Cause of Plastic Container Deformation in Microwave Oven

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Discussion Overview

The discussion revolves around the deformation of a plastic container in a microwave oven, exploring the causes of this phenomenon, particularly when the container is heated with a lid on. Participants examine the implications of steam pressure, thermoplastic properties, and potential safety concerns regarding food contamination.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant describes their experience of a plastic container imploding in the microwave and questions the role of depressurization and molecular deformation of the plastic.
  • Another participant explains that steam can form in a closed container, leading to high pressure that may soften the plastic, potentially causing it to deform when the steam cools and creates a vacuum.
  • Some participants note that certain containers are designed to be microwave-safe and may have features like vents to prevent pressure buildup.
  • Concerns are raised about the potential for chemicals to leach into food when plastic is heated, with some participants discussing the implications of thermoplastic deformation and surface treatments.
  • One participant expresses skepticism about the health risks associated with plastic deformation and leaching, suggesting that the amounts are negligible.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the mechanisms of deformation, the safety of using plastic containers in microwaves, and the potential health risks associated with chemical leaching. No consensus is reached on these points.

Contextual Notes

The discussion includes various assumptions about the properties of different plastics, the conditions under which they deform, and the safety of using certain containers in microwaves. Specific definitions of "microwave-safe" and the implications of steam pressure are not fully resolved.

Who May Find This Useful

This discussion may be of interest to individuals concerned about food safety, those using microwaves for cooking, and anyone interested in the properties of materials used in kitchenware.

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Greetings,

Earlier this afternoon I managed to ruin a perfectly good plastic container in a microwave oven by unwittingly deforming it. I curious as to why it happened.
All I did was warm it (and the chilli inside it) for about a minute with the lid firmly secure. I've been told this was a stupid move and to never attempt warming things with the lid in place ever again. But I'm not convinced. Upon removing it, I noticed that it had imploded. Depressurization doesn't make sense in this case, unless my knowledge of EM interactions is laughably bad. A lot of Googling turned up molecular deformation of the plastic itself as a possibility (but none of the aforementioned lid warning), but why would the plastic deform in the shape of a cavity and not with the lid removed?
 
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A lot of containers are thermoplastic, they are formed by heating the plastic and injecting into a mold.

Water gets heated by microwaves and a high pressure steam will form in a closed container.
For example, don't put uncracked eggs in the microwave.
They will explode.

The steam is hot enough to make the plastic soften.
Some escapes the container, if it dosn't blow the lid off and make a mess in the first place. When the microwave shuts down the steam cools quickly creating a vacuum.
Then the container cools below its forming temp giving the imploded shape.

With lid off the hot gas quickly leaves the container so that it doesn't get a chance to heat the plastic.
 
NoTime said:
A lot of containers are thermoplastic, they are formed by heating the plastic and injecting into a mold.

Water gets heated by microwaves and a high pressure steam will form in a closed container.
For example, don't put uncracked eggs in the microwave.
They will explode.

The steam is hot enough to make the plastic soften.
Some escapes the container, if it dosn't blow the lid off and make a mess in the first place. When the microwave shuts down the steam cools quickly creating a vacuum.
Then the container cools below its forming temp giving the imploded shape.

With lid off the hot gas quickly leaves the container so that it doesn't get a chance to heat the plastic.

Smart guy ^ ^ ^
 
NoTime said:
A lot of containers are thermoplastic, they are formed by heating the plastic and injecting into a mold.

Water gets heated by microwaves and a high pressure steam will form in a closed container.
For example, don't put uncracked eggs in the microwave.
They will explode.

The steam is hot enough to make the plastic soften.
Some escapes the container, if it dosn't blow the lid off and make a mess in the first place. When the microwave shuts down the steam cools quickly creating a vacuum.
Then the container cools below its forming temp giving the imploded shape.

With lid off the hot gas quickly leaves the container so that it doesn't get a chance to heat the plastic.

Thanks a lot, that makes perfect sense.
 
NoTime said:
A lot of containers are thermoplastic, they are formed by heating the plastic and injecting into a mold.

Water gets heated by microwaves and a high pressure steam will form in a closed container.
For example, don't put uncracked eggs in the microwave.
They will explode.

The steam is hot enough to make the plastic soften.
Some escapes the container, if it dosn't blow the lid off and make a mess in the first place. When the microwave shuts down the steam cools quickly creating a vacuum.
Then the container cools below its forming temp giving the imploded shape.

With lid off the hot gas quickly leaves the container so that it doesn't get a chance to heat the plastic.

That was a very practical response, you must be an engineer. :wink:
 
Or a Chef! ;)
 
I manage to do a little of both.
Thanks for the kind words :smile:
 
Some plastic containers (like styrofoam that to go food is often served in) will just start to melt due to the heat.

In the cases where a closed lid makes a difference, the other factor is that a lot of heat will be trapped inside the container, causing it to soften quite a bit.

If the container is micro-wave compatable, then a nearly closed lid might seal up and become concave as the air/steam inside cools, but not the rest of the container which is stiffer than the lid. With a fully closed lid, the lid will be popped loose (hopefully), or the container will expand.
 
There are microwaveable plastic containers, if heating with a lid on is important (as it si in the case of chilli). TupperwareTM makes a bowl with a lid that has a vent in it and will release pressure without splattering all over the place. That one's called an "Oval Cooker", but there are many kinds of microwavable plastic bowls.

It's true you should never nuke any plastic bowl without the words "microwave safe" associated with it. This time you just got deformed plasticware; next time it might explode in there. And you probably ended up eating some plastic with that chilli.
 
  • #10
Personally, I am worried about chemically altering the plastic and having chemicals leach into the food.
 
  • #11
DaveC426913 said:
Personally, I am worried about chemically altering the plastic and having chemicals leach into the food.

Why? What is leaching? It is thermoplastic deformation.


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  • #12
Why? What is leaching? It is thermoplastic deformation.
Could be a surface treatment or other compound mixed in with the plastic that vents if heated.
 
  • #13
Eh, its so minute its not going to kill you. :-p
 
Last edited:

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