Applicability of FTIR PAS depth profiling for the study of coated papers

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 th e chemical composition of biodegradable, starch-based paper coatings was in vestigated 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 mu m. FTIR/PAS spectra were recorded using step-scan phase modulatio n, and the spectral information was processed using photoacoustic (PA) magn itude/phase spectra calculations and generalised two-dimensional (G2D) spec tral correlation analysis. Variation of the phase modulation frequencies ma de it possible to differentiate the characteristic bands arising from the b ase 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 p ure oleate, oleate mixed with starch and oleate-starch mixture in contact w ith the calcium carbonate-containing base paper. On the basis of this infor mation, the specific depth profile of the coating layer was obtained by ana lysis of magnitude spectra, phase angles and a G2D correlation spectrum. Ea ch of these approaches indicated that the long chain surfactant (sodium ole ate) is enriched in the outermost surface layer of the coating. In the atte mpt to calculate the thickness of the coating layer from the phase data, th e overlapping of the signals did not allow the traditional equation to be u sed as such and a nevi baseline correction method was introduced to elimina te the overlapping. The advantage of this method is that no me;measurements of pure components are required. However, the method can be applied only f or well-resolved and linearly overlapped bands. FTIR/PAS is concluded to yi eld a completely new type of qualitative information, which can be utilised in the development of paper coatings. (C) 1999 Elsevier Science B.V. All r ights reserved.