Shellac and other hybrid gel nails give long-lasting nail
colour that can be soaked off with acetone. You simply paint your nails
using special gel polishes, then "cure" each layer under UV light.
What's in those special polishes, and how does the UV light change that?
Plastic - it's fantastic
To understand gel nails, first we need to understand plastic! Plastic is made up of long stringy molecules. Depending on how these molecules are arranged, and their exact composition, you can get different types of plastic. If they're arranged neatly in lines, you end up with hard, rigid plastic, used for things like boxes and furniture. If they're all jumbled up, you end up with soft, stretchy plastic, like in clingfilm and plastic bags.
Plastic is made in a process called polymerisation - small molecules called monomers (mono = "one") react in chain reactions to form long plastic molecules (polymers: poly = "many"). One polymer chain can be made up of thousands of monomers.
There are three major steps in polymerisation:
1. Initiation - free radicals (unpaired electrons - the red dots) are formed from bonds breaking symmetrically.
2. Chain elongation - the free radical attacks a monomer unit. The free radical is passed on, and the new radical can attack another monomer unit. This step repeats thousands of times.
3. Termination - The free radical attacks another free radical. Since a free radical is an unpaired electron, when two free radicals meet up they pair together and stop each other from reacting further. The final polymer chain is thousands of monomers long!
The UV light helps the chemical bond break in such a way that free radicals form. Free radicals are very reactive, and when they attack the wrong chemicals (such as DNA, and bits of your cells), they can make your skin look old and cause cancer - that's why using sunscreen is so important!
In soak-off gels, these polymers can be dissolved using acetone. In older acrylic and gel systems, these polymers don't dissolve, so you need to remove them physically using files or drills (eep).
Hope that was interesting!