THE SOLUTION TO THE PLASTICS LITTER PROBLEM

Plastics are known for their strength, toughness and versatility.   These characteristics account for the wide-spread use of plastics in construction, transportation and packaging.   In particular, thin polyethylene (PE) films are remarkably strong which is why they are so useful as shopping bags.   Indeed it is not surprising that PE is the largest volume commodity plastic in the world.

The useful properties of PE derives from the very large molecules out of which it is made.   As long as the molecular size is maintained, PE articles will retain their strength and toughness.   For many end uses, a long service life is desirable.   In other cases however, a shorter controlled life time is preferred.

In the case of PE shopping bags, for instance, a limited lifetime is necessary, especially in the case of bags that may end up as litter.   By definition, such bags are scattered so widely so as to make collection for recycling impractical.   Conventional PE bags when littered, will eventually become brittle and break up into small fragments as a result of the effects of warm temperatures and/or exposure to ultraviolet (UV) wavelengths in sunlight.   This degradation, however, can take months or even years.

New technology has evolved which will cause PE films to become brittle and disintegrate far more rapidly than normal.   This technology called TDPA - Totally Degradable Plastic Additives which includes DCP – Degradable Compostable Plastic and AGP – Agricultural Plastic involves the addition of small amounts of non-toxic materials into conventional PE resins.   TDPA technology speeds up the normal process by which PE molecules become much smaller as a result of the influence of heat and /or UV light.   This in turn converts the tough, stretchy PE film into a very brittle plastic that breaks up into fragments, and eventually into tiny particles too small to see.

The TDPA enhanced deterioration continues no matter how small the particles become.   The time scale for degradation is now months instead of years.   The only ultimate products are carbon dioxide and water as they are for all ordinary PE materials.   No new chemical processes and no new degradation products are introduced by the TDPA technology.   The important issue is that TDPA technology produces a controlled lifetime film by creating plastic materials that degrade by photo, chemical or thermal means at controlled rates.

It should be noted that similar TDPA technology is used to make bio-degradable compost bags and agricultural mulch.   The PE moleculesin a film will, as a result of partial oxidative degradation, become smaller and thus bio degradable when in contact with arable soil or in a commercial compost environment at molecular weights much lower than in the virgin plastic but much higher than the values required for water solubility.   Converting non bio-degradable PE into a material that bio-degrades readily in repeated testing in commercial composting operations is a remarkable breakthough.  

TDPA technology provides for the kind of controlled lifetime that is necessary for many short term plastics applications.

QUESTIONS FREQUENTLY ASKED ABOUT DEGRADATION

1. What happens to the degradation process when micro-organisms are not present (if this is possible – say in soils deprived of oxygen)?

There are some 80,000 described species of true fungi and some 2,000 species of known bacteria but it is felt that this represents only a small proportion of the full bacterial species count.   While fungi proliferate by sporulation (astronomical number of tiny spores are produced; they are found in air samples above all parts of the earth), soil is the greatest source and reservoir of micro-organisms are actually or potentially present in any bit of soil.   They survive nd are active in proportion to the extent to which conditions suit them.

Bacteria grow by cell division and if nothing interrupted the process, a single bacterial cell with the common generation time of 20minutes would result in the production of 24 x 10 tons of cells (~4 thousand times the weight of the earth) in 48hours.   Fortunately, the necessary conditions do not exist for nearly as long as 48hours.   Nevertheless, it has been calculated that there are 3 to 5 tons of micro organisms pre acre-foot of arable land, and that 1 gram of micro-organisms contains 10 individuals.

The kinds of micro-organisms that are important in composting and agricultural mulch situations are fungi and aerobic bacteria.   They both require oxygen and water, in the form of vapour for the fungi, and liquid water for the bacteria.   The nature and proportion of the species of each that cause the preoxidized (thermal or photo- and thermal) TDPA polyethylene film to biodegrade will be determined by the prevailing conditions.  

It is not easy to think of an oxygen-starved environment in either a compost or a mulch situation but if that were to occur, the organisms would simply wait until more oxygen diffused into their micro-environment.

The landfill situation is rather different.   At or near the top or active face, there is of course water and oxygen.   In the case of kitchen or garden waste, for example, not very much bio-degradation occurs at this time.   As more and more stuff is put in the dump, the supply of water and oxygen becomes restricted until this rather slow aerobic bio-deterioration ceases.

This is why it is possible to read newspapers that have been buried in a landfill site for 25years and to recognize carrots and other foodstuffs that, like paper, are inherently bio-degradable.   Anaerobic bacteria can and do operate in the absence of oxygen but rather slowly.   Since the carbon in bio-degradable materials cannot be oxidized to CO when there is no oxygen, it is reduced to form methane (CH) or natural gas.

This of course is where natural gas comes from but it takes a very long time to form.   In fact, some natural gas is produced in mature landfill sites and it is sometimes collected for fuel.

TDPA film which is used to as landfill cover can be broken up mechanically in the compaction process at the landfill site.   It is likely to have some exposure to sunlight and it will certainly undergo some thermo-oxidative degradation until eventually, deep in the landfill, there is no more oxygen available.

The molecular fragments which remain may then be subject to anaerobic bio-deterioration, but rather slowly just like the rest of the bio-degradable materials – as described above.   Conventional plastic waste (ie not containing the formulation package) will resist degradation for many decades longer.

2. When TDPA film is ingested by micro-organisms, is the only residue Co2 and water?

Aerobic micro-organisms, like people oxidize carbon to Co2 in order to obtain energy as well as to build cell materials; the hydrogen in food is oxidized to water.  The inorganic materials – those with no carbon bonded to hydrogen and/or other atoms – are not involved.

Thus any mineral fillers, metal salts and the like that were present in the plastic to start with will still be there after all the bio-degradation of the oxidized plastic has occurred.   Metals, for example, tend to end up as metal oxides, which is how they exist in nature in the first place.   But note that any part of a metal compound will undergo bio-degradation, assuming that conditions for the bio-degradation of anything exist.

3. Is the end result of degradation, bio-degradation?

Talking specifically about TDPA film, the answer is yes.   The micro-organisms cannot derive any energy from conventional polyethylene films because these films are not even water-wettable, and the polyethylene molecules are too large.   Oxidative degradation of TDPA polyethylene by heat or sunlight (either or both) is greatly enhanced compared to conventional polyethylenes because of the components in the formulation package.

Oxidative degradation causes a reduction in the size (and molecular weight) of the polyethylene molecules and this causes the film to become very brittle and water-wettable.   Oxidative degradation continues and eventually the smaller and smaller polyethylene molecules become small enough that microbial deterioration takes over.   The speed at which this sequence occurs can be varied from very fast (for composting) to somewhat slower (for agricultural mulch) to still slower in a landfill cover product, by altering the formulation package.

4. Is the amount of plastic matter in carrier bags after degradation reduced to nil?

YES, after the degradation/bio-degradation sequence has occurred.