Clay/Epoxy Nanocomposites

Abstract

Nanocomposites are a novel class of composite materials where one of the constituents has dimensions in the range between 1 and 100 nm. Recent and ongoing research on polymer/inorganic nanocomposites has shown dramatic enhancements in stiffness, strength and thermal properties over those of polymers, without compromising on density, toughness or processibility. Such improvement in properties has been estimated to correspond to approximately 30–50% in weight savings in advanced high speed propulsion and space systems. This in turn results in substantial improvement in system performance and cost reduction. Major differences in behaviour between conventional and nanostructured materials result from the fact that the latter have much larger surface (or interface) area per unit volume. Since many important chemical and physical interactions are governed by surfaces, a nanostructured material can have substantially different properties from a larger-dimension material of the same composition. In the case of fibres or foils the area per unit volume is inversely proportional to the fibre diameter or the foil thickness, respectively. Thus, the smaller these dimensions are, the larger is the surface area per unit volume. Reinforcing efficiency requires high aspect ratios of the particulate constituent which is provided by nanofibers or nanotubes (1-dimensional) and nanofoils (2-dimensional). A general comprehensive review of the various types of nanocomposites was given by Komarneni [1]. One particular class of nanocomposites with great potential are organic/clay nanocomposites [2]. The crystalline structure of some clays, especially smectite clays, is layered and amenable to forming organic/clay nanocomposites because of the weak bonding (van der Waals) between layers. Smectite clays and other layered inorganic materials can be broken down into submicron size disk-like particles of approximately 10 : 1 aspect ratio consisting of a stack of nanometer thick layers of very high stiffness and strength. These layers are approximately 1 nm thick and have aspect ratios in the range of 100–1000. A layer of montmorillonite clay is 1 nm thick and has a specific gravity of 2.5. The layered clay particles have an aggregate specific gravity of 1.98. The clay particles or their layers can be incorporated into a polymer matrix to form an organic/inorganic composite. Polymer/clay composites can be divided into three categories: Conventional particulate composites; intercalated nanocomposites; exfoliated nanocomposites. In a conventional particulate composite the clay particles (tactoids) exist in their original aggregated state with no insertion (intercalation) of polymer matrix between the layers. In this state the particles can impart only marginal enhancement of properties to the matrix. In intercalated nanocomposites the polymer is inserted (intercalated) into the clay structure between the layers in a crystallographically regular fashion. The nanocomposite is interlaid by only a few molecular layers of polymer and the properties of the particle resemble those of the ceramic host. In an exfoliated nanocomposite the individual 1 nm thick clay layers are separated and dispersed in a continuous polymer matrix with average distances between layers depending on the clay concentration. An exfoliated nanocomposite has properties governed primarily by the matrix. This type of composite was developed by the Toyota group who synthesised a nylon/clay nanocomposite [3]. Nanocomposites consisting of exfoliated silicate nanolayers in polystyrene and epoxy matrices have also been synthesised and studied. Generally, exfoliated nanocomposites where the individual silicate layers are completely separated and dispersed in the matrix, exhibit better properties than intercalated nanocomposites of the same particle concentration. A key to fabrication of clay nanocomposites with optimised properties is the intercalation or exfoliation of the silicate layers of the clay particles. Procedures of course vary with the type of matrix used. The Toyota group, for example, managed to produce a nylon/clay hybrid nanocomposite by mixing the clay mineral with ε-caprolactam. The result was Nylon-6 with exfoliated and dispersed clay nanolayers. A procedure for intercalating the same polymer (ε-caprolactam) and polymerizing it between silicate layers has been described by others. Exfoliation of montmorillonite clay in epoxy resin has been achieved by mixing the epoxide (DGEBA, Epon 828), and the curing agent (polyether amine, Jeffamine D2000) and the clay. Intercalation and exfoliation of clays in epoxy has been studied more recently using X-ray diffraction and differential scanning calorimetry (DSC). It was pointed out that exfoliation takes place in the early stages of resin curing before gelation. The extent of exfoliation depends on the rate of curing and sequence of gelation between the layers (intragallery) and outside the clay particles (extragallery). If the resin between layers cures faster than the outside resin and reaches full cure before gelation of the outside resin, the clay will exfoliate. One of the advantages of polymer/clay composites with intercalated or exfoliated layers is their enhanced thermomechanical behaviour. The Toyota group reported substantial increases in modulus and strength of Nylon-6/clay nanocomposites over those of pure Nylon [4]. A three-fold increase in modulus at 120 °C was reported for a moderate clay particle concentration of 5% by weight. In the case of epoxy/clay nanocomposites increases in both modulus and strength were measured over those of pure epoxy. The relative increases were much more pronounced in the case of a rubbery matrix below Tg. The tensile strength and modulus increase nearly linearly with clay concentration. More than a 10-fold increase in strength and modulus is realised by the addition of only 15% by weight (approximately 8% by volume) of the exfoliated nanoclay. The pronounced property enhancement for the rubbery matrix is attributed to the fact that the higher strain in the rubbery matrix tends to align the clay layers better in the loading direction. Research to date has shown that polymer/clay nanocomposites offer enhanced thermomechanical properties with inexpensive raw materials and not very involved processing procedures. Further development and optimization of these materials from the mechanical point of view require the measurement and application/development of models for prediction of properties, such as stiffness, strength, fracture toughness and coefficient of thermal expansion. A thorough study of processing, microscopic assessment of their structure, characterization and modelling of the mechanical behaviour of clay/epoxy and graphite/epoxy nanocomposites was performed at Northwestern University under the directorship of I.M. Daniel [5–13].