Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H11/00—Pulp or paper, comprising cellulose or lignocellulose fibres of natural origin only
  • D21H11/16—Pulp or paper, comprising cellulose or lignocellulose fibres of natural origin only modified by a particular after-treatment

Definitions

• the present invention relates to a method of treating cellulose fibres.
• the invention further involves the production of paper from said treated fibres and the paper obtainable therefrom.
• the invention also relates to the use of a .cellulose derivative as an additive to an acidic bleaching stage.
• wet strength of a paper In the field of papermaking, various methods of improving the wet strength of paper are known by retaining wet strength agents to the cellulosic fibres in the pulp suspension while forming the paper.
• the wet strength of a paper relates to its ability to maintain physical integrity and to resist tearing, bursting, and shredding under use, especially under wet conditions.
• a further important property of wet strengthened paper is the softness, especially for tissue paper or the like. The softness can be described as the tactile sensation perceived when holding or rubbing a paper across the skin.
• WO01/21890 discloses a method of modifying cellulose fibres to provide high wet strength to a paper. This method, however, involves adding electrolyte to the pulp suspension and treating it at a temperature of at least 100 0 C which restricts flexibility and the use of this process.
• the present invention intends to provide an energy-efficient and simple method for producing paper with increased wet strength and softness as well as other advantageous properties imparted by the modification of the fibres. It is a further object of the present invention to provide a method which can be used with conventional existing equipment and machinery.
• the present invention relates to a method of modifying cellulose fibres comprising providing a pulp suspension of cellulose fibres, adding a cellulose derivative during the bleaching of said cellulose fibres to at least one acidic bleaching stage.
• no electrolyte is added in conjunction with the addition of cellulose derivative, except for optional addition of acid or base to adjust the pH.
• Addition of a pH regulating base or acid may be made in an amount of about 0.001 to about 0.5 M if the electrolyte is univalent.
• Addition of e.g. Ca 2+ or other bivalent electrolyte could in some cases increase the risk of precipitation of calcium oxalate.
• Equipment used in the bleaching process may then become clogged with such precipitates derived from electrolytes since the pulps may naturally contain e.g. oxalic acid. The electrolyte, however, does not significantly influence the modification of the fibres.
• the pH of the pulp suspension in the acidic bleaching stage suitably is from about 1 to about 7, more preferably from about 2 to about 6, and most preferably from about 2 to about 4.
• the temperature during the acidic bleaching suitably is from about 30 to about 95, preferably from about 60 to about 90 0 C.
• the dry content of cellulose fibres in the pulp suspension is from about 1 to about 50, more preferably from about 15 to about 30, and most preferably from about 5 to about 15 wt%.
• the bleaching suitably is performed for about 0.1 to about 10, preferably from about 1 to about 5, and most preferably from about 1 to about 3 hours.
• the acidic bleaching stage at which the cellulose derivative is added may be during any of the stages where the pulp is treated with chlorine dioxide, ozone, peracid, or other acidic bleaching treatment stages, preferably during the treatment with chlorine dioxide.
• acidic stages integrated in the bleaching process or sequences of acidic bleaching stages such as washing steps, acidification, or acidic chelating stages are here also meant to be included in the bleaching treatment during which a cellulose derivative may be added.
• CMC-treated pulp in which CMC was added to an acidic bleaching stage was significantly increased as well as the relative wet strength when a wet strength agent subsequently was added to the paper furnish in a papermaking process.
• the present method can thus also impart enhanced softness properties of the produced paper.
• RWS stands for the relative wet strength
• WS is the wet tensile index
• DS is the dry tensile index of a paper.
• RWS is often a good measure of the softness of a paper; the higher the RWS, the higher the softness of the paper.
• the modification with a cellulose derivative may also influence the effect of any subsequent addition of paper chemicals to the pulp furnish which in turn may influence both the necessary dosage of the paper chemicals to the pulp furnish as well as the quality of the obtained paper product. It has also been seen that sizing, retention and dewatering can be improved as a result of the modified cellulosic fibres in papermaking processes.
• any further paper chemicals suitable in the production of paper may be added to the pulp furnish containing the modified bleached cellulose fibres.
• Such chemicals may include e.g. dry strength, wet strength agents, retention agents, sizing agents etc.
