Biocomposites from Renewable Resources

Keywords

Starch • Glycerol • Thermoplastic starch • Nanocellulose • Reinforcement • Biodegradation

Starch is inherently non-thermoplastic but is made thermoplastic using plasticizers and/or chemical modifications, and the modified starch has been used as matrix for composites. In one such study, starch was reinforced with bacterial cellulose, and the tensile properties, resistance to biodegradation, and moisture absorption were studied [09Wan]. Starch was plasticized with 30 % glycerol and made into 10-20 % solutions. Bacterial cellulose sheets cultured from Acetobacter xylinum X-2 were added into the solution and made into composite sheets with an average thickness of 0.5 mm. The amounts of fibers in the starch were 7.8, 15.1, and 22 wt%. Tensile properties of the BC-reinforced starch fiber composites are shown in Table 71.1 [09Wan]. Morphological analysis of the fractured surface of a starch composite containing 22 % bacterial cellulose showed that the BC fibers were present in a layered fashion as seen in Fig. 71.1. Such a layered structure was typical of bacterial cellulose. Pullout length of fibers from the matrix was low suggesting good fiber — matrix interaction [09Wan]. The presence of bacterial cellulose also increased the resistance of the fibers to moisture absorption. Degradation by soil burial tests showed that the weight loss of the composites was similar to that of unreinforced starch, and about 30 % weight loss had occurred after 30 days of burial. However, the bacterial cellulose-reinforced composites had slightly higher strength retention than the starch films. In a similar study, bacterial cellulose containing nanofibrils with diameters between 10 and 100 nm was mixed (1 or 5 %) with starch containing 30 % glycerol. Later, the mixture was heated at 120 °C for 20-30 min and later injection molded into composites in the form of tensile bars [09Mar]. More than six times increase in strength and modulus were obtained for composites containing 5 % nanocellulose compared to the thermoplastic starch [09Mar].

Green coir fibers were milled into lengths of about 10 mm and mixed with starch plasticized with 30 % glycerol. Composites were developed by injection molding and later heated (annealed) at 60 °C for 12 h to improve properties. The addition of the coir fibers increased the tensile strength to 10-11 MPa compared to 3 MPa without the reinforcement. Similarly, Young’s modulus increased to 374 MPa from 176 MPa due to the presence of the coir fibers [11Ram]. Coir-reinforced composites had substantially lower moisture absorption and water take-up than the thermoplas­tic starch matrix. Although composites were successfully developed from starch and coir fibers, the stability of the composites and changes in tensile properties at high humidities or in aqueous environments were not reported. Due to the hydro­philic nature of starch and coir fibers, it is very likely that the composites will have poor performance properties at high humidities or under aqueous environments and therefore have limited applications.

Fibrous materials derived from various sources and in different configurations were used to reinforce thermoplastic starch [04Ave]. Increasing the amount and length of the fibers in the matrix was found to increase the transition temperatures due to improved interfacial bonding and strong hydrogen bond interactions. Ligno — cellulose fibers were found to provide higher degradation temperature than cellu­lose fibers, and the addition of biodegradable synthetic polyesters did not vary the properties of the composites [04Ave].

Curaua fibers in stretched sliver form and those treated with concentrated alkali were used as reinforcement for commercially available cornstarch-based biode­gradable resin containing polycaprolactone [07Gom]. Fibers used in the study had tensile strength of 913 MPa, fracture strain of 3.9 %, and modulus of 30 GPa. Three methods (direct, prepreg sheet, and preforming) were used to fabricate the composites. Tensile properties of the composites obtained using the three methods are listed in Table 71.2. As seen from the table, the prepreg method of developing composites provided the highest tensile properties. Further, alkali treatment enhances the fracture strain without considerably changing the tensile strength. Composites obtained using the direct method after alkali treatment showed sub­stantial increase (nearly twice) in modulus as seen in Table 71.3.

References

[04Ave] Averous, L., Boquillon, N.: Carbohydr. Polym. 56, 111 (2004)

[07Gom] Gomes, A., Matsuo, T., Goda, K., Ohgi, J.: Compos. Part A 38, 1811 (2007)

[09Mar] Martins, I. M.G., Magina, S. P., Oliveira, L., Freire, C. S.R., Silvestre, A. J.D., Neto, C.

P., Gandini, A.: Compos. Sci. Technol. 69, 2163 (2009)

[09Wan] Wan, Y. Z., Luo, H., He, H., Liang, Y., Huang, Y., Li, X. L.: Compos. Sci. Technol. 69, 1212 (2009)

[11Ram] Ramirez, M. G.L., Satyanarayana, K. G., Iwakiri, S., Muniz, G. B., Tanobe, V., Flores — Sahagun, T. S.: Carbohydr. Polym. 86, 1712 (2011)