Challenges for Wood–Plastic Composites: Increasing Wood Content and Internal Compatibility ^†

Abstract

Wood–plastic composites (WPCs) are interesting materials as the biobased content is determined by the inclusion of wood particles regenerated from residual wood sources or biomass products. At present, the aim is to increase the wood content in WPCs above 60%, while it is currently limited to around 40%. The rationale behind this is based on the need for an increase in the performance of WPCs, the relatively cheap price of wood and the aim to augment the biobased content. Most studies are presently carried out with a maximum of 50% wood particles (preferably ranging from around 30 to 40%), while there are only very few sources where the wood concentration is increased to 70%. The formulations are not yet optimized and there are problems in interface compatibility, leading to weak mechanical properties. Problems in the augmentation of the wood content have to be further controlled, e.g., aggregation, dimensional stability and water absorption. Alternative approaches for the treatment of wood chips before (or during) compounding with the polymer matrix should therefore be developed. As the water resistance is mainly related to the control of the surface properties of the hydroscopic wood particles, possible solutions should consider the better protection of the individual wood particles’ surfaces against water ingress, the better development of the wood–polymer interface and the prevention of the formation of a continuous network with contacting wood particles. Therefore, this overview suggests various processing routes together with their industrial potential based on various sources from the literature, including the effects of compatibilizers and additives, the spray coating of wood particles, chemical pretreatment, physical modifications and the thermal treatment of wood fillers.

1. Introduction

Wood–plastic composites (WPCs) are emerging as innovative materials that combine the best properties of wood and plastic by blending wood fibers or wood flour with thermoplastic resin such as polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC). The resulting material exhibits characteristics of both wood and plastic, offering high durability and flexible processability. Their ability to be molded into various shapes and sizes makes them suitable for a versatile range of applications, including decking, fencing, automotive parts and furniture. As the demand for sustainable materials is continuously growing, WPCs are expected to play a significant role in the future.

One of the most significant advantages of WPCs compared to traditional wood and plastic products is their environmental sustainability, related to processing and use [1]. By utilizing recycled sources for wood and plastic, WPCs help to reduce the demand for virgin wood materials, minimizing waste and promoting resource recovery. The other fiber materials derived from residual biomass may also serve as potential candidate fillers (e.g., seasonal crops, plants). As such, the design of WPCs reduces the environmental impact associated with the production and disposal of traditional wood and plastic products that would otherwise end up in landfills. Within the larger scope of a circular economy, WPCs contribute to the conservation of natural resources and reduced deforestation as they provide a direct replacement for traditional wood products. Moreover, WPCs are highly durable and resistant to rot, decay and insect damage, which extends their lifespans and reduces the need for frequent replacement. Enhanced durability contributes to better sustainability and lowers the overall environmental impact by reducing the consumption of materials over time. In parallel, the carbon footprint of WPCs is lower than that of pure plastic or wood products due to the selection of incorporated components and lower processing energy [2], while their production and lifecycles involve low greenhouse gas emissions and the low release of volatile organic compounds after multiple instances of recycling [3].

Within the context of further enhancing the sustainability of WPCs, the biobased content should be further augmented by increasing the amount of wood (or alternative biomass) filler. The increased biocontent in WPCs will enable further reductions in the carbon footprint, waste reduction and lower energy consumption and contribute to more sustainable resource use and biodegradability. However, associated technical challenges in processing and performance will occur and should be addressed. Therefore, this study summarizes some of the occurring problems and the remediation measures to be considered to enhance the performance of WPCs with high wood filler content.

2. Technical Challenges

2.1. Limitations in Biobased Filler Content

The motive behind maximizing the content of wood fillers in WPCs is based on the relatively cheap price of recycled wood and the augmentation of the biobased content, in parallel with an increase in the performance of the WPC. It is crucial to maintain a good balance to ensure that the product retains its desired mechanical properties and processability (see Section 2.2). The maximum content of wood fillers in WPCs can vary depending on the specific application and desired properties. Generally, the wood filler content can range from 10% to 70% (w/w). Higher wood filler content, typically above 50%, is often more desirable to maximize the benefits of wood fillers [4]. However, there is little literature available where the wood concentration in WPCs is increased above 60%: most studies are conducted with a maximum of 50% wood particles (preferably ranging from 30 to 40%) [5] and the use of appropriate coupling agents for optimized mechanical properties. Only a few compositions of WPCs can be found where the concentration of wood fillers was increased to 70% [6], but the formulation was not further optimized and problems in interface compatibility and water sorption leading to weak mechanical properties were reported.

