Using Aqueous Cleaners Safely
James Unmack and Barbara Kanegsberg, November 2004
An-all-too-common scenario is reported. A components manufacturer adopts a
new, safe, water-based process. No hazardous ingredients are indicated on the
MSDS, and the product comes with all manner of environmentally-related certifications.
The company safety department has approved the chemistry, the local regulatory
agency indicates that it is environmentally acceptable, and the requisite daunting
array of qualification and acceptance testing has been performed. Why are some
employees complaining of skin irritation?
Use of aqueous systems has gradually increased. Aqueous cleaning processes
are an integral part of a number of critical applications such as biomedical
devices, wafer fabrication, microelectronics, optics, and navigation systems.
Cleaning chemistries and cleaning equipment have increased in sophistication;
they are more readily adapted to precision applications.1 In addition, lubricants,
photoresists, and other materials that ultimately have to be removed from
the product, are increasingly being reformulated for removal with
water-based processes.
Costs and environmental constraints also contribute to adoption of aqueous
cleaning. While most facilities recognize the potential worker safety hazards
associated with solvents, aqueous cleaners may present a different set of
challenges compared to solvent cleaners, particularly in critical
cleaning applications.
Just as the aqueous process must be optimized to the expected soil mix and
soil loading, materials of construction, component configuration, and component
mix, aqueous cleaning processes, like all cleaning processes, have to be
managed to minimize employee exposure and to avoid introducing unintended
environmental
risks.
Aqueous cleaners present a somewhat different set of challenges relative
to solvent cleaners. These safety challenges are perhaps best understood
in the
context of the manner in which aqueous cleaning processes work to remove
soils. With solvent cleaning, the emphasis is on solubility of the soil
of interest
in the solvent; solubility parameters are commonly considered first. Other
physical parameters such as boiling point or operating temperature, evaporation
rate cleaning force, and time of exposure are also important. However,
there are times when immersion alone in ambient temperature solvent
results in
sufficient efficacy of cleaning.
With aqueous cleaning, immersion in the cleaning solution at ambient
temperature is typically not an efficient way to remove soil. Instead,
the interaction
of the cleaning chemistry with other process parameters is more important
in determining process effectiveness. The factors include:
* Chemical composition, concentration
* Temperature
* Physical force or agitation
* Time of exposure
Understanding, optimizing, and controlling these parameters is a key
to optimizing the cleaning process and to minimizing hazards to employees.
In addition, aqueous cleaning processes have characteristics that
require designing the fabrication facility so as to minimize physical
and electrical
impacts
on the employee. For example, many aqueous processes involve automated
transfer of components. In addition, aqueous processes are electrically
conductive.
For both aqueous and organic solvent based processes, careful management
is required, from receipt and storage, through initial blending
and use of the
cleaning agents, to ultimate safe disposal of the spent cleaning
agent.
Chemical Composition, Concentration
Aqueous cleaning agents are complex blends of water and various
chemicals.2 Water alone is effective in removing inorganic soils;
and water under
pressure can often blast soils away. To enhance the cleaning
ability, organic and
inorganic additives are often added to water. These additives
serve many functions including
adjustment of pH, improvement of wettability (ability to penetrate
closely-spaced components), and improvement in efficacy of cleaning,
minimization of
foaming, water softening, corrosion prevention, and biological
activity inhibition.
The pH, the inverse log of the hydrogen ion concentration, is
typically adjusted to provide the appropriate cleaning characteristics.
A
pH level of seven
is most compatible with living organisms; but it is often
less than useful for
removal of soils. A low pH (acidic) is typically used for
removal of inorganic compounds. High pH (alkaline) cleaners are useful
in removing
organic contaminants;
and such cleaning agents pose greater hazards to employees.
To
avoid such hazards, companies may adopt policies mandating
cleaning agents
with the
pH closer to
neutral. When this happens, efficacy of cleaning may suffer.
To compensate for this loss in cleaning capability, near-neutral
aqueous
cleaning
agents may be formulated with increased varieties and concentration
of additives.
