EP2622132B1

EP2622132B1 - Method for improving runnability of a wet paper web, use of a solution and paper

Info

Publication number: EP2622132B1
Application number: EP11770850.3A
Authority: EP
Inventors: Kristian Salminen; Elias Retulainen; Janne Kataja-Aho
Original assignee: Stora Enso Oyj; Kemira Oyj
Current assignee: Stora Enso Oyj; Kemira Oyj
Priority date: 2010-10-01
Filing date: 2011-09-30
Publication date: 2018-04-04
Anticipated expiration: 2031-09-30
Also published as: US20130299109A1; EP2622131A1; CA2813148A1; CN103201426B; CN103140626A; CN103201426A; FI20106021A; CA2813148C; FI125713B; PT2622132T; BR112013007244A2; FI20106021L; WO2012042115A1; BR112013007876A2; ES2674881T3; FI20106021A0; EP2622132A1; ES2791997T3; EP2622131B1; WO2012042116A1

Description

The present invention relates to a method for improving runnability of a wet paper web according to the preamble of the enclosed claim.

BACKGROUND OF THE INVENTION

Economical production of paper requires a good runnability of a paper machine. The paper machine runnability is often evaluated by the number of web breaks in proportion to production speed. To attain good runnability, the paper must run well with a low number of web breaks in each sub-process along the entire paper machine line. In practice, it is common for one of the sub-processes to cause most of the web breaks, leading to a poor total efficiency. It has been noticed that many of the runnability problems occur when the paper web is still in the wet state, especially during the transfer from the press section to the dryer section. Thus, a good runnability of the beginning part of the paper machine when the paper is still wet is advantageous if a high production efficiency of the entire papermaking line is to be attained.
The wet paper runnability may be increased by increasing the strength of the wet web. A number of solutions are known for increasing the strength of the wet paper web, such as increasing the beating degree of the pulp, varying the overall furnish composition or web tension in process. However, many of these solutions cause, at the same time, deterioration of the properties of the final produced paper or increase significantly the production costs. For example, an increase in the beating degree may increase curling and adversely affect the properties of final paper.
Traditional wet strength additives, which are used to increase the wet strength of the dried final paper web, do not enhance strength of the wet paper web, i.e. the strength of never dried wet webs. This is because wet strength additives typically require heating and curing before they show strength improving properties.
Paper machines producing paper grades whose strength before drying is a critical factor may have high efficiency but their average production speed may be significantly lower than their nominal speed. The speed of these paper machines could be raised if the strength of the wet paper web could be increased.
Fillers, such as clay, calcium carbonate, calcium sulphate or talc are used in paper making to reduce cost of paper and to improve optical properties of paper. Fillers are added to the stock before paper machine. For coated paper grades coating pigments, which comprise the same minerals, may partly enter to the paper via the broke, which is recycled back to paper making process. The content of fillers and coating pigments is typically measured through ash content measurement by burning the stock or paper sample in 525 °C.
The base paper for uncoated fine paper and for coated fine paper is made from softwood and hardwood and its ash content is typically 18 - 24 %. The base paper for 100 % softwood based uncoated fine paper and for coated fine paper has an ash content typically 10 - 17 %. An important limiting factor preventing the increase of filler content in fine papers is the wet web runnability.
Known methods are disclosed in DE 19520092 and US 2005/0155731 . GB 1163842 discloses a method for manufacturing paper, board or the like, where an aqueous dispersion of an additive is added to a wet web containing a flocculant for the additive in the said dispersion.
An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.
An object of the present invention is to provide an effective and simple method for improving the strength of a wet paper web or the like.
An object of the present invention is to increase filler content of paper in order to reduce papermaking cost.
These objects are attained with a method having the characteristics presented below in the characterising part of the independent claim.
Typical method according to the present invention for improving runnability of a wet paper web or the like is defined in claim 1. The method comprises, inter alia,

adding to a stock of fibres a cationic polymer,
forming a wet paper web from the stock of fibres, and
applying an anionic polymer solution to the wet paper web.

