2.3. Thermal properties

For the second half of the absorption curve, where Mt/Mm > 0.6, Springer [37] proposed the

<sup>¼</sup> <sup>1</sup> <sup>−</sup> exp <sup>−</sup> <sup>7</sup>:<sup>3</sup> <sup>D</sup>:<sup>t</sup>

Figure 9 shows a comparison between the Fick's law predicted and the experimental moisture absorption test results of TPS composites reinforced with 60 wt% of different fibers (banana, bagasse, DPF, and flax). The four curves show that the experimental data are in a good agreement with the predicted values. Thus, TPS/lignocellulosic fiber composites follow a

h2 � �<sup>0</sup>:<sup>75</sup> " #

(19)

Mt Mm

2.2.4. Effect of moisture absorption on mechanical properties of composites

lowers the moisture absorption [30, 39–41].

fibers, (c) DPFs, and (d) flax fibers.

Mechanical properties deterioration is one of the drawbacks of moisture absorption on polymer/natural fiber composites. This deterioration is attributed to the swelling of the cellulose fibers. Due to this swelling, development of shear stress at the fiber/matrix interface occurs. Therefore, this leads to debonding of the fibers, delamination, and loss of structural integrity [30, 38]. Fiber surface chemical treatments are proven to promote the moisture resistance by reducing fiber hydrophilicity. Moreover, these treatments improve the interfacial adhesion between matrix and fiber thus, tightens the water penetration pathways and consequently,

Figure 9. Experimental and predicted moisture absorption of TPS composites reinforced with 60% (a) BFs, (b) bagasse

following approximation:

56 Composites from Renewable and Sustainable Materials

Fickian behavior.

Thermogravimetric analysis (TGA) is a widely used thermal technique due to its high accuracy in determining the decomposition temperature and thermal stability of materials. TGA measures the rate and amount of weight change of a material as a function of time or temperature in a controlled atmosphere. This technique proved to be useful in studying the thermal characteristics of polymeric materials such as thermoplastics, composites, films, and fibers [43]. The thermogravimetric graph for thermoplastic starch matrix is shown in Figure 11. The authors in references [13–16] reported that the thermal decomposition of thermoplastic starch occurs in three main steps. The initial weight loss that occurred in the TGA curve between room temperature and 100°C represents the first step. This weight loss was attributed to the evaporation of water. It can be observed as a small peak around 100°C in the DTG curve. The second step appeared as a peak in the DTG curve around 200°C. This peak was attributed to the evaporation of glycerin. The last step appeared as a major peak around 330°C in the DTG curve. This peak was attributed to thermal decomposition of starch.

TGA and DTG curves for different lignocellulosic fibers are comparable due to their chemical composition similarity. On the other hand, chemical surface treatments affect the thermal stability of the fibers [40, 45]. Darwish et al. [16] studied the effect of NaOH treatment on the thermal stability of BFs. Their results showed that TGA and DTG curves were shifted to the right after treatment (Figure 12). This shift indicated that the treated fiber exhibited an increased thermal stability relative to the untreated fiber.

Figure 11. (a) TGA and (b) DTG of TPS matrix. Adopted from Refs. [13, 14, 44].

Figure 12. (a) TGA and (b) DTG of untreated and treated BFs. Adopted from Ref. [44].

TGA and DTG curves were plotted by the authors in references [13–16] to analyze the thermal degradation of starch-based composites reinforced with different contents of NaOH-treated lignocellulosic fibers (bagasse, flax, DPF, and banana) (Figure 13). The authors reported that the decomposition of TPS/natural fiber composites has two main peak regions. From the DTG curves, peaks, Group I represents the first decomposition region which is attributed to the decomposition of starch. These peaks appeared at the temperature range of 300–340°C.The second decomposition region appeared in the DTG curves as peaks Group II in the temperature range of 340–400°C. These peaks are attributed to the decomposition of the lignocellulosic fibers.

The authors in references [11–14] reported that the small peak that is related to the evaporation of water disappeared at high fiber contents. They attributed this disappearance to the improvement of the moisture absorption resistance of the composites with higher fiber contents. Also, the authors noted that as the fiber content increases, the residual weight decreases, which imply that starch-based matrix contains more inorganic inclusions. The authors noticed that the onset degradation temperature of starch increased with increasing the fibers' content. They assigned this improvement to the possible formation of hydrogen bond linkage between starch High-Content Lignocellulosic Fibers Reinforcing Starch-Based Biodegradable Composites: Properties and Applications http://dx.doi.org/10.5772/65262 59

right after treatment (Figure 12). This shift indicated that the treated fiber exhibited an

TGA and DTG curves were plotted by the authors in references [13–16] to analyze the thermal degradation of starch-based composites reinforced with different contents of NaOH-treated lignocellulosic fibers (bagasse, flax, DPF, and banana) (Figure 13). The authors reported that the decomposition of TPS/natural fiber composites has two main peak regions. From the DTG curves, peaks, Group I represents the first decomposition region which is attributed to the decomposition of starch. These peaks appeared at the temperature range of 300–340°C.The second decomposition region appeared in the DTG curves as peaks Group II in the temperature range of 340–400°C. These peaks are attributed to the decomposition of the lignocellulosic

The authors in references [11–14] reported that the small peak that is related to the evaporation of water disappeared at high fiber contents. They attributed this disappearance to the improvement of the moisture absorption resistance of the composites with higher fiber contents. Also, the authors noted that as the fiber content increases, the residual weight decreases, which imply that starch-based matrix contains more inorganic inclusions. The authors noticed that the onset degradation temperature of starch increased with increasing the fibers' content. They assigned this improvement to the possible formation of hydrogen bond linkage between starch

increased thermal stability relative to the untreated fiber.

58 Composites from Renewable and Sustainable Materials

Figure 11. (a) TGA and (b) DTG of TPS matrix. Adopted from Refs. [13, 14, 44].

Figure 12. (a) TGA and (b) DTG of untreated and treated BFs. Adopted from Ref. [44].

fibers.

Figure 13. TGA and DTG curves for TPS composites reinforced with (a) flax fibers, (b) DPFs, (c) banana fibers, and (d) bagasse fibers. Adopted from Refs. [13–16, 46].

and fibers. The mentioned studies [13–16] reported that with increasing the fiber content, the onset degradation temperature of the composites increased and the weight loss decreased. Thus, the thermal stability of the composites improved with increasing the fibers content. This improvement was attributed to the higher thermal stability of the cellulosic fibers when compared with starch and the good compatibility of both polysaccharides. Moreover, proper selection of the surface treatment technique of the fibers plays a prominent role in improving the thermal stability of the resultant composite.

In short, the parameters obtained from the thermal characterization of TPS/lignocellulosic fiber composites are useful in identifying the temperature limits of treatment, processing, or operating both matrix and fibers. Moreover, these parameters are useful in comparing the thermal performance of various TPS/lignocellulosic fiber composites.
