Civil Engineering Reference
In-Depth Information
could be obtained by enzymatic hydrolysis, and that, depending on the treatment and
on their sequence, MFC coexist with NCC in the obtained suspensions.
Because of the natural advantage of an abundance of hydroxyl groups at the surface
of NCC, dif erent chemical modii cations and functionalizations have been attempted,
including esterii cation, etherii cation, oxidation, silylation, polymer grat ing, etc.
Noncovalent surface modii cation, including the use of adsorbing surfactants and poly-
mer coating, has been also studied. All chemical functionalizations have been mainly
conducted to introduce stable negative or positive electrostatic charges on the surface
of NCC to obtain better dispersion (NCC obtained at er sulphuric acid hydrolysis
introduce labile sulphate moieties that are readily removed under mild alkaline condi-
tions) and to tune the surface energy characteristics of NCC to improve compatibility.,
h is proved particularly ef ective when used in conjunction with nonpolar or hydro-
phobic matrices in a nanocomposite approach. h e main challenge for the chemical
functionalization of NCC is to conduct the process in such a way that it only modii es
the surface of NCC, while preserving the original morphology to avoid any polymor-
phic conversion and to maintain the integrity of the crystal [43]. Cellulose nanocrystals
have better mechanical properties than the majority of the commonly used reinforcing
materials, while of ering additional exceptional advantages, such as biodegradability
and biocompatibility, high stif ness and low density. For these reasons, they are perfect
candidates to be used as reinforcements phase in thermoplastic and/or thermosetting
matrices in a nanocomposite approach.
6.2.3
Bacterial Cellulose (BC)
Bacterial cellulose (BC), also called bacterial nanocellulose (BNC), microbial cellu-
lose, or biocellulose, is formed by aerobic bacteria, such as acetic acid bacteria of the
genus
Gluconacetobacter
, as a pure component of their bioi lms. h ese bacteria are
wide-spread in nature where the fermentation of sugars and plant carbohydrates takes
place. In contrast to MFC and NCC materials isolated from cellulose sources, BC is
formed as a polymer and nanomaterial by biotechnological assembly processes from
low-molecular weight carbon sources, such as d-glucose. h e biofabrication approach
opens up the exciting option to produce cellulose by fermentation in the sense of white
biotechnology and to control the shape of the formed cellulose bodies, as well as the
structure of the nanoi ber network during biosynthesis. h e bacteria are cultivated in
common aqueous nutrient media, and the BNC is excreted as exopolysaccharide at the
interface to the air. h e resulting form-stable BNC hydrogel is composed of a nanoi ber
network (i ber diameter: 20-100 nm) enclosing up to 99% water. Cellulose derived
from bacteria species has the advantage of being free from wax, lignin, hemicellulose
and pectin, which are present in plant-based cellulosic materials. h is BNC proved to
be very pure cellulose with a high weight-average molecular weight (M
w
), high crystal-
linity, and good mechanical stability. h is highly crystalline structure of BC is a prop-
erty that is favorable for composite production, in particular resulting in a high Young's
modulus value for BC. It was found that BC possesses a Young's modulus of about 114
GPa with theoretical values between 130 GPa and 145 GPa, depending on its crystal-
linity [51] . h ese values exceed that of synthetic glass i bers (about 70 GPa) and aramid
i bers (about 67 GPa), considering that BC has a lower density (1.25 g cm
-3
) than glass
