*2.1. Materials*

Industrially manufactured birch and beech plywood, with a thickness of 10 mm, and solid wood were sourced from Frischeis GmbH (Stockerau, Austria). Polypropylene (Daplen KSR 4525) and polyamide 6 (Grilon BZ 3) were provided by Borealis (Vienna, Austria) and EMS-Chemie AG (Domat, Switzerland), respectively.

#### *2.2. Plywood Composites*

Sixteen boards of each species, birch and beech, respectively, with a dimension of 297 × 146 mm were prepared using a circular saw and overmolded with PP as shown in Figure 1a. Injection molding was performed with an injection molding machine (Engel ES 1350/200 HL-V, Schwertberg, Austria) with a screw diameter of 70 mm. Each species was overmolded at three different injection temperatures to investigate the effect of the injection temperature on the interfacial adhesion and the mechanical properties of the wood-polymer composites. For this, cylinder temperatures were set to an average value of 220 ◦C, 240 ◦C and 260 ◦C, respectively. The volumetric flow rate was 15 cm3/s at an injection pressure of 170 bar, injection time of 2 s and the cycle time was 80 s. The way point of the feed screw was set to 35 mm, which corresponds to a changeover point at 95 cm<sup>3</sup> after an injection time of 10 s and a holding pressure of 40 bar.

In order to investigate mechanical properties under three different load conditions, specimens were cut using a circular saw (Figure 1a–d). For tensile tests perpendicular to the plane of the board (σP), specimens with a dimension of 50 × 20 mm (Figure 1b), and for tensile tests perpendicular to the edge (σE) specimens with a dimension of 120 × 20 × 10 mm

were produced (Figure 1c), with one specimen consisting of 2 individual parts. These two parts were welded together with a welding mirror. For this purpose the temperature of the welding mirror was set to 200 ◦C and the welding time was 2 s. Specimens for tensile shear (σS) tests, performed according to DIN EN 302-1, had a dimension of 120 × 20 × 4 mm, with the overlap length of the overmolded areas being 10 mm [34] (Figure 1d). In total 121 birch and 118 beech samples were prepared for this study. *Polymers* **2021**, *13*, x FOR PEER REVIEW 4 of 16

**Figure 1.** Schematic of the overmolded plywood boards (**a**) and schematic representation of the specimens for the tensile strength perpendicular to the plane of the board σP (**b**), tensile strength perpendicular to the edge σE (**c**) and tensile shear strength σS (**d**). **Figure 1.** Schematic of the overmolded plywood boards (**a**) and schematic representation of the specimens for the tensile strength perpendicular to the plane of the board σ<sup>P</sup> (**b**), tensile strength perpendicular to the edge σ<sup>E</sup> (**c**) and tensile shear strength σ<sup>S</sup> (**d**).

#### In order to investigate mechanical properties under three different load conditions, *2.3. Solid Wood Composites*

specimens were cut using a circular saw (Figure 1a–d). For tensile tests perpendicular to the plane of the board (σP), specimens with a dimension of 50 × 20 mm (Figure 1b), and for tensile tests perpendicular to the edge (σE) specimens with a dimension of 120 × 20 × 10 mm were produced (Figure 1c), with one specimen consisting of 2 individual parts. These two parts were welded together with a welding mirror. For this purpose the temperature of the welding mirror was set to 200 °C and the welding time was 2 s. Forty board specimens were cut with a dimension of 140 × 140 mm from each solid wood species, planed to a thickness of 4 mm and overmolded with PP or PA 6, respectively, to a final thickness of 8 mm (Figure 2a). Injection molding was performed with an injection molding machine (Wittmann Battendfeld Smart Power 120/750 B 8) having a screw diameter of 70 mm. *Polymers* **2021**, *13*, x FOR PEER REVIEW 5 of 16

having a screw diameter of 70 mm. For PA 6, the cylinder temperature was set to an average value of 260 °C. The **Figure 2.** Schematic of the overmolded solid wood (**a**) and representation of the specimens for the tensile shear strength σS (**b**) according to ÖNORM EN 302-1 [34]. **Figure 2.** Schematic of the overmolded solid wood (**a**) and representation of the specimens for the tensile shear strength σ<sup>S</sup> (**b**) according to ÖNORM EN 302-1 [34].

