Difference between revisions of "Dry cleaning" - New World Encyclopedia

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[[Image:Drycleanmachine.JPG|thumb|300px|A modern dry cleaning machine.]]
 
[[Image:Drycleanmachine.JPG|thumb|300px|A modern dry cleaning machine.]]
  

Revision as of 17:51, 28 November 2007

A modern dry cleaning machine.

Dry cleaning is any cleaning process for clothing and textiles using an organic solvent rather than water. The solvent, generally known as dry cleaning fluid, is typically tetrachloroethylene (called perchloroethylene or perc in the industry). Dry cleaning is necessary for cleaning items that would otherwise be damaged by water and soap or detergent. It may also be sought if washing by hand—another alternative for some delicate fabrics—is seen as laborious.

History

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-19th century by French dye-works owner Jean Baptiste Jolly, who noticed that his tablecloth became cleaner after his maid spilled kerosene on it, and from this observation developed a service to clean other people's clothes in this manner, which he termed "nettoyage à sec," or "dry cleaning" in English.[1]

Early dry cleaners used petroleum-based solvents, such as gasoline and kerosene. Concerns over 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, which resulted in heavy regulation of dry cleaners.

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) as a standard, colloquially called "perc," as the ideal solvent. It is stable, nonflammable, and has excellent cleaning power and is gentle to most garments.

Process

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). This 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 lb (9-36 kg) 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 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 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, a catalyst may be applied to the soil; 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 very well by perchloroethylene, whereas 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. Dry cleaners have a variety of catalysts to choose from to remove different types of spots.

Garments are also checked carefully 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), and will shed 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. Garments are checked carefully 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 will 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 fabrics comes off. It takes approximately ten to twelve 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 (e.g., 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 the extraction process. This process 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 regarded as cheap and, at the time, believed to be harmless.)

Since this number is still short of 100 percent, and because dry-cleaning doesn't 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/finishing.

Solvent processing

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 (lint, fasteners, buttons, coins etc) from entering the solvent pump.

Next the solvent passes through a filter unit which removes lint and insoluble suspended soils from the solvent. Several different types are used, 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 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 then further processed to recover any solvent trapped in the muck. 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 which 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 soil from redepositing on garments. Depending on the machine's design, either an anionic or cationic detergent is used.

Dry cleaning wastes

Cooked muck

Cooked Powder Residue—the waste material generated by cooking down or distilling muck. Cooked powder residue 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 then 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.

Health and Safety

Perc is classified as a probable human carcinogen by the International Agency for Research on Cancer, and exposure carries risks to both those who work in the dry cleaning industry as well as consumers. 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.[2] 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.[3]

Women who work in dry cleaners are 2-4 times more likely to have miscarriages than women in other professions.[4] 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.[5]

Environment

Perc is classified as a hazardous air contaminant by the EPA and must be handled as a hazardous waste. 70 percent of all perc used in the United States ends up in the environment, contaminating ground and drinking water. [4] Because of this, dry cleaners that use perc must take special precautions against site contamination; landlords are becoming increasingly reluctant to allow dry cleaners to operate in their buildings. When released into the air, perc can contribute to smog when it reacts with other volatile organic carbon substances.[6] 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.[3]

Solvents used

Modern

  • Perchloroethylene—In use since the 1940s, perc is the most common solvent, the "standard" for cleaning performance, and most aggressive cleaner. It can cause color bleeding/loss, especially at higher temperatures, and may destroy special trims, buttons, and beads on some garments. Better for oil-based stains (which account for about 10 percent of stains) than more common water-soluble stains (coffee, wine, blood, etc). Known for leaving a characteristic chemical smell on garments. Nonflammable.
  • Hydrocarbon—This is most like standard dry cleaning, but the processes use hydrocarbon solvents such as Exxon-Mobil’s DF-2000 or Chevron Phillips' EcoSolv. These petroleum-based solvents are less aggressive than perc and thus require a longer cleaning cycle. While flammable, these solvents do not present a high risk of fire or explosion when used properly. Petroleum, however, is a non-renewable resource and its use in dry cleaning (and all other applications) is not sustainable. Hydrocarbon also contains volatile organic compounds (VOCs) that contribute to smog.[3]
  • Liquid silicone(decamethylcyclopentasiloxane or D5)—gentler on garments than Perc and does not cause color loss. Requires a license be obtained to utilize the property of GreenEarth Cleaning. Degrades within days in the environment to sand and trace amounts of water and CO2. Produces nontoxic, nonhazardous waste. Toxicity tests by Dow Corning shows the solvent to increase the incidence of tumors in female rats (no effects whatsoever were seen in male rats), but further research concluded that the effects observed in rats were not relevant to humans because the biological pathway that results in tumor formation is unique to rats.[7](170.6 °F/77 °C flash point)
  • Liquid CO2—Consumer Reports rated this method superior to even conventional methods, but the Dry cleaning and Laundry Institute commented on its "fairly low cleaning ability" in a 2007 report.[8] Machinery is very expensive—up to $90,000 more than a perc machine—making it very difficult for small business owners to afford. Most cleaners with these machines keep traditional machines on-site for the heavier soiled textiles. CO2 cleaning is also used for fire- and water-damage restoration due to its effectiveness in removing toxic residues, soot and associated odors of fire.
  • Modified hydrocarbon blends (Pure Dry)
  • Glycol ethers (dipropylene glycol tertiary-butyl ether) (Rynex)—not as effective as perchloroethylene.
  • Wet cleaning—Not a solvent, but rather a system that uses water and biodegradable soap. Computer-controlled dryers and stretching machines ensure that the fabric retains its natural size and shape. Wet cleaning is claimed 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 U.S.

Historical

  • Stoddard solvent—Very flammable and explosive, 100°F/38°C flash point.
  • Carbon tetrachloride—Toxic and corrosive
  • Trichloroethane—Overly aggressive and harsh
  • Valclene 113 Freon-113—Ozone destroying CFC

See also

  • Martinizing Dry Cleaning, a trademarked one hour dry cleaning process
  • List of environment topics
  • Fabric Restoration
  • GreenEarth Cleaning

Notes

  1. How Dry Cleaning Works. - HowStuffWorks. Retrieved October 16, 2007.
  2. “Tetrachloroethylene (PCE).” - Ohio Bureau of Health Retrieved October 16, 2007.
  3. 3.0 3.1 3.2 "Wet is the New Dry." by Linda Immediato. LA Weekly: La Vida. Retrieved October 16, 2007.
  4. 4.0 4.1 “Breaking the ‘perc’ habit: dirty secrets of dry cleaning” - Fair Companies Retrieved October 16, 2007. Cite error: Invalid <ref> tag; name "fair" defined multiple times with different content
  5. “Material Safety Data Sheet: Tetrachloroethylene.” - Mallinckrodt Baker, Inc Retrieved October 16, 2007.
  6. “Chemicals in the Environment: Perchloroethylene.” - U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics. Retrieved October 16, 2007.
  7. “Fact Sheet: D5 in Dry Cleaning.” - Silicones Environmental, Health and Safety Council. December 2004. Retrieved October 16, 2007.
  8. Drycleaning and Laundry Institute. "The DLI White Paper: Key Information on Industry Solvents." The Western Cleaner & Launderer, August 2007.

External links

All links retrieved October 16, 2007.

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