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Dry cleaning is any cleaning process for clothing and textiles using a chemical solvent other than water. The solvent used is typically tetrachloroethylene (perchloroethylene), which the industry calls "perc" or "PERC". It is used to clean delicate fabrics that cannot withstand the rough and tumble of a washing machine and clothes dryer; it can also eliminate labor-intensive hand washing.
The ancient Romans used ammonia (derived from urine) and fuller's earth to launder their woolen togas. Fullonicae were very prominent industrial facilities, with at least one in every town of any notability, and frequently the largest employer in a district. These laundries obtained urine from farm animals, or from special pots situated at public latrines. The industry was so profitable that fuller's guilds were an important political constituency, and the government taxed the collection of urine.
Modern dry cleaning uses non-water-based solvents to remove soil and stains from clothes. The potential for using petroleum-based solvents such as gasoline and kerosene was discovered in the mid-19th century by French dye-works owner Jean Baptiste Jolly, who noticed that his tablecloth became cleaner after his maid spilled kerosene on it. He subsequently developed a service cleaning people's clothes in this manner, which became known as "nettoyage à sec"—i.e., dry cleaning.
Flammability concerns led William Joseph Stoddard, a dry cleaner from Atlanta, 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), or "perc" for short, as the ideal solvent. It has excellent cleaning power and is stable, nonflammable, and gentle to most garments. Perc, however, was incidentally the first chemical to be classified as a carcinogen by the Consumer Product Safety Commission (a classification later withdrawn).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 adopted regulations to reduce perc emissions from dry cleaning operations; the same year, the U.S. Environmental Protection Agency (EPA) followed suit. The EPA updated its regulation in 2006 to reflect the availability of improved emission controls.
The dry cleaning industry is in the process of replacing perc with other chemicals and/or methods. Between 2006 and 2011, the dry cleaning industry had an average market of $7.5 billion, 22,000 firms, and employed approximately 150,000 workers. A typical dry cleaning firm employs 7 workers and brings in $330,000 per year.
Traditionally, the cleaning process was carried out at centralized factories; high street cleaners' shops received garments from customers, sent them to the factory, and then had them returned to the shop, where the customer could collect them. This was due mainly to the risk of fire or dangerous fumes created by the cleaning process. At this time, dry cleaning was carried out in two different machines—one for the cleaning process itself and the second to dry the garments.
Machines of this era were described as vented; their fumes and drying exhausts were expelled to the atmosphere, the same as modern tumble-dryer exhausts. This not only contributed to environmental contamination but also much potentially reusable perc was lost to the atmosphere. Much stricter controls on solvent emissions have ensured that all dry cleaning machines in the Western world are now fully enclosed, and no solvent fumes are vented to the atmosphere. In enclosed machines, solvent recovered during the drying process is returned condensed and distilled, so it can be reused to clean further loads or safely disposed of. The majority of modern enclosed machines also incorporate a computer-controlled drying sensor, which automatically senses when all traces of perc have been removed. This system ensures that only the smallest amount of perc fumes will be released when opening the door at the end of the cycle.
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 (referred to as the "basket" or "drum"), which constitutes 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 (20 to 80 lb).
During the wash cycle, the chamber is filled approximately one-third full of solvent and begins to rotate, agitating the clothing. The solvent temperature is maintained at 30 degrees Celsius (86 degrees Fahrenheit), as a higher temperature may damage it. During the wash cycle, the solvent in the chamber (commonly known as the "cage" or "tackle box") is passed through a filtration chamber and then fed back into the "cage". This is known as the cycle and is continued for the wash duration. 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 is separated from the solvent and then 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.
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-color 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 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.
Not all stains can be removed simply by dry cleaning. Some need to be treated with spotting solvents—sometimes by steam jet or by soaking in special stain-remover liquids—before garments are washed or dry cleaned. Also, garments stored in soiled condition for a long time are difficult to bring back to their original color and texture. Natural fibers such as wool, cotton, and silk of lighter colors should not be left in dirty or soiled condition for long amounts of time as they absorb dirt in their texture and are unlikely to be restored to their original color and finish.
A typical wash cycle lasts for 8–15 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 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 wherein 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 the 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 350-450 rpm, causing much of the solvent to spin free of the fabric. Until this time, the cleaning is done in normal temperature, as the solvent is never heated in 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 (60-63 °C/140-145 °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 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 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 and finishing.
Working solvent from the washing chamber passes through several filtration steps before it is returned to the washing chamber. The first step is a button trap, which prevents small objects such as lint, fasteners, buttons, and coins from entering the solvent pump.
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 disk filters", which remove the muck from the filter by centripetal 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 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.
Since the solvent recovery is less than 100%, and because dry cleaning does not remove water-based stains well, entrepreneurs have developed the wet cleaning process, which is, in essence, cold-water washing and air drying, using a computer-controlled washer and dryer. In general, wet cleaning is regarded as being in its infancy, although low-tech versions of it have been used for centuries.
The international GINETEX laundry symbol for dry cleaning is a circle. It may have the letter P inside it to indicate perchloroethylene solvent, or the letter F to indicate a flammable solvent (Feuergefährliches Schwerbenzin). A bar underneath the circle indicates that only mild cleaning processes should be used. A crossed-out empty circle indicates that dry cleaning is not permitted.
Cooked powder residue is the name for the waste material generated by cooking down or distilling muck. It is a hazardous waste and will contain solvent, powdered filter material (diatomite), carbon, non-volatile residues, lint, dyes, grease, soils, and water. This material should be disposed of in accordance with local law.
The waste sludge or solid residue from the still contains solvent, water, soils, carbon, and other non-volatile residues. Still bottoms from chlorinated solvent dry cleaning operations are hazardous wastes. Still bottoms from machines using other solvents may also be hazardous due to toxic constituents in fabric dyes and notions.
Filters used in perchloroethylene based machines are also regulated hazardous waste in the United States.
Water will separate from the dry cleaning solvent, and must be removed. This waste water, from machines using perchloroethylene, is a regulated hazardous waste in the United States.
Perc is classified as carcinogenic to humans by the United States Environmental Protection Agency and must be handled as a hazardous waste. To prevent it from getting into drinking water, dry cleaners that use perc must take special precautions.
When released into the air, perc can contribute to smog when it reacts with other volatile organic carbon substances. California declared perchloroethylene a toxic chemical in 1991, and its use will become illegal in that state in 2023. A recent study conducted at Georgetown University shows that perc is retained in dry-cleaned clothes and that perc levels increase with repeat cleanings.
Some alternatives such as CO2 offers a solution to perc, however CO2 is inferior in removing some forms of grime.
Various consumer-grade products in the marketplace today, such as Procter & Gamble's Dryel, allow portions of the dry cleaning process to be performed at home using home laundry machines. Even though the use of these products does not follow all the steps of the commercial process, such use does work for certain types of garments.
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