Reinforcement Effects on the Properties of Wood-Veneered Wood Fiber/Fabric/High-Density Polyethylene Laminated Composites

Abstract

This study explores the lamination performance of wood-plastic composite boards designed for indoor decoration, aiming to enhance adhesion between a wood fiber/high-density polyethylene (HDPE) composite board and poplar wood veneer by incorporating fabrics (canvas or polyester fibers) as an intermediate layer. Traditional adhesives, such as polyvinyl acetate (PVAc) and isocyanate, were utilized to create decorative panels with a multi-interface sandwich structure. The impacts of factors such as the hot-pressing temperature, wood fiber content in the substrate, and fabric type on the performance of the panels were systematically investigated. The results indicated that the hot-pressing temperature of the substrate had minimal effect on the lamination performance. Panels that used canvas fabric as the intermediate layer and bonded with a mixed adhesive of PVAc and isocyanate exhibited superior surface bonding strength (2.76 MPa), bending properties (strength = 49.21 MPa; modulus = 3.92 MPa), and tensile properties (strength = 31.62 MPa; modulus = 1.51 GPa). The enhanced performance was attributed to the covalent bonding formed by isocyanate with canvas fabric, polyester fibers, and wood veneer, whereas PVAc primarily established physical bonds through penetration.

1. Introduction

Indoor environments have become central to human social activities, including living, working, studying, entertaining, and socializing. Despite the comfort and personalization offered by modern indoor spaces, occupants are inevitably exposed to pollution originating from decorative materials and furniture [1,2]. Among materials that are widely used indoors, wood-based panels and epoxy resin adhesives are prevalent; however, these materials can release volatile harmful gases, posing potential risks to human health [3,4].

Wood-plastic composites (WPCs) are composite materials made from plant fibers (such as wood, bamboo, hemp, and straw) and thermoplastic polymers (including polyethylene and polypropylene (PP)), which are the primary polymers examined in this study. These materials are nontoxic, recyclable, and environmentally friendly [5]. WPCs have successfully replaced traditional man-made boards in various applications, such as outdoor public facilities, automotive interiors, and building exteriors [6,7,8,9]. However, the poor texture of WPC surfaces limits their application as indoor decorative materials. The use of wood veneer for surface decoration significantly enhances the application field and adds value to WPCs. WPCs exhibit dual material properties due to the combination of wood fibers (WFs) and polymers: the strong polarity of WFs contrasts with the nonpolarity of polyolefin materials. During the extrusion and hot-pressing processes, a layer of the plastic matrix gathers on most of the composite’s surface. This plastic layer remains intact even after cooling and solidification. The plastic surface is smooth and non-porous, preventing it from forming covalent bonds with adhesives typically used for traditional man-made boards. Moreover, the plastic layer on the composite material greatly hinders the adhesive’s ability to bond with the wood–plastic surface, making traditional adhesive-based laminating and facing methods infeasible [10,11].

Chemical methods are among the most direct and effective methods for modifying the surface of composites. For example, treating the composite surface with strong oxidizing acids results in coarse etching and the generation of polar groups, improving compatibility with polar adhesives [12,13]. Coupling agents, commonly employed for wood and nonpolar polymers, such as PE and PP, can also serve as reagents for the surface modification of WPC materials. These agents enhance adhesion by forming new chemical bonds between the adhesive and the WPC material. However, the treatment temperature and amount of coupling agent significantly affect the modification effect. At elevated temperatures, the coupling agent forms a rigid network, primarily comprising covalent bonds with the hydroxyl (-OH) groups on the surface of the WF. Nevertheless, high temperatures may lead to material deformation. Similarly, an excessive amount of coupling agent can lead to self-aggregation and cross-linking, whereas an insufficient amount may result in inadequate local surface modification [14,15,16,17].

Surface treatment of WPC materials using physical methods has proven effective. Plasma treatment, widely used for modifying polymer surfaces [18,19], is equally applicable to WPCs due to their polymeric composition [20]. In addition, techniques such as mechanical sanding, flame burning, and ultraviolet irradiation achieve different degrees of surface modification. The primary method of mechanical sand treatment of WPC surfaces is to increase their surface roughness and surface area. This facilitates the formation of a hook-and-anchor structure for the adhesive layer, thereby improving bonding strength. The flame method and ultraviolet irradiation also promote oxidation and activation of the surface of WPC materials, introducing new polar groups on the molecular chains. Additionally, ultraviolet treatment etches the wood component, further increasing surface roughness. Several surface treatment methods can thus improve the bonding strength of WPC materials.

