WO2011042604A1

WO2011042604A1 - Method for manufacturing a web, and the web

Info

Publication number: WO2011042604A1
Application number: PCT/FI2010/050776
Authority: WO
Inventors: Isko Kajanto; Maria Frolova
Original assignee: Upm-Kymmene Corporation
Priority date: 2009-10-06
Filing date: 2010-10-06
Publication date: 2011-04-14
Also published as: FI20096025A0; FI20096025A; FI121999B

Abstract

The invention relates to a method for manufacturing a web from fiber-based pulp, and to the corresponding web. According to the invention a fiber-based pulp is manufactured and fibers of the fiber-based pulp are treated with two oppositely charged polyelectrolytes, the first one of which is cationic and the second one of which is anionic, substantially in connection with the manufacture of the fiber-based pulp. The tensile strength index of the fiber-based pulp is more than 60Nm/g before treating the fibers. A web is formed from the fiber-based pulp by web formation.

Description

METHOD FOR MANUFACTURING A WEB, AND THE WEB

FIELD OF THE INVENTION

The invention relates to the method defined in the preamble of claim 1 and to the web defined in the preamble of claim 10.

BACKGROUND OF THE INVENTION

Known from the prior art are different meth- ods for manufacturing a web and paper from pulp. In addition, different methods for improving the properties of paper are known. In papermaking, the aim for certain products is a square weight as low as possible, yet at the same time a specific high strength, which has been problematic to carry out. In papermaking, conventionally the objective has been to produce strong paper that measures up to the required properties with minimal use of fibers. It is known from the prior art to increase the strength of paper by the se- lection of the right fibers, by the right type of grinding of the fibers, by controlling the drying and by the use of additives that increase the strength. The strength of paper is generally a function of three things: the actual strength of the fibers, the fiber length and the bonding strength between the fibers. With strong fibers, the bonding strength is often the limiting factor. The density of paper also affects the strength of the paper. In practice, the limit for tensile index of paper has been approximately lOONm/g, while the density of paper has been approximately 700kg/m³ at the maximum.

Publications US20080023164 , WO0032702,

US5061346 and JP2007277795 disclose methods for treating fiber-based pulp designated for papermaking with several polymers to increase the strength of the paper to be formed.

OBJECTIVE OF THE INVENTION

An objective of the invention is to resolve the above-mentioned problems and to disclose a new type of a method for manufacturing a web and for improving its properties. In addition, an objective of the invention is to disclose a new type of a web, e.g. a paper web, which has the improved properties, especially the strength.

SUMMARY OF THE INVENTION

The method and the web according to the in- vention are characterized by what has been presented in the claims.

The invention is based on a method for manufacturing a web from fiber-based pulp by forming a fiber-based pulp, treating the fibers, e.g. cellulose fibers, of the fiber-based pulp with two oppositely charged polyelectrolytes, the first one of which is cationic and the second one of which is anionic, substantially in connection with the manufacture of the fiber-based pulp, and forming a web from the fiber- based pulp by web formation. According to the invention, the tensile strength index of the fiber-based pulp is arranged to be more than 60Nm/g before treating the fibers.

In addition, the invention is based on a web formed from fiber-based pulp by web formation so that fibers of the fiber-based pulp are treated with two oppositely charged polyelectrolytes, the first one of which is cationic and the second one of which is anionic, substantially in connection with the manufacture of the fiber-based pulp. According to the invention, the tensile strength index of the fiber-based pulp is more than 60Nm/g before treating the fibers.

The invention is based specifically on polye- lectrolytic multilayer coating of the fibers in the fiber pulp to increase the bonding ability of the fibers, and/or on forming polyelectrolyte complexes in the fiber pulp to increase the strength of the web to be formed. In coating the fibers, the oppositely charged polymer layers are absorbed on the surface of the fibers to provide a different surface charge and bonding ability as compared to conventional fibers. By modifying the bonding strength of the fibers a stronger web, and thereby a paper product, is provided.

