Dry cleaning

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Dry Cleaning is any cleaning process for clothing and textiles using a solvent other than water.

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 by the owner of a French dye-works, Jean Baptiste Jolly. He noticed that a tablecloth in his home became cleaner after his maid had spilt kerosene from a gas lamp onto it, and from this observation went on to develop a service to clean other people's clothes in this manner, which he termed 'Dry Cleaning'.

Early dry cleaners used these 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, because they were much less flammable than petroleum solvents and had much greater cleaning power. By the mid-1930s the dry cleaning industry began using tetrachloroethylene (perchloroethylene) as a standard, coloquially called "perc", as the ideal solvent. It is stable, nonflammable, and has excellent cleaning power while being gentle to garments.

Solvents used

Modern

• Perchloroethylene — Most common solvent, known more commonly as "Perc", the "standard" for cleaning performance, and most aggressive cleaner (use caution, may cause color bleeding/loss, especially at higher temperatures).

• High flash point hydrocarbons DF-2000 (140°F/60°C flash point) — Slightly less flammable and explosive than Stoddard Solvent, not as effective as perchloroethylene.

• Modified hydrocarbons blends (Pure Dry)

• Glycol ethers (dipropylene glycol tertiary-butyl ether) (Rynex) — not as effective as perchloroethylene.

• Cyclic Silicone decamethylcyclopentasiloxane, GreenEarth — not as aggressive as perchloroethylene. Gentler cleaning through lowest surface tension of current solvents. Cost per gallon is similar to perc in most states with environmental surcharges. Solvent mileage higher than perc and hydrocarbon products. Requires a license be obtained to utilize the intellectual property of GreenEarth Cleaning. Degrades within days in the environment to SiO2 ( sand ), CO2 , and H2O.

• Supercritical CO₂ — Inconsistent performance; machinery is very expensive.

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

Process

A dry cleaning machine is somewhat similar to 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 (36 KG) of garments.

During the wash cycle the chamber is filled approximately 1/3rd 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 condensor. The ideal flow rate is one gallon of solvent per pound 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 judgement 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 and perspiration 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.

Garments are also checked carefully for foreign objects; items such as plastic pens will melt 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 coloured textiles to avoid colour transfer. Garments are checked carefully for dry cleaning textile compatibility, including the fasteners; many decorative fasteners are either not dry cleaning solvent proof, or will not withstand the mechanical action of the cleaning cycle. These will be removed and restiched after the cleaning, or protected with a small padded protector as appropriate. Finally, fragile items such as feather bedspreads or tasselled rugs or hangings may be enclosed into a loose mesh bag. Perchloroethylene is around 1.7Kg/L at room temperature (70% heavier than water), and the sheer weight of absorber solvent may cause the textile to fail under centrifugal 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 much longer wash cycle of at least 25 minutes because of the much slower rate of solvation of solvent soluble soils (e.g oily stains). A drycleaning surfactant "soap" may also be added.

At the end of 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 adsorbed 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% of the solvent used in the cleaning process.

The extraction begins by draining the solvent out of the washing chamber cycle 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 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 to 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.

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, odorless and ready for pressing/finishing.

Solvent processing

Working solvent from the washing chamber passes through several filtration steps before it is returned to 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 (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 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 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%-1.5%) 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.

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.

Environment

Perchloroethylene is toxic, and some believe that long-term exposure can cause liver and kidney damage, though no study has conclusively proven that. There are other solvents including:

• Wet Cleaning — This is 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 purportedly clean 99.9% of "dry clean only" garments safely, including leather; suede; most tailored woolens, silks and rayons. (Neckties seem to be the one exception.)
• Silicone and Liquid CO2 Solvents — Relatively new approaches to dry cleaning have been developed based on both liquid carbon dioxide (usually obtained as a recycled byproduct of other industrial processes) and silicone. Dry cleaners using these solvents are currently few in number.
• Alternative Petroleum Solvents — This is more like standard dry cleaning, but the processes use alternative hydrocarbon solvents such as Exxon DF-2000 or Chevron-Phillips' EcoSolv. Alternative solvents tend to be much less effective than perc, and very flammable creating a substantial explosion and fire hazard.

References