Outgassing (also known as offgassing) occurs when gas is released from adhesives, encapsulants, lubricants, sealants, and other materials. Essentially, volatile products used in the production of polymer materials can become trapped in the polymer matrix. Under certain conditions these volatile materials can transition into the gaseous state and escape the polymer compound.This may seem harmless, however, these escaped gases can condense on nearby equipment, potentially obscuring sensitive optics or electronics. All polymer compounds have a potential to outgass, however, some products are formulated specifically to reduce this potential even under extreme conditions.
This post is dedicated to expanding on a previous post dedicated to NASA outgassing standards and answer the following questions:
- How Does Outgassing Occur?
- When Is Outgassing An Issue?
- What Are Low-Outgassing Materials?
- How Is Outgassing Tested?
How Does Outgassing Occur?
Outgassing occurs under nearly all conditions. Materials that use volatile products to make them are likely to give off small amounts of gas even in normal conditions (e.g. room temperature and standard atmospheric pressure). However, the amount of gas released under normal conditions is probably negligible for most purposes/ applications.
Outgassing does however pose a problem under extreme environment conditions. The severity of outgassing can be increased due to specific chemical exposures or physical interactions. Conditions that increase outgassing potential include:
- High heat
- High moisture
- Low pressure/ vacuum
Especially when these conditions are sustained for long exposure periods, outgassing could become an issue for many applications.
When Is Outgassing An Issue?
As suggested above, adhesive or sealant outgassing is really an issue within two types of applications:
- The environment is so extreme that outgassing is substantially increased.
- The bonded or nearby components are so sensitive that even minimal outgassing that occurs under normal conditions is an issue.
Unfortunately, aerospace applications meet both of these criteria. Extreme vacuum conditions and massive temperature swings, such as those experienced in space, substantially increase the severity of polymer outgassing. The released volatile compounds might condense on the surface or nearby sensitive equipment, potentially affecting the performance and precision of these components. And for obvious reasons performance and precision are quite important in aerospace applications...
If a spacecraft's parts were joined by high-outgassing adhesives and sealants, problems would almost certainly result due to volatile products finding their way onto the surface of nearby display and electronic equipment.
The same goes for certain industrial, medical, and military applications where electronic and display equipment are also being used. Although these industries might not experience conditions quite as extreme as those experienced in space, the exact precision of equipment often prohibits the use of high-outgassing adhesives and sealants.
Outgassing can also be an issue for humans. Volatile compounds are normally detrimental to human health and as a result many household items pose a threat to humans via outgassing. The graphic below by CustomMade.com outlines some of the most dangerous sources of household outgassing.
With all the issues and problems mentioned above, it becomes more and more important to use low-outgassing materials.
What Are Low-Outgassing Materials?
Low-outgassing materials are products that have been specifically formulated to limit the degree of outgassing even under extreme conditions. Low-outgassing adhesives and sealants normally have the following qualities:
- No Solvents Or VOCs - Although many types of adhesives and sealants utilize solvents, these systems are absolutely unacceptable for low-outgassing applications. These components are the most likely to outgass and affect nearby equipment. Systems that don't use solvents are normally described as 100% solids (even though they are still liquid prior to polymerization).
- High Cross link Density - High cross link density results in a tightly constructed polymer matrix. A tight polymer matrix is able to more effectively prevent volatile components from escaping into the air.
- High Molecular Functionality - Monofunctional molecules are commonly used to manipulate the properties of polymer adhesives. However, because these molecules only have one reactive site they are less likely to become apart of the polymer matrix. These unreacted molecules are then available to outgassing. Instead multifunctional molecules are used to build low-outgassing materials.
- Ease Of Polymerization - At 100% polymerization outgassing is theoretically no longer an issue. For this reason resin systems that easily react are normally chosen for low-outgassing applications. Polymerization completion can be further improved via a heat curing process.
How Is Outgassing Tested?
To address the issue of outgassing in aerospace applications, NASA developed a specific set of requirements and a testing method to minimize outgassing as a potential issue. The testing method is called ASTM E595 and is conducted as follows:
- The material is exposed to 50% relative humidity at 25C for 24 hours and then weighed.
- The material is then placed in a heated vacuum chamber and exposed to 125C (257F) and at minimum 5x10-5 torr for another 24 hours. Any volatiles that may outgas during this process are able to escape through a port hole in the vacuum chamber and condensate on a cooled plate.
- The material is then weighed to determined Total Mass Lost (TML) and the condensate is weighed to determine Collected Volatile Condensable Materials (CVCM).
- OPTIONAL: The material is then again exposed to 50% relative humidity for 24 hours and weighed again to determine Water Vapor Regained (WVR).
With these three values (TML, CVCM and WVR) we can determine whether the material has passed or failed (basically whether or not the material is fit for use with spacecraft equipment).
- If CVCM <0.1% and TML < 1%, the material passes.
- If CVCM <0.1% and TML > 1 %, the material can pass if the TML-WV <1%.
- If CVCM > 0.1% or TML-WVR > 1%, the material fails.
If the material meets NASA outgassing requirements, it could qualify for use in spacecraft equipment, electronic and display equipment, and other commercial applications (including medical, LED, and solar power applications). Originally, the ASTM E595 method is designed for testing of spacecraft-related materials. But now, it can also be employed to test materials that will be used in industrial products and applications. NASA outgassing standards can be adopted for testing of materials that will be used together with highly sensitive and precision equipment. It has now become a widely accepted standard.
Aside from adhesives and sealants, other materials can also be tested using the ASTM E595 method. These are coatings, foams, tapes, insulations, elastomers, fabrics, tie cords, lubricants, and shrink tubings. Further testing after ASTM E595 can also be done to ensure that the materials meet the requirements or if their performance is already satisfactory for a specific application.