**3.7. Differential scanning calorimetry (DSC) analysis**

Differential scanning calorimetry (DSC) can be used to measure melting temperature (*T*m), crystallization temperature (*T*c), and crystalline content (X*c*) of materials. **Figure 13** presents the heating and cooling thermograms for neat matrices and modified polyolefine composites which contain 50 wt.% fillers. The *T*<sup>m</sup> of neat matrices and the composites was obtained from the maximum of the endothermic melting peak (heating curves). The *T*m of polyolefines was not significantly changed by the addition of bio-fillers. The peak area is reduced by 50% due to the filler, as expected.

**Figure 13.** DSC heating (2nd run) and cooling curves of neat matrix and modified composites (50% filler) of (a) PP/RH, (b) PE/RH, and (c) PE/SD without and with compatibilizer.

To characterize the polyolefine crystallization, the crystallization peaks of the cooling run were analyzed, confer **Table 2**. *T*c of neat matrices and the composites was obtained from the maximum of the exothermic crystallization peak. The crystallization is influenced by the fillers: For PP matrix without filler, the crystallization peak was observed at *T*<sup>c</sup> = 113°C, while for the PP matrix composites, these exothermic peaks shifted to higher temperatures (*T*c = 120°C).

This shift to earlier crystallization in the cooling run is due to the nucleating ability of the compatibilizer MAPP, which is enriched in the interphase, and perhaps the cellulosic filler surface, creating a transcrystalline interphase morphology, which improves the material properties [24].

The *T*c of polyethylene systems (*T*c = 116°C) does not increase significantly upon addition of the fillers (*T*<sup>c</sup> = 118°C). As the neat PE crystallizes only 10°C below *T*m and PE is a fast crystallizing thermoplast (i.e., the width of the crystallization peak is narrow), nucleation influences *T*c less than in PP, where the difference *T*m − *T*c = 30°C.


**Table 2.** Thermal properties determined by DSC.

the maximum of the endothermic melting peak (heating curves). The *T*m of polyolefines was not significantly changed by the addition of bio-fillers. The peak area is reduced by 50% due

**Figure 13.** DSC heating (2nd run) and cooling curves of neat matrix and modified composites (50% filler) of (a) PP/RH,

To characterize the polyolefine crystallization, the crystallization peaks of the cooling run were analyzed, confer **Table 2**. *T*c of neat matrices and the composites was obtained from the maximum of the exothermic crystallization peak. The crystallization is influenced by the fillers: For PP matrix without filler, the crystallization peak was observed at *T*<sup>c</sup> = 113°C, while for the PP matrix composites, these exothermic peaks shifted to higher temperatures (*T*c = 120°C).

This shift to earlier crystallization in the cooling run is due to the nucleating ability of the compatibilizer MAPP, which is enriched in the interphase, and perhaps the cellulosic filler surface, creating a transcrystalline interphase morphology, which improves the material

(b) PE/RH, and (c) PE/SD without and with compatibilizer.

properties [24].

to the filler, as expected.

16 Composites from Renewable and Sustainable Materials

#### **3.8. Thermogravimetric analysis (TGA)**

Thermogravimetric analysis is becoming an increasingly useful tool for material characterization, particularly in the development of new materials. It is essential to monitor not only the final properties of the composites but also the basic raw materials through the processing procedure to the final product. Optimization of the processing temperature and time with an understanding of matrix, reinforcing element and interface between matrix and reinforcing phase, can lead to the best balance of composite properties such as modulus, thermal stability, and damping behavior [25]. Thermogravimetric analysis (TGA) can determine the moisture content, thermal cleavage, thermal degradation temperature, and thermal stability of composite materials [26, 27]. The differential thermogravimetric analysis (DTG) i.e. the slope of the TGA data curve, permits a more detailed analysis of thermal decomposition.

#### *3.8.1. Fillers and Polyolefines*

**Figure 14** shows the results of the thermal analysis of the used polyolefines and fillers performed in nitrogen and air atmosphere. The TGA and DTG curves of the fillers in nitrogen as well as air atmosphere show a slight weight loss between 40 and 100°C, indicating the vaporization of water. A second weight loss from approximately 150–500°C is due to the decomposition of the three major constituents of bio-fillers, namely cellulose (275–350°C), hemicellulose (150–350°C), and lignin (250–500°C) [26, 28–30]. At 700°C, rice husk and saw dust leave a greater char content, decomposing only by about 65 and 80% in nitrogen cf. **Table 3**. The ash in the RH is mainly composed of silica (96%) [26]. In an air atmosphere, rice husk degradation shifted to lower temperature values and split into two processes (320 and 438°C). The second may be associated with thermal oxidation degradation of char. Therefore, the residues of RH filler at 700°C in the air atmosphere (20%) were lower than that in nitrogen atmosphere (36%).

**Figure 14.** TGA curves of stock materials: fillers saw dust and rice husk as well as polyolefines PP and PE in nitrogen and air atmospheres.


