Testing Adhesives for Resistance to Hydrofluoric Acid

Specialty Master Bond epoxy systems can be formulated to provide superior acid 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.

In this experiment, we focused on testing our epoxies for their resistance to hydrofluoric acid. The compounds Master Bond used for testing are a variety of two component epoxies, and one component epoxies with good overall chemical resistance. For the first round of testing, which involved exposure to 10% hydrofluoric acid, the products tested were EP62-1HT, EP62-1BF, EP125, and EP17HT. For the second round of testing, which involved exposure to 20% hydrofluoric acid, the same products were tested. For both tests, these products were compared against the resistance of a generic or standard non-chemically resistant epoxy. A few thin castings 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 either 10% or 20% hydrofluoric acid. Then, we continued recording frequent weight measurements. Below you will see the results of soaking for more than one year in each concentration. The castings were weighed periodically, and the graphs shown below demonstrate the percentage of weight change after 30 and 58 weeks.

For the purpose of comparison, please note that for each concentration, 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 hydrofluoric acid than the other epoxies tested. It demonstrated a significantly greater change in weight over time at both 10% and 20%. At 20% concentration levels, after the 30 week measurement, the casting ultimately dissolved in hydrofluoric acid and therefore we have no value to reference at 58 weeks.

Acids can etch the surface of an epoxy (resulting in loss of weight), or they can cause swelling of the sample (resulting in weight gain). In general, a weight change of around 5% or less (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 hydrofluoric acid 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 hydrofluoric acid is critical, many other factors must be considered in addition to th.jpge chemical resistance. Depending on the concentration level of hydrofluoric acid, each of the epoxies in the charts above offers a distinct set of performance properties. For example, at both 10% and 20% concentrations, if high temperature resistance and structural strength are needed, EP17HT may be a good option. If a thixotropic paste viscosity and excellent electrical insulation are needed, EP125 can be considered. These products all need to be processed or cured differently but the key in all cases is the addition of heat for extended time periods, which enhances/optimizes the overall chemical resistance.

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