Method for producing paper, board or the like

Abstract

The invention relates to a method for producing paper, board or the like. The method comprises obtaining a thick stock comprising cellulose fibres, forming a fibrous web and drying of drying the fibrous web. A strength agent system, which comprises at least one synthetic or natural polymer, or their mixture, and microfibrillated parenchymal cellulose material originating from non-wood sources, is added to the thick stock. The invention relates also to paper or board produced by the method.

Claims

1. A method for producing paper, board or the like, which comprises obtaining a thick stock comprising cellulose fibres, forming a fibrous web, drying the fibrous web, wherein a strength agent system, which comprises at least one synthetic or natural polymer, or their mixture, and microfibrillated parenchymal cellulose material originating from non-wood sources is added to the thick stock.

2. The method according to claim 1, wherein the microfibrillated parenchymal cellulose material originates from vegetables, such as sugar beet pulp, potato pulp, cassava pulp, sweet potato; fruits, such as citrus peel; cellulose pith, such as bagasse pith, corn pith, bamboo pith; and mixtures thereof.

3. The method according to claim 1, wherein the microfibrillated parenchymal cellulose material is obtained by mechanical treatment, such as homogenization, without preceding treatment steps, which are selected from oxidation, extraction and/or washing.

4. The method according to claim 1, wherein the microfibrillated parenchymal cellulose material has a Brookfield viscosity of at least 10 Pas, preferably at least 100 Pas, measured at concentration of 1.0 weigh-% at 100 rpm shear rate, and/or a turbidity value less than 1000 NTU, preferably in the range of 100-700 NTU, measured at 0.1 weight-% concentration, and/or a net charge in the range of 0.001-1.99 meq/g, preferably 0.01-1.50 meq/g, more preferably 0.05-1.0 meq/g, at pH 7.5.

5. The method according to claim 1, wherein the Brookfield viscosity of the microfibrillated parenchymal cellulose material is in the range of 500-2000 Pas, preferably 800-1400 Pas, more preferably 900-1300 Pas.

6. The method according to claim 1, wherein the turbidity of the microfibrillated parenchymal cellulose material may be in the range 375-525 NTU, preferably 490-510 NTU.

7. The method according to claim 1, wherein the strength agent system is added in such amount that the microfibrillated parenchymal cellulose material is added in amount of 1-100 kg, preferably 5-70 kg, more preferably 10-50 kg, even more preferably 15-50 kg, given as dry per ton of dry solids of the fibre stock.

8. The method according to claim 1, wherein the strength agent system comprises 50-99.9 weight-%, preferably 70-99.9 weight-%, more preferably 80-99.5 weight-%, even more preferably 85-90 weight-%, of microfibrillated parenchymal cellulose material, calculated from the dry solids content.

9. The method according to claim 1, wherein that the strength agent system comprises a natural polymer, which is selected from a group consisting of cationic or amphoteric starch, chitin, guargum, carboxymethyl cellulose, and any mixture thereof.

10. The method according to claim 1, wherein the strength agent is a synthetic polymer selected from cationic polyacrylamide (C-PAM), glyoxalated polyacrylamide (G-PAM), amphoteric polyacrylamide, polydiallyldimethylammonium chloride (poly-DADMAC), polyacrylic amide (PAAE), polyvinyl amine (PVAm), polyethylene oxide (PEO), polyethyleneimine (PEI) or a mixture of two or more of these polymers.

11. The method according to claim 10, wherein the average molecular weight of the synthetic polymer is in the range 100 000-20 000 000 g/mol, typically 300 000-8 000 000 g/mol, more typically 300 000-1 500 000 g/mol.

12. The method according to claim 10, wherein the synthetic polymer is a cationic synthetic polymer which has a charge density of 0.5-2 meq/g, preferably 0.3-1.9 meq/g, more preferably 0.4-1.35 meq/g.

13. The method according to claim 10, wherein the synthetic polymer is a cationic synthetic polymer which is a copolymer of methacrylamide or acrylamide and at least one cationic monomer.

