Post-Consumer Plastics in the Production of Wood-Plastic Composites: Comparison
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Wood-plastic composites (WPCs) are produced by combining a polymeric matrix with wood particles, with this matrix potentially composed of either virgin or post-consumer polymers. The manufacturing process of wood-plastic composites encompasses the combination of polymeric matrices, such as polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC), together with lignocellulosic material, which can be fragmented into wood chips or particles. 

  • wood-plastic composites (WPC)
  • polymer
  • recycled plastic
  • plastic waste

1. Introduction

Wood-plastic composites (WPCs) are produced by combining a polymeric matrix with wood particles, with this matrix potentially composed of either virgin or post-consumer polymers [1]. These composites find extensive application across various sectors, such as construction, furniture manufacturing, transportation, technology, and other areas [2].
Polymeric-origin composites are manufactured from at least two components: a matrix, usually taking the form of a polymeric phase, and a reinforcing or filling agent. These components do not homogeneously mix, resulting in a well-defined interface between phases. Full fusion into a solid solution does not occur, resulting in a clear demarcation between the matrix and the reinforcement. In many cases, the production of a composite aims to achieve specific characteristics and properties that would not be feasible to attain individually for each component. The primary function of the matrix is to transfer stress to the reinforcing agent. These reinforcements can take the form of particulate, lamellar, or fibrous materials (continuous or discontinuous). The interface, in turn, is identified as the interface region between the matrix and the reinforcing agent, marking a discontinuity of limited dimensions within the material where properties experience a sudden alteration from one phase to the other [3]. In various situations, to facilitate the incorporation of loads into thermoplastic polymers, it becomes necessary to resort to compatibility agents, also recognized as coupling agents [4]. Additionally, it is possible to use residues from wood and plastic consumption, further contributing to the sustainability of the manufacturing process [5].
The manufacturing process of wood-plastic composites encompasses the combination of polymeric matrices, such as polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC), together with lignocellulosic material, which can be fragmented into wood chips or particles. These proportions can vary between 30% and 70%, as mentioned in [6]. The subsequent blend undergoes stages in equipment such as extruders and injectors, which are responsible for producing both the composites and their respective artifacts [7]. The extrusion procedure that combines wood and plastic results in a material that retains its original appearance but presents superior attributes such as enhanced robustness, increased flexibility, and greater resistance to climatic adversities. These qualities, stemming from the properties of the employed plastics, lead to reduced maintenance needs and extended durability, as pointed out by [8].
Plastics can be classified into two categories: thermoplastics and thermosetting, delineated by their preparation process and thermal response. Thermoplastics exhibit the ability to soften and flow when heated, allowing for molding according to the desired shape. This transition characterizes a reversible physical change. In contrast, thermosetting polymers emerge from polymerization that leads to the formation of cross-links between chains, rendering them permanently rigid, a phenomenon known as curing. Consequently, they become impervious to subsequent melting, insoluble, and not amenable to recycling [9].
The growing demand for plastic products is driven by the application, convenience, durability, and versatility these materials offer society and current consumption patterns. However, it is regrettable that the inadequate disposal of these products is resulting in significant and, in some cases, irreversible damage to the environment [10]. Improper plastic disposal leads to adverse impacts on the environment and human health, resulting in soil, freshwater, and ocean contamination. Furthermore, the increasing ingestion of plastic nanoparticles by humans and other species through food and drinking water raises uncertainties about their overall effects. Improper plastic disposal has a devastating impact on natural ecosystems and contributes to climate change, as carbon dioxide emissions increase annually due to the growing production and incineration of plastic waste [11].
Pollution by plastics is particularly concerning on a global scale in aquatic environments. Due to the low density of plastics, they tend to float, disperse, and slowly accumulate, persisting in the environment for prolonged periods. This highlights the urgency of developing and implementing more sustainable approaches to plastic production, use, and lifecycle management [11]. Furthermore, it is crucial to explore options that promote circular economy models, avoiding significant negative social and environmental impacts [12].

2. What Are the Main Post-Consumer Plastics Commonly Used as Substitutes for Virgin Plastics in the Production of these Composites?

