NO174724B - Procedure for making paper and cardboard - Google Patents

Abstract

Paper or cardboard is prepared by providing an aqueous cellulose suspension containing a cationic polymer and then adding cationic starch or a high molecular weight synthetic cationic polymer, after which the suspension is subjected to shear forces, after which an inorganic material selected from the group consisting of bentonite or colloidal silica is added. The process is of particular value when the suspension is prepared from mechanically derived pulp and / or bleached pulp, and when the product is newsprint or cardboard.

Description

Paper or board is made by providing a thick pulp, thinning the thick pulp to form a thin pulp, and then dewatering the thin pulp to form a sheet which is then dried. A thicker pulp can be produced either by adding water to dry pulp, or in an integrated paper machine or mill by diluting an unwatered pulp.

It is standard practice to improve the process or the product's quality by adding different additives at one or more of the aforementioned production steps.

If e.g. if the pulp from which the paper or cardboard is made is impure, inorganic materials such as alum, talc or bentonite are usually added before dewatering during the production of the pulp. Such treatment will usually have the effect of reducing or eliminating problems caused by glue or other sticky substances.

If it is necessary to improve the strength of the final paper sheet, it is common to add a so-called dry-strength resin, e.g. a cationic starch, in the mass to be dewatered.

Furthermore, it is common to add cationic polymers to the mass to be dewatered in order to improve dewatering and/or retention.

Methods for improving retention are described in U.S. Pat. patent 4,388,150 and includes adding cationic starch and colloidal silicic acid to the mass before dewatering. Such methods have been commercialized under the trademark "Com-posil" (trademark).

Methods that provide improved dewatering, retention, drying and production of paper are described in European patent 235893 and include adding a first synthetic cationic polymer before a shearing step and bentonite after this shearing step. Such methods have been commercialized under the trademark "Hydrocol" (trademark).

Although this method in most cases gives very good results, there is still room for improvement with certain types of pulp, particularly impure pulp, and for certain end products such as newsprint and cardboard.

In the present invention, paper or cardboard is produced by a method which includes providing an aqueous cellulose mass, subjecting this to one or more shearing steps selected from cleaning, mixing and pumping steps, after which a main polymer selected from cationic starch and high-molecular cationic polymers with a molecular weight above 500,000 and cationic starch, before one of the shearing steps and then adding an inorganic substance selected from bentonite and colloidal silicic acid after the shearing step, dewatering the mass to produce a sheet and drying it. The method is characterized by the fact that it includes a preliminary polymer addition step which either (a) involves adding the mass before or after adding the main polymer, a low molecular weight water-soluble synthetic cationic polymer with a lower molecular weight than that used for the main polymer, and (b) adding a water-soluble cationic polymer as a dewatering agent to the cellulose pulp, and then dewatering the cellulose pulp (in the presence of the dewatering agent), after which the dewatered pulp is diluted to an aqueous cellulose pulp.

The preferred method according to the present invention includes adding a low molecular weight water solvent. lig synthetic cationic polymer.

The addition of the low molecular weight cationic polymer in the thin mass before adding the main polymer leads to an improvement in the further processing and in the final properties achieved by adding the main polymer before a shearing step and bentonite or colloidal silicic acid after the shearing step. It has e.g. has been shown, depending on the other conditions, that it can lead to reduced problems that are rinsed with water or other sticky materials and can lead to better qualities with regard to wet and/or dry strength, drivability, dewatering, dusting, opacity and other qualities.

In this first aspect of the invention, the aqueous cellulose pulp can either be produced from a dried pulp, or in an integrated plant by diluting a dewatered pulp, both parts in a commonly known manner.

In another aspect of the invention, the cellulose pulp is produced by diluting a dried pulp in an integrated paper machine and where the dewatering of the pulp is promoted by adding a dewatering aid to a pulp to be dewatered, and where said dewatering aid includes a water-soluble cationic polymer. This polymer can be any of the synthetic polymers described below that can be used as the cationic main polymer.

