Outgassing of space materials, screening tests for COTS components

Outgassing mechanisms, analytical approach

Desorption

Desorption is the release into vacuum of molecules that were adsorbed on top of a materials.

Molecules are initially linked to the material by weak forces (physisorption) or stronger forces (chemisorption). The random thermal movements of a molecule may allow it to leave the resulting potential wall to vacuum.

Evaporation/sublimation

Evaporation is the passage of contaminant from its condensed phase (liquid or solid) to gas phase. The contaminant is thus implicitly assumed to be present as a thick layer on top of the material (many monolayers). For a pure chemical compound evaporation follows zero‐order Langmuir law. 

Diffusion

Contaminant molecules can be located deep inside a material. The random motion of these molecules within the material is called diffusion. Those reaching the vacuum exposed surface of the material can further be desorbed to vacuum.

3. OUTGASSING SOURCES-MATERIALS

Materials outgassing, main sources….

• Polymer or macromolecular materials, comprising  low relative molecular weight molecules, that can contain concentration of volatile variety of mixed chemical compounds in a complex formulations. Weakness of the linkages between molecular structures in polymers also make them prone to outgassing.

• Certain metallic materials can also outgass, such us Zn that can sublimate under vacuum at heating (150ºC) or Cd (100ºC), but at lower order of magnitude compared with polymers, metallic depositions can be critical and must be avoided also. 

Standarized type of space materials prone to outgassing:

Optical materials, adhesive tapes, coatings and varnishes, paints, inks, lubricants and greases, pottings, sealants, foams, reinforced plastics, fabrics, rubbers and elastomers, thermoplastic materials, thermoset materials, cables , ceramic and glass materials, miscellaneous, metallic materials, surface anodized treatments, etc.

Electronic and microelectronic materials /COTS

• Plastic materials made of complex compounds. Formulations can include i.e : epoxy resin, silicones, urethanes,  hardening compounds, accelerators, fillers, flame retardants, couplers, stress relief additives, mold release additives, coloring, and ion–getters among others. LCP with metal treatments.
• Typically proprietary formulation of a specific parts resin (i.e: encapsulating), and various types of additives, which provide the desired properties (i.e: for the packaged device).

Main factors determining outgassing levels, 

Material processing  

Material is not validated itself, validation of particular processing for specific application is required for space use or concrete mission.  

• Chemical nature of materials.

• Formulation.

• Manufacturing process.

• Curing parameters / process and other treatments (i.e. baking).

• Environmental conditions/storage.

• Preparation/application methods.

typical chemical compounds outgassed /deposited 

epoxy, silicone, polyamide , dyciandiamide,  polyurethane, acrylic, fluorocarbons, polycarbonates, acetals, ester compounds, hydrocarbons, water vapour , etc..

• Residual concentration of monomers non reacted after polymerization process.

• Concentration of volatile additive or agent compounds present in the polymer (flame retardant, plastificier, estabilizer agent, hardeners, etc.).

• Contaminants, compounds resulted from chemical reactions, absorbed or trapped during material processing.

• Excess of humidity  (water vapour could be critical contaminant)

4. INFLUENCE OF OUTGASSING IN SPACECRAFT

Detrimental effects of outgassing on materials and components

• Degradation of functional properties of materials (i.e : outgassing of functional additives). 

• Molecular or chemical contamination on component surfaces, affecting the performance (i.e:degradation of optical, thermal, electrical properties).

• Perturbation of the spacecraft medium (i.e: voltage breakdowns, arcs, heat transfer problems)

SYNERGISTIC EFFECTS ->  Interactions between a material and the different environments to which it is exposed are quite often synergistic and not simply additive; i.e. the sum of both interactions is larger than each of the effects separately.

(i.e: radiation UV, P+, e– molecular degradationoutgassing increase)

5. OUTGASSING TEST AND BAKE OUT IMPLEMENTATION

Outgassing testing 

Screening outgassing tests

Std. ref.: ECSS-Q-ST-70-02C / ASTM E 595

Implementation of general criterium for material acceptance/rejet, in the materials selection step for space application, and reject those with high levels of outgassing.  The general standard limits are:  

• 1,00%  Total mass loss (TML/RML).

• 0,10% sample mass, for Collected Volatile Condensable Material  (CVCM).

• More stringent values can be used, for specific critical cases (i.e: in optical applications, CVCM < 0,01%— “Ultra Low OutgassingTM requirements” < 0.1 % TML and < 0.01 % CVCM).

  The test is performed at one standard temperature of reference: 125ºC sample, and 25ºC for collector plate for condensables, on samples in the order of 100-300 mg mass.

