Outgassing is defined as “the release of a gas that was dissolved, trapped, frozen, absorbed, or adsorbed in some material” (Strong, John). Most rubber compounds contain small amounts of oil and other ingredients that will become volatile under vacuum conditions.
Evidence of outgassing is shown as a thin film deposited on surrounding surfaces. Other compounds exhibit some degree of weight loss in the form of water vapor, which may act as a contaminant in certain applications. While this process is inevitable, it is accelerated at elevated temperatures and in high vacuum situations. Optical and electrical contact applications are a special concern in this situation, as they incorporate sensitive surfaces that must remain uncontaminated.
“Post curing” elastomeric compounds such as Viton®, fluorosilicone, and silicone prior to service removes many of the unwanted volatiles and improves physical properties by increasing the degree of crosslinking.
Post curing is a process that removes volatiles by diffusion and evaporation, carried out at a temperature higher than the service temperature for the part. Once the volatiles are “vaporized,” they are no longer available to outgas. Insufficient or poor post cure can result in a reduction in strength, elongation, and compression set properties accompanied by chemical decomposition, as well as “smoke,” bubbling, delamination, and unsightly sticky surface deposits.
Compounds such as nitrile and natural rubber don’t usually benefit as much from this process as the previous compounds.
Apple Rubber Products engineers are currently working on ultra-low outgassing materials made of fluorocarbon and fluorosilicone, which will be sent to a test lab for analysis. With the data we receive, we will be able to compare the standard material to the ultra-low outgassing version, noting similarities and differences in both outgassing properties as well as physical properties. Stay tuned for the findings!
(Selected References: Outgassing of Silicone Elastomers J. Rothka, R. Studd, K. Tate and D. Timpe ARLON – Silicone Tech Div)