Production of modified pulp

Abstract

A method for producing a modified lignocellulosic pulp having improved wet-strength properties and a modified lignocellulosic pulp obtainable by the method.

Claims

1. A method for producing a modified lignocellulosic pulp, comprising the steps: (i) providing a dry lignocellulosic pulp, particularly a pulp having a dryness of at least about 85 weight-%; (ii) adding a dicarboxylic acid anhydride with a molecular weight of 220 g/mol or less to the dry pulp in an amount and under conditions to obtain a modified pulp having a negative surface charge of at least about 8 μmol/g based on the dry pulp weight; and (iii) beating the pulp from step (ii).

2. The method of claim 1, wherein in step (iii) the Schopper Riegler degree of the pulp is increased by at least 2° SR.

3. The method of claim 2, wherein in step (iii) the Schopper Riegler degree of the pulp is increased by up to 70° SR.

4. The method of claim 1, wherein the pulp has a Schopper Riegler degree of at least about 16° SR after step (iii).

5. The method claim 1, wherein the modified pulp has increased wet-strength, compared to an unmodified reference pulp, particularly an increased wet tensile strength index, e.g. a wet tensile strength index of at least 5 kNm/kg after beating when measured according to ISO 1924-3.

6. The method of claim 1, wherein the dicarboxylic acid anhydride is added in an amount of between about 1 kg to about 55 kg per ton pulp based on the dry pulp weight.

7. The method of claim 1, wherein a modified pulp having a relative negative surface charge from about 15% to about 40 is obtained.

8. The method of claim 1, wherein said pulp is an unbeaten pulp, particularly selected from a kraft pulp, or a sulfite pulp, particularly selected from a softwood pulp, or a hardwood pulp, particularly a northern bleached softwood kraft (NBSK) pulp or a eucalyptus kraft pulp, particularly an NBSK pulp.

9. The method of claim 1, wherein the dicarboxylic acid anhydride is maleic acid anhydride.

10. The method of claim 1, wherein the pulp is kept at a temperature of about 100° C. or less during the time period where a reaction between the pulp and the dicarboxylic acid anhydride takes place.

11. The method of claim 1, wherein the dicarboxylic acid anhydride is added to the pulp in a gaseous or liquid distribution medium, in which the dicarboxylic acid anhydride is homogenously distributed.

12. The method of claim 11, wherein the distribution medium is a gaseous distribution medium comprising the dicarboxylic acid anhydride in gaseous form, and wherein the pulp is provided as disintegrated pulp, e.g. as fluffed pulp and/or flash-dried pulp.

13. The method of claim 11, wherein the distribution medium is a liquid distribution medium, particularly an aprotic organic solvent with a boiling point of at least about 80° C. under atmospheric pressure comprising the dicarboxylic acid anhydride in dissolved form, and wherein the pulp is provided in form of a sheet.

14. The method of claim 1 further comprising the step of producing a fiber-based product from the modified pulp.

15. The method of claim 14, wherein a paper product produced from the modified pulp has an increased wet strength compared to a paper product produced from unmodified pulp reference, wherein the increased wet strength is observed without subsequent addition of a wet-strength resin.

Description

FIGURE LEGENDS

(1) FIG. 1 shows a schematic depiction of a device for carrying out the process according to the present invention.

(2) FIG. 2 shows the wet tensile strength of a reference pulp and a maleic acid anhydride treated pulp as a function of the PAE addition.

(3) FIG. 3a shows the wet tensile strength of a reference pulp and a maleic acid anhydride treated pulp as a function of beating revolutions in a PFI mill.

(4) FIG. 3b shows the wet tensile strength of a reference pulp and a maleic acid anhydride treated pulp as a function of the delta beating degree (° SR); i.e. the difference of the beating degree after and before beating for a reference pulp and maleic acid anhydride treated pulp.

(5) FIG. 4 schematically illustrates the surface charge and wet strength of a number of northern bleached softwood kraft (NBSK) pulps treated with maleic acid anhydride.

