Material Outgassing in Space Engineering

Material outgassing is the release of gases, vapours or volatile compounds from a material when it is exposed to reduced pressure or vacuum. The effect is particularly important in spacecraft because released molecules can migrate through the vacuum environment and condense on colder or contamination-sensitive surfaces.

Adhesives, coatings, polymers, lubricants, tapes, composite materials, elastomers, inks and some additively manufactured materials can release water, residual solvents, plasticisers, oils or other volatile substances. These contaminants can deposit on optics, detectors, thermal-control surfaces, solar cells, electrical contacts and precision mechanisms.

Material outgassing is therefore not simply a chamber-cleanliness concern. It is a materials-selection, design, contamination-control and mission-assurance issue.

ECSS addresses outgassing principally through the product-assurance and contamination-control framework. ECSS-Q-ST-70-02 defines a thermal-vacuum outgassing screening method for space materials. ECSS-Q-ST-70-01 addresses cleanliness and contamination control across the project lifecycle. ECSS-E-ST-10-04 provides wider space-environment context, including the induced molecular environment created by the spacecraft itself.

Outgassing evidence can support material approval, Declared Materials List review, contamination analysis and thermal-vacuum campaign planning. It does not prove complete spacecraft suitability on its own. The applicable limits depend on the material quantity, location, operating temperature, nearby sensitive surfaces, mission environment and customer requirements.

Important: A material passing a screening limit is not automatically suitable for every spacecraft application. Location, quantity, geometry, temperature, view factors and contamination sensitivity still matter.

Material outgassing is the release of gaseous or volatile species from a material.

In vacuum, molecules that were absorbed, adsorbed, trapped or chemically retained can leave the material. The released species can travel through the vacuum environment and:

• Leave the spacecraft.

• Remain temporarily within an enclosed volume.

• Strike another surface.

• Condense on a colder surface.

• Re-evaporate when the receiving surface becomes warmer.

• Become altered by ultraviolet radiation or atomic oxygen.

• Contribute to a wider molecular-contamination environment.

These terms are related but should not be used as if they are identical.

Outgassing is a source mechanism. Contamination is the potential consequence.

Outgassing control aims to:

• Select materials appropriate for vacuum service.

• Reduce molecular-contamination sources.

• Protect optics, sensors and thermal-control surfaces.

• Support contamination modelling.

• Maintain chamber and hardware cleanliness.

• Prevent unacceptable functional or performance degradation.

• Generate traceable evidence for material approval and verification.

Condensed molecular films can reduce optical transmission, increase scattering or change reflectance. Even a small deposit can be significant for:

• Cameras.

• Star trackers.

• Telescopes.

• Spectrometers.

• Ultraviolet instruments.

• Infrared detectors.

• Laser systems.

• Optical windows and mirrors.

The risk can be greater where the receiving surface is cold because volatile species are more likely to remain deposited.

Contamination can alter the solar absorptance and infrared emittance of thermal-control surfaces. This can affect the spacecraft thermal balance and create temperatures different from those predicted during design.

Sensitive items include:

• Optical solar reflectors.

• Radiator surfaces.

• Thermal-control coatings.

• Multilayer insulation surfaces.

• Instrument apertures.

Outgassing and contamination can contribute to:

• Electrical leakage paths.

• Arcing or discharge in high-voltage equipment.

• Noise on electrical contacts or slip rings.

• Increased mechanism friction.

• Lubricant loss.

• Obstruction of precision mechanisms.

• Degradation of solar-cell performance.

• Corrosion where unsuitable residues are present.

Early material review is cheaper than discovering a contamination issue after integration. Screening can identify unsuitable or poorly controlled materials before they enter higher-value assemblies.

However, screening should not become a box-ticking exercise. A test result without application context can give false confidence.

Material outgassing control supports confidence that contamination-sensitive equipment will retain its intended optical, electrical, mechanical and thermal performance throughout the mission.

The principal ECSS references can include:

• ECSS-Q-ST-70-01: Cleanliness and contamination control.

• ECSS-Q-ST-70-02: Thermal-vacuum outgassing test for screening space materials.

• ECSS-Q-TM-70-52: Kinetic outgassing guidance used for more detailed contamination modelling.

• ECSS-E-ST-10-04: Space environment.

• ECSS-Q-ST-70-04: Thermal testing under vacuum for materials, processes, mechanical parts and assemblies.