2.2. Performance with High Wood Filler Content

The increase in wood filler content poses important implications and limitations for the current technologies, affecting the WPC’s properties and performance as follows.

• Mechanical Properties: It is noticed that the high wood filler content can enhance certain properties like the stiffness, tensile strength and thermal stability, but it may reduce other properties, such as impact resistance, flexibility and toughness [7].
• Moisture Sensitivity: The moisture absorption of hydrophilic wood fillers can lead to local swelling, warping and reduced dimensional stability. Particularly at high filler content, the formation of a continuous network with contacting wood particles enhances the pathways for the diffusion of water and moisture throughout the composite [8].
• Processing: The viscosity of the melt increases at high wood filler content, making processing techniques like extrusion and injection molding more challenging [9].
• Thermal Stability: As wood fillers are thermally less stable and can degrade at temperatures of around 220 °C, high processing temperatures should be avoided, and the types of plastics that can be used under these processing conditions become limited.
• Interface Adhesion: The poor adhesion between the hydrophilic wood fibers and the hydrophobic polymer matrix may lead to weak interfaces and reduced strength [10].

3. Remediation Strategies

The various strategies to improve the performance of WPCs with high wood filler content in relation to their technical challenges are summarized in Figure 1, and the remediation routes are further discussed below. In particular, novel ideas for the treatment of the wood chips before (or during) compounding with the polymer matrix should be implemented. As water resistance is mainly related to control of the surface properties of the hygroscopic wood particles, possible solutions should consider the better protection of the individual wood particles against water ingress, the better development of the wood/polymer interface and the prevention of the formation of a continuous network with contacting wood particles. Therefore, it is proposed that several methods for surface treatment at the level of individual wood particles should be primarily considered, while the more prominent application of an external coating or co-extrusion layer is not considered in this study. Although these technologies are not yet common industrial practice for WPC processing, they may be derived from cross-over domains (e.g., natural composites and papermaking).

3.1. Compounding with Compatibilizers and Additives

The most straightforward route includes the addition of compatibilizers and additives, where polymers or inorganic materials are mixed into the compound to act as a coupling agent or lubricant. Many research works are focused on the improvement of the matrix–lignocellulosic filler interactions to produce highly filled composites with satisfying performance properties. A non-limitative overview of possible compatibilizers discussed in the literature is provided in Table 1. Blending with copolymers and/or grafted copolymers (e.g., maleic anhydride–poly(vinyl chloride ((PVC-g-MA), maleic anhydride-modified polyethylene (PE-g-MA) or polyethylene-co-glycidyl methacrylate (PE-co-GMA)) has resulted in better miscibility and compatibilizing effects. Alternatively, the addition of small concentrations of other copolymers in the blend (e.g., PHA, PLA, PP-MA) has resulted in improved compatibility. Poly[methylene-(polyphenyl isocyanate)] (PMPPIC), aminopropyl-triethoxysilane, maleated polypropylene (MAPP) and copper metallic complexes have proven to be effective coupling agents [11]. Recently, polyurethanes and isocyanates were used as a promising approach due to their versatile structures and functionalities [12]. Compatibility with a polyurethane interlayer between PVC and wood was formed in situ, where the inner layer consisted of urethane linkages, which increased the compatibility between the wood filler and polyurethane, and the outer layer was a soft segment that was compatible with the matrix as it acted a plasticizer for PVC [13].

Besides traditional methods (e.g., silanes, maleic anhydrides), novel types of biobased additives were also recently explored. A novel green strategy for the use of decayed wood in WPCs with improved mechanical properties and physical or chemical bonding properties utilized chitosan as an additive, where WPCs with recycled wood content were prepared with nano-calcium carbonate, activated carbon and 4% (wt/wt) chitosan, which no further need for additives [14]. Moreover, the recovered pyrolysis liquid from hardwood enhanced the compatibility when added in concentrations of 1 to 8% (wt/wt), while reducing the water absorption of WPC samples substantially by up to 25% [15]. The use of nanopowders (e.g., SiO₂, TiO₂, nanoclay, nanocalcium carbonate) has been investigated for the improved miscibility and compatibility of wood flour independently of the polymer matrix; these included HDPE, LDPE, PP and PVC, resulting in enhanced mechanical properties, UV resistance and flame retardance. However, the industrial use of nanopowders remains critical.

