Dry Cleaning is any cleaning process for clothing and textiles using a solvent other than water.
Early dry cleaners used petroleum based solvents such as gasoline and kerosene. Concerns over flammability lead 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 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 flammable 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.
- Perchloroethylene — Perfect solvent, unmatched cleaning performance
- High flash point hydrocarbons DF-2000 (140*F 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 effective as perchloroethylene. Very Expensive and requires royalties be paid to GE.
- Supercritical CO2 — Inconsistent performance, machinery is very expensive.
- Stoddard Solvent — Very flammable and explosive, 100*F flash point.
- carbon tetrachloride — Toxic and corrosive
- trichloroethane — Overly aggressive and harsh
- Valclene 113 Freon-113 — Ozone destroying CFC, otherwise very nice and non toxic.
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”). 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 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, 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 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).
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.
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 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 a 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 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.
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.
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 including: