Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

[s]Lots of polymers dissolve very easily in certain solvents. For example, if you place a polystyrene coffee/tea cup into a petri dish with a thin layer of acetone (I think) the whole thing just sinks in and totally disappears into a goo.

A similar thing will happen, but obviously not to the same extent, if the adhesives contain anything that will react with the surface of the helmet.

I don't know anything specific, but the stickers that come with the helmet are going to be made with an adhesive that won't react, or that does react but the rate of reaction is so slow that the helmet will be knackered before the consequences of the reaction are noticeable.

I thought if you got polystyrene balls and put them in a bucket of petrol they dissolved, and if you kept doing it till no more would dissolve the petrol/polystrene mix would be thick and sticky. And you would have made napalm.