• ECSS-Q-ST-20: Quality assurance.

• ECSS-E-ST-10-02: Verification.

• ECSS-E-ST-10-03: Testing.

The applicable issue of each document must be confirmed against the project requirements.

ECSS-Q-ST-70-02 provides a standardised thermal-vacuum method for screening materials used in spacecraft, launchers, payloads and associated equipment.

The test is commonly associated with:

• TML: Total Mass Loss.

• RML: Recovered Mass Loss.

• CVCM: Collected Volatile Condensable Material.

These values describe different aspects of mass loss and condensable material. They should not be interpreted as a complete prediction of in-orbit contamination.

The current internal copy of ECSS-Q-ST-70-01 Rev.1 states that outgassing screening should be performed in accordance with ECSS-Q-ST-70-02. It also states that outgassing data older than ten years is not admissible for material selection and that screening should be performed by a trusted test house capable of providing the required conformity evidence.

That requirement is materially stricter than treating any historic database value as permanently valid.

The acceptable outgassing position can depend on:

• Quantity of material.

• Material temperature.

• Receiving-surface temperature.

• Direct or indirect view of sensitive items.

• Available volume.

• Venting.

• Mission duration.

• Vacuum exposure.

• Location inside or outside the spacecraft.

• Optical sensitivity.

• Cryogenic surfaces.

• Cleaning and bakeout history.

• Project contamination budget.

Customers can impose limits more stringent than the general ECSS screening criteria. Optical, scientific, cryogenic and contamination-sensitive missions can require:

• Lower CVCM limits.

• Kinetic outgassing data.

• Species identification.

• Witness samples.

• Quartz crystal microbalance monitoring.

• Residual gas analysis.

• Contamination transport modelling.

• Dedicated bakeout.

• Material-specific approval.

TML is the percentage of the original specimen mass lost during the specified test exposure.

It can include:

• Absorbed water.

• Residual solvents.

• Low-molecular-weight constituents.

• Plasticisers.

• Process residues.

• Other volatile substances.

TML shows how much mass has left the sample under the test conditions. It does not show where that material goes or whether all released species are harmful.

RML adjusts the mass-loss result to account for recovered water vapour.

This helps distinguish moisture-related mass loss from other volatile losses. RML is often more useful than raw TML when comparing materials that absorb different amounts of atmospheric moisture.

CVCM is the percentage of the original specimen mass collected on a controlled condensing surface.

CVCM is particularly important where released material could deposit on:

• Optics.

• Detectors.

• Cold surfaces.

• Radiators.

• Solar cells.

• Thermal-control coatings.

A low CVCM result reduces concern but does not eliminate the need to assess geometry, temperature and mission sensitivity.

Screening results are generated under defined laboratory conditions. Flight conditions can differ in:

• Temperature.

• Duration.

• Material quantity.

• Surface area.

• Vacuum level.

• Geometry.

• View factor.

• Radiation exposure.

• Receiving-surface temperature.

The screening test provides comparable data. It is not a full mission simulation.

Define:

• Applicable standard and issue.

• Material identity.

• Manufacturer and product designation.

• Batch or lot.

• Cure state.

• Specimen preparation.

• Test objective.

• Required measurements.

• Acceptance criteria.

• Reporting and certification requirements.

The submitted specimen should represent the material as used.

This can require control of:

• Mixing ratio.

• Cure temperature.

• Cure duration.

• Thickness.

• Substrate.

• Cleaning process.

• Post-cure.

• Storage.

• Environmental exposure.

• Time between preparation and testing.

Testing an uncured or incorrectly prepared sample can produce data that does not represent the flight configuration.

The specimen is weighed and conditioned in accordance with the approved method. Balance accuracy, environmental conditions and handling controls are important because the measured mass changes can be small.

The sample is exposed to the prescribed vacuum and temperature conditions. Volatile species released by the sample can be collected on a temperature-controlled collector.

The sample and collector are reweighed under controlled conditions. TML, RML and CVCM values are calculated as required by the method.

Results should be checked against:

• Applicable limits.

• Material identity.

• Sample preparation.

• Test validity.

• Repeatability.

• Measurement uncertainty.

• Customer requirements.

• Intended use.

The report or certificate should provide enough information to link the result to the exact material, batch, preparation and test configuration.

Potential concerns include residual solvents, incomplete cure, mixing errors and unreacted constituents.

