Introduction

In recent times, biodegradable polymers like Polylactic acid (PLA) have received increasing attention due to progression of environmental issues caused by conventional petroleum-based plastics [1,2,3]. Applications of PLA include medicines, coatings, food products, packaging materials, disposable cups and plates [4, 5]. The cost of PLA is much higher when it is used as short-life products, and therefore, it is extremely important to blend it with cheaper sustainable fillers like agricultural residues [2, 6]. Moreover, it has been reported that in order to produce fully renewable and biodegradable composites, both polymeric matrix and reinforcement must be derived from renewable resources [7, 8]. One readily available agricultural residue in Uganda and all over the world is rice husks [9, 10]. Over 20 mass% of paddy rice is husk [11, 12]. Rice husks are normally disposed by either landfilling or open burning, both of which methods are associated with negative environmental effects [9, 13]. Additionally, rice husks’ open burning leads to air pollution, and the accruing airborne particles can cause respiratory diseases in humans [14]. Research incorporating rice husks in PLA has exponentially increased because of accruing advantages such as less environmental impact and enhanced mechanical properties [15,16,17,18,19,20]. The role of polylactic acid matrix in the rice husks fiber-reinforced PLA composites is to bind the reinforcing fibers (rice husks) together and give the composite component its shape, and determine its surface quality [21]. Rice husk usage as filler in PLA matrix accounts for only a small percentage of the total production of agricultural fiber-reinforced PLA composites [3]. There are clear gaps in quantifying energy required for their processing, utilization and disposal. The complexity in the synergy between processing, utilization and disposal of rice husks fiber-reinforced PLA composites could be solved by obtaining their combustion characteristics and kinetics behaviors [22, 23].

It is of extreme importance to study the combustion characteristics and kinetic behavior of a material and the ICTAC Kinetic Committee recommended use of multiple heating rates to obtain more reliable combustion characteristic and kinetic parameters as opposed to using a single heating rate [24]. Thermogravimetric analysis (TGA) is the best way to obtain combustion characteristics and kinetic parameters because of its simplicity and accuracy [25,26,27,28,29]. Combustion characteristic parameters focus on combustion reaction mechanisms, while kinetic parameters focus on activation energies of a given material [25]. High activation energy is directly related to better flame retardancy because of formation of an appropriate char residue that increases resistance against flammability [30, 31]. It should be noted that the Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW) have been widely utilized in the literature for obtaining kinetic parameters of different materials [32,33,34,35,36]. In recent years, pyrolysis technology has attracted researchers’ attention because of its minimal environmental pollution, wide application and easy industrialization operation [37]. Processing and utilization of rice husks fiber-reinforced PLA composites could bring some pollution to the atmosphere. Therefore, the rational processing and utilization of rice husks fiber-reinforced PLA composites requires more attention for environmental considerations. On this basis, the research on thermal analysis of rice husks fiber-reinforced PLA composites is imminent. Moreover, only one study has analyzed the pyrolysis characteristics of rice husks fiber-reinforced PLA composites. Ndazi and Karlsson [38] carried out a kinetics study on rice hulls/PLA composites using the Horowitz and Metzger model to understand the influence of hydrolytic degradations on the hulls. They reported activation energies in a 161.60−359.90 kJ mol−1 range, depending on the hydrolytic degradation temperature. Higher molecular mass products were reported to require more energy to decompose and vice versa; hence, the decrease in activation energy was related to molecular mass degradations.

There is more research required to fill the gap in kinetics studies of rice husks fiber-reinforced PLA composites, considering the vast availability of rice husks wastes and to the best of our knowledge, detailed investigations on the kinetics of thermal degradation of rice husks/clay fiber-reinforced PLA composites have never been done before. Moreover, there have been no attempts to determine the synergy of reaction combustion characteristics, kinetic and thermodynamic parameters of PLA composites. Therefore, this work aimed at generating a detailed analysis of the combustion, kinetics and thermodynamic analyses of rice husks/clay fiber-reinforced PLA composites using thermogravimetric analysis at different heating rates. The kinetic parameters were obtained from TG experimental data using Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW) methods.

Experimental

Materials

Polylactic acid 4032D (Huaian Ruanke Trade Co. Ltd., Huaian, China), with a specific gravity 1.24 g cm−3 and melt flow index (MFI) of 7 g/10 min (210 °C/2.16 kg), was used as matrix material. K98 rice husks of 13% moisture content were obtained from Tororo district (latitude 0.45°, longitude 34.05°) in Eastern Uganda. Kaolin was collected from Buwambo clay deposits in Uganda (latitude 0.50°, longitude 32.55°). Sodium Hydroxide (NaOH) (CAS number 1310-73-2) with a molecular mass of 40 g mol−1 was supplied by Lab Access Ltd, Kampala, Uganda.

Surface modification

Part of the K98 rice husks was used as received. This formed the raw rice husks. Alkaline surface modification was done for the other part of rice husks using Sodium hydroxide (NaOH) to form modified rice husks. This involved soaking husks in 4 mass% NaOH solution for 1 h at a liquor ratio of 20:1. Alkali-modified rice husks were then washed in reverse osmosis water until a neutral PH was obtained. The samples were dried for 48 h at room temperature and then, oven-dried at 60 °C overnight to achieve a moisture content of ≤ 5% before storage for characterization. Kaolin was used as received.

Preparation of fiber-reinforced PLA composites

Prior to sample fabrication, PLA matrix pellets, rice husks and clay were held in an oven at 100 °C for 1 h. Rice husks and clay were then each separately ground to ≤ 100 μm using a Retsch Planetary Ball mill PM 100 machine (Hann, Germany). Size reduction was done to ensure homogeneous mixing of reinforcement, filler and the matrix materials. Formulations of rice husks/clay fiber-reinforced PLA composites with different clay filler contents are shown in Table 1. Fiber contents ranged between 10  and  30 mass% [39, 40], while clay contents ranged between 1  and 5 mass% [41]. This enabled studying the effect of both clay loading at a uniform fiber content and rice husks loading at a uniform filler content. PLA was melted with different fiber and filler formulations in a compression molding machine to obtain composites. Uniform mixing between fiber, filler and matrix materials was achieved by a motor attached to the compression rig. Residence time for composites preparation at 195 °C was 10 min. Compression was effected by use of a hand-screw jerk for 10 min under ≈7 MPa loading. Developed samples were then air-cooled for 10 min and stored before mechanical and thermal characterization.

