WO2008099064A1

WO2008099064A1 - Method of producing paper and cardboard

Info

Publication number: WO2008099064A1
Application number: PCT/FI2008/050064
Authority: WO
Inventors: Antti Korpela; Erkki Saharinen; Eero Hiltunen
Original assignee: KESKUSLABORATORIO - CENTRALLABORATORIUM Oy AB; Keskuslaboratorio Centrallaboratorium Oy AB
Current assignee: KESKUSLABORATORIO - CENTRALLABORATORIUM Oy AB; Keskuslaboratorio Centrallaboratorium Oy AB
Priority date: 2007-02-14
Filing date: 2008-02-14
Publication date: 2008-08-21
Anticipated expiration: 2009-08-14
Also published as: FI20075100A7; FI20075100A0; FI20075100L

Abstract

A method of modifying the pulp which is used for manufacturing paper or cardboard. According to the method, the pulp is crosslinked chemically, after which it is used to produce paper or cardboard. According to the present invention, the crosslinlcing is carried out by using the dry method, and the crosslinked pulp is then refined to a chosen drainability. With the invention, it is possible to decrease the grammage of the products which are produced from the pulp, without degrading the properties of the products.

Description

Method of producing paper and cardboard

The present invention relates to the improvement of the properties of fibrous pulp. The invention relates especially to a method of modifying the fibrous pulp which is used in the manufacturing of paper, cardboard and corresponding fibrous products, according to the preamble of Claim 1.

According to such a method, the pulp is first refined to a predefined drainability level, after which the fibrous product is prepared from the fibrous pulp at the paper or cardboard machine.

In the manufacturing of paper and cardboard, the aim is to minimize the consumption of raw material in order to reduce the costs of production. The grammage of the products to be manufactured is determined by the requirements of the process of manufacturing paper and cardboard, their further preparation and end uses. For instance, the grammage of printing papers must be high enough for the papers to withstand the papermaking and printing without their breaking, and in order to render them sufficiently opaque to ensure good printability and good visual appearance. The cardboard must, for its part, be thick enough to achieve a good flexural strength, which is required in package usage.

The density and the opacity of paper and cardboard depend essentially upon the properties of the raw materials. Some of the positive characteristics of the so-called chemical pulps that are widely used by the paper and cardboard industry are their high brightness (bleached qualities), good strength and purity. A collective name of the chemical pulp masses could be the term "pulp"; typical pulps are as known particularly sulphate and sulphite pulps. The major paper technological weakness of the pulps is the tendency of the fibres to collapse during wet pressing and drying. As a result of the collapse, the thickness and the opacity of the paper and cardboard web are reduced. At the same time, the dewatering of the web is slowed, leading to higher consumption of energy for drying the web. In several paper and cardboard machines slow dewatering is the most important single factor limiting the speed of the process.

The purpose of the present invention is to eliminate the disadvantages associated with the known solutions and to generate a completely new solution for treating chemical pulp mass or pulp, which are used in the production of fibre products, in which case the properties of the products generated from the pulps are improved, especially their optical properties, their dewatering and their bulk (density).

The present invention is based on the idea that the treatment of cellulose-bearing fibrous pulp, especially chemical pulp, with a crosslinking material, the refining of the generated pulp and the using of the refined pulp for producing a fibrous product, are all combined. According to the present invention, the fibrous pulp is brought to the crosslinking stage in at least essentially dry conditions, i.e. the solution is carried out by using the "dry method".

More specifically, the present invention is characterized by what is stated in the characterizing part of Claim 1.

Considerable advantages can be achieved with the present invention. Thus, from the cellulose-bearing fibre pulps, especially from the chemical pulps, which are crosslinked and then refined, it is possible to produce paper and cardboard the density, light-scattering, opacity and dewatering properties of which are considerably better than before. By applying the present invention, it is possible to adjust and widen the traditional range of the properties of the pulp which is used in paper and cardboard making. The method enables a reduction in the grammage of the printing papers and cardboards containing pulps and thereby saves raw material costs during their production process.

