Abstract
The processing temperature of poly(ethylene terephthalate) waste (rPET) and ultra-high molecular weight polyethylene (UHMWPE) was lowered by blending to avoid decomposition of rice husk fiber (RHF) during the manufacturing of wood plastic composite (WPC). Maleic anhydride (MA) grafted blends of HDPE/LDPE/UHMWPE-g-MA and rPET/LDPE-MA were prepared. Those blends were flowable at 240 °C. They were employed as blended matrices to make HDPE/LDPE/UHMWPE-g-MA/rPET/LDPE-MA/RHF wood composite at processing temperatures not exceeding 240 °C. The study of RHF loading on the WPC performance revealed that melt flow index (MFI) and mechanical performances measured by impact, flexural, and tensile properties were weakened, but heat distortion temperature (HDT) was enhanced at high RHF loading. When the stabilizer content did not exceed 2 phf, the toughness and ductility were improved. Surface treatment of RHF by MA and dicumyl peroxide (DCP) enhanced the interfacial surface adhesion, but the toughness and ductility of the WPC were reduced at high MA/DCP dosing. The formation of crosslink structure via peroxide free radical initiated reaction at the MA grafted branch chains was the prime suspect for the inferiority of the mechanical performances.
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Manufacturing of Wood Plastic Composite from Polyethylene/Poly(ethylene terephthalate) Waste/Ultra High Molecular Weight Polyethylene Blends and Rice Husk Fiber: Raw Materials Preparation and Preliminary Processing Investigation
Utai Meekum
The processing temperature of poly(ethylene terephthalate) waste (rPET) and ultra-high molecular weight polyethylene (UHMWPE) was lowered by blending to avoid decomposition of rice husk fiber (RHF) during the manufacturing of wood plastic composite (WPC). Maleic anhydride (MA) grafted blends of HDPE/LDPE/UHMWPE-g-MA and rPET/LDPE-MA were prepared. Those blends were flowable at 240 °C. They were employed as blended matrices to make HDPE/LDPE/UHMWPE-g-MA/rPET/LDPE-MA/RHF wood composite at processing temperatures not exceeding 240 °C. The study of RHF loading on the WPC performance revealed that melt flow index (MFI) and mechanical performances measured by impact, flexural, and tensile properties were weakened, but heat distortion temperature (HDT) was enhanced at high RHF loading. When the stabilizer content did not exceed 2 phf, the toughness and ductility were improved. Surface treatment of RHF by MA and dicumyl peroxide (DCP) enhanced the interfacial surface adhesion, but the toughness and ductility of the WPC were reduced at high MA/DCP dosing. The formation of crosslink structure via peroxide free radical initiated reaction at the MA grafted branch chains was the prime suspect for the inferiority of the mechanical performances.
DOI: 10.15376/biores.18.1.1293-1329
Keywords: Wood plastic composite; MA grafted blended matrices; Processing Temperature; Performance Properties
Contact information: Institute of Engineering, Suranaree University of Technology, Maung, Nakorn Ratchasima, Thailand; *Corresponding author: umsut@g.sut.ac.th
GRAPHICAL ABSTRACT
INTRODUCTION
For the material science aspect, wood-plastic composites (WPCs) are materials manufactured from thermoplastics reinforced with natural fibers. For industrial applications, they are used as a substitute or alternative for natural wood, the usage of which is widely prohibited or legally controlled by many countries. Building and decorative materials, such as outdoor decks, floors, windows, and doors, are found to be their main applications. Polyolefins, such as poly(vinyl chloride) (PVC), poly(propylene) (PP), and poly(ethylene) (PE), are commonly used as matrices. In science and engineering considerations, there are advantages and disadvantages to these WPC matrices. For example, PP is easy to process, but it has a low ultraviolet radiation (UV) resistance. The PE offers decent melt processing, but it has a low service temperature. The PVC has an outstanding resistance to environmental deterioration, but it is harmful to operator due to the emitted toxic fume during melt extrusion. There have been many research studies and attempts to overcome these weaknesses.
