Application of FT-IR-ATR Spectroscopy to Evaluate the Penetration of Surface Sizing Agents into the Paper Structure

It is widely recognized that the surface properties of paper depend on the fibrous matrix and the final surface treatment applied to the paper. Regarding chemical paper surface treatments, an important issue is the evaluation of the penetration of chemical compounds into the fibrous matrix, as the chemicals can potentially cause changes in the intrinsic properties of paper. The work presented here aimed to use Fourier transform infrared (FT-IR) spectroscopy to study paper surface sizing, namely, the penetration of the sizing chemicals into the paper structure. Two different surface sizing formulations were applied to paper produced from Eucalyptus globulus bleached pulp (reference paper): both contain 90% (w/w) cationic starch, but one contains 10% (w/w) poly(styrene-co-maleic anhydride) whereas the other contains 10% (w/w) poly(styrene-co-butyl acrylate). The surface-sized paper sheets were further manually delaminated, so that the top surfaces as well as the internal layers could be analyzed by FT-IR spectroscopy. A non-surface-sized sample was taken as the reference. From the spectroscopic results, it was possible to detect the presence of the copolymers on the paper top surfaces, despite the application of only small amounts of these chemicals in the surface sizing. However, the chemicals were not found in the layers closest to the surface (30−40 μm from the top), leading to the conclusion that the penetration of the sizing formulations into the fibrous matrix was insignificant (at least up to this distance). Infrared spectroscopy data also showed that the calcium carbonate added as a filler was always present at higher concentration in the analyzed inner layers than at the top surface, for the reference paper as well as the sized papers

Straining and relaxation properties of wet paper during heating

The influence of increasing temperature on the strength and relaxation of wet press-dry paper was studied using a tensile tester equipped with a special heating chamber. The heating chamber made fast heating possible without detectable moisture loss. The results showed that temperature had a significant influence on the straining, relaxation and re-straining behavior of wet paper. The majority of observed changes due to increased temperature seem to originate from the softening of wet fibers. The observed short time scale phenomena in wet paper have practical significance for fiber webs dried under tension in paper machines. Straining–relaxation–de-straining cycles were used to analyze the effect of heating on the work of straining and apparent plastic and elastic work. Heating affected the amount of mechanical energy absorbed by the sample and the amount of elastic energy recoverable in a straining–relaxation–de-straining cycle. Increased temperature reduced the work of straining and both elastically and plastically absorbed energy. The hysteresis work of the examined wet papers was estimated to correspond to a 1–22 mK temperature change. This suggests that temperature changes in wet paper induced by straining play no role in practice. After mechanical conditioning, tensile stiffness in the re-straining of wet paper depended only marginally on temperature, whereas in initial straining the effect of temperature was clearly stronger. The linear thermal expansion coefficient of wet paper in the machine direction was estimated and the influence of moisture content on the linear thermal expansion coefficient of paper was found to be relatively small