By Mavyn Holman
R&D Manager
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.
For a
listing of some new RT-ECAs, download our latest piece “Room Temperature Curing ECAs” on epotek.com.
