Cleaning With Alcohol and Cyclohexane
Cleaning With Alcohol and Cyclohexane
Cleaning With Alcohol and Cyclohexane
Peter G. Davis
Forward Technology Industries
Time has run out for many managers responsible for replacing ozone depleting substances (ODS)
in their cleaning processes, and they must now choose between a variety of alternative
technologies. Recently, many of these managers have chosen cleaning processes involving
Alcohol or Cyclohexane. These non-ODS solvents have a proven record of excellent results
for precision cleaning applications, and can be implemented for relatively low cost in safe
systems that clean using the same processes as conventional vapor-degreasing equipment.
Typical applications for alcohols include removal of particle contamination and inorganic films
such as salts, fingerprints, and highly activated fluxes from:
For electronics cleaning, alcohol is used extensively for printed circuit board defluxing and
cleaning handling soils from critical electronic assemblies like gyroscope and cesium beam
clock components. Alcohol offers excellent cleaning results, and parts can usually pass
dielectric tests immediately after cleaning. As alcohol is already pre-qualified for many
milspec requirements, it can be immediately substituted for ODS processes in many critical
military applications.
For precision mechanical components and optics devices, alcohol is recognized for its superior
particle removal capabilities. Alcohol’s polar molecular chemistry results in repulsion forces
between particles and the surface. These forces tend to shield the effect of van der walls forces
and dissipate electrostatic forces to the extent that alcohol is approximately twice as effective for
1
removal of micron-sized particles than water.
In the medical field, changing to some non-ODS solvents can require an extensive approval
process. Accordingly, many medical component manufacturers look for aqueous alternatives,
1
A. Patzkò, B. Vàrkonyi, and F. Szàntò, Acto Phys. Chem., 17(1-2):91 (1971).
Property of the Pacific Northwest
Pollution Prevention Research Center
1326 Fifth Avenue, Suite 650
254 Precision Cleaning ‘94 Proceedings
Seattle, WA 98101
(206)223-1151
only to find that implementing a sanitary or pyrogen-free process can be very difficult. Alcohol,
on the other hand, is already widely accepted in the medical industry as a cleaning agent for
small-scale wiping and rinsing. This acceptance allows managers of medical companies to adopt
alcohol for larger-scale cleaning operations without undertaking an exhaustive approval process.
Additionally, alcohol can be implemented in a sanitary or pyrogen-free cleaning process much
more simply than aqueous alternatives. Common examples of medical devices cleaned in
alcohol processes include catheter components, angioplasty devices, surgical blades, prosthetic
joints, implantable heart valves, and surgical optics.
Alcohol is known for its excellent rinsing properties, and is frequently used for rinsing and
drying parts cleaned with other solvents. For example, precision communications hardware
components contaminated with a variety of light oils and particulate have been effectively
cleaned with a hydrocarbon solvent in an ultrasonic bath, followed by isopropyl alcohol (IPA)
rinsing. In another example, alcohol was effective in spot-free drying of high-vacuum
components cleaned in an aqueous process.
CYCLOHEXANE AS AN ALTERNATIVE
Cyclohexane, a solvent with physical properties similar to alcohol and employed in identical
equipment, is effective for removing organic films (oil and grease), and is frequently used to
replace processes using 1,1,1 -trichloroethane in precision applications.
For certain applications involving multiple soils, alcohol and cyclohexane can be combined in
azeotropic form to take advantage of the cleaning capabilities of each component material. This
azeotrope has been shown to be effective in removing a wide variety of contaminants.
OTHER SOLVENTS
Similarly, these solvents may be combined with other special solvents or blends to remove
particular soils. For example, a blend of 80% cyclohexane-IPA azeotrope with 20% DuPont
Axarel 38 as the cleaning agent, followed by a series of cyclohexane-IPA azeotrope rinse steps,
has been used effectively to deflux circuit boards.
While alcohol (or cyclohexane) is effective for all of the aforementioned applications, there are
certainly other technologies that are also effective. Yet alcohol is often chosen because it offered
distinct advantages. For example, as compared with aqueous processes, the advantages of
alcohol and cyclohexane include:
l Better Solvency: For removing soils like high activated flux from
circuit boards.
Low Solvent Use: While a large aqueous cleaner could use thousands
of gallons of de-ionized water per week, even the
largest alcohol cleaner typically uses less than 50
gallons of solvent per week.