Analyzing the studies found that different types of post-consumer plastics were used as waste in the evaluated experiments. Among the most prevalent typologies, polypropylene (PP) was highlighted in five studies [14[13][14][15][16][17],15,16,19,22], and high-density polyethylene (HDPE) in another five studies [16,19,20,21,22][15][16][17][18][19]. Additionally, polyethylene terephthalate (PET) was used in a specific study [24][20]. In addition to these specific categories, other types of plastic waste were also addressed in a single study [23][21]. These results highlight the diversity of post-consumer plastics employed in the analyzed research, indicating that the use of plastic waste in composites is not limited to a single type of plastic, emphasizing the importance of considering a wide range of plastic waste to explore effective recycling and reuse strategies (Figure 1).
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Figure 1.
Types and number of citations of post-consumer plastics used as waste in the analyzed studies.
Each type of plastic waste possesses distinct characteristics that contribute to its specific application. HDPE stands out for its chemical resistance, while LDPE has favorable electrical properties. PVC is recognized for its excellent chemical resistance, and PP is valued for its good heat resistance. ABS, on the other hand, excels in exceptional abrasion and chemical resistance, as well as its ease of molding and machining. Regarding PS, its good mechanical properties and thermal insulation make it a common choice in food and pharmaceutical packaging applications. In addition to these mentioned plastics, others such as PBS, PC, PES, PET, PF, PHBV, PLA, PPC, and UPE have also been used as matrices in wood-plastic composites (WPCs), demonstrating significant improvements in the mechanical and thermophysical properties of the composite materials [26][22].
The analyzed studies revealed common points in the use of plastic waste in composites. Firstly, there is a preference for recycled polypropylene (r-PP) and recycled high-density polyethylene (r-HDPE), demonstrating an environmental concern for reusing post-consumer materials. Additionally, plant-based fillers, such as crushed wood waste, pine nut shells, corn husks, hemp fibers, and wood chips, were incorporated to improve mechanical properties and reduce reliance on virgin materials.

3. What Are the Percentage Compositions of Wood and Plastic Employed in These Composites?

The studies reviewed showed a wide variation in the use of wood and plastic in composite compositions. In one study, the composites contained WPC residues with approximately 45% HDPE and additives, while the crushed recycled wood fiber represented about 50% of the composition [23][21]. Another study had composite formulations ranging from 5% to 35% recycled polypropylene powder, while the wood content ranged from 60% to 85% by weight of the total mixture [17][23]. Recycled PP ranging from 30% to 100% in cycles 1, 3, and 6, combined with variations of 40% to 70% wood flour, were used in another study [18][24]. Additionally, composites were evaluated with 5% to 10% recycled PETE, compared with 43% to 45% wood fiber and 45% to 50% polymethyl methacrylate [24][20]. Furthermore, four different plastic/fiber weight ratios ranging from 0% to 100% were considered in a study [25]. The composites in another study ranged from 20% to 55%, while the wood content varied from 40% to 60% [16][15]. Finally, variations from 0% to 100% of polypropylene and wood about the total weight of the composite were analyzed in other studies [14][13]. The maximum and minimum variations in the percentage of plastic and wood can be visualized in Figure 2.
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Figure 2. Maximum and minimum percentages of plastic and wood used in composite production studies [14,16,17,18,23,24,25].
Maximum and minimum percentages of plastic and wood used in composite production studies [13][15][20][21][23][24][25].
The possibility of integrating a wide variety of materials is evident, encompassing diverse polymeric matrices, including both recycled and virgin options, as well as various fillers and residues. Composites offer an exceptionally flexible platform for incorporating multiple constituents, encompassing different materials, recycled and virgin polymeric matrices, as well as a range of fillers and residues.

4. What Shaping Processes Are Commonly Employed in the Manufacturing of These Composites?

Shaping methods play a crucial role in composite production, allowing materials to be molded and formed. Several studies have used different shaping methods to manufacture their composites. One study employed the extrusion method, which involves melting the components in an extruder and passing the material through a die to produce the desired shape [16,17,22,23][15][17][21][23]. Another study used injection and vulcanization methods, which involve injection of the molten material into a mold and subsequently curing it to achieve the desired properties [15,18][14][24]. A study opted for hot molding, which involves heating and shaping the composite material in a mold [21][19]. Another study utilized the compression method, which involves applying pressure to compact the composite material in a mold [14,25][13][25]. The main shaping methods used can be visualized in Figure 3.
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Figure 3.
Shaping methods employed in studies involving composite production experiments.
Among the shaping methods used in the production of composites, extrusion, injection, vulcanization, hot molding, and compression are commonly employed, with extrusion being the most prioritized shaping method. These shaping methods offer different approaches to composite production, allowing control over the resulting materials’ shape, properties, and performance. The variety of shaping methods also highlights the versatility of the application of these composites in different industrial sectors.
In addition to choosing the ideal method of shaping for material production, the quality of the shaping process plays a fundamental role in composite manufacturing, directly influencing a series of key characteristics of the final product. A well-executed shaping process is essential to ensure uniform distribution of components, eliminate unwanted air bubbles, and create solid interfaces between different phases. These aspects hold paramount importance, as the uniform distribution of fillers and other additives directly impacts the mechanical properties such as strength, rigidity, and toughness of the resulting composite. Moreover, proper shaping also helps minimize potential defects that could lead to weak points in the material, affecting its durability and performance over time. Therefore, careful attention to the shaping method not only ensures the structural integrity of the composite but also significantly influences the crucial mechanical and functional attributes for its success in specific applications.

