What is Outgassing?

When Materials Release Their Secrets

Engineers face countless invisible challenges in product development. Outgassing ranks among the most critical yet frequently overlooked phenomena that can compromise product integrity. This silent process occurs when materials release trapped gases under specific environmental conditions, potentially causing contamination, reduced performance, or even complete system failure.

For aerospace, defense, and medical device engineers, understanding outgassing isn't merely academic – it's essential for mission success and patient safety. This comprehensive guide explores what outgassing is, why it matters, and how to effectively manage it in your engineering designs.

Materials don't outgas equally. Some release significant amounts of volatile compounds, while others remain relatively stable. Understanding this fundamental difference helps engineers select appropriate materials for specific applications.

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Why Outgassing Matters in Engineering Applications

The consequences of outgassing vary dramatically depending on the application environment and requirements. In sensitive engineering contexts, outgassing can create serious problems that compromise system performance.

Critical Consequences in Key Industries

Aerospace applications face particularly severe outgassing challenges:

• Optical surface contamination causing degraded instrument performance
• Electrical arcing in high-voltage equipment
• Thermal control surface degradation leading to temperature regulation failures
• Mechanical interference in precision mechanisms
• Signal attenuation in sensitive communication systems

Medical device applications encounter equally concerning issues:

• Chemical contamination of sterile environments
• Degradation of device effectiveness over time
• Potential toxicity from released compounds
• Compromised sensor accuracy
• Reduced product lifespan

These real-world impacts demonstrate why outgassing deserves serious consideration during engineering design. One microscopic layer of contamination can mean the difference between mission success and failure.

The Science Behind Outgassing

Outgassing follows predictable physical and chemical principles. Understanding these fundamentals helps engineers anticipate and mitigate potential issues before they manifest.

Key Factors Influencing Outgassing

Three primary factors determine outgassing behavior:

The relationship between these factors follows an Arrhenius-type behavior, where outgassing rates approximately double with each 10°C (18°F) temperature increase. This temperature sensitivity underscores the importance of thermal management in sensitive applications.

Common Outgassing Mechanisms

Several distinct mechanisms contribute to material outgassing:

1. Desorption: Surface-bound gas molecules release from material surfaces
2. Diffusion: Gas molecules migrate through the material's internal structure
3. Decomposition: Chemical breakdown creates new gaseous compounds
4. Permeation: External gases pass through material boundaries

Each mechanism requires different mitigation approaches. For example, desorption can often be addressed through pre-treatment processes, while decomposition might necessitate alternative material selection.

How Engineers Measure Outgassing

Quantifying outgassing characteristics requires standardized testing methods that provide comparable results across different materials and applications.

ASTM E595: The Industry Standard Test

The most widely recognized test for outgassing is ASTM E595, which measures two critical parameters:

1. Total Mass Loss (TML): Percentage of original mass lost during testing
2. Collected Volatile Condensable Materials (CVCM): Percentage of original mass that recondenses

The test exposes material samples to 125°C (257°F) at a pressure of 10^-6 Torr for 24 hours, then measures both values. For space applications, NASA typically requires TML < 1.0% and CVCM < 0.1%.

Additional parameters sometimes measured include:

• Water Vapor Regained (WVR): Moisture reabsorbed after testing
• Recovered Mass Loss (RML): TML minus WVR, representing non-recoverable loss

These test results provide engineers with quantitative data for material selection decisions in outgassing-sensitive applications.

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Materials and Their Outgassing Behavior

Material selection represents the most powerful tool engineers have to control outgassing. Different material classes exhibit vastly different outgassing characteristics.

Comparative Outgassing of Common Engineering Materials

Materials demonstrate predictable outgassing behavior based on their chemical composition and structure:

This variation explains why material selection demands careful consideration in outgassing-sensitive applications. The right material choice can eliminate potential problems before they begin.