volumetric flow rate was 40 cm3/s at an injection pressure of 360 bar, the injection time

was 2 s and the cycle time was 64 s. The changeover point was set to 12 cm3, with a holding pressure and time of 100 bar and 2 s, respectively. For PP, the cylinder temperature was set to an average value of 260 °C, the volumetric flow rate to 15 cm3/s, the injection pressure was 170 bar, the cycle time 66 s and injection *2.4. Mechanical Properties of Wood-Polymer Composites*  Mechanical tests were performed using a universal testing machine (Zwick/Roell Z20, Ulm, Germany). Prior to mechanical tests, all samples were stored under standard For PA 6, the cylinder temperature was set to an average value of 260 ◦C. The volumetric flow rate was 40 cm3/s at an injection pressure of 360 bar, the injection time was 2 s and the cycle time was 64 s. The changeover point was set to 12 cm<sup>3</sup> , with a holding pressure and time of 100 bar and 2 s, respectively.

time 5 s. The changeover point was set at 12 cm3, with a holding pressure and time of 100 bar and 1 s, respectively. In total 108 specimens with a dimension of 110 × 20 × 8 mm were produced using a circular saw (Figure 2b). In total 50 birch and 58 beech samples were prepared. climate conditions (20 °C ± 2 °C, 65% ± 5% relative humidity) according to standard ISO 554 [35] until constant mass was reached. All tests were stopped after a 30% load reduction of the maximum force (Fmax) was reached or failure occurred within 90 ± 30 s. For plywood composites tensile tests perpendicular to the plane of the board (σP) were performed with clamps originally designed for testing internal bond strength of For PP, the cylinder temperature was set to an average value of 260 ◦C, the volumetric flow rate to 15 cm3/s, the injection pressure was 170 bar, the cycle time 66 s and injection time 5 s. The changeover point was set at 12 cm<sup>3</sup> , with a holding pressure and time of 100 bar and 1 s, respectively. In total 108 specimens with a dimension of 110 × 20 × 8 mm

particle and fiber boards, [36] which were used to attach the wooden part to the testing machine as shown in Figure 3a. A pre-force of 10 N was applied before testing at a

52 188 [37], depicted in Figure 3b. After a pre-force of 10 N was applied, the specimens were loaded at a constant speed of 0.3 mm/min. σS was determined following DIN EN 302-1 [34] (Figure 3c) with an applied pre-force of 20 N at a constant speed of 0.4 mm/min. For solid wood composites, the tensile shear strength (σS) of birch and beech wood composites was assessed according to DIN EN 302-2 [34]. The samples were loaded with

**Figure 3.** Schematic of the test setup for overmolded plywood specimens for strength measurements according to DIN 52 188 [37] and DIN EN 302-1 [34]: (**a**) test setup for tensile strength perpendicular to the plane of the board σP, (**b**) test setup for tensile strength perpendicular to the edge σE, and (**c**)

To investigate the penetration of the polymer into the wood structure on a microscopic level, two samples per combination were analyzed by means of SEM (Hitachi TM3030, Tokyo, Japan). To analyze the interphase of a cross-section of the overmolded samples, specimens with a dimension of about 3 × 8 mm were cut using a double-bladed circular saw. To obtain a smooth surface without any disturbing artefacts, the area of

*2.5. Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS)* 

a pre-force of 10 N and tested at a constant crosshead speed of 0.6 mm/min.

test setup for tensile shear strength σS.

interest was cut with a razor blade.

were produced using a circular saw (Figure 2b). In total 50 birch and 58 beech samples were prepared. *2.4. Mechanical Properties of Wood-Polymer Composites*  Mechanical tests were performed using a universal testing machine (Zwick/Roell

**Figure 2.** Schematic of the overmolded solid wood (**a**) and representation of the specimens for the

#### *2.4. Mechanical Properties of Wood-Polymer Composites* Z20, Ulm, Germany). Prior to mechanical tests, all samples were stored under standard climate conditions (20 °C ± 2 °C, 65% ± 5% relative humidity) according to standard ISO

tensile shear strength σS (**b**) according to ÖNORM EN 302-1 [34].

