Strategies for Achieving Microcellular LDPE Foams in Extrusion

Abstract

This paper describes the fundamental process design for achieving microcellular foams using low-density polyethylene (LDPE) in extrusion. Microcellular foams are classified as foams with cell densities larger than 10⁹ cells/cm³ and cell sizes in the order of 10 micrometers. Supercritical CO₂ was used as a blowing agent in microcellular foaming due to its high volatility, which greatly increases thermodynamic instability. Our previous studies have indicated that microcellular foams cannot be produced from pure LDPE in a conventional microcellular extrusion system because of the high activation energy for cell nucleation. To increase the cell-nuclei density, an attempt was made at reducing the free energy for bubble nucleation by heterogeneous cell-nucleation. LDPE blends, with a small amount of polystyrene (PS) and/or a nucleating agent, were employed to induce heterogeneous cell-nucleating spots. The amount of the PS phase was varied to determine the optimum content. Furthermore, the melt strength of LDPE was increased by crosslinking. Microcellular LDPE foams have been successfully obtained in extrusion and the materials and processing windows have been clearly identified. The amount of injected CO₂ was varied in order to investigate its effect on the cell-population density.