Depth profiling of the chemical composition of paper and paper coatings is of essential importance for the development of paper qualities tailored to the demands set by recycling and new printing technologies. In this work the chemical composition of biodegradable, starch-based paper coatings was investigated by Fourier transform photoacoustic spectroscopy (FTIR/PAS). The coatings were prepared by mixing solutions of cationised starch and anionic surfactant (sodium oleate) and were applied in thicknesses ranging from 20 to 60 μm. FTIR/PAS spectra were recorded using step-scan phase modulation, and the spectral information was processed using photoacoustic (PA) magnitude/phase spectra calculations and generalised two-dimensional (G2D) spectral correlation analysis. Variation of the phase modulation frequencies made it possible to differentiate the characteristic bands arising from the base paper and the coating layer. Closer study of the coating spectra showed, however, that the position of the carbonyl salt band of sodium oleate is influenced by the environment, i.e., the position of band is different in pure oleate, oleate mixed with starch and oleate-starch mixture in contact with the calcium carbonate-containing base paper. On the basis of this information, the specific depth profile of the coating layer was obtained by analysis of magnitude spectra, phase angles and a G2D correlation spectrum. Each of these approaches indicated that the long chain surfactant (sodium oleate) is enriched in the outermost surface layer of the coating. In the attempt to calculate the thickness of the coating layer from the phase data, the overlapping of the signals did not allow the traditional equation to be used as such and a new baseline correction method was introduced to eliminate the overlapping. The advantage of this method is that no measurements of pure components are required. However, the method can be applied only for well-resolved and linearly overlapped bands. FTIR/PAS is concluded to yield a completely new type of qualitative information, which can be utilised in the development of paper coatings.
Halttunen, M., Tenhunen, J., Saarinen, T., & Stenius, P. (1999). Applicability of FTIR/PAS depth profiling for the study of coated papers. Vibrational Spectroscopy, 19(2), 261-269. https://doi.org/10.1016/S0924-2031(98)00097-6