14. The method according to claim 1, wherein a cationic retention polymer is added to the stock after the addition of the strength agent system.

15. The method according to claim 14, wherein the cationic retention polymer is cationic polyacrylamide having an average molecular weight of 4 000 000-18 000 000 Da, preferably 4 000 000-12 000 000 Da, more preferably 7 000 000-10 000 000 Da, and/or having a charge density of 0.2-2.5 meq/g, preferably 0.5-1.5 meq/g, more preferably 0.7-1.2 meq/g.

16. The method according to claim 1, wherein the stock comprising cellulose fibres is obtained by using furnish from kraft pulping, chemithermomechanical pulping or by repulping recycled fibres.

17. Use of microfibrillated material originating from non-wood sources as strength agent in manufacture of paper, board or the like.

18. A paper or board product, having a density in the range of 150-800 kg/m.sup.3, produced by a method according to claim 1.

Description

EXPERIMENTAL

[0063] Some embodiments of the present invention are described in the following non-limiting examples.

Example 1

Production of Fibrillated Parenchymal Cellulose Based on Potato Pulp or on Sugar Beet Pulp

Batch Extraction Followed by Fibrillation:

[0064] Concentrated potato pulp from a starch factory was purified in a water or lye wash. Here, the potato pulp was taken to a 25 g/L suspension and heated to 60-90 C. Alternatively compressed sugar beet pulp with 26 weight-% dry content from a sugar factory was first washed in lye. Here, pulp was taken to a 25 dry g/L suspension and heated to 70-80 C. For both potato and sugar beet NaOH was added thereafter with gentle stirring. During this time, the hydrated potato/sugar beet clippings lost their solid-like morphology and broke down into a dark brown viscous mass. After 120 minutes of stirring, the reaction was cooled down and optionally filtrated through a steel screen with 0.25 mm pore size. The lye-washed pale grey cellulosic potato mass was further washed with copious amounts of water. The obtained material was dispersed into water and fibrillated using a high speed grinder or homogenizer at pH 8-10. The raw material, extraction parameters and fibrillation method for different samples were as follows:

[0065] RefMFC: Wood based MFC produced via homogenization, used as reference and produced according to WO 2015/166141.

[0066] MFC 35: The sample was neutral extracted dried potato, extracted at a constant pH of 7. The sample was fibrillated with Atrex, 4 pass at 1800 rpm, after washing the product with copious amounts of water. The sample contains 4 ppm biocide.

[0067] MFC 37: The sample was base extracted dried potato, extracted at a constant NaOH concentration of 0.05 M. The sample was fibrillated with Atrex, 4 pass at 1800 rpm, after washing the product with copious amounts of water. The sample contains 4 ppm biocide.

[0068] MFC 38: The sample was base extracted dried potato, extracted at a constant NaOH concentration of 0.05 M. The sample was fibrillated with Atrex, 4 pass at 1800 rpm, after washing the product with copious amounts of water. The sample contains 4 ppm biocide.

[0069] MFC 41: The sample was base extracted pelleted sugar beet, extracted at 0.05 M NaOH (4% solids). The sample was fibrillated with Atrex, 4 pass at 1800 rpm. The sample contains 200 ppm of Fennocide BZ26 as a biocide. Solids content 3.7%.

[0070] MFC 42: The sample was base extracted potato fibre, extracted at 0.05 M NaOH (4% solids). The sample was fibrillated with Atrex, 4 pass at 1800 rpm. The sample contains 200 ppm of Fennocide BZ26 as a biocide. Solids content 3.4%.

Continuous Counter Current Extraction Followed by Fibrillation:

[0071] MFC 43: The sample was base extracted fresh sugar beet clippings, extracted by using a counter current reactor. The extraction was done by adding the clippings as they were received (24%). This sample was extracted using a steady flow of 0.3 M NaOH. The sample was then washed with hot water, again, using the counter current reactor. After the hot water wash, the sample pH was 10 and solids content was roughly 10%. The sample was fibrillated with Atrex, 4 pass at 1800 rpm. After fibrillation (4 pass), the solids content was roughly 20%. The sample contains 200 ppm of Fennocide BZ26 as a biocide. Solids content 20.5%.

