Rotary peeled veneers of Malabar neem (Melia dubia Cav.) (2,5 mm - 3 mm thickness) were used for this study. The species is one of the fast growing tree species extensively raised in several parts of India and prominently used for plywood manufacturing. The average moisture content of the veneers was in range of 9 % to 13 %.

Profile extruded 3 mm thick WPC sheets were used for the study. The sheets were provided by the Spectrus Sustainable Solutions Pvt. Ltd., Bengaluru, India having composition of polypropylene (55 %wt), bamboo flour (40 %wt) and maleic anhydride grafted PP coupling agent (orvac / P613, dupont make, 5 %wt). The mechanical properties of the WPC sheet material were determined in the laboratory and are given in Table 1. The sheets were used as a bonding material instead of traditional formaldehyde-based adhesives.

Table 1:
Properties of WPC sheet prepared from PP-bamboo particles. Values in the parenthesis denote standard deviation.

The hybrid composites were fabricated by sandwiching WPC sheets between wood veneers (Figure 1). The orientation of the core veneer was varied to make two different types of hybrid composite namely plywood and laminated veneer lumber (LVL). In plywood configuration, the grain direction of core veneer was oriented perpendicular to the top and bottom veneers. In LVL configuration, the grain direction of all the veneers was parallel to each other. In this study, composites with three ply configurations were prepared. The assembly of veneers and WPC sheets was placed in a hydraulic press, preheated at 155 ºC -160 ºC. The assembly was pressed at a specific pressure of 10 kg/cm² for 15 min. The pressed boards were allowed to cool down under pressure till the temperature reached to 65 ºC -70 ºC to avoid warping in the board due to differential cooling. Three boards of each plywood and LVL configuration were manufactured with dimensions 300 mm×300 mm×10mm. The prepared boards were conditioned at the 21 ºC - 25 ºC and 60 % -70 % relative humidity for 24 h. Thereafter, test specimens were extracted from the boards.

Figure 1:
Schematic representation showing assembly of Veneer-WPC based boards.

The panels were tested for various physical properties namely density, volume fraction ratio, adhesion of plies (knife test), water absorption (WA), thickness swelling (TS), and volumetric swelling (VS) were measured after exposure to water for 2 h and 24 h. Mechanical properties such as modulus of rupture (MOR), modulus of elasticity (MOE), compressive strength (CS), tensile strength (TSS), glue shear strength (GSS), internal bond strength (IB), were evaluated using universal testing machine (UTM) (Shimadzu-autograph-AX) with 50 kN capacity. All the tests were carried as per recommendations given in Indian standards BIS 1734 (1983) and BIS 14616 (1999) and for each test, five replicates were taken. Statistical analysis was carried out using SPSS statistical software (IBM 2019) and t-statistics was used to determine the statistically significant differences in properties of WPC bonded ply and LVL configuration panels. The fractured surfaces of specimens used for testing tensile strengths of LVL and plywood were examined using scanning electron microscope (SEM) to understand the penetration and surface interface between WPC and wood veneers of the hybrid composites. Additionally, heat deflection temperature (HDT) measurements were carried out to observe the temperature at which the sample of ply/LVL and WPC sheet deflects. The test was carried out with loading stress of 455 kPa, heating rate of 3 ºC/min. Test was conducted for two replicates of each sample with span length of 100 mm and deflection of 0,25 mm.

