Testing Adhesives for Resistance to Skydrol

Many Master Bond epoxy systems are formulated with superior chemical resistance. We continually test our materials by exposing them to specific chemicals over a long period of time. A common way of testing the chemical compatibility of an epoxy is immersing a sample in a chemical and measuring its change in weight over time. A significant loss or gain in weight would indicate a decreased ability of a material to stand up to chemical exposure. These tests allow us to more accurately recommend the right product based on specific application requirements.

IIn this experiment, we focused on testing our epoxies for their resistance to Skydrol. The compounds Master Bond used for testing are a variety of two component epoxies and one component UV curing systems with good overall chemical resistance. For the first round of testing, which involved exposure to Skydrol for 10 months, the products tested were UV16, UV22DC80-1, EP42HT-2AO-1 Black, UV26 and EP46HT-2AO Black. For the second round of testing, which involved exposure to Skydrol for two years, the products tested were UV25, EP42HT-2, and EP125. For the third round of testing, which involved exposure to Skydrol for more than two and a half years, the products tested were EP41S-1HT, EP41S-5, EP62-1HT and EP62-1 (100:5). For the first two tests, these products were compared against the resistance of a generic or standard non-chemically resistant epoxy. For all the tests, a few thin castings, roughly 2 inches in diameter and around 0.125 inches thick, were made for each product and cured in accordance with their specifications. Once the cured samples were created and initial weight was recorded, the castings were immersed in Skydrol. Then, we continued recording frequent weight measurements. Below you will see the results of soaking for these various time intervals. The castings were weighed periodically, and the graphs shown below demonstrate the percentage of weight change over time.

For the purpose of comparison, please note that for the 10 month and two-year exposures, a casting of a standard epoxy was also tested under these same conditions which served as a reference. As can be seen in the graphs, the standard epoxy was markedly less resistant to Skydrol than the other epoxies tested. It demonstrated a significantly greater change in weight over time and the casting ultimately dissolved in Skydrol after approximately one month.

Chemicals can etch the surface of an epoxy (resulting in weight loss) or they can cause swelling of the sample (resulting in weight gain). In general, a weight change of less than 4-5% (gain or loss) can be considered excellent, especially since these tests may be more rigorous compared to actual service conditions. It is also worth noting that in the context of a bonded joint or a potted assembly the exposure to Skydrol might not be as severe or direct as in the above test conditions.

Please note, when choosing an epoxy for an application where the resistance to Skydrol is critical, many other factors must be considered in addition to the chemical resistance. All the products tested performed well, even with prolonged exposure to Skydrol. However, depending on the time of exposure to Skydrol, each of the epoxies in the charts above offers a distinct set of performance properties. If optical clarity and fast cure is needed, and there is full access to a light source (with no shadows or blocks), UV25 or UV26 can be considered. For longer term exposures, if a high glass transition temperature is needed, EP125 is an excellent choice. EP41S-5 may be considered if electrical insulation and low outgassing are the priorities. These products all need to be processed or cured differently but the key in all cases is the addition of heat for enhancing/optimizing chemical resistance especially to Skydrol.

Disclaimer: The findings in this article are not meant to be used for specification purposes.