Special Section “Fabrication and Properties of Engineering Plastic Foams”

Engineering plastics are a group of plastics that are commonly used in industry because of their low density and enhanced mechanical and thermal properties. They can replace metals in many advanced fields, such as aerospace, aircraft and automobiles. With the introduction of cellular structure, engineering plastic foams present a further reduction of density as well as adding the properties of thermal insulation, sound insulation and mechanical damping. The traditional foaming methods for engineering plastics mainly use chemical foaming agents or combustible physical agents. Due to the high temperature resistance of engineering plastics, the chemical foaming method requires an inorganic foaming agent with a high decomposition temperature. Lower decomposition of the inorganic foaming agent results in a high foam density, for example, a 10–30% mass reduction. Physical foaming agents such as pentane, butane and acetone are also used in the extrusion foaming of PC, PET and PEI. Unfortunately, these physical foaming agents are flammable and exhibit poor cell nucleation ability, resulting in a cell size as large as 200–300 μm or even larger. In recent years, the microcellular foaming technology using supercritical CO2 or N2 fluids has received much attention from both academy and industry. CO2 and N2 are inert gases, which are environmentally more friendly, inexpensive and easy to obtain compared to the flammable agents, and can achieve supercritical state under mild conditions. Supercritical fluids have high solubility and high gas diffusivity in engineering plastic matrices, which is beneficial for inducing a large number of cell nuclei through a pressure quenching or temperature rising process. Therefore, the supercritical fluid physical foaming technology can induce a microcellular structure or even a nanocellular structure in engineering plastic foams, which endows the foams with excellent mechanical, thermal insulation and dielectric properties. This gives engineering plastic foams prospects for important applications in structural, acoustic, thermal insulation, flame-retardant and dielectric fields. For this special section, 17 articles from experts in the field of polymer foaming who focus on various engineering material systems such as ABS, UHMWPE, PEI, PEEK, PEN, etc., and various foaming technologies, such as extrusion foaming, injection foaming and autoclave foaming, to prepare engineering plastic foams with adjustable cell structures have been included. This special section also reports some novel polymer physical foaming technologies, such as micro-extrusion foaming of PEEK fiber and in-situ foaming and 3D printing of biomimetic porous PEI structure. In addition to the preparation of various engineering plastic foams, this section also focuses on the mechanical and functional properties of the foams.