Use of extrusion for synthesis of starch-clay nanocomposites for biodegradable packaging films

Abstract

One of the worst pollution menaces of modern times is plastic packaging, because of its poor degradability. Packaging materials based on starch utilize the benefits of natural polymerization, abundant availability of raw material, and fast biodegradability. However, the highly hydrophilic nature and poor mechanical properties of starch based films limit their application. This problem was sought to be overcome by forming a nanocomposite of starch and layered silicate clay. This study utilizes melt extrusion processing to synthesize starch-clay nanocomposites for biodegradable packaging films and investigate the effects of chemical compatibility of starch, plasticizer and nanoclay and melt extrusion conditions on the structure and properties of composite films. In the first part of the study, the influence of clay type, clay content, starch source and amylose content was investigated. Starch-montmorillonite (MMT) hybrids showed an intercalated nanostructure due to the compatibility of the two components and led to cast film with higher tensile strength and better water vapor barrier properties as compared to starch-organically modified montmorillonite (I30E) hybrids, as well as native starch only. With increase in clay content (0-21 wt%), significantly higher (15-92%) tensile strength (TS) and lower (22-67%) water vapor permeability (WVP) were obtained. The results indicated that nanocomposite technology could be applied to improve the properties of starch-based packaging films. The barrier and mechanical properties of nanocomposite films did not vary significantly with different starch sources (corn, wheat and potato starch), whereas films from regular corn starch showed better properties than either high amylopectin or high amylose-based nanocomposite films. The second part of the study investigated the effects of glycerol content (0-20 wt%) and three plasticizers (glycerol, urea, formamide) on the structure and properties of the starch-clay nanocomposite films. With decreasing glycerol content, the extent of clay exfoliation increased. Films with 5% glycerol exhibited the lowest WVP, and the highest TS and glass transition temperature (Tg). The use of urea and formamide improved the dispersion of clay platelets. Compared to glycerol and urea, formamide has an intermediate hydrogen bond forming ability with starch. However, at the same level of plasticizer (15 wt%), formamide plasticized nanocomposite films exhibited the lowest WVP, highest TS and Tg. Results indicated that a balance of interactions between starch, clay surface modifications and plasticizers might control the formation of nanocomposite structure, and in turn affect the performance of the nanocomposite films. The last part of the study investigated the effects of extrusion conditions (screw configuration, barrel temperature profile, screw speed and barrel moisture content) on the structure and properties of the starch-clay nanocomposite films. Increasing the shear intensity significantly improved the exfoliation and dispersion of clay platelets. The combination of lowest barrel moisture content (20%) and high shear screw configuration exhibited almost complete clay exfoliation and the lowest WVP and highest TS of all treatments. Increasing the barrel temperature also improved clay exfoliation and performance of films. The results suggested that, when polymer and clay are chemically compatible, optimization of process conditions (shear intensity, temperature etc.) can enable significant improvement in clay exfoliation and dispersion and the performance of nanocomposite films.