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in ash content of the specimen from 525 °C to 900 °C (Table 6 ). Filler retention values as a function of PAM pretreatment and filler contents were determined and they are shown in Table 6 as well. Effects of accelerated ageing The acidity–alkalinity of the handsheets, measured as pH, i. e. a measure of the potential activity of hydrogen ions, is affected by many other factors, including organic components of specimens and accelerated ageing conditions. As a result of ageing effect and the corresponding pH values of handsheets are given in Table 7 . Table 6

unmodified PCC. In the present work these increments were of 8%, 8% and 6%, respectively. Contrary to the aforementioned published results the bulk did not vary when using the modified filler, which is a positive achievement. Three different pilot trials were conducted and papers with different densities were produced. The filler content obtained in these trials was in general smaller than that obtained in the laboratory experiments, the latter with filler retention values always superior to 93% (Table 1). These differences are due to the distinct operating

tear strength than those produced with unmodified PCC, while the optical properties are not significantly affected, at filler levels ranging from about 5 to 25% (Zhao et al. 2005). Laleg et al. (2008) treated PCC particles with anionic polymer dispersions (latex) and found that the resultant slurries could improve paper internal resistance PAPER CHEMISTRY Nordic Pulp & Paper Research Journal Vol 29 no (2) 2014 241 and breaking length for filler levels in the handsheets in the range of 10 to 25%, besides improving filler retention, sizing performance

effect of water quality on 2: 134 Hartler, N. washing of unbleached kraft pulps McKague, A . B. Reactions of lignin model dimers 2:84 Kang, G. with chlorine dioxide Reeve, D. W. Middleton, S. R. Filler retention and the specific sur- 3: 156 Scallan, A . M. face area of fibers Milanova, E. On the determination of residual 1:4 Dorris, G. M. alkali in black liquors Norden, H. K Cake filtration with application to 4:208 Kauppinen, P. dewatering of pulp Nordstrom, B. Influence of sheet anisotropy, 1 :53 Norman, B. formation, Z-toughness and tensile

(Vicron-type) and precipitated cal- cium carbonate were used. A series of reference sheets and a series of sheets containing 4.2% cationic starch (CS) were made. Starch retention was aided by the addition of 0.3% anionic polyacrylamide (A- PAM) to ensure that CS which was not adsorbed onto the fibers and fillers in the stock was precipitat- ed onto the stock components in the form of a polyelectrolyte complex (6). By using such a dual retention aid system a high filler retention (> 90%) is also secured. Two series of reference sheets without CS were made. In

procedures of the CPPA Technical Section. Filler addition To ensure a reproducible level of filler retention in the handsheets a method based on previous work was used for filler addition (16). Cont. p. 29 Nordic Pulp and Paper Research Journal no. 111 992 sequential addition, (DC), where acidification was not performed. Oxygen bleaching was performed in a 50 1 auto- clave equipped with a stirring mechanism. Chlorine dioxide treatment at 3.5% consistency was carried out in the ozone reactor. Temperature was 20°C. When other consistencies were used, the

water consistency, turbidity, and cationic demand were all significantly affected by the enzyme dosing strategy. A significantly increased level of turbidity, indicating a greater amount of non-retained dissolved and colloidal substances, was seen, in particular for the sample refined during the enzymatic hydrolysis without the following deactivation step. The data for the first pass and filler retention, as well as the retention levels of AKD and starch, determined from their concentrations in the headbox stock and white water samples (Table 5), show that

correlation has been found between the reduction in strength at a certain filler content and the amount of filler retained in the sheet at a certain charge of filler, fig. 10. Average filler retention,% Fig. 10. The responses to filler addition of the papers containing different mechanical pulps differ some- what. The values plotted have been normalized to a reference level of 20 % filler. The fact that these differences correlate with filler retention suggests that the reason for these differences is probably experi- mental and not a result of the character of

pigments do not have any appreciable bonding capability to cellulosic fibers. Several methods for enhancing the bonding capacity of fillers with fibers have been proposed to help improve paper strength and allow for higher filler contents. Natural polymers and their derivatives, such as starch, cellulose, carboxymethylcellulose (CMC), chitin and guar gum, have been examined for improving filler bondability, paper strength or filler retention (Shen et al. 2009). Starch is widely used by several industries, such as adhesives and paper. The benefits of starch

disturb the fibre- fibre and fibre-filler retention by interfering with add- ed cationic chemicals. The formation of different kinds of polyelectrolyte complexes (Dickinson, Eriksson 1990; Lindstrom 1989), are examples of interfering re- actions which may be detrimental to paper production. If the degree of closure of the system is high, deposits forming on rolls and wires may also cause severe dis- turbances in paper machine runability. The anionic character of the detrimental substances is due to dissociated acidic groups such as carboxyl and sulfonic acid