Low-Outgassing Material Options

For applications where outgassing must be minimized, several specialized material options exist:

• Low-outgassing silicones: Modified formulations that meet NASA outgassing standards
• PTFE and other fluoropolymers: Inherently stable polymer structures
• Specialty epoxies: Formulated specifically for space and medical applications
• Vacuum-compatible lubricants: Designed for stability in reduced pressure environments
• Outgassing-resistant adhesives: Specially formulated bonding agents

These materials often carry premium pricing but provide essential performance characteristics for critical applications. The investment typically pays dividends through improved reliability and longevity.

Designing to Minimize Outgassing Effects

Material selection represents only one part of the outgassing control strategy. Design choices and processing methods also play critical roles in managing this phenomenon.

Effective Design Strategies

Smart engineering design can dramatically reduce outgassing impacts:

1. Material minimization: Reduce the total volume of outgassing-prone materials
2. Thermal isolation: Separate temperature-sensitive components from potential contamination sources
3. Strategic venting: Direct outgassing products away from sensitive surfaces
4. Surface treatments: Apply barrier coatings to reduce permeation and diffusion
5. Contamination barriers: Implement physical shields between outgassing sources and sensitive areas

Each strategy addresses different aspects of the outgassing challenge. The most effective approach typically combines multiple techniques tailored to the specific application requirements.

Manufacturing Processes That Reduce Outgassing

How materials are processed significantly impacts their outgassing behavior:

• Pre-baking: Controlled heating under vacuum to remove volatile compounds before assembly
• Extended curing: Complete polymerization of thermosets and adhesives
• Clean room processing: Minimizing contamination during manufacturing
• Specialty cleaning: Removing surface contaminants that could later outgas
• Material conditioning: Stabilizing materials before final assembly

These processes add steps to manufacturing workflows but deliver substantial benefits in outgassing-sensitive applications. The investment in proper processing pays dividends in improved product performance and reliability.

Testing and Verification Approaches

Confirming that outgassing has been effectively controlled requires appropriate testing strategies throughout the development process.

Laboratory Testing Methods

Several testing approaches help engineers verify outgassing performance:

1. Thermal vacuum testing: Exposing assembled products to application-representative environments
2. Quartz crystal microbalance (QCM): Measuring molecular contamination rates in real-time
3. Residual gas analysis (RGA): Identifying specific compounds released during outgassing
4. Optical witness samples: Monitoring contamination on representative surfaces
5. Long-term stability testing: Evaluating performance over extended periods

These methods provide quantitative data on outgassing behavior, allowing engineers to validate material selections and design choices before deployment.

When to Test for Outgassing

Outgassing testing should occur at strategic points in the development process:

• Material selection phase: Screening candidate materials
• Component qualification: Verifying individual part performance
• Sub-assembly testing: Evaluating interactions between components
• Final product verification: Confirming overall system performance
• Lot acceptance testing: Ensuring manufacturing consistency

This systematic approach identifies potential issues early, when they can be addressed cost-effectively. Waiting until final assembly to discover outgassing problems can lead to expensive redesigns and schedule delays.

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Real-World Applications and Solutions

Understanding outgassing principles becomes most valuable when applied to practical engineering challenges. Different applications require tailored approaches to manage outgassing effectively.

Satellite and Space Systems

Space applications face the most demanding outgassing requirements due to the vacuum environment and extreme temperature cycles:

• Solar array performance depends on maintaining clean optical surfaces
• Star trackers require contamination-free optics for accurate navigation
• Precision mechanisms must operate reliably for years without maintenance
• Thermal control surfaces must maintain stable properties throughout mission life

These challenges drive the use of specialized materials and extensive pre-flight testing to ensure reliable performance in the unforgiving space environment.

Medical Implantable Devices

Implantable medical devices present unique outgassing considerations related to biocompatibility and longevity:

• Patient safety depends on using non-toxic, stable materials
• Device longevity requires minimal material degradation over time
• Sensor accuracy demands stable material performance
• Sterilization processes must not induce unexpected outgassing behavior

These requirements necessitate rigorous material qualification and specialized processing techniques to ensure patient safety and device effectiveness.

