Batch foaming of SAN/clay nanocomposites with scCO 2 : A very tunable way of controlling the cellular morphology Laetitia Urbanczyk a , Cédric Calberg b , Christophe Detrembleur a , Christine Jérôme a, * , Michaël Alexandre a a Centre for Education and Research on Macromolecules (CERM), University of Liege, Building B6, 4000 Liège, Belgium b Department of Applied Chemistry, University of Liege, Building B6, 4000 Liège, Belgium article info Article history: Received 23 February 2010 Received in revised form 17 May 2010 Accepted 19 May 2010 Available online 8 June 2010 Keywords: Foam CO 2 Clay abstract This paper aims at elucidating some important parameters affecting the cellular morphology of poly (styrene-co-acrylonitrile) (SAN)/clay nanocomposite foams prepared with the supercritical CO 2 tech- nology. Prior to foaming experiments, the SAN/CO 2 system has first been studied. The effect of nanoclay on CO 2 sorption/desorption rate into/from SAN is assessed with a gravimetric method. Ideal saturation conditions are then deduced in view of the foaming process. Nanocomposites foaming has first been performed with the one-step foaming process, also called depressurization foaming. Foams with different cellular morphology have been obtained depending on nanoclay dispersion level and foaming conditions. While foaming at low temperature (40  C) leads to foams with the highest cell density (w10 12 e10 14 cells/cm 3 ), the foam expansion is restricted (dw0.7e0.8 g/cm 3 ). This drawback has been overcome with the use of the two-step foaming process, also called solid-state foaming, where foam expansion occurs during sample dipping in a hot oil bath (dw0.1e0.5 g/cm 3 ). Different foaming parameters have been varied, and some schemes have been drawn to summarize the characteristics of the foams prepared e cell size, cell density, foam density e depending on both the foaming conditions and nanoclay addition. This result thus illustrates the huge flexibility of the supercritical CO 2 batch foaming process for tuning the foam cellular morphology. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Conventional polymeric foams are very attractive mostly because of their lower cost per volume unit compared to unfoamed mate- rials, but also for their sound or heat insulating properties, cush- ioning ability, etc. However, the foams mechanical properties are generally weaker than plain materials, thus limiting their range of applications. Microcellular foams, defined as foams with cell diam- eter lower than 10 mm and cell density greater than 10 10 cells/cm 3 , usually show better mechanical properties over conventional foams (cell size >300 mm and cell density <10 6 cells/cm 3 ) [1]. A lot of works and reviews report that supercritical CO 2 is a very efficient foaming agent for microcellular foam processing [1e4]. In addition, CO 2 is environmentally friendly, non toxic, cheap, abun- dant, and its supercritical parameters are easily attainable (T c ¼ 31.1  C, P c ¼ 73.8 bar). Batch and continuous foaming of polymers with supercritical fluids, and more especially with scCO 2 , have been extensively described in the literature [4,5]. Basically, the foaming process consists of three main steps. First, the fluid is solubilized into the polymer and the mixture forms a homogeneous phase. Then, a thermodynamic instability is suddenly applied to the system via a pressure drop, or a temperature increase, leading to sudden polymer/fluid immiscibility. The system reacts against this perturbation by inducing phase demixion, which usually occurs in the form of cell nucleation. Then, CO 2 molecules migrate towards the nucleated cells and participate to cell growth. Foam expansion occurs until the rising viscosity of the polymer (through cooling, deplasticization, crystallization and/or strain hardening) restricts any additional deformation, or until all the fluid available is used [6,7]. As illustrated in a previous work [8], polymers can be filled with a small amount of nanoparticles, such as lamellar nanoclays, in order to enhance several material properties like reduced flam- mability and gas permeability, and improved mechanical resis- tance. Actually, these nanofillers are not only beneficial for material properties, but they are also known to promote the heterogeneous nucleation of extra cells during foaming. Therefore, polymer/clay nanocomposite foams are usually characterized by small cell size and high cell density, which is highly desirable to achieve better mechanical resistance [4]. A lot of works dealing with scCO 2 - assisted foaming of several nanocomposites systems have already * Corresponding author. Tel.: þ32 43663491; fax: þ32 43663497. E-mail address: c.jerome@ulg.ac.be (C. Jérôme). 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.2010.05.037 Polymer 51 (2010) 3520e3531