984 zyxwvutsrqponm L. Wigberg and K. Kolar: Adsorution of Cationic Starch on Fibres from Mechanical Pubs zyxw Adsorption of Cationic Starch on Fibres from Mechanical Pulps Lars Wagberg and Katarina Kolar SCA Research AB, Box zyxwvut 3054, 850 03 Sundsvall, Sweden zyxwv Key Words: Adsorption / Cationic Starch / Colloides / Dissolved Solids / Electrostatic Charge / Neutralisation / Thermomechanical Pulps The present investigation, in which the adsorption of cationic potato and corn starch on fibres from Thermo Mechanical Pulps (TMP) has been studied, has been divided into two parts. In the first part, the adsorption of differently charged cationic potato starches onto the fibre fraction of a peroxide-bleached TMP was investigated. The results show that the adsorption can be viewed as an ion-exchange process in which the anionic charges on the fibres are neutralised by cationic charges on the starch. Experiments with different electrolyte concentrations show that the starch adsorption on this kind of fibre follows theories for polyelectrolyte adsorption. When Dissolved and Colloidal Substances (DCS) from the TMP is introduced into the experiments these conclusions are no longer valid since the complexation between the starch and the DCS becomes the dominating process. The results from the second part show the large influence of cellulosic fines and DCS on the amount of starch that can be adsorbed on the fibres. Since both fines and DCS have a higher charge density and a higher specific surface area than the fibres, they can adsorb significantly larger amounts of starch and may hence increase the total amount of starch adsorbed on the fibres, provided the fines and DSC can be retained to the fibres. The fines fraction is retained to the fibres by a bridging type of mechanism whereas the DCS-fraction is retained through charge neutralisation of the charges on the DCS-fraction by the charges on the starch. Potato starch was somewhat more efficient than corn starch in neutralising the effect of the DCS-fraction. Introduction Many of the most common types of printing papers con- tain large amounts of mechanical pulps. These pulps are produced in a process where the fibres are liberated from the wood through the combined action of mechanical forces and heat. The pulp produced in this way contains virtually all components that can be found in the wood apart from a small fraction (< 10% by weight) which is dissolved and/ or dispersed in the water phase used during the processing. This dissolved and/or dispersed fraction contains mainly dissolved carbohydrates from the wood, both uncharged and negatively charged, and a negatively charged colloidal phase mostly containing dispersed lipophilic extractives [I -41. Despite the relatively small fraction of these com- ponents, they constitute a major obstacle when trying to apply cationic, polymeric performance chemicals during paper production zyxwvutsr [5 - 81. Cationic starch, added to increase the strength of the paper, can serve as an example of such a performance chemical. The use of inexpensive inorganic fillers has become more and more common, both to lower the production cost of the paper and to improve its optical properties. Since the fillers lower the paper strength there is an increasing need to add dry strength additives to the paper in order to maintain the strength. It is therefore necessary to determine how ad- ditives such as cationic starch may be adsorbed by cellulosic fibres both in the presence and absence of Dissolved and Colloidal Substances (DCS). An improper use of the dry strength additive will result in inferior paper properties and environmental problems due to an increased discharge of chemicals from the paper mill. Considerable work has been done concerning the adsorp- tion of cationic starch on fibres from bleached chemical pulps [9- 151. In general, these studies show that the ad- sorption of the cationically charged starch can be viewed as an ion-exchange process [9, 12, 141 where the anionic charges on the fibres are compensated by cationic charges on the starch. This has indeed been explained using recent theories of polyelectrolyte adsorption [I61 where it was shown that a charge compensation regime, i.e. compensa- tion of charges on the fibres by charges on the polyelec- trolyte (starch), was reached at very low charges of the starch. Some studies [lo, 141 have also shown the importance for the starch adsorption of the fine material in pulps used for paper production. This can naturally be discussed in terms of available surface area of the material and, since fines, i.e. fragments from the cellulosic fibres, have a much larger specific surface area than the fibres, they can adsorb a larger amount, in mg/g, of cationic starch. WBgberg and Bjorklund [I41 also showed that different fines were able to adsorb different amounts of cationic starch, mostly due to differences in morphology. In the case of mechanical pulps, the situation is even more complex since these pulps contain about 35% fines, i.e. material smaller than 70 pm, compared to only 10% in the chemical pulps, and they also contain a considerable amount of DCS-material [17]. In addition, the fibres con- tain a higher amount of anionically charged hemicellulose bound to the fibres, which means that the charge is much higher on these fibres, 85 - 110 peq./g, than on the bleach- ed chemical pulps, where it is only 10- 25 peq./g [18]. All this means that these pulps can bind or complex much higher amounts of cationic polymers such as cationic starch, but due to the interaction between the starch and the DCS-material it is uncertain whether starch will be wasted in the effluents or will end up in the paper where it can im- prove the strength properties. Several workers have studied this interaction [5, 6, 19-21] and it is evident that the in- teraction between the DCS-material is very complex and that it is very difficult to find general laws describing this Ber. Bunsenges. Phys. Chem. 100, 984-993 (1996) No. 6 zyxwvutsr 0 VCH Verlagsgesellschaft mbH, 0-69451 Weinheim, 1996 0005-9021/96/0606-0984 $ 15.00+.25/0