High performance LDPE/thermoplastic starch blends: a sustainable alternative to pure polyethylene F.J. Rodriguez-Gonzalez 1 , B.A. Ramsay 2 , B.D. Favis * Department of Chemical Engineering, Centre de Recherche Applique ´e Sur les Polyme `res (CRASP), E ´ cole Polytechnique de Montre ´al, P.O. Box 6079, Succursale Centre-ville, Montre ´al, Que ´., Canada H3C 3A7 Received 8 July 2002; received in revised form 25 November 2002; accepted 27 November 2002 Abstract Thermoplastic starch (TPS), as opposed to dry starch, is capable of flow and hence when mixed with other synthetic polymers can behave in a manner similar to conventional polymer – polymer blends. This paper presents an approach to preparing polyethylene/thermoplastic starch blends with unique properties. A one-step combined twin-screw/single screw extrusion setup is used to carry out the melt – melt mixing of the components. Glycerol is used as the starch plasticizer and its content in the TPS is varied from 29 to 40%. Under the particular one-step processing conditions used it is possible to develop continuous TPS (highly interconnected) and co- continuous polymer/TPS blend extruded ribbon which possess a high elongation at break, modulus and strength in the machine direction. The PE/TPS (55:45) blend prepared with TPS containing 36% glycerol maintains 94% of the elongation at break and 76% of the modulus of polyethylene. At a composition level of 71:29 PE/TPS for the same glycerol content, the blend retains 96% of the elongation at break and 100% of the modulus of polyethylene. These excellent properties are achieved in the absence of any interfacial modifier and despite the high levels of immiscibility in the polar – nonpolar TPS – PE system. The 55:45 blend possesses a 100% continuous or fully interconnected TPS morphology, as measured by hydrolytic extraction. This highly continuous TPS configuration within the blend should enhance its potential for environmental biodegradation. The elongation at break in the cross direction of these materials, although lower than the machine direction properties, also demonstrates ductility at high TPS concentrations. At a glycerol content of 36% in the TPS, the blends demonstrate only very low levels of sensitivity to moisture. A high degree of transparency is maintained over the entire concentration range due to the similar refractive indices of PE and TPS and the virtual absence of interfacial microvoiding. Effective control of the glycerol content, TPS concentration and processing conditions can result in a wide variety of morphological structures including spherical, fiber-like, highly continuous and co-continuous morphologies. These various blend morphologies are shown to be the determining parameters with respect to the observed mechanical properties. This material has the added benefit of containing large quantities of a renewable resource and hence represents a more sustainable alternative to pure synthetic polymers. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Thermoplastic Starch; Polymer Blends; Morphology; Properties 1. Introduction Starch is a natural carbohydrate storage material accumulated by green plants in the form of granules. It is composed of linear polysaccharide molecules (amylose) and branched molecules (amylopectin) and is an inexpensive, renewable and natural polymer. It can be added to synthetic polymers to lower the cost of the final product. Numerous studies have shown that the addition of dry starch granules to low density polyethylene (LDPE) follows the general trend for filler effects on polymer properties [1,2]. The modulus increases due to the stiffening effect of the starch granules and the elongation decreases as the starch content is increased. In pioneering work, Griffith [3] reported on the preparation of starch-filled polyethylene (PE) composites. Ductile films could be prepared at low starch loading, but the films became paper-like when starch content exceeded 15% [3]. In a fashion typical of that observed for mineral fillers, the addition of granular starch to PE results in a severe reduction of tensile strength (s max ) and elongation at 0032-3861/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S0032-3861(02)00907-2 Polymer 44 (2003) 1517–1526 www.elsevier.com/locate/polymer 1 Centro de Investigacio ´n en Quı ´mica Aplicada, Saltillo, Me ´xico 25100. 2 Polyferm Canada, RR#1, Harrowsmith, Ont., Canada K0H 1V0. * Corresponding author. Tel.: þ 1-514-340-4818; fax: þ1-514-340-4159. E-mail address: basil.favis@polymtl.ca (B.D. Favis).