Dry strength composition, its use and method for making of paper, board or the like

Abstract

A dry strength composition for manufacture of paper, board or the like is disclosed. The dry strength composition includes, as a mixture, at least one anionically derivatized polysaccharide, and cationic starch having an amylopectin content ≥80 weight-%. The anionically derivatized polysaccharide and the cationic starch provide the composition with a charge density in a range of 0.1-1.5 meq/g, when measured at pH 2.8, and −0.1-−3 meq/g, preferably −0.3-−2.5 meq/g, more preferably −0.5-−2.0 meq/g, when measured as an aqueous solution, at pH 7.0. Further disclosed are a use of the composition and a method for manufacturing paper, board or the like.

Claims

1. A dry strength composition for manufacture of paper, board or the like, which comprises, as a mixture: at least one anionically derivatized polysaccharide, and cationic starch having an amylopectin content ≥80 weight-%, wherein the anionically derivatized polysaccharide and the cationic starch provide the composition with a charge density in a range of: 0.1-1.5 meq/g, when measured at pH 2.8, and −0.1-−3 meq/g, preferably −0.3-−2.5 meq/g, more preferably −0.5-−2.0 meq/g, when measured as an aqueous solution, at pH 7.0.

2. The composition according to claim 1, wherein the anionically derivatized polysaccharide comprises anionically derivatized celluloses, anionically derivatized starches, or any combinations thereof, preferably the anionically derivatized polysaccharide comprises carboxymethylated cellulose.

3. The composition according to claim 2, wherein the anionically derivatized polysaccharide comprises carboxymethylated cellulose, which has: a degree of carboxymethyl substitution >0.2, preferably 0.3-1.2, more preferably 0.4-1.0, even more preferably 0.5-0.9, and/or a charge density value <−1.1 meq/g, preferably in a range of −1.6-−4.7 meq/g, more preferably −2.1-−4.1 meq/g, even more preferably −2.5-−3.8 meq/g, when measured at pH 7, and/or viscosity in a range of 100-30 000 mPas, preferably 200-20 000 mPas, more preferably 500-10 000 mPas, measured from a 2 weight-% aqueous solution at 25° C., by using a Brookfield LV DV1, and/or ash content <35 weight-% of dry material, preferably <30 weight-%, more preferably <25 weight-%, at 525° C., 4 h.

4. The composition according to claim 1, wherein the anionically derivatized polysaccharide comprises anionic microfibrillar cellulose.

5. The composition according to claim 1, wherein the cationic starch has: an amylopectin content ≥85 weight-%, preferably ≥90 weight-%, more preferably ≥95 weight-%, and/or a substitution degree of 0.025-0.3, preferably 0.03-0.16, more preferably 0.045-0.1.

6. The composition according to claim 1, wherein the dry strength composition comprises the anionically derivatized polysaccharide and the cationic starch in weight ratio (dry/dry) of 10:90-90:10, preferably 30:70-70:30, more preferably 40:60-60:40.

7. The composition according to claim 1, wherein the dry strength composition is in form of a dry particulate material.

8. The composition according to claim 1, wherein the dry strength composition is in form of an aqueous solution, preferably having a viscosity of <10 000 mPas, preferably <8000 mPas, more preferably <6000 mPas, at solids content of 2 weight-% and at pH 7.0, at 25° C., measured by using a Brookfield LV DV1.

9. Use of a dry strength composition according to claim 1 for improving strength properties of a paper, board or the like.

10. A method for manufacturing of paper, board or the like, comprising: obtaining a fibre stock comprising cellulosic fibres, adding a cationic coagulant and/or a cationic strength agent to the fibre stock, and introducing to the fibre stock a dry strength composition comprising: at least one anionically derivatized polysaccharide, and cationic starch having an amylopectin content ≥80 weight-%, wherein the anionically derivatized polysaccharide and the cationic starch provide the composition with a charge density in a range of: 0.1-1.5 meq/g, when measured at pH 2.8, and −0.1-−3 meq/g, preferably −0.3-−2.5 meq/g, more preferably −0.5-−2.0 meq/g, when measured as an aqueous solution, at pH 7.0, and optionally, introducing a retention aid to the fibre stock.

11. The method according to claim 10, wherein the dry strength composition is introduced to the fibre stock by adding the at least one anionically derivatized polysaccharide and the cationic starch as aqueous solutions separately but simultaneously.

12. The method according to claim 10, wherein the dry strength composition is introduced to the fibre stock through a single inlet to which separate aqueous solutions of the at least one anionically derivatized polysaccharide and the cationic starch are fed.

13. The method according to claim 10, wherein the dry strength composition is introduced to the fibre stock as an aqueous mixture, which comprises the at least one anionically derivatized polysaccharide and the cationic starch.

14. The method according to claim 10, wherein the dry strength composition comprising the at least one anionically derivatized polysaccharide and the cationic starch is in form of a dry particulate mixture, wherein the dry mixture is dissolved into water in order to obtain an aqueous dry strength composition, the aqueous dry strength composition is optionally diluted, and after the optional dilution the aqueous dry strength composition is introduced to the fibre stock.

15. The method according to claim 10, wherein the cationic strength agent, preferably the cationic starch, is added to the fibre stock before introduction of the dry strength composition.