Table 1. Overview of common compatibilizers and additives in WPCs for improved particle compatibility and hydrophobic protection.

Polymer Matrix	Filler	Compatibilizer	Reference
PE	wood flour	polyethylene-co-glycidyl methacrylate	[16]
	wood chips	aliphatic copolyamide	[17]
PE, PP	wood flour	anhydride-modified ethylene copolymer, ethylene elastomer, anhydride-modified polyethylene and ethylene copolymer	[18]
PP	saw dust (Scots pine)	hardwood distillate (pyrolysis liquid)	[15]
PE, PP, PVC	wood veneer	methylenediphenyl-4,4′-diisocyanate	[19]
	wood flour Phragmites karka	exfoliated nanoclay (evt. + polyethylene-co-glycidyl methacrylate)	[20]
PVC	bamboo particles	fatty acid, glycerol ester, PE wax	[21]
	olive pit flour	chlorinated polyethylene	[22]
	wood flour	chlorinated polyethylene	[23]
	wood flour	poly-1,4-butylene glycol adipate diol, polyurethane prepolymer	[13]
	wood flour	terpolymers (P(MAA-BA-MMA)) of methyl acrylate, butyl acrylate and methyl methacrylate	[24]
	wood flour	chitosan (evt. + nano calcium carbonate (NCC) + activated carbon (AC))	[14,25]
	alfa fibers	maleic anhydride–poly(vinyl chloride)	[26]
	wood sawdust	silanes: chlorosilanes, aminopropyltrimethoxysilanes	[27]

Table 1. Overview of common compatibilizers and additives in WPCs for improved particle compatibility and hydrophobic protection.

Polymer Matrix	Filler	Compatibilizer	Reference
PE	wood flour	polyethylene-co-glycidyl methacrylate	[16]
	wood chips	aliphatic copolyamide	[17]
PE, PP	wood flour	anhydride-modified ethylene copolymer, ethylene elastomer, anhydride-modified polyethylene and ethylene copolymer	[18]
PP	saw dust (Scots pine)	hardwood distillate (pyrolysis liquid)	[15]
PE, PP, PVC	wood veneer	methylenediphenyl-4,4′-diisocyanate	[19]
	wood flour Phragmites karka	exfoliated nanoclay (evt. + polyethylene-co-glycidyl methacrylate)	[20]
PVC	bamboo particles	fatty acid, glycerol ester, PE wax	[21]
	olive pit flour	chlorinated polyethylene	[22]
	wood flour	chlorinated polyethylene	[23]
	wood flour	poly-1,4-butylene glycol adipate diol, polyurethane prepolymer	[13]
	wood flour	terpolymers (P(MAA-BA-MMA)) of methyl acrylate, butyl acrylate and methyl methacrylate	[24]
	wood flour	chitosan (evt. + nano calcium carbonate (NCC) + activated carbon (AC))	[14,25]
	alfa fibers	maleic anhydride–poly(vinyl chloride)	[26]
	wood sawdust	silanes: chlorosilanes, aminopropyltrimethoxysilanes	[27]

3.2. Particulate Coating

The coating of single wood particulate fillers through spraying with polymeric or natural compounds can offer a simpler alternative than chemical modification. After cleaning and drying to remove any moisture and impurities, the wood particles are sprayed with a coating material that can include either a polymer, coupling agent or other additives, enhancing the compatibility between the wood particles and polymer matrix. The spray coating mainly ensures that the coating material is evenly distributed on the wood particles, leading to a more consistent composite material.

