Fabrication of Polypropylene/ CNF Composite Foam by Foam Extrusion With Chemical Blowing Agent

Chemical foaming processes have attracted considerable attention because of their easy implementation in the fabrication of polymeric foams. However, the relatively low gas concentrations and the residues from chemical blowing agents (CBAs) make it difficult to obtain fine, well‐controlled cellular structures. This study examines the addition of cellulose nanofibers (CNF) to overcome these limitations in polypropylene foams produced by extrusion with NaHCO 3 ‐based CBA. Polypropylene/CNF masterbatches were compounded in a quad‐screw extruder, and foamed strands were fabricated with various CNF and CBA contents. Rheological measurements and polarized optical microscopy observations showed that CNF formed a network structure and acted as a crystallization nucleating agent. X‐ray computed tomography revealed that CNF decreased the cell diameter and increased the cell density at all CBA concentrations. At low CBA contents, CNF mainly suppressed cell growth and coalescence, whereas CNF and PP spherulites acted as efficient cell nucleating agents at high CBA contents. Homogeneous cells with low CBA loadings are expected to reduce the amount of CBA residues, thereby improving material recyclability. Additionally, a spike‐like cell morphology was observed with the cell wall composed of spherulites and a gas‐penetrated amorphous phase. This structure provides a new foam design method for achieving a high specific surface area.