Biodegradable Plastic
Three
generations of starch-based plastics are recognized. The first generation
consists of a synthetic polymer. Starch is only used as a filling material
it�s polymeric properties are not made use of. An example are "biodegradable"
plastic bags. These bags are not fully biodegradable, though, since they
consist of mainly non-biodegradable synthetic polymers like polyethylene or
polypropene and only 5-20 percent starch. Under special conditions the starch
degrades and the plastic falls apart into small particles, that will prevail
for many years although they are not visible.
In the second
generation the starch is used for its polymeric properties. It is blended with
hydrophilic synthetic polymers and contributes to the strength of the
material. 50-80% starch can be used in these plastics, but still a large part
is not biodegradable.
Biodegradable
plastics |
Product Name |
Constituent |
Micro-organisms Based: |
BIOPOL |
copolymer of
polyhydroxybutyrate and valeric acid (PHB/V) |
Chemical
Synthesis Based: |
Poly Lactic Acid |
polylactic acid |
Plockcelton |
polycaprolactone (~NH-(CH2)5-C=O
~) |
BIONOLLE |
aliphatic polyester |
Natural
Product Based: |
MATER-BI |
starch (60%) / PVA alloy |
NOVON |
starch (90-95%) + additive |
AMIPOL |
starch (100%) |
The third
generation is a truly biodegradable plastic, that does not contain synthetic
polymers at all. To improve some of the properties of the plastic, the
biopolymer may be modified, but no synthetic materials are necessary.[42]
The barrier
properties that are required for a film depend on it�s use. Fresh fruits or
vegetables have to be able to breath, so a film with too low an oxygen and/or
carbon dioxide permeability can not be used. Foods which are rich in
polyunsaturated fat, however, are sensitive to oxygen and need a film with a
high oxygen barrier. Often the barrier against water is the most important
function of a film, since aw is an important factor for the shelf life of a
product (microbial growth, chemical reactions, crispiness).[33]
Properties of some biodegradable plastics
Material |
Film preparation |
Moisture barrier |
Oxygen barrier |
Mechanical properties |
Cost, $/kg |
Cellophane |
Aqueous suspension |
+/- |
+ |
+ |
5 |
NC-W/ Cellophane |
NC-W coating |
+ |
+ |
+ |
5 |
Cellulose Acetate |
Extrusion |
+/- |
|
+/- |
3-5 |
Starch/PVOH |
Extrusion |
- |
+ |
+ |
3-7 |
PHB/V |
Extrusion |
+ |
+ |
+/- |
7-13 |
PLA |
Extrusion |
+/- |
- |
+ |
2-11 |
-
NC-W = Nitrocellulose-wax
PVOH = Polyvinylalcohol
PHB/V = Polyhydroxybutyrate/valerate
PLA = Polylactic acid
|
Foods coated with a film that
is highly impermeable to fat will absorb less fat when fried.[33]
Edible coatings can also
protect the outside structure of a food during mechanical handling, for
example extruded or frozen foods.[33]
Biodegradable plastics can made
by micro-organisms or by man. The latter can be from natural products (e.g.
starch or proteins) or synthetic polymers. Some commercial biodegradable
plastics are presented in table 1.
In table 2 some properties are
given of a number of biodegradable plastics.
|