Ultrastrong, flexible and lightweight anisotropic polypropylene foams with superior flame retardancy

Abstract Novel polypropylene (PP) foams with both greatly enhanced mechanical properties and superior flame-retardant performance are urgently needed in the area of automobiles, packaging, electrics and military. In this work, ultrastrong, flexible and lightweight anisotropic PP foams with superior flame retardancy have been fabricated by foaming of PP and intumescent flame retardant (IFR) composites through the drawing-assisted supercritical carbon dioxide (scCO2) continuous extrusion-foaming technique. The as-prepared PP/IFR foams with only 25 wt% IFR loading and an ultralow density of ∼0.08 g cm−3 could reach the HF-1 level in the standard horizonal burning tests due to the uniform IFR dispersion resulting from the plasticization effect of scCO2 coupled with the in-situ bubble stretching during foaming, and also exhibited greatly enhanced mechanical properties mainly owing to their anisotropic cellular structure and oriented molecular chains, reaching a record specific stiffness as high as ∼1813 MPa/(g cm−3). More importantly, this technique is environment-friendly without the use of any volatile organic solvents, and can go into practical mass production with a speed up to ∼0.56 m s−1.