5. What Is the Mechanical Behavior of These Composites?

Among the studies analyzed, some evaluated the tensile and flexural strengths of composites manufactured from recycled plastics. When comparing these studies (Figure 4), it is observed that the composites produced exhibit variation in results due to the different fillers, matrices, and shaping methods used. For the tensile tests, the minimum values range from 15.70 MPa [23][21] to 38.55 MPa [25], while the maximum values range from 32.00 MPa [23][21] to 44.39 MPa [25]. In the flexural tests, the minimum values range from 6.17 MPa [14][13] to 34.83 MPa [15][14], and the maximum values range from 17.96 MPa [14][13] to 47.84 MPa [15][14].
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Figure 4. Minimum and maximum results found for tensile and flexural strength [14,15,18,23,25]. (*) Authors who added compatibilizers.
Minimum and maximum results found for tensile and flexural strength [13][14][21][24][25]. (*) Authors who added compatibilizers.
Studies have highlighted the importance of incorporating an interfacial compatibilizer to enhance the mechanical properties, particularly tensile and flexural strength, of wood-plastic composites (WPCs) [15,23][14][21]. Without the use of compatibilizers, WPCs exhibit a tensile strength ranging from 15.70 MPa to 27.0 MPa [23][21]. However, by incorporating an interfacial compatibilizer at a 5% weight ratio, the tensile strength can be increased to a range of 19.0 MPa to 32.0 MPa [23][21]. Furthermore, the addition of the MAPE compatibilizer leads to an improvement in flexural strength, increasing it from a range of 17.0 MPa to 27.0 MPa to 32.0 MPa compared with compatibilized WPCs [23][21].
In another study, the addition of a compatibilizer to biomass/plastic composites, even when different agricultural residues are used as fillers, resulted in an increase in tensile strength from 36.80 MPa to 47.84 MPa and from 34.83 MPa to 43.06 MPa [15][14]. Another study demonstrated that composites with chemically treated fillers exhibited significantly higher tensile strength than those with untreated fillers [22][17]. Moreover, finer particle sizes showed higher tensile strength than coarser ones. However, the presence of coarse particles had a negative impact on the flexural strength of the composites.
Therefore, the main factors identified by the authors as potential influencers on the strength of the composites are the use of compatibilizers and the particle size of the raw materials. From the review, it is evident that the use of a compatibilizer improves the flexural properties of the composites and contributes to improved interfacial bonding between the matrix and fillers, aiding in stress transfer and consequently increasing strength.
In addition to the previously addressed aspects, it is important to recognize that the strength of composites is influenced by a range of other crucial factors. While compatibilizers and the particle size of raw materials play a significant role in enhancing mechanical properties, there are other variables that also play an essential part.
The selection of matrix and filler materials has a substantial impact on the strength of composites. Carefully choosing polymers and fillers with intrinsic strength properties can directly contribute to overall composite strength improvement. Additionally, the uniform and efficient distribution of fillers within the matrix is crucial to ensuring stress is evenly distributed, avoiding weak points and potential failures.
The manufacturing process also plays a vital role in determining composite strength. Factors like temperature profile, pressure, curing time, and mixing techniques can directly impact the final properties of the composite material. An improper process can result in poor dispersion and adhesion between the matrix and fillers, leading to reduced strength.
Furthermore, environmental influence and composite usage conditions also play a relevant role in long-term strength. Exposure to adverse weather conditions, such as humidity, temperature variations, and UV radiation, can affect material durability and, consequently, its mechanical strength.
In summary, the strength of composites is a complex interplay of various factors, including material selection, filler distribution/dispersion, manufacturing process, and environmental conditions. Therefore, a comprehensive approach that considers all these aspects is essential to achieving composites with optimized mechanical strength and long-term durability.

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