Carbohydrate Polymers 82 (2010) 1082–1089 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Effects of LDPE and glycerol contents and compounding on the microstructure and properties of starch composite films  Heartwin A. Pushpadass a,1 , Pratik Bhandari a , Milford A. Hanna b,a,∗ a Department of Biological Systems Engineering, University of Nebraska-Lincoln, United States b Industrial Agricultural Products Center, University of Nebraska-Lincoln, United States article info Article history: Received 9 February 2010 Received in revised form 16 June 2010 Accepted 18 June 2010 Available online 30 June 2010 Keywords: Compounding Extrusion Films LDPE Properties Starch abstract Corn starch–low density polyethylene (LDPE) blends, at ratios of 95:5, 90:10 and 85:15, were processed into thin films by either single-step twin-screw extrusion or by a two-step process involving compound- ing (pelleting) of the ingredients before film formation. The microstructure, X-ray patterns, and the tensile, thermal and water vapor permeability (WVP) properties of the films, prepared by both methods, were evaluated and compared. SEM micrographs of the single-step processed films were characterized by the presence of cracks and a discontinuous interface between starch and LDPE. Consequently, their tensile and WVP properties were impaired. Compounding, on the other hand, improved the dispersion of LDPE on to the starch matrix and the interface between the starch and LDPE phases by lowering the vis- cosity of the thermoplastic starch (TPS) melt. As a result, the stress transfer between the starch and LDPE phases was improved, enhancing the tensile and water vapor barrier properties of the films considerably. In general, the tensile strengths and moduli increased as the LDPE content increased from 5 to 10%, but then decreased with further increase in LDPE content to 15%. Addition of glycerol decreased the tensile strengths and moduli but did not necessarily improve the tensile strains. Differential scanning calorime- try scans and Fourier-transform infrared spectra did not indicate any significant interactions between the two immiscible polymers even after compounding. Phase separation was observed in the TPS. The water vapor barrier properties of the composites were improved by 7.3–25.4% after compounding, but were adversely affected by glycerol content. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Starch is a widely used polymer in thermoplastic applica- tions because of its biodegradability, abundant availability and low cost. Plasticized and gelatinized starch, processed at high temperatures and shear, will flow and form films like synthetic polymers. But, the hydrophilic nature of starch is a major con- straint that limits the development of solely starch-containing films. To make starch-based films a greater barrier to moisture and to improve their strength and flexibility, thermoplastic starch (TPS) is blended with synthetic polymers (Swanson, Shogren,  This study was conducted at the Industrial Agricultural Products Center. ∗ Corresponding author at: Industrial Agricultural Products Center, University of Nebraska-Lincoln, United States. Tel.: +1 402 472 1634; fax: +1 402 472 6338. E-mail address: mhanna1@unl.edu (M.A. Hanna). 1 Now at: National Dairy Research Institute, Bangalore 560 030, India. Fanta, & Imam, 1993). Starch–polyethylene, starch–poly(lactic acid), starch–poly(caprolactone), starch–polyhydroxybutyrate, starch–poly(vinyl alcohol) (PVA) and starch–poly(glycolic acid) co-polymers are commonly reported (Bastioli, Bellotti, Giudice, & Gilli, 1993). Among them, starch-filled polyethylene plastics have been widely researched (Matzions, Bikiaris, Kokkou, & Panayiotou, 2001; Otey, Westhoff, & Doane, 1980, 1987; Roper & Koch, 1990; Wang, Jiugao, & Jinglin, 2006). Griffin (1977) first described a patented process of making starch–low density polyethylene (LDPE) blown films containing unsaturated fatty acids and their derivatives. Similarly, Otey et al. (1980) showed that starch–poly(ethylene-co-acrylic acid) mix- tures containing up to 60% starch could be extrusion-blown into thin films. Later, Swanson, Westhoff, and Doane (1988) studied the effect of starch modification on starch–LDPE films containing ethy- lene acrylic acid (EAA), and reported that hydroxylpropylated and acetyl derivatives of starch had higher tensile strengths and elonga- tions than those of native starch-filled films. The granular size and 0144-8617/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2010.06.032