Civil Engineering Reference
In-Depth Information
Nanocrystalline cellulose (NCC), also designed as cellulose nanocrystals (CNC) or
nanowhiskers (CNW) (Section 6.2.2), is typically a rigid rod-shaped monocrystalline
cellulose domain, 1-100 nm in diameter and from tens to hundreds of nanometers in
length. h is has morphological and structural characteristics, including entanglement
and geometrical dispersion, strongly dependent on the species, cultivar and agronomi-
cal factors (e.g., plant maturity, characteristics of the soil). h e yield of the extraction
process (i.e., the quantity of nanocellulose obtained from a given weight of macroi ber),
depends on both the crystallinity of the specii c plant i ber and the procedure adopted
for extraction [8]. Dealing with source materials, in a number of instances, CNCs were
extracted from cellulosic materials obtained as a by-product/waste of plant i ber crops.
h e extraction of CNC from plant i bers can be performed by means of enzymatic pre-
treatment or through acid hydrolysis, in both cases with the aim to remove the amor-
phous cellulose and form highly crystalline cellulose [9]. h e most common extraction
method involves a i rst chemical treatment leading to the production of holocellulose
by the gradual removal of lignin, while the subsequent sulphuric acid hydrolysis pro-
cess allows obtaining cellulose nanocrystals in an aqueous suspension. h is method
originated from the classical works of Ranby [10-12] and Battista [13]. h is can be
regarded as a top-down approach.
Other possibilities exist of applying the reverse process, therefore of a bottom-
up approach, by developing nanocellulose from the action of bacteria, in particular
Gluconacetobacter xylinum.
h is allows obtaining cellulose whiskers of unprecedented
purity and crystallinity [14]. In some cases, bacterial nanocellulose, whose merits and
possibilities are discussed in detail in Section 6.2.3, proved interesting nevertheless for
the manufacturing of biomedical devices, such as scaf olds, for which the biocompat-
ibility of the obtained material is essential, which can be more easily controlled with
the bottom-up approach. h e costs incurred in applying this method, as well as the
high level of hydrophilicity, are obvious limits to the wide dif usion of bacterial nano-
cellulose, although in other senses its hydrophilic character can serve other purposes,
for example, for the modii cation of the surface of natural i bers, in view of their easier
insertion in a polymer matrix [15]. It is also worth considering that water soluble poly-
mers such as, for example, poly(vinyl alcohol), can be incorporated into bacterial cellu-
lose by adding them to the culture medium [16]. A further possibility, which is inherent
to the production of bacterial cellulose, is tailoring the dimensional characteristics of
the nanostructure to some specii c needs, for example, water and/or oil repellence, to
obtain specii c macrostructures, such as aerogel membranes [17].
To obtain other specii c functional properties, a second reinforcement (metal-
lic, ceramic, carbon-based or biological) can be added into cellulose nanoreinforce-
ment. Nanocellulose added to metallic reinforcement (Section 6.3.1) can have, e.g.,
as an objective the achievement of antibacterial action by using nanosilver particles
[18] . h is application appears to be particularly promising, although this ef ect is by no
means exclusive of silver nanoparticles, and investigations on suitable dimensions and
geometries of particles for materials optimization are still lacking in this i eld. When
the objective is biocompatibility, a more common procedure is the addition to cellulose
of ceramic particles, whose interaction with nanocellulose will be investigated more
in depth in Section 6.3.2. For example, hydroxyapatite can be precipitated on bacte-
rial cellulose [19], while, aiming at producing more structural nanocomposites, silica
