Method for increasing the strength properties of a paper or board product

Abstract

A method is disclosed for increasing strength properties, preferably burst strength and SCT strength, of a paper or board product. The paper or board product is manufactured from a fibrous web produced by a multilayer headbox, where an aqueous layer is formed between at least a first and a second fibre layer formed from fibrous stock suspension(s), and where feed water for the aqueous layer includes at least one cationic polymer. The method of the invention includes adding an anionic additive selected from a group comprising anionic synthetic organic polymers, anionic polysaccharides, and any of their combinations to the feed water before formation of the aqueous layer.

Claims

1. A method for increasing strength properties of a paper or board product, manufactured from a fibrous web produced by a multilayer headbox, where an aqueous layer is formed between at least a first and a second fibre layer formed from fibrous stock suspension(s), and where feed water for the aqueous layer comprises at least one cationic polymer, and wherein the method comprises adding an anionic additive selected from the group consisting of copolymers of acrylamide and anionic monomers, a carboxymethyl cellulose having a degree of carboxymethyl substitution in a range of 0.4-1.2, and any of their combinations to the feed water before formation of the aqueous layer.

2. The method according to claim 1, wherein the feed water further comprises a cellulosic fibre material selected from unrefined cellulosic fibres, refined cellulosic fibres, microfibrillated cellulose fibrils and/or nanocellulose fibrils.

3. The method according to claim 2, wherein the feed water comprises 1-15 weight % of cellulosic fibre material, based on produced paper or board product.

4. The method according to claim 1, wherein a consistency of the feed water is lower than a consistency of the fibrous suspension(s) forming the first and the second fibre layer.

5. The method according to claim 1, wherein the anionic additive has a weight average molecular weight >100 000 g/mol, or a charge density of −0.05-−5 meq/g dry polymer, or both.

6. The method according to claim 1, wherein the anionic additive is a carboxymethylated cellulose having: a charge density value below −1.1 meq/g dry polymer, measured at pH 7, or viscosity in a range of 30-30 000 mPas, measured from 2 weight-% aqueous solution at 25° C. by using Brookfield LV DV1, or both.

7. The method according to claim 1, wherein the copolymer of acrylamide has a weight average molecular weight <5 000 000 g/mol.

8. The method according to claim 1, wherein the copolymer of acrylamide has an anionicity in a range of 2-70 mol %.

9. The method according to claim 1, wherein the anionic additive is added in an amount maintaining a sum of added charges from cationic polymer(s) and anionic additive net cationic.

10. The method according to claim 1, wherein the anionic additive is added separately from the at least one cationic polymer.

11. The method according to claim 1, wherein the at least one cationic polymer is cationic starch or cationic synthetic strength polymer.

12. The method according to claim 1, wherein the at least one cationic polymer is cationic starch added in an amount of 3-20 kg/t.

13. The method according to claim 1, further comprising adding one or more auxiliary agents to the feed water, the one or more auxiliary agents being selected from alum or anionic microparticles.

14. The method according to claim 13, wherein the anionic microparticles are anionic silica microparticles or bentonite microparticles.

15. The method according to claim 1, wherein the at least first and/or the second layer comprises anionic microparticles and cationic synthetic flocculant.

16. The method according to claim 1, wherein the at least first and/or the second layer comprises recycled cellulosic fibres.

Description

EXPERIMENTAL

(1) An embodiment of the invention is described more closely in the following non-limiting example.

Example 1

(2) Technical performance of anionic additive in a water layer of a multilayer headbox together with cationic starch was tested with a pilot paper machine and by using recycled furnish. Characteristics of paper testing devices and methods which were employed are given in Table 1. Chemicals used in Example 1 are described in Table 2.

(3) The furnish used in the pilot paper machine trial comprised refined recycled fibres at top layer and unrefined recycled fibres at back layer and refined recycled fibres to water layer.

