Why plastic covers on containers are compressed when microwaved?

Question

These situations happen a lot around me since I always bring my lunch to the school. It is not just plastic warp, but my plastic container lid too. Unlike Why does plastic wrap grow, then shrink, then grow again when microwaved?, it happens even when I am only microwaving my lunch, which doesn't contain a lot of liquid. It also happened when I microwaved my steaks, which are partly submerged in meat juices and other sauces, in a glass bowl with a plastic wrap on the top. The plastic warp "wrapped" against the steaks, but when I started to remove the wrap, there was no hot gas shooting out.

I am not getting the answer the aforementioned question provided since I don't understand how can the air inside the microwave and inside the container decrease so rapidly, and the "superheating" they mentioned is only about liquid at around boiling temperature. So could any one help me out here?

Answer 1

All food contains some water - it's mostly the water molecules that absorb the microwave energy.

As the water heats up, some will evaporate. Since there is already some air in the space above the food, the addition of water vapor will increase the pressure - and this will result in some "venting", with part of the air-vapor mixture escaping from the container.

Once you stop the microwave, the production of vapor stops (at least it slows down a lot). The air above the food cools down, leading to condensation of the vapor. But since a lot of the air escaped during the heating phase (see previous paragraph), now you have less volume of gas left - and the gas that is left will be cooler and occupy a smaller volume. Both these thinks lead to a "shrinking down" of the plastic cover.

Answer 2

There are a few things going on in your microwave. First, the microwave heats by vibrating water molecules. Second some water turns to steam and expands, this expansion will cause the container to swell (no matter what type of container) sealed lids tend to pop off when sufficient internal pressure is achieved. Third you are heating plastics up with the steam and contact with moist foods; many plastics tend to contract when critical temperatures are hit.

You may notice with some plastics, they will shrink in some areas and seem to expand in others; this is due to pressure differentials. The shrinking zones have little or no pressure and the expanded are pressurized.

Superheating is another phenomenon that happens when water is vaporized then is the vapor (steam) is heated. All matter has latent heat energy, as matter is heated it goes through phase change from solid, to liquid to vapour and finally plasma. The temperature doesn't change as phase change occurs no matter the energy applied but once water is vapourized it can be super heated by applying more heat and it will carry that extra energy until released as in a steam turbine generator.