Biodegradable dry cleaning solvent

A method for dry-cleaning garments which comprises treating them with an azeotropic solvent of propylene glycol tertiary-butyl ether and water.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The present invention relates to a novel dry-cleaning solvent and a method 
for effecting dry-cleaning using said solvent. More particularly, the 
present invention relates to a solvent which is comparable or superior to 
perchloroethylene in its attributes and benefits, but which does not 
suffer from the serious environmental, health and occupational negatives 
and problems associated with the use of perchloroethylene. 
DESCRIPTION OF THE PRIOR ART 
The most widely used dry-cleaning solvent is perchloroethylene, which is 
commonly referred to and will be referred to sometimes hereinafter as 
("Perc"), which is a chlorinated hydrocarbon-based solvent. It is the 
dry-cleaning solvent of choice throughout North America, Europe and Asia. 
In addition to Perc's use in the dry-cleaning industry, it has found 
extensive use as a degreasing agent in the metals industry, in 
scouring/milling and in various "clean room" applications in the 
semiconductor and electronics industry. The industrial uses of Perc are 
approximately tenfold greater than its use as a solvent for dry-cleaning. 
While Perc has been found to be an effective dry-cleaning agent due to the 
fact that it does not damage synthetic fabrics or cause shrinkage to 
fabrics containing naturally occurring fibers, such as wool, as well as 
being non-flammable and possessed of a relatively low boiling point, which 
permits its being reclaimed and purified by means of ordinary 
distillation, it does present a number of other problems which present 
drawbacks to its use. 
In particular, perchloroethylene presents a number of health and 
environmental hazards which would militate against its continued use, 
provided a substitute solvent of comparable quality were available which 
were free of the aforementioned hazards. 
Since Perc is heavier than water, its disposal represents a significant 
environmental risk since it will sink to the bottom of the aquifer, lake, 
river, etc., with possibly resultant contamination of the water supply. 
Additionally, Perc vapors have been implicated as having a deterious 
effect on the central nervous system. In addition, due to its being a 
highly chlorinated molecule, Perc has been identified as being a 
significant health hazard to cattle, and as a cause of skin cancer, 
particularly melanoma, due to the action of the chlorine in Perc depleting 
oxygen from the ozone layer. Furthermore, and of particular import, is the 
fact that Perc is not biodegradable and, hence, will, over a period of 
time accumulate, presenting a significant industrial waste disposal 
hazard. 
As the nature and seriousness of the foregoing problems became more and 
more manifest with the passage of time and with the completion of various 
research and clinical investigations into the nature of Perc and its 
mechanisms of action, the use of alternative solvents has been sought, but 
none have met with any degree of commercial success since they could not 
match the results obtained by Perc as a dry-cleaning agent. 
However, at this point in time, when environmental concerns are being 
rigorously monitored and policed by domestic and foreign governments by 
means of legislation and civil and even criminal prosecution, the need for 
a substitute solvent for Perc for dry-cleaning operations, as well as 
other operations, has become a matter of some degree of urgency. 
A difficulty in identifying a replacement dry-cleaning solvent for Perc is 
that it must meet so many requirements, both as to its efficacy as a 
dry-cleaning agent, i.e., non-shrinking with respect to about 160 types of 
fabric, dye-fast or non-bleeding with respect to about 900 types of dyes, 
a high flashpoint to render it non-flammable and non-combustible, the 
ability to separate from water, effective detergency, distillable, 
reclaimable, usable with existing dry-cleaning equipment, etc., as well as 
its being non-polluting to the water supply and the ozone layer, 
biodegradable, non-toxic, non-carcinogenic, etc. 
One proposed solvent substitute, namely, propylene glycol monomethyl ether, 
which is disclosed in EP 0479 146 A2 as possessing many desirable 
properties, was found to be wanting in that it causes damage to weaker 
dyes and to fine yarns and to delicate fabrics, such as acetates, due to 
its pronounced tendency to accumulate water. Water accumulation or 
water-miscibility is also a decided negative from another aspect in that 
it significantly impairs the efficiency of the dry-cleaning process since 
the dry-cleaning equipment is burdened with the handling of excessive 
quantities of water and the solvent stock is diluted and must be brought 
back to a correct ratio for stability reasons. 
In line with the foregoing, it is an object of the present invention to 
provide a solvent which possess comparable, if not superior chemical and 
physical properties when compared to Perc in dry-cleaning, degreasing of 
metals, cleaning of electronic components and the scouring and milling of 
woolens, while, simultaneously, protecting the environment, public health 
and safety from the many known negatives associated with the use of Perc. 
It is a further object of the present invention to provide a dry-cleaning 
solvent which has a specific gravity less than that of water. 
