Dry cleaning

Dry cleaning is any cleaning process for clothing and textiles using a solvent other than water. Clothes are instead soaked in a water-free liquid organic solvent typically inside a specialised dry-cleaning machine. The most commonly used solvent is Perchloroethylene, although other solvents such as hydrocarbon mixtures and decamethylcyclopentasiloxane are also used. Historical solvents include carbon tetrachloride, trichloroethylene, trichlorotrifluoroethane, trichloroethane and n-propyl bromide.
Most natural fibers can be washed in water but some synthetics react poorly with water and should be dry cleaned if possible. If not, this could result in changes in texture, colour, strength, and shape. Additionally, certain specialty fabrics, including silk, acetate and rayon, may also benefit from dry cleaning to prevent damage.

History

In the beginning of the 19th century, 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. Allegedly Jolly got his idea after an accident in 1825, perceiving that his table cloth became cleaner after an oil lamp's content was spilled on it. The oil inside was probably camphene. Over the 1800s, benzene-based compounds were also used as solvents. Early names for commercial dry cleaning were "french cleaning" and "chemical cleaning".
Flammability concerns led William Joseph Stoddard, a dry cleaner from Atlanta, to develop in 1924 Stoddard solvent as a slightly less flammable alternative to gasoline-based solvents. It was the dominant dry-cleaning solvent in the US until the 1950s when perchloroethylene became the dominant solvent.
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. Carbon tetrachloride was first used as a stain remover in the early 1890s in Germany. TCE was introduced in 1930, it had the downside of being incompatible with acetate dyes.
In 1930, chemists Sylvia Stoesser, John Grebe, and J. Lawrence Amos of Dow Chemical suggested using perchloroethylene in place of flammable hydrocarbon solvents in dry cleaning, following requests from industry representatives. A washing machine was purchased for the laboratory for research and testing, and all its rubber components were replaced with neoprene, which is resistant to organic solvents. Perchloroethylene's use in dry cleaning was adopted as early as 1933, when it was beginning to be considered the "ideal dry cleaning solvent." In the same year, a dry cleaning machine specifically for use with perchloroethylene and trichloroethylene was developed. By the mid-1930s, the dry cleaning industry had started to adopt perchloroethylene as the main solvent. It has excellent cleaning power and is nonflammable and compatible with most garments. Because it is stable, perchloroethylene is readily recycled by distillation.

Mechanism and process

Dry cleaning solvents selectively dissolve stains on the article. The solvents are non-polar and tend to selectively extract many compounds that cause stains. Some of these stains would otherwise only dissolve in aqueous detergent mixtures at high temperatures, potentially damaging delicate fabrics. Non-polar solvents are also good for some fabrics - especially natural fibres - as the solvent does not interact with any polar groups within the fabric. Water binds to these polar groups, resulting in the swelling and stretching of proteins within fibers during laundering. Also, the binding of water molecules interferes with weak attractions within the fiber which results in the loss of the fiber's original shape. After the laundry cycle, water molecules will evaporate. However, the original shape of the fibers has already been distorted and this commonly results in shrinkage. Non-polar solvents prevent this interaction, protecting more delicate fabrics. The usage of an effective solvent coupled with mechanical friction from tumbling effectively removes stains.
A dry cleaning machine is similar to a combination of a domestic washing machine and clothes dryer. Garments are placed in the washing or extraction chamber, which constitutes the core of the machine. The washing chamber contains a horizontal-axis, perforated drum that rotates within an outer shell. The shell holds the solvent while the rotating drum holds the garment load. During the wash cycle, the chamber is filled approximately one-third full of solvent and begins to rotate, agitating the clothing. During the wash cycle, the solvent in the chamber is passed through a filtration column and then fed back into the chamber. The solvent is then removed and sent to a distillation unit consisting of a boiler and condenser. The condensed solvent is fed into a separator unit where any remaining water and water-soluble impurities are separated from the solvent, and the refined solvent is fed into the clean solvent tank. The ideal flow rate is roughly 8 liters of solvent per kilogram of garments per minute, depending on the size of the machine.
A typical wash cycle lasts for 3–15 minutes depending on the type of garments and degree of soiling. During the first three minutes, solvent-soluble soils dissolve into the perchloroethylene and loose, insoluble soil comes off. It takes 10–12 minutes after the loose soil has come off to remove any ground-in insoluble soil from garments. Machines using hydrocarbon solvents require a wash cycle of at least 25 minutes because of the much slower rate of solvation of solvent-soluble soils. To enhance the solvent's cleaning power, small amounts of special dry-cleaning detergent may be added to the working solvent. These detergents emulsify hydrophobic soils and keep soil from redepositing on garments. Depending on the machine's design, either an anionic or a cationic detergent is used.
At the end of the wash cycle, the machine starts a rinse cycle where the garment load is rinsed with freshly distilled solvent dispensed from the clean solvent tank. This clean solvent rinse prevents discoloration caused by soil particles being deposited back into the garment from the "dirty" working solvent. After the rinse cycle, the machine begins the extraction process, which recovers the solvent for reuse. Modern machines recover over 90% of the solvent employed. The extraction cycle begins by draining the solvent from the washing chamber and accelerating the basket to 350–450 rpm, causing much of the solvent to spin free of the fabric. Until this time, the cleaning is done at normal temperature, as the solvent is never heated during the dry cleaning process. When no more solvent can be spun out, the machine starts the drying cycle.
During the drying cycle, the garments are tumbled in a stream of warm air that circulates throughout the basket, evaporating traces of solvent left after the spin cycle. The air temperature is controlled to prevent heat damage to the garments. The exhausted warm air from the machine then passes through a chiller unit where solvent vapors are condensed and returned to the distilled solvent tank. Modern dry cleaning machines use a closed-loop system in which the chilled air is reheated and recirculated. This results in high solvent recovery rates and reduced air pollution. In the early days of dry cleaning, large amounts of solvent vapors were vented to the atmosphere via exhausts because it was regarded as cheap and believed to be harmless. After the drying cycle is complete, a deodorizing cycle cools the garments and removes further traces of solvent by circulating cool outside air over the garments and then through a vapor recovery filter made from activated carbon and polymer resins. After the aeration cycle, the garments are clean and ready for pressing and finishing.
Not all stains can be removed by dry cleaning. Some need to be treated with spotting solventssometimes by steam jet or by soaking in special stain-remover liquidsbefore garments are washed or dry cleaned. Also, garments which have been stored in soiled condition for a long time are difficult to bring back to their original color and texture, since irreversible chemical reactions may occur over time.

Machinery

Dry-cleaning machines are classified into several generations:

First generation machines consisted of two separate units, used until the late 1960s. One unit was solely for the washing and other one was for drying, dry-cleaned clothes would be manually taken to the dryer unit by the operator. 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.
Second generation machines combined both washer and dryer functions, still vented vapours outside. Such machines are still called "dry-to-dry".
Third generation machines included mechanisms to reduce vapour emissions and recover the solvent.
Fourth generation machines included internal vapour recycling by capturing solvent vapours from the air inside the machine, often having carbon filters to clean used solvent. These machines were almost fully close-circuit. 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.
Fifth generation machines added special sensors to capture higher concentrations of solvent in the air. This generation of machines have automatic locks on the lid to prevent opening unless the solvent inside the chamber has been reduced to below 300 parts per million concentration in the air inside.