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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for cleaning articles,
particularly articles that have been buffed with a hydrocarbon wax. The
method and apparatus involve contacting a soiled article with a liquid
chlorinated benzotrifluoride compound at a temperature of about 70.degree.
to about 100.degree. C., followed by washing the benzotrifluoride compound
off the article with liquid methylene chloride.
In the metalworking industry, metal parts are commonly buffed to enhance
their appearance. The buffing compound, or rouge, typically consists of
various metal oxides dispersed in a waxy substance. To remove the buffing
compound from the parts, the parts were usually cleaned in a vapor
degreaser where they were contacted with vapors of 1,1,1-trichloroethane
or trichloroethylene. Because these solvents boil at 74.degree. and
86.degree. C., respectively, they are ideal for melting and removing
buffing compounds from the parts, yet they do not become so hot that they
"set" the buffing compound or oxidize the metal surface (as
perchloroethylene would, which boils at 121.degree. C.).
However, 1,1,1-trichloroethane has been found to cause depletion of ozone
in the stratosphere and its use in degreasers has now been made illegal.
Trichloroethylene is also outlawed in many states as it is a photochemical
oxidant. At the present time no good substitutes have been found to
replace these two solvents.
SUMMARY OF THE INVENTION
I have discovered that articles can be effectively cleaned with a liquid
chlorinated benzotrifluoride compound followed by washing the liquid
chlorinated benzotrifluoride compound off the article with liquid
methylene chloride. While the prior process for cleaning articles involved
the use of a single vaporized solvent, the process of this invention
involves the use of a first liquid to do the cleaning, followed by a
second liquid to remove the first liquid. Unlike the previously used
degreasing solvents, regulations do permit the use of the solvents of this
invention for cleaning purposes.
The process of this invention offers a number of advantages over the prior
cleaning process. For example, while the prior process heated the parts to
74.degree. C., the boiling point of 1,1,1-trichloroethane, the parts leave
the process of this invention at a temperature of only 40.degree. C., the
boiling point of methylene chloride. Thus, it is not necessary to cool the
parts and they can be handled directly.
I have found that existing degreasing machines can be easily retrofitted to
accommodate the cleaning process of this invention. In addition, it is
possible to adjust the temperature of the benzotrifluoride compound to the
optimal temperature for cleaning any particular article.
In one embodiment of this invention, a portion of the methylene chloride is
vaporized to provide a vapor around the benzotrifluoride liquid, thus
rendering the combined benzotrifluoride and methylene chloride vapors
nonflammable. The combined vapors are then condensed for recovery; the
condensed liquids are easily separable because their boiling points are
widely separated (40.degree. C. for methylene chloride, 139.degree. C. for
parachlorobenzotrifluirde (PCBTF), and 173.degree. C. for
3,4-dichlorobenzotrifluoride (DCBTF)). Because the two liquids have low
heats of vaporization (142 BTU/lb for methylene chloride and about 100
BTU/lb for the benzotrifluoride compound), distillation does not require
the expenditure of a great deal of energy.
BRIEF DESCRIPTION OF THE DRAWING
FIG. I is a diagrammatic view showing a vapor degreaser for performing the
process of this invention, where the articles are cleaned one at a time.
FIG. II is a diagram illustrating an automatic degreasing machine for
performing the process of this invention, where articles to be cleaned
continuously pass through the machine on a conveyor.
In FIG. I, degreasing apparatus 1 consists of a large container 2 in which
are three smaller tanks, 3, 4, and 5. In tank 3 is placed the chlorinated
benzotrifluoride liquid according to this invention. The liquid in tank 3
can be heated by, for example, steam pipes 6 and 7. (Electrical or other
means of heating could also be used.) While articles to be cleaned can be
dipped into this tank, the liquid from tank 3 can also be pumped by pump 8
through line 9 and out sprayer 10 onto the article.
Tank 4 contains liquid methylene chloride. After the articles have been
contacted with the chlorinated benzotrifluoride compound, the articles are
dipped into tank 4 where the methylene chloride washes the chlorinated
benzotrifluoride compound off the articles. Overflow from tank 4 passes
into tank 5. Tank 5 also contains methylene chloride, which can be heated
by steam pipes 11 and 12 to its boiling point, 104.degree. C., to vaporize
it and provide a methylene chloride vapor over the chlorinated
benzotrifluoride liquid and thereby render the chlorinated
benzotrifluoride vapors nonflammable. The combined chlorinated
benzotrifluoride and methylene chloride vapors are condensed by coils 13
which, typically, contain water chilled to less than 15.degree. C. The
condensed vapors pass through line 14 into water separator 15 where any
condensed water is decanted off through line 16. The condensed organic
liquids are then returned through line 17 to tank 4. Periodically, it is
necessary to remove solids from the bottom of the tanks and purify the
methylene chloride, which can be done by distillation.
In FIG. II, degreasing machine 18 consists of an enclosed passageway 19
having an open entrance 20 and an open exit 21. Inside passageway 19, at
the center, are three tanks, 22, 23, and 24. Tank 22 contains a liquid
chlorinated benzotrifluoride compound according to this invention. Steam
pipes 25 and 26 heat the liquid chlorinated benzotrifluoride compound to
the desired temperature for cleaning the articles. A pump, 27, pumps the
liquid benzotrifluoride compound through line 28 and out spray nozzles 29
onto articles 30 that pass through the degreaser on conveyor 31, entering
through entrance 20 and leaving through exit 21. Liquid benzotrifluoride
compound that drips from the articles strikes splash pan 32 and flows back
into tank 22.
