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Review

Circular Economy Solutions: The Role of Thermoplastic Waste in Material Innovation

by
Princess Claire D. Ochigue
1,
Maricar A. Aguilos
2,*,
Arnold A. Lubguban
3,4 and
Hernando P. Bacosa
1
1
Department of Environmental Science, School of Interdisciplinary Studies, Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
2
Department of Forestry and Environmental Resources, North Carolina State University, Raleigh, NC 27606, USA
3
Center for Sustainable Polymers, Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
4
Department of Chemical Engineering and Technology, Mindanao State University-Iligan Institute of Technology, Iligan 9200, Philippines
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(2), 764; https://doi.org/10.3390/su17020764
Submission received: 31 December 2024 / Revised: 15 January 2025 / Accepted: 16 January 2025 / Published: 19 January 2025
(This article belongs to the Section Waste and Recycling)
Figure 1
<p>Outline of the paper based on archival literature on recycled plastics.</p> ">
Figure 2
<p>Ultimate tensile strength (UTS) of composites as a function of flax fiber volume fraction (V<sub>f</sub>). Adapted from Singleton et al. [<a href="#B54-sustainability-17-00764" class="html-bibr">54</a>]. Error bars indicate the variability in UTS measurements, which becomes more pronounced at higher fiber content.</p> ">
Figure 3
<p>The hardness of the material shows a gradual increase with the increasing weight fraction of quartz particulate. Adapted from Sayuti et al. [<a href="#B62-sustainability-17-00764" class="html-bibr">62</a>].</p> ">
Figure 4
<p>Mechanical performance comparison of 2/1- and 3/2-laminates with varying metal volume fractions and the presence or absence of inherent adhesion-promoter layers (IAPL), highlighting flexural modulus, bending strength, and bending elongation. Adapted from Nestler et al. [<a href="#B66-sustainability-17-00764" class="html-bibr">66</a>].</p> ">
Figure 5
<p>Glass-fiber-reinforced polymer (GFRP) composite (<b>a</b>) impact strength, (<b>b</b>) hardness value, (<b>c</b>) flexural strength, and (<b>d</b>) compressive strength increase with increasing weight of plastic filler material. Adapted from Mahmood et al. [<a href="#B75-sustainability-17-00764" class="html-bibr">75</a>].</p> ">
Figure 6
<p>Comparative mechanical properties of PET-reinforced glass fiber samples (PET-0 to PET-4) showing yield stress, stress at break, strain at yield stress, and strain at break. Adapted from Monti et al. [<a href="#B76-sustainability-17-00764" class="html-bibr">76</a>].</p> ">
Figure 7
<p>Schematic representation of the interactions between polylactic acid (PLA), coconut fiber, and natural rubber. Adapted from Kaisone et al. [<a href="#B95-sustainability-17-00764" class="html-bibr">95</a>].</p> ">
Figure 8
<p>(<b>a</b>) Compressive strength and (<b>b</b>) volume expansion of rigid polyurethane foam (RPUF) incorporating varying weight percentages of pulverized polyvinyl chloride (PVC) and polyethylene terephthalate (PET) fillers. Adapted from Ochigue et al. [<a href="#B98-sustainability-17-00764" class="html-bibr">98</a>].</p> ">
Figure 9
<p>Effect of thylene glycidyl methacrylate (E-GMA) content on the impact strength of rHDPE/rPET (75/25 wt%) blends. Adapted from Salleh et al. [<a href="#B102-sustainability-17-00764" class="html-bibr">102</a>].</p> ">
Figure 10
<p>Effect of plastic waste replacement on the compressive strength of paving blocks. Adapted from Sandjaya et al. [<a href="#B112-sustainability-17-00764" class="html-bibr">112</a>].</p> ">
Figure 11
<p>The shrinkage of the polymer depends on the amount of wood filler and its thermal treatment, with variations observed for untreated filler (1) and fillers modified at 180 °C (2) and 220 °C (3). Adapted from Mukhametzyanov et al. [<a href="#B127-sustainability-17-00764" class="html-bibr">127</a>]. The different colored symbols represent the following: black indicates untreated filler, red indicates filler modified at 180 °C, and blue indicates filler modified at 220 °C. These dis-tinctions were already indicated in the figure title. However, to enhance clarity, we have updated the figure with the following modifications: (1) black for untreated filler, (2) red for filler modified at 180 °C, and (3) blue for filler modified at 220 °C.</p> ">
Versions Notes

Abstract

:
Plastics play an indispensable role in modern society, yet their long-term durability poses severe environmental challenges, with mismanaged waste polluting ecosystems worldwide. The transition to a circular economy emphasizes the importance of recycling and resource recovery to mitigate these impacts. While conventional disposal methods like mechanical and chemical recycling or incineration face limitations such as quality degradation, high costs, or pollutant emissions, value-added approaches present an innovative solution. This review explores the potential of integrating recycled plastic waste into composite materials to enhance performance and sustainability. Focusing on diverse strategies, the paper highlights the use of recycled plastics in combination with fibers, wood, metal, concrete, glass, rubber, textiles, and foam. These composites demonstrate superior mechanical, thermal, and chemical properties, enabling applications across industries like construction, automotive, aerospace, and furniture. Furthermore, various roles of plastic waste—such as filler, reinforcement, matrix, or additive—are analyzed to showcase advancements in material innovation. By presenting methodologies and outcomes from recent research, this paper underscores the potential of recycled plastics in creating high-performance materials, supporting sustainable development and circular economic goals.

1. Introduction

Plastics have become essential in modern society, fulfilling critical roles across various sectors including packaging, medical devices, and construction [1,2,3]. This widespread utilization has driven an exponential rise in global plastic production [4], highlighting the material’s integral role in contemporary industry. The longevity of plastics, while beneficial, creates significant challenges for their disposal [5]. The lack of effective disposal methods has led to a severe environmental crisis, with plastic waste clogging waterways [6], overflowing landfills [7], leaching into soils [8], and dispersing through the air [9], contaminating nearly every natural resource. These escalating environmental concerns have shifted global focus toward sustainable materials and the adoption of a circular economic approach, emphasizing the importance of recycling and resource recovery.
Various solutions for addressing plastic waste accumulation include mechanical recycling, chemical recycling, and incineration, however, each with its own drawbacks [3], [7,10]. For instance, mechanical recycling often leads to degraded material quality and limited applications [11], chemical recycling can be costly and energy-intensive [12], and incineration, while reducing waste volume, generates pollutants and does not recover material value [13]. Conversely, promoting value addition presents a more effective and straightforward option [3]. This approach not only repurposes plastic waste into high-value products, enhancing material properties and functionality, but also mitigates environmental impact by reducing waste and conserving raw materials [14].
As of recent estimates, approximately 9% of plastic waste is recycled, encompassing both mechanical and chemical recycling processes [15]. Roughly 19% of plastic waste undergoes incineration, and 50% of plastic waste ends up in landfills [16,17]. The remaining 22% is mismanaged, including practices such as open burning, dumping into oceans, or disposal in unsanitary landfills [18]. This situation highlights the urgent need for improved waste management practices and increased investment in recycling technologies to mitigate the environmental impact of plastic waste.
Recently, there has been a significant increase in the production of advanced, functional, and value-added materials derived directly from recycled plastic waste [19]. One innovative approach gaining attention involves using waste plastics as fillers to create composite materials for construction [20]. A composite material consists of two or more distinct constituent materials with significantly different physical or chemical properties [21]. The resulting composite exhibits unique properties that differ from those of the individual materials [22]. One constituent material, known as the reinforcing phase (such as fibers, particles, or flakes), is embedded within another material, referred to as the matrix, to enhance the overall performance of the composite [23,24,25].
Various studies have investigated the use of diverse types of plastics as composite materials, with significant research focused on combining plastics with natural fibers [26]. This area of study explores how blending plastics with fibers such as hemp, jute, or flax can enhance the mechanical properties and sustainability of composites. From there, researchers have expanded the integration of plastics with a variety of materials to create advanced composites with enhanced properties [27]. Plastics combined with wood fibers result in durable wood-plastic composites used in decking and furniture [28], while metal-plastic composites offer improved strength and corrosion resistance for engineering applications [29]. In concrete, plastic fibers enhance tensile strength and reduce cracking, contributing to more resilient construction materials [30]. Glass fibers embedded in plastics create lightweight yet strong composites ideal for aerospace and automotive uses [31]. Combining plastics with rubber produces flexible and durable materials for automotive and cushioning applications, and the integration of plastics with textiles results in enhanced durability and water resistance for outdoor and industrial fabrics [32]. These diverse combinations showcase the versatility of plastics in creating high-performance materials across various industries.
This paper explores various strategies to enhance the value of plastic waste, presenting a sustainable model for plastic waste management that holds significant potential for circular economic benefits. A comprehensive review of studies is provided, focusing on the integration of plastic waste into other materials (fiber, wood, metal, concrete, glass, rubber, textiles, and foam) to create innovative composites. Additionally, different research approaches are analyzed, employing plastic waste as filler, compatibilizer, coating, reinforcement, interlayer, matrix material, additives, and binding agents. This analysis highlights the advancements, methodologies, and outcomes associated with the use of plastics to improve the mechanical, thermal, and chemical properties of diverse materials, as outlined in the flow presented in Figure 1.