When dewatering a pulp in an integrated papermaking plant for the production of a wet pulp which can then be diluted for further processing, it is common not to add a dewatering agent as the dewatering often takes place satisfactorily without additional costs. In the present aspect of the invention, however, it is desirable to add a dewatering agent as this improves the dewatering and/or retention and provides an unwatered mass that already contains cationic polymer, and the addition of this has beneficial effects in the subsequent treatment with the above-mentioned main polymer and the inorganic additive. For example, the amount of main polymer needed to obtain optimal processing can be reduced, and the total amount of dewatering agent and main polymer can be approximately the same as the optimal amount of main polymer if the pulp has not been treated with dewatering agent. By using a dewatering agent, the process can be improved both with regard to pulp dewatering and in the sheet formation step itself, while the total amount of polymer used is essentially unchanged and the same applies to the quality after the final processing.

The amount of dewatering polymer is usually at least 0.005 or 0.01%, often at least 0.03 or 0.05%, but it is usually unnecessary to add more than 0.3, and at most 0.5%. It is often preferred to have amounts between 0.1 and 0.2%. These percentages are based on the dry weight of the pulp.

The synthetic polymeric dewatering agent can be a dewatering-promoting, relatively low-molecular weight polymer, e.g. any of those indicated below as the lower molecular weight polymer than the main polymer, but is usually a relatively high molecular weight polymer, e.g. with a molecular weight common to dehydrating agents and retention aids. For example, the polymer will usually be an essentially synthetic linear cationic polymer with a molecular weight above 500,000, and preferably with an intrinsic viscosity of 4 dl/g. One can thus use any of the polymers described in EP 0235893.

Intrinsic viscosities are here measured in a commonly known manner by determining the viscosity of the solutions using a level viscometer at 25°C in 1 molar NaCl buffered to a pH of approx. 7 using sodium phosphate.

Regardless of whether the thick starting mass is prepared by diluting a wet mass that has been dewatered in the presence of a dewatering agent, it is preferred in the present invention to add the described low molecular weight soluble synthetic cationic polymer before the main polymer.

It is preferred that the rest of the process should correspond to the previously stated "Hydrocol" process, and it can thereby be carried out as stated in EP 235893, by using a synthetic cationic polymer with a molecular weight of at least 500,000 before one of the shearing steps and bentonite afterwards. The materials and conditions described in EP 235893 can be used in the present invention, with the modification that the mass includes that the low molecular weight polymer is added before the main polymer. Alternatively, but less preferably, the bentonite may be replaced by colloidal silicic acid or another suitable material in the form of fine particles, or the synthetic polymer may be replaced by cationic starch.

Sometimes smaller amounts of the main polymer than recommended in EP 235893 can give good results in the present invention, e.g. quantities of less than 300 g/h, e.g. 50 g/h (0.005%) to 250 g/h, more particularly approx. 100 g/t, based on the dry weight of the pulp.

Alternatively, the method may correspond to that described in U.S. Pat. patent 4,388,150 with the addition of cationic starch to the pulp before adding the colloidal silicic acid (which may be modified as WO86/5826).

The low molecular weight polymer can be present in the thick mass and then diluted to the thin mass or can be added directly to the latter. For example, one will usually dilute the thick mass for the production of the thin mass by using back water. It is desirable to add the low molecular weight polymer before, or immediately after or during the dilution with back water and then add the main polymer to the thin mass after the addition of the low molecular weight polymer.

The latter polymer must have a molecular weight that is sufficiently lower than the molecular weight of the main polymer that it will provide other process or performance advantages. The present invention thus does not include a method where both the low molecular and the high molecular polymer are essentially primary cationic retention aids. The invention is thus limited to methods where the low molecular weight polymer provides a different process advantage than the other polymer. Generally, the low molecular weight polymer will have an intrinsic viscosity below 2 dl/g and usually have a molecular weight below 500,000. The molecular weight is usually above 50,000 and often above 100,000.

A preferred relatively low molecular weight polymer is polyethyleneimine. A suitable quality of this type of polymer is the product sold under the trademark Polymin SK. Other suitable polymers are polymers and copolymers of diallyldimethylammonium chloride, and of dialkylaminoalkyl(meth)acrylates and dialkylaminoalkyl(meth)acrylamides (both usually as acid addition salts or as quaternary ammonium salts) as well as polyamines and polydicyandiamide-formaldehyde polymers. One can also use amphoteric synthetic polymers.

