3.1. Influence of Melt Temperature
The first parameter investigated in this paper is the melt temperature in injection molding, as this is one of the major processing parameters. Also, increasing the melt temperature is a widely used approach to overcome high viscosity in injection molding, especially for wood–plastic composites where the wood is, on the one hand, decreasing in apparent viscosity and, on the other hand, thermally sensitive, as we investigated for compounding [
5]. Therefore, it is important to characterize the melt temperature’s influence on the properties in injection molding.
The elastic modulus (
Figure 1) shows an increase with increasing wood particle share (due to the higher modulus of the wood particles compared with the polypropylene matrix). The melt temperature does not show a big influence on the elastic modulus overall—a slight decrease can be seen at the higher temperatures, but this difference is not very pronounced and is in the range of single-digit percentage values. In the case of the tensile strength, the same trend as that for the elastic modulus can be seen. With the increasing wood particle share, the tensile strength increases (due to the reinforcing effect of the wood and the presence of a compatibilizer that facilitates the proper adhesion of the matrix to the wood particles). While for temperatures up to 210 °C, the tensile strength is not influenced, at 230 °C, a decrease in tensile strength can be seen. The higher the wood content, the larger the reduction in the tensile strength, which leads to the conclusion that here some thermal degradation of the wood can be seen. This is in accordance with data from the literature on thermal wood modification, where wood was modified in a similar temperature range under air. The authors found some changes in hemicellulose content and an increase in carbonyl and carboxyl groups at treatment temperature of up to 210 °C [
17]. With this change in wood surface chemistry, there are less hydroxyl groups for the compatibilizer (MAPP) to interact with on the wood surface, therefore reducing the load transfer capability of the interphase between wood and polymer matrix. The results shown in
Figure 1 support this finding, as the interphase plays a more important role at higher strains, i.e., the region where the composite strength is measured, where a higher decrease can be seen, in contrast to the elastic modulus, which is measured at low strains, where the influence of load transfer plays a minor role.
Another trend of a decreasing property with an increasing melt temperature is presented for impact strength in
Figure 2. For the composites with 10 wt% of wood, a constant decrease with increasing temperature can be seen, while for the higher wood shares the impact strength decreases only at a melt temperature of 230 °C. This effect could be due to the fact, that at 10 wt% of wood, the influence of the wood on the impact strength is lower than for the higher wood shares, but that with the additional degradation due to the temperature, and coming with that a degradation of the wood particle surface and therefore less stress transfer as explained above, impact behavior can be lowered. This is supported by the lightness of the specimens (
Figure 2), which also decreases with increasing melt temperature, therefore indicating a thermal degradation of the wood particles, regardless of the wood particle share in the composites. This is also shown in
Figure 3, where the different specimens produced with the different wood shares and melt temperatures are depicted. With increasing melt temperature, the specimens become darker, which is also in accordance with the findings in the literature [
18]. Also, with increasing wood content, a difference in appearance can be seen; while for the composites with 10 wt% of wood, the plastic matrix still is somewhat dominating the appearance, at 30 and 50 wt%, the wood particles dominate the surface. This different appearance for the composites with 10 wt% is also the reason for the 30 wt% composites showing greater lightness (
Figure 2).
The question remains of whether the degradation of the tensile and impact strength at higher melt temperatures is caused by wood particle size reduction or the degradation of the interface between the wood and the polypropylene. Differences in wood particle size reduction due to the melt temperature are very unlikely, as at higher temperatures the viscosity of the polypropylene is reduced, and thus less shear is applied to the wood particles in the melt. This is also supported by the wood particle size measurements (
Figure 4), where a reduction in the particle size with the wood content was found, due to the increased wood–particle interactions, i.e., the solid–solid interactions, which give rise to breakage of the wood particles, as well as the overall higher apparent viscosity at higher wood content. Within one wood content, there is only low variation in wood particle size, but this is irregular and does not show any trend with the melt temperature. From that, we can conclude that it has to be the interface that is negatively influenced by the melt temperature—most likely that the thermal degradation is responsible for reducing the interaction between the compatibilizer and the wood particle surface, thus resulting in reduced strength properties. As stated above, in the case of the elastic modulus, this is less visible, as the modulus is recorded at low strains where less force is transferred from the matrix to the particles, while for the strength values, higher strains and therefore higher load transfer happens, which indicates a more pronounced influence of the interface.
