Co-Extruded Wood-Plastic Composites: Their Structure, Properties, and Applications

Abstract

Wood-plastic composites (WPCs) are innovative materials prepared by mixing wood fibers, thermoplastics, and additives through extrusion, injection molding, or compression molding processes. Co-extruded WPCs (Co-WPCs) are multilayer composites, in which regular WPCs are coated with a protective shell layer through coextrusion. The core-shell structure of Co-WPCws provides a way to overcome the shortcomings of WPCs by modifications specific to the composite surfaces. With the development of coextrusion technology, Co-WPCs show promise to become one of the important leading directions of the WPC industry. Based on the special characteristics of the core-shell structure, the properties of Co-WPCs were reviewed in this chapter, including mechanical properties, dimensional stability, weather resistance, flame retardancy, etc. Furthermore, the applications of Co-WPCs were comprehensively presented. Finally, the problems and challenges in the development of Co-WPCs were put forward, and the key points of future research were also expounded.

1. Introduction

Wood-plastic composites (WPCs) are composite materials made primarily from polar wood fibers and non-polar thermoplastic polymers, produced through melting, compounding, including extrusion, hot pressing, injection molding, and other molding processes [1, 2, 3]. WPCs have excellent comprehensive performance such as anti-corrosion, durability, waterproofing, and moisture resistance, as well as low cost, environmental friendliness, and excellent mechanical properties. They have been widely used in areas, such as architectural decoration, interior design, outdoor landscaping, and so on. The main drawbacks of WPCs are the moisture absorption caused by the hydrophilic wood flour, which increases the degradation and loss of properties of the WPCs [4, 5]. Thermoplastic materials are typically hydrophobic, hence WPCs with injection-molded plastic-rich surfaces exhibit less water absorption and improved weathering resistance [6, 7]. Therefore, applying a layer rich in plastic to WPCs is a viable way to lessen the composites’ absorption of moisture and improve their resilience to weathering. WPCs’ flammability because of the organic nature of its primary ingredients (thermoplastics) is another drawback. Adding fire retardants to bulk composites is the standard approach of reducing the flammability of WPCs [8]; however, adding a large amount of fire retardants frequently degrades the mechanical qualities of WPCs and raises costs. The inside condition of the composites has no bearing on the initial damage to WPCs since external factors, including moisture, ultraviolet (UV) radiation, and combustion sources, primarily affect the surface of WPCs. Hence, surface modification rather than bulk materials should be the main emphasis of WPC protection. Moreover, by incorporating additives only into the outer layer, the necessary amount of additives is reduced. As a result, applying a functional protective surface layer to WPCs is a cost-effective and efficient way to enhance their durability.

Coextrusion is a process where two or more polymeric materials are combined through a feedblock or die to create a single multilayer structure [9, 10]. Various polymers can be merged to form a unified structure with improved properties. Coextrusion has emerged as a leading plastic processing technology for producing customizable multilayer composites, commonly utilized in manufacturing tubing, multilayer sheets, wire coatings, films, and core-shell profiles [11]. Co-WPCs are typically manufactured with core-shell structures to introduce a cap layer to WPCs, as illustrated by the Co-WPC sample shown in Figure 1. The coextrusion system comprises two extruders for the core layer and shell layer melts, along with a specifically designed die for transforming the two melts into the desired core-shell profile. The Co-WPCs combine the different performance advantages of the surface layer and core layer materials. The optimization of overall performance within the core-shell structure enhances its added value and provides diverse functionalities. Therefore, Co-WPCs have the potential to replace traditional wood-plastic composites in various application fields, particularly in outdoor environments with extreme conditions and areas with special functional requirements.

Figure 1.

Scheme of the coextrusion system used to produce Co-WPCs [12].

Researchers first prepared WPCs with a core-shell structure, known as Co-WPCs, using coextrusion technology in 2007 [13]. With the development of the wood-plastic industry over the years, Co-WPCs have matured and gradually become a research hotspot in the field of WPCs, showing promising development potential. In recent years, while demand for traditional WPC products has slowed down, the development of Co-WPC products has surged, with its market share gradually increasing to over 50%. Co-WPCs are likely to become the mainstream trend in the WPC industry. The presence of the shell layer not only imparts multifunctionality to Co-WPCs but also provides them with diverse decorative characteristics such as rich colors and surface textures. Therefore, Co-WPCs are expected to exhibit a trend toward diversification in development. This chapter summarizes the structure design, properties, and applications of Co-WPCs.

2. Structure design of Co-WPCs

As people’s requirements for the performance of WPCs and their products continue to increase, traditional single-structure wood-plastic composites can no longer meet the special performance, functionality, and appearance requirements of wood-plastic products. Co-WPCs can combine materials with different characteristics to fully leverage the inherent properties of various materials, producing products for specific purposes with short molding cycles and low energy consumption. Different types of composite products can be produced by co-extruding materials with different structures. Coextrusion of materials with different characteristics can give a product excellent properties from several different materials. By employing suitable structure design, high added-value co-extruded wood-plastic composite products can be manufactured to meet various performance and application requirements.

2.1 Fully and non-fully wrapped Co-WPCs

The shell layer of Co-WPCs is typically fully coated on the surface of the core layer, blocking the contact between the core layer material and the external environment, and serving a protective function for the core layer. Research has shown that the mechanical and thermal properties of Co-WPCs with fully coated surfaces are superior to those of semi-coated materials [14]. Therefore, adopting a semi-coated structure on the surface to save costs is not worthwhile. The typical structures of fully and non-fully wrapped Co-WPCs are shown in Figure 2.

Figure 2.

Fully (left sample) and non-fully (right sample) wrapped Co-WPCs [14].

The conventional non-fully wrapped Co-WPCs often rely solely on the adhesive properties between the surface layer and the core layer after heating and melting, but the limitations of adhesion between inner and outer layer materials, as well as differences in material properties such as shrinkage/expansion rates and weather resistance in different usage environments, often lead to detachment and peeling between the core layer and surface layer at the transitional region of semi-coated bonding, resulting in issues such as shortened lifespan and product vulnerability. By incorporating a fixed groove design, the coating material extends beyond just attaching to the board surface, reaching into the interior cross-section of the core layer material, significantly increasing the contact area, and effectively preventing coating layer detachment. These embedded non-fully wrapped Co-WPCs, compared to traditional non-fully wrapped Co-WPCs, offer the advantage of structural stability over traditional fully coated boards and cost savings.

