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.
Highlights
Cellulose nanofibrils are the structural backbone of plants[1−4] and a versatile building block, enabling the transition to a biobased society.[5−12] New materials are intensively developed on an academic level with commercialization at risk, missing a reproducible, robust, and economically viable manufacturing of nanocellulose at a larger scale
Despite some attempts, Ankerfors’[21] observation of a limited process understanding is still current. It is a paradox in research on cellulose nanofibrils that a focus on material research is jeopardizing the chance for a wider impact by failing to provide knowledge-based developments for process engineering at a larger scale. In response to these needs, we have developed the Hyper Inertia Microfluidizer (HIMF) as a general-purpose experimental study and process platform with the aim to facilitate research of basic mechanisms that can lead to technology developments and innovation
From our ex situ fiber and fragment analysis and in situ fiber-motion observations, we derive the mechanisms of cellulose comminution by hyper inertia microfluidization. These findings enable a rational design of a fiber comminution process and a truly sustainable production of nanocelluloses at reliable quality
Summary
Cellulose nanofibrils are the structural backbone of plants[1−4] and a versatile building block, enabling the transition to a biobased society.[5−12] New materials are intensively developed on an academic level with commercialization at risk, missing a reproducible, robust, and economically viable manufacturing of nanocellulose at a larger scale. Homogenization, for example, is developed for emulsification of liquid−liquid systems, where the size of the dispersed phase is reduced to small and stable droplets under hyper inertia flow conditions, i.e., flows with strong accelerations and high velocities.[17,18] Herrick et al.[19] and Turbak et al.[20] successfully utilized the apparatus for the comminution of cellulose fibers to their fibril level They highlighted the potential of nanocellulose[19,20] and pointed out the need of a systematic investigation for describing the fiber comminution.[19] the following research focused mostly on nanocellulose application[21,22] and characterization,[23] neglecting the development of a rigorous mechanistic process understanding. Larger developments were undertaken to Received: May 24, 2021 Revised: December 10, 2021 Published: January 4, 2022
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