(4) TABLE-US-00001 TABLE 1 Paper testing devices and standards used in Example 1. Measurement Device Standard Basis weight Mettler Toledo ISO 536 Short compression test, Lorenzen & Wettre ISO 9895 SCT Burst strength Lorenzen & Wettre ISO 2758 Tensile strength Lorenzen & Wettre ISO 1924-3 (geometrical)

(5) TABLE-US-00002 TABLE 2 Chemicals used in Example 1 Composition/Product, Abbreviation Manufacturer Description Starch Cationic starch, cooked cationic charge density 0.27 meq/g dry APAM Copolymer of acrylamide and 8 mol-% anionic, MW acrylic acid, Kemira Oyj ~0.5 Mg/mol Finland CPAM Cationic polyacrylamide Dry polymer dissolved at flocculant, HMW, Kemira 0.5% concentration Oyj Finland Silica Colloidal silica sol/FennoSil Anionic colloidal silica sol 5000, Kemira Oyj Finland

(6) In the pilot paper machine trial chemicals were added in following dosing points: Refined recycled fibres to water layer before feed pump, starch to water layer before feed pump just after the addition of refined recycled fibres, APAM to water layer after feed pump, retention CPAM to top and back ply furnish before screen and colloidal silica to top and back ply furnish after screen.

(7) Before testing the paper samples, sheets were pre-conditioned for 24 h at 23° C. in 50% relative humidity, according to ISO 187.

(8) Test points and indexed strength results are presented in Table 3. Results showed that APAM dosed together with cationic starch and fibers in water layer improved clearly the strength properties of the recycled paperboard. Especially it was found that APAM is able to provide a local maximum for both SCT strength and tensile strength with improved burst strength. SCT and burst strength are the main strength specifications for recycled board.

(9) TABLE-US-00003 TABLE 3 Test points and indexed strength results. Dosages as dry. All points include CPAM 300 g/t and colloidal silica 450 g/t in top and back ply. SCT Burst Tensile Test point Index CD index index Refined recycled fibres 7%, Starch 12 kg/t 100 100 100 Refined recycled fibres 7%, Starch 12 kg/t, 105.8 107.1 103.9 APAM 0.4 kg/t Refined recycled fibres 7%, Starch 12 kg/t, 110.4 113.8 106.8 APAM 0.8 kg/t Refined recycled fibres 7%, Starch 12 kg/t, 101.7 116 101.8 APAM 1.2 kg/t

Example 2

(10) Technical performance of anionic additive in an aqueous layer of a multilayer headbox together with cationic starch was tested with a dynamic handsheet former. The test furnish was recycled fiber made from European testliner board sheets.

(11) Test fibre stock was made to simulate recycled fibre. Central European testliner board, having ash content about 15% and comprising about 5% surface size starch, was used as raw material. Dilution water was prepared from tap water, where Ca.sup.2+ concentration was adjusted to 520 mg/l with CaCl.sub.2), and conductivity to 4 mS/cm with NaCl. Testliner board was cut to 2×2 cm squares. 2.7 l of dilution water was heated to 70° C. The testliner squares were wetted for 10 minutes in dilution water at 2% concentration before disintegration in Britt jar disintegrator with 30 000 rotations.

(12) Disintegrated pulp was diluted to 0.8% consistency for first and second fibre layers by adding dilution water.

(13) A part of the disintegrated pulp, which was intended to be used in water layer, was further refined in Valley Hollander at 1.75% consistency until refining degree SR 60 was reached. Water layer was obtained by dilution of refined fibres to 0.4% consistency by using dilution water.

(14) Consistency determinations were made according to SCAN-M1:64 standard using ashless white ribbon filter paper Whatman 589/2 in the Büchner funnel.

(15) Chemicals used in Example 2 and their preparation are described in Table 4.

(16) TABLE-US-00004 TABLE 4 Test chemicals for Example 2 and their preparation. Chemical Composition, name Product name, Supplier Properties Preparation C-Starch Cationic potato starch 30 min cooking at 97° C. cationic substitution DS 0.035 at 1% concentration A-starch Anionic starch Brookfield LV 30 min cooking at 97° C. anionic substitution DS 0.015 DVI viscosity at 1% concentration (about −0.07 meq/g) 7800 mPas at 2%, 25° C. CPAM-2 10 mol-% cationic 6M g/mol Dissolving at 0.5% conc. polyacrylamide molecular 60 min, dilution to 0.05% weight conc. Silica-2 Structured silica 8% dry solids Diluted to 0.5% FennoSil 2180, Kemira CMC-1 CMC, Brookfield LV dissolved in 50° C. at 1% DS 0.7 (about −4 meq/g), DVI viscosity conc. for 60 min 300 000 g/mol 190 mPas at 2% 25° C. CMC-2 CMC Brookfield LV dissolved in 50° C. at 1% DS 0.4 (about −2 meq/g), DVI viscosity conc. for 60 min. 400 000 g/mol 16 000 mPas at 2% 25° C. APAM anionic polyacrylamide, 8 mol- diluted to 1% conc. % acrylic acid, ~500 000 g/mol

(17) Test fibre stock was added to dynamic hand sheet former Formette by Techpap. Chemical additions were made to mixing tank of Formette according to Table 5. All chemical amounts are given as kg dry chemical per ton dry fibre stock. Drum was operated with 1000 rpm, mixer for pulp 400 rpm, pulp pump 1100 rpm/min, all the pulps were sprayed.

