ES2398361T3 - Method to treat cellulose fibers - Google Patents

Abstract

Method of modifying cellulose fibers, comprising providing a pulp suspension of cellulose fibers and adding a cellulose derivative during bleaching of said cellulose fibers in at least one acid bleaching stage, in which the pH of the suspension of Paste is from about 1 to about 4 and the temperature varies from about 30 to about 95 ° C

Description

Method to treat cellulose fibers

The present invention relates to a method of treating cellulose fibers. The invention also involves the production of paper from said treated fibers and the paper obtainable from them. The invention also relates to the use of a cellulose derivative as an additive in an acid bleaching step.

Background of the invention

In the field of papermaking, various methods of improving the wet strength of the paper by retaining wet strength agents in the cellulosic fibers of the pulp suspension during paper formation are known. The wet strength of a paper refers to its ability to preserve its physical integrity and resistance to tearing, bursting and disintegration during use, especially in wet conditions. An additional important property of wet strength paper is softness, especially tissue paper or the like. Softness can be described as the perceived tactile sensation when a paper is held or rubbed on the skin.

WO 01/21890 describes a method of modifying cellulose fibers to provide high wet strength to a paper. However, this method involves adding an electrolyte to the pulp suspension and treating it at a temperature of at least 100 ° C, which limits the flexibility and use of this process.

The present invention aims to provide a simple and energy efficient method for producing paper with greater wet and soft resistance as well as with other advantageous properties imparted by fiber modification. A further object of the present invention is to provide a method that can be used with existing conventional equipment and machinery.

Description of the invention

The present invention relates to a method of modifying cellulose fibers comprising providing a pulp suspension of cellulose fibers and adding a cellulose derivative during the bleaching of said cellulose fibers in at least one step of acid bleaching.

Preferably, no electrolyte is added together with the addition of the cellulose derivative, except the optional addition of an acid or base to adjust the pH. The addition of an acid or base to regulate the pH can be done in an amount of about 0.001 to about 0.5 M if the electrolyte is monovalent. The addition, for example, of Ca2 + or other divalent electrolyte could in some cases increase the risk of calcium oxalate precipitation. The equipment used in the bleaching process can then be clogged with said electrolyte-derived precipitates since the pastes may naturally contain, for example, oxalic acid. However, the electrolyte does not significantly influence fiber modification.

The pH of the pulp suspension in the acid bleaching step is conveniently 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 acid bleaching is conveniently from about 3 to about 95 ° C, preferably from about 60 to about 90 ° C.

Preferably, the dry content of cellulose fibers 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% by weight.

Bleaching is conveniently performed for a time of about 0.1 to about 10, preferably about 1 to about 5, and most preferably about 1 to about 3 hours. The acid bleaching stage during which the cellulose derivative is added can be any of the stages in which the pulp is treated with chlorine dioxide, ozone, peracid or other acid bleaching treatment steps, preferably the treatment stage with chlorine dioxide. In this context, acid stages integrated in the process or bleaching sequences of acid bleaching stages, such as washing steps, acidification or acid chelate addition stages, during which a derivative can be added can also be included in the bleaching treatment of cellulose.

It has been found that the adsorption of cellulose derivatives on cellulose fibers, particularly the adsorption of CMC (carboxymethyl cellulose) on the fibers, causes a significantly higher surface charge compared to wood fibers not treated with CMC.

This may be the explanation for which the wet strength of a paper produced from the CMC-treated pulp was significantly increased, in which CMC was added in an acid bleaching stage, as well as the relative wet strength when subsequently added a wet strength agent to the pulp suspension in a papermaking process.

The present method can also impart increased softness properties to the paper produced. The smoothness of a sheet of paper can be estimated, at least indirectly, by means of the relative value of the wet strength, which is defined as the ratio between the wet tensile index and the dry tensile index according to the formula RWS (%) = (WS / DS) x100, in which RWS is the relative wet strength, WS is the wet tensile index and DS is the dry tensile index of a paper. Frequently RWS is a good measure of the smoothness of a paper: the higher the RWS, the greater the smoothness of the paper.

Modification with a cellulose derivative can also influence the effect of any subsequent addition of paper chemicals to the pulp suspension, which in turn can influence the necessary dosage of paper chemicals to the pulp suspension as well as on the quality of the paper obtained.

