Several types of solvents are currently used for dry cleaning; perchlorethylene (perc), petroleum, and new hydrocarbon petroleum distillates specially formulated just for the dry cleaning industry within the last decade. While all perform essentially the same function, their structures and properties are different.
A solvent must meet certain criteria in order to be used for dry cleaning. For example, an acceptable dry cleaning solvent must be free of objectionable odors, and certainly must not leave residual odor in garments after drying. In addition, the solvent should be able to be safely heated to its boiling point for distillation purposes so that it may be continually cleaned and recycled.
In order to be used for safe and effective dry cleaning, a solvent must have the capability to dissolve solvent-soluble substances. This “solvent power” must fall in a range that will effectively remove solvent-soluble soils (fats, oils and greases) without risking any damage to common textile fibers and dyes.
Solvents that are appropriate for use in dry cleaning perform a number of functions. Dry cleaning solvents dissolve solvent-soluble soils, such as oils, waxes, and greases. They also act as a carrier for insoluble soils. Solvents carry detergent, which in turn carries water to remove water-soluble soils. Lastly, in combination with mechanical action, solvents produce a flushing action on fabrics to aid in cleaning.
There are two types of dry cleaning machines used in the industry for cleaning. One type is the dry-to-dry machine, in which the clothes are put in dry and come out dry, ready to finish. The other type is the transfer unit, in which the clothes are cleaned and extracted in one machine and dried in another machine before finishing. This transfer unit type is being phased out of existence giving way to the newest, most advanced dry-to-dry machines which are completely self contained, closed machines which have little or no emissions into the atmosphere. There are various options to choose from, such as filtration systems and model sizes. These options apply to all types of solvents.
The actual cleaning process for dry cleaning is similar to the washing process. Clothes are separated by weight, finish, and color. Heavyweight clothes are separated from lightweight clothes. Delicate clothes are further separated to be cleaned by themselves. And, finally, light colors and dark colors are cleaned separately.
The clothes are cleaned in machines that look like large, over-sized front load washing machines. The difference, though, is that the cleaning solvent is used over and over again, continually being recycled through filtering and distillation.
As the solvent passes from the wheel to the filters and back to the wheel (where the clothes are), the solvent passes through several filters removing solvent-soluble soils, such as oils and fats, as well as solvent-soluble dyes.
With each load of clothes cleaned, some distillation takes place. As the clothes are dried, the solvent vapors are passed over condensation coils where the solvent is again returned to its clear liquid form and reused. Additionally, a portion of solvent from each load can, in certain machine models, be pumped into a still where it is heated and turned into vapors which are condensed and returned to a clear liquid form.
Finally, after the clothes are removed from the machine, they are checked for any additional spot cleaning necessary, steam finished, and then ready to return to the customer.