Abstract
This study investigated the effects of Iroko wood flour (WF) and nano-titanium dioxide (TiO2) concentration on the properties of polypropylene (PP)-based composites, including accelerated weathering resistance, tribological behavior, thermal stability, physical characteristics, mechanical strength, morphological features, color changes, and surface roughness. The results showed that the presence of WF and TiO2 significantly influenced the density, hardness, thermal stability, crystallinity, coefficient of friction, and wear rate of the composites. Both fillers positively impacted the tensile strength, flexural strength, and flexural modulus of the composites, although the elongation at break values decreased. TiO2 addition enhanced thermal stability and protection against UV radiation, whereas using wood flour negatively affected color properties. Moreover, the surface roughness of the composites was affected by weathering time and wood flour content. These findings highlight the potential of WF and TiO2 as effective fillers for enhancing PP-based composites’ properties and weathering resistance.
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Polypropylene-based Composites Reinforced with Waste Tropic Wood Flours: Determination of Accelerated Weathering Resistance, Tribological, and Thermal Properties
İbrahim Halil Başboğa *
This study investigated the effects of Iroko wood flour (WF) and nano-titanium dioxide (TiO2) concentration on the properties of polypropylene (PP)-based composites, including accelerated weathering resistance, tribological behavior, thermal stability, physical characteristics, mechanical strength, morphological features, color changes, and surface roughness. The results showed that the presence of WF and TiO2 significantly influenced the density, hardness, thermal stability, crystallinity, coefficient of friction, and wear rate of the composites. Both fillers positively impacted the tensile strength, flexural strength, and flexural modulus of the composites, although the elongation at break values decreased. TiO2 addition enhanced thermal stability and protection against UV radiation, whereas using wood flour negatively affected color properties. Moreover, the surface roughness of the composites was affected by weathering time and wood flour content. These findings highlight the potential of WF and TiO2 as effective fillers for enhancing PP-based composites’ properties and weathering resistance.
DOI: 10.15376/biores.18.4.7251-7294
Keywords: Wood-plastic composite; Iroko wood flour; UV aging; Tribological properties
Contact information: Department of Forest Industry Engineering, Faculty of Forestry, Bursa Technical University, 16310, Bursa, Türkiye; *Corresponding author: ibrahim.basboga@btu.edu.tr
INTRODUCTION
Wood-plastic composites (WPCs) refer to materials obtained by incorporating wood or other lignocellulosic materials into thermoplastic matrices in varying sizes and proportions (Klyosov 2007; Behravesh et al. 2010). Over the past decade, there has been a growing interest in producing composite materials. Carus and Eder (2015) anticipate a substantial growth trajectory, projecting a remarkable increase from a modest 10,000 tonnes in 2012 to a significant 100,000 tonnes by 2020, owing to enhanced technical attributes, reduced costs, and strengthened supplier networks facilitating robust customer support. Furthermore, the global WPC market projection is predicted to reach 9,953.8 million US dollars by 2028, with a value of approximately 4,033.5 million US dollars in 2018 and exhibit a healthy growth rate of over 9.5% during the forecast period of 2021 to 2030 (Johnson 2020). These statistics highlight the remarkable growth and promising future of the WPC market.
Numerous studies have focused on the utilization of wood flours, residues from various wood species, agricultural and industrial waste, and numerous lignocellulosic materials in the production of WPCs (Mengeloglu and Kabakci 2008; Başboğa et al. 2020, 2022; Çavuş 2020; Çavuş and Mengeloğlu 2020; Tasdemir et al. 2020; Boran Torun et al. 2021; Dönmez Çavdar et al. 2021; Mrówka et al. 2021). These studies have contributed significantly to understanding the potential feedstock materials and their utilization in the production of WPCs. Extensive research has investigated the characteristics of WPCs produced by blending various thermoset or thermoplastic polymers with different lignocellulosic fillers. These studies explored the potential of WPCs as environmentally friendly alternatives to traditional materials in various industries, including construction, automotive, and packaging. The combination of polymers with lignocellulosic fillers offers the advantage of utilizing renewable resources while enhancing the composites’ mechanical properties, thermal stability, and overall performance. Understanding the synergistic effects between different polymers and fillers is essential for optimizing WPCs’ formulation and manufacturing processes, thus enabling their widespread usage as sustainable materials.
