Torque Rheological Properties of Agro-Waste-Based Polypropylene Composites: Effect of Filler Content and Green Coupling Agent Koay Seong Chun, 1 Salmah Husseinsyah, 2 Chan Ming Yeng 1 1 School of Engineering, Taylor’s University Lakeside Campus, No.1, Jalan Taylor’s, 47500 Subang Jaya, Selangor, Malaysia 2 Division of Polymer Engineering, School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia This study investigates the effect of filler content and green coupling agent (GCA) on the torque rheological properties of polypropylene (PP)/cocoa pod husk (CPH) composites. The GCA was prepared from coconut oil and it was used to improve the properties of PP/CPH composites. The processing torque of PP/CPH compo- sites was increased with increasing of filler content and the presence of GCA. The power law index (n) of PP/CPH composites were less than 1.0. Thus, the com- posites melt showed pseudoplastic character and it exhibited shear thinning effect. The values n of PP/CPH composites increased with higher filler content and after filler modification using GCA. This indicated that the pseudoplasticity of composites were affected by filler– matrix interaction. The viscosity of PP/CPH composites increased after increased filler content and the presence of GCA. The change of viscosity was related to filler–filler interaction and filler–matrix adhesion. The increase of fil- ler content and addition of GCA also increased the acti- vation energy (E a ) of PP/CPH composites. This indicated that the energy of fusion process of PP/CPH composites increased with higher filler content and the addition of GCA. J. VINYL ADDIT. TECHNOL., 00:000–000, 2016. V C 2016 Society of Plastics Engineers INTRODUCTION Nowadays, thermoplastic composite materials made from agro-waste and thermoplastic have gained great inter- est among the industries and researchers due to their eco- nomic advantage, ecological awareness, and accumulation of agro-waste materials [1–5]. To date, numerous combina- tions of agro-waste and thermoplastic materials have been made into composite materials and already enthusiastically used in building applications (e.g., deck and wooden fit- tings), consumer products (e.g, packaging tray and utensils), and automotive parts (e.g., door panel) [2, 4–7]. In the pres- ent, this research was focused on producing thermoplastic composites by combining polypropylene (PP) and cocoa pod husk (CPH). The CPH was a major waste material in the cocoa industry which was readily abundant and widely available in Malaysia [8–10]. Generally, the CPH waste does not have any market potential and burning became a common disposal method for these CPH wastes [10]. For this reason, the CPH needs utilization as filler in producing thermoplastic composite materials and this will benefit the environment, economy and technology. Certainly, the incompatible between thermoplastic mate- rial and agro-waste filler is a main factor influencing the properties of composites. This is because the agro-waste fil- ler mainly contains cellulose, hemicellulose, and lignin which contain plenty of polar hydroxyl groups and leads to weak adhesion and poor wettability with most thermoplas- tic materials [11–15]. Hence, different kinds of coupling agents, such as maleated polymer [6, 8, 16, 17], silane- based coupling agent [9, 18–21], fatty acid and its deriva- tives [4, 5, 9, 11] are usually used to impart better adhesion between agro-waste filler and thermoplastic material. In present work, a green coupling agent (GCA) was synthe- sized from coconut oil fatty acid. Generally, the GCA was a type of glycidyl fatty acid ester that was produced by react- ing sodium fatty acid from coconut oil and glycidyl chlo- ride. The GCA consists a reactive oxirane group as head and fatty acid group as tail in the structure. Ideally, the oxir- ane’s head can covalent bonded on natural filler surface leaving the fatty acid tail provide an organophilic character to natural filler (as shown in Fig. 1). This provides the natural filler better wetting to the thermoplastic matrix and further enhances the filler–matrix adhesion. The new GCA show several benefits compared to synthetic coupling agents, such as low cost and it is made from a sustainable resource. Currently, there are many published literatures related to effect of coupling agents on improving the filler– matrix adhesion and enhancing the physical-mechanical Correspondence to: K.S. Chun; e-mail: seongchun.koay@taylors.edu.my DOI 10.1002/vnl.21561 Published online in Wiley Online Library (wileyonlinelibrary.com). V C 2016 Society of Plastics Engineers JOURNAL OF VINYL & ADDITIVE TECHNOLOGY—2016