Frequently Asked Questions About Outgassing

General Outgassing Questions

What is outgassing and why should engineers care about it?

Outgassing is the process where materials release trapped gases when exposed to certain environmental conditions like reduced pressure or elevated temperatures. Engineers should care because these released gases can condense on critical surfaces, causing contamination that may interfere with optical, electrical, or mechanical functions. In aerospace, defense, and medical applications, outgassing can lead to system failures, compromised performance, and even safety issues.

What types of gases are typically released during outgassing?

The most common gases released during outgassing include:

• Water vapor (most common)
• Atmospheric gases (oxygen, nitrogen, carbon dioxide)
• Residual solvents from manufacturing processes
• Plasticizers from polymers
• Unreacted monomers or curing agents
• Organic compounds from lubricants
• Degradation products from material breakdown

The specific gases depend on the material composition, processing history, and environmental conditions.

How long does outgassing continue after a product is manufactured?

Outgassing rates typically follow an exponential decay curve. Initial outgassing can be quite rapid, with rates decreasing over time as volatile compounds are depleted. Some materials may outgas significantly for days or weeks, while others might continue at lower rates for years. The timeline depends on:

• Material type and composition
• Processing and pre-treatment methods
• Storage conditions
• Environmental factors (temperature, pressure)

For critical applications, material conditioning or pre-baking processes can accelerate initial outgassing before the product enters service.

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Testing and Measurement

What is ASTM E595 and why is it important for outgassing?

ASTM E595 is the industry standard test method for evaluating outgassing properties of materials. It measures two critical parameters:

1. Total Mass Loss (TML): The percentage of a material's mass lost during testing
2. Collected Volatile Condensable Materials (CVCM): The percentage of original mass that recondenses on a collector plate

The test exposes samples to 125°C (257°F) at a pressure of 10^-6 Torr for 24 hours. This standardized approach allows engineers to compare materials consistently and select those appropriate for their application. NASA typically requires TML < 1.0% and CVCM < 0.1% for space applications.

Beyond ASTM E595, what other tests can measure outgassing?

Several other testing methods provide valuable outgassing data:

• Thermogravimetric Analysis (TGA): Measures mass changes as temperature increases
• Residual Gas Analysis (RGA): Identifies specific compounds released during outgassing
• Quartz Crystal Microbalance (QCM): Provides real-time measurement of molecular deposition
• Optical Witness Samples: Monitors contamination effects on representative surfaces
• Custom Thermal Vacuum Testing: Simulates application-specific conditions
• Long-Duration Testing: Evaluates outgassing effects over extended timeframes

The appropriate test depends on your specific application requirements and performance concerns.

How do outgassing requirements differ between industries?

Outgassing requirements vary significantly across industries:

Each industry has developed specialized testing protocols tailored to their unique application environments.

Material Selection

Which materials have the lowest outgassing properties?

Materials with inherently low outgassing properties include:

• Metals: Stainless steel, aluminum, titanium, gold
• Ceramics: Alumina, beryllia, silicon nitride
• Glasses: Fused silica, optical glasses, glass-ceramics
• Specific Polymers: PTFE (Teflon), polyimides (Kapton), certain epoxies
• Specialty Formulations: Low-outgassing silicones, space-grade adhesives

For polymers specifically, those with high molecular weight, high crosslinking density, and minimal additives typically demonstrate lower outgassing characteristics.

What makes silicones particularly challenging for outgassing control?

Silicones present unique outgassing challenges because:

• They contain inherently low molecular weight siloxanes that can migrate and volatilize easily
• Many formulations include processing aids and unreacted components
• Their porous molecular structure allows gases to permeate and diffuse readily
• Typical cure processes may not fully complete all cross-linking reactions
• They absorb and release atmospheric moisture efficiently

Despite these challenges, specially formulated low-outgassing silicones are available for critical applications. These materials undergo additional processing and formulation optimization to minimize volatile content.

How do adhesives compare to mechanical fasteners regarding outgassing?