Depending on the nature and concentration of those additives,
there can be an increased potential contact and inhalation
hazards.
Aqueous surfactants are generally grouped by their charge
and may be non-ionic, anionic, cationic, or amphoteric
(Zwitterionic). Non-ionics, for example,
include ethylene or propylene oxide, alcohol ethoxylates,
and
glycol
ethers. Zwitterions
include sultaines and betaines. Some additives are multi-functional.
Carbonates, for example, contribute to detergency, soil
holding or water-conditioning, and corrosion inhibition.
Preservatives are often used in the aqueous cleaning solutions
to inhibit biological activity or retard oxidation of
active ingredients. Some
preservatives may
cause adverse or allergic reactions to workers. Various
compounds are used as preservatives and antioxidants.
Many of these
compounds are
effective at very low concentration, often less than
0.01%, and so
are not required
to be
listed on the material safety data sheet.
Attempting to discern all possible additives from the
MSDS may not be possible. To formulate aqueous cleaning,
an
array of additives
must be
appropriately
blended; and not all additives may appear on the MSDS.
Even when trade secret protection is not claimed, the
ingredients in the
additive
package
may not
be revealed in product information or on material safety
data sheets. Unknown or undefined additive packages
do not
convey
hazards to
the user. If a
supplier of cleaning chemistry refuses to provide adequate
information regarding the
chemical content, it is prudent to select an alternate
supplier.
Proprietary additive packages may contain hazardous
ingredients that are protected by trade secrets.
If the toxic is
below 1%, it does
not have
to be listed.
Families of chemicals with related toxicity have
to be listed, even if each is under 1%. However, where
a large
variety
of additives are blended,
the
issue of where a family begins may become clouded.
As more additives are included,
the potential for chemical interaction and for employee
exposure issues both increase.
Temperature
Aqueous cleaners often use elevated temperatures
to accelerate grease and oily dirt removal and
provide faster drying.
In general, higher
operating temperatures
result in more effective soil removal. For one
thing, some soils liquefy at higher temperatures. In addition,
chemical
reactivity
increases
with increasing temperature. As a very rough rule
of
thumb, with each ten
degrees
Celsius
increase
in temperature, the reaction rate doubles.
Higher temperatures increase the hazards to workers
in terms of skin exposure and inhalation exposure.
Protection
against
thermal
injury
is based on
keeping the temperature of the living tissue
below 106°F.
The temperature of living tissue is determined
by the balance of the rate at which heat is conveyed
to
the tissue and the rate at which the heat is
carried away by the blood flow. Individual differences
to withstand heat are
due to varying thickness of skin
and degree of calluses. The ability to withstand
immersion or contact may be diminished by poor
peripheral circulation,
such as may occur with diabetes.
The elevated temperatures used in aqueous cleaners
can be injurious to exposed flesh. The following
temperatures are
given as general
guidelines. (See Table
1) Neither OSHA nor NIOSH have published
standards or
guidelines for the prevention of thermal
injury from immersion or
contact. The maximum
temperature
for total
immersion of bare hands should be limited
to 110°F (43°C). Bare hands
should not be used to grasp or handle metal parts if the metal is more than
110°F. Flocked or knit lined rubber gloves will protect hands to 135°F
(57°C) for prolonged immersion or hot metal parts coming out of the cleaner.
Most polymers will melt if the temperature exceeds 165°F (74°C). Melting
a glove onto a hand makes a nasty injury and must be avoided. Metal and other
surfaces with high thermal conductivity may produce an injury on momentary
contact at temperatures above 130°F (54°C) and must be protected against
contact. Normal reflexes will generally allow a person to withdraw from a 130°F
hot surface without injury. Where physical constraints limit the withdrawal
response or damage to critical parts is a risk, the maximum temperature for
accessible metal surfaces should be limited to about 110°F.
Barriers, insulation, or shielding are the preferred
way to guard such surfaces. When guarding a
hot surface is not practicable, a warning sign
should be used.
The above guidelines refer to water. In nearly
all cases, aqueous cleaning is performed
in a mixture of water with
organic chemicals
(or solvents)
and/or inorganic chemicals (e.g. salts).