There are numerous reports on PP/wood flour WPC made with various wood flour particle sizes (Hubbe and Grigsby 2020). The mechanical properties of composites depend on the fiber aspect ratio (Stark and Rowlands 2003). Additionally, the effect of fiber types on the mechanical properties of WPCs were studied and published (Zaini et al. 1996; Febrianto et al. 2006; Khan et al. 2009). Improving their long-term properties by grafting, crosslinking and matrix blending, fiber modification, and adding high performance fillers were among the most typical methods that were studied and found in previous literature (Bengtsson and Oksman 2006; Lei et al. 2007; Clemons 2010). Silane grafting followed by a water crosslink reaction of the polymer matrix to form a loosely macro crosslink chain, especially with PP and PE, has received much attention in both industrial applications and fundamental research. The chemical mechanism of the crosslink reaction was demonstrated (Zhou et al. 2009). The published work has revealed that macroscopic crosslinking can provide obvious advantages, such as easy processing, low capital investment, and favorable properties in the processed materials (Sirisinha and Kawko 2005; Meekum and Khongrit 2018). Vinyl silane was chemically grafted onto the polymer chain by free radicals using peroxide as an initiator. Then, it was hydrolyzed and condensed to create –Si–O–Si– bonds between the chains and/or bonding between the wood and polymer. The macro crosslink, via silane bridges, results in outstanding performance properties (Meekum 2014; Meekum and Kingchang 2017).
The flammability, fracture toughness, and impact resistance of WPCs are important for different bending loaded applications. To achieve the highest load bearings possible, various modifications such as toughening of the matrix with a stiffer polymer, rubber toughening, hybridization of the natural fiber with engineering reinforcement, and/or filling with inorganic particles have been studied (Hristov et al. 2004). Polymer blends have been successful in improving the properties of WPCs such as impact strength, tensile strength, environmental stress cracking, low temperature impact properties, and more. (Wang et al. 1995; Pearson et al. 2000; Lei and Wu 2010; Sudár et al. 2016; Zadeh et al. 2017). Treatment or modification of the fiber/filler is one of the main ways to enhance the properties of WPCs (Schirp et al. 2014; Koohestani et al. 2017).
The usage of environmentally friendly materials and the circular economy are current mega trends for industrial product marketing, especially for interior and exterior construction applications. Accordingly, the utilization of polymeric waste and cellulosic fiber by-product from agro-industry for manufacturing of WPC have gained attention by both researchers and industries especially for the bio-circular green economic concept (Adhikary et al. 2008; Leu et al. 2012; Chen and Ahmad 2017).
In this research, WPC was manufactured from the polymer blends of virgin high density PE(HDPE)/low density PE(LDPE)/poly(ethylene terephthalate)(PET) obtained from drinking bottle waste, which was toughened with ultra-high molecular weight PE (UHMWPE) and reinforced with rice husk fiber (RHF). UHMWPE and PET are known as high melting temperature polymers. Processing temperatures over 300 °C are typically employed. RHF, a type of cellulosic fiber, is easily decomposed at a critical temperature above 250 °C. Hence, in the manufacturing of WPC having PET, UHMWE, and RHF as main ingredients, lowering the processing temperature of PET and UHMWPE without thermal decomposition of RHF was one of the main important challenges of the research work. The matrix blending innovation was the key to overcoming those challenges. Preparation and characterization of the blended matrices raw material were evaluated, reported, and justified in this work. One of the ambitious industrial applications from this work is interior and exterior construction products manufactured by extrusion processes. The basic research procedures to obtain the most appropriate WPC formulation, both mechanical performance and economic aspects, were also included in this publication.