• the cellulose fibres may be derived from any type of soft or hard wood-based or nonwood-based material, e.g. pre-bleached, half-bleached or unbleached sulphite, sulphate or soda pulps or unbleached, half-bleached or pre-bleached mechanical, thermomechanical, chemo-mechanical and chemo-thermomechanical pulps, and mixtures thereof.
• nonwood materials can be mentioned e.g. bagasse, kenaf, grass fibres, sisal or the like.
• the cellulose derivative preferably an alkyl cellulose derivative, and most preferably a carboxymethyl cellulose derivative, is water-soluble or at least partly water- soluble or water-dispersible, preferably water-soluble or at least partly water-soluble.
• the cellulose derivative is ionic.
• the cellulose derivative can be anionic, cationic or amphoteric, preferably anionic or amphoteric.
• suitable cellulose derivatives include cellulose ethers, e.g. anionic and amphoteric cellulose ethers, alkali cellulose, cellulose metal complexes, graft copolymer cellulose preferably anionic cellulose ethers.
• the cellulose derivative preferably has ionic or charged groups, or substituents.
• Suitable ionic groups include anionic and cationic groups.
• suitable anionic groups include carboxylate, e.g. carboxyalkyl, sulphonate, e.g. sulphoalkyl, phosphate and phosphonate groups in which the alkyl group can be methyl, ethyl propyl and mixtures thereof, suitably methyl; suitably the cellulose derivative contains an anionic group comprising a carboxylate group, e.g. a carboxyalkyl group.
• the counter-ion of the anionic group is usually an alkali metal or alkaline earth metal, suitably sodium.
• Suitable cationic groups of cellulose derivatives according to the invention include salts of amines, suitably salts of tertiary amines, and quaternary ammonium groups, preferably quaternary ammonium groups.
• the substituents attached to the nitrogen atom of amines and quaternary ammonium groups can be same or different and can be selected from alkyl, cycloalkyl, and alkoxyalkyl groups, and one, two or more of the substituents together with the nitrogen atom can form a heterocyclic ring.
• the substituents independently of each other usually comprise from 1 to about 24, preferably from 1 to about 8 carbon atoms.
• the nitrogen of the cationic group can be attached to the cellulose or derivative thereof by means of a chain of atoms which suitably comprises carbon and hydrogen atoms, and optionally O and/or N atoms.
• the chain of atoms is an alkylene group with from 2 to 18 and preferably 2 to 8 carbon atoms, optionally interrupted or substituted by one or more heteroatoms, e.g. O or N such as alkyleneoxy group or hydroxy propylene group.
• Preferred cellulose derivatives containing cationic groups include those obtained by reacting cellulose or derivative thereof with a quaternization agent selected from 2, 3-epoxypropyl trimethyl ammonium chloride, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride and mixtures thereof.
• the cellulose derivatives of this invention can contain non-ionic groups such as alkyl or hydroxy alkyl groups, e.g. hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxylbutyl and mixtures thereof, e.g. hydroxyethyl methyl, hydroxypropyl methyl, hydroxybutyl methyl, hydroxyethyl ethyl, hydroxypropyl and the like.
• the cellulose derivative contains both ionic groups and non- ionic groups.
• Suitable cellulose derivatives according to the invention include carboxyalkyl celluloses, e.g. carboxymethyl cellulose, carboxyethyl cellulose, carboxy- propyl cellulose, sulphoethyl carboxymethyl cellulose, carboxymethyl hydroxyethyl cellulose ("CM-HEC”), carboxymethyl cellulose wherein the cellulose is substituted with one or more non-ionic substituents, preferably carboxymethyl cellulose ("CMC").
• suitable cellulose derivatives and methods for their preparation include those disclosed in U.S. Pat. No. 4,940,785, which is hereby incorporated herein by reference.
• degree of substitution or "DS”, as used herein, mean the number of substituted ring sites of the beta-anhydroglucose rings of the cellulose derivative. Since there are three hydroxyl groups on each anhydroglucose ring of the cellulose that are available for substitution, the maximum value of DS is 3.0.
• the cellulose derivative has a degree of substitution of net ionic groups ("DSM") up to about 0.65, i.e. the cellulose derivative has an average degree of net ionic substitution per glucose unit up to about 0.65.
• the net ionic substitution can be net anionic, net cationic or net neutral.
• net anionic groups the average number of anionic groups minus the average number of cationic groups, if any, per glucose unit
• DS N is the same as the degree of substitution of net anionic groups
• DS NC the degree of substitution of net cationic groups
• the cellulose derivative has a degree of substitution of carboxyalkyl groups ("DS C A") up to about 0.65, i.e. the cellulose derivative has an average degree of carboxyalkyl substitution per glucose unit up to about 0.65.
