Dry cleaning is any cleaning process forclothing andtextiles using asolvent other thanwater. Clothes are instead soaked in a water-free liquid organic solvent (usuallynon-polar, as opposed to water which is apolar solvent) typically inside a specialised dry-cleaning machine. The most commonly used solvent isPerchloroethylene (known as "PCE" or "perc" for short), although other solvents such ashydrocarbon mixtures anddecamethylcyclopentasiloxane are also used. Historical solvents includecarbon tetrachloride,trichloroethylene,trichlorotrifluoroethane,trichloroethane andn-propyl bromide.
Most natural fibers can be washed in water but some synthetics (e.g.,rayon) react poorly with water and should be dry cleaned if possible.[1] If not, this could result in changes in texture, colour, strength, and shape. Additionally, certain specialty fabrics, including silk, acetate and rayon, may also benefit from dry cleaning to prevent damage.
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French dye-works operator Jean Baptiste Jolly[2][a] developed his own method usingkerosene andgasoline to clean fabrics.[2] He opened the first dry cleaning service in Paris in 1845.[4]
Flammability concerns led William Joseph Stoddard, a dry cleaner fromAtlanta, to develop in 1924Stoddard solvent (white spirit) as a slightly lessflammable alternative to gasoline-based solvents.[5] It was the dominant dry-cleaning solvent in the US until the 1950s when perchloroethylene became the dominant solvent.[6]
The use of highly flammable petroleum solvents caused many fires and explosions, resulting in government regulation of dry cleaners.[citation needed]
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AfterWorld War I, dry cleaners began usingchlorinated solvents. These solvents were much less flammable than petroleum solvents and had improved cleaning power.[citation needed] Early solvents werecarbon tetrachloride andtrichloroethylene (TCE). Carbon tetrachloride was first used as a stain remover in the early 1890s in Germany. TCE was introduced in 1930, it had the downside of being incompatible with acetate dyes.[7]
In 1930, chemistsSylvia Stoesser, John Grebe, and J. Lawrence Amos ofDow Chemical suggested using perchloroethylene in place of flammable hydrocarbon solvents in dry cleaning, following requests from industry representatives. A washing machine was purchased for the laboratory for research and testing, and all its rubber components were replaced withneoprene, which is resistant to organic solvents.[8] Perchloroethylene's use in dry cleaning was adopted as early as 1933, when it was beginning to be considered the "ideal dry cleaning solvent." In the same year, a dry cleaning machine specifically for use with perchloroethylene and trichloroethylene was developed.[9] By the mid-1930s, the dry cleaning industry had started to adopt perchloroethylene as the main solvent. It has excellent cleaning power and is nonflammable and compatible with most garments. Because it is stable, perchloroethylene is readily recycled by distillation.[10]
Dry cleaningsolvents selectivelydissolve stains on the article. The solvents arenon-polar and tend to selectively extract many compounds that cause stains. Some of these stains would otherwise only dissolve in aqueous detergent mixtures at high temperatures, potentially damaging delicate fabrics. Non-polar solvents are also good for some fabrics - especially natural fibres - as the solvent does not interact with any polar groups within the fabric. Water binds to these polar groups (hydroxyls in thecellulose for example), resulting in the swelling and stretching of proteins within fibers during laundering. Also, the binding of water molecules interferes with weak attractions within the fiber which results in the loss of the fiber's original shape. After the laundry cycle, water molecules will evaporate. However, the original shape of the fibers has already been distorted and this commonly results in shrinkage. Non-polar solvents prevent this interaction, protecting more delicate fabrics. The usage of an effective solvent coupled with mechanical friction from tumbling effectively removes stains.
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-axis, perforated drum that rotates within an outer shell. The shell holds the solvent while the rotating drum holds the garment load. During the wash cycle, the chamber is filled approximately one-third full of solvent and begins to rotate, agitating the clothing. During the wash cycle, the solvent in the chamber is passed through a filtration column and then fed back into the chamber. The solvent is then removed and sent to adistillation unit consisting of aboiler andcondenser. The condensed solvent is fed into a separator unit where any remaining water and water-soluble impurities are separated from the solvent, and the refined solvent is fed into the clean solvent tank. The ideal flow rate is roughly 8 liters of solvent per kilogram of garments per minute (very approximately one gallon per pound of garments), depending on the size of the machine.
