Surface application of yellowing inhibitors into paper

Papermakers are highly interested in upgrading mechanical paper by retarding or inhibiting the lignin color reversion. Yellowing can be prevented by screening UV light on the surface and adding yellowing inhibitors into papers. Surface treatment of paper in a size press by an inhibitor-starch mixture has more advantages compared to other methods. A novel microscopy technique using scanning electron microscopy is developed to identify and measure the polymer penetration into sized sheets.; The kinetics of starch adsorption on wood fibers was elucidated. An initial high adsorption of cationic starch on pulp fibers is off set by high desorption rates. Furthermore the adsorption of hydroxyethyl ether starch increases smoothly with time due to polymer penetration into the macropores. At high salt concentrations, cationic starch does not make strong bonds with negatively charged fibers and as a result the desorption rate increases and the maximum adsorption approaches zero. The presence of salt has little effect on the adsorption of non-ionic hydroxyethyl ether starch.; Preferential adsorption of amyIose compared to amylopectin on cellulosic fibers is related to the size exclusion mechanism involved in polymer penetration into macropores.; The presence of cationic starch clusters is confirmed. The clusters initially adsorb on the fiber surface; however due to a higher desorption rate they are gradually replaced by individual polymers. The cluster size decreases with increasing shear in the presence of salt in cationic starch solutions. The cluster desorption rate for pulp fibers is higher than that for glass substrates. This might be due to different surface chemistry or roughness between pulp fibers and glass substrates. The maximum adsorption on glass substrates increases by increasing the cluster size. Hydroxyethyl ether starch does not form clusters and shear and the presence of salt have a minor effect on the polymer size.; Starch pickup by paper during surface sizing depends on the liquid absorption at the puddle and counter pressure of the trapped air in the pores. Starch viscosity, paper velocity and nip load have no effect on the thickness of the starch layer across the sized paper. However, for thick boards penetration increases by increasing the nip load due to lower sheet thickness at higher nip load. For sized board, the starch penetration is less for cationic starch than for hydroxyethyl ether starch due to higher viscosity and adsorption.; For the sized sheets with yellowing inhibitor/starch mixtures, the paper brightness after aging is affected by inhibitor concentration in the solution, since operating conditions have little effect on pickup. Using cationic starch slightly increases the brightness for boards, while it has little effect on paper brightness.