• the carboxyalkyl groups are suitably carboxymethyl groups and then DS CA referred to herein is the same as the degree of substitution of carboxymethyl groups ("DS C M")-
• DS N i, DS NA , DS NC and DS CA independently of each other are usually up to about 0.60, suitably up to about 0.50, preferably up to about 0.45 and more preferably up to 0.40
• DS N ,, DS NA , DS NC and DS C A independently of each other are usually at least 0.01 , suitably at least about 0.05, preferably at least about 0.10 and more preferably at least about 0.15.
• the ranges of DS N) , DS NA , DS NC and DS CA independently of each other are usually from about 0.01 to about 0.60, suitably from about 0.05 to about 0.50, preferably from about 0.10 to about 0.45 and more preferably from about 0.15 to about 0.40.
• Cellulose derivatives that are anionic or amphoteric usually have a degree of anionic substitution ("DS A ") in the range of from 0.01 to about 1.0 as long as DS N
• Cellulose derivatives that are cationic or amphoteric can have a degree of cationic substitution ("DSc") in the range of from 0.01 to about 1.0 as long as DSNI and DSN C are as defined herein; suitably from about 0.02, preferably from about 0.03, and more preferably from about 0.05 and suitably up to about 0.75, preferably up to about 0.5, and more preferably up to about 0.4.
• the cationic groups are suitably quaternary ammonium groups and then DS 0 referred to herein is the same as the degree of substitution of quaternary ammonium groups ("DSQN").
• DS A or DS 0 can of course be higher than 0.65 as long as DSN ⁇ and DSN C , respectively, are as defined herein. For example, if DS A is 0.75 and DS 0 is 0.15, then DS NA is 0.60.
• the water-soluble cellulose derivatives suitably have a solubility of at least 85 % by weight, based on total weight of dry cellulose derivative, in an aqueous solution, preferably at least 90 % by weight, more preferably at least 95 % by weight, and most preferably at least 98 % by weight.
• the cellulose derivative usually has an average molecular weight which is at least 20000 Dalton, preferably at least 50000 Dalton, and up to about 1000000 Dalton, preferably up to about 50000 Dalton.
• the cellulose derivative is suitably added in an amount of from about 0.5 to about 50, preferably from about 5 to about 20, and most preferably from about 5 to about 10 kg/t dry cellulose fibres.
• the invention also relates to a paper obtainable by a method comprising dewatering on a wire a pulp furnish of modified bleached cellulose fibres produced according to a method as described herein and forming a paper of said dewatered pulp furnish. 7
• the wet strength is strongly increased when CMC has been adsorbed in a final chlorine dioxide stage compared to addition of CMC to the stock or with no CMC addition.
• the increase in wet strength of the produced paper was up to 65%.
• the aim of the experiment was to adsorb CMC to fibres in a final acidic bleaching stage which here was a chlorine dioxide stage. Even though not necessary, calcium chloride was used to enhance the adsorption.
• the used pulp was a five stage Elemental Chlorine Free bleached softwood pulp of full brightness having a final brightness of 90% ISO.
• a reference pulp was treated as the CMC modified pulp according to the invention but without the charge of CMC.
• the final chlorine dioxide stage was performed at 80 0 C for 180 minutes at a 10 wt% pulp consistency.
• the chemical charges were: 10 kg/t chlorine dioxide, as active chlorine 7 kg/t, 18 kg/t calcium chloride calculated as Ca 2+ based on the weight of dry pulp.
• the end pH of the chlorine dioxide stage was 2.8.
• the used CMC was Finnfix WRH from Noviant. The degree of substitution was 0.5 and the molecular weight 1*10 6 .
• Wet strength agent Kenores XO was added at a charge of 15 kg/t dry pulp to the bleached pulp suspension. The strength properties of the CMC treated pulp were evaluated at different beating degrees ( 0 SR). The beating was performed in a lab-scale PFI beater. The strength properties of beaten CMC-treated pulp were compared with the non-CMC treated beaten reference pulp and with a pulp to which CMC was added to the pulp suspension subsequent to the bleaching. The analyzed pulp had a final brightness of 90 % ISO that had been bleached with a final chlorine dioxide stage (in laboratory scale).
• RWS relative wet strength

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