It has also been seen that the gluing, retention and drainage can be improved in papermaking processes as a result of the modified cellulosic fibers.

Any paper chemicals can be added to the pulp suspension containing the modified bleached cellulose fibers in the paper production process. Such chemicals may include, for example, wet strength agents, retention agents, sizing agents, etc.

Cellulose fibers can be obtained from any type of material based on coniferous or hardwood wood and not based on wood, for example, raw, pre-bleached or semi-bleached soda, sulfate or bisulfite pastes or mechanical, thermomechanical, chemical-mechanical pastes or crude, semi-bleached or pre-whitened chemothemomechanics, and mixtures of these types of pastes. Examples of non-woody materials may be mentioned, for example, bagasse fibers, kenaf, herbs, etc.

The cellulose derivative, preferably an alkyl cellulose and most preferably carboxymethyl cellulose, is soluble in water or at least partially soluble in water or dispersible in water, preferably soluble in water or at least partially soluble in water. Preferably, the cellulose derivative is ionic. The cellulose derivative can be anionic, cationic or amphoteric, preferably anionic or amphoteric. Examples of suitable cellulose derivatives include cellulose ethers (eg, anionic and amphoteric cellulose ethers), alkyl celluloses, cellulose metal complexes and cellulose graft copolymers, preferably cellulose ethers. The cellulose derivative preferably has ionic or charged groups, or substituents. Examples of suitable ionic groups include carboxylate groups (for example, carboxyalkyl), sulfonates (for example, sulfoalkyl), phosphates and phosphonates in which the alkyl group can be methyl, ethyl, propyl and mixtures thereof, conveniently methyl; conveniently the cellulose derivative contains an anionic group comprising a carboxylate group, for example, a carboxy alkyl group. The opposite ion of the anionic group is conveniently an alkali metal or alkaline earth metal ion, conveniently sodium.

Examples of suitable cationic groups of cellulose derivatives according to the present invention include amine salts, conveniently tertiary amine salts, and quaternary ammonium groups, preferably quaternary ammonium groups. The substituents attached to the nitrogen atom of the amines or quaternary ammonium groups may be the same or different and may be selected from alkyl, cycloalkyl and alkoxyalkyl groups, and one, two or more of the substituents together with the nitrogen atom may form a heterocyclic ring. Substituents, independently of one another, usually comprise 1 to about 24, preferably 1 to about 8 carbon atoms. The cationic group nitrogen may be attached to cellulose or cellulose derivative by means of a chain of atoms conveniently comprising carbon and hydrogen atoms and optionally oxygen and / or nitrogen atoms. Usually the chain of atoms is an alkylene group with 2 to 18, preferably 2 to 8 carbon atoms, optionally interrupted or substituted by one or more heteroatoms, for example, oxygen or nitrogen, forming an alkyleneoxy or hydroxypropylene group. Preferred cellulose derivatives containing cationic groups include those obtained by reacting cellulose or a cellulose derivative with a quaternization agent selected from 2,3-epoxypropyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride and mixtures thereof.

The cellulose derivatives of this invention may contain non-ionic groups, such as alkyl or hydroxyalkyl groups, for example, hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl and mixtures thereof, for example, hydroxyethylmethyl, hydroxypropylmethyl, hydroxybutylmethyl, hydroxyethylethyl, hydroxypropyl, etc. In a preferred embodiment of the invention, the cellulose derivative contains ionic and non-ionic groups.

Examples of suitable cellulose derivatives according to the present invention include carboxy alkyl cellulose, for example, carboxymethyl cellulose (CMC), carboxymethyl cellulose, carboxypropyl cellulose, sulfoethylcarboxymethyl cellulose, carboxymethylhydroxyethyl cellulose (CM-HEC), carboxymethyl cellulose in which the cellulose is substituted for one or more. ionic, preferably carboxymethyl cellulose. Examples of suitable cellulose derivatives and methods for their preparation include those described in US Patent No. 4,940,785, which is incorporated herein by reference.

