In many critical aerospace and semiconductor applications, low-outgassing materials
must be specified in order to prevent contamination in high vacuum environments.
Outgassing occurs when a material is placed into a vacuum (very low atmospheric pressure)
environment, subjected to heat, and some of the material's constituents are volatilized
(evaporated or "outgassed").
ASTM TEST METHOD E595
Although other agency-specific tests do exist (NASA, ESA, ESTEC), outgassing data for
comparison is generally obtained in accordance with ASTM Test Method E595-93, "Total Mass
Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment".
In Test Method E595, the material sample is heated to 125°C for 24 hours while in a vacuum
(typically less than 5 x 10-5 torr
or 7 x 10-3 Pascal). Specimen mass is measure before and
after the test and the difference is expressed as percent total mass loss (TML%). A small
cooled plate (at 25°C) is placed in close proximity to the specimen to collect the volatiles
by condensation ... this plate is used to determine the percent collected volatile condensable
materials (CVCM%). An additional parameter, Water Vapor Regained (WVR%) can also be determined
after completion of exposures and measurements for TML and CVCM.
ASTM Test Method E595 data is most often used as a screening test for spacecraft materials. Actual
surface contamination from the outgassing of materials will, of course, vary with environment
and quantity of material used. The criteria of TML < 1.0% and CVCM < 0.1%
has been typically used to screen materials from an outgassing standpoint in spaceflight
applications. Semiconductor applications may often be more sensitive to contamination
during certain processes and may have even lower limits of acceptability.
Goddard Space Flight Center (NASA) maintains an on-line searchable reference of Outgassing
Data for Selecting Spacecraft Materials at
http://outgassing.nasa.gov/.
Note: This provided link was correct as of August 2003. When you reach the site, use
the SEARCH function to find the material information you need. Related reference information
is also available at that site.
The paragraphs below describe specific materials and their uses in these industry applications . . .
LOW-OUTGASSING MATERIALS
Celazole® PBI (PolyBenzImidazole)
Celazole® is the highest temperature-capable
plastic available. However, it is very brittle (almost ceramic-like) and
quite difficult to machine. That said, is it frequently used for bushings,
bearings, rollers, and spacers in extreme environments. Its outgassing
values are listed as 2.50% TML, 0.00% CVCM, 0.40% WVR.
Vespel® (Polyimide)
DuPont Vespel® SP-1 is one of the most-used
high-temperature plastic materials used in applications where high-purity
and electrical properties are needed. Vespel is frequently used in
ultra-clean semiconductor and chemical applications. It is also one of
the most expensive materials sold, but is flight-approved for NASA, USAF
and other aerospace agencies. Its NASA outgassing values are listed as 1.09% TML,
0.00% CVCM, 0.40% WVR.
Duratron® XP (Polyimide)
Duratron® XP is the first real alternative
to Vespel ... it was developed specifically to replace Vespel in extreme
applications at a slightly lower price. It contains less than 1% metallic
impurities as measured using the ICP-MS test standard. Duratron XP is
ideal for use in high-energy gas plasma etch and strip processes. Outgassing
values for Duratron XP are 0.75% TML, 0.00% CVCM, 0.49% WVR.
Torlon® 4203 unfilled PAI (PolyAmide-Imide)
Unfilled Torlon® 4203 has high dielectric properties and low
thermal expansion, and is much less expensive than some advanced polymers.
Torlon 4203 is typically used for insulators, spacers, and mechanical
parts up to 520°F. Its outgassing values are listed
as 1.85% TML, 0.00% CVCM and 0.49% WVR.
Torlon® 5530 glass-filled PAI (PolyAmide-Imide)
Torlon 5530 (30% glass-filled) is typically used for applications where dimensional
stability over a wide temperature range is needed, as with temperature
test sockets, nests, and fixtures. Its outgassing values are listed
as 0.58% TML, 0.00% CVCM (% WVR is not shown). NOTE: Torlon's moisture absorption
is a bit high, so critical dimensional stability can be an issue.
