Effective use of FWAs in papermaking by controlling their interactions with other chemicals

The trends for fine papers are toward higher brightness and opacity, beside a lower grammage. Alkaline sizing and use of calcium carbonate are part of solution to this challenge. However, the increase of brightness via high filler content is limited by filler characteristics and by decrease of paper strength. The fluorescent whitening agents (FWAs) offer to papermakers the opportunity to obtain high brightness levels which cannot be achieved economically with standard systems, but their effectiveness could be strongly impaired by cationic chemicals, which are always present in the alkaline papermaking systems. The objective of our work was to analyze the interactions of the FWAs with common chemicals of the alkaline papermaking system for fine papers and to find out solutions for improving their effectiveness. In this order, the following chemicals were considered: cationic starch (CS) as retention aid for AKD size and as dry strength additive; cationic and anionic polyacrylamides (C-PAM, A-PAM) as flocculant additive for filler and fines, bentonite (B) as a microparticle component of retention system; cationic chitosan (CH), a multifunctional biopolymer that could substitute cationic starch and polyacrylamide. These chemicals were tested with regard to their effect on the effectiveness of a fluorescent whitening agent - Ciba Tinopal ABP liquid (T-ABP), and on the other paper characteristics. Interaction of the FWA with chemical additives was studied by evaluating the UV absorption of T-ABP solutions of constant concentration and with variable addition of chemicals, corresponding to the common dosages in papermaking system. The UV spectra indicated a decrease of the absorption at 340nm as a function of the cationic starch addition and only a slight reduction for C-PAM. In the case of T-ABP solution treated with chitosan, a very complex transformation is indicated by UV spectra, which leads not only to a change of absorbance at 340 nm, but also to a shift of absorption maximum toward a higher wavelength (370 nm). Increase of chitosan addition results in decrease of the both absorbencies (340 nm and 370 nm). On the other hand, anionic bentonite alone stimulates UV adsorption of T-ABP and limits interaction of the chitosan with T-ABP when is used in a microparticle system. The effectiveness of the Tinopal was evaluated by measuring the fluorescent index of the lab paper sheets obtained under alkaline sizing conditions and using the above mentioned chemicals. The results have shown that brightness improvement gained by Tinopal ABP addition could be correlated with decrease in UV adsorption of Tinopal due to interaction with cationic chemicals. At the same time, the investigations considered other important paper properties; such are opacity, sizing level, tensile and surface strength, which are influenced by cationic chemicals application. Finely, the study allows us to find out two combinations of additives -CS/CPAM/B and CH/B, which are effective for sizing, filler retention and dry strength, without significant reduction of the FWA effectiveness.