Obtaining high density polyethylene (HDPE) based microcellular foams is a topic of interest due to the synergistic properties that can be obtained by the fact of achieving a microcellular structure using a polymer with a high number of interesting properties. However, due to the high crystallinity of this polymer the production of low density microcellular foams, by a physical foaming process, is not a simple task. In this work, the solution proposed to manufacture these materials consists on using crosslinked HDPEs, with different crosslinking degrees. In addition, the foaming conditions, i.e. the foaming time and the foaming temperature, have also been modified with the aim of analyzing and understanding the mechanisms taking place during the foaming process to control this process and therefore, to obtain cellular materials with low density and improved cellular structures. Results indicate that there is a time and a foaming temperature optima for which the nucleation rate and the cell growth rate prevails over other phenomena like cell coalescence and gas diffusion out the material. Therefore, with these foaming conditions the lowest densities and the best cellular structures are obtained. Moreover, when the gel content increases the amount of gas available for foaming increases and the rheological properties improve, as the polymers show strain hardening. As a consequence, the relative density decreases and the cell nucleation density increases. After this detailed study, it was concluded that cellular materials with relative densities of 0.37 and with cell sizes of approximately 2 microns can be produced using a saturation pressure of 5 MPa, a foaming time of 30 s, a foaming temperature of 160 ºC and a crosslinked HDPE with a gel content of 50 %.
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