Non-Destructive Depth Profile Analysis of Conversion in Thick-Layer UV Curable Resin Using Laser Micro-Raman Spectrometer

Abstract

A new method to analyze conversion vs. depth (depth profile of conversion) in a thick-layer of UV curable resin non-destructively, using a laser micro-Raman spectrometer, has been investigated. A few methods exist to analyze the depth profiles of the curing state in a thick-layer of UV curable resins, but these methods are destructive. On one hand, laser micro-Raman spectrometry can be applied to non-destructive analysis inside a thick-layered film, but the polymerization of a sample by the irradiation of strong laser light was of concern, when it was applied to a measurement of the conversion of UV curable resin. Furthermore, if a long-wavelength laser is used in order to avoid polymerization of sample, Raman scattering at a deep part of the sample might be too weak to analyze the conversion. In this study, by confirming the influences of these issues, laser micro-Raman spectrometry was investigated as a depth-profiling method. We selected 780 nm as the long wavelength of the laser, and it was determined that the irradiation of a urethane methacrylic UV curable resin for 216 s did not promote polymerization of the sample. Then, a 1.86 mm thick cured sample of the UV curable resin was tested using the laser micro-Raman spectrometer through laser irradiation for 60 s. The Raman spectra at several depths in the cured sample were recorded. Because the spectrum of the deepest part was as clear as the others, it was determined that all of the spectra throughout the depth were acceptable to measurements of the conversions. From areas of the peaks on the spectra at 1635 cm^-1 of C=C stretching vibration in the methacrylic group, the value of the conversion at each depth in the cured sample was calculated, and the depth profile was obtained. This study showed that laser micro-Raman spectrometry can be applied to the non-destructive analysis of the depth profile of conversion in a thick-layer of UV curable resins.