Structural foam and manufacture thereof

A method of manufacturing a structural foam body having an anisotropic cell structure, the method comprising the steps of:

• (a) forming an initial foam body by conducting a first, primary expansion step in which heat is applied to an unfoamed polymer embryo at an elevated temperature above the glass transition temperature Tg of the polymer so as to expand the polymer embryo into the initial foam body, wherein the embryo comprises styrene-acrylonitrile as the polymer and a blowing or expansive agent;
• and
• (b) forming a final foam body having an anisotropic cell structure from the initial foam body by conducting a second expansion step in a vacuum chamber, by:
• (i) in a first phase of step (b), applying a negative pressure anisotropically to the initial foam body in the vacuum chamber while the polymer remains above the glass transition temperature Tg of the polymer from heat retained from the heat of step (a), the polymer cooling during the first phase, so as to expand the initial foam body into the final foam body having the anisotropic cell structure, and
• (ii) in a second phase of step (b), cooling the final foam body in the vacuum chamber under the anisotropically applied negative pressure to a temperature below the glass transition temperature Tg of the polymer so as to set the anisotropic cell structure,
• wherein the initial and final foam bodies are each in the form of a sheet each having a thickness direction between opposite major surfaces thereof and wherein an anisotropy index of the anisotropic cell structure is provided such that cell dimensions in the thickness direction of the final foam sheet are from 1.1 to 1.4 times greater than cell dimensions in a width or length direction of the final foam sheet as a result of the negative pressure being applied in the thickness direction of the initial foam sheet.