2. Types of Thermoplastics

Plastics are mainly classified into two categories: thermoplastic and thermosetting plastic [2]. Thermoplastics—such as polyethylene terephthalate (PET), high-density polyethylene (HDPE), polyvinyl chloride (PVC), low-density polyethylene (LDPE), polystyrene (PS), and, polypropylene (PP)—are widely used in composites due to their ability to be melted and reformed multiple times, making them highly recyclable and ideal for circular economy applications [33]. In contrast, thermosetting plastics like epoxy and polyester resins form rigid, irreversible cross-linked structures upon curing, which traditionally made them difficult to recycle [34]. The salient properties of the common recyclable plastics are presented in Table 1.
The unique properties of thermoplastics, particularly their recyclability and thermal behavior, set them apart from other types of plastics. Thermoplastics can be repeatedly melted and reshaped without significantly degrading their properties, which allows for recycling scrap materials into new products [35]. This characteristic is particularly advantageous in waste management and sustainability, as it enables the recovery of valuable materials from discarded products. For instance, Ahmed et al. [36] demonstrated that polylactic acid (PLA) waste can be 3D printed into new composite materials, showcasing the potential for recycling thermoplastics into functional products. This process reduces waste and contributes to the development of lightweight materials with desirable mechanical properties. However, the recycling of thermoplastic waste presents certain challenges. The presence of additives, fillers, and contaminants in recycled thermoplastics can affect the quality and performance of the final products. Ugarte et al. [37] explored different strategies for recycling polyurethane waste, emphasizing the importance of understanding the properties of recycled materials to optimize their performance in new applications. This highlights the need for careful sorting and processing of thermoplastic waste to ensure that the recycled materials meet the required specifications for various applications.
Another significant advantage of thermoplastics is their lower processing temperatures than thermosetting plastics, which can lead to energy savings during manufacturing. Oladele et al. [38] noted that thermoplastic composites are gaining increased attention due to their advantages, such as lower manufacturing costs, high strength, and reprocessing flexibility. These properties make thermoplastics particularly attractive for applications in industries such as automotive and aerospace, where weight reduction and cost-efficiency are critical. Despite these advantages, the recycling of thermoplastic waste is not without its challenges. Uzosike et al. [39] highlighted that while most thermoplastics can be recycled, the focus should be on specific solid thermoplastic wastes, such as PET, HDPE, LDPE, and PP, which account for a significant portion of packaging materials. The durability of these materials allows them to retain their properties even after multiple recycling cycles, making them suitable for various applications. However, the recycling process must be carefully managed to avoid contamination and degradation of the material properties. Furthermore, the development of multi-composition thermoplastic materials poses additional challenges for recycling. Lin et al. [40] pointed out that recycling strategies for single-composition thermoplastics are well-established, but the simultaneous recycling of multi-composition materials remains a prominent issue. This complexity necessitates innovative recycling technologies and processes to recover valuable materials from mixed waste streams effectively.
Overall, thermoplastics offer significant advantages in terms of recyclability, processing efficiency, and mechanical performance, making them a vital component of sustainable material innovation. However, challenges related to contamination, multi-composition recycling, and the presence of additives must be addressed to fully realize the potential of thermoplastic waste in the circular economy. Continued research and development in recycling technologies and material processing will be essential to enhance the sustainability and performance of thermoplastic composites.
Table 1. Properties of different plastic polymers.
Table 1. Properties of different plastic polymers.
Kinds of PlasticPolyethylene Terephthalate [41,42]High-Density Polyethylene [43,44,45]Polyvinyl Chloride [46,47]Low-Density Polyethylene [48,49,50]Polystyrene [51]Polypropylene [52,53]
Density (g/cm3)1.410.945–0.9651.3–1.70.941–0.9651.04–1.070.90–0.91
Melting Point (°C)220110–140210111265164–170
Flexural Strength (MPa)96.5–124.145616.018.134.6916–33
Tensile Strength (MPa)58.6–72.42610.2717.519.930.0–39.0

3. Recycled Plastic in Composite Material

3.1. Natural Fiber-Reinforced Plastic Composites

The combination of recycled plastics and natural fibers results in composites that not only utilize waste materials but also enhance the mechanical properties of the resulting products. For instance, studies have shown that natural fibers, when effectively combined with recycled polymers, can improve flexibility, toughness, and overall mechanical strength. Singleton et al. [54] highlighted that with the variability of flax fiber volume fractions (Vf), the composite strength becomes more pronounced. A clear trend of increasing tensile strength with higher Vf is observed in Figure 2, indicating the reinforcing effect of flax fibers in the composite matrix. At 0% fiber content, the ultimate tensile strength (UTS) is 27.1 MPa, reflecting the baseline strength of the matrix material. With the introduction of flax fibers, the UTS improves progressively, reaching 31.0 MPa at 10% Vf and 36.3 MPa at 18% Vf. Similarly, Ali et al. [55] demonstrate that the introduction of recycled plastic polymers can further improve the hardness value of date palm fiber, thereby optimizing their performance.
Moreover, the environmental benefits of using natural fibers in conjunction with recycled plastics are significant. These composites contribute to waste reduction by repurposing plastic waste and utilizing renewable resources, thus promoting a circular economy. The review by Sriprom et al. [56] highlights that incorporating natural fibers into recycled expanded polystyrene foam waste not only improves mechanical properties but also addresses the environmental impact of plastic waste. This aligns with the findings of Hossain et al. [52], who advocate for using natural fibers in automotive applications, emphasizing their sustainability and potential to replace conventional materials.
A specific example of a thermoplastic that has been successfully utilized in conjunction with natural fibers is polypropylene (PP). Scholten et al. [57] reported that composites made from recycled newspaper fibers reinforced with PP exhibit Young’s modulus that matches or exceeds that of traditional fiber-reinforced composites, demonstrating the mechanical advantages of using recycled materials. Furthermore, Zhang et al. [58] explored the characteristics of recycled polypropylene composites reinforced with old newspaper fibers, emphasizing the potential of utilizing waste materials to enhance composite properties.
In addition to mechanical and environmental advantages, the processing methods for these composites are critical for their performance. The work of Castro et al. [59] illustrates how the production of composites from recycled polymers and açaí fibers can yield materials with enhanced mechanical properties while being environmentally friendly. Furthermore, Maiti et al. [60] discuss the importance of optimizing processing techniques to achieve a balance between performance and biodegradability in fiber-reinforced composites. This is particularly relevant as the demand for sustainable materials continues to rise in various industries, including automotive and construction. The potential applications of these eco-friendly composites are vast. They can be utilized in non-structural components in the automotive sector, as highlighted by Scholten et al., who investigate the use of recycled waste paper as fiber reinforcement for polypropylene [57]. Additionally, the development of sandwich composites using natural fibers and recycled materials, as discussed by Balcıoğlu et al. [61], shows promise for applications requiring lightweight yet strong materials. The versatility of these composites makes them suitable for a range of industries, from packaging to construction.

