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With the increase of dissolved and colloidal substances and inorganic ions resulting from the increased degree of whitewater closure, whether various chemicals added in the wet end of papermaking can still work well is an issue greatly concerned by papermakers. In this study, the conductivities of pulp slurries made from a BCTMP (sometimes calcium carbonate filler was also added) were raised to low (500μS/cm) or high level (5000μS/cm), by the addition of sodium or calcium chloride. Using ultraviolet-visible spectrometry as the main method, the adsorption performances of various wet-end chemicals including a fluorescent whitening agent (FWA), a methyl violet (MV) tinting dye, a polyaminoamide-epichlorohydrin.(PAE) wet strength resin, a cationic starch (DSA-CS) dry strength agent, an alkyl ketene dimmer (AKD) internal sizing agent, and a cationic polyacrylamide (CPAM) retention agent, on the BCTMP fiber, under such low or high conductivity papermaking systems, were investigated. Among them, the key part of this work was the comparison of FWA adsorption onto the BCTMP fiber and GCC filler.Firstly, the adsorption of FWA on the BCTMP fiber was studied. The results showed, no matter in the sodium or calcium chloride adjusted system, the adsorption of FWA onto the BCTMP in the high conductivity system was always higher than that in the low one. Comparing the NaCI and the CaCl2adjusted systems with equal conductivity and at the same dosage level of FWA, the adsorption of FWA in the CaCl2system was always higher than that in the NaCl one. This was demonstrated to coincide with the results by paper dipping method, where it was found that paper dipped in the high conductivity FWA solution always had higher whitening effect than that dipped in the low condition. Similarly, the dipping method also showed that calcium had better effect in promoting the whitening effect of FWA, although this happened more clearly in the low conductivity condition (under the high conductivity condition, the effects of calcium and sodium were almost the same).In the following step, considering that the use of filler in papermaking always affects the performances of other wet-end chemicals, the distribution of FWA adsorbed onto fiber and filler was studied. The result showed that in a suspension consisted of70%fiber and30%GCC (ion-exchanged water was used for dilution), most of the FWA was resided in water, i.e., only small portions of FWA was adsorbed by the solids. Further analysis showed that the amount of FWA adsorbed onto the fiber was clearly higher than that adsorbed onto the filler. However, if the calculation was based on the amount of FWA adsorbed onto the unit weight of fiber or filler, then the filler would have higher adsorption ability than the fiber. Again, NaCl or CaCl2was used to raise the conductivity of the fiber/filler system to low (500μS/cm) and high (5000μS/cm) level, and the adsorption of the FWA on the fiber and filler in the four different systems, including Na-H, Na-L, Ca-H and Ca-L, where Na and Ca meant sodium chloride and calcium chloride adjusted system, H and L meant high and low conductivity respectively, were compared. The results showed that, with the increase of FWA dosage, the amount of FWA adsorbed by fiber, the amount by filler, and the amount of residual FWA in the water, were all increased. However, considering the percentages, it should be reclaimed that with the increase use of FWA, the percentage of residual FWA in water was increasing, and the percentage of FWA adsorbed by fiber was nearly changed, while the percentage of FWA adsorbed by the filler was surprisingly decreasing sharply. Investigation on the effect of conductivity showed that, no matter for the NaCl or the CaCl2system, increasing the conductivity prompted the adsorption of the FWA on the paper solids, however, it was also found that the increases of adsorbed amount and adsorption ratio were all for the filler, in that for the fiber, the adsorbed amount and adsorption ratio changed very little. Comparing the effects of sodium and calcium ions under the same low conductivity conditions (500μS/cm), the result demonstrated that calcium can promote the adsorption of FWA onto the paper solids, in that the amount and percentage of FWA resided in water were reduced, but in more detail, then increases of the adsorbed amount and percentage of the FWA on the solids were also for the filler only, in that those for the fiber were decreased. Comparing the effects of sodium and calcium ions again but under the same high conductivity conditions (5000μS/cm), it was found that other results were almost similar to those found in the low conductivity conditions, except that now calcium ion significantly reduced the adsorbed amount of FWA on fiber. Therefore, it can be concluded that high conductivity promoted the adsorption of FWA, but the contribution was mainly to filler, for fiber, high conductivity can even impair the adsorption of FWA onto it.Finally, the adsorption behaviors of some other typical additives onto fiber, under low and high conductivity conditions, were investigated. The results showed for MV and AKD. high conductivity promoted their adsorption onto fiber, for PAE, conductivity had little effect, and for DSA-CS and CPAM. high conductivity reduced their adsorption. Comparison the effects of sodium and calcium under the same conductivity level, the results demonstrated that calcium can significantly promote the adsorption of MV onto fiber at the low conductivity side, however, when it came to the high side, the effect of calcium on MV adsorption became negative. For AKD, either in the low or high conductivity, increasing the amount of calcium promoted its adsorption onto fiber, and for PAE. DSA-CS and CPAM. again either in the low or the high conductivity condition, all the effects of calcium on promoting their adsorption onto fiber were found to be negative.The results obtained in this research can be helpful in optimizing the use of wet-end additives and thus improving cleaner production of paper in high conductivity conditions.
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