METHODS

(6) 1. Charge Determination

(7) The negative surface charge and total negative charge of pulp fibers were analyzed by using polyelectrolyte adsorption, described e.g. in Winter et. al. “Polyelectrolytes Adsorbed on the Surface of Cellulosic Materials” (J. Colloid Interface Sci. 111(2), 1986, p. 537-543). To the pulp suspension a controlled amount of sodium hydrogen carbonate, NaHCO.sub.3 from Merck KGaA was added to a molar concentration of 10 mM for 15 min while stirring. The addition of the buffer resulted in a pH of 8.3. The pulp fibers were then washed with deionized water for minimum three times or until the pH was 7.5 and the conductivity was below 5 μS/cm in the filtrate.

(8) For determination of the negative surface charge of a pulp, polyelectrolyte adsorption isotherms on fibers were made using poly(diallyldimethylammonium) chloride, (polyDADMAC Alcofix 132 from Allied Colloids) ultrafiltrated using a filter with cut-off of 300000 Da in order to remove the low-molecular weight fraction.

(9) PolyDADMAC was added to the fibers and then mixed with a magnetic stirrer for 30 min prior to filtration. The filtrate was then titrated with potassium polyvinyl sulfate (KPVS from Wako Pure Chemical Industries) using a particle charge detector, (PCD 03 equipment from BTG Mütek) in order to detect the zero point and calculate the negative fiber surface charge expressed in μmol/g.

(10) For determination of the negative total charge of a pulp, polyelectrolyte adsorption isotherms on fibers were made using hexadimethrine bromide, (Polybrene from Sigma Aldrich) with a molecular weight of 4,000-6,000 g/mol.

(11) Polybrene was added to the fibers and then mixed with a magnetic stirrer for 30 min prior to filtration. The filtrate was then titrated with KPVS using the particle charge detector in order to detect the zero point and to calculate the total negative fiber charge expressed in μmol/g.

(12) 2. Preparation of Paper Hand Sheets

(13) Paper hand sheets were prepared for strength evaluation using unmodified reference pulps and pulps modified with maleic acid anhydride using different treatment protocols.

(14) The sheets were prepared from pulp diluted with water containing NaHCO.sub.3. A solution was prepared containing 1% NaHCO.sub.3 and then the pH of the solution was adjusted to 7.5. The pulp was soaked in the solution for at least 4 h and thereafter disintegrated. All pulps, both modified and the reference pulps, were disintegrated in a standard laboratory disintegrator according to ISO 5263-1 operated with 30000 revolutions. Prior to sheet forming the pulps were beaten in a standard PFI mill according to ISO 5264-2. If nothing else mentioned the PFI beating was performed using 4000 revolutions.

(15) Hand sheets with a basis weight of 65 g/m.sup.2 were then prepared according to ISO 5269-1. The prepared sheets were heated at a temperature of 105° C. for 10 min before measuring the strength properties. This was made in order to simulate the drying in the paper machine.

(16) 3. Wet Strength Measurements

(17) The wet tensile strength properties of the paper hand sheets were measured according to ISO 1924-3. Paper strips were soaked in water prior to wet tensile strength testing. The soaking time was 15 s.

(18) The wet strength of a pulp herein refers to the wet strength of sheets prepared from the pulp according to the procedures described above, i.e. by disintegrating the pulp in a standard laboratory disintegrator according to ISO 5263-1 operated with 30000 revolutions followed by beating the pulp 4000 revolutions in a PFI mill according to ISO 5264-2, then forming a sheet according to ISO 5269-1, heating the sheet at 105° C. for 10 min and finally testing its wet strength according to ISO 1924-3.

(19) 4. Measurements of the Schopper Riegler Beating Degree

(20) Measurements of the Schopper Riegler (SR) beating degree were carried out in accordance with ISO 5267-2.

EXAMPLES

Example 1

(21) (Comparative Example, Prior Art)

(22) A bleached softwood kraft pulp (NBSK) was examined with regard to its charge as described in the section “Methods”. The negative surface charge was 4 μmol/g and the relative negative surface charge was 15%.

