In-situ Analysis of Chemical Structure ofAPI Adhesive Using FT-NIR Spectroscopy

Abstract

adhesive evaporates gradually, API adhesive turns from liquid-state into solid-state, and finally adhesive layer forms. This period usually takes one day and gen-erally is called cure of the adhesive. Actually, cross-linking reactions of NCO could continue for many days even though the adhesive is in solid-state, and this period is called post-cure. In the post-cure period, adhesive layer absorbs moisture from ambient air then provides water for the urea generating reaction, in this term urethane generating reaction also continues. The post-cure process is very important for API adhe-sive because most of the chemical linkages, especially urethane, are establishes and the adhesive layer's me-chanical properties improve in this period. But until now there still no definitive conclusion reveals how many days the post-cure reaction could last 5) . Fourier transform infrared (FT-IR) spectrometer is normally used to analyze the chemical structure. Bas-ing on FT-IR spectra, the generation of urea and ure-thane linkages were confirmed by Patel and Gao 7, 8) . However, in FT-IR measuring process, it is difficult to repeat analysis of the same sample. On the other hand, Fourier transform near infrared (FT-NIR) spec-trometer has the advantages such as simple pretreat-ment, damage treatment less and in-situ analysis com-pared to FT-IR, which detects the overtone and combi-nation tone of bond's vibration 9, 10) . Therefore, FT-NIR was used to analyze the in-situ chemical structure of API adhesive layer in this study. 2. 2 nd derivatization Derivatization is normally utilized as one effective analysis method for IR spectrum. By using 1 st derivati-zation the resolution is enhanced and the change of gra-dient in IR spectrum is calculated. 2 nd derivative spec-trum is achieved by performing another derivatization on 1 st derivative spectrum, in which the negative peak shows up at the same position as raw IR spectrum. Es-pecially for NIR spectrum, in which bands overlapping occur frequently, 2 nd derivatization could be used to separate the overlapping bands and eliminate the base-line draft effect. 3. 2D correlation spectroscopy Two-dimensional (2D) correlation spectroscopy for IR is one analytical technique to study the molecular in-teractions basing on time-resolved IR spectra and started by Noda 11) . The 2D correlation concept comes from 2D NMR, however, 2D correlation spectroscopy for IR is different from NMR's. 2D NMR is calculated from spectra with multiple pulse excitations, but in IR detection this procedure is difficult to be accomplished. In 2D correlation spectroscopy for IR the excitations are various temperature, time etc., which are easy to be collected. In this study the 2D correlation spectra were calculated by using 2D shige 12) , synchronous and asynchronous spectrum are obtained separately. In syn-chronous spectrum the auto-peak appears on diagonal, by which the change of band in IR spectrum correspond-ing to excitation is observed. Cross-peak generates from two correlative bands, therefore, basing on the signs (positive or negative) of cross-peaks in synchro-nous spectra and asynchronous spectra two bands' changing rates can be confirmed. As for FT-NIR spec-trum the 2D correlation spectroscopy is inappropriate to be performed directly on raw spectrum in which bands of different chemical linkages overlap signifi-cantly. In this study 2 nd derivatization was used to sepa-rate the overlapping bands in FT-NIR spectrum, fur-ther, 2D correlation spectroscopy was performed on 2 nd derivative spectrum.