FI97307B - Process for making paper and cardboard - Google Patents

Description

METHOD OF MANUFACTURE PAPER AND PAPERBOARD

Paper or board is made by making a thick stock, diluting the thick stock to form a dilute stock, filtering that stock to form a sheet, and drying the sheet. The thick stock can be made either by mixing water with the dried pulp or by diluting the filtered pulp in a combined plant.

10 It is common practice to improve the efficiency of a process or the quality of a product by adding various additives in one or more of these steps.

For example, if the pulp from which the thick stock is made is impure, the normal way to prepare it for filtration is to add an inorganic substance such as alum, talc or bentonite in the defibering or thickening steps. These treatments can have the effect of minimizing problems due to resin and other sticky substances.

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Generally, a dry resin *, for example cationic starch, is added to the pulp to be filtered if necessary to improve the strength of the finished sheet.

It is common practice to add cationic polymers to the filtrate to improve the permeability and / or retention V '.

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Methods for improving retention are described in U.S. Patent No. 4,388,150, which relates to the addition of cationic starch and colloidal silicic acid to a stock prior to leaching. Such methods have been commercialized under the trade name "Composil" (trademark).

• · «i *: 35 Methods for improved permeability, retention,; · Drying and formation, are described in EP 235893, and relate to the addition of a synthetic cationic polymer first before the shear step and the addition of bentonite 2 after the shear step. Such methods have been commercialized under the trade name "Hydrocol" (trademark).

Although this method gives very good results in most cases, some stockings, especially impure stockings and some end products such as newsprint and board, have room for improvement.

According to the invention, paper or board is produced by a process comprising forming an aqueous suspension of cellulose of at least 25% by weight of mechanical pulp, passing the suspension through one or more cutting steps selected from cleaning, mixing and pumping steps, adding at least 0.005 p % Of the main polymer, which is a high molecular weight, water-soluble synthetic cationic polymer having an intrinsic viscosity of at least 4 dl / g, before one of the shear steps and after which the bentonite is added, the suspension is allowed to leach to form a sheet and the sheet is dried; and the infiltration is improved by adding to the suspension before,; : addition of main polymer from low molecular weight, water soluble! a synthetic cationic polymer having an intrinsic viscosity of less than 2 dl / g in an amount of 0.01 to 0.2% by weight.

25 The permeability properties obtained by adding the main poly. the sea before the shear step and the bentonite after the shear step »· · * · · * *, can be improved by adding a low molecular weight cationic polymer to the dilute stock before adding the main polymer. Depending on other conditions, it may result in V * 30, for example due to resin and other sticky substances. problems and can lead to improved wet and / or dry strength, improved runnability, dusting, opacity and improved other properties of the paper.

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The aqueous suspension of cellulose can be made either from the dried pulp or in a combined plant by diluting the filtered pulp, all in a conventional manner.

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The intrinsic viscosities of the polymer have been obtained in a standard manner from the determination of solution viscosities with a suspended level viscometer from solutions at 25 ° C in 1 molar NaCl buffered to a pH of about 7 using sodium phosphate.

It is preferred that after the addition of the low molecular weight polymer, the rest of the process be similar to the "Hydro-col" process and thus should otherwise be performed as in EP 235893, using a synthetic cationic polymer having a molecular weight of at least 500,000 and an intrinsic viscosity of more than 4 dl / g, before one of the cutting steps and after the bentonite. The materials and processing conditions described in EP 235893 can be used in the invention of a modification in which the suspension contains a low molecular weight polymer prior to the addition of the main polymer.

Sometimes lower amounts of major polymer than recommended in EP 235893 can give good results in this invention, e.g. amounts less than 300 g / t: e.g. 50 g / t (0.005%) to 250 g / t , in particular more than 100 g / t,; calculated on the dry weight of the stock.

The low molecular weight polymer may be present in the thick. in a stock diluted to form a dilute stock, Γ · * 'or may be added to the dilute stock. For example, a thick stock is usually diluted to make a dilute stock using circulating water. It is desirable to add a low molecular weight polymer immediately before or during dilution with circulating water or to add the main polymer. - 41 «, ** ·. Ri after addition of low molecular weight polymer to the dilute stock->» «.