(23) Paper hand sheets were prepared from the pulp and examined with regard to their wet strength characteristics as described in section “Methods”. The wet tensile strength index was determined as being 0 kNm/kg.

(24) This example shows that standard bleached softwood kraft pulp has a low negative surface charge and that this pulp has no wet strength.

Example 2

(25) (Comparative Example, Prior Art)

(26) A bleached eucalyptus kraft pulp (BEK) was examined with regard to its charge characteristics as described in the section “Methods”. The negative surface charge was 3 μmol/g and the relative negative surface charge was 5%.

(27) Paper hand sheets were prepared from the pulp and examined with regard to their wet strength characteristics as described in section “Methods”. The wet tensile strength index was determined as being 0 kNm/kg.

(28) This example shows that standard bleached eucalyptus kraft pulp has a low negative surface charge and that this pulp has no wet strength.

Example 3

(29) Sheets of a bleached softwood kraft pulp (NBSK) were treated with maleic acid anhydride using acetone as a distribution medium. The sheets were impregnated by soaking in a solution of 5% (w/w) maleic acid anhydride, purity: 99%, (Sigma Aldrich) in acetone, pro analysi, (Honeywell Specialty Chemicals). The impregnation was accomplished by placing a sheet in the liquid for about 5 minutes and then allowing excess of liquid to drain by placing the sheet on a PE-film in horizontal position. After evaporation of the acetone at room temperature, the sheet was placed in a plastic bag, which was closed and kept in a laboratory drying oven at 70° C. for 1 h.

(30) The treated pulp was examined with regard to its charge as described in the section “Methods”. It showed a negative surface charge of 40 μmol/g and a relative negative surface charge of 28%.

(31) Paper hand sheets were prepared from the pulp and examined with regard to their wet strength characteristics as described in section “Methods”. The wet tensile strength index was determined as being 18.6 kNm/kg.

(32) This example shows that the negative surface charge of bleached kraft fibers can be increased considerably by treatment with maleic acid anhydride accomplished by dissolving maleic acid anhydride in an aprotic solvent, adding the solution to the fibers, evaporating the solvent and keeping the fibers at a temperature above the melting point of maleic acid anhydride. This treatment results in high wet strength of paper produced from the modified pulp.

Example 4

(33) Sheets of a bleached eucalyptus kraft pulp (BEK) were treated with maleic acid anhydride using acetone as a distribution medium. The sheets were impregnated by soaking in a solution of 5% (w/v) maleic acid anhydride, purity: 99%, (Sigma Aldrich) in acetone, pro analysi, (Honeywell Specialty Chemicals). The impregnation was accomplished by placing a sheet in the liquid for about 5 min and then allowing excess of liquid to drain by placing the sheet on a PE-film in horizontal position. After evaporation of the acetone at room temperature, the sheet was placed in a plastic bag, which was closed and kept in a laboratory drying oven at 70° C. for 1 h.

(34) The treated pulp was examined with regard to its charge as described in the section “Methods”. It showed a negative surface charge of 11 μmol/g and a relative negative surface charge of 16%.

(35) Paper hand sheets were prepared from the pulp and examined with regard to their wet strength characteristics as described in section “Methods”. The wet tensile strength index was determined as being 6.7 kNm/kg.

(36) This example shows that the negative surface charge of bleached eucalyptus kraft fibers can be increased by treatment with maleic acid anhydride accomplished by dissolving maleic acid anhydride in an aprotic solvent, adding the solution to the fibers, evaporating the solvent and keeping the fibers at a temperature above the melting point of maleic acid anhydride. This treatment results in improved wet strength of paper produced from the modified pulp.

Example 5

(37) Sheets of a bleached softwood kraft pulp (NBSK) were conditioned at 50% relative humidity (RH) and 23° C. The sheets were disintegrated by the aid of a Kamas H01 hammer mill equipped with a sieving plate with Ø 5 mm holes. Stripes with a length of 60 cm and a width of 5 cm were cut from the pulp sheets and then inserted into the hammer mill at a feeding rate of 2.2 m/minute. The rotor speed was 4000 rpm. Fluffed pulp consisting of substantially free fibers was prepared. The pulp was dried at 70° C. in a drying oven for 2 h.

