How moisture works in drycleaning
Part 1
ince water-soluble soil is not soluble in
drycleaning solvents alone, it is usually removed from garments
by spotting, either before or after drycleaning. Since spotting
is a hand operation and very costly in time and chemicals, the
name of the game is to remove as much of the water-soluble soil
and spots as possible in the drycleaning washer. At the least,
we would like to have the spots loosened or brought to the
surface after cleaning so we can easily remove them.
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Moisture will also avoid caramelized
sugar stains which can develop on fibers made of cotton, linen,
rayon and acetate or blended with these cellulose materials
which were exposed to an alkaline condition and heat above
140°F.
Also, silk, wool and other animal fibers
do not need the alkalinity — just heat above 175°F
will caramelize the sugar contained in most beverages,
including alcoholic. Some fruit juices will caramelize at
140°F without the presence of alkalinity.
To understand water-soluble soil and spot
removal, we must begin with the study of humidity, which is the
study of the wetness of the air.
The air around us always contains some
moisture in the form of a gas and is invisible. If we place
water in an open container, it will gradually disappear as it
evaporates by changing from a liquid to a gas and passing into
the air.
The amount of water air can hold varies
with the air temperature. Warm air will hold more moisture than
cold air. When air has more moisture than it can hold, the
excess moisture becomes visible as rain, snow, dew, fog or
frost. “Saturated” air contains all the water it
can hold.
The “absolute humidity” of
the air is the actual amount of water present in the air and is
expressed as grains of water (weight of the water vapor) per
cubic foot of air. Therefore, if one cubic foot of air contains
three grains of water, its absolute humidity is three grains of
water vapor.
“Relative humidity” (RH)
expresses the relationship between the amount of water actually
in the air compared to the total amount of water the air can
hold at any given temperature. It is expressed as a percentage.
As drycleaners, we must understand
relative humidity since it can affect soil removal, shrinkage,
wrinkling and graying. It is mathematically determined by
dividing the amount of moisture in the air, at a given
temperature, by the amount of moisture the air can hold, at
that temperature, and multiplying by 100.
Example: If one cubic foot of air
contains four grains of moisture at 70°F, what is the RH of
the air? Given: One cubic foot of air can hold a maximum of 8
grains of moisture at 70°F.
Formula: Percent RH = 4 divided by 8 x
100, or 50 percent.
The relative humidity is 50 percent, and
the absolute humidity is four grains of moisture.
The water-holding capacity of air
increases sharply with a corresponding increase in temperature.
The water-holding capacity of air approximately doubles for
each 20°F rise in temperature between 0°F and
120°F.
Most fibers have the capacity to absorb
water vapor from the atmosphere. Air deposits its moisture
content on the fibers as it passes through them (process of
adsorption).
The physical properties of these fabrics
are affected by the amount of moisture they absorb. If it
absorbs too little moisture, the fabric will feel harsh and
stiff; it will easily retain a static charge, and it will
become difficult to process and be coated with lint or pills.
If it absorbs too much moisture, the fabric will feel limp and
damp, mildew may grow on it, it may shrink or wrinkle and it
will not hold a crease.
The amount of moisture in a fabric is
controlled by relative humidity, not absolute humidity.
Adsorbed moisture greatly affects the
properties of textiles because most of the moisture is absorbed
into the fibers and causes them to swell. This is the
difference between drycleaning and wetcleaning/laundry since a
dry solvent will not cause fibers to swell or pants to
completely lose their creases. Of course, some fibers are
non-absorbent (thermoplastics such as polyester, nylon and
acrylic) and do not swell.
In dry weather, in a heated room (not a
drycleaning plant: humid), fabrics lose much of their moisture
to the surrounding air. In a damp room or warm, damp weather,
they pick up moisture from the surrounding air. In the
drycleaning washer, therefore, water-soluble soil removal
(WSSR) increases noticeably during the summer months and falls
off again during the winter months.
At 75°F and 90 percent relative
humidity, the following fibers hold the indicated amount of
moisture:
Wool, 3 ounces per pound.
Rayon, 2.8 ounces per pound.
Silk, 2.5 ounces per pound.
Cotton, 1.8 ounces per pound.
Acetate, 1.6 ounces per pound.
This difference in moisture content
between the conditioned percent regain and the desired percent
regain for high water-soluble soil removal (WSSR) is what must
be supplied by the moisture contained (or added) in the charged
solvent (anionic detergent) or by emulsion additions (cationic
detergent/water blend).
