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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for cleaning articles by vapor
degreasing; and more particularly to a method of removing organic
materials from metallic and electrical materials with a solvent employing
various blends including dibromomethane and appropriate stabilizers.
2. Background
The utilization of vapor degreasing techniques has found wide acceptance in
industry in the past. This method of cleaning typically involves the
heating of a solvent to a boil and the generation of a vapor layer into
which the object to be cleaned is placed. This vapor layer is a mixture of
air and solvent, where the air has reached the saturation point with the
solvent. The vapor is generated by boiling the liquid. It condenses upon
the object placed in the vapor layer and drips off of the object into a
holding tank. This condensation dissolves the hydrocarbon contaminants and
removes it from the object, thereby cleaning it.
This technique is a preferred method of cleaning precision parts, such as
electronics, machined metallic parts, etc., since vapor cleaning leaves
virtually no residue upon the parts.
Degreasing has been found particularly valuable for the removal of flux
from soldered circuit boards. A typical circuit board consists of a thin
plate normally of epoxy resin or similar material reinforced with glass
fibers carrying electrical connectors on one or both sides consisting of
thin strips of copper or other electrically conductive material.
Electronic components are normally placed on the side opposite the
conductors and by means of leads passing through holes in the board are
attached to the connectors by crimping the leads followed by soldering.
Soldering is typically carried out by first coating the connector side of
the board with a flux and then passing the side of the board over a
surface of molten solder. The fluxes employed in the use consist for the
most part of rosin. Increasing popular in the trades are rosin fluxes
which are activated by the addition of ionic materials. Such activators
insure better solder bonds, especially on slightly corroded connectors and
leads.
A choice of solvent for removal of the flux is restricted by the insuring;
that the solvent will not attack the board or the various material of
construction or any of the electronic components contained thereon.
Alcohols have been used in the past, however, their use is limited because
of the severe danger of fire. Non-flammable materials employed in the past
as a solvent medium include CFC 113, 1,1,1 Trichloroethane,
perchloroethylene, 1,1,1 Trichloroethylene and various
hydrochlorofluorocarbons, such as "Gensolve" (manufactured by Allied
Chemical).
Such vapor degreasing technique employing same or similar materials is
taught by U.S. Pat. No. 3,881,949 which issued on May 6, 1975 to Carl
Martin Brock.
The solvents described above and in the Brock patent are typically on the
Clean Air Act list of ozone depleting chemicals and are being phased out
of production. It is obvious from the foregoing that a suitable
replacement is necessary to try to fill in for these banned ozone
depleting chemicals.
U.S. Pat. No. 4,056,403 issued to Robert J. Cramer et al on Nov. 1, 1977
describes a method in which a number of non ozone depleting chemicals,
including methylene bromide or dibromomethane are used in cleaning
polyurethane foam generating equipment. Cramer et al teach a method
wherein a solvent composition described therein is used for cleaning
polyurethane foam generating apparatus or a segment thereof and noting
particularly the cleaning operation where rinsing or flushing the entire
foaming apparatus takes place after the foam forming chemicals have been
exhausted. The solvents taught may be periodically injected under pressure
through the mixer portion of the foaming apparatus in order to purge it of
residual unreacted or partially foam forming materials. The method
described in this patent would be totally ineffective because its
composition does not include the appropriate stabilizers which would
prevent the dibromomethane from becoming acid and thereby attacking the
metal surfaces which might be placed into the vapor layer. Use of hot
saturated vapors of a liquid halogenated hydrocarbon, including
dibromomethane is taught in U.S. Pat. No. 4,193,838 which issued to Robert
J. Kelly et al on Mar. 18, 1980. This patent teaches the maintenance of a
pool of hot saturated vapors of a halogenated acyclic hydrocarbon then
placing pieces of coated paper stock, which had been coated with "hot
melt" coatings such as are used on consumer items and milk cartons, etc.
