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  Plastics Engineering

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 itís 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 itís 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.

 

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