[0072] MFC 46: The sample was base extracted fresh sugar beet clippings, extracted by using a counter current reactor. The extraction was done by adding the clippings as they were received (24%). This sample was extracted using a steady flow of 0.5 M NaOH. The sample was then washed with hot water, again, using the counter current reactor. After the hot water wash, the sample pH was 10 and solids content was roughly 10%. The sample was fibrillated with Atrex, 4 pass at 1800 rpm. After fibrillation (4 pass), the solids content was roughly 20%. The sample contains 200 ppm of Fennocide BZ26 as a biocide. Solids content 19.3%.

[0073] The raw material information, extraction parameters, and fibrillation methods are summarized in Table 1.

[0074] The resulting microfibrillated parenchymal cellulose material was characterized using turbidity and viscosity measurements, and the characteristic values are summarised in Table 2.

TABLE-US-00001 TABLE 1 Summary of the raw material information, extraction parameters, and fibrillation methods. NaOH conc. Extraction Sample Raw material/form (mol/l) method Fibrillation method MFC 35 Sugar beet pulp/ 0 Batch Atrex 4 passages, dry pellet 3.6% MFC 37 Potato pulp/dry 0.05 Batch Atrex 4 passages, powder 3.6% MFC 38 Potato pulp/dry 0.05 Batch Atrex 4 passages, powder 3.6% MFC 41 Sugar beet pulp/ 0.05 Batch Atrex 4 passages, dry pellet 3.6% MFC 42 Potato pulp/dry 0.05 Batch Atrex 4 passages, powder 3.6% MFC 43 Sugar beet pulp/ 0.3 Contin- Atrex 4 passages, Fresh cutting uous 10% MFC 46 Sugar beet pulp/ 0.5 Contin- Atrex 4 passages, Fresh cutting uous 10%

TABLE-US-00002 TABLE 2 Summary of the turbidity and viscosity results. Turbidity Viscosity 50 rpm Viscosity 100 rpm Sample (NTU) (Pas) (Pas) MFC 35 188 115 MFC 37 175 MFC 38 249 309 198 MFC 41 402 648 421 MFC 42 189 955 610 MFC 43 229 262 172 MFC 46 268 442 260

[0075] Transmission electron microscopy was used to evaluate dimensions of a typical product. The microfibrillated parenchymal cellulose material based on potato pulp is shown in FIG. 1 and the microfibrillated parenchymal cellulose material based on sugar beet pulp is shown in FIG. 2.

Example 2

Characterization Methods

[0076] The viscosity of the microfibrillated parenchymal cellulose material was measured by Brookfield DV3T viscosimeter (Brookfield Engineering Laboratories, Middleboro, USA) equipped with a vane geometry (V-72, diameter 21.67 mm, length 43.38 mm). The material was diluted with water to a concentration of 1.0 weight-%, and agitated for 10 min before degassing in vacuum to remove the entrapped air bubbles. The temperature was adjusted to 20 C. prior to viscosity measurement. The viscosity of the samples was measured at 50 and 100 rpm shear rates.

[0077] Turbidity of dilute aqueous dispersions of microfibrillated parenchymal cellulose material was measured with HACH P2100 turbidimeter. The material was diluted with water to a concentration of 0.1 weight-%, and agitated for 10 min before degassing in vacuum to remove the entrapped air bubbles. The temperature was adjusted to 20 C. prior to the turbidity measurement where the emission of light scattered from particles of a sample was detected.

Example 3

Strength Compostions for OCC and Kraft Pulps

[0078] Three different samples of microfibrillated parenchymal cellulose material were tested as component for strength compositions comprising also synthetic strength aid polymers with OCC (Old Corrugated Cardboard) and Kraft pulps. Both Dynamic and Rapid Kothen sheet formers were used.

