3.2. Thickness and Density of Plywood Samples
The physical properties of plywood samples, which were made by combining veneers of different wood species and different types of treatment in one panel, are summarized in
Table 3. The thinnest and thickest plywood samples were 6.7 mm and 7.4 mm and were made of only densified alder veneer (A
D–A
D–A
D–A
D–A
D) and only non-densified birch veneer (B
N–B
N–B
N–B
N–B
N), respectively. Between all other samples, the thickness of the panels differed insignificantly and was in the range of 6.9–7.2 mm. The thickness of the plywood panels in this study did not exceed the tolerances for unsanded panels according to European standard EN 315 [
37]. The lowest density of 583 kg/m
3 was found in plywood (A
N–A
N–A
N–A
N–A
N) from non-densified alder veneer, as expected. The greatest densities of 779 and 832 kg/m
3 were found in plywood made of non-densified birch veneer (B
N–B
N–B
N–B
N–B
N) and densified birch veneer (B
D–B
D–B
D–B
D–B
D), respectively. The lowest
WA values of 29.3% and 30.4% were found in plywood samples made of densified (B
D–B
D–B
D–B
D–B
D) and non-densified birch veneer (B
N–B
N–B
N–B
N–B
N), respectively. The highest
WA values of 41.9% and 43.0% were found in plywood samples made of densified (A
D–A
D–A
D–A
D–A
D) and non-densified (A
N–A
N–A
N–A
N–A
N) alder veneer, respectively. The smallest and greatest values of
TS of 7.2% and 12.1% were observed in plywood made from non-densified (A
N–A
N–A
N–A
N–A
N) and densified (A
D–A
D–A
D–A
D–A
D) alder veneer, respectively.
The ANOVA analysis (
Table 4) showed that combinations of wood species and having different types of veneer treatment in one panel significantly affected the thickness, density and
WA of plywood samples. In addition, the combination of a densified and non-densified veneer in one panel affected the thickness and density of plywood samples to almost the same extent, taking into account approximately the same influencing factors (
F = 10.355 and
F = 13.334). Meanwhile, the combination of veneers of different wood species in one panel had a stronger effect on the density (
F = 101.404) and a weaker effect on the thickness (
F = 9.464) of plywood samples.
TS did not depend on the mixing of wood species (
F = 1.527) in one panel, but significantly depended on the mixing of densified and non-densified veneers (
F = 31.045) (
Table 4).
A graphic illustration of the effect of combining veneers of different wood species and different treatments in one panel on the thickness, density,
WA and
TS of plywood panels is presented in
Figure 1. The smallest average thickness (6.7 mm) was in plywood samples made from a densified alder veneer, and the largest (7.4 mm) was in samples made from a non-densified birch veneer (
Table 3,
Figure 1). Decreasing the share of alder veneer in the panel increased the thickness of the plywood samples, although this increase was insignificant. For the lower proportion (40%) of alder veneer in one panel, the thickness of samples with outer layers of non-densified (A
N–B
N–B
N–B
N–A
N) or densified (A
D–B
D–B
D–B
D–A
D) alder veneer was lower (7.0 mm and 6.9 mm, respectively) than the thickness of samples with outer layers of non-densified (B
N–A
N–B
N–A
N–B
N) or densified (B
D–A
D–B
D–A
D–B
D) birch veneer (7.2 mm and 7.0 mm, respectively); however, the difference between them was insignificant (
p > 0.05). For the higher proportion (60%) of alder veneer in one panel, the thickness of samples (A
N–B
N–A
N–B
N–A
N) with outer layers of non-densified alder veneer was lower (7.0 mm) than the thickness of samples (B
N–A
N–A
N–A
N–B
N) with outer layers of non-densified birch veneer (7.2 mm). For the same conditions, the thickness of samples (A
D–B
D–A
D–B
D–A
D) and (B
D–A
D–A
D–A
D–B
D) with outer layers of densified veneers was equal (7.0 mm). This effect of veneer wood species on the thickness of plywood samples was mainly explained by the difference in the density of alder and birch wood. Birch veneer, having a higher density, is compressed less (8.2%) than alder veneer (11.5%), and, accordingly, formed thicker plywood samples. Moreover, the effect of the birch veneer on the formation of panel thickness was more pronounced when a denser birch veneer was placed in the outer layers of the panel. The smallest thicknesses (6.7–7.2 mm) of plywood samples were in samples made from densified veneer, and the greatest thicknesses (7.0–7.4 mm) were in the samples made from non-densified veneer (
Table 3,
Figure 1); however, the difference between them was significant. Plywood, in which the share of densified veneer (D) was greater, had a smaller thickness than plywood with a greater share of non-densified veneer (N), although the difference between them was insignificant.
