Dry cleaning is any cleaning process for
clothing
Clothing (also known as clothes, apparel, and attire) are items worn on the body. Typically, clothing is made of fabrics or textiles, but over time it has included garments made from animal skin and other thin sheets of materials and natural ...
and
textile
Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, different fabric types, etc. At first, the word "textiles" only referred to woven fabrics. However, weaving is not the ...
s using a
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
other than
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
.
Dry cleaning still involves liquid, but clothes are instead soaked in a water-free liquid solvent.
Tetrachloroethylene
Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and many other names (and abbreviations such as "perc" or "PERC", and "PCE"), is a chlorocarbon with the formula Cl2C=CCl2 . It is a colorless li ...
(perchloroethylene), known in the industry as "perc", is the most widely used solvent. Alternative solvents are
1-bromopropane
1-Bromopropane (''n''-propylbromide or nPB) is an organobromine compound with the chemical formula CH3CH2CH2Br. It is a colorless liquid that is used as a solvent. It has a characteristic hydrocarbon odor. Its industrial applications increased dr ...
and petroleum spirits.
Most natural fibers can be washed in water but some synthetics (e.g.,
viscose
Rayon is a semi-synthetic fiber, made from natural sources of regenerated cellulose, such as wood and related agricultural products. It has the same molecular structure as cellulose. It is also called viscose. Many types and grades of viscose ...
,
lyocell
Lyocell, originally trademarked in 1982 as Tencel, is a form of regenerated cellulose. It consists of cellulose fibers, made by dissolving pulp and then reconstituting it by dry jet-wet spinning. The fiber is used to make textiles for clothing a ...
,
modal, and
cupro
Cuprammonium rayon is a rayon fiber made from cellulose dissolved in a cuprammonium solution, Schweizer's reagent.
It is produced by making cellulose a soluble compound by combining it with copper and ammonia with caustic soda. The solution is pa ...
) react poorly with water and must be dry-cleaned.
History
Dry cleaning originated with American entrepreneur
Thomas L. Jennings. Jennings referred to his method as “dry scouring”.
French dye-works operator Jean Baptiste Jolly
developed his own method using
kerosene
Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
and
gasoline
Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organi ...
to clean fabrics.
He opened the first dry-cleaners in Paris in 1845.
Flammability concerns led William Joseph Stoddard, a dry cleaner from
Atlanta
Atlanta ( ) is the capital and most populous city of the U.S. state of Georgia. It is the seat of Fulton County, the most populous county in Georgia, but its territory falls in both Fulton and DeKalb counties. With a population of 498,715 ...
, 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
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fightin ...
, dry cleaners began using
chlorinated
In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transformation important, e.g. in the production of polyme ...
solvents. These solvents were much less flammable than petroleum solvents and had improved cleaning power.
Shift to tetrachloroethylene
By the mid-1930s, the dry cleaning industry had adopted
tetrachloroethylene
Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and many other names (and abbreviations such as "perc" or "PERC", and "PCE"), is a chlorocarbon with the formula Cl2C=CCl2 . It is a colorless li ...
(perchloroethylene), or PCE for short, as the solvent. It has excellent cleaning power and is nonflammable and compatible with most garments. Because it is stable, tetrachloroethylene is readily recycled.
Infrastructure
From the customer's perspective, dry cleaning businesses are either "plants" or "drop shops". A plant does on-site cleaning. A drop shop receives garments from customers, sends them to a large plant, and then has the cleaned garments returned to the shop for collection by the customer. This cycle minimized the risk of fire or dangerous fumes created by the cleaning process. At this time, dry cleaning was carried out in two machines—one for the cleaning process and the second to remove the solvent from the garments.
Machines of this era were described as "vented"; their drying exhausts were expelled into the atmosphere, the same as many modern tumble-dryer exhausts. This contributed to environmental contamination, and much potentially reusable PCE 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. Most modern enclosed machines also incorporate a computer-controlled drying sensor, which automatically senses when all detectable traces of PCE have been removed. This system ensures that only small amounts of PCE fumes are released at the end of the cycle.
Mechanism
left, 220 px, Structure of cellulose, the main constituent of cotton. The many OH groups bind water, leading to swelling of the fabric and leading to wrinkling, which is minimized when these materials are treated with tetrachloroethylene and other dry cleaning solvents.
In terms of mechanism, dry cleaning selectively solubilizes stains on the article. The
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
s are non-polar and tend to selectively extract compounds that cause stains. These stains would otherwise only dissolve in aqueous detergents mixtures at high temperatures, potentially damaging delicate fabrics.
Non-polar solvents are also good for some fabrics, especially natural fabrics, as the solvent does not interact with any polar groups within the fabric. Water binds to these polar groups which results in the swelling and stretching of proteins within fibers during laundering. Also, the binding of water molecules interferes with weak attractions within the fiber, resulting in the loss of the fiber's original shape. After the laundry cycle, water molecules will dry off. 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.
