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I recently bought plastic multi-coloured clothes pegs, which have slowly disintegrated in the sun except for the yellow ones. Why?
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Peter Holness
Hertford, Hertfordshire, UK
The phenomenon that destroyed the non-yellow clothes pegs is called 鈥渦nzipping鈥�. Many years ago, a chemistry colleague told me about it. His explanation was that the sun emits high-energy photons capable of breaking chemical bonds. This explains things like sunburn and curtains faded by sunlight.
Most plastics are polymers, which are made from smaller units called monomers that link together through chemical bonds. Polyvinyl chloride (PVC), for example, consists of repeating vinyl chloride subunits. In turn, the subunits consist of carbon, hydrogen and chlorine atoms. Sunlight has been known to unzip PVC by dislodging hydrogen and chlorine atoms, creating hydrochloric acid and causing further erosion of plastic subunits. Unzipping potentially affects other plastics, too. It can be slowed or prevented by introducing certain chemical additives to the material. So, one possible explanation for the survival of the reader鈥檚 yellow pegs is stabilising additions of this sort.
Another explanation for the disintegrating pegs involves both light absorption and heat. The colour of the plastic affects the amount of light reflected or absorbed, and hence heat, with darker colours absorbing more than lighter shades, such as yellow. But without chemical and spectroscopic analysis, it is impossible to know whether the reader鈥檚 yellow pegs were protected by their colour or additives, or perhaps a combination of both.
To answer this question 鈥� or ask a new one 鈥� email lastword@newscientist.com.
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