It is still a further object of the present invention to provide a 
dry-cleaning solvent which minimizes or eliminates shrinkage of woolen 
garments, prevents or limits the bleeding of dyes, and which is able to 
treat acetates, silks, virgin wool and other delicate fabrics gently so as 
to avoid damage. 
It is still yet another object of the present invention to provide a 
dry-cleaning and degreasing solvent which is non-flammable and which has a 
sufficiently low boiling point to allow it to be reclaimed and purified 
via conventional distillation processes. 
The foregoing objects and other objects and advantages of the present 
invention will be apparent to persons of ordinary skill in the art from 
the following description of the specific embodiments of the invention. 
SUMMARY OF THE INVENTION 
As a result of experimental investigation, it has been found that a 
particular propylene glycol ether, when mixed with certain weight 
percentages of water to form an azeotropic mixture, serves to provide a 
highly effective dry-cleaning solvent which performs as well as, or better 
than, Perc, and without its attendant environmental, health and 
occupational safety problems. An azeotropic mixture of water and the 
particular propylene glycol ether which has been identified has the 
advantage of behaving like a single substance. A mixture of chemicals is 
azeotropic if the vapor composition is identical to that of the liquid 
phase. This means that the distillate of an azeotrope is theoretically 
identical to the solvent from which it is distilled. 
DETAILED DESCRIPTION OF THE INVENTION 
It has been found that when propylene glycol tertiary-butyl ether (PTB) is 
used in the dry-cleaning of garments, it possesses all of the attributes 
associated with perchloroethylene and none of its drawbacks. Furthermore, 
propylene glycol tertiary butyl ether also has certain significant 
advantages not possessed by perchloroethylene. For instance, by using an 
azeotropic mixture of PTB and water, the water component of the binary 
azeotrope is effectively tied-up, thus avoiding the tendency of woolen 
garments to shrink in water, while simultaneously preventing damage to 
acetates. Further, the water component lowers the boiling point of PTB to 
96.degree. C. and raises its flashpoint. 
It has been found, for example, that when propylene glycol tertiary-butyl 
ether is mixed with water in certain weight percentages, an azeotrope is 
formed which is preferred in the dry-cleaning of garments. 
It has also been determined that a solvent comprising a mixture of PTB and 
water is effective in degreasing of metals, in the cleaning of electronic 
components, as well as being effective in the scouring and milling of raw 
wool. An azeotropic ratio of PTB and water is not needed in these types of 
processes, but is nonetheless preferred. This is especially so in the 
scouring of raw wool, which involves the pulling of oils and fatty acids, 
e.g. lanolin, from wool and in the milling of the yarns formed therefrom. 
Such scouring and milling operations are conducted in dry-cleaning 
machinery. 
A particular advantage of the PTB-water azeotrope of the present invention 
in dry-cleaning is that it does not behave like a typical mixture, but, 
rather, its behavior is the same as a single substance. This permits a 
better defined separation upon distillation at a lower boiling point and 
also facilitates its being reclaimed, more effectively, at a level of 99% 
or greater, and also enhances its purification using conventional 
distillation techniques. 
Of particular note, from an economic as well as an operational standpoint, 
is the ability of PTB to separate from water. This is of particular 
significance in dry-cleaning since garments entering a dry-cleaning plant 
contain water in the form of moisture. If water were not separable from 
the PTB solvent, the PTB azeotrope would be diluted with free water and, 
thus, the dry-cleaning process, and its efficiency, would be seriously 
compromised, as would the reclaimability of PTB. For example, from the 
perspective of performance, the PTB will enhance the ability to dry and 
clean woolen and cotton garments since those types of garments will be 
subjected to little, if any, shrinkage due to the fact that PTB has a 
limited degree of miscibility with water. Additionally, this limited 
degree of miscibility avoids dilution of the solvent stock with its 
attendant problems, which are not inconsiderable when one considers the 
need to replenish the solvent. 
The PTB azeotrope is a very effective dry-cleaning solvent since its 
detergency action breaks down solvent-soluble stains, which account for 
15% of all stains found in garments and which are caused by fatty acids. 
The detergency of the solvent occurs by lifting the soiled area from a 
surface and by displacing it with surface active materials that have a 
greater affinity for the surface than they do for the soiled area. It can 
also deal most effectively with water-soluble stains, which account for 
more than 80% of stains encountered in dry-cleaning, such as, for example 
stains from fruit, blood, urine, sweat, etc. It has also been found to 
limit the bleeding of dyes and to avoid the shrinkage of man-made 
polymers, such as acetates. 