This comparison highlights important differences:

For outgassing-sensitive applications, engineers often use qualified low-outgassing adhesives or mechanical fasteners with minimal surface treatments.

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Design and Manufacturing

What processing techniques can reduce outgassing in manufactured parts?

Several manufacturing approaches effectively reduce outgassing:

1. Pre-baking/Vacuum Baking: Heating materials under vacuum conditions to drive off volatiles before assembly
2. Extended Cure Cycles: Ensuring complete polymerization reactions in adhesives and composites
3. Post-Cure Treatments: Additional thermal processing to stabilize material properties
4. Proper Solvent Selection: Using solvents that completely evaporate during processing
5. Clean Room Processing: Minimizing contamination introduction during manufacturing
6. Specialized Cleaning: Removing surface contaminants that could later outgas
7. Material Conditioning: Exposing materials to application environments before final assembly

The optimal approach depends on specific material properties and application requirements.

How does temperature affect outgassing rates?

Temperature dramatically influences outgassing rates:

• Outgassing typically follows an Arrhenius relationship, approximately doubling with each 10°C (18°F) temperature increase
• Higher temperatures increase molecular mobility, allowing faster diffusion
• Thermal expansion creates more free volume within materials, facilitating gas movement
• Some materials may undergo chemical changes at elevated temperatures, creating new volatile compounds
• Temperature cycling can create "pumping" effects, accelerating outgassing

This temperature sensitivity makes thermal management crucial in outgassing-sensitive applications. Understanding the maximum expected operating temperature is essential for material selection and design decisions.

What design strategies help mitigate outgassing effects?

Effective design approaches to manage outgassing include:

• Material Minimization: Reducing the overall volume of outgassing-prone materials
• Strategic Placement: Locating outgassing sources away from sensitive components
• Thermal Isolation: Preventing heat transfer to potential outgassing sources
• Controlled Venting: Directing outgassing products away from critical surfaces
• Barrier Layers: Incorporating thin metal or impermeable coatings
• Getter Materials: Including materials that actively absorb outgassed compounds
• Surface Treatments: Sealing porous surfaces to reduce gas transmission
• Redundant Sealing: Implementing multiple barrier approaches for critical areas

The optimal strategy combines multiple approaches tailored to specific application requirements.

Application-Specific Questions

How does outgassing impact optical systems in space applications?

Outgassing creates several specific challenges for space-based optical systems:

• Condensation on optical surfaces reduces light transmission and creates scattered light
• Even nanometer-thick contamination layers can degrade optical performance
• UV radiation can polymerize condensed compounds, creating permanent deposits
• Thermal cycling causes repeated condensation/vaporization cycles, potentially redistributing contaminants
• Precision optical surfaces may experience interference effects from even minimal contamination

These issues have led to specialized design approaches for space optics, including selective heating of optical surfaces, molecular absorbers near optical paths, and extensive ground testing in representative environments.

What outgassing considerations are unique to medical implantable devices?

Medical implantable devices face distinctive outgassing challenges:

• Long-term biocompatibility concerns for outgassed compounds
• Potential toxicity or immune response to volatile chemicals
• Consistent performance throughout the device's service life
• Effects of body temperature (37°C/98.6°F) on outgassing rates
• Interaction with bodily fluids and tissue environments
• Sterilization process impacts on material stability
• Regulatory requirements for material characterization

These factors drive extensive biocompatibility testing, specialized material selections, and robust manufacturing processes for implantable medical devices.

How does vacuum exposure accelerate outgassing compared to atmospheric conditions?

Vacuum conditions dramatically accelerate outgassing through several mechanisms:

1. Pressure Differential: The absence of atmospheric pressure allows volatile compounds to escape more readily
2. Boiling Point Reduction: Many compounds that remain liquid at atmospheric pressure will volatilize in vacuum
3. No Diffusion Barrier: Without surrounding atmospheric molecules, outgassed compounds move away quickly
4. Surface Desorption: Surface-bound molecules release more easily without atmospheric pressure
5. Continuous Removal: Vacuum systems actively remove released gases, maintaining the pressure differential

These factors explain why materials that perform adequately in atmospheric conditions may exhibit significant outgassing issues when exposed to vacuum environments. This is particularly relevant for space, semiconductor manufacturing, and certain analytical instrument applications.