TABLE-US-00003 TABLE 3 Strength and retention polymers used in the tests. Abbreviation Tradename Supplier Purpose Chem1 Fennobond 3300 Kemira Oyj Strength aid Chem2 Fennobond 46 Kemira Oyj Strength aid Chem3 Fennopol K3400P Kemira Oyj Retention chemical Chem4 Fennopol K4230P Kemira Oyj Retention chemical Chem5 Fennosil 320 Kemira Oyj Retention chemical

[0079] Dynamic sheet former tests were performed as follows:

[0080] Sheets of paper or board with a grammage of approximately 100 g/m.sup.2 were made using a Dynamic Sheer Former (A.D.F supplied by Techpap SAS, France) with the wire speed 1400 m/s. Sheet dimension was 2592 cm and dryness of the sheet was 10-15%. Pulp suspension was formed in a mixing chest and microfibrillated cellulose material was added. Suspension was mixed for 30 seconds after which other chemicals were added in 10 s intervals. Retention polymers, Chem4 with dosage of 400 g/ton and Chem5 with dosage of 320 g/ton, were added last, 20 s and 10 s before the start of sheet making. The obtained sheet was couched on a roll press at 2 bar, pressed between blotting paper for 5 minutes at 10 bar in a plate press. Thereafter the sheet was dried restrained in a STFI dryer at 140 C. for 9 minutes. The sheets were conditioned in a climate room according to ISO 187: 1990.

[0081] The sheet properties were measured according to the standards listed in Table 4.

TABLE-US-00004 TABLE 4 Tested sheet properties. Measured sheet property Standard Unit Grammage ISO 536: 2012 g/m.sup.2 Thickness ISO 534: 2011 m Density ISO 534: 2011 g/cm.sup.3 Bulk ISO 534: 2011 cm.sup.3/g Air Permeance Bendtsen ISO 5636-3: 2013 ml/min method Tensile Index CD ISO 1924-3: 2005 Nm/g Tensile Energy Absorption ISO 1924-3: 2005 Nm/g Index CD Z-strength ISO 15754: 2009 KPa Burst strength ISO 2759: 2014 KPa Short-Span Compressive ISO 9895: 2008 Nm/g Test (SCT)

[0082] The obtained results are summarized in Tables 5 and 6. Dosage of Chem1 is given as kg/ton.

TABLE-US-00005 TABLE 5 Results for furnish comprising OCC pulp. Tensile energy Formulation, Dose of Chem1 Tensile Index absorption Air Addition Order [kg/ton] Bulk Density Grammage (CD) SCT (CD) Burst (kPa) index (CD) J/kg permeability KP1 Chem4 + Chem5 1.56 0.64 90.06 18.48 1.28 209.142 257.73 177.00 Ref. KP3 Chem1 0.5 1.59 0.63 91.47 18.91 1.39 220.489 277.30 186.17 KP4 Chem1 0.9 1.57 0.64 91.87 19.72 1.43 225.947 266.87 166.67 KP5 Chem1 1.25 1.56 0.64 91.23 20.10 1.50 238.602 279.03 152.33 KP6 MFC42 + Chem1 0.1 1.55 0.64 92.39 20.34 1.47 220.347 258.20 113.00 KP7 MFC42 + Chem1 0.3 1.56 0.64 91.10 20.58 1.44 229.513 266.41 115.17 KP8 MFC42 + Chem1 0.5 1.55 0.65 92.67 21.02 1.50 241.478 275.32 113.17 KP9 MFC41 + Chem1 0.5 1.54 0.65 93.72 20.42 1.53 233.097 267.52 105.58 KP10 MFC41 + Chem1 1 1.53 0.65 93.13 20.78 1.55 240.025 257.65 100.27 KP11 MFC41 + Chem1 1.5 1.53 0.66 95.53 21.24 1.61 244.868 283.32 90.78 KP12 MFC43 + Chem1 1 1.53 0.65 93.29 20.25 1.54 244.878 327.02 118.67 KP13 MFC43 + Chem1 1.5 1.54 0.65 93.54 22.42 1.63 250.261 304.74 110.83 KP14 MFC43 + Chem1 2 1.53 0.65 93.32 22.65 1.63 246.572 356.67 104.28

