Defining a strain-induced time constant for oriented low shear-induced structuring in high consistency MFC/NFC-filler composite suspensions

摘要

Micro and nanofibrillated cellulose is an essentially one-dimensional high aspect-ratio particle material, which can undergo two-dimensional layer (band) structuring under shear. Controlling the evolving rheological properties in aqueous suspension is essential for industrial applications in composite materials. This study focuses on an as yet considered to be unreported phenomenon of structure hardening under low shear. The timescale of the quasi gelation-controlled structure formation under low shear is studied using the large gap vane-in-cup geometry of the Brookfield viscometer. It is proposed that localized structure forms within continuous shear bands, similar to quasi liquid-crystal formation. By extrapolating a characteristic structure growth parameter to the rotation speed at which it becomes zero, the strain-induced structure time constant, t(gel), can be obtained as (gamma) over dot(= f (Omega)) = 1/t(gel) for the range Omega = 10-100/min. The time constant of low shear structure formation is shown to be separable from the static viscoelastic structure build under oscillation in concentrated composite suspension using plate-plate geometry, which is manifest by a Weissenberg normal force response on switching to applied shear, when the time constant of structuration t(gel) is long. (C) 2015 Wiley Periodicals, Inc.