The WPCs are commonly preferred for applications in outdoor environments, such as garden furniture and wetted surfaces. Combining wood fibers or other lignocellulosic materials with thermoplastic matrices offers several advantages that make WPCs well-suited for these specific applications. Their inherent resistance to moisture, decay, weathering, and ability to withstand harsh outdoor conditions make them an ideal choice for outdoor settings. Additionally, WPCs provide an aesthetically pleasing alternative to traditional materials because they can mimic the appearance of natural wood while offering enhanced durability and low maintenance requirements. The utilization of WPCs in these contexts addresses the demand for sustainable materials and provides functional and visually appealing solutions for various outdoor applications.
Many studies have been conducted to determine the weathering properties of WPCs (Du et al. 2010; Teacǎ et al. 2013; Peng et al. 2014; Chen et al. 2016; Badji et al. 2017; Aydemir et al. 2019; Boran Torun et al. 2021; Dönmez Çavdar et al. 2021; Mengeloğlu and Çavuş 2021). Determining the aging properties of WPCs holds paramount importance in assessing these materials’ long-term performance and durability. Aging processes, such as exposure to environmental factors, ultraviolet (UV) radiation, moisture, temperature variations, and mechanical stress, can significantly influence WPCs’ structural integrity and functional properties. Understanding the effects of aging on WPCs is crucial for ensuring their suitability in outdoor applications with challenging weather conditions and prolonged exposure to UV radiation. Studying aging characteristics provides valuable insights into the degradation mechanisms, dimensional stability, color changes, mechanical strength, and overall service life of WPCs. Moreover, this knowledge aids in developing effective strategies for material formulation, manufacturing processes, and protective treatments to enhance WPCs’ long-term durability and performance in real-world applications.
Despite extensive research conducted on various attributes of WPCs, the number of studies aimed at determining coefficient of friction (CoF) and wear rate (WR) properties remains limited. The majority of previous studies in this field have focused on composites produced by incorporating lignocellulosic fillers with thermoset polymers (Dwivedi and Chand 2008; Yousif and El-Tayeb 2008/2010; Nirmal et al. 2010; Ahalwan and Yousif 2013; Latha et al. 2016; Richard et al. 2017; Ranakoti et al. 2019; Mylsamy et al. 2020). Furthermore, in another study, three different liquid solutions were impregnated into wood, and their wear properties were examined (Hamdan et al. 2010). Even fewer publications have been dedicated to the study of tribological properties of thermoplastic polymers and their composites incorporating natural fillers, including materials such as polyoxymethylene (Li et al. 2008; Xiang et al. 2012), polyethylene (Brostow et al. 2016; Yang et al. 2019; Al-Maqdasi et al. 2022), polyvinyl chloride (Jiang et al. 2017; Jiang et al. 2018), polypropylene (Aurrekoetxea et al. 2008; Bajpai et al. 2012; Mysiukiewicz and Sterzyński 2017; Ibrahim et al. 2019; Mazzanti et al. 2021), and polylactic acid (Mysiukiewicz and Sterzyński 2017). Despite the limited number of studies on the subject, a previous investigation indicated that the addition of wood flour reduced the CoF compared to the neat polymer (Aurrekoetxea et al. 2008). The potential usage of WPCs as sliding or frictional materials in bearing production is highlighted by the decreased CoF resulting from the inclusion of natural fibers. The two main components of WPCs, polymer and wood, are commonly already employed in bearing production (Mysiukiewicz and Sterzyński 2017).