The molecular weight of the low molecular weight polymer should be sufficiently lower than the molecular weight of the parent polymer to provide a variety of process or performance advantages. In general, the low molecular weight polymer has an intrinsic viscosity 4 of less than 2 dl / g and usually has a molecular weight of less than 500,000. The molecular weight is usually more than 50,000 and often more than 100,000.

The preferred relatively low molecular weight polymer is polyethyleneimine. A suitable grade of this type of polymer is a substance sold under the trade name Polymin SK. Other suitable materials include polymers or copolymers of diallyldimethylammonium chloride, dialkylaminoalkyl (meth) acrylates and dialkylaminoalkyl (meth) acrylamides (both usually in the form of acid addition or quaternary ammonium salts) and polyamide polymers as well as polyamides. Amphoteric synthetic polymers can be used.

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A relatively crude stock containing significant amounts of resin and / or having a high cationic need can be used in the process according to the invention. For example, it may require at least 0.1% Polymin SK to provide improved retention when Polymin SK is used in a conventional manner as a retention aid. Polymin is. : : trademark. Such pulps are those which contain more than 25% by weight, usually more than 50% by weight of mechanically obtained pulps and / or deinked pulps. By mechanically .. 25 pulps we mean groundwood, pressure ground, hot milled, chemical milled or any other • · · mechanically obtained fibers in high yield.

• · · • · When using these relatively raw pulps, the method is particularly useful when the pulp is intended for use in the manufacture of newsprint, and for this purpose the pulp is generally mainly filler-free or ♦ ♦♦ contains only small amounts of filler, for example 0 to 15% and often 0 to 10% based on the dry weight of the stock. However, the advantages are also achieved if the stock contains an amount of filler corresponding to 30% of the amount of filler in the finished paper.

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The method is also useful in the manufacture of paperboard, which also uses similar raw pulps with little or no filler. In these cases, an alternative or additional feature of the low molecular weight polymer may be to improve the strength of the paperboard, and a low molecular weight, water-soluble synthetic cationic dry resin may be used as the polymer for this purpose. Amphoteric polymers are particularly suitable for this purpose.

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The amount of low molecular weight polymer is generally in the range of 0.01 to 0.2% or 0.05 to 0.2% based on the dry weight of the stock, and the optimum can be found by routine experimentation. Prior to treatment with the low molecular weight polymer, the cationic need for the pulp (as measured by titration with the main cationic polymer) is often greater than 400 g / t, and the low molecular weight polymer is added to the pulp or before the pulp in an amount below 300 g / t before the main polymer is added.

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The method according to the invention has been found to provide; Improvements in performance due to improved resin and / or tack removal, improved paper quality such as opacity and dusting properties, improved wet strength or runnability during manufacture. In addition, the performance of the process, when evaluated on the basis of leaching properties, is improved by adding another polymer compared to a process without that polymer, for example to the process described in the EP patent publication. 30 in 235893 or U.S. Patent 4,388,150.

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

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Example 1

A 100% blended waste paper stock with a consistency of 0.5% was prepared. Leaching experiments on the stock were performed using a modified Shopper-Riegler Grinding Degree Assay-5 device to measure the time taken for 600 ml of circulating water to leach from the stock sample being measured. The stock was cut and the permeability was measured. In one experiment, no additions were made before or after the surgical phase. In other experiments, bentonite 10 was added after surgery and polymer A and / or B was added before surgery. If both polymers A and B were added, then B was added well before polymer A. The results are as follows.

15 Table 1

table 1

Polymer B Polymer A Bentonite Leachability 20 0 0 0 74 0 0.04% 0.2% 32: 0.02% 0.04% 0.2% 18 0.04% 0.04% 0.2% 13 ·; 0.04% 0 0.2% 51 25 ------- »*”. Example 2 • · · - - 1 ---

A method similar to that of the previous example was carried out using a pulp containing a large amount of mechanical fiber and more specifically containing a ratio of 50:50 ground and bleached kraft pulp and a consistency of 1.0%. In addition to measuring the infiltration time as in the previous example, a resin count (in particles / ml by the Alien method) was performed. The following results were obtained.

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Table 2

Table 2 5 Polymer Polymer Bento- Leaching- Resin Percentage BA riveting resin reduction 0 0 0 80 5.8 x 106 0 0.025% 0.2% 49 1.7 x 106 70% 10 0.025% 0.025% 0.2% 35 1.2 x 106 79% 0.05% 0.025% 0.2% 31 5.1 x 105 91% These examples clearly show the importance of adding polyethyleneimine (e.g. amount 0.01 to 0.1%, generally about 0 .02 to 0.07%), i.e. it reduces the amount of high molecular weight (e.g. intrinsic viscosity above 4) cationic retention aid required for good leaching and retention and neutralizes the effect of high cationic need and especially high resin stock.

