EP1240389B1

EP1240389B1 - Method for modifying cellulose-based fiber material

Info

Publication number: EP1240389B1
Application number: EP00966646A
Authority: EP
Inventors: Tom Lindström; Gunborg Glad-Nordmark; Gunnel Risinger; Janne Laine
Original assignee: Svenska Traforskningsinstitutet; STFI Skogsindustrins Tekniska Forskningsinstitut AB
Current assignee: Svenska Traforskningsinstitutet; STFI Skogsindustrins Tekniska Forskningsinstitut AB
Priority date: 1999-09-22
Filing date: 2000-09-20
Publication date: 2010-06-02
Anticipated expiration: 2020-09-20
Also published as: ES2346634T3; WO2001021890A1; SE9903418D0; AU7696000A; DE60044504D1; EP1240389A1; ATE470011T1

Abstract

A method for modifying cellulose fibers; which are treated for at least 5 minutes with an aqueous solution of CMC or CMC-derivative containing electrolyte at a temperature of at least approximately 100 DEG C, and the pH during the treatment is approximately 1.5-4.5, or higher than 11; or the concentration of electrolyte is approximately 0.001 - 0.5 M, if the electrolyte has monovalent cations, or approximately 0.0002 - 0.5 M, if the electrolyte has divalent cations, and cellulose fibers that have been modified according to this method, and use of the modified cellulose fibers for the manufacture of rayon fibers. A method for the manufacture of paper with a high wet strength, whereby an aqueous suspension of cellulose fibers is produced, and the cellulose fibers are modified as described above, and wet-strength agent is added to the aqueous suspension; and paper with a high wet strength manufactured according to this method.

Description

This invention concerns the technical field of paper manufacture, in particular chemical additives during paper manufacture.
The use of carboxymethyl cellulose, hereafter referred to as "CMC", as dry-strength agent or as an additive during the grinding of paper pulp is described by, for example, B.T. Hofreiter in "Pulp and Paper Chemistry and Chemical Technology", Chapter 14, Volume III, 3rd. edition, New York, 1981; W.F. Reynolds in "Dry strength additives", Atlanta 1980; D. Eklund and T. Lindstrom in "Paper Chemistry - an introduction", Grankulla, Finland 1991; J.C. Roberts in "Paper Chemistry"; Glasgow and London 1991.
CMC is anionic and thus has a low affinity for cellulose fibers, since these are anionically charged. Aluminium salts can be used to retain these additives, as has been described by, for example, L. Laurell in "Svensk Papperstidning", 55th. annual edition, 1952, no. 10, page 366.
J.W. Hensley and C.G. Inks (Text. Res. Journal, June 1959, page 505) have described the failure of CMC to be adsorbed to cellulose fibers in electrolyte-free environments and the consequent limitation of its use to what are known as "acidic" paper manufacturing methods in which aluminium salts are used. Adsorption to the fiber material becomes extremely poor when CMC is used in systems that are free of aluminium salts, something that is not compatible with modem paper manufacture. The reason for this is that the presence of anionic polymers, such as CMC, in the stock system interferes with cationic additives of functional or process chemicals by forming what are known as polyelectrolyte complexes. This is a well known phenomenon, and paper manufacturers often refer to such substances as "anionic trash".
Modem paper manufacturing processes in which extremely closed process water systems are used are particularly sensitive for disturbing anionic substances, since a build-up of such substances occurs in the system.
These facts have resulted in the development of cationic additives that have a significantly better affinity for the anionically charged cellulose fibers. Such additives currently have what is essentially a monopoly in the market for dry-strength agents.
In addition to its use as a dry-strength agent, the use of CMC together with wet-strength resin has been described in US-A-3 058 873 . This document specifies a synergistic action between the addition of CMC and wet-strength resin when these additives are used at the same time during paper manufacture. This depends on the fact that CMC can be precipitated on the fibers in the same way as what is known as "anionic trash" can be retained on fibers with the aid of cationic chemical additives. The wet-strength agent is retained through colloidal precipitation. Optimal precipitation of CMC occurs when a stochiometrically neutral complex of CMC and the wet-strength agent is obtained, something that makes the process sensitive to disturbances in the chemistry of the stock. This leads to an unstable process, since the retention of the wet-strength agent will depend on the variability in the incoming raw material and the concentrations of dissolved and colloidal material in the process water.
It would be desirable to be able to achieve a method by which adsorption of CMC to the cellulose fibers could be significantly improved. In this way, the effect of CMC as dry-strength agent during paper manufacture could be improved, among other things. Improved adsorption of CMC to the cellulose fibers would also improve the retention of, and thus the effect of, wet-strength agents.
One problem that is considered to be solved with the present invention is that of achieving such a method.
This problem is solved by the method according to claim 1 presented here. In more detail, the present invention concerns a method whereby cellulose fibers are treated for at least 5 minutes with an aqueous electrolyte-containing solution of CMC or a derivative of CMC, whereby the temperature during the treatment is at least 100 °C and at least one of the following conditions applies:

