Abstract
This article presents an experimental investigation on developing wood plastic composites reinforced with continuous glass fibers in an extrusion process. The main role of wood component in wood plastic composite is mainly as a filler to reduce cost, and not reinforcing. Adding wood component causes a noticeable reduction in strength and toughness. This will hinder their usage in load-bearing applications. In this study, a novel method for production of hybrid wood plastic composites is introduced. This manufacturing process takes the advantages of a cost-effective and flexible production line. First, E-glass rovings were impregnated with a polymer in an especially designed (impregnating) die. Then, six melt impregnated glass rovings were integrated with wood plastic composites in a single step via an extrusion process. Another unique die was designed and manufactured to feed and position the impregnated glass rovings into a round shaped wood plastic composite profile. Wood content was changed in three levels to investigate its effect on the processability and the final properties of the hybrid composites. The experimentation on the extruded hybrid wood plastic composites proved the feasibility of the process and the relative uniformity of the roving positions in the extrudate. Three-point bending tests were conducted to evaluate the flexural properties. Improvements in strength, modulus, and toughness were dramatic.
Introduction
The incorporation of cellulosic materials into thermoplastics results in a new class of materials called wood plastic composites (WPCs). Recently, WPCs have experienced an outstanding growth due to their advantages such as ability to use recycled materials, recyclability, and reasonable increased stiffness. 1 The main usage of WPCs, nowadays, is in building industry such as decking, roofing, and door and window frames. 2 Generally, low strength and toughness (or impact strength) of these composites hindered their usage in the load-bearing applications. Further addition of wood to the composite, while making them more cost-effective and stiff, significantly compromises their strength and impact tolerance. There are many potential structural applications in material replacement for WPCs, an example of them is pallet. 3 To introduce WPC as a structural material, it is necessary to improve their mechanical properties, convincingly.
Literature survey shows that the research studies on the mechanical properties of WPCs are subjectively categorized to: the compatibilization of wood particles and polymer surfaces,4–9 type and geometry of wood,9–15 the effect of processing parameters,15–22 the type of polymer matrix,22–25 and usage of impact modifier, e.g. rubber.26–28 Recently, short glass fibers were incorporated into WPC29–37 to produce hybrid WPCs. Hybridizing glass fibers with wood flour (WF) leads to a composite having a balance of performance and cost.
The results of the published research works on improving the properties of WPCs are not still convincing in regards to the requirements for load-bearing applications. On the way toward strengthening WPCs to be used in structural application, recently WPCs were externally reinforced by fiber-reinforced polymer and metallic sheets. Significant improvements on flexural and tensile strength were obtained. In previous works,38–42 reinforcement sheets were bonded on top and bottom of the WPC surfaces by epoxy adhesives. Also, local heating at the interface of WPC and sheet was another way for bonding in the mentioned research works. Regarding that these reinforcing methods are semi-continuous, they are expensive and time consuming for mass production. Also by this method, surface texture of WPCs is lost. Besides, the application of this method for various profiles is questionable.
Profile extrusion is the most common process for production of WPCs. To benefit the ease and low-cost process of extruding, one objective of this study was to reinforce WPC in this process. Applying long fiber reinforcements can increase the mechanical properties of the composites much higher than the short fibers.15,43 This is related to the higher efficiency of stress transfer between fibers and matrix. But mechanical properties could be improved if the two following conditions are satisfied: (a) the fibers must be well distributed within matrix and agglomeration is minimized and (b) the breakage of long fibers during processing should be minimized. In the extrusion process of polymer composites, the two above conditions are incompatible with each other, where good mixing requires a severe compounding which consequently causes the fibers to break down to shorter ones.
Embedding continuous fibers in WPC profiles is the most effective usage of reinforcements to enhance properties, in a high order of magnitude. However, the method of embedding and attaining an acceptable product where the fibers are well positioned is highly challenging. The idea of continuous reinforcing in an extrusion process of WPCs is presented in this study where a feasibility study is carried experimentally. As a case study, a circular-shaped profile with a diameter of 14 mm is produced.
Materials and procedure
High-density polyethylene (HDPE) grade 5620 was used as the polymeric matrix, supplied by Arak Petrochemical Corporation, Iran. WF of oak species, sieved with a mesh size of 40 (420 µm), was used as the filler in three levels of weight ratios: 50, 60, and 70 wt%. Prior to extruding, WFs were dried in an oven at a temperature of 80°C for 24 h to minimize the moisture content. E-glass roving 2400 tex was used as the reinforcing fibers. Since the interface quality between the glass fibers and WPC presents a significant effect on the mechanical properties, an attempt was made to improve compatibility of the glass roving with the matrix (WPC). For this purpose, the roving was melt impregnated with the same HDPE grade via a laboratory impregnating die (Figure 1). The glass roving was passed through a melt chamber with a filament die located at the exit with a diameter of 2 mm. Glass weight percentage was measured to be 62 wt% via burning 10 g of the impregnated glass roving (IMGR) in an oven.
