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

Abstract

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 _ c(5 f (X)) 5 1/t gel for the range X 5 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.

Characterising exfoliated few-layer graphene interactions in co-processed nanofibrillated cellulose suspension via water retention and dispersion rheology

“…5, followed by high frequency vibration-induced elastic behaviour [40]. This observation clearly indicates that the stronger aggregation structure in the NFC medium becomes ever more more rigid under vibration, following a hardening response [43,49,51]. Fascinatingly, the greater the refining time, the weaker the initial static structure, as seen before in Fig.…”

“…Firstly, a weaker static loose floc/agglomeration structure, included in the NFC gel-like matrix, yields and then the system retains a stronger particle-particle aggregation interaction until the strain is sufficient, secondly, to break this down into a more purely viscous state [8,42]. The decoupling of the weaker first stage floc behaviour followed by the particle-particle aggregation structure reveals an interesting trend in respect to refining time, in that the refining leads to big differences in respect to the weaker floc structure, but less so in respect to the particle-particle aggregation [32,41,43 ].Notable is the fact that the behaviour of the viscoelastic response is similar whether the graphite has been coprocessed with NFC (R NFC NGF and R NFC EG) or the separately processed NFC has simply been added to the water processed graphite (R W NFC + NGF and R W NFC + EG ). This tells us that the weakening of the floc structure, as we saw earlier linked with increasing water retention, is dependent on the particle number and hydrophilicity.…”

“…Yield stress is perhaps the most important rheological property of complex suspensions, as it needs to be exceeded in order that they can flow, which is challenging to evaluate for such wall slip prone materials as NFC-containing suspensions, which display shear banding to minimise stress when sheared [8,12,42]. Therefore, we use both static yield stress, (τs 0 ) obtained from the LVE region conducted with plate-plate geometry, and dynamic yield stress (τd 0 ) from flow curves conducted from vane in cup geometry [14,43]. The dynamic yield stress (τd 0 ) is defined as the minimum stress required for maintaining the flow, while static yield stress (τs 0 ) is defined as the stress required for initiating flow, and the latter often has the higher value for NFC suspensions [4,10,43].…”

“…Therefore, we use both static yield stress, (τs 0 ) obtained from the LVE region conducted with plate-plate geometry, and dynamic yield stress (τd 0 ) from flow curves conducted from vane in cup geometry [14,43]. The dynamic yield stress (τd 0 ) is defined as the minimum stress required for maintaining the flow, while static yield stress (τs 0 ) is defined as the stress required for initiating flow, and the latter often has the higher value for NFC suspensions [4,10,43].…”

“…The steady state shear curves of graphene suspensions, Fig. 3, produced from graphite in water, and graphite in surfactant solution, reveal that the suspensions have shear thinning behaviour at low shear rate, and a slight tendency to transient dilatancy at higher shear rate [41,43,45,48]. The Ostwald-de Waele expression (Eq.…”