Cleaning is loosely defined
as the process of removing unwanted contaminants or dirt from a surface. It is
differentiated from finishing in that the cleaning process does not alter the
surface physically or chemically. A properly cleaned surface is just the same
as it was prior to cleaning, except for the missing dirt.
Contamination takes on
a variety of forms, each of which requires a specific cleaning mechanism for effective
removal. Furthermore, these contaminants are found on parts having various compositions
and shapes, which require that the cleaning mechanism be applied in a particular
way to be effective.
Contaminants
Contaminants may be soluble, insoluble or mixed in nature. Soluble contaminants
are soluble in a suitable and practical solvent. Solvents may include water and
hydrocarbons as well as aqueous chemistries, which cause or promote the dissolution
of the soluble contaminant. In short, a soluble contaminant is one that can be
totally dissolved by or mixed with a solvent so that it can be rinsed or swept
away.
Insoluble contaminants
are those that can’t be dissolved using practical cleaning chemistry. These
contaminants require physical removal by force.
Mixed contaminants are,
as the term implies, a mixture of soluble and insoluble contamination, which may
require a combination of cleaning processes and techniques to remove.
Removing
Soluble Contaminants
Soluble contaminants may be either liquid or solid in nature. Oils and greases
are examples of soluble liquids, while salt and gelatin are examples of soluble
solids.
The challenge in removing
soluble contaminants is finding an appropriate solvent to dissolve the contaminant
without damaging the substrate. When the substrate is relatively robust, as in
the case of steel, stainless steel and many composite materials, there is relatively
little worry about damaging the substrate. In the case of softer metals, such
as copper, brass and aluminum, carefully choosing the solvent to prevent substrate
damage may be a major issue.
Please note that today’s “solvents”
include water-based formulations as well as traditional solvents, including hydrocarbons.
Many of the traditional solvents have been eliminated by the Montreal Protocol
and may no longer be considered as practical cleaning alternatives.
The solvent cleaning process
involves dissolving the soluble contaminant and diluting it until it is no longer
present on the surface. In some cases, it is necessary to provide repeated application
of fresh solvent or a secondary “rinsing” solvent, such as water, to
totally remove the contaminant and solvent residue. Cleaned parts are then dried
to physically remove or evaporate any residue of pure, volatile solvent, leaving
a clean, dry surface.
Traditional
Solvents
Although
the arsenal of traditional solvents is considerably smaller than it was in years
past, these very effective materials remain a viable alternative in many applications
where the presence of water is either intolerable or troublesome. Traditional
solvents find favor in applications for cleaning electrical assemblies that have
open electrical coils. Their benefit is that they are nonconductive and do not
cause corrosion. They are also favored in applications requiring extremely low
surface tension to allow deep penetration into otherwise inaccessible areas.
Traditional solvents are
unique in that they can be used in either liquid or vapor form. As a liquid, the
solvent is used in an immersion dip or is sprayed onto the parts. As a vapor,
parts are suspended in solvent vapor in a confined area of a vapor degreaser.
Solvent vapors condense on the relatively cool part surface to dissolve and rinse
away contaminants.
Aqueous
Chemistries
Aqueous chemistries are the most widely used solvents. Although water has long
been recognized as the “universal solvent,” the range of materials that
are directly and quickly soluble in water alone is limited. The dissolution of
contaminants found in the form of water-soluble solids is largely accomplished
by water itself. This is often a relatively slow process, but it can be hastened
by the introduction of chemical additives and mechanical energy. Water-miscible
contaminants may also be diluted and ultimately cleaned away by water alone. More
often than not, chemical additives are helpful in promoting this process as well.
Dissolution of non-water-miscible liquid contamination requires the use of appropriate
chemical additives to both separate and isolate the removed contaminant to prevent
redeposition.
The most common chemical
additives are those that promote one of six cleaning mechanisms: wetting; emulsification;
solubilization; saponification; deflocculation; and sequestration.
Wetting is essential
to the removal of both soluble and insoluble contaminants. It is how the cleaner,
through the use of surface active agents, loosens the substrate-soil bond by the
displacement of oil and the lowering of surface and interfacial tensions. Wetting,
in essence, is the first requirement of cleaning because it brings the cleaning
chemistry into intimate contact with the liquid or solid contaminant. If the cleaner
doesn’t come into intimate contact with the contaminant, it can’t be
removed.
Wetting agents reduce the
surface tension of the cleaning liquid, allowing it to penetrate between the contaminant
and the substrate. Surface active agents promote preferential wetting of the substrate,
allowing the cleaning agent to “wedge” its way between the contaminant
and the substrate. The contaminant, once separated from the substrate, is then
swept away and removed.