There are, of course, many other options for managers who cannot use aqueous processes. While
each of these options has its advantages and disadvantages, managers who choose alcohol or
cyclohexane over these other options are frequently motivated by cost. The cost of alcohol, for
example, is typically $2-3 per gallon. As a comparison at the other end of the cost spectrum.
perflourocarbons (PFCs) also have good rinsing properties, but typically cost $14 per pound, or
about $196 per gallon. Similarly, alcohol and cyclohexane disposal can be accomplished by
incineration. The pick-up and handling cost to incinerate the alcohol is typically $2 per gallon.
when the solvent is recycled as a fuel in industrial processes.
Another factor that influences managers1 decisions to use alcohol or cyclohexane is the relative
simplicity of the cleaning process. System designs are identical to conventional vapor-
degreasers from a cleaning process perspective. Operators using a conventional vapor degreaser
see little difference from a process standpoint when the equipment transition is made. Of course,
equipment for alcohol does differ from conventional equipment in that it is designed for use with
low flash point substances.
Finally, a major reason managers turn to cleaning with solvents like alcohol is that these solvents
deliver the cleanliness results they require. For example, cleanliness testing was recently
conducted in a four-stage cleaning system using an isopropyl alcohol-cyclohexane azeotrope in
the first tank followed by a series of alcohol rinses. The system is used to clean high vacuum
components. Some of the components are cleaned with an aqueous process and then rinsed with
the alcohol, and some of the components are cleaned by the alcohol-cyclohexane azeotrope. The
process test parameters were as follows:
The major contaminants of concern were fluorine, sodium, chlorine, silicon, zinc, and calcium.
ESCA analysis showed micro-layer contamination by these elements less than 0.5 %. The
operator’s cleanliness specification is 2%.
Given that companies selected alcohol cleaning because they were able to get good cleaning
results at low cost, it is important to understand how the low flashpoint issue was addressed.
In the United States, equipment designed to use solvents with low flash points must be
"explosion-proof." "Explosion-proof" is a term that has an officially recognized meaning as
defined by the National Fire Protection Association (NFPA). NFPA sets standards for the
design, installation, and operation of equipment that uses liquids that are “flammable.” Solvents
like alcohol and cyclohexane are classified as Class 1 B Flammable in accordance with the NFPA
definition of flammable:
“Any liquid having a flash point below 100°F (38°C) and having a vapor pressure
not exceeding 40 psia (2069 mm Hg) at 100°F.”
Properly applied, the NFPA standards allow widespread use of flammable substances with
relatively little safety risk. For example, very few people stop to think about the risk of fire
when they fill their automobile with gasoline, yet gasoline is a particularly flammable substance.
Fortunately, gasoline dispensing mechanisms conform to applicable fire protection standards,
and the risk of fire is very low. More importantly, the risk of fire is outweighed by the benefits
of using gasoline as automobile fuel.
to keep the vapor concentration outside the system well below the lower
flammable limit, and
to prevent spark or flame initiation inside the system.
SUMMARY
Hundreds of facilities throughout the world have successfully eliminated ozone depleting
substances from their cleaning processes by switching to alcohol or cyclohexane. Managers
who have chosen alcohol have recognized that systems can be designed to operate safely.
Alcohol has a proven record of excellent results for precision cleaning applications, and can be
implemented for relatively low cost in systems that clean using the same processes as
conventional CFC equipment.
Abstract:
Precision cleaning and drying with isopropyl alcohol (IPA) and
cosolvents are discussed. But, cleaning with IPA is not new and it
is a widely accepted and effective solvent in many industrial
a p p l i c a t i o n s . What is new is a technology which safely deals with
volatile organic solvents like IPA at their boiling points, so that
they can be utilized in vapor degreasing and cleaning operations.
Other solvents such as v o l a t i l e silicones, IPA/hydrocarbon
azeotropes and low boiling alcohols can also be used. This new
technology, coupled with proven, high performance CFC alternatives
for cleaning and fast evaporating solvents for drying, will be of
great value f o r p r e c i s i o n c l e a n i n g o f s m a l l p a r t s , complex
geometries and water sensitive components and devices. Parts whose
shape and size are likely to entrap water in aqueous and semi-
aqueous processes can be effectively cleaned and dried, residue free
with this familiar vapor degreasing process. since the process uses
no water, there are no problems associated with water spotting, nor
with waste water management.
The process involves immersing parts in a selected cleaner or
solvent to loosen and remove oils, greases, waxes and other soils.
Then parts are rinsed and dried in IPA vapors. This is accomplished
in a single tank where parts are cleaned in a boiling blend of IPA
and cosolvent and dried in IPA vapors above the liquid surface.
Alternatively, this is accomplished in two tanks where each solvent
is contained separately, one used for washing and the other for
rinsing and drying. In both cases, the parts emerge clean, dry and
residue free.
