchez et al. [27] showed that the fiber dispersion is not dependent on

#### *Properties of High-Density Polyethylene-Polypropylene Wood Composites DOI: http://dx.doi.org/10.5772/intechopen.101282*

the polyolefin type, and reprocessed PP showed more pseudo-plasticity loss than the reprocessed PP HDPE. Bhaskar et al. [28] used recycled PP to produce WPC and compared the mechanical properties with virgin PP WPC. Recycled PP with less than 50% fiber load had WPC with good properties [29]. Combining shell core with recycled polyolefin could lead to cost-effective advantageous WPC [30]. Catto et al. [31] showed that recycled polyolefin is a viable alternative due to its comparable physical and mechanical properties to virgin polyolefin. Adding virgin PP to recycled HDPE improves WPC properties [32]. De Oliveira Santos et al. [33] showed that using recycled PET below the melting temperature could enhance the composites processing and mechanical properties. Another study [34] reported that virgin and recycled polymers give similar mechanical properties and water uptake.

The reuse of polyolefins also has environmental advantages. It contributes to decreasing global warming and using non-renewable fossil hydrocarbons [10]. However, the emissions of reprocessed products weaken these advantages. Rangavar et al. [35] reported that recycled PP leads to WPC with similar or even better properties than the virgin polymer in addition to economic and environmental benefits. Thus, recycled polymers exhibit a high potential to replace virgin polymers for producing fiber-filled thermoplastic composites [36].

This study assessed the effect of the variation of PP and PEHD proportions on WPC properties. We used two different combinations of these two polymers (20% HDPE + 80% PP and 80% HDPE + 20% PP) to simulate recycled plastics. We did not use any grafting agent in all formulations to better assess their interaction and the effects of their simultaneous presence on the WPC properties.
