ABSTRACT
Polypropylene (PP) has many desirable and beneficial properties, such as a high tensile modulus, high melting point, low density, excellent chemical resistance, and easy recycling (Vasile and Seymour, 1993). These outstanding properties and a low material cost have made linear PP foams a potential substitute for other thermoplastic foams, such as polystyrene (PS) and polyethylene (PE), in various industrial applications. However, it is challenging to produce linear PP foams with a high expansion ratio due to its weak melt strength and melt elasticity. It is found that the cell walls are not strong enough to bear any extensional force during bubble growth, and thus the bubbles are prone to coalesce and collapse during foam processing. Consequently, the foamed PP products usually have a high open-cell content and nonuniform cell distribution (Burt, 1978; Park and Cheung, 1997; Zhai et al., 2008a), and thus are not good for practical applications.