A) the pH of the aqueous solution during the treatment lies in the interval of approximately 1.5 - 4.5; or
B) the pH of the aqueous solution during the treatment is higher than approximately 11; or
C) the concentration of the electrolyte in the aqueous solution lies in the interval of approximately 0.001 - 0.5 M if the electrolyte has monovalent cations, or in the range approximately 0.0002 - 0.25 M if the electrolyte has divalent cations.

It is preferable if condition C applies together with either condition A or condition B.
A method for modifying cellulose fibers with a cellulose derivative such as CMC is described in the published international patent application WO 99/57370 . This method is performed at a pH of 6 - 13 and a temperature of up to 100 °C, preferably in the approximate range of 20 - 80 °C. It is specified (on page 7, lines 29-30) that the temperature does not constitute a critical factor. There is nothing specified or even implied that a temperature over 100 °C would involve significant advantages for the adsorption.
In association with the present invention it has become apparent that CMC is not adsorbed onto cellulose fibers unless an electrolyte is simultaneously present, and that a higher concentration of electrolyte and high valencies of the counter-ious are advantageous for the adsorption. It has further become apparent that it is necessary to resort to elevated temperatures in order to obtain a sufficiently good adsorption. It has further become apparent not only that the adsorption is irreversible when the concentration of CMC is reduced, but also that essentially ion-free conditions can be reached, with the pulp in its Na-form, without CMC being desorbed to any significant degree. This is a very surprising fact, since according to conventional techniques, essentially no CMC is adsorbed onto cellulose-based fibers under such conditions.
It is thus possible, according to the present invention, to achieve a pulp/paper process in which the pulp is treated for a certain time at a high temperature under such electrolytic conditions that promote the adsorption of CMC. The final pulp receives a higher number of carboxyl groups than the original pulp, which gives a paper that is considerably stronger than paper made using pulp that has been produced using conventional techniques.
The cellulose fibers that are used with the present invention include all types of wood-based fibers, such as bleached, half-bleached and unbleached sulfite, sulfate and soda pulps, together with unbleached, half-bleached and bleached mechanical, thermomechanical, chemo-mechanical and chemo-thermomechanical pulps, and mixtures of these. Both new fibers and recycled fibers can be used with the present invention, as can mixtures of these. Pulps from both softwood and hardwood trees can be used, as can mixtures of such pulps.
Pulps that are not based on wood, such as cotton linters, regenerated cellulose, kenaf and grass fibers can also be used with the present invention.
The preferred concentration of CMC is approximately 0.02 - 4 % w/w, calculated on the dry weight of the fiber material. A more preferred concentration is approximately 0.04 - 2 % w/w, and the most preferred concentration of additive is approximately 0.08 - 1% w/w.
The concept "CMC" is used here to include, in addition to carboxymethyl cellulose, various derivatives thereof. The preferred molar degree of substitution is approximately 0.3 - 1.2 and the preferred viscosity is approximately 25 - 8,000 mPa at a concentration of 4%. A higher viscosity is preferred, since it has become clear that the irreversibility of the adsorption is higher for higher molecular weights.
A high concentration of pulp is particularly desirable if the adsorption is not quantitative, since the loss of CMC can thus be reduced and CMC solution can easily be reintroduced into the reaction vessel. Treatment of pulp preferably takes place as a separate treatment step at high pulp concentration, but it can naturally also be carried out at the same time as, for example, digesting, or during a bleaching step. As high a concentration of pulp as possible is thus desired, but this is naturally limited by practical conditions during the conduct of the method. The preferred concentration of pulp is approximately 3 - 50%, a more preferred concentration interval is approximately 5 - 50%, and the most preferred concentration interval is approximately 10 - 30%. Such high concentration mixes are known to one skilled in the arts within the relevant technical field, and are suitable for use in association with the present invention.
A preferred range of pH is approximately 2 - 4, in particular approximately 2.5 - 3.5.
A higher concentration of electrolyte and a higher valence of the cation increase the affinity of CMC for the pulp. The preferred concentration interval for salts with monovalent cations, such as Na₂SO₄, is approximately 0.002 - 0.25 M, in particular within the range approximately 0.005 - 0.1 M. The preferred concentration interval for salts with divalent cations, such as CaCl₂, is between approximately 0.0005 - 0.1 M, in particular approximately 0.02 - 0.05 M.
The preferred adsorption period is approximately 5 - 180 min, a more preferred adsorption period is approximately 10 - 120 min and the most preferred adsorption period is approximately 15 - 60 min.
The preferred temperature is in excess of approximately 100 °C, a more preferred temperature is in excess of approximately 120 °C and the most preferred adsorption temperature is up to approximately 150 °C. The method according to the invention is thus carried out at a pressure in excess of atmospheric pressure. Suitable equipment and working conditions for this will be obvious for one skilled in the arts.
The pulp can be washed or diluted directly after the treatment, or it can be dried in the normal manner.
The present invention also concerns a method for production of paper with a high wet strength, whereby