16. The method according to claim 10, wherein addition of the cationic coagulant and/or the cationic strength agent increases original zeta potential value of the fibre stock to a first zeta potential value, which is in a range of −15-+15 mV, preferably −10-+10 mV.

17. The method according to claim 10, wherein the introduction of the dry strength composition, which comprises the at least one anionically derivatized polysaccharide and the cationic starch, decreases the obtained first zeta potential value by 1.5-10 mV, preferably by 2-5 mV.

18. The method according to claim 10, wherein the dry strength composition is introduced to thick stock.

19. The method according to claim 10, wherein the fibre stock has a pH value at least 4.5, preferably at least 5, and the dry strength composition has an anionic net charge at the pH of the fibre stock.

Description

EXPERIMENTAL

[0083] Chemicals and Measurement Methods Used in the Examples

[0084] Following methods were used in the examples for analysing the characteristics of aqueous polymer/polysaccharide solutions: [0085] Dry solids content was analysed by using Mettler Toledo HR73, at 150° C. [0086] Viscosity was analysed by using Brookfield LV DV1, equipped with small sample adapter, at 25° C., using spindle S18 for solutions with viscosity <500 mPas and spindle S31 for solutions with viscosity 500 mPas or higher. The highest feasible rotation speed for the spindle was used. [0087] pH of the solution was analysed by using a calibrated pH-meter. [0088] Charge density was determined at pH 7.0 by charge titration, using polyethylene sulfonate solution as titrant and using Mütek PCD-03 for end point detection. pH of the polymer solution was adjusted to pH 7.0 with 10 weight-% aqueous sodium hydroxide solution or with 10 weight-% aqueous sulphuric acid solution before the charge density determination. [0089] Ash content (525° C.) was measured by using standard ISO 1762, 4 h.

[0090] Preparation of Polysaccharide Solutions by Using Carboxymethylcellulose, Sodium Salt Products (CMC-Na)

[0091] A number of different carboxymethylcellulose, sodium salt products, CMC1-CMC5, were dissolved in water by mixing with a mechanical mixer, 700 rpm, for 3 h at 23° C. Characteristics of the products are given in Table 1.

TABLE-US-00001 TABLE 1 Characteristics of CMC-Na products CMC-Na Ash content, Dry content Viscosity Charge density, product [%] [%] [mPas] pH [meq/g dry] CMC1 19 2.0 270 6.8 −3.7 CMC2 19 2.0 640 6.4 −3.9 CMC3 19 2.0 3050 6.8 −3.9 CMC4* 19 1.0 360 7.0 −3.9 1.9 6360 7.0 −3.9 CMC5 28 1.9 7710 6.9 −1.8 *CMC4 viscosity measured at two dry content levels, providing different viscosity values. The pH and charge density remain the same irrespective of the solids content.

[0092] Preparation of Cationic Starch, Starch-A

[0093] 171 g cationic waxy potato starch, Starch-A, dry content 82 weight-%, was suspended in 829 g of water in a reactor equipped with a jacket for heating, a condenser and agitator. Slurry was heated to 98° C. while agitating at 500 rpm. It was kept at that temperature for 45 min with constant agitation on. The formed starch solution, when cooled, had concentration of 14.5 weight-%, pH of 8.3, viscosity of 1200 mPas and charge density (at pH 7.0) of 0.43 meq/g dry material.

[0094] Names, compositions and short description of the properties of the chemicals used in the examples are given in Table 2.

TABLE-US-00002 TABLE 2 Chemicals used in the examples. Name Composition/Product Description APAM-1 Copolymer of acrylamide and MW ca. 0.5 Mg/mol 8 mol-% acrylic acid, anionic Starch-A Cationic waxy potato starch charge density 0.4 meq/g; DS 0.07; amylopectin content >95%; cooked Starch Cationic potato starch charge density 0.2 meq/g; DS 0.035; amylopectin content 80%; cooked at 97° C. for 30 min, at 1% concentration SCPAM Copolymer of acrylamide and Solution polymer, MW ca. 10 mol-% ADAM-Cl, cationic 0.8 Mg/mol CPAM Copolymer of acrylamide and MW 7 000 000 g/mol, dry 10 mol-% ADAM-Cl, cationic polymer dissolved at 0.5% concentration CMC1 CMC-Na, anionic dissolved at 80° C. for 2 h CMC3 CMC-Na, anionic dissolved at 80° C. for 2 h CMC4 CMC-Na, anionic dissolved at 80° C. for 2 h CMC5 CMC-Na, anionic disintegrated at 80° C. for 2 h

[0095] Preparation of Dry Strength Composition

[0096] A series of aqueous dry strength compositions were prepared using the following general procedure.

[0097] Dry strength compositions with different proportions of polysaccharide (CMC, Na-salt) and cationic starch (Starch-A), different dry content and different pH value were prepared using dissolved starch solution and dissolved polysaccharide solution, prepared as described above. Dry strength compositions with low dry content were prepared by dilution with de-ionized water.

[0098] Dry strength compositions were prepared, and their properties were measured, as given in Table 3. All percentages and values are calculated and given per dry material.

APPLICATION EXAMPLES

[0099] Examples 1-8 were performed for providing information about the behaviour and effect of different dry strength compositions. Tables 4 and 5 give methods and standards used for pulp characterisation and sheet testing in the Examples.