Wood-plastic boards produced by traditional extrusion methods exhibit low impact toughness, susceptibility to breakage, and other shortcomings. Moreover, as the content of wood flour increases, the strength and modulus initially rise to a certain extent before sharply declining, with a more pronounced decrease in toughness. Lowering the wood flour content can improve impact toughness to a certain degree, but the strength and modulus may not meet requirements. Therefore, the issue of balancing impact toughness and strength modulus in wood-plastic boards remains a challenge for the wood-plastic industry. In response to this problem, by producing sandwich-structured Co-WPCs with a low-density polyethylene core structure, compared to wood-plastic composites without extruded core structures, bending strength can be increased by 10–50%, bending modulus by 5–50%, and impact strength by 20–80%. This enhancement makes it possible to simultaneously strengthen and toughen wood-plastic composite materials mechanically.

2.2 Co-WPCs with solid wood core

Currently, Co-WPCs prepared through extrusion technology have relatively low mechanical strength. Solid wood materials exhibit higher comprehensive mechanical performance compared to wood-plastic composite materials; however, they are prone to warping and deformation after absorbing moisture, easily decay, and are challenging to utilize efficiently. In response to these issues, scholars have proposed using wood-plastic composite materials as the surface layer and fast-growing artificial forest wood and reconstituted wood (referred to as solid wood) as the core material. Co-WPCs with solid wood core formed through coextrusion exhibit better anti-creep properties, non-brittleness, lightweight, high strength, cost-effectiveness, and excellent durability. In comparison to traditional Co-WPCs, Co-WPCs with solid wood core-only require one extruder and can be prepared by a specific coextrusion die and solid wood transport device to achieve simple or complex cross-sectional Co-WPCs (Figure 3) [15]. Co-WPCs with solid wood core demonstrate excellent comprehensive performance and are environmentally friendly, with costs falling between wood-plastic and wood-based multilayer boards, providing high cost-effectiveness. By designing mortise and tenon structures, the outer-shell-layer polyvinyl chloride (PVC) wood-plastic composite (WPVC) and the inner-core-layer laminated veneer lumber (LVL) could form a mechanical interlock, which could effectively improve the interface bonding property between the two layers [16]. They are not only suitable as upgraded replacement products for traditional preservative wood and engineered panels but also exhibit outstanding advantages in high-quality building doors and windows, construction templates, large-span wooden components, architectural components, multifunctional wall structures, green buildings, and other applications requiring materials with high environmental, load-bearing, waterproof, moisture-resistant, decay-resistant, and weather-resistant properties, offering broad application prospects.

Figure 3.

Schematic of the fabrication of Co-WPCs with solid wood core [15].

2.3 Specially structured Co-WPCs

To enhance the performance of Co-WPCs to meet different application scenarios, other specially structured Co-WPCs have been developed. The specially structured Co-WPCs include Co-WPCs with multilayered core, Co-WPCs with compartmentalized surface structures, Co-WPCs with metal core, co-extruded wood-plastic-rebar composite materials, and co-extruded core foaming composite materials. However, these special structural co-extruded wood-plastic composite materials have been reported only in some patents and have yet to appear in academic papers and practical applications.

3. Main properties of Co-WPCs

Compared to ordinary WPCs, Co-WPCs exhibit better toughness, but there will be varying degrees of reduction in strength and modulus, especially when different functional fillers are added to the shell layer, which significantly impact the mechanical performance. Mechanical performance has always been a key issue in Co-WPC research. Additionally, the higher polymer content in the shell layer results in higher temperature dependency of Co-WPCs, making it necessary to study the influence of external environmental factors (such as temperature, humidity, and external forces) on their dimensional stability. Furthermore, during long-term indoor and outdoor use, Co-WPC products have certain requirements for flame resistance and weather resistance. It is also important to consider the balance between flame resistance, weather resistance, and other properties to avoid significant decreases in individual performance. In summary, this section systematically summarizes and outlines the main characteristics of Co-WPCs, focusing on their mechanical properties, dimensional stability, flame resistance, and weather resistance.

3.1 Mechanical properties of Co-WPCs

Since the core and shell layers of Co-WPCs control their mechanical properties, it is convenient to tailor the mechanical performance of Co-WPCs by adjusting the thickness of the shell layer and the quality of the core and shell layers [12, 17]. By filling the core and shell layers with varied amounts of wood flour, reinforced fibers, inorganic particles, and hybrid fillers, or by employing recycled thermoplastics as matrices, different characteristics of the layers can be achieved [12, 17, 18, 19, 20]. Additionally, by applying an external force to the Co-WPCs, the shell layer can limit the deformation of the core layer and stop surface cracks from spreading to the center of the composites. Therefore, when Co-WPCs are subjected to external stress, a stronger and more flexible shell layer would aid in preserving their integrity and minimizing damage. Co-WPCs with a core-shell structure exhibit special mechanical qualities that depend on several variables.