As described in more detail below, by using the present invention, the drainability (dewatering) of the treated fibres is improved. The reason seems to be that the crosslinked fibre absorbs less water than that which is non-crosslinked. The cross-bridges prevent the fibres from swelling and the water from being absorbed into the fibre. It also seems that a crosslinked fibre does not easily change its shape, for instance by spreading out or flattening out during the wet pressing and during the drying, which also means that the removal of the water in the fibrous network is easier.

The present invention will now be described and explained in more detail with the help of the accompanying drawings:

Figure 1 shows a process scheme of the crosslinking of pulp, and Figures 2a and 2b show the effect of this crosslinking on the properties of birch sulphate pulp; the percentage changes in the properties are shown at two different levels of pulp refining (in Figure 2a 1000 revolutions, and in Figure 2b 4000 revolutions), the reference point (0 %) being dried birch sulphate pulp.

As described above, according to the present invention, the pulp comprising cellulose is mixed with the crosslinker/crosslinkers, after which the crosslinking of the cellulose molecules is generated in essentially dry conditions in order to modify the cellulose, and thereafter the pulp which is treated in this way is refined and used in a way which is known per se to form a web. The controlling parameters characteristic to the manufacturing of the product are the level of crosslinking, and the degree and the method of refining. The crosslinking reaction between the cellulose and the crosslinker is carried out using the dry method, in which method the dry matter content of the fibrous material is increased to over 85 % by weight, especially at least 90 % by weight, prior to the heating treatment or corresponding treatment, during which treatment the crosslinker is brought to react with the hydroxyl groups of the cellulose.

The crosslinkers are organic compounds comprising at least two groups which react with the hydroxyl groups of the cellulose molecules. When the crosslinker reacts with two parallel cellulose chains and forms bonds with them, a so-called cross-bridge is formed between the chains. The cross-bridges stabilize the fibres to protect them against deformations caused by stress, and always tend to restore the cellulose chains and thus the whole fibre to the same position and shape which the cellulose chains and fibres had at the time of the formation of the cross-bridges. The cross-bridges stabilize the structure of the fibres in such a way that the contraction of the fibre, which normally takes place during the refining of the pulp, during the wet pressing of paper and cardboard, and during the drying stage, is at least partially prevented. Consequently, the thickness of paper and cardboard made from crosslinked fibres is considerably higher than that of standard paper and cardboard. At the same time, the optical properties, for instance the brightness, the light scattering ability, and opacity, are considerably improved. The increased thickness of the paper, improved optical properties, and maintenance of strength properties at a sufficiently high level enables a reduction in the grammage of the paper products, and thus savings in the raw material costs during their production. Together, the reduced water absorbency of the chemical pulp fibres resulting from the crosslinking and the improved structural stability of the fibres accelerate the removal of water from the fibres and the pulp. The quicker dewatering improves the runability of paper at the paper machine - and cardboard at the cardboard machine - and in many cases makes it possible to raise the process speed of the paper and cardboard machines and decrease the amount of energy needed for drying.

In the examples below, the crosslinking of the chemical pulp was carried out by using hydroxy-methylated 4.5 dihydroxyethylen urea (abbr. "mDMDHEU"). However, the present invention is not restricted to the use of this compound. As experts obviously know, during the crosslinking of cellulose molecules it is possible to use any compound that comprises at least two reactive groups, which form a covalent bond with the hydroxyl groups of the cellulose, or any other compound which reacts in the pulp in such a way that cross-bridges are formed between the cellulose chains. In addition to the above-mentioned ethylene urea derivatives, it is possible to use as crosslinkers for instance many poly carboxylic acids, such as succinic acid, maleic acid or citric acid or mixtures thereof. Particularly good results are achieved with poly carboxylic acids which have three reactive groups. Crosslinking often requires a catalyst. Any catalyst which accelerates the crosslinking reaction speed, such as MgCl₂, or an acid or alkali catalyst, may be utilised.