The physical properties of plywood and LVL are given in Table 2. The average value of moisture content of prepared plywood and LVL was observed to be 3,91 % and 4,08 %, respectively. Air dry density average value of the prepared hybrid composites was found to be 750 kg/m³ and 720 kg/m³ for plywood and LVL, respectively. Khali et al. (2017) prepared plywood using veneers taken from different progenies of the same species and conventional urea formaldehyde resin, pressed at 17,5 kg/cm². The density of the prepared plywood varied from 500 kg/m³ to 520 kg/m³. Similarly, Prakash et al. (2019) reported density of LVL prepared using Malabar neem (Melia dubia Cav.) and phenol formaldehyde, pressed at 16 kg/cm² to be 600 kg/m³. The hybrid ply as well as LVL prepared in this study exhibited higher density as compared to conventional composites using the same species. The density of the Malabar neem (Melia dubia Cav.) wood ranges between 390 kg/m³ to 460 kg/m³ (Kumar et al. 2018, Sharma et al. 2019). The higher density of hybrid composite is attributed to the high density of WPC sheet (≈ 1010 kg/m³) instead of a thin layer of formaldehyde-based resins. There was no statistically significant difference in M.C. % and density of hybrid plywood and LVL (P>0,05) which was on the expected lines.

Water absorption after 2 h and 24 h of soaking for hybrid WPC plywood was 7,34 % and 19,29 % respectively. Water absorption values for hybrid WPC-LVL composite after 2 h and 24 h of soaking were 7,89 % and 18,06 %, respectively. Water absorption was nearly the same in both types of composites irrespective of time of immersion. Lustosa et al. (2015) manufactured the LVL using HDPE and reported that the water absorption after 2 h and 24 h was ranged in between 17,78 % -19,77 % and 43,82 % -49,48 %, respectively, which was substantially higher as compared to hybrid WPC-LVL composite prepared in this study. Lower moisture absorption by the composites may be attributed to the effective encapsulation of core veneer by thick WPC layer and filling of pores/crevices present on wood veneer by wood fibres present in the WPC restricting free movement of water (Fang et al. 2014). WPC itself is reported to absorb negligible amount of moisture i.e. 3 % -4 % even on long-term repeated cycle of wetting and drying (Gunjal et al. 2020).The ability of WPC’s to absorb moisture drastically reduces as the wood fibres are entangled with polymers which are hydrophobic in nature. As a result, the total moisture uptake capacity of the hybrid ply and LVL composites prepared by incorporating WPC reduces significantly.

Table 2:
Physical properties of Plywood and LVL hybrid composites. Values in the parenthesis denote standard deviation.

Thickness swelling on water absorption is an important parameter for veneer based panels products. Hybrid plywood exhibited thickness swelling of 3,68 % and LVL exhibited 4,33 %. After 24 h of water immersion, thickness swelling was 5,09 % in plywood panel and 6,80 % in LVL panel. The thickness swelling in plywood configuration was significantly lower (P<0,05) than LVL panel after 2 h and 24 h of exposure to water though the water absorption was not significantly different. In principle, both water absorption and thickness swelling should have been similar in both types of composites. Slight difference in thickness swelling may be attributed to the natural variation in density of veneers used in fabricating the panels and it is expected that the prolonged exposure to water may lead to uniform swelling. Tenorio et al. (2011) reported uniform thickness swelling in plywood and LVL made from melina (Gmelina arborea Roxb.) wood.

Mechanical properties of plywood and LVL are given in Table 3. The average value of MOR in the case of plywood panel was 57,19 MPa and was in close agreement with the MOR of conventional plywood from the same species (Khali et al. 2017). However, in case of LVL, MOR average values of hybrid composite were much lower (63,79 MPa) than PF bonded LVL of the same species (106,8 MPa) as reported by Prakash et al. (2019) and was not differing significantly from MOR of plywood configuration (Table 3). The low MOR of LVL is attributed to the poor flexural strength of WPC (58,42 MPa) as compared to MOR of Malabar neem (Melia dubia Cav.) wood along the grain (89,44 MPa) (Chauhan and Sethy 2016). The thick layer of WPC is expected to contribute significantly to overall MOR of the composites as the strength of the composite laminate is expected to depend on the strength of the components of the composites, their relative volume fractions and orientation of each layer. The contribution of each layer of the laminate in the flexural strength of the composite is different with top and bottom layer influencing the most. The difference in plywood and LVL configuration panel was only the orientation of the core veneer which is on the neutral axis on bending influencing very little on overall strength of the composites. Therefore, the bending strength of both configurations were statistically similar despite of nearly six-fold difference in MOR of the wood along the grain (89,44 MPa) and across the grain (15 MPa). Del Menezzi et al. (2016) studied the mechanical properties of LVL bonded with expanded polystyrene (EPS) and reported that increase in amount of EPS had a negative effect on the MOR and MOE of the composites, also the increased amount of wood content resulted in improved flexural performance of EPS bonded LVL. Also, the amount of compression achieved during the fabrication of the veneer composites plays a crucial role in modelling the mechanical properties (Kurt and Cil 2012). In the current study, the pressure of 10 kg/cm² was used for only achieving uniform heat transfer through the material as well as improving surface bonding between polymer and the veneers and not for achieving higher compression of the assembly as the higher pressure may result in spilling of the melted WPC during the fabrication process.

