Fabrication and characterization of bio-degradable starch-clay nano-composite films

Abstract

Biodegradable materials based on natural polysaccharides, particularly starch, can be used to fabricate starch based nano-composite film to reduce environmental problem. Starch can be produced at low cost and at large scale. Potato is an abundant and cheap agricultural source of starch. Starch is not a real thermoplastic material but, in the presence of a plasticizer, high temperatures and shearing, it melts and fluidizes like plastics. Plasticizing agents and nanofillers, like hectorite, montmorillonite, kaolinite clays, can improve mechanical properties of the films, resulting in materials with better flexibility and extensibility. The aim of the present work was to investigate the mechanical properties of thermoplastic starch reinforced with different types of plasticizers and clays at different concentration. With the purpose of producing potato starch nano-composite films, four types of plasticizer such as glycerol, ethylene glycol, sorbitol, formamide and three types of clays such as hectorite, montmorillonite, kaolinite at various concentration (2, 5 and 10 wt%) were used. Nano-composite films were prepared using casting process at 70 °C. Tensile tests results infer that materials based on potato starch reinforced with nano clay particles can significantly improve the mechanical properties which can be used as an interesting biodegradable alternative for synthetic plastic materials, and after addition of three type of clays showed different mechanical behavior. The nano-composite film prepared using hectorite clay is better than montmorillonite clay and montmorillonite clay is better than kaolinite clay. Dynamic Light Scattering measurements were performed at room temperature to determine particle size and zeta potential for starch and different type of clays. The degree of crystallinity of the starch-clay nano-composite was analyzed by X-ray Diffraction (XRD) which showed that starch-plasticizer crystalline structure was affected by clay integration. XRD analysis provided information of crystallite size of different types of clay. Crystallite size was found 6.5 nm, 26.1 nm, 25.9 nm for hectorite, montmorillonite, kaolinite clay respectively. When clay is used into starch matrix starch peaks turns to broad peak. In addition, both XRD and Fourier Transform Infrared Spectroscopy (FTIR) spectra showed a strong interaction between the clay and the starch molecules. The lower peak frequency of C–O group in starch represents the stronger the interaction between starch and plasticizers. The peak for pure starch powder appeared at 1170 -1 cm but after addition of plasticizer the peak shifted at downward frequency (1150cm-1), which indicates the presence of strong hydrogen bond. Scanning Electron Microscope (SEM) micrographs shows that clay particles were homogeneously dispersed in starch-clay nano-composite films at optimum clay concentration. Thermo gravimetric analysis (TGA) and Differential thermal analysis (DTA) showed that Starch decomposed at 292 °C and charred with residue of 22%. Starch-glycerin film exhibits maximum decomposition temperature of 296 °C. Starch-glycerin-clay nano-composites film exhibit better thermal stability compared with starch-plasticizer film. Again, starch-glycerin-hectorite nano-composite film displayed decomposition temperature at 316 °C whereas nano-composite films prepared using montmorillonite and kaolinite decompose at 306 °C and 311 °C, respectively. Starch film, where no plasticizer and clay is used, shows highest water uptake percentage and it decreases with addition of different plasticizer and further decreases with addition of different type of clays.