New Scientist - USA (2013-06-08)

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THE LAST WORD


Bag breakdown


I found this plastic shopping bag,
which was full of other plastic bags,
stored in the bottom of a wardrobe
(see photo). Though the bag is
labelled as “100% degradable”,
I wonder why it should start to
degrade in the absence of light or
moisture, and especially why the
red parts should degrade first.

n It looks as though the
questioner has an oxo-degradable
bag. These bags decompose by
oxidation, which can proceed
in the absence of sunlight or
moisture. A metal, usually
manganese or iron, is added to
the bag to catalyse the natural
oxidation process, which simply
chops up the polyalkenes that
make up the plastic product into
shorter-chain molecules.
But when plastic finds its way
into the oceans, this process
makes the pollution less obtrusive
but still results in a plastic soup
of microfragments, typically a
couple of millimetres in diameter.
Toxins and persistent organic
pollutants stick to the
microscopic particles, which are
consumed by zooplankton and
filter feeders, such as mussels.
Persistent organic pollutants
(commonly called POPs) therefore
enter the food chain and become
concentrated by the time they
reach the flesh we eat. Plastic
floating in the oceans can also
carry alien species to new habitats
and can kill marine life entangled
in it. To its credit, oxo-degradable
plastic does reduce this ecological

impact in comparison with
non-biodegradable forms.
Mike Follows
Willenhall, West Midlands, UK

n Many plastic shopping bags
are made from polyethylene.
The thinner bags tend to be high-
density polyethylene and the bag
in the photograph appears to be
one of those. Polyethylene is a
hydrocarbon and is hydrophobic,
so it cannot be printed on in its
raw form. The surface must be
treated so the dyes will stick.
Corona treatment is often used,
usually just after the polyethylene
film is produced and before it is
made into rolls. Corona treatment
uses a high voltage to create a
plasma, or “glow discharge”,
that breaks the long polymer
molecules and partly oxidises the
surface. This tends to separate
charges in and around the surface
molecules, making them polar,
less hydrophobic and so able to
accept printing. Corona treatment,
however, starts the degradation
of the polymer itself. I have also
noticed that printed plastic
shopping bags tend to degrade
quicker than non-printed bags.

Your correspondent notes that
the bag is labelled degradable so it
is possible that a low level of iron,
manganese or cobalt stearate has
been added to aid degradation.
These can work by thermal
degradation: light is not necessary,
but a warm environment helps.
It is not possible to tell what dyes
were used, but the red colour
must contain either an organic
or metal-based compound that
accelerates the degradation.
Greg Cash
Senior research fellow, polymers
University of Queensland, Australia

Hot youth


All of the radioactive elements that
made up the early Earth started out
in the hot ash of ancient supernova
explosions. This means they have
been working through their
half-lives for at least 5 billion years
since our planet was born. How much
hotter was Earth’s interior then?
What would the heat from nuclear
reactions mean for tectonic activity
and the evolution of life in our
world’s feverish youth? (Continued)

n The initial heat of the Earth was
caused by the collisions of smaller
objects that came together to
make it. Gravitational attraction
accelerated objects towards each
other, and when they struck their
kinetic energy converted to heat.
As time went on, the amount
of energy contributed by objects
falling onto the proto-Earth
became larger as the planet and
its gravity became larger. Towards
the end of the process, any object

hitting the Earth would contribute
at least 60 kilojoules for every
gram of its mass. If that heat were
confined to the colliding object,
it would heat it to thousands of
degrees. This heat did not have
time to radiate away as the planet
grew, so the result was a hot Earth.
Besides this, the Earth produced
heat through radioactive decay.
Early on, the amount of heating
per unit volume was greater than
at present. Today, heating is almost

all due to the decay of uranium-238,
thorium, and potassium-40, in
roughly equal measure. But
4.5 billion years ago, the amount
of uranium-235 was close to the
amount of uranium-238 we have
today (instead of being a minor
component) and uranium-235
produces heat much faster than
uranium-238. There was also about
13 times as much potassium-40
as there is today. The presence of
these nuclides helped to heat up
the interior of the Earth during the
first billion years of its existence.
Eric Kvaalen
Les Essarts-le-Roi, France

This week’s question


noT His cup of Tea
I like drinking tea, but my husband
hates it. Are we tasting the same
flavours? If so, why the difference?
Patricia Lloyd
Cardiff, UK

“The initial heat of the earth
was caused by the collision
of smaller objects that
came together to make it”

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