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Bag breakdownI 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 FollowsWillenhall, 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 CashSenior research fellow, polymersUniversity of Queensland, Australia

Hot youthAll 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 KvaalenLes Essarts-le-Roi, France

This week’s questionnoT His cup of TeaI like drinking tea, but my husband hates it. Are we tasting the same flavours? If so, why the difference?Patricia LloydCardiff, 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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