Production of paper and paper board

Abstract: Paper or paper board is made by providing an aqueous cellulosic suspension containing a cationic polymer and then adding cationic starch or a high molecular weight synthetic cationic polymer, subjecting the suspension to shear, and then adding inorganic material selected from bentonite or colloidal silica. The process is of particular value when the suspension is formed from a mechanically derived pulp and/or deinked pulp and when the product is to be newsprint or board.

Description: Paper or paper board is made by providing a thick stock, diluting the thick stock to form a thin stock, draining the thin stock to form a sheet and drying the sheet. The thick stock can be made either by mixing water into dried pulp or, in an integrated mill, by diluting a drained pulp. It is standard practice to improve the process performance, or the product quality, by including various additives at one or more of these stages. For instance, if the pulp from the which the thick stock is made is impure, the normal way of preparing it for drainage is by adding inorganic material, such as alum, talc or bentonite, at the pulping or thick stock stages. These treatments can have the effect of minimising problems due to pitch and other sticky materials. If it is necessary to improve the strength of the final sheet it is common to include a dry strength resin, for instance a cationic starch, in the stock that is to be drained. It is standard practice to include cationic polymers in the stock that is to be drained in order to improve drainage and/or retention. Processes for improving retention are described in US-A-4,388,150 and involve the addition of cationic starch and colloidal silicic acid to the stock before drainage. Such processes have been commercialised under the trade name "Composil" (trade mark). Processes that give improved drainage, retention, drying and formation are described in EP-A-235,893 and involve adding a first synthetic cationic polymer before a shear stage and bentonite after that shear stage. Such processes have been commercialised under the trade name "Hydrocol" (trade mark). Although this process gives very good result in most instances, there is room for improvement with some stocks, especially impure stocks, and for some end products, for instance newsprint and board. In the invention, paper or paper board is made by a process comprising providing a cellulosic suspension of which at least 25% by weight is formed by mechanically derived pulp, subjecting this to one or more shear stages selected from cleaning, mixing and pumping stages, adding a main polymer which is a high molecular weight , water soluble , synthetic cationic polymer having molecular weight above 500,000 and intrinsic viscosity above 4 dl/g, before one of the shear stages and adding bentonite after that shear stage, draining the suspension to form a sheet and drying the sheet, and in this process drainage is improved by adding to the suspension before the main polymer a low molecular weight water soluble synthetic cationic polymer having molecular weight lower than the molecular weight of the main polymer, and having intrinsic viscosity below 2 dl/g in an amount in the range 0.01 to 0.2% based on the dry weight of the suspension. The inclusion of the low molecular weight cationic polymer in the thin stock before addition of the main polymer leads to improvement in the drainage characteristics obtained by the addition of the main polymer before a shear stage and bentonite after that shear stage. Depending upon the other conditions, it can also lead to reduced problems due to pitch and other sticky materials and can lead to improved wet and/or dry strengths, runability, linting, opacity and other paper qualities. The aqueous cellulosic suspension can be made either from dried pulp or, in an integrated mill, by diluting a drained pulp, all in conventional manner. Intrinsic viscosities herein derived in standard manner from determination of solution viscosities by suspended level viscometer of solutions at 25°C in 1 Molar NaCl buffered to pH about 7 using sodium phosphate. It is preferred that the remainder of the process, after adding the low molecular weight polymer, should be similar to the "Hydrocol" process and, thus, should be otherwise conducted as in EP-A-235,893, using a synthetic cationic polymer having molecular weight at least 500,000 and IV above 4 dl/g before one of the shear stages and bentonite after. The materials and processing conditions described in EP-A-235,893 can be used in the invention, subject to the modification that the suspension includes the low molecular weight polymer before addition of the main polymer. Sometimes lower amounts of the main polymer than are recommended in EP-A-235,893 can give good results in the present invention, for instance amounts of less than 300 g/t eg 50 g/t (0.005%) to 250 g/t, especially above 100 g/t