Civil Engineering Reference
In-Depth Information
applied an acid hydrolysis step before pumping the sulphite pulp through the homog-
enizer. h e sulphuric acid treatment, combined with mechanical dispersion, resulted in
the production of MFC with i ner i bril structures than it was the case for MFC obtained
only by a mechanical treatment. h e former produced diameters below 50 nm, but their
lengths were still in the micrometre range. Another treatment that has been used in
combination with mechanical shearing is the enzymatic hydrolysis. Henriksson
et al.
[34] treated cellulosic wood i ber pulps with pure C-type endoglucanase in order to
facilitate the disintegration of MFC.
At er disintegration, MFC is typically obtained as a suspension in liquid, usually
water. During homogenization, the suspension changes from a low viscosity to a high
viscosity medium. Normally a 2 wt% i ber suspension is used for the preparation of
MFC. At higher concentrations, the increased viscosity during processing becomes too
high, to allow forward pumping of the suspension. h e morphology of constitutive
nanoparticles is generally characterized using microscopic techniques and it depends
from the source and the production technique. Cellulose microi brils are long and l ex-
ible nanoparticles presenting lateral dimensions in the order of 3 to 100 nm, and length
generally in the micrometre scale depending on the source of cellulose, dei brillation
process and pretreatment [36, 37](Figure 6.1a). h e properties of cellulose microi brils
dif er greatly from the properties of wood pulp i bers, mainly because of the large spe-
cii c surface area and high aspect ratio of the i brils. In aqueous suspensions the i brils
form a highly entangled network, which behaves as a pseudo plastic gel [28]. h e rheo-
logical properties enable the use of the i bril materials as thickeners or stabilizers in
suspensions or emulsions in many applications, such as foods, paints, cosmetics and
pharmaceuticals [29] . h e Young's modulus of cellulose i brils has been reported to be
138 GPa and the extremely good strength properties and good thermal stability of the
i brils make them suitable for use as reinforcement in nanocomposites [39] and paper
products. Homogeneous thin i lms of cellulose i brils can be prepared upon drying and
these i lms can be utilized, for example, in paper coatings. Films from hydrophobized
i brils can be used for water-repellent and self-cleaning material applications. Initially,
cellulose microi brils were proposed to be used in biomedical applications, such as drug
carriers. Films from antimicrobial grat ed microi brils have shown antibacterial behav-
ior, so the i lms could be used, for example, for wound healing or air i ltration applica-
tions. Some other applications, e.g., in foods, cosmetics, are proposed during the years
for this kind of material, while, because of its properties such as high strength, l exibil-
ity and aspect ratio, many research groups have focused their attention on the use of
MFC as a reinforcing phase in nanocomposites.
6.2.2
Nanocrystalline Cellulose (NCC)
In the 1950s, Ranby reported for the i rst time that colloidal suspensions of cellulose can
be obtained by controlled sulphuric acid-catalysed degradation of cellulose i bers [10-
12] . h is work was inspired by the studies of Nickerson and Habrle [40], who observed
that the degradation induced by boiling cellulose i bers in acidic solution reached a
limit at er a certain time of treatment. Transmission electron microscopy (TEM)
images of dried suspensions revealed for the i rst time the presence of aggregates of