We learned last month that the
drycleaning solvent can hold water only in proportion to its
anionic detergent concentration. Therefore, it is very
difficult to condition a “dry” load in a low
charged solvent. Very little water-soluble soil removal (WSSR)
can occur until the load acquires a moisture content equal to
approximately 70 percent RH. Hence, a dry load will pull a
large proportion of the water out of a weak charge and still
not get enough to obtain water-soluble soil removal (WSSR)
during the first few minutes of the drycleaning wash cycle.
Remember, a good anionic detergent charge is 1.5 percent to 1.7
percent.
Caution: I do not recommend adding
moisture to the drycleaning washer by way of spray-spotting the
garments. This was acceptable during the “polyester
era” when the load was almost 100 percent polyester.
Today, with unserviceable dyes and natural fibers, you can
easily get dye crocking, dye loss, fabric abrasion,
re-deposition, severe wrinkling and shrinkage.
Since the spray spotting detergent is
both water-soluble and solvent-soluble, it is safer to hang the
sprayed garment until it is completely dry before drycleaning
it. Water must be added directly to the SOLVENT for its
conditioning.
Water must be added to the charged
solvent to restore the solvent RH. It takes a small amount of
moisture to raise solvent RH above the danger point (over 75
percent RH in a weak anionic detergent charge). Water is added
to the solvent/detergent by either hand or automatic moisture
control device.
What do we mean by solvent relative
humidity?
A detergent-charged drycleaning solvent
acts toward moisture very much as air does. It can hold
moisture, but the moisture in the solvent retains a certain
vapor pressure, just as in air. Therefore, a charged solvent
eventually equalizes with the air above it. When this
equilibrium is attained, the vapor pressure of the moisture
leaving the solvent is equal to the vapor pressure of the
moisture in the air. Thus, the solvent has the same relative
humidity as the air above it, and is called “solvent
relative humidity” (SRH).
If we were to place a piece of rayon or
wool fabric into solvent which is at equilibrium with an
atmosphere of 70 percent RH, it would absorb exactly the same
amount of moisture as it would if we let it hang in the air
above the solvent.
Dry garments rapidly pick-up moisture
from the conditioned solvent in the drycleaning washer. Until
the moisture content is built up inside the drycleaning washer,
very little water-soluble soil removal (WSSR) will be obtained.
It might take as much time as 15 minutes
(in perchloroethylene, hydrocarbon or Stoddard) to overcome the
low initial moisture content of the load. When the load has
been exposed to high atmospheric humidity before cleaning, as
would occur on a humid summer day, it will not pick-up much
moisture from the solvent. Under these conditions, not much
water would have to be added to the solvent/detergent, and
water-soluble soil removal (WSSR) would begin shortly after the
cleaning cycle starts.
Therefore, we can conclude that garments
exposed to low relative humidity before cleaning require a
longer washing time to get the same amount of water-soluble
soil removal (WSSR) as could be obtained if the same garments
had been exposed to a higher relative humidity.
Next month we will continue the
discussion on solvent relative humidity and water-measuring
control which automatically adds the desired amount of water
safely to the anionic charged solvent. Also, we’ll see
how the cationic detergent/water blend works on water-soluble
soil. You can then decide which is the better method. I have
already done that years ago.
Note: My spotting video, “The
Caplan Method of Stain Removal,” which includes my
comprehensive text (edited by Hal Horning) and handy spotting
board reference, is available in English, Spanish and Korean
(video only). A special disc for South America equipment is
also available through Golomb Group (e-mail: dm@golombgroup.com
or phone (800) 679-5856). A lecture and demonstration are
presented similar to my classes over the years at IFI and SDA
schools. This video and text are ideal for training
inexperienced spotters as well as a good review for experienced
spotters. Digesting, bleaching, oxidized oil stains and
caramelized sugar stains are discussed and demonstrated. An
article on “Removing Spots in the Cleaning Machine (for
perc and petroleum/hydrocarbon) is included on the text book.
Also available from Golomb Group is my
video on step-by-step shirt finishing which includes my
comprehensive text in loose-leaf form outlining each procedure
for single operator and two-operator cabinet shirt unit using a
cabinet sleeve press. Proper forming of the collar using heated
collar formers is demonstrated. Each lay is demonstrated for
top quality and production with very little effort. Attractive
detailing and packaging of the hung shirt, padding, steam
pressures and timing are all discussed. A unique wash formula
for whiter whites and brighter colors and removal of grease and
body oils is included in the textbook.
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