Dry cleaning is any cleaning process for clothing and textiles using a solvent other than water.
Dry cleaning still involves liquid, but clothes are instead soaked in a water-free liquid solvent (usually non-polar, as opposed to water which is a polar solvent). Tetrachloroethylene (perchloroethylene), known in the industry as "perc", is the most widely used solvent. Alternative solvents are 1-bromopropane and hydrocarbons.
Most natural fibers can be washed in water but some synthetics (e.g., viscose, lyocell, modal, and cupro) react poorly with water and should be dry-cleaned if possible.
History
The ancient Greeks and Romans had some waterless methods to clean textiles, involving the use of powdered chemicals and absorbent clay (fuller's earth). By the 1700s, the French were using turpentine-based solvents for specialized cleaning.
Modern solvent-based dry cleaning may have originated in 1821 with American entrepreneur Thomas L. Jennings. Jennings referred to his method as "dry scouring".
French dye-works operator Jean Baptiste Jolly developed his own method using kerosene and gasoline to clean fabrics. He opened the first dry cleaning service in Paris in 1845.
Flammability concerns led William Joseph Stoddard, a dry cleaner from Atlanta, to develop in 1924 Stoddard solvent (white spirit) as a slightly less flammable alternative to gasoline-based solvents.
The use of highly flammable petroleum solvents caused many fires and explosions, resulting in government regulation of dry cleaners.
Shift to chlorinated solvents
After World War I, dry cleaners began using chlorinated solvents. These solvents were much less flammable than petroleum solvents and had improved cleaning power. Early solvents were carbon tetrachloride and trichloroethylene (TCE), but they gradually were phased out as their adverse health effects became more known. TCE may still occasionally be used for spot cleaning of difficult stains.
By the mid-1930s, the dry cleaning industry had started to use tetrachloroethylene (also called perchloroethylene or PCE) as the solvent. It has excellent cleaning power and is nonflammable and compatible with most garments. Because it is stable, tetrachloroethylene is readily recycled, but it is persistent if released into the environment.
Infrastructure
From the customer's perspective, dry cleaning businesses are either "plants" or "drop shops". The former does on-site cleaning, while a drop shop receives garments from customers, sends them to a large plant, and then has the cleaned garments returned to the shop for pickup by the customer. The latter setup minimized the risk of fire or dangerous fumes created by the cleaning process. At the time, dry cleaning had been accomplished by using two separate machines—one for the cleaning process and the second to remove the solvent from the garments.
Machines of this era were described as "vented"; their drying exhausts were expelled into the atmosphere, the same as many modern tumble-dryer exhausts. This contributed to environmental contamination, and much potentially reusable solvent was lost to the atmosphere. Today, much stricter controls on solvent emissions have ensured that all dry cleaning machines in the Western world are fully enclosed, and no solvent fumes are vented to the atmosphere. In enclosed machines, solvent extracted during the drying process is recovered and purified by distillation, so it can be reused to clean further loads or safely disposed of. Most modern enclosed machines also incorporate a computer-controlled drying sensor, which automatically senses when all detectable traces of PCE have been removed. This system ensures that only small amounts of PCE fumes are released at the end of the cycle.
Mechanism
In terms of mechanism, dry cleaning selectively solubilizes stains on the article. The solvents are non-polar and tend to selectively extract many compounds that cause stains. These stains would otherwise only dissolve in aqueous detergent mixtures at high temperatures, potentially damaging delicate fabrics.