an aqueous suspension of cellulose fibers is produced;
the cellulose fibers are modified by treatment for at least 5 minutes with an aqueous solution of CMC or a CMC derivative containing electrolyte, whereby
the temperature during the treatment is at least approximately 100 °C
and at least one of the following conditions apply:
1. A) the pH of the aqueous solution during the treatment lies in the interval of approximately 1.5 - 4.5; or
2. B) the pH of the aqueous solution during the treatment is higher than approximately 11; or
3. C) the concentration of the electrolyte in the aqueous solution lies in the interval of approximately 0.001 - 0.5 M if the electrolyte has monovalent cations, or in the range of approximately 0.0002 - 0.25 M if the electrolyte has divalent cations; and
wet-strength agent is added to the aqueous suspension of cellulose fibers.

It has actually also become clear that cellulose fibers treated according to the present invention, when treated with wet-strength agent, provide a much higher wet strength than the strength that can be explained by the higher adsorption of wet-strength agent to the fibers.
This may be due to the fact that it is more advantageous to retain the wet-strength agent evenly distributed over the fiber surfaces, as occurs according to the present invention, than it is to have it as a colloidal precipitation, as occurs according to US-A 3 058 873 .
A paper can be defined as wet-strengthened in this context when the geometric mean value of the wet strength divided by the dry strength exceeds 0.15.
Mixtures of compatible wet-strength agents and other chemicals used in paper production can be used within the scope of the present invention, as can what are known as "debonding agents."
The preferred concentration of wet-strength agent used as additive to the stock is up to approximately 2% wiw, calculated on the [weight of ] dry fibers, a more preferred concentration is approximately 0.02 - 1.5 % and the most preferred concentration is 0.05 - 0.8 %.
Wet-strength agents that can be used include all cationic polymeric resins. These include, for example, those wet-strength agents that give permanent wet strength: urea-formaldehyde resins, melamine-formaldehyde resins and polyamide-amine resins. Examples of wet-strength agents that give temporary wet strength are polyethylene imine, dialdehyde starch, polyvinyl amine and glyoxal polyacrylamide resins.
According to one embodiment of the present invention, a method is also provided for making paper with a high wet strength but low dry strength, a method that can be used, for example, for producing paper structures that are strong when wet and absorbent. What are known as "debonding agents" are used in this embodiment, and preferred debonding agents are quaternary ammonium salts with fatty acid chains that can be retained by electrostatic attraction to the negatively charged groups on the surfaces of the fibers. The result is a paper with a wet strength /dry strength ratio that preferably exceeds 0.1, a more preferred value exceeds 0.2 and the most preferred value exceeds 0.3.
It has also become clear that if fibers treated according to the present invention are dried, a pulp is produced that when repulped gives fibers with a higher water retention ability. In other words, the treatment of this nature has given a fiber that has been keratinized to a lesser degree during drying. Such fibers demonstrate also a higher reactivity during subsequent chemical treatments, for example, when manufacturing rayon fibers.
Other embodiments of the present invention are described in more detail with the aid of examples of embodiments, the only purpose of which are to illustrate the invention and are in no way intended to limit its scope.