The shell layer feeding rate and extrusion speed can be controlled to alter the shell thickness, which can then be used to alter the ratio of shell to core materials in Co-WPCs. When the wood flour loading in the shell layer was fixed, Yao and Wu [11] discovered that Co-WPCs with thicker shell layers exhibited higher impact strength and a lower modulus. When Jin and Matuana [12] co-extruded WPCs with PVC cap layers, the thickness of the PVC cap layers had a negative impact on the flexural modulus but a negligible influence on the flexural strength of the Co-WPCs. Co-Kim et al. [17] prepared Co-WPCs with shell layers that varied in thickness and core layers that were classified as weak, moderate, and strong. The relative magnitudes of the modulus and strength of the shell and core layers determined the variation in the modulus and strength of the Co-WPCs with different shell layer thicknesses. When the shell modulus and strength were greater than the core layer, a thicker shell enhanced the Co-WPCs’ modulus and strength; conversely, when the core layer’s characteristics were superior, the opposite was noted. When the shell thickness was increased for the weak, moderate, and strong core systems, the impact strength of the Co-WPCs rose as well. Co-WPCs’ shell layer was typically filled with less material than their core layer when the preparation process and composite performance were taken into account. The impact properties of Co-WPCs are typically enhanced in low-filled shell layers, leading to lower strength and modulus. Increasing the thickness of the shell layer has been observed to improve impact performance while reducing modulus and strength. However, the strength of Co-WPCs may be increased compared to core-only materials in certain cases due to the fully enclosed structure. Ultimately, the impact of shell thickness on Co-WPCs is influenced by the properties of the shell layer. Enhancing the thickness of a high-performance shell layer is advantageous for improving the mechanical properties of Co-WPCs. Typically, the thickness of the shell layer is less than a quarter of the core thickness, and for reasons related to cost and protective efficiency, a thinner shell layer is recommended.

The core layer is a significant component of Co-WPCs, serving as their fundamental and primary element. By adjusting the composition of high-density polyethylene (HDPE) to low-density polyethylene (LDPE), modifying process parameters such as temperature and wood flour content, incorporating recycled polymers, and introducing materials like glass fiber, the qualities of the core layer can be customized. Research indicates that Co-WPCs exhibit superior mechanical properties when equipped with a high-quality core, even with identical shell layers. In contrast to core-only materials, the enhancement in Co-WPCs is more pronounced with a low-quality core; therefore, Co-WPCs featuring satisfactory mechanical characteristics can be achieved by incorporating a weaker core through suitable core-shell designs. Instead of utilizing WPCs as core layers, the integration of laminated veneer lumber (LVL) with exceptional mechanical properties results in the production of high-strength WPCs-LVL co-extruded composites. The high strength and minimal deformation of WPCs-LVL make them ideal materials for structural applications.

The constituents of the shell layer are crucial in determining the collective mechanical performance of Co-WPCs, with the ability to attain desirable mechanical properties through adjustments in the shell layer quality [14]. Variations in the shell layer quality were achieved by altering wood flour content [11, 12, 21], incorporating reinforced fibers [22, 23], introducing inorganic particles [9, 24, 25, 26], and utilizing hybrid fillers [18]. An overview of the primary fillers employed in the shell layers of Co-WPCs can be found in Table 1.

		Mechanical performance
Fillers	Percentage (wt%)	Flexural strength	Flexural modulus	Impact strength	References
Wood flour	0–27.5; 0–25, 35; 0–20	↑a; ↑, ↓b; ↑	↑; ↑, ↑; ↑	-; ↓, ↓; ↓	[12]; [11]; [20]
CNTs	5	↑	↑	—	[19]
SGFs	0–40	↑	↑	↓	[23]
BFs	0–30	↑	↑	↓	[2]
Talc	0–50	↑	↑	↓	[25]
SiO2	0–20	↑	↑	↑	[26]
CaCO3	0–25	↑	↑	↓	[24]
TPCC/WF	12/0–15, 12/25	↑, ↓	↑, ↓	↓	[18]

Table 1.

Effect of shell fillers on the mechanical performance of Co-WPCs [27].

^a

↑ = Increase.

^b

↓ = Decrease.

3.2 Dimensional stability of Co-WPCs

Wood materials have a high polarity, leading to significant dimensional changes upon moisture absorption, but minimal impacts from temperature fluctuations. In contrast, polymers typically absorb moisture at less than 1%, resulting in negligible effects on dimensional changes due to humidity, but considerable size deformations with temperature variations. Furthermore, polymers can exhibit creep deformation under prolonged external forces or gravity. WPCs are composed of polar wood fibers and non-polar polymers; their dimensional stability includes moisture-induced expansion and contraction, thermal expansion, and contraction due to temperature fluctuations, as well as creep deformation under long-term loading. Considering the diverse factors of humidity, temperature, and load based on different regions and seasons is essential to understand the comprehensive impact on the dimensional stability of WPCs. Given the unique multilayer structure of Co-WPCs, investigating their dimensional stability is particularly crucial.

3.2.1 Water absorption

The presence of abundant hydroxyl groups and the distinctive structure of wood flour render it hydrophilic. Wood-plastic composites (WPCs) combine hydrophilic wood flour with hydrophobic plastics, with the assumption that the wood flour is enclosed within the plastic matrix. However, traditional highly filled WPCs often encounter challenges in fully encapsulating the wood flour, allowing moisture to penetrate the composite easily. When water is absorbed, the wood flour swells, leading to the disruption of the interface between the wood flour and the matrix. Subsequently, microcracks may form as the water evaporates (as depicted in Figure 4), resulting in diminished mechanical properties of the WPCs and permitting further water ingress [5, 28].

Figure 4.

Schematic diagram of the water-induced failure of WPCs. (A) WPCs with wood flour capped well by a plastic matrix (B). Soaked WPCs with water in the interface of the wood flour and matrix (C). Dried WPCs with a broken wood flour/matrix interface [5].

Applying a pure hydrophobic polymer coating to WPCs can significantly enhance their resistance to moisture [29]. Jin and Matuana [12] utilized coextrusion to encapsulate WPCs with a PVC shell and studied the water absorption characteristics of the resulting composites. The inclusion of shell layers led to a reduction in both the rate and overall moisture content absorbed by the WPCs. While the introduction of wood flour in the shell layer increased the water absorption of Co-WPCs, the thickness of the shell layer itself did not impact their water absorption properties. This correlation was also found by Mei et al. [30]. However, Hao et al. [20] observed that the addition of wood flour content ranging from 0 to 20 wt% in the shell layer did not significantly alter the water absorption or thickness of Co-WPCs. However, Co-WPCs containing 20 wt% wood flour still exhibited higher water absorption compared to other Co-WPCs. In a separate study, Kim et al. [18] developed Co-WPCs with shell layers filled with treated precipitated calcium carbonate (TPCC) and wood fiber (WF). The water absorption of these Co-WPCs was unaffected by the TPCC content in the shell layer but showed a direct relationship with the WF content. This was attributed to the effective dispersion of TPCC, enhanced interactions between TPCC and HDPE in the shell, and the hydrophilic nature of WF.

