1. Introduction
Natural fibers and wood flour used in biocomposite materials provide many advantages in terms of economy, sustainability, and environmental health. Biocomposites are materials derived from a biological source and contain one or more phases [
1]. Reinforcing natural fibers, wood flour used as fillers, natural resins, and natural rubbers are widely used in biocomposite materials. These materials, found in nature, have high specific hardness and strength. They are also lightweight, cheap, renewable, recyclable, and biodegradable materials, so they are highly preferred in the composite industry [
2]. In addition, these materials are alternative ecological materials that can be used in composite production due to their low density and satisfactory physical, mechanical, thermal, and aesthetic properties [
3]. Additionally, they do not release a large amount of carbon dioxide into the atmosphere when burned. Natural fiber and particle-reinforced composites can be used in transportation (automobiles, train wagons, and aviation), military applications, the construction and building industries, packaging, and consumer products [
4].
Composite materials made by incorporating wood as a natural fiber with a plastic matrix are called wood–plastic composites (WPCs). In studies related to thermoplastic matrix WPC materials, it is seen that various wood species, such as radiata pine, yellow pine, kenaf, red pine, western pine, spruce, and poplar, are used [
5,
6,
7,
8]. Wood particle decomposes at low temperatures. If the melting temperature of the matrix material exceeds the wood particle decomposition temperature, the wood particle will be thermally degraded by the elevated temperatures and lose its properties. Therefore, the matrix material should melt at a temperature below the wood particle decomposition temperature, preferably 200–220 °C. Since wood is an anisotropic material, it is susceptible to deformation, bending, and cracking due to the dimensional instability caused by changes in ambient humidity [
9]. For this reason, the moisture absorption of the selected matrix material should not be high. Considering all these limitations, polypropylene, which has a suitable melting temperature, low water-absorption rate, and is resistant to living organisms such as bacteria and fungi, is generally preferred. Polypropylene is a promising matrix material for the formation of natural–synthetic polymer composites when combined with fibrous natural polymers suitable for admixtures and reinforcing elements [
10].
In addition to extrusion and injection molding, studies are now being carried out on hot-pressing technology for plastic shaping of flat WPC sheets [
11]. Yue et al. impregnated Chinese fir wood with boric phenol formaldehyde (BPF) resin at a concentration of 30% and then hot pressed it at different compression ratios. According to the test results, they stated that the higher the compression ratio, the better the mechanical properties and burning performance of the BPF-densified sample [
12]. Cheng et al. modified poplar wood by combining impregnation of borate-containing phenol-formaldehyde resin and transverse compression of varying densities. They investigated the effects of this combined treatment on fire resistance. The test results showed that the combination of BPF resin impregnation and compression improves the fire resistance of poplar wood [
13].
Polymer materials can readily burn when exposed to heat in an oxygenated environment. When WPC materials are exposed to direct sunlight, their covalent bonds can be damaged, in which case the mechanical properties are weakened, and they become brittle [
14,
15]. At the same time, bacteria and fungi can form rapidly within the wood fiber. This situation reduces the durability and lifespan of the wood. To prevent these issues and ensure the flame resistance of WPC materials, this study used tetrabromobisphenol A (TBBPA) as a flame-retardant additive due to its low toxicity, UV resistance, and antimicrobial activity. TBBPA is a white powder with a 67.7% Br content, suitable for processing temperatures of polypropylene and wood flour with a melting point of 113–178 °C [
16]. Additionally, antimony trioxide (ATO) was used to increase the reaction rate of the TBBPA flame-retardant additive in this study. Antimony trioxide is a white powder additive with the chemical formula Sb
2O
3. It is commonly used as a synergist for brominated flame retardants, enhancing their effectiveness. Antimony trioxide is particularly effective in combination with aromatic products, improving flame resistance and helping to reduce the flammability of various materials. Its role in flame retardancy makes it valuable in applications such as plastics, textiles, and electronics, where fire safety is a critical concern [
17,
18].
Beech wood is known for its hardness, good mechanical properties, especially high compressive strength, strong fiber structure, and easy availability. Due to its durable structure, beech plywood is preferred in various applications, such as flooring, toy production, and furniture manufacturing. Therefore, beech wood (Fagus sylvatica), belonging to the Fagaceae family found in Turkey, was preferred in this study.
