Dry cleaning is any cleaning process for clothing and textiles using an organic solvent rather than water. The solvent is generally known as dry cleaning fluid. Dry cleaning is useful for cleaning items that would otherwise be damaged by water and soap or detergent. It may also be sought by those who wish to avoid the trouble of washing by hand, as in the case of delicate fabrics.
The dry cleaning fluid most commonly used is tetrachloroethylene, more generally known as perchloroethylene or "perc." This solvent, however, produces toxic waste and is classified as a probable human carcinogen and Toxic Air Contaminant by the U.S. Environmental Protection Agency EPA. To avoid the problems of using perc, some businesses have switched to other solvents, such as hydrocarbons, liquid silicone, and liquid carbon dioxide. Others offer the a "wet cleaning" process that involves the use of water and biodegradable soap. The use of perc is being phased out in California.
Dry cleaning uses non-water-based solvents to remove dirt and stains from clothes. The potential for using petroleum based solvents in this manner was first discovered in the mid-nineteenth century by French dye-works owner Jean Baptiste Jolly, who noticed that his tablecloth became cleaner after his maid spilled kerosene on it. From this observation, he developed a service to clean other people's clothes in this manner, which he termed "nettoyage à sec," or "dry cleaning" in English.
Early dry cleaners used petroleum-based solvents such as gasoline and kerosene. Concerns about flammability 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 led to many fires and explosions, because of which dry cleaners were heavily regulated.
After World War I, dry cleaners began using various chlorinated solvents. These solvents were much less flammable than petroleum solvents and had much greater cleaning power. By the mid-1930s, the dry cleaning industry had adopted tetrachloroethylene (perchloroethylene), colloquially called "perc," as a standard solvent. It is stable, nonflammable, has excellent cleaning power, and is gentle on most garments.
A dry cleaning machine is somewhat 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 dry cleaning 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. Depending on the size of the machine, the basket capacity will be between 20 and 80 pounds (lb) (9-36 kilograms) of garments.
During the wash cycle, the chamber is filled approximately 1/3 full of solvent and begins to rotate to agitate the clothing. The solvent temperature is controlled at 85°F (29.4°C), as a higher temperature may extract dye from the garments, causing color loss. During the wash cycle, the chamber is continually 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 eight liters 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, the operator may apply a catalyst to it. Oil-based soils (such as grease, oil, or lipstick) typically are removed very well by perchloroethylene, whereas water-based soils (such as coffee, wine, perspiration, and blood) 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. Dry cleaners have a variety of catalysts to choose from to remove different types of spots.
The garments have to be checked for foreign objects; such items 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), shedding dye during the solvent immersion; these will not be included in a load along with lighter-based colored textiles to avoid color transfer. In addition, the solvent used must be distilled to remove any impurities that may transfer to the clothes.
The garments are also checked for dry-cleaning textile compatibility, including the fasteners. Many decorative fasteners either are not dry-cleaning-solvent proof or will not withstand the mechanical action of the cleaning cycle. These may be removed and restitched after the cleaning, or protected with a small padded protector as needed. Finally, fragile items, such as feather bedspreads or tasseled rugs or hangings may be enclosed into a loose mesh bag. The density of perchloroethylene is around 1.7 g/cm³ at room temperature (70 percent 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 from the fabrics comes off. It takes approximately 10-12 minutes after the loose soil has come off to remove all of the ground-in insoluble soil from the 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 (such as oily stains). 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 of garments caused by soil particles being absorbed back onto the garment surface from the "dirty" working solvent.
After the rinse cycle, the machine begins an extraction process, which recovers dry-cleaning solvent for reuse. Modern dry cleaning machines can recover approximately 99.99 percent of the solvent used in the cleaning process.
The extraction cycle begins by draining the solvent out of the washing chamber and accelerating the basket to speeds of 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 its drying cycle.
During the drying cycle, the garments are tumbled in a continuous stream of warm air (145°F/63°C) that circulates through the basket, evaporating any traces of solvent left behind after the spin cycle. The temperature of the air is carefully controlled to prevent over-drying and heat damage to the garments. The warm air then passes through a chiller unit, where the 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 then reheated and recirculated. This results in very high solvent recovery rates and reduces air pollution. (In the early days of dry cleaning, large amounts of perchlorethylene were vented to the air, because it was cheap and, at the time, was believed to be harmless.)