The equipment, designed and manufactured, for melt impregnation of glass roving.
Equipment
A schematic view of the process is illustrated in Figure 2. A laboratory, counter-rotating twin-screw extruder with a screw diameter of 62.5 mm and an L-to-D ratio of 22 was utilized. The temperatures of zones 1–3 were set at 170°C in all the experiments. The temperatures of zone 4 and the die were set according to the WPC compositions to yield appropriate surface quality. To investigate the effect of extrusion speed, two levels, 2 and 5 r/min, were considered in this study.
Schematic view of the extrusion process used in this study.
Die design
Regarding the high viscosity of WPC melt, it was predicted that in the extrusion process of hybrid WPC, the IMGRs could be pulled in by the melt flow with no extra puller facility (self-pulling system) as opposed to the thermoset pultrusion system, where a puller is required to pull the product due to presence of a low viscose resin.
A die with the similar concept to the wire coating die was designed and manufactured. Since the melt flow turns 90° with respect to the extruder, this configuration made flow balancing complicated. Literatures survey indicated that few research works investigated the design methodology of dies for WPCs.44–46 Based on the few published works, the die shown in Figure 3 was designed, balanced, and manufactured. The IMGRs (2) are entered into the die from a central hole at first segment of the mandrel (3) and then passes through a guide (8) where six orifices of 2 mm in diameter were drilled on the guide.
Schematic of the designed die: (1) WPC melt; (2) IMGR; (3) mandrel; (4) deflector; (5) flow balancer; (6) adjusting screw; (7) adjusting ring; (8) IMGR guide; and (9) shaping die.
To take the full advantage of the high mechanical strength of the reinforcements in flexural loading, IMGRs had to be located as close as possible to the surface of the WPC profile. The flow deflector (4) with α1 = 67°, two flow balancers (5) with α2 = 50° and 60°, three adjusting screws (6), and adjusting ring (7) were used to obtain a balanced flow.
For start-up, six IMGRs were manually passed into the guide. An adjustable ring was implemented to yield a uniform flow as it affects the uniformity of IMGRs positioning across the section of extrudate.
Three-point bending tests were conducted utilizing a universal testing machine, Instron 5500R. Span length and crosshead speed were set at 240 mm and 5 mm/min, respectively. To compare the mechanical properties, unreinforced composites were also extruded at the same conditions.
Results and discussions
Process parameters
Table 1 presents the summary of the results and the final appropriate adjustment of the parameters in this study. As it is shown, via decreasing the WF content, the temperatures had to be decreased to yield a suitable surface quality. A larger entering angle, α2, was resulted in for the composites with 60 and 70 wt% wood content to yield a balanced flow, which can be attributed to their plug flow behavior.
46
In Figure 4, samples of the produced hybrid continuous glass fiber/WPC (HWPC) profiles are shown.
Extruded HWPCs at 2 r/min: (a) HWPC50; (b) HWPC60; and (c) HWPC70. Parameters and the process-ability
As illustrated, for HWPC60 and HWPC70 with 60 and 70 wt% wood contents, respectively, the process resulted in a good surface quality. In the case of WPC with 50 wt% wood content, the obtained surface quality was highly poor, attempts to improve the quality failed. Literatures indicate that at a low WF content WPC, a surface fracture can occur in the extrusion processing.46,47 It is to be noted that no calibrator was used in this study as it could further complicate the process. Besides that, a higher amount of wood content is preferred due to its economic advantage.
As stated in this study, WPC melt flow could pull the IMGRs into the die, favorably. At a higher WF content (60 and 70 wt%), WPC melt behaves more like solid (plug flow) through the land section. Therefore, the melt pulling force could overcome the friction between the IMGRs and the guiding orifices. But for the WPC with 50 wt% wood content, the melt strength seemed to be too low to pull the roving in, and the melt slipped over the IMGRs.
Figure 5 shows HWPC60 extruded at a speed of 5 r/min. By increasing extrusion RPM, or flow velocity, the melt residence time in the land section of shaping die became too short to produce a strong skin, and a blistered surface was resulted.
48
As the length of the land section was maintained unchanged in this study, further research is required to investigate the effect of high-speed extrusion and the die land length on the production of the hybrid composite.
Gross surface texture at a higher extrusion speed for HWPC60.
