Can the degree of cure of a polymer-based ink applied to a Mylar film be readily determined by Fourier Transform Infrared Spectroscopy (FT-IR)? (The ink is screened onto the Mylar film and then exposed to UV light to cure the ink.) We tested this because the determination of percent cure is an important quality control (QC) tool that can also be used to optimize the product manufacturing process.
In an experiment, spectra were collected using an FT-IR spectrometer and an Attenuated Total Reflection (ATR) sampling technique accessory with a zinc selenide crystal. The ATR sampling technique was chosen for this analysis to enhance the spectral response of the inked surface and to minimize the response of the bulk Mylar film base. The inked side of the Mylar film sample was simply pressed onto the surface of the ZnSe crystal of the ATR accessory. No sample preparation was required. Spectra were collected at 4 cm-1 with 32 sample scans (40 second sample collection time).
For this particular ink, the peak of interest was at 810 cm-1 and was related to free acrylate monomer. As the ink cured, there was less of this free monomer, so the intensity of this peak decreased. In order to quantify the degree of polymerization, an internal standard peak was needed. For this ink, the peak at 830 cm-1 was unrelated to the cure chemistry and remained unchanged. It was a simple exercise to monitor the ratio of these two peak heights in order to determine the amount of polymerization that occurred during the cure process.
We looked at six spectra from the polymerization process with percent of cure ranging from 0 to 87%. For these six spectra autoscaled on the 830 cm-1 absorbance, the band at 810 cm-1 varied in intensity. Where the peak heights of the 830 cm-1 and 810 cm-1 bands were nearly identical, the ink was uncured. Where the peak height of the 810 cm-1 band was lowest with respect to the 830 cm-1 absorbance the ink was 87% cured.
Plot figures showed a linear relationship between percent cure and the ratioed peak area. The physical properties of the cured ink were then related to the percent cure to determine the optimal manufacturing process for the UV cured ink based upon the QC determination of the spectral band ratios.
We concluded that the use of FT-IR with ATR sampling provides a fast and easy determination of the quality and state of UV curable polymerization in inks.
Read the Monitoring the UV Cure Process of a Polymer Based Ink by FT-IR application note to see the spectrum of the sample prior to UV irradiation, the six spectra plot lines, a simple macro that computes the ratioed peak height of 810 cm-1/830 cm-1 for each sample spectrum using a base-line correction point at 895 cm-1 and the linear relationship plot.
To get more info and resources about Fourier Transform Infrared Spectroscopy, visit our FTIR Learning Center.