The dispersion of a layered silicate into an epoxy matrix often increases the material strength and stiffness, but reduces resin toughness. This innovation is a method to selectively place organically modified clay within specific regions of an epoxy blend, where the clay provides maximum benefit to the material performance. By this process, the material yield stress was observed to increase by 40 to 100%, depending on the blend composition. The toughness of the material, as defined by the area under the stress-strain curve, was observed to increase or remain unchanged.
Dispersing layered silicate clay into a polymeric matrix influences matrix properties in vastly differing manners, especially with regard to mechanical properties. Factors influencing the variation in mechanical performance may include resin properties, silicate loading, degree of silicate dispersion, and the chemistry of the organic modifier on the clay. For example, tradeoffs in material properties have been observed, such as an increase in strength and modulus leading to reduced ductility and toughness.
Several researchers have noted that this trend does not hold when a tensile load is applied at a temperature above the nanocomposite glass transition temperature (Tg). Sometimes it results in enhancements to the material strength that greatly exceed those resulting from clay addition to a more rigid, glassy matrix. However, epoxy resins used for aerospace applications are used below their Tg, and have a Tg far above room temperature. Therefore, improving the material properties by using the resin above Tg is not an option.
Additional studies have shown an increase in the nanocomposite toughness of high Tg epoxies blended with rubber tougheners. The novel feature of the innovation is that by forcing the clay into the toughening portion of the resin blend, dramatic increases in material strength and modulus were observed without the corresponding reduction in material toughness, as defined by the area under the stress-strain curve.