Typically the solution comprising an anionic polymer in dissolved form is used for improving runnability of a wet paper web by applying the solution to the web.
Now it has been surprisingly found out that when an anionic polymer solution is applied, preferably by spraying, to the wet paper web, which comprises fibres having at least partially cationic surface charge, the interactions between the fibres are improved as well as the strength of the wet paper web. It is assumed, without willing to be bound by theory, that the applied anionic polymer is adsorbed or attached by the electronic interactions on the fibre surface with cationic charge. The increase in wet paper web strength could be affected by the molecular level interactions between the chemicals and fibres. This may promote the amount of fibre-fibre interactions and the strength of the bonds that are formed between the fibres. It is further speculated that that the increased molecular level interaction between fibres, whether of electrostatic or chemical nature, explains the strength increase of the wet paper web rather than formation of covalent bonds. Application of an anionic polymer solution to the wet paper web is thus made in order to improve the interactions of the fibres with each other and tensile strength and/or residual tension of the wet paper web.
It is also observed that the application of an anionic polymer solution to the wet paper web may enable an increase in the filler content of paper. When the web wet strength or tension after straining is improved by use of the present invention, a high filler content in the base paper may be used, corresponding to ash content e.g. over 25 % for uncoated fine paper and for coated fine paper base paper made from softwood and hardwood mixture. Correspondingly, a high filler content in the base paper may be used for 100% softwood based uncoated fine paper and for coated fine paper base paper, the high filler content corresponding to an ash content over 18%. An improvement in initial wet web strength and dry strength would enable ash content increase also for other paper and board grades, such as ash content increase to 5 - 17 % for newsprint grades, or ash content increase to 8 - 14 for SC and LWC base paper.
Improvement in strength of the initial wet paper web may also be utilised by changing to cheaper raw material mixture for the stock. For example, less old corrugated container (OCC) and more collected paper from households to make test liner or fluting board grade. The ash content of recycled fibre based fluting or test liner board may be increased over 15%.
Another way to utilise the improved initial wet web strength is to reduce line loads at the press, which improves dry strength and decreases porosity of paper. This would be beneficial for printability.
The anionic polymer solution may be applied to the wet fibre web and the application may be performed in any suitable manner, for example by spraying or coating. The anionic polymer solution may also be applied by using film transfer, such as film transfer to a press belt, foam layer application or feeding of anionic polymer solution from a separate headbox. Preferably the application of the anionic polymer solution is performed by spraying. It has been found out that the spraying of the anionic polymer solution to the fibre web provides many surprising advantages. Spraying of the polymer solution does not influence the formation of the paper web, whereby there is no negative effects to be noticed in the final paper properties. On the other hand, it has also been noticed that the retention of the anionic polymer solution to the web is better than by addition the polymer solution to the stock. This means that the used amount of the anionic polymer can be kept low, and chemical losses may be minimised. It has been observed that when the anionic polymer solution is added by spraying, the polymer is evenly distributed through the whole web. No difference in amount of the polymer can be observed between the surfaces and the core part of the web.
Preferably, the applying of the anionic polymer to the wet paper web does not have an effect on the density of final dry paper. This might indicate that the application of anionic polymer increases the strength of fibre-fibre bonds in the wet paper web but it might not increase the number of these bonds. Also surprisingly, the spraying of the anionic polymer may increase the air permeability of the final dried paper, even by 35% on average.
The anionic polymer solution is a solution that comprises a dissolved anionic polymer in a solvent, typically water. The anionic polymer solution may comprise the anionic polymer both in dissolved and dispersed form, as long as the amount of dissolved anionic polymer is sufficient for obtaining the desired effect. According to one embodiment of the invention the anionic polymer solution, which is used in this invention may be free from discrete polymer particles. The anionic polymer solution may comprise one anionic polymer or it may comprise a mixture of different dissolved anionic polymers, for example a mixture of two or three anionic polymers. This embodiment means applying to the wet paper web anionic polymer solution, which comprises a mixture of different anionic polymers, for example a mixture of two or three anionic polymers. In case the anionic polymer solution comprises a plurality of different dissolved anionic polymers, the anionicity of the polymers may be different from each other. In other words, the solution may comprise two or more dissolved anionic polymers having different anionicity.
According to one embodiment two or more anionic polymer solutions may be applied on the wet paper web sequentially, preferably by spraying. This means that a first anionic polymer solution is applied to the wet paper web and after this a second and optional following anionic polymer solutions are applied to the wet paper web. The application is preferably performed by spraying.
