3.1. Thermal Decomposition Characteristics
The thermogravimetric decomposition process of raw materials at a given heating rate of 20 °C /min is shown in
Figure 1. The thermal degradation of cellulosic materials occurring in the composite material processing process may influence the composite performance [
30,
31,
32]. At a lower temperature (below 100 °C), BFs and waste HDPE almost did not decompose in the TG curves, and the residual mass was nearly 100%. Wood flour had a weight loss of ~7 % as a result of the loss of adsorbed water and crystal water from the fibers [
33]. The residue of BFs, WF, and waste HDPE was 97.1%, 15.5%, and 3.9%, respectively, at the end of test, which were quite different. BFs had only 3% weight loss in the entire pyrolysis process, which was consistent with the literature [
34], and the residual mass was the largest among the three raw materials, indicating that BF had a good high temperature resistance. At a given heating rate, the pyrolysis of wood mainly went through four stages with increasing temperature. The first stage is from 100 to 250 °C, where the weight loss of wood materials was very small, because only a portion of the cellulose was dehydrated at lower temperature to produce dehydrated cellulose, which was a slow process. The second stage was from 250 °C to 450 °C, which is the main stage of samples pyrolysis. In this range, hemicellulose and cellulose in the wood pyrolysis reaction produce bio oil, dehydrated cellulose further generated small molecular gas and coke, resulting in significant weight loss, and the weight loss rate reached a maximum value at about 350 ° C. The last area corresponded to the slow decomposition of lignin and the final residue, and some carbon and ash were generated [
35,
36,
37]. The two weightless peaks in
Figure 1 of waste HDPE, with complex components, were the decomposition of wood fiber and HDPE. The main structure of HDPE was C-C as the main chain, and its pyrolysis process is a free radical chain reaction. According to the TG-DTG curves of materials in
Figure 1, it can be deduced that the second part was mainly the fracture of the C-C main chain in HDPE, which is the pyrolysis macromolecular polymers into small molecules. The weight loss rate of continuous BFs was the lowest relative to the others, because basalt fibers were mainly composed of SiO
2 as the main component, Al
2O
3 as the secondary, and other oxide ceramic components, which leads to basalt fibers showing excellent high temperature resistance performance [
38]. The initial decomposition temperature
T0 (obtained by the corresponding temperature when the weight loss percentage was 5%) of the three raw materials at the differential heating rates β, the peak temperature
TP (the temperature at the maximum point on the DTG curve), WL (ratio of weight loss percentage relative to
T0 and
TP is the marked sample, respectively), and residual amount are shown in
Table 3. As the weight changes of BFs were so small that the mass loss rates were less than 5 % at 800 °C, the following studies on the decomposition and average
Ea of BFs were no longer conducted.
Table 3 presents the thermal decomposition parameters of the raw materials at the indicated heating rates. The heating rate of the raw material temperature lagged behind that of environmental temperatures, meaning the
T0 and
TP increased. The temperatures when DTG reached the peaks of WF and waste HDPE at a heating rate of 5 °C/min were 342.0 and 447.5 °C, respectively. Once the heating rate approached 20 °C/min, the maximum
TP of WF and waste HDPE appeared at 357.4 °C and 469.6 °C, respectively, which were higher than that at 5, 10, and 15 °C/min.
The thermogravimetric decomposition process of wood plastic composites and WPCs reinforced with varying amounts of BFs at a heating rate of 20 °C/min is shown in
Figure 2. With reference to
Figure 2, the general trend of TG and DTG curves for WPCs and WPCs reinforced with BFs was similar, and the two significant weight loss zones of the composites were in the 250–350 °C and 440 –540 °C ranges. The first weight loss stage of wood-plastic composites was mainly pyrolysis of the mixture of cellulose and hemicellulose in wood flour, and the second was the fracture of the C-C main chain in HDPE [
35]. The coke produced by the pyrolysis of hemicellulose and cellulose covered the surface of HDPE and basalt fibers, which prevented further pyrolysis of the composite materials, and the salt minerals in basalt fibers also have some chemical reactions during this stage; for example, FeO was oxidized to Fe
2O
3, which makes the pyrolysis of composites more difficult [
23]. Thermal decomposition parameters,
T0 and
TP, and residual amounts of composites with different BF contents with varying heating rates are listed in
Table 4. When the heating rate grew from 10 °C/min to 30 °C/min,
T0 of all composites tended to move towards a high temperature side, and the temperature relative to maximal weight loss rate also tended to increase, which was consistent with the view of Kim et al. [
39]. This is because the outside-to-inside heat transfer of the particles takes a longer time at a higher temperature rise rate [
40], which reduces the heat transfer efficiency, so the
TP rises when the maximum weight loss rate is attained. In addition, there was no distinct difference in residual mass among the four heating rates observed, indicating that the change in heating rate made no obvious impact on residue. Compared with A2 (without loading BFs in composites), the
T0 and
TP of composite materials with BF addition shifted to higher temperatures, and with a higher BF content, the
T0 and
TP values were higher. In addition, the content of BF mainly affected the change in the residual amount of the thermal decomposition process of composites, and the residual levels of WF/BF/HDPE composites were significantly greater than those of WF/HDPE composites; furthermore, the composite residuals gradually increased with the increasing BF content, indicating the filling of BFs is beneficial in improving the thermal stability of composites.
