Compressive behaviour of nanoclay modified aerospace grade epoxy polymer A. Jumahat 1,2 , C. Soutis* 1 , F. R. Jones 3 and A. Hodzic 1 The effect of nanoclay on the compressive response of an aerospace grade epoxy polymer was studied. The resin was modified with montmorillonite clay type nanomer I.30, and compressive tests were performed on the optimised specimen geometry. A series of nanocomposite with 1– 5 wt-% nanoclay content was fabricated using mechanical stirring and three-roll mill methods. The degree of dispersion of the clay nanoplatelets was examined using TEM. Static uniaxial compression tests were conducted. The compressive stress–strain curves showed that the presence of nanoclay improved the compressive strength and stiffness, promoted higher plastic hardening behaviour after yielding and enhanced the fracture toughness (area under s–e curve) of the epoxy polymer. The fracture surfaces of the broken specimens were observed using SEM with the aim to identify critical failure mechanisms that contributed to the polymer toughening. Rule of mixtures, Halpin–Tsai and modified Halpin–Tsai models were employed to estimate the compressive modulus of the clay–epoxy nanocomposite system. Keywords: Nanocomposites, Nanoclay, Epoxy resin, Compressive properties, Fracture, Stiffness prediction Introduction The addition of nanosize fillers to a polymer resin can produce a number of desirable effects. The most practical reasons for the use of nanofillers are the following: (i) to stiffen the matrix (increase the elastic mod- ulus) without sacrificing the strain to failure (ii) to improve dimensional stability and thermal properties (iii) to enhance yield stress and strength (iv) to improve resistance to crack initiation and propagation (fracture toughness). 1–7 In order to achieve these property improvements, the selected nanofillers usually need to have a higher elastic modulus and a lower coefficient of thermal expansion than the matrix. Several types of nanofillers are commercially available and commonly used for developing nanocomposites, 1–14 such as nanosilica, carbon nanotubes and montmorillo- nite organoclay, as shown in Fig. 1. Clay nanoplatelets are the most widely used nanoparticles in a variety of different polymer matrixes for a wide range of applications. A nanosize clay particle has an elastic modulus of 172 GPa, and once dispersed in the resin, the particles provide an enormous amount of surface area (750 m 2 g 21 ). 3,6,7,10 A large surface area of dispersed nanoclay plays an important role in the confinement of the polymer chain mobility under stress. 3,6,7,10 This gives significant improve- ments in stiffness, strength, dimensional stability and thermomechanical behaviour of the polymer. 6,7,10 The prerequisite for the improvement of properties of this type of nanocomposite is of course the complete exfoliation of clay particles in the resin. However, a processing technique that produces complete dispersion and exfoliation of silicate layers in the polymer matrix still remains a technical challenge. The degree of exfoliation depends on the type of clay structure and its surface treatment, type of epoxy and hardener (including viscosity of the resin), resin–clay interaction, clay content and processing method. The most widely used manufacturing methods for polymer nanocompo- sites are direct mechanical stirring, in situ polymerisa- tion, high shear mixing, melt blending, high pressure mixing, sonication and combination of the above. 1–14 Yasmin et al. 6 reported that high shear mixing using a three-roll mill machine was the best method to exfoliate the stacked layers of silicate clay in a low molecular weight epoxy resin. This method applies external shear forces generated between the adjacent rollers to increase the d-spacing of the silicate platelets (Fig. 2). This paper aims to modify the properties of an aerospace grade epoxy polymer by the inclusion of nanoclay particles. The effect of processing methods on the quality of the produced nanocomposites was studied. Furthermore, the influence of morphological structure (intercalated and/or exfoliated nanocomposite) on the compressive properties was investigated. Measuring the compressive properties provides many challenges, since premature failures due to Euler buckling, specimen misalignment and other imperfections can occur. Several models were adopted to predict the compressive elastic 1 Department of Mechanical Engineering, Aerospace Engineering, University of Sheffield, Sheffield S1 3JD, UK 2 Faculty of Mechanical Engineering, Universiti Teknologi MARA, Shah Alam, Selangor 40450, Malaysia 3 Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, UK *Corresponding author, email c.soutis@sheffield.ac.uk ß Institute of Materials, Minerals and Mining 2012 Published by Maney on behalf of the Institute Received 14 February 2011; accepted 2 April 2011 DOI 10.1179/1743289811Y.0000000028 Plastics, Rubber and Composites 2012 VOL 41 NO 6 225