TABLE-US-00003 TABLE 3 Characteristics of dry strength compositions. Charge density by Dry Dry Mütek strength Starch A CMC-Na content Viscosity at pH 7.0 composition CMC-Na [wt-%] [wt-%] [%] [mPas] pH [meq/g] Comp-A CMC1 50 50 3.4 550 7.2 −1.7 Comp-B CMC1 65 35 3.4 700 7.4 −1.0 Comp-C CMC4 50 50 1.8 680 7.2 −1.7 Comp-D CMC4 65 35 1.8 490 7.3 −1.1 Comp-E CMC2 58 42 4.0 3100 6.8 −1.4 Comp-F CMC3 58 42 2.9 2890 7.2 −1.4 Comp-G CMC3 65 35 4.6 16620 7.2 −1.1 Comp-H CMC4 58 42 1.6 480 7.3 −1.3 Comp-I CMC5 60 40 4.0 high viscosity 7.2 not determined Comp-J CMC5 50 50 3.4 26000 7.2 −0.7 Comp-K CMC5 40 60 3.0 12600 7.0 −0.9

TABLE-US-00004 TABLE 4 Pulp characterization methods. Property Device/Standard pH Knick Portamess 911 Turbidity (NTU) WTW Turb 555IR Conductivity (mS/cm) Knick Portamess 911 Charge (μeq/l) Mütek PCD 03 Zeta potential (mV) Mütek SZP-06 Consistency (g/l) ISO 4119

TABLE-US-00005 TABLE 5 Sheet testing devices and standard methods used for produced paper sheets. Measurement Device Standard Basis weight Mettler Toledo ISO 536 Ash content, 525° C. — ISO 1762 Scott bond Huygen Tappi T 569 Z-directional tensile (ZDT) Lorentzen & Wettre ISO 15754 Taber, bending stiffness Lorentzen & Wettre Tappi T 489 om-08 Tensile strength, elastic modulus Lorentzen & Wettre ISO 1924-3 Bulk Lorentzen & Wettre ISO 534 Short span compression test Lorentzen & Wettre ISO 9895 (SCT)

Example 1

[0100] Example 1 simulates preparation of tissue paper, fine paper, kraft paper or surface layer for multi-ply board.

[0101] Test fibre stock was chemical hardwood pulp, which was bleached birch kraft pulp refined at 2% consistency to 25° Shopper Riegler (° SR) in Valley Hollander. Pulp was diluted with deionized water, which conductivity was adjusted to 1.5 mS/cm level by addition of NaCl.

[0102] In handsheet preparation the used chemicals were added to the test fibre stock in a dynamic drainage jar (DDJ) under mixing, 1000 rpm. Cationic strength chemicals were diluted before dosing to 0.2 weight-% concentration. Anionic strength chemicals and retention chemical were diluted to 0.05 weight-% concentration before dosing. The used strength chemicals and their addition times are given in Table 6. In addition to the strength chemicals the retention chemical, CPAM (see Table 2), was dosed at dosage of 0.03 kg/t 10 s prior to sheet making. All chemical amounts are given as kg dry active chemical per ton dry fibre stock.

[0103] Handsheets having basis weight of 80 g/m.sup.2 were formed by using Rapid Kothen sheet former with 1.5 mS/cm conductivity in backwater, adjusted with NaCl, in accordance with ISO 5269-2:2012. The handsheets were dried in vacuum dryers for 6 minutes at 92° C., at 1000 mbar. Before testing the handsheets were pre-conditioned for 24 h at 23° C. in 50% relative humidity, according to ISO 187.

[0104] Results for Example 1 are also presented in Table 6. It is seen that dry strength compositions Comp-A, Comp-B and Comp-C are providing improved tensile index and elastic modulus values in comparison to results achieved in reference Test 2 using only starch as cationic strength agent. Needed CMC addition for same strength result is less with the new anionic composition according to the invention compared to CMC alone used in test 8. Excess amount of CMC may not retain to the sheet and can therefore cause additional cationic demand in the water circulation. This risk may be minimised with the present invention. Furthermore, an elastic modulus improvement may be accomplished, which is important in order to achieve good bending stiffness for multi-ply board.

TABLE-US-00006 TABLE 6 Hand sheet tests of Example 1: chemical additions and measured results. Time [s] −60 −30 −30 −30 −30 Elastic modulus Tensile Bulk Test Starch Comp-A Comp-B Comp-C CMC1 [Gpa] index [cm.sup.3/g] 1 (ref.) — — — — — 5.7 55 1.38 2 (ref.) 15 — — — — 5.9 75 1.35 3 15 0.6 — — — 6.1 81 1.35 4 15 1.2 — — — 6.2 82 1.33 5 15 2.4 — — — 6.3 84 1.33 6 15 — 2.4 — — 6.3 86 1.34 7 15 — — 2.4 — 6.4 85 1.33 8 (ref.) 15 — — — 1.2 6.2 81 1.33

Example 2

[0105] This example simulates preparation middle ply of multi-ply board, such as folding box board or liquid packaging board. Test sheets were made with Formette-dynamic hand sheet former manufactured by Techpap.

[0106] Test fibre stock was made from 80% of bleached dried CTMP having Canadian standard Freeness (CSF) of 580 ml and from 20% of dry base paper broke from manufacture of folding box board. Test pulp was disintegrated according to ISO 5263:1995, at 80° C. Test fibre stock was diluted to 0.6% consistency with deionized water, pH was adjusted to 7, and NaCl salt was added to obtain conductivity of 1.5 mS/cm.