EXPERIMENTAL
Materials
Polymeric materials
Commercial blown film grade HDPE (HD7000F) was kindly supplied by PTT Global Chemical Plc. (Bangkok, Thailand). LDPE (LD1905F) was kindly supplied by SCG Chemical Co. Ltd. (Bangkok, Thailand). Both PEs were employed for matrices blending. PET flake from post-consumer drinking bottles were collected from a local disposal site (Nakorn Ratchasima, Thailand). It was machine shredded into small flakes and thoroughly cleaned by detergent water. It was denoted as rPET. UHMWPE (SUNFINE UH900) as toughener was supplied from Asahi Chemical Industry Co., Ltd. (Osaka, Japan). The recommended melt processing temperatures and HDTs for the polymeric materials employed in this work are summarized in Table 1.
Filler, fiber reinforcement, and additives
The talc filler (Jetfine® 8CF) with the averaged particle size of 1.1 µm was supplied from Imerys Talc Luzenac France (Luzenac sur‐Ariège, France). Dicumyl peroxide (DCP) was employed as a free radical initiator. It was supplied by Thai Poly Chemical Ltd. (Sumutsakorn, Thailand). 99% purity maleic anhydride (MA) as co-initiator was purchased from Acros Organics (Glee, Belgium). A mixed powder at 1:1 by weight containing Tris(2,4-di-tert-butylphenyl) phosphate (Irgafos 168) and Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl) propionate (Inganox 1076) was employed as the heat/processing stabilizer. Those two chemicals were supplied from Ciba Specialty Chemicals Corp (Tarrytown, NY, USA). All chemicals employed were used as received. Rice husk hull from a local rice mill (Nakorn Ratchasima, Thailand) was ground into a fine powder rice husk fiber (RHF) with a hammer mill machine. The RHF was obtained and collected via a sieving machine with the mesh size ranging from 100 – 500 microns. The RHF was vacuum dried at 105 °C for at least 3 h prior to melt compounding into WPC.
Table 1. Summary of the Recommended Melt Processing Temperature for HD7000F, LD1905F, UHMWPE (SUNFINE UH900), and PET
HDPE/LDPE/UHMWPE-g-MA and rPET/LDPE-g-MA preparation
Table 2 summarizes the formulation and also batch size for preparing HDPE/LDPE/UHMWPE-g-MA and PET/LDPE-g-MA as the matrices blends of the WPC in this work. For the HDPE/LDPE/UHMWPE-g-MA melt compounding, the calculated batch size quantity of 1000 g of HDPE, 500 g of LDPE, 12.0 g of MA pellet, and 1.2 g of DCP was placed in a sealed plastic bag. The mixture of ingredients was placed in the oven at 100 °C for 2 min. After MA and DCP were completely liquidized, the plastic pellets mixture were immediately transferred into a high-speed mixer chamber (LabTech Engineering Co., Ltd, Samutprakarn, Thailand). Then, 400 g of talc and 250 g of UHMWPE powder were added into the chamber. All solid ingredients were mixed inside the high-speed mixer chamber for 2 min at approx. 5,000 rpm. The polymer pellets mixture was then constantly fed via a single screw feeder into a co-rotation twin screw extruder equipped with a section of three triple kneader disks and 25 mm in diameter and L/D = 20 screws (Brabender Model PL2100; Brabender®GmbH & Co. KG, Duisburg, Germany). The barrel temperature profiles were electrically controlled at 230, 240, 250, 265, and 230 °C, from the feed to die zones, respectively. The screw speed was constantly controlled at 25 rpm. The extrudate strand was cooled by air blowing.