The high demand for tropical wood species in Turkey includes Iroko (Chlorophora excelsa), Dahoma/Dabema, Sapelli, Sipo, Acajou (Akaju/Khaya), Ayous, Limba (White frake), and Afrormosia timber (Ekşioğlu 2022). Iroko wood is highly regarded for its exceptional mechanical and physical properties, making it widely utilized for structural components (Ouinsavi et al. 2005; Geert and Kuilen 2010). Following European standards, Iroko wood is classified as strength class D40, as defined in EN 338 (2009). The utilization of Iroko wood in construction and other industries is rising because of its robust nature and ability to withstand heavy loads and environmental factors. The remarkable mechanical and physical properties of Iroko wood make it a desirable choice for structural elements, contributing to the overall strength and durability of the constructed components. Iroko wood finds applications in decorative veneers, furniture production, interior and exterior decorations, solid parquet manufacturing, boat building, manufacturing of industrial kitchen materials, industrial or heavily used flooring, as well as in flooring for docks and piers, staircase construction, and furniture components. Iroko timber exhibits remarkable resistance to natural conditions, such as water, moisture, and sunlight, making it a preferred choice for outdoor furniture used extensively in parquet, boat, yacht, ship, and deck construction (Ekşioğlu 2022). The sawing process of these woods generates a substantial amount of sawdust. Additionally, cutting and furniture manufacturing produces unused small wood particles as waste, leading to wastage.
This research aimed to examine the impact of incorporating Iroko wood flour as filler in the production of wood-plastic composites (WPCs) on their technological properties. Furthermore, the study investigated the effects of incorporating nano TiO2, which has demonstrated UV radiation resistance in previous studies (Hazarika and Maji 2013), combined with Iroko wood flour as a filler. Additionally, while there have been several types of research on the determination of tribological properties in metal and polymer composites, more studies need to focus on assessing these properties in WPCs. The utilization of natural fibers within WPCs, leading to improved CoF properties (Aurrekoetxea et al. 2008) and the potential of TiO2 as a promising lubricating filler for enhanced engine efficiency (Birleanu et al. 2022), have served as sources of inspiration for this study. Hence, this study also aims to evaluate the influence of fillers on the friction coefficient and wear rate of WPCs.
EXPERIMENTAL
Materials
In this study, commercial polypropylene (PP) (product code: EH-102) was used as a thermoplastic matrix and purchased from PETKİM Petrochemical Company in İzmir, Türkiye. The general properties of PP coded with EH-102 are presented in the PETKİM Petrochemical Company data sheet (Petkim 2016).
Wood flours of the Iroko (Chlorophora excelsa) tree, which is a tropical species, were used as lignocellulosic filler. The Iroko waste flours were obtained from a company that produces industrial kitchens, which is a company operating in the Kısıkköy furniture region in İzmir, Türkiye. The sawdust and the leftover pieces that emerged while sawing the timber and during the production of furniture were utilized as the filler. Iroko wood wastes with an average air-dry density of 0.575±0.026 g/cm3 were granulated into different mesh-sized flour by Wiley mill and dried before production. Iroko flours (WF) were screened and passed through a 20-mesh sieve and retained on an 80-mesh sieve were used. The WF dimensions were approximately in the size range of 0.71 to 0.177 mm. Nano-sized titanium dioxide (TiO2) was purchased from KIMETSAN Ltd. Co. (Ankara, Türkiye). TiO2 was used to increase the resistance of the WPCs against UV rays. The general properties of TiO2 are given in Table 1.
Table 1. General Properties of TiO2
To increase the interface interaction between the hydrophobic polymer matrix and the hydrophilic lignocellulosic filler, maleic anhydride grafted polypropylene (MAPP) (Licomont AR 504 by Clariant, Muttenz, Basel, Switzerland) was used as a coupling agent, and Paraffin-wax (K.130.1000) was used as a lubricant. The general properties of coupling agent and lubricant are presented in Table 2.