Table 1 Rice husks/clay fiber-reinforced PLA composites formulation
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Thermogravimetric analysis

An Eltra Thermostep non-isothermal Thermogravimetric analyzer, Haan, Germany, was used to determine mass loss of the developed rice husks/clay fiber-reinforced PLA composites with increase in temperature. The samples were heated from 40 to 600 °C at three different heating rates of 16, 25 and 34 °C min−1. High-purity compressed air (Oxygen/Nitrogen = 21:79, > 99.99%) was used for cleaning the crucibles and chamber prior to TGA experimentation. Nitrogen gas was used as the purge gas for pyrolysis experimentation. The flow rate was maintained at 1 L min−1, and the sample masses averaged 1.2 g. TGA also provided combustion explanations in terms of proximate analysis (moisture content, ash content, fixed carbon and volatile matter), differential thermogravimetry (DTG), peak temperatures, char residues and mean reactivity of the developed rice husks/clay fiber-reinforced PLA composites.

Combustion characteristics parameters

For each heating rate (16, 25 and 34 °C min−1), ignition temperature, \(T_{{\text{i}}}\) and burnout temperature, \(T_{{\text{f}}}\) were obtained. Ignition temperature is a measure of difficulty to ignite as low ignition temperature suggests easy ignition, and vice versa. Burnout temperature is the temperature of the combustible substances in the completely burnt state. The ignition and burnout temperatures of the developed rice husks/clay fiber-reinforced PLA composites were determined according to the tangent method proposed by Liu et al. [42]. The flammability index, F, reflects the reactivity of the early stages of combustion and is expressed by Eq. (1).

$$F = \frac{{\left( {\frac{\rm{d}\alpha }{{\rm{d}t}}} \right)_{\max } }}{{T_{{\text{i}}}^{2} }}$$
(1)

where, \(\left( {\frac{\rm{d}\alpha }{{\rm{d}t}}} \right)_{\max }\) is the maximum combustion rate. The larger the value of flammability index, the more combustible a material is. The combustion characteristic index, S, reflects how quick the combustion rate is and is expressed by Eq. (2) [25].

$$S = \frac{{\left( {\frac{{\rm d}\alpha }{{{\rm d}t}}} \right)_{\max } \left( {\frac{{\rm d}\alpha }{{{\rm d}t}}} \right)_{{{\text{mean}}}} }}{{T_{{\text{i}}}^{2} T_{{\text{f}}} }}$$
(2)

where, \(\left( {\frac{\text{d}\alpha }{{\text{d}t}}} \right)_{{{\text{mean}}}}\) is the average combustion rate. The larger the value of the combustion characteristic index, the better the combustion characteristics, and the more intense combustion is [43].

Kinetics modelling

From the Arrhenius equation (\(k = A\text{e}^{{\left( {\frac{\text{ - E}}{\text{RT}}} \right)}}\)), the rate constant in the kinetic equation is closely related to temperature. A is a pre-exponential factor and T is the absolute temperature. A non-isothermal process can be regarded as an isothermal process within an infinitesimal short time interval [44]. As such, the kinetic equation for an isothermal homogeneous phase reaction can be adopted for the pyrolysis of the developed rice husks/clay fiber-reinforced PLA composites under a programmed heating condition by the means of thermal analysis [see Eq. (3)].

$$\frac{\rm{d}\alpha }{{\rm{d}t}} = k \times f\left( \alpha \right)$$
(3)

where \(k\) is the rate constant of the reaction, \(\alpha\) is the immediate mass loss ratio during composites’ pyrolysis and is given by Eq. (4), \(f\left( \alpha \right)\) is a function that reflects the apparent kinetics of pyrolysis of substances, which is expressed as shown in Eq. (5).

$$\alpha = \frac{{\left( {m_{0} - m} \right)}}{{\left( {m_{0} - m_{{\text{f}}} } \right)}}$$
(4)

where \(m_{0} ,m, m_{0}\) are the initial, instantaneous and final mass of the sample, respectively.

$$f\left( \alpha \right) = \left( {1 - \alpha } \right)^\text{n}$$
(5)

where n is the order of the reaction.

Combining Eqs. (3)−(5) gives:

$$\frac{{\rm d}\alpha }{{{\rm d}t}} = A\text{e}^{{\left( {\frac{ - \text{E}}{{\text{RT}}}} \right)}} \left( {1 - \alpha } \right)^{{\rm n}}$$
(6)

For a given heating rate, \(\beta = \frac{{\rm d}T}{{\rm d{}t}}\), Eq. (7) can be obtained as the non-isothermal reaction rate.

$$\frac{{\rm d}\alpha }{{{\rm d}T}} = \frac{{\rm d}\alpha }{{{\rm d}t}} \times \frac{{\rm d}t}{{{\rm d}T}}$$
(7)

Therefore, substituting Eq. (6) into Eq. (7) gives:

$$\frac{{\rm d}\alpha }{{{\rm d}T}} = \frac{A}{\beta }\text{e}^{{\left( {\frac{ - {\text{E}}}{{\text{RT}}}} \right)}} \left( {1 - \alpha } \right)^{{\rm n}}$$
(8)

In the current study, the data obtained from TG/DTG were used to determine the kinetic parameters [activation energy (E), pre-exponential factor (A) and order of reaction (n)], based on Arrhenius equation. Further, these kinetic parameters can be estimated graphically by integrating Eq. (8) and then applying mathematical approximation for exponential term. In the present study, two integral forms namely Kissinger–Akahira–Sunose (KAS) model and Ozawa–Flynn–Wall (OFW) methods were used to calculate the apparent activation energy (E) at specific conversion time (\(\alpha\)) [25, 45, 46].

