Environmental Engineering Reference
In-Depth Information
blend. They are intended to break the synthetic polymer chains to the point where
they became digestable by micro-organisms, thus making starch accessible.
Degradation begins at low rate and is accelerated when exposed to degradation
conditions (compost, etc.). In most cases, the material is biofragmented but not
fully biodegraded or bio-assimilated when exposed to degradation. In order to
reduce water absorption of starch blends, and therefore their mechanical proper-
ties, the addition of zein seems to be efficient [45].
Examples of commercial starch-containing blends include Ecopolym (Polychim)
and Ecostar (St Lawrence Starch Company). In the former, the synthetic polymer
is polyethylene and starch is present at 10%. This is associated to one catalyst that
promotes the decomposition by oxidation and by cleavage of chains due to the
produced radicals. The latter product associates PE with a mixture of starch and
auto-oxidant unsaturated fatty acids. The global content of starch is between 6 and
15%. The degradation process then follows two mechanisms: the starch is frag-
mented then assimilated by micro-organisms; and the interaction between the
auto-oxidants and the metallic complexes from soil or water yields peroxides that
attack the synthetic polymer chains. These types of products are normally used in
mulch films, bags and packing.
Other systems have been investigated. The combination of starch and polyester
has been claimed to be fully biodegradable [46]. Others are partially biodegrada-
ble such as the starch/polyethylene/poly-ε-caprolactone blends [47] and their
derivatives or the combinations of starch and modified polyesters.
The product Mater-Bi, produced by Novamont who have revolutionised the
starch-based biomaterials sector over the last two decades, deserves a particular
mention. The commercial success of this biodegradable and biocompostable plas-
tic relies on two main factors: the economy of scale that allows costs to be reduced,
and the diversity of formulations to develop different end-products (plastic bags,
tableware, toys, etc.). More than 210 references in Chemical Abstracts are available
on this (registered) keyword, and the number of patents related to different formu-
lations and developments is also impressive. Mater-Bi can essentially be described
as a blend of starch with a small amount of other biodegradable polymers and addi-
tives. The actual compositions are only known by a very few people.
6.4.3
Composites with Natural Fibres
6.4.3.1
Wood-Plastic Composites
Vegetal fibres (including wood fibres) represent a good replacement solution for
glass and carbon fibres in the reinforcement of composites based on a thermoplas-
tic matrix. The advantages of vegetal fibres are both economical and ecological:
• they are inexpensive and less abrasive for the processing equipment;
• they are of low density, lightening the whole composite material; and
• they have a limited environmental impact.