3.2. Pure Metals-Reinforced Plastic Composites

The incorporation of thermoplastic waste into metal matrices can enhance the mechanical properties of composites, providing a balance between strength and weight. Research has shown that composites made from recycled thermoplastics and metals exhibit improved tensile strength and impact resistance compared to their virgin counterparts. For instance, Sayuti et al. [62] demonstrate that the addition of silicon particles to aluminum composites can enhance their mechanical properties, suggesting that similar methodologies could be applied to thermoplastic–metal composites to achieve superior performance, as shown in Figure 3. Furthermore, Klein et al. [63] highlight the lightweight potential of fiber-reinforced plastics combined with metal sheets, indicating that such hybrid structures can significantly reduce weight while maintaining structural integrity.
Additionally, Osiecki et al. [64] highlighted that metal-reinforced composites, particularly those utilizing aluminum and magnesium alloys, demonstrate excellent fatigue resistance and corrosion resistance, making them ideal for lightweight structural applications. This is particularly relevant in aerospace engineering, where the need for materials that are both strong and lightweight is critical. Furthermore, the microstructural characteristics of the reinforcements play a crucial role in determining the mechanical behavior of pure metals reinforced plastic composites. The study by Wang et al. [65] indicated that the arrangement and orientation of the reinforcing fibers significantly impact the mechanical properties, particularly in terms of anti-shock and high-temperature resistance. This finding underscores the necessity of careful design and engineering of the composite structure to achieve optimal performance.
Moreover, the synergistic effects of combining different materials can lead to composites with superior properties. For example, hybrid laminates that incorporate both metal and fiber-reinforced thermoplastic layers have been shown to improve lightweight construction properties and damage tolerance [66]. Figure 4 highlights the mechanical performance of 2/1- and 3/2-laminates with varying metal volume fractions and the presence or absence of inherent adhesion-promoter layers (IAPL). In 2/1-laminates, a lower metal volume fraction enhances bending strength and bending elongation but reduces the flexural modulus due to the greater influence of the fiber-reinforced plastic layer. Conversely, 3/2-laminates exhibit increased flexural modulus but reduced bending strength and elongation under similar conditions. Micro-bending tests reveal failure primarily in the fiber-reinforced thermoplastics, with carbon fiber-reinforced laminates showing localized failure due to limited impregnation caused by higher packing density and reduced wettability compared to glass fibers. These observations underscore the potential of hybrid laminates to optimize lightweight structural applications by balancing stiffness, strength, and failure mechanisms. This innovation allows for the development of materials that can withstand dynamic loading conditions while maintaining a low weight, which is essential for modern engineering applications.
A notable example of a thermoplastic successfully used in metal composites is PP. Kada et al. [67] investigated the tensile properties of short carbon fiber-reinforced polypropylene composites, demonstrating that including carbon fibers significantly enhances the mechanical properties of the PP matrix. This study highlights the potential of combining PP with metal reinforcements to achieve composites that not only retain the advantageous properties of the thermoplastic but also benefit from the strength and durability of metals. In the context of metal reinforcements, aluminum matrix composites (AMCs) have garnered considerable attention. Sun et al. [68] explored the mechanical behavior of silicon carbide (SiC) fiber-reinforced aluminum matrix composites, revealing that the load-transfer mechanisms at the interface between the matrix and the reinforcement are crucial in determining the overall mechanical performance. The findings suggest that similar principles could be applied to thermoplastic–metal composites, where the interface quality between the thermoplastic matrix and metal reinforcements is critical for optimizing mechanical properties. Furthermore, Lee and Kim [69] examined the mechanical properties of metallic glass matrix composites synthesized by powder consolidation, noting that the addition of ductile agents can enhance the plasticity of the composite. This concept can be extended to thermoplastic–metal composites, where the ductility of the thermoplastic matrix may contribute to improved toughness and resistance to failure under stress.
The environmental benefits of using thermoplastic waste in metal–plastic composites are substantial. Geyer et al. emphasize the importance of recycling plastics to mitigate the environmental impact associated with plastic waste, noting that recycling can delay, but not entirely eliminate, the final disposal of plastics [70]. Materials can degrade in quality after multiple recycling cycles or become unsuitable for further recycling due to contamination or inherent material limitations [71]. By incorporating thermoplastic waste into metal matrices, manufacturers can not only reduce the volume of plastic waste but also create valuable materials that contribute to a circular economy. This aligns with the findings of Hackert et al., who discuss the advantages of hybrid metal composites in terms of both mechanical properties and sustainability [72]. The use of recycled materials in composite production can significantly lower the carbon footprint associated with manufacturing processes, making these composites an attractive option for environmentally conscious industries.
The processing techniques used to create metal–plastic composites from thermoplastic waste are crucial for optimizing their properties. Various methods, such as powder metallurgy, extrusion, and additive manufacturing, have been explored to facilitate the effective integration of thermoplastics with metals. Ortiz-Cañavate [73] highlights the advancements in additive manufacturing technologies that have shown tremendous potential in automotive applications, which could be adapted for producing metal–plastic composites. Additionally, the work of Plettke et al. [74] emphasizes the importance of developing new joining techniques for effectively combining metal and thermoplastic components, which is essential for ensuring the durability and performance of the resulting composites.