based on the dry weight of the stock. The low molecular weight polymer can be present in the thick stock that is diluted to form the thin stock or it may be added to the thin stock. For instance generally the thick stock is diluted to form the thin stock by use of white water. It is desirable to add the low molecular weight polymer before, or immediately after or during, the dilution with white water and to add the main polymer to the thin stock, after the addition of the low molecular weight polymer. The low molecular weight polymer should have a molecular weight sufficiently lower than the molecular weight of the main polymer that it will provide different process or performance benefits. The low molecular weight polymer has intrinsic viscosity below 2 dl/g and usually has 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 polyethylene imine. A suitable grade of this type of polymer is the material sold under the trade name Polymin SK. Other suitable materials are polymers and copolymers of dially dimethyl ammonium chloride, of dialkylaminoalkyl (meth) acrylates and of dialkylaminoalkyl (meth) acrylamides (both generally as acid addition or quaternary ammonium salts), as well as polyamines and polydicyandiamides-formaldehyde polymers. Amphoteric synthetic polymers may be used. The process according to the invention utilises a relatively crude stock containing significant amounts of pitch and/or having high cationic demand. For instance it may require at least 0.1% Polymin SK to give improved retention when the Polymin SK is used in conventional manner as retention aid. Polymin is a trade mark. Such stocks are those containing more than 25% by weight, usually more than 50% by weight, of mechanically derived pulps and/or deinked pulps. By mechanically derived pulps we mean groundwood, pressure refined groundwood, thermo-mechanical, chemi-thermo mechanical or any other high yield mechanically derived fibres. When using these relatively crude pulps, the process is of particular value when the stock is to be used for the manufacture of newsprint, and for this purpose stock is generally substantially unfilled or only contains small amounts of filler, for instance 0 to 15% and often 0 to 10% based on the dry weight of the stock. Benefits are however also achieved if the stock contains filler in amounts to give up to 30% filler in the final paper produced. The process is also of value in the manufacture of board, again from similar crude pulps containing little or no filler. In these instances an alternative or additional property of the low molecular weight polymer may be to improve the strength of the board and for this purpose a low molecular weight water soluble synthetic cationic dry strength resin may be used as the polymer. Amphoteric polymers are particularly suitable for this purpose. The amount of low molecular weight polymer is in the range 0.01 or 0.05 to 0.2%, based on the dry weight of the stock, and the optimum can be found by routine experimentation. Often the pulp, before treatment with the low molecular weight polymer, has a cationic demand (as measured by titration with the main cationic polymer) of above 400 g/t and the low molecular weight polymer is included in the stock, or ahead of the stock, in an amount to reduce the cationic demand of the thin stock to below 300 g/t before adding the main polymer. The process of the invention is found to give an improvement in the performance since it can give improved pitch and/or stickies removal, improved paper quality such as opacity and linting characteristics improved wet strength or runnability during manufacture. Furthermore the performance of the process when assessed in terms of the drainage characteristics is improved by the incorporation of the second polymer, as compared to a process without that polymer, for instance a process is described in EP-A-235,893 or US-A-4,388,150. In the following examples, Polymer A is a polymer of IV 7 dl/g formed from 75% acrylamide and 25% dimethylaminoethyl acrylate, MeCl quaternised, and Polymer B is a modified polyethyleneimine as sold under the trade name Polymin SK. Example 1 A 100% mixed waste stock having a consistency of 0.5% was prepared. Drainage tests were conducted on the stock using a modified Shopper Riegler freeness tester, the time for 600 mls of backwater to drain from the stock sample being measured. The stock was subjected to shear and the drainage was measured. In one test no additions were made before or after the shear. In other tests bentonite was added after the shear and Polymer A and/or Polymer B was added before the shear. When both Polymers A and B were added, B was added considerably ahead of Polymer A. The results are as follows: Polymer B Polymer A Bentonite Drainage 0 0 0 74 0 0.04% 0.02% 32 0.02% 0.04% 0.02% 18 0.04% 0.04% 0.02% 13 0.04% 0 0.02% 51 Example 2 A process similar to the preceding example was conducted using a stock having a high