Kissinger–Akahira–Sunose (KAS) method

In the KAS method, mathematical approximation for exponential term is assumed and after approximation and rearrangement, the solution is given by Eq. (9) [47].

$$\ln \left( {\frac{{\beta_{{\text{i}}} }}{{T_{{{\upalpha }_{{\text{i}}} }}^{2} }}} \right) = \ln \left( {\frac{{A_{{\upalpha }} E_{{\upalpha }} }}{R \cdot g\left( \alpha \right)}} \right) - \frac{{E_{{\upalpha }} }}{{RT_{{{\upalpha }_{{\text{i}}} }} }}$$
(9)

A plot of \(\ln \left( {\frac{{\beta_{{\text{i}}} }}{{T_{{{\upalpha }_{{\text{i}}} }}^{2} }}} \right)\) against \(\frac{ - 1}{{T_{{{\upalpha }_{{\text{i}}} }} }}\) for a given value of conversion, \(\alpha\), yields a straight line with slope \(\frac{{ - E_{{\upalpha }} }}{R}\) and an intercept \(\ln \left( {\frac{{A_{{\upalpha }} E_{{\upalpha }} }}{R.g\left( \alpha \right)}} \right)\), from which \(E_{{\upalpha }}\) can be calculated.

Ozawa–Flynn–Wall (OFW) method

The OFW method uses the Doyle’s approximation [48], and OFW equation is expressed by Eq. (10) [49].

$$\ln \beta_{{\text{i}}} = \ln \left( {\frac{{A_{{\upalpha }} E_{{\upalpha }} }}{R.g\left( \alpha \right)}} \right) - 5.332 - 1.052\frac{{E_{{\upalpha }} }}{{RT_{{{\upalpha }_{{\text{i}}} }} }}$$
(10)

where \(g\left( \alpha \right)\) is constant at a given value of conversion. A plot of \(\ln \beta_{i}\) against \(\frac{1}{{T_{{{\upalpha }_{{\text{i}}} }} }}\) for a given value of conversion, \(\alpha\), yields a straight line with slope \(\frac{{ - 1.052E_{{\upalpha }} }}{R}\) and an intercept \(\ln \left( {\frac{{A_{{\upalpha }} E_{{\upalpha }} }}{R.g\left( \alpha \right)}} \right) - 5.332\).

Thermodynamic analysis

The OFW method was used to obtain average thermodynamic characteristics of the developed rice husks/clay fiber-reinforced PLA composites, including change in Gibbs free energy (ΔG), change in enthalpy (ΔH) and change in entropy (ΔS) using Eqs. (11)−(13) [50,51,52].

$$\Delta G = E_{{\upalpha }} + \left( {RT_{{\text{m}}} } \right)\ln \left( {\frac{{K_{{\text{B}}} T_{{\text{m}}} }}{hA}} \right)$$
(11)

where \(A = \frac{{\beta E_{{\upalpha }} \text{e}^{{\left( {\frac{{E_{{\upalpha }} }}{{RT_{{\text{m}}} }}} \right)}} }}{{RT_{{\text{m}}}^{2} }}\)

$$\Delta H = E_{{\upalpha }} - \left( {RT_{{\text{m}}} } \right)$$
(12)
$$\Delta S = \left( {\frac{\Delta H - \Delta G}{{T_{{\text{m}}} }}} \right)$$
(13)

where \(K_{{\text{B}}}\) is the Boltzmann constant, \(h\) is the Plank’s constant and \(T_{{\text{m}}}\) is the peak temperature during combustion.

Results and discussions

Proximate composition

Proximate composition of neat PLA and the rice husks/clay fiber-reinforced PLA composites is shown in Table 2. It is clear that increasing heating rates from 16 to 25 to 34 °C min−1 led to increasing ash and volatile matter compositions, while moisture contents and fixed carbon were subsequently reduced. Additionally, NaOH modification of rice husks used as reinforcement in PLA matrix led to decreasing volatile matter and increasing fixed carbon, ash and moisture compositions in the accruing PLA composites. Average volatile matter compositions ranged between 83.17–91.56% and between 81.95–90.76% for PLA composites developed with raw rice husks and modified rice husks, respectively. Such high volatile matter contents indicate the composites’ suitability for the pyrolysis process [52]. It should be noted that increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) and increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) generally decreased volatile matter compositions. The average fixed carbon compositions ranged between 2.30–5.64% and between 2.57–6.46% for PLA composites developed with raw rice husks and modified rice husks, respectively. Increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) generally decreased fixed carbon contents, while an increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) generally led to an increase in fixed carbon contents of the developed rice husks/clay fiber-reinforced PLA composites.

Table 2 Proximate analysis for the developed rice husks/clay fiber-reinforced PLA composites
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Average ash contents ranged between 5.40–9.56% and between 5.85–10.46% for PLA composites developed with raw rice husks and modified rice husks, respectively. Low ash contents in PLA composites are favorable for the pyrolysis process because low ash content minimizes risks of fouling at high temperature [52, 53]. The obtained ash contents in developed composites show that the formation of silica ash or silicon carbide signal that greater energy is expected to initiate combustion [54]. Based on this, higher flame retardancy is expected more in modified rice husks fiber-reinforced PLA composites. It should be noted that increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) and increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) generally increased ash contents of the developed composites. This is extremely important to note because neat PLA had no ash compositions and clay as well as rice husks acted as impurities that are ash-forming compounds [55]. For moisture contents, values ranged between 0.62–1.00% and between 0.69–0.96% for PLA composites developed with raw rice husks and modified rice husks, respectively. Reduced moisture content of the modified rice husks PLA composites is due to the fact that alkali pretreatment enhanced removal of hemicelluloses which implies that the amount of bonding sites is reduced and the lumen structure of fibers is compressed [56]. The low moisture, below the authorized limit (≤ 10%), signals the composites’ appropriateness for the pyrolysis [50, 57]. Moreover, this is expected because rice husks inherently have low moisture which makes the accruing PLA composites be able to resist moisture absorption as well as attain better mechanical properties [54]. Increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) generally decreased moisture contents, while an increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) generally led to an increase in moisture contents of the developed rice husks/clay fiber-reinforced PLA composites.