According to one embodiment the anionic polymer solution comprises synthetic anionic polymer, such as anionic polyacrylamide, or carboxymethyl cellulose (CMC) or other anionic high molecular weight polymers, such as anionic starch, anionic guar gum or alginate. This anionic polymer solution is applied on the wet paper web. Carboxymethyl cellulose is an anionic polymer produced by introducing carboxylmethyl groups to the cellulose chain, the degree of substitution and the chain length of the cellulose backbone affecting its properties. When the degree of substitution exceeds 0.3, carboxymethyl cellulose becomes water soluble.
According to one embodiment of the invention the anionic polymer solution may also be obtained by using an amphoteric polymer, provided that its net charge is anionic at the pH of the papermaking process.
Typically the concentration of the anionic polymer solution is < 1 weight-%, more typically 0.05 - 1 weight-%, even more typically 0.2 - 0.6 weight-%
According to one embodiment of the present invention a solution of a synthetic anionic polymer is applied as the anionic polymer solution to the wet paper web. The synthetic anionic polymer is selected from the group comprising a completely or partly hydrolyzed polyacrylamide, or a homopolymer or a copolymer comprising at least one anionic monomer, such as (meth)acrylic acid, maleic acid, crotonic acid, itaconic acid, vinylsulphonic acid, 2-acrylamide-2-methylpropanesulfonic acid, styrene sulfonic acid, vinyl phosphonic acid or ethylene glycol methacrylate phosphate. Also non charged monomers can be included. Preferably the synthetic anionic polymer is a copolymer of acrylamide with one or more anionic comonomers. Also glyoxylated anionic acrylamide copolymers are suitable for use in the present invention. The synthetic anionic polymer can be either in acid or in salt form, and it can be linear, branched or slightly crosslinked.
In this application a copolymer means a polymer, which is composed of at least two different monomers. The number of different monomers, which form the copolymer, may be higher than two, for example three or four.
The weight average molecular weight (MW) of the synthetic anionic polymer is typically >100,000 g/mol, more typically >1,000,000 g/mol.
According to one embodiment of the invention the application of the anionic polymer solution on the wet paper web may be performed by spraying a synthetic anionic polymer solution having a concentration in the range of 0.3 - 0.5 weight-%. Preferably, the solution of synthetic anionic polymer is sprayed to the wet paper web. A solution suitable for use in the present invention may be obtained, for example, by dissolving a synthetic anionic polymer powder in water in order to form a 0.3 - 0.5 % w/w solution. The solution may also be obtained by dissolving an emulsion or dispersion of a synthetic anionic polymer.
The anionic polymer may be applied, preferably by spraying, in amount ≤ 2 g/m², typically 0.05 - 1.5 g/m², more typically ≤ 1 g/m², most typically 0.05 - 1 g/m², preferably 0.05 - 0.5 g/m², more preferably 0.05 - 0.3 g/m² to the wet paper web.
Normally the surface charge of natural unmodified cellulosic, lignocellulosic or wood fibres, which are used in papermaking, is anionic. According to one embodiment of the invention the surface charge of the fibres is manipulated to be at least partially cationic, i.e. the fibres may have surface areas that have a cationic charge, even if they may have other surface areas that have anionic charge. This may be obtained by modifying at least part of the fibres in order to at least partially change their surface charge. For example, the surface charge of the fibres, which are used for making the paper web, may be modified, partially or wholly, by adding a cationic polymer to the stock.
The cationic polymer, which is added to the stock, may be any cationic polymer suitable to be used in the stock. Cationic polymer, which is added to the stock of fibres, may be selected from the group comprising chitosan, a cationised polysaccharide, such as cationic starch or cationic guar gum, or a cationic synthetic polymer, such as cationic acrylamide copolymers, vinylamine copolymers or polyamidoamine. Also glyoxylated cationic polyacrylamides may be used as cationic polymer in the present invention. Cationic polymer may also be a cationic starch graft co-polymer that is described for example in US 7,786,238 B2 . Preferably the cationic polymer, which is added to the stock, is cationic starch.
When cationic polymer, which is added to the stock of fibres is cationic acrylamide polymer, it may be formed from water soluble ethylenically unsaturated monomers or from a monomer blend, which includes cationic monomers. Preferably the cationic acrylamide polymer has an apparent intrinsic viscosity of at least 1.0 dl/g more preferably at least 1.5 dl/g.
According to one embodiment of the invention cationic polymer may be an amphoteric polymer, provided that its net charge is cationic at the pH of the papermaking process.
In the embodiment of the invention where synthetic cationic polymers are added to the stock, the addition is preferably made to the thick stock, which has consistency of > 2 %, preferably 3 - 5 %. Synthetic cationic polymers are typically and preferably added to the thick stock in amount of 0.5 - 5 kg/t. Preferably the synthetic cationic copolymers that are added to the stock have low charge density, for example cationic acrylamide copolymers are typically low charge cationic acrylamide copolymers having charge < 1.7 meq/g preferably < 1.2 meq/g at pH 4.
According to one embodiment of the invention the surface charge of the fibres is modified by adding cationised polysaccharide, which is cationic starch, to the stock of fibres. The cationised polysaccharide, which is cationic starch, is added to the thick stock, having consistency of 2 - 5 % between pulp storage tower and short circulation. Cationic starch may be any cationic starch suitable to be used in paper making, such as potato, rice, corn, waxy corn, wheat, barley or tapioca starch. Starches having an amylopectin content > 75 % are advantageous. Typically cationic starch comprises cationic groups, such as quaternized ammonium groups, and the degree of substitution (DS), indicating the number of cationic groups in the starch on average per glucose unit, is typically 0.01 - 0.20, preferably 0.01 - 0.06. Typically cationic starch has a charge of 0.06 - 1.04 meq/g, preferably 0.06 - 0.35 meq/g. The starch may be used in amount of 2 - 20 kg/ton pulp, typically 7 - 12 kg/ton pulp.