Introduction:
Cleaning processes for parts a n d t o o l s i n i n d u s t r y a r e b e i n g
p r o f o u n d l y a f f e c t e d b y s e v e r a l t r e n d s w h i c h a r e now shaping
precision cleaning operations. Environmental regulations and global
agreements are forcing industry to seek alternatives for CFC’s and
other ozone depleting substances used for cleaning, stripping and
degreasing. As a consequence, t h e i n t r o d u c t i o n o f numerous
alternatives in both cleaning chemicals and process equipment has
caused confusion because of the multitude of combinations now
possible. Numerous manufactured parts have been developed with
unique design criteria specifically intended to facilitate new
cleaning and drying techniques. Many of the alternative cleaners
call for multiple washes and water rinses , phase separations and new
waste handling practices , all of which are unfamiliar to many users.
Since there is no single solution, there is also no single selection
process which can completely satisfy manufacturers besieged with the
need to change. Some have chosen to first select a chemistry which
works well with their particular soils and substrates, and have
Discussion:
For years isopropyl alcohol (IPA) has been the solvent of choice for
cleaning and drying semiconductor wafers in the final and moat
c r i t i c a l s t a g e s o f t h e i r f a b r i c a t i o n . Drying in circulating hot air
or other gases necessitates very high purity and careful submicron
filtration to prevent the risk of contaminating these high value
wafers. The basis for selecting IPA was that it is easily vaporized
at low temperature to create a particle free environment of IPA
vapors, which are capable of extracting water entrapped in the fine
line geometries of microelectronic devices. The physical properties
of IPA are listed in Table 1.
Thus, t h e u s e o f I P A a n d t h e e q u i p m e n t r e q u i r e d f o r i t s s a f e
handling in the vapor state are now well understood. Although IPA
alone is an excellent dehydrating agent, its capacity for dissolving
a broad range of soils is limited. A large number of cleaners have
emerged since the beginning of the ozone crisis. Many are well
proven and meet a variety of selection criteria imposed by end
users.
No single solvent will meet all the cleaning needs of industry. IPA
i s n o d i f f e r e n t , and may lack the potency for specific cleaning
These alternatives, which at one time would have been dismissed, are
now viable because of the advances made in equipment and safeguards
designed to handle flammable solvents in the vapor state. The
equipment and the safe handling techniques for IPA apply also to
these blends, opening up their chances for replacing ODC's in vapor
degreasing equipment specifically designed to handle these and other
low boiling solvents.
Many ODC alternatives have been developed over the last few years
(Precision Cleaning, Feb. 1994), and some have gained a respected
place in precision cleaning applications. All of the alternatives
fall into the several categories listed in Figure 2.
Hydrocarbons are synthetic and naturally derived products which are
non-polar, a p r o t i c solvents, p a r t i c u l a r l y s u i t e d t o a t t a c k a n d
r e m o v e s o i l s l i k e o i l s , greases and waxes which are chemically
similar to the solvents themselves. Some of these are blends of
natural and synthetic hydrocarbons together with other materials
such as ester, ethers and glycols. The higher boiling members of
this category have low vapor release, reducing the risk to workers
and the impact on the environment. However they pose a drying
problem in cleaning parts with complex geometries because of the
potential entrapment of solvent in blind holes and tight clearances.
Semi-aqueous systems are baaed on surfactants (soaps, detergents,
alkaline materials, etc.) blended with the hydrocarbons in the class
mentioned above. The surfactant is added to improve their activity
as a broad range cleaner and to facilitate rinsing in water. A
typical cleaning process, depicted in Figure 3, requires multiple
washes in the cleaner, followed by multiple rinses in water (often
DI water) to free the part of residual solvent and surfactant. The
rinse water requires continual phase separation to isolate and
remove soils from the water which could be recirculated through the
system. Parts drying, potential water spotting, corrosion and waste
management issues of the solvent and the rinse water must be
addressed when designing systems for these alternatives.
