Properties of natural rubber reinforced with cellulose nanofibers based on fiber diameter distribution as estimated by differential centrifugal sedimentation.

Cellulose nanofibers (CNFs) with different degrees of fibrillation are prepared by the mechanical fibrillation of kraft pulp using wet disk milling, and dispersions of the prepared CNFs were subjected to differential centrifugal sedimentation (DCS) in order to estimate the diameter distributions of the CNFs. The low-fibrillated CNFs (fiber diameter (d): >10 μm) had a weak reinforcing effect on natural rubber (NR), while the medium-fibrillated CNFs (d: 0.1-10 μm) dramatically improve the initial modulus and decrease the elongation at break. The high-fibrillated CNFs (d: <0.1 μm) enhanced the tensile strength even further while maintaining the elongation at break. The reinforcing mechanism of the NR composites reinforced by the CNFs (NR-CNFs) was confirmed by field-emission scanning electron microscopy imaging, dynamic mechanical analysis, and toluene uptake measurements. It was concluded that these characteristic mechanical properties of the NR-CNFs were determined by the morphologies of the CNFs. The branching structure of the medium-fibrillated CNFs affected high improvement of the initial modulus, and the network formed by the high-fibrillated CNFs were involved in enhancement of the tensile strength without compromising viscoelastic properties. Understanding the effect of their diameter distribution can potentially reduce the production cost of CNFs and thus expand their applicability.