48 Journal of Research Updates in Polymer Science, 2013, 2, 48-56 E-ISSN: 1929-5995/13 © 2013 Lifescience Global Optimization of Process Parameters for Generation of Nanocellular Polymer Foams Subhendu Bhattacharya * , Rahul Gupta and Sati N. Bhattacharya Rheology and Materials Processing Centre (RMPC), School of Civil, Chemical & Environmental Engineering, RMIT University, Melbourne, Australia Abstract: High melt strength polypropylene nanocomposites, PPNC/Cloisite 20A (clay) with exfoliated and intercalated morphologies were prepared and subsequently foamed in a batch setup under different foaming conditions. The foaming parameters were varied to relate the foam cell structure to these parameters and determine the efficiency of clay in producing fine cell foams. A Box Benkhen design approach was used initially to determine the effect of processing parameters on foam cell morphology and also to perform optimization studies. The optimization process helped in identifying the range of operating conditions needed to minimize foam cell sizes. Saturation pressure and temperature and foaming time and temperature are the four processing variables used in these studies. Nanocellular foam cells were effectively generated for the first time in Polypropylene nanocomposites. Keywords: Nanocellular, DOE, optimization, batch foaming. INTRODUCTION Considerable amount of work has been devoted to the generation of foams characterized by smaller cell sizes and narrower distribution specifically in production of sub microcellular and nanocellular foams [1-2]. The foams with reduced cell sizes have been found to provide improved mechanical and insulating properties as compared to the larger microcellular foams. The addition of a filler to improve nucleation rate propelled by the low energy of activation required in heterogeneous nucleation does provide a way to generate sub micron and nanocellular foams [3]. The internal structure of the material and its rheological characteristics play an important role in determining the potential of a polymer to be used in making fine cell foams. Hence the production of fine cell foams is dependent on the polymer structure, its rheological behaviour, the effect of filler and careful control of processing conditions. The available window for generation of fine cell foams is very small and hence proper control of process parameters is essential for generation of fine cell foams. As such experimental design is difficult to use in the case of generation of fine cell foams. The available processing window for generation of nanocellular foams is unknown hence without knowing proper limits of the parameters affecting cell size it would be impossible to perform an optimization study for controlling cell sizes. Also selection of proper independent parameters are important to ensure that there are no cross interactions and the results are accurate and scalable. *Address correspondence to this author at the Rheology and Materials Processing Centre (RMPC), School of Civil, Chemical & Environmental Engineering, RMIT University, Melbourne, Australia; Tel: +917925307798; E-mail: subhendu.bhattacharya@gmail.com EFFECTS OF PROCESSING PARAMETERS ON FOAM CELL STRUCTURE The HMS –PP and PPNCs were saturated in the autoclave under different saturation temperature and pressure respectively. Gas solubility studies using supercritical CO 2 have revealed that the highest amount of solubility is attained at lower temperatures and higher pressures respectively [5]. The increase in the saturation temperature imparts increasing amount of kinetic energy to the gas molecules inside the polymer samples, which results in an increased tendency of the gas to escape from the polymer samples resulting in lower solubility and vice versa. Similarly the pressure of the gas used to saturate the polymer sample solubility increases due to increased potential energy of the gas molecules accompanied by the higher penetration power of the gas molecules. Interestingly the pressure differential generated during the subsequent depressurization of the sample is also higher at higher saturation pressure causing an improvement in the degree of supersaturation [7]. Higher solubility of blowing agent within the polymer samples causes supersaturation at lower foaming temperature and improves the nucleation rate within the polymer sample since the nucleation rate is directly proportional to the amount of gas dissolved in the polymer due to the presence of a competing mechanism between cell nucleation and growth. The quicker super saturation of the sample at lower temperature also additionally increases the tendency of the gas to escape from the sample with an increase in the degree of superheat. All these factors when combined cause a reduction in foam cell size by reducing the amount of gas available for foam cell growth and nucleation.