Fiber-based pulp in this context refers to any fiber—based pulp or fibrous pulp which may be formed from chemical pulp, mechanical pulp, chemi-mechanical pulp, recycled pulp, botanical pulp, pulp of synthetic fibers and their mixtures or derivatives. The fiber- based pulp may contain different fillers and additives.

A web in this context refers to any fiber- based web, fibrous web, paper web or equivalent web. The web may be formed from any fiber-based pulp or fibrous pulp. The web may contain different fillers and additives.

Web formation in this context refers to any manner of forming a web that is known per se in the art .

In one embodiment of the invention a multi- layer coating which contains two oppositely charged polyelectrolytes is formed on the surface of the fibers in the fiber-based pulp.

In one embodiment of the invention the fiber- based pulp is treated by forming in the pulp polyelec- trolyte complexes which contain two oppositely charged polyelectrolytes, substantially in connection with the manufacture of the fiber-based pulp. In one embodiment the polyelectrolyte complexes are formed in the fiber- based pulp substantially in connection with the treatment of the fibers, e.g. as a secondary reaction in the treatment.

In one embodiment a multilayer coating, e.g. a double layer coating or more than one layer coating, and, at the same time, polyelectrolyte complexes are formed on the surface of the fibers of the fiber-based pulp.

In one embodiment of the invention the tensile strength index of the fiber-based pulp is more than 80Nm/g before treating the fibers. In one embodiment the density of the fiber-based pulp is more than 700kg/m³ before treating the fibers. These properties of the fiber-based pulp may be achieved e.g. by the right type of grinding or by the addition of nanofi- bers .

In the context of the invention the following standards were used in the determinations: the manufacture of the sheets ISO 5269-1:2005; the determination of physical properties ISO 5270; and the tensile strength ISO 1924-2.

In one embodiment of the invention the treat- ment of the fibers in the fiber-based pulp is made in two steps, wherein the fibers are treated cationically in a first step and anionically in a second step.

In one embodiment of the invention the first cationic polyelectrolyte is cationic starch.

In one embodiment of the invention the second anionic polyelectrolyte is carboxy-methyl cellulose.

In one embodiment of the invention nanofi- bers, e.g. microfibril cellulose, are added to the fiber-based pulp. By the nanofibers, the strength and tensile strength index values of the web to be formed may be increased. The nanofibers may be added to the fiber-based pulp before treating, i.e. coating, the fibers. Alternatively, the fibers of the fiber-based pulp and the nanofibers may be treated with the cati- onic polyelectrolyte before mixing them together, and the obtained joint pulp may be treated with the anionic polyelectrolyte. In a second alternative embodiment the nanofibers may be added in connection with the anionic polyelectrolyte, e.g. in such a manner that the anionic polyelectrolyte contains nanofibers. In a third alternative embodiment the fiber-based pulp is first treated with the cationic polyelectrolyte, after that the nanofibers are added to the pulp, and then the pulp is treated with the anionic polyelectrolyte .

Nanofibers in this context refer to fibers which are mainly nanosize-class fibers. The nanofibers may also include microsize-class fibers.

The term "microfibril cellulose" refers to a set of isolated cellulose microfibrils or microfibril bundles derived from a cellulose raw material. The aspect ratio of microfibrils is typically high: their length may be more than one micrometer and the number- average diameter is typically less than 200nm. The diameter of the microfibril bundles may also be larger, but generally less than Ιμπι. The smallest microfibrils are similar to the so-called elementary fibrils, the diameter of which is typically 2 to 12 nm. The dimensions of the fibrils or fibril bundles depend on the raw material and the decomposition method. There are many widely used synonyms for microfibril cellulose, for example: nanocellulose, nanofibrillated cellulose (NFC) , nanofibrillar cellulose, cellulose nanofiber, nano-scale fibrillated cellulose, microfibrillated cellulose (MFC) or cellulose microfibrils.

In one embodiment nanofibers are added to the fiber-based pulp in an amount of 0 to 10 w-%. In one embodiment of the invention the web has a tensile strength index of more than lOONm/g, preferably more than 120Nm/g, in at least one direction of the web plane, e.g. in the machine direction,. after the treatment with two oppositely charged polye- lectrolytes according to the invention. In one embodiment the tensile strength index of the web is more than lOONm/g, preferably more than 120Nm/g, as determined from a sheet made by the method IS05269-1 : 2005.