To meet the various needs of WPCs, surface bonding methods must also exhibit versatility. When the surface characteristics of the bonding target material differ greatly from those of the WPC substrate, introducing other specific materials can increase the bonding strength. Fabric materials, characterized by their porous structure, demonstrate strong adhesive interactions with wood–plastic boards, providing favorable conditions for the penetration of adhesives. Previous studies shown that the melting of HDPE on the surface of the wood–plastic substrate into the gaps between the fabric fibers forms an interlocking structure, resulting in a robust bond between the fabric and the wood-plastic base material. When the penetration depth was optimized, the fabric surface retained sufficient fiber gaps, enabling the adhesive to bond effectively with the wood veneer. Building on this foundation, the present study investigates several fabric-faced board materials with high surface bonding strength as base materials. Commonly used milky glue and isocyanate adhesive for wood-based panels were employed to explore the bonding [21,22] mechanism and related properties of veneers. These findings provide more possibilities for indoor applications of WPC materials.

2. Materials and Methods

2.1. Materials

Wood veneer: Poplar wood veneer was prepared via rotary cutting, with a thickness ranging from 1.0 mm to 1.2 mm. The veneer was provided by YiMu Wood Industry, located in Linyi, Shandong Province, China. HDPE (No. 5000 S) with a density of 0.949−0.953 g/L, melt flow index of 0.8−1.1 g/10 min, and crystallinity of 71% was purchased from Sinopec (China petrochemical corporation), Daqing, China. Maleated polyethylene (MAPE) was used as a reinforcing agent for the combination of wood fibers and HDPE, which, with a graft percentage of 0.9%, was purchased from Shanghai Sunshine New Technology Development Co., Ltd., Shanghai, China. PVAc (Polyvinyl acetate adhesive), an emulsion copolymer with a solid content of 35%, was supplied by Harbin Minglang Glue Industry Co., Ltd., Harbin, China. Diphenylmethane diisocyanate (No. 44V20L), containing 30.5%–32.5% isocyanate, was obtained from JiaCheng Plastic Raw Materials Co., Ltd., Linyi, China. Fabrics used for veneering were sourced from a local wholesale clothing market, with their basic characteristics listed in

Table 1. Parameters of the textile fabrics used for veneering in this study.

Fabric	Thickness	Melting (Decomposition) Temperature	Chemical Composition	Appearance
Polyester fiber fabric	0.48 mm	Melting: 205 °C	HO-CH2-CH2-O [-OC-Ph-COOCH2CH2O-]n
Canvas	0.70 mm	Decomposition: 310 °C	Mainly cellulose

.

2.2. Preparation Parameters of the Fabric-Decorated WF/HDPE Substrate

Poplar wood was ground in the laboratory into wood fibers of 40–80 mesh size. The wood fibers were dried at 103 °C until their moisture content was reduced to 3.0%. The HDPE, WF, and MAPE were then blended in different mass fractions (WF/HDPE = 5/5, 6/4, 7/3) using a high-speed mixer (SHR-10A, Zhangjiagang Tonghe Plastic Machinery Co., Zhangjiagang, China). The resulting mixture was compounded in a corotating twin-screw extruder (JSH30, Nanjing Rubber & Plastic Machinery Plant Co., Ltd., Nanjing, China) and cut into small granules. These particles were processed in a single-screw extruder (SJ45, Nanjing Rubber and Plastic Machinery Plant Co., Ltd., Nanjing, China) to prepare WF/HDPE composite boards with a thickness of 4 mm and a width of 200 mm.

The fabric was spread over the surface of the WF/HDPE composite board, which was then placed in a thermocompressor (SL-6; Harbin Special Plastic Production Co., Harbin, China) for simultaneous heating and pressing (Figure 1, step 1). Initially, the board was hot-pre-pressed, followed by hot-pressing, with the parameters detailed in

Table 2. Preparation parameters of the fabric-decorated WF/HDPE substrate.

Fabric	WF/HDPE	Temperature (°C)
Canvas	5/5	140, 160
	6/4	140, 160
	7/3	160
Polyester fiber	5/5	140, 160
	6/4	140, 160

. After hot-pressing, the fabric-veneered WF/HDPE panel was cooled to ambient temperature under a pressure of 5 MPa. Owing to the insufficient HDPE, effective interlocking structures between the polyester fabric and WPC (WF/HDPE = 7/3) could not be formed. Therefore, WPC with a WF/HDPE ratio of 7/3 was excluded from subsequent experiments.

