Plastic litter is unsightly and appears never to rot away. Chemists were
pressed to do something about it by making polymers that will decompose and by
redesigning well-known ones so that they self-destruct. But is this really the
answer?
THE CHEMICAL giant ICI produces what appears to be the perfect plastic.
Bacteria make it from sugar, and once we have finished with it, other bacteria
in the soil digest it back to carbon dioxide. This remarkable plastic is
called polyhydroxybutyrate (PHB), and is tradenamed Biopol by ICI. It has
recently won environmental awards in the US and Germany. Wella, the German
hair-care company, now sells its Sanara brand of shampoo in bottles made of
Biopol and reports increased sales as a result.
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Yet Peni Walker of Friends of the Earth says that her organisation and
other environmental groups now oppose degradable plastics on the grounds that
plastic waste is best recycled, an opinion they share with the British
Plastics Federation, a trade association of plastics manufacturers. However,
the US, Swedish and Italian governments have passed laws making degradability
compulsory for plastics in certain types of packaging. What is going on?
Making plastics rot
Microbes or light
THERE are two important ways of degrading a plastic and thereby helping it
to rot away. One is to make it out of a material which microbes digest in a
process called biodegradation. The other option is to make the plastic
sensitive to sunlight which fractures its chemical bonds and breaks it down by
a process known as photodegradation. To make plastics photodegradable,
chemists implant a material in the plastic that will absorb sunlight and
become sufficiently reactive to attack the molecules from which the plastic is
composed.
One other means of photodegradation is to incorporate a so-called carbonyl
unit, CO, into the backbone of the polymer (polymers are molecules composed of
long chains of repeating molecular units). When a 鈥減ackage鈥� of light called a
photon hits the carbonyl unit, it is absorbed. The sudden influx of energy can
be off-loaded to the polymer molecule, but carries so much energy that it
breaks chemical bonds, dismembering the polymer chain. The fracturing of the
chains makes the plastic brittle so it falls apart.
Biodegradation is very different. This relies on the plastic being seen as
a source of food by microbes such as bacteria, fungi and yeast. The enzymes
they produce to digest food break down the carbon chains of the polymer, the
time it takes depending on such factors as moisture, temperature and oxygen
availability. Natural polymers biodegrade in a matter of days, but most
artificial ones take microbes years, often decades, to digest.
Synthetic polymers resist biodegradation if they cannot be attacked by
enzymes. Natural polymers, such as the fatty acids and waxes, have the
chemical grouping -CH2-CO- which enzymes can easily snip out of the
chain. Chemists found that they could make plastics resistant to enzymes by
altering the CH2 groups that join together to form polymer chains.
They succeeded by replacing one of the two hydrogens with other atoms or
groups, such as chlorine in PVC (polyvinyl chloride), benzene in polystyrene,
or the methyl group in polypropylene. Even polythene, which is composed
entirely of CH2 links, is attacked only slowly by microorganisms if
there are more than 35 such units in the chain.
Armed with these possibilities, chemists now make degradable plastics in
four broad ways. To promote biodegradability, they either make the plastic
wholly edible to microbes 鈥� as in the case of PHB 鈥� or pack a plastic skeleton
with starch that can be devoured, leaving an indigestible but fragile polymer
scaffold that then disintegrates. To promote photodegradability, they can
either incorporate weak links into the polymer chains so that they break on
exposure to sunlight, or implant additives that attack the polymer after
absorbing sunlight. Each kind of plastic is now being produced.
Contrary to public opinion, plastics are seldom indestructible, and in the
past chemists wrestled with ways of halting rather than promoting degradation
鈥� generally by putting in additives to prevent oxidation and attack on the
polymer chains. A small amount of degradation can render a plastic unfit for
the job it was designed to do. Even without such additives, complete decay is
slow, taking between decades and centuries.