*Polymers* **2021**, *13*, x FOR PEER REVIEW 5 of 16

Mechanical tests were performed using a universal testing machine (Zwick/Roell Z20, Ulm, Germany). Prior to mechanical tests, all samples were stored under standard climate conditions (20 ◦C ± 2 ◦C, 65% ± 5% relative humidity) according to standard ISO 554 [35] until constant mass was reached. All tests were stopped after a 30% load reduction of the maximum force (Fmax) was reached or failure occurred within 90 ± 30 s. 554 [35] until constant mass was reached. All tests were stopped after a 30% load reduction of the maximum force (Fmax) was reached or failure occurred within 90 ± 30 s. For plywood composites tensile tests perpendicular to the plane of the board (σP) were performed with clamps originally designed for testing internal bond strength of particle and fiber boards, [36] which were used to attach the wooden part to the testing

For plywood composites tensile tests perpendicular to the plane of the board (σP) were performed with clamps originally designed for testing internal bond strength of particle and fiber boards, [36] which were used to attach the wooden part to the testing machine as shown in Figure 3a. A pre-force of 10 N was applied before testing at a constant crosshead speed of 1 mm/min. σ<sup>P</sup> was calculated according to DIN 52 188 [37], by dividing Fmax through the calculated interface area. σ<sup>E</sup> was determined following DIN 52 188 [37], depicted in Figure 3b. After a pre-force of 10 N was applied, the specimens were loaded at a constant speed of 0.3 mm/min. σ<sup>S</sup> was determined following DIN EN 302-1 [34] (Figure 3c) with an applied pre-force of 20 N at a constant speed of 0.4 mm/min. machine as shown in Figure 3a. A pre-force of 10 N was applied before testing at a constant crosshead speed of 1 mm/min. σP was calculated according to DIN 52 188 [37], by dividing Fmax through the calculated interface area. σE was determined following DIN 52 188 [37], depicted in Figure 3b. After a pre-force of 10 N was applied, the specimens were loaded at a constant speed of 0.3 mm/min. σS was determined following DIN EN 302-1 [34] (Figure 3c) with an applied pre-force of 20 N at a constant speed of 0.4 mm/min. For solid wood composites, the tensile shear strength (σS) of birch and beech wood composites was assessed according to DIN EN 302-2 [34]. The samples were loaded with a pre-force of 10 N and tested at a constant crosshead speed of 0.6 mm/min.

**Figure 3.** Schematic of the test setup for overmolded plywood specimens for strength measurements according to DIN 52 188 [37] and DIN EN 302-1 [34]: (**a**) test setup for tensile strength perpendicular to the plane of the board σP, (**b**) test setup for tensile strength perpendicular to the edge σE, and (**c**) test setup for tensile shear strength σS. **Figure 3.** Schematic of the test setup for overmolded plywood specimens for strength measurements according to DIN 52 188 [37] and DIN EN 302-1 [34]: (**a**) test setup for tensile strength perpendicular to the plane of the board σP, (**b**) test setup for tensile strength perpendicular to the edge σE, and (**c**) test setup for tensile shear strength σS.

*2.5. Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS)*  To investigate the penetration of the polymer into the wood structure on a microscopic level, two samples per combination were analyzed by means of SEM (Hitachi For solid wood composites, the tensile shear strength (σS) of birch and beech wood composites was assessed according to DIN EN 302-2 [34]. The samples were loaded with a pre-force of 10 N and tested at a constant crosshead speed of 0.6 mm/min.

#### TM3030, Tokyo, Japan). To analyze the interphase of a cross-section of the overmolded samples, specimens with a dimension of about 3 × 8 mm were cut using a double-bladed *2.5. Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS)*

circular saw. To obtain a smooth surface without any disturbing artefacts, the area of interest was cut with a razor blade. To investigate the penetration of the polymer into the wood structure on a microscopic level, two samples per combination were analyzed by means of SEM (Hitachi TM3030, Tokyo, Japan). To analyze the interphase of a cross-section of the overmolded samples, specimens with a dimension of about 3 × 8 mm were cut using a double-bladed circular saw. To obtain a smooth surface without any disturbing artefacts, the area of interest was cut with a razor blade.