TABLE-US-00006 TABLE 6 Results for furnish comprising Kraft pulp. Tensile energy Formulation Dose of Chem1 Tensile Index absortion Air Addition Order [kg/ton] Bulk Density Grammage (CD) SCT (CD) Burst (kPa) index (CD) J/kg permeability KP1 Chem4 + Chem5 1.44 0.69 96.79 43.97 2.52 526.931 706.67 206.33 Ref. KP3 Chem1 0.5 1.44 0.69 98.26 47.66 2.71 557.644 890.10 172.67 KP4 Chem1 0.9 1.42 0.71 100.97 50.67 2.82 586.580 943.34 149.50 KP5 Chem1 1.25 1.43 0.70 100.36 51.26 2.84 614.527 957.00 159.17 KP6 MFC42 + Chem1 0.1 1.44 0.70 99.99 48.82 2.77 598.092 832.06 129.33 KP7 MFC42 + Chem1 0.3 1.43 0.70 99.43 50.59 2.71 611.063 909.70 112.95 KP8 MFC42 + Chem1 0.5 1.43 0.70 98.86 51.30 2.75 590.799 945.06 123.92 KP9 MFC41 + Chem1 0.5 1.38 0.73 102.34 48.76 2.90 612.385 909.53 91.88 KP10 MFC41 + Chem1 1 1.39 0.72 101.99 49.23 2.98 605.866 866.32 95.97 KP11 MFC41 + Chem1 1.5 1.39 0.72 101.76 51.24 3.06 640.495 878.52 82.70 KP12 MFC43 + Chem1 1 1.40 0.71 101.09 51.31 2.99 599.390 870.42 89.32 KP13 MFC43 + Chem1 1.5 1.40 0.72 103.24 51.74 2.82 626.951 964.08 96.30 KP14 MFC43 + Chem1 2 1.43 0.70 100.96 57.15 3.14 645.812 1113.72 71.68

[0083] Rapid Kothen sheet forming tests were performed as follows:

[0084] Hand sheets were formed with Rapid Kothen sheet former according to standard ISO 5269/2. The pulp suspension was stirred at a constant stirring rate. Microfibrillated parenchymal cellulose material, dose 20 kg/t, and a synthetic cationic polymer was added into the suspension. Stirring of furnish was performed at 1000 rpm with propeller mixer. All sheets were dried in vacuum dryer 5 min at 1000 mbar pressure and at 92 C. temperature. After drying the sheets were pre-conditioned for 24 h at 23 C. in 50% relative humidity. Sheet basis weight was 100 g/m.sup.2 in air conditioned state. Basis weight was adjusted by retention polymer dosage 100 g/t for kraft pulp and 300 g/t for OCC, to keep the retention constant.

[0085] The results are summarized in Tables 7 and 8.