Table 3:
Mechanical properties of Plywood and LVL hybrid composites.

Values in the parenthesis denote standard deviation.

MOE of the hybrid WPC- plywood was 6,89 GPa which was significantly lower than MOE of the hybrid WPC- LVL was 8,73 GPa. This is on expected lines as the orientation of core veneer is going to influence the overall MOE of the composites unlike flexural strength. MOE of Malabar neem (Melia dubia Cav.) along the grain is nearly 20 times higher than across the grain. However, MOE of LVL in this study was observed to be slightly lower that the MOE reported in conventional LVL from same species (Prakash et al. 2019). Low MOE of the hybrid LVL as compared to conventional composites is mainly attributed to the higher proportion of low modulus WPC present in the composites as the modulus of the composites depends on the modulus of the individual component of the composites and their relative proportions (Chauhan et al. 2005). The modulus of elasticity of WPC sheet was 3,98 GPa whereas MOE of Malabar neem (Melia dubia Cav.) veneer along the grain is 11 GPa and across the grain is 1,06 GPa. The MOE of the composites was theoretically estimated based on rule of mixture (Equation 1).

(1)

Where, MOE_C = MOE of composite (GPa), MOE_V = MOE of veneer (GPa), MOE_wpc = MOE of WPC (GPa), Vol_v = volume of veneer (mm³), Vol_wpc = volume of WPC (mm³), Vol_C = volume of composite (mm³).

The predicted MOE for plywood configuration was found to be 6,16 GPa and for LVL configuration it was 8,10 GPa which were in close agreement with the observed MOE of composite panels. Using WPC instead of pure polymer in binding veneers provides added advantages in terms of higher volume proportion of natural material in the overall composites formulations. The volume faction of polymer and wood can be estimated by the following (Equation 2) (Ashok 2015).

(2)

Where, wv = weight of veneer (g), wb = weight of bamboo (g), wp = weight of polymer (g), ρv = density of veneer (g/cm³), ρb = density of bamboo (g/cm³), ρb = density of polymer (g/cm³). The volume fraction of overall wood component (wood veneers and wood in WPC) was estimated to be 68,39 % when WPC with 40 % wood content was used.

The average value of compressive strength parallel to grain (CS_║) for hybrid plywood was 35,50 MPa whereas, the hybrid WPC-LVL showed significantly higher CS_║ i.e.44,88 MPa. Tensile strength (TSS_║) of hybrid plywood was not significantly differing from hybrid LVL. Glue shear strength average value of hybrid WPC-plywood and LVL was 1,25 MPa and 1,15 MPa, respectively. GSS mainly reflects the strength of bond against slippage between WPC layer and veneer on tensile force. Chang et al. (2017) reported GSS of 1,50 MPa in plywood prepared with HDPE which is in close agreement with the current study.

Figure 2:
Internal bond sample after delamination.