Troubleshooting and Solutions

How can engineers identify if outgassing is causing a performance problem?

Several diagnostic approaches can help identify outgassing as the root cause:

1. Visual Inspection: Look for fogging, films, or deposits on surfaces
2. Performance Trends: Note if issues worsen with temperature increases or over time
3. Environmental Testing: Reproduce the issue in controlled thermal-vacuum conditions
4. Witness Samples: Place clean surfaces near suspected sources to capture condensate
5. Chemical Analysis: Identify specific compounds on affected surfaces
6. Material Review: Evaluate materials in the system against outgassing data
7. Process Evaluation: Examine if manufacturing changes correlate with performance issues

Often, multiple lines of evidence are necessary to confirm outgassing as the specific cause of a problem.

What remediation options exist if outgassing is discovered in an existing design?

When outgassing issues are identified in existing designs, several remediation approaches may help:

• Bake-out Procedures: Controlled heating to accelerate initial outgassing before regular use
• Material Substitution: Replacing problematic materials where design allows
• Additional Shielding: Implementing barriers between outgassing sources and sensitive areas
• Surface Treatments: Applying conformal coatings or sealants to reduce emissions
• Operating Procedures: Modifying usage parameters (temperature limits, duty cycles)
• Environmental Controls: Adjusting storage or operational environments
• Scheduled Maintenance: Implementing cleaning procedures to remove accumulated contaminants

The feasibility of these approaches depends on the specific design constraints, accessibility, and performance requirements.

What resources are available to help engineers select low-outgassing materials?

Engineers can access several valuable resources for low-outgassing material selection:

• NASA Outgassing Database: Comprehensive testing results for thousands of materials
• ESA (European Space Agency) Materials Database: European standards and testing results
• Material Manufacturer Data: Specialized low-outgassing product lines with test reports
• Industry Standards Organizations: ASTM, ISO, and MIL-SPEC material guidelines
• Testing Laboratories: Custom testing services for specific applications
• Manufacturing Partners: Expertise from companies specializing in low-outgassing applications
• Technical Literature: Peer-reviewed papers and case studies on material performance

Working with experienced manufacturing partners like Modus Advanced can provide additional guidance tailored to specific application requirements, combining theoretical knowledge with practical manufacturing expertise.

Working With Modus Advanced on Outgassing Challenges

At Modus Advanced, we understand the critical nature of outgassing in aerospace, defense, and medical device applications. Our engineering team brings extensive experience in material selection, design optimization, and manufacturing processes that minimize outgassing impacts.

Our vertically integrated capabilities allow us to control the entire production process, ensuring consistent quality and performance in outgassing-sensitive applications. From material selection through final assembly, we maintain rigorous quality standards that deliver reliable products every time.

With AS9100 and ISO 9001 certifications, we meet the demanding requirements of aerospace, defense, and medical device industries. Our team of engineers – making up more than 10% of our staff – works directly with customers to solve complex technical challenges, including outgassing control.

Mastering the Invisible Challenge

Outgassing remains one of engineering's less visible but most impactful challenges. The gases released from materials can compromise performance, reduce reliability, and even cause complete system failure in critical applications.

By understanding the fundamental science of outgassing, selecting appropriate materials, implementing effective design strategies, and employing proper manufacturing processes, engineers can successfully manage this phenomenon. The result: products that perform reliably in even the most demanding environments.

For engineers working on aerospace, defense, or medical device applications, outgassing deserves careful consideration throughout the development process. The effort invested in controlling this phenomenon pays dividends through improved product performance, extended lifespans, and enhanced reliability.

Have questions about managing outgassing in your specific application? Contact our engineering team today to discuss your challenges and discover how our expertise can help bring your innovations to market faster and more reliably.