bioplasticsMAGAZINE_0905

Basics
Basics of
Starch-Based Materials
Starch is a reserve of energy for plants and is widely
available in cereals, tubers and beans all over the
planet. The present annual production of starch
worldwide is about 44 million tonnes and comes mainly
from corn, where worldwide production is about 700 million
tonnes, as well as from wheat, tapioca, potatoes etc.. Today
the main uses of starch available annually from corn and
other crops, produced in excess of current market needs in
the United States and Europe, are in the pharmaceutical and
paper industries. Starch is totally biodegradable in a wide
variety of environments and can permit the development of
totally biodegradable products for specific market demands.
Biodegradation or incineration of starch products recycles
atmospheric CO 2 sequestered by starch-producing plants
and does not increase potential global warming.
All of these reasons aroused a renewed interest in
starch-based plastics over the last 20 years. Starch graft
copolymers, starch plastic composites, starch itself, and
starch derivatives have been proposed as plastic materials.
Starch consists of two major components: amylose (Fig. 1),
a mostly linear a-D-(1,4)-glucan; and amylopectine (Fig. 2), an
a-D-(1,4) glucan that has a-D-(1,6) linkages at the branch
point. The linear amylose molecules of starch have a
molecular weight of 0.2–2 million, while the branched
amylopectine molecules have molecular weights as high as
100–400 million.
In nature starch is found as crystalline beads of about
15–100 mm in diameter, in three crystalline design
modifications: A (cereal), B (tuber), and C (smooth pea and
various beans), all characterised by double helices - almost
perfect left-handed, six-fold structures, as elucidated by X-
ray-diffraction studies.
Starch as a filler
Crystalline starch beads can be used as a natural filler in
traditional plastics [1]; they have been used particularly in
polyolefines. When blended with starch beads, polyethylene
films biodeteriorate on exposure to a soil environment. The
microbial consumption of the starch component, in fact,
leads to increased porosity, void formation, and loss of
integrity of the plastic matrix. Generally, starch is added at
fairly low concentrations (6–15%); the overall disintegration
of these materials is obtained, however, by transition metal
compounds, soluble in the thermoplastic matrix, used as
pro-oxidant additives to catalyse the photo and thermooxidative
processes [2].
Starch-filled polyethylenes containing pro-oxidants have
been used in the past in agricultural mulch film, in bags,
and in six-pack yoke packaging. According to St. Lawrence
Starch Technology, regular cornstarch is treated with a
silane coupling agent to make it compatible with hydrophobic
polymers, and dried to less than 1% of water content. It is
then mixed with the other additives such as an unsaturated
fat or fatty-acid autoxidant to form a masterbatch that is
added to a commodity polymer.
The polymer can then be processed by convenient
methods, including film blowing, injection molding, and
blow molding. The non compliance of these materials with
the international standards of biodegradability in different
environments and the increasing concern for micropollution
that can be enhanced by their fragmentability, together with
the potential negative impact on recyclability of traditional
plastics, and their limited performances with time, have not
permitted serious consideration of this technology as a real
industrial and environmental option.
Thermoplastic starch
There are two different conditions for loss of crystallinity
of starch: at high water volume fractions (>0.9) described
as gelatinization; and at low water volume, fractions (