The proper positioning of IMGRs into the extrudate is another important parameter affecting the mechanical properties. To detect the position of the IMGRs inside the produced extrudate, arbitrary specimens of 300 mm in length was selected and cut into pieces of 60 mm in length. Figure 6 shows the positions of IMGRs for the produced HWPCs. The distance of IMGRs from the rod center at each section was measured via an appropriate software. Figure 7 shows the average distance of the IMGRs from the center of cross sections, along the 300 mm of extruded HWPCs. The miniature unbalanced flow could be responsible for the non-uniform distances of IMGRs from the center. It must be mentioned that, while the designed die seemed to be balanced for the unreinforced WPC, it was shown that further attempt was needed to balance the die while inserting the continuous glass rovings. It revealed that the position of the guiding holes had to be changed to a larger distance to locate the IMGRs closer to the surface. Exploring the relation between the orifice positions on the guide and the final location of IMGRs through the extrudate could be a subject of research interest.
The positions of IMGRs in the extrudates: (a) HWPC60 and (b) HWPC70. Average distance of IMGRs from the rod center along the HWPCs.
Flexural tests
Figure 8 shows the ultimate flexural strength of the unreinforced and the produced hybrid WPCs. Embedding the continuous glass fiber into the WPCs resulted in an outstanding increase in flexural strength by 120% and 132%, respectively, for HWPC60 and HWPC70. Figure 9 shows flexural modulus calculated from the stress–strain curve slope at the strain range 0.1–0.4%. An improvement in flexural modulus by 19% and 28% were obtained for HWPC60 and 70, respectively. The mechanical properties of WPCs with 60 wt% of wood content are higher than those of WPCs with 70 wt% wood content. But it seems that reinforcing effect of continuous fibers is higher for the higher wood content WPCs (compared to the unreinforced WPCs). Due to the enormous increase in mechanical strength and toughness, and adequate increase in modulus, when applying continuous fibers, it seems that usage of a higher wood content of WPC is preferred for its lower cost. It is known that highly filled WPCs (above 60 wt% of wood content) exhibit a low strength and specially toughness, despite an increase in stiffness. When embedding the continuous fibers, even at a low percentages (a few rovings: here, six rovings), the strength increases so significantly that the inherent weakness of the highly filled WPCs is compensated to a large extent. Hence, adding more wood presents more cost effective HWPCs.
Flexural strength of the extruded unreinforced and HWPCs. Flexural modulus of the extruded unreinforced and HWPCs.
An interesting observation in bending tests was the evidence of non-catastrophic failure in the reinforced WPCs against the catastrophic fracture in the unreinforced ones. Figure 10 shows the flexural load–deflection curves of the both composites. As illustrated, on the contrary to the unreinforced WPC60 that exhibited a sudden breakage (sudden drop of the force at the highest load), HWPC60 exhibited a gradual loss of load from the peak, point B to point C. This behavior enables the HWPC to be used for the applications where safety requirement is more severe. Besides, it is evident that the HWPC60 endures higher load at point B than that of Point C, which is a signature of a tough material. Figure 11 shows the pictures of the fractured specimens at points A and C, respectively, for WPC60 and HWPC60.
Flexural load–deflection curves obtained from the three-point bending test for unreinforced WPC60 and HWPC60. Fractured specimens: (a) at point A for WPC60 and (b) at point C for HWPC60.
Toughness is the ability of the material to absorb energy and is characterized by the area under the load–deflection curve. The areas measured for WPC60 and HWPC60 shows an outstanding improvement, by 1970% (about 20 times larger), for the HWPCs. The fracture behavior of these hybrid composites opens a wide spectrum of structural applications where load-bearing requirements are to be met.
Conclusions
Manufacturing and evaluating the HWPCs in extrusion process was experimentally investigated. Impregnated glass fibers were first produced in a twin-screw extruder via an especially designed die and then fed into to another extrusion system to produce hybrid WPCs. Three levels of wood loading were applied and flexural tests were performed. The experimental results present the following conclusions:
The results showed that the WPCs with the wood contents of 60 and 70 wt% behave well in the process, while the WPC with a wood content of 50% causes inappropriate extrudate for reinforcing purpose. The main reason was the ability of the higher wood content WPCs in pulling the melt IMGRs into its matrix (due to their plug flow behavior). Some deviations were observed regarding the position of the glass rovings in the perfect and planned locations. The deviations of reinforcement fibers from the desired diameter along 300 mm length of the extrdates were acceptable. Three-point bending tests on the extruded unreinforced and hybrid composites showed 2.19 and 2.32 times improvement on the flexural strength, respectively, for HWPC60 and HWPC70. This reinforcing mechanism was more effective for higher wood content WPCs. An outstanding improvement of about 20 times for the absorbed energy was obtained via continuous reinforcing the WPCs.
Footnotes
Funding
This research was funded by the Tarbiat Modares University, Iran.