Solid state foaming of nanocomposites pp.1089-1102
Authors: (Guglielmotti A., Lucignano C., Quadrini F., University of Rome “Tor Vergata”, Department of Mechanical Engineering, Italy)
Abstract: Porosity in dense engineering materials is often undesirable for load-bearing applications. However, porosity is highly beneficial for weight and cost reduction, damping, thermal insulation and specific strength. Moreover, they are extensively used in several industrial application: automotive, aeronautic, naval. In the last years, the attention was focused on new materials because they combine good mechanical properties with low weight. The method for making foams is straightforward: generating bubbles and stabilizing them within the matrix. There are two major foaming methodologies: soluble foaming (or physical foaming) and reactive foaming (or chemical foaming). The former is generally applied to thermoplastics and involves the mixture of a polymer melt and a blowing agent. The latter is typical for thermosets and involves the addition of reactants for the gas evolution. In both cases, the same three steps are necessary: gas implementation, gas expansion and foam stabilization. Different organic solvents (hexane or cyclohexane) are used as physical blowing agents. Alternatively a chemical foaming agent can be adopted so that a gas is released during the cross-linking reaction. In this case, an epoxy foam is synthesized with an optimum ratio of epoxy amine blowing agent. Moreover, the polymer matrix is always processed in a liquid state by means of plasticators (for thermoplastics) or mixers (for thermosets). In this chapter a new foaming process is described for thermosetting resins. Uncured resin tablets are fabricated pressing commercial powders in a steel mold at room temperature and used as foam precursors. Then the tablets foam when heated in a muffle at high temperature. No blowing agent was added because the foaming mechanism depends on the uncured resin boiling point. The foaming temperature is set to be high enough to rapidly produce the resin boiling but not excessive to avoid the thermal degradation. During boiling, the resin polymerizes and the bubbles froze in the final structure.