We used an imaging infrared microscope to study the curing process of a mixed epoxy. We looked for domains of the epoxy components, resin and hardener, as an example of a possible adhesive failure mode.
The two components of the 5-minute epoxy were mixed and smeared on a front surface mirror for reflection-absorption analysis. Infrared images of the smear were collected in reflectance on an imaging infrared microscope. Twenty-three Images of the smear were collected over a field of view of 500 x 550 micrometers at a spatial resolution of 25 microns per pixel over 8 minutes. Each individual map was recorded in 2.2 seconds.
Correlation images were created where the data set was correlated with reference spectra of the pure resin, pure hardener, and the well mixed epoxy.
Infrared correlation images of the uncured mix are presented here, correlated with reference spectra of the pure components and a reference spectrum of the mixed resin and hardener. In the correlation images, red indicates areas of high correlation with the reference spectrum, and blue areas of low correlation.
We can clearly see domains of high resin concentration, high hardener concentration, and well mixed components. Surprisingly, these images show that the mixture is fairly non-homogenous on a micro scale, even though the two components were mixed prior to application.
A similar set of correlation images for the last image of the run are shown below, for the cured mixture. The cured mixture still shows domains of high correlation with the individual components in the mixture and the uncured mixture.
A comparison of the initial to final correlation images shows that even though changes in the images can be observed from top to bottom, the cured mixture still appears as a non-homogenous mixture.
One problem with the previous set of images is that the color scale of each image was scaled independently, which may make it difficult to compare images across the whole data set. Below we set a constant color scale across a series of images correlated with a reference spectrum of the cured mixture, from the first image in the data set to the last.
You can see areas of red growing into the images as the epoxy cures, which areas are highly correlated with the reference spectrum of the cured mixture. Even though we can observe the epoxy curing from left to right, we can clearly see domains in the final image which are still poorly cured, attributed to the non-homogeneity in the initial mix. We estimate that only ~50% of the area in the final image correlates to the cured epoxy.
More details and additional examples of adhesive analysis with FTIR spectroscopy are shown in our webinar Analysis of Automotive Adhesives Using FTIR Spectroscopy. You can download the recorded 30-minute webinar, at no charge, which is available on demand until March, 2017.
Access the webinar now: Analysis of Automotive Adhesives Using FTIR Spectroscopy