These pieces of paper stock are placed in the vapor pool after which they
are agitated. Again it is noted that this method would be ineffective at
cleaning circuit boards and other metallic materials because of the acidic
nature of the non stabilized compound utilized therein which would tend to
destroy the object rather than just clean it.
Obviously it is the object of the present invention to provide materials
which are suitable for vapor cleansing of electronic and metallic parts
without the use of ozone depleting chemicals with a material that would
not be appropriate for the cleaning of metal.
SUMMARY OF THE INVENTION
It is the primary purpose of the present invention to develop an
environmentally safe "drop in" substitute for 1,1,1 TCE and CFC solvents
which are utilized at the present in vapor degreasing equipment. It has
been determined that dibromomethane because of its non-flammability, high
solvency and very low ozone depleting potential is a very appropriate
material for use in such solvents.
In the present invention, stabilized mixtures of dibromomethane will be
added to a conventional vapor degreaser such as Baron-Blakeslee or Branson
models. The thermostat would then be set at 210 degrees Fahrenheit with
the included solvent allowed to reach this temperature after approximately
1/2 hour. At the temperature of 210 degrees Fahrenheit, the dibromomethane
boils and this temperature must be reached obviously before any vapors
appear. When the design temperature of 210 degrees Fahrenheit is attained,
a vapor layer will appear above the solvent as a "fog". This fog
constitutes the principal feature of cleaning by the vapor method. When
the fog appears, an object can be placed into that vapor layer and the
dibromomethane will condense onto the object. When such condensation takes
place, any oil, grease, rosin, flux or similar organic material which was
on the subjects will then be dissolved and will drip back down into the
boiling solvent and thereby be removed, effectively cleaning the item
deposited into the fog. The vapors from the solvent will not contain any
of the removed contaminants and therefore be ready to clean more objects
of any hydrocarbon soluble contaminants.
Most vapor degreasers include a cold side where excess solvent vapors can
condense and then be recirculated back to the boiling side. This
arrangement helps insure that there is fresh solvent boiling at all times.
When the solvent becomes too contaminated with oil, grease or flux, the
boiling point of the dibromomethane will increase. When the boiling point
of the dibromomethane reaches 225 degrees Fahrenheit, it is time to change
out the old material and replace it with fresh dibromomethane.
It has been found that dibromomethane can be stabilized with a mixture of
three low boiling solvents to prevent it from turning acidic and releasing
free bromine into the air. The solvents are nitromethane, 1,2 butylene
oxide and 1,3, dioxolane. It was also discovered that excessive pitting
and corrosion would appear on metals placed into the vapor layer unless
appropriate stabilizers as indicated have been added. It has been
determined that the appropriate ratio of the stabilizers is approximately
0.5% nitromethane, 0.5% of 1,2 butylene oxide and 3-4% 1,3 dioxolane. It
has also been found that mixtures of dibromomethane and various terpene
hydrocarbons and terpene alcohols as well as mineral spirits, glycol
ethers, alcohols, and ketones can be used in conjunction with the above
identified stabilizers. It has also been determined that some mixtures
will make cloudy solutions indicating that the two components are not
necessarily completely compatible. By mixing two solvents together, a wide
variety of soils or contaminants can be removed. Some of the acceptable
combinations are described in Table 1 below.
TABLE 1
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SOLUTION VAPOR
PRODUCT CLARITY LAYER
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Dibromomethane/limonene/stabilizers
Clear Yes
Dibromomethane/Dipentene/stabilizers
Clear Yes
Dibromomethane/mineral spirits/
Clear Yes
stabilizers
Dibromomethane/glycol ethers/
Cloudy Trace
stabilizers
Dibromomethane/acetone/stabilizers
Cloudy Trace
Dibromomethane/Isopropyl Alcohol/
Cloudy Trace
stabilizers
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A better understanding of the present invention can be had by reference to
the following description particularly to several included examples to
outline the vapor degreasing solvent which effectively meets the object
outlined above.