3.3. Glass-Fiber-Reinforced Plastic Composites

The integration of recycled plastics with glass fibers has emerged as a significant innovation in composite materials, particularly for construction and infrastructure applications. This combination not only enhances mechanical properties but also contributes to sustainability by reducing reliance on virgin materials. The mechanical performance of these composites is largely influenced by the type and quality of the recycled plastic used, as well as the characteristics of the glass fibers. In addition to mechanical strength, the thermal properties of pure metals reinforced plastic composites can also be enhanced through the incorporation of metals. Mahmood reported (shown in Figure 5) that the addition of glass fibers to polymer matrices improves stiffness, toughness, and heat-distortion temperature, which is critical for applications subjected to elevated temperatures [75]. This is particularly beneficial in automotive applications, where components must withstand varying thermal conditions without compromising structural integrity.
Research indicates that the incorporation of glass fibers into recycled thermoplastic matrices can significantly improve the stiffness and durability of the resulting composites. For instance, studies have shown that composites made from recycled polyethylene terephthalate (PET) reinforced with short glass fibers exhibit enhanced mechanical performance, with optimal fiber content leading to improved tensile strength and impact resistance, as shown in Figure 6 [76,77]. In the figure, PET 0 serves as the baseline composite with no additives, while PET 1–4 incorporates varying polar moieties of ethylene copolymers to assess their impact on the structural properties of the glass-fiber-reinforced composite. These additives are expected to influence fiber-matrix bonding and dispersion, thereby affecting the composite’s mechanical properties. The effective dispersion and bonding of glass fibers within the recycled PET matrix are crucial for achieving these enhancements, as they facilitate load transfer and improve the overall structural integrity of the composite [77]. Moreover, the economic advantages of using recycled materials are noteworthy. Combining recycled plastics with glass fibers reduces materials costs and addresses the growing legislative pressures to increase recycling rates and minimize waste [78]. This is particularly relevant in construction, where the demand for high-performance materials is coupled with the need for sustainable practices. Recycled glass fibers have been explored as a viable alternative to virgin fibers, providing a cost-effective solution while maintaining desirable mechanical properties [79].
The challenges associated with the recycling of plastics, particularly in terms of contamination and degradation during processing, can be mitigated through careful selection and treatment of the recycled materials. For example, the use of coupling agents has been shown to enhance the interfacial adhesion between the glass fibers and the recycled polymer matrix, leading to improved mechanical properties [77,79]. Additionally, the thermal recycling of glass fibers from waste materials has been investigated as a method to recover high-quality reinforcement for new composite applications [80].

3.4. Textiles-Reinforced Plastic Composites

The utilization of recycled plastics in textile composites has garnered significant attention due to its potential to enhance durability and resistance to wear, making it particularly suitable for applications in clothing, sports equipment, and industrial fabrics. By incorporating recycled materials, these composites not only contribute to sustainability efforts but also improve the functional properties of textiles, thereby meeting the increasing demand for high-performance materials in various sectors.
Recycled PET is one of the most widely used recycled plastics in textile applications. Its inherent properties, such as good thermal and chemical resistance, lightweight nature, and durability, make it an ideal candidate for producing high-quality textile fibers [81], [82]. The mechanical recycling of PET, which is predominantly sourced from post-consumer bottles, allows for the creation of fibers that can be utilized in a range of textile products, including athletic wear and technical textiles [83,84]. The effective transformation of PET into functional textiles not only reduces environmental impact but also promotes a circular economy by extending the lifecycle of plastic materials [85,86]. Moreover, the integration of recycled textiles into composite materials has been shown to enhance their mechanical properties. For instance, using textile waste as reinforcement in composites can yield improved strength and durability compared to traditional materials [87,88]. This is particularly relevant in the context of sports equipment, where durability and resistance to wear are critical. The combination of recycled textiles with other materials can result in composites that are not only lightweight but also exhibit superior performance characteristics, making them suitable for high-stress applications [89].
The impregnation of thermoplastics into textile reinforcements is critical for achieving optimal mechanical properties. Studer et al. [90] investigated the effects of fabric architecture, compaction, and permeability on the melt impregnation of thermoplastics, finding that novel heating systems could enable rapid production cycles for fabric-reinforced parts. This advancement is crucial for industries that require high-volume production, such as automotive and aerospace, where efficiency and performance are paramount. Moreover, incorporating embedded sensor networks within textile-reinforced thermoplastic composites has opened new avenues for smart materials. Weck et al. [91] discussed how these materials could offer load-adapted characteristics and additional functionalities, such as monitoring structural health while maintaining high toughness and formability. The thermoplastic matrix simplifies the recycling process compared to thermosetting matrices, further enhancing the sustainability of these composites. A specific example of a thermoplastic used in textile-reinforced thermoplastic composites is polyamide 6,6 (PA 6,6), combined with carbon fibers to create commingled hybrid yarns. Hasan et al. [92] demonstrated that this approach not only improves the mechanical properties of the composites but also addresses the challenges associated with the high viscosity of thermoplastic melts during processing. Minimizing the mass transfer distance of the thermoplastic melt makes the manufacturing process more efficient, leading to better performance outcomes.
Developing sustainable textile composites also addresses the pressing issue of textile waste management. The textile industry is one of the largest contributors to waste, and innovative approaches to recycling and reusing textile materials are essential for reducing environmental impact [86]. By valorizing textile waste through its incorporation into composite materials, manufacturers can mitigate the negative effects of textile disposal while simultaneously creating high-value products [89,93]. This approach aligns with the principles of the circular economy, emphasizing the importance of reusing materials to minimize waste and promote sustainability [83,85].

3.5. Foam and Rubber-Reinforced Plastic Composites

The integration of recycled plastics with foam and rubber materials has emerged as a significant innovation in the development of shock-absorbing and insulating composites. Due to their enhanced mechanical properties and environmental benefits, these composites are increasingly utilized across various industries, including packaging, automotive, and construction. The use of recycled materials not only addresses the growing concern of plastic waste but also contributes to creating high-performance materials that meet the demands of modern applications.
Foam composites with recycled rubber exhibit remarkable energy absorption and durability. Super-elastic foams maintain elasticity under cyclic loading, making them suitable for shock-resistant applications [94]. Additionally, natural rubber enhances the compressive strength, elasticity, and interfacial adhesion of composite foams, improving overall performance as shown in Figure 7 [95].
In the automotive sector, the utilization of recycled tire rubber in foam composites has gained traction due to its lightweight nature and excellent insulating properties. Buddhacosa et al. [96] discussed the benefits of incorporating tire-derived rubber particles into syntactic foams, noting that this approach increases the density and mechanical strength of the resulting materials. The low void content achieved through advanced manufacturing processes further enhances the compression strength and modulus of these composites, making them suitable for high-stress applications in vehicles. Moreover, the valorization of recycled tire rubber extends to the development of polystyrene composites [97]. The mechanical, fire-retarding, and acoustic insulation properties of polystyrene composites are enhanced with recycled tire materials. The incorporation of ground tire rubber not only improves the performance characteristics of the composites but also aligns with the principles of circular economy by promoting the reuse of waste materials. Additionally, the study conducted by Ochigue et al. [98] further supports the notion that the mechanical properties of rigid foams can be significantly improved through strategic reinforcement of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) thermoplastic waste. The research focused on the effects of incorporating various types of plastic fibers into rigid polyurethane foams, revealing that the addition of fibers led to enhanced compressive strength and stiffness (Figure 8). This improvement is attributed to the effective load transfer between the fibers and the foam matrix, which increases the overall structural integrity of the composite. These findings align with the broader trend in material innovation, where combining different materials is utilized to achieve superior mechanical performance.
The construction industry has also benefited from the integration of recycled plastics in foam and rubber composites. Wicaksono et al. [99] explored the use of recycled polypropylene and low-density polyethylene as thermoplastic binders in particulate composites for building applications. Their research indicates that the addition of recycled plastics enhances the mechanical properties of the composites, making them suitable for various construction applications where insulation and shock absorption are critical.

4. Functional Components of Plastic Waste-Reinforced Composites

In this section, the different functional components or roles of plastic waste in composite materials will be discussed, the various plastic waste-reinforced composites (PWRC) will be categorized according to the roles they can fulfill. These components include compatibilizers, reinforcements, matrix materials, binding agents, fillers, coatings, interlayers, and additives. Each plays a specific role in enhancing the properties and performance of the resulting composites, making them suitable for a wide range of applications.

