Title, for example teeth fillings or glass glue. How or why is it getting hard / solid just with UV light?
Thanks!
In: Physics
It’s not necessarily that things harden especially readily in UV light, it’s that we select certain chemicals that do because it’s useful to us.
Something that hardens in UV will remain liquid in most conditions until we supply a relatively easy and safe source of UV.
Compounds that harden in visible light aren’t particularly useful to us, as they’d harden before we could use them.
Compounds that harden with difficult or dangerous components like another liquid, or radiation aren’t much use to us either.
Compounds that stay liquid until we supply a safe external stimulus are useful, so we see their application all around us
The resin contains things called “plasticizers”, which are basically any component that’s in there to keep something soft/liquid. These plasticizers sort of “accompany” the long chains of polymers, and help keep them separated so the long chains can move around each other and not get tangled or stuck together.
The plasticizers in some resins are formulated so that when hit with UV light, the light will cause them to get excited in such a way that they break apart. When the plasticizers break apart, they aren’t good at “accompanying” the long polymer chains, so the polymer chains become tangled with one another, and then they may even begin cross-linking (imagine joining 2 chains together… but in the middle, not end-to-end).
What you’re left with is a mess of tangled and cross-linked long chain molecules — or cured resin.
The UV light causes a chemical reaction, which makes the liquid hard.
You can imagine a bit like a photo film. Where the light hit, the film turns dark, as a chemical reaction is happening where the light hit. For your UV hardening polymer, it’s not the color that change, but it turns from liquid to hard if light hits on it.
Latest Answers