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why do plastic containers not get dry in the dishwasher?

Posted on 2004-08-29
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Last Modified: 2008-03-06
I often put Rubbermaid and Tupperware plastic containers through my dishwasher (standard cycle with heated dry), and even though the ceramic dishes and steel flatware come out completely dry, the plastic still needs to be dried by hand or air dried. Why would this be? What is it about plastic that causes it to dry so slowly, even under heat?

Just curious.
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Question by:graham_charles
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by:gn0
ID: 11928654
conductivity -- the ceramics / steel are much better conductors of heat compared to plastics / rubber .... so they will dry faster ..... they will conduct the heat faster into the droplets of water holding on to the surface of the object.... whereas with plastics heat will be conducted more slowly....

GN.
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by:aburr
ID: 11928674
larger drops of water also tend to stick to the plastic.
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by:Arthur_Wood
ID: 11933422
gn0 close - what is really involved is called the Heat Capacity - Ceramics tend to hold the heat (from the hot water), longer than does the plastic.  Since the Ceramic plates remain HOT, the water droplets will evaporate, while the low heat capacity of the Plastic means that they do not retain enough heat to cause the larger adhering water droplets to evaporate.

AW

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by:gn0
ID: 11937829
I think its a combination of the 3 - but mainly the amount of water that will hold to the surface (as aburr said) - non-stick (fully plastic type) materials would dry faster ..... the higher heat capacity of the metal / creamic would help - however this is not the most significant cause for the results, since, as u can calculate - the heat capacity of the metal / ceramics is not that much compared to the heat capacity + latent heat of vaporisation of the water that will hold to the surface.

GN.
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thanasis57 earned 500 total points
ID: 11939356
There are two parameters to consider:
A) Heat capacity
B) Heat conductivity

A) The heat exchanged by the temperature change of a body is given by
Q=c*m*deltaT,
where c is the per mass heat capacity, m its mass, and deltaT the temperature difference from heating/cooling.

If we consider that deltaT is common for all types of containers (they are heated in the same dry cycle at the same temperature), we are left with c and m as probable reasons of different behaviour.

The higher the c, the more heat will be stored in the material.
The higher the mass, also more heat will be stored in the container's material.

So,
a) if your plastic has a lower c than that of metals and ceramics, and (ie steel 450, glass 840 J/(kg.T))
b) if it has thinner walls (as it usually happens for thus type of containers) which means smaller m

they will have less thermal energy to give to the water drops in order to transform them to steam.

B) But the most important reason is heat conductivity, ie the the heat flux within a body.

If you check out site: http://www.hukseflux.com/thermal%20conductivity/thermal.htm,
you will see that stainless steel (16 W/m.K), glass (0.93 W/m.K), pyrex (1005 W/m.K) have much higher heat conductivities than polymers used in food industry (ie PVC 0.016 W/m.K).

The higher the heat conductivity of a material, the higher the rate that heat from inside the body will replenish the surface heat taken away by water, so water will evaporate much more quickly.

Hope that helps
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