Turku et al. [8] incorporated various fillers, including aluminum trihydroxide (ATH), zinc borate (ZB), melamine, graphite (G), and titanium dioxide (TiO₂), into the shell of polypropylene (PP)-based Co-WPCs. The hydrophilic ZB filler increased the water absorption of Co-WPCs, while the other fillers resulted in reduced wettability of the composite. Additionally, the group introduced pulp cellulose (PC), a blend of PC/MFC (microfibrillated cellulose), and wood flour of different sizes into the shell of Co-WPCs. They discovered that Co-WPCs loaded with wood flour exhibited the highest water absorption and thickness swelling, whereas PC-filled Co-WPCs demonstrated lower moisture absorption. MFC-loaded Co-WPCs showed a similar water absorption trend as PC-filled Co-WPCs but a higher tendency for thickness swelling. The higher crystallinity of PC and MFC compared to wood flour, along with the impermeability of cellulose crystals to water, likely contributed to the reduced moisture absorption of PC- and MFC-filled Co-WPCs. The impact of incorporating graphite (G), expandable graphite (EG), carbon nanotubes (CNTs), carbon black (CB), and carbon fibers (CFs) into the shell of Co-WPCs on water absorption performance was also studied [22]. Co-WPCs containing EG exhibited the highest water absorption due to the lamellar and expanded structure of EG particles, while other fillers did not negatively affect the wettability of the composite. Furthermore, Butylina et al. [31] added Fe₃O₄, TiO₂, and zinc oxide (ZnO) to the Co-WPC shell and analyzed the wettability of the composites. The inclusion of TiO₂ increased water absorption by 43%, whereas iron oxide and ZnO did not impact composite wettability. However, TiOO₂-filled Co-WPCs showed minimal thickness swelling due to their low density and high porosity.

Typically, moisture absorption tests on Co-WPCs are conducted at 20°C, despite WPCs being exposed to temperatures as high as around 60°C during midday. In a study by Zhao et al. [5], the water absorption performance of Co-WPCs enclosed in an HDPE shell was compared to that of uncoextruded WPCs at 60°C. The Co-WPC exhibited reduced water absorption and enhanced dimensional stability compared to the uncoextruded WPC. Additionally, the Co-WPC maintained higher flexural properties than the uncoextruded WPC. Therefore, applying a hydrophobic shell layer through coextrusion onto WPCs can offer protection to the composites in environments with high temperatures and humidity.

3.2.2 Creep properties

During long-term use, the creep behavior resulting from the movement and slippage of polymer matrix molecular chains in WPCs can significantly impact their macroscopic properties and service life. Given the higher polymer content in the shell layer, Co-WPCs exhibit more pronounced time- and temperature-dependent creep behavior during extended use [2, 20, 26]. Increasing the wood fiber content can effectively reduce the creep deformation of WPCs; studies have shown that increasing the wood fiber content (by mass fraction) from 50–70% can reduce the 24-hour creep deformation by 58% [32]. Similarly to mechanical properties, incorporating rigid particles such as silica or wood fibers in the shell layer can significantly reduce the creep deformation of the shell layer, aiming to decrease the overall long-term creep deformation of Co-WPCs. This is primarily attributed to the ability of rigid silica or wood fibers to effectively inhibit polymer chain slippage and rearrangement, thus enhancing their creep resistance [33, 34]. Compared to micron-sized silica, nano-sized silica can achieve a higher reduction in the creep deformation of the shell layer and Co-WPCs, mainly due to the reinforcing effect of nano-sized particles. Surface modification of nano-sized silica with silane can further enhance the creep resistance of Co-WPCs. However, while shell reinforcement has a limited impact on the overall creep resistance of Co-WPCs, increasing the wood fiber content in the core layer, which is the main component of Co-WPCs, can also effectively reduce the overall creep deformation of Co-WPCs.

3.2.3 Thermal expansion properties

When the temperature rises, materials frequently expand thermally because their free volume increases. The slope of the linear part of the dimensional change-temperature curve served as a proxy for the linear coefficient of thermal expansion (LCTE) of composites [35]. The depth of the atomic bond energy function and the reinforcing capacity of fillers in composites are connected with LCTE. A lower LCTE value is preferred for composite structural applications in order to guarantee thermal dimensional stability [36, 37]. The linear coefficient of thermal expansion of polymers is generally higher than 1 × 10–4°C-1, significantly exceeding those of metals and ceramics at 2 × 10–5°C-1. In contrast, the thermal expansion coefficient of natural biomass materials ranges from 5 × 10–6 to 5 × 10–5°C-1 [38, 39]. Therefore, the presence of wood fibers can effectively reduce the thermal expansion behavior of the polymer matrix. The addition of glass fibers [40], talc, basalt fibers (BFs), TPCC/wood flour, and talc/BFs to the shell layers of Co-WPCs all resulted in a reduction in the LCTE of the composites.

The complete encapsulation of the core layer by the shell layer structurally also to some extent restrains the thermal expansion behavior of Co-WPCs. Nevertheless, a difference in the linear coefficient of thermal expansion (LCTE) between the core and shell layers can lead to stress accumulation at their interface, exacerbated by higher temperatures [35]. This accumulated stress has the potential to disrupt the core-shell structure of Co-WPCs, ultimately leading to decreased mechanical properties, water absorption performance, flame resistance, and weathering characteristics. Hence, achieving a similar LCTE between the shell and core layers is advantageous for Co-WPCs. Additionally, the extrusion process of WPCs results in significant thermal expansion anisotropy due to the oriented arrangement of wood fibers. This means that the coefficient of thermal expansion (CTE) along the direction of the oriented wood fibers is much smaller than those in the thickness and other directions [41]. This indicates that wood fibers with different aspect ratios achieve a reduction in the coefficient of thermal expansion of Co-WPCs by limiting the deformation of the polymer matrix.