TG and DTG results

Figure 1b–e shows the TG curves for PLA composites developed with raw rice husks, while Fig. 1g–j shows the TG curves for PLA composites developed with NaOH modified K98 rice husks at three different heating rates of 16, 25 and 34 °C min−1. The respective TG curves show the typical appearance of pyrolysis of PLA composites and from them, the thermal phases for each of the heating rates can be located [58]. Thermal decomposition of the developed rice husks/clay fiber-reinforced PLA composites took place in three main stages. The first stage is attributed to moisture evaporation from their surface, and it occurred up to around 300 °C, even though sizable mass loss was only clear from about 290 °C [59]. During this stage, mass loss (≤ 8%) of some light components, including inbound water and light volatile components, especially in the rice husks occurs [57, 60, 61]. From the findings of Braga et al., [62], lignocellulosic materials with less than 10% water content are feasible for combustion and this agrees well with the moisture contents of the rice husks/clay fiber-reinforced PLA composites presented in Table 1. A second stage due to hemicellulose and cellulose decomposition is then observed until around 420 °C [59, 63, 64]. An increase in heating rate leads to a clear shift of this stage to the right-hand side, signaling that higher temperatures are required to cause decomposition of hemicellulose and cellulose [65, 66]. This trend could be due to the principle of time–temperature superposition [23, 26]. It is also possibly because at the lower heating rate (16 °C min−1), sufficient time is available for heating because of the linear temperature profile between the outer surface and inner core of the composite material, while at higher heating rates (25 and 34 °C min−1), the temperature gradient is sufficient between outer and inner core of composite material [44, 67,68,69,70]. Finally, depolymerization of PLA and clay takes place until char residues are left at 600 °C [71]. The ability of the rice husks/clay fiber-reinforced PLA composites to reach such high temperatures due to the inherent lignin in rice husks and the accruing silica formation [72].

Fig. 1
figure 1

TG curves for neat PLA (a, f) and fiber-reinforced PLA developed with: (b–e)- raw rice husks; (g–j)- modified rice husks (b, g-10% husks; c, h-1% clay; d, i-5% clay; e, j-30% husks)

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From Fig. 1, an observation of a relatively smaller degradation in the temperature range from 105 to 150 °C in case of PLA composites was noted due to the presence of moisture in the rice husks fibers. However, this step could not be seen in case of neat PLA. Neat PLA shows degradation in a single stage between around 290 and 400 °C with the maximum degradations occurring at 388.34, 396.62, and 406.34 °C at 16, 25 and 34 °C min−1, respectively. The thermal degradation behavior of composites is extremely important as it can decrease the mechanical properties of the composites, thus effectively rendering the composite products useless in some applications [73]. From Fig. 1b–e, g–h, it can be clearly seen that the char residues obtained by combustion of rice husks/clay fiber-reinforced PLA composites increased with the addition of rice husks as compared to neat PLA. For example, at 25 °C min−1, at the same clay filler composition (5 mass%), char residues in the raw rice husks (20 mass%) PLA composites (see Fig. 1d) were 13.81% at 600 °C while for modified rice husks (20 mass%) PLA composites (see Fig. 1i), char residues were 14.77%. Increasing char residues with alkali modification are because the modification process roughens rice husks surface, which facilitates superior interlocking and adhesion between PLA matrix and rice husks fiber [71, 74]. It should be noted that increase in clay composition from 1 to 5 mass% (see Fig. 1c–d, h–i) led to increasing char residues. The increase in char residues with increasing clay loading is expected because clay particles allow for stabilizing and barrier effects [75]. As such, enhanced thermal stability in the developed rice husks/clay fiber-reinforced PLA composites is obtained [71]. Char residues lead to formation of a char layer with more thermal stability and physical integrity, which results in an improvement on flame retardancy of developed fiber-reinforced PLA composites [76, 77].

The effect of increase in rice husks loading can be observed in Fig. 1b, g well as Fig. 1e, j for raw and modified rice husks at 10–30 mass% rice husks loading, respectively. At 25 °C min−1 heating rate in 10 mass% reinforced PLA composites, 6.44% char residues could be noticed at 600 °C for raw rice husks composites (see Fig. 1b) while for modified K98 rice husks PLA composites, char residues increased to 7.63% (see Fig. 1g). Meanwhile, at 30 mass% rice husks loading, the highest char residues were observed in the composites (see Fig. 1e, j. High char residues are a depiction of enhanced flame retardancy in fiber-reinforced composites [78, 79]. It should be noted that increase in char residues was higher with incorporation of modified rice husks because the alkali introduced during the modification process was converted to irrecoverable salts or incorporated as salts, which provided higher thermal stability to the rice husks during the alkali pretreatment [58, 80]. Moreover, it could also be because of removal of low thermally stable components (hemicellulose and pectins) in the husks [81].

