Q: I often clean paint brushes in white spirit and then keep the white spirit in glass jars with plastic lids. After a while, typically a few weeks, I find that the lids have bent inwards. They have not simply softened and sagged as they are quite hard. The effect is quite marked, with a depression of over 5 millimetres in a lid with a diameter of 7 centimetres. How and why?
A: The lid plastic is very probably a polyolefin, such as polyethylene, and the white spirit solvent is a mixture of paraffins. The plastic lid will absorb the solvent vapour, and be slightly softened by it, but it will not dissolve. It will, however, swell. As the relatively small solvent molecules migrate through the plastic, they will eventually reach the other side and evaporate. Therefore the concentration of solvent molecules will decrease from the inside of the lid to the outside. Therefore the inner surface will swell more than the outer surface and will deform the originally flat disc of plastic into a spherical shape. In other words, the lid will bend inwards.
This may give the impression of being sucked inwards. My guess is, however, that the contrary is the case: the atmosphere inside the jar will be compressed by the deformation of the lid. Were the questioner to fill the depression in the lid with soapy water and then perforate the lid in the middle with a sharp spike, he would see escaping air blow bubbles into the water and the lid would bulge inwards even more.
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