Representative Volume Element and Morphological-Image Based Analyses of Polypropylene/Nanoclay Composites

Abstract

In recent years, the usage of polymer nanocomposites has increased rapidly but although their functional properties are generally superior, often the relevant mechanical properties do not give sufficient consistency to satisfy the expectations of the end users. In most cases, the property variations come from the manufacturing methods and the lack of knowledge of the individual effects of processing parameters. As a result, the theoretical predictions also get adversely affected. The present paper describes firstly a systematic approach towards determining the optimised formulation and melt compounding of the nanomaterials, including the apparent dual roles of compatibilisers, and secondly numerical analyses that are carried out based on the obtained morphological structures of polypropylene (PP)/clay nanocomposites. An optimisation of manufacturing parameters is carried out by implementing the Taguchi design of experiments methodology. Clay type and content, compatibiliser content and PP type are varied to produce the combinations of factors that maximise (not simultaneously) the tensile/flexural moduli and strengths as well as the impact strengths of prepared nanocomposites. Following this step, nanocomposites with good individual mechanical properties are manufactured using a globally suboptimised combination and used for the subsequent analysis. Two-dimensional 3×3 array representative volume element (RVE) models are implemented to predict the elastic moduli of PP/clay nanocomposites in terms of clay content, aspect ratio and dispersion pattern of clay platelets.In addition, the effects of intercalated or exfoliated structures are also considered with the aspect ratios of 10 and 100 as the benchmarks. Finally, an image-based extended object-oriented finite element (OOF) modelling technique is implemented by mapping the real micro/nanostructures of clay platelets. The information on the overall material behaviour is gathered from the real morphological data and the fundamental material characteristics of the constitutive phases. Captured morphological images from scanning or transmission electron microscopy, combined with image analysis, are used to generate the necessary geometric information regarding the micro/nanostructures. The material properties are obtained from conducted tests and published literature. The numerical results from both methods predicting the elastic moduli of PP/clay nanocomposites are compared with the experimental data and the available theoretical models for composites. Very good agreement is shown to exist establishing the viability of this kind of numerical approach, especially that of OOF which is based on image analysis of captured real morphological data.