Environmental Engineering Reference
In-Depth Information
but, despite their linear or slightly branched structure, they have two disadvantages.
First, their structure contains relatively fragile bonds or fragile configurations that
break with high temperatures. This is the case for the glycosidic bond or the C-O-C
bond in the saccharide cycle of the polysaccharides. The tertiary structure of proteins
is also altered with heat. The second disadvantage is that they form numerous inter-
molecular hydrogen bonds, resulting in some cases in very rigid structures. A lot of
energy is necessary to break these bonds and make the macromolecules flow; the
polysaccharides would decompose under the action of this energy before the poly-
mer could attain their molten state. This is particularly true in the case of cellulose,
and is the reason why paper burns and does not melt at high temperatures. In the case
of starch, thermoplasticity occurs only with the help of an external plasticiser (water,
glycerol, etc.) that disrupts the hydrogen bonding of the biopolymer.
In the following sections, we describe the technologies needed to overcome
these and other disadvantages. Following treatment, natural polymer can be turned
into profitable materials for industry. We have limited ourselves in this chapter to
the main biopolymers currently used in commercial products.
6.3.1
Cellulose
The ribbon-like structure of the cellulose molecule (Figure 6.1) favours its organi-
sation in oriented packs of about 50-100 molecules. If the organisation is com-
pletely regular, crystallites are formed. In nature the crystallinity rate approximates
50% in most species (wood, cotton, etc.). The same molecule therefore has both
crystalline and amorphous regions. The fibrous structure of cellulose is main-
tained even after the chemical processes of pulping from wood. Indeed, pulping
attacks and dissolves the LCC, that is, the middle lamella. The cell walls, which
essentially comprise cellulose fibres, are therefore recovered without severe
chemical degradation. Such a fibrous structure is useful for common bulk materi-
als such as paper or absorbing cushions (the so-called non-wovens). Pure cellu-
loses are also used in high-added-value applications such as hydrogels, stationary
phases for chromatography or pharmaceutical formulations.
The potential of cellulose increases greatly after chemical modification. The
esterification or the etherification of its hydroxyl groups leads to new biopolymers
with very different properties, described in the following section.
We first consider a particular case of the modification of the cellulose molecule:
regeneration. Two types of regeneration can be distinguished: with and without
chemical modification.
6.3.1.1
Regeneration of Cellulose with Chemical Modification
During viscose processes, a reaction occurs between cellulose and CS
2
in an alka-
line solution to form a viscous solution of cellulose xanthate. The resulting solu-
tion is filtered to eliminate solid particles, before being extruded to form continuous
