Crystallization Behavior of Starch-Filled Polypropylene Wanjun Liu, Ya-Jane Wang, Zhenhua Sun Department of Food Science, University of Arkansas, 2650 N. Young Avenue, Fayetteville, Arkansas 72704 Received 20 March 2003; accepted 1 October 2003 ABSTRACT: Starches of different granule sizes, including corn, rice, and amaranth starches, were used to prepare starch-filled polypropylene (PP) and the effect of starch granule size on crystallization behavior PP was investigated. Differential scanning calorimetry and scanning electron mi- croscopy were used to monitor the energy changes of the crystallization of the melt and to characterize the morphol- ogy of PP/starch composites, respectively. Little interaction was observed between starch and PP despite the difference in starch granule size. The crystallization temperature of PP decreased with the addition of starch and this decrease became more apparent with increasing starch granule size. During nonisothermal crystallization, the dependency of the relative degree of crystallinity on time was described by the Avrami equation. The addition of starch decreased the over- all crystallization rate of PP, which was attributed to an increase in the activation energy of crystallization under nonisothermal conditions according to the Kissinger equa- tion. An increase in starch granule size of starch would increase the crystallization activation energy of PP and con- sequently decrease its crystallization rate. © 2004 Wiley Peri- odicals, Inc. J Appl Polym Sci 92: 484 – 492, 2004 Key words: polypropylene (PP); starch; crystallization; melt- ing; morphology INTRODUCTION Polypropylene (PP) is a thermoplastic and widely used in automobile, electrical equipment, furniture, and packaging industries because of its excellent and versatile properties. Many fillers were incorpo- rated into PP matrix for cost reduction and it was later discovered that those fillers contributed func- tionality to the composites, such as improving the stiffness and mechanical properties and modifying the crystallization of the polymer. 1–6 The structure and properties of the composites were affected by the source and structure of fillers and the interaction between the filler particles and polymeric matrix. 1– 4,6 Inorganic substances are the main filler materials used during the past decades. However, with an increased interest in biodegradable polymers, natu- ral biopolymers, such as starch and cellulose, are being evaluated in the development of biodegrad- able products. 6,7 Hamdan et al. 8 reported that PP and sago starch were not compatible after noting that the tensile properties of the PP/sago starch blends decreased with increasing sago starch con- tent even though sago starch granules were well dispersed in the PP matrix. 8 To improve the com- patibility between PP and starch, Weil 9 proposed using maleated PP instead of PP and the resultant PP/starch composites exhibited improved tensile strength. Li et al. 10 and Bagheri 11 reported that starch had interactions with PP under processing conditions, thus inhibiting thermal degradation of PP and improving the flame-retardant properties of PP. However, few studies reported the effect of starch on the crystallization behavior of starch-filled PP. This study was undertaken to understand the changes in crystallization behavior and morphology of PP/starch composites by differential scanning calo- rimetry (DSC) and scanning electron microscopy (SEM), respectively. The effect of starch granule size on crystallization of PP was also considered. EXPERIMENTAL Materials Polypropylene, PP6219, with a melt flow rate (MFR) of 2.2 g/10 min and a density of 0.92 g/cm 3 , was pur- chased from Amoco Inc. (Houma, LA). Common corn starch was obtained from Cerestar USA, Inc. (Ham- mond, IN), native rice starch was a gift from A&B Ingredients Inc. (Fairfield, NJ), and amaranth starch was provided by Nu World Amaranth (Naperville, IL). Because of a high residual protein content, native rice and amaranth starches were further purified ac- cording to the method of Yang et al. 12 Correspondence to: Y.-J. Wang (yjwang@uark.edu). Contract grant sponsor: Agricultural Experiment Station, University of Arkansas. Contract grant sponsor: Arkansas Rice Research and Pro- motion Board. Journal of Applied Polymer Science, Vol. 92, 484 – 492 (2004) © 2004 Wiley Periodicals, Inc.