Table 2. General Properties of MAPP and Paraffin-wax
Polymer Composite Production by Injection Molding
The manufacturing of waste Iroko-filled and PP-based WPCs was completed in two stages. In the first stage, composite pellets were produced by the extrusion method, and then in the second stage, composite test samples were produced from these pellets by the injection molding method. The manufacturing schedule of the study is given in Table 3.
Table 3. The Manufacturing Schedule of WPCs
Iroko waste wood flours (WF) were dried in an oven so that the resulting moisture content was close to zero before manufacturing. Depending on the formulation given, PP, WF, TiO2, MAPE, and paraffin wax were dry-mixed in a high-intensity mixer to produce a homogeneous blend. In all groups, MAPP was used as 3% of the total weight, while paraffin wax was used as an extra 3% of the total weight. Subsequently, the blends were compounded under heat. Compounding was completed with the help of a laboratory-type extruder with a single screw and five different temperature zones. The screw speed was 60 rpm, and extruder temperatures were 195, 190, 185, 180, and 180 °C from the feed zone to the die zone. A water pool was used to cool the extruded compounds. The cooled mixtures were granulated into pellets in the pellet machine. The pellets were kept in a drying oven at 103 °C (± 2) to reach an oven-dry weight before manufacturing WPC samples by injection molding. The standard test samples were produced with HDX-88 Injection Molding Machine. The dried pellets were converted into WPC test specimens under heat and pressure by injection molding method. The temperatures were 180, 190, and 200 °C from the feed zone to the die zone, and the pressure was 102 kg/cm2. The injection speed and screw speed were 80 mm/s and 40 rpm, respectively. The WPC specimens were injected with a cooling time of about 30 s. Before testing, test specimens were conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for 72 h in a climate chamber. At least 5 samples were tested for the determination of each property of WPCs.
Determination of Physical Properties
Density was measured according to ASTM D792 (2008) water displacement technique using test specimens in the size of 20 mm × 20 mm × 4 mm. The hardness properties of samples were determined accordance with ASTM D2240 (2017) by ENPQIX EHS5D Durometer (Shore D) (Polygon Co., Shenzhen, China).
Determination of Thermal and Morphological Properties
To determine the thermal properties of the PP-based composites, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were conducted with Shimadzu TGA-50 thermal analyzer and Shimadzu DSC-60, respectively. The TGA of the samples was performed at a heating rate of 10 °C/min under nitrogen with 100 mL/min flow rate. The samples were heated from room temperature to 600 °C. The temperature increased from 10 to 200 °C at a heating rate of 10 °C/min during the DSC analysis. The analysis was performed as both cooling and heating according to the isotherm points. The DSC analysis was performed on approximately 10 mg of the sample under a dry nitrogen atmosphere with a 100 mL/min flow rate. The degree of crystallinity (Xc %) was specified from the second melting enthalpy values using the following Eq. 1,
(1)
where Xc (%) is the crystallinity value, ΔHm (J/g) is the melting enthalpy of the specimens, ΔHc (207 J/g) is the enthalpy value of melting of a 100% crystalline form of polypropylene (PP) (Erem et al. 2013; Alsan 2016; Bazan et al. 2021; Rivera-Armenta et al. 2022), and (1-α) is the weight fraction of polymer into the composite material.
The morphology of the composites was characterized using a scanning electron microscope (SEM) (Nova NaNoSEM 650, FEITM, Hillsboro, OR, USA) with the help of the Everhart–Thornley detector (ETD). Before the analysis, the composite samples were dipped into liquid nitrogen and then broken in half to prepare the fractured surfaces. The gold powders were sputtered to the fractured surfaces by Desk V Coater (Desk V, Denton Vacuum Co., Moorestown, NJ, USA) at 10 mA for 120 s to provide electrical conductivity. In addition, a compositional back-scattered detector (CBS), which is one of the back-scattered electron (BSE) detectors, was used for contrast depending on the atomic number to determine the presence of TiO2 in the WPCs. The SEM analyses were also conducted on unweathered and weathered samples to show the effects of accelerated weathering.