[0107] Pulp mixture was added to Formette. Chemical additions were made to mixing tank of Formette according to Table 7. All chemical amounts are given as kg dry chemical per ton dry fibre stock. Water was drained out after all the pulp was sprayed. Drum was operated with 1400 rpm, mixer for pulp 400 rpm, pulp pump 1100 rpm/min, number of sweeps 100 and 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 5 bar pressure with 2 passes having new blotting paper each side of the sheet before each pass. Dry content was determined from the pressed sheet by weighting part of the sheet and drying the part in oven for 4 hours at 110° C. Sheets were dried in restrained condition in drum dryer. Drum temperature was adjusted to 92° C. and passing time to 1 min. Two passes were made. First pass with between blotting papers and second pass without. Before testing in the laboratory sheets were pre-conditioned for 24 h at 23° C. in 50 relative humidity, according to ISO 187.

[0108] Table 7 presents the test program and handsheet results. Z-directional tensile for Tests 2-4 and 2-5 show that the results were improved with the addition of dry strength compositions Comp-G and Comp-H compared to the addition of cationic starch alone, even at high dosages (Tests 2-2, 2-3). Elastic modulus was also improved in MD and CD direction when dry strength compositions according to the invention were used. Bulk was not reduced compared to Test 2-1. In general, a common challenge in production of multi-plyboard is to improve z-directional strength without losing bulk significantly. It seems that this problem can be effectively solved with the dry strength compositions according to the invention comprising anionically derivatized polysaccharide and cationic starch.

TABLE-US-00007 TABLE 7 Dynamic hand sheet test program and results. Time [s] −60 −30 −30 pressing dryness ZDT E-mod CD E-mod MD bulk Test Starch Comp-G Comp-H [%] [kPa] [Gpa] [Gpa] [cm.sup.3/g] 2-1 (ref.) — — — 37 146 0.22 2.3 3.2 2-2 (ref.) 5 — — 39 186 0.22 2.3 3.3 2-3 (ref.) 15 — — 35 225 0.23 2.4 3.4 2-4 15 2.4 — 41 279 0.26 2.6 3.2 2-5 15 — 2.4 36 279 0.26 2.5 3.3

Example 3

[0109] This example simulates preparation of middle ply of multi-ply board, such as folding box board or liquid packaging board. Test sheets were made with Rapid Kothen hand sheet former.

[0110] Test fibre stock was made from 90% CTMP and 10% hardwood pulp. CTMP was bleached dried CTMP having CSF of 580 ml. CTMP was disintegrated according to ISO 5263:1995, at 80° C. Hardwood (HW) pulp was bleached birch kraft pulp refined at 2% consistency to 25° SR in Valley Hollander. Test fibre stock was diluted to 0.6% consistency with deionized water, pH was adjusted to 7, and NaCl salt was added to obtain conductivity of 1.5 mS/cm.

[0111] In handsheet preparation chemicals were added to the test fibre stock in a dynamic drainage jar under mixing with 1000 rpm. Cationic strength chemicals were diluted before dosing to 0.2 weight-% concentration. Anionic strength chemicals and retention chemicals were diluted to 0.05 weight-% concentration before dosing. The strength chemicals added and their addition times are given in Table 8. The retention chemical CPAM (see Table 2) was dosed 0.03 kg/t 10 s prior to sheet making. All chemical amounts are given as kg dry chemical per ton dry fibre stock.

[0112] Handsheets having basis weight of 80 g/m.sup.2 were formed in the same manner as in Example 1.

[0113] Results for Example 3 are shown in Table 8. It can be seen that strength compositions Comp-A, Comp-B, Comp-C and Comp-D provide improved Z-directional tensile (ZTD) and Scott bond values in comparison to that what is gained with reference test 3-1, where cationic starch alone was used as cationic strength agent. Compositions Comp-A, Comp-B, Comp-C and Comp-D provided also better Z-directional tensile and Scott bond values than CMC1 alone in Test 3-14 at same dosage level of 2.4 kg/t. In general, bulk of the produced sheets is typically decreasing with increased strength properties when bonds are generated between the fibres. From the results of Table 8 it can be seen, however, that the reduction of bulk remained low, clearly below 5%, when compositions according to the invention were used.

TABLE-US-00008 TABLE 8 Handsheet tests of Example 3: chemical additions and measured results. Time [s] −60 −30 −30 −30 −30 −30 Bulk ZDT Scott Bond Test Starch Comp-A Comp-B Comp-C Comp-D CMC1 [cm.sup.3/g] [kPa] [J/m.sup.2] 3-1 (ref.) 15 — — — — — 2.19 525 190 3-2 15 1.2 — — — — 2.17 600 224 3-3 15 2.4 — — — — 2.16 638 243 3-4 15 — 1.2 — — — 2.18 574 212 3-5 15 — 2.4 — — — 2.19 619 240 3-6 15 — — 1.2 — — 2.14 633 241 3-10 15 — — 2.4 — — 2.15 662 261 3-11 15 — — — 1.2 — 2.18 589 236 3-12 15 — — — 2.4 — 2.16 668 260 3-14 (ref.) 15 — — — — 2.4 2.15 608 219

Example 4

[0114] This example simulates preparation of middle ply of multi-ply board, such as folding box board or liquid packaging board. Test sheets were made with Formette-dynamic hand sheet former manufactured by Techpap.

