A PAPER-MAKING METHOD AND A COMBINATION OF INGREDIENTS TO BE USED IN IT

Abstract: The retention properties of an aqueous cellulose fibre pulp and the bonding of the sheet prepared by a paper-making meth­ od from this pulp within a pH range of 4-8 are improved by adding to the pulp, either together or separately, an organic natural or synthetic polymer and a Ti, Zr, Sn or B compound as inorganic oligomer or as a compound which in an aqueous liquid hydrolyses to form such an oligomer.

Description: The present invention relates to making paper by forming a pulp suspension in water, and removing water from the pulp suspension to form a fiber web or sheet. The present invention relates more particularly to such a method in which water is removed from a pulp suspension which contains an organic polymer and an inorganic oligomer. Paper-making methods are known, e.g. from international Application No. PCT/SE 8L/00401, in which water is removed from a pulp suspension which contains, as the organic polymer, a cationic or amphoteric guar gum or a cationic starch, and, as the inorganic oligomer, a colloidal silicic acid. In these known paper-making methods the ratio of the guar gum to the silicic acid, calculated as SiO₂, has been 0.01-25.1 and the ratio of the cation-active starch to the silicic acid has been 1-25;1. These known bonding agent systems are, however, relatively expensive, and they are strongly dependent on the pH. It has been shown experimentally that their action decreases considerably when the pH drops below six. These prior known bonding agent systems also do not yield a good result when paper is made from pulps which contain groundwood. The present invention provides a paper-making method and a bonding agent combination intended for use in the method, by means of which it is possible to make paper having properties at least as good as those obtained by using the above-mentioned known bonding agent systems, and the action of which is not dependent on fluctuations of the pH in the process, or on whether the paper is made using neutral sizing or under acid conditions. The paper-making method and a bonding agent of the present invention also make it possible to make paper from all kinds of pulp, such as groundwood pulp, bleached or unbleached cellulose, and filler-free or filler-containing pulp. Thus, by using the method and bonding agent system of the invention it is possible to make newsprint, SC-quality paper, fine paper, cardboard, liner, bag paper, etc. The bonding agent combination of the present invention is, furthermore, one in which the inorganic oligomer, or the compound forming the oligomer, is a product having an economical price. According to the present invention, paper is made by a method which comprises forming an aqueous cellulose pulp suspension containing a combination of cationic starch and titanyl sulphate at 0.1-15% based on the dry weight of the pulp, and at a pH of 4-8, the weight ratio of the cationic starch and titanyl sulphate being 0.2:1 to 20:1 and dewatering the said suspension to form a fibre web or sheet. The invention also includes an aqueous pulp suspension comprising the aforesaid combination of cationic starch and titanyl sulphate at 0.1 - 15% based on the dry weight of the pulp and having pH of 4 - 8. Thus, it has now surprisingly been observed that, whejn the colloidal silica sol used in the above-mentioned known paper-making processes and bonding agent combinations is replaced by titanyl sulphate, the pH-dependence of the retention decreases substantially and the action of the bonding agent system remains good within a very wide pH range of 4-8. The cationic starch and titanyl sulphate are added to the pulp suspension either together or separately, and in such an amount that the pulp suspension contains the combination of the cationic starch and titanyl sulphate at 0.1-15% of the dry weight of the pulp. The cationic starch and the titanyl sulphate advantageously amount to 0.4-2% of the dry weight of the pulp. In the method according to the present invention, the cationic starch and titanyl sulphate can be added either together or separately, in which case any pulp constituent can, for example, be pretreated with one or both constituents, or the pulp can be treated as a whole. The paper-making method according to the invention is also independent of the order in which the above-mentioned constituents are added, and of the point at which they are added. Thus, the cationic starch and titanyl sulphate can be added, for example, to the circulating water of the paper-making process in order to precipitate the solids present in it. The hydrolysis of titanyl sulphate can take place either entirely afer the catching, or it can be carried out completely or in part in advance, for example by allowing water to react under controlled conditions with the titanyl sulfate. Calculated as TiO₂, the titanyl sulphate is preferably used at 0.1-1.4% of the dry weight of the pulp suspension. By means of the method and constituent combination according to the present invention, a better retention, both filler retention (= ash retention) and overall retention, better dewatering and good forming and high strength are obtained, as compared with former bonding agent systems. The invention is described below in greater detail with reference to the accompanying examples and drawings. Example 1 The strength of the floc formed by a cellulose (degree of grinding 20° SR) treated with one constituent combination according to the invention, titanyl sulfate (TiOSO₄) and a cationic starch, and a filler was evaluated in a dynamic dewatering vessel (Britt Dynamic Jar tester) by varying the