At higher temperatures, organic chemicals
more readily volatilize, increasing inhalation
exposure.
Problems from exposure to eyes are more
likely to be exacerbated at higher
temperatures.
Physical Force and Agitation
In addition, to achieve the appropriate
cleaning activity, aqueous processes
are more likely
to depend on mechanical
force such
as high pressure spray
in air, immersion spray, and ultrasonic
cleaning. The same force that can remove
soil from product can adversely impact
the
worker. Pressure sprays above 30 psi
(200 kPa) should never
be directed
at skin. Higher
pressure sprays
have
the potential to cut through gloves.
Ultrasonic cleaning has become an important
factor in many cleaning operations,
both solvent and aqueous-based.
Ultrasonic
action
can enhance the effect
of chemical reactions. This so-called
sono-chemistry effect is thought
to be related
to the localized high pressures and
temperatures at the
site of implosion of the cavitation
bubble.3 While ultrasonic cleaning is not normally
associated with employee safety issues,
secondary vibrations can
result in a very
high noise level.
To avoid problems down the line,
it is wise to monitor the noise
level due both
to ultrasonic
cleaning and
to other
chemical and
mechanical aspects of the process
during the equipment
evaluation phase. In
fact,
involving
the safety
advisors early on can have positive
impacts on employee safety and
save money
by avoiding the need to rework
or retrofit the cleaning system.
Specialized Environments
The elimination of product contamination
must be balanced against personnel
safety. Cleaning
systems
are often
used in cleanrooms
where personnel
are dressed in hoods, masks,
and gloves. Cleanroom garments must
be appropriately
selected
to maximize contamination control
while avoiding potential interference
with
the vision and
mobility of personnel,
thus minimizing the
risk of injury
while working with cleaning systems
and other process equipment.
Process Management and Employee
Safety
Because aqueous cleaning
is inherently a process rather
than a solvency
phenomenon, assuring
employee safety
is related
to understanding
the overall cleaning
process and to controlling
that process.
For example, water in combination
with heat and a nutrient
source (including components
of the
cleaner
and organic
soils), can
promote growth of
microbes. In some cases,
benign bacteria are a
functional part
of the cleaning
system, as with systems
having on-board bioremediation.
Inherent to the concept
of a biodegradable product
is a potential to support
the growth
of microbes.
While
there
are exceptions,
if the
pH is high enough,
if
the water is
hot enough,
bacteria and molds are
unlikely to grow. Many formulations,
particularly those
containing non-ionic
surfactants which tend to act as bacteriostats,
have been
widely and safely used.
However, they may lose
their bacteriostatic
effectiveness as they age through
oxidation or evaporation.
While bacteria growth in process baths is not unknown, the key is not to be alarmist but rather to understand the potential for such occurrences and to take prudent steps to prevent, control, and minimize the impact. Preventing potential problems due to biological growth is a matter of appropriate, system design, bath monitoring, and system monitoring.
Bath monitoring can include
noting changes in pH
or obtaining simple
cell cultures.
When such contamination
occurs,
it would be prudent
to change
the bath and
to take steps to prevent
re-occurrence. Beyond
the issue of employee
safety, if bio-contamination
modifies
the
additive package, efficacy
of cleaning
may be adversely impacted.
Bio-contamination can
also
be introduced through
improperly maintained
filtration systems,
particularly in rinse
water. Appropriate
maintenance of filters
is essential.
If such
contamination occurs,
depending on the filter
in use, a pH change
or treatment
with household bleach
is generally
sufficient
to correct
the situation.
The potential impact
on the employee depends
on the system
in question.
A system which
creates mists
would produce
more concern
for inhalation
issues
than would
a simple dip tank.
This is true not
only for
biological
agents but more importantly
for
chemical constituents
of the cleaning bath.
It is possible
to minimize
impacts of the cleaning
agent
additive package
while maximizing efficacy
of cleaning through
a collaboration
of safety,
environmental, and
process advisors
(in-house or consultant)
with the technical
staff of the cleaning
chemistry
supplier.