In their study, Çavuş and Mengeloğlu examined the melt flow index (MFI) and mechanical properties of the WPCs produced by adding wax, olive pomace, and red pine wood flour into PP. They found that there was no significant change in the melt flow index, but the density increased with the increase in the filler, and the filler reduced the tensile strength and elongation at break [
19]. Arao et al. compared WPC materials with PP matrix, 50% wood flour, MAPP, and flame retardants such as ammonium polyphosphate (APP), melamine polyphosphate (MPP), and aluminum hydroxide in different proportions. They reported that PP containing 10% APP by weight increased self-extinguishing properties in the UL94 flammability test, while PP containing 30% APP by weight did not self-extinguish, and flame retardants caused a decrease in mechanical properties [
20]. Pokhrel et al. produced WPC by using four different types of wood flour in a PP matrix and examined the mechanical properties of the resulting composites [
21]. Bledzki and Faruk worked on the effect of wood filler geometry on the physical and mechanical properties of PP/wood flour WPC materials [
22]. Stark and Berger investigated the effects of wood flour types and particle sizes on the mechanical properties of polypropylene containing wood flour. They reported that as the wood flour content increased, the tensile and bending modulus, density, heat-deflection temperature, and notched impact energy of the composites increased, while the tensile and bending strength, tensile elongation, mold shrinkage, melt flow index, and unnotched impact energy decreased [
23]. Ito et al. stated that the mechanical properties of polypropylene-based WPC were improved by the addition of wood flour [
24]. Ayrılmış et al. conducted a study in which they produced WPC by adding wood flour and olive cake into polypropylene. They used MA-g-PP (maleic anhydride-grafted polypropylene) to enhance compatibility between the matrix and the fillers. They noted that there was no significant change in the water resistance and bending properties of the WPC material, and they reported that the coupling agent had no effect on the bending strength [
25]. Isa et al. worked on WPC materials containing wood flour. They found that the tensile and bending properties of composites containing wood flour were 10% higher than those of composite materials without wood flour [
26].
In this study, the physical, mechanical, and flammability properties of wood–plastic composite (WPC) materials containing beech wood were investigated. Polypropylene (PP) was used as the matrix. Two flame retardants, TBBPA and ATO, were utilized, with ATO intended to enhance the effect of TBBPA. First, TBBPA and ATO were added to the PP in varying ratios to produce a masterbatch through an extrusion process. Then, wood flour and the masterbatch containing the optimal ratios of TBBPA and ATO were fed through different zones during the injection process to produce WPC samples. These procedures were carried out to ensure homogeneous distribution of the materials and to improve their flammability properties.
3. Results and Discussion
Sieve analysis was performed to determine the particle size distribution of wood flour. The literature review showed wood flour particle sizes in the range of 50 to 700 μm were used in WPC production [
40]. The beech-wood flour used in our study consists of particles with a maximum size of 180 µm. The graphs created according to the determined particle size/wood flour ratio value data are shown in
Figure 2. The largest proportion was found in wood flour with the largest particle size (180 µm). As the particle size decreases, the wood flour ratio also decreases.
When wood flour was kept in the oven at 23 °C for 7 days, its moisture content was calculated to be 5.56%. In order to dry quickly, wood flour was kept in the oven at temperatures of 120 °C and 140 °C for 15, 30, and 60 min (
Figure 3), and then, the optimum temperature and time were determined by calculating the moisture contents according to Equation (2). The standard deviation was not calculated because only one sample was used for each test. At this determined temperature and time, WPC granules were also subjected to a drying process, and their moisture content was calculated. Considering the applied temperature and time, 140 °C for 15 min was preferred for quick drying.
It was observed that the increase in temperature and time during the drying process was inversely proportional to the moisture content of the material, and the moisture content decreased depending on the increase in temperature and time (
Table 5). It was also determined that the increase in temperature did not affect the color and appearance of the wood flour. The moisture content of the wood flour decreased significantly after the drying process. The moisture content of the WPC granules was calculated as 0.29%.
It was stated in the sources that the moisture content of unprocessed and oven-dried wood flour should be a maximum of 12% [
41]. Based on this ratio, the moisture content values of wood flour and WPC samples are considered suitable.
