The glues used to fix extension lashes to your natural lashes are only sold to professionals, and they are much stronger than glues you could buy for home use.
There are basically two types of professional glue. One which is called ‘non-flexible’ glue may come in black or clear versions and needs a sealant to be applied at the end of the treatment. The more modern type, called ‘flexible’ glue, is usually black and has includes rubber in its ingredients. Flexible glue doesn’t require any sealant. It also dries more quickly than the other type.
The most important ingredient in every make of glue is cyanoacrylate. This ingredient is what gives the glues their bonding strength. It’s the main ingredient of super-glues.
I referred earlier to the glue drying, but that isn’t strictly correct when talking about these cyanoacrylate based glues. What they do is ‘cure’. Far from losing more moisture and drying, they actually take in moisture in order to form a hard bond. So it’s necessary for there to be exactly the right amount of humidity in the atmosphere when the glue is applied. If the air is too dry, then the curing process can’t work properly and the glue can remain tacky. Then the bond is weak and the lash is at risk of falling off after a few days. The opposite problem is caused by too much moisture. This causes the glue to over cure, as they call it, becoming brittle and losing resilience, which may also lead to lashes falling off. The sort of humidity we have in the United Kingdom is not a problem most of the time.
The scientific explanation is that cyanoacrylate polymerises in the presence of water and starts to form long, strong chains which then join together the two surfaces being bonded. The glue has to be applied very thinly so that the reaction takes place smoothly throughout the material. Too thick and a crust forms on top which stops action continuing below.
The curing of the glue can also be affected by temperature. The room needs to be around 20° or so. If you are comfortably warm, the glue is probably happy to.
One reason why you should keep your eyes closed the whole time during treatment is because of the glue’s attraction to moisture. Fumes from the glue would be attracted to the moisture on your eyeballs and could then cause temporary reddening. One recommendation which has been put forward is that you shouldn’t talk during the eyelash extension process, because talking causes you to involuntarily flicker your eyeballs, thus opening them very slightly.