Processing Influence on the Properties of Injection-Molded Wood Plastic Composites

Abstract

Wood–plastic composites (WPCs) utilize wood particles as the reinforcing phase. These particles are susceptible to thermal degradation, which can happen while processing the WPCs in usual thermoplastic processes. In this work, we investigated the influence of different processing parameters in injection molding and their influence on WPC properties. To achieve that, WPCs with wood contents ranging from 10 to 50 wt% were processed using different process settings, and then characterized using mechanical testing and appearance changes. We found that the melt temperature showed a major influence, due to degrading the interface between the wood and the polymer matrix, while other parameters, like mold temperature and dwell pressure, showed only minor influence. Overall, the WPCs exhibited good process stability and, with proper process settings, their performance can be improved.

1. Introduction

Wood–plastic composites (WPCs) are composites utilizing wood as the reinforcing phase, where thermoplastic (or sometimes thermosetting) polymers are usually used as a matrix. Typically, other additives, e.g., compatibilizers for improving the interaction between wood and the matrix polymer or stabilizers, as well as other reinforcements like inorganic fillers, can be added to tailor the material’s properties. The wood particles, often referred to as saw dust or wood fibers, are usually either a byproduct from lumber processing, or are produced in wood mills from trees which cannot be used for lumber production. In such wood mills, a variety of different wood grades and pulp fibers are produced, which are utilized for various applications [1,2,3]. The processing of such wood–plastic composites can be performed via commonly used thermoplastic material processes, like injection molding or extrusion [4].

In the processing of wood–plastic composites, different mechanisms can influence the different constituents of the composite or their interactions, which further influences the composite’s properties. When using wood as a reinforcement, one aspect to be considered is the sensitivity to temperature, as wood exhibits thermal decomposition within the range of temperatures necessary for processing the respective plastics. This was shown in earlier works from our group, where the influences of compounding and recycling were investigated [5,6]. Another work investigated the difference between injection molding and extrusion, finding higher levels of the properties with injection molding, due to the higher pressures and shear applied [7]. The process of injection molding itself has not been widely researched for WPCs. Some literature can be found on foaming in injection molding [8,9], and on the differences in properties between compression- and injection-molded WPCs [10]. Also, overmolding (ply)wood with plastic has been investigated, giving additional insight into the interfacial interaction between wood and plastic [11]. Another publication described the issues and challenges in the injection molding of a complex part. Besides proper mold and sprue design, a minimum mold temperature is necessary to fill the mold. Further, pre-drying is of essence when producing compact WPC parts without surface defects [12]. Another work reported, alongside formulation influences, on the influence of melt temperature on the properties of polyethylene-based WPCs. The authors found that with increasing temperature, a slight reduction in elastic modulus and strength could be found [13].

In the available literature, there is a lack of investigation into how processing parameters can influence WPC properties. This is not only relevant for primary processing to mold products, but also in regard to reprocessing production waste and even in the recycling of such materials. Therefore, the aim of this work was to investigate the influence of processing parameters on the properties of injection-molded wood–plastic composites. For that, different parameters like melt and mold temperature, as well as residence time, were varied, and their effects on the mechanical properties and the color of the wood–plastic composites were characterized.

2. Materials and Methods

2.1. Materials

For the matrix polymer, we used a polypropylene (PP) homopolymer injection molding grade, with a melt flow rate of 8 g/10 min (grades HD120MO, Borealis, Vienna, Austria). The MFR values were given for 230 °C and 2.16 kg piston weight. To improve the interaction between the PP and the wood particles, a maleic anhydride grafted polypropylene (MAPP, type Exxelor PO1020, Exxon Mobile Chemical Company, Houston, TX, USA) was used as a compatibilizer. For the reinforcement, softwood particles with a 125–180 µm particle size (type CB200, LaSoLe Est., Percoto, Italy) were chosen. The different composite formulations produced in this study are given as weight percentages (wt%) of the components.

2.2. Composite Processing

With these materials, three formulations were realized, i.e., 10 wt% wood + 1 wt% MAPP + 89 wt% PP, 30 wt% wood + 3 wt% MAPP + 67 wt% PP and 50 wt% wood + 5 wt% MAPP + 45 wt% PP. For the sake of clarity, the formulations are referred to as WPC with 10, 30, and 50 wt% of wood, respectively. To produce these mixtures, a co-rotating, intermeshing 24 mm twin-screw extruder (TSE24MC, ThermoPrism, Karlsruhe, Germany), with a processing length of 40 L/D, a throughput of 12 kg/h at a screw speed of 300 rpm, and a maximum barrel temperature of 220 °C was applied. The wood particles were dosed into the already molten PP (with MAPP) after 8 L/D by means of a side-feeder. The extruded strands were cooled in a water bath and afterwards cut to 4 mm long granules using a strand cutter. Subsequently, a drying step (hot-air-drying hopper Luxor 120, Motan, Konstanz, Germany) was carried out at 80 °C for at least 6 h.

