Nail shellac is a long wearing form of nail polish that is quick drying and resists chipping designed to be worn for 2 weeks prior to removal. It is made from a photocured polymer similar to that used for dental bonding, known as polymethyl methacrylate (PMMA) or Plexiglas. Three thin films are applied followed by exposure to UV radiation that cures the polymer. The UV radiation can only penetrate to a limited depth, thus the coats must be thin. Methacrylate monomers are polymerized with benzoyl peroxide as the initiator. The benzoyl peroxide induces hemolytic bond cleavage forming 2 oxygen radicals that decompose to carbon dioxide plus a phenyl radical adding one methacrylate double bond. The process repeats until 2 carbon-centered radicals combine to complete the polymerization. Once the monomers are completely polymerized, the shellac has been created. The thin film over the nail plate can be clear or pigmented to any desired shade. Nail shellac, also know as nail gel, is the most popular form of nail adornment in professional salons at present.
The biggest problem with nail shellac is removal. After 2 weeks, the new natural nail growth appears at the proximal nail fold the shellac begins to chip irregularly. The bond between the nail shellac the natural nail is stronger than the bond between the nail and nail beds making it impossible to peel, bite, or scrap off. At commercial nail salons, the nails are soaked in nail polish remover to loosen the shellac. This damages the natural nail plate, which is the biggest draw back to nail shellac. For this reason, dermatologists may observe onychoschizia, or lamellar nail plate peeling, in patients who wear gel nails. This results from the acetone induced nail dehydration and can be minimized by the use of urea or lactic acid containing moisturizers that open up water binding sites on the nail plate and facilitate temporary rehydration.