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BIODEGRADABLE PLASTICS (1997)

Antonín SLEJŠKA

Ecology is one of the best examples of scientific discipline, which rediscovers uniformity and homogeneity of the world. The general definition of ecology says, that it is the branch of biology, that studies organisms in terms of their relationships with other organisms and their local environment (Dictionary of Science). I divide ecology to two subbranches, but only for myself. The first is theoretical ecology, which casts about for coherences of everything with everything. The second is practical ecology, which tries to harmonise the relationships of mankind with the other nature. In the present time the main province (field, range) of practical ecology is looking for the most perfect possibilities of making use or disposing metabolites of human society. One of the metabolites are plastic products.

Treatment of used plastic products

Table 1

There are these potentialities (contingencies) of treatment of used plastics. Synthetic polymers should be treated mainly by reusing, whilst the best way of treatment of natural polymers is in the present biodegradation (e.g. composting), combustion or in some cases mechanical utilisation. Dumping is the worst way of treatment for every waste.

In this lecture (article) I will speak mainly about biodegradable plastics, but before it I’d like to say a few words on enzymology.

In nature, organic substrates are degraded by enzymatic processes. Enzymes are secreted mainly by microorganisms.

Enzymes are intracellular, which are effective in a cell and extracellular, which are secreted into environment and break large macromolecules which cannot go through a cell-wall. Enzymes are also divided on exoenzymes, which hydrolyse end-groups of macromolecules, and endoenzymes, which break the inner bonds of macromolecules.

Microorganisms have formed their enzymatic systems for millions of years. We are able to change them genetically, but the genetically changed microbes are mostly uncompetitive with microbes occurring in nature. This is one of the reasons why it is so difficult to cultivate microorganisms which would be able to deteriorate polymers. Another reason is that enzymes are too large to penetrate into synthetic plastics. Therefore if we want manufacture biodegradable polymers, we either have to use natural raw materials such as cellulose, starch, polyesters (e.g. polyhydroxyalkanoate, polylactate, polycaprolactone) or blends of synthetic and natural polymers.

Plastics

Table 2

In this table you can see a simple conception of kinds and properties of polymers with respect to biodegradable polymers. The plastics are divided into three groupes according their composition in the table. There are pure synthetic polymers, pure natural polymers and blends of synthetic and natural polymers. The greatest advantages of synthetic polymers are a number of possibilities of reusing, good and variable properties and low price. The main advantages of natural polymers are renewability of raw materials, low toxicity and biodegradability.

In the course of designing a new type of commercial plastic, we have to consider these main parts of fate of the material.

Main parts of a polymer cycle

Table 3

1. During manufacturing of products from biodegradable polymers may be problems with degrading of macromolecules, with cohering of different types of macromolecules, with the strength of the polymer during extruding and other ‘critical’ technological processes. (But it isn’t my speciality).

2. For the second stage is important to know, that the biodegradable plastics have usually worse mechanical properties. They mainly are not stable against water, fotodegradation and of course against biodegradation. Biodegradable plastics made from pure natural polymers are untoxical, some are even eatable (e.g. plastics made from pure starch). From these and others properties of biodegradable plastics we can conclude that they are eligible for products with short application time e.g. bags for municipal waste esp. organic waste, vending dishes (such as coffee cups, table-cloths for one use etc.)

3. From the way of using we can approximately judge (assess) the final stage of the exact plastic product and its treatment. So if we will take into account the whole route of a polymer than we will able to decide, which kind of polymer or blend of polymers to use for an exact purpose. But it is great and eternal balancing among hundreds advantages and unsuitabilities.

Biodegradation takes place when a polymer is placed in one of these environments:

Biodegradative environments

COMPOSTS

WILD NATURE

AND SOIL

FRESHWATER OR MARINE SEDIMENTS

(LANDFILLS)

(ANIMAL BODIES)

(Table 4)

Plastics are degraded aerobically in wild nature, anaerobically in sediments and landfills and partly aerobically and partly anaerobically in composts and soil.

Intensity of biodegradation is dependent on the environment and if we want know how a plastic will behave in real conditions, we have to simulate them. At the present there are these methods for testing biodegradability of plastics:

In these tests we can measure mass decrease of a plastic, changes of mechanical properties of the plastic, oxygen consumption, end products and so on. Measurement of end products is considered as the best.

Endproducts of biodegradations

(Table 5)

Carbon dioxide and water are produced during aerobic biodegradation and carbon dioxide, water and methane are produced during anaerobic biodegradation.

In nature, biodegradation is effected by several environmental factors, e.g. temperature, light, nutrients, pH, oxygen and water content, presence of enzymes, microbes, meio- and macro-organisms. When testing, how ever, the conditions should be kept as constant as possible in order to ensure the repeatability of the results.

During biodegradation toxic by-products can be produced. It is a reason, why we should measure eco- and phyto-toxicity before, during and after any biodegradability testing.

xEnhancing of possible biodegradation can be reached by:

- adding of a biodegradable compound into a synthetic polymer (or using only a pure natural biodegradable polymer),

- adding nutrients for microorganisms,

- ensuring a good access of enzymes into a polymer structure,

- adding of initiators of hydrolysis into a polymer, etc.

Testing of biodegradability of plastics is based on simulation of conditions, in which it is presupposed to be disposed, and measuring of end products of degradation, weight losses of the plastics, oxygen demand, changes of mechanical properties of the plastics, etc. Biodegradability tests can be divided into aerobic and anaerobic tests or into screening and real-life tests. Toxicity testing should be performed before the start and after the end of any biodegradability test.