The combination of wood species in one panel had a much stronger effect (
F = 101.404) on the density of plywood samples than the combination of the type of treated veneer (
F = 13.334) (
Table 4). Plywood samples (A
N–A
N–A
N–A
N–A
N) made from non-densified alder veneer had the lowest density (583 kg/m
3), while samples (B
D–B
D–B
D–B
D–B
D) made from densified birch veneer had the highest density (832 kg/m
3). The predominance of non-densified alder veneer in one panel (60%) provided a lower density of 684 kg/m
3 and 661 kg/m
3 for plywood samples (B
N–A
N–A
N–A
N–B
N) and (A
N–B
N–A
N–B
N–A
N), respectively, in comparison to the density of 779 kg/m
3 for a birch-only plywood (B
N–B
N–B
N–B
N–B
N). However, these densities were greater than the density of 583 kg/m
3 obtained for the alder-only plywood (A
N–A
N–A
N–A
N–A
N). A similar trend was observed for the densified veneer. It is obvious that the density of the birch veneer was greater than the density of the alder veneer. The effect of the type of veneer treatment (N and D) and its combination on the density of plywood samples was not as strong as the effect of the wood species of the veneer (
Table 4). With the same schemes of veneer assembly of different species in one panel, plywood samples with a densified veneer in the outer layers provided a greater density of plywood samples compared to samples comprising non-densified veneers. With the same assembly scheme of veneer and different wood species in one panel, increasing the proportion of densified veneer in the panel, in particular in the outer layers, reduced the density of sample (B
D–A
N–A
N–A
N–B
D) to 673 kg/m
3, compared to the density of 684 kg/m
3 obtained for sample (B
N–A
N–A
N–A
N–B
N), which was made from non-densified veneer only. However, the difference between the densities was insignificant. Meanwhile, the addition of densified veneer in the outer layers of sample (A
D–B
N–B
N–B
N–A
D) led to an increase in density to 726 kg/m
3 compared to the density of 709 kg/m
3 obtained for sample (A
N–B
N–B
N–B
N–A
N), which was made from non-densified veneer only. The higher
CR and
DR of alder veneer than birch veneer explained this (
Table 2).
The combination of different wood species and different types of veneer treatment in one plywood panel significantly affected the
WA of plywood samples. In addition, the combination of veneers from different wood species in one panel had a much stronger effect (
F = 51.493) on
WA than the type of veneer treatment used (a combination of densified and non-densified veneer in one panel) (
F = 4.148) (
Table 4). The lowest and highest
WA values of 29.3% and 43.0% were in plywood samples made from densified birch veneers (B
D–B
D–B
D–B
D–B
D) and non-densified alder veneers (A
N–A
N–A
N–A
N–A
N), respectively. With an increase in the proportion of alder veneer in the inner layers of birch plywood made from non-densified veneer (B
N–A
N–B
N–A
N–B
N and B
N–A
N–A
N–A
N–B
N), the
WA increased and its value was significantly higher (36.3% for B
N–A
N–B
N–A
N–B
N and B
N–A
N–A
N–A
N–B
N) than the value 30.4% for the
WA of a birch-only plywood made from non-densified veneer (B
N–B
N–B
N–B
N–B
N). However, the addition of birch veneers to the inner layers of an alder plywood resulted in a reduction in the
WA of plywood made from alder only. A similar trend was observed for plywood panels made from densified veneer (
Table 3,
Figure 1). An increase in the proportion of densified veneer in one panel led to a decrease in the
WA of plywood samples. This was explained by the density of plywood samples. It is well known that there is a relationship between
WA and panel density [
38]. The dependence of
WA on the density of plywood panels is shown in
Figure 2a. As the density of plywood samples increases, their
WA decreases, as the number and size of available cavities through which water can enter the sample decreases. Several authors [
27] also showed that
WA is related to panel density, with a higher density resulting in a lower number of pores and, consequently, a lower
WA.
