Hysteresis behavior of nanocellulose-reinforced natural rubber composite sheets

Composite sheets consisting of natural rubber (NR) and phosphorylated cellulose nanofibers (P-CNFs) were prepared, and their hysteresis behaviors during loading/unloading cycles in tensile testing were investigated to accumulate fundamental data of the NR/P-CNF composites. An NR latex was mixed with an aqueous P-CNF dispersion with or without zinc methacrylate (Zn-mAA) used as a water-resistant improver. The NR/P-CNF and NR/P-CNF/Zn-mAA mixtures were oven-dried for preparation of flake-like NR/P-CNFs and NR/P-CNFs/Zn-mAA master-batches. NR/P-CNFs (100/20 by dry mass) and NR/P-CNFs/Zn-mAA (100/20/17.5 by dry mass) composite sheets were prepared from the master-batches via kneading and cross-linking. Tensile and thermomechanical properties of the NR sheet were improved by compounding with P-CNFs or P-CNFs/Zn-mAA. Additional post-heating or annealing treatment at 70°C for 2 h further improved the tensile properties especially at 50% and 100% strains. Hysteresis behavior of the composite sheets was analyzed by repeating the loading/unloading cycles in strain range of 0−100%. Linear relationships between the loading energy and either the corresponding unloading energy or its hysteresis loss were experimentally obtained for all composite sheets, and therefore their hysteresis behaviors can be predicted from the loading energy, which is beneficial in practical use. However, further studies at molecular and/or nano-sized levels of the NR/P-CNF composites during loading/unloading cycles are needed to clarify the detailed mechanisms to explain the hysteresis behaviors obtained in this study.