Selected Morphological and Functional Properties of Extruded Acetylated Starch−Polylactic Acid Foams

Abstract

Native starch and polylactic acid (PLA) are biodegradable polymers that can be used in packaging materials. Their hydrophilic properties hinder their widespread use. Hydrophobicity can be improved significantly by acetylating native starch. This study was conducted to determine the morphological and functional properties of acetylated starch−PLA foams. Acetylated corn starch, with a degree of substitution (DS) of 2.3, and acetylated potato starch, with a DS of 1.07, were extruded with 5, 10, or 15% PLA in a twin screw co-rotating extruder at 150, 160, or 170 °C barrel temperatures and 130, 150, or 170 rpm screw speeds. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) were used to analyze the morphological properties of the extruded foams. A central composite response surface design was used to analyze the effects of acetylated starch type, PLA content, barrel temperature, and screw speed on the specific mechanical energy requirements of preparing extruded foams and the radial expansion ratios and compressibilities of the extruded foams. DSC showed that glass transition temperatures (Tg's) and melting temperatures (Tm's) of DS 2.3 corn starch−PLA foams were between the Tg's and Tm's of DS 2.3 corn starch and PLA. The Tg's of DS 1.07 potato starch−PLA foams were higher than those of acetylated potato starch and PLA, while the Tm's were closer to that of the PLA, when acetylated potato starch was the predominant phase in the blends. XRD showed that both acetylated starch and PLA lost crystallinity during extrusion. The X-ray pattern of the DS 1.07 potato starch−PLA foam was similar to those of DS 1.07 potato starch and PLA. FTIR spectroscopy confirmed no new bonds were formed in either DS 2.3 corn starch−PLA or DS 1.07 potato starch acetate−PLA foams. The type of acetylated starch, PLA content, barrel temperature, and screw speed had significant effects on the specific mechanical energy requirements, radial expansion ratios, and compressibilities of the acetylated starch foams.