Paint stripper

Paint stripper, or paint remover, is the generic name given to solvent mixtures designed to remove paint and other finishes and also to clean the underlying surface. The principal active ingredient is usually dichloromethane. Formulations with orange oil (or other terpene solvents), N-methylpyrrolidone, esters such as dibasic esters (often dimethyl esters of shorter dicarboxylic acids, sometimes aminated, for example, adipic acid or glutamic acid), aromatic hydrocarbons, dimethylformamide, and other solvents are known as well. The formula differs according to the type of paint and the character of the underlying surface. Nitromethane is another commonly used solvent. Dimethyl sulfoxide is a less toxic alternative solvent used in some formulations.

Paint strippers come in a liquid, or a gel ("thixotropic") form that clings even to vertical surfaces.

The principle of paint strippers is penetration of the paint film by the molecules of the active ingredient, causing its swelling; this volume increase causes internal strains, which, together with the weakening of the layer's adhesion to the underlying surface, leads to separation of the layer of the paint from the substrate.

Other components
Various co-solvents are added to the primary active ingredient. These assist with penetration into the paint and its removal and differ according to the target paint. Ethanol is suitable for shellac, methyl ethyl ketone is used for cellulose nitrate, and phenol and cresols are employed in some industrial formulas. Benzyl alcohol is used as well.

Activators increase the penetration rate; for dichloromethane water is suitable, other choices are amines, strong acids or strong alkalines. The activators role is to disrupt the molecular and intermolecular bonds in the paint film and assist with weakening it. Their composition depends on the character of the paint to be removed; mineral acids are used for epoxy resins to hydrolyze their ether bonds. Alkaline activators are usually based on sodium hydroxide. Some cosolvents double as activators. Amine activators, alkalines weaker than inorganic hydroxides, are favored when the substrate could be corroded by strong acids or bases.

Surfactants assist with wetting the surface, increasing the area of where the solvent can penetrate the paint layer. Anionic surfactants (eg. dodecyl benzene sulfonate or sodium xylene sulfonate) are used for acidic formulas, cationic or nonionic are suitable for alkaline formulas. Paint strippers containing surfactants are excellent brush cleaners.

Thickeners are used for thixotropic formulas to help the mixture form gel that adheres to vertical surfaces and to reduce the evaporation of the solvents, thus prolonging the time the solvent can penetrate the paint. Cellulose-based agents, eg. hydroxypropyl cellulose, are commonly used for mixtures that are not extremely acidic or basic; under such conditions cellulose undergoes hydrolysis and loses effectivity, so fumed silica is used for these instead. Another possibility is using waxes (usually paraffin wax or polyethylene or polypropylene derivates), or polyacrylate gels.

Corrosion inhibitors are added to the formula to protect the underlying substrate and the paint stripper storage vessel (usually a tin can) from corrosion. Dichloromethane decomposes with time to hydrochloric acid, which readily reacts with propylene oxide or butylene oxide and therefore is removed from the solution. Chromate-based inhibitors give the mixture a characteristic yellow color. Other possibilities include polyphosphates, silicates, borates and various antioxidants.

Sequestrants and chelating agents are used to "disarm" metal ions present in the solution, which could otherwise negatively impart the efficiency of other components, and assist with cleaning stains, which often contain metal compounds. The most common sequestrants used in paint strippers are EDTA, tributyl phosphate, and sodium phosphate.

Colourants are added in order to make the substance look different from the competitors, and/or to make it easier to see to which areas the remover has been applied.

Alternatives
Hot air guns are an alternative to chemical paint strippers.