A typical wash cycle lasts for 3–15 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 any 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. To enhance the solvent's cleaning power, small amounts of special dry-cleaningdetergent (0.5–1.25%) may be added to the working solvent. These detergents emulsifyhydrophobic soils and keep soil from redepositing on garments. Depending on the machine's design, either ananionic or acationic detergent is used.[10]
At the end of the wash cycle, the machine starts a rinse cycle where the garment load is rinsed with freshly distilled solvent dispensed from the clean solvent tank. This clean solvent rinse prevents discoloration caused by soil particles being deposited back into the garment from the "dirty" working solvent. After the rinse cycle, the machine begins the extraction process, which recovers the solvent for reuse. Modern machines recover over 90% of the solvent employed.[6] 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 at normal temperature, as the solvent is never heated during the 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 (typically around 60–63 °C; 140–145 °F) that circulates throughout 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 solvent vapors were vented to the atmosphere via exhausts (See#Machinery) 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 further traces of solvent by circulating cool outside air over the garments and then through a vapor recovery filter made fromactivated carbon and polymer resins. After the aeration cycle, the garments are clean and ready for pressing and finishing.
Not allstains can be removed 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 which have been stored in soiled condition for a long time are difficult to bring back to their original color and texture, since irreversible chemical reactions (such as oxidation) may occur over time.[citation needed]
Dry-cleaning machines are classified into several generations:[6][11][12]
From the customer's perspective, dry cleaning businesses are either "plants" or "drop shops". The former does on-site cleaning, while a drop shop receives garments from customers, sends them to a large plant, and then has the cleaned garments returned to the shop for pickup by the customer. The latter setup minimized the risk of fire or dangerous fumes created by the cleaning process. An older dry-cleaning setup was the "coin-op" machines which were operated by the customers themselves. Coin-op machines were mostly seen in North America and went into decline in late 20th century.[10]
It is estimated that 50% to 70% of dry cleaners in the US were using PCE as of 2012[update].[13] Alternative solvents are available, but these may require major changes in equipment, procedures, and operator training. Flammable solvents may require installation of expensivefire-suppression systems. Because PCE has been the longtimede facto solvent for dry cleaning, there is considerable interest in finding a "drop-in" substitute solvent which could be used with minimal changes to existing equipment and procedures.[13]
Perchloroethylene (PCE or perc, tetrachloroethylene) is the main solvent in dry cleaning and it has been in use since the 1930s. PCE is the most common solvent, the standard for cleaning performance. It is a highly effective cleaning solvent with aKB-value of 90,[14] and it is thermally stable, nonflammable, recyclable, and has very low toxicity and a pleasant smell. PCE is recycled bydistillation at its boiling point (121 °C). Unlike the related dry-cleaning solventcarbon tetrachloride, perchloroethylene is not anozone-depleting substance.[15] Perchloroethylene can cause color bleeding/loss, especially at higher temperatures. In some cases it may damage special trims, buttons and beads on some garments. It is better for oil-based stains than more common water-soluble stains. It does not leave smell on dry-cleaned clothes.
