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Why Doesn’t Plastic Dry as Easily as Glass in the Dishwasher?

An Abstract reader recently sent me this question: “Why doesn’t plastic dry in the dishwasher? Or why doesn’t it dry as quickly/easily as glass? This drives me totally nuts.” Good question! That phenomenon drives me nuts too.

The answer appears to have a lot to do with the amount of energy (in the form of heat) that different sorts of tableware are able to absorb.

Ounce for ounce, plastics (such as children’s sippy cups) may be able to hold more heat than glassware or ceramic bowls. However, cups and plates made from ceramic and glass are denser and have more mass (i.e., they are heavier) than those made from plastic. This increased mass allows glassware and ceramics to absorb and hold more energy (heat) in the dishwasher – so they stay hot longer than plasticware. That’s why ceramic plates are usually hotter than plastic ones when you unload the dishwasher.

The fact that glasses and ceramics stay hotter for longer has two effects: first, water is more likely to condense on plastics (or at least not evaporate), because they have a cooler surface; second, any water on the hotter glass and ceramic surfaces is more likely to evaporate.

If you have any additional questions for the Abstract to answer, let us know in the comments!

Note: Many thanks to Dr. Doug Irving, assistant professor of materials science and engineering at NC State, for taking the time to talk with me about this subject. Any errors in the above post are mine alone.

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  1. I like Stells’s explanation and think it’s a combination of factors.
    I’m noticing that a plastic bottle from a humming bird feeder has been sitting there at room temp for almost 2 weeks, upside down, with drops inside it while it’s ‘cousin’ the glass bottle (slightly thicker walled) has was dry after a day.
    I’m going to sit it upright H2O vapour lighter than air to see what happens.
    Next trial might be playing it like a flute!

  2. The surface energy of the material is also relevant. i.e. the plastics are lower surface energy, which means the water beads up on them. The glass and ceramic cups and dishes have higher surface energy, so the water spreads out, giving it more surface area so the water evaporates more quickly.

  3. This drives me batty. Plastic dishware, in general, tends to be flimsier and as a result is designed with more curves and rolls for strength (e.g. the edge of a tupperware bowl or the bottom of plastic cup) where water can accumulate. That contributes to the poor drying as well.

  4. In this instance, I think mass is not the reason – it is a confounded variable. The real reason is the distribution of mass. Due to the thickness of the ceramic plates and bowls, their surface area to volume ratio is much higher than that of plastic, so the heat retention is much higher. The heat retention is also higher because of the density – even if the ceramic bowls and plates were the same thickness as the plastic ones, they would have more mass per square cm.

    Also, watch out about the “ounce for ounce” and then going on to talk about mass – one is weight, the other is mass (though I do understand that the former is also useful as an idiomatic expression, just making sure).

    What are your thoughts?