Process
![Drycleanmachine](https://upload.wikimedia.org/wikipedia/commons/2/26/Drycleanmachine.JPG)
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 (22 to 88 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 or lower, 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
Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heat ...
unit consisting of a
boiler
A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...
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
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptab ...
pens may dissolve in the solvent bath, damaging the textiles. Some textile
dyes are "loose" and will shed dye during solvent immersion. Fragile items, such as feather bedspreads or tasseled rugs or hangings, may be enclosed in a loose mesh bag. The
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of perchloroethylene is around 1.7 g/cm
3 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 by dry cleaning. Some need to be treated with spotting solventssometimes by steam jet or by soaking in special stain-remover liquidsbefore 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.
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 where the garment load is rinsed with freshly distilled solvent dispensed from the 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
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
, 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 perchloroethylene were vented to the atmosphere because it was regarded as cheap and believed to be harmless.
![Dry clean rack](https://upload.wikimedia.org/wikipedia/commons/9/93/Dry_clean_rack.jpg)
After the drying cycle is complete, a deodorizing (
aeration
Aeration (also called aerification or aeriation) is the process by which air is circulated through, mixed with or dissolved in a liquid or other substances that act as a fluid (such as soil). Aeration processes create additional surface area in ...
) 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
![Saver40](https://upload.wikimedia.org/wikipedia/commons/a/a8/Saver40.jpg)
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
A filter cake is formed by the substances that are retained on a filter. Filter aids, such as diatomaceous earth or activated carbon are usually used to form the filter cake. The purpose is to increase flow rate or achieve a smaller micron filtr ...
(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 filter
A disc filter is a type of water filter used primarily in irrigation, similar to a screen filter, except that the filter cartridge is made of a number of plastic discs stacked on top of each other like a pile of poker chips. Each disc is cover ...
s", which remove the muck from the filter by
centrifugal force
In Newtonian mechanics, the centrifugal force is an inertial force (also called a "fictitious" or "pseudo" force) that appears to act on all objects when viewed in a rotating frame of reference. It is directed away from an axis which is parall ...
while it is back washed with solvent.
After the lint filter, the solvent passes through an
absorptive cartridge filter. This filter, which contains activated clays and charcoal, 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. 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.
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
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, t ...
soils and keep soil from redepositing on garments. Depending on the machine's design, either an anionic or a cationic detergent is used.
Symbols
The international
GINETEX The (GINETEX) is the international association for textile care labeling, based in Clichy, France. GINETEX relies on 22 national members in 23 nations spanning Europe, South America, Africa, and Asia
Asia (, ) is one of the world's most n ...
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 an item should not be dry cleaned.
File:Professionelle reinigung.svg, Professional cleaning symbol
File:Professionelle reinigung (F).svg, Dry clean, hydrocarbon solvent only (HCS)
File:Professionelle reinigung (F)s.svg, Gentle cleaning with hydrocarbon solvents
File:Professionelle reinigung (F)ss.svg, Very gentle cleaning with hydrocarbon solvents
File:Professionelle reinigung (P).svg, Dry clean, tetrachloroethylene
Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and many other names (and abbreviations such as "perc" or "PERC", and "PCE"), is a chlorocarbon with the formula Cl2C=CCl2 . It is a colorless li ...
(PCE) only
File:Professionelle reinigung (P)s.svg, Gentle cleaning with PCE
File:Professionelle reinigung (P)ss.svg, Very gentle cleaning with PCE
File:Nicht chemisch reinigen.svg, Do not dry clean
Solvents used
Perchloroethylene
Perchloroethylene
Tetrachloroethylene, also known under the systematic name tetrachloroethene, or perchloroethylene, and many other names (and abbreviations such as "perc" or "PERC", and "PCE"), is a chlorocarbon with the formula Cl2C=CCl2 . It is a colorless li ...
(PCE, or tetrachloroethylene) 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. It is thermally stable, recyclable, and has low toxicity. It can, however, 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 (which account for about 10% of stains) than more common water-soluble stains (coffee, wine, blood, etc.). The toxicity of tetrachloroethylene "is moderate to low" and "Reports of human injury are uncommon despite its wide usage in dry cleaning and degreasing".
Hydrocarbons
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
s are represented by products such as
Exxon-Mobil's DF-2000 or
Chevron Phillips
Chevron Phillips Chemical Company, LLC is a petrochemical company jointly owned by Chevron Corporation and Phillips 66. The company was formed July 1, 2000 by merging the chemicals operations of both Chevron Corporation and Phillips Petroleum ...
' EcoSolv, and Pure Dry. These
petroleum
Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
-based solvents are less aggressive but also less effective than PCE. Although combustible, risk of fire or explosion can be minimized when used properly. Hydrocarbons are however pollutants. Hydrocarbons retain about 10-12% of the market.