Determination of Tribological Properties
Wear tests were performed with the help of a pin-on-disc machine. An AISI 1040 steel disc with 10-mm thickness and 60-mm diameter, and a hardness value of 50 to 55 HRC was used as a counter-disc material against WPCs. The ASTM G99-17 (2017) standard generally recommends a ground surface roughness of 0.8 µm (Ra) or less. Surface roughness values of steel discs were determined between 0.259 to 0.443 (Ra, μm). Wear tests were conducted at room temperatures (23 °C ± 2 and 48 ± 2% humidity) under dry conditions adapted from the ASTM G99-17 (2017) standard. Tribological tests were performed at two different loads (30 and 60 N) and 1.0 m/s sliding speeds. Wear (Ko) was calculated using the following Eq. 2,
(2)
where Δm is the average weight loss (g), L is the distance (m), F is the load (N), and ρ is the density (g/cm3).
Determination of Mechanical Properties
The mechanical strength (flexural (ASTM D790 2010), tensile (ASTM D638 2010), and impact (ASTM D256 2010) properties) of the WPC samples was determined according to the relevant standards. The performing of mechanical tests of composites was detailed by Başboğa et al. (2020). The variation ratios of flexural and tensile strengths (Eq. 3), the variation ratios of flexural modulus (Eq. 4), and the variation ratios of impact strength (Eq. 5) were calculated:
(3)
(4)
(5)
In the above equations, ‘before’ refers to the average strength and modulus values measured before weathering, and ‘after’ refers to the average strength and modulus values measured after weathering. IS refers to impact strength.
Accelerated Weathering Properties
All composite groups were exposed to accelerated weathering conditions in the accelerated weathering test chamber (Atlas UV Test, Mount Prospect, IL, USA). To simulate outdoor aging, accelerated outdoor testing was performed according to procedure 1 based on the ASTM G154 (2011) standard. The WPCs were exposed to UV light for 672 h with variable cycles of temperatures and humidity. The test was performed using 340 nm fluorescent at 0.89 W/m2/nm irradiance, 8 h of UV light at 60 (± 3) °C, followed by a 4-h condensation treatment cycle at 50 (± 3) °C. Because the first hours are important for the changes in the sample’s surface exposed to the accelerated weathering test, measurements were realized on the samples every 24 h for 168 h. Afterwards, surface properties were determined every 168 h and a total of 8 measurements were made from the beginning. Color and surface roughness measurements were recorded as surface properties.
The color coordinates (L*, a*, and b*) were determined over an 8 mm diameter spot from five different points (every time from the same points) with 10° observer angle. Color measurements were realized with the help of a Minolta CM-2600 D spectrophotometer (Konica Minolta, Tokyo, Japan), with 5 different samples (unweathered and weathered) for each group and a total of 25 different measurements. Total color change (ΔE*) was calculated with the help of Eq. 6 below according to ASTM D2244-22 (2009):
(6)
where ΔL*is the change in lightness and darkness values after weathering (L2*- L1*), Δa* is the change in red and green color values after weathering (a2* – a1*), and Δb*is the change in yellow and blue color values after weathering (b2* – b1*).
The surface roughness of the control and weathered groups samples were determined with a stylus-type diamond tip profilometer (Mitutoyo Surftest SJ-310, Sakado, Japan) according to guidelines provided by ISO 21920-2 (2021) standard, which was revised ISO 4287:1997 (ISO-21920-2 2021). To detect the roughness on the surface, measurements were carried out periodically at the same intervals (24, 48, 72, 96, 120, 144, 168, 336, 504, and 672 h) on the surfaces of the samples exposed to UV degradation. Measurements were realized from 3 different points for arithmetic average roughness (Ra) values on surfaces of 5 different samples from neat PP and wood and nano-TiO2 filled wood-plastic composite groups. A total of 15 measurements were taken for each group. A cutoff length (λc) of 8 mm, a cutoff wavelength of 8 µm, and a tracing length of 16 mm, was employed for the roughness test.