Biodegradation
Recipes for microbe fare
THE POLYMER polyhydroxybutyrate (PHB) is ideal food for microbes. It is not
made from petroleum but is a natural polymer made by a wide range of microbes,
such as the bacterium Alicaligenes eutrophus, which form the polymer as a
convenient way of storing energy in the same way that human beings store
energy as fat. It was first discovered in 1926 at the Pasteur Institute in
Lyon. PHB is composed from chains of 3-hydroxybutanoic acid. This molecule has
an acid group at its head and a hydroxy group at its tail. When molecules
react head-to-tail, they form an ester linkage, and as this reaction is
repeated the chain grows and eventually we have thousands of units linked to
form a polymer.
The growing chain can be modified by adding another hydroxyacid, such as
3-hydroxypentanoic acid. In this way, the properties of the polymer can be
tailored to make it suitable either for moulded articles such as shampoo
bottles, or thin films for plastic envelopes or carrier bags. However, PHB is
not cheap, and a container made of Biopol is seven times more expensive than
polythene. PHB is now in production and ICI hopes to be making 5000 tonnes a
year by the mid-1990s. Most of the PHB that the company makes will be used for
packaging, agricultural products and for items of personal hygiene that we
flush away.
The other kind of biodegradable plastic is starch, again a natural polymer
food store, which is composed of carbohydrate units linked together. Shopping
bags said to be biodegradable contain up to 15 per cent starch, the rest
being a matrix of polythene. Microbes digest the starch and leave a flimsy
plastic lace that disintegrates mechanically. Ferruzzi, an Italian firm which
dominates the European corn-starch industry, has launched a polymer which is
more than 50 per cent starch, and the Italians hope eventually to raise the
content to 90 per cent. Novon, a biodegradable plastic made by Warner Lambert,
an American company based in New Jersey, is reputed to be composed entirely
from starch.
In addition to shopping bags, starch-filled plastic film is suitable for
envelopes for magazines, hospital laundry sacks and agricultural mulch
(sheeting spread over fields to prevent frost damage to young crops, to
control weeds, to halt soil erosion and conserve soil moisture). At present,
high-starch plastics cost up to four times as much as polythene, but the price
should fall with increased production, and while consumers regard
biodegradability favourably, they are happy to pay the extra cost. Currently,
Europeans use 100 000 tonnes each year of such degradable plastic, and
consumption is forecast to reach a million tonnes annually by 1995.
Polyvinyl alcohol is one plastic that poses no litter problem. It simply
dissolves in water where it eventually degenerates into carbon dioxide. It
makes ideal packaging for such things as swimming pool chemicals, detergents,
descalers, seed strips, sanitary items and even oxygen tenting, and about l00
tonnes are manufactured for these purposes in Britain each year, says Keith
Taylor, development director of Hoechst, the company which makes the polymer.
Photodegradation
Scorching plastics to bits
ONE item of plastic litter that has upset many people is the piece of
polythene used to hold 鈥渟ix-packs鈥� of beer and lager. In the US it is illegal
to use such six-pack holders that do not degrade. Some states, including
Florida, have banned them. Furry animals, birds or fish poking their heads
through one of the holes may be trapped and spend the rest of their lives
dragging it round with them. Degradable six-pack holders are made of a co-
polymer of ethene and carbon monoxide that has been known for more than 40
years. (A co-polymer consists of chains that are not of a single repeating
unit but of two kinds of unit bonded together chemically.)
Polythene is the only cheap polymer that has the potential to be fully
biodegraded, and to make it susceptible to attack by microbes, it has to be
weakened so that it will decompose into short chains that enzymes can digest.
This can be done by co-polymerising it with a small percentage of carbon
monoxide in the ethene gas from which polythene is made. Carbonyl groups 鈥�
which predispose the polymer to attack by sunlight 鈥� are then built into the
long chains. Polymers containing 1 per cent of carbonyl groups lose their
strength after two days in the Sun, a process that takes one morning if the
content is raised to 13 per cent. Without carbonyl groups, it takes about 300
days of sunlight to achieve the same effect.