In the case of insoluble
contaminants, it is important to first wet the particles of contamination so that
mechanical energy can be delivered by means of the liquid cleaning medium to displace
and remove the contaminants.
Once wetting takes place,
surfactants allow the process of emulsification to occur.
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Various types
of cleaning methods
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Emulsification is
the dispersion of two mutually immiscible liquids, such as oil and water, one
into the other. The extent of emulsification achieved is dependent primarily on
the oil encountered and the choice of surfactants. Other factors, including pH
and temperature, also affect emulsification. Emulsification also requires the
presence of mechanical energy, in the form of agitation, spray or ultrasonics,
to initiate the formation of the emulsion and effectively break down the size
of the droplets.
Emulsification does not
substantially change the character of the materials being emulsified. The components
of an emulsion remain insoluble in one another on a micro scale, although in the
large picture they appeared to have mixed. In essence, the emulsifying agent or
surfactant surrounds and prevents regrouping of the emulsified material. Emulsification
is also needed to remove contaminants from the cleaning site once they are in
solution.
Solubilization is
the process by which the solubility of a substance, such as oil, is increased
in a certain medium, such as water, by the proper selection of surface-active
agents and/or co-solvents. Solubilized contaminants actually go into solution
with the cleaning chemistry and no longer retain their discrete characteristics.
It is common for a cleaning agent to be able to solubilize a certain amount of
contamination while additional contaminant is held in suspension by emulsification.
Saponification is
the reaction of any organic oil containing reactive fatty acids with free alkali
to form soap. Alkaline cleaners containing saponifiers rely on this process to
remove some oils, including vegetable and animal fats and their derivatives. The
soaps that are generated are easily removed by subsequent rinsing with water.
Deflocculation causes
the breakdown of contaminants into very small particles that are then dispersed
in the liquid cleaning medium and swept away. This process is similar to emulsification
except it happens on a larger scale.
Sequestration is
a process where undesirable ions, such as Ca+2 or Mg+2, and heavy metals are de-activated,
preventing them from reacting with material that normally would form insoluble
products. The classic example is the hard water scum formed when soaps are used.
The scum formed is the reaction between the Ca+2 or Mg+2 ions in hard water with
soap. When the water is softened, the Ca+2 or Mg+2 ions become tied or sequestered
and are unable to react.
Sequestering agents, although
they may not provide a cleaning mechanism, are important to prevent the formation
of compounds that can’t be removed following the cleaning process itself.
Co-Solvents
Co-solvent cleaning uses a mixture of miscible solvents to achieve the benefits
of multiple solvents. One example is a mixture of water and a water-miscible solvent,
such as alcohol, ethylene glycol or butyl cellusolve. There are many other miscible
co-solvent combinations in common use. In many cases, the mixture of solvents
is beneficial to not only gain the positive effects of one solvent but also to
minimize or eliminate the negative properties, such as flammability, of another
solvent. Co-solvent mixtures seldom exhibit all of the characteristics of both
solvents when used individually, but in combination, they can be more effective
than either used alone.
Co-solvent formulations
are applied in much the same way as aqueous cleaners and other solvents. It is
possible to use either the co-solvent mixture itself or only one of the components
of the co-solvent mixture as a rinse. Many co-solvent mixtures containing water
use pure water as the rinse.
Semi-Aqueous
Semi-aqueous cleaning uses a solvent in conjunction with water to clean. Terpene
and d-limonene are examples of solvents used in semi-aqueous cleaning. In most
cases, the solvent is not inherently water-miscible but is rendered water-emusifiable
by the addition of wetting agents, emulsifiers and other chemical additives. The
primary cleaning agent is the solvent. In the cleaning process, the solvent is
used either alone or partially mixed with water depending on the contaminant being
removed. The cleaning mechanism is dissolution of the contaminant by the solvent
as well as by the chemical additives and water, if present in the cleaning stage.
Once the cleaning process is complete, the remaining chemical residue is rinsed
off using pure water.
Semi-aqueous cleaning offers
the benefits of solvent cleaning with the convenience of a water rinse. There
are many examples of semi-aqueous cleaning, including emulsion cleaners, which
are commonly a mixture of a hydrocarbon solvent and water.
Cleaning
Techniques
Suitable cleaning equipment is required to implement the cleaning mechanisms described
previously. The purpose of this equipment is to provide a convenient means of
accomplishing the cleaning process as well as other functions, including the separation
and collection of removed contamination. In addition, the cleaning equipment may
enhance the cleaning mechanism through the addition of heat and mechanical energy.
