[iacopo.russo (OP)]

Ben wrote to us to ask:

Why don't plastic food containers dry in a dishwasher? All porcellain, glass and metal items get properly cleaned and subsequently dried, but the drying part fails consistently with the usual food containers of various brands and age.

What do you think?

[Zer0]

Perhaps the surface has something to do with it.
Maybe Plastic ain't as smooth as Others.
( Not sure Atall, Wild Guessing )
🙏

[alancalverd]

Interesting how the simplest of everyday observations has drawn a complete blank from all the resident geniuses, but everyone has an opinion on dark matter, the evolution of consciousness, and the quantisation of time!

And just for the record, I don't know either.

[Colin2B]

Perhaps the surface has something to do with it.
Maybe Plastic ain't as smooth as Others.

Dishwashers (including me when I do them by hand) rely on retained heat to do the final drying. Glass & ceramics retain heat, plastics don’t.

PS Alan knew it all along, but was just toying with you 

[wolfekeeper]

That is a factor, but I think it's more to do with the fact that plastic is, comparatively, a thermal insulator. Ceramics, metal and glass are much better conductors of heat so they can conduct the heat to help evaporate the water away. Plastic items tend to  act as thermal insulators which stops the heat from the rest of the item from reaching any drops of water that haven't flowed off.

[alancalverd]

For once, I agree with the wolfman. The specific heat capacity of plastics is higher than most metals and the conductivity much lower, so they don't get as hot when splashed with hot water, nor do they release their heat quickly at high enough temperature  to evaporate the surface water in what is still a steamy atmosphere.

Also worth considering the surface tension effect. Metals and ceramics can be fully wetted with a small  amount of residual surfactant, so the remaining water spreads into a uniform thin film with a large surface to volume ratio. Polyethylene in particular is very difficult to wet so the water forms large droplets that take longer to evaporate.

[Bored chemist]

Practically nothing has a high enough heat capacity to store the energy needed to evaporate a significant amount of water.

The surface tension difference has another vitally important effect.
It lets the water drain off ceramics and metals well.

[Colin2B]

The surface tension difference has another vitally important effect.
It lets the water drain off ceramics and metals well.

That’s a good point, the plastic boxes we put in are generally horizontal or with only a small slope whereas plates and cutlery are vertical. I don’t think it needs to evaporate much water as there is often only a few drops.
When you open the dishwasher when hot there is a lot of steam so no chance of evaporation. When it has cooled it is dry inside even on the horizontal surfaces of the machine. Is there a separate dry heat cycle?
Time to talk to a manufacturer methinks.

[alancalverd]

Practically nothing has a high enough heat capacity to store the energy needed to evaporate a significant amount of water.

It's not only heat capacity but thermal diffusivity that determines how much heat the object acquires during the wash cycle, and its surface temperature a few minutes afterwards. The surface water layer at the end of the cycle won't be much different between materials since they all get sprayed by the same mechanism. 

[alancalverd]

Is there a separate dry heat cycle?

Some washers have a dry heat cycle, sometimes fan-assisted, but most just use a very hot final rinse. Then as the whole unit cools down, the surface water equilibrates to saturation at whatever the air temperature is when you open the box.

[hdsbcvjhjhg]

plastic food container preserve the food for long time

[wolfekeeper]

Practically nothing has a high enough heat capacity to store the energy needed to evaporate a significant amount of water.

Absolutely. Although it helps to have hot water, for the water to evaporate away on its own, it would have to be supercritical. I don't know about you, but my dishwasher is only run at 70C, not 373C.

The surface tension difference has another vitally important effect.
It lets the water drain off ceramics and metals well.

That's what rinse aid does of course. It's a mild detergent that the dishwasher adds during the final rinse. It wets all the surfaces and most of the water forms a film and flows off the material. Some tablets say they have built-in rinse aid. Although there may be small traces left after the main wash, it's mostly a lie and you should basically always use separate rinse aid. Rinse aid considerably helps get plastic dry and spot-free.

So the answer to the question may well be: because he's not using rinse aid.

[Colin2B]

So the answer to the question may well be: because he's not using rinse aid.

We use separate rinse aid for the very reason you gave and still get water on plastic.

I began to wonder whether it might be something about the surface of plastic allowing water to condense so I searched on net and found a number of consumer reports which say the same thing as this one:

““Adam Hofmann, Director of Engineering, Dishwasher Advanced Systems at GE Appliances, gave Reader’s Digest the inside scoop. These are the items you should never out in your dishwasher.
“Plastic items are more difficult to dry because they have a lower thermal mass than ceramic or metal items in the dish load, causing them to cool faster than the other items during the dry portion of the cycle,” says Hofmann. “Once the surface of a dish cools, the moisture on the surface is less likely to evaporate into the air and be removed by the dishwasher dry system.””

I guess he must be wrong.

[vhfpmr]

I have some polythene water bottles that I use on the bike, and some  polyethylene terephthalate squash bottles that I use when I'm walking. When I've finished with them I rinse them out under the cold tap, empty them, drain them, then leave them on the worktop to dry.

The PE bottles take a few hours, the PET bottles take about a week.

[Zer0]

OffTopic -

This might sound Dumb...

But does the Water have to be Heated?

Washing machines & Dish washers won't/don't work with lukewarm or cold water?

Say, what if Hair dryers just spewed out Air at room temperature & not copper coiled hot air?
The drying process would take longer, that's all, Right?

Ps - i got nothin against Heat, I'm all for it...i cannot stand the Cold.
✌️

[alancalverd]

Finally came up with an argument that convinces me, at least.

Detergent molecules have a lyophilic (grease-loving) end and a hydrophilic (water-loving) end, thus dispersing grease lumps into the water. Problem with polyethylene is that it looks like grease, so (a) it's very difficult to completely remove food residues because the detergent can't distinguish between soil and substrate, and (b) any residual detergent will hang on to the plastic at one end, and water  at the other.

[Bored chemist]

I have some polythene water bottles that I use on the bike, and some  polyethylene terephthalate squash bottles that I use when I'm walking. When I've finished with them I rinse them out under the cold tap, empty them, drain them, then leave them on the worktop to dry.

The PE bottles take a few hours, the PET bottles take about a week.

Are the  the same size and shape?

[alancalverd]

But does the Water have to be Heated?

Dishwasher tablets and solutions are designed to work at higher temperatures and deliver a higher pH than handwashing detergents. Their ability to scour and saponify grease is greater than you can achieve with a simple surfactant, and is positively harmful to bare hands.   

[Bored chemist]

Finally came up with an argument that convinces me, at least.

Detergent molecules have a lyophilic (grease-loving) end and a hydrophilic (water-loving) end, thus dispersing grease lumps into the water. Problem with polyethylene is that it looks like grease, so (a) it's very difficult to completely remove food residues because the detergent can't distinguish between soil and substrate, and (b) any residual detergent will hang on to the plastic at one end, and water  at the other.

Well, it convinces you, but it's plainly wrong.
If that happened then the detergent would be doing its job as a wetting agent and the water would spread out, rather than beading up in droplets.

[Zer0]

Thanks Alan!

Hmm...so I've been washing & drying stuff incorrectly ever since.
😑
DuH!

I only heat water for 3 months...Geyser.
Winters for Bathing.
& For cooking, that's all.

BTW - is this OP a new International participation or collaboration invitation from/for the Russians by any chance?
🤔