This EArly-concept Grant for Exploratory Research (EAGER) grant provides funding to test whether interfacially-adsorbed particles can stabilize polymer foams. In aqueous systems, particles that adsorb or adhere to air/water surfaces are known to stabilize foams. The hypothesis of this proposal is that similar stabilization can be achieved when the bulk fluid is not water, but a molten polymer. The EAGER proposal seeks to evaluate whether particle stabilization is technically feasible in the real world using a commercial polymer and a foam process similar to that used industrially. Two kinds of experiments will be conducted. The first will be with polystyrene foams stabilized by spherical silica particles; these experiments will test whether particles can adsorb at the air/polymer interface even when the bulk fluid has a very high viscosity. The second will be with polypropylene foams stabilized with fluorinated particles; these will test whether particle-stabilized foams will survive even if kept under molten conditions for extended periods.
Polymer foams such as polystyrene foam or polyurethane foam are commonly used for insulation, cushioning, packaging, and for reducing the weight of structural parts. Foam bubble coalescence restricts the range of materials that can be foamed and the processing conditions under which foaming can occur. Particle stabilization provides a new method for stabilizing foams with the potential for conducting foaming operations in a wider parameter space, and also foam materials traditionally regarded as unfoamable. The research to be conducted during this grant will clearly establish whether the concept of particle-stabilized foams is viable commercially; if so, the results will provide an strong foundation for further fundamental research on the mechanisms of particle-stabilization.
Polymer foams are commonly used as lightweight materials for structural, insulating, and cushioning applications. It is common wisdom that during foaming, the molten plastic must have adequate melt strength. Otherwise foam bubble coalescence leads to foam collapse. But this is not true for aqueous foams: soap and water give stable foams even without any melt strength, because such foams are stabilized by interfacial phenomena. We hypothesized that particles that are partially-wetted by a polymer melt will stabilize a foam of that polymer. Furthermore, because the mechanism of stabilization is interfacial in nature, the foam will remain stable even if the polymer itself remains molten. In this project, we started with a polylactic acid, a plastic widely-recognized as being difficult to foam. The effect of adding polytetrafluoroethylene (PTFE) particles to the foaming behavior were studied. We find that addition of a few percent of PTFE particles creates stable foams whereas without PTFE, not a single foam bubble survives. Electron microscopy confirmed that the foams are stable due to an interfacial mechanism: each foam cell is covered with a "shell" of particles that protects it against coalescence.In summary, the ideas developed here can be used to the range of polymers that can be foamed and the parameter space within which foaming operations can be conducted. Finally, we also reviewed the literature on nanofiller-containing foams and concluded that the improvements in foam properties due to nanofiller are typically modest, but with flexible polyurethane foams, substantial improvements are possible.