XPS spectra were recorded to determine the penetration depth of the polymer into the microstructure of wood as well as chemical interactions between polymer and wood and to gain a deeper understanding into interfacial adhesion. Six solid wood specimens of each combination having a cross section of 8 × 4 mm were cut. The analysis was performed using an XPS system (Nexsa, Thermo-Scientific, Waltham, MA, USA) using an Al K<sup>α</sup> radiation source operating at 72 W and an integrated flood gun. A pass energy of 200 eV, "Standard Lens Mode", CAE Analyzer Mode and an energy step size of 1 eV for the survey spectrum were used. The diameter of the X-ray beam was 100 µm. A line scan was performed where four analysis points were placed in the wood-polymer interphase (Figure 4, −2 to +1). Starting from the first analysis point (Figure 4, 0), two spots were placed in the wood direction (Figure 4, −1 to −2) and one measuring spot in the polymer direction (Figure 4, +1) at a distance of 200 µm, respectively. As a reference, additional analysis points were placed in the wood substrate and in the polymer bulk, respectively. Prior to analysis the surface was cleaned by sputtering with Ar-clusters (1000 atoms, 6000 eV, 1 mm raster size) for 60 s. High-resolution spectra of C1s, N1s and O1s of 6 specimens were examined, acquired with 50 passes at a pass energy of 50 eV and an energy step size of 0.1 eV. These were analyzed using software package Thermo Avantage (v5.9914, Build 06617) with Smart background and Simplex Fitting algorithm by using Gauss-Lorentz Product. Peak profiles of C1s and O1s were deconvoluted. scan was performed where four analysis points were placed in the wood-polymer interphase (Figure 4, −2 to +1). Starting from the first analysis point (Figure 4, 0), two spots were placed in the wood direction (Figure 4, −1 to −2) and one measuring spot in the polymer direction (Figure 4, +1) at a distance of 200 µm, respectively. As a reference, additional analysis points were placed in the wood substrate and in the polymer bulk, respectively. Prior to analysis the surface was cleaned by sputtering with Ar-clusters (1000 atoms, 6000 eV, 1 mm raster size) for 60 s. High-resolution spectra of C1s, N1s and O1s of 6 specimens were examined, acquired with 50 passes at a pass energy of 50 eV and an energy step size of 0.1 eV. These were analyzed using software package Thermo Avantage (v5.9914, Build 06617) with Smart background and Simplex Fitting algorithm by using Gauss-Lorentz Product. Peak profiles of C1s and O1s were deconvoluted.

XPS spectra were recorded to determine the penetration depth of the polymer into the microstructure of wood as well as chemical interactions between polymer and wood and to gain a deeper understanding into interfacial adhesion. Six solid wood specimens of each combination having a cross section of 8 × 4 mm were cut. The analysis was performed using an XPS system (Nexsa, Thermo-Scientific, Waltham, MA, USA) using an Al Kα radiation source operating at 72 W and an integrated flood gun. A pass energy of 200 eV, "Standard Lens Mode", CAE Analyzer Mode and an energy step size of 1 eV for the survey spectrum were used. The diameter of the X-ray beam was 100 µm. A line

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**Figure 4.** Micrograph of the overmolded solid wood specimens for X-ray photoelectron spectroscopy analyses (XPS) used to determine the elementary distribution of C, O and N and for high-resolution deconvoluted XPS spectra within the samples cross section, indicating the position of the X-ray beam. **Figure 4.** Micrograph of the overmolded solid wood specimens for X-ray photoelectron spectroscopy analyses (XPS) used to determine the elementary distribution of C, O and N and for high-resolution deconvoluted XPS spectra within the samples cross section, indicating the position of the X-ray beam.

#### *2.6. Statistics*  In this study a one-way analysis of variance with an error level of 0.05 was calculated *2.6. Statistics*

using Excel 2016 (Microsoft, Redmond, WA, USA) to statistically evaluate the effect of the injection temperature and the wood species on the mechanical properties. In this study a one-way analysis of variance with an error level of 0.05 was calculated using Excel 2016 (Microsoft, Redmond, WA, USA) to statistically evaluate the effect of the injection temperature and the wood species on the mechanical properties.

#### **3. Results and Discussion**  *3.1. Mechanical Properties of Wood-Polymer Composites*  **3. Results and Discussion**