Table 2. Blending Formula of (a) HDPE/LDPE/UHMWPE-g-MA and (b) rPET/LDPE-g-MA
For the rPET/LDPE-g-MA preparation, similar compounding procedures were adopted. rPET flake was vacuum dried at 100 °C for 8 h prior to melt compounding. The calculated batch size consisted of 1000 g of vacuum dried rPET flake, 500 g of LDPE, 5.0 g of MA pellet, and 0.50 g of DCP. These were placed in a sealed plastic bag. Then, the ingredient was placed in the oven at 100 °C for 2 min. After MA and DCP were completely liquidized, the mixture ingredient was immediately transferred into a high-speed mixer chamber and vigorously mixed for 2 min. The polymer pellets mixture was then melt compounded on a co-rotation twin screw extruder at barrel temperature profiles of 230, 240, 250, 265, and 230 °C, from feed to die zones, respectively. The rPET/LDPE-g-MA strand was cooled and pelletized.
WPC manufacturing and specimen preparation
The WPC manufacturing was performed on a twin screw compounder as schematically summarized in Fig. 1. In accordance with the designed WPC formulation, the solid mixture comprised of HDPE/LDPE/UHMWPE-g-MA, rPET/LDPE-g-MA, RHF and talc was pre-blended and vacuum dried at 90 °C for 3 h. Then, it was vigorously mixed by a high-speed mixer. The solid ingredient was then constantly fed via single screw feeder into a co-rotation twin screw extruder at the barrel temperature profiles of 220, 225, 230, 240, and 230 °C from feed to die zones, respectively. The melt compounding was achieved at a screw speed of 25 rpm. The resultant WPC strand was cooled down and pelletized. The wood composite pellet was lowered in moisture content at 90 °C in a vacuum oven for 3 h before injection molding into a standard test specimen. The injection molding machine, CLF-80T (Chuan Lih Fa Machinery Works Co. Ltd., Tainan, Taiwan), was employed. The barrel temperature profile was electrically controlled at 190, 200, 210, and 220 °C from the feed to nozzle zones, consecutively. The mold temperature was set at 45 °C with a cooling time of 25 s. The obtained test specimens were annealed at atmospheric room temperature for 24 h prior to performing testing.
Fig. 1. Schematic diagram for manufacturing WPC and test specimens
Material Characterization and Testing
The mechanical properties include three-point flexural bending, tensile, and Izod impact strengths (both notched and unnotched modes). Testing was carried out in accordance with ASTM D790-10 (2010), ASTM D638 (2014), and ASTM D256-10e1 (2010), respectively. A universal testing machine (Instron Model 5565, Norwood MA, USA) with load cell of 5 kN was employed. The flexural span length at 80 mm and crosshead speed of 15 mm/min were assigned. The standard test specimen size of 3.2 mm x 12.7 mm x 125 mm was used. For the tensile testing, the strain rate at 15 mm/min by means of the crosshead speed was also electronically controlled. The ASTM Type I dumbbell shape standard specimen made from the above injection molding process was employed. The pendulum impact testing machine (Instron Ceast Model 9050) equipped with striking impactors was employed. The impactor having the striking energy of 2.16 and 11.0 Joules was used for notched and unnotched impact strength measurements, respectively. The injected molded standard test specimen size of 3.2 mm x 12.7 mm x 60 mm was employed. The V-notch on the sample was made by using the standard notching machine.
The heat distortion temperature (HDT) was examined using an Atlas testing machine (HDV1, Atlas Material Testing Technology LLC, Mount Prospect, IL, USA), and ASTM D648-07 (2007) was followed with a 455-kPa standard load. The exact specimen dimension to those flexural testing was employed. The melt flow index (MFI) was performed in accordance with ASTM D1238-13 (2013) using a Kayeness melt flow indexer (Dynisco, Inc., Franklin, USA) at 230, 240 or 250 °C and 5.0 kg load. The durability of the RHF wood samples by means of % water absorption and % thickness swelling after 1- and 7-day water submersion periods, were measured in accordance with ASTM D570-98(2010)e1 (2010). The specimen having the identical dimension to the unnotched impact testing was employed. The morphological observation by mean of scanning electron microscope (SEM) was conducted at the accelerating voltage of 15 keV (JSM 6400, JEOL, Tokyo, Japan). The Mettler Toledo TGA/DSC1, Star System (Schwerzenbach, Switzerland) was used to measure the decomposition temperature (Td) of RHF. The IR spectra at 1 cm-1 resolution was acquired on the thin film sample. The T27/Hyp2000 FTIR was equipped with attenuated total reflection (ATR) sampling technique (Bruker Scientific LLC, MA, USA).
