Additive for paper making

#The present invention concerns an additive for paper making, which is added to the fiber pulp prior to the web formation step in a paper making process. By means of the additive it is possible to impart to the paper i.a. a reduced tendency for dusting. In addition, the additive has been shown to facilitate water removal in the web formation stage, to improve filler retention, and to affect advantageously the removal of harmful substances, which have accumulated in the water circulation system of the paper machine. The additive also increases the strength of the finished paper, both dry strength and wet strength. By using the additive for paper making according to the invention improvements have been seen also in the printability of the paper, i.a. as regards its applicability for ink-jet printing. The term paper as used in this application also includes the term paperboard.

#A problem that is encountered in finished paper is its high tendency for dust formation, the reason for which are fiber particles and filler particles released from the paper surface. The dust gives rise to problems already at the drying stage in paper making, but above all in the machines and equipments handling paper, such as in printing equipment. The printing methods as such are developed which means high machine speeds and long printing series. High speeds aggravate dust formation, and long printing series reduce standing times, during which it would be possible to carry out a cleaning of the equipments.

#In order to solve the dusting problem it is known to use methods, which is a rule are based on chemical compounds to be added to the fiber pulp, prior to the paper web formation. The use of mineral and micro waxes, of sizing agents, such as AKD and ASA dispersions, of wet strength resins and of pulp sizing starch is known. Irrespective of these known measures, paper dusting is still a significant problem.

#The reduction of the molecular size has been carried out advantageously by oxidizing, such as by peroxide oxidation. But also other oxidants such as hypochlorites and persulfates can be used as well as degradation method. Also other commonly known degradation methods e.g. degradation by acids can be used as a degradation method. The reduction of the molecular size is appropriately carried out so that the viscosity of a 5% starch suspension at 60° C. is in the range 10–500 mPas (Brookfield). The viscosity is advantageously 40–300 mPas, and especially advantageously in the range 100–200 mPas. Oxidation to viscosity level of 10–500 mPas can be achieved for example using a hydrogen peroxide dose of 0.02–2% from the starch dry matter in slightly alkaline reaction conditions. The desired degree of degradation is also bound to the desired cationic charge of the end product, since an increase in the cationic charge decreases the viscosity of the end product. There is also interdependence between the molecular size and the cationic charge which affects the behaviour of the starch in the paper machine i.e. the retention of highly degraded molecules is improved by increased cationic charge level of the molecules.

#Starch which has been processed to have the desired viscosity level is thereafter processed with a quaternary nitrogen compound according to the present invention so that its cationic charge level will be in the range <1.5 meq/g (meq/g means in this application milliequivalent of charges per gramme of dry, nonaqueous polymer), preferably in the range 0.36–1.46 meq/g, advantageously 0.72–1.46 meq/g and especially advantageously in the range 0.72–1.10 meq/g, whereby the nitrogen contents when using the quaternary cationizing chemical, will be correspondingly in the range 0.50–2.04%, 1.01–2.04% and 1.01–1.54%. The product is advantageously prepared using solution cationization, in which the starch is fed into the cationization process in granular form, the process conditions are chosen so that the starch dissolves completely during the process. Essential process quantities in this respect are the concentration of the starch to be cationized, suitable alkalinity and increased temperature. The alkali dose (NaOH) is siutably in the range of approximately 1.5–3% of the starch dry matter, and the temperature suitably in the range of approximately 60–80° C. The dry matter content of the reaction mixture should advantageously be over 50%, which gives i.a. a good yield for the end product. A suitable quaternary cationizing chemical is 2,3-epoxypropyltrimethylammonium chloride, which should be used in an amount of approximately 10–40% of the amount of starch.

#The spirit of the invention also comprises the use of other suitable cationization methods e.g. dry cationization method, in which the moisture content is typically below 30% and in which starch remains powder like. Also cationization can be alternatively done prior to the oxidation step.