By Mavyn Holman
Epoxy Technology, Inc.
Fall has arrived in New England and winter will be here before we know it. Unfortunately, that means cold temperatures, snow and more importantly for our adhesives - low humidity. Did you know that maintaining proper humidity levels in your manufacturing environment can play a critical role in ensuring proper curing and performance of your adhesive? This is particularly the case for room temperature curing electrically conductive adhesives (RT-ECAs).
In order for any ECA to achieve optimal electrical conductivity, it must be cured properly for maximum cross-link density. For this to happen, the proper mix ratio and curing temperatures need to be followed according to the product’s datasheet recommendation(s). With standard ECAs cured at elevated temperatures, the cure and cross-linking develop quickly. This helps to maintain homogeneity (and proper ratio of components) during the cure, and allows for an end product that is fully cross-linked. For a RT-ECA, the cure happens much more slowly and there is more time for the cure to be affected. With these materials, a well maintained manufacturing environment is essential. It is recommended that the manufacturing floor maintain a temperature of 23⁰C-27⁰C with a relative humidity (RH) of 40-60%. Lower temperatures will slow down the reaction rate and potentially lead to a less complete cure. The importance of humidity may not be as obvious but, it is equally as important.
As it turns out, the slow curing rate of RT-ECAs is what makes them susceptible to issues when cured at low humidity levels. While the RT-ECA is curing, the lowest surface tension components of the adhesive migrate to the higher surface tension interfaces (air). When this happens in excess, the internal mix ratio is altered, preventing the adhesive from cross-linking properly. It may eventually harden, but it will not provide good electrical conductivity.
How does higher humidity levels help prevent this from occurring? Excessive migration of low surface tension components can be prevented by forming a thin amine carbonate barrier layer. This effectively lowers the surface tension and stops the migration. Under the right conditions, this barrier layer forms naturally in the presence of three components: amine, carbon dioxide and water. Amine is present in the adhesive itself and low levels of carbon dioxide are found in the atmosphere. Sufficient humidity levels are needed to supply the final component of the equation - water.
The chart below shows the effect of humidity level in the environment on the volume resistivity (VR) for one of our RT-ECAs, EPO-TEK® EJ2189. The blue line represents a 3-4 day room temperature cure only. The red line shows the results of a subsequent 150⁰C/1 hour post cure that is imparted to the same samples, originally cured at room temperature.
In summary, low ambient humidity can severely prevent proper VR development during the room temperature curing process. Humidity levels >40% RH are required to bring the VR into the proper specification range for the product. Unfortunately, a thermal post cure will not improve VR after a poor initial room temperature cure. In fact, if the initial room temperature cure is carried out at very low ambient humidity level, the separation of components may be severe enough that a thick layer of non-conductive components will separate to the surface of the adhesive. Thermally post curing this layer will further lock in the non-conductive layer and actually lead to even poorer conductivity.
Remember: To obtain the best performance when curing RT-ECAs, a user should maintain a temperature range of 23˚C to 27˚C and humidity levels between 40% to 60% RH during the full cure cycle.