The addition of some form
of mechanical energy is beneficial, if not essential, to the successful application
of nearly any cleaning mechanism. Without mechanical energy, the cleaning effect
stagnates once the activity of the cleaning agent in immediate contact with the
contaminant is saturated or consumed. Mechanical energy provides mixing to further
the cleaning process as well as physical displacement of dissolved or dislodged
contaminants. Nearly all cleaning equipment uses either spray or immersion technology
as the means to deliver the cleaning agent to the surface.
Spray
Cleaning
In
its simplest form, spray cleaning involves delivery of a liquid cleaning agent
to the surface through the use of a pump and nozzle. This same pump and nozzle
arrangement delivers mechanical energy to the cleaning site as the stream of spray
impinges on the surface.
In general, spray cleaning
is highly effective on any surface that can be “seen” directly by the
spray. Different effects can be achieved by changing the pressure of the spray,
the spray pattern and the volume of cleaner sprayed to provide more or less impingement
on the part. Since the cleaning agent is usually recirculated, the collection
of insoluble contaminants is easily accomplished using a suitable filter placed
in line with the pump.
Spray cleaning is an effective
means of cleaning parts that have a variety of contaminants and configurations.
It is especially useful in removing large amounts of contaminant that would quickly
saturate equipment using immersion-cleaning techniques.
Because of its effectiveness
and simplicity, spray cleaning is probably the most widely used cleaning technology.
In some cases, it is possible to clean, rinse and dry parts in a single processing
chamber, reducing space and material handling requirements.
On the down side, spraying
liquid is a relatively inefficient way to deliver mechanical energy to the cleaning
site. Energy expended in accelerating liquid not impinging directly on the surface
is wasted. Also, atomized liquid evaporates rapidly, necessitating frequent replenishment
of its water component and the addition of considerable heat to maintain the required
cleaning temperature.
Immersion
Cleaning
Immersion
cleaning involves immersing the part directly in a liquid cleaning agent. Immersion
alone, of course, does not inherently supply any mechanical energy. Mechanical
energy is supplied by a number of auxiliary means that may be used alone or in
combination.
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Ultrasonic cavitation and implosion
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Agitation of the cleaning
solution is one way to enhance cleaning by adding mechanical energy. The goal
of agitation is to deliver mechanical energy directly to the part surface where
the cleaning is taking place. The selection of the appropriate means of agitation
depends primarily on the configuration of the part and the degree of cleanliness
required
Simple stirring
of the cleaning liquid is commonly accomplished with rotary stirrers or paddles.
In general, this form of agitation is gentle and does not significantly enhance
cleaning in most applications unless the chemistry is very aggressive. This method
is not effective in enclosed or hidden areas of a part.
Bubbled air is a
gentle form of agitation with limited effect. Bubbled air has the advantage of
being relatively inexpensive to initiate and easy to apply. Bubbled air is most
useful in rinsing.
Turbulation, or
“spray under immersion” as it is often called, involves the use of high-velocity
jets of liquid created by appropriate nozzles submerged in the cleaning tank.
These high-velocity jets can be effective if the nozzles (eductors) can be positioned
relatively close to the surface. Like spray in air, turbulation is not particularly
efficient, often requiring pumps in the 10’s of horsepower to be effective
in even a relatively small tank. Although not strictly a line-of-sight technology,
the effectiveness of turbulation diminishes rapidly outside the direct effect
of the high-velocity jet.
Ultrasonic agitation
is more than just high-frequency mechanical agitation. High-frequency sound waves
create small cavities (bubbles) in the cleaning liquid that collapse (implode),
releasing considerable energy. Due to the nature of the phenomenon, this action
is mostly concentrated at the interface between the cleaning solution and the
contaminant being removed. Ultrasonic waves can penetrate thin layers of metal
and propagate around corners to clean work pieces inside and out. Ultrasonic cleaning
is usually not appropriate for thick buildups of contaminant.
Part agitation is
another way of imparting energy to the cleaning site. Parts are literally moved
up and down or side to side while immersed in the cleaning liquid to create shear
forces between the liquid and the part surface. The more rapid the agitation,
the more effective agitation becomes. Because of hydraulic “pumping”
of the cleaning liquid through internal passages, part agitation can also be an
effective means of cleaning inside some parts with appropriate configurations.
A successful
cleaning process requires careful selection of both the cleaning
chemistry and equipment. Various cleaning chemistries use different
cleaning mechanisms to attack and remove soils from the surfaces.
Knowing the nature of the contaminant is helpful in selecting a
cleaning chemistry that will effectively remove it. The cleaning
equipment facilitates the application of the cleaning chemistry
and enhances its effect. In the search for a cleaning process, one
must consider both the cleaning mechanism and the cleaning technique.
