Water-assisted compounding of cellulose nanocrystals into polyamide 6 for use as a nucleating agent for microcellular foaming

Cellulose nanocrystals (CNCs) are a biorenewable filler and can be an excellent nucleating agent for the development of microcellular foamed polymeric nanocomposites. However, their relatively low degradation temperature limits their use with engineering resins like polyamide 6 (PA6) in typical melt processing techniques such as injection molding, compounding, and extrusion. A water-assisted extrusion compounding process was investigated to directly compound CNC suspensions with PA6 without the need of predrying the CNCs. By using water as a plasticizer and reducing the processing temperature by 30 °C, this process can mitigate the degradation of CNCs during compounding. The effects of the CNCs on the mechanical properties, crystal type, and microstructure of solid and microcellular foamed specimens were characterized. The CNCs primarily acted as a nucleating filler, affecting both the matrix crystal structure and, in foamed composites, the cell structure. The CNCs nucleated the α-crystalline form of PA6 and also acted as a foam cell nucleator, increasing cell density by an order of magnitude while significantly reducing cell size. The weight reduction of the foamed specimens was about 15%. Adding small amounts of CNCs also increased matrix orientation in the solid injection molded specimens. These factors helped to improve the mechanical performance, especially the modulus of elasticity. During water-assisted compounding, thermal hydrolysis of PA6 occurred and generated carbon-carbon double bonds, as evaluated by FTIR. However, the molecular weight reduction caused by hydrolysis was less than 5%. The total molecular weight reduction was around 18%, combined with the melt extrusion and injection molding processes.