The additives and fillers used in plastic materials are important components that affect the physical properties of the material. In this study, while the density of PP/TBBPA/ATO 2 was 1.1214 gr/cm
3, the density of the WPC samples increased with the addition of wood flour (
Table 6). The density of the WPC 1 sample with 10% wood flour added is 1.1767 gr/cm
3, while the density of the WPC 5 sample with 30% wood flour added is 1.2737 gr/cm
3. As can be understood from this, as the amount of wood flour increases, the density of the WPC samples also increases.
In the study by Çavuş and Mengeloğlu, where they used olive pomace and red pine wood flour in a PP matrix, it was observed that the density increased as the amount of wood flour increased [
19]. The same result has been obtained and reported in many studies [
42].
In WPC materials, water absorption may increase due to the wood flour or other filler materials they contain. Generally, water absorption increases as the exposure of the wood pores in the material to water and the exposure time increase.
The water-absorption rate of WPC samples was determined by the water-absorption test and calculated according to Equation (3). The standard deviation was not calculated because only one sample was used for each WPC group.
The sample thickness and the duration of water exposure are two important test parameters. It was observed that the amount of water absorption decreased with increasing sample thickness. Additionally, the samples exposed to water for 48 h absorbed more water than those exposed for 24 h.
As the amount of wood flour increased in WPC samples, the amount of water absorption increased (
Table 7). The water-absorption rate of WPC 1, which was 1 mm thick and kept in water for 24 h, was 0.24%, while in WPC 5, this rate increased to 1.37%. This rate was 0.66% in WPC 1, which was kept in water for 48 h, and it increased to 2.18% in WPC 5. In samples with a thickness of 2 mm, as the amount of wood flour increased, the amount of water absorption also increased similarly. However, as the thickness increased, the water absorption showed a decrease. The percentage increase in water absorbed in B samples was less than that in A samples.
The melt flow index (MFI) indicates the workability of the material and its performance in the production process, making it one of the most important parameters in injection molding. It was seen that the MFI value of the matrix material, i.e., homopolymer polypropylene, increased with the addition of TBBPA and ATO additives.
The effect of these two additives on viscosity was clearly observed in PP/TBBPA/ATO 2. Thus, it was revealed that TBBPA [
43] and ATO caused a decrease in viscosity, in other words, an increase in flowability. The addition of wood flour and the increase in the amount of wood flour in WPC samples decreased the flowability (
Table 8). The MFI value of the PP/TBBPA/ATO 2 sample was 80.58 g/10 min, while it was 43.15 g/10 min for WPC 1 and 13.19 g/10 min for WPC 5.
In PP/TBBPA/ATO 2, which does not contain wood flour but contains only TBBPA and ATO, the tensile strength and % elongation ratio are higher; these values are 32.8 MPa and 8.61%, respectively. The elasticity modulus is the lowest; this value is 1681 MPa. It was determined that with the increase in the wood flour ratio in WPC samples, the tensile strength and the % elongation decreased, the elasticity modulus increased, and the samples showed brittle fracture. In the WPC 1 sample, where the wood flour rate is 10%, the tensile strength is 24 MPa, the modulus of elasticity is 1846 MPa, and the % elongation rate is 4.22, while in the WPC 5 sample, where the wood flour rate is 30%, the tensile strength is 20.4 MPa, the modulus of elasticity is 2602 MPa, and the % elongation rate is 2.33 (
Table 9). Lignocellulosic wood fillers usually improve the tensile modulus of thermoplastic composites [
44].
In the study conducted by Kurt and Mengeloğlu [
45], it was reported that the tensile strength of the PP/pine wood flour/MAPP mixture with 25% ammonium polyphosphate (APP) as flame retardant was 15.68 MPa. In contrast, when compared to the WPC 5 sample without any compatibilizer, our sample showed a tensile strength of 20.4 MPa, indicating an approximately 33% higher value. This result revealed that the WPC 5 sample obtained without a compatibilizer is more advantageous in terms of tensile strength.
