posted on 2022-01-04, 17:38authored byJakob D. Redlinger-Pohn, Christophe Brouzet, Christian Aulin, Åsa Engström, Anastasia V. Riazanova, Claes Holmqvist, Fredrik Lundell, L. Daniel Söderberg
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.