Analysis of Water-Based Adhesives Using Internal Reflectance and Curve-Fitting

Abstract

Within our laboratory a need arose to develop a rapid, quantitative analysis for the constituents in water-based, poly(vinyl acetate) (PVA) adhesives. These materials exhibit several problems for infrared quantitative analysis. The first problem area is reproducibility in the preparation of the sample for infrared measurement. Water, as the base, precludes the use of the more common window materials (i.e., KBr, KCl) due to their water solubility. One simple remedy to this problem is to cast a film onto a secondary support, typically a glass plate, remove the dried film, and obtain a spectrum by direct transmission. Alternatively, non-water-soluble window materials such as AgCl may be used. These materials permit the film to be cast directly onto the window surface and eliminate at least a portion of the sample handling. Although these two methods are well suited for obtaining a qualitative IR spectrum of the adhesive constituents, quantitation remains a problem. To quantitate a mixture, spectral bands must be selected which are preferably unique to the components of interest or which at least increase linearly in absorbance with concentration. Spectra obtained by direct transmission through a polymer film are generally too intense to meet these criteria. While ultrathin films offer a possible solution, film uniformity and surface defects can become significant problems as the thickness of the film is decreased. One method which alleviates the reproducibility problems in the aforementioned techniques is Attenuated Total Reflectance (ATR). An advantage of internal reflectance spectroscopy is that the penetration depth of the infrared radiation, which is wavelength dependent but reproducible for a given angle of incidence, generally does not exceed a few micrometers. By maintaining the sample thickness greater than the effective penetration depth, only the sample surface in contact with the ATR crystal is important. Consequently, surface defects can be minimized by normal maintenance of the crystal.