RESULTS AND DISCUSSION
Characterization and Testing of Raw Materials
The main raw materials for manufacturing WPC were HDPE/LDPE/UHMWPE-g-MA, rPET/LDPE-g-MA, RHF, and talc. They were in-house prepared. One of the main challenges for this innovative research was utilizing as low a processing temperature as possible, without sacrificing the mechanical properties, to manufacture WPC from high processing temperature polymeric matrices to avoid fiber reinforcement decomposition. As summarized in Table 1, the recommended processing temperature of PET and UHMWPE is normally above 300 °C. However, the degradation temperature (Td) of cellulosic natural fiber is typically below 240 °C. On the fiber with high lignin content, the Td below 200 °C is common. Accordingly, the upper limit melt compounding temperature for manufacturing WPC from high melting point polymer matrices with the natural fiber reinforcement is restricted by its Td. Thermal decomposition of natural fiber during WPC manufacturing will cause dramatic mechanical/physical properties deterioration. One of the alternative solutions to overcome this processing burden is by compatible blending the high melting point polymer matrix with low melting point polymer constituent as adopted in this work. By such a method, the flow ability, compatibility, and mechanical properties of the resultant blends and eventually the final WPC product must be confirmed by testing.
Figure 2 shows the TGA thermogram of RHF reinforcement and talc filler employed in this work. The Td window of RHF was 200 to 250 °C. Above 250 °C, the RHF decomposition process indicated by the weight loss was obviously observed. The allowed upper limit for processing of RHF as reinforcement for WPC manufacturing must be below 250 °C. For the talc filler, the TGA thermogram revealed that the employed talc filler showed no sign of the weight loss at temperatures up to 400 °C.
Figure 3 contains micrographs of the hammer mill ground RHF and talc filler employed for manufacturing the WPC. The fiber lengths of the ground RHF ranged from 145 to 500 m. Rice hull fiber was the main composition of RHF. For the talc filler, the thin flat flake with an approximate width of 10 m was visualized.
Fig. 2. TGA thermogram of ( ) RHF and (- -) Talc Filler
Fig. 3. SEM Photos of (a) RHF and (b) Talc filler
For the polymer processing aspect, MFI is the most common and basic melt flowability or rheological measurement of polymer under the static torque loading. It is a fundamental material parameter used by polymer engineers to select the most suitable processing method to fabricate the polymeric into product. Table 3 summarizes the flowability measured as MFI at 5.0 kg standard load with the assigned melting temperatures at 230, 240, and 250 °C.
Table 3. MFI Test Result at the Given Temperature of the Raw Material for WPC
HDPE and LDPE melted and flowed at all given temperatures, but very low flowability of rPET was observed at only the melt temperatures above 250 °C. In contrast, UHMWPE indicates no flow (NF) at all assigned temperatures. These MFI results confirmed that the processable temperature of rPET and UHMWPE employed as matrix and toughener, respectively, for manufacturing the WPC in this study must be higher than 250 °C. Especially for the UHMWPE, melt processing temperature beyond 300 °C must be engaged.