The present invention is not limited only to crosslinking of pulp, but it is possible to crosslink any natural fibre which comprises cellulose and which is used in the production of paper, cardboard or a corresponding product by means of a wet- webbing technology. The fibre raw material can be sourced from hardwood or softwood or even species of annual or perennial grasses, or recycled waste fibres. Pulp, especially made of hard- or softwood, is a good example of a raw material where crosslinking has great commercial potential. Examples of chemical defibering methods are those carried out in alkaline conditions, such as the conventional sulphate method and the continuous cookings, sulphite/sulphide cookings in neutral or acidic conditions, and cookings performed using organic cooking chemicals, such as the organosolv and Milox processes. It is known that mechanical and chemi-mechanical pulps can be manufactured from the above-mentioned raw materials by using any of the following methods: the mechanical pulp method, and the refined mechanical pulp method, and the CMP and CTMP processes.

Hydroxy-methylated 4.5 dihydroxyethylen urea, in turn, is one example of those crosslinkers that form cross-bridges between the cellulose chains used under the process conditions, the arrangement of which conditions does not require any or only moderate changes in conventional process solutions for pulp, paper and cardboard making.

Typically, the percentage of the chemicals which are used for the crosslinldng of the dry matter of the pulp is <10 %, most preferably < 2 %, especially preferably 0.1 % - 1.5 %.

After crosslinking, the crosslinked pulp is refined to different degrees in order to adjust the properties of the pulp. In the refining, as areas of the fibre surfaces are exposed by the process and become bare, they adhere well to other fibres.

The drainability achieved in the refining is chosen depending on the desired end product. Generally, the drainability is approximately 300-700 ml CSF or approximately 15.8-22 ⁰SR, however, these are not absolute limit values, but, depending on the strength properties desired, it is also possible to continue the refining further.

It is known that the refining can be carried out at a low consistency (2-6 % consistency, LC), at medium consistency (10—20 % consistency, MC), or at high consistency (30-35 % consistency, HC). It is possible to affect the drainability with the method chosen. All the basic methods mentioned are usable for producing the modified pulp prepared in the present invention.

The present invention does not apply to solutions where the exclusive purpose of the crosslinking is to improve both the absorbing capacity and the drainability of the pulp. Examples of these solutions are different kinds of adsorbent substances, such as the filling materials in nappies and filter papers. The use of crosslinked celluloses in these products is already known III. Characteristic of the solutions in question is that the substances used in them are unrefined.

It is possible to carry out the crosslinldng in association with the drying, as shown in Figure 1. The process scheme shown in the figure applies to a preferred embodiment of the present invention, which is easy to apply in factory environments.

In the case of the process scheme, the pulp, for instance ordinary birch sulphate pulp, is crosslinked by using the dry crosslinking method, in which the pulp is first absorbed by employing an aqueous solution comprising the crosslinker and the catalyst, whereafter the pulp is dried and heated. The formation of cross-bridges takes place during the heating. According to this solution, the chemical pulp and the crosslinking agent are fed into the pulp container 1 and then mixed with each other in order to distribute the chemicals as evenly as possible throughout the pulp. The pulp which is removed from the container is led into a dewatering unit 2, where the excess water is separated from the pulp for instance by compressing the pulp. This water is then recirculated back into the pulp container 1. The pulp, which at this stage has a dry matter percentage of typically approximately 30-50 % by weight, is led into the drying stage 3 where approximately 50-95 % of the remaining water is removed from it.

A conventional drum dryer can be used as the drying unit but other known drying methods are possible, too. The drying is carried out at room temperature or at an elevated temperature, but most suitably under 120 ⁰C. After that, the dried pulp, the dry matter percentage of which is over 85 % by weight, especially at least 90 % by weight, most suitably even 92 % by weight or higher, is fed into the heating zone, where it is heated at an elevated temperature in order to generate the crosslinking reactions. The heating temperature and time always vary depending on the chemicals used and the moisture content of the pulp. After the heating, the crosslinked pulp is refined in the refiner 5.