In the Izod impact test, the impact strength of PP/TBBPA/ATO 2 was found to be 1.82 kJ/m
2, and it was determined that the impact strength increased depending on the addition of wood flour and the increase in the amount of wood flour. In the WPC 1 sample, the impact strength was 1.83 kJ/m
2, while in the WPC 5 sample, this value increased to 1.98 kJ/m
2 (
Table 10). Wood has the capacity to absorb energy during impact. Thus, wood flour showed this effect.
In their study, Çavuş and Mengeloğlu found that the Izod (notched) impact strength of the composite material they produced by adding 20% red pine flour, wax, and MAPP to PP was 1.80 kJ/m
2 [
19]. In the WPC 3 sample, which contains the same amount of wood flour, the impact strength is higher at 1.86 kJ/m
2.
Pokhrel et al. produced WPC materials using 80% PP and 20% of four different types of flour. They used white cedar, white pine, spruce fir, and red maple as wood flour. They found that the Izod impact strength of the WPC materials ranged from 0.018 to 0.023 kJ/m
2 [
21]. It is evident that higher impact strength results were obtained in this study, where beech-wood flour was used.
Since PP/TBBPA/ATO 2 has a more ductile structure compared to the WPC samples, it is an expected result that the amount of deflection is high, and the modulus of elasticity is low. The bending strength of the PP/TBBPA/ATO 2 sample was approximately 30% lower than that of the WPC samples. While the bending strength of PP/TBBPA/ATO 2 was 29.3 MPa, this value increased to 45.3 MPa in the WPC 1 sample. These results indicate that the addition of wood flour enhances the bending strength of the material. However, as the wood content increases, the samples become more rigid, which can lead to a decrease in bending strength. This increase in rigidity may also result in brittle behavior in the material. Consequently, while the bending strength of WPC 1 was determined to be 45.3 MPa, this value decreased to 41.3 MPa in WPC 5. Additionally, the increase in the amount of wood flour raised the modulus of elasticity and reduced deflection. In WPC 1, the elastic modulus was 1340 MPa, and the deflection was 5.2%, whereas in WPC 5, these values were 2140 MPa and 2.4%, respectively (
Table 11). The lowest modulus value was observed in PP/TBBPA/ATO 2. This is due to the fact that lignocellulosic woody structures possess a higher elastic modulus than polymers [
46,
47].
In their study, Pokhrel et al. found that the bending strength of WPC materials was in the range of 40–45 MPa. Similar results were obtained in this study, where beech-wood flour was used [
21].
When examining the PP/TBBPA/ATO 2 sample, it is evident that TBBPA and ATO have a significant effect on hardness. The hardness of the PP/TBBPA/ATO 2 sample, which does not contain wood flour, is higher than that of the WPC samples. This indicates that flame-retardant additives increase hardness more effectively than wood flour [
2]. However, in general, the inclusion of wood components in polymer materials tends to increase hardness. Therefore, increasing the amount of wood flour in WPC samples led to an increase in hardness. The Shore A hardness of the PP/TBBPA/ATO 2 sample was 84.2, while the hardness values for the WPC 1 and WPC 5 samples were 75.8 and 81, respectively (
Table 12).
The results obtained from the abrasion test are shown in
Table 13. The weight loss and wear rate of the PP/TBBPA/ATO 2 sample are higher than those of WPC samples. The weight loss and wear rate of the PP/TBBPA/ATO 2 sample are 0.0017 g and 2.473 × 10
−6 cm
3/Nm, respectively, while these values are 0.0012 g and 1.755 × 10
−6 cm
3/Nm for the WPC 1 sample. Wood flour increased the abrasion resistance of WPC materials. As the amount of wood flour increased, the weight loss and wear rate decreased. In the WPC 5 sample, the weight loss and wear rate decreased and were 0.0007 g and 0.981 × 10
−6 cm
3/Nm, respectively. Materials with higher hardness generally tend to have better abrasion resistance, which is clearly seen here.
Ibrahim et al. produced WPC materials using the injection-molding method with wood flour (WF) at weight ratios of 5%, 15%, 25%, 35%, 45%, and 55%, incorporating malleated polypropylene. They achieved the lowest weight loss with a 60% reduction in WPC materials containing 25% wood flour compared to PP. In other words, while obtaining optimal wear resistance in WPC materials with 25% wood flour, they found lower values with higher wood flour content [
48].