(38) 0.8 g maleic acid anhydride, purity: 99%, (Sigma Aldrich), was placed in a steel tray. 20 g fluffed pulp was placed in another tray with a bottom made from a 4 mesh net. The two trays were then joined together and a lid, was used to enclose the pulp. As the pulp lay on a net it was not in direct contact with the maleic acid anhydride but could still be reached if the chemical was gasified.

(39) FIG. 1 shows a scheme of the device. The tray 10 with maleic acid anhydride 12 was positioned below the tray 14 with the net 16 and the fluffed pulp 18 closed by lid 20. The tray 10 with maleic acid anhydride 12 was placed on the hot plate 22 of a heating device 24. The temperature of the plate was 220° C., which caused evaporation of the maleic acid anhydride and treatment of the fluffed pulp by the gasified maleic acid anhydride. The fluffed pulp 18, being at a distance from the hot plate 22, had a temperature of approximately 50-60° C. After 20 h, the pulp was collected.

(40) The treated pulp was examined with regard to its charge as described in the section “Methods”. It showed a negative surface charge of 25 μmol/g and a relative negative surface charge of 20%.

(41) Paper hand sheets were prepared from the pulp and examined with regard to their wet strength characteristics as described in section “Methods”. The wet tensile strength index was determined as being 12.1 kNm/kg.

(42) This example shows that the negative surface charge of bleached kraft fibers can be increased by treatment with gasified maleic acid anhydride and that this increase results in improved wet strength of paper produced from the pulp.

Example 6

(43) In this example, triacetin was tested as an alternative to using acetone and similar distribution media. Triacetin is associated with a number of advantages, such as being a comparatively inexpensive, odorless, and harmless distribution medium.

(44) Sheets of bleached softwood kraft pulp (NBSK) were treated with maleic acid anhydride using triacetin as a medium. The sheets were sprayed with a solution consisting of 50 weight-% maleic acid anhydride, (purity: 99%, Sigma Aldrich) and 50 weight % triacetin, (purity: 99%, Sigma Aldrich). The spraying was accomplished by spraying both sides of a pulp sheet by the aid of a spray bottle. About 83 g solution was added per kg pulp, which resulted in an addition of 41.5 g maleic acid anhydride/kg pulp. Several treated sheets were stacked on top of each other and pressed together. The sheets were then placed in a plastic bag and stored at 70° C. for three days.

(45) The treated pulp was examined with regard to its charge as described in the section “Methods”. It showed a negative surface charge of 21.5 μmol/g and a relative negative surface charge of 18%.

(46) Paper hand sheets were prepared from the pulp and examined with regard to their wet strength characteristics as described in section “Methods”.The wet tensile strength index was determined to 6.8 kNm/kg.

(47) This example shows that the negative surface charge of bleached kraft fibers can be increased by spraying a pulp sheet with maleic acid anhydride dissolved in a high boiling aprotic solvent and that this treatment results in improved wet strength of paper produced by the pulp.

(48) In this experiment, triacetin was sprayed onto the sheets with a less than optimum sheet coverage. The improved wet strength was observed even without complete immersion of pulp sheets into a liquid distribution medium. Thus, triacetin can be a useful alternative to acetone and similar distribution media in industrial processes.

Example 7

(49) Sheets of bleached softwood kraft pulp (NBSK) were treated with maleic acid anhydride using acetone as a distribution medium. The sheets were impregnated by soaking in a solution of 2% (w/w) maleic acid anhydride, purity: 99%, (Sigma Aldrich) in acetone, pro analysi, (Honeywell Specialty Chemicals). The impregnation was accomplished by placing a sheet in the liquid for about 5 min and then allowing excess of liquid to drain by placing the sheet on a PE-film in horizontal position. After evaporation of the acetone at room temperature, the sheet was placed in a plastic bag, which was closed and kept in a laboratory drying oven at 70° C. for 1 h.