Here the term "degree of substitution" (DS) means the number of substituted sites of the β-anhydroglucose rings of the cellulose derivative. Since three hydroxyl groups are available for replacement in each cellulose anhydroglucose ring, the maximum value of the degree of substitution is 3.0. According to a

preferred embodiment of the invention, the cellulose derivative has a degree of substitution of net ionic groups (DSNI) of up to about 0.65, that is, the cellulose derivative has an average degree of net ionic substitution per glucose unit of up to approximately 0.65. The net ion substitution can be net anionic, net cationic or net neutral. When the net ionic substitution is net anionic, there is a net excess of anionic groups (net anionic groups = average number of anionic groups minus average number of cationic groups, if any, per unit of glucose) and DSNI is equal to the degree of replacement of net anionic groups (DSNA). When the net ionic substitution is net cationic, there is a net excess of cationic groups (net cationic groups = average number of cationic groups minus average number of anionic groups, if any, per unit of glucose) and DSNI is equal to the degree of replacement of net cationic groups (DSNC). When the net ion substitution is net neutral, the average number of anionic and cationic groups, if any, per glucose unit is the same and DSNI as well as DSNA and DSNC are zero. According to another preferred embodiment of the invention, the cellulose derivative has a degree of substitution of carboxyalkyl groups (DSCA ") of up to about 0.65, that is, the cellulose derivative has an average degree of substitution of carboxyalkyl per unit. of glucose up to about 0.65. The carboxy alkyl groups are conveniently carboxymethyl groups and then DSCA is the same as the degree of substitution of carboxymethyl groups (DSCM). In accordance with these embodiments of the invention DSNI, DSNA, DSNC and DSCA are usually, independently of one another, up to about 0.60, conveniently to about 0.50, preferably up to about 0.45 and more preferably up to about 0.40 , while DSNI, DSNA, DSNC and DSCA are usually, independently of each other, at least 0.01, conveniently at least about 0.05, preferably at least about 0.10 and more preferably at least about 0 ,fifteen. The ranges of DSNI, DSNA, DSNC and DSCA are usually, independently of one another, from about 0.01 to about 0.60, conveniently 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 an anionic substitution degree (DSA) in the range of about 0.01 to about 1.0 provided that DSNI and DSNA are those defined herein, conveniently from about 0, 05, preferably from about 0.10 and more preferably from about 0.15, to about 0.15, conveniently to about 0.75, preferably to about 0.5 and more preferably to about 0.4. Cellulose derivatives that are cationic or amphoteric may have a cationic substitution degree (DSC) in the range of about 0.01 to about 1.0 as long as DSNI and DSNC are those defined herein, conveniently from about 0, 02, preferably from about 0.03 and more preferably from about 0.05, and conveniently to about 0.75, preferably up to about 0.5 and more preferably up to about 0.4. The cationic groups are conveniently quaternary ammonium groups and then DSC has the same value as the degree of substitution of quaternary ammonium groups (DSQN). In the amphoteric cellulose derivatives of this invention DSA or DSC may, of course, be greater than 0.65 as long as DSNA and DSNC, respectively, are those defined herein. For example, if DSA is 0.75 and DSC is 0.15, then DSNA is 0.60.

In an aqueous solution, the water-soluble cellulose derivatives conveniently have a solubility of at least 85% by weight, based on the total weight of dry cellulose derivative, preferably at least 90% by weight, more preferably by at least 95% by weight and most preferably at least 98% by weight.

The cellulose derivative usually has an average molecular weight of at least 20,000 Da, preferably at least 50,000 Da, to about 1,000,000 Da, preferably up to about

50,000 Da.

The cellulose derivative is conveniently added in an amount of about 0.5 to about 50, preferably about 5 to about 20 and most preferably about 5 to about 10 kg / ton of dried cellulose fibers.

The invention also relates to a paper obtainable by a method comprising stripping a pulp of modified bleached cellulose fibers produced in a mesh produced in accordance with the method described herein and forming a paper from the said slurry pulp suspension.

Having thus described the invention, it should be apparent that it can be varied in many ways. Such variations should not be considered as deviation from the basis and scope of the present invention and it is intended that all of these modifications, as should be apparent to those skilled in the art, be included within the scope of the claims. The following examples will further illustrate how the described invention can be carried out without limiting the scope thereof. Unless otherwise specified, all parts and percentages refer to parts and percentages by weight.