[0115] Test fibre stock was made from 80% of bleached dried CTMP having CSF of 580 ml and from 20% of dry base paper broke from manufacture of folding box board. Test pulp was disintegrated according to ISO 5263:1995, at 70° C. Test fibre stock was diluted to 0.6% consistency with deionized water, salt mixture was added to obtain conductivity of 1.5 mS/cm and pH was adjusted to 7 with sulfuric acid. Salt mixture contained 70% calcium acetate, 20% sodium sulphate and 10% sodium bicarbonate.

[0116] Pulp mixture was added to Formette. Chemical additions were made to mixing tank of Formette according to Table 9. All chemical amounts are given as kg dry chemical per ton dry fibre stock. Water was drained out after all the pulp was sprayed. Drum was operated with 1000 rpm, mixer for pulp 400 rpm, pulp pump 1100 rpm/min, number of sweeps 29 and 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 9 bar pressure with 2 passes having new blotting paper each side of the sheet before each pass. Dry content was determined from the pressed sheet by weighting part of the sheet and drying the part in oven for 4 hours at 110° C. Sheets were cut to 15 cm*20 cm size. Sheets were dried in restrained condition in STFI restrained dryers for 10 min at 130° C. Before testing in the laboratory sheets were pre-conditioned for 24 h at 23° C. in 50% relative humidity, according to ISO 187. In this example, tensile index was geometric mean value calculated from square root of MD tensile index*CD tensile index.

[0117] Results of Example 4 are presented in Table 9.

TABLE-US-00009 TABLE 9 Dynamic hand sheet test program and results of Example 4. Zeta potential Dryness time [s] −60 −60 −60 −30 −30 −30 −30 −30 (after cationic), pressing Tens. ZDT Bulk Test Starch Starch-A SCPAM APAM-1 Comp-H CMC5 Comp-J Comp-K [mV] [%] index [kPa] [cm.sup.3/g] 4-1 (ref.) — — — — — — — — −13 49 15 105 3.0 4-2 (ref.) 5 — — — — — — — 0 47 8 102 3.3 4-3 (ref.) 14 — — — — — — — 5 32 19 182 3.2 4-4 (ref.) 14 — — 0.9 — — — — 5 52 18 163 3.3 4-5 (ref.) 14 — — 1.8 — — — — 5 52 18 144 3.3 4-6 14 — — — 0.7 — — — 5 45 19 180 3.2 4-7 14 — — — 1.4 — — — 5 49 20 176 3.2 4-8 14 — — — 2.4 — — — 5 49 24 213 3.2 4-9 (ref.) 14 — — — — 0.5 — — 5 52 11 154 3.2 4-10 (ref.) 14 — — — — 1.0 — — 5 51 11 160 3.2 4-11 14 — — — 2.4 — — — 5 49 24 213 3.2 4-12 14 — — — — — 1.4 — 5 48 21 185 3.2 4-13 14 — — — — — 2.4 — 5 52 22 203 3.1 4-14 14 — — — — — — 1.4 5 49 22 195 3.1 4-15 14 — — — — — — 2.4 5 55 21 180 3.3 4-16 — 6.4 — — 0.7 — — — 11 49 15 112 3.4 4-17 — 6.4 — — 1.4 — — — 11 51 15 112 3.4 4-18 — 1.9 1.9 — 1.4 — — — 6 50 15 120 3.3

[0118] It can be seen from Table 9 that an increase in starch addition turns zeta-potential of the stock cationic, which may cause reduced dryness after pressing (see tests 4-2, 4-3). Starch addition with strength compositions Comp-H, Comp-J and Comp-K according to the invention improve the z-directional strength in comparison to tests where starch alone was used or where starch together with separate addition of APAM-1 or CMC5 were used.

[0119] Compositions according to invention provide also adequate dryness after pressing which is needed for good speed in drying. It is also surprising that a good tensile strength and z-directional tensile values are obtained at bulk levels that are over 3 cm.sup.3/g. It is known that at bulk levels over 3 cm.sup.3/g the contact area between the fibres is limited, and lower tensile index values could be normally expected. Anionically derivatized polysaccharide used in the compositions according to the invention, possibly due to its molecular weight, gives a unique strength effect in this respect.

[0120] Other cationic strength agents may be also suitable for the system according to invention. Strength results depend also from cationic component of the strength composition, see tests from 4-16 to 4-18. Preferably cationic chemistry changes zeta-potential of the fibre stock to −5 from +10 mV after the addition of the cationic strength agent.

Example 5

[0121] This example simulates preparation of testliner and fluting board.

[0122] Test fibre stock was OCC (old corrugated containers) made from central European testliner containing 15% ash. OCC was disintegrated according to ISO 5263:1995, at 80° C. Disintegrated OCC was diluted to 0.8% consistency with water containing 520 mg/l calcium from calcium chloride. Conductivity of test fibre stock was adjusted to 4 mS/cm by sodium chloride addition.