4.1. Compatibilizers

The role of plastic waste as a compatibilizer in PWRC is pivotal in enhancing the interfacial adhesion between the polymer matrix and the waste materials, thereby improving the overall mechanical properties of the composites. Compatibilizers are essential for addressing the immiscibility of different polymer phases, especially when incorporating heterogeneous plastic waste into a composite matrix. This section delves into the various compatibilization strategies and their implications for the performance of PWRC.
One of the primary mechanisms through which compatibilizers enhance composite performance is by reducing interfacial tension between incompatible phases. For instance, maleated polypropylene (MAPP) has been widely utilized as a compatibilizer in composites reinforced with natural fibers, such as wood particles. The presence of MAPP significantly improves the dispersion of the reinforcement within the polymer matrix, leading to enhanced mechanical properties, including tensile strength and impact resistance. The effectiveness of MAPP is attributed to its ability to chemically bond with both the hydrophobic polymer matrix and the hydrophilic fiber, thus creating a more cohesive interface [100]. This principle can be extended to PWRC, where the incorporation of plastic waste can be optimized through the use of suitable compatibilizers.
In addition to maleated polyolefins, other natural and environmentally friendly compatibilizers have been explored. For example, epoxidized soybean oil has been shown to function effectively in ternary blends of biodegradable polymers, enhancing compatibility through its reactive hydroxyl groups that interact with both the polymer matrix and the lignocellulosic fillers [100]. This dual action of plasticization and compatibilization is particularly beneficial in PWRC, where the goal is to maintain the sustainability of the composite while improving its mechanical performance.
The choice of compatibilizer can also influence the thermal properties of the composites. Studies have indicated that the addition of compatibilizers such as maleic anhydride can lead to improved thermal stability and mechanical properties in EVA/starch composites, highlighting the importance of selecting appropriate compatibilizers based on the specific polymer blend [101]. Furthermore, the use of glycidyl methacrylate as a compatibilizer has demonstrated significant improvements in the impact resistance of recycled HDPE/PET blends shown in Figure 9, showcasing its potential in PWRC applications [102]. The figure shows the impact strength of rHDPE/rPET (75/25 wt%) blends with varying ethylene glycidyl methacrylate (E-GMA) content. Without E-GMA (0%), the impact strength is low at 1.92 kJ/m2 due to poor interfacial adhesion. Adding 2.5% E-GMA significantly increases the impact strength to 5.18 kJ/m2, a 160% improvement, indicating enhanced compatibility between the two phases. Further increasing E-GMA to 5% boosts the impact strength to 7.96 kJ/m2, nearly a 300% increase compared to the non-compatibilized blend, demonstrating E-GMA’s effectiveness in improving mechanical properties through better interfacial adhesion.
Moreover, the use of compatibilizers can mitigate the adverse effects of excessive plastic waste inclusion, which can otherwise lead to decreased mechanical performance. For instance, while PET plastic waste can enhance the bearing capacity of sandy soil composites, excessive amounts can reduce shear strength. The strategic use of compatibilizers can help optimize the balance between waste inclusion and mechanical performance, ensuring that the composites meet the required engineering [103].
The advancement of compatibilization techniques is also noteworthy. Recent studies have explored the use of multiblock copolymers as next-generation compatibilizers, which can provide enhanced interfacial adhesion and mechanical performance in polymer [104]. These innovative compatibilization strategies are crucial for addressing the complexities associated with mixed plastic waste, which often comprises a variety of polar and nonpolar polymers [59,105].

4.2. Reinforcements

Plastic waste, particularly from PET bottles, has been effectively used to reinforce sandy soils, demonstrating significant improvements in soil stability and load-bearing capacity. Alshkane et al. [106] emphasizes that the incorporation of PET fibers not only reduces the environmental burden of plastic waste but also enhances the mechanical properties of weak soils, making them suitable for construction applications. This dual benefit of waste reduction and material enhancement positions plastic waste as a valuable resource in geotechnical engineering.
In addition to soil stabilization, plastic waste has been explored in the development of innovative building materials. Cestari et al. [107] report on the upcycling of polymers and natural fibers to create composites with high compressive resistance, suitable for construction. The study highlights the potential of using recycled high-density polyethylene (rHDPE) in combination with other materials to produce robust building components. This innovative approach not only utilizes waste materials but also contributes to the circular economy by promoting the reuse of plastics in construction.
The durability of wood–plastic composites manufactured from recycled plastics has also been a focus of some research. Turku et al. [108] investigated the weathering effects on these composites, revealing that the mechanical performance can be maintained over time, thus ensuring the longevity of structures built with such materials. This durability is crucial for construction applications, where material longevity directly impacts sustainability and lifecycle costs. Furthermore, the integration of plastic waste into masonry bricks has shown promising results. Mahyoub et al. [109] discusses the use of mixed plastic and glass waste in brick production, noting that the thermal conductivity of the resulting materials decreases with increasing plastic content. This characteristic is advantageous for energy-efficient building designs, as it can contribute to better thermal insulation properties.
The innovative use of plastic waste extends to the creation of paver blocks, where non-recyclable plastic wastes are incorporated into concrete mixtures. Vaccaro et al. [110] demonstrate that while the addition of macro plastic fibers may reduce the compressive strength of concrete, it significantly enhances toughness and post-crack resistance. This finding underscores the potential of plastic waste to improve the performance of traditional construction materials, thereby fostering a more sustainable approach to building practices.
Moreover, the acoustic and thermal performance of buildings can be enhanced through the use of recycled plastic waste in interlocking bricks. These bricks are lightweight, non-corrosive, and resistant to weather, making them suitable for various construction applications [111]. The ability to utilize waste plastics in this manner not only addresses waste-management issues but also contributes to the development of high-performance building materials.

4.3. Binding Agent

The use of plastics as binding agents in material innovation presents a promising avenue for enhancing the performance and sustainability of various composite materials. In construction, the incorporation of plastic waste as a binding agent has shown significant potential. For instance, research has demonstrated that low-density polyethylene (LDPE) can be effectively utilized as a binding material in the production of paving blocks. By replacing up to 40% of fine aggregate with melted LDPE, studies have reported improvements in compressive strength and reductions in water absorption [112,113]. Figure 10 illustrates the variation in compressive strength of paving blocks with increasing plastic waste content as a replacement for fine aggregate. The regression line shows a general downward trend, indicating that higher percentages of plastic replacement reduce compressive strength. However, an increase in strength is observed at the optimum replacement level of 20%, where the compressive strength peaks before declining again. This suggests that, at this point, the plastic waste enhances the material’s properties, likely due to its binding effect, before exceeding the threshold that compromises structural integrity. This innovative approach not only enhances the mechanical properties of the paving blocks but also contributes to the recycling of plastic waste, thus addressing environmental concerns associated with plastic disposal. Moreover, the use of recycled plastics in asphalt mixtures has gained traction as a method to improve the performance of road materials. The addition of plastic waste to bituminous mixes has been shown to enhance binding properties, increase the melting point of bitumen, and extend the lifespan of road surfaces [114]. This is particularly important in regions where traditional bitumen is scarce or expensive. The incorporation of plastic waste not only provides a sustainable alternative to conventional materials but also helps mitigate the environmental impact of plastic waste [115].
The mechanisms by which plastics enhance binding properties in composites are multifaceted. In the case of asphalt mixtures, the presence of plastic waste increases the bonding forces between aggregates and bitumen, leading to improved stiffness and durability of the mixture [115]. This is attributed to the chemical interactions between the plastic and bitumen, which enhance the overall cohesion of the material. Additionally, the use of plastic waste as a binding agent can improve the resistance of paving blocks to wear and environmental degradation, making them suitable for various applications in construction and infrastructure [112,113].
The integration of plastic waste as a binding agent in construction materials aligns with sustainability goals by promoting the circular economy. By utilizing waste plastics, researchers and manufacturers can reduce the reliance on virgin materials and minimize the environmental footprint of construction activities. This approach not only addresses the issue of plastic pollution but also contributes to the development of eco-friendly building materials [114]. Furthermore, the use of waste plastics in construction can lead to cost savings, as it reduces the need for expensive raw materials and disposal costs associated with plastic waste [115].
As the demand for sustainable construction materials continues to grow, further research into the optimization of plastic waste as binding agents is essential. Investigating the effects of different types of plastics, their compatibility with various aggregates, and the long-term performance of these materials will be crucial for advancing this field [112,113]. Additionally, exploring innovative processing techniques, such as the use of enzymatic degradation for recycling plastics into high-performance binding agents, could enhance the applicability of plastic waste in construction [116].