TABLE-US-00007 TABLE 7 Rapid Kothen sheet test results for OCC furnish. Formulation, Tensile energy Addition Chem1/Chem2 Tensile Burst absortion index Air Order dosage Bulk Density Grammage Index Z-strength SCT (kPa) J/kg permeability KP1 Chem3 1.66 0.60 97.38 26.74 586.4 1.89 157.370 319.84 359.00 Ref. KP2 Chem1 + 1.5 1.63 0.61 99.88 27.84 633.0 1.99 172.038 343.56 382.50 Ref. Chem3 KP3 Chem1 + 3 1.64 0.61 101.34 29.33 644.3 2.13 170.767 399.92 364.24 Ref. Chem3 KP6 Chem2 + 1.5 1.60 0.63 100.64 27.85 619.7 2.03 175.741 365.72 290.50 Ref. Chem3 KP7 Chem2 + 3 1.61 0.62 99.67 29.30 636.7 2.14 174.800 389.81 295.50 Ref. Chem3 KP8 Chem1 + 1.5 1.60 0.62 102.18 30.85 608.0 2.16 180.983 436.65 202.75 MFC41 + Chem3 KP9 Chem1 + 3 1.65 0.61 102.60 29.85 663.6 2.24 193.854 396.37 214.75 MFC41 + Chem3 KP12 Chem2 + 1.5 1.62 0.62 99.91 28.38 627.8 2.11 183.433 386.70 200.75 MFC41 + Chem3 KP13 Chem2 + 3 1.59 0.63 99.54 29.34 638.5 2.18 188.313 381.51 191.75 MFC41 + Chem3 KP14 Chem1 + 1.5 1.62 0.62 101.94 28.99 626.2 2.24 186.599 392.86 213.75 MFC42 + Chem3 KP15 Chem1 + 3 1.60 0.62 103.43 30.94 688.2 2.29 188.590 440.73 227.25 MFC42 + Chem.3 KP18 Chem2 + 1.5 1.59 0.63 100.07 29.63 636.3 2.09 179.823 419.41 200.75 MFC42 + Chem3 KP19 Chem2 + 3 1.60 0.63 100.23 29.88 658.1 2.17 195.299 390.23 197.50 MFC42 + Chem3 KP20 MFC41 + 1.5 1.59 0.63 101.91 28.91 636.5 2.14 186.014 370.07 219.00 Chem1 + Chem3 KP21 MFC41 + 3 1.59 0.63 98.49 30.69 675.1 2.11 201.887 450.35 247.50 Chem1 + Chem3

TABLE-US-00008 TABLE 8 Rapid Kothen sheet test results for Kraft furnish. Formulation, Tensile energy Addition Chem1/Chem2 absortion Air Order dosage Bulk Density Grammage Tensile Index Z-strength SCT Burst (kPa) index J/kg permeability KP1 Chem3 1.61 0.62 97.52 69.77 778.7 3.43 559.077 1236.288 469.00 KP2 Chem1 + 1.5 1.54 0.65 98.40 74.61 839.5 3.50 564.575 1392.813 528.50 Chem3 KP3 Chem1 + 3 1.55 0.64 98.85 81.68 843.9 3.75 576.244 1646.625 651.25 Chem3 KP6 Chem2 + 1.5 1.54 0.65 100.94 72.22 788.0 3.57 602.963 1267.589 438.00 Chem3 KP7 Chem2 + 3 1.52 0.66 101.28 77.35 802.7 3.52 583.539 1532.619 486.50 Chem3 KP8 Chem1 + 1.5 1.54 0.65 103.18 70.41 793.5 3.73 589.173 1228.992 319.75 MFC42 + Chem 3 KP9 Chem1 + 3 1.56 0.64 102.33 74.73 802.2 3.45 612.693 1488.168 373.75 MFC42 + Chem3 KP12 Chem2 + 1.5 1.53 0.66 99.77 75.57 811.4 3.55 571.593 1487.542 326.00 MFC42 + Chem3 KP13 Chem2 + 3 1.52 0.66 100.59 80.75 831.8 3.67 612.522 1627.861 316.00 MFC42 + Chem3 KP14 MFC42 + 1.5 1.53 0.65 97.39 76.82 852.7 3.64 563.274 1526.204 342.00 Chem1 Chem3 KP15 MFC42 + 3 1.54 0.65 98.61 79.80 3.64 615.129 1561.072 417.50 Chem1 Chem3

Example 4

Performance of the Strength Composition Comprising Microfibrillated Parenchymal Cellulose Material

[0086] Performance of strength composition comprising microfibrillated parenchymal cellulose material was compared to a reference strength composition comprising wood based microfibrillated cellulose. Hand sheets were formed according to Rapid Kothen procedure, as described above and using OCC furnish. Amount of cationic polymer (Chem1, Chem2) in the strength composition was 3 kg/t pulp. Amount of retention polymer (Chem3) was 300 g/t pulp.

[0087] The results are summarised in Table 9.

