Biodegradation of thermoplastic starch/eggshell powder composites
Introduction
In the last decade, thermoplastic starch (TPS) has been of interest as a polymer that could replace petroleum-derived plastics because it has a low production cost, is readily available, is fully biodegradable and is a renewable agricultural resource (Roz et al., 2006, Teixeira et al., 2009). Cassava roots have become economically important products of Thailand. Generally, cassava roots are processed as animal feed or are fermented for producing alcoholic beverages. Therefore, the preparation of TPS by using cassava starch not only increases the market value of cassava roots but also helps to solve the accumulation of petroleum-derived plastics in nature that have become a worldwide environmental problem. However, TPS has limitations including its poor long-term stability, poor mechanical properties and poor water resistance (Lopez et al., 2013, Roz et al., 2011). Several research groups have attempted to improve the properties of TPS. Many fillers have been used and some have been effective for improving some of the properties of TPS such as fiber (Gilfillan et al., 2012, Kaewtatip and Thongmee, 2012), clay (Kaewtatip and Tanrattanakul, 2012, Wang et al., 2009), protein (Corradini et al., 2011, Kaewtatip et al., 2013), brucite (Moreira, Pedro, Glenn, Marconcini, & Mattoso, 2013), fly ash (Ma, Yu, & Wang, 2007) and carbon nanotubes (Fama, Pettarin, Goyanes, & Bernal, 2011). However, the hydrophilic characteristics of TPS can cause incompatibility with fillers and some of the fillers are nonbiodegradable materials. Currently, most research has been focused on the development of a bio-filler for TPS that will not only improve the compatibility between the TPS and the filler but also can facilitate biodegradation by microorganisms.
Eggshell has been considered to be a good candidate for use as a bio-filler for preparing polymer composites because it is inexpensive, abundant, has low density, is environmentally friendly and is a renewable resource (Kang et al., 2010, Toro et al., 2007a). Eggshell is a major waste product of the food industry and is produced in large quantities. Eggshell contains about 95% of calcium carbonate and 5% of organic materials (Ji et al., 2009, Kang et al., 2010). Eggshell has potential uses for many applications such as a fertilizer, animal feed, for coating pigments, as a catalyst and adsorbent (Cho and Seo, 2010, Tsai et al., 2006, Yoo et al., 2009). One of its most useful applications is as a filler for synthetic plastics e.g. poly(styrene-b-ethylene/butylene-b-styrene) (Kang et al., 2010), polypropylene (PP) (Toro et al., 2007a, Toro et al., 2007b), epoxy resins (Ji et al., 2009) and low density polyethylene (LDPE) (Shuhadah & Supri, 2009). Toro, Quijada, Arias, et al. (2007) compared the effect of commercial calcium carbonate and eggshell on the properties of a PP composite. They found that eggshell disperses well in the PP matrix due to the higher surface/volume ratio for eggshell than that of commercial calcium carbonate. The toughness of the epoxy resin was improved by using eggshell (Ji et al., 2009). However, there have been few reports on the use of eggshell with starch-based materials. Takamine, Bhatnagar, and Hanna (1995) studied the effect of eggshell on the properties of corn starch extrudates. They suggested that eggshell brought about nucleation sites for water vaporization as the material exited the die of the extruder. Moreover, eggshell was used as a nucleating agent for preparing starch foam (Xu & Hanna, 2007). It was found that eggshells decreased the cell size and increased the cell population.
The aim of this work was to study the effect of eggshell on the properties of TPS and compare it with the use of commercial calcium carbonate. The morphology of their fractured surfaces, the thermal properties and water absorption of the composites was studied. The surface morphology and weight loss after burial in soil was also investigated.
Section snippets
Materials
Pregelatinized cassava starch (PD 10370) was kindly supplied by Siam Modified Starch Co., Ltd. The starch was dried at 105 °C for 48 h in an oven and kept in a desiccator prior to use. The chicken eggshell was obtained from waste at a local market. Eggshells were washed in water to remove any impurities, dried in a hot air oven and ground by a Motar grinder, FRITSTSCH (Germany). After grinding, the eggshell powder was dried in a hot air oven until a constant weight was achieved and kept in a
Characterization of fillers
The physical properties of commercial calcium carbonate and eggshell powder such as density, surface area and oil absorbent value has been reported in many articles (Ji et al., 2009, Toro et al., 2007a, Tsai et al., 2006). Thus, the morphology and XRD patterns of commercial calcium carbonate and eggshell powder were studied. Fig. 1(a) presents the SEM micrograph of the commercial calcium carbonate. The shape of the commercial calcium carbonate was plate-like, whereas the morphology of the
Conclusions
In this study eggshell powder was used to improve the properties of TPS and was compared with adding commercial calcium carbonate. The results from the weight loss and surface morphology after the soil burial test for 15 days showed that both the eggshell powder and commercial calcium carbonate delayed the biodegradation of TPS. From the SEM micrographs, the eggshell powder was well dispersed and embedded in the TPS matrix, while the agglomeration of commercial calcium carbonate appeared to be
Acknowledgements
We thank the Prince of Songkla University and the Faculty of Science for their financial support and Siam Modified Starch Co., Ltd. for their pregelatinized cassava starch. We would like to thank Dr. Apon Numnuam for suggestions about the chicken eggshell. Thanks also to Dr. Brian Hodgson for assistance with the English.
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