Statistical Analysis
Design-Expert® Version 7.0.3 (State-Ease, Inc., Minneapolis, MN, USA) and Minitab 19 (Pennsylvania State University, State College, PA, USA) statistical software packages were utilized to specify the interaction of waste Iroko wood flour and TiO2 amounts on the technological properties of PP-based composites and to discover the extent of statistical significance of impact of filler amounts on surface roughness parameter following the weathering cycles. Two-way analysis of variance (ANOVA) tests were performed to observe the effects of the filler amounts on the technological properties of the samples and effect of weathering and filler amounts on physical properties. For the surface roughness analyses, stepwise regression method was preferred at the 95% confidence level.
RESULTS AND DISCUSSION
In this study, mechanical (tensile, flexural, and impact strength), physical (density and hardness), thermal (TGA, DSC), morphological (SEM), tribological, color change, and surface roughness properties of all PP-based composite groups filled with waste Iroko wood flours (WF) and TiO2 were determined. All properties were examined under separate headings, and the findings were statistically analyzed and presented graphically under these headings.
Density of WPCs
The average density values of WPCs are given in Table 4.
Table 4. Average Density Values of WPCs
*The numerical value in the parenthesis is standard deviation
A density interaction graph showing the effects of fillers on the density properties of WPCs is also presented in Fig. 1. When the density interaction graph in Fig. 1 was examined, it was determined that both fillers had a significant effect on the density values (P < 0.0001). As shown in the interaction graph, the density values show an increase with the addition of WF and TiO2. Even the usage of TiO2 at low amounts had a significant effect on the density values. The density value of PP, which is a polymer matrix, is given as 0.905 g/cm3 in the factory data sheet. In addition, the average density values of the control group samples (without filler) were determined as 0.893 g/cm3. Moreover, the density value of TiO2 is given as 3,900 g/cm3 in the factory data sheet. TiO2 has a high-density value, even at low levels of usage in the polymer matrix with a much lower density; it was effective on the density values because the density of TiO2 was much higher than the polymer matrix. From this point of view, the presence of nanomaterial in the WPCs has been demonstrated, and it has been possible to say that a large part of the nanomaterial is contained in the matrix while mixing in the high-speed mixer.
Fig. 1. WF and TiO2 loadings effects on density of PP-based composites
Furthermore, SEM images were taken at 5000× with the help of a compositional back-scattered detector (CBS) to determine the presence of TiO2 in the WPCs. The SEM images of all TiO2 usage amounts of WPC groups with the highest percentage of wood flour (40%) are presented in Fig. 2.
When the SEM images of the W40T0 group (without nanomaterial) in Fig. 2-a are examined, it can be seen that there are no reflecting or back-flare images. Only wood flour pieces that had pulled out of the matrix are visible. However, when the images in Fig. 2-b/c/d are examined, it can be seen that the intensity of the flare in the images rose with the increase in the usage amount of the nanomaterial. These images supported the presence of TiO2 in WPCs. However, some small agglomerations were determined in some areas of fractured surfaces of WPCs containing nanomaterials at high levels, such as 6% and 9%. These images support the increment of density values of WPCs with TiO2 loading.