The combination of veneers of different wood species in one panel had no effect on the
TS, while the type of veneer treatment had a significant effect on the
TS (
Table 4). The lowest (7.2%) and highest (12.1%) values of
TS were found in alder plywood samples made from non-densified (A
N–A
N–A
N–A
N–A
N) and densified (A
D–A
D–A
D–A
D–A
D) veneers, respectively. The mixed-species plywood samples using only densified veneers had a higher
TS than samples using only non-densified veneers (
Table 3,
Figure 1). Increasing the share of densified veneer in one panel led to an increase in the
TS of the mixed-species plywood samples. In this case, this was explained by the increase in the density of plywood samples. This is well known from sources in the literature [
12,
27,
38]. However, in this study, no clear dependence of
TS on sample density was found (
Figure 2b).
The surface roughness of plywood panels with outer layers of non-densified and densified birch and alder veneer was also compared (
Figure 3). In addition to the fact that plywood with outer layers of alder veneer had lower values of MOR and MOE (
Table 5), the surface of such plywood also had a greater surface roughness compared to the birch plywood (
Figure 3). It was found that for plywood made of non-densified alder veneer (A
N–A
N–A
N–A
N–A
N), the roughness parameters
Ra and
Rq were 19.4% and 5.4% greater, respectively, than the similar parameters for plywood made of non-densified birch veneer (B
N–B
N–B
N–B
N–B
N). On the contrary, the roughness parameter
Rz for plywood made of non-densified alder veneer was 18.3% lower than for plywood made of non-densified birch veneer. For plywood made of densified alder veneer, the roughness parameters
Ra,
Rz and
Rq were 41.6%, 0.4% and 14.2% higher than similar parameters for plywood made of densified birch veneer, respectively. This was logical, as birch wood has less porosity than alder wood. However, the differences in
Ra,
Rz and
Rq found between non-densified/densified birch and alder veneers were statistically insignificant. Nevertheless, the thermal compression led to a decrease in the surface roughness values for both the birch and alder veneers. In our previous work [
10], we also found that the surface of wood veneers became smoother and roughness values decreased significantly due to the thermal densification process.
3.3. Mechanical Properties of Plywood Samples
The mechanical properties of plywood samples, which were made by combining veneers of different wood species and different types of treatment in one panel, are summarized in
Table 5. The highest (119 MPa) and lowest (73 MPa) values of MOR were observed in plywood samples (B
D–B
D–B
D–B
D–B
D), a panel made of densified birch veneer, and (A
D–B
N–B
N–B
N–A
D), a panel with outer layers of densified alder veneer and inner layers of non-densified birch veneer, respectively. The highest (12,405 MPa) and lowest (7934 MPa) values of MOE were found in plywood samples (B
N–B
N–B
N–B
N–B
N), a panel made of non-densified birch veneer, and (A
N–B
N–B
N–B
N–A
N), a panel with external layers of non-densified alder veneer and internal layers of non-densified birch veneer, respectively. The highest (3.8 MPa) and lowest (2.2 MPa) shear strength values were observed in plywood samples (A
D–B
N–A
D–B
N–A
D) and (B
D–A
D–B
D–A
D–B
D), respectively. Thus, the highest shear strength was observed between densified alder veneer and non-densified birch veneer (A
D–B
N–A
D–B
N–A
D). The lowest shear strength was found between a densified birch and densified alder veneer (B
D–A
D–B
D–A
D–B
D).