Interestingly, average value of IB for hybrid WPC-plywood (1,77 MPa) was significantly lower than average value of IB in hybrid WPC-LVL (2,75 MPa) and mostly wood failure was observed (as shown in Figure 2) indicating effective bonding between wood and WPC layer which is attributed to the deeper penetration of WPC in veneers. Further in order to investigate the bond strength, knife test was carried as per specifications given in BIS 1734 (1983) on the dry and wet (boiled in water for 2 h) specimens by pushing a sharp knife with its cutting edge parallel to the grain of the face veneer. After insertion, the knife was pulled upwards. The specimens showed excellent bond as the penetration of knife was difficult and after prising upwards the veneer breaks off instead of completely pulling out (Figure 3). Since, WPC melts at higher temperature i.e. 160 ºC - 170 ºC, boiling in water does not have any detrimental effect on the bonding mechanism of these hybrid composites compared to the conventional plywood and LVL especially prepared using urea formaldehyde adhesives.

Figure 3:
Knife delamination test.

The SEM images of the fractured surfaces of the hybrid LVL and plywood are shown in Figure 4. It was observed that the wood veneers were embedded/bonded in the matrix of WPC. The polymers penetrated deeply inside the voids. During the fabrication process the external heat causes the WPC to melt and flow into the voids present in the veneers created during the peeling, resulting in a strong mechanical interlocking between cells of the wood and the polymer. Images further confirmed that there was no de-bonding observed where veneer surfaces and WPC interacts. Chang et al. (2017) studied interfacial bonding mechanism of poplar plywood bonded with HDPE films and reported that there was a poor cohesion between the HDPE and the wood cells which was attributed to their poor compatibility. However, such phenomenon was not observed in the SEM images and there was no delamination at the interphase of WPC and veneer. This suggests that the coupling agent added in the WPC may enhance the adherence of the polymer matrix to the wood veneer substrate which results in an excellent bonding strength of the final product. Also, addition of coupling agent during the fabrication of WPC’s plays a vital role in enhancing the wood - polymer interaction resulting in satisfactory adhesion between fibre and polymer matrix (Chauhan et al. 2016, Karmarkar et al. 2007, Nandi et al. 2013, Poletto 2017). The thick layer of the WPC imparts better performance due to presence of cross- linked/ polymerized structures, providing superior resisting against failures. The polymer flow in the pores and crevices present in the veneer would also result in mechanical locking/attachment of WPC with veneer. The overall volume of polymer and wood present in material can significantly affect physical and mechanical properties of the hybrid composites (Lustosa et al. 2015).

Figure 4:
SEM micrographs showing bonding of thermoplastic polymer (up) penetration of WPC in pores of veneer (down) bonding interface between wood veneer and WPC.

The heat deflection observed with respect to change in temperature for polypropylene, WPC sheet and hybrid composite is represented in Figure 5. HDT test result revealed that a deflection of 0,25 mm was observed when temperature was 120 ºC, 137,8 ºC and 195,2 ºC for PP, WPC sheet and hybrid composite specimens, respectively. Reinforcing polymer with natural fibers is known to result in higher HDT indicating increased stiffness at higher temperatures. Rojanathavorn et al. (2014) studied HDT of WPC manufactured using PP and HDPE polymers, reinforced with Ironwood (Xylia xylocarpa (Roxb.) Taub.) at different fibre content loading and reported that with increase in wood content resulted in improvement of thermal stability of WPC. HDT of both plywood and LVL configuration composites were similar. The higher heat deflection temperature of hybrid composites can be attributed to presence of wood veneers which restrict the conduction of heat through the material as compared to WPC specimens. WPC used as a binding agent along with presence of veneer can result in improved thermal stability when exposed to higher temperature as compared to composites bonded using pure polymers. The deflection of hybrid composite with increased temperature also suggest that such plywood or LVL can potentially be thermoformed at elevated temperatures which is not observed in conventional plywood/LVL.

Figure 5:
Heat deflection curve of WPC and hybrid composite.

^♠Corresponding author: shakti32@gmail.com, shakti@icfre.org