Trichloroethylene
Trichloroethylene
The chemical compound trichloroethylene is a halocarbon commonly used as an industrial solvent. It is a clear, colourless non-flammable liquid with a chloroform-like sweet smell. It should not be confused with the similar 1,1,1-trichloroethane, w ...
is more aggressive than PCE but is very rarely used. With superior degreasing properties, it was often used for industrial workwear/overalls cleaning in the past. TCE is classified as carcinogenic to humans by the
United States Environmental Protection Agency
The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
.
Supercritical
Supercritical is an alternative to PCE; however, it is inferior in removing some forms of grime. Additive surfactants improve the efficacy of .
Carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
is almost entirely nontoxic. The greenhouse gas potential is also lower than that of many organic solvents.
The dry cleaning process involves charging a sealed chamber which is loaded with clothes using gaseous carbon dioxide from a storage vessel to approximately . This step in the process is initiated as a precaution to avoid thermal shock to the cleaning chamber. Liquid carbon dioxide is then pumped into the cleaning chamber from a separate storage vessel by a hydraulic, or electrically driven pump (which preferably has dual pistons). The pump increases the pressure of the liquid carbon dioxide to approximately . A separate sub-cooler reduces the temperature of the carbon dioxide by below the boiling point in an effort to prevent cavitation which could lead to premature degradation of the pump.
''
Consumer Reports'' rated supercritical superior to conventional methods, but the Drycleaning and Laundry Institute commented on its "fairly low cleaning ability" in a 2007 report. Supercritical is, overall, a mild solvent which lowers its ability to aggressively attack stains.
One deficiency with supercritical is that its electrical conductivity is low. As mentioned in the Mechanisms section, dry cleaning utilizes both chemical and mechanical properties to remove stains. When solvent interacts with the fabric's surface, the friction dislocates dirt. At the same time, the friction also builds up an electrical charge. Fabrics are very poor conductors and so usually, this build-up is discharged through the solvent. This discharge does not occur in liquid carbon dioxide and the build-up of an electrical charge on the surface of the fabric attracts the dirt back on to the surface, which diminishes its cleaning efficiency. To compensate for the poor solubility and conductivity of supercritical carbon dioxide, research has focused on additives. For increased solubility, 2-propanol has shown increased cleaning effects for liquid carbon dioxide as it increases the ability of the solvent to dissolve polar compounds.
Machinery for use of supercritical is expensiveup to $90,000 more than a PCE machine, making affordability difficult for small businesses. Some cleaners with these machines keep traditional machines on-site for more heavily soiled textiles, but others find plant enzymes to be equally effective and more environmentally sustainable.
Other solvents: niche, emerging, etc.
For decades, efforts have been made to replace PCE. These alternatives have not proven economical thus far:
*
Stoddard solvent – flammable and explosive,
flash point.
*
CFC-113
1,1,2-Trichloro-1,2,2-trifluoroethane, also called trichlorotrifluoroethane or CFC-113, is a chlorofluorocarbon. It has the formula . This colorless, volatile liquid is a versatile solvent.
Atmospheric reactions
CFC-113 is a very unreactive chl ...
(Freon-113), a
CFC. Now banned as ozone-unfriendly.
*
Decamethylcyclopentasiloxane ("liquid silicone"), called D5 for short. It was popularized by GreenEarth Cleaning. It is more expensive than PCE. It degrades within days in the environment.
*
Dibutoxymethane
Dibutoxymethane is an oligoether (more than one -O- grouping) or acetal containing two butyl groups and a methylene grouping. It is used in cosmetics, as a cleansing agent, or solvent. It reduces the formation of soot and nitrogen oxides when a ...
(SolvonK4) is a bipolar solvent that removes water-based stains and oil-based stains.
*Brominated solvents (
n-propyl bromide
1-Bromopropane (''n''-propylbromide or nPB) is an organobromine compound with the chemical formula CH3CH2CH2Br. It is a colorless liquid that is used as a solvent. It has a characteristic hydrocarbon odor. Its industrial applications increased dr ...
, Fabrisolv, DrySolv) are solvents with higher
KB-values than PCE. This allows faster cleaning, but can damage some synthetic beads and sequins if not used correctly. Healthwise, there are reported risks associated with nPB such as numbness of nerves. The exposure to the solvents in a typical dry cleaner is considered far below the levels required to cause any risk.
Environmentally, it is approved by the U.S. EPA. It is among the more expensive solvents, but it is faster cleaning, lower temperatures, and quick dry times.
See also
*
Fabric restoration
*
List of laundry topics
*
Wet cleaning
Wet cleaning refers to methods of professional cleaning that, in contrast to traditional dry cleaning, avoids the use of chemical solvents, the most common of which is tetrachloroethylene (commonly called perchloroethylene or "perc"). Environment ...
Notes
References
External links
Hazard Summaryprovided by the
United States Environmental Protection Agency
The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
.
NIOSH Safety and Health Topic: Drycleaning
{{Authority control
Clothing industry
Occupational safety and health
Cleaning methods