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The “solvent-free one-pot synthesis” method has been developed as an effective, low-cost process to prepare silica/epoxy nanocomposites. A uniformly dispersed, strongly adhered silica/epoxy system is achieved with greatly improved thermal and mechanical properties.
DR. NOPPHAWAN PHONTHAMMACHAI, RESEARCH ENGINEER
DR. CHAOBIN HE, PRINCIPAL SCIENTIST INSTITUTE OF MATERIALS RESEARCH AND ENGINEERING (SINGAPORE)(Published on June-July 2010 - JEC Magazine #58)
Due to the current global demand for new low-cost, high-performance lightweight materials for fuel economy and low emissions (CO2 and NOx reductions) in the aerospace and automotive industries, high-strength and lightweight carbon-fibre-reinforced polymers (CFRP) are emerging as promising sustainable materials to replace metals. To achieve these requirements, nanocomposites (polymers incorporating a few percentages of nanometre-sized fillers) are incorporated into CFRPs to provide higher strength, lower density and lower production costs, while combining insulating, acoustic, damping and conductive properties into one structural component.
The proper dispersion of nano-fillers and their adhesion to polymer matrices are key challenges for the preparation of high-performance nanocomposites. Many attempts have been made to overcome these issues, such as the development of a synthesis method for mono-dispersed filler/solvents, the modification of silica surfaces and the dispersion of nano-fillers in epoxy compositions under severe conditions (high temperature and pressure), in which a large amount of filler is required to obtain nanocomposites with high mechanical and thermal properties (5-30% by weight of the total composite).
Key advantages of the “solvent-free one-pot synthesis” process
However, integrating these processes into the industrialscale production of nanocomposites is difficult due to their manufacturing complexity, high processing cost and highviscosity products. These methods use solvents in the preparation step, which need to be removed and recycled/disposed of, thus posing environmental, health and safety issues and involving additional solvent removal/disposal costs. Moreover, the production of CFRPs using such processes as resin infusion moulding (RIM), resin transfer moulding (RTM) or vacuum-assisted resin infusion moulding (VARIM) involves many types of mould shapes that require low-viscosity nanocomposites for easy resin flow into the mould cavities before curing. Therefore, a convenient, low-cost process has to be developed to prepare epoxy-based nanocomposites that combine a uniform dispersion of nano-fillers, high adhesion between filler and epoxy matrix, a lower degree of filler content and low viscosity to target the high-performance matrix required for CFRPs.
An all-in-one method
The “solvent-free one-pot synthesis” process has been developed as an all-in-one method (effective, convenient and low cost) to prepare silica/epoxy nanocomposites. In this particular process, the steps of silica formation, surface functionalization and silica dispersion in epoxy compositions are combined into one step under mild conditions (25-50°C, mechanical stirring), where strong adhesion between silica, epoxy matrix and curing agent is homogeneously formed. This process produces mono-dispersed silica nanoparticles (60-100 nm) with a high aspect ratio (higher than 1) (Figure 1) with various degrees of silica loading (1-4 wt % in the total composite).
The mechanical properties of silica/epoxy nanocomposites are significantly improved over the neat epoxy system (49, 20, 17% increases in fracture toughness, flexural modulus and tensile modulus) through the incorporation of only 3 wt% silica.
The TGA analysis (Figure 2) shows a composite structure with high thermal stability, in which the degradation of the homogeneous composite composition occurred as a single transition at 375°C. Due to their low viscosities (~ 3000- 7,000 mPa.s), the nanocomposite prepared using the present technology can easily be applied to any type of moulding system.
Here are few examples and update of these two techniques which can be seen in many application sectors now.