(50) Untreated reference pulp and pulp treated with maleic acid anhydride was examined with regard to its charge and wet-strength characteristics as described in the section “Methods”. The beating degree in Schopper Riegler (SR) was measured according to ISO 5267-1.

(51) Additionally, Kymene 25 X-Cel, a polyaminoamide epichlorohydrin (PAE) from Solenis was added in this example. Sheets were prepared with additions of 5, 10 and 20 kg of PAE per ton pulp, respectively, both to treated and untreated pulp suspension. The adsorption time for PAE was 1 min prior to sheet forming. The prepared sheets were heated at 105° C. for 10 minutes before measuring the strength properties. Soaking time when measuring wet tensile strength was 15 seconds.

(52) The treated pulp without addition of PAE showed a negative surface charge of 11.4 μmol/g. The wet tensile strength index of a hand paper sheet made from this pulp was determined to 11.1 kNm/kg.

(53) This example shows improved wet strength of paper produced from the treated pulp. As can be seen in FIG. 2 it also shows an increase in the wet tensile strength index after addition of PAE both to the reference pulp and the maleic acid anhydride treated pulp. However, the treated pulp shows a higher strength than the untreated reference pulp which indicates an additive effect of maleic acid anhydride treatment of the pulp and PAE addition during paper preparation.

Example 8

(54) Sheets of bleached softwood kraft pulp (NBSK) were treated with maleic acid anhydride using acetone as a distribution medium. The sheets were impregnated by soaking in a solution of 5% (w/w) maleic acid anhydride, purity: 99%, (Sigma Aldrich) in acetone, pro analysi, (Honeywell Specialty Chemicals). The impregnation was accomplished by placing a sheet in the liquid for about 5 min and then allowing excess of liquid to drain by placing the sheet on a PE-film in horizontal position. After evaporation of the acetone at room temperature, the sheet was placed in a plastic bag, which was closed and kept in a laboratory drying oven at 70° C. for 1 h.

(55) The treated pulp was examined with regard to its charge as described in the section “Methods”. It showed a negative surface charge of 40 μmol/g and a relative negative surface charge of 28%.

(56) Paper hand sheets were prepared from the pulp and examined with regard to their wet strength characteristics as described in section “Methods”. The wet tensile strength index was determined as being 18.6 kNm/kg.

(57) In order to evaluate the influence of beating, the procedure described in the section “Methods” was modified by using several different numbers of revolutions when beating the pulp in a PFI mill. The pulp was beaten 0, 1000, 2000, 3000, 4000 and 7000 revolutions in the PFI mill. The results are shown in FIG. 3a. The wet tensile strength characteristics as a function of the delta beating degree (° SR) are shown in FIG. 3b.

(58) This example, with unbeaten and PFI beaten pulp, respectively, shows that some beating is needed to achieve a wet strong paper and that in the investigated range of beating degree the wet tensile strength index can be further increased by increasing the beating degree.

Example 9

(59) A number of bleached softwood kraft pulps, more particularly northern bleached softwood kraft (NBSK) pulps, were treated with maleic acid anhydride. The treatment with maleic acid anhydride involved soaking of sheets with acetone as distribution media, as described above in Example 3, and treatment of fluffed pulp with gasified maleic acid anhydride, as described above in Example 5. The dryness of the pulps was about 90 weight-% or higher during treatment.

(60) Surface charge and wet strength of the respective pulps were determined as described in section “Methods”.

(61) FIG. 4 schematically illustrates the relation between wet strength and negative surface charge for these maleic acid anhydride treated NBSK pulps. An unmodified reference NBSK pulp is also included in FIG. 4, i.e. the point with surface charge 4 μmol/g and no wet strength (cf. Example 1).

(62) This example indicates that a considerable improvement of the wet strength of the pulp can be achieved by pulp treatment with maleic acid anhydride in accordance with the present invention. An interval with no wet strength at low surface charges is followed by an interval with high, increasing wet strength, as illustrated by the approximate dashed line in FIG. 4.