Testing Adhesives for Resistance to Phosphoric Acid

Many Master Bond epoxy systems are formulated with superior chemical resistance. In the lab, 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 by 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 adhesive based on specific application requirements.

In this experiment, we focused on testing our epoxies for their resistance to phosphoric acid. The compounds Master Bond used for testing are a variety of one and two component epoxies with good overall chemical resistance. For the first round of testing, which involved exposure to 10% phosphoric acid, the products tested were EP17HT, EP21ARHT, EP35SP, EP41S-5, EP46HT-1, EP62-1BF, EP62-1HT and EP125. For the second round of testing, which involved exposure to 20% phosphoric acid, the same products were tested. These were compared against the resistance of a generic or standard non-chemically resistant epoxy. A few thin castings of a predetermined size and shape were made for each product and cured in accordance with their specifications. Once the cured samples were created and initial weight was recorded, we submerged them in either 10% or 20% phosphoric acid at room temperature ~ 75° F. Then we continued recording frequent weight measurements. Below you will see the results for 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 over time.

For comparison, 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 not as resistant to phosphoric acid as the other epoxies tested. It demonstrated a significantly greater change in weight over time, and at a 10% concentration level, the casting ultimately broke down in phosphoric acid after approximately 20 weeks (about 4 and a half months).

In general, a swelling of less than 4-5% 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 phosphoric 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 phosphoric acid is critical, many other factors must be considered in addition to the chemical resistance. Each of the epoxies in the charts offers a distinct set of performance properties. For example, if electrical insulation and heat resistance are needed, EP125 may be a good option. If lower viscosity is a higher priority, EP21ARHT can be considered. At higher concentrations of phosphoric acid EP62-1BF and EP17HT are excellent candidates.

These products all need to be processed or cured differently and although some of them can be cured at room temperature alone, it is critical to add heat for enhancing/optimizing chemical resistance especially to phosphoric acid.

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