The spray coating of hydrophobic substances can be performed with prepared emulsions, where, e.g., a small amount of biobased oil can provide sufficient hydrophobic surface properties in combination with the original roughness of the wood particle surface. These techniques were developed to coat spruce wood with surfactant-free emulsions based on tung oil, linseed oil or a linseed oil-based long oil alkyd resin [28], which were effective at concentration levels as low as 0.04 wt% oil content, roughly equivalent to 0.04 g/m², and led to static water contact angles reaching up to >130°. Stabilized emulsions or microemulsions of biobased wax were also developed [29], where carnauba wax showed better hydrophobization in comparison with traditional paraffin wax. The named dispersions can be sprayed on wood surfaces, with covalent bonding between the nanoparticles and cellulose fibers of the wood. Other biobased products, such as lignin grafted with methyl methacrylate (lignin–MMA), were sprayed from a 10 wt% solution in 1, 4-dioxane, where the total concentration of the grafting product on the wood flour was 1 to 5%, followed by drying. The resulting dispersion of treated wood flour in the polymer matrix was improved, together with improved mechanical properties [30].

Other hydrophobic agents, such as alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA) (see Figure 2), are well known to create hydrophobicity in paper coatings and can be similarly applied to coat single wood chips. After first spraying the wood particles with pure ASA (3% wt/wt) and curing them (130 °C), the water repellence of UF-bonded particleboards was improved [31]. Unlike conventional paraffin wax, the anhydride was covalently anchored to the wood surface by esterifying with the surface hydroxyl groups, while wettability studies confirmed that the surface polarity was increased after the anhydride treatment. Moreover, AKD (5% wt/wt) spraying provided enhanced compatibility between poplar wood fibers and the polymer matrix, and the better hydrophobicity and increase in mechanical properties demonstrate that AKD is an effective coupling agent for wood fiber/PP composites [32].

3.3. Chemical Pretreatment

Chemical pretreatment methods rely on separate chemical modifications through the grafting of wood particulates and/or the modification of the wood composition through a chemical processing step. The reactivity of wood particulates, mainly near the cellulose functional groups, allows for direct surface modification through polymer grafting. The covalent (permanent) attachment of functional polymer groups, e.g., through reaction with aminosilane, melamine and acetic anhydride [33], will modify the water sensitivity and can also affect the polymerization of the matrix. In particular, the amino-alkyl functional oligomeric siloxane and melamine, as well as acetylated wood flour composites, showed decreased equilibrium moisture content and a reduced speed of water absorption, while the reactivity of grafted precursors means that they can further bind with the matrix during chemical reactions. Alternatively, biobased fatty acid derivates are reactive and could be used in an esterification reaction with octanoïc acid and octanoïc anhydride on cellulose [34]; they were later used during high-pressure molding at elevated temperatures.

Interestingly, the chemical pretreatment of wood particles changes the composition of the wood in parallel with its hydrophilic/hydrophobic surface nature, as the different constituents of hemicellulose, cellulose and lignin can be balanced for improved wettability. The chemical washing steps of wood flour were initially performed with soap solutions and NaOH [35] or sodium sulfate [36]. The light delignification of wood demonstrated that that the oxygen-rich composition on the outer surface was reduced, and the coverage of lignin increased, resulting in better mechanical properties and hydrophobicity [37]. In parallel, the selective removal of lignin under alkaline treatment and the removal of hemicellulose under the acid treatment of lignocellulosic fillers improved the interfacial compatibility and mechanical strength of PVC-based WPCs, as the degree of crystallinity of the filler increased [38]. Moreover, selective hydrolysis and ammoxidation led to a decrease in the hemicellulose content in the lignocellulosic fillers. Besides this, hydrolysis and ammoxidation favor the formation of amide bonds in ammoxidized particles; these changes enhanced the contact angles, decreased the work of adhesion and decreased the surface free energy of WPC samples filled with the modified particles, in comparison with WPC samples that contained unmodified wood fillers [39].

3.4. Physical Pretreatment

The plasma treatment of wood particles requires a separate pretreatment step that involves the modification of the wood surface through combination with reactive compounds. The water repellence of wood surfaces can particularly be improved after atmospheric dielectric barrier discharge (DBD) treatment, where ethylene, methane, chlorotrifluoroethylene and hexafluoropropylene have been used as DBD reagents [40]. During plasma treatment, a polymerized film forms on the wood surface after the nucleation of plasma-derived initiators at the wood surface; the deposition of a low-surface-energy polymer film on the already rough surface of the wood is supposed to further stimulate the highly hydrophobic nature of surface-treated wood.