Another kind of plastic that needs to be degradable is the film used as
agricultural mulch. Plastic film makes ideal mulch with holes through which
the crop can grow, but it must rot away by the end of the growing season and
be absorbed into the soil. The best way to ensure this is to make it
photodegradable by adding compounds which accelerate oxidation of the polymer
when it is exposed to light.
Iron and nickel dithiocarbamates are compounds that are excellent at
triggering polymer degradation. They act as so-called 鈥減hotosensitisers鈥�. They
can absorb a photon of light and become extremely reactive agents called free
radicals, which have unpaired electrons. These can only recover their
stability by snatching hydrogen atoms from the polymer. In doing so, they
oxidise the polymer and a chain of reactions begins which ultimately snaps a
polymer bond. The stability of a polymer can be regulated by the amount of
photosensitiser used. In this way, a polymer sheet used as a mulch can be
timed to decompose and disappear at the end of the time of harvesting. What
becomes of the polymer fragments in the soil is unknown, however.
Although environmentalists blame the litter problem on the long life of
synthetic polymers, chemists saw longevity as a virtue. They took great pains
to find materials called stabilisers that can be added to plastics to make
them more durable. Without stabilisers, most polymers lose their strength in a
few years, although complete degradation takes much longer. Designing
degradable plastics is much more than just leaving out the stabilisers, as we
have seen. Yet just as the new plastics are coming on to the market, the
environmental lobby now advocates stable plastics that we can reuse or
recycle. Supermarkets, which hand out 5 thousand million plastic bags a year
in Britain alone, are happy to go along with this. Some stores now encourage
shoppers to reuse bags by offering them a small discount.
Biodegradability or photodegradability makes little sense if we dispose of
our plastic waste by burying or burning it. This is how 85 per cent is
eradicated, even in Europe, the most environmentally aware economy. Nor do
bio- and photo-degradation help with the other way of dealing with plastics,
which is to recycle. We recycle paper and glass, so why not plastic? The
reason is that plastic is not a uniform material like paper and glass. Two
plastic bottles may look the same, but if one is made of PVC and the other of
polythene, then melting them down together will not give us a material that we
can turn into new bottles. However, we can make other useful things from a
blend of plastics.
In the US, 鈥渂each clean-ups鈥� have been organised in which thousands of
people have collected the plastic flotsam and jetsam from the shore. They take
this to 鈥渞ecycling鈥� plants where it is melted down and turned into park
benches, which they then take back to the seaside. An aesthetically pleasing
exercise maybe, but a waste of energy if the motor fuel consumed is
considered.
And in any case, the park benches may not last very long because mixtures
of plastics are much weaker than individual plastics. To get round this
problem, we need to add large amounts of stabilisers, chemicals which prevent
degradation.
Degradability is the ideal solution for plastics which cannot be collected
economically for disposal by other means. Such degradable plastics are
recommended for agricultural use, such as baler twine, bird netting, sapling
protectors and mulch films. At sea, there is a good case for degradable
fishing nets, and for any plastic that is disposed of overboard as waste.
The debate about the degradability of plastics has led to a reappraisal of
the true cost of plastic, from 鈥渃radle to grave鈥�, and part of the sum is how
we dispose of it when we have no further use for it. Recycling of some
plastics is possible, and leading the field is polyethylene terephthalate
(PET), the plastic from which many bottles and jars are fabricated. This
polymer is also the basis of polyester fibre and has the advantage of not
creasing. Reclaimed PET bottles could in theory be recycled to make suits and
dresses, but the material is more likely to be used for insulation in duvets,
pillows and anoraks, or tufting for carpets and rugs.
According to Norman Billingham of the University of Sussex, plastics are
not as environmentally unfriendly as people think. Replacement of other
materials by plastics has brought great economic benefits. He says that the
damage plastics cause can be overstated. Oil burned as fuel contributes
directly to environmental damage, both by adding toxic pollution to the
atmosphere and by generating carbon dioxide, the most important contributor to
global warming. Oil converted to plastics does not have these direct effects.
