Environmental Engineering Reference
In-Depth Information
sodium chloroacetate instead of chloroacetic acid. The modification of the rheo-
logical properties of sodium CMC depends on the DS and on the degree of poly-
merisation. Commercial sodium CMC has DS values between 0.6 and 0.9 and is
water-soluble. Sodium CMC is widely used in many applications, not as a bioma-
terial itself but as a component of many materials. These include additive in tex-
tiles (e.g. ironing, printing, anti-shrinking), paper (e.g. retention of fillers, coating,
glue for wallpaper) and cement (e.g. retarder agent).
Methyl cellulose
(MC) is a yellowish or white solid, with no odour or flavour.
It is obtained by reaction of alkali cellulose with methylene chloride. It is soluble
in water with DS values of between 1.4 and 2. MC and its derivatives (mixed
ethers with hydroxyethyl, hydroxypropyl and hydroxybutyl) form gels when the
water solutions are heated. The temperature of gelatinisation depends on the DS,
concentration, degree of polymerisation and the presence of salts or organic sol-
vents. MC is present in the formulation of different materials and formulations in
a similar manner to those of CMC.
Ethyl cellulose
(EC) is a water-insoluble cellulose ether. It is produced by the
reaction of alkali cellulose with ethylene chloride and has film-forming and ther-
moplastic properties. As a plastic, it can be processed by extrusion and injection.
It is hard, stiff and with good resistance to impact. It is soluble in the molten state
with other thermoplastics. As for its film-forming properties, it is used in the for-
mulation of varnishes, inks and glues. It can also form removable coatings.
6.3.3
Starch
Starch is the main energy reserve of superior vegetal plants. It is found in large
quantities in wheat, potato, corn and manioc. Starch is a homopolymer (99%) of
d-anhydroglucopyranose units. Two different configurations exist: amylose and
amylopectin (Figure 6.18). The branched polymer is present in the proportion
70-80% [27]. Native starch is present under the form of partially crystalline (25-
40%) granules (up to 100 µm diameter), demonstrating a complex structure which
has been the subject of thousands of scientific papers.
Starch grains are insoluble in water at room temperature. At 50-60°C, starch
absorbs water reversibly and hydrogen bonding is reduced. Above 60°C (the exact
temperature depends on the native source) the structure of starch is modified irre-
versibly, crystallinity disappears and gelatinisation occurs.
The paper industry is the main non-food outlet for starch and consumes 17% of
the European starch production. Starch-cellulose-starch bonds are created and
contribute to the internal cohesion of paper sheets.
Thermoplastic materials may be obtained by extrusion in the presence of water
at temperatures of around 160-200°C. If an external plasticiser (i.e. glycerol or
sorbitol) is added [28], the glass transition temperature decreases. Even though
these materials are fully biodegradable, their affinity with water is a huge incon-
venience when considered to replace traditional plastic materials. Blends [29] and