mechanical fibre content, and in particular being a 50:50 groundwood:bleached kraft pulp having a consistency of 1.0%. In addition to measuring the drainage time as in the previous example, a pitch count was made (in particles/ml by the Allen method). The following results were obtained. Polymer B Polymer A Bentonite Drainage Pitch Count Percentage Pitch Reduction 0 0 0 80 5.8 x 10 6 0 0.025% 0.2% 49 1.7 x 10 6 70% 0.025% 0.025% 0.2% 35 1.2 x 10 6 79% 0.05% 0.025% 0.2% 31 5.1 x 10 5 91% These examples clearly demonstrate the value of adding, for instance 0.01 to 0.1%, generally around 0.02 to 0.07%, polyethylene imine so as to reduce the amount of high molecular weight (for instance IV above 4) cationic retention aid that is required for good drainage and retention and so as to counteract the effect of stock having high cationic demand and, high pitch count. Example 3 Newsprint is made using a stock based on 3% kraft, 17% magnefite, 38% thermomechanical pulp and 42% groundwood, and to which 20% broke has been added. High molecular weight polymer is added, in some tests, just before the last shear stage and bentonite is added, in some tests, after the last shear stage. Low molecular weight polymer is added to the thin stock soon after it is diluted from the thick stock. In these tests the low molecular weight polymer is polymer K which is a solution polymer of about IV 1 dl/g and formed from about 20% acrylamide and 80% by weight diallyl dimethyl ammonium chloride. The high molecular weight polymers are L, which is 70% acrylamide, 30% methyl chloride quaternised dimethylaminoethyl acrylate IV 8, and polymer M which is 95% acrylamide and 5% methyl chloride quaternised dimethylaminoethyl acrylate IV 11. The drainage rate for each of the treated suspensions is measured, with the best results being those that have the highest drainage figure. The results are as follows: Polymer K High MW Polymer Bentonite Drainage 0 0 0 205 0.2% 0 0 195 0.2% 0 0.2% 300 0.2% 0.05%L 0.2% 335 0.2% 0.05%M 0.2% 340 0 0.05%M 0.2% 325 These results clearly demonstrate the benefit in the manufacture of newsprint from adding high molecular weight cationic polymer immediately before shear and bentonite after shear even when the high molecular weight polymer only has a relatively low cationic charge, and they also show that a useful result can be obtained when the high molecular weight polymer is replaced by a low molecular weight polymer having molecular weight above 500,000, but that best results are obtained using a combination of both.

Claims: A process in which paper or paper board is made by forming an aqueous cellulosic suspension of which at least 25% by weight is formed from mechanically derived pulp, passing the suspension through one or more shear stages selected from cleaning, mixing and pumping stages, adding a main polymer which is a high molecular weight, water soluble, synthetic cationic polymer having intrinsic viscosity above 4 dl/g to the suspension before one of the shear stages and adding bentonite after that shear stage, draining the suspension to form a sheet and drying the sheet, characterised in that drainage is improved by adding to the suspension before the addition of the main polymer, a low molecular weight water soluble synthetic cationic polymer having intrinsic viscosity below 2 dl/g in an amount in the range 0.01 to 0.2% based on the dry weight of the suspension. A process according to any preceding claim in which at least 50% by weight of the cellulosic suspension is formed from mechanically derived pulp. A process according to either preceding claim in which the product is newsprint. A process according to any preceding claim in which the low molecular weight polymer has molecular weight 100,000 to 500,000. A process according to any preceding claim in which the low molecular weight polymer is selected from polyethylene imine, polyamines, polycyandiamide formaldehyde polymers, amphoteric polymers, and polymers of monomers selected from diallyl dimethyl ammonium chloride, dialkylaminoalkyl (meth) acrylates and dialkylaminoalkyl (meth) acrylamides. A process according to any preceding claim in which the suspension to which the low molecular weight polymer is added has a cationic demand, as measured on the main cationic polymer, of at least 400 g/t and the amount of low molecular weight polymer that is added reduces the said cationic demand to below 300 g/t. A process according to any preceding claim in which the suspension that is drained to form the paper or paper board is a thin stock formed by dilution of a thick stock and the main polymer is added to the thin stock and the low molecular weight polymer is present in the thick stock.

Inventors: LANGLEY JOHN GRAHAM, LANGLEY, JOHN GRAHAM
Applicants: CIBA SPEC CHEM WATER TREAT LTD, CIBA SPECIALTY CHEMICALS WATER TREATMENTS LIMITED
Application year: 1989
Main classification: D21H23
IPC codes: D21H23, D21H17, D21H11, D21H21