JEC COMPOSITES MAGAZINE - Issue #38 - January/February 2008 - 61

Surface-modified silica nanoparticles in epoxy and vinylester resins Surface-modified silica nanoparticles have been available in industrial quantities for a couple of years, as concentrates in epoxy resins like DGEBA, DGEBF or reactive diluents [1]. More recently, a nanoparticle-containing tetrafunctional epoxy resin has become available. Vinylester resins containing silica nanoparticles are very close to commercialization. These silica nanoparticles are manufactured by a sol-gel process and within the resin. The commercial products are transparent, lowviscosity resins which contain 40% nanoparticles. They can be blended with standard resins easily, and no expensive or complex dispersing equipment is needed. Picture 2 shows these spherical particles at different concentrations. curing, but when fracture occurs, there is void formation between the particle and resin matrix – macroscopically leading to an increased toughness. The same mechanism was discovered for silica nanoparticles [7], and this toughening effect appears to increase with decreasing particle size. Thus, nanoparticles give a larger toughness increase than micron-sized glass particles. These property improvements are found with fibre-reinforced composites as well, as reported previously [3-5]. Three different systems were investigated: first carbon-fibre-reinforced epoxy resin cured with an anhydride, typically used for sporting goods or filament-wound articles. The second system was glass-fibre-reinforced epoxy resin, cured with an amine which was chosen from a typical wind-turbine blade formulation. Finally, carbon-fibre-reinforced tetrafunctional epoxy resin, cured with aromatic amines, which is characteristic for high-end applications such as aerospace. The first composite parts and prepregs using the benefits of this technology are now being introduced to the market, and glassfibre-reinforced vinylester resin systems are currently being investigated. Synergy between rubber toughening and nanoparticles Laminates were tested using the falling dart method. Although laminates using nanoparticles showed no improvement, the GIc of these laminates was much higher than for the control. This created the idea to combine rubber toughening and the addition of silica nanoparticles to achieve the best of both worlds: very tough materials, even when impacted at high speed, with a higher modulus than the unmodified system. The previous contradiction, having a tough and stiff composite, could be resolved with the combination of these two technologies. Moreover, a synergy was discovered for epoxy resins: the toughness increase was larger than expected [3-5]. The same synergy exists for vinylester resins. Rubbertoughened vinylester resins were modified with surfacemodified silica nanoparticles and cured with an amineaccelerated peroxide. The commercially available materials Derakane 8084 and DION 9500 were used. Flexural strength was found to increase from 52 to 79 MPa, and flexural modulus from 790 to 1650 MPa, with an addition level of 10% nanoparticles. Toughness was increased as well. A higher addition level of 20% nanoparticles gave only minor further improvements: a flexural strength of 83 MPa, and a flexural modulus of 1790 MPa. As in epoxy resins, it seems that an addition level of approximately 10% of nanoparticles is sufficient for most applications. Confirming these first bulk resin results for glass-fibre-reinforced composites is the subject of our current research work. Picture 2: TEM picture of silica nanoparticles in amine-cured epoxy resins at different concentrations As these particles have an average size of 20 nm and a very narrow particle size distribution, they are transparent. The silica nanoparticles do not undergo sedimentation, even in solvent-based impregnating solutions. In injection processes like RTM or VARTM, resin viscosity is not increased significantly. Due to their small size, the silica nanoparticles are not filtered out, but easily pass through close-meshed fabrics. Thus, even large parts can be infiltrated by the modified resins without problems. A variety of properties can be improved in fibre-reinforced thermosets by using these silica nanoparticles: tensile modulus and stiffness, bending strength and bending modulus. If higher addition levels are applied, shrinkage can be reduced by 2030% [3-5]. Toughness is also increased, which is rather unusual for a functional filler. The toughening mechanisms have been identified. Firstly, crack growth may be affected by the crack deflection caused by the nanoparticles [6]. Secondly, it was well-known for more than 20 years that glass microspheres can be used to toughen thermosetting resins such as epoxies or vinylesters. Here, the matrix shrinks onto the particles during No38 January - February 2008 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #38 - January/February 2008

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #38 - January/February 2008