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Example 3 i Newsprint is prepared using pulp with:: a base of 3% kraft pulp, 17% magnetite, 38% hot milling and 42% groundwood, to which 20% scrap has been added. ·; j 25 The high molecular weight polymer is added, in some experiments, just before the last shear step, and the bentonite is added, in some experiments, after the last shear step. The low molecular weight polymer is added to the dilute .. stock soon after it has been diluted from the thick * »· * ... 30 stock.

• · · • · * «: In these experiments, the low molecular weight polymer is polymer K,!" ': Which is a solution polymer with an intrinsic viscosity of about 1 dl / g and consisting of about 20% acrylamide and 80 to 35% by weight. The high molecular weight polymers are L with 70% acrylamide, 30% dimethylaminoethyl acrylate quaternized with methyl chloride, intrinsic viscosity 8, and polymer M with 95% dimethyl acrylamide and 5% methyl acrylate, quaternary acylate.

The infiltration rate of each treated suspension is measured, with the best results being those with the highest infiltration number. The results are as follows.

Table 3

Table 3

Polymer K High Mole Bentonite Leachability 10 bulk polymer 000 205 0.2% 0 0 195 0.2% 0 0.2% 300 15 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 show the advantage obtained in the production of newsprint by adding a high molecular weight polymer immediately before the cutting step and bentonite after cutting, even when the high molecular weight polymer has only a relatively small cationic charge, and those '1' also show that a useful result can be obtained when; · '! A high molecular weight polymer with a molecular weight greater than ΓΓ: 500,000 is replaced by a lower molecular weight polymer. but that the best results are obtained by using a combination of both.

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Claims (8)

A process for making paper or paperboard in which a cellulosic aqueous suspension is formed, of which at least 25% by weight consists of mechanical pulp, the suspension being passed through one or more shear stages selected from the cleaning, mixing and pumping stages to the suspension. is added at least 0.005% by weight of the main polymer with large molar mass, which is a water-soluble synthetic cation polymer, whose intrinsic viscosity is at least 4 dl / g, before one of the shear stages, and after this shear stage, bentonite is added, the suspension is allowed to drain to form the sheet and the sheet is dried , characterized in that the drainage is improved by adding to the suspension before the addition of the main polymer a water-soluble synthetic cationic polymer with a low molar mass, whose intrinsic viscosity is below 2 dl / g, in an amount ranging from 0.01 to 0.2 v. %. 20 Process according to claim 1, characterized in that at least 50% by weight of the cellulose suspension consists of; mechanically recovered pulp. 3. Method according to claim 1 or 2, characterized in that the product is newsprint. * · · T A process according to any one of claims 1-3, characterized in that the molar mass of the polymer with small, molar mass is 100,000 - 500,000. A process according to any one of claims 1-4, characterized in that the polymer having a small molar mass has »> · ·“ '; are selected from the polymers polyethyleneimine, polyamines, polycyanamide-formaldehyde polymers, amphoteric polymers and polymers whose monomers are selected from the group of diallyldimethylammonium chloride, diallylaminoalkyl (meth) acrylates and dialkylaminoalkyl (meth) acrylamides. 6. A process according to any one of claims 1-5, characterized in that the cation requirement, measured by cation main polymer, for the suspension to which the low mole polymer is added is at least 400 g / t and the added small mole polymer. lowers the amount of cation needed below 300 g / h. Process according to any one of claims 1-6, characterized in that the suspension which is allowed to drain 10 for the production of paper or paperboard is a weak stock which has been fattened by diluting a thick stock, and the main polymer is added. tili the weak stock and the polymer with small molar mass are included in the thick stock. 15 8. A process according to any of claims 1-6, characterized in that the suspension which is allowed to drain for the production of paper or paperboard is a weak stock which has been fattened by diluting thick stock, and the main polymer is added to the weak one. the stock and polymer having a small mole mass are added to the weak stock. • · * «·« • * *, V: • * • I • · • · · «· ·» f · • · · «·» '· # • · · i ":