[0123] In handsheet preparation chemicals were added to the test fibre stock in a dynamic drainage jar under mixing with 1000 rpm. Cationic strength chemicals were diluted before dosing to 0.2% concentration. Anionic chemicals and retention chemicals were diluted to 0.05% concentration before dosing. The strength chemicals added and their addition times are given in Table 10. Retention chemical CPAM (see Table 2) was dosed 0.15 kg/t 10 s prior to sheet making in Test 5-1. In other tests, from 5-2 to 5-13 retention polymer dosage was adjusted to obtain a retention level, which was required to maintain basis weight, when a constant amount of fibre stock was used. All chemical amounts are given as kg dry chemical per ton dry fibre stock.

[0124] Handsheets having basis weight of 80 g/m.sup.2 were formed in the same manner than in Example 1.

[0125] Short span compression test (SCT) index results for Example 5 are presented in Table 10.

TABLE-US-00010 TABLE 10 Handsheet tests of Example 5: chemical additions and measured results. Time [s] −60 −30 −30 −30 −30 −30 SCT index Test Starch Comp-A Comp-B Comp-C Comp-D CMC1 [Nm/g] 5-1 (ref.) — — — — — — 20.4 5-2 (ref.) 10 — — — — — 22.2 5-5 10 1 — — — — 22.4 5-6 10 2 — — — — 22.7 5-7 10 — 1 — — — 22.3 5-8 10 — 2 — — — 22.9 5-9 10 — — 1 — — 22.3 5-10 10 — — 2 — — 22.7 5-11 10 — — — 1 — 22.7 5-12 10 — — — 2 — 22.8 5-13 (ref.) 10 — — — — 1 21.0

[0126] From the results in Table 10 it can be seen that use of starch with strength compositions Comp-A, Comp-B, Comp-C and Comp-D give improved SCT strength values compared to use of starch alone (Test 5-2) or to use of separate additions of starch and CMC1 (Test 5-13). It can be seen from results that products having charge more positive than −1 meq/g at pH 7 show improvements in SCT strength. It seems that CMC type in the dry strength composition may preferably have a higher molecular weight for SCT strength.

Example 6

[0127] Preparation of Cationic Starch Component, Starch-B

[0128] 45 g cationic waxy corn starch, Starch-B, dry content 88 weight-%, was sludged in 1955 g of water in a reactor equipped with a jacket for heating, a condenser and agitator. Slurry was heated to 98° C. while agitating by 500 rpm and kept at that temperature for 60 min with agitation on. The formed starch solution, when cooled, had concentration of 2.0 weight-%, pH of 7.1, viscosity of 180 mPas and charge density at pH 7.0 of 0.26 meq/g dry material.

[0129] Preparation of Dispersion of Anionically Derivatized Microfibrillar Cellulose (MFC)

[0130] Anionically derivatized microfibrillar cellulose was dispersed in water by mixing. The formed MFC dispersion had dry content of 2.0 weight-%, viscosity of 1170 mPas, and charge density at pH 7.0 of −0.20 meq/g dry material.

[0131] Preparation of Dry Strength Compositions Comprising Cationic Starch-B and MFC

[0132] A series of aqueous dry strength compositions were prepared by mixing different proportions of MFC dispersion and Starch-B solution, prepared as defined above. Dry strength compositions were prepared, and their properties were measured, as given in Table 11. All percentages and values are calculated and given per dry material.

TABLE-US-00011 TABLE 11 Properties of starch solution, MFC dispersion and dry strength compositions prepared in Example 6. Charge density by Dry Mütek Starch-B MFC content Viscosity at pH 7.0 Composition [wt-%] [wt-%] [%] [mPas] pH [meq/g] Starch-B 100 0 2.0 180 0.26 Comp-L 50 50 2.0 870 7.4 0.03 Comp-M 20 80 2.0 1800 7.4 −0.11 Comp-N 10 90 2.0 3520 7.5 −0.15 MFC 0 100 2.0 1170 −0.20

[0133] Viscosity results show that poly-ion-complex forms between MFC and cationic starch, when the charge density at pH 7.0 is within the range according to the invention. This is evidenced by viscosity values: viscosities of compositions Comp-N and Comp-M are higher compared to viscosity of Starch-B solution alone, or MFC dispersion alone.

Example 7

[0134] Test fibre stock was made from 60% of bleached dried CTMP and from 40% of dry base paper broke from manufacture of folding box board. Test fibre stock was disintegrated according to ISO 5263:1995, at 70° C., and had CSF of 540 ml. Test fibre stock was diluted to 0.6% consistency with deionized water, and a salt mixture containing 70% calcium acetate, 20% sodium sulphate and 10% sodium bicarbonate was added to obtain conductivity of 1.5 mS/cm. pH was adjusted to 7 with sulfuric acid.

[0135] Dynamic drainage jar, DDJ (Paper Research Materials, Inc., Seattle, Wash.), was equipped with 60M wire screen, which had 210 μm diameter screen holes. Consistency of the furnish is approximately 0.6% and the sample volume was 500 ml in the experiment. Stirring speed was 1000 rpm and stirring was started 60 s before drainage. Used chemicals were added before the drainage, addition times are indicated in Table 12, as negative times. The retention chemical CPAM (see Table 2) was dosed 0.1 kg/t 15 s prior the drainage. Test 7-1 was a 0-test without any chemical addition. All chemical amounts are given as kg dry chemical per ton dry fibre stock.

