Mechanisms of Cellulose Fiber Comminution to Nanocellulose by Hyper Inertia Flows

Abstract

Nanocelluloses are seen as the basis of high-performance materials from renewable sources, enabling a bio-based sustainable future. Unsurprisingly, research has initially been focused on the design of new material concepts and less on new and adapted fabrication processes that would allow large-scale industrial production and widespread societal impact. In fact, even the processing routes for making nanocelluloses and the understanding on how the mechanical action fibrillates plant raw materials, albeit chemically or enzymatically pre-treated, are only rudimentary and have not evolved significantly during the past three decades. To address the challenge of designing cellulose comminution processes for a reliable and predictable production of nanocelluloses, we engineered a study setup, referred to as Hyper Inertia Microfluidizer, to observe and quantify phenomena at high speeds and acceleration into microchannels, which is the underlying flow in homogenization. We study two different channel geometries, one with acceleration into a straight channel and one with acceleration into a 90°bend, which resembles the commercial equipment for microfluidization. With the purpose of intensification of the nanocellulose production process, we focused on an efficient first pass fragmentation. Fibers are strained by the extensional flow upon acceleration into the microchannels, leading to buckling deformation and, at a higher velocity, fragmentation. The treatment induces sites of structural damage along and at the end of the fiber, which become a source for nanocellulose. Irrespectively on the treatment channel, these nanocelluloses are fibril-agglomerates, which are further reduced to smaller sizes. In a theoretical analysis, we identify fibril delamination as failure mode from bending by turbulent fluctuations in the flow as a comminution mechanism at the nanocellulose scale. Thus, we argue that intensification of the fibrillation can be achieved by an initial efficient fragmentation of the cellulose in smaller fragments, leading to a larger number of damaged sites for the nanocellulose production. Refinement of these nanocelluloses to fibrils is then achieved by an increase in critical bending events, i.e., decreasing the turbulent length scale and increasing the residence time of fibrils in the turbulent flow.

Cavitation Fibrillation of Cellulose Fiber

“…The BT process is comparable to the recently documented fibrillation in short microchannels. 6 In microchannel fibrillation, fibers are initially fragmented by tension forces arising during the acceleration. We estimated the acceleration and velocities for BT ( Supporting Information ) and find these low compared to microchannel flows.…”

“…We estimated the acceleration and velocities for BT ( Supporting Information ) and find these low compared to microchannel flows. 6 Therefore, we can exclude fragmentation by tension for BT. The treatment effect in BT can be accounted to cavitation damaging, which appears inefficient for the fibrillation of cellulose fibers into CNFs.…”

“… 56 The linear increase of RSC with NF for the UH treatment can, hence, be explained by the efficient and direct fibrillation of the cellulose fiber and fiber fragments into nanocellulose at the elementary scale. That is unique for sonication treatment, and differs, for example, from microfluidization, for which Redlinger-Pohn et al 6 recently documented the limitation leading to the production of fibril aggregates, i.e., incomplete fibrillated cellulose fibers captured by RSC being below the NF ( Figure 5 , gray diamond). At intermediate NF , RSC for CMCe and TCe and UH are slightly above the linear increase (dashed line), corresponding to an opening of the cellulose structure in the remaining fibers and fragments that are not accepted by the NF .…”

“…Including a safety margin, we decided on C m = 0.4%, which is also identical to the concentration used in our previous study of fibrillation by hyper-inertia flows. 6 A subset of experiments were performed at a higher concentration of C m = 2%, what is used in industrial homogenization. 45 The dispersions of DCe, CMCe, and TCe were prepared from their flocculated pulp at C m of 41.6, 22.2, and 15.6%, respectively.…”

“…They documented that short CNTs are indeed suspected to break from scission, while long CNTs, however, break by bending. Bending failure was recently discussed by Redlinger-Pohn et al 6 as a leading mechanism in hyper-inertia fibrillation (i.e., microfluidization and homogenization), but it appears to us that it is not considered in cavitation fibrillation.…”