For the rPET/LDPE-g-MA blend, the MFI of 0.021 g/10 min was observed at 240 °C and it was increased to 7.332 g/10 min at 250 °C. Hence, the minimum processable temperature of the rPET/LDPE-g-MA blend at 240 °C was suggested. Similarly, the HDPE/LDPE/UHMWPE-g-MA blend at 240 °C was at 0.056 g/10 min and it marginally increased to 0.073 when the melt temperature was increased to 250 °C. For the screw operated polymer processing machines, melting is induced by conduction heat (heated barrel) and viscous heat induced by torque shearing between screw/polymer and fluid/barrel during the screw revolution. This is the main and important source of melting energy. For such thermodynamic principle, melt compounding of the WPC comprised of rPET/LDPE-g-MA, HDPE/LDPE/UHMWPE-g-MA, and RHF reinforcement to carry out manufacturing on the twin screw extruder at a temperature that does not exceed 240 °C is manageable. Hence, the thermal decomposition of the RHF reinforcement can be avoided.
For the rPET/LDPE-g-MA blend, the MFI of 0.021 g/10 min was observed at 240 °C and it was increased to 7.332 g/10 min at 250 °C. Hence, the minimal processable temperature of the rPET/LDPE-g-MA blend at 240 °C was suggested. Similarly, the HDPE/LDPE/UHMWPE-g-MA blend at 240 °C was at 0.056 g/10 min, and it marginally increased to 0.073 when the melt temperature was increased to 250 °C. For the screw operated polymer processing machines, melting is induced by conduction heat (heated barrel) and viscous heat induced by torque shearing between screw/polymer and fluid/barrel during the screw revolution is the main and important source of melting energy. For such thermodynamic principle, melt compounding of the WPC comprised of rPET/LDPE-g-MA, HDPE/LDPE/UHMWPE-g-MA, and RHF reinforcement to manufacture on the twin screw extruder at a temperature that does not exceed 240 °C is manageable. Hence, the thermal decomposition of the RHF reinforcement can be minimized.
Infrared Analysis
Both the HDPE/LDPE/UHMWPE-g-MA and rPET/LDPE-g-MA blends were manufactured via single-step melt compounding on the twin screw extruder. Then, the MA would be randomly grafted onto individual polymer chains. The presence of MA-grafted polymer chains was characterized by FTIR. The possible unreacted MA residual in the blends was removed by precipitation method prior to conducting the test. The polymer blend was dissolved in hot xylene and then precipitation in methanol. The FTIR spectra at the wave number 1750 to 2000 cm-1 of HDPE/LDPE/UHMWPE-g-MA and HDPE, and rPET/LDPE-g-MA and LDPE are presented in Fig. 4(a) and 4(b), respectively.
The strong asymmetric C=O stretching at 1791 cm-1 was obviously apparent in the HDPE/LDPE/UHMWPE-g-MA spectra. Also, a medium intensity peak attributable to symmetric C=O stretching at 1867 cm-1 was evidenced. These two IR peaks are the most commonly seen for unsaturated cyclic anhydrides such as maleic anhydride. In rPET/LDPE-g-MA spectra, the C=O stretching at 1791 and symmetric C=O stretching at 1867 cm-1 were also in evidence with relatively lower intensity than the spectra found in the HDPE/LDPE/UHMWPE-g-MA. The interference of strong C=O stretching from the ester group from rPET would be the main reason for lowering the C=O anhydride peaks intensity. Obeying these IR characterization results, it could be suggested confidently that the MA was grafted onto the polymer chains in both HDPE/LDPE/UHMWPE-g-MA and rPET/LDPE-g-MA blends.
Fig. 4. FTIR spectra (a) HDPE/LDPE/UHMWPE-g-M and (b) rPET/LDPE-g-MA
Scanning Electron Microscopy
Miscibility or compatibility is a prime engineering aspect in polymer blends, especially when blending two different generic polymers. SEM is the most common method to observe blend morphology. Figures 5(a) and 5(b) are the SEM photos of HDPE/LDPE/UHMWPE-g-MA at X400 and X1000 magnification, respectively. The HDPE/LDPE/UHMWPE-g-MA blend shows the homogenous material. It is absolute miscible blends. The photos also reveal the “fibril like skin” fracture traces of the UHMWPE toughener. Figures 5(c) and 5(d) are fracture surface SEM photos of rPET/LDPE and rPET/LDPE-g-MA blends at X1000 magnification, respectively. The phase separation between rPET and LDPE was obvious on the rPET/LDPE blend. This evidence indicated poor compatibility between rPET and LDPE. However, for the rPET/LDPE-g-MA blend, homogenous disbursement of very tiny LDPE droplets within the rPET as major continuous phase was evident. This SEM information supported that better compatibility between rPET and LDPE blend can be enhanced by the MA grafting process.
