Chemical testing was carried out on a number of compounds to assess their compatibility for close contact with LEDs in Nemalux fixtures. This test is accelerated stress test, designed in a way that it will produce noticeable degradation on relatively short time-scales ( This test is unrelated to chemical compatibility of compounds with encapsulation materials.
The chemical compound of interest is applied directly to the LED surface where possible, and a low volume glass vial is sealed over top of the LED and test chemical using a compatible sealant such as Arctic Silver epoxy or 3M TC-2810 epoxy. In cases where the chemical compound of interest is an aerosol, the compound is sprayed into a glass vial and the vial immediately placed into an already-prepared adhesive ring around the LED in order to trap as much of the aerosol components as possible. The sealant is allowed to cure fully before powering the LEDs at a current setpoint representative of their use in a product. The test fixture is powered on continuously, with notes and photos taken at intervals to track degradation of the light output.
The compounds in Table 1 were tested, resulting in variable degradation on the light output from the LEDs. Two degradation mechanisms are noted; a decrease in light output and/or a change in colour of the emitted light from broad spectrum white to a much bluer tone.
Of the compounds tested, the chemicals causing the worst degradation of the LEDs were found to be aerosol-sprayed coatings;. This was not found for the same compound when applied through air gun spraying the liquid compound from a 4L canister, indicating the propellant and solvents unique to the aerosol can are the culprit. If aerosol cans are used to coat LEDs, these compounds should evaporate over time, as long as the fixture is not closed up prior to complete drying of the coating. It is recommended that the aerosol applied coatings should dry for at least 24 hours at a temperature of 15°C or above.
Over a medium timer frame, degradation was found when individual components of 2-part epoxy compounds were applied. This is not representative of an installed product, but as cure will always be partially incomplete, epoxy compounds are not recommended for direct application to the LEDs. The medium time scale indicates it could be an issue in some cases.
Over a very long time period the thermal paste and primer were also seen to cause some degradation. The very long time frames indicate that if these are not applied directly to the LEDs, there is a low chance of degradation in a luminaire. Alternate materials should be used if possible, however.