2.3. Preparation of Wood-Veneered WF/HDPE Composite Panels

The treated poplar wood veneers were positioned on the top and bottom surfaces of the WF/HDPE substrate, which was coated with fabric and adhesive. A five-layer sandwich structured board (shown in Figure 1, step 2) was hot-pressed in a thermocompressor at 80 °C and 5 MPa for 10 min. The thickness of the veneered composite panel was controlled using two 6 mm thick steel bars placed between the hot press boards. After hot-pressing, the veneered WF/HDPE board was cold-compressed to room temperature. A control group of untreated wood veneer and WF/HDPE board was prepared under identical conditions. The veneered WF/HDPE boards were then stored at 20 °C and 65% relative humidity for 24 h prior to testing.

2.4. Surface Bonding Strength Test

The surface bonding strength of the veneered WF/PP board was tested in accordance with the vertical drawing method, as shown in Figure 2. The test samples measured 50 mm × 50 mm × 6 mm (length × width × thickness). A circular area of 1000 mm² was isolated at the center of each sample by cutting through the veneer and bonding layer. This circular section of wood veneer was bonded to the upward-fixed head with polyurethane hot melt adhesive 3731 (Minnesota Mining and Manufacturing Corporation (3M), Shanghai, China). The remaining part of the sample was then secured by its edges. Each group comprised 12 samples, and the loading rate was 2 mm/s (second). The experiments were conducted using a universal mechanical testing machine (RGT-20A; Shenzhen Reger Instrument Co., Shenzhen, China).

2.5. Scanning Electron Microscopy

The bonding morphology between the fabric and adhesive was observed using scanning electron microscopy (SEM; JSM7500F, JEOL DATUM Shanghai Co., Ltd. Shanghai, China). Sample slices were prepared via microtomy to observe the permeability of the molten intermediate film into the poplar veneer. The samples were coated with gold and examined at an accelerating voltage of 5 kV.

2.6. Attenuated Total Reflection–Fourier Transform Infrared Spectroscopy

The surfaces of the wood veneer and fabric were stripped. The interface between the fabric and wood veneer was analyzed using infrared spectroscopy. The chemical characteristics of treated and untreated poplar veneers were assessed through attenuated total reflection–Fourier transform infrared spectroscopy (Nicolette 6700, Nicolette Company, Long Beach, CA, USA). Scans were recorded within the range of 4000−400 cm⁻¹ at a resolution of 4 cm⁻¹, with two spectra recorded for each sample.

2.7. Bending Strength Tests

The bending properties of the samples were tested with a universal mechanical testing machine (RGT-20A; Shenzhen Reger Instrument Co., Shenzhen, China) in accordance with ISO 16978:2003 [23]. The support span was set to 20 times the specimen thickness. The specimen width was 50 ± 1 mm, and its length was calculated as 20 times the thickness of plus 50 mm. The loading rate of the upper press head was maintained at 10 mm/min.

3. Results and Discussion

3.1. Surface Bonding Strength

The selected substrates were WPC boards with fabric veneers, which exhibited relatively strong adhesive strength. However, cracking or separation at the bonding interface between the fabric and the WPC board remained possible. Due to the presence of the fabric middle layer, failure during the surface bonding strength test may occur either at the interface between the wood veneer and the fabric or between the fabric and the WPC board. If the bonding strength between the fabric and the WPC board is weaker than that between the fabric and the wood veneer, the measured value primarily reflects the bonding strength of the fabric with the WPC board. Both types of interfacial failure are used to evaluate the surface bonding strength of the veneered board, serving as reference indicators for its veneering performance and product usability.

3.1.1. Surface Bonding Strength of Wood Veneered WF/HDPE Composite Board with PVAc

The surface bonding strength of wood veneers bonded with PVAc adhesive and fabric as the interlayer material was illustrated in Figure 3. The samples incorporating canvas interlayers demonstrated a bonding strength of approximately 2 MPa, while those with polyester fiber interlayers achieved around 1 MPa. Under identical temperature conditions and WPC ratios, the bonding strength between the canvas fabric and the wood veneer surpassed that of the polyester fibers. This enhanced bonding was due to the textured surface of the canvas fabric’s cotton fibers, which contain numerous -OH groups on their surface, resulting in higher polarity, thereby facilitating more favorable interactions with the polar white glue.