However, while
the
cleaning agent itself
may pose no problem
to employee safety
or
to the environment,
the in-use situation
can be another matter.
One must consider
the impact
of oils, particles,
and ionic contamination
on the
makeup and behavior
of
the bath while in
use. This goes
beyond
mists that
might be generated.
In contract
build
situations,
unexpected reactivity
in the process bath
can occur,
with
the potential
for acute employee
exposure problems.
Certainly, such issues
can occur in solvent
cleaning
systems as
well. The problem
is that aqueous
systems are often
assumed to
be non-harmful.
In
both aqueous
and solvent systems,
emissive situations
are to be avoided.
In solvent systems,
costs and
environmental
mandates
often impel
the purchase
of very well-contained
systems which keep
the cleaning agent
away from
the
employee. With
aqueous systems, local exhaust
and ventilation
may be the preferred
solutions.
Electrical Safety
All aqueous solutions
are electrically
conductive and
ionic solutions
conduct electricity
very easily.
For personnel
safety, all electrical
outlets within
five feet of
any potential spray
or splash should
be protected
by ground fault
circuit
interrupters
(GFCI).
No electrical
outlet should
be located
within
a spray hazard
zone.
The ground path
for the process
equipment
should
be tested
to assure
a competent ground. The
impedance
to ground should
be
sufficiently
low to
limit the
potential above ground
and to facilitate
the
operation
of the
overcurrent
devices in the circuit.4
Noise
Power cleaning
systems
generate noise. The
noise
is usually
controlled by
enclosure.
Where
the noise would
adversely
impact
other activities
in
the
area,
additional control
can be achieved
through
a variety
of engineered
insulation
and isolation
applications.
Ultrasound
is rapidly
attenuated
in air
and is
generally
not a
concern for
worker
exposure, but
subharmonics
may
be
audible
and may at times
be more
than
just an annoyance.
Sound
pressure
levels
greater than
85 decibels
are hazardous
to hearing
for
continuous
exposures.
Conclusion
A safe
aqueous
process
begins
with
selection of the
appropriate
cleaning
chemistry
and
a responsible cleaning
agent
supplier. Aqueous
cleaning
is
usually a multistep
process
involving
wash,
rinse, and dry.
There
may be hazards
to
the product
and/or
the
operators
at
each
step.
With knowledge
of
the chemistry
and
process, hazards
may
avoided, eliminated,
or
at least guarded
against.
One
must not
ignore the
issue of
variable responses
among individuals.
Some individuals
may respond
adversely to
lower levels
of individual
or blended
chemicals. In
fact, our
understanding of
the impact
of blends
is not
nearly as
complete as
is the
impact of
individual chemicals.
When one
adds in
the additional
factors of
temperature, mechanical
force, and
exposure time,
it is
not surprising
that, as
described in
the introductory
scenario, an
individual employee
might react
adversely to
a safe, well-designed
process.
Each
cleaning application
is unique;
and each
process must
be tailored
to the
specific requirements
for surface
quality and
contamination control.
In the
same manner,
aqueous cleaning
processes must
be managed
thoughtfully so
as to
minimize employee
safety issues.
Aqueous cleaning
systems can
provide a safe
alternative to
solvent cleaning
when properly
selected for
the contaminant
and substrate
and the
hazards to
the operators
are adequately
addressed.
References:
1
B. Kanegsberg. “Aqueous Cleaning for High-Value Processes,”A2C2
Magazine,
November, 1999.
2 B. Kanegsberg. “Cleaning is More than Dipping and Scrubbing, Presentation
and Proceedings,” CleanTech,
2003.
3
F. John
Fuchs. “The Fundamental Theory and Application of Ultrasonics
for Cleaning,” in Handbook for Critical Cleaning, Kanegsberg & Kanegsberg,
ed.,
CRC
Press,
2001.
4
National Electrical
Code Requirements
for Wiring
Devices National
Electrical Code
NFPA No.
70-1996 p.250-51
Effective Grounding
Path
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
