Highly anisotropic crosslinked HDPE foams with a controlled anisotropy ratio: Production and characterization of the cellular structure and mechanical properties

Abstract In this paper a two-step foaming process was used to produce highly anisotropic foams with controlled anisotropy ratio. Anisotropic cellular materials are of great interest due to the enhanced mechanical properties that they show in the anisotropy direction. Therefore, the production of foams with high and controlled anisotropy ratios is an essential topic when it comes to design materials with good mechanical performance. In this work, high density polyethylene (HDPE) foams with anisotropy ratios larger than 3 have been produced. A foaming process consisting of two consecutive heating steps: pre-foaming and foaming, has been employed. Foaming kinetics at four different pre-foaming temperatures (145 °C, 155 °C, 170 °C and 180 °C) and at a constant foaming temperature (180 °C) have been performed. Results showed that the pre-foaming temperature is the key parameter that allows controlling the anisotropy ratio of the final foam. A relationship between the pre-foaming temperature, the number of cells and the anisotropy ratio has been established. Low pre-foaming temperatures lead to low numbers of cells, which eventually results in high anisotropy ratios. Mechanical properties in compression were measured and compared with the anisotropy of the cellular structure, being that the elastic modulus increases with the anisotropy ratio.