FI125713B - A method for improving the runnability of a wet paper web and paper - Google Patents

Description

METHOD FOR IMPROVING THE WET PAPER FABRICABILITY AND PAPER

The present invention relates to a method for improving the runnability of a wet paper web and to paper according to the preambles of the appended claims.

BACKGROUND OF THE INVENTION

Economical production of paper requires good runnability of the paper machine. The runnability of a paper machine is often estimated by the number of web breaks in relation to the production speed. To achieve good runnability, the paper must pass through very few web breaks in each subprocess of the paper machine line. In practice, it is common for one of the subprocesses to cause most of the outages, resulting in poor overall efficiency. It has been found that many runnability problems occur when the paper web is still wet, especially when moving from the press section to the drying section. Thus, good runability of the initial part of the papermaking machine while the paper is still wet is advantageous if high production efficiency is to be achieved throughout the papermaking line.

The runnability of wet paper can be increased by increasing the strength of the wet web. Solutions for increasing the strength of a wet paper web are known, such as increasing the pulverization rate of the pulp, changing the total stock composition or web tension in the process. However, many of these solutions simultaneously result in the final degradation of the properties of the paper produced or significantly increase the cost of production. For example, increasing the grinding rate may increase curvature and adversely affect the properties of the final paper.

Conventional wet strength agents used to increase the wet strength of the dried final web do not improve the wet paper web strength, i.e., the strength of wet webs that have never been dried. This is because wet strength agents typically require heating and storage maturation before exhibiting strength enhancing properties.

Paper machines that produce paper grades where strength before drying is a critical factor can be very efficient, but their average production rate may be significantly lower than their rated speed. The speed of these paper machines could be increased if the strength of the wet paper web could be increased.

Fillers such as clay, calcium carbonate, calcium sulfate or talc are used in papermaking to reduce the cost of paper and improve the optical properties of paper. The fillers are added to the pulp before the paper machine. In coated paper grades, coating pigments containing the same minerals may be partially introduced into the waste paper, which is recycled back into the papermaking process. The filler and coating pigment content is typically measured by ash content measurement by burning a pulp or paper sample at 525 ° C.

The base paper of uncoated fine paper and coated fine paper is made from coniferous and hardwood and typically has an ash content of 18-24%. The ash content of 100% softwood-based uncoated fine paper and coated fine paper is typically 10-17%. An important limiting factor that prevents an increase in the filler content of fine papers is the wet web runnability.

The object of the present invention is to reduce or even eliminate the disadvantages of the prior art.

It is an object of the present invention to provide an effective and simple method for improving the strength of a wet paper web or the like.

It is an object of the present invention to increase the filler content of paper to reduce the cost of papermaking.

These objects are achieved by a method and arrangement, the features of which are set out below in the characterizing parts of the independent claims.

A typical method of improving the wet paper web or similar runnability according to the present invention comprises: - adding a cationic polymer to the pulp, - forming a wet paper web from the pulp, and - adding an anionic polymer solution to the wet paper web.

A typical paper of the present invention is prepared using the process of the invention.

Typically, a solution comprising the anionic polymer in dissolved form is used to improve the runnability of the wet paper web by adding the solution to the web.

It has now surprisingly been found that when anionic polymer solution is added, preferably by spraying, to a wet paper web comprising fibers having at least partially cationic surface charge, the interactions between the fibers and the strength of the wet paper web are improved. Without wishing to be bound by theory, it is assumed that the added anionic polymer adsorbs or attaches itself to the surface of the fiber having a cationic charge by electrical interactions. The increase in wet paper web strength could be influenced by molecular level interactions between chemicals and fibers. This may enhance the amount of fiber-to-fiber interactions and the strength of bonding between the fibers. Furthermore, it is assumed that the increased inter-fiber interactions at the molecular level, which are either electrostatic or chemical in nature, explain the increase in wet paper web strength rather than the formation of covalent bonds. Thus, the anionic polymer solution is added to the wet paper web to improve the interaction of the fibers with each other and the tensile strength and / or residual tension of the wet paper web.

It has also been found that adding an anionic polymer solution to a wet paper web may allow for an increase in the filler content of the paper. When the wet strength or tension of the web after stretching has been improved using the present invention, a higher filler content equivalent to, e.g., more than 25% ash in uncoated fine paper and coated fine paper blended with softwood and hardwood can be used. Similarly, a high filler content in base paper can be used for 100% softwood-based uncoated fine paper and coated fine paper base paper, with a high filler content of more than 18% ash. Improving the original wet web strength and dry strength would allow increasing the ash content in other paper and board grades, such as newsprint grades to 5-17%, or SC and LVVC base paper ash content 8-14.

The improvement in the strength of the original wet paper web can also be realized by changing the raw material blend to a cheaper one. For example, less recycled corrugated board (OCC) and more household recycled paper for testliner or fluting quality. The ash content of the recycled fiber-based fluting or testliner may be increased by more than 15%.

Another way to take advantage of the strength of the original wet web is to reduce the line load on the press, which improves dry strength and reduces the porosity of the paper. This would be useful for printability.