In one embodiment the polyelectrolytes are mixed to the fiber-based pulp and allowed to act therein for a certain time to provide absorption on the surface of the fibers. The amount of the first polyelectrolyte must be high enough to provide the change in the charge of the fibers to a cationic state. This may be observed by measuring the zeta-potential . The amount of the second polyelectrolyte must be sufficient to return the fiber charge back to an anionic state. In one embodiment the treatment is performed without any washing steps between the treatment steps. Alternatively, a washing step may be used.

In one preferred embodiment the mixing of the polyelectrolytes to the fiber-based pulp is made near the head box of the web-making, i.e. web formation.

In one embodiment of the invention the method is used in the improvement of the strength of the web.

In one embodiment of the invention the web is used in the formation of a paper product.

The method according to the invention may be used in the manufacture of different webs and paper products. The method may be utilized in the manufacture of conventional paper grades, e.g. in the manufacture of strong fine paper, or alternatively in the manufacture of special paper grades, e.g. in the manu- facture of thin and strong glassine paper such as release paper. The method and the web according to the invention provide considerable advantages as compared to the prior art.

Thanks to the invention, the strength of ini- tially strong pulps and thereby also the strength of the web to be formed can be improved.

Thanks to the invention, a web and a paper product are provided that have strength properties up to 20 to 30 % superior than those of webs and papers containing the corresponding untreated fibrous pulp, while the square weight remains on the same level.

An industrially applicable easy, quick and useful manner of manufacturing a web and a paper product having a good strength is achieved by the inven- tion.

DETAILED DESCRIPTION OF THE INVENTION

In the following, the invention will be described by means of a detailed example of an embodi- ment .

Example 1

In this test paper webs (60g/m²) were manu- factured from fiber-based pulp.

The fibers of the fiber-based pulp were treated with two polyelectrolytes before forming the web. The fiber-based pulp used was baled pine pulp ground to a value SR 35 by a Valley grinder. The tensile strength index of the untreated pulp was 9lNm/g. The cationic polyelectrolyte used was cationic starch, Raisamyl 150, and the anionic polyelectrolyte used was carboxy-methyl cellulose (CMC) , Finnfix RM. A washing step was not used during the polyelectrolyte treat- ments, simulating the manufacture of real paper. In the tests the fibrous pulp was treated . with cationic starch as well as cationic starch and carboxy-methyl cellulose. The amount of the carboxy- methyl cellulose was half of the amount of the cati- onic starch. The following amounts of the cationic starch/CMC were used in the tests (mg of polyelectro- lyte/g of fibrous pulp): 10/5, 20/10, 30/15 and 50/25.

The treatment was made by first adding the cationic starch to the pulp at room temperature and mixing for approximately lOmin, and after that adding the carboxy-methyl cellulose to the pulp at room temperature and mixing again for approximately lOmin. The thickness of the pulp before the chemicals were added was 2g/l. Handmade test sheets were formed according to the method ISO 5269-1:2005. The sheets were dried.

In the tests it was discovered that a double coating was provided on the surface of the fibers when the fibers were treated with cationic starch and carboxy-methyl cellulose. It was discovered in the tests that, without intermediate washing steps, the unad- sorbable chemicals may form in the fibrous pulp polye- lectrolyte complexes that have an additional effect on the strength of the resulting product.

In the tests it was discovered that by treat- ing the fibers with cationic starch and carboxy-methyl cellulose, better tensile strength index values were provided for the web than by using only cationic starch in the treatment of the fibers. The tests provided paper web products formed from the double- coating treated fibers, the tensile strength indexes of which were ranging between 120 and 130 Nm/g (ISO 5269-1:2005 and ISO 1924-2:2008) and the densities of which were ranging between 700 and 800 kg/m³.

The tests were reproduced in a corresponding manner. The reproductions provided the corresponding results. In addition, dirty paper machine conditions were simulated by means of increased electrical conductivity. It was discovered that it did not have any effects on the results.