4.4. Fillers and Additives

The utilization of plastics as fillers and additives in material innovation has gained prominence due to their ability to enhance the mechanical, thermal, and functional properties of composite materials. This discussion will explore the roles of plastics as fillers and additives, focusing on their impact on various composite systems, including polyvinyl chloride (PVC), polylactic acid (PLA), and epoxy composites. The integration of plastic fillers not only improves material performance but also contributes to sustainability by promoting the recycling of plastic waste.
Fillers are essential components in composite materials, serving to enhance mechanical properties, reduce costs, and modify the physical characteristics of the matrix. For instance, the incorporation of calcite-rich waste particulates into PVC composites has been shown to improve tensile strength while also affecting the absorption of water and plasticization behavior [117]. However, it is important to note that high filler content can lead to increased hardness and stiffness, often resulting in a loss of ductility and tensile strength compared to virgin polymers [117]. Similarly, the use of agricultural waste, such as linseed cake, as a filler in PLA composites has demonstrated significant improvements in mechanical properties. The addition of these lignocellulosic fillers can enhance the tensile modulus of the composite, although the stiffening effect of the crystalline structure may limit the overall ductility [96]. This highlights the importance of optimizing filler content and type to achieve the desired mechanical performance while maintaining processability.
The particle size and morphology of fillers play a crucial role in determining the performance of plastic-based composites. Smaller particle sizes of fillers, such as fly ash, can enhance interfacial interactions and improve the mechanical properties of PVC composites [46]. The spherical shape of fillers can also contribute to better energy dissipation during deformation, leading to improved toughness and flexibility in the composite [46]. Conversely, the morphology of fillers can influence the overall mechanical behavior, as irregularly shaped fillers may lead to stress-concentration points that can compromise the integrity of the composite [118].
In addition to serving as fillers, plastics can also function as additives that modify the properties of the matrix. For example, the incorporation of biochar as an additive in epoxy composites has been shown to enhance thermal conductivity and mechanical strength by blocking molecular motion within the polymer matrix [119,120]. This effect is particularly beneficial in applications requiring improved thermal management, such as electronic packaging and automotive components. Moreover, the use of recycled polymer aggregates as additives in concrete has demonstrated the potential for improving the mechanical properties of self-compacting concrete. The addition of these aggregates can enhance the workability and durability of the concrete while also addressing the environmental concerns associated with plastic waste [118,121]. This approach aligns with the principles of sustainable construction, promoting the circular economy by reusing waste materials.
The integration of plastics as fillers and additives in composite materials not only enhances performance but also addresses environmental challenges associated with plastic waste. By utilizing recycled plastics, researchers and manufacturers can reduce the reliance on virgin materials and minimize the environmental impact of production processes [122]. Future research should focus on optimizing filler types, sizes, and processing conditions to maximize the benefits of plastic fillers while minimizing potential drawbacks, such as agglomeration and processing difficulties [123,124].
The incorporation of plastics as fillers and additives in composite materials has been extensively studied, revealing significant improvements in the physicomechanical properties of the resulting materials. This discussion synthesizes various studies that highlight the enhancements achieved through the use of different plastic fillers and additives, focusing on their impact on mechanical strength, thermal stability, and overall material performance.
One of the key benefits of incorporating plastic fillers is the enhancement of mechanical properties. For instance, Sahoo et al. [125] demonstrated that the addition of coconut shell powder (CSP) to recycled polyvinyl chloride (r-PVC) significantly improved the surface hardness and tensile strength of the composites. The study found that composites with 40 wt% CSP exhibited the highest surface hardness, attributed to improved interfacial compatibility between the CSP and the r-PVC matrix, which enhanced resistance to applied forces. Similarly, the incorporation of coconut shell particles in eco-composite materials resulted in increased tensile modulus and strength, as the stability of the filler allowed for better stress transfer from the polymer matrix [125].
In another study, Abreu et al. explored the effect of clay mineral addition on the properties of bio-based polymer blends. The introduction of montmorillonite clay resulted in increased stiffness and modulus, demonstrating the effectiveness of mineral fillers in reinforcing polymer matrices [126]. This aligns with findings from Mukhametzyanov et al., who reported that thermally modified fillers in polylactic acid (PLA) composites led to higher ultimate strength compared to untreated fillers, indicating that the processing conditions of fillers can significantly influence mechanical performance [127]. The tensile strength curves for the composite are shown in Figure 11. The graphs indicate that as the filler content increases, the tensile strength of the composite decreases. This reduction is due to weaker interactions between the wood particles and the binder. However, composites with thermally modified filler exhibit higher tensile strength compared to those with untreated filler. Based on the tests, the optimal filler-to-polymer ratio is determined to be 50/50% for the PLA/filler composition.
The thermal stability of composites is another critical aspect affected by the incorporation of plastic fillers. For example, the use of modified silicate fillers in polyvinyl chloride composites has been shown to enhance both physicomechanical and thermal properties, including improved thermal stability and fire resistance. This is particularly important in applications where materials are exposed to high temperatures or flammable environments. Członka et al. investigated the effects of incorporating eggshells into polyurethane (PU) matrices, finding that the addition improved mechanical properties while also reducing water uptake and enhancing dimensional stability in aqueous environments [128]. This demonstrates the dual benefit of using natural fillers not only to improve mechanical performance but also to enhance the durability of the composites.
The economic aspect of using plastic fillers and additives is also noteworthy. Paciorek-Sadowska highlighted that the addition of natural fibers to polyurethane foams not only improved mechanical properties but also reduced production costs, making the composites more economically viable [129]. This is consistent with the findings of Thathsarani et al., who emphasized the potential of biomass boiler ash as a reinforcing filler for polyamide composites, which not only improved mechanical properties but also reduced overall production costs [130]. Moreover, the use of agricultural waste materials, such as corncake in PU composites, has been shown to enhance mechanical properties while promoting sustainability by utilizing waste products from the agricultural sector [129]. This aligns with the broader trend of integrating waste materials into composite formulations to create environmentally friendly products.