#### *3.1. Mechanical Properties of Wood-Polymer Composites*

#### 3.1.1. Plywood-Polymer Composites 3.1.1. Plywood-Polymer Composites

Figure 5 displays the results of all test configurations at the different injection temperatures for birch and beech plywood, respectively, overmolded with PP. Regarding the influence of the injection temperature on the mechanical properties of these woodpolymer composites no statistically significant effect was present, with the exception of σ<sup>E</sup> for the overmolded birch plywood specimens, for which higher injection temperatures resulted in higher σE. Chang et al. [25] reported that the hot-pressing temperature and pressure exhibit an inflexion point, i.e., a certain pressure and temperature, at which penetration of the polymer into the wood structure with partly damaged cells and cracks and into the vessels is highest, thus resulting in the highest strength. Furthermore, no significant difference between birch and beech plywood for all test configurations was Figure 5 displays the results of all test configurations at the different injection temperatures for birch and beech plywood, respectively, overmolded with PP. Regarding the influence of the injection temperature on the mechanical properties of these wood-polymer composites no statistically significant effect was present, with the exception of σ<sup>E</sup> for the overmolded birch plywood specimens, for which higher injection temperatures resulted in higher σE. Chang et al. [25] reported that the hot-pressing temperature and pressure exhibit an inflexion point, i.e., a certain pressure and temperature, at which penetration of the polymer into the wood structure with partly damaged cells and cracks and into the vessels is highest, thus resulting in the highest strength. Furthermore, no significant difference between birch and beech plywood for all test configurations was observed.

observed. σ<sup>P</sup> was on average (over all three injection temperatures) 3.16 ± 0.91 MPa and 2.89 ± 0.68 MPa for birch and for beech plywood, respectively. The highest σ<sup>P</sup> for birch and beech plywood specimens was observed at an injection temperature of 240 ◦C (3.31 ± 1.15 MPa and 3.02 ± 0.49 MPa), similar to results reported by Chang et al. [25]. The tensile strength perpendicular to the edge was on average 5.08 ± 1.44 MPa and 4.78 ± 1.01 MPa for birch and beech plywood, respectively, with the highest values achieved at 260 ◦C for birch and 240 ◦C for beech plywood specimens (6.04 ± 1.04 MPa and 5.14 ± 1.24 MPa). Liu et al. [38] investigated the surface bond strength of engineered plywood in a similar fashion. Poplar veneers (*Populus tomentosa* Carriére) were bonded with chlorinated PP films on a wood fiber PP composite core layer (80% wood fiber and 20% PP) using a hot-pressing procedure. They observed surface bonding strength values of the veneers on the composite core layer, which is comparable with σP, of approx.

1.75 MPa, which was significantly lower compared to our study. The higher values were attributed to two reasons. On the one hand, the poplar veneers used have a significantly lower tensile strength perpendicular to the grain of about 1.7–2.8 MPa compared to birch (~7.0 MPa) and beech (~7.0–10.7 MPa) wood [39]. On the other hand, they prepared the PPbonded plywood using a hot-pressing process, in which the pressure applied was about 5 MPa at a temperature of 110 ◦C, which is much lower compared to those used in our study. Improved penetration of PP into the wood at 170 bar (17 MPa) and 360 bar (36 MPa) for PA 6, respectively, the pressures used in this study, could eradicate the damage caused in the wood structure due to compressive failure of the top layers. *Polymers* **2021**, *13*, x FOR PEER REVIEW 7 of 16

**Figure 5.** Strength properties of 121 birch plywood specimens and 118 beech plywood specimens overmolded with PP at three different injection temperatures. σP is the average tensile strength perpendicular to the plane of the board, σE is the average tensile strength perpendicular to the edge, σS is the average tensile shear strength and n is the number of the samples tested. The whiskers show minimum and maximum values. X is the mean value. ° indicate values of statistical outliers and—is the median. **Figure 5.** Strength properties of 121 birch plywood specimens and 118 beech plywood specimens overmolded with PP at three different injection temperatures. σ<sup>P</sup> is the average tensile strength perpendicular to the plane of the board, σ<sup>E</sup> is the average tensile strength perpendicular to the edge, σ<sup>S</sup> is the average tensile shear strength and n is the number of the samples tested. The whiskers show minimum and maximum values. X is the mean value. ◦ indicate values of statistical outliers and—is the median.

σP was on average (over all three injection temperatures) 3.16 ± 0.91 MPa and 2.89 ± 0.68 MPa for birch and for beech plywood, respectively. The highest σP for birch and beech plywood specimens was observed at an injection temperature of 240 °C (3.31 ± 1.15 MPa and 3.02 ± 0.49 MPa), similar to results reported by Chang et al. [25]. The tensile strength perpendicular to the edge was on average 5.08 ± 1.44 MPa and 4.78 ± 1.01 MPa for birch and beech plywood, respectively, with the highest values achieved at 260 °C for birch and 240 °C for beech plywood specimens (6.04 ± 1.04 MPa and 5.14 ± 1.24 MPa). Liu et al. [38] investigated the surface bond strength of engineered plywood in a similar fashion. Poplar σ<sup>S</sup> of birch and beech plywood samples were on average 3.81 ± 0.76 MPa and 3.83 ± 0.69 MPa, respectively, with highest values observed at an injection temperature of 260 ◦C for birch and 220 ◦C for beech of 3.87 ± 0.68 MPa and 3.92 ± 0.94 MPa, respectively. Bekhta et al. [28] reported shear strength values for birch and beech plywood bonded with PA 6 and polyethylene (PE) higher than 3 MPa and 1.7 MPa, respectively. However, PP overmolded birch and beech plywood showed higher σS, which is thought to be mainly influenced by the different process parameters used in this study.

