Strength agent, its use and method for increasing strength properties of paper

Abstract

The invention relates to a strength agent for paper, board or the like. The strength agent comprises a first component, which is refined cellulosic fibers having a refining level of >70? SR, and a second component, which is a synthetic cationic polymer having a charge density of 0.1-2.5 meq/g, determined at pH 2.7, and an average molecular weight of >300 000 g/mol. The invention relates also to a use of the strength agent and to a method for increasing strength properties of paper, board or the like.

Claims

1. A strength agent for paper, or board, which agent is formed by mixing a first component with a second component before the strength agent is added to a fibre stock, or which agent is a combination formed by separate but simultaneous addition of the first component and the second component to the fibre stock, wherein the strength agent comprises: the first component, which is mechanically refined cellulosic fibres having a refining level in the range of 70-98? SR, the second component, which is a synthetic cationic polymer, which is a copolymer of methacrylamide or acrylamide and at least one cationic monomer, and has a charge density of 0.1-2.5, determined at pH 2.7, and an average molecular weight of >300000 g/mol, wherein the strength agent comprises refined cellulosic fibres and synthetic cationic polymer in a weight ratio range of approximately 12.5:1 to 50:1.

2. The strength agent according to claim 1, wherein the cellulosic fibres have a refining level of 75-90? SR.

3. The strength agent according to claim 1 wherein the first component consists of cellulosic fibres, which are obtained by kraft pulping and which have been subjected to mechanical refining.

4. The strength agent according to claim 1, wherein the cellulosic fibres are bleached softwood fibres obtained by kraft pulping.

5. The strength agent according to claim 1, wherein the synthetic cationic polymer has a charge density of 0.2-2.5 meq/g.

6. The strength agent according to claim 1, wherein the synthetic cationic polymer has an average molecular weight of 300000-6000000 g/mol.

7. The strength agent according to claim 1, wherein the cationic monomer is selected from a group consisting of methacryloyloxyethyltrimethyl ammonium chloride, acryloyloxyethyltrimethyl ammonium chloride, 3-(methacrylamido) propyltrimethyl ammonium chloride, 3-(acryloylamido) propyltrimethyl ammonium chloride, diallyldimethyl ammonium chloride, dimethylaminoethyl acrylate, dimethylaminoethyl methacrylate, dimethylaminopropylacrylamide and dimethylaminopropylmethacrylamide.

8. The strength agent according to claim 1, wherein it comprises cationic or amphoteric starch with a substitution degree in the range of 0.01-0.5.

9. The strength agent according to claim 1, wherein it comprises 70-98 weight-%, of refined cellulosic fibres and 1.4-7.8 weight % of synthetic cationic polymer.

10. A method for increasing strength properties of paper or board, comprising; obtaining a fibre stock, adding to the fibre stock a strength agent comprising a first component and a second component according to claim 1.

11. The method according to claim 10, wherein the fibre stock comprises mineral filler.

12. The method according to claim 10, wherein the first component of the strength agent is added to the stock, and thereafter the second component of the strength agent.

13. The method according to claim 10 wherein the second component of the strength agent is added to the stock, and thereafter the first component of the strength agent.

14. The method according to claim 10, wherein the strength agent or any of its component is added to the thick fibre stock, which has a consistency of at least 20 g/l.

15. The method according to claim 14, wherein the consistency is at least 25 g/l.

16. The method of claim 14, wherein the consistency is at least 30 g/l.

Description

EXPERIMENTAL

(1) General principle of manufacturing hand sheets with Rapid K?then hand sheet former is as follows:

(2) Sheets are formed with Rapid K?then sheet former, ISO 5269/2. Fibre suspension is diluted to 0.5% consistency with tap water, which conductivity has been adjusted with NaCl to 550 ?S/cm in order to correspond the conductivity of real process water. The fibre suspension is stirred at a constant stirring rate at 1000 rpm in a jar with a propeller mixer. Strength agent according to the present invention for improving the strength properties of the final sheet is added into the suspension under stirring 60 s before drainage. All sheets are dried in vacuum dryer for 5 min at 1000 mbar pressure and at 92? C. temperature. After drying the sheets are pre-conditioned for 24 h at 23? C. in 50% relative humidity before testing the tensile strength of the sheets.

(3) For Zeta potential measurement fibre suspension is diluted to 0.5% consistency with tap water, which conductivity has been adjusted with NaCl to 550 ?S/cm in order to correspond the conductivity of real process water.

(4) Measurement methods and devices used for characterisation of hand sheet samples are disclosed in Table 1.

(5) TABLE-US-00001 TABLE 1 Measured hand sheet properties and standard methods and device used for measurements. Measurement Standard, Device Grammage ISO 536, Mettler Toledo Tensile strength ISO 1924-3, Lorentzen & Wettre Tensile tester Scott bond T 569, Huygen Internal Bond tester Zeta potential M?tek SZP-06

Example 1

(6) Hand sheets were formed as described above. Sheet basis weight was 80 g/m.sup.2.

(7) The fibre suspension comprised 50 weight-% of long fibre fraction, which was pine kraft pulp, SR 18, and 50 weight-% short fibre fraction, which was eucalyptus pulp, SR18.

(8) The strength agent comprised:

(9) 1) a first component, which was pine kraft pulp with refining level of SR 90. The refining of the pine kraft pulp was performed with Valley-beater, 1.64 weight-%, calculated as dry fibre, and

(10) 2) a second component which was cationic polyacrylamide, average molecular weight 800 000 g/mol, charge density 1.3 meg/g.

(11) The results of Example 1 are given in Table 2. All the dosages are given as kg/pulp ton and as active component.

(12) TABLE-US-00002 TABLE 2 Results of Example 1 1.sup.st 2.sup.nd Tensile Scott Zeta Test component component index Bond, potential, Point dose dose [Nm/g] [J/m2] [mV] 1 38.1 150 ?91 2 50 42.1 171 ?87 3 2 44.1 228 ?30 4 50 1 44.3 228 ?58 5 50 2 49.2 260 ?33 6 50 4 48.1 258 6

(13) From Table 2 it can be seen that the strength agent according to the invention comprising both refined cellulosic fibres and synthetic cationic polymer improves the tensile index and Scott Bond values of the obtained paper. It is also seen that when strength agent is used, lower amounts of synthetic cationic polymer yield similar results than higher amount of synthetic cationic polymer alone. This may indicate that by using the present invention, lower amount of synthetic cationic polymers can be used, which have positive effect on overall process economy, as usually the synthetic polymers are the expensive components in manufacture of paper or board.

(14) 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.