Copyright © 2007 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Biobased Materials and Bioenergy Vol. 1, 71–77, 2007 Dispersion of Wood Microfibers in a Matrix of Thermoplastic Starch and Starch–Polylactic Acid Blend Ayan Chakraborty 1 , Mohini Sain 1 2 ∗ , Mark Kortschot 1 , and Sean Cutler 3 1 Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada M5S 3E5 2 Faculty of Forestry, University of Toronto, Toronto, Canada M5S 3B3 3 Department of Botany, University of Toronto, Toronto, Canada M5S 3B2 The successful dispersion of cellulose fibers of submicrometer diameter in polymers has been restricted to solution-cast films so far. In this work, the dispersion of microfibers in biopolymers was investigated by melt-mixing using conventional processing equipment. Thermoplastic starch and a blend of starch and polylactic acid (PLA) were used as matrix materials. A suspension of cellulose microfibers less than 1 m in diameter was prepared in water. This microfiber suspension was poured into molten thermoplastic starch to obtain fiber loadings up to 2%. The composites were compression molded into thin films roughly 0.25 mm thick. there was a 10% increase in tensile strength and a 50% increase in stiffness with each percentage increase in microfiber loading in the starch polymer. Similar improvement in tensile properties was also noted for a polymer system prepared by blending starch and PLA. Laser confocal microscopy images were analyzed to quantify microfiber dispersion at different composite processing parameters. This was the first work where successful dispersion of cellulose fibers of submicrometer was achieved in a composite prepared solely by the melt-mixing process. Keywords: Cellulose Microfiber, Microfibrils, Micro- and Nano-Composite, Biocomposite, Thermopalstic Starch, Packaging, Automotive. 1. INTRODUCTION In recent years, there is a growing trend in using natu- ral resources to isolate cellulose reinforcements for use in composites. These novel reinforcing agents have diameters in the range of micro- and nanometers. A major thrust in research in producing such submicrometer cellulose chains has been targeted at isolating cellulose microfibrils generated during photosynthesis. The diameters of these microfibrils vary substantially depending on the source and range from a few nanometers up to 38 nm. 1 Microfib- ril generation has mostly been limited to plant sources, like sugar beet and potato tuber cells. 2–4 In addition to microfibrils, there is substantial ongoing research directed toward isolating cellulose whiskers, i.e., crystallites of cel- lulose having very high crystallinity, and using them as reinforcing agents. For this purpose, studies have been done on animal sources like tunicin, 5 chitin, 6 and bacterial cellulose. 7 However, studies on the generation of cellu- lose microfibrils and crystallites from wood sources have been relatively limited. Nevertheless, microfibrils from wood pulp fibers were produced by Taniguchi 8 by using ∗ Author to whom correspondence should be addressed. a supergrinder that broke the fibers apart into diameters of 100 nm and less. The most notable use of wood pulp in this regard has been in generating “microfibrillated cel- lulose” (MFC), produced by opening up and unraveling of the fibers through homogenization. 9–12 This produced a mesh of smaller fibrils and microfibrils. However, none of these methods succeeded in isolating these wood microfib- rils as individual entities separate from the cell wall. This paper discusses the reinforcing potential of “micro- fibers” generated from bleached softwood kraft pulp, as reported by Chakraborty et al., 13 in biopolymers derived from natural resources. A microfiber is defined as a fiber consisting of continuous cellulose chains with trace lignin and hemicellulose content and having a diameter between 0.1 to 1 m, with a minimum corresponding length of 5–50 m. Therefore, as opposed to the microfibrils or MFC, the microfibers are discrete fibers free at both ends and have a distinct aspect ratio (length/diameter). In the longitudinal direction, it consists of alternating crystalline and amorphous zones of cellulose. Laterally, however, the chains are linked together by a combination of hydro- gen bonding, amorphous cellulose molecules, and traces of lignin and hemicellulose not removed through the pulping processes. J. Biobased Materials and Bioenergy 2007, Vol. 1, No. 1 1556-6560/2007/1/071/007 doi:10.1166/jbmb.2007.008 71