The physical conditioning of wood particulates also introduces grinding and sieving processes to select appropriate ranges of particle sizes and size distributions, as they both influence the organization of the fillers in the product after molding. The percolation threshold limit (i.e., a critical value for the random organization of the wood particles and the formation of a long-range connectivity network between the particles) highly depends on the particle aspect ratio, while it has also been demonstrated that the tensile and flexural properties, as well as the impact strength in general, increased with an increasing particle size and decreased with an increasing cross-sectional particle size [41].

3.5. Thermal Pretreatment

The thermal and hydrothermal treatment of wood chips has been explored in parallel with knowledge of the effects of heat treatment for regular wood products [42], where both the composition and surface morphology of the treated wood is altered. For thermally treated wood chips at temperatures between 120 and 180 °C under saturated steam in a digester, it was demonstrated that the water absorption decreased with the increasing severity of the thermal treatment, while significant influences on the water diffusion mechanism in the composites were modeled [43]. In particular, the compatibility of the treated wood chips with a polymer matrix was noted through a reduction in the number of holes and broken fiber ends [44]. Hydrothermally treated wood flakes in a hot water digester at 170 °C allowed the selective extraction of hemicellulose with different efficiencies depending on the sizes of the wood flakes: as the chip size was decreased, the amounts of extractives and hemicellulose decreased in the wood, while the amounts of cellulose and lignin increased, yielding consequent influences on the hygroscopic properties and interfacial bonding with the polymer matrix [45]. The hot water extraction of wood flour from pine chips indeed demonstrated that the hemicellulose was removed, and the performance of the formulated WPC with treated wood flour showed improved mechanical properties [46].

4. Industrial Feasibility and Patent Review

The industrial state-of-the-art and relevance of processing techniques for WPCs with high wood content and improved mechanical properties can be illustrated through a selection of relevant patents in the field, as shown in

Table 2. Overview of patent literature and industrial use of various compatibilizers in WPCs.

Patent	Assignee	Title	Compatibilizer	Reference
US 5,827,462 US 5,866,264 US 6,011,091 US 6,117,924	CPG International LLC	Balanced cooling of extruded synthetic wood material Renewable surface for extruded synthetic wood material Vinyl based cellulose reinforced composite Extrusion of synthetic wood material	lubricants (zinc stearate or calcium stearate)	[47] [48] [49] [50]
US 5,474,722	Eovations LLC	Oriented thermoplastic and particulate matter composite material	polyurethane	[51]
US 4,376,144	Flexsys America LP	Treated fibers and bonded composites of cellulose fibers in vinyl chloride polymer characterized by an isocyanate bonding agent	isocyanate	[52]
US 4,851,458	Rehau AG	Use of cellulose fibers for structurally modifying polyvinyl chloride articles	undisclosed structure	[53]
US 5,981,631	Wood Composite Technologies	Process for the production of composites of co-mingled thermoset resin bonded wood waste blended with thermoplastic polymers	fatty acid, rosin	[54]
US 3,245,867	Dow Chemical	Wood particle board and a method of making the same	magnesium chloride, bromide, sulfate	[55]
US 2007/0259995	Galata Chemicals LLC	Compatibilizers for composites of PVC and cellulosic materials	maleated styrene/acrylonitrile copolymer	[56]

. Compatibilization through the addition of lubricants and coupling agents in the compounding state is the most prominent, although it requires the adaptation of the compounding processes and infrastructure. Very few new types of sizing agents have been developed on the market, and they are mostly based on common technologies such as reactive compounding, zinc stearate lubricants, wax additives or amide lubricants.

5. Conclusions

There is very little literature available where the wood concentration in WPCs is increased above 60%. Based on the technological feasibility of enhancing the performance of WPCs with high wood filler content, possible recommendations for industrial implementation can be proposed depending on the complexity of the processes.

• The following techniques are the most straightforward: the spray coating of individual wood chips (natural oils, wax, ASA, AKD), the use of copolymer additives (PHA, maleated styrene/acrylonitrile copolymer, chlorinated PE) and the addition of selected compatibilizing agents (chitosan/chitin, acrylic-based terpolymers) in the compound.
• The thermal pretreatment of the wood seems efficient, but it requires collaboration over the processing chain and long-term development.
• The chemical pretreatment of the wood particles seems efficient, but it requires the extensive testing and optimization of the processes. The removal of hemicellulose fractions seems critical in improving the water resistance.