Coatings can contain solvents, binders, pigments and additives. Cure thickness and process control can affect results.

Plasticisers, processing aids and low-molecular-weight constituents can be released under vacuum.

Lubricant volatility can affect mechanisms and nearby sensitive surfaces. The correct lubricant quantity and application method matter.

The backing, adhesive, printed ink and release residues can all contribute to outgassing.

Composites

The fibre is often not the primary concern. Resins, adhesives, core materials, bonding films and process residues require review.

Printed polymers can contain unreacted feedstock, trapped gases or process residues. Surface area, porosity and post-processing can influence outgassing.

Printed circuit boards, conformal coatings, soldering residues, staking compounds, wire insulation and component adhesives can all contribute to the contamination load.

Cell seals, potting compounds, insulation, harnessing and module materials require assessment. Material outgassing screening is distinct from battery leak, venting and safety testing.

A material should not be assessed only by its individual TML or CVCM value.

A contamination source has an unobstructed path to a sensitive surface.

Molecules can reach a sensitive surface after reflection or transport through adjacent volumes. Indirect view should not be assumed to mean no contamination risk.

Small or poorly vented volumes can retain released species. The local concentration can be more important than the total spacecraft mass.

Cold surfaces can act as strong contamination sinks. A material acceptable near room-temperature hardware can be unsuitable near a cryogenic detector.

Hotter materials generally release volatile species more rapidly. Temperature history must therefore be considered when applying screening data.

The Declared Materials List should identify:

• Material designation.

• Manufacturer.

• Application.

• Location.

• Quantity.

• Processing.

• Approval status.

• Supporting data.

• Restrictions or deviations.

Identify which hardware could be affected and what level of deposition is acceptable.

Confirm that data:

• Relates to the correct material.

• Relates to an appropriate cure state.

• Is within the permitted age.

• Comes from an acceptable source.

• Covers the required metrics.

• Meets the applicable criteria.

For sensitive applications, screening values can support a wider model considering:

• Outgassing flux over time.

• Surface temperature.

• Geometric view factors.

• Residence time.

• Sticking probability.

• Re-emission.

• Mission duration.

• Contaminant transport.

Bakeout can reduce removable volatile content. It does not convert an unsuitable material into a suitable one.

A bakeout plan should define:

• Temperature.

• Vacuum or purge condition.

• Duration.

• Hardware limits.

• Monitoring.

• Completion criteria.

• Post-bake handling.

• Recontamination controls.

Evidence should be linked to material approval, contamination requirements and the project verification system.

Commercial products can change formulation while retaining a similar trade name. Manufacturer, grade, lot and revision matter.

A screening result for a fully cured laboratory sample might not represent a thick bondline or a poorly cured production part.

Historic database values can relate to a different formulation, batch or preparation method. Current ECSS material-selection requirements also place limits on the age of acceptable data.

A material passing a generic threshold can still be unacceptable near a highly sensitive optical or cryogenic surface.

A small amount of a marginal material can be acceptable in one location. A large quantity of the same material can dominate the contamination budget.

Bakeout is not a repair for poor material selection. It can reduce volatile content but can also damage temperature-sensitive hardware or redistribute contamination.

A test item with uncontrolled outgassing can contaminate the chamber, instrumentation or subsequent customer hardware.

Outgassing screening under ECSS-Q-ST-70-02 is a specialised service. It should not be described as available unless the equipment, method, competence, reporting and approval position are confirmed.

Material selection should be reviewed before design release and procurement commitment.

Record the manufacturer, grade, batch, cure system and processing route.

Prepare specimens using the same materials and processes intended for flight hardware.

Do not assume historic screening data remains acceptable indefinitely.

Assess how much material is used and what sensitive surfaces it can influence.

Use screening data as an input to contamination control, not as the complete answer.

Protect tested and baked hardware from recontamination during storage, transport and integration.

Reassess evidence when there is a change to:

• Formulation.

• Supplier.

• Manufacturing location.

• Cure process.

• Thickness.

• Cleaning method.

• Material quantity.

• Hardware location.

Material waivers and deviations should identify the technical justification, risk and approval authority.

A controlled outgassing and contamination package can include:

• Declared Materials List.

• Declared Processes List.

• Cleanliness and Contamination Control Plan.

• Contamination Requirements Specification.

• Materials Selection Report.

• Outgassing Test Specification.

• Test Procedure.

• Test Report or Certificate of Conformity.