3.3 Weather resistance

As the use of WPCs in outdoor settings rises, they face exposure to factors like ultraviolet light, moisture, fungal growth, heat, and other environmental stressors. The yearly costs associated with the degradation of WPCs amount to millions of dollars [42]. Enhancing the weathering resistance of WPCs is crucial for their advancement and broadening their range of applications.

3.3.1 Accelerated ultraviolet weathering

Typically, accelerated ultraviolet weathering tests involve exposing materials to several cycles of UV radiation, water spraying, and condensation to replicate sunlight, rain, and dew conditions [4]. These tests are designed to imitate the natural outdoor weathering process of WPCs over an extended period in a shorter timeframe within a laboratory setting. When exposed to outdoor elements, WPCs may experience color fading due to natural weathering, leading to a significant decline in both the visual appeal and functional longevity of the material. Analysis of color involves assessing attributes, such as lightness (L*), the red-green coordinate (a*), and the yellow-blue coordinate (b*) [29]. Color changes (ΔE_ab) are typically determined using the following formula:

$$\Delta E_{ab}={\left(\Delta L^2+\Delta a^2+\Delta b^2\right)}^{1/2}$$E1

In Eq. (1), ΔL, Δa, and Δb represent the variances in lightness (L*) and chromaticity coordinates (a* and b*) observed before and after the weathering process.

After 1000 hours of weathering, Co-WPCs with an HDPE cap exhibited less color change compared to uncapped WPCs, while Co-WPCs with a PP cap experienced greater color alteration than uncapped WPCs. The susceptibility of PP to UV degradation surpasses that of HDPE, leading to more pronounced discoloration in Co-WPCs with a PP cap. Matuana et al. [4] delved further into the degradation mechanism of HDPE-coated Co-WPCs. Photooxidation altered the surface chemistry of uncapped WPCs, resulting in darkening of the composites. Subsequently, the loss of pigmented wood components and increased surface roughness contributed to a lighter appearance of uncapped WPCs. Conversely, in Co-WPCs, the hydrophobic HDPE shell layer created through coextrusion prevented the loss of pigmented components, thereby significantly mitigating discoloration (as illustrated in Figure 5). The discoloration observed in Co-WPCs mainly stemmed from the photooxidation of wood constituents at the core-shell interface. Applying a transparent plastic shell layer via coextrusion on WPCs can serve as a safeguard against discoloration caused by weathering. The integration of UV absorbers or photostabilizers into the shell layers could offer added protection, potentially minimizing discoloration in Co-WPCs (as outlined in Table 2) [29, 31, 42, 43, 44, 45]. Additionally, adding metal oxide-based pigments (such as titanium dioxide, iron oxide, zinc oxide, etc.) to the shell layer can enhance color stability while improving mechanical performance [31, 43].

Figure 5.

Discoloration of uncapped WPCs (left column) and Co-WPCs (right column) after various weathering times (0, 192, 432, 744, 1392, and 1952 h) [4].

Additives	Basic composition	Percentage (wt%)	ΔL	ΔEab	References
TiO2	HDPE/Compatibilizer; PP/cellulose pulp; PP/cellulose pulp	1; 3; 10	a ↓; ↓	↓; ↓	[29]; [31, 43]; [32]
Photostabilizer	HDPE/Compatibilizer	0.6	↓	↓	[29]
Fe3O4	PP/cellulose pulp	3	↓	↓	[31, 43]
ZnO	PP/cellulose pulp	3	b ↑	↑	[31, 43]
Melamine	PP/cellulose pulp	10	↓		[44]
Graphite-	PP/cellulose pulp	10	↓		[44]
ATH	PP/cellulose pulp	10	↓		[44]
ZB	PP/cellulose pulp	10	↓		[44]
CB	PP/cellulose pulp	3	↓		[45]
EG	PP/cellulose pulp	3	↓		[45]
CNTs	PP/cellulose pulp	3	↓		[45]
CFs	PP/cellulose pulp	3	↓		[45]

Table 2.

Summary of shell layer additives and their effects on the color protection of Co-WPCs [27].

^a

↓ = Decrease.

^b

↑ = Increase.

3.3.2 Freeze-thaw weathering

With the expanding range of applications for WPCs, it is essential to consider the impact of extreme conditions on their lifespan. In regions with high latitudes, WPCs are subjected to UV light degradation as well as the effects of freeze-thaw cycles [46]. Exposure to lower temperatures can lead to WPCs becoming brittle, while increased moisture uptake can compromise interfacial adhesion, resulting in a significant loss of mechanical properties [46, 47]. Freeze-thaw weathering tests involve subjecting samples to cycles of immersion in water at 23°C for 70 ± 1 hours, freezing at −20°C for 24 hours, and drying at 70°C for 70 ± 1 hours. Turku and Kärki [48] examined the impact of incorporating various fire retardants (melamine, aluminum trihydrate, graphite, zinc borate, and TiO2) at a 10% concentration into the shell of Co-WPCs after five freeze-thaw cycles. They observed surface cracks and reduced mechanical properties in all composites, indicating that the fire retardants had minimal effect on the freeze-thaw resistance of Co-WPCs. Furthermore, melamine and zinc borate displayed instability in Co-WPCs, leading to partial leaching from the surface. The researchers then introduced different carbon fillers, including carbon black, graphite, expandable graphite, carbon nanotubes, and carbon fibers, at a 3% concentration into the shell of Co-WPCs. They found that Co-WPCs filled with carbon black, carbon nanotubes, and carbon fibers maintained a significant portion of their mechanical performance. Furthermore, carbon black and carbon nanotubes were effective reinforcements for Co-WPCs when compared to carbon fibers.

3.4 Fire resistance of Co-WPCs

The flammability of WPCs presents safety concerns that restrict their potential applications. Conventional methods for enhancing composite fire resistance involve incorporating fire retardants into the bulk materials. Cone calorimetry is commonly employed to evaluate flame-retardant properties and determine parameters, such as heat release rate (HRR), peak heat release rate (PHRR), ignition time (IT), total heat release (THR), total smoke release (TSR), average specific extinction area (ASEA), average carbon monoxide (CO) and carbon dioxide (CO2) values, average effective heat of combustion (AEHC), and average mass loss rate (AMLR), to evaluate the fire performance of composites. The peak heat release rate (PHRR) is particularly significant as it indicates fire intensity and growth rate.