TABLE-US-00009 TABLE 9 Results of Example 4. Formulation, Air Test No Addition Order Bulk Density Grammage Tensile Index Z-strength SCT Burst Ash permeability 1 Chem3 1.66 0.60 99.92 26.04 548.2 1.86 236.72 17.4 369.8 2 Chem1 + 1.65 0.60 101.62 27.43 583.1 2.01 259.20 17.6 437.3 Chem3 3 Chem2 + 1.65 0.61 99.98 28.49 593.0 2.03 267.99 17.2 342.3 Chem3 4 Chem1 + 1.67 0.60 101.43 30.58 595.1 2.17 305.14 16.6 268.0 RefMFC + Chem3 5 Chem2 + 1.69 0.59 99.93 31.08 622.9 2.14 301.80 16.5 257.5 RefMFC + Chem3 6 Chem1 + 1.66 0.60 98.30 31.63 624.0 2.16 320.99 17.6 235.8 MFC38 + Chem3 7 Chem2 + 1.61 0.62 100.77 32.17 645.9 2.24 313.75 17.0 197.5 MFC38 + Chem3

[0088] From these results it can be seen that strength composition according to the present invention and comprising microfibrillated parenchymal cellulose material gave higher enhancement especially in Z-strength and burst values compared to the reference.

Example 5

Performance of the Strength Composition Comprising Microfibrillated Parenchymal Cellulose Material

[0089] Performance of a strength composition comprising cationic synthetic polymer and microfibrillated parenchymal cellulose material was tested in pilot paper machine using a dry recycled fibre furnish. The strength system comprised microfibrillated parenchymal cellulose added in amount of 20 kg/ton dry pulp and cationic polymer Chem2, as defined above, added in amount of 3 kg/ton dry pulp.

[0090] The stock used for the test was prepared as follows: ca. 75 kg dry paper was slushed into 3 m.sup.3 local tap water, temperature 8-10 C.

[0091] Targeted white water conditions were: conductivity: 8000 mS/cm, calcium level 800 ppm Ca.sup.2+, charge level: 1500 meq/1.

[0092] The anionic trash levels in stock were adjusted by addition of calcium propionate (Caldic, Espoo, Finland), Na.sub.2SO.sub.4 (Algol, Espoo, Finland) and anionic carboxmethyl cellulose Staflo Exlo (Akzo Nobel, Gothenburg, Sweden). These additions were made to the pulp as follows: first 3.72 g/l of calcium propionate is added to reach conductivity of 3500 mS/cm, followed by addition of 4.67 g/l of Na.sub.2SO.sub.4 and 3.5 mg/l of carboxymethyl cellulose, in this order.

[0093] The prepared paper stock was subjected to mixing, pumping and a mild refining (resembling a deflaking step) at 2.25 weigh-% feed consistency in an Esser-Wysch 13A refiner, 4.4 kWh/t.

[0094] The stock was fed to a pilot paper machine and diluted to headbox consistency with white water. The volume of the white water was ca 8.5 m.sup.3 per pulped batch as follows: pulper 2 m.sup.3; four mixing chests 5 m.sup.3; white water circulation ca. 1.5 m.sup.3.

[0095] The produced stock was formed into a web on the wire, speed 30 m/min, while passing four vacuum boxes, one vacuum roll and two nips, 50 MPa and 70 MPa and 11 drying cylinders having average effect of 120 kW at a production rate of ca. 90 kg/h.

[0096] The results of Example 5 are summarized in Table 10.

TABLE-US-00010 TABLE 10 Strength enhancement measured in %. CHEM 2 MFC CHEM 2 + MFC Tensile index, GM, % 2 6 8 TEA index, GM, % 4 14 20 SCT index, GM, % 2 6 7 Burst index, % 0 5 7 CMT30 index MD, % 5 0 2 Z-strength, % 5 12 11

[0097] Although certain embodiments and examples have been described in detail above, those having ordinary skill in the art will clearly understand that many modifications are possible in the embodiments and examples without departing from the teachings thereof. All such modifications are intended to be encompassed within the below claims of the invention.