Cell morphologies and mechanical properties of cellulose nanofiber reinforced polypropylene foams

The pressure quench physical foaming method of carbon dioxide was applied to cellulose nanofiber reinforced polypropylene composites to fabricate ultra-high-strength and lightweight materials. The effects of cellulose nanofiber content and foaming conditions on cell morphology and mechanical and thermal properties are examined. The product is prepared from refined pulp and polypropylene powder by a melt compounding technique using a twin-screw extruder. At a foaming temperature of 155°C, which is below the polypropylene melting point, the bubble number density of cellulose nanofiber reinforced polypropylene foam is much higher than that of neat polypropylene foam because of the bubble nucleating effect of cellulose nanofiber. However, at 160°C, the bubble number density increases as fine bubbles are generated because of the viscosity increment by cellulose nanofiber. The effect of bubble nucleation and viscosity increment of the cellulose nanofiber reinforced polypropylene matrix contributes to enhance the low density property of the composites. Furthermore, the unique features of cellulose nanofiber such as high strength and low thermal expansion improve the mechanical and thermal properties of the polypropylene foam. As a result, cellulose nanofiber reinforced polypropylene foams which had a lower density and a higher flexural modulus, flexural stress, and heat deflection temperature than the neat polypropylene solid were obtained