Cationic starch adsorption onto cellulosic pulp in the presence of other cationic synthetic additives

•Synthetic polycations interfere with cationic starches at adsorption onto fiber.•The addition of synthetic polycations after cationic starch results in higher adsorption amounts.•The replacement of cationic starch depends on molecular properties of the starch itself and of the synthetic polycation. The production of paper requires a lot of different raw materials and additives in order to achieve the desired paper properties in a cost-efficient way. Cationic starches are widely used in the wet end process to develop high paper strength and several other additives are also applied to optimize the paper properties or the production process. The aim of this work was to investigate the adsorption behavior of cationic starches on the fiber in the presence of other polycations. Cationic potato starches with three different degree of substitution (DS) of 0.03, 0.065 and 0.09 were compared in the investigations as well as a cationic maize, potato and waxy potato with the same DS (DS=0.065). The used synthetic polycations (synth. PC) were polyvinylamine, polyethylenimine, polyacrylamide and poly(diallyldimethylammonium chloride) with three different molar mass. It was found that cationic starches and synthetic polycations compete against each other during fiber adsorption. The higher the cationic charge of the synthetic polycation was, the less cationic starch adsorption occurred. Adding the cationic starch to the suspension before the synthetic polycation minimized this effect. It has been figured out, that a cationic potato starch with a middle DS (0.065) built up the toughest adsorption layer and was less affected by replacement due to synthetic polycations than the other cationic starches. The smaller the pDADMAC, the more cationic starch was adsorbed to the fiber due to the increased pore diffusion of the synthetic polycation.