Abstract
We have investigated the steady-shear and viscoelastic properties of composite dispersions of cellulose nanofibrils (CNFs) with medium or high charge density and two different nanoclays, viz. rod-like sepiolite or plate-like bentonite. Aqueous dispersions of CNFs with medium charge density displayed significantly lower steady-state viscosity and storage modulus but higher gelation threshold compared with CNFs with high charge density. Dynamic light scattering (DLS) results showed that the apparent hydrodynamic radius of bentonite particles increased when CNFs were added, implying that CNFs adsorbed onto the amphoteric edges of the plate-like bentonite particles. The sepiolite network in CNF–sepiolite dispersions was relatively unaffected by addition of small amounts of CNFs, and DLS showed that the hydrodynamic radius of sepiolite did not change when CNFs were added. Addition of CNFs at concentrations above the gelation threshold resulted in drastic decrease of the steady-shear viscosity of the sepiolite dispersion, suggesting that the sepiolite network disintegrates and the rod-like clay particles are aligned also at low shear rate. The relative change in the rheological properties of the clay-based dispersions was always greater on addition of CNFs with high compared with medium charge density. This study provides insight into how the rheology of CNF–nanoclay dispersions depends on both the nanoclay morphology and the interactions between the nanoclay and nanocellulose particles, being of relevance to processing of nanocellulose–clay composites.
Highlights
Cellulose nanofibrils (CNFs), extracted from wood, marine animals, and bacteria, are a sustainable nanomaterial that combines high specific stiffness, high surface area, low density, optical transparency, and flexible surface chemistry (Klemm et al 2011; Moon et al 2011; Usov et al 2015)
Composite dispersions of CNFs and two different nanoclays, i.e., plate-like bentonite and rod-like sepiolite, displayed complex rheological behavior that depended on both the morphology of the clay particles and the interactions between the nanoclay and CNF particles at quiescent conditions and under shear
Rheological studies on composite dispersions of CNFs and nanoclays suggested that the sepiolite network was relatively unaffected by addition of small amounts of CNFs, but addition of CNFs at concentrations above the gelation threshold promoted breakup of the sepiolite network and assisted in alignment of the rod-like clay particles at low shear rate
Summary
Cellulose nanofibrils (CNFs), extracted from wood, marine animals, and bacteria, are a sustainable nanomaterial that combines high specific stiffness, high surface area, low density, optical transparency, and flexible surface chemistry (Klemm et al 2011; Moon et al 2011; Usov et al 2015). Nanoclays (Bailey et al 2015) are commonly used as an additive in CNF-based composite materials to improve, e.g., their mechanical properties (Gabr et al 2013; Wang et al 2014) or fire retardancy (Liu et al 2011; Wicklein et al 2015), and to tailor the selectivity and flux of membranes (Zheng et al 2014). Comprehensive studies on the steady-shear and viscoelastic properties of such CNF–nanoclay dispersions are rare, and previous rheological studies have mainly focused on dispersions of plate-like nanoclays and moderately charged or uncharged nanocellulose particles (Abend and Lagaly 2000; Li et al 2015; Ming et al 2016; Nechyporchuk et al 2014). The effect of the CNF content on the viscosity under shear and the storage modulus at quiescent conditions of the nanoclay dispersions is found to be related to the interactions between the CNFs and the different types of clay particle
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