From the standpoint of health and safety, which as previously mentioned are 
of significant importance in the present era, the PTB azeotrope is 
non-flammable, non-combustible, non-carcinogenic, non-toxic and, of the 
utmost import, it is biodegradable and weighs less than water, i.e., its 
specific gravity is less than that of water. 
The PTB azeotrope dries at a relatively low temperature, namely, about 
55.degree. C., which is well within the drying requirements for fabrics 
constructed of fine yarns so as to avoid damage thereto by excessive heat. 
In preparing the PTB azeotropic solvent of the present invention, PTB and 
water, are combined, most preferably at the azeotropic ratio. 
The mixture is first heated until it boils at its azeotropic boiling point, 
which in the case of PTB and water, is about 96.degree. C. The heated 
mixture should then be cooled to molecularly bind the water to the fullest 
extent. This results in the formation of the PTB-water azeotrope. The 
resultant azeotrope can now be distilled, with the azeotrope recoverable 
after distillation. The number of distillations which can be done are 
indefinite in number, with a recovery rate of the PTB being in excess of 
99%. 
A departure from the azeotropic ratio of about 82%, by weight, of PTB and 
about 18%, by weight, of water will result in the shrinkage of woolen 
garments if the amount of PTB is decreased, due to the presence of a 
greater quantity of unbound water in the composition or, conversely, if 
the quantity of PTB is increased, damage to acetate fabrics can occur, 
accompanied by increased bleeding of dyes. It is therefore preferable that 
the quantity of PTB be maintained at less than 90%, by weight, and even 
more preferably less than 85%, by weight. At those percentages, it is 
still an effective dry-cleaning solvent. Most preferably, as stated 
previously, is the use of about 82%, by weight, of PTB and about 18%, by 
weight, of water which is the azeotropic ratio and which provides the best 
dry-cleaning results combined with the most efficient and cost-effective 
dry-cleaning operation. 
While PTB can quite successfully and efficiently clean garments made of all 
types of textile fabrics without the need for additional agents, such as 
detergents and fabric softeners, it is desirable to include in the 
formulation one or more surfactants to enhance the detergency action of 
the PTB, by means of reducing the surface tension of the azeotrope. 
Exemplary surfactants are fatty alcohol polyethylene glycol ethers and 
linear primary alcohol eynoxylates. While fabric softeners are not 
necessary to achieve effective dry-cleaning, they are beneficial and serve 
to enhance the dry-cleaning process. 
The following examples are set forth to illustrate more clearly the 
principle and practice of the present invention. It is to be understood, 
of course, that the invention is not limited to the specific examples.

EXAMPLE 1 
One of the most significant properties that a dry-cleaning solvent should 
possess is limited fiber shrinkage to ensure that fibers comprising the 
garment do not shrink excessively. Excessive shrinkage, naturally, deforms 
the garment rendering it unsuitable for future wear. Accordingly, the 
dry-cleaning solvent which is employed must not excessively shrink the 
component fibers which comprise the fabric of the garment. In contemporary 
usage, garments containing virgin wool and acetates, such as the lining 
found in men's jackets, can ill-afford shrinkage beyond established norms. 
A shrinkage test was conducted with respect to virgin wool by taking a 
series 4".times.4" patterned virgin wool swatches and immersing them in 
separate containers containing each of the azeotropic solvents set forth 
in Table I below. Approximately 10 minutes of mechanical action was 
applied to ensure that the wool fibers became totally saturated. The test 
swatch was then removed and dried at a constant temperature not exceeding 
55.degree. C. The test swatch was then compared with a control material to 
identify any changes in the fibers to ensure that the patterns had not 
changed their dimensions. 
Each of the test solvents was then analyzed to identify any fiber loss. The 
maximum shrinkage should not exceed 2% on the first immersion test and is 
usually expected to be less than 0.25% in any subsequent immersion test. 
TABLE I 
______________________________________ 
% Shrinkage 
Solvent on 1st Immersion 
______________________________________ 
PM 2% 
(propylene glycol methyl ether) 
PNP 2% 
(propylene glycol n-propylether) 
DPM 2% 
(dipropylene glycol methyl ether) 
PERC 2% 
(perchloroethylene) 
PTB 1/2% 
(propylene glycol tertiary-butyl ether) 
______________________________________ 
EXAMPLE 2 
The shrinkage test conducted in Example 1, was repeated with 4".times.4" 
swatches of acetate fabric. The results are set forth below in Table II. 