Figure 3 shows that the hot-pressing temperature of the substrate did not significantly influence the surface bonding strength of the secondary laminated veneers. This is attributed to the considerable thickness of the fabrics, which permits only partial penetration by the HDPE in the WPC, leaving ample space for effective adhesive permeation.

In the case of veneers with canvas interlayers, the bonding strength between the WF/HDPE composite board and the canvas fabric was notably greater than that between the canvas fabric and the wood veneer. As a result, failure primarily occurred in the veneered panel (Figure 4a). When the WF/HDPE substrate ratio is adjusted to 7/3, a significant reduction in the surface bonding strength of the laminated board was observed. This decrease was attributed to the higher content of WFs in the substrate, which weakened the bond between the canvas fabric and the WPC. When the adhesive’s bonding strength exceeds the adhesion between the canvas and the wood–plastic substrate, the fabric may detach from the composite base. Furthermore, the bonding strength between white glue and polyester fibers was relatively low, with failures predominantly occurring at the interface between the adhesive layer and the polyester fibers (Figure 4c).

3.1.2. Surface Bonding Strength of Wood Veneered WF/HDPE Composite Board with PVAc-Isocyanate

The bonding of fabric layers to wooden veneer surfaces was achieved via a compound adhesive consisting of PVAc and isocyanate, with the surface bonding strength depicted in Figure 5. The incorporation of isocyanate clearly increased the adhesive strength between the canvas and the wooden veneer. At a hot-pressing temperature of 140 °C, the content of WFs had no significant influence on the bonding strength of the veneered panels, consistent with the behavior observed for PVAc. However, at 160 °C, a notable decrease in bonding strength was observed in panels with a relatively high WF content (WF/HDPE = 7/3). This reduction in bonding strength was attributed to weaker interfacial adhesion between the canvas fabric and the WPC at elevated WF concentrations. When the bonding strength between the fabric and the wood veneer surpassed that between the fabric and the WPC, separation of the fabric from the wood–plastic substrate can occur.

A comparison of the data in Figure 5a,b revealed that the surface bonding strength of the polyester fiber-laminated composite board with the substrate hot-pressed at 160 °C was greater than that of the samples with the substrate hot-pressed at 140 °C. This enhancement was ascribed to the enhanced fluidity of HDPE at 160 °C, which allowed for greater penetration into the polyester fiber fabric, forming more robust bonds. Additionally, the bonding strength between the PVAcisocyanate adhesive and the polyester fibers was higher at this temperature, contributing to the overall increase in bonding strength. In contrast, at a substrate hot-pressing temperature of 140 °C, less HDPE penetrates the fabric, leading to a weaker bond between the fabric and the wood–plastic board. During the bonding strength testing, the interface between the fabric and the wood-plastic board tended to fail before the interface with the wooden board, resulting in lower bonding strength.

The failure observed in the adhesive layer, as depicted in Figure 4, revealed that the specimens bonded with the PVAc adhesive primarily exhibited failure within the adhesive layer itself. The wooden veneer did not readily remain on the fabric surface, and the failure edge was clean and well-defined. In contrast, the samples bonded with the PVAc-isocyanate adhesive contained a significant amount of wooden veneer fibers remaining on the fabric, indicating that failure predominantly occurred within the wooden veneer. This suggests a strong bond between the fabric and the veneer, with the adhesive layer effectively holding the materials together.

3.2. Bonding Mechanism of the Fabric and Adhesive

Upon peeling the wooden veneer from the fabric surface, SEM analysis of the exposed fabric revealed that when PVAc (Figure 6a,c) was used as the adhesive, the bond between the adhesive and the fibers was relatively weak. The adhesive formed a film-like structure that intertwined with the fabric fibers. This behavior was ascribed to primary component of PVAc, a copolymer emulsion. The bonding mechanism of this adhesive relies on the evaporation of moisture from the emulsion during hot-pressing, leading to the formation of a cross-linked structure among the adhesive particles. This resulted in glue films that facilitate adhesion between the bonded materials. Consequently, when the surface wooden veneer was removed under external force, some flaky adhesive film remained on the fabric surface. Although the fabric fibers were entangled with the flaky adhesive film, their boundaries remain distinct, indicating that no chemical reactions have occurred. Thus, PVAc primarily utilized a penetration-curing mechanism to bond the fabric to the wooden veneer [24,25].