Kinetic /dynamic outgassing tests

Std. ref.: ECSS-Q-TM-70-52A / ASTM 1559

Mesurement of outgassing/deposition rate characteristics with time, under selected values of different heating sample and cold condensable collector temperatures outgassing long term predictions. The method include the use of Vacuum balances / QCM Quartz Crystal Microbalances (ESA / ASTM configurations). 

Data from this tests allow to compute contamination models and simulate contamination processes in the designed systems, as well as provide further information of outgassing characteristics more than simple data obtained from screening tests.

i.e: Kinetic Outgassing test on polymer for electronics

Material: Uralane 5753. (polyurethane). Thermally Conductive Potting/Staking Compounds. Conductive heat transfer from electronic assemblies. 

Mass sample: 4851.96 mg. Sample size: (4 x 4 x 0,3) cm3. Sample Tª: (EFFUSSION CELL): 60ºC.  

CVCM Tª QCMs: QCM1: -183ºC.  QCM2: -40ºC. QCM3: 0ºC .QCM4: 20ºC 

Temperature time: 144 h. 

Vacuum : 10-10 – 10-8 torr. View factor: -QCM: 415 cm2. QCMs sensibity: 4.43×10-9 g/cm2/Hz. 

Kinetic /dynamic outgassing tests: standard method ESA

Bake out under vacuum 

The purpose of this activity, is to evacuate the excess of possible volatile compound of the material or molecular contamination on hardware to reduce or eliminate the potential outgassing or in a cleaning process for the removal of residual contamination after solvent application (i.e: IPA) or humidity excess. A material or component is heated under vacuum for a certain period of time (normal practice is between to heat 70-125 ºc under mid – high vacuum , 10-2 – 10-5 mbar , for 72 h or more). 

It can be done also in controlled atmosphere. 

6. STANDARD TEST METHOD APPLICATION: ECSS-Q-ST-70-02C / ASTM E 595

Screening outgassing test ECSS-Q-ST-70-02C / ASTM E 595

Aplicability

• Determination of outgassing screening material properties, not for outgassing simulations.
• Reliable for operating temperatures below 50ºC in general. 
• The validity of this test, depends on the environmental conditions during the lifetime of the material as well as the vicinity of critical or sensitive surfaces (i.e: for higher temperatures is questionable).
• Not reliable for temperatures > 50ºC for extended period of time , or > 125ºC for short period of time—dedicated test progamm is required.

Test standard conditions

• SAMPLE MASS: 100-300 mg (metal parts excluded)

• SAMPLE TEMPERATURE: 125ºC.

• COLLECTOR COLD PLATE TEMPERATURE: 25ºC.

• TEST DURATION: 24h.

• VACUUM PRESSURE: high vacuum range,  10-6 – 10-7 mbar.

In order to control the balance of water vapour absorbed/desorbed, samples are conditioned before and after the test at 22ºC , 55 HR%, 24h.

Test parameters measured : in % of the tested mass sample 

TML%: Total Mass Loss (total mass outgassed just after test)

RML%: Recovered Mass Loss (total mass outgassed after  24 h water vapour mass recovering)

WVR%: Water Vapour Regained (TML% -RML%)

CVCM%: Collected Volatile Condensable Material (mass of condensed product on collector plates

Determination of CVCM nature by spectrometry techniques  (i.e: FTIR, GC-MS) 

Test process illustration

Test sample configuration

Sample basic preparation

Introduction of COTS samples in specimen cups for test

7. OUTGASSING TEST FACILITIES

Screening outgassing test ECSS-Q-ST-70-02C / ASTM E 595

Outgassing test facilities : vacuum technology application

Outgassing test facilities: DMAVAC system –vacuum chamber

Outgassing test facilities: ESA vertical system- vacuum chamber

Outgassing test facilities: Examples of condensable products procuced by outgassed samples (25ºC)

Outgassing test facilities: HIGH TEST VACUUM GENERATION: Turbomolecular pump

Outgassing test facilities: HIGH TEST VACUUM GENERATION: pump features

• Test chamber able to reach 10-5 mbar < 1 h, vacuum range 10-6 – 10-7 mbar is obtained after 3-4 hour running with turbomolecular pump. 
• Pumping speed S = 2100 l/s.
• Thoughput Thp = 30 mbar · l · s-1 : capacity of the pump to recover the vacuum level < 10-5 mbar when pressure is increased during initial outgassing of samples (higher rate in the first 1-2 hour of the test run) 
• Range 10-6 – 10-7 mbar , is valid to reproduce extreme vacuum conditions in deep space, (< order of 10-12 mbar).

8. OUTGASSING TEST DATA AND EVALUATION OF E-COMPONENTS AND COTS PROPOSED FOR SPACE APPLICATIONS

Screening outgassing test ECSS-Q-ST-70-02C / ASTM E 595

Test experience on several E-components / COTS materials