Moisture Contamination
Barbara Kanegsberg and Mantosh Chawla, June 2002
Water is an invaluable tool in achieving reliable contamination control and
appropriate surface quality. Sometimes, however, water itself is a contaminant.
y´or critical components or parts, water may be the best approach for
removing other soils. Outgassing of lower boiling organic solvents is a concern;
however, higher boiling solvents can, like water, be trapped in blind holes
or absorbed into plastics or other synthetics.
The consequences of contamination by either water or solvents include
corrosion, gradual breakdown of the product during use, and degradation
of coatings. If
the coating failure is primarily aesthetic, the main consequence will be
customer dissatisfaction. If the coating is required for critical
surface characteristics,
as in biomedical implantables, even if the product is used in an environment
which is itself primarily water, residual underlying moisture at the coating
stage can shorten product lifetime.
Water can also interact with other fluids found in the completed product.
Water can, for example, interact with the flotation fluids used in some
gyroscopes. Residual water can be a problem in other areas as well;
moisture retained
in
motion picture film has been associated with the breakdown of master copies
of classic movies.
This doesn’t necessarily mean that all water must be removed from all
products. In some cases, as in lyophilization, the quantity of water must be
controlled, not necessarily minimized to below any imaginable detection limit.
Water can contaminate and degrade organic solvents and refrigerants.
Certain esters, when combined with water, can break down to acids
and alcohols.
In addition to the more obvious changes in solubility characteristics,
the breakdown
products may have a less desirable toxicological profile than the original
solvent.This may impact solvent
accessibility. For example, one of the issues involving de-listing of t-butyl
acetate as a volatile organic
compound involves
questions by a California regulatory agency concerning the toxicity
of a breakdown product of t-butyl acetate, t-butyl alcohol. Classic chlorinated
solvents such
as perchloroethylene and methylene chloride can break down in the presence
of water to form acids.
Acidic solvents are particularly damaging where there is extensive
exposure of the liquid to metal surfaces, as in liquid/vapor phase
cleaning systems.
It is not unknown for costly cleaning systems to be ruined by acidic
solvent.
In the case of classic chlorinated solvents, stabilizers are added
to prevent solvent breakdown. Water and acid content can be monitored
by
relatively
straighforward tests conducted in the manufacturing plant. With
more complex blends or azeotropes,
there is the potential for solvent modification if one or more
components partitions to the aqueous phase. In general, exposure
of the solvent
to water can be limited
by using appropriate equipment and process design. With some of
the more esoteric solvents, such as parachlorobenzotrifluoride (PCBTF),
water
may be present
as a result of the manufacturing processes. The best approach is
likely to be to work with a reliable supplier and obtain solvent
certification
data
indicating water content of the batch in question.
The question of controlling and detecting water contamination in
a product is more complex. Often the emphasis is placed on presumptive
water removal
rather than detection. Water may be removed by physical drying
methods or displaced by organic compounds which are themselves
presumed to
be more readily
evaporated.
Sometimes vacuum drying for hours or even days is used, with
the presumption
that all residual organic solvents and water are removed. This
presumption may be backed up by product performance data or accelerated
stress
testing of the product itself. Potential problems with this approach
include
reproducibility (particularly when the assembly or surface preparation
approach changes)
and documentation. In some cases, surfaces treated to remove
moisture may be stored
at a temperature lower than the dew point temperature, thus causing
condensation and “contamination” of the surface.
Over the next few months, we’ll cover some approaches to detection of
water including the Kauri-Butanol method, Standard Gravimetric method, electrical
impedence, near infrared, and millimeter wave (MMW) techniques. We’ll
also address the strengths and weaknesses of these techniques and discuss
some of the applications in which these detection methods are used.
Barbara Kanegsberg at BFK Solutions, 16924 Livorno Dr., Pacific Palisades, CA
90272, 310-459-3614; barbara@bfksolutions.com;
contact Mantosh Chawla atPhotoemission Tech., (PET), 3255 Grande Vista Dr., Newbury
Park, CA, 91320; 805-499-7667
