Civil Engineering Reference
In-Depth Information
h is pulping method facilitates the separation of purii ed cellulose with minimal deg-
radation and superior quality pulps [48, 49].
9.1.2.2
Cellulose Nanoi bers
9.1.2.2.1 Isolation of Cellulose Nanoparticles
Isolation of cellulose nanoparticles has traditionally been performed mainly by
mechanical or chemical methods. Intensive mechanical treatments impose high shear
forces to cellulose i bers and allow the extraction of microi brils and microi bril aggre-
gates with high aspect ratio. h e product obtained is generally called
microi brillated
cellulose (MFC).
Pressure, cavitation, shear and impact forces of the homogenizer had
broken down the cell walls of the microi bers and liberated the desired nanoi brils
[50-52]. MFC is now a commercial product available from various companies such
as Daicel (Japan), Rettenmaier (Germany), Innventia AB (Sweden), UPM Kymmene
and VTT (Finland) and Borregaard (Norway) [7]. h e manufacture of MFC is now
generally performed by a mechanical treatment consisting of rei ning and high-pres-
sure homogenizing steps [53-55]. Mechanical treatment by cryocrushing is another
method used for isolation of microi brils. h e process implies freezing cellulose i bers
using liquid nitrogen and subsequently subjecting them to high shear forces [29]. High
impact forces applied to the frozen i bers induce ice crystals to exert pressure on the
cell walls causing them to rupture and liberate microi brils [31] . Grinding processes
in which the i bers slurry is passed between a static grind stone and a rotating stone
(~1500 rpm), induce shearing forces that can also result in i ber i brillation and nanoi -
ber isolation [24-27]. Independently of the mechanical method used to obtain MFC, it
is ot en reported that MFC suspensions are not homogeneous and that they consist of
microi brils and microi bril aggregates. Suspensions may also contain a certain amount
of larger i ber fragments and uni brillated i bers [56, 57].
Cellulose microi brils are composed of crystalline cellulosic domains and less
ordered amorphous domains located at the surface and along their main axis. Upon
contact with strong acid solutions amorphous domains are preferentially cleaved,
whereas crystalline regions that have a higher resistance to acid attack (cellulose nano-
crystals) remain essentially intact. Strong acid hydrolysis promotes transversal cleavage
of noncrystalline fractions of cellulose microi brils, leading to the so-called
cellulose
nanocrystals
or
nanowhiskers,
which are rod-like particles with diameters in the range
of 2-20 nm and 100-600 nm in length [7].
9.1.2.2.2 Bacterial Cellulose (BC)
A promising approach for a relatively simple and eco-friendly obtention of cellulose
microi brils that has received great attention in the last decade is the microbrial route,
in which specii c aerobic bacteria secrete cellulose microi brils with nanometric widths
as an extracellular primary metabolite. Bacteria able to do so are those belonging to the
genera
Acetobacter, Agrobacterium, Alcaligenes, Pseudomonas, Rhizobium, Aerobacter,
Achromobacter, Azotobacter, Salmonella
or
Sarcina
[50, 51] . h e ef ectiveness of micro-
bial cellulose production depends mainly on the strain used, the composition of the
culture medium, the fermentation temperature, oxygen supply, and carbon source used
(for example glucose). h e recovery of BC (generally at er 12-14 days of fermentation)