Initially, the proximity of the heat source to the WPC surface rendered the internal fire retardants ineffective in preventing combustion. Additionally, the high concentration of fire retardants added to WPCs adversely impacted their mechanical properties [8]. Incorporating a fire-retardant shell layer through coextrusion onto WPCs can prevent combustion without the need for excessive fire-retardant content. Conversely, a pure plastic shell layer negatively influenced the flammability of Co-WPCs, as the peak and heat release rates of plastics surpassed those of wood flour [49, 50, 51]. Consequently, Co-WPCs with pure plastic shell layers were not recommended due to fire safety concerns. Wu et al. [2] implemented basalt fibers at 10%, 20%, and 30% weight content in the shell of Co-WPCs, with findings revealing a reduction in PHRR, THR, HRR, ASEA, AEHC, and AMLR as basalt fiber content increased. Compared to Co-WPCs with a pure HDPE shell, the 30% basalt fiber-loaded variant exhibited a 19.7% decrease in PHRR. The improved fire resistance of basalt fiber-filled Co-WPCs was attributed to incomplete combustion within the core-shell composites, although these materials remained more flammable than core-only composites. Another study by Huang et al. introduced talc at weight percentages of 5%, 15%, 25%, 35%, and 50% into the HDPE shell layer of Co-WPCs, noting enhanced flame resistance at higher talc loadings (35% and 50%). Only the 50% talc-filled Co-WPCs exhibited comparable flammability to core-only WPCs loaded with 55% wood flour (Table 3).

Fire retardants	Percentage; (wt%)	HRR; (kW/m^2)	PHRR; (kW/m^2)	IT; (s)	THR; (MJ/m^2)	TSR; (m^2/m^2)	ASEA; (m^2/kg)	AEHC; (MJ/kg)	AMLR; (g/s)	References
BF	0–30	↓^a	↓		↓		↓	↓	↓	[2]
Talc	0–5, 15–50	↑^b, ↓	↑, ↓		↑, ↓	↑, ↓		↑		[49]
Graphite	10		↓	↑	—			↑	↓	[8]
TiO2	10		↓	↑	↓			↑	↓	[8]
Melamine/APP	10/10; 20/20	↓; ↓		-; ↑	-; ↓	↑; ↑	↑; ↑		↑; ↑	[52]
Melamine/CNTs	10/2	↓		↓	↓	↓	↓		↑	[52]
Melamine/Natural graphite	10/10; 10/20; 20/20	↓; ↓; ↓		↑; ↑; ↑	-; ↓; ↓	↓; ↓; ↓	↓; ↓; ↓		↑; ↑; ↑	[52]
Melamine/Expandable graphite	10/5	—		↓	—	↓	↓		↑	[52]
ATH	10	↓	↓	↑	↓	↓	↑	↑	↓	[8, 53]
Melamine	10	↓	↓	↑	↓	↓	↓	↑	↓	[8, 53]
ZB	10	↓	↓	—	↓	↑	↑	↓	↓	[8, 53]
APP	10	↓		↓	↓	↓	↑			[53]
Natural graphite	10	↓		↑	↓	↓	↑			[53]
APP/Natural graphite	5/5	↓		↑	↓	↓	↑			[53]
APP/Expandable graphite	5/5	↓		↑	↓	↓	↑			[53]

Table 3.

Flammability properties of Co-WPCs after adding flame retardants [27].

Turku et al. [8] conducted a study examining the impact of incorporating five fire retardants—aluminum trihydroxide (ATH), zinc borate (ZB), melamine, graphite, and titanium oxide (TiO₂)—into the shell of PP-based Co-WPCs on the flammability characteristics of the composite materials. The addition of fire retardants to Co-WPCs led to enhancements in peak heat release rate (PHRR), ignition time (IT), and average mass loss rate (AMLR) compared to unfilled Co-WPCs. Among the formulations tested, Co-WPCs with melamine exhibited the lowest PHRR, while the inclusion of graphite resulted in the most significant decrease in IT (indicating the longest ignition time). Building upon these findings, Nikolaeva and Kärki [52] introduced ammonium polyphosphate (APP), natural graphite, expandable graphite, and carbon nanotubes (CNTs) individually into the shell of melamine-filled Co-WPCs. Notably, Co-WPCs containing 20 wt.% melamine and 20 wt.% natural graphite demonstrated the most substantial improvement in flammability performance. The incorporation of 20 wt.% melamine and 20 wt.% APP also contributed to enhancements in PHRR, IT, total heat release (THR), and mass loss rate (MLR) of the Co-WPCs, although it notably increased smoke production and CO emissions. Interestingly, the APP-filled Co-WPCs exhibited a lower PHRR compared to melamine-filled counterparts, yet displaying the shortest ignition time (IT) [53]. To address this issue and enhance the ignition time of the APP-filled Co-WPCs, natural graphite was incorporated into the formulation, as standalone natural graphite exhibited the longest IT. The combination of APP and natural graphite effectively improved the flame-retardant properties of the Co-WPCs, synergistically enhancing flame resistance. Although natural graphite did not independently enhance flame resistance in the Co-WPCs, it notably affected the ignition time in isolation. These results underscore the synergistic flame resistance benefits achieved by incorporating a blend of different fire retardants. Notably, the studies mentioned did not compare the flammability performance of the Co-WPCs with core-only WPCs.

4. Applications of Co-WPCs

After years of development, WPCs in Europe have reached a mature stage in the market, with outdoor flooring and automotive interiors accounting for about 90% of the total market share. While outdoor wall panels, fences, furniture, and other aspects hold a relatively small market share, they have shown strong growth momentum. In China, WPCs mainly include doors and windows, interior wall panels, and outdoor pallets. WPC product series are widely used in the construction industry, with many traditional building facilities made of wood and steel gradually being replaced by WPCs, such as decking, railings, fences, window frames, and exterior wall cladding. However, when used outdoors, issues such as moisture absorption, aging and fading, cracking and deformation, insect infestation, and mold growth caused by external factors such as water and sunlight remain important factors limiting the outdoor service life of ordinary WPC products. The use of a functionally co-extruded shell layer can effectively address and prevent the aforementioned problems.