A dry crosslinking method that resembles the one described above is commonly used in the textile industry to diminish the tendency of textiles containing cotton to shrink and crinkle (the so-called "easy-care" treatment) 121. No suggestion is found in this publication that it could be used in modifying conventional paper or cardboard pulp. However, it should be stressed that the present invention is not limited to applying the dry crosslinking method described in the above example, rather that this example represents a solution, the implementation of which does not require substantial process changes in the production of pulp, and thus it is the most usable crosslinking solution for most paper mills. An alternative method of bringing the pulp into contact with crosslinking chemicals is a method where the solution is sprayed into a wet compressed pulp, wherein it is allowed to be absorbed evenly. In that case, a separate compressing (dewatering) stage is not needed. In a solution such as this, the dry matter percentage of the pulp is generally at a maximum of 90 % by weight. All known techniques for drying pulp can be applied to the drying of the pulp in this solution. Also, the heating can be carried out using several alternative techniques, for instance furnace, cylinder or blast heating.

For a successful crosslinking, it is essential that during the heating stage all of the pulp that is heated reach a sufficiently high temperature, which depends on the crosslinker employed and the type of catalyst used, and that this temperature is maintained for long enough to allow most of the crosslinking reactions to take place. When using compounds such as the mDMDHEU and other ethylene urea-based compound types and catalysts such as MgCl₂, a preferable heating temperature is approximately 130-150 ⁰C and the heating time is generally 0.1—30 minutes, typically approximately 1-10 minutes, especially approximately 2—5 minutes, for instance approximately 3-10 minutes. When other chemicals are used, different temperature ranges and different treating periods are used.

The process of crosslinking can be applied to market pulps, i.e. pulps that are dried and transported to be used elsewhere. Moreover, crosslinking can also be employed advantageously in so-called integrated paper mills using the pulps they produce themselves, as many of these paper mills dry their pulp before it is taken to refining. The reason for this practice is to improve the dewatering properties, the bulk and optical properties of the pulp which are affected by the plain drying of the pulp. The effect can, however, be significantly enhanced by crosslinking.

There now follows a description of an example which comprises a dry crosslinking process and the refining of the crosslinked pulp, in which example the effect of the crosslinking on the properties of the refined pulp is described. The example introduces the main stages of the dry crosslinking of cellulose, i.e. the absorbing, drying and heating of the chemicals. As experts obviously know, the optimal absorbing, drying and heating conditions entirely depend on both the crosslinker used and its catalyst. Experts also know that by changing the refining method, for instance the specific arris load of the refining, it is possible to considerably affect the length of the fibre and the flexibility of the fibres, the quantity and quality of the fines generated and thus the properties of the crosslinked pulp. In the example, a conventional refining method which is typical of birch sulphate pulps, was used. Example

Bleached birch pulp was absorbed at a 3 % consistency into an aqueous solution, into which 5g/l of crosslinker (mDMDHEU active substance) and 2 g/1 catalyst (MgCl₂ active substance) had been added. The pH of the solution was adjusted to the range of 5-6. During the absorbing (duration 10 minutes) the pulp was mixed in order to ensure that fibres absorbed the chemicals evenly. After the absorbing, the pulp was filtered to a consistency of 20 %. At this point, the percentage of the crosslinker (active substance) of the pulp was 2.5 % of the dry matter quantity of the pulp. The quantity of the catalyst, in turn, was 1 % of the dry matter quantity of the pulp. The filtered pulp was dried at a temperature of 110 ⁰C. After that, the pulp was heated to a temperature of 150 °C for a period of 10 minutes, by which time the crosslinking was complete. After the crosslinking, the pulp was slushed in water and refined. The bulk and the optical properties of the refined pulp were excellent.