The UL94 test was conducted on all PP/TBBPA/ATO samples to determine the optimum ratio of flame retardants and on all produced samples to assess their flammability properties. UL94 test results are shown in
Table 14. The UL94 test conducted on PP/TBBPA/ATO samples indicated that samples with thicknesses of 0.8 mm, 1.6 mm, and 3.2 mm did not burn, but all exhibited dripping, thus classifying them as V2. It was observed that they did not burn the cotton placed underneath. Additionally, it was found that as the sample thickness increased, the burning time and amount of dripping decreased.
Arao et al. produced WPC materials containing a PP matrix, 50% wood flour, MAPP, and ammonium polyphosphate (APP). They set the APP content at 10% and 30%. They reported that the material with 10% APP self-extinguished in the UL94 flammability test, while the material with 30% APP did not show self-extinguishing properties [
20]. In the WPC samples in this study, it was observed that while the burning time increased, the amount of dripping decreased. Although samples WPC 1, 2, and 3 had burning times that qualified them for V0 classification, they were categorized as V2 due to dripping. For samples WPC 4 and 5, it was noted that, despite the flame being drawn, they continued to burn, and the cotton burned. Additionally, as the amount of wood flour in the WPC samples increased, the burning rate also increased.
The glow wire test was conducted on all PP/TBBPA/ATO samples to determine the optimum ratio of flame retardants and on all produced samples to assess burning time and flame resistance. The glow wire test results are shown in
Table 15.
All PP/TBBPA/ATO samples and WPC 1, WPC 2, and WPC 3 samples extinguished as soon as the flame was withdrawn at 960 °C. It was observed that the samples dripped but did not burn the paper. It was determined that the WPC samples containing wood flour burned longer. It was determined that the WPC 4 and WPC 5 samples continued to burn for more than 30 s when the flame was withdrawn and burned the paper by dripping. WPC 4 and WPC 5 samples could not pass the 960 °C test temperature and 850 °C test temperature depending on the burning time. When the same samples were subjected to a temperature of 750 °C, it was observed that the samples did not burn but produced a significant amount of smoke. It was determined that as the total additive ratio of TBBPA and ATO increased, the burning temperature increased while the burning time decreased.
The GWIT test results indicate that samples 1 and 5 from the PP/TBBPA/ATO group extinguished flames at 725 °C, while samples 2, 3, and 4 extinguished at 875 °C. Samples 6 and 7, on the other hand, extinguished at 675 °C. This variation in flame-extinguishing temperatures suggests differing thermal stability and effectiveness of flame retardants among the samples. Such differences could be attributed to the specific compositions of each sample. It was observed that as the wood flour ratio increased, the amount of dripping decreased.
Both the UL94 and glow wire tests indicated that PP/TBBPA/ATO 2 provided the optimum results in terms of burning time and ignition temperature (GWIT) among the PP/TBBPA/ATO samples. Therefore, the flame-retardant additive ratios in the production of WPC samples were based on the PP/TBBPA/ATO 2.
Candan et al. conducted a study on the fire performance of laminated veneer lumber (LVL) panels treated with different fire-retardant chemicals. They treated beech veneers using a mixture of borax and boric acid as well as monoammonium phosphate and diammonium phosphate. The fire test results revealed that the LVL panels treated with diammonium phosphate exhibited the lowest ignition temperature at 220 °C, while the panels treated with a borax–boric acid mixture reached the highest ignition temperature at 420 °C [
49]. On the other hand, the WPC samples containing TBBPA and ATO obtained in this study showed a flame resistance of up to 725 °C, offering a significantly higher flame resistance than LVL panels.
Determining the heat-deflection temperatures (HDT) of the samples plays a critical role in determining the usability areas of plastic and composite materials. WPC materials containing a high percentage of wood flour delay degradation and provide better thermal stability to the composite material. This makes the material more durable and long-lasting in various applications. For this reason, the HDT-A test was applied to all WPC samples (
Figure 4). It was determined that the heat-deflection temperatures of the samples increased with the increase in the wood flour content of the WPC samples (
Table 16). While the heat-deflection temperature of the WPC 1 sample containing 10% wood flour was 72.28 °C, this temperature increased to 86.71 °C in the WPC 5 sample containing 30% wood flour. Increasing hardness generally enhances the material’s resistance to heat, which can lead to a rise in the heat-deflection temperature. As can be seen from the results, such an increase was observed here.
