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Dry cleaning

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

History

Early dry cleaners used petroleum based solvents such as gasoline and kerosene. Corcerns over flamability lead William Joseph Stoddard, a dry cleaner from atlanta, to develop Stoddard solvent as a slightly less flammable alterative to gasoline based solvents. The use of highly flammable petroleum solvents lead 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 flamable than petroleum solvents and had much greater cleaning power. By the mid-1930s the dry cleaning industry settled on perchloroethylene (perc) as the ideal solvent. It is stable, nonflammable, and has excellent cleaning power while being gentle to garments.

Solvents used

Modern

  • Perchloroethylene -- Perfect solvent, unmatched cleaning performance
  • High flashpoint hydrocarbons DF-2000 (140*F flash point) -- Slightly less flamable and explosive than Stoddard Solvent, not as effective as perc.
  • Modified hydrocarbons blends (Pure Dry)
  • Glycol ethers (dipropylene glycol tertiary-butyl ether) (Rynex) -- not as effective as perc.
  • Cyclic Silicone decamethylcyclopentasiloxane, GreenEarth -- not as effective as perc. Very Expensive and requires royalties be paid to GE.
  • Supercritical CO2 -- Inconsistant performance, machinery is very expensive.

Historical

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 (refered to as the "basket"). 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 capcity will be between 20 and 80 lb of garments.

During the wash cycle the chamber is filled approximatly 1/3rd full of solvent and begins to rotate to agitate the clothing. The solvent temperature is controlled at 85*F, 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. The ideal flow rate is one gallon of solvent per pound of garments per minute.

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 disolve into the perchloroethylene and loose insoluble soil from fabrics comes off. It takes approximatly 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 solution of solvent soluble soils (e.g oily stains).

At the end of wash cycle, the machine starts a rinse cycle and the garmet 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 drycleaning machines can recover approximatly %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 garmets are tumbled in continious stream of warm air (145*F) 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 overdrying 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, an deodorizing (aeration) cycle starts to cool the garmets and remove the last traces of dry cleaning solvent, cool outside air is circulated 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 it 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 a ultra fine mesh to support a thin layer of filter powder (made from diatomacious earth and activated clays). Some machines use powderless filters which are capable a removing soil particles greater than 30 micron's 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 backwashed with solvent.

After passing through the lint filter, the solvent passes through an adsorptive cartridge filter, this filter is made from actived 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 disolove hydrophilic soils and keep soil from redepositing on garments. Depending on the machines 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 drycleaning operations are hazardous wastes.

Environment

Perc 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 that have not been associated with this negative publicity, including:

  • Wet cleaning with biodegradable soap and water.
  • Silicon and liquid CO2 solvents.
  • Alternative hydrocarbon solvents.

To be more precise... (Information from "Envirnoment Friendly"-page.)

  1. Wet Cleaning — This is a system that uses biodegradable soap and water. 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.)
  1. 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.
  1. Alternative Petroleum Solvents — This is more like standard dry cleaning, but the processes use alternative hydrocarbon solvents such as Exxon D-2000 or Chevron-Phillips' EcoSolv. Alternative solvents tend to be much less effective than perc, and very flamable creating a substantial explosion and fire hazard.

See also

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