Eli5: How come paint dries on walls but not in a paint can?
In: Chemistry
There’s two main kinds of paint:
Solvent based paints. A common solvent (something than can dissolve things) is water, so you may have heard of water based paints. In water based paints, the the dye is mixed in with the water. These paints dry as the water evaporates away, leaving just the dye behind. The water can evaporate really quickly when spread thinly on a surface. In a container, it will slowly evaporate as it can only evaporate from the surface. In a sealed can, it will evaporate, but because the can is sealed, the humidity rises. Once the humidity in the case rises, no more water can evaporate. Sometimes other solvents other than water are used, but the idea is the same evaporation. Just don’t drink or inhale these other solvents, not safe like water.
The second kind of paints are oil based paints. The dye is mixed into a special type of oil. These don’t evaporate, but rather a chemical reaction occurs with oxygen in the air to harden the oil into a solid. The plastic, rubber like layer you’ve seen some paints turn into. Basically the same idea again for why a cam doesn’t “dry”. A thin layer on a wall exposes all the oil to oxygen in the air, quickly “drying” all of it. In a container, really only the top layer is exposed to oxygen so you get a film on the top. In sealed can, the top will react with the oxygen, but it will quickly run out of oxygen in the small amount of air trapped in the can.
Exposure to air.
If you leave a can of paint open and don’t stir it the top layer will dry too (or depending on the type of paint some evaporates and it slowly gets thicker)
Liquids dry through evaporation. With an airtight lid, there’s no evaporation. If you opened a can of paint and left it with its lid open, it would eventually dry out
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