Figure 2b–e shows the DTG curves for PLA composites developed with raw rice husks, while Fig. 2g–j shows the DTG curves for PLA composites developed with NaOH modified K98 rice husks at three different heating rates of 16, 25 and 34 °C min−1. DTG curves show decomposition maximums of single peaks, owing to the degradation of cellulose in the rice husks/clay fiber-reinforced plastic composites [82]. The trend obtained in rice husks/clay fiber-reinforced PLA composites in the current study is similar to that reported on other PLA composites by Tran et al., [17]. In the DTG curves, the point of highest intensity corresponds to peak temperature at which the respective reaction occurs most dominantly in the specific rice husks/clay fiber-reinforced PLA composites. All the developed PLA composites had DTG results of  ≤ 2.04% min−1 which is much less than the 2.77−3.12% min−1 range realized in neat PLA. This signals that due to the rice husks and clay in PLA matrix, the rate of decomposition of the developed composites reduced as a function of time for the second temperature zone observed in the TG curves. In this case, rice husks and clay acted as flame retardants to thermal decomposition of the developed PLA composites. The second stage of degradation shows the peak temperature ranges for the developed rice husks/clay fiber-reinforced PLA composites. These high peak temperatures are extremely important in application, because the composites will be able to withstand these temperatures before severe degradation. Moreover, increase in heating rate leads to increasing peak temperatures as observed by the shifts of the peaks to the right, due to shorter reaction time at higher heating rates, so the temperature required for degradation is higher [52, 57, 60, 61, 67, 72]. This might be because when the heating rate is high, equilibrium reaches slowly, and consequently, higher peak temperature is observed, probably due to the slower diffusion of heat to the PLA composites [26]. For example, increasing heating rate of 10 mass% raw rice husks fiber-reinforced PLA composites from 16 to 25 °C min−1 to 34 °C min−1 led to respective increases in peak temperature from 390.01 to 399.48 °C and to 408.01 °C (see Fig. 2b).

Fig. 2
figure 2

DTG curves for neat PLA (a, f) and fiber-reinforced PLA developed with: (b–e)- raw rice husks; (g–j)- modified rice husks (b, g-10% husks; c, h-1% clay; d, i-5% clay; e, j-30% husks)

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Clay addition into rice husks fiber-reinforced PLA was noted to increase peak temperatures because clay has no heating value [9] (see Fig. 1c–d, h–i for raw and modified rice husks PLA composites, respectively). However, inclusion of rice husks in PLA matrix decreased its pyrolysis rate, hence decreasing peak temperatures of accruing PLA composites compared to neat PLA (396.62 −406.34 °C) [58, 81, 83]. From DTG analysis, it is obvious that peak temperatures decreased with increase in rice husks alkali concentration. The decrease in peak temperatures with increasing rice husks loading is because agricultural fibers degrade at temperatures lower than that of PLA matrix [84, 85]. As such, thermal stability of PLA composites is slightly reduced with increasing rice husks composition. Even then, thermal stability of the composites developed in this study is still higher than that required for various applications, including automotive industries [73]. Actually, according to Kumar and Das, [73], it is necessary to use thermogravimetric analysis to understand the thermal behavior of biocomposites when intended for application in automotive industry. The minimal differences and shifts in peak temperatures for both raw and modified rice husks/clay fiber-reinforced PLA composites necessitated further separate investigation on the pyrolysis kinetics to understand their activation energies as well as thermodynamic characteristics (see “Kinetics analysis” section). Other researchers also reported similar decreases with increasing agricultural fiber loading in polymer composites [54, 80, 86, 87]. Reduction in peak temperatures with alkali modification was noted possibly because 4 mass% NaOH concentration is high for rice husks, causing an excess removal of materials covering the cellulose surface, hence damaging of the fiber structure [88, 89].

Combustion characteristics parameters

Table 3 presents the combustion characteristic parameters of neat PLA and accruing rice husks/clay fiber-reinforced PLA composites. Increase in heating rate for both neat PLA and PLA composites led to an increase in ignition, peak and burnout temperatures. This result confirms the results for thermal stability presented in Figs. 1, 2. For example, increasing heating rate of 10 mass% raw rice husks fiber-reinforced PLA composites from 16 to 25 °C min−1 as well as to 34 °C min−1 led to respective increases in ignition, peak and burnout temperatures from 352.78, 390.01 to 407.45 °C to 364.48, 399.48–415.07 °C as well as to 372.42, 408.01–424.40 °C, respectively. For 10 mass% modified rice husks fiber-reinforced PLA composites, ignition, peak and burnout temperatures from 330.61, 376.89 to 394.0.46 °C to 345.01, 384.84–400.23 °C as well as to 356.46, 394.89 and 408.16 °C, respectively. Increasing ignition, peak and burnout temperatures signal enhanced thermal stability since higher temperatures are required to onset degradation of the rice husks [90]. This result was more pronounced in raw rice husks fiber-reinforced PLA composites (see Table 3). Lower thermal stability in modified rice husks fiber-reinforced PLA composites was possibly because during alkali modification, some thermally stable constituents of rice husks were removed from the surface of the rice husks [91].

Table 3 Combustion characteristics parameters for the developed rice husks/clay fiber-reinforced PLA composites
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It is also clear that increase in clay loading from 1 to 5 mass% increased both ignition and burnout temperatures. This is expected as increasing clay contents are noted to delay the onset of combustion and hence increase thermal stability of such composites [71]. For example, at a 25 °C min−1 heating rate, when raw rice husks (20 mass%) were used as reinforcement, ignition temperature increased from 355.70 to 359.69 °C with increase in clay loading from 1 to 5 mass%, respectively. Burnout temperatures at these condition also increased from 407.73 to 410.28 °C with increase in clay loading. The effect of increasing rice husks loading from 10 to 30 mass% was decreasing ignition and burnout temperatures in the respective rice husks/clay fiber-reinforced PLA composites. For example, for a heating rate of 25 °C min−1, ignition and burnout temperatures of raw rice husks PLA composites (at 20 mass% clay filler loading), decreased from 364.48 to 415.07 °C to 349.48 °C and 400.23 °C, respectively, when rice husks loading was increased from 10 to 30 mass%. The reason for this trend is the silica formed during combustion of the rice husks in the plastic composites, which prevents heat from spreading [9, 92, 93].