In the tests it was discovered that the method according to the invention for manufacturing a paper web provides approximately a 30% increase in the strength of the paper web as compared with a reference web formed in a conventional manner from the corresponding untreated pine fiber pulp.

Example 2

The fibrous pulp according to the invention is manufactured, mixed with microfibrillated cellulose that contains nanofibers to increase the strength.

The fibrous pulp is first treated with cati- onic starch. To the treated fibrous pulp, 0 to 10 w-% of microfibrillated cellulose is added, and after that the obtained pulp is treated with carboxy-methyl cel- lulose.

On the basis of the conducted tests and other analyses performed it was surprisingly discovered that an initially strong fibrous pulp may be treated with two oppositely charged polyelectrolytes to provide improvement of strength also in initially strong pulps, wherein the tensile strength index of the pulp is more than 60Nm/g before treating the fibers. The method in accordance with the invention is suitable in different embodiments to be used for manufacturing the most different kinds of webs.

The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the inventive idea defined by the claims.

Claims

1. A method for manufacturing a web from fiber-based pulp by forming a fiber-based pulp, treating the fibers of the fiber-based pulp with two oppositely charged polyelectrolytes, the first one of which is cationic and the second one of which is anionic, substantially in connection with the manufacture of the fiber-based pulp, and forming a web from the fiber- based pulp by web formation, cha ra ct e ri z ed in that the tensile strength index of the fiber-based pulp is arranged to be more than 60Nm/g before treating the fibers.

2. The method according to claim 1, cha r - a ct e r i z ed in that a multilayer coating that contains two oppositely charged polyelectrolytes is formed on the surface of the fibers in the fiber-based pulp.

3. The method according to claim 1 or 2, cha ra ct e ri z ed in that the fiber-based pulp is treated by forming polyelectrolyte complexes in the pulp substantially in connection with the manufacture of the fiber-based pulp.

4. The method according to any one of claims 1 to 3, cha ract e r i z ed in that the treatment of the fibers in the fiber-based pulp is made in two steps, wherein fibers are treated cationically in a first step and anionically in a second step.

5. The method according to any one of claims 1 to 4, cha ract e r i z ed in that the first cati- onic polyelectrolyte is cationic starch.

6. The method according to any one of claims 1 to 5, cha r acte r i zed in that the second anionic polyelectrolyte is carboxy-methyl cellulose.

7. The method according to any one of claims 1 to 6, cha ract e r i z ed in that nanofibers are added to the fiber-based pulp.

8. The method according to any one of claims 1 to 7, cha ra ct e r i z ed in that the tensile strength index of the web is arranged to be more than lOONm/g.

9. The method according to any one of claims 1 to 8, charact e ri z ed in that the method is used in the improvement of the strength of the web.

10. A web formed from fiber-based pulp by web formation so that fibers of the fiber-based pulp are treated with two oppositely charged polyelectrolytes , the first one of which is cationic and the second one of which is anionic, substantially in connection with the manufacture of the fiber-based pulp, cha ra c t e r i z ed in that the tensile strength index of the fiber-based pulp is more than 60Nm/g before treating the fibers.

11. The web according to claim 10, cha r a ct e r i z e d in that a multilayer coating that contains two oppositely charged polyelectrolytes is formed on the surface of the fibers in the fiber-based pulp.

12. The web according to claim 10 or 11, cha r a ct e r i z e d in that the fiber-based pulp is treated by forming polyelectrolyte complexes in the pulp substantially in connection with the manufacture of the fiber-based pulp.

13. The web according to any one^of claims 10 to 12, cha ra ct e r i zed in that the first cationic polyelectrolyte is cationic starch.

14. The web according to any one of claims 10 to 13, cha ract e r i zed in that the second anionic polyelectrolyte is carboxy-methyl cellulose.

15. The web according to any one of claims 10 to 14, cha ra c t e r i z ed in that nanofibers are added, to the fiber-based pulp.

16. The web according to any one of claims 10 to 15, cha ra ct e r i z ed in that the web has a tensile strength index of more than lOONm/g.

17. The web according to any one of claims 10 to 16, cha ra ct e r i z ed in that the web is used in the formation of a paper product.