Heating rates significantly impact the thermogravimetric process of materials (
Table 3 and
Table 4). With increasing heating rates, the thermogravimetric curves reach higher temperatures, causing the extent of the weight loss of materials to vary. Generally, it is considered that the difference in the degree of weight loss under different heating rates are due to the differences in the reaction process and secondary reactions of WPC at varying heating rates, resulting in the discrepancy concerning wood flour carbonization. In addition, with increasing heating rates, the main reaction ranges of the materials are widened.
The
T0 and
TP values of all materials changed with the increased heating rates, as the actual temperature of the materials lagged behind the surrounding temperatures with the increased heating rates during testing. To exclude the effects of the heating rate,
T0 and
TP were retrieved from the linear extrapolation
.
Figure 3 shows the decomposition characteristic parameters with 6.7% BF content (B3). According to its
T0 and
TP values under different heating rates, linear extrapolation methods were adopted to determine the initial
T0 and
TP values [
41].
Table 5 lists
T0,
TP, and residual mass of composites after linear fitting. It is shown that there is no significant change in
T0 and
TP among wood plastic composite materials with different wood flour concentrations. Compared with wood plastic composites, the addition of BFs has little impact on the
T0 and
TP of materials after eliminating the influences of heating rates, indicating that the BF content within the scope of the test had little influence on the material characteristics of the pyrolysis process temperature, but the addition of BF increased the residual mass of the composites.
3.2. Kinetics of WPC
Upon assessment of the pyrolysis kinetics of the materials, the activation energy (
Ea) tendencies of the materials were assessed.
Figure 4 presents representative iso-conversional plots drawn by the modified C-R and F-W-O methods for waste HDPE and wood-plastic composites (A2), respectively. Assuming a reaction order of n = 1 according to the modified C-R method, the connectivity of
and
at different conversion rates ranging between 0.1 and 0.9 is shown in
Figure 4, and the kinetic parameters calculated with varying heating rates are listed in
Table 6. The fitting lines of different conversion rates were close to parallel, suggesting that they had an approximate
Ea and, consequently, the pyrolysis process could be described as a first-order reaction. From the TG curves, the conversion rate was relatively slow when the pyrolysis conversion rate of materials was between 0 and 10%, and in addition, Yao et al. [
30] found that the reaction mechanism was altered under conditions of high conversion rates when studying the thermal decomposition of different natural fiber materials. As such, we focused on conversions ranging from 0.2 to 0.8 as opposed to the complete process. The correlation coefficients (
R2) of kinetic parameters computed with the improved Coats-Redfern method with different conversion rates (α = 0.2–0.8) were greater than 0.9 (in
Table 6), which indicates that the C-R method can better describe the pyrolysis process. The average
Ea value of WF was close to that of waste HDPE, at 182.7 and 178.9 kJ/mol, respectively. By comparing four kinds of WPCs with different ratios of wood flour, it can be found that, as the proportion of WF increased, the
Ea values also continued to increase. When the WF content accounted for 5 wt.% in the composites, the activation energy calculated by the C-R method was 115.8 kJ/mol, and when the wood flour content accounted for 20%, the
Ea value was 171.3 kJ/mol. This is because the pyrolysis carbonization of wood flour can form carbon on the plastic matrix in WPC materials, which prevents heat transfer, so that the reaction activation energy of the composite material increases.
3.3. Kinetics of WPC Reinforced with BFs
Activation energy trends of wood-plastic composites reinforced with basalt fibers are expressed in the plots of iso-conversional improved C-R and F-W-O methods in
Figure 5. The F-W-O plot for B3 is listed in
Figure 5a, and
Figure 5b illustrates the plot of the modified C-R method. The fitted lines were nearly parallel as the conversion rate varied from 0.5 to 0.8 and the specific activation energy values are presented in
Table 6. The average
Ea obtained by the modified C-R method of wood plastic composites loaded with 3.3 wt% BFs was relatively low at 145.5 kJ/mol, but the differences in average
Ea values with incremental BF contents became obvious. The average
Ea of BF/WF/HDPE composites increased to 204.6 kJ/mol after the fiber loading level increased to 6.7 wt.%.