Tank 23 contains liquid methylene chloride, which is pumped by pump 33
through line 34 and out nozzles 35 onto articles 30 to be cleaned, washing
the liquid benzotrifluoride compound off articles 30 and down into tank
23. In tank 24, methylene chloride is vaporized by steam pipes 36 and 37
to provide methylene chloride vapor within degreasing machine 18 in order
to make the vapors of chlorinated benzotrifluoride compound nonflammable.
The combined vapors are condensed by coils 38, typically containing cold
water, and the condensed vapors flow through lines 39 and 40 into tank 41.
From tank 41 the condensed vapors can be pumped by pump 42 through line 43
and out nozzles 44 to provide additional methylene chloride rinsing of the
articles. Overflow from tank 41 passes through line 45 back to tank 23.
The methylene chloride in tank 23 overflows into tank 24 and methylene
chloride from tank 24 is removed through line 44 to methylene chloride
still 47, which separates the methylene chloride from the chlorinated
benzotrifluoride compound. (While the methylene chloride and
benzotrifluoride compounds are miscible, they do not form an azeotrope.)
The methylene chloride from still 47 passes through line 48 back to tank
41 and the chlorinated benzotrifluoride compound passes from methylene
chloride still 47 back through line 49 to benzotrifluoride still 50.
Distilled benzotrifluoride compound passes from benzotrifluoride still 50
through line 51 into one end of tank 22, and less pure benzotrifluoride
compound in tank 22 passes through line 52 back to benzotrifluoride still
50. Waste products in the benzotrifluoride liquid can be removed from
benzotrifluoride still 50 through line 53.
Almost any type of small part can be cleaned with the method and apparatus
of this invention. Examples of typical parts include lipstick cases,
watchbands, jewelry, pens, and small plated metal articles. The parts are
typically contaminated with a buffing compound, usually a hydrocarbon wax,
that is soluble in the benzotrifluoride compound. However, articles that
contain non-soluble contaminants can also be cleaned using the process and
apparatus of this invention. For example, the article can be heated with
the benzotrifluoride solvent to a temperature above the boiling point of
the methylene chloride and, when the article is dipped in, or sprayed
with, the methylene chloride, the methylene chloride will boil on the
surface of the article and blast the soil off.
The benzotrifluoride compounds of this invention have the formula
##STR1##
where n is 1, 2, or 3. The preferred benzotrifluoride compound is PCBTF or
DCBTF as those compounds are commercially available. The other isomers are
known to those skilled in the art. The benzotrifluoride compound should be
pure and should not contain contaminants that may attack the equipment.
How much benzotrifluoride compound should be used depends upon the size of
the part to be cleaned, the amount of soil on the article, and the
solubility of the soil. The benzotrifluoride compound should be heated to
about 70.degree. to about 100.degree. C., and preferably to about
75.degree. to about 90.degree. C.; lower temperatures do not clean well,
and higher temperatures may cause corrosion problems.
Because the vapors of the benzotrifluoride compound are-flammable, it is
preferable to mix them with methylene chloride vapors to exclude enough
oxygen to create a nonflammable vapor mixture above the benzotrifluoride
compound. The mixed vapors can then be condensed to provide a soil-free
methylene chloride for final rinsing of the articles. Sufficient methylene
chloride should be vaporized not only for nonflammability, but also to
provide enough condensate for rinsing the articles.
It is preferable to use methylene chloride that has been stabilized. For
example, about 0.5 wt % propylene oxide can be added to the methylene
chloride to act as an acid acceptor, reacting with hydrochloric acid to
prevent the corrosion of equipment. About 65 ppm (by weight) of
cyclohexene can be added to the methylene chloride to prevent its
oxidation to formyl chloride and then to formic acid, which can release
hydrochloric acid. Diisopropyl amine in an amount of about 25 ppm (wt) can
be added to control the pH, which should be kept above 7 but, in order to
prevent the corrosion of copper-containing metals, should not be too high.
Mixed amylenes can be added to act as high-temperature stabilizers to
prevent oil on the parts from alkylating to form carbon; about 0.3 wt %
can be used. Preferably, a mixture of different stabilizers is used;
methylene chloride can be purchased as a commercial product that already
contains a mixture of stabilizers (e.g., "M-Clene D" from Occidental
Chemical Corp.). See, for example, U.S. Pat. Nos. 3,900,524, 3,923,912,
3,860,665, 3,968,250, 3,864,408, 3,887,628, and 3,898,195, herein
incorporated by reference, for additional information on methylene
chloride stabilizers.
The following examples further illustrate this invention.
EXAMPLE
Using ASTM Test D1545-89, the solubility of various waxes and resins in
25.degree. C. PCBTF and DCBTF was determined. The kinematic viscosity (in
centipoises) was also determined at the solubility limit. The following
table gives the wt % solubility (the number in parenthesis is the
viscosity).
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PCBTF DCBTF
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Paraffin Wax (60.degree. C. melt point)
<5 <5
Paraffin Wax (at 60.degree. C.)
>70 >70
AMOCO INDOPOL H100 >70 >70
(Polybutenes)
Durez Resin 29095 53 45
(Phenolic) (1332) (154)
Goodyear Wingtack 58 53
Extra (384) (373)
(Polyterpene
Hydrocarbon)
Goodyear Vitel 31 25
PE 200 (9948) (12580)
(Phthalate Ester)
Goodyear Vitel 34 26
PE 307 (8617) (4422)
(Phthalate Ester)
Hercules Stay- 60 55
belite Ester 10 (502) (221)
(Hydrogenated Rosin
Glyceride)
Monsanto Gelva >70 >70
GMS 788
(Acrylate
Copolymers In
Solvent)
Shell Krayton 32 31
D 1107P (31900) (59249)
(Styrene Rubber
Block Polymers)
______________________________________
The table shows that PCBTF and DCBTF are good solvents for the wax and a
variety of resins.
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