team called “declumping” the fibrillation of the bulk cellulose
diven by by both AC and HC (Figure 1). Indeed, ultrasonically processed
cellulose was completely fibrillated into individually separated fibrils
less than 100 nm thick due to the more intense cavitational collapse and
absence of fluid flow of acoustic cavitation, whereas hydrodynamic
cavitation resulted into full cellulose fiber-fiber detachment with some
fibers at nano level and most of the fibers of micrometer thickness.
Both forms of cavitation reduced cellulose crystallinity from 87% to
38%. Such dominant presence of amorphous domains enhances the
flexibility and plasticity of the material, and lowers both stiffness
and elasticity.42 The particle size decreased to 1.36
µm and 0.3 µm for the hydrodynamically and ultrasonically processed
cellulose samples, respectively. whose thermal stability was
significantly higher of the newly obtained nanocelluloses due to
decrystallization.
We briefly remind that the cavitation bubbles generated in water via
acoustic or hydrodynamic cavitation upon collapse locally release
short-lived (µs duration) shockwaves of extreme pressures (1000-2000
atm) and temperatures (5000 K) that are ideally suited for the
extraction of natural products.43
Two years later, Paquin and co-workers in Quebec reported that by
carrying out the aminoxyl-radical mediation oxidation of a diluted
suspension (1 wt%) of bleached hardwood kraft pulp in a 45 L
flow-through sonoreactor, a 87.5% decrease in energy consumption (in
comparison to the process in batch reactor) and a 95% higher production
rate of oxidized fibers (compared to reaction without any ultrasound in
batch mode) are achieved. These results, the team concluded, suggest the
possibility of scaling up the process on industrial scale directly in
continuous mode.44
The original discovery of Pandit and Pinjari using no added chemicals to
fibrillate crystalline cellulose, however, remained without further
investigation until the teams of Meneguzzo and Pagliaro demonstrated the
process on semi-industrial scale by processing via HC more than 30 kg of
citrus (lemon and grapefruit) processing waste in 120 L
water.45