FTIR analysis revealed the characteristic region distributions of the materials (
Figure 5). In the FTIR examination of homopolymer polypropylene, peaks were observed at 2950 cm
−1 and 2910 cm
−1 for the methyl group (CH
3); at 2870 cm
−1, 2840 cm
−1, and 1460 cm
−1 for the methylene group (CH
2); and at 1375 cm
−1 for CH
3, while a CH
2 peak was noted in the range of 400–1000 cm
−1 [
50].
In the FTIR analysis of TBBPA, peaks were identified at 1551 cm
−1 (C=C−X), 1475 cm
−1, 1393 cm
−1 (symmetric bending of CH
3), 1318 cm
−1 (out-of-plane deformation of OH), 1278 cm
−1 and 1243 cm
−1 (in-plane deformation of OH), 1159 cm
−1 (C−OH), and 737 cm
−1 (aromatic group CH stretching), and the vibrations in the range of 500–700 cm
−1 correspond to C−Br stretching [
51]. When comparing the PP/TBBPA/ATO samples with PP, an increase in peaks in the 1000–1400 cm
−1 range was observed. This increase can be attributed to the chemical presence of TBBPA and ATO additives as well as the contribution of the metal hydroxide content.
The FTIR analysis of wood flour showed a peak for hydroxyl (OH) in the 3000–3500 cm
−1 range, a characteristic peak for CH
4 in the 2800–3000 cm
−1 range, carbonyl functional group double peaks in the 1650–1750 cm
−1 range, and vibrations related to C−O−C and C−C stretching frequencies between 1000–1500 cm
−1 [
52]. When examining the FTIR spectrum of the WPC sample, it was observed that, although not as sharp as that of wood flour, there was still a hydroxyl (OH) peak in the 3000–3500 cm
−1 range. Additionally, the peaks in the 1650–1750 cm
−1 range exhibited a broader trend due to chemical interactions, resulting in reduced sharpness. Conversely, the peaks in the 500–1500 cm
−1 range showed increased sharpness and formed composite peaks as a result of these chemical interactions.
According to the TGA analysis results in
Figure 6, it was determined that wood flour began to decompose at 278 °C and reached its maximum degradation temperature at 400 °C. Lignocellulosic materials such as wood and plants are composed of cellulose, hemicellulose, and lignin [
53]. Studies have reported that hemicelluloses decompose in the temperature range of 275–350 °C, lignin at 360 °C, and cellulose in the range of 350–400 °C [
54,
55].
For polypropylene (PP), the initial degradation temperature was found to be 283 °C, while the maximum degradation temperature was 455 °C [
56,
57]. When examining the TGA graph of the WPC samples, it was observed that the addition of flame-retardant additives increased the initial degradation temperature of wood flour, as degradation occurred at 301 °C. Additives with aromatic rings, such as TBBPA, tend to decompose at higher temperatures due to their strong bonding. While pure wood decomposes rapidly, in WPC samples, the presence of additives results in a delay in degradation, corresponding to a weight loss in the range of 60% to 70%. Furthermore, it was observed that as the amount of wood flour increased, the final residue amount also increased. The residue amount was 6% in WPC 1 and 17% in WPC 5 (
Table 17).
The SEM images of the PP/TBBPA/ATO 2 (a and b) and the WPC 5 (c and d) samples are shown in
Figure 7. The particle sizes of ATO range from 100 to 900 nm, while TBBPA’s particle sizes are between 1 and 3 microns. In the SEM image shown in
Figure 7b, small particles are observed that belong to ATO, whereas the larger particles are associated with TBBPA. The SEM images in
Figure 7c,d show the beech-wood flour within the PP matrix.
In the SEM images of the PP/TBBPA/ATO 2 sample (a and b), a homogeneous distribution of flame-retardant additives is observed. This is an important factor that enhances the flame-resistance performance of the WPC material. In the SEM images of the WPC 5 sample (c and d), a homogeneous distribution is achieved, and a strong interfacial interaction between PP and wood flour is noted. This strong interaction plays a crucial role in improving the mechanical properties and durability of the material. The production process significantly affected these characteristics.