Why That Sippy Cup Won’t Dry in the Dishwasher

Q: Why do plastic items in a dishwasher take so much longer to dry than similar items made of metal, glass or ceramic?

A: At least two physical processes are at work in the dishwasher. The first is perhaps easier to intuit: The dense and heavy metal, glass and ceramic items absorb and hold more heat during the washing process, and retain it for a longer time.

These items radiate heat, which evaporates residual water for a longer time after the heat is turned off inside the dishwasher.

In fact, many dishwashers no longer have a heating element for drying, instead relying on the heat from items exposed to the very hot washing water to dry the load of dishes. But plastic items quickly dissipate heat and tend to stay wet.

The second system involves the interaction of water and plastic at the surface where they intersect, a process called wetting out. How well a surface wets out depends on its surface energy compared with that of the liquid.

Substances like glass, metal and ceramic are held together with high-energy bonds, higher in energy than the bonds of water. When the lower-energy water surface meets the higher-energy solid surface, the water spreads in a thin, even film. Such substances are called hydrophilic.

But the kinds of plastic typically used in plastic containers, like polypropylene, have a lower surface energy and are called hydrophobic. On such surfaces, the water forms comparatively large droplets instead of spreading out. It takes more heat or more time to evaporate these droplets.