Semitron® ESd 500HR (filled PTFE)
Semitron® ESd 500HR is antistatic/conductive PTFE.
This material is relatively clean, readily machinable, dissipates static
electricity reliably ... as a result it is used in test handling equipment,
fixtures, and other applications where static generation may cause failures
and/or errors in production environments. This material has low outgassing values
of 0.04% TML, 0.00% CVCM and 0.01 % WVR. PTFE has good mechanical properties up
to approximately 500°F.
Neoflon® PCTFE (PolyChloroTetraFluoroEthylene)
PCTFE exhibits high chemical resistance, low and high temperature capability,
resistance to most chemicals (including strong acids and bases),
low friction, electrical and thermal insulation, and "slipperiness".
PCTFE has the lowest outgassing values of any thermoplastic material we
sell ... 0.01% TML, 0.00% CVCM, 0.00% WVR.
PEEK (PolyEtherEtherKetone)
PEEK is pure, easily machinable, chemically resistant, stable, and also has
relatively low outgassing values ( 0.31% TML, 0.00% CVCM, 0.06 % WVR).
PEEK has good mechanical properties, but will not take temperatures
over 350°F, so it may not have the mechanical or thermal performance needed.
Techtron® PPS (PolyPhenylene Sulfide)
Techtron® PPS is easily machined to close
tolerance, has excellent mechanical, thermal and chemical stability and
has one of the lowest outgassing values of any thermoplastic material we
offer ( 0.04% TML, 0.00% CVCM ... % WVR is not shown ). Techtron
PPS is generally a bit less expensive than PEEK or Torlon, but again,
will not take as high temperatures.
Ultem® PEI (PolyEtherImide)
Ultem® has good dielectric properties and low
thermal expansion, and is considerably less expensive than some other polymers.
PEI is also clean and stable, but is not particularly resistant to
chemicals or solvents ... it has outgassing values of 0.40% TML,
0.00% CVCM and 0.06 % WVR. PEI has good mechanical properties up
to approximately 410°F.
Semitron® ESd 410C (filled PEI)
Semitron® ESd 410C is antistatic/conductive PEI.
This material is relatively clean, readily machinable, dissipates static
electricity reliably ... as a result it is used in test handling equipment,
fixtures, and other applications where static generation may cause failures
and/or errors in production environments. This material has outgassing values
of 0.46% TML, 0.00% CVCM and 0.17 % WVR. PEI has good mechanical properties up
to approximately 340°F.
Ertalyte® PET-P (Polyethylene Terephthalate)
Ertalyte® offers the dimensional stability
of acetal with the wear resistance of nylon. Ertalyte®
PET-Polyester is clean, chemically resistant, stable, and also has
relatively low outgassing values ( 0.13% TML, 0.00% CVCM ... % WVR
is not shown ). PET-P is considerably less expensive than most of
the other materials listed above, but may not have the mechanical
or thermal performance needed for all applications.
Semitron® ESd 225 (filled acetal)
Semitron® ESd 225 is antistatic/conductive acetal.
This material is relatively clean, readily machinable, dissipates static
electricity reliably ... as a result it is used in test handling equipment,
fixtures, and other applications where static generation may cause failures
and/or errors in production environments. This material has low outgassing values
of 1.00% TML, 0.05% CVCM and 0.60 % WVR. Acetal has good mechanical properties up
to approximately 180°F.
Trademark Acknowledgments:
TORLON is a registered trademark of Solvay Advanced Polymers.
NEOFLON is a registered trademark of Daikin America.
DURATRON, ERTALYTE and SEMITRON are registered trademarks of Quadrant DSM Engineering Plastic Products.
VESPEL is a registered trademark of DuPont.
ULTEM is a registered trademark of the General Electric Company.
CELAZOLE is a registered trademark of Celanese Advanced Materials, Inc.
PEEK is a trademark of Victrex plc.
©
Copyright Boedeker Plastics, Inc.
2008