Fig. 5. SEM Photos of polymers blended matrix (a) HDPE/LDPE/UHMWPE-g-MA at X400, (b) HDPE/LDPE/UHMWPE-g-MA at X1000, (c) rPET/LDPE at X1000, and (d) rPET/LDPE-g-MA at X1000, respectively
Effect of RHF Contents on WPC Performance
RHF is an agroindustry waste that is abundantly available. For the economic and ecological point of view, utilizing this cellulosic waste as high quantity as possible into WPC and commercializing the wood composite as construction material is one of the high value-added challenges. Nevertheless, for the sake of mechanical performance, the optimal matrix to fiber reinforcement ratio is one of the prime considerations for manufacturing of the WPC material. By employing the rPET/LDPE-g-MA and HDPE/LDPE/UHMWPE-g-MA blended matrices at 50:50 weight ratio in the WPC manufacturing, the RHF reinforcement contents ranged from 30 to 50 phr corresponding to the total weight of the blended matrices were initially investigated. 20 phr of talc filler was equally added into each formulation. Five WPC formulations as demonstrated in Table 4 were assigned. The WPC specimen was successfully compounded and then injection molded at temperatures that did not exceed 240 °C.
Table 4. WPC Formulations with Various RHF Contents
Figure 6 illustrates the fractured surface SEM photos of the WPC reinforced with RHF at the contents of 30, 35, 40, and 50 phr. The fine fibril-like traces were visualized on the matrix phase of the WPC. This fibril scar resulted from the presence of UHMWPE toughener that was blended in the HDPE/LDPE/UHMWPE-g-MA matrix. Also, there was no obvious indicator for the phase separation within the blended matrices. There was no doubt that the matrix surface area portion was reduced upon increasing the RHF reinforcement loading. The fiber pull-out or fiber break down footprints were also observed on the fractured surface. With closer observation at the RHF/matrices interface bonding, the SEM photos revealed that reasonably good surface adhesion between RHF and the MA-grafted blended matrices was established. Nevertheless, the diminishment of surface adhesion seemingly occurred when the RHF reinforcement contents were increased, especially at 50 phr of RHF loading. The effect of reinforcement overloading of RHF on the WPC material would be suspected. RFH has the lowest bulk density in comparison to other polymeric and filler ingredients. Then, for the volumetric quantity consideration on the obtained WPC at 50 phr and above of RHF, the volume fraction of RHF must be much higher than the polymeric fraction.
Fig. 6. SEM at X400 magnification for WPC reinforced with (a) 30 phr (b) 35 phr, (c) 40 phr, and (d) 50 phr of RHF
Regarding the basic “rule of mixtures” for mechanical performance consideration of the composite material, lower mechanical performance of RHF must be the domain fraction of the manufactured WPC. Accordingly, material performance inferiority especially for the mechanical properties would be experienced.
Flow ability of the manufactured WPC at melt temperatures not exceeding 250 °C was one of the main challenges of this work. Figure 7 shows the measured MFI at 230/5.0 of the WPC reinforced with 30 to 50 phr of RHF, respectively. For the first instance, under the static shear loading at 5.0 kg, all of the manufactured WPC samples had ability to flow at 230 °C melt temperature.
