Gels consist of oligomors which are short chains of a few thousand monomers, the photoinitator and other additives to give colour, limit yellowing etc. When UVA hits the photoinitiators they break down to become free radicals and generate heat in what chemists call an exothermic reaction (heat producing). It's the heat and those crazy free radicals that helps the oligomers to connect into much longer chains to form polymers.
Before curing, the oligomers in the gel are unorganised - like a box of dominoes emptied on a table. After curing, the polymers are more organised and the cured gel is smaller - this difference in size is what we call skrinkage. If you imagine making a low wall of dominoes, you can understand that this takes less space on the table. If the dominoes are glued together, you can also appreciate that the wall would be much stronger than the glued unorganised dominoes which don't touch each other very much.
Some gels contain more photoinitators and some contain less. The decision is affected by what curing time is required, the amount of UVA provided to cure, and the chemical efficiency of the exothermic reaction. Using fewer photoinitators means the gel requires a longer curing time, or needs to be used with UV lamps that produce more UVA light on the nail (either by making the UV bulbs closer to the fingernails, or by using more powerful UV bulbs).
Are the chemists trying to kill us?
So we know why heat is produced, but perhaps the most interesting question is why do some gel burn? Did the chemist deliberately produce a material that hurts customers?!
Well know that if the gel is not applied correctly in too thick a layer, that curing will generate more heat. So it makes sense that if the gel is applied in several thin layers the heat will be reduced. But several thin layers of cured gel are also stronger than one thick layer (like rings in a tree, more thin rings produces stronger wood).
We also know that not all customers feel the heat in the same way. Customers with thinner nail plates have less of a barrier and protection against the heat reaching the nerve ends under the nail plate, and feel the heat more. I also suspect that men are more sensitive to pain than women, from my experience being a guinea-pig for gel tests!
But what about those gels that seem to generate too much heat even when applied correctly on customers who don’t have thin nail plates? An answer for this may be found in an old article by Doug Schoon. In this article, Doug mentioned that most gel lamps used in the USA were 8W, while in Europe it was more common to use 36W.
Now imagine that the chemist developed a gel for an 8W lamp, and put in just enough photoinitiators so the gel cured in a specific time. But then the gel was used in Europe with a 36W UV lamp that generated more than 4 times the amount of UVA light - that would make the chemical reaction faster and generate too much heat!
What this illustrates is the importance of matching the UV lamp to the gel (or developing the gel to match the UV lamp). If this UV lamp and the gel is not matched correctly, either you could suffer under-curing, over-curing and in some cases this would generate excessive heat.
So in summary, there are several possible reasons why a gel may burn:
1. Gel is applied too thickly
2. Customer has a thin nail plate
3. Customer may be more sensitive to heat
4. A UV lamp with too much UVA power is being used
5. The chemist was not very good, or the product wasn't tested properly ;-)
FootnoteDeveloping the unique IKON.iQ gels with our chemist helped me to better understand the difficulties to produce the perfect gel. Most low-heat gels in the market have a medium to thin viscosity, and some do yellow during weeks of wear. Trying to produce a higher viscosity gel that was good for sculpting, generated low heat, didn’t yellow and cured in a 36W UV lamp within two minutes was very difficult.
But more difficult still was trying to create a white sculptor gel that could be applied in one layer, would cure in two minutes and that would hold the c-curve shape during and after curing (i.e. very little shrinkage). From our experience, no other manufacturer had created a white gel with all these features.
We did finally succeed with all of our goals, but it took nearly two years and many chemical formulations. We were lucky to work with an exceptional and very patient chemist and also that as the company owners, we were willing and able to wait for as long as it took to create the gels we wanted.