3.2. Influence of Residence Time
The darkening effect is not only found with increasing melt temperatures, but also with an increased residence time (facilitated by a longer cooling time, which results in keeping the melt longer at the temperature in the cylinder of the injection molding machine) of the wood–plastic composites in the melt. We only looked at composites with 30 wt% wood here, as the lower wood content (10 wt%) still shows a more plastic appearance, while at 50 wt% the wood–particle interactions are much higher, therefore giving rise to a much darker specimen already at the lowest residence time setting.
As shown in
Figure 5, the lightness is reduced with increasing residence time, as the thermal degradation can take place for longer, but with a reduced effect at the higher residence times. We believe this behavior to be due to the fact that the surface of the wood particles starts to degrade at a certain temperature, but when the degradation mechanisms happen for a longer time, there is a saturation as the degradation reaches its thermodynamic equilibrium. At the investigated temperature, the tensile strength values yielded were within the margin of error compared to the standard setting (50 s residence time), i.e., approx. 40 MPa of tensile strength with a maximum scattering of ±1 MPa.
3.3. Influence of Dwell Pressure
Another influencing factor to be considered in injection molding is the dwell pressure, as this packs the melt in the mold and can, in some cases, lead to higher packing. A known effect of the use of wood particles in plastic processing is that, in these particles, the lumen (i.e., the empty space in the middle of the wood cells) collapses, and therefore the density of the wood in the composite is higher than that in solid wood. The maximum here is the density of the cell wall (approx. 1.5 g/cm
3), but usually wood particles exhibit densities of about 1.3 g/cm³ after injection molding, as found from experience and shown in the literature [
15]. Here, we only plotted the tensile strength and the composite density (
Figure 6). Again, the sample with 30 wt% of wood was chosen for the investigation, as in the higher wood contents, wood–particle interactions are much higher, and due to that there are limitations in the range of dwell pressures which can be applied, as these need to be high enough to fill the mold, and for the sample containing 10 wt% of wood the effects are less pronounced due the low wood content.
It can be seen that with the higher dwell pressure, a slight increase in tensile strength can be found, which we expect to result from packing more material into the mold. The composite density does not reflect that trend; the differences here are much less pronounced, as the difference between the densities with the different dwell pressures at melt temperatures up to 210 °C are negligible. The only difference found is that at 230 °C melt temperature, where the density is reduced—again indicating the thermal degradation of the wood particles and its influence on the interface between wood and matrix—where a higher dwell pressure yields a significantly higher composite density. We expect this effect to originate from the fact that the thermal degradation of the wood particles is producing some gases at the interface, and the higher dwell pressure helps to collapse these gas voids, at least partially.
3.4. Influence of Mold Wall Temperature
In the next section, the influence of the mold wall temperature was investigated. Here, we also added the unreinforced polypropylene, as, in contrast to the melt temperature, we expected it to have an influence on the cooling of and therefore on the structure formation in the material.
The elastic modulus (
Figure 7) of the different materials increases slightly with increasing mold wall temperature, and the same trend can be seen for the tensile strength. To investigate this in more detail, we performed linear fits of the data points and evaluated the slope of each fit as a measure for the increase in each property (
Table 1). It can be seen that with the increasing wood content, the slope decreases, which means that this effect is expected to originate from the polypropylene matrix and not from the wood reinforcement. To support this, we investigated the microstructure of the materials by cutting thin slices with the microtome and using polarized light microscopy to visualize the crystalline structure. This could only be performed for the unreinforced composites and the composites with 10 wt% of wood, as at higher wood contents it was not possible to cut a thin enough slice for transmitted light microscopy, as these slices were either thick enough for cutting but also thick enough that the wood particles restricted light going through the piece, or thin enough to lose their mechanical integrity while cutting.
Looking into the generated micrographs (
Figure 8), we see that with increasing mold wall temperature, the spherulite sizes also increase. This indicates a slightly higher crystallinity with higher mold wall temperatures, which also translates into an increase in mechanical properties. Nevertheless, it must be stated here that these influences are rather low and will most likely not be utilizable in terms of materials engineering and application. For the impact strength (
Figure 9), no clear trend is visible in relation to the mold wall temperature.