Fig. 7. MFI at 230/5.0 of WPC reinforced with 30 to 50 phr RHF contents, respectively
Obviously, the measured MFIs of the WPC pellet under the static shear loading were evidently low. Upon increasing the RHF contents from 30 to 50 phr, the MFIs were undoubtedly reduced from approx. 0.054 to 0.028 g/10 min. For the short fiber reinforced thermoplastic composite materials, the flow ability of the molten composite is typically retarded or resisted by the quantity and also the size of fiber reinforcement. Therefore, increasing in the melt viscosity (lowering in MFI) is commonly found when the fiber loading and/or fiber size is increased. Nevertheless, the obtained MFI values are preferably and manageably handled by extrusion based processors as typically adopted for the industrial manufacturing process of the WPC products.
HDT is the common parameter to verify the upper limit service temperature of polymeric products. In the production of items from WPC materials, PE matrix is generally categorized as having good toughness but low service temperature. Outdoor applications at temperatures above 50 °C would not be recommended for PE-based WPCs. In this work, HDT at 455 kPa standard load was performed on the manufactured WPC specimens at the RHF loading from 30 to 50 phr, and the results are presented in Fig. 8.
As expected, HDT gradually increased from 79.1 to 90.5 °C when the RHF loading was increased from 30 to 45 phr, respectively. HDT decreased by a few degrees at 50 phr of RHF loading. In the composite material with sufficient matrix/fiber interfacial adhesion, the HDT is typically enhanced by increasing the reinforcement loading. However, the upper limit enhancement would be overcome by the matrix/fiber adhesion droughting effect provoked by the “fiber overloading” of the material. According to the above SEM analysis, good surface adhesion between (rPET/LDPE-g-MA)/(HDPE/LDPE/UHMWPE-g-MA) blended matrices and RHF at 30 to 40 phr loading were manifested. Therefore, HDT improvement with the RHF loading of the manufactured WPC could be theoretically explained. By overloading RHF at 50 phr, the interfacial bonding was diminished and hence the upper limit of HDT was established. As for the engineering and commercial applications consideration, the maximum service temperature, by means of HDT, of the (rPET/LDPE-g-MA)/(HDPE/LDPE/UHMWPE-g-MA) wood composite reinforced with 45 phr of RHF was at approx. 90.5 °C. This figure would be far more acceptable for this WPC to be commercialized as the construction material for both interior and exterior applications.
Fig. 8. HDT of WPC at various RHF contents
Fracture toughness under accelerated force/energy striking of the (rPET/LDPE-g-MA)/(HDPE/LDPE/UHMWPE-g-MA)/RHF wood composites test specimens having the RHF contents from 30 to 50 phr were characterized by notched and unnotched impact strengths. The test results versus the RHF contents were plotted and are presented in Fig. 9. It can be seen that the notched impact strengths marginally declined from 1.67 to 1.47 kJ/m2 when the RHF loading was increased from 30 to 50 phr. The exact trend, reducing from 6.07 to 4.24 kJ/m2, was clearly apparent for the unnotched impact strength test results. For unfilled or unreinforced polymeric materials, the dissimilarity trend between notched and unnotched impact strength is commonly manifested as notched sensitive phenomenon. As for composite materials design, the notched sensitive effect would be commonly resolved by reinforcing the material with high stiffness fiber. Polyethylene is not categorized as notched sensitive material. According to the impact strengths data obtained from the (rPET/LDPE-g-MA)/(HDPE/LDPE/UHMWPE-g-MA)/RHF wood composites, the weakening of the impact strengths with increasing the RHF loading would have a two- fold explanation. RHF is a highly brittle reinforcement material. Accordingly, the toughness, by means of impact strength especially tested on the notched mode, of the manufactured wood composites must be reduced at high RHF loading. Also, as justified from SEM characterization, the interfacial adhesion between blended matrices and RHF seem to be diminished at high RHF loading. As such events, the impact strengths inferiority of the wood composites was experienced when the RHF reinforcement contents were overloaded.