Maximum and average combustion rates for neat PLA were 3.12–0.30% min−1, respectively. Incorporation of rice husks and clay led to decreasing values because they acted as flame retardants in PLA matrix to reduce its ability to combust [94]. Maximum and average combustion rates of raw rice husks fiber-reinforced PLA composites were higher than those for modified rice husks fiber-reinforced PLA composites. For example, for 10 mass% raw rice husks fiber-reinforced PLA composites at 16 °C min−1, maximum and average combustion rates were 1.98–0.29% min−1, respectively. For 10 mass% modified rice husks fiber-reinforced PLA composites at 16 °C min−1, maximum and average combustion rates decreased to 1.79–0.23% min−1, respectively. Clearly, alkali modification of rice husks enhanced flame retardancy of accruing composites due to enhanced adhesion between the fibers, clay and PLA matrix materials [95]. It should be noted that increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) and increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) generally decreases both maximum and average combustion rates of the developed composites. For example, for 10 mass% raw rice husks fiber-reinforced PLA composites at 16 °C min−1, maximum and average combustion rates were 1.98–0.29% min−1, respectively, but these values decreased to 1.79–0.23% min−1, respectively. Similarly, for raw rice husks fiber-reinforced PLA composites filled with 1 mass% clay at 16 °C min−1, maximum and average combustion rates were 1.97–0.22% min−1, respectively, but these values decreased to 1.89–0.22% min−1, respectively.

Flammability index, combustion characteristic index and mean reactivity of both raw and modified rice husks fiber-reinforced PLA composites decreased with increase in heating rate (see Table 3). This trend was expected since it takes longer to transfer heat from the external environment to the interior of the composites, thereby creating a hysteresis effect [25]. Moreover, the low flammability indices, combustion characteristic indices and mean reactivities obtained indicate that the rice husks/clay fiber-reinforced PLA composites generally have very poor combustion performance, possibly due to the silica formation during combustion of rice husks embedded materials which creates a flame retardancy effect [96, 97]. The flammability indices, combustion characteristic indices and mean reactivities obtained for the PLA composites are much lower than those for neat PLA (2.00 × 10−5 –2.44 × 10−5% min−1 °C−2, 0.87 × 10−8 –1.79 × 10−8% min−2 °C−3 and 6.97 × 10−3 –8.04 × 10−3% min−1 °C−1, respectively) which signals that rice husks and clay improved flame retardancy of accruing PLA composites [96]. This confirms the TGA results obtained Figs. 1, 2. From Table 3, it can be observed that increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) and increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) generally decreases flammability indices, combustion characteristic indices and mean reactivities of the developed PLA composites. For example, for 10 mass% raw rice husks fiber-reinforced PLA composites at a heating rate of 16 °C min−1, flammability index, combustion characteristic index and mean reactivity were 1.59 × 10−5% min−1 °C−2, 1.15 × 10−8% min−2  °C−3 and 5.09 × 10−3% min−1 °C−1, respectively, but these values decreased to 1.59 × 10−5% min−1 °C−2, 0.89 × 10−8% min−2 °C−3 and 4.78 × 10−3% min−1 °C–1, respectively. Similarly, for raw rice husks fiber-reinforced PLA composites filled with 1 mass% clay at 16 °C min−1, flammability index, combustion characteristic index and mean reactivity were 1.65 × 10−5% min−1 °C−2, 0.91 × 10−8% min−2  °C−3 and 5.19 × 10−3% min−1 °C−1, respectively, but these values decreased to 1.55 × 10−5% min−1 °C−2, 0.87 × 10−8% min−2  °C−3 and 4.90 × 10−3% min−1 °C−1, respectively.

Kinetics analysis

Figure 3b–e shows the variation in degree of conversion for PLA composites developed with raw rice husks, while Fig. 3g–j shows the the variation in degree of conversion for PLA composites developed with NaOH modified K98 rice husks at three different heating rates of 16, 25–34 °C min.−1. This was done for better understanding of degradation of rice husks/clay fiber-reinforced PLA composites [98]. In order to compute the kinetic parameters, the same values of conversion rate (α) in the range of 0.2–0.8 were considered for the three heating rates [45]. This range was considered because during the fitting of data using the KAS and OFW methods, the lowest conversion value (0.1) and the highest conversion value (0.9) did not fit well because of lower correlation values [50, 99]. As the temperature reaching the composite sample rises, conversion rates increased due to reduction in original mass of the rice husks/clay fiber-reinforced PLA composites [see Eq. (4)]. The trend followed by the conversion curves is similar to that presented in TG and DTG curves, which present a decomposition in original mass (see Figs. 1, 2). Moreover, increase in heating rate tends to shift the conversion curve to the right, signaling higher temperatures are required to decompose constituents of rice husks/clay fiber-reinforced PLA composites at higher heating rates. Similar depictions have been presented elsewhere [100]

Fig. 3
figure 3

Conversion curves for neat PLA (a, f) and fiber-reinforced PLA developed with: (b–e)- raw rice husks; (g–j)- modified rice husks (b, g-10% husks; c, h-1% clay; d, i-5% clay; e, j-30% husks)

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Kinetic parameters of the developed rice husks/clay fiber-reinforced PLA composites, such as activation energy and pre-exponential factor for pyrolysis, were determined based on the thermogravimetric analysis data obtained under different heating rates (16, 25 and 34 °C min−1). Using the KAS method, activation energies for progressive conversion values need to be calculated according to Eq. (9). Figure 4b–e shows the linear plots of ln (β/T2) vs 1/T for the conversion values within 0.2–0.8 for PLA composites developed with raw rice husks, while Fig. 4g–j shows linear plots of ln (β/T2) vs 1/T for the conversion values within 0.2–0.8 for PLA composites developed with NaOH modified K98 rice husks. The calculated activation energies and pre-exponential factors from the slopes and intercepts of the KAS plots are listed in Table 4.