Figure 6b,d display the adhesive interface of specimens bonded with a PVAc-isocyanate composite adhesive. Notably, a significant amount of residual adhesive remained on the fabric fibers, yet the adhesive morphology differs markedly from that of pure PVAc. Upon the addition of isocyanate, the composite adhesive bonded closely with the fabric fibers, almost entirely enveloping their surfaces. This was due to the high content of -OH groups on the cotton fibers, which can react with the -NCO groups of isocyanates to form covalent bonds, as depicted in Figure 7. While the polyester fibers contain fewer -OH groups, they can still react with isocyanate, as shown in Figure 8. Therefore, the incorporation of a PVAc-isocyanate composite adhesive enabled strong adhesion to the fabric fiber surfaces [26,27].

As depicted in Figure 9, the -OH absorption peaks of the canvas laminate samples treated with PVAc-isocyanate adhesive, typically observed in the 3100–3500 cm⁻¹ range, were significantly reduced. Additionally, the characteristic alcohol -OH peak of cellulose at 1025 cm⁻¹ was also notably decreased [22]. A strong new carbonyl (C=O) stretching vibration peak emerged at 1731 cm⁻¹, which was attributed to the reaction between -NCO groups in isocyanates and -OH groups, resulting in the formation of carboxyl groups. Moreover, a minor characteristic peak corresponding to the residual isocyanate N=C=O appeared at 2276 cm⁻¹ [28].

The bonding strength between the polyester fibers and PVAc-isocyanate adhesive was relatively low because of the limited presence of -OH groups at the ends of the polyester molecular chains (Figure 8). These -OH groups can react with isocyanates to form covalent bonds. In this study, the polyester fabric we used has a low content of -OH groups, as evidenced by the lack of significant characteristic peaks in the infrared spectrum in the 3000–3500 cm⁻¹ range (Figure 10). Additionally, the characteristic peak at 1024 cm⁻¹ exhibited a relatively small area. After bonding, peaks in the region 3000–3500 cm⁻¹ were observed, corresponding to the stretching vibrations of N-H groups in the isocyanate, while the peak at 2278 cm⁻¹ was associated with the stretching vibrations of N=C=O groups. These findings indicated that the isocyanate had not fully reacted with the polyester fibers, leaving residual isocyanate on the fabric surface. As the bonding mechanism of PVAc did not involve chemical reactions with the fabric surface to form covalent bonds, infrared spectroscopy was not a suitable method for characterizing adhesion at the interface.

3.3. Bonding Mechanism Between Adhesive and Wood Veneer

PVAc is widely utilized adhesive in wood processing and bonding. It adheres to wood primarily through the formation of an anchorage structure resulting from its penetration into the wood. The bonding mechanism between wood and PVAc has been extensively studied, as noted in the literature, and thus will not be reiterated here [21,29]. Additionally, isocyanates react with -OH groups on the wood surface to form covalent bonds, as depicted in Figure 11 [22,30].

Building upon the research in Section 3.2, which examined the bonding mechanism of fabric and adhesive, the interaction between wood veneer and the fabric can be summarized as follows (Figure 12). PVAc adheres the fabric to the wood veneer through its penetrating action. PVAc-isocyanate forms covalent bonds with -OH groups present on both the wood veneer and fabric fibers, facilitating adhesion. This mechanism also possesses a certain degree of penetrating effect.

3.4. Mechanical Properties of Decorated Boards

3.4.1. Bending Properties of the Decorated WF/HDPE Board

The bending strength and modulus of the substrate were substantially enhanced after bonding with a solid wood veneer, as shown in Figure 13 and Figure 14. This improvement was attributed to the superior bending performance of the solid wood veneer, which, when applied to both the upper and lower surfaces of the fabric sandwich panel, reinforced the panel’s overall enhancement of the bending properties. Additionally, the adhesive type had a minor effect on the bending strength of the panel, with stronger veneer adhesion resulting in a more pronounced strengthening effect. However, its impact on the bending modulus is negligible. This is because of the linear phase of bending modulus calculation, during which interlaminar shear forces do not compromise the adhesive layer. As a result, the solid wood veneer and substrate act as a single entity, maintaining consistent rigidity of the material.