(18) First fibre layer of 47 g/m.sup.2 (back ply) was formed first. Then water layer with 6 g/m.sup.2 of refined recycled pulp (SR 60) was formed, and finally was formed the second fibre layer of 47 g/m.sup.2 (top ply). All the water was drained at the end. Scoop time was 60 s. Sheet was removed from drum between wire and 1 blotting paper on the other side of the sheet. Wetted blotting paper and wire were removed. Sheets were wet pressed at Techpap nip press with 4.5 bar pressure with 2 passes having new blotting paper each side of the sheet before each pass. Sheets were cut to 15×20 cm rectangles. Sheets were dried in restrained condition in STFI restrained dryers 10 min at 130° C.

(19) TABLE-US-00005 TABLE 5 Chemical additions in Example 2. Layer top/back top/back water layer water layer water layer water layer water layer Time [s] −20 −10 −40 −30 −15 −15 −15 Chemical Test CPAM-2 Silica-2 Pulp, SR 60 C-Starch APAM CMC-2 CMC-1 No [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] 1 (ref.) 0.1 0.15 60 2 (ref.) 0.1 0.15 60 20 3 0.1 0.15 60 20 3 4 0.1 0.15 60 20 1.5 5 0.1 0.15 60 20 3 6 0.1 0.15 60 20 1.5 7 0.1 0.15 60 20 3

(20) Before testing in the laboratory sheets were pre-conditioned for 24 h at 23° C. in 50% relative humidity, according to ISO 187. Basis weight was measured according to ISO 536 and bulk according to ISO 534. Z-directional tensile (ZDT) was measured according to ISO 15754. Short span compression strength (SCT) was measured in cross direction (CD) according to ISO 9895. Bursting strength (Burst) was measured according to Tappi T 569. SCT and burst were indexed by dividing strength value by basis weight of the sheet.

(21) The test results are presented in Table 6. Test 1 is a comparative example without strength agents, and Test 2 is a comparative example with cationic starch but without anionic additive. Test 3 with APAM as anionic additive shows improvement in Z-directional tensile, in burst and in SCT values. Tests 4-7 with CMC as anionic additive indicate improvement in burst and SCT values. The Z-directional tensile is dependent on bulk. For multi-ply board improving the ratio of Z-directional strength to bulk is important. The ratio was improved in Tests 3-7 compared to comparative Tests 1-2. Test 5 improved the bulk when Z-directional tensile was constant compared to Test 1.

(22) TABLE-US-00006 TABLE 6 Test results for Example 2. Bulk Z-directional tensile Burst index SCT (CD) index Test [cm.sup.3/g] [kPa] [kPam.sup.2/g] [Nm/g] 1 1.8 500 2.27 15.5 2 1.8 540 2.38 16.5 3 1.8 550 2.43 16.6 4 1.8 590 2.45 17.1 5 2.0 500 2.73 22.9 6 1.9 600 2.38 17.2 7 1.8 620 2.49 16.9

Example 3

(23) Technical performance of anionic additive in a water layer of a multilayer headbox together with cationic starch was tested with a dynamic handsheet former. Central European testliner board, having ash content about 17% and comprising about 5% surface size starch, was used as raw material for furnish.

(24) The anionic additive was anionic starch A-starch, see Table 4. The example 3 was carried out with similar procedure than in Example 2, but conductivity was adjusted to 3 mS/cm. The chemical additions, sheet ash and SCT (CD) strength results are presented in Table 7. It is seen that test 9 and test 10, which are according to invention, resulted good SCT-strength and improved ash retention to the sheet.

(25) Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.

(26) TABLE-US-00007 TABLE 7 The chemical additions and results of Example 3. Layer top/back top/back water layer water layer water layer Time [s] −20 −10 −40 −30 −15 Ash SCT (CD) CPAM-2 Silica-2 Pulp, SR 60 C-Starch A-Starch 525° C. index Test [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] [%] [Nm/g] 8 0.1 0.15 60 12.4 13.6 9 0.1 0.15 60 15 1.75 13.7 13.9 10 0.1 0.15 60 15 2.75 14.2 15.5