Oxo-biodegradable bags are designed to have a useful shelf life and to degrade over time when exposed to oxygen and sunlight, says Michael Stephen

If the researchers from the University of Plymouth’s International Marine Litter Research Unit (Biodegradable plastic bags still intact after three years, 29 April) had been polymer scientists who understood the process of abiotic degradation, they would have understood that an oxo-biodegradable shopping bag contains stabilisers to give the product a useful service life and which could therefore delay the onset of degradation for two years or more depending on how the particular bag was designed.

They would also have understood that oxo-biodegradable bags are not intended to degrade in special conditions such as landfill or when buried deep in soil, but are intended to degrade if they become litter in the open environment on the surface of land or sea with an abundance of oxygen and usually exposed to sunlight.

The experiments performed at Plymouth were not, therefore, a fair test of the product, because they had folded it tightly so as to exclude most of the oxygen, and placed it in a dark environment under a pontoon. Even then it would eventually have degraded, and biodegraded until there was nothing left, much more quickly than ordinary plastic.

Michael Stephen

Chairman, Oxo-biodegradable Plastics Association

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