Fig. 2. SEM Images (taken at 5000 magnification) of WPCs containing 40% Iroko Flour and different amounts of TiO2 taken with a CBS detector: a-) W40T0, b-) W40T3, c-) W40T6, and d-) W40T9
The density values of the WPCs prominently increased with the addition of wood flours. It is believed that a higher density of lignocellulosic Iroko wood flour, which has a high cell wall density, might be responsible for the increased density of the WPCs (Mengeloglu and Karakus 2008; Mengeloğlu et al. 2015).It is reported that the usage of lignocellulosic materials as a filler increases the density of PP-based (Steckel et al. 2007; Özdemir et al. 2013; Mengeloğlu et al. 2015; Mazzanti et al. 2016; Basalp et al. 2020; Mengeloğlu and Çavuş 2021) and HDPE-based (Ramezani Kakroodi et al. 2013; Başboğa et al. 2020) composites. PP-based WF and TiO2-filled composites were produced in the density range of 0.893 to 1.076 g/cm3. The highest average density value was obtained in the composite group, in which fillers were used at the highest level, while the lowest value was obtained in the neat PP group. Higher increases in density values were observed when wood flour and TiO2 were used at the highest amounts. Moreover, this is generally explained by the rule of mixtures in the literature (Matuana et al. 1998; Mengeloǧlu and Karakuş 2008; Mengeloğlu and Çavuş 2021). Composite materials obtained by combining high-density fillers and low-density polymer matrix have a higher density compared to the polymer itself (Çavuş and Mengeloğlu 2020; Mengeloğlu and Çavuş 2021). There are similar studies in the literature in which the density values of composites increase with the increment in the amount of filler (Klyosov 2007; Mengeloğlu et al. 2015; Çavuş and Mengeloğlu 2016; Mengeloğlu and Çavuş 2021).
Hardness Properties of WPCs
The hardness (Shore D) values of WPCs were determined by an ENPQIX EHS5D durometer (Polygon Co., Shenzhen, China), and the average values are summarized in Table 5.
Table 5. Average Hardness Values of WPCs
*The numerical value in the parenthesis is standard deviation
An interaction graph showing the effects of fillers on the hardness properties of composites is given in Fig. 3. Considering the interaction graph, it was determined that both fillers were significantly effective on the hardness values of the composite materials (P < 0.0001). The hardness values were enhanced with the usage of both fillers. The hardness values continued to rise with increasing amount of fillers used. Similar results were also reported by Çavuş (2017). The highest hardness value was determined as 73.9 in the W40T9 group containing the highest amount of fillers, and the lowest hardness value was 61.3 in the pure polymer (W0T0) group without fillers.
Fig. 3. WF and TiO2 loadings effects on hardness of PP-based composites
Thermal and Morphological Properties of WPCs
The TGA analyses were performed on six different groups (W0T0, W20T0, W40T0, W40T3, W40T6, and W40T9). The thermal degradation results of WF and TiO2– reinforced PP-based composites are summarized in Table 6.
Table 6. TGA Results of WF and TiO2-reinforced PP-based Composites
When Table 6 was examined, single-stage thermal degradation was observed in the control group without filler (W0T0) in the TGA analysis. Considering the values in Table 6, the decomposition for the W0T0 group started at approximately 273.7 °C and ended at approximately 490.7 °C. A similar result was reported by Esmizadeh et al. (2020). The maximum thermal degradation occurred at 469.7 °C for the W0T0 group samples, and the amount of residue after 600 °C was determined as 1.32%. If the W20T0 and W40T0 groups were examined, in which only wood flour was used as filler in different proportions, the thermal degradation occurred in two stages in the Derivative-TGA (drTGA) graphs in these groups, and two peaks were obtained on the graph (Fig. 4).
Fig. 4. DrTGA results of just WF reinforced PP-based composites
Considering the first peaks obtained in the DrTGA graphs in Fig. 4, the first decompositions were at 234.9 and 228.2 °C for W20T0 and W40T0, respectively. The first decomposition end-temperatures were approximately 376.0 and 383.5 °C, respectively. These first peaks formed because of the wood flours main components, cellulose, hemicellulose, and lignin in the WPCs. While the thermal degradation range of hemicelluloses was between 190 and 410 °C (Chen et al. 2020), for celluloses it was between 250 and 400 °C (Várhegyi et al. 1994). It is reported that lignin has a wider thermal degradation temperature range (between 105 and 800 °C) (Chen et al. 2020; Foong et al. 2020). Maximal degradation was observed at 343.9 °C with a 12.3% extent and at 348.9 °C with a 17.3% extent for the W20T0 and W40T0 groups, respectively. The degradation temperatures of WPCs produced by adding wood flour to the polymer matrix decreased. This occurred because wood flour started to decompose before the polymer matrix. The second degradation began at 398.1 and 397.7 °C and finished at 505.4 and 504.9 °C for W20T0 and W40T0, respectively. Maximal degradation was determined at 478.6 and 479.7 °C for W20T0 and W40T0, respectively. With the addition of wood flour, the maximum decomposition temperatures increased by approximately 10 °C compared to the control group. Considering the remaining residue amounts at 600 °C, the amount of residue also increased with the usage of wood flour compared to the control group. Furthermore, the increase in this residue amount continued with the increase in wood flour usage.