veneers (*Populus tomentosa* Carriére) were bonded with chlorinated PP films on a wood fiber PP composite core layer (80% wood fiber and 20% PP) using a hot-pressing procedure. They observed surface bonding strength values of the veneers on the

significantly lower compared to our study. The higher values were attributed to two reasons. On the one hand, the poplar veneers used have a significantly lower tensile strength perpendicular to the grain of about 1.7–2.8 MPa compared to birch (~7.0 MPa) and beech (~7.0–10.7 MPa) wood [39]. On the other hand, they prepared the PP-bonded plywood using a hot-pressing process, in which the pressure applied was about 5 MPa at a temperature of 110 °C, which is much lower compared to those used in our study. Improved penetration of PP into the wood at 170 bar (17 MPa) and 360 bar (36 MPa) for

#### 3.1.2. Solid Wood-Polymer Composites 3.1.2. Solid Wood-Polymer Composites

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the wood structure due to compressive failure of the top layers.

influenced by the different process parameters used in this study.

Figure 6 summarizes the mean values, standard deviation and sample number for birch and beech solid wood, overmolded with PA 6 and PP, respectively. There was no statistically significant difference between the birch and beech solid wood composites. σ<sup>S</sup> for birch-PA 6 were on average 5.71 ± 1.13 MPa, while σ<sup>S</sup> for beech-PA 6 was slightly higher (6.36 ± 1.47 MPa). For PP-composites a slightly lower tensile shear strength was observed for birch solid wood compared to beech (2.33 ± 0.44 MPa and 2.54 ± 0.83 MPa, respectively). The observed maximum values for σ<sup>S</sup> for the PA 6-composites were 8.65 MPa and 9.74 MPa and for PP-composites 2.98 MPa and 4.19 MPa for birch and beech, respectively. Compared to literature, the measured maximum values were similar, with tensile shear strengths of 9 MPa and 3.5 MPa reported for beech wood rods overmolded with PA 6 and PP, respectively [16]. However, a perfectly aligned longitudinal fiber orientation of the specimens results in fewer cut vessels and fibers and thus fewer open lumens into which polymer could penetrate, which in turn results in less mechanical interlocking and thus in a lower average tensile shear strength. For comparison, typical values for bonded birch and beech wood specimens (melamine-urea-formaldehyde (MUF), polyurethane (PU) and phenol-resorcinol-formaldehyde (PRF)) with commercially adhesives do exceed 10 to 11 MPa [31,32]. Figure 6 summarizes the mean values, standard deviation and sample number for birch and beech solid wood, overmolded with PA 6 and PP, respectively. There was no statistically significant difference between the birch and beech solid wood composites. σ<sup>S</sup> for birch-PA 6 were on average 5.71 ± 1.13 MPa, while σS for beech-PA 6 was slightly higher (6.36 ± 1.47 MPa). For PP-composites a slightly lower tensile shear strength was observed for birch solid wood compared to beech (2.33 ± 0.44 MPa and 2.54 ± 0.83 MPa, respectively). The observed maximum values for σS for the PA 6-composites were 8.65 MPa and 9.74 MPa and for PP-composites 2.98 MPa and 4.19 MPa for birch and beech, respectively. Compared to literature, the measured maximum values were similar, with tensile shear strengths of 9 MPa and 3.5 MPa reported for beech wood rods overmolded with PA 6 and PP, respectively [16]. However, a perfectly aligned longitudinal fiber orientation of the specimens results in fewer cut vessels and fibers and thus fewer open lumens into which polymer could penetrate, which in turn results in less mechanical interlocking and thus in a lower average tensile shear strength. For comparison, typical values for bonded birch and beech wood specimens (melamine-urea-formaldehyde (MUF), polyurethane (PU) and phenol-resorcinol-formaldehyde (PRF)) with commercially adhesives do exceed 10 to11 MPa [31,32].