4.1 Formworks

Formwork is a temporary structure used in concrete construction, serving as a critical tool for the formation of concrete structures. It consists primarily of panels, supporting structures, and connectors. The panels act as load-bearing plates directly in contact with newly poured concrete; the supporting structure serves as a temporary framework supporting the panels, concrete, and construction loads; and the connectors, as accessories, join the panels and supporting structure into a cohesive unit. Formwork must be constructed according to design specifications to ensure that concrete structures and components are formed in the prescribed positions and geometric dimensions, maintaining their correct alignment, and capable of withstanding their own weight as well as external loads acting upon them.

The commonly used materials for formwork in construction are plastic, plywood, and steel. However, plastic formwork has low rigidity and a high coefficient of thermal expansion; steel formwork is heavy, difficult to dismantle, prone to rust, and has high usage costs; plywood has a low number of uses and leads to significant wood wastage. In comparison to these materials, wood-plastic composite materials used for formwork offer the following advantages: high strength, good abrasion resistance, and physical and mechanical properties such as compression and impact resistance comparable to hardwood; high dimensional stability and good surface quality that remains flat even after multiple uses; low surface polarity, preventing concrete adhesion and facilitating easy form removal; resistance to aging and corrosion, insect-proof, impermeable to water, mud, and oil during casting, thus improving concrete quality; made from recycled waste plastic and agricultural and forestry residues, environmentally friendly, and cost-effective; favorable social and economic characteristics, aligning with sustainable development strategies and effectively alleviating the scarcity of resources such as steel and wood; excellent processing properties, allowing for multiple sawing, planning, and bonding processes, convenient fixing, and easy maintenance; lower density compared to steel, facilitating transportation and handling. As a result, the use of wood-plastic composite materials for formwork has gained increasing attention in recent years.

4.2 Architectural decoration field

4.2.1 Wallboard

In architectural decoration materials, wall panels include interior wall protective panels and exterior wall decorative panels that possess excellent noise reduction and thermal insulation characteristics. They not only effectively maintain the surface of the wall but also serve as decorative elements. Compared to wall panels made of other materials, co-extruded wood-plastic wall panels offer advantages, such as anti-slip properties, durability, crack resistance, waterproofing, termite resistance, corrosion resistance, non-polluting, insulating, heat-insulating, flame-retardant, and easy maintenance. In recent years, co-extruded wood-plastic wall panels have become increasingly popular and are widely used in daily life. For instance, co-extruded wood-plastic wall panels can be used in modular kitchens, wall panels, bathroom cabinets, exterior wall claddings, prefabricated houses, signage, and display boards.

The decorative function of co-extruded wood-plastic wall panels can provide architectural and esthetic value, significantly enhancing the utilitarian value of building structures. Through coextrusion technology, wall panels with different colors, textures, and functionalities can be easily produced. Co-extruded wall panels can also be designed in various shapes, such as flat panels, long panels, circular panels, circular flat panels, and wave panels, meeting the demand for diverse decorative styles.

4.2.2 Ceiling

In recent years, with the rapid development of China’s economy and the improvement of people’s living standards, interior decoration of residences and public buildings has become a burgeoning industry. To enhance esthetics, suspended ceiling decoration for interior ceilings and related corridor ceilings during renovations is becoming increasingly common. Traditionally, the materials used to make ceiling panels include wood, gypsum board, plastic board, aluminum alloy board, among others. Each of these ceiling panels has significant drawbacks: wooden ceiling panels are prone to cracking and deformation over long-term use, and their production consumes a considerable amount of wood, posing challenges to ecological preservation; plastic ceiling panels are expensive, prone to degradation, discoloration, deformation, and have low strength and a short lifespan; aluminum alloy ceiling panels offer high strength and modulus, resist deformation during use, but are heavy, challenging to install, and lack esthetic appeal; gypsum ceiling panels have weak strength, low impact resistance, and being made of thermosetting gypsum, do not degrade, posing serious environmental pollution concerns when disposed. In recent years, wood-plastic composite materials made mainly from discarded plastics and agricultural and forestry waste have gained attention due to their low density, high strength, recyclability, and ease of processing, among other advantages. Co-extruded wood-plastic materials with superior comprehensive performance have also been widely applied in the ceiling field.

4.2.3 Floor

Co-extruded wood-plastic flooring not only retains the traditional advantages of mold resistance, insect resistance, and other physical properties of wood-plastic materials but also possesses wear resistance, scratch resistance, stain resistance, and weather resistance. Experimental data have shown that the wear resistance and scratch resistance of co-extruded wood-plastic flooring are more than five times stronger than first-generation wood-plastic composite, effectively preventing damage from scratches caused by hard objects. Additionally, the robust surface layer of wood-plastic co-extruded flooring can resist the penetration of colored liquids and oily liquids, making the surface very easy to clean. This surface layer also enhances the resistance of wood-plastic flooring to sunlight, rain, snow, acid rain, and seawater, giving the flooring an exceptionally long lifespan. Co-extruded wood-plastic flooring primarily comes in solid core, round hole, and square hole structures.

4.2.4 Skirting line

Baseboards are installed at the junction of the floor and the wall, serving to provide visual balance and enhancing the overall esthetic appeal of the interior space. Utilizing their linear design, material, and color, baseboards can complement and harmonize with the indoor environment, adding a decorative touch. Baseboards help strengthen the connection between the wall and the floor, reducing wall deformation and preventing damage from external impacts. Additionally, baseboards are easy to clean, making it convenient to wipe away dirty water or spills during floor cleaning. In addition to their protective function for the wall, baseboards play a significant role in enhancing the overall visual appeal of home decor. They act as the boundary line of the floor, often attracting the eye naturally, with a typical thickness of 5–10 mm when extended from the wall during installation. Co-extruded wood-plastic composite materials, which can be easily optimized for hardness and waterproofing, are also used in the production of baseboards.

