Dry cleaning

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. 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.

Shift to tetrachloroethylene

By the mid-1930s, the dry cleaning industry had adopted tetrachloroethylene (perchloroethylene), or PCE for short, as the solvent. It has excellent cleaning power and is stable, nonflammable, and gentle to most garments. PCE, however, was incidentally the first chemical to be classified as a carcinogen by the U.S. Consumer Product Safety Commission. Tetrachloroethylene is currently considered a potential carcinogen by the Environmental Protection Agency and the National Institute for Occupational Safety and Health (NIOSH). In 1993, the California Air Resources Board adopted regulations to reduce PCE 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 PCE with other solvents remains of interest as it will ultimately disappear from the market.

Infrastructure

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, and the second to dry the garments.

Machines of this era were described as vented; their 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 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. The majority of 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

The mechanism of dry cleaning can be explained by selective solubility, often known as "like dissolves like" among chemists. Selective solubility refers to the phenomena of how polar solutes dissolve in polar solvents and non-polar solutes only dissolve in non-polar solvents.

Dry-cleaning utilizes non-polar solvents so that stains that would normally not dissolve in water, easily dissolve. This eliminates the needs for a high temperature and tumbling, resulting in less damage to 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 laundry. 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

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 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 periods 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 where 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.

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

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.

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

PCE is classified as carcinogenic to humans by the United States Environmental Protection Agency and must be handled as a hazardous waste. To prevent it from getting into drinking water, dry cleaners that use PCE must take special precautions.

When released into the air, PCE can contribute to smog when it reacts with other volatile organic compounds. California declared perchloroethylene a toxic chemical in 1991, and its use will become illegal in that state in 2023.

The most popular alternative to PCE is decamethylcyclopentasiloxane ("liquid silicone"), called D5 for short. It is popularized by GreenEarth Cleaning.

CO₂ represents an alternative to PCE, however CO₂ is inferior in removing some forms of grime. Research has pursued additive surfactants to improve the efficacy of CO_2.

Perchloroethylene

Perchloroethylene (PCE, or tetrachloroethylene) has been in use since the 1940s. PCE is the most common solvent, the "standard" for cleaning performance. It is a most aggressive cleaner. It can cause color bleeding/loss, especially at higher temperatures, and may damage special trims, buttons, and beads on some garments. Better for oil-based stains (which account for about 10% of stains) than more common water-soluble stains (coffee, wine, blood, etc.). Known for leaving a characteristic chemical smell on garments. Nonflammable. PCE is becoming less popular due to its ground contamination problems and potential health effects.

Hydrocarbons

Hydrocarbons are 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 PCE and require a longer cleaning cycle. Although combustible, these solvents do not present a high risk of fire or explosion when used properly. Hydrocarbon also contains volatile organic compounds (VOCs) that contribute to smog. Pure Dry is another brand.

Dibutoxymethane

Dibutoxymethane (SolvonK4) is a bipolar solvent that removes water-based stains and oil-based stains. Dibutoxymethane is considered to be one of the best replacements solvents for PCE as cleaning performance is very similar.

Liquid silicone

Liquid silicone (decamethylcyclopentasiloxane or D5) is gentler on garments than PCE and does not cause color loss. Though more environmentally friendly, it is more expensive. It degrades within days in the environment to silicon dioxide and trace amounts of water and carbon dioxide (CO₂). It produces nontoxic, nonhazardous waste. Toxicity tests by Dow Corning show the solvent to increase the incidence of tumors in female rats (no effects were seen in male rats), but further research concluded that the effects observed in rats are not relevant to humans because the biological pathway that results in tumor formation is unique to rats.

Brominated solvents

Brominated solvents (n-propyl bromide, Fabrisolv, DrySolv) are solvents with a 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 as an alternative to hazardous solvents used in the past. It is among the more expensive solvents, but due its faster cleaning, lower temperatures, and quick dry times, it's considered to have the same or lower costs overall for the entire process.

Carbon dioxide

Supercritical CO₂, which is the liquid form of carbon dioxide above a critical temperature and critical pressure, contains a lot of potential, as supercritical CO₂ is not only mild but the usage of carbon dioxide as a dry-cleaning solvent provides a means to reduce carbon dioxide levels in the atmosphere.

Advantages

Carbon dioxide is almost entirely nontoxic and carbon dioxide cleaned clothing does not give off volatile compounds. This resolves health-related concerns regarding traditional solvents and reduces environmental impact. Furthermore, the greenhouse gas potential is also lower than that of many organic solvents.

Carbon dioxide cleaning can also be used for fire- and water-damage restoration due to its effectiveness in removing toxic residues, soot and associated odors of fire.

Concerns

Consumer Reports rated this method superior to conventional methods, but the Drycleaning and Laundry Institute commented on its "fairly low cleaning ability" in a 2007 report. Supercritical CO₂ is, overall, a mild solvent which lowers its ability to aggressively attack stains.

Another concern with supercritical CO₂ is that the conductivity of this solvent is poor. As mentioned in the Mechanisms section, dry cleaning utilizes both chemical and mechanical properties to effectively remove stains. Friction occurs when the solvent interacts with the surface of the fabric and this dislocates some 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. This results in a poor efficiency in the washing process overall. To make up for the poor solubility and conductivity of supercritical carbon dioxide, current research is 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 molecules while other antistatic agents are also being researched.

Furthermore, machinery is expensive — up 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 the heavier soiled textiles, but others find plant enzymes to be equally effective and more environmentally sustainable.

Glycol Ethers

Glycol ethers (dipropylene glycol tertiary-butyl ether, Rynex, Solvair, Caled GenX and Impress) are proposed as environmentally friendly competitors to PCE with processing advantages including lower cycle times, reduced detergent use, and better cleaning.

Modified Alcohol

Modified alcohol (SENSENE) is an environmentally friendly alternative for PCE and HCS with processing advantages. Better cleaning than PCE, less greying than with HCS, reduced finishing effort and very low odor.

Historical