**Table 3.** Residues of polymers and fillers as well as their composites at 700°C in nitrogen and air atmosphere in %.

PE and PP decompose at temperatures above 400°C, cf. **Figure 14**. In nitrogen atmosphere, thermal degradation of PP and PE occurs very rapidly at 468 and 488°C, respectively. In air, the thermal resistance of both polyolefines (PP and PE) starts at 250°C. While PP is fully degraded at 380°C, PE decomposed more gradually and is fully degraded at 470°C. The maximum decomposition rates of PP and PE in the air occurred at 371 and 399°C, respectively. Both PP and PE decomposed almost completely. Residues of both polyolefines at 700°C were <2% in nitrogen and <1% in air atmosphere.

#### *3.8.2. Composites*

**Figure 15** shows the TGA and DTG curves of the polyolefines, fillers, and the composites with filler content ranging from 30 to 50 wt.% in nitrogen and air atmosphere. The TGA or DTG analyses performed in nitrogen atmosphere show two regions of thermal decomposition that consist of the superposition of the profiles of filler and polyolefines. As the filler is less thermally stable than the matrix polymer, when filler loading is increased, the thermal stability of the composites slightly decreases, whereas the final ash content increased (**Table 3**).

**Figure 14.** TGA curves of stock materials: fillers saw dust and rice husk as well as polyolefines PP and PE in nitrogen

**Residue in nitrogen atmosphere Residue in air atmosphere**

*Filler content 30% 40% 50% 30% 40% 50%* PP/RH 11.0 12.7 14.1 5.5 6.9 8.0 PE/RH 10.9 12.2 18.8 5.5 6.7 9.4 PE/SD 4.5 7.4 8.8 1.0 1.0 1.3

**Table 3.** Residues of polymers and fillers as well as their composites at 700°C in nitrogen and air atmosphere in %.

PE and PP decompose at temperatures above 400°C, cf. **Figure 14**. In nitrogen atmosphere, thermal degradation of PP and PE occurs very rapidly at 468 and 488°C, respectively. In air, the thermal resistance of both polyolefines (PP and PE) starts at 250°C. While PP is fully degraded at 380°C, PE decomposed more gradually and is fully degraded at 470°C. The maximum decomposition rates of PP and PE in the air occurred at 371 and 399°C, respectively. Both PP and PE decomposed almost completely. Residues of both polyolefines at 700°C were

**Figure 15** shows the TGA and DTG curves of the polyolefines, fillers, and the composites with filler content ranging from 30 to 50 wt.% in nitrogen and air atmosphere. The TGA or DTG

PP 1.8 0.8 PE 1.8 1.0 RH 35.7 19.8 SD 19.6 7.8

<2% in nitrogen and <1% in air atmosphere.

and air atmospheres.

18 Composites from Renewable and Sustainable Materials

*3.8.2. Composites*

**Figure 15.** TGA of fillers, plastics, and the composites in nitrogen (left) and air (right) of PP/RH (top), PE/RH (middle), and PE/SD (bottom) composites, inset in left images: DTG.

In the case of the two studied PE composites decomposed in air, the TGA/DTG curves of the composites correspond to the superposition of the separate components only up to 400°C, whereas the charring behavior is different. The final reduction is only reached at 500°C, 60°C above the temperature where both PE and filler are completely degraded thermal oxidatetively . That may be due to a thermally stable material (char), formed during the oxidation or thermal degradation of hemicellulose, cellulose, and lignin, providing a thermal shielding and acting as diffusion barrier on the polyethylene decomposition process. Moreover, the second peak of decomposition of olefinic products or their oxidation degradation products containing functional groups such as C=O, O–H, and C–O–C which formed in the first stage of polyethylene degradation appears at 438°C. This second main peak of PE degradation is in superposition with the second main peak of the bio-fillers due to degradation of char (**Figure 15a**). Therefore, the second-stage degradation of both polyethylene and the bio-fillers is competitive at above 438°C, that can also lead to retard the degradation of the composite. However, there is a one-step degradation of polypropylene in air atmosphere and PP fully degraded at 380°C. Therefore, RH char did not affect the degradation of polypropylene.

**Figure 16.** Effect of compatibilizers on TGA of 50% filler composites in nitrogen atmosphere.

The RH-filled PE leaves a bigger residue than the SD-filled PE, as expected from the TGA analysis of the fillers. The residues of rice husk composites (10.9–18.8% in nitrogen and 5.5– 9.4% in air atmosphere) were much higher than those of saw dust composites (4.5–8.8% in nitrogen and 1.0–1.3% in air atmosphere) due to high ash content of rice husk compared to saw dust. The residue is in all cases 0–10% smaller than the value calculated from the residues of the separate components, and only in the saw dust composites in air, it is a third of the expected value. The filler/matrix interaction leads to a more profound decomposition maybe due to a wick effect.

**Figure 16** shows the TGA curves of the composites at 50 wt.% filler without and with compatibilizers (2 wt.% for PP matrix composites and 4 wt.% for PE matrix composites). The thermal stability and degradation temperature of the composites with compatibilizers [PP/RH (MA), PE/RH (MA), and PE/SD (MA)] were slightly higher than those of the composites without compatibilizers (PP/RH, PE/RH, and PE/SD). The improved thermal stability of the composites with compatibilizers is due to enhanced interfacial interaction and additional intermolecular bonding (ester and hydrogen bonds) between hydroxyl groups of rice husk, saw dust and the anhydride functional groups of compatibilizers.
