Evolution of the morphology of HIPS particles G. Patricia Leal, Jose ´ M. Asua * Institute for Polymer Materials (POLYMAT) and Grupo de Ingenierı ´a Quı ´mica, Departamento de Quı ´mica Aplicada, Facultad de Ciencias Quı ´micas, University of the Basque Country, Apdo. 1072, ES-20080 Donostia-San Sebastia ´n, Spain article info Article history: Received 4 June 2008 Received in revised form 16 October 2008 Accepted 25 October 2008 Available online 30 October 2008 Keywords: High impact polystyrene Phase inversion Salami morphology abstract The formation and evolution of the particle morphology during the production of high impact poly- styrene in bulk were investigated. Evidence about the morphology of the system during phase inversion and the subsequent evolution of the particle morphology was obtained by transmission electron microscopy in reactions in which the type and concentration of initiator, agitation speed and poly- butadiene content were varied. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction High impact polystyrene (HIPS) is a multiphase material whose properties are largely determined by its morphology. HIPS is produced by polymerizing styrene in which some amount of polybutadiene (PBD) has been dissolved. Initially, the system is homogeneous, but as the concentration of polystyrene (PS) increases, phase separation occurs leading to a system in which the continuous phase is a solution of PBD in styrene and the dispersed phase is a solution of PS in styrene. The stability of the dispersion is greatly improved by the formation of poly(butadiene-graft- styrene) copolymers, which is promoted by using initiators able to create radicals on the PBD backbone. As polymerization proceeds, the amount of PS in the system increases and that of styrene decreases. Therefore, the volume fraction of the PS-rich phase increases and that of the PBD-rich phase decreases. At one point, a phase inversion occurs and the PS-rich phase becomes the continuous phase. The dispersed rubber particles present a complex morphology with PS occlusions. This morphology is often referred to as salami morphology. After phase inversion, polymerization proceeds until final conversion, trying to preserve the salami morphology. Excellent reviews of the process have been published [1–4]. Kinetic models for the polymerization rate, MWD and grafting [5– 7], which can be used for simulating the dynamics of continuous reactors [8–11], are available. The effect of the morphology on the properties of the HIPS has been extensively studied [12–15]. However, in spite of importance of the HIPS morphology, the effect of the process conditions on the morphology has not been exten- sively studied. It has been reported that particle size decreased with agitation rate [16–19]. Conflicting results on the effect of the PBD content on particle size have been reported [16,19]. Increasing initiator concentration yielded larger particle sizes [16,20]. Reiss and Graillard [16] and Moore [21] reported that the size of the rubber particles increased with the molecular weight of the PBD. Moore [21] found that salami morphologies were obtained with a first stage polymerization temperature of 60  C, whereas very small particles with no salami morphology were obtained at 100  C. These small particles do not provide impact strength to the HIPS. However, the effect of temperature could be counteracted by increasing the PBD concentration. The results discussed above are difficult to interpret in mecha- nistic terms as the evolution of the morphology has not been studied in detail. The final morphology of the HIPS is the result of both the phase inversion and the evolution of the morphology during the second stage of the polymerization. However, no experimental evidence about the way in which phase inversion occurs has been reported. In addition, the evolution of the particle morphology after phase inversion has been scarcely studied and the studies were mostly restricted to the size of the polymer particles [18,22–24]. Lee et al. [22] used laser light scattering to monitor the time evolution of the rubber particle size distribution finding that the observed PSD became narrower and the average particles size decreased during polymerization until a stable particle size distribution was reached. The authors claimed that this evolution was linked to the phase inversion process that, because of * Corresponding author. Tel.: þ34 9 43 018 181; fax: þ34 9 43 017 065. E-mail address: jm.asua@ehu.es (J.M. Asua). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2008.10.035 Polymer 50 (2009) 68–76