Towards functionally graded cellular microstructures

Many materials require functionally graded cellular microstructures whose porosity (i.e. ratio of the void to solid volume of a material) is engineered to meet specific requirements. Indeed numerous applications have demonstrated the engineering potential of porous materials (e.g. polymeric foams) in areas ranging from biomaterial science through to structural engineering. Although a huge variety of foams can be manufactured with homogenous porosity, for heterogeneous foams there are no generic processes for controlling the distribution of porosity throughout the resulting matrix. Motivated by the desire to create a flexible process for engineering heterogeneous foams, this paper reports how ultrasound, applied during some of the foaming stages of a polyurethane (PU) melt, affects both the cellular structure and distribution of the pore size. The experimental results allowed an empirical understanding of how the parameters of ultrasound exposure (i.e. frequency and acoustic pressure) influenced the volume and distribution of pores within the final polyurethane matrix: the data demonstrates that porosity (i.e. volume fraction) varies in direct proportion to the acoustic pressure magnitude of the ultrasound signal. The effects of ultrasound on porosity demonstrated by this work offer the prospect of a manufacturing process that can adjust the cellular geometry of foam and hence ensure that the resulting characteristics match the functional requirements.