The effect of the quantity of crosslinker and of the degree of refining on the properties of the pulp is demonstrated in the accompanying drawings (Figures 2a and 2b). The percentage changes of the properties are shown at two different degrees of refining (revolutions 1000 and 4000). The reference point (0 %) is dried birch sulphate pulp.

It should be pointed out that in the cases of Figures 1 and 2, PFI refining was used, which is common in laboratories but which differs substantially from real refining. The case described in the previous paragraph including its conventional refining refers to a case where the pulp was refined with a laboratory/pilot refiner which corresponds well to a paper mill refiner.

Changing of pulp properties by crosslinking in a way which is described in example 1 makes it possible to increase the thickness of a paper product while maintaining a standard grammage or, alternatively, reducing the grammage while a standard paper thickness is maintained. The present invention does not limit in any way the use of generally known means by which the properties of paper products have traditionally been adjusted. For instance, the fall in strength which is caused by crosslinking can be preferably compensated by raising the amount of dry strong glue or by increasing for instance the amount of long stock reinforcement pulp. For a person skilled in the art it is obvious that raising the bulk and increasing the optical properties by using crosslinking leads to considerably more savings in the costs of raw materials of paper and cardboard making than would be possible if compensating actions were taken. Experts know, too, that the impact of improved dewatering enabled by crosslinking together with improved air permeability decreases the amount of energy needed for drying paper and cardboard. The pulp produced according to the present invention is used in for instance the manufacturing of paper of qualities required for printing; these include writing papers and copying papers, and cardboards.

References:

1. Westervelt, P.H., Elston, C. Cross-linked fibers for papermaking applications. TAPPI Proceedings, 1995, Nonwovens conference, pp 79-85.

2. Peter, M., Rouette, H. K. Grundlagen der Textil Veredlung. Handbuch der Technologie, Verfahren, Maschinen. 13. ϋberarbeitete Auflage. DfV Deutscher Fachverlag 1989.

Claims

Claims:

1. A method of modifying the pulp used in paper and cardboard making, according to which method - the pulp is crosslinked chemically, after which

- paper or cardboard is manufactured from the pulp, characterized in that

— the pulp is a chemical cellulose pulp, which is crosslmked by using the dry method and - the crosslinked pulp is then refined to a predefined drainability.

2. The method according to Claim ^characterized in that the crosslinker is a water soluble organic compound.

3. The method according to Claim 1 or 2, c h a r a c t e r i z e d in that the crosslinker is an organic material that comprises at least two reactive groups, which form a covalent bond with the hydroxyl groups of the cellulose.

4. The method according to Claim 3, characterized in that the crosslinker is a polyvalent organic acid.

5. The method according to any of Claims 1-4, characterized in that a water soluble catalyst is used to assist the crosslinking.

6. The method according to any of the preceding claims, characterized in that the pulp which is absorbed along with the crosslinker and the catalyst is, before the refining, brought into conditions in which the formation of cross-bridges between the cellulose molecules takes place.

7. The method according to any of the preceding claims, characterized in that the crosslinking is combined with the drying of the pulp.

8. The method according to any of the preceding claims, characterized in that the properties of the crosslinked pulp are controlled by the refining.

9. The method according to any of the preceding claims, characterized in that an ethylene urea derivative such as hydroxy-methylated 4.5 dihydroxyethylen urea is used as the crosslinker.

10. The method according to any of the preceding claims, characterized in that the percentage of the chemicals used for the crosslinking, which is calculated from the dry matter of the pulp, is <10 %, most preferably < 2 %, especially preferably 0.1 % - 1.5 %

11. The method according to any of the preceding claims, characterized in that the dry matter percentage of the fibrous pulp is over 85 % by weight, especially at least 90 % by weight, most suitably even 92 % by weight or higher.

12. The method according to any of the preceding claims, characterized in that the fibrous pulp is produced by using the sulphate or the sulphite method.

13. The method according to any of the preceding claims, characterized in that the pulp is used for the manufacturing of printing paper qualities, including writing or copying papers, or cardboards.