The anionic polymer solution may be applied to the wet paper web when the dryness of the web is < 50 %, typically < 40 %, more typically < 30 %, most typically 8 - 15 %. When the pulp suspension enters the headbox and thus the paper machine, its dryness level is typically more or equal to 0.3 % and less than 2 %. The first water removal from the web is driven by gravity when the web enters the wire section from the headbox. As paper travels further in the wire section, water removal is assisted by different vacuum units. After the wire section, the dryness of the paper is typically 20 %. The dryness of paper increases to 40 - 50 % during wet pressing. The applying of the anionic polymer solution is preferably conducted before the last vacuum zone of the wire section, preferably by spraying.
According to one embodiment of the invention the applying of the anionic polymer solution is performed to the wet paper web before press section of a paper machine.
The present invention is advantageous for improving strength of the wet paper web when producing wood-free uncoated and coated paper grades. The present invention is also suitable for improving strength of the wet paper web when producing paper grades including super calendered (SC) paper, ultralight weight coated (ULWC) paper, light weight coated (LWC) paper and newsprint paper, but not limited to these. Typical coated magazine paper, such as LWC, comprises mechanical pulp around 40 - 60 weight-%, bleached softwood pulp around 25 - 40 weight-% and fillers and/or coating agents around 20 - 35 weight-%. SC paper comprises mechanical pulp around 70 - 90 weight-% and long fibered cellulose pulp around 10 - 30 %. Especially paper webs that are to be used for making recording substrates for the inkjet printing are suitable to be treated according to the method of the present invention. The paper web may comprise fibres from hardwood trees or softwood trees or a combination of both fibres. The fibres may be obtained by any suitable pulping or refining technique normally employed in paper making, such as thermomechanical pulping (TMP), chemimechanical (CMP), chemithermomechanical pulping (CTMP), groundwood pulping, alkaline sulphate (kraft) pulping, acid sulphite pulping, and semichemical pulping. The paper web may comprise only virgin fibres or recycled fibres or a combination of both. The weight of the final paper web is 30 - 800 g/m², typically 30 - 600 g/m², more typically 50 - 500 g/m², preferably 60 - 300 g/m², more preferably 60 - 120 g/m², even more preferably 70 - 100 g/m².
In some embodiments the paper web may comprise fibres originating from non-wood material, such as bamboo, sugar cane bagasse, hemp, wheat or rice straw.
According to another embodiment, the application of the anionic polymer solution to the wet paper web is preceded or followed by application of cationic or amphoteric polymer solution. This kind of sequential application of polymers to the wet paper web, preferably through spraying, may produce a marked improvement of dry and wet paper web strength. Anionic and cationic polymer solutions may also be premixed together before their application, preferably by spraying, to the wet paper web. In this embodiment, addition of a cationic polymer to the stock of fibres is not compulsory, but it may be performed.
For example, polymer solution(s) of carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), modified polyvinyl alcohol, guar gum and/or chitosan are applied on wet paper web before the press section before or after applying the anionic polymer solution to the wet paper web. Carboxymethyl cellulose, polyvinyl alcohol, modified polyvinyl alcohol, guar gum or chitosan may be applied one at the time or two or three of them may be applied sequentially. CMC and chitosan improve the wet web strength when the web has a dryness level above approximately 55%, and PVA improves the wet web strength also at lower web dryness levels. Polyelectrolyte multilayers of anionic and cationic polymers increase the molecular contact area in the fibre-fibre joints and thus increase the strength of dry paper. Now it has been noticed that the layering of polymers, for example two layers, may improve also the strength of the wet paper web significantly. This shows that the layering of polymers also increases the interactions between fibres in the wet state. It is also plausible that the spraying of anionic polymer to the outermost layer may reduce the adhesion between the wet web and the anionic centre roll on a paper machine. During the generation of polymer bi- or multilayers on the paper machine by spraying the amount of sprayed polymers is advantageously minimised.
According to still another embodiment one or more layers of chemical solutions are applied to the wet paper web before the press section or drying section. The addition of a cationic polymer to the stock of fibres is not compulsory, but it may be performed. Cationic polymers are as defined earlier in this text. The chemical solutions are preferably applied to the wet paper web by spraying, as described in the application, but they may be applied by coating, film transfer, foam layer application or feeding from a separate headbox. The chemical solution that is applied to the web, e.g. by spraying, may be a solution of carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), chitosan or guar gum. Guar gum is here understood as a galactomannan. It is a polysaccharide comprising galactose and mannose. The backbone of the guar gum is a linear chain of β 1,4-linked mannose residues to which galactose residues are 1,6-linked at every second mannose, forming short side-branches. Guar gum may be applied to the web in form of native guar gum, anionic guar gum or cationic guar gum. For example, native, cationic or anionic guar gum may be applied to the wet paper web, which is formed without using addition of a cationic polymer to the stock. In another example, native or anionic guar gum may be applied to the wet paper web, which is formed from stock into which cationic polymer, such as cationic guar gum, is added.