Injection molding was carried out to yield a universal test specimen (specimen geometry determined according to ISO-527 [14]) using a conventional injection molding machine (Victory 80, Engel, Schwertberg, Austria). As we investigated the influence of injection molding processing, we varied the melt temperature between 170 and 230 °C in 20 °C steps, as well as varying the mold temperature (15, 40 and 70 °C) and the dwell pressure (225 and 445 bar). To increase the residence time of the material in the barrel, we increased the cooling time from the starting value of 50 s (the standard setting for all other injection molding trials) up to 1800 s (i.e., 30 min).

2.3. Materials Characterization

For characterization, tensile properties were recorded in accordance with ISO-527, with a crosshead speed of 1 mm/min until the determination of the elastic modulus, and afterwards with 5 mm/min until the break of the samples on a 20 kN universal test machine (Z020, Zwick-Roell, Ulm, Germany). Strain was recorded by a macro-extensometer clamped onto the samples. For each sample series, 5 replicates were tested. Unnotched Charpy impact properties were recorded on 5 replicates of a prismatic specimen (80 × 10 × 4 mm³), punched out from the parallel part of the universal test specimen in accordance with ISO-179 [15] on a pendulum impact tester (5113.300, Zwick-Roell, Ulm, Germany). Color measurements were performed on the shoulders of the universal test specimen using a spectro-guide sphere gloss (BYK Additives and Instruments, Wesel, Germany) to measure color in the Lab color sphere. Three replicates were measured per sample. Density was measured according to ISO 1183-1 [16] (in pure ethanol) using a density kit (YDK01) and a scale (M-pact AX224), both obtained from Sartorius, Göttingen, Germany. Prismatic specimens measuring approx. 40 × 10 × 4 mm³ (halves of the specimen used for impact testing) were used, and for each mixture three replicates were tested.

The fiber length of the selected samples was determined by measuring at least 500 fibers on microscopic pictures of the wood particles (microscope BX61, Olympus, Tokio, Japan), after their extraction from the parallel part of the universal test specimens. Extraction was carried out in boiling xylene (100 mL xylene, technical grade, per 1 g of WPC sample) under reflux for 4 h. After that, the whole sample was vacuum-filtered and washed with hot xylene to yield the wood particles. After drying the wood particles (at 105 °C for 2 h in a hot air cabinet), the samples were spread on microscopic slides and fixed using an aqueous mounting agent (Aquatex^®, Merck kgaA, Darmstadt, Germany) before measuring. To investigate the crystallinity, crossed polarizers in the above-mentioned microscope were used to investigate 20 µm thin sections in transmitted light. The thin sections were prepared using a microtome equipped with a stainless steel blade (RM2255, Leica, Wetzlar, Germany), and were fixed to a microscopic slide with the same aqueous mounting agent as above.

All the results generated are given in the following sections as the average value, plotting one standard deviation in the positive and negative direction of the value as error bars. In cases where these error bars are not visible, the error bars are smaller than the symbol size used.

3. Results & Discussion

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/cm³), 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. Slopes from linear fits of the elastic modulus and tensile strength vs. the wall temperature (Figure 7) in correlation to the wood content.

Wood Content [wt%]	Slope (Elastic Modulus) [MPa/wt%]	Slope (Tensile Strength) [MPa/wt%]
0	3.50	0.0619
10	2.15	0.0228
30	3.60	0.0404
50	1.49	0.0167

). 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.

4. Conclusions

In this paper, we investigated the effects of different injection molding parameters on the properties of WPCs with different amounts of wood particles. We found that the dwell pressure only had a minor influence in the investigated region, as the lumens of the wood particles had already collapsed to a certain extent because of the lower pressure and the processing during compounding. The melt temperature showed a major influence on the WPC properties, mainly on the tensile strength, which was due to the influence of the interface, as the wood particles did not change size at different injection molding temperatures. Also, the melt temperature influenced the lightness of the composites due to the darkening of the wood through thermal degradation. With increasing mold temperature (and constant residence time), the mechanical performance of the WPC was increased slightly due to crystallization effects of the matrix. This effect was more pronounced at lower wood contents, as the crystallization only took place in the matrix.

Overall, we found that the effects in injection molding were only minor, which showed the process stability of the WPC in the investigated wood shares up to 50 wt%. Also, although the single effects were only minor, in combination they could make a difference to the performance of the WPC, and therefore could be utilized to improve WPC performance via injection molding settings.