[0136] At the moment of drainage, the stirring was stopped, and filtrate hose was opened. 200 g of the screened material was taken as sample. 100 g of the sample was filtrated through white ribbon filter paper in Bühner funnel equipped with vacuum. Material on filter paper was weighed after drying. 100 g of filtrate was taken.

[0137] Filtrate consistency was calculated from weight of material on filter pad divided by feed sample weight (100 g).

[0138] Ash content of 525° C. was measured from furnish and from dried filtrate pads. First pass ash retention was calculated by using following formula:

Ash Retention=100%*(FeedAsh*FeedCons−FiltrateAsh*FiltrateCons)/(FeedAsh*FeedCons)

where

[0139] FeedAsh, FiltrateAsh denotes the ash content of the feed and the filtrate, respectively; and

[0140] FeedCons, FiltrateCons denotes the consistency of the feed and the filtrate, respectively.

[0141] Zeta-potential was measured from the feed sample after addition of chemicals. Determination of the charge density was made by filtering 20 ml DDJ filtrate through black ribbon filter paper gravimetrically in a funnel and measuring the charge with Mütek PCD titration.

[0142] Determination of soluble starch was made from DDJ filtrate. To a sample of 25 ml filtrate was added to 10 ml of 10 weight-% HCl. Mixture was stirred for 10 min in 50 ml beaker with magnetic stirrer, and then filtrated by gravitation in a funnel with black ribbon filter paper. 5 ml of obtained filtrate from the mixture was taken, and 0.5 ml iodine reagent (7.5 g Kl/l+5 g I.sub.2/l), was added. After 2 min reaction time, absorbance value was measured at 610 nm by Hach Lange DR 900 spectrophotometer. Zeroing of the spectrophotometer was done by using the sample before iodine reagent addition. Raisamyl 50021 cationic starch was used as reference to make calibration equation for starch content determination. Test pulp starch content was determined by same method than DDA filtrate starch content. Blank test for HCl-iodine solution absorbance was made to subtract baseline absorbance from the results.

[0143] The obtained test results are given in Table 12. Usually a good charge level for a retention system is from −400 to −10 μeq/1, and a good Zeta-potential value is <−2 mV for avoiding foaming and poor retention of cationic starch. Filtrate starch value can be used to indicate the total starch retention, including starch from broke and/or from wet-end starch. Usually a filtrate starch value <50 mg/l is a suitable level for avoiding deposits and slime formation.

[0144] Tests 7-3, 7-5, 7-6, 7-11, 7-12 and 7-16 use COMP-H (see Table 3) as dry strength composition. In reference Test 7-7 the dry strength composition comprises CMC4, added in amount of 0.12 kg/t, and Starch-A, added in amount of 2.28 kg/t, resulting a charge density of +0.18 meq/g for the composition at pH 7. In reference Test 7-8 the dry strength composition comprises CMC4, added in amount of 2.28 kg/t, and Starch-A, added in amount of 0.12 kg/t, resulting a charge density of −3.8 meq/g for the composition at pH 7.

TABLE-US-00012 TABLE 12 Chemical additions and measured results for Example 7. −30 Dry CMC4 in Starch-A in −60 Strength −30 −30 Strength Strength Filtrate Zeta- Ash Time [s] Starch-1 Comp. Starch-A CMC4 Comp. Comp. Charge Starch potential retention Test [kg/t dry] [kg/t dry] [kg/t] [kg/t] [kg/t] [kg/t] [μeq/l] [mg/l] [mV] [%] 7-1 (ref.) — — — — — — −16 19 −11.7 54 7-2 (ref.) — — — — — — −11 21 −12.4 58 7-3 (ref.) — 2.4 — — 1.0 1.4 −34 20 −30.3 56 7-4 (ref.) 12 — — — — — −3 59 5.9 59 7-5 12 1.4 — — 0.6 0.8 −14 45 — 57 7-6 12 2.4 — — 1.0 1.4 −17 43 −3.9 58 7-7 (ref.) 12 2.4 — — 2.28 0.12 >50 66 7.2 58 7-8 (ref.) 12 2.4 — — 0.12 2.28 −38 42 −18.7 55 7-9 (ref.) 12 — — 0.6 — — −12 43 — 56 7-10 (ref.) 12 — — 1.0 — — −16 40 — 55 7-11 3 2.4 — — 1.0 1.4 −25 23 — — 7-12 8 2.4 — — 1.0 1.4 −27 30 — — 7-13 (ref.) 12 — 0.8 — — — −1 57 — — 7-14 (ref.) 12 — 1.4 — — — >50 64 — — 7-15 (ref.) 12 — 3.5 — — — >50 73 — — 7-16 12 6 — — 2.5 3.5 −29 40 — —

[0145] It is seen from Table 12 that in Tests 7-3 and 7-4, where only cationic strength agent or dry strength composition are used alone, there may be problems of low ash retention (Test 7-3) or of positive Zeta-potential (Test 7-4).

[0146] The Tests 7-5, 7-6 and 7-16 according to the invention show good charge, good filtrate starch content and good ash retention. Variation in cationic strength dosage is seen in Tests 7-11 and 7-12.

