Products and manufacturing processes can be sensitive to outgassed chemical compounds, resulting in the use of adhesives, encapsulants, and sealants that have been specially formulated for low-outgassing performance. The most stringent outgassing requirements are best met by products formulated to pass an industry-standard outgassing test known as ASTM E595-07. Low-outgassing adhesives don’t typically require any sacrifice in other mechanical or physical properties. What sets low-outgassing products apart from their generic counterparts is that the low-outgassing grades have passed the ASTM test.
Developed by NASA to screen low-outgassing materials for use in space, the test determines the volatile content of material samples placed in a heated vacuum chamber. Samples to be tested are first preconditioned at 50 percent relative humidity for 24 hours and weighed. They then go into the test chamber for another 24 hours with the temperature set at 125 °C and the vacuum at a minimum of 5 x 10-5 torr. During that time in the test chamber, volatiles that outgas from the sample escape through a port in the test chamber and condense on a cooled (25 °C) collector plate. The sample and condensate on the collector plate are then weighed to determine the total mass lost (TML) by the sample and the amount of collected volatile condensable materials (CVCM) on the collector plate.
Materials pass or fail the test based on these TML and CVCM measurements. If the CVCM exceeds 0.1 percent, the material fails. The material will also fail if the TML exceeds 1 percent, though the TML may be offset by water vapor regained (WVR) by the sample in a subsequent measurement: If CVCM <0.1 percent and TML 1 percent, the material fails.
Other than space systems, the most common applications for low-outgassing products are those that involve optical or electro-optical components, which can be clouded or fogged by outgassed chemical compounds.
The compliant adhesive products available today cover a wide range of capabilities for those times when nothing else will do (see figure). When looking for low-outgassing products, some adhesives, such as two-part epoxies, can be specially formulated to achieve outgassing levels well below what ASTM E595 requires. Other types of adhesive chemistries have traditionally not been able to pass ASTM E595. The adhesives that outgas most severely are those that cure through the action of solvents or moisture. These would include a variety of pressure-sensitive and contact adhesives as well as cyanoacrylates.
The outgassing potential of the individual adhesive grades can vary substantially. This variance results from the optimization of each grade’s physical and mechanical properties through modifications to the underlying adhesive chemistry. The more flexible grades within the epoxy family tend to be more prone to outgassing, possibly because they have a lower crosslink density than their more rigid counterparts.
Many adhesive formulations cure well at room temperature, developing better than adequate mechanical and physical properties for their intended use. Yet, the addition of a heat cycle will optimize desirable physical properties such as low outgassing. In low-outgassing applications, heat can even be essential. Heat tends to improve the crosslink density, which in turn limits outgassing. A typical cure schedule would be room temperature overnight, followed by 2 to 5 hours at 60-80 °C.
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