A preferred method according to the present invention uses a relatively impure starting mass which contains significant amounts of resin and/or has a high cation requirement. The starting material can e.g. require at least 0.1% Polymin SK to provide improved retention when Polymin SK is used conventionally as a retention agent. Polymin is a trademark. Such output masses will e.g. be those that contain more than 25% by weight, usually more than 50% by weight mechanically produced pulp and/or de-blackened pulp. By mechanically produced pulp is meant ground wood pulp, pressure-refined wood pulp, thermo-mechanical, chemical-thermo-mechanical or otherwise mechanically and with high yield produced fibres.

In such cases, the low molecular weight polymer will be chosen primarily to reduce the need for cations and/or to avoid resin problems and/or dusting problems.

When using these relatively impure pulps, the process will be particularly valuable when the pulp is to be used for the production of newsprint, and for this purpose fillers will usually not have been added or only small amounts, e.g. from 0 to 15%, often from 0 to 10% based on the dry weight of the pulp. However, benefits will also be achieved when the pulp contains fillers in quantities of up to 30% in the finished paper product.

The method is also of value in the production of cardboard, often from similarly uncleaned pulps that contain little or no filler. In these cases, an alternative or additional property for the low-molecular polymer would be to improve the strength of the board, and for this a low-molecular water-soluble synthetic cationic resin with high dry strength can be used. Amphoteric polymers are particularly well suited for this purpose.

The amount of the low molecular weight polymer is up to 0.5%, usually in the range of 0.01 or 0.05 to 0.2%, based on the dry weight of the pulp, and the optimum amount can be found by routine experimentation. Often, before treatment with the low-molecular polymer, the mass will have a cation requirement (as measured by titration with the main cation polymer) of over 400 g/h, and the low-molecular polymer is added to the mass in an amount that reduces the cation requirement in the thin mass to below 300 g/ t before adding the main polymer.

It has been found that the method according to the present invention provides improved processing of the pulp as it can provide improved removal of resin and/or other sticky substances, improved paper quality such as opacity and dusting properties, improved wet strength and processing during manufacture. Furthermore, the process is improved with respect to dewatering properties by the addition of the second polymer, compared to a process without this polymer, e.g. a method of the type described in EP 235893 or U.S. patent 4,388,150.

In the following examples, polymer A is a polymer with an intrinsic viscosity of 7 dl/g, prepared from 75% acrylamide and 25% dimethylaminoethyl acrylate, MeCl quaternized, while polymer B is a modified polyethyleneimine sold under the trademark Polymin SK.

Example 1

A 100% mixed waste mass with a consistency of 0.5% was produced. Dewatering tests were carried out on this mass using a modified Shopper Riegler freeness test apparatus, measuring the time it took for 600 ml of bottom water to be dewatered from the mass to be measured. The pulp was then subjected to shear and dewatering was measured. In one sample there were no additions either before or after the shearing effect. In other samples, bentonite was added after the shearing step and polymer A and/or polymer B was added before shearing. When both polymer A and B were added, polymer B was added significantly earlier than polymer A.

The results were as follows.

Example 2

An experiment was carried out in the same way as described in example 1, but using a pulp with a high content of mechanical fibres, and the pulp was a 50:50 malt bleached kraft pulp with a consistency of 1.0%. In addition to measuring the dewatering time as in the aforementioned example, a determination of the number of resin particles (as particles/ml using the Allen method) was carried out.

The following results were obtained.

These examples clearly show the value of adding, e.g. from 0.01 to 0.1%, usually from 0.02 to 0.07% polyethyleneimine, thereby reducing the amount of the high molecular weight (eg with intrinsic viscosity above 4) cationic retention aid required for good dewatering and retention, so that the effect of masses with a high cation demand, and especially a high content of resin particles, can be counteracted.

Example 3

Newsprint is produced using a pulp of 3% kraft, 17% magnefit, 38% thermomechanical pulp and 42% ground pulp to which 20% waste paper has been added. High molecular polymer was added, in some samples just before the last shearing step and bentonite was added, in some samples after the last shearing step. The low molecular weight polymer was added to the thin mass immediately after it had been prepared by diluting the thick mass.