4.5. Coatings and Interlayers

The use of plastics as coatings and interlayers in material innovation has gained significant attention due to their ability to enhance the performance, durability, and functionality of various substrates. This discussion will explore the roles of plastics in coatings and interlayers, focusing on their mechanical properties, adhesion characteristics, and applications in diverse fields, including electronics, construction, and food packaging.
Plastics are increasingly utilized as coatings to improve the surface properties of materials. For instance, Cui et al. [131] demonstrated that low-temperature atmospheric plasma deposition could create highly transparent multifunctional bilayer coatings on polymers. These coatings exhibited enhanced mechanical properties due to the formation of carbon bridges that dissipate energy during plastic deformation, thereby reducing the likelihood of crack propagation. The study highlights the importance of coating thickness and intrinsic properties, such as yield stress, in determining the performance of the coating under mechanical stress. Similarly, Dong et al. investigated the dual precursor atmospheric plasma deposition of transparent bilayer protective coatings on plastics. Their findings indicated that the incorporation of a reactive organic precursor prevented overoxidation of the polymer substrate, which is crucial for maintaining adhesion and mechanical integrity. The study emphasized that increased molecular bridging within the coating significantly improved adhesion, thereby enhancing the overall durability of the coated substrate [132].
Plastics also play a critical role as interlayers in laminated structures, particularly in glass applications. Laminated safety glass, which incorporates a plastic interlayer such as polyvinyl butyral (PVB), provides enhanced safety and structural integrity. Gupta’s research on laminated glass performance under static and dynamic loading conditions highlighted the importance of the interlayer in transferring shear forces between glass plies, thereby preventing shattering and maintaining the integrity of the structure [133]. The PVB interlayer’s viscoelastic properties allow it to absorb energy during impact, which is essential for safety applications. Moreover, Kamarudin et al. examined the buckling behavior of laminated glass panels, emphasizing the role of the PVB interlayer in enhancing the mechanical performance of the glass under compression. Their findings demonstrated that the interlayer significantly contributes to the overall strength and stability of the laminated structure, making it suitable for various architectural applications [134].
In food packaging, the use of plastic coatings can significantly improve barrier properties against moisture, gases, and contaminants. Debeaufort et al. explored the influence of gelatin-based coatings crosslinked with phenolic acids on polylactic acid (PLA) film barrier properties. The study found that the coatings enhanced the barrier characteristics of PLA films while providing antioxidant bioactivity, making them suitable for food preservation [135]. This highlights the potential of biodegradable coatings in reducing plastic waste while maintaining functionality.
The development of nanostructured coatings has further expanded the applications of plastics in coatings and interlayers. Zhang et al. [136] investigated graphene oxide/urushiol-formaldehyde polymer composite coatings, which exhibited remarkable liquid and gas barrier properties, as well as excellent mechanical stiffness and strength. These properties make such coatings ideal for anticorrosion applications, demonstrating the versatility of plastics in enhancing material performance.
One of the primary benefits of using plastics as coatings is the enhancement of mechanical properties. For example, Wu et al. [137] investigated the plastic deformation induced by nanoindentation tests on ZrN/Si3N4 multilayer coatings. They found that the orientation of ZrN nanograins and their interaction during indentation led to significant mechanical improvements, indicating that the multilayer structure can effectively distribute stress and enhance hardness. This study underscores the importance of coating architecture in optimizing mechanical performance. Similarly, Książek and Tchórz [138] studied the mechanical properties and erosive wear behavior of HVOF sprayed Al2O3-15 wt.%TiO2 coatings with a NiAl interlayer on Al–Si cast alloy. Their findings revealed that the interlayer effectively mitigated stress concentrations at the coating/substrate interface, enhancing the overall mechanical durability of the coating. This highlights the critical role of interlayers in improving the performance of coatings under mechanical loading.
The adhesion properties of coatings are crucial for their performance in practical applications. Kim et al. [139] explored the effects of interlayer thickness on the adhesion properties of CrZrN coatings. Their results indicated that increasing the interlayer thickness improved the critical load required for coating delamination, thereby enhancing the adhesion strength of the coating system. This finding is significant for applications where mechanical stability and durability are paramount. In another study, Zhang et al. [140] examined the microstructure and mechanical properties of laser-welded joints between dual-phase steel and stainless steel with preset nickel coatings. They reported that the presence of the interlayer improved the shear strength of the joints, optimizing plasticity and toughness, which are essential for maintaining structural integrity under load. This demonstrates the potential of interlayers to enhance adhesion and mechanical performance in multi-material systems.
Wear resistance is another critical aspect influenced by plastic coatings. Vereschaka et al. [141] investigated the wear resistance and plastic properties of composite coatings with a nanolayer structure. Their study found that the nanolayer architecture significantly improved wear resistance by distributing stress more evenly across the coating, thus reducing localized wear and extending the service life of the coated materials. This is particularly relevant in applications where coatings are subjected to abrasive conditions. Moreover, Huang et al. [142] discussed the weakening effect of plastic yielding in thin hard coating systems. They emphasized that enhancing the plasticity onset threshold of the substrate is crucial for improving the tribological performance of coating/substrate systems. By optimizing the substrate properties, the coatings can better withstand mechanical stress without cracking. This highlights the interplay between substrate and coating properties in determining overall performance.
The application of plastics as coatings and interlayers extends to advanced materials, such as organic photovoltaics. Farahat et al. [143] developed a new perylene diimide ink for interlayer formation in organic photovoltaic devices. Their research demonstrated that the interlayer facilitated electron transfer while maintaining compatibility with the underlying photoactive layer, significantly improving device efficiency. This illustrates the versatility of plastic coatings in enhancing the functionality of electronic materials. In the context of cutting tools, Li et al. [144] examined the performance of TiAlSiN-coated cemented carbide tools enhanced by inserting Ti interlayers. Their findings indicated that the multilayer coatings significantly improved cutting performance against challenging materials, showcasing the potential of interlayers to optimize mechanical properties in demanding applications.

5. Conclusions

The growing environmental challenges posed by plastic waste require urgent and innovative solutions to mitigate its impact and promote sustainability. Integrating plastic waste into value-added composite materials offers a promising approach to addressing the waste-disposal crisis and the demand for advanced materials across various industries. By repurposing plastic waste as fillers, reinforcements, or matrix components, we can enhance the mechanical, thermal, and chemical properties of a wide range of materials, from construction to automotive and aerospace applications. The combination of plastics with natural fibers, wood, metal, glass, rubber, textiles, and other materials not only improves material performance but also contributes to reducing the environmental footprint of plastic waste. Despite challenges such as the degradation of material quality in mechanical recycling or the high energy requirements of chemical recycling, the promotion of value-added solutions presents a more sustainable path forward, aligning with the principles of a circular economy. Continued research and development are essential to optimize these strategies, ensuring that plastic waste can be effectively transformed into high-value products that benefit both industry and the environment. By adopting such approaches, we can advance towards a more sustainable future where plastic waste is no longer a burden but a resource for innovation.

6. Recommendations and Future Prospects

There is a need to examine emerging thermoplastic recycling techniques beyond mechanical and chemical methods, such as enzymatic, solvent-based, or bio-recycling, which may offer higher efficiency and lower environmental impacts. Additionally, a comprehensive life cycle assessment (LCA) of thermoplastic waste-based composites would be valuable to understand the sustainability of these materials, comparing the environmental footprints of various recycling and repurposing strategies. Economic viability should also be a focus, with a review of the cost-benefit dynamics of using recycled thermoplastics, considering factors such as production costs, market demand, and barriers to adoption. Integrating thermoplastic waste with bio-based or hybrid materials, including biodegradable polymers or natural fibers, presents another potential research direction, enhancing the sustainability and performance of composites. Moreover, the use of thermoplastic waste in additive manufacturing, particularly in 3D printing, remains underexplored and could be a key area for future review, focusing on its potential for localized, sustainable production. Regulatory and policy frameworks also significantly advance circular economy models; a future review could investigate how regulations and incentives influence the adoption of recycled thermoplastic materials. In addition, advanced functionalization of thermoplastic waste to improve properties such as fire resistance, UV stability, and antimicrobial qualities could provide new avenues for material innovation. Finally, tailored circular economy models for specific industries, such as automotive, packaging, electronics, and construction, should be explored, highlighting unique challenges and opportunities for integrating thermoplastic waste into high-performance applications.