Examples

Example 1: This example according to known technology demonstrates how the conditions in the chemical environment affect the amounts of different types of CMC that are irreversibly adsorbed. The CMC preparations that were used were commercially available preparations from Metsä-Serla: Finnfix WRH with a DS of 0.56 and a viscosity of 530 mPa at a concentration of 2%, and Cekol FF2 with a DS of 0.7 - 0.85 and a viscosity of 25 mPa at a concentration of 4%. The pulp was a bleached, long-fibered, undried softwood sulfate pulp from Metsä-Serla/Husum's factories. The adsorption trials were conducted at a pulp concentration of 2%. The pulp was washed with 0.01 M HCl after the treatment and then transferred to its Na-form in de-ionised water. The pulp was washed after 2 hours with de-ionised water. The amount of CMC adsorbed was determined by conductometric titration. The amount of CMC that was added was 40 mg/g. "DS" is used to denote the degree of molar substitution for the CMC used.

Table 1
Chemical conditions	Temp (°C)	Adsorption time (hours)	Amount of Finnfix WRH adsorbed (mg/g)	Amount of Cekol FF 2 adsorbed (mg/g)
De-ionised water	80	2	0
0.1 M NaCl	23	2	4.8
0.1 M NaOH 0.1 M NaCl	80	2		16.1
0.1 M NaCl	80	2	8.9	2.9
0.1 M NaCl	80	72	11.4
0.05 M CaCl₂	80	2		17.4

Table 1 shows that the presence of electrolyte is necessary to obtain adsorption. It is also clear that the adsorption is higher at higher temperatures. Higher alkalinity is also advantageous for the adsorption. The degree of molar substitution or the molecular weight of CMC is not critical, but the adsorption increases when the degree of substitution decreases.

Example 2: This example shows that a very high relative amount of CMC can be irreversibly bound to a bleached undried softwood sulfate pulp (Metsä-Serla/Husum factories) by the selection of a high temperature and a high electrolyte concentration. The experiment was performed by treating the pulp at 120 °C or at 150 °C for 2 hours in 0.05 M CaCl₂ buffered with 0.001 M NaHCO₃. The amounts of CMC adsorbed (FinnFix WRH and Cekol FF2) were measured both after washing the pulp with de-ionised water (Ca-form) and after washing the pulp with 0.01 M HCl, de-ionised water, adjusting its pH value using NaOH to a pH of 8 and equilibrating it with an 0.001 M NaHCO₃ buffer for 2 hours (Na-form). As the table shows, a smaller amount of CMC is desorbed when the pulp has been transferred into its Na-form. "WRV" is an abbreviation for "Water Retention Value" and is a measure of the ability of the pulp to retain water (here the Na-form was measured at 3,000 g and 15 minutes in de-ionised water).

Table 2
Type of CMC	Treatment temperature (°C)	Dosage (mg/g)	Ionic form	Amount adsorbed (mg/g)	WRV (%)
	120	0	Na	0	135
Finnfix WRH	120	20	Ca	17.7
Finnfix WRH	120	20	Na	14.8	173
Finnfix WRH	120	40	Ca	27.3
Finnfix WRH	120	40	Na	23.4	195
Finnfix WRH	150	40	Ca	26.3
Finnfix WRH	150	40	Na	23.1	182
Cekol FF2	120	20	Ca	15.5
Cekol FF2	120	20	Na	11.5	146
Cekol FF2	120	40	Ca	22.4
Cekol FF2	120	40	Na	18.5	156
Cekol FF2	150	40	Ca	19.1

Example 3: CMC (Finnfix WRH) was adsorbed onto a bleached undried softwood sulfate pulp (Metsä-Serla/Husum factories) at different pH values in de-ionised water at 120 °C. The pulp had been transferred to its Na-form before the pH was adjusted. The amount of CMC added was 20 mg/g. The results in Table 3 show that a certain amount is adsorbed at 120 °C, but better adsorption is achieved if electrolyte is present during the treatment (compare with Table 2).