Dibromomethane when properly stabilized and blended with various solvents
may be utilized as a vapor degreasing solvent in standard sump type or
ultrasonic vapor degreasing equipment as a replacement for 1,1,1
trichloroethane and chlorofluorocarbon solvents. A vapor degreasing
solvent should have the following characteristics for proper cleaning of
metal, plastic, elastomers, and circuit boards. 1) it must be properly
stabilized. 2) be non-flammable and should have an ozone depletion
potential of less than 0.2. It should also have a high solvency with a
Kauri-Butanol value above 70. Additionally, it should include an
evaporation rate of at least 3 and on evaporation leave behind no residue.
The solvent should have a latent heat evaporation of 31 Kcal/mole so as to
facilitate condensation of the solvent on the chiller side of a standard
degreasing system.
One preferred embodiment, is a standard sump type vapor degreaser 5 gallons
of a 90% dibromomethane mixture stabilized by the addition of 0.5%
nitromethane, 0.5%, 1,2 butylene oxide, 4% 1,3 dioxolane and 5% dipentene
mixture. The total being blended and added together. The thermostat on the
vapor degreaser is adjusted to 210 degrees Fahrenheit and the system was
allowed to equilibrate. After the mixture inside the solvent reservoir
reached 210 degrees Fahrenheit, the mixture began to boil. A vapor layer
of approximately 7 inches deep was observed inside the unit. Enough vapor
was being evolved so as to condense and be circulated from the chilled
side of the degreaser and be returned to the boiling side.
A circuit board having approximately 1 gram of rosin flux was immersed into
the observed vapor layer. Dibromomethane was then seen to condense onto
the circuit board and dissolve the rosin flux. After approximately one
minute had passed, the circuit board was removed from the vapor layer. The
observed circuit board contained no rosin flux. The flux had been removed
by the solvent vapor and dripped off into the boiling solvent. The same
procedure outlined above was tried for pieces of sheet metal containing
light mineral oils, silicone oils, lithium greases and other types of
industrial release fluids. Similar results were obtained.
In another embodiment of the present invention, five gallons of the above
composition were added to a vapor degreaser with dipentene being replaced
by p-menthane. Again, the thermostat was adjusted to 210 degrees
Fahrenheit and the system was allowed to equilibrate. After the solvent
blend reached 210 degrees Fahrenheit, a vapor layer approximately 7 inches
deep was again observed inside the solvent reservoir. A steel bolt was
placed inside the vapor layer which had lithium soap based grease smeared
on it. After 1 one minute has passed, the bolt was removed from the vapor
layer. All of the lithium soap based grease had been removed and the part
was now completely clean.
Other approaches included using the same mixture as above with mineral
spirits added in place of the dipentene as part of the total solvent
mixture added to the boiling sump of a vapor degreaser. Again, similar
results were obtained.
Finally, mixtures of 5% C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11 and
C12 alcohols with one OH group were individually substituted for the
dipentene in the initial example. Hereto, similar results were obtained.
Thus, it can be seen from the foregoing that a properly stabilized mixture
of dibromomethane and various solvents can effectively be utilized as a
vapor cleaning solvent in the effective cleaning of organic materials from
the surfaces of electrical and metallic parts.
Corrosion tests were also performed as oxidation is a potential problem as
with all solvent cleaners. In performing these tests, strips of copper and
steel measuring 1" wide by 6" long and of 20 mil thickness were buffed on
a belt sander to remove any oxide films. Fifty milliliters of the above
mentioned solvents were placed in a cylindrical Pyrex glass container and
strips were placed in so that 75% of the surface was immersed in the
solvent. A sample container filled with tap water was used as a control
for the test. This would insure that there were no alloys in the metal
strips that would have been prevented oxidation. The openings of the
sample containers were all sealed with cork stoppers to reduce
evaporation. After an appropriate time, the strips were removed and it was
determined that all of the solvent types described above all were found to
be non-corrosive.
While but selected embodiments of the present invention have been shown, it
will be obvious to those skilled in the art that numerous modifications
may be made without departing from the spirit of the present invention,
which shall be limited only by the scope of the claims appended hereto.
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