Fig. 4
figure 4

Kinetic plots for neat PLA (a, f) and fiber-reinforced PLA developed with: (b–e)- raw rice husks; (g–j)- modified rice husks (b, g-10% husks; c, h-1% clay; d, i-5% clay; e, j-30% husks) by KAS method

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Table 4 Kinetic parameters for the developed rice husks/clay fiber-reinforced PLA composites
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Using the OFW method, activation energies for progressive conversion values need to be calculated according to Eq. (10). Figure 5b–e shows the linear plots of ln (β) vs 1/T for the conversion values within 0.2–0.8 for PLA composites developed with raw rice husks, while Fig. 5g–j shows linear plots of ln (β) vs 1/T for the conversion values within 0.2–0.8 for PLA composites developed with NaOH modified K98 rice husks. The calculated activation energies and pre-exponential factors from the slopes and intercepts of the OFW plots are listed in Table 4.

Fig. 5
figure 5

Kinetic plots for neat PLA (a, f) and fiber-reinforced PLA developed with: (b–e)- raw rice husks; (g–j)- modified rice husks (b, g-10% husks; c, h-1% clay; d, i-5% clay; e, j-30% husks) by OFW method

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For both raw and modified rice husks fiber-reinforced PLA composites using both KAS and OFW methods, R2 values were close to 1, which signalled accuracy of the fitting method for the values of the three heating rates [101]. From Table 4, it is evident that both activation energy and pre-exponential factors generally increased with increase in conversion rate because as temperatures increased, changes in mass of the PLA composites became very minimal and therefore, more energy was required for a reaction or for transformation to occur. Moreover, for the KAS method, high pre-exponential factors (between 1.38 + 03 min−1 and 1.10E+03 min−1) for KAS method and between (8.18E+10  min−1 and 17.33E+11 min−1) for OFW method depict low frequency of molecular collisions in the reaction mixture [51]. Additionally, the range for the obtained pre-exponential factors for both KAS and OFW methods was quite narrow, which indicated the reliability of computed activation energy values [52].

Average activation energy from KAS method for raw rice husks fiber-reinforced PLA composites ranged between 137.83–143.99 kJ mol−1 while those for modified rice husks fiber-reinforced PLA composites ranged between 124.51–133.95 kJ mol−1. From OFW method, average activation energies for raw rice husks fiber-reinforced PLA composites were higher and ranged between 141.24–146.92 kJ mol−1 while those for modified rice husks fiber-reinforced PLA composites ranged between 128.17–134.58 kJ mol−1. Similarly, using KAS, average pre-exponential factors of raw rice husks fiber-reinforced PLA composites ranged between 4.72E+02 and 1.00E+02 min−1 while those for modified rice husks fiber-reinforced PLA composites ranged between 1.38 E+02–2.70E+02 min−1. Using OFW, average pre-exponential factors for raw rice husks fiber-reinforced PLA composites ranged between 3.10E+11–7.33E+11 min−1 while those for modified rice husks fiber-reinforced PLA composites ranged between 8.18E+10–1.55E+11 min−1. It should be noted that neat PLA had activation energies of 139.22–142.64 kJ mol−1 using the KAS and OFW method. These higher values indicate the decrease in thermal stability with incorporation of rice husks due to poor adhesion with PLA matrix [102].

More specifically, from the KAS method, it was noted that increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) generally increased activation energy from 139.38 to 143.99 kJ mol−1 in raw rice husks PLA composites. Similarly, using OFW method, increase in clay composition from 1to 5 mass% at constant rice husks fiber loading (20 mass%) generally increased activation energy from 142.41 to 145.83 kJ mol−1 in raw rice husks PLA composites. Evidently, clay incorporation enhanced activation energy by acting as a barrier, constraining the mobility to chain, which consequently hindered the decomposition process of the developed composites [83]. Meanwhile, using the KAS method, increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) led to reducing activation energy from 143.56 to 137.83 kJ mol−1 in raw rice husks fiber-reinforced PLA composites and from 133.95 to 124.51 kJ mol−1 in modified rice husks fiber-reinforced PLA composites. A similar trend was noted when using the OFW method. From this, increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) led to reducing activation energy from 146.92 to 141.24 kJ mol−1 in raw rice husks fiber-reinforced PLA composites and from 137.50 to 128.17 kJ mol−1 in modified rice husks fiber-reinforced PLA composites. The decreasing trend in activation energies with increasing rice husks loading is due to cleavage of PLA chains during composites fabrication [103].

The variation of activation energy with increase in conversion rate (α) for neat PLA and rice husks/clay fiber-reinforced PLA composites is shown in Fig. 6, depicting that the activation energy of both composites developed with raw husks (see Fig. 6b–e) and those developed with modified rice husks (see Fig. 6g–j) is highly dependent on conversion rate. This also confirmed the complexity of the process of the PLA composites’ pyrolysis, since their pyrolysis reaction is not a one-step reaction [2, 104,105,106]. The low correlation coefficient (R2) of conversion rate values less than 0.2 and greater than 0.8 was not considered. From Fig. 6, it is very clear that increase in conversion rate led to increasing activation energy, due to thermal degradation of different components of PLA composites with increasing temperature. This trend is similar to other studies based on agricultural residue fiber-reinforced PLA composites [2, 101, 107]. The lower molecular weight compounds decayed at moderate energy and lower temperatures, while degradation of higher molecular weight compounds needed more energy at greater temperature [23, 25]. It should be noted that OFW method presented higher activation energy values compared to the KAS method, but the variations among these methods at each conversion rate were very minimal. The reason for this is that the KAS method considers both heating rate and temperature at each instant, while OFW method depends only on heating rates applied [102]. This was consistent with the other literature on agricultural residue fiber-reinforced PLA composites [100].

Fig. 6
figure 6

Activation energy versus conversion rates for neat PLA (a, f) and fiber-reinforced PLA developed with: (b–e)- raw rice husks; (g–j)- modified rice husks (b, g-10% husks; c, h-1% clay; d, i-5% clay; e, j-30% husks)

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Thermodynamic parameters

Average thermodynamic parameters of the developed fiber-reinforced PLA composites calculated using the OFW method are presented in Table 5. It is clear that pre-exponential factors (A), ΔH (enthalpy of reaction) and ΔS (change in entropy) increased with increase in conversion rate while ΔG (Gibb’s free energy) showed an opposite trend as increasing conversion rates led to decreasing ΔG values.