The thermal properties of WPC groups containing wood flour at the highest rate and TiO2 at different levels were determined to observe the effects of TiO2 on the thermal properties of WPCs. DrTGA graphs of WPCs groups containing 40% wood flour and different amounts of TiO2 are given in Fig. 5.
Fig. 5. DrTGA results of PP-based composites contain 40% WF and different amounts of TiO2
From Fig. 5, it can be seen that the WPCs decomposed in two stages, and generally, close peaks were obtained. It was observed that the starting and ending temperatures of the peaks obtained in the groups in which the nanomaterial was used were close to each other. An increment was observed in the initial decomposition temperature with the usage of TiO2, and it was approximately 15 to 20 °C. The maximum thermal degradation temperature was determined at 350 °C for the first peak and at 479 °C for the second peak. The increment in the amount of TiO2 in the WPCs also caused an increase in the amount of residue at 600 °C. In the W40T9 (23.42% residue at 600 °C) group, which contained the maximum amount of wood flour and nanomaterials, the amount of residue at 600 °C increased almost twice that compared to the W40T0 group (12.96% residue at 600 °C), which contained only 40% wood flour and without nanomaterial. In previous studies, it was stated that the decomposition for pure TiO2 started at 200 °C and continued up to 950 °C (Özbey 2018). It is thought that the thermal stability of TiO2 is higher than wood flour and that it wraps around the wood during its usage at high levels, which is caused by slowing down the degradation and decreasing the mass loss (%). In addition, it was reported that the thermal stability of the nanocomposite increased with the usage of nanoparticles in the lignocellulosic matrix (Rahman et al. 2017) and WPCs (Kaymakci 2019).
The DSC analyses were performed on eight different groups (W0T0, W20T0, W20T3, W20T6, W40T0, W40T3, W40T6, and W40T9). The crystallinity ratios of WPCs were calculated with the help of Eq. 1, which is presented in the “Methods” section, and are summarized in Table 7.
Table 7. DSC Results of WF and TiO2 Reinforced PP-based Composites
(1-α) is the weight fraction of polymer into the composite material, Tm is the melting temperature, Xc (%) is the crystallinity value, ΔHm is the melting enthalpy of the specimens, and ΔHc is the enthalpy value of melting of a 100% crystalline form of PP
Table 7 shows there was no significant change in melting temperatures, and they were close to each other. However, when the crystallinity ratio of the composites is examined, the crystallinity of the composites decreased with increasing amount of fillers to the polymer. While the melting temperatures of the composites did not change, decreases were determined in the crystallinity ratios (Xc) compared to the control group without filler (W0T0). While both fillers tended to decrease the crystallinity of the polymer, it is possible to say that wood flour loading reduced the crystallinity of the composites at a higher amount than TiO2 loading with the results of the DSC analysis. While the highest crystallinity (Xc) ratio was determined in W0T0, containing just the coupling agent and lubricant, the lowest value was determined in the W40T9 group containing the maximum amount of both fillers.
Tribological Properties of WPCs
Generally, the tribological properties are described with the friction coefficient (CoF) and the specific wear rate (WR) (Tai et al. 2012; Hou et al. 2018; Yetgin 2020). Tribological tests of Iroko wood flour and TiO2 filled PP-based WPCs were conducted under two different loads (30 and 60 N) at one sliding speed (1.0 m/s). The WRs calculated with Eq. 2 and the CoF values of the composites are presented in Table 8.
Table 8. Results of Friction Coefficient and Wear Rate of WPCs