Examples

The objective of the experiment was to adsorb CMC on fibers in a final stage of acid bleaching which, in this case, was a stage of chlorine dioxide. Although not necessary, calcium chloride was used to increase adsorption. The

Paste used was a five-stage bleached conifer paste without elemental chlorine, with a final ISO whiteness of 90%. A reference paste was treated the same as the CMC modified paste according to the invention but without the CMC loading. The final chlorine dioxide stage was performed at 80 ° C for 180 minutes at a consistency of 10% by weight. The chemical load was: 18 kg of chlorine dioxide / ton of dry pulp (7 kg of active chlorine / ton of dry pulp) and 18 kg of calcium chloride (calculated as Ca2 +) / ton of dry pulp. The final pH of the chlorine dioxide stage was 2.8. The CMC used was Finnfix WRH, from Noviant. The degree of substitution was 0.5 and the molecular weight 1x106. Kenores XO wet strength agent was added to the bleached pulp suspension at a load of 15 kg / ton of dry pulp. The resistance properties of CMC-treated pulp at different degrees of refining (ºSR) were evaluated. The refining was performed in a PFI laboratory refining. Be

10 compared the strength properties of refined CMC treated pulp with those of the reference pulp treated with unrefined CMC and with a paste to which CMC was added to the pulp suspension after bleaching. The analyzed paste, which had been bleached with a final stage of chlorine dioxide (laboratory scale), had a final ISO whiteness of 90%.

Diagram 1

15 Wet strength depending on the degree of refining

As can be seen in diagram 1, the wet strength is greatly increased when CMC has been adsorbed in a final stage of chlorine dioxide compared to the addition of CMC to the pulp or without the addition of CMC. In this experiment, the increase in wet strength of the paper produced was 65%.

Diagram 2 Relative wet strength depending on the degree of refining

In diagram 2, the relative wet strength (RWS) is plotted as a function of the degree of refining (ºSR) for CMC adsorbed in the final acid stage of chlorine dioxide and the addition of CMC to the pulp suspension in the process of papermaking as well as a reference without adding CMC. As is evident from diagram 2, the relative wet strength is considerably increased in a paper obtained by the addition of CMC in the acid chlorine dioxide stage.

Claims (14)

one.

 Method of modifying cellulose fibers, which comprises providing a pulp suspension of cellulose fibers and adding a cellulose derivative during the bleaching of said cellulose fibers in at least one stage of acid bleaching, in which the pH of the Paste suspension is from about 1 to about 4 and the temperature ranges from about 30 to about 95 ° C.

2.

Method according to claim 1, wherein the cellulose derivative is added in a final acid bleaching stage.

3.

 Method according to any of claims 1-2, wherein the cellulose derivative is a carboxyalkylcellulose.

Four.

 Method according to any of claims 1-3, wherein the cellulose derivative is carboxymethyl cellulose.

5.

 Method according to any of claims 1-4, wherein the final acid bleaching stage is a chlorine dioxide stage.

6.

 Method according to any of claims 1-5, wherein the cellulose derivative is added in an amount of about 0.5 to about 50 kg / ton of dried cellulose fibers.

7.

Method according to any of claims 1-6, wherein the dry content of cellulose fibers in the suspension is from about 1 to about 50% by weight.

8.

Method according to any of claims 1-7, wherein a wet strength agent is subsequently added to the bleached pulp suspension.

9.

Method according to any of claims 1-8, wherein a dry strength agent is subsequently added to the bleached pulp suspension.

10.

 Method according to any of claims 1-9, wherein the final bleaching stage is performed at a pH of about 2 to about 4.

eleven.

A method of producing paper comprising providing a bleached pulp suspension according to any one of claims 1-10, stripping said pulp suspension into a mesh and forming a paper from said dripped pulp suspension.

12.

 Paper obtainable by the method according to claim 11.

13.

 Use of a cellulose derivative as an additive to a suspension of cellulose fibers that is treated in an acid bleaching step at a pH of about 1 to about 4 and at a temperature of about 30 to about 95 ° C.

14.

 Use of a cellulose derivative according to claim 13, wherein the bleaching stage is an acidic chlorine dioxide stage.