Gas barrier properties of PP/EPDM blend nanocomposites

Abstract

Nanocomposites of polypropylene (PP)/ethylene-propylene-diene rubber (EPDM) blend with montmorillonite-based organoclay were prepared in a solvent blending method. Solvent blending of PP and EPDM in a composition of 50:50 formed a two phase morphology in which EPDM appeared as dispersed phase with irregular shape. The size of dispersed phase reduced significantly to almost spherical domains by addition of the nanoclay. For better dispersion of nanoclay in the PP/EPDM blend, an antioxidant was used as compatibilizer. State of nanoclay dispersion was evaluated by X-ray diffraction (XRD), and also, by a novel method using permeability measurements data in a permeability model. The measured d-spacing data proved a good dispersion of nanoclay at low clay contents along with compatibilizer. The permeability model for flake-filled polymers was used to estimate the aspect ratio of nanoclay platelets in the blend nanocomposites. Oxygen and carbon dioxide barrier property of the PP/EPDM blend improved about two-fold by adding only 1.5 vol% organoclay. Differential scanning calorimetry measurements indicated a decrease in crystallinity up to 27% suggesting a reduction in spherulites growth. However, the melting temperature remained unchanged. The increase in barrier property of the blend, despite a decrease in crystallinity, indicated the dominant role of organoclay platelets in barrier improvement. According to the permeability model, very high barrier property could be obtained if the aspect ratio of the flakes or platelets of the organoclay could be significantly increased in the blend. Scanning electron micrographs of fracture surface of nanocomposite membranes broken at very low temperatures, exhibited a very ductile surface indicating a good compatibility of PP and EPDM rubber and also, a possible contribution of nanoparticles to deformation mechanisms, such as extensive shear yielding in the polymer blend.

Introduction

Polyolefins, such as polypropylene and polyethylene find extensive use in packaging. Improvements in poor barrier properties of polyolefins will be beneficial for the current applications and should also lead to applications in pharmaceuticals, electronic packaging and particularly packaging of food products, which are sensitive to oxygen. Most schemes to improve polyolefins gas barrier property involve either addition of higher barrier plastics via a multilayer structure or high barrier surface coatings, however, these approaches are not cost effective. The emerging field of polymer-layered nanocomposites is unique in that it addresses shortcomings of polyolefins for both packaging and engineered applications requiring desirable mechanical, thermal as well as good barrier properties. Polymer-layered silicate nanocomposites are plastics containing low levels of dispersed platelet minerals with at least one dimension in the nanometer range. A small clay loading does not affect the processibility and does not substantially increase the specific gravity of the polymer. Also, due to the fine particle size of the fillers, the film clarity, for all practical purposes, will not be reduced. The most common mineral used in nanocomposites is montmorillonite. Nonpolar, hydrophobic polyolefins, such as polypropylene and polyethylene lack a strong interaction with polar, hydrophilic clay particles of montmorillonite. To improve the interaction between polymer and clay, montmorillonite is treated usually with ammonium alkyl compounds and the treated clay is called organoclay. Many approaches have been tried to prepare organoclay/polyolefin nanocomposites, including in situ polymerization of ethylene in the presence of layered silicates [1], [2], solvent blending [3] and melt compounding [4], [5], [6], [7], [8]. However, melt blending is the most convenient process for preparation of the nanocomposites. Although, in situ polymerization may form an exfoliated nanocomposite but the clay platelets would recover the stacked structure in the subsequent melt forming process and mechanical properties would not significantly improved [1], [2]. Only one recent article was found on the solvent blending of polyethylene/montmorillonite reporting a good dispersion of clay in polyethylene [3]. Compatibilizers (usually maleated PP or PE) are believed effective for melt blending of clay and polyolefin in order to enhance the interaction between clay and polymer [9], [10]. In solvent blending process that was used for preparation of nanocomposites in our study, maleic anhydride grafted polypropylene was unsuccessful, and indeed, it caused agglomeration of the clay instead of resulting in good dispersion. One recent report used an antioxidant as compatibilizer for polypropylene/montmorillonite in a melt compounding process claiming a relatively successful result [11]. So far, most papers on clay/polyolefin nanocomposites concentrate on the state of dispersion of nanoclay in melt compounding and mechanical properties in particular [2], [5], [6], [8], [12], [13], [14], [15], [16]. However, to our best knowledge none of these reports have studied the barrier properties. In this paper, we have studied diffusion of oxygen and carbon dioxide gases across the 50:50 blend of polypropylene (PP) and ethylene-propylene-diene (EPDM) rubber membranes containing organophilic montmorillonite nanoclay and an antioxidant as compatibilizer. The nanocomposite was prepared in a solvent blending process. The effect of nanoclay content and presence of compatibilizer on the state of dispersion are discussed by using XRD and a gas permeation model. The effect of nanoclay in barrier improvement of PP/EPDM blend against oxygen and carbon dioxide was investigated. Crystallinity of polypropylene and morphology of PP/EPDM/organoclay systems are discussed.