Figure 15 and Figure 16 demonstrate that the bending strength and modulus of panels with polyester fabric sandwich facings were significantly enhanced compared to WPC panels with only polyester fiber facings. This enhancement was ascribed to the reinforcing effect of the solid wood veneer on the overall bending strength of the panels. Meanwhile, panels bonded with PVAc-isocyanate adhesives exhibit greater bending strength, whereas the bending modulus remained largely consistent. The analysis of Figure 13 and Figure 14 highlights that the primary factors influencing the bending strength of faced panels include the adhesive bond strength between the facing veneer and the substrate, the WF content of the substrate, and the pressing temperature. In contrast, the type of fabric used in the sandwich construction had a minimal impact on the bending strength. The bending modulus of faced panels was predominantly affected by the WF content of the substrate and the pressing temperature of the substrate.

The bending failure conditions of the faced panels were depicted in Figure 17. Panels bonded with PVAc adhesive are shown in Figure 17a,c, while those bonded with PVAc-isocyanate adhesive are displayed in Figure 17b,d. Notable morphological differences in bending failure were evident between the two adhesive types. For panels bonded with PVAc, veneer failure extended beyond the substrate failure area. In contrast, for panels bonded with PVAc–isocyanate, the veneer failure pattern aligned closely with that of the WPC substrate. During the yield failure stage of the faced panels, the increased interlaminar shear forces within the adhesive layer induced localized debonding at the interface between the facing veneer and the substrate. This resulted in asynchronous failure of the facing veneer and the WPC substrate, leading to variations in bending strength. Figure 18 presents the stress-displacement curves from the bending tests. When PVAc was used as the adhesive, the stress curve initially demonstrated a yielding stage followed by a continued increase, corresponding to partial debonding in the adhesive layer between the wooden veneer and the substrate. The linear phase preceding this behavior was used to calculate the elastic modulus.

3.4.2. Tensile Properties of Decorated WF/HDPE

Figure 19 and Figure 20 illustrate a significant improvement in the tensile properties of WPC panels with canvas sandwich layers after surfacing with wooden veneers. This enhancement was attributed to the superior tensile performance of the wooden veneers, which reinforced the overall panel when applied to both sides of the substrate. Samples bonded with PVAc-isocyanate adhesive exhibited even greater tensile performance, ascribed to the higher bonding strength of the adhesive. This stronger bond created a more integrated material structure, amplifying the reinforcement effect on the wooden veneer under tensile stress. Meanwhile, panels with polyester fiber sandwich layers displayed a similar pattern, as shown in Figure 21 and Figure 22, where the fabric type had minimal influence on tensile performance. However, the pressing temperature and WF content of the substrate significantly affected the tensile properties of the surfaced panels. Higher pressing temperatures resulted in lower substrate density and fiber and molecular chain orientation, leading to reduced tensile performance of the surfaced panels. Conversely, a higher WF content in the substrate correlated with improved tensile performance of the surfaced panels.

4. Conclusions

This study investigated the use of natural and synthetic fiber fabrics as intermediate layers to enhance the decorative lamination of WF/HDPE composites using traditional wood adhesives. This approach aimed to expand the application of WPCs in interior decoration and furniture. Experimental results indicated that using PVAc-isocyanate as the adhesive for laminating fabric-covered wood–plastic panels to wood veneers yields superior bonding strength (2.76 MPa) and physicomechanical properties. The specific conclusions were as follows:

a. Panels laminated with PVAc-isocyanate adhesive exhibited superior surface bonding strength (2.76 MPa). In contrast, those bonded with PVAc alone showed lower surface bonding strength (1.93 MPa).

b. The bonding mechanism of PVAc with wood veneer and fabric layers involved initial penetration into the pores and interstices of the fabric fibers, followed by curing to form an adhesive film. Similarly, the bonding mechanism of PVAc-isocyanate with wood veneers and fabric layers involved penetration into the surfaces of both materials. Additionally, the isocyanate component reacted with -OH groups on the surfaces of wood veneers and fabric fibers, forming covalent bonds that enhanced the bonding strength.

c. After secondary lamination with real wood veneer on fabric-covered WF/PP boards, the physicomechanical properties of the panels significantly improved. Panels laminated with PVAc-isocyanate adhesive demonstrated greater bending properties (strength = 49.21 MPa; modulus = 3.92 MPa) and tensile properties (strength = 31.62 MPa; modulus = 1.51 GPa), compared to those laminated with PVAc alone.