Dry cleaning

Dry cleaning is any cleaning process for clothing and textiles using a chemical solvent other than water. The solvent used is typically toxic 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. "Perc" is the most widely used solvent across the world, although new solvents have become more popular in the United States and Europe; including hydrocarbons, Green Earth, and Solvon K4.

History

Modern dry cleaning use of non-water-based solvents to remove soil and stains from clothes was reported in 1855. The potential for using petroleum-based solvents such as gasoline and kerosene was recognized by French dye-works operator Jean Baptiste Jolly, who offered a new service that became known as nettoyage à sec—i.e., dry cleaning.[1][2] Flammability concerns led William Joseph Stoddard, a dry cleaner from Atlanta, to develop Stoddard solvent (white spirit) 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.

On March 3, 1821, Thomas L. Jennings became the first African-American to be granted a United States patent, for his cleaning process called "dry scouring," which was the precursor to dry cleaning.[3]

Shift to tetrachloroethylene

By the mid-1930s, the dry cleaning industry had adopted tetrachloroethylene (perchloroethylene), or "perc" for short, as the 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 U.S. Consumer Product Safety Commission. Tetrachloethylene is currently considered a potential carcinogen by the Environmental Protection Agency and the National Institute for Occupational Safety and Health (NIOSH).[4][5] In 1993, the California Air Resources Board adopted regulations to reduce perc emissions from dry cleaning operations; the same year, the EPA did the same. The U.S. EPA updated its regulation in 2006 to reflect the availability of improved emission controls.

The replacement of perc with other solvents remains of interest as perc will ultimately disappear from the market.

Infrastructure

In the United States 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. By these numbers, dry cleaning firms on average each employ 7 workers and bring in $330,000 per year.[6]

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 many 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 detectable 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.

Process

A modern dry cleaning machine with touchscreen and SPS control, manufacturer EazyClean, type EC124, photo taken prior to installation
Series 3 Dry cleaning machine with PLC control, manufacturer, BÖWE Textile cleaning Germany

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 when left in dirty or soiled condition for long times 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 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.

Many dry cleaners place cleaned clothes inside thin clear plastic garment bags

After the drying cycle is complete, a deodorizing (aeration) 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.

Solvent processing

A Firbimatic Saver Series. This machine uses Activate Clay Filtration instead of distillation. It uses much less energy than conventional methods.

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 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 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.

Symbols

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 is recommended. A crossed-out empty circle indicates that dry cleaning is not permitted.[7]

Dry-cleaning waste

Wastes are potentially hazardous, and restrictions often apply to disposal.

Cooked powder residue is the name for the waste material generated by cooking down or distilling muck. It will contain solvent, powdered filter material (diatomite), carbon, non-volatile residues, lint, dyes, grease, soils, and water. The waste sludge or solid residue from the still contains solvent, water, soils, carbon, and other non-volatile residues. Used filters are another form of waste as is waste water.

Toxicity and environmental effects

Perc is classified as carcinogenic to humans by the United States Environmental Protection Agency[8] 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.[9] California declared perchloroethylene a toxic chemical in 1991, and its use will become illegal in that state in 2023.[10]

CO2 represents an alternative to perc, however CO2 is inferior in removing some forms of grime.[11]

Solvents used

Modern

Historical

Home dry cleaning

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. However this is not at all similar to professional dry cleaning and may not clean the garment properly.

See also

References

  1. ↑ "How Dry Cleaning Works". Science.howstuffworks.com. Retrieved 2006-03-30.
  2. ↑ "How To Setup a Dry Cleaning Business". Jalingo.co. Retrieved 2015-08-19.
  3. ↑ "U.S. Congress Resolution H. Res. 514 Honoring Thomas Jennings of New York City as the first African-American to be granted a patent by the United States". U.S. Government Printing Office. Retrieved 2015-03-21.
  4. ↑ http://www3.epa.gov/airtoxics/hlthef/tet-ethy.html
  5. ↑ http://www.cdc.gov/niosh/topics/cancer/npotocca.html#t
  6. ↑ "Drycleaning & Laundry Services Industry Analysis". PellResearch.com. Retrieved 2013-03-14.
  7. ↑ "Professional textile care symbols". GINETEX - Swiss Association for Textile Labelling. Retrieved 2013-07-18.
  8. ↑ EPA Releases Final Health Assessment for TCE September 2011. Accessed 2011-09-28.
  9. ↑ U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics. “Chemicals in the Environment: Perchloroethylene.” 1994-08. Retrieved on 2007-08-15.
  10. ↑ "California bans dry cleaning chemical". msnbc.com. 2007-01-25.
  11. ↑ "Dry-cleaning with CO2 wins award [Science] Resource". Resource.wur.nl. 2010-10-12. Retrieved 2013-03-14.
  12. ↑ Immediato, Linda. "Wet is the New Dry." LA Weekly: La Vida. 2007-01-17. Retrieved on 2007-08-15.
  13. ↑
  14. ↑ Silicones Environmental, Health and Safety Council. "Fact Sheet: D5 in wetCleaning." December 2004. Accessed 2007-07-30, unavailable 5 July 2013.
  15. ↑ "HAZARD EVALUATION 1-Bromopropane" July 2003. Accessed 2014-Jan-22
  16. ↑ "Occupational exposure to perchloroethylene in dry-cleaning shops in Tehran, Iran." April 2009. Accessed 2014-Jan-22
  17. ↑ Drycleaning and Laundry Institute. "The DLI White Paper: Key Information on Industry Solvents." The Western Cleaner & Launderer, August 2007.

External links

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