TABLE II 
______________________________________ 
% Shrinkage 
Solvent on 1st Immersion 
______________________________________ 
PM 3% 
(propylene glycol methyl ether) 
PNP 3% 
(propylene glycol n-propyl ether) 
DPM 2-5% 
(dipropylene glycol methyl ether) 
PERC 2% 
(Perchloroethylene) 
PTB 1/2% 
(propylene glycol tertiary-butyl ether) 
______________________________________ 
It is evident from an examination of the results tabulated in Tables I and 
II that propylene glycol tertiary-butyl ether azeotropic mixture resulted 
in the smallest percentage of shrinkage in both virgin wool and acetate 
fabrics and, in fact, reduced shrinkage by about 400% or greater compared 
with the other solvents, including Perc, when employed with virgin wool, 
and an even greater percentage when employed with acetate fabrics. 
EXAMPLE 3 
The bleeding of dyestuffs is the bane of the dry-cleaners existence. The 
variety of dyestuffs, their differing chemical structures, the degree to 
which they are soluble or insoluble in the particular dry-cleaning solvent 
employed, etc., present manifold problems which must be met, addressed and 
solved before a new dry-cleaning solvent can be introduced successfully. 
Dye-bleeding tests were conducted by taking test swatches of virgin wool, 
1".times.1", and immersing them in separate containers filled with each of 
the azeotropic solvent mixtures indicated in Table III below. Ball 
bearings were added to each of the containers to increase the impact of 
mechanical action on the dyes in an effort to dislodge the dyes from the 
fabric. The increased mechanical action was applied for a period of 10 
minutes. Thereafter, the test swatch and the ball bearings were removed 
from the solvent. Colorimeter tests employing a Bausch Lomb Spec-20 
colorimeter were conducted on the solvent remaining, which serves to 
indicate the relative quantity of dye removed by the test swatch. The 
results are set forth below in Table III with respect to the various 
solvents tested on virgin wool swatches which had been dyed red, green, 
yellow, blue and purple, respectively. The greater the value, the greater 
the degree of dye-bleeding. 
TABLE III 
______________________________________ 
Dye Bleeding 
Solvent 
Red Green Yellow Blue Purple 
______________________________________ 
PM 8 7 7 8 8 
PNP 6 4 4 5 6 
DPM 6 3 5 5 6 
Perc 2 2 1 1 3 
PTB 2 1 1 2 1 
______________________________________ 
EXAMPLE 4 
In similar fashion to Example 3 above, swatches of various colored acetate 
fabrics were tested to determine dye bleeding in the below-listed 
solvents. The results are set forth in Table IV below. 
TABLE IV 
______________________________________ 
Dye Bleeding 
Solvent 
Red Green Yellow Blue Purple 
______________________________________ 
PM 9 8 9 9 8 
PNP 9 8 8 8 8 
DPM 8 8 8 9 8 
Perc 1 1 1 2 2 
PTB 2 1 1 2 2 
______________________________________ 
It is clearly evident from Tables III and IV that the azeotropic solvent of 
the present invention, namely, propylene glycol tertiary-butyl ether 
(PTB), is far superior to PM, PNP and DPM, and is comparable to Perc, as 
respects dye bleeding, whether the fabric employed is virgin wool or 
acetate. In point of fact, the solvent of the present invention was in 
each instance, regardless of fabric type or dye color, significantly more 
effective in preventing the bleeding of dyes when compared with the 
non-Perc solvents. 
EXAMPLE 5 
A stain removal test was conducted with respect to cotton by taking a 
series of 12".times.12" test panels of cotton and applying thereto 
standard stain items as set forth below in Table V, which were then 
cleaned with a Perc azeotropic mixture containing soap. Another set of 
test panels similarly stained were cleaned with the PTB azeotropic mixture 
of the present invention without soap. It will be understood by those 
skilled in the art that the purpose of the solvent is to act as a carrier 
for detergents, soaps, water, etc. and that most stains are removed by 
"spotting" prior to the dry-cleaning process. 
TABLE V 
______________________________________ 
Type of Stain Perc w/ Soap 
PTB w/o soap 
______________________________________ 
Shoe polish 50% 50% 
Lipstick 60% 70% 
Face Powder 100% 100% 
Ketchup 40% 70% 
Salad Dressing 
70% 80% 
Animal Fat 80% 80% 
Mascara 90% 90% 
Mayonnaise 90% 90% 
Coffee 30% 60% 
Ink 30% 40% 
Motor Oil 80% 80% 
Syrup 80% 90% 
______________________________________ 
It is evident with respect to each of the stains enumerated, which are 
quite typically encountered by dry-cleaners, that the PTB azeotropic 
mixture of the present invention performed as well as or better than Perc, 
which is the most prevalent solvent employed in dry-cleaning today. 
The foregoing examples are intended to be illustrative only and are not to 
be deemed as in any way limiting the scope of the appended claims.