In our daily life, we use plastics almost
everywhere, in bottles, bags, toys, gadgets etc. We use terms such as polythene, polyvinyl chloride etc. What exactly are these? These are type of
plastics – which everyone knows. But what are plastics made of? Plastics is a
sub-category of polymers and a lot
of materials fall under the category of plastics.
Polymers are substances containing a molecule repeated
again and again by the same type of bondage. Poly is the prefix used when
writing a polymer’s name. The actual substance is everything following the
prefix poly. For example, the substance repeated in polythene is ethene (C2H4).
Polymers exist in natural and man-made forms. Starch, cellulose are natural
polymers while polythene etc. are man-made.
Polymers are formed by the process of polymerization
of singular molecules. For example, when ‘n’ molecules of ethene are heated at
high temperature in the presence of catalysts such as Hydrogen Fluoride and Hydrogen
Sulphate, Polythene (also called Polyethylene) is formed.
There are different polymers such as polyethylene,
polypropylene, polyvinyl chloride, polystyrene, polyvinyl acetate,
polyisoprene, polychloroprene and PET (polyethylene terephthalate). PET is a
very widely used material.
Polyethylene terephthalate (PET) is a clear, tough
and shatterproof plastic. PET is made with the carbon molecules of carbon,
hydrogen and oxygen formulating as (C10H8O4)n.
Looking at its properties, it has low permeability to oxygen, water, and carbon
dioxide which is a reason for its specific uses. It is also a strong, synthetic
fibre which is why apart from being a plastic, it is classified in polyesters.
Polyethylene Terephthalate is mainly used due to the
properties it exhibits. Some of its properties are extremely helpful for packaging.
PET has its melting point at 490˚C which is why it does not have any specific
problems with normal conditions. In case, it does catch fire, it is self
extinguishable. It is generally a hard and strong material and while being
highly ductile, is stiff too. It does not even start to deform/ lose shape even
at high temperatures below its melting point. PET can be added with a range of
colours which is why products made of PET are coloured. When PET is having
fewer layers, it is a highly transparent material but as layers of it are added
together, it becomes more opaque. Most commercial uses use translucent PET.
PET exhibits good chemical resistance to almost all
types of substances except to alkalis and aromatic hydrocarbons (hydrocarbons
with alternating double and single bonds forming cyclic hydrocarbons such as
benzene – C6H6).
As PET is very less permeable to carbon dioxide, it
is used to store liquids like soft drinks, juices, beer and as single serve
water bottles. PET is used for microwaving food as well. PET has a
comparatively high melting point hence completely safe for microwaving food
substances.
Other than the above uses, it is also used for
storing some types of chemicals in laboratories. PET can be used for making
other products too. Some of these are polyester clothings, sleeping bag
insulations and carpeting too. But most of the products other than containers
that are made out of PET are second-generation products or products made out of
recycled PET.
Polyethylene terephthalate as the name suggests must
be made by the reaction between one compound containing ethylene and another
containing terephthalate. It is made by the reaction of monoethylene glycol
(MEG) and purified terephthalate acid (PTA) or dimethyl terephthalate (DMT)
(usually PTA). When PTA, MEG are mixed into a slurry mixture and are taken
through the esterification process, water is released as a bi-product and
unreacted MEG is recovered. Then, the pre-polymerization process which takes
the mixture to a stage before actual polymerization starts; takes place where
exhausted gas is removed through outlet. Then, the polymerization process takes
place from which some gases are again released and Polyethylene terephthalate
is formed.
The following equation shows the formation of PET:
n C8H6O4
+ n C2H4(OH)2
→ n C10H8O4 + 2n H2O
PTA
+ MEG →
PET + Water
PET is considered in one of the recyclable plastics. It is identified as recyclable plastic number 1. The others are HDPE (High Density Poly Ethylene – 2), PVC (Poly Vinyl Chloride – 3), LDPE (Low Density Poly Ethylene – 4), PP (Poly Propylene – 5), PS (Poly Styrene – 6) and others (7).
Out of all bottles, 95 percent are made
up of either PET or HDPE whereas on an overall basis, PET is about 48 percent
of all plastics. Although PET is recyclable, only 25 percent of all PET is
recycled and the rest keeps on piling up in landfills forever as PET is not
biodegradable but is only recyclable. In the following graph, we can see that
wasted PET has been gradually increasing from 1995 to 2005 but recycling is
still lagging and is not increasing along with the increase in waste of PET.
Consumption of PET cannot be reduced easily
as population and use is growing. Hence, out of “Reduce, Reuse and Recycle”, Reduce
is less applicable. Reuse and Recycle are applicable but neither there are
enough resources to recycle PET nor reusing can go on for a long time – disposal
of PET will take place some day.
All PET other than ‘the little’ that is
recycled is dumped into either bare land or into water bodies. This is
polluting the planet a lot. A survey also says that due to these landfills, the
tourism industry in several countries is facing a problem. Other techniques are
being discussed to reduce landfills and water body pollution. Some of these
techniques are incineration and flake production. When these techniques were
compared, it was shown that landfills created the most harm to environment
while least was when PET was incinerated.
The incineration mentioned above is currently
practiced. This is done under the category of waste-to-energy incinerations.
Plastics have high energy values which is why they are used in waste-to-energy
incineration. PET releases 10,900 Btu (British Thermal Units) of energy per
pound which is equal to 11,500,108 Joules.
These environmental issues of PET are
increasing daily. Disposal and recycling of PET is becoming a larger and larger
problem. This is because consumption is more but recycling plants are less. PET
is still being accumulated in landfills.
Economically and production-wise, PET is
still the largest produced plastic in the world. Due to its more availability,
PET’s cost is much lesser than several other plastics. This has given an
economic boost as PET is consumed more. However,
its low cost has promoted the culture of “use and throw”. Because of the
convenience it provides, people forget ethics of limited usage.
PET is a great polymer and plastic that
has been discovered and introduced in the society but its use has to be limited
before it destructs the nature with further landfills. If this is not done, the
‘world’ in “Polymers and our world” will be destroyed! A rough claim of PepsiCo
is that they have successfully created PET using natural materials which is
100% bio-degradable. In future, this would be a very useful discovery…
-Suhas
27/1/14
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