rate of rotation of the mixer. The pulp used was pine cellulose, and the filler was kaolin (English China Clay). A compound which hydrolyses in an aqueous solution to an oligomer, i.e. titanyl sulfate, was mixed at about 2.7 percent by weight with a 10-percent (by weight) kaolin slurry half an hour prior to the carrying out of the test. Diluted pulp and kaolin slurry treated in the manner described above were poured into the Britt Jar, which was stirred at a rate of 1500 revolutions per minute. After this, the rate of rotation was adjusted to the desired value. The cationic starch which was used as the organic polymer was added at 10 seconds. The mixture was stirred for another 101 seconds, and the removal of water was started. In all tests, the pH was adjusted to 7, the solids content in the slurry was 0.5%, and the weight ratio of cellulose and kaolin was 50:50. The cationic starch was used at 1% by weight, and titanyl sulfate, calculated as TiO₂, was added at 0.4% of the solids content of the slurry. The control substance was the same cationic starch by itself. The results are shown in Figures 1a and 1b, which depict the ash retention (1a) and total retention of the pulp suspension treated with titanyl sulfate and cationic starch and of the pulp suspension treaded with only a cationic starch, in percent, as a function of the rate of rotation. Example 2 This example compares the pH-dependence of the retention action of titanyl sulfate and silica sol when they were used together with a cationic starch. The pulp used was pine cellulose (degree of grinding 20° SR) and the filler was kaolin. Titanyl sulfate, and respectively silica sol, was mixed as a solution of about 1.5 percent (by weight) with a 10-percent (by weight) kaolin slurry half an hour before the test was started. The pH of the slurry thus obtained and of the cellulose slurry was adjusted to the desired value. The pH was adjusted by using sodium hydroxide or sulfuric acid. The diluted pulp and the kaolin slurry treated in the above manner were poured into a Britt Jar, which was stirred at a rate of 1500 revolutions per minute. The rate of rotation was thereafter adjusted to 900 revolutions per minute. At 10 seconds the cationic starch was added, the stirring was continued for another 10 seconds, and removal of water was started. The solids contents of the slurry to be tested was at all measuring points 0.5 percent by weight, and the weight ratio of cellulose and kaolin was 50:50. The cationic starch was used at 1% by weight, titanium sulfate, calculated as TiO₂, was used at 0.4% by weight, and silica sol, calculated as SiO₂, was used at 0.3% by weight of the solids content of the slurry. Thus, the titanyl sulfate and the silica sol were used in equel molar proportions. The results are shown in Figures 2a and 2b, which depict the ash retention (2a) and total retention (2b), in percent as a function of the pH, of a pulp suspension treated with titany sulfate and a cationic starch, a pulp suspension treated with silical sol and a cationic starch, and a pulp suspension treated with only cationic starch. It can be seen from Figures 2a and 2b that, when titanyl sulfate was used, the improvement of the retention between pH-values of 4 and 7 was almost independent of the pH. The retention of a bonding agent system containing silica sol and a cationic starch, known per se , was strongly dependent on the pH. Example 3 This example illustrates the effect of the adding method on the ash retention of titanyl sulfate and silica sol, as a function of the pH. Method A corresponds to the method presented in Examples 1 and 2. In method B, kaolin, cellulose and a cationic starch were mixed with each other half an hour before the test was carried out. The slurry thus obtained was poured into a tester in which the rate of rotation was 1500 revolutions per minute. Thereafter the rate of rotation was adjusted to 900 revolutions per minute. The mixture was stirred for 10 seconds and the pH was adjusted to the desired value by using sodium hydroxide or sulfuric acid. The titanyl sulfate, and respectively the silica sol, was also added at the same time. After a further stirring of 10 minutes the removal of water was started. The amounts of the constituents used were the same as in Example 2. The results are shown in Figure 3. Figure 3 shows that method B is better when titanyl sulfate is used. Method A, on the other hand is better suited for silica sol. With both method A and method B, a better filler retention is obtained by using titanyl sulfate than by using silica sol. Example 4 The purpose of this example is to describe the effect of the amount of titanyl sulfate on the filler retention. The tests were carried out in the same manner as in Example 3 (methods A and B) at a Ph of 6―7. The amount of titanium sulfate, calculated as TiO₂, was varied between 0.1 and 1.4% of the solids content of the slurry being tested. The results are shown in Figure 4, which depicts the effect of the titanyl sulfate amount ant the adding method on the ash retention. It can be seen that by using adding method A the filler retention does not change significantly when the TiO₂ content is 0.1―0.7% by weight of the solids. In adding method B, the optimum batch, calculated as TiO₂, is 0.2―0.4% by weight of the solids. When large amounts are used, retention clearly deteriorates. Comparative Example 1 This comparative example comparative describes the synergistic effects of various compounds which hydrolyse in water to oligomers, and combinations of the same, on the ash retention, when they were used together with a cationic starch. The experiments were carried out in the manner of