Evolution of cellular morphologies and crystalline structures in high-expansion isotactic polypropylene/cellulose nanofiber nanocomposite foams

Abstract

Herein, the development of cell morphology and crystalline microstructure of injection-molded isotactic polypropylene/cellulose nanofiber (PP/CNF) composite foams with 2-10-fold expansion ratios was investigated through scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), and small-angle X-ray scattering (SAXS). Compared with isotactic polypropylene (iPP) foams, the added CNF improved the cell morphology and resulted in a great reduction in cell size. Additionally, the PP lamella orientation and crystal type were notably altered during the core-back FIM process. As the expansion ratio increased, the original isotropic lamellae in the iPP foams were transformed into an oriented lamellar structure and then further transformed into a typical shish-kebab structure, while hybrid shishkebab structures were simultaneously generated in the high-expansion PP/CNF nanocomposite foams. Accordingly, the highest content of b-crystals was observed in the low-expansion iPP foams. In contrast, the b-crystal content in PP/CNF composites decreased monotonously as the expansion ratio increased, which resulted from the combined effects of CNF's nucleating ability for a-crystals and the more dominant extensional flow effect assisted by the added CNF.

Reinforcement of polypropylene by cellulose microfibers modified with polydopamine and octadecylamine

“…These mechanical properties depended on the degree of substitution (DS) and were maximized at the optimal DS value of 0.44. Wang et al studied the foaming of ASA‐CNF/isotactic polypropylene (iPP) composites 18–22 . They confirmed the formation of a network‐like structure of CNFs in the iPP matrix by rheological measurements and achieved an 18‐fold expansion ratio by combining the materials with a core‐back injection molding process.…”

Lightweight investigation of long chain branching polypropylene/cellulose nanofiber composite foams

“…Conventionally, the foam injection molding (FIM) using supercritical fluid (SCF) as a physical blowing agent has been investigated in the way of ecofriendly foaming technology 9 . This method is widely studied with various thermoplastics, for instances, PP, 5,10–15 high‐density polyethylene, 16 acrylonitrile butadiene styrene, 17 and poly(lactic acid) (PLA), 18 and the mechanical, morphological, and thermal properties of their foams are also evaluated. However, the foamability of PP is poor due to the low melt strength and lack of strain hardenings, which can lead to the cell breakings and coalescences 18–20 .…”

“…However, the foamability of PP is poor due to the low melt strength and lack of strain hardenings, which can lead to the cell breakings and coalescences 18–20 . PP modified by various fibers and particles, such as carbon fiber (CF), 5,7,8 glass fiber (GF), 10 talc, 11 and natural fibers, 12,13 have been reported that the melt strength and cell nucleation can be improved. Furthermore, the incorporation between general PP and long chain branching 14,20–22 or crosslinking 22,23 PP could generate strain hardening and suppress the cell breakings and coalescences.…”

“…Furthermore, the incorporation between general PP and long chain branching 14,20–22 or crosslinking 22,23 PP could generate strain hardening and suppress the cell breakings and coalescences. In these studies, the uniform cellular structure of PP was achieved by using each method 5,7,8,10–13,20–23 …”

“…In particular, cellulose nanofiber (CNF) are also applied widely as their lightweight and high specific strength filler features. Though many research have been conducted on polymer/CNF composites, for instance, PP/CNF, 12,13,27–30 PLA/CNF, 31–34 and polyamide (PA)/CNF, 35 it is difficult to disperse hydrophilic CNF into hydrophobic polymers uniformly due to the weak interfacial adhesion properties. Yang et al studied the dispersion evaluation of PP/CNF composites using thermogravimetric analysis 36 .…”