PA 6, respectively, the pressures used in this study, could eradicate the damage caused in

σS of birch and beech plywood samples were on average 3.81 ± 0.76 MPa and 3.83 ± 0.69 MPa, respectively, with highest values observed at an injection temperature of 260 °C for birch and 220 °C for beech of 3.87 ± 0.68 MPa and 3.92 ± 0.94 MPa, respectively. Bekhta et al. [28] reported shear strength values for birch and beech plywood bonded with PA 6 and polyethylene (PE) higher than 3 MPa and 1.7 MPa, respectively. However, PP overmolded birch and beech plywood showed higher σS, which is thought to be mainly

**Figure 6.** Average tensile shear strength σS of birch solid wood specimens and beech solid wood overmolded with PA 6 and PP, respectively; n is the number of the samples. The whiskers show minimum and maximum values. X is the mean value. ° indicate values of statistical outliers and is the median. **Figure 6.** Average tensile shear strength σ<sup>S</sup> of birch solid wood specimens and beech solid wood overmolded with PA 6 and PP, respectively; n is the number of the samples. The whiskers show minimum and maximum values. X is the mean value. ◦ indicate values of statistical outliers and—is the median.

The differences between PA 6 and PP composites can be explained by the different polarity of both polymers. It is supposed that the polar PA 6 [40] exhibits a good adhesion and/or sound bonding with the wood surface, which promotes higher strength values. In addition, the high temperature during the molding process (260 °C) degrades free hydrophilic groups of wood polymers, mainly the hemicelluloses [41,42]. The effects of thermally modified wood fibers on the adhesion to thermoplastics were also reported by Follrich et al. [43]. It can be assumed, that exposure to elevated temperatures leads to a more hydrophobic character of the wood surface enhancing the interfacial compatibility to hydrophobic polymers, which results in improved interfacial interactions. The differences between PA 6 and PP composites can be explained by the different polarity of both polymers. It is supposed that the polar PA 6 [40] exhibits a good adhesion and/or sound bonding with the wood surface, which promotes higher strength values. In addition, the high temperature during the molding process (260 ◦C) degrades free hydrophilic groups of wood polymers, mainly the hemicelluloses [41,42]. The effects of thermally modified wood fibers on the adhesion to thermoplastics were also reported by Follrich et al. [43]. It can be assumed, that exposure to elevated temperatures leads to a more hydrophobic character of the wood surface enhancing the interfacial compatibility to hydrophobic polymers, which results in improved interfacial interactions. Furthermore, the higher strength for PA 6-composites in contrast to PP-composites can also be explained by the higher cohesive strength of PA 6.

#### *3.2. Wood-Polymer Interfaces*

#### 3.2.1. Morphology of Wood Polymer Composites by SEM

Figure 7 shows cross sections of representative birch and beech solid wood composites. During the overmolding process, the melted polymers penetrated into the wood structure through the sliced vessels and fibers. For the specimens overmolded with PA 6, in comparison to PP-composites only minor penetration of the melt into the wood substrate was observed. Due to the high pressure used (360 bar) during the injection process, the melt

flow was mainly directed in radial direction (Figure 7(1a–2b)). PP composites were fabricated at much lower injection pressure (170 bar), hence, the melt penetrated the wood cells in both directions, for birch and beech, respectively (Figure 7(3a–4b)). Furthermore, it was observed that the outer cellular structure (approximately 100 µm up to 200 µm) is stabilized by the polymer that penetrated the wood by filling the lumens of vessels and tracheids. Additionally, only a few micro gaps along the interface between the wood and the polymer were observed, which is interpreted as an indicator of good adhesion between the materials. In addition, wood rays and also the transition zone from early to late wood have a structurally reinforcing effect. As Mattheck and Kubler [44] presented, the many rays oriented perpendicular to the grain behave like beams, that lead to an increasing compressive strength of the wood structure. These compressed areas generate an increased interface and thus improved mechanical interlocking between the polymer and the wood surface. According to Sretenovic et al. [21] the mechanical interlocking influences the stress transfer from the polymer to the wood structure in wood fiber composites. In addition, Smith et al. [45] reported that both the porosity of the wood structure and the processing parameters are influencing mechanical interlocking. Furthermore, Gupta et al. [46] showed that there are strong correlations between surface roughness and interfacial adhesion. process, the melt flow was mainly directed in radial direction (Figure 7(1a–2b)). PP composites were fabricated at much lower injection pressure (170 bar), hence, the melt penetrated the wood cells in both directions, for birch and beech, respectively (Figure 7(3a–4b)). Furthermore, it was observed that the outer cellular structure (approximately 100 µm up to 200 µm) is stabilized by the polymer that penetrated the wood by filling the lumens of vessels and tracheids. Additionally, only a few micro gaps along the interface between the wood and the polymer were observed, which is interpreted as an indicator of good adhesion between the materials. In addition, wood rays and also the transition zone from early to late wood have a structurally reinforcing effect. As Mattheck and Kubler [44] presented, the many rays oriented perpendicular to the grain behave like beams, that lead to an increasing compressive strength of the wood structure. These compressed areas generate an increased interface and thus improved mechanical interlocking between the polymer and the wood surface. According to Sretenovic et al. [21] the mechanical interlocking influences the stress transfer from the polymer to the wood structure in wood fiber composites. In addition, Smith et al. [45] reported that both the porosity of the wood structure and the processing parameters are influencing mechanical interlocking. Furthermore, Gupta et al. [46] showed that there are strong correlations between surface roughness and interfacial adhesion.