Example 2 at a pH of 6―7 , in such a way that part of the titanyl sulfate was replaced by silica sol or zirconium chloride, tin chloride or boric acid. For comparison, the action of each of the above-mentioned compounds separately together with a cationic starch was tested. The results are shown in Figure 5, which depicts the ash retention of the different compounds and compound combinations in percent. The results show that silica sol, zirconium chloride and titanyl sulfate are good retention aids even alone together with a cationic starch, but used together at suitable ratios they have a synergistic action. Tin chloride and boric acid do not, when used alone with a cationic starch, serve as retention aids, but when they are used together with titanyl sulfate the ash retention improves. Example 5 This example describes the effect of titanyl sulfate and silica sol on the rate of dewatering when they were used together with starch. A 50 µm screen was attached to the lower part of a plastic graduated glass having a volume of 500 ml and a diameter of 70 mm. 500 ml of a slurry containing 0.25% by weight kaolin, 0.25% by weight pine-birch cellulose, and a cationic starch 1% by weight of the solids content of the slurry was poured into the tester. The pH of the slurry had been adjusted to 6. Titanyl sulfate or silica sol was added at 0.3% of the solids, the contents were mixed by turning the graduated glass upside down five times within 15 seconds. The bottom bung was opened and the quantity of water which flowed out was measured as a function of the time. The results are shown in Figure 6, and they show that titanyl sulfate improves dewatering better than does silica sol. Example 6 Sheets were prepared in a laboratory sheet mold by batching bleached pine sulfate (degree of grinding 20° SR) 1.7 g and filler kaolin 1.7 g per one sheet, except that at testing points 2 and 3 the batching of kaolin was 3.4 per sheet and 5.1 g per sheet. Both batching method A and method B (cf Example 3) were tested in the batching of the additives. The pH of the pulp suspension at the sheet-making stage was 7-8. At all testing points, with the exception of testing points 1―3, the amount of cationic starch was 1.0%, calculated on the basis of the dry weight of the pulp and the filler. The results are shown in Table 1 below. Comparative Example 2 The working of the combinations of constituents according to the invention was investigated by using the pulp composition of another SC-paper mill: 12% bleached cellulose 48% thermomechanical pulp 40% talcum. The ash retention was measured in accordance with Example 3, by using batching method B. The short-chain polyacrylamides (PAM) were batched in the same way as the cationic starch. The measured pH was 5.5, and the control was a mildy cationic polyacrylamide (PAM) generally used as a retention aid in the making of SC-paper. The results are shown in Table 2, which also shows the combinations of constituents and the amounts of constituents used, indicated in % by weight of the solids content of the slurry. It can be observed that by using the combinations of constituents according to the invention, a considerably better ash retention is achieved than by using mildly cationic, cationic short-chain, strongly cationic short-chain or catonic short-chain PAM, either with or without TiOSO₄. The examination of the results is complicated by the variation of the ash content from one testing point to another. For this reason the bonding strength is shown in Figure 7 as a function of the ash content. The results show that also by using a laboratory sheet mold a better ash retention is obtained by using a cationic starch and titanyl sulfate i.e. higher content of ash by using a certain filler batching, than by using a cationic starch and silica sol. As regards strengths, the systems work in the same manner, and the difference as compared with only starch is slight. Under dynamic conditions starch alone does not, however, work properly as a retention aid, as shown by Examples 1―3. However, each bonding agent system yields a clear improvement over the situation in which no starch at all is used.

Claims: 1. A method of making paper which comprises forming an aqueous cellulose pulp suspension containing a combination of cationic starch and titanyl sulfate at 0.1-15% based on the dry weight of the pulp, and at a pH of 4-8, the weight ratio of the cationic starch and titanyl sulfate being 0.2-20:1, and dewatering the said suspension to form a fibre web or sheet. 2. A method according to claim 1, in which the pulp suspension contains a combination of cationic starch and titanyl sulfate, at 0.4-2% based on the dry weight of the pulp. 3. A method according to claims 1 or 2, in which the pulp suspension also contains an oligomeric Si compound and/or an oligomeric P compound. 4. An aqueous pulp suspension for paper-making comprising a combination of cationic starch and titanyl sulfate at 0.1-15% based on the dry weight of the pulp and having a pH of 4-8, the weight ratio of the cationic starch and titanyl sulfate being 0.2-20:1. 5. A pulp suspension according to claim 4 in which the titanyl sulfate, calculated as TiO₂, comprises 0.1-1.4% of the dry weight of the pulp suspension. 6. A bonding agent combination for addition to an aqueous pulp suspension or to the circulating water of the paper-making process, which combination comprises cationic starch and titanyl sulfate in a weight ratio of cationic starch to titanyl sulfate from 0.2-20:1.

Inventors: JOKINEN, OLLI JUHANI, PETANDER, LARS HARALD, VITRA, PIRKKO JOHANNA
Applicants: KEMIRA OY
Application year: 1984
Main classification: D21H17
IPC codes: D21H17