Technical Applications Engineer
Epoxy Technology, Inc.
Here is an interesting question that came in to Technical
Services recently:
“We recently switched
from a low temperature minimum alternate cure to the recommended data sheet cure
and the epoxy cured darker than usual. Is the higher cure temperature degrading
the epoxy?”
Our short answer was no. Here’s a short explanation why: Color variation related to cure profiles is a well-known phenomenon and is most commonly seen in epoxies with imidazole-based epoxy hardeners (Part B’s). In general, higher cure temperatures lead to a darker cured epoxy color. Certain imidazoles have a higher activation energy required to complete the reaction. Higher cure temperatures more fully activate the adhesive curing mechanism and results in a darker colored cured product. In some cases a higher temperature cure and darker color can even lead to increased percent polymerization and higher strength.
Epoxy Technology is very familiar with cured epoxy color changes
for many of our adhesives as it relates to various cure temperature. As an example, let’s look at a photo
illustrating this phenomenon with one of our custom formulated products:
Color Change at Various Cure Temperatures
EPO-TEK
108-115-LH
EPO-TEK 108-115-LH is a custom formulated, low halogen epoxy
for sealing wafer level MEMS devices. Our customer was concerned after an
EPO-TEK technical application engineer recommended a cure temperature increase
(for enhanced performance) that resulted in a color change from what they were
used to (orange) to dark brown. (Our
application engineers often recommend alternate cure schedules in order to
optimize material performance based on a specific application and that can include
a higher temperature cure.) Upon
explanation and the short curing experiment shown above, our customer accepted
this new curing profile for 108-115LH and has received better performance of
the material in their end product.
To recap, it is important to note that this color change is not
always a quality issue and, in this case, does not indicate the epoxy has degraded.
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