Current Nanoscience, 2006, 2, 351-357 351 1573-4137/06 $50.00+.00 © 2006 Bentham Science Publishers Ltd. Stress-Induced Reduction of Water Uptake in Clay-reinforced Epoxy Nanocomposites Erol Sancaktar* and Jason Kuznicki Department of Polymer Engineering, The University of Akron, Akron, OH 44325, USA Abstract: Exfoliated nano-clay/epoxy composites typically contain approximately 1 nm thick layers of clay dispersed in the polymer matrix. Owing to the platy morphology of the silicate layers, exfoliated clay nanocomposites can exhibit dramatically improved barrier and mechanical properties that are not available with conventional composite materials. Since epoxy applications may exist in areas of high moisture content and under mechanically induced stress, the effect of such stressing on water uptake by epoxy-clay nanocomposites is of interest. In this work, low viscosity liquid aromatic diglycidyl ether of bisphenol A (DGEBA) epoxy resin, Epon 815C, was mixed with Montmorillonite nanoclay to produce an exfoliated clay – epoxy resin system containing 0.5% nanoclay by weight. These samples were immersed in water in stressed condition (flexural stress) to assess the effect of stress on the nanocomposite epoxy system’s water uptake behavior. Application of the flexural stress affected the water uptake barrier properties for the nanoclay/epoxy nanocomposites, with the stress acting to decrease the rate of absorption as well as to decrease the equilibrium moisture content in the 0.5% loaded nanocomposite. The results revealed up to 33% reduction in water uptake for the stressed samples. Keywords: Epoxy nanoclay composites, Nanocomposite barrier properties, Water uptake, Stress effect. INTRODUCTION The use of organically modified nanoclays, or organoclays incorporated into epoxy systems has sparked new interest in creating better epoxy systems. In incorporating nanoclay particles into thermoset epoxy resins, the chemical crosslinking which changes the conformation of the molecule chains is the key. As the number of bonds between chains increases, the inter-chain distance decreases causing shrinkage in the bulk system with an overall increase in chain length as well as the formation of a three dimensional network. Park and Jana [1] report that this entropy change and the resulting elastic forces drive the exfoliation and intercalation in nanoclay epoxies. The successes and benefits shown by many others in this area are summarized in a recent review of the methods and properties of exfoliated polymer-clay nanocomposites [2]. Models that consider how the addition of nanoclay benefits moisture diffusion through polymers have surfaced in the work of Yano, Usuki, Okada, Toshio and Kamigaito [3] and Drozdov, Christiansen, Gupta and Shah [4]. The main difference in the theories is the concept of interaction of water molecules with the clay layers. Yano, Usuki, Okada, Toshio and Kamigaito assumed no interaction between the clay in the nanocomposite and water molecules diffusing through it. Drozdov, Christiansen, Gupta and Shah, however, assumed a hydrophilic clay particle that attracts and immobilizes water molecules onto its surface. Many applications involve epoxy systems under stress and in humid areas. The protection of the adherend-adhesive *Address correspondence to this author at the Department of Polymer Engineering, The University of Akron, Akron, OH 44325, USA; Tel: 330- 972-5508; Fax: 330-258-2339; E-mail: erol@uakron.edu interface from moisture is a key part to the stability of the joint. The study aims to compare moisture diffusion through nanoclay filled epoxy systems in stressed and unstressed modes. The degree of exfoliation, dispersion, and orientation of the dispersed clay phase in a stressed and unstressed epoxy matrix will be characterized by x-ray diffraction and TEM to provide supporting evidence for our moisture diffusion findings. EXPERIMENTAL METHODS AND MATERIALS Epon 815C supplied by Resolution Performance Products in Houston, Texas was combined with 30B Montmorillonite nanoclay from Southern Clay Products (Gonzales, Texas). The curing agent used was Epi-cure 3223, or Diethylenetri- amine (DETA) also by Resolution Performance Products. Low viscosity (5-7 Poise) liquid aromatic diglycidyl ether of bisphenol A (DGEBA) epoxy resin, Epon 815C, was mixed with nanoclay at 60°C for 6 hours. The epoxy-clay mixture was then mixed with curing agent DETA (Diethylenetri- amine) at 80°C for 4 minutes. Film samples for testing were cast using a Teflon and PET compression mold. The nanoclay-filled and neat uncured epoxy systems prepared were poured into the mold and allowed to gel while under 34.5 MPa of pressure to ensure an average thickness of 0.15 mm. The samples gelled at room temperature, then were removed and cured at 120°C for 3 hours. The film samples were cut and then sanded to remove any cracks or chips with 400 grit sandpaper into 2 cm by 3 cm rectangles prior to testing. Unstressed samples were secured by a weight to a flat aluminum surface and stressed samples were secured by plastic clips to aluminum cylinders to induce flexural stress in such a way that the inner surface under compression was not exposed to water. The applied stress levels were determined by the diameter of the cylinders used, and