Aqueous systems essentially consist of detergent solutions for parts
washing, followed by water rinsing. This category of cleaners is
becoming widely accepted in industrial cleaning because of their
s a f e t y , ease of operation and low coat. Water baaed cleaners are
effective in removing light soils like machining and cutting oils,
These s o l v e n t s h a v e b e e n u s e d i n c o n j u n c t i o n w i t h I P A i n
commercially available vapor cleaning and degreasing equipment
produced by S&K Products International. The Model IG-200 is a one
tank design for single stage operation, and Model IG-300 is a two
tank design for two stage operation. A v a r i e t y o f p r e c i s i o n p a r t s
have been successfully cleaned by this technology, parts such as:
Precision bearings
Glass photo masks
Aerospace guidance devices
Multichip modules and hybrid devices
Surgical equipment
Intimate contact medical implant devices
Precision optics
Gyroscopes
Circuit board assemblies
Fiber optic connectors
Microelectronic components
Conclusions:
A new cleaning method which employs isopropyl alcohol and other
solvents in a familiar vapor degreasing configuration has been
developed. Because the process relies on vapor cleaning, precision
o
Boiling Point (1 atm) 82.3 C (180°F)
o o
Flash Point (CC) 12 C (53.6 F)
ODP zero
CFC ALTERNATIVES
CLEANING and DEGREASING
by
Robert L. Polhamus
However, one major obstacle still remains to the complete elimination of solvent
cleaning with aqueous or semi-aqueous substitution. There are certain applications and
materials to which water is detrimental. These are usually high precision needs where
water will deteriorate the performance of the product by producing surface changes and
The chemical properties that have made ODS desirable cleaning chemicals
centered around; adequate solubility for the soils encountered, low toxicity, non-
flammability, and reclaimability. These characteristics, in conjunction with relatively low
usage cost and a simple process, allowed these chemicals to become a dominant factor in a
wide range of cleaning applications. It does not appear that in the near future there will be
substances available that can combine all of these characteristics. However, a process has
been developed that incorporates two chemicals that , when their properties are combined,
produce a suitable replacement.
The Alternative Cleaning Technology (ACT) process utilizes a two phase system
with liquids of different physical properties. One liquid provides the cleaning power
whereas the second liquid is used to rinse the cleaning chemical and is subsequently dried
off the surface. Much of the development work on this process has utilized isopropyl
alcohol as the primary cleaning agent and a fully fluorinated hexane, referred to as a
perfluoronated chemistry (PFC), as a rinsing, drying, and inerting agent.
The cleaning power of isopropyl alcohol is well documented, but due to its
flammability, it has not achieved wide acceptance. The perfluoronated materials have been
The ACT process is very similar to the conventional vapor degreasing process.
The equipment consists of a two sump design where the left hand sump produces a vapor
blanket and the right hand sump provides immersion in pure distillate chemical for
precision cleaning.
Unlike the single liquid vapor degreasing process where the boil sump and
immersion sump are comprised of the same chemical, in this system the alcohol and PFC
remain distinct due to their immiscibility. The alcohol, being much less dense than the
PFC, forms an upper layer in each sump with the PFC on the bottom. The beneficial PFC
properties of having a higher density and lower boiling point are the key to the process.
In the boil sump, the PFC material is boiled and percolates through the alcohol
level to produce a high density vapor blanket above both sumps. The boiling point of the
PFC is around 50 deg. C. The boiling point of the isopropyl alcohol is around 82 deg. C.
The vapor blanket is composed of approximately 90% PFC and 10% alcohol. At this
concentration the vapor is non-flammable. Since the vapor has displaced all atmosphere
above the sumps we have eliminated the flammability hazard of using alcohol at elevated
temperatures.
The reservoir overflow enters the immersion sump where the alcohol and PFC
exist in separate layers. The PFC sinks to the bottom, and through a standpipe, is returned
to the boil sump. The alcohol level overflows a weir separating the boil and immersion
sumps and carries soils removed in the immersion sump into the boiling sump. This
completes the distillation/purification cycle that maintains a clean immersion sump.
The cleaning process is similar to the conventional degreaser in that the product is
introduced into the area above the boil sump for a pre-clean vapor rinse or can be
immersed directly into the boil sump for boiling agitation cleaning. Although the PFC is
boiling and the alcohol is not, the PFC vapors that pass through the alcohol create a strong
The ACT equipment depicted in the BASIC SYSTEM DESIGN schematic has
several features designed to control vaporous emissions that are not usually found on
conventional vapor degreasers. Past industry data has shown that liquid drag-out caused
by low vapor zone residence times and rapid transportation of product in and out of the
machine are the largest source of losses. In some instances over 70% of losses can be
contributed to these factors. The next largest source is usually vapor loss at the vapor/air
interface through atmospheric air exchanges in the freeboard area. Finally, the last
common source of emissions is the drag-out produced by a micro layer of condensate on
the product surface. The ACT design addresses these aspects.
