In situ composites: eect of elongational ¯ow velocity on thermotropic liquid crystalline co-polyester ®brillation in thermoplastic/TLCP systems P.M. Thangamathesvaran a,b ,XiaoHu a, *, Kam Chiu Tam b , Chee Yoon Yue b a School of Materials Engineering, Nanyang Technological University, Singapore 639 798, Singapore b School of Mechanical & Production Engineering, Nanyang Technological University, Singapore 639 798, Singapore Received 16 July 1999; received in revised form 14 June 2000; accepted 15 August 2000 Abstract This study focuses on the TLCP ®brillation process in a thermoplastic (PP) matrix during melt blending. The process of blending includes a single extrusion process in a twin-screw extruder with compatiblization and injection moulding of the extrudates. The TLCP, VA950 in situ ®brillation, compatiblization with PP and retainment of ®brils in the injection-moulded samples has already been achieved in our earlier study. However, the ®brillation process remains a complex issue. There are many factors like viscosity ratio, capillary number, composition, and ¯ow speed which will aect the ®brillation. The elongational ¯ow seems to be a more importantfactoraectingtheTLCP®brillation.Thepresentstudyaimstoidentifytheimportantfactorswhichin¯uencetheTLCP ®brillation at their near-critical conditions. The morphological features and mechanical properties show that the compatiblization and elongational ¯ow speed are very important for eective ®brillation. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Polymers; B. Interfacial strength; D. Scanning Electron Microscopy (SEM); E. Extrusion; In situ composites 1. Introduction Blends of thermotropic liquid crystalline polymers (TLCPs) with thermoplastics have attracted interest over the last few years because of their potential in generating in situ TLCP ®bril composites [1±3]. Recently, studies on hybrid composites comprising of thermoplastics, TLCPs and inorganic ®llers as whiskers are becoming more important for eective reinforce- ment [4,5]. One important aspect of considerable inter- est in the development of TLCP-based blends is the control of morphology during processing. By control- ling the morphology one can control the mechanical performance of the system. The successful formation of in situ composites based on thermoplastics like poly- propylene and TLCP is a challenging one owing to the vast dierence in their glass transition temperatures, melting temperatures and their inherent incompatibility. It is well proved that the two-stage processing results in a much better product for overall performance than the product obtained from single-stage direct injection moulding.Two-stageprocessingminimizestheskin-core morphological dierence and anisotropy, and results moreover in a smooth surface on the micro-composite [6,7]. The two-stage process involves extrusion and injection moulding. Also, there are special but compli- cated co-extrusion processes for the blending of PP- based in situ composites [6]. The extrusion process in which in situ ®brillation occurs is very important for obtaining an optimum ®nal product. The draw ratio (which is the ratio of the square of the die diameter and square of the ®nal extrudate diameter) is the factor which is commonly used to de®ne the resultant extru- date. In general, TLCP ®brillation will be better with increase in draw ratio and hence the mechanical perfor- mance of the resultant product. However, the draw ratio depends on many factors like processing tempera- ture, screw RPM, die diameter, L/D ratio and material feed rate. In addition most of the incompatible systems show variation in extrudate diameter and hence large variation in draw ratio. On the other hand, draw speed is the on line process parameter which can be controlled and can be used to reproduce the product morphology 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(00)00189-5 Composites Science and Technology 61 (2001) 941±947 www.elsevier.com/locate/compscitech * Corresponding author. E-mail address: asxhu@ntu.edu.sg (X. Hu).