Given that dry cleaning does not remove water-based stains very well, a few brave 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.
After the drying cycle is completed, a deodorizing (aeration) cycle starts to cool the garments and remove the last traces of dry cleaning solvent, by circulating cool outside air over the garments and then through a vapor recovery filter made from activated carbon and polymer resins. At the end of the aeration cycle, the dry-cleaned garments are clean and ready for pressing or finishing.
The 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 that prevents small objects (such as lint, fasteners, buttons, coins) from entering the solvent pump.
Next, the solvent passes through a filter unit that removes lint and insoluble suspended soils from the solvent. Several different types of filters are used, most of which have an ultra-fine mesh to support a thin layer of filter powder (made from diatomaceous earth and activated clays). Some machines use powderless filters that are capable of removing soil particles larger than 30 micrometers from the solvent.
As the machine is used, a thin layer of filter cake (called muck) accumulates on the surface of the lint filter. The muck is removed regularly (commonly once per day) and further processed to recover any solvent trapped in it. Many machines use "spin disc filters," in which the muck is removed from the filter surface by centrifugal action while the filter is back-washed with solvent.
After passing through the lint filter, the solvent passes through an adsorptive cartridge filter. This filter is made from activated clays and charcoal and removes fine insoluble soil and non-volatile residues along with dyes from solvent. Finally, the solvent passes through a polishing filter that removes any traces of soil not removed by the previous filters. The clean working solvent is then returned to the working solvent tank.
To enhance cleaning power, small amounts of detergent (0.5 to 1.5 percent) are added to the working solvent and are essential to its functionality. These detergents help dissolve hydrophilic soils and keep the soil from redepositing on garments. Depending on the machine's design, either an anionic or cationic detergent is used.
Cooked Powder Residue: This is the waste material generated by cooking down or distilling muck. Cooked powder residue will contain solvent, powdered filter material (diatomite), carbon, non-volatile residues, lint, dyes, grease, soils, and water. This material is a hazardous waste and should be disposed of in accordance with local laws.
Sludge: The waste sludge or solid residue from the still. Still bottoms contain solvent, water, soils, carbon, and other non-volatile residues. Still bottoms from chlorinated solvent dry cleaning operations are hazardous wastes.
Wet cleaning is a system that uses water and biodegradable soap, thus it differs from dry cleaning. Computer-controlled dryers and stretching machines ensure that the fabric retains its natural size and shape. Wet cleaning is said to clean a majority of "dry clean only" garments safely, including leather; suede; most tailored woolens, silks and rayons. (Neckties seem to be the one exception.) Most perc cleaners use wet cleaning on some garments, but there are only about 20 exclusive wet cleaners in the United States.
Perc is classified as a probable human carcinogen by the International Agency for Research on Cancer, and exposure carries risks to both consumers and those who work in the dry cleaning industry. It has been estimated that people who wear one dry-cleaned garment one day a week over a 40 year period could inhale enough perc "to measurably increase their risk of cancer" by as much as 150 times what is considered "negligible risk." Perc exposure has been shown to cause liver cancer in mice and kidney cancer in male rats. According to California's South Coast Air Quality Management District (SCAQMD), people who live near dry cleaners have a higher risk of cancer than those who live near oil refineries or power plants.
Women who work in dry cleaners are 2-4 times more likely to have miscarriages than women in other professions. Prolonged exposure to chemical fumes or direct contact can cause liver, kidney, and central nervous system damage, especially to those with preexisting conditions. Consuming alcohol can enhance these effects.
Perc is classified as a hazardous air contaminant by the U.S. Environmental Protection Agency (EPA) and must be handled as a hazardous waste. As much as 70 percent of all perc used in the United States ends up in the environment, contaminating ground and drinking water. Because of this, dry cleaners who use perc must take special precautions against site contamination. Also, when released into the air, perc can react with other volatile organic carbon compounds and contribute to smog.
California passed a ban on perc in January 2007, beginning a 15-year phase-out of chemicals and equipment. By 2023, no dry cleaner in the state will be allowed to operate with perc.
All links retrieved August 25, 2013.
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