4.3 Landscape architecture

There are many forms of application for wood-plastic composite materials in paving design, such as garden pathways, courtyard paving, observation platforms, wooden boardwalks, and more. In comparison to standard wood-plastic composite materials, co-extruded wood-plastic flooring features an added protective layer, clear wood grain, and natural colors. Furthermore, the four-sided coextrusion fully protects the core layer of the board, making it more resistant to wear, scratches, and stains. In high-traffic areas, it effectively prevents scratches from foot traffic and damage from hard objects, enduring various harsh tests from nature. Additionally, co-extruded wood-plastic flooring offers users high practical value, esthetic enjoyment, and visual appeal.

In architecture, a wall refers to a vertical spatial partition structure used to enclose, divide, or protect a certain area. Nearly all significant building materials can be used for constructing walls: wood, stone, brick, concrete, metal, glass, and even vegetation. By utilizing wood-plastic composite materials in wall construction, the aging resistance of the walls can be greatly enhanced. Additionally, the repair methods for wood-plastic composite materials are relatively straightforward, which can significantly improve the practical performance of the walls. Therefore, applying wood-plastic composite materials in wall construction can enhance both the performance and esthetics of the walls. Co-extruded wood-plastic composite materials have advantages in weather resistance and visual appeal compared to standard wood-plastic composite materials, making them highly suitable for use in walls.

Railings are a key component in landscape construction, primarily used for personal safety and the protection of equipment, facilities, and surroundings. Their applications include lake-side railings, riverbank railings, mountain path railings, and road guardrails. Railings have high safety requirements, and wood-plastic composite materials possess strong impact resistance, ensuring the strength and stability of the railings. Furthermore, wood-plastic composite materials are highly malleable, meeting the esthetic needs of railings. They can be designed with different patterns, shapes, and colors according to the requirements of landscape design. As society increasingly agrees on the importance of ecological environment and forest conservation, high-end raw wood will be replaced by high-quality weather-resistant outdoor composite materials. Co-extruded wood-plastic composite materials not only have excellent weather resistance but also provide a lasting tactile experience, meeting the higher standards and requirements of various landscape projects to create a harmonious, peaceful, clear, and serene ecological landscape.

Pavilions in landscape design are often designated for relaxation, with most pavilions being wooden structures. By utilizing wood-plastic composite materials with strong malleability, exquisitely designed small pavilions can be created, incorporating light wood tones paired with natural textures to connect seamlessly with long leisure boardwalks, achieving eco-friendly innovative design. Furthermore, wood-plastic composite materials can provide a sense of comfort and tranquility in terms of auditory perception. From a scientific perspective, wood-plastic composite materials can absorb some of the more harsh sound waves, creating a quiet and comfortable ambiance when people converse in spaces constructed using wood-plastic composite materials.

Pergolas are important urban landscapes that can both separate and connect two distinct spaces. In modern society, some exquisitely designed pergolas are made of wood structures, and by using wood-plastic composite materials, the natural wood texture can be replicated. Pergolas constructed from co-extruded wood-plastic composite materials do not degrade or peel after long-term exposure to rain, exhibiting strong stability while maintaining the texture and appearance of wood, making them popular among people. Therefore, we can see wood-plastic pergolas being utilized in many residential areas and parks. For instance, in a lattice wood structure pergola, sunlight can filter through the structure, creating a warm and inviting atmosphere for those seated within.

By optimizing material and structural design, co-extruded wood-plastic composite materials have greatly enhanced weather resistance, stability, and a rich wood texture compared to standard wood-plastic composite materials. In landscape design, a large number of street furniture items, such as seats, benches, trash cans, flower boxes, tree pots, and walkways, are made from co-extruded wood-plastic composite materials.

5. Conclusions

Co-extruded WPCs (Co-WPCs) with core-shell structures are versatile and can be tailored to meet various requirements for different application scenarios. Common non-fully wrapped structures are not recommended, unless cost savings outweigh the loss in properties. Non-fully wrapped Co-WPCs with a fixed groove design or a sandwich structure are acceptable. Co-WPCs with a solid wood core and specially structured Co-WPCs are suitable for high-performance applications. Applying a protective outer layer to WPCs can effectively reduce moisture absorption and degradation caused by weathering. The incorporation of fillers into the outer layer has been found to enhance the mechanical properties, durability, flame resistance, and dimensional stability of Co-WPCs. However, optimizing the performance of Co-WPCs involves balancing various factors. For instance, a fully hydrophobic plastic coating can reduce water uptake but may compromise tensile and flexural strength. While fillers can improve dimensional stability, they may also complicate the manufacturing process. The enhanced weathering resistance of Co-WPCs may result in a darker appearance due to the inclusion of carbon fillers. Although most reinforced Co-WPCs exhibit increased tensile and flexural strength, their impact resistance tends to decrease. Co-WPCs have been successfully applied in formwork, architectural decoration field such as wallboard, ceiling, floor, and skirting line, and landscape architectures including outdoor flooring, wall, guardrail, pavilion, porch frame, and landscape pieces. Future research should focus on tailoring Co-WPCs to specific applications, such as enhancing flame resistance for indoor use, strengthening mechanical properties for construction applications, and increasing outdoor durability. The development of multifunctional fillers capable of improving multiple performance aspects of Co-WPCs is a key area for advancement. Co-WPCs are widely applied in daily life and are considered the most promising biomass composite material with great application potential.

Acknowledgments

This work is supported by the following grants and programs: 1. Yunnan Agricultural Joint Research Key Project (202301BD070001-153); 2. Yunnan Provincial Applied and Basic Research Grants (202201AT070058); 3. National Natural Science Foundation of China (32060381); 4. The High Level Innovative One-Ten-Thousand Youth Talents of Yunnan Province (YNWR-QNBJ-2020-203); 5. “111” Project (D21027).

Conflict of interest

The authors declare no conflict of interest.