JEC COMPOSITES MAGAZINE - Issue #51 - September 2009 - 55

Nano-toughening structural epoxy adhesives Toughening of high-performance structural epoxy adhesives using dispersed organic nanoparticles enhances their performance and longevity, pushing the boundaries of adhesives in composite applications. Incorporating fibres in a structural adhesive can also bring some toughness improvement, but this is often offset by a higher viscosity, making mixing and application difficult, in addition to a higher density of the adhesive which increases the weight of the final part. OLIVIER DE VERCLOS, R & T, & DUNCAN HOWLAND, R & T, HUNTSMAN ADVANCED MATERIALS Nanoparticles Nanoparticles are now well-known additives in thermoset materials, where they have been used to enhance properties such as stiffness, temperature resistance, thermal and electrical conductivity, and scratch resistance. The interesting properties achieved through the use of nanoparticles are due mainly to the very high surface area of the particle-matrix interface and the accompanying interfacial effects. The size of each particle is close to the molecular level, making the surface-to-volume ratio extremely high. In practice, this means that the particles predominantly display surface interaction properties, and minimal bulk properties. Another benefit of such a huge surface area is that only small additions (a few percent) of nanoparticles are necessary to bring about dramatic changes in properties. This contrasts with more traditional modifiers such as fibres or mineral fillers, where high addition levels often have a negative effect on viscosity and, in some cases, mechanical properties. Several approaches to improve the modulus-toughness balance of brittle systems have been described in the literature, to varying degrees of success. Mineral particles such as nanoclays and silica were found to be less effective than a special dispersed organic nanophase having soft and hard domains. The organic nanophase offers a highly effective method to delocalise strain in the material, and thus prevent brittle fracture. tructural thermoset materials used in composite materials and adhesives display excellent mechanical properties such as rigidity, strength and temperature resistance, but are relatively brittle. This means that they fail before reaching their ultimate strength due to brittle fracture. This contrasts with ductile deformation seen in many metals and thermoplastics, which occurs after reaching ultimate strength. Small imperfections in a composite material or adhesive initiate cracking under load, which leads to crack propagation and then material failure. S Fig. 1: Brittle and tough materials (Courtesy of Professor H. E. H. Meijer, Technical University of Eindhoven, The Netherlands). Toughening and brittle fracture Increasing the flexibility of a composite material can decrease brittleness and avoid brittle fracture. Flexibilizers are well known in epoxy systems. However, increasing flexibility reduces the modulus (rigidity), strength and temperature resistance of the material. The loss of other mechanical properties has made this an unsuitable method for increasing toughness in many structural materials. Thus, there is a need for new methods to increase the toughness of thermosetting materials without impacting negatively their strength, modulus or temperature resistance. Toughening epoxy adhesives Huntsman has developed a nanoparticle technology which forms a dispersed nanophase in situ during the curing of the epoxy and hardener. This differs from a system containing mineral nanoparticles, which requires very careful dispersion and stabilisation. To form a dispersed phase in situ, the curing No51 September 2009 / jec composites magazine 55

JEC COMPOSITES MAGAZINE - Issue #51 - September 2009

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #51 - September 2009