Outgassing
Barbara Kanegsberg and Ed Kanegsberg, June 2005
IN OUTGASSING, ONE OR MORE VAPOR PHASE CONTAMINANTS that have been
trapped in a product or component are released. At first glance, this might
seem like a non-problem. After all, if a contaminant is adsorbed into the
materials of construction and then released, the product becomes cleaner
as outgassing progresses. In that sense, one might think of outgassing as
the obverse of airborne molecular contamination (AMC) [1]. However, outgassing
itself has the potential for contamination.
Depending on the environment and on the use of the product, outgassing
may result in catastrophic product failure. For one thing, the released
vapor
is not always removed from the vicinity of the component or product. If
the component is in a sealed environment, the outgassing chemical
may react with
other materials of construction, resulting in materials compatibility problems
or corrosion. As a general rule, outgassing and reactivity are enhanced
as the temperature increases. Decreased pressure also enhances the
removal of
vapors. Outgassing problems have been observed in sealed navigation systems
where solvent residue reacts with a flotation fluid or with metal parts.
Outgassing can produce clouding of optics. Avoiding outgassing is essential
in deep-space exploration, where the systems must perform reliably over
long periods of time and where possibilities for rework are at best
problematic.
Obviously, one must also minimize outgassing from medical devices.
Whether or not the product is sealed, release of solvents from implantable
medical devices is inherently undesirable. During our recent Medtec workshop,
we provided an example where a proposed solvent was considered preferable
to the currently-used solvent because, under similar conditions, less
of the proposed solvent was found to outgas using gas chromatography.
An attendee
correctly commented that the lower level of solvent was not per se necessarily
sufficient to demonstrate acceptability, given the variable differences
in toxicity among solvents. This is an important point. Further, one
cannot necessarily extrapolate potential impact of outgassing solvents
based on
toxicity characteristics. Even vapors of mild or relatively safe solvents
can exhibit undesirable effects.
While a contaminant has very occasionally been shown to have beneficial effects, it is probably prudent to assume that solvents should be removed to as low a level as possible. It would also probably be prudent to supplement analytical testing with at least in vitro studies, particularly for very critical biomedical applications. By the way, relatively non-volatile (i.e. high boiling point) solvents and water are included in this discussion.
Minimizing outgassing of process solvents involves process control. One
aspect is careful selection of the cleaning agent or process agent
so that minimal
solvent is retained. In addition, removal of the solvent can be expedited
by drying, by a process referred to as a bake-out. Often a forced
air or vacuum oven is used. Vacuum drying is an important aspect
of contamination
control, particularly for components with blind holes, complex geometries,
or porous surfaces. The drying process itself has to be controlled.
The time must be adequate to remove the solvent, with the understanding
that
absolute
solvent removal is impossible. The temperature must be adequately
high to coax the solvent away from the part, but not so high as to
produce
substrate
damage. When trying to remove high boiling point solvents or water,
more time and higher temperatures are frequently needed. The drying
chamber
itself
must not significantly add to contamination. Contaminants from the
drying chamber include particles and vacuum pump oil.
One additional approach might be to clean using the new chemistry, but not make it the last step in the process. After cleaning, the chemistry could be displaced with successive rinse steps using chemistries where there is known historical performance data.
Cleaning solvents are one obvious source of outgassing, but there
are others. Incompletely or inappropriately cured adhesives may
continue to react.
In this case, the outgassing material itself may do damage and
in addition,
the materials of construction continue to be altered. Further,
the complexity of most adhesives makes it difficult to assess potential
toxicity issues.
The solution is to assure that complete curing has occurred prior
to assembling the product or medical device.
Reactive chemicals, reactive processes, and adsorbed chemicals are part of many complex products. Outgassing can be controlled by awareness of the potential problem, careful process design and control, and judicious analytical testing.
Reference:
1 B. Kanegsberg, M Chawla, “Airborne Molecular Contamination (AMC)
Parts 1 to 5,” A2C2 Magazine, (January – April,
2001).
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.