The number of fish and sea-birds killed by plastic items discarded at sea is
dwarfed by the number dying in oil spills.
Grasping the nettle
Degradables and nature
ASKING chemists to solve the problem of plastic litter did not address the
real cause 鈥� uncaring human behaviour. However, society demanded a chemical
answer to the problem and chemists complied. The resulting plastics could now
interfere with recycling schemes. Another unforeseen problem is that as
plastics biodegrade, they break down into smaller fragments that animals and
birds may eat.
Some people in the plastics industry claim that society will soon have to
make a choice between degradable plastics and recycling of plastics 鈥� that we
cannot have both. However, ICI says that PHB can be reused and recycled along
with other plastics. Photodegradable plastics pose the greater problem 鈥� if we
want to recycle plastics then we must prevent our waste being contaminated
with them. Waste plastics can be turned into sacks, park benches, roofing,
drain pipes, fencing, even road surfacings, but calamities could occur if
items made from such materials contain degradable elements and break up in
sunlight.
Most plastic in Europe and the US ends up in rubbish tips. Surprisingly,
plastics account for only a small proportion of all our rubbish in terms of
volume and weight. Archaeologist William Rathje of the University of Arizona
has investigated rubbish sites and found that plastics account for less than 5
per cent by weight and 12 per cent by volume. He also finds that in the
airless conditions of such sites, fewer bacteria can thrive and so there is
only slow degradation of paper and waste food, and almost none of plastics.
Only 25 per cent at most of what goes into a landfill site is biodegraded and
he found that rotting almost ceases after 15 years. Thereafter, there is
little decomposition.
Another of Rathje鈥檚 more startling findings, in a tip at Phoenix, Arizona,
was the lack of degradation of paper. He found newspapers dating back to 1952
that looked 鈥渉ot off the press鈥�.
Plastic waste evokes strong emotions, often because there seems so much of
it. However, it accounts for only 3 per cent of annual oil production. And
much of that could be reclaimed as energy if another option for disposal is
chosen: incineration. Plastic waste still contains about the same amount of
energy as the oil from which it came. Municipal incinerators consume more than
70 per cent of the domestic waste in Denmark and Japan, and other countries
are turning to this as the preferred method of disposal. In Britain, less than
10 per cent ends up this way. If this option is chosen, then the plastic part
of waste becomes important because it helps in the incineration of other damp
domestic rubbish. Some people have misgivings about incineration, however,
fearing that poisons such as dioxin escape through the chimney of the
incinerator if the temperature is not high enough. An alternative method of
municipal composting is used in Holland and Austria.
Martin Dennison, head of plastic waste management at Shell, points out that
of the 23 million tonnes of plastics manufactured in Europe in 1989, 40 per
cent was used for packaging. This can result in direct savings in unexpected
ways. For instance, changing from glass to plastic bottles for drinks in
aircraft has been estimated to save about $1000 per aircraft per year
in fuel bills. Moreover, Dennison argues that degradability is not the answer
to plastic waste. Unloved plastic can be environmentally more friendly than
paper, which is often advocated as the biodegradable and renewable resource
that we should prefer.
A life on the ocean wave
MOST OF the plastic litter which defaces beaches around the world comes
from ships and rigs. One study of a cargo ship with a crew of 46 revealed that
during a 6-week period, sailors threw overboard 320 cardboard boxes, 165 crisp
packets, 19 plastic bags, 2 plastic drums, 2 metal drums, 245 bottles, 5
drinking glasses, 29 fluorescent tubes, 370 plastic beer can holders and 5176
cans. Fishing vessels also discard damaged nets.
Annex V of the International Convention for the Prevention of Pollution
from Ships came into force at the start of 1989 and will outlaw this kind of
carelessness. It has been accepted by 41 nations including Britain, the US,
Germany, France, Italy, China, Japan and the USSR. The signatories to the
treaty only sail 60 per cent of the world鈥檚 ships. Annex V regulates the
disposal of garbage at sea and its rules get tighter the nearer a ship is to
land. Plastic waste must not be discharged at sea but returned to land.
