In-situ chemical structure analysis of aqueous vinyl polymer solution-isocyanate adhesive in post-cure process by using Fourier transform near infrared spectroscopy

API (aqueous vinyl polymer solution-isocyanate) adhesive was prepared from PVA (polyvinyl alcohol) aqueous solution, SBR (styrene-butadiene rubber) latex and pMDI (polymethylene polyphenyl polyisocyanate). A paper sample was prepared by sandwiching API adhesive between two pieces of filter paper, which could be regarded as the model for an actual bonded structure in a practical application. In a post-cure process an in-situ chemical structure analysis of the adhesive layer in the paper sample was carried out using FT-NIR (Fourier transform near infrared) spectroscopy. The absorption bands of urea and urethane, two primary products generated from the chemical reactions of isocyanates, were assigned on a 2nd derivative spectrum at 5005 cm-1 and 5065 cm-1, respectively. Band intensities of urea and urethane increased with time during the post-cure process, indicating the advance of chemical reactions in the adhesive layer. DSC was utilized to analyze the thermal properties of the API adhesive film sample, together with the cross-linking density in the API adhesive film sample which was tested using DMA. In addition, FT-IR spectroscopy was used to confirm the chemical reactions of isocyanate in the API adhesive thin film sample.