Hair sprays are formulated to hold hair in place and keep it shiny, without flaking off, without failing in humid conditions, and yet retain the ability to be washed out of the hair easily for at least 48 hours.
Complex polymers are needed to perform all of these tasks well.
The basic building blocks of these polymers are the same ones found in acrylic paints and white glue, but they are put together in such a way that they are flexible, shiny, non-clogging in the spray can nozzle, dry quickly, and remain waterproof yet able to be washed out of the hair.
The two main polymers used are
Vinyl Acetate/Crotonates/Vinyl Neodecanoate Copolymer,
(a form of vinyl acetate) and
Octylacrylamide/Acrylates/Butylaminoethyl Methacrylate Copolymer,
a form of methacrylate.
These polymers are carried in a base of water and alcohol (usually denatured), and the liquified gas dimethyl ether which acts as the propellant. The dimethyl ether boils away as a gas when the spray head is pushed down, propelling the rest of the ingredients out in a fine mist.
The emulsifier aminomethyl propanol serves several purposes in hair sprays. It acts as a "buffering" agent, controlling the acidity of the mixture to make it neutral ("pH balanced"). It also helps keep the polymers mixed with the water and alcohol, and controls the water-solubility of the final mist, giving it the humidity-resistance needed. It also helps to form the polymers into a gel.
Sodium benzoate is used as a preservative, although it also functions as a corrosion inhibitor.
Cyclopentasiloxane is a volatile silicone used to make the film water repellent, and to eliminate tackiness. It is a thickener; it acts as a lubricant and gives the resulting coating a silkier feel.
Sodium PCA is a moisturizing agent, drawing moisture from the air and holding it.
Some sprays include vitamins such as tocopherols (Vitamin E) or panthenol, which is metabolized in the skin to become pantothenic acid, a B vitamin.
Since hair does not metabolize (it's dead, Jim), these perform the functions of anti-oxidants (tocopherols), and add shine and moisture (panthenol), rather than perform their normal vitamin roles. Moisture helps prevent damage during combing.
Hydrolyzed soy proteins are also sometimes added. These are more commonly known as soy sauce and MSG (monosodium glutamate), an amino acid.
Butylene glycol is sometimes used as a humectant (moisturizer) and preservative if the spray contains food items such as vitamins and proteins. It also helps to retain scents.