In these samples, the low molecular weight polymer was a polymer K where the polymer in solution had an intrinsic viscosity of 1 dl/g and was prepared from 20% acrylamide and 80% by weight diallyldimethylammonium chloride. The high molecular weight polymers are L, which is 70% acrylamide, 30% methyl chloride quaternized dimethylaminoethyl acrylate with an intrinsic viscosity of 8, and polymer M which is 95% acrylamide and 5% methyl chloride quaternized dimethylaminoethyl acrylate with an intrinsic viscosity of 11. The dewatering rate of each of the treated pulps was measured, and the best results were obtained with those who had the highest unwanted numbers. The results are as follows.

These results clearly show the advantage in the manufacture of newsprint of adding a high molecular weight cationic polymer immediately before the shearing step and bentonite after the shearing step, even when the high molecular weight polymer often only has a relatively low cationic charge. The results show that it can also be advantageous to replace the high molecular weight polymer with a low molecular weight polymer with a molecular weight above 500,000, but that the best results are obtained by using a combination of both.

Claims (14)

1. Process for the production of paper or cardboard by preparing an aqueous cellulose pulp, passing it through one or more shearing steps selected from cleaning, mixing and pumping steps, adding a main polymer selected from cationic starch and high molecular weight water-soluble cationic polymers to the pulp before one of the shearing steps and adding an inorganic material selected from bentonite and colloidal silicon dioxide after the shearing step, dewatering the mass to produce a sheet and drying it, characterized in that the method includes a preliminary polymer addition step selected from (a) adding to the mass before or after addition of the main polymer, a low molecular weight water-soluble synthetic cationic polymer with a molecular weight that is lower than the molecular weight of the main polymer, and (b) adding a water-soluble, cationic , polymeric dewatering agent to a cellulose pulp, and then dewaters the pulp and dilutes the dewatered pulp into an aqueous cellulose pulp. 2. Method according to claim 1, characterized in that the main polymer is a high molecular weight linear water-soluble cationic polymer with a molecular weight above 500,000. 3. Method according to claim 2, characterized in that the inorganic material is bentonite. 4. Method according to any of the preceding claims, characterized in that at least 25% by weight of the cellulose pulp is produced from mechanically derived pulp and/or de-blackened pulp. 5. Method according to any of the preceding claims, characterized in that newsprint or cardboard is produced. 6. Method according to any of the preceding claims, characterized in that the main polymer is a synthetic polymer with an intrinsic viscosity of at least 4 dl/g, or a cationic starch, and that a low molecular water-soluble, synthetic cationic polymer with a lower molecular weight is added to the mass before adding the main polymer. 7. Method according to claim 6, characterized in that the low molecular weight polymer has an intrinsic viscosity below 2 dl/g. 8. Method according to claim 6, characterized in that the low molecular weight polymer has a molecular weight of from 100,000 to 500,000. 9. Method according to any one of claims 6 to 8, characterized in that the low molecular weight polymer is selected from polyethyleneimine, polyamines, polycyandiamide-formaldehyde polymers, amphoteric polymers and polymers of monomers selected from diallyldimethylammonium chloride, diallylaminoalkyl(meth)acrylates and dialkylaminoalkyl (meth)acrylamides. 10. Method according to any one of claims 6 to 9 in the production of cardboard, characterized in that the low molecular weight polymer is an amphoteric dry strength cation resin. 11. Method according to any one of claims 6 to 10, characterized in that the mass to which the low molecular weight polymer is added has a cation requirement, as measured on the main cationic polymer, of at least 400 g/t, and that the amount of low molecular weight polymer is added in such large quantities that the said cation requirement is reduced to below 300 g/t. 12. Method according to any one of claims 6 to 11, characterized in that the mass which is dewatered for the production of paper or cardboard is a thin mass produced by diluting a thick mass, and that the main polymer is added to the thin mass while the low molecular weight polymer is present in the thick mass. 13. Method according to claim 12, characterized in that the mass that is dewatered for the production of paper or cardboard is a thin mass produced by diluting a thicker one, and in that the main polymer is added to the thin mass and that the low molecular weight polymer is added to the thin mass or the thicken the pulp in an amount from 0.01 to 0.5% based on the dry weight of the pulp. 14. Method according to any of the preceding claims, characterized in that the mass which is dewatered for the production of paper or cardboard is produced by diluting an unwatered mass which has been produced by dewatering a cellulose mass containing a dewatering aid, which is a water-soluble, cationic , synthetic, polymeric dewatering aid with a limit viscosity above 4 dl/g.