Plywood-like structure of injection-moulded polypropylene Masayuki Yamaguchi a, * , Yuta Irie a,1 , Panitha Phulkerd a , Hiroki Hagihara a , Soichiro Hirayama a , Shintaro Sasaki b a School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292 Japan b Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology,1-1 Asahidai, Nomi, Ishikawa 923-1292 Japan article info Article history: Received 26 June 2010 Received in revised form 17 September 2010 Accepted 7 October 2010 Available online 15 October 2010 Keywords: Molecular orientation Polypropylene Injection-moulding abstract Injection-moulded products having unique structure, in which the direction of molecular orientation in the skin layer is perpendicular to that in the core layer, are developed employing isotactic polypropylene with a nucleating agent. The extraordinary three-layered structure with b trigonal crystal form in the core layer, which shows higher impact strength than the conventional a monoclinic form, leads to high level of toughness. Moreover, an injection-moulded product having five-layered structure is also demonstrated in this paper. Because of the complicated crack propagation nature due to the abrupt change of molecular orientation, which avoids fractured pieces with sharp-edge, the products with plywood-like structure will be employed in various applications to improve the safety. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Plywood is composed of piled wooden sheets in which adjacent plies have their wood grain at right angles to each other for marked strength, resistance of cracks, shrinkage and twisting. The same concept will be applicable to plastic products, because polymer chains orient to one direction in most of conventional plastic products. However, the difficulty of assembling process, leading to poor cost performance, prevents from prevailing the method. Meanwhile, our research group has clarified that isotactic polypropylene (PP) containing a small amount of N,N 0 -dicyclo- hexyl-2,6-naphthalendicarboxamide as a b-modification nucle- ating agent shows extraordinary molecular orientation at sheet processing recently [1,2]. PP crystallizes on the surface of the needle crystals of the nucleating agent, in which chain-axis (c-axis) of PP orients perpendicular to the long axis of the needle crystals. Since the needle crystals of the nucleating agent align to the applied flow direction by hydrodynamic force, PP chains orient perpendicular to the flow direction at the sheet processing [1]. The anomalous molecular orientation is, however, affected by mixing condition, resin temperature, and cooling condition [2]. In other words, the direction of molecular orientation is adjustable by control of pro- cessing conditions. Although the b nucleating ability of N,N 0 -dicy- clohexyl-2,6-naphthalenedicarboxamide was revealed by numerous researchers [3e16], the extraordinary orientation, leading to new application of b crystalline PP, has not been clarified. In general, injection-moulded products of polymeric materials, including PP, have skin-core structure, in which high level of molecular orientation to the flow direction is attained in a skin layer by flow-induced crystallization [17e22]. On the contrary, a core layer has weak molecular orientation because of low shear rate. In this paper, we developed a novel method to obtain injec- tion-moulded PP having plywood-like structure, considering the difference in crystallization kinetics and applied shear rate between skin and core layers. 2. Experimental 2.1. Materials Commercially available isotactic polypropylene (PP) (Sumitomo Chemical, FLX80E4, MFR ¼ 10 [g/10min]) was employed in this study. The melting point is approximately 165  C, because it is a propylene homopolymer. Further, the stereoregularity is more than 95% [mmmm%, pentads], and the weight average molecular weight is approximately 209,000, as mentioned in detail in the preceding paper [2]. PP with 0.05 wt% of N,N 0 -dicyclohexyl-2,6-naphthalenedicarbox- amide (New Japan Chemical, NJ Star TR NU-100) were extruded by a corotating twin-screw extruder (Technovel, KZW15TW-45MG-NH) either at (1) 200  C (sphere) or (2) 280  C (needle) to control the shape of the nucleating agent [1,2]. The temperature profile in the extruder * Corresponding author. Tel.: þ81 761 51 1621; fax: þ81 761 51 1625. E-mail address: m_yama@jaist.ac.jp (M. Yamaguchi). 1 Present address Nitta Corp. Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.10.007 Polymer 51 (2010) 5983e5989