M&M's are known as the melt in your mouth candy, but sometimes they have a tendency to melt in your hand, your pocket, etc. Similarly, all lash adhesives dry on your lashes not in your bottle, this is due to a chemical called hydroquinone.
So what is it? Hydroquinone (pronounced hydro-queen-own or hydro-quin-own) is an organic compound found in many facial and skin care products.
How does it work? In short, it helps the adhesive last longer by controlling the reaction in the bottle. Cyanoacrylate (the main ingredient in lash adhesive) undergoes a chain reaction called polymerization when exposed to moisture (you can read more about that here). Moisture is necessary for the adhesive to cure but if it gets inside the adhesive bottle the adhesive will polymerize (or cure) inside the bottle. This could ruin a whole bottle. It's important to do our best to prevent that from happening, but with moisture in the air it’s almost impossible to prevent completely. This is where hydroquinone comes into play. The hydroquinone antioxidant is able to quench or stop the reaction inside the bottle.
During the adhesive curing process are free radicals created. Free radicals are very reactive and way too small to see with your eyes or even a microscope. Free radicals are like a high energy child running around touching everything in their path. They cannot be stopped until they have touched everything in the house. In this case, free radicals cause the chain reaction (or polymerization) to keep going until there is no more cyanoacrylate left. You don’t want this to happen inside the bottle of adhesive or you will have a bottle full of sticky, dry adhesive. Hydroquinone is called a free radical scavenger or antioxidant (not the kind of antioxidant you find in blueberries). Antioxidants are like the perfect toy your child needs to relax and stop the chaos.
Hydroquinone finds the free radicals that will cure the adhesive and stops the polymerization or curing process. This helps control the reaction in the bottle and allows the adhesive to last longer in the bottle. Although it is allowed by the United States in products at concentrations up to 4% by weight, our adhesives only contain less than or equal to 0.5% by weight.
Source: Mcintire, J. M., Jr, T. H. W., and Co, E. K. Patent US3527841 - Alpha-cyanoacrylate adhesive compositions. Google Books .