Ea is a key factor used to ascertain the reaction rate, indicative of the minimum energy necessary to reach an activating molecule from the reactant molecule in the chemical reaction [
42]. Different reactions are matched with different activation energies. Besides, a lower reaction
Ea results in more highly activated molecules and faster reaction rates at a given temperature. Conversely, a higher average
Ea results in a slower reaction rate. The modified Coats-Redfern, Flynne-Walle-Ozawa, and Friedman methods were adopted for ascertaining the
Ea of overall composite materials. The average
Ea values of WF/BF/HDPE composites calculated by modified Coats-Redfern and F-W-O methods were significantly greater than those of WF/HDPE composites. Moreover, the average
Ea increased, which indicates that the addition of BFs improves the thermal stability of the composite, increases the difficulty of thermal decomposition, and enhances the fire safety performance. This is because the BF is resistant to high temperatures, which enables the BF/WF/HDPE composites to have a better thermal stability from the surface and higher
Ea. Through the evaluation of the decomposition activation energy for wood-plastic composites and WF/BF/HDPE composites, it can be judged that composites coated with BFs enjoy prominent edges. However, it is worth mentioning that the
Ea values computed with the Friedman and Kissinger methods are quite different from those obtained by the other two methods, indicating that the Friedman and Kissinger models are not suitable for basalt fiber-reinforced wood-plastic composites. Scholars have made pyrolysis kinetic models of natural materials and fiber-reinforced polymer composites, and the
Ea values are consistent with different kinetic models [
30,
43,
44,
45]. The differences of
Ea in composite materials may be due to the mixture of basalt fibers and wood flour, which leads to changes in the pyrolysis process. In the initial stage of the pyrolysis process (
α = 0.1–0.4), the accelerated decomposition process of hemicellulose, cellulose, and HPDE is prominent, which corresponds to the carbonization of cellulose and fracture of the HDPE backbone. Beyond
α = 0.4, the fitting lines of materials were parallel and close. This is because the carbonization of wood flour in the initial stage hindered the transfer of heat, and basalt fibers still maintained a stable state at this stage, so the energy required for the pyrolysis reaction gradually increased, resulting in
Ea values higher than in the initial stage.
The linear relationship between
and
at different conversion rates obtained via the F-W-O method is displayed in
Figure 4 and
Figure 5. The specific results of
Ea and correlation coefficients are also listed in
Table 6. The iso-conversional F-W-O method, defined as a specific integral method devoid of a functional model, can avert the trouble of choosing the reaction mechanism function until the activation energy is solved, without causing errors related to the hypothesis about the reaction mechanism function. Consequently, it is capable of examining the feasibility of the activation energy retrieved from the Coats-Redfern model method. Thus, the value of
Ea calculated by the F-W-O method was not significantly different from that computed with the modified Coats-Redfern method. The activation energies retrieved with the F-W-O method were relatively higher, but the variation in
Ea was the same as that obtained by the modified Coats-Redfern method. This change was similar to the literature [
45,
46], which proves that the kinetic parameters obtained by these two models are reliable. The activation energy calculated by the Friedman and Kissinger methods was quite different from the value obtained by the modified Coats-Redfern method, which shows that WPCs loaded with basalt fiber cannot be accurately modelled with the Friedman and Kissinger models.
Figure 6 shows the changes in activation energy in raw materials and composite materials with respect to the conversion rate.
Figure 4a clearly shows that the
Ea values of wood flour and waste HDPE increase as the conversion rate increases, and the corresponding activation energy range (153–234 kJ/mol) of HDPE during the entire pyrolysis process is relatively narrow compared with that of wood flour (59–224 kJ/mol). Low
Ea with low rates of conversion and high
Ea with high rates of conversion of polymer composite materials (in
Figure 6b, c) are presented in their pyrolysis processes, which imply different decomposition mechanisms [
47]. It is obvious that, in
Figure 6c, the
Ea values of WPCs reinforced with basalt fibers at any conversion rate over the full pyrolysis range are significantly higher than those of other WPCs without basalt fibers loading. These results demonstrate that polymer composites reinforced with basalt fibers show better thermal stability compared with other WPCs.