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 鈥�100% degradable鈥�, I wonder why it should start to degrade in the absence of light or moisture, and especially why the red parts should degrade first.
鈥� 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.
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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
鈥� 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 鈥済low discharge鈥�, 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