Table 3

Type of CMC pH during treatment Amount adsorbed (mg/g) WRV (Na-form) pH = 8.0

WRH 3 8.6 136

WRH 8 5.2 141

WRH 12 8.9 151
Example 4: This example shows that the adsorbed amount of CMC is adsorbed to the cellulose fibers so strongly that it remains on the fibers even after a prolonged period of leaching. The bleached sulfate pulp from Example 2 was treated with 40 mg/g Finnfix WRH for 2 hours at 120 °C in 0.1 M NaCl. The amount adsorbed after this treatment was 7.7 mg/g. After leaching the pulp in de-ionised water for 19 hours, the adsorbed amount was 7.4 mg/g.
Example 5: This example shows that selecting a high temperature and a high concentration of electrolyte at the adsorption step gives a pulp that has a lower water retention ability than that obtained if the CMC is adsorbed onto the pulp at a lower temperature.
The experiment was performed by treating an undried softwood sulfate pulp (Metsa-Serla/Husum factories) at 23 °C, 80 °C and 120 °C for 12 hours in 0.05 M CaCl₂ buffered by 0.001 M NaHCO₃. The amount of adsorbed CMC (Finnfix WRH; 20 mg/g) was measured after washing the pulp with 0.01 M HCl, de-ionised water, adjusting its pH with NaOH to a pH of 8 and equilibrating it with 0.001 M NaHCO₃ buffer for 2 hours (Na-form). WRV is an abbreviation for "Water Retention Value" according to the definition given earlier.
Table 10 shows that the increase in WRV per mg/g of adsorbed CMC is considerably lower if the CMC has been adsorbed at a higher temperature (120 °C) than if it has been adsorbed at a lower temperature. This is particularly advantageous if it is to be easy to de-water the pulp on the paper machine. The ability of the pulp to retain water, however, does not reflect the strength of the paper that is manufactured from the pulp under consideration. Table 10

Temperature during treatment (°C) WRV (%) Amount Finnfix WRH adsorbed (mg/g) (WRW - WRW_ref) per mg/g adsorbed CMC

Reference 129 - -

23 200 4.8 14.8

80 204 8.3 9.0

120 173 14.8 3.0

Claims

Method for modifying cellulose fibers, characterized in that the cellulose fibers are treated for at least 5 minutes with an aqueous electrolyte-containing solution of CMC or a CMC derivative, whereby
- the temperature during the treatment is at least 100 °C, and at least one of the following conditions apply:
A) the pH of the aqueous solution during the treatment lies in the interval of approximately 1.5 - 4.5, preferably in the region 2 - 4; or

B) the pH of the aqueous solution during the treatment is higher than approximately 11; or

C) the concentration of the electrolyte in the aqueous solution lies in the interval of approximately 0.001 - 0.5 M, preferably 0.005 - 0.1 M, if the electrolyte has monovalent cations, or in the range of approximately 0.0002 - 0.25 M, preferably 0.0005 - 0.1 M, if the electrolyte has divalent cations.
Method according to claim 1, characterized in that the pH of the aqueous solution during the treatment lies in the interval of approximately 1.5 - 4.5 and the concentration of the electrolyte in the aqueous solution lies within the interval of approximately 0.001 - 0.5 M if the electrolyte has monovalent cations, or approximately 0.0002 - 0.25 M if the electrolyte has divalent cations.
Method according to claim 1, characterized in that the pH of the aqueous solution is higher than 11 and the concentration of the electrolyte in the aqueous solution lies within the interval of approximately 0.001 - 0.5 [M] if the electrolyte has monovalent cations, or approximately 0.0002 - 0.25 M if the electrolyte has divalent cations.
Method according to claim 1, characterized in that the cellulose fibers are treated for approximately 5 - 180 minutes.
Method according to claim 1, characterized in that the temperature during the treatment is at least approximately 120 °C, and preferably up to approximately 150 °C.
Method for manufacturing paper with a high wet strength, characterized in that
- an aqueous suspension of cellulose fibers is produced;

- the cellulose fibers are modified according to any one of the preceding claims; and

- wet-strength agent is added to the aqueous suspension of cellulose fibers.
Method according to claim 6, characterized in that a debonding agent is also added to the aqueous suspension of cellulose fibers.