EXPERIMENTAL

Example 1

Commercial never-dried bleached softwood kraft pulp from a Finnish paper mill is used as raw material. The pulp has a Canadian Standard Freeness (CSF) value of 500 ml, measured according to standard SCAN-C 21:65.
Wet handsheets having grammage of 60 g/m2 are formed by using standard for preparation of laboratory sheets with recirculated white water SCAN-CM 64:00. Cationic starch, cooked for 30 min at T=97°C is added to the furnish in the sheet former separately for each sheet. The amount of added cationic starch is 10 kg/t (Raisamyl 135 starch DS 0.035, BASF). Reference is made without cationic starch.
Anionic polymer solution is sprayed onto the formed handsheets attached onto a wire with a vacuum underneath. The vacuum enhances the penetration of anionic polymer solution into the wet paper sheet during spraying. The concentration of the sprayed anionic polyacrylamide (Fennopol A 8500 R from Kemira Oyj) is 0.3 % w/w consistency and mixed at room temperature over night before spraying. Reference sheets are sprayed with water.
After spraying, the handsheets are wet pressed. The wet pressing is done at two different pressure levels, 50 kPa and 350 kPa to reach two different dryness levels for the wet handsheets. Wet samples are cut to a width of 20 mm with a sample length of 100 mm. Wet samples are stored in an air-proof condition in a plastic bag at a temperature of 7°C in order to maintain the level of dryness.
Mechanical properties of dry and wet paper samples are determined with an impact device that used a velocity of 1.0 m/s (VTT Finland, FAST TENSILE TEST RIG IMPACT). The impact device and test method are described in following references: Kurki, M., Kouko, J., Kekko, P., Saari, T., Laboratory scale measurement procedure of paper machine wet web runnability: Part 1, Paperi ja Puu, 86(2004)4; and Kouko, J., Salminen, K., Kurki, M., Laboratory scale measurement procedure of paper machine wet web runnability: Part 2, Paperi ja Puu, 89(2007)7-8.
The results are seen in Figure 1, showing the effect of studied application method on the initial wet strength of paper. The following abbreviations are used in Figure 1:

CS= cationic starch
A-PAM= anionic polyacrylamide

Claims

Method for improving runnability of a wet paper web or the like, comprising
- adding to a stock of fibres a cationic polymer, which is selected from
- cationised polysaccharide, which is cationic starch, added to a thick stock, which has a consistency of 2 - 5 %, in amount of 2 - 20 kg/t; or

- a cationic synthetic polymer, which is selected from cationic acrylamide copolymers, added to a thick stock, which has consistency of > 2 %, in amount of 0.5 - 5 kg/t, and

- forming a wet paper web from the stock of fibres,
characterised in
applying an anionic polymer solution comprising
- carboxymethyl cellulose (CMC), or

- a synthetic anionic polymer selected from the group comprising a completely or partly hydrolyzed polyacrylamide; or a homopolymer or a copolymer comprising at least one anionic monomer, such as (meth)acrylic acid, maleic acid, crotonic acid, itaconic acid, vinylsulphonic acid, 2-acrylamide-2-methylpropanesulfonic acid, styrene sulfonic acid, vinyl phosphonic acid or ethylene glycol methacrylate phosphate,
in amount of 0.05 - 1.5 g/m² to the wet paper web when the dryness of the web is 8-15%.
Method according to claim 1, characterised in that the synthetic anionic polymer is a copolymer of acrylamide with one or more anionic comonomers.
Method according to claim 1 or 2, characterised in applying the anionic polymer in amount 0.05 - 0.5 g/m² to the wet paper web.
Method according to any of preceding claims, characterised in the application of the anionic polymer solution is performed by spraying, by coating, by using film transfer, by foam layer application or by feeding the anionic polymer solution from a separate headbox.
Method according to claim 4, characterised in performing the application of the anionic polymer solution on the wet paper web by spraying a synthetic anionic polymer solution having a concentration in the range of 0.3 - 0.5 weight-%.
Method according to any of preceding claims, characterised in applying to the wet paper web anionic polymer solution, which comprises a mixture of different anionic polymers, for example a mixture of two or three anionic polymers.
Method according to claim 1, characterised in applying two or more anionic polymer solutions on the wet paper web sequentially, preferably by spraying.
Method according to claim 1, characterised in applying of the anionic polymer solution to the wet paper web before press section of a paper machine.
Method according to claim 1, characterised in applying of the anionic polymer solution to the wet paper web before the last vacuum zone of the wire section.
Method according to claim 1, characterised in by adding to the stock of fibres cationic acrylamide polymer, formed from water soluble ethylenically unsaturated monomers or from a monomer blend including cationic monomers, the cationic acrylamide polymer preferably having an apparent intrinsic viscosity of at least 1.0 dl/g more preferably at least 1.5 dl/g.
Method according to any of preceding claims, characterised in that the application of the anionic polymer solution to the wet paper web is preceded and/or followed by application of cationic or amphoteric polymer solution to the wet fibre web, preferably through spraying.
Method according to claim 1, characterised in applying a polymer solution of polyvinyl alcohol (PVA), modified polyvinyl alcohol, guar gum and/or chitosan on the wet paper web before the press section before or after applying the anionic polymer solution to the wet paper web.
Method according to claim 1, characterised in improving strength of the wet paper web when producing wood-free uncoated paper, wood-free coated paper, super calendered (SC) paper, ultralight weight coated (ULWC) paper, light weight coated (LWC) paper or newsprint paper.