The ANOVA analysis showed (
Table 6) that combining veneers of different wood species in one panel had a significant effect on the MOR and MOE, while combining different types of veneer treatment had an insignificant effect on the MOR and MOE. The mixed-species plywood panels manufactured from densified veneers had a higher MOR and MOE than panels made from non-densified veneers (
Table 5,
Figure 4). The mixed-species plywood panels made with outer layers comprising birch veneers (non-densified or densified) had higher MOR and MOE than panels made with outer layers comprising alder veneers (non-densified or densified). At the same content of birch and alder veneers in one panel (samples that included non-densified veneers B
N–A
N–B
N–A
N–B
N and A
N–B
N–B
N–B
N–A
N, or samples that included densified veneers B
D–A
D–B
D–A
D–B
D and A
D–B
D–B
D–B
D–A
D), plywood with outer layers of birch veneer had about 22% and 18% higher MOR and MOE, respectively, when compared to plywood with outer layers of alder veneer. The birch plywood panels made from non-densified or densified veneers had higher MOR and MOE than alder panels made from non-densified or densified veneers. Birch plywood samples made from non-densified (B
N–B
N–B
N–B
N–B
N) or densified (B
D–B
D–B
D–B
D–B
D) veneer had 31.5% and 34.8% higher MOR, and 34.7% and 32.1% higher MOE than alder plywood samples made from non-densified (A
N–A
N–A
N–A
N–A
N) or densified (A
D–A
D–A
D–A
D–A
D) veneers, respectively. This was explained by the higher strength and density of birch wood compared to alder wood. However, the difference between the MOR values for mixed-species plywood samples (B
N–A
N–A
N–A
N–B
N, B
N–A
N–B
N–A
N–B
N, B
D–A
D–A
D–A
D–B
D and B
D–A
D–B
D–A
D–B
D) made from non-densified or densified veneers was insignificant, so in practice, during the manufacture of birch plywood, alder veneers can be used to form the inner layers without deteriorating the MOR of the plywood. Similar results were obtained in the research conducted by Bal [
19], in which seven-ply LVL panels were manufactured from fast-growing poplar veneers and used as the inner layers, while eucalyptus veneers were used as the outer layers. The author demonstrated that using two eucalyptus veneers on the faces significantly increased the MOE (30%) and MOR (12%) in comparison to poplar-only LVL panels. In another work [
26] where seven-ply LVL panels were manufactured by combining higher-density Austrian pine veneers and lower-density Lombardy poplar veneers, it was found that as the share of pine veneers increased in the mixed-species panels, the MOR and MOE increased by up to 40% and 69% on average, compared to panels manufactured only with poplar. Xue and Hu [
17] also found that the strength of the LVL made of birch veneers on the outer surface was much greater than the LVL made of poplar veneers.
Figure 5 shows the linear dependence of the MOR and MOE of plywood samples on their density. These dependences were not strong, which indicates that, in addition to density, other factors, such as the veneer thickness, species and type and amount of adhesive used, affected the MOR and MOE. In another work [
39], it was also observed that the MOR and MOE of plywood panels made from densified and non-densified veneers increased when the density increased.
With an increase in the proportion of alder veneer in the inner layers of birch plywood made from non-densified veneers (B
N–A
N–B
N–A
N–B
N and B
N–A
N–A
N–A
N–B
N), the MOR and MOE increased, although its values were lower (97 MPa and 9765 MPa for B
N–A
N–B
N–A
N–B
N, respectively; 101 MPa and 11033 MPa for B
N–A
N–A
N–A
N–B
N, respectively) than the MOR and MOE values of 110 MPa and 12405 MPa, respectively, obtained for plywood made from birch alone (B
N–B
N–B
N–B
N–B
N). The addition of birch veneer to the inner layers of an alder plywood also led to an increased MOR and MOE. A similar trend was observed for plywood panels made from densified veneers. An increase in the proportion of densified veneer in one panel contributed to an increase in the MOR and MOE. Densified veneer had a higher density than non-densified veneer. It is known that MOR and MOE increases with increasing density [
39]. This was in good agreement with the linear relationship between density and both MOR and MOE obtained in this study (
Figure 5).