The ACT system is designed with two independent internal lift mechanisms to
move the product vertically within the unit. These are illustrated in the schematic. This
eliminates the operator from the process and allows slow controlled motion to reduce
vapor blanket collapse. In addition, since this step is automated, an operator no longer
needs to perform the tedious task of suspending the parts baskets in the various process
areas. This allows extended residence times in the vapor zone that are often cut short
when manual operation is involved. Automation of the process has shown to contribute
significantly to emission reduction. An external hoist is capable of performing this same
function, however, the internal hoist provides an additional benefit. The internal hoist
allows the cover to be closed for the duration of the processing. Since there is no robot
arm to interfere with the cover, the ACT cover is closed and pneumatically sealed during
the entire cleaning process. By sealing the lid there exists further insurance against
emission losses caused by process parameters such as spray and vapor collapse. The
closed lid also reduces the opportunity for air exchanges within the freeboard area, further
reducing emissions.
The last area of emissions to be addressed is the micro layer of condensate trapped
on the product surface. In a conventional degreaser, the final vapor rinse is done to flood
off drag-out from the immersion sump and to vaporize liquid condensed on the surface.
During this rinse/dry phase the condensation of vapor on the parts causes the part
temperature to approach the boiling point of the liquid. Since the temperature of the parts
cannot reach the boiling point of the fluid, eventually an equilibrium between condensate
and vapor is established producing a microscopic layer of liquid which remains on the
surface When the product is removed from the system, this liquid generally flashes off
very quickly in either the freeboard of the degreaser or outside the machine. In most cases,
this material is lost to the atmosphere.
The ACT system has been designed with the ability to superheat the parts to raise
their temperature above the boiling point of the PFC helping to eliminate this micro layer.
During the cleaning process, the parts are immersed in the alcohol level within the
immersion sump. After the cleaning is performed the parts are removed from the sump
coated with alcohol. The alcohol is not volatile enough at the vapor blanket temperature
to quickly or adequately dry the parts. PFC condensate from the condensate reservoir is
sprayed onto the parts to displace the alcohol from the surface. As shown in the
schematic, the spray takes place within the vapor zone just above the liquid level in each
sump The liquid spray is immediately followed by a superheated vapor that condenses on
the parts and heats the parts to a temperature above the PFC boiling point. This
significantly reduces the micro layer of PFC condensate found on the surface. After going
through several cycles of liquid and superheated vapors, the parts are raised into the
freeboard area where any trace amounts of liquid flash off. These vapors immediately fall
back down into the vapor zone where they are contained. Since the cover has been closed
and sealed during this entire step, there are no air exchanges within the freeboard area that
could contribute to PFC rich atmosphere being removed from the unit.
Due to the inclusion of-the aforementioned design and process features, the overall
emissions of the unit can be maintained at a very low level. Actual process losses will
depend on part configuration and adherence to good procedures, but should be controlled
within environmental and operational cost considerations.
The ACT system has been designed around specific chemical properties and is not
dedicated to a particular chemical. Other chemicals can be used as cleaning agents as long
as they have the properties of, being less dense than PFC, are essentially immiscible in
PFC, and have a boiling point higher than PFC. If a flammable chemical is chosen its vapor
pressure at the boiling point of PFC must be low enough to produce a non-flammable
concentration, Volatile Methy Silicone chemistry is an example of a product that meets
Next generation chemicals are being developed now that may replace the PFC in
the system. These chemicals will require the characteristics of, high liquid density, low
toxicity, non-flammability, preferably low boiling points and heat of vaporization.
Improved environmental compatibility will also be a desired characteristic. Although PFC
have Global Warming potential over which to be concerned, the controlled usage within
this process makes them acceptable substitutes for ODS. Future chemistries with even
better environmental compatibility will enjoy the benefits of this technology.
There remain some critical industrial applications where the aqueous and semi-
aqueous cleaning process is unacceptable. Many of these applications still rely on Ozone
Depleting Substances (ODS) in their manufacturing process. Utilization of a two-phase
system incorporating a cleaning solvent and an inert rinsing/drying agent will provide
necessary cleaning while eliminating ODS from the process. Proper equipment design and
management will produce an environmentally compliant process in the near term while
next generation solvents are being developed. Equipment design parameters w-ill be
compatible with the chemical properties of next generation solvents allowing the process
to be implemented in the present with applicability to the future.
SOLVENT CLEANER/DEGREASERS
SEMI-AQUEOUS/SOLVENT EMULSIONS
AQUEOUS CLEANERS
SOLVENTS
Ross Gustafson
Abstract
Introduction
The final phase-out date for the use of chlorinated solvents is rapidly
approaching. Many manufacturers are still looking for effective
alternative solvents for these chemicals in their cleaning applications.
The use of terpenes, as an alternative, has been widely documented
and accepted.