Friday, January 17, 2014

Epoxy Color Change as a Result of Cure Profile

By Brian Bruce
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.