Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: iacopo.russo on 06/10/2021 13:51:35

Title: Why don't plastic food containers dry in a dishwasher?
Post by: iacopo.russo on 06/10/2021 13:51:35
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?
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Zer0 on 11/10/2021 10:04:38
Perhaps the surface has something to do with it.
Maybe Plastic ain't as smooth as Others.
( Not sure Atall, Wild Guessing )
🙏
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 11/10/2021 23:50:05
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Colin2B on 12/10/2021 00:06:06
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  ;D
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: wolfekeeper on 12/10/2021 01:36:32
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 12/10/2021 10:18:33
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Bored chemist on 12/10/2021 11:40:31
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Colin2B on 12/10/2021 14:36:01
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 12/10/2021 17:26:10
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. 
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 12/10/2021 17:32:26
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: hdsbcvjhjhg on 13/10/2021 02:59:16
plastic food container preserve the food for long time
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: wolfekeeper on 13/10/2021 05:59:39
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.
Quote
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Colin2B on 13/10/2021 08:51:51
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: vhfpmr on 13/10/2021 15:50:44
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Zer0 on 14/10/2021 19:33:41
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.
✌️
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 14/10/2021 19:34:20
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Bored chemist on 14/10/2021 19:39:25
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?
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 14/10/2021 19:39:52
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.   
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Bored chemist on 14/10/2021 19:42:26
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.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Zer0 on 14/10/2021 19:57:56
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?
🤔
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 14/10/2021 23:31:57
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.
I visualised the problem as the formation of inverted micelles, with water droplets surrounded by detergent molecules that stick to the plastic.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 14/10/2021 23:35:08
Hmm...so I've been washing & drying stuff incorrectly ever since.
😑
If you have been washing up by hand with cold water, you have been wasting water, detergent, and your own time.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Bored chemist on 15/10/2021 08:36:43
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.
I visualised the problem as the formation of inverted micelles, with water droplets surrounded by detergent molecules that stick to the plastic.
And, because they stick to the plastic, they spread out so that as many of them as possible are stuck to it.
That's how detergents and wetting agents work.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: alancalverd on 15/10/2021 11:13:09
So here's a development of the hypothesis.

Agreed we see water droplets on the surface of washed plastics because the residual water doesn't wet the plastic. I also note that detergents are less effective at removing grease from plastics than from inorganic materials: could just be that used plastics have more microscopic scratches but arguably the lyophilic end of the detergent molecule has some affinity for the plastic and sticks to it rather than dissolving in the grease. So during the washing phase the surfactant doesn't form the desired micelles, and the rinsing phase removes it because the hydrophilic end preferentially dissolves in the large excess of water leaving the hydrophobic plastic plus grease to form droplets.

On the other hand clean metals and ceramics have a low contact angle with water even in the absence of surfactant, so if they are degreased they will sustain a thin coating which evaporates easily.
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Bored chemist on 15/10/2021 13:36:26
The grease sticks preferentially to the ("oily")  plastic for essentially the same reasons that it sticks to the "oily" end of the detergent molecule.
That's why the detergents struggle to get plastic clean.

However, with or without a layer of grease, the water beads up on the plastic.
It doesn't do so on clean  ceramic or metal.

Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: Zer0 on 15/10/2021 18:32:55
Hmm...so I've been washing & drying stuff incorrectly ever since.
😑
If you have been washing up by hand with cold water, you have been wasting water, detergent, and your own time.

I've done Worse!
😑
I use a Washing Machine.

So i wasted Electricity which I'm aware comes from a Coal Plant, thereby polluting the environment.

Wasted bucket loads of non drinking water, which might have been useful for gardening or atleast drinkable for other animals.

Wasted gel liquid & detergent resources, almost like i made it rain on my dumbness.

Perhaps i should have just bought new clothes.
With covid19 there are boards up on shopping windows offering 50% discount.
Even like buy 2 & get 1 free.

Query - Is there any line of clothing that does not require cleaning & ironing?
(Except for raincoats & banana leaves)
Title: Re: Why don't plastic food containers dry in a dishwasher?
Post by: wolfekeeper on 16/10/2021 02:45:30
To be honest, I'm not finding that plastic dries significantly worse than ceramics. I find that if the object is set so the water flows off, it dries very well, or if there's a place for the water to collect, it will still be there at the end, and that's irrespective of the material.

I did have a big problem previously, but this went away when I started using rinse aid. AFAIK dishwashers usually have a setting for adjusting the amount of rinse aid used. If you're getting excessive drying marks then increase this. Also make sure you set things on a tilt so that the water rolls off.