Combinations of different wood species and types of veneer treatment in one panel significantly affected the shear strength of plywood samples (
Table 6). In addition, the effect of wood species was stronger (
F = 12.048) than the effect of the type of veneer treatment used (
F = 9.562). It was found that there was no significant difference (
p > 0.05) in shear strength values (2.8 MPa and 2.9 MPa, respectively) between birch (B
N–B
N–B
N–B
N–B
N) and alder (A
N–A
N–A
N–A
N–A
N) plywood panels made from non-densified veneers. However, a significant difference (
p ≤ 0.05) in shear strength values (3.2 MPa and 2.5 MPa, respectively) was observed between birch (B
D–B
D–B
D–B
D–B
D) and alder (A
D–A
D–A
D–A
D–A
D) plywood panels made from densified veneers. When the cut was along the birch (the second sheet-layer of veneer in one panel), the strength was higher, while the strength was lower when the cut was along the alder (the second sheet-layer of veneer in one panel). However, in this study, all plywood composed of densified and non-densified veneers still met the requirements (>1.0 MPa) of EN 314-2 [
32]. This showed that all the combinations with high-density birch veneer and low-density alder veneer bonded well.
From the obtained experimental results, it can be observed that the addition of alder veneer to the inner layers of birch plywood had a positive effect on the shear strength, increasing it compared to the strength of a birch-only plywood made from non-densified veneer. Similarly, the addition of birch veneer to the inner layers of an alder plywood made from both non-densified or densified veneers led to an increased shear strength.
Higher values of shear strength were observed in mixed-species plywood panels with alternating sheets of densified and non-densified veneers in adjacent layers (panels B
D–A
N–A
N–A
N–B
D, A
D–B
N–B
N–B
N–A
D, B
D–A
N–B
D–A
N–B
D and A
D–B
N–A
D–B
N–A
D). In our previous work [
27], it was also observed that using densified veneers increased the mechanical performance of plywood panels, but worsened the
TS and
WA of panels.
Some authors [
1,
40] have affirmed that the density of wood is an important factor that affects the formation of an adhesive bond between veneers. In their opinion, low-density woods will absorb a larger quantity of adhesive, due to its higher porosity. Based on this, it can be assumed that when non-densified veneer is in contact with another non-densified veneer (N–N), more adhesive is absorbed by the two surfaces to be bonded and less adhesive remains on the surfaces to be bonded. Starvation bonding may occur in this case. When densified veneer is in contact with non-densified veneer (D–N), it can be expected that less adhesive will be absorbed by the densified surface (due to its lower porosity) and more adhesive will remain between the bonded surfaces. This will ensure a good bonding strength and prevent “hungry” bonding. In addition, it should be taken into account that birch and alder veneers have a different density and porosity.
Microscopic images of a bond line in birch and alder plywood are presented in
Figure 6 and
Figure 7.
Figure 6 shows that the birch wood had a smaller percentage of vessels compared to the alder (
Figure 6A and
Figure 7A). Wagenführ [
41] stated that the number of vessels in birch wood ranges from 40 to 60 per 1 mm
2, while in the case of alder wood the number of vessels is 75–145 per 1 mm
2; there was also a difference in the porosity of the woods, with 59% in birch compared to 71% in alder. The arrangement of the anatomical elements and their morphometric parameters influenced the resulting penetration of the glue from the bond line into the deeper layers of the veneer. The bond line in non-densified birch veneers was thin and the glue did not penetrate into the deeper layers of the glued veneers (
Figure 6B,C). The penetration of glue into the deeper layers of the densified veneer was additionally prevented by a layer of compressed fibers and vessels (
Figure 6F). The densification process of the veneer surface layers resulted in deformed anatomical elements and eliminated lumens. The densification of the surface layers of birch veneers was also manifested by a reduction in the surface roughness (
Figure 3).
Figure 7 shows a bond line in alder plywood made from non-densified and densified veneers. The alder wood had a higher number of vessels than the birch, and the wood fibers were thin-walled (
Figure 7A,B). The higher porosity of the alder wood, thus, enabled an easier penetration of the glue into the deeper layers of the veneer (
Figure 7B,C). In the case of densified alder veneers, there was also a distinct layer of deformed fibers and vessels. The densification of the surface layers, as in the case of birch, prevented the penetration of the glue into the deeper layers of the veneer, and the bond line was, therefore, thinner. In both types of wood panels, it was possible to observe distinct microscopic cracks that occurred when the veneer was peeled off (
Figure 6A,B and
Figure 7D).