• Material Safety Data Sheet.

• Manufacturer technical data.

• Cure and processing records.

• Bakeout Procedure.

• Contamination Analysis Report.

• Verification Control Document.

• Nonconformance Reports.

• Requests for Waiver.

Selected materials meet the applicable screening and project requirements.

Evidence is current, traceable and representative of the material as used.

Predicted and measured contamination remains within the allocated limits.

Optical, thermal, electrical and mechanical performance is not unacceptably degraded.

Approved data can be traced to the actual material, process and flight configuration.

Requirements, test evidence, analyses, deviations and approvals are formally closed.

The final hardware configuration has an acceptable contamination-risk position supported by controlled evidence.

Resonate Testing can help customers identify contamination concerns before thermal-vacuum testing and define what material, configuration and cleanliness information should be reviewed.

Independent review can challenge assumptions concerning materials, cure state, contamination-sensitive surfaces, bakeout and test preparation.

Outgassing and contamination considerations can be incorporated into the wider thermal-vacuum test strategy, including handling, chamber protection, temperature limits and post-test inspection.

Support can include review of material lists, customer specifications, test procedures, contamination requirements and reporting expectations.

Resonate can help identify where specialist ECSS-Q-ST-70-02 screening, detailed contamination analysis or an approved external laboratory is required.

Outgassing evidence can be integrated into the wider programme covering materials approval, environmental testing, verification closure and flight readiness.

Capability limitation: This page must not imply that Resonate Testing is an ESA trusted outgassing laboratory or currently provides accredited ECSS-Q-ST-70-02 material screening unless that capability and approval status have been formally confirmed.

Your Test Facilitator. Not simply a test facility.

Outgassing can result from absorbed moisture, residual solvents, plasticisers, oils, unreacted constituents and other volatile compounds leaving a material.

No. Materials can release substances at higher pressures. Vacuum can make the release and transport mechanisms more significant for space hardware.

Adhesives, coatings, polymers, lubricants, tapes, elastomers, composites, inks, printed materials and electronic assembly materials commonly require review.

TML measures the total specimen mass lost. CVCM measures the portion collected as condensable material on a controlled collector.

No. Screening is one part of material approval. Quantity, location, temperature, mission, sensitive surfaces and customer requirements still apply.

Not automatically. Bakeout can reduce removable volatile material but does not correct every material or contamination problem.

Sometimes, if the data matches the exact material and preparation and is acceptable under the project requirements. Data age, source and formulation control must be checked.

A DML is the controlled record of materials used within the product, including identity, application, quantity, approval status and supporting evidence.

A QCM measures very small deposited masses by monitoring changes in the resonant frequency of a quartz crystal.

RGA identifies gaseous species present within a vacuum system. It can help investigate leaks, background gases and outgassing behaviour.

No. Material outgassing screening assesses material mass loss and condensable products under a defined method. TVAC testing assesses hardware under vacuum and temperature conditions. The activities can be related but have different objectives.

Request deeper review when the hardware contains sensitive optics, cryogenic surfaces, new materials, large quantities of polymers, unknown coatings, unusual printed materials or customer-defined contamination limits.

• Cleanliness and Contamination Control Under ECSS.

• Material Selection for Space Hardware.

• Declared Materials Lists Explained.

• Vacuum Bakeout for Space Hardware.

• Molecular Contamination Modelling.

• Particulate Contamination Control.

• Thermal Vacuum Testing Under ECSS.

• Thermal Cycling Under Vacuum.

• Space Hardware Test Preparation.

• Vacuum Leak Testing.

• Radiation Effects on Space Materials.

• Qualification Testing Under ECSS.

• Acceptance Testing Under ECSS.

• Protoflight Testing.

• Verification Planning Under ECSS.

• Verification Evidence and Closure.

• Test Readiness Reviews.

• AIT Plans.

• Test Specifications.

• Test Procedures.

• Test Reports.

• Nonconformances and Requests for Waiver.

Material outgassing should be reviewed before contamination-sensitive hardware enters thermal-vacuum testing or final integration.

Share the material list, hardware description, known coatings and adhesives, sensitive surfaces, customer requirements, cleaning history and any existing outgassing evidence. Resonate Testing can help identify the practical next step and determine whether specialist material screening or a wider contamination analysis is required.

Contact Resonate Testing to discuss material outgassing, TVAC preparation and contamination-risk planning.