When I open my dishwasher, which has a dry heat/dry finish, the china, glassware and cutlery are dry, but plastic containers are still covered in drops of water. This is regardless of position or proximity in the wash. Why is the plastic wet, when the rest is dry?
• There are several possible reasons why plastic things don’t dry as well as crockery. The answer most often given is that plastics have a lower than crockery at the same temperature. But in fact the opposite is true. Plastics need an input of about 1.5 kilojoules per kilogram to warm up by 1 kelvin compared with 0.8 kilojoules per kilogram for glass. But plastics are much less dense, so the heat capacity is indeed lower for plastics of the same volume. This difference is not enough to explain the very marked difference in drying, however.
More important is that glass and china objects tend to be thicker and so can store more heat. As the amount of surface water needing to be evaporated is much the same irrespective of material then, by virtue of its greater mass, a hot glass contains more heat than a hot plastic cup and so will be the first to evaporate all the water.
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Another big difference is in , which is 0.2 watts per metre per kelvin for plastics and 1 for glass. This means that more heat passes to the water and evaporation is much faster from a drinking glass than from a plastic cup. This also explains why thin aluminium objects, such as pie cases, dry very well: aluminium has a thermal conductivity of 250 watts per metre per kelvin.
The heat capacity argument alone would suggest that plastic cups and aluminium pie cases would be equally wet. But because thermal conductivity is also a key factor, the specific heat capacity becomes much less important.
Hugh Hunt, Cambridge, UK