Author Contributions

Conceptualization, P.C.D.O., A.A.L. and H.P.B.; Validation, P.C.D.O. and H.P.B.; Formal analysis, P.C.D.O.; Investigation, P.C.D.O.; Resources, H.P.B.; Data curation, P.C.D.O.; Writing—original draft preparation, P.C.D.O., H.P.B. and M.A.A.; Writing—review and editing, P.C.D.O., M.A.A. and H.P.B.; Visualization, P.C.D.O.; Supervision, H.P.B. and A.A.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

P.C.D.O. acknowledges the support from the DOST-SEI Accelerated Science and Technology Human Resource Development Program (ASTHRDP) to pursue graduate studies at the MSU–Iligan Institute of Technology.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Outline of the paper based on archival literature on recycled plastics.
Figure 1. Outline of the paper based on archival literature on recycled plastics.
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Figure 2. Ultimate tensile strength (UTS) of composites as a function of flax fiber volume fraction (Vf). Adapted from Singleton et al. [54]. Error bars indicate the variability in UTS measurements, which becomes more pronounced at higher fiber content.
Figure 2. Ultimate tensile strength (UTS) of composites as a function of flax fiber volume fraction (Vf). Adapted from Singleton et al. [54]. Error bars indicate the variability in UTS measurements, which becomes more pronounced at higher fiber content.
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Figure 3. The hardness of the material shows a gradual increase with the increasing weight fraction of quartz particulate. Adapted from Sayuti et al. [62].
Figure 3. The hardness of the material shows a gradual increase with the increasing weight fraction of quartz particulate. Adapted from Sayuti et al. [62].
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Figure 4. Mechanical performance comparison of 2/1- and 3/2-laminates with varying metal volume fractions and the presence or absence of inherent adhesion-promoter layers (IAPL), highlighting flexural modulus, bending strength, and bending elongation. Adapted from Nestler et al. [66].
Figure 4. Mechanical performance comparison of 2/1- and 3/2-laminates with varying metal volume fractions and the presence or absence of inherent adhesion-promoter layers (IAPL), highlighting flexural modulus, bending strength, and bending elongation. Adapted from Nestler et al. [66].
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Figure 5. Glass-fiber-reinforced polymer (GFRP) composite (a) impact strength, (b) hardness value, (c) flexural strength, and (d) compressive strength increase with increasing weight of plastic filler material. Adapted from Mahmood et al. [75].
Figure 5. Glass-fiber-reinforced polymer (GFRP) composite (a) impact strength, (b) hardness value, (c) flexural strength, and (d) compressive strength increase with increasing weight of plastic filler material. Adapted from Mahmood et al. [75].
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Figure 6. Comparative mechanical properties of PET-reinforced glass fiber samples (PET-0 to PET-4) showing yield stress, stress at break, strain at yield stress, and strain at break. Adapted from Monti et al. [76].
Figure 6. Comparative mechanical properties of PET-reinforced glass fiber samples (PET-0 to PET-4) showing yield stress, stress at break, strain at yield stress, and strain at break. Adapted from Monti et al. [76].
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Figure 7. Schematic representation of the interactions between polylactic acid (PLA), coconut fiber, and natural rubber. Adapted from Kaisone et al. [95].
Figure 7. Schematic representation of the interactions between polylactic acid (PLA), coconut fiber, and natural rubber. Adapted from Kaisone et al. [95].
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Figure 8. (a) Compressive strength and (b) volume expansion of rigid polyurethane foam (RPUF) incorporating varying weight percentages of pulverized polyvinyl chloride (PVC) and polyethylene terephthalate (PET) fillers. Adapted from Ochigue et al. [98].
Figure 8. (a) Compressive strength and (b) volume expansion of rigid polyurethane foam (RPUF) incorporating varying weight percentages of pulverized polyvinyl chloride (PVC) and polyethylene terephthalate (PET) fillers. Adapted from Ochigue et al. [98].
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Figure 9. Effect of thylene glycidyl methacrylate (E-GMA) content on the impact strength of rHDPE/rPET (75/25 wt%) blends. Adapted from Salleh et al. [102].
Figure 9. Effect of thylene glycidyl methacrylate (E-GMA) content on the impact strength of rHDPE/rPET (75/25 wt%) blends. Adapted from Salleh et al. [102].
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Figure 10. Effect of plastic waste replacement on the compressive strength of paving blocks. Adapted from Sandjaya et al. [112].
Figure 10. Effect of plastic waste replacement on the compressive strength of paving blocks. Adapted from Sandjaya et al. [112].
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Figure 11. The shrinkage of the polymer depends on the amount of wood filler and its thermal treatment, with variations observed for untreated filler (1) and fillers modified at 180 °C (2) and 220 °C (3). Adapted from Mukhametzyanov et al. [127]. The different colored symbols represent the following: black indicates untreated filler, red indicates filler modified at 180 °C, and blue indicates filler modified at 220 °C. These dis-tinctions were already indicated in the figure title. However, to enhance clarity, we have updated the figure with the following modifications: (1) black for untreated filler, (2) red for filler modified at 180 °C, and (3) blue for filler modified at 220 °C.
Figure 11. The shrinkage of the polymer depends on the amount of wood filler and its thermal treatment, with variations observed for untreated filler (1) and fillers modified at 180 °C (2) and 220 °C (3). Adapted from Mukhametzyanov et al. [127]. The different colored symbols represent the following: black indicates untreated filler, red indicates filler modified at 180 °C, and blue indicates filler modified at 220 °C. These dis-tinctions were already indicated in the figure title. However, to enhance clarity, we have updated the figure with the following modifications: (1) black for untreated filler, (2) red for filler modified at 180 °C, and (3) blue for filler modified at 220 °C.
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Ochigue, P.C.D.; Aguilos, M.A.; Lubguban, A.A.; Bacosa, H.P. Circular Economy Solutions: The Role of Thermoplastic Waste in Material Innovation. Sustainability 2025, 17, 764. https://doi.org/10.3390/su17020764

AMA Style

Ochigue PCD, Aguilos MA, Lubguban AA, Bacosa HP. Circular Economy Solutions: The Role of Thermoplastic Waste in Material Innovation. Sustainability. 2025; 17(2):764. https://doi.org/10.3390/su17020764

Chicago/Turabian Style

Ochigue, Princess Claire D., Maricar A. Aguilos, Arnold A. Lubguban, and Hernando P. Bacosa. 2025. "Circular Economy Solutions: The Role of Thermoplastic Waste in Material Innovation" Sustainability 17, no. 2: 764. https://doi.org/10.3390/su17020764

APA Style

Ochigue, P. C. D., Aguilos, M. A., Lubguban, A. A., & Bacosa, H. P. (2025). Circular Economy Solutions: The Role of Thermoplastic Waste in Material Innovation. Sustainability, 17(2), 764. https://doi.org/10.3390/su17020764

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