Furthermore, the higher strength for PA 6-composites in contrast to PP-composites can

Figure 7 shows cross sections of representative birch and beech solid wood composites. During the overmolding process, the melted polymers penetrated into the wood structure through the sliced vessels and fibers. For the specimens overmolded with PA 6, in comparison to PP-composites only minor penetration of the melt into the wood substrate was observed. Due to the high pressure used (360 bar) during the injection

*Polymers* **2021**, *13*, x FOR PEER REVIEW 9 of 16

also be explained by the higher cohesive strength of PA 6.

3.2.1. Morphology of Wood Polymer Composites by SEM

*3.2. Wood-Polymer Interfaces* 

**Figure 7.** Representative SEM micrographs recorded on the cross-section of the overmolded specimens for birch solid wood (**1a**–**2b**) and beech solid wood (**3a**–**4b**); b represents always the detailed inset of a. (**1a**), (**1b**), (**3a**) and (**3b**) show samples overmolded in radial direction and (**2a**), (**2b**), (**4a**) and (**4b**) represents samples overmolded in tangential direction. **Figure 7.** Representative SEM micrographs recorded on the cross-section of the overmolded specimens for birch solid wood (**1a**–**2b**) and beech solid wood (**3a**–**4b**); b represents always the detailed inset of a. (**1a**), (**1b**), (**3a**) and (**3b**) show samples overmolded in radial direction and (**2a**), (**2b**), (**4a**) and (**4b**) represents samples overmolded in tangential direction.

> Beyond the stabilized interface the wood structure exhibits a zone of compressive failure, caused by plastic deformation of the wood structure during injection molding (Figure 7(1a–4a)). These compressed zones range approximately 0.4 to 1.1 mm into the wood structure depending on the species. Müller et al. [47] reported that a pressure of about 12 MPa is required to densify a diffuse-porous wood structure (e.g., birch and beech) perpendicular to the grain. Corresponding to the thickness of the compressed zone, the overmolding procedure causes almost homogeneous densification across the Beyond the stabilized interface the wood structure exhibits a zone of compressive failure, caused by plastic deformation of the wood structure during injection molding (Figure 7(1a–4a)). These compressed zones range approximately 0.4 to 1.1 mm into the wood structure depending on the species. Müller et al. [47] reported that a pressure of about 12 MPa is required to densify a diffuse-porous wood structure (e.g., birch and beech) perpendicular to the grain. Corresponding to the thickness of the compressed zone, the overmolding procedure causes almost homogeneous densification across the overmolded surface for both birch and beech. However, the compression zone of birch wood is much larger than the compression zone of beech wood, which is caused by its lower compression strength perpendicular to the grain [39,48,49]. A lower ratio of strength perpendicular to the grain to the injection pressure, leads to higher densification of the wood substrate, causing the formation of a so-called weak boundary layer, which influences the strength of wood polymer composites. In birch wood samples, failure occurs mainly in the weak boundary layer, which corresponds to the results of the mechanical tests, both for solid wood and plywood overmolded with PA 6 and PP as well as findings, as reported by Chang et al. [25]. Furthermore, it is clearly shown, that mainly vessels are compressed in this range. In contrast, tracheids and wood fibers are compressed mainly in the peripheral areas up to a depth of 200 to 600 µm.