Table 5 Average thermodynamic parameters for the developed rice husks/clay fiber-reinforced PLA composites
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The ΔH (enthalpy of reaction) is the energy exchanged between reactants and products during the chemical reaction [52]. Average ΔH ranged between 135.79–141.33 kJ mol−1 and between 122.92 –132.02 kJ mol−1 for PLA composites developed with raw rice husks and modified rice husks, respectively. Evidently, rice husks pre-treatment decreased the enthalpy of reaction in the developed rice husks/clay fiber-reinforced PLA composites which points to the fact that modified rice husks fiber-reinforced PLA composites required lower energy to decompose, compared to raw rice husks fiber-reinforced PLA composites, probably due to the difference in the chemical composition [50]. Additionally, it should be noted that increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) generally increased enthalpy values by about 2.47%, while increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) led to reducing enthalpy values by over 6.89%. This is expected because clay particles allow for stabilizing and barrier effects while rice husks addition rendered PLA polymer molecules immobilized at the interface, hence decreasing mobility of polymer molecules [75, 108]. Low energy barrier (≤ 6.00 kJ mol−1) between the activation energies (see Table 5) and the ΔH values indicated that initiation occurs easily, due to the fact that the lower difference between ΔH and activation energy favors the complex formation [52, 67, 109, 110].

Gibbs free energy reveals the overall energy change of the system, and lower ΔG values indicate favorable decomposition [51, 111]. Average ΔG values ranged between 163.09 –167.78 kJ mol−1 and between 160.00 –167.11 kJ mol−1 for PLA composites developed with raw rice husks and modified rice husks, respectively. Rice husks pre-treatment decreased Gibbs free energy for the developed rice husks/clay fiber-reinforced PLA composites which pointed to the fact that modification increased favorability for reaction with an extreme assessment of the heat flow and disorder change [112]. Increase in clay composition from 1 to 5 mass% at constant rice husks fiber loading (20 mass%) generally increased Gibbs free energy, while increase in rice husks fiber content from 10 to 30 mass% at constant clay filler amount (3 mass%) led to reducing Gibbs free energy.

The change in entropy (ΔS) is a measure of disorders, and a negative ΔS value depicts decreased disorders of products formed through bond dissociations, i.e., ordered product formation [50, 51, 67]. The range for average ΔS values was between  − 0.05 and − 0.04 kJ mol−1 K−1 and between  − 0.06 and − 0.05 kJ mol−1 K−1 for PLA composites developed with raw rice husks and modified rice husks, respectively. The low and negative value obtained for raw and treated rice husks fiber-reinforced PLA composites revealed that extended time is required for thermal decomposition of active material in the composites [110, 112, 113]. This was even more pronounced after alkali pre-treatment of rice husks.

Pre-exponential factor is a depiction of frequency of molecular collisions during combustion and average values ranged between 1.743E+11 –7.26E+11 min−1 and between 5.06 E+10– 9.85E+10 min−1 for PLA composites developed with raw rice husks and modified rice husks, respectively. The alteration in the pre-exponential factor with increasing conversion rate showed that developed rice husks/clay fiber-reinforced PLA composites have a multifaceted fraction and formed complex reaction mechanisms at the time of pyrolysis [50]. Actually, increase in conversion rate led to increasing pre-exponential factor values, which indicated the reliability of the computed activation energy values [52]. The results showed that raw rice husks fiber-reinforced PLA composites had a higher pre-exponential factor than modified rice husks fiber-reinforced PLA composites, which indicates that the decomposition will be higher in the former.

Conclusions

In this study, the thermal degradation behavior, pyrolysis kinetics and thermodynamic characteristics of the developed rice husks/clay fiber-reinforced PLA composites were investigated using thermogravimetric analysis under three heating rates (16, 25 and 34 °C min−1). TGA provided mass loss and DTG results for the developed PLA composites. Kissinger–Akahira–Sunose (KAS) and Ozawa–Flynn–Wall (OFW) methods were used for pyrolysis kinetics analysis. Average moisture, ash, volatiles and fixed carbon compositions for the developed rice husks/clay fiber-reinforced PLA composites were in ranges 0.62–1.00%, 5.40–10.46%, 81.95–91.56% and 2.43–6.67%, respectively. Increase in heating rate for both neat PLA and PLA composites led to an increase in ignition, peak and burnout temperatures, which signaled enhanced thermal stability. Incorporation of rice husks and clay led to decreasing maximum and average combustion rates of PLA because they acted as flame retardants in PLA matrix to reduce its ability to combust. Flammability indices, combustion characteristic indices and mean reactivities of PLA composites are much lower than those for neat PLA, and they decreased with increasing heating rates since it takes longer to transfer heat from the exterior to the interior of the PLA composites, creating a hysteresis effect. The combustion kinetics revealed mean activation energies of 124.51–143.99 kJ mol−1, calculated from the KAS method, and 128.17–146.92 kJ mol−1 calculated from the OFW method. There was a high relation between conversion degree and activation energy, which indicated complexity of the rice husks combustion process. Enthalpy, Gibbs free energy and Entropy changes of the developed rice husks/clay fiber-reinforced PLA composites were in ranges 122.92–141.33 kJ mol−1, 160.00–167.78 kJ mol−1 and 0.04–0.06 kJ mol−1 respectively. The low energy barrier (≤ 6.00 kJ mol−1) between the activation energies and ΔH values for the process indicated that initiation occurs easily. These findings revealed that thermal stability, kinetic parameters, combustion and thermodynamic characteristics of rice husks/clay fiber-reinforced PLA composites are better, compared to conventional polymer products.