Until Annex V is signed by all nations, and all ships comply with it, the
problem of plastic waste may best be solved by using degradable plastics.
However, the problem of discarded trawling nets will require a different type
of degradability, perhaps relying on slow chemical attack by water and
dissolved oxygen.
The energy debate
IN 1988, the German Federal Office of the Environment in Berlin compared
the cost to the environment of plastic and paper carrier bags and came up with
the surprising results shown in the Figure. Plastic bags were not only much
cheaper but were more environmentally friendly. Making paper bags produced
more than twice as much atmospheric pollution and 200 times as much waste
water as making plastic bags. The message was clear: people should be
encouraged to use and reuse plastic bags.
But what about things that we would never consider reusing, such as
disposable drinking cups? Does polystyrene still win out over paper? According
to Martin Hocking of the University of Victoria in British Columbia,
polystyrene again beats paper if aspects of the environment are taken into
consideration. According to Hocking, to make a polystyrene cup weighing only
1.5 grams, we need 3.2 grams of oil and 0.1 gram of other chemicals. A paper
cup holding the same volume of liquid weighs 10 grams and to make it we need
33 grams of wood, 4.1 grams of oil and 1.8 grams of chemicals.
If this analysis based on raw materials does not convince us, then the
energy required to manufacture the two cups should, says Hocking. To make a
tonne of polystyrene cups (about 650 000), we need 5000 kilograms of steam and
180 kilowatt hours of electricity. To make a tonne of paper cups (100 000) we
need 10 000 kilograms of steam and 6400 kilowatt hours of electricity.
However, this part of his analysis has been challenged by the paper industry,
which points out that half the energy it uses comes from burning wood waste
and not oil. Even so, the general assumption that plastics are inherently
wasteful of a natural resource, while paper is environmentally friendly and
uses only renewable resources, does not stand close scrutiny.
The energy bill is not the only hidden environmental cost. Papermaking
requires a lot of water, more than 500 times that required to make the same
weight of polystyrene. And what of the chemical waste; surely there is less of
that from the making of paper? Sadly no. Making the polystyrene uses metal
salts which are discharged into waste water and into rivers, but making paper
also requires chemicals such as chlorine and sulphides. Makers of paper cups
discharge more than twice as much waste into the environment as makers of
polystyrene ones.
Are we not in danger of forgetting the other reason that moulded
polystyrene cups and fast-food containers became unpopular in the first place?
To make this lightweight material requires a so-called 鈥渂lowing agent鈥� and in
the past, manufacturers used chlorofluorocarbon (CFC) gases, now known to
damage the ozone layer. However, polystyrene can be 鈥渂lown鈥� using a
hydrocarbon gas such as pentane, which does not damage the ozone layer,
although it adds to global warming. Paper, on the other hand, is made from
wood which removes the greenhouse gas, carbon dioxide, from the atmosphere.
However, the saving is illusory because half the power needed to make the
paper cup generally comes from burning fossil fuel. And in any event, the
carbon in the paper will eventually return to the atmosphere, its much
acclaimed biodegradability being the very reason people advocated it in the
first place.
Further Reading
ICI provides a useful book entitled Packaging 鈥� An Introduction to
Ecological Assessments, and the Shell International Petroleum Company has
published Plastics: A Reusable Resource by Martin Dennison (free). Peter
Klemchuk offers an excellent, if technical, assessment of the various kinds of
polymer decomposition in Polymer Degradation and Stability, volume 27, pl83
(1990). William Rathje sums up his findings, on rubbish tips, in the December
1990 issue of The Atlantic Monthly. Science published 鈥淧aper versus
polystyrene: a complex choice鈥�, by Martin Hocking, 1 February 1991, and a hot
debate ensued in its Letters pages.