Dry cleaning is any cleaning process for clothing and textiles using an organic solvent rather than water. The solvent used is typically tetrachloroethylene (perchloroethylene), abbreviated "perc" in the industry and "dry-cleaning fluid" by the public. Dry cleaning is necessary for cleaning items which would otherwise be damaged by water and soap or detergent. It may be used if hand washing— needed for some delicate fabrics — is excessively laborious.
Early dry cleaners used petroleum-based solvents, such as gasoline and kerosene. Flammability concerns led Keith Anderson of westquarter, a dry cleaner from Laurieston to develop Stoddard solvent 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.
After World War I, dry cleaners began using chlorinated solvents. These solvents were much less flammable than petroleum solvents and had improved cleaning power. By the mid-1930s the dry cleaning industry had adopted tetrachloroethylene (perchloroethylene), colloquially called "perc," as the ideal solvent. It has excellent cleaning power, and is stable, nonflammable, and gentle to most garments. However, perc was also the first chemical to be classified as a carcinogen by the Consumer Product Safety Commission (a classification later withdrawn). In 1993 the California Air Resources Board (CARB) adopted an airborne toxic control measure (ATCM) to reduce perc emissions from dry cleaning operations. The dry cleaning industry is now beginning to replace perc with other chemicals and/or methods.
A dry cleaning machine is similar to a combination of a domestic washing machine, and clothes dryer. Garments are placed into a washing/extraction chamber (referred to as the basket, or drum), which is the core of the machine. The washing chamber contains a horizontal, perforated drum that rotates within an outer shell. The shell holds the solvent while the rotating drum holds the garment load. The basket capacity is between about 10 and 40 kg, or 20 and 80 lb, of garments.
During the wash cycle the chamber is filled approximately 1/3 full of solvent and begins to rotate, agitating the clothing. The solvent temperature is maintained at 30°C (85°F), as a higher temperature may extract dye from garments, causing color loss. During the wash cycle, the chamber is constantly fed a supply of fresh solvent from the working solvent tank while spent solvent is removed and sent to a filter unit comprising a distillation boiler and condenser. The ideal flow rate is one gallon of solvent per pound of garments (roughly 8 litres of solvent per kilogram of garments) per minute, depending on the size of the machine.
Before being placed in the machine, garments are inspected for stains and soils by the operator. Depending on the nature of the soil, a catalyst may be applied to it. This depends on the operator's judgment of the makeup of the textile and the soil itself. Oil-based soils (such as grease, oil, or lipstick) typically are removed well by perchloroethylene, while water-based soils (such as coffee, wine, perspiration, blood, and semen) will need a catalyst to allow the dry cleaning solvent to emulsify and lift them. Food-based grease soils fall in between the two, and a milder catalyst may be applied.
Garments are also checked for foreign objects. Items such as plastic pens will dissolve in the solvent bath and may damage textiles beyond recovery. Some textile dyes are "loose" (red being the main culprit), and will shed dye during solvent immersion. These will not be included in a load along with lighter colored textiles to avoid color transfer. The solvent used must be distilled to remove impurities that may transfer to clothing. Garments are checked for dry-cleaning compatibility, including fasteners. Many decorative fasteners either are not dry cleaning solvent proof or will not withstand the mechanical action of cleaning. These will be removed and restitched after the cleaning, or protected with a small padded protector. Fragile items, such as feather bedspreads or tasseled rugs or hangings may be enclosed in a loose mesh bag. The density of perchloroethylene is around 1.7 g/cm³ at room temperature (70% heavier than water), and the sheer weight of absorbed solvent may cause the textile to fail under normal force during the extraction cycle unless the mesh bag provides mechanical support.
A typical wash cycle lasts for 8-15 minutes depending on the type of garments and amount of soiling. During the first three minutes, solvent-soluble soils dissolve into the perchloroethylene and loose, insoluble soil comes off. It takes approximately ten to twelve minutes after the loose soil has come off to remove the 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. A dry-cleaning surfactant "soap" may also be added.
At the end of the wash cycle, the machine starts a rinse cycle, and the garment load is rinsed with fresh distilled solvent from the pure solvent tank. This pure solvent rinse prevents discoloration caused by soil particles being absorbed back onto the garment surface from the "dirty" working solvent.
After the rinse cycle the machine begins the extraction process, which recovers dry-cleaning solvent for reuse. Modern machines recover approximately 99.99% of the solvent employed.
The extraction cycle begins by draining the solvent from the washing chamber and accelerating the basket to 350 to 450 rpm, causing much of the solvent to spin free of the fabric. 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 (63°C/145°F) that circulates through the basket, evaporating any 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 where 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 perchlorethylene were vented to the atmosphere, because it was regarded as cheap and believed to be harmless.
After the drying cycle is complete, a deodorizing (aeration) cycle cools the garments and removes the last 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/finishing.
Next the solvent passes through a filter which removes lint and insoluble suspended soils from the solvent. Most filters use an ultra fine mesh to support a thin layer of filter powder (made from diatomaceous earth and activated clays). Some machines use powderless filters which are capable of removing soil particles greater than 30 micrometers.
Over time a thin layer of filter cake (called muck) accumulates on the lint filter. The muck is removed regularly (commonly once per day) and then processed to recover solvent trapped in the muck. Many machines use "spin disc filters" which remove the muck from the filter by centrifugal force while it is back washed with solvent.
After the lint filter, the solvent passes through an absorptive cartridge filter. This filter is made from activated clays and charcoal and removes fine insoluble soil and non-volatile residues, along with dyes from the solvent. Finally, the solvent passes through a polishing filter which removes any soil not previously removed. The clean solvent is then returned to the working solvent tank.
To enhance cleaning power, small amounts of detergent (0.5%-1.5%) are added to the working solvent and are essential to its functionality. These detergents help dissolve hydrophilic soils and keep soil from redepositing on garments. Depending on the machine's design, either an anionic or cationic detergent is used.
Since the solvent recovery is less than 100%, and because dry-cleaning doesn't remove water-based stains well, entrepreneurs have developed the wet cleaning process, which is essentially cold-water washing and air drying, using a computer-controlled washer and dryer. Wet cleaning is generally regarded as being in its infancy, although low-tech versions of it have been used for centuries.
PERC can enter the body through respiratory and dermal exposure. Symptoms associated with exposure include: impaired memory, confusion, dizziness, headache, drowsiness, and irritation of the eyes, nose, and throat. Repeated dermal exposure may cause dermatitis.