Inhalation of high concentrations of perchloroethylene can producenarcotic andhallucinogenic effects, thus perchloroethylene is regarded as a mild neurotoxin and these effects are completely reversible upon the cessation of exposure. The toxicity of perchloroethylene is moderate to low and reports of human injury are uncommon despite its wide usage in dry cleaning and degreasing.[16] Perchloroethylene is classified as "probably carcinogenic to humans" (Group 2A, same classification asred meat[17] and hot beverages[18]) by theInternational Agency for Research on Cancer (IARC). There is a suspicion that it is carcinogenic to humans in long term, but the evidence is limited since most of the evaluated dry-cleaners had heavy smoking and drinking habits which are known carcinogens and were exposed to many other chemicals at the workplace.[19] A study published in 2011 investigated cancer rates among dry cleaners exposed to perchloroethylene for many years and laundry workers who were not exposed to perchloroethylene as thecontrol group, based on a total of more than nine thousand people, found no difference in the cancer rates between the two groups: there was no significant increase in the incidence of esophageal, cervical, liver, kidney and bladder cancers, which were previously suspected to be caused by perchloroethylene, between the two groups.[20] The exposure to perchloroethylene in a typical dry cleaner is considered far below the levels required to cause any risk.[21]
Hydrocarbon mixtures have KB values between 27-45.[10] Hydrocarbons have been used in dry cleaning since the early years (19th century). Early hydrocarbon solvents used werekerosene and the less flammableStoddard solvent (also called "white spirit") is a mixture of over 65% C10 or higher hydrocarbons, has a relatively lowflash point of 38 °C (100 °F).[22] High flashhydrocarbons, characterized as having aflash point higher than 60 °C (140 °F), are considered to be safer than traditional hydrocarbon solvents.[13]: 18–19 Examples includeExxon-Mobil's DF-2000 orChevron Phillips' EcoSolv, and Pure Dry. Thesepetroleum-based solvents are less aggressive but also less effective than PCE. Although hydrocarbons are combustible, risk of fire or explosion can be minimized when they are used properly; a fire-suppression system may also be required. Hydrocarbons are considered to be volatile organic (VOC) pollutants.[13]: 18–19
Decamethylcyclopentasiloxane (colloquially known as "siloxane" or "liquid silicone", trademarked Siloxane D5),[13]: 25 was initially popularized by GreenEarth Cleaning in the late 1990s.[23] It is more expensive than PCE (chemical and machinery-wise) and requires aGreenEarth licence to be used by dry-cleaners.[13] It is insufficient in dissolving oils and grease, compared to other dry-cleaning solvents. It has the lowest KB value (13) among all dry-cleaning solvents.[14] It is less aggressive than PCE and can be used to clean more delicate garments.[13]: 25 Dry-cleaning machines for decamethylcyclopentasiloxane recycle the solvent by extensive filtration, unlike most other dry-cleaning systems for PCE, hydrocarbons and some other solvents.
It is marketed as an eco-friendly product that degrades quickly in the environment, but is controlled in the European Union due to itspersistent, bioaccumulative and toxic characteristics.[24] It does not degrade in nature, it is highly toxic to the aquatic life and has higher bioaccumulative properties than perchloroethylene.[13]: 26 Its use in dry cleaning will be prohibited in the European Union after June 2026 due to its environmental effect.[25]
For decades, efforts have been made to replace PCE. These alternatives have not proven popular thus far:
Working solvent from the washing chamber passes through severalfiltration 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. After the lint filter, the solvent passes through anabsorptive cartridge filter. This filter, which contains activated clays andactivated charcoal, removes fine insoluble soil residues, non-volatile residues, and dyes from the solvent. Finally, the solvent passes through a polishing filter, which removes any contaminants not previously removed. The clean solvent is then returned to the working solvent tank.
Over time, a thin layer offilter 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 "spindisk filters", which remove the muck from the filter bycentrifugal force while it is back washed with solvent. "Cooked powder residue" is the name for the waste material generated by cooking down or distilling muck. It will contain residual 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 residual solvent, water, soils, carbon, and other non-volatile residues. Used filters are another form of waste, as is waste water, which are also subject to regulation by local environmental authorities (such asEnvironmental Protection Agency in the United States).[13]
Garments should be carefully checked for foreign objects before being placed in the machine. Depending on the solvent used, items such asplastic pens may dissolve in the solvent bath, damaging the entire batch of textiles, certain textiledyes are "loose" and will shed dye during solvent immersion.
Fragile items, such as feather bedspreads or tasseled rugs or hangings, may be protected by enclosing them in a loosemesh bag. Thedensity of perchloroethylene is around 1.62 g/cm3 at room temperature (62% heavier than water), and the sheer weight of absorbed solvent may cause the textile to fail under typical forces during the spin extraction cycle, unless the mesh bag provides mechanical support.
The internationalGINETEXlaundry 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 (German:Feuergefährliches Schwerbenzin). A bar underneath the circle indicates that only mild cleaning processes are recommended. A crossed-out empty circle indicates that an item should not be dry cleaned at all.[40]
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In Britain America the discovery was for long attributed to a supposed Paris tailor by name of Jolly-Belin [...] Actually the discoverer of drycleaning was not named Jolly-Belin but Jean-Baptiste Jell
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