[0147] Reference Tests 7-7 and 7-8 show that when the dry strength composition comprises anionically derivatized polysaccharide (CMC4) and cationic starch (Starch-A) in amount that produces a charge ratio outside the defined range, the obtained results deteriorate. Test 7-7 is net cationic which generates cationic filtrate charge and too high filtrate starch. Test 7-8 generates low ash retention compared to the dry strength composition according to the invention.

[0148] Reference Tests 7-9 and 7-10 show that separate use of cationic strength agent and anionic polysaccharide does not provide desired results. Especially the obtained ash retention is low.

[0149] Reference Tests 7-13, 7-14 and 7-15 show results when cationic amylopectin starch (Starch-A) is added without mixing with anionically derivatized polysaccharide. It is seen that t generates too high cationic charge to the filtrate.

Example 8

[0150] This example demonstrates drainage and dewatering results obtainable.

[0151] Test fibre stock was made from 70% of bleached dried CTMP and from 30% of dry base paper broke from manufacture of folding box board. Test pulp was disintegrated according to ISO 5263:1995, at 70° C., and had a CSF value of 450 ml. Test fibre stock was diluted to 0.8% consistency with deionized water, and a salt mixture containing contained 70% calcium acetate, 20% sodium sulphate and 10% sodium bicarbonate was added to obtain conductivity of 1.5 mS/cm. pH was adjusted to 7 with sulfuric acid.

[0152] Dynamic Drainage Analyzer (DDA) Test

[0153] A Dynamic Drainage Analyzer, DDA, (AB Akribi Kemikonsulter, Sweden) was used to measure drainage. DDA's vacuum and stirrer were calibrated and necessary adjustments to the settings were made. DDA was connected to a computer for measuring the time between vacuum application and the vacuum break point. A change in the vacuum expresses the drainage time of a wet fibre web until air breaks through the thickening web indicating the drainage time. A drainage time limit was set to 30 seconds for the measurements.

[0154] In drainage measurements, 500 ml of the stock sample was measured into the reaction jar. The drainage test was performed by mixing the sample stock with the stirrer at 1000 rpm for 40 s while the chemicals to be tested were added in the predetermined order. Test chemical addition times are indicated in Table 13 as negative times, calculated backwards from the start of the drainage. A wire with 0.25 mm openings was used in the drainage test. 300 mbar vacuum for 30 s after drainage was used. Drainage time was recorded. Filtrate turbidity was measured immediately. Wet sheet was weighted to calculate dry content after forming. Wet pressing of the sheets was completed individually immediately after drainage test in Lorenz & Wettre wet press for 1 min at 4 bar pressure, 2 blotting papers both sides of the sheet. Pressed sheet was weighted. The sheet was reweighted after 5 min drying in Lorenz & Wettre hot plate dryer to calculate dry content after pressing. Relative retention was calculated from dry weight of the sheet compared to dry weight of the 0-test (Test 8-1) sheet.

[0155] Comp-Ref was used as a reference composition. Comp-Ref is made by mixing cationic amylopectin starch and anionic polyacrylamide at 50:50 weight ratio, and corresponds to conventional polyelectrolyte complexes used in paper and board making. Charge of COMP-Ref was +0.2 meq/g at pH 2.7 and −0.6 at pH 7. Used silica was colloidal silica having about 5 nm particle size.

[0156] Chemical additions, dosing times and the measured results are given in Table 13.

[0157] It is seen from Table 13 that the filtrate turbidity is improved when dry strength composition Comp-H is used (see Tests 8-3, 8-4, 8-5) in comparison to reference Tests 8-1 and 8-2. In comparison to results obtained with Comp-Ref in Tests 8-11, 8-12 the dry strength composition according to invention outperforms in both drainage time, forming dryness as well as in press dewatering.

[0158] Furthermore, results for Tests 8-4 and 8-5 show improvement in dryness after forming and in relative retention. It can also be seen that the dry strength composition in general provides improved dryness after forming as well as pressing and also an improved relative retention (Test 8-6, 8-7, 8-8).

[0159] The dry strength composition according to the present invention works well with conventional retention systems also, which make the composition suitable for a variety of different chemical systems used in paper and board making. In is seen from Table 13 that the dry strength composition can be combined with conventional retention system comprising CPAM and silica, and a good drainage and retention performance can be obtained (Tests 8-9, 8-10).

TABLE-US-00013 TABLE 13 Chemical additions, dosing times and results for Example 8. −60 −30 −30 −15 −10 Drainage Dryness after Dryness after Relative Time [s] Starch Comp-H Comp-Ref CPAM Silica time, Turbidity, forming, pressing, retention, Test [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] [kg/t dry] [s] NTU [%] [%] [%] 8-1 (ref.) — — — — — — 124 21 50 100 8-2 (ref.) 12 — — — — 9 115 21 51 100 8-3 12 1.6 — — — 9 114 21 51 102 8-4 12 2.4 — — — 9 103 22 51 103 8-5 12 4 — — — 9 98 22 52 102 8-6 — 1.6 — — — 7 — 24 57 111 8-7 — 2.4 — — — 7 — 23 52 106 8-8 — 4 — — — 7 — 23 53 108 8-9 12 0.8 — 0.15 0.3 8 — 22 55 107 8-10 12 1.6 — 0.15 0.3 8 — 22 52 108 8-11 (ref.) 12 — 1.6 — — 9 — 20 50 101 8-12 (ref.) 12 — 2.4 — — 9 — 21 49 102

[0160] 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.