Shellac is a product made from secretions of the lac insect, Kerria lacca. Chemically, shellac is a terpenic acid attached to a hydroxy acid. These shellac “units” (monomers) can spontaneously form polymers in solution. Polymerization of shellac monomers turns a “soft form” of shellac into a z’hard form” resin. Polymerization creates problems for applications such as pharmaceutical manufacture that require a uniform coating. What is a drug manufacturer to do? Add ammonium ions.
Shellac monomers will form a salt with ammonium ions that is resistant to polymerization. Unlike pure shellac which dissolves in ethanol but not water, the ammonium salt is water soluble. This allows a shellac coating to be applied to pharmaceuticals as an ammonium salt dissolved in water.
Many drugs must be absorbed through the intestines but are degraded in the acidic environment of the stomach. A shellac coating allows these drugs to pass through the stomach unscathed and into the intestines where the shellac coating disintegrates and releases the drug.