The anionic polymer solution may be added to the wet fibrous web by any suitable means, for example by spraying or coating. The anionic polymer solution can also be added using film transfer to the press belt, foam layer addition or feed of the polymer solution from a separate headbox. Preferably, the addition of the anionic polymer solution is effected by injection. It has been found that injecting an anionic polymer solution onto a fibrous web has many surprising advantages. Injection of the polymer solution does not affect the paper web formation, so that there is no discernible effect on the properties of the final paper. On the other hand, it has also been found that the retention of the anionic polymer solution in the web is better than adding the polymer solution to the pulp. This means that the amount of anionic polymer used can be kept low and the chemical losses can be minimized. It has been found that when anionic polymer solution is added by injection, the polymer is uniformly distributed throughout the web. No difference can be observed in the amount of polymer between the surfaces of the web and the core.

Preferably, the addition of the anionic polymer to the wet paper web does not affect the final dry paper density. This may mean that the addition of the anionic polymer increases the strength of the fiber-to-fiber bonds on the wet paper web, but does not necessarily increase the number of these bonds. Unexpectedly, the spraying of the anionic polymer can also increase the air permeability of the final dry paper by up to 35% on average.

Anionic polymer solution is a solution comprising dissolved anionic polymer in a solvent, typically water. The anionic polymer solution may comprise the anionic polymer in both dissolved and dispersed form, as long as the amount of dissolved anionic polymer is sufficient to produce the desired effect. According to one embodiment of the invention, the anionic polymer solution used in the present invention may be free of discrete polymer particles. The anionic polymer solution may comprise a single anionic polymer or may comprise a mixture of various dissolved anionic polymers, for example a mixture of two or three anionic polymers. If the anionic polymer solution comprises several different dissolved anionic polymers, the anionicity of the polymers may be different relative to one another. In other words, the solution may comprise two or more dissolved anionic polymers with different anionicities.

According to one embodiment, two or more anionic polymer solutions can be added sequentially to a wet paper web. This means that the first anionic polymer solution is added to the wet paper web, and then the second and optional subsequent anionic polymer solutions are added to the wet paper web. The addition is preferably effected by injection.

In one embodiment of the present invention, the anionic polymer solution comprises a synthetic anionic polymer, such as anionic polyacrylamide, or carboxylmethylcellulose (CMC) or other anionic high molecular weight polymers such as anionic starch, anionic guar gum or alginate. Carboxylmethylcellulose is an anionic polymer made by adding carboxylmethyl groups to the cellulose chain, the degree of substitution and the length of the cellulose backbone affecting its properties. When the degree of substitution exceeds 0.3, the carboxylmethylcellulose becomes water soluble.

According to one embodiment of the invention, the anionic polymer solution can also be obtained by using an amphoteric polymer provided that its net charge at the pH of the papermaking process is anionic.

Typically, the concentration of the anionic polymer solution is <1% by weight, more typically 0.05 to 1% by weight, even more typically 0.2 to 0.6% by weight.

According to one embodiment of the present invention, the solution of the synthetic anionic polymer is added to the wet paper web as an anionic polymer solution. For example, the synthetic anionic polymer may be wholly or partially hydrolyzed polyacrylamide, or a homopolymer or copolymer comprising at least one anionic monomer such as (meth) acrylic acid, maleic acid, crotonic acid, itaconic acid, vinylsulfonic acid, 2-acrylic acid, 2-acrylic acid, 2-acrylic acid, ethylene glycol methacrylate phosphate. Uncharged monomers may also be included. Preferably, the synthetic anionic polymer is a copolymer of acrylamide, together with one or more anionic comonomers. Glyoxylated anionic acrylamide copolymers are also suitable for use in the present invention. The synthetic anionic polymer may be in either acid or salt form and may be straight chain, branched, or slightly crosslinked.

As used herein, copolymer means a polymer consisting of at least two different monomers. The number of different monomers forming the copolymer may be greater than two, for example three or four.

The synthetic anionic polymer typically has a weight average molecular weight (MW) of> 100,000 g / mol, more typically> 1,000,000.

Preferably, the synthetic anionic polymer solution is sprayed onto a wet paper web. A suitable solution for use in the present invention is obtained, for example, by dissolving the synthetic anionic polymer powder in water to obtain a 0.3-0.5% w / w solution. The solution may also be obtained by dissolving an emulsion or dispersion of the synthetic anionic polymer.

The anionic polymer may be added, preferably by injection, in an amount of <2 g / m 2, typically 0.05-1.5 g / m 2, more typically <1 g / m 2, most typically 0.05-1 g / m 2, preferably 0.05-0 , 5 g / m 2, more preferably 0.05-0.3 g / m 2 for a wet paper web.

In general, the surface charge of natural unmodified cellulose, lignocellulosic or wood fibers used in papermaking is anionic. According to one embodiment of the invention, the surface charge of the fibers is treated such that it is at least partially cationic, i.e. the fibers may have surface areas having a cationic charge, although they may have other surface areas having an anionic charge. This can be achieved by modifying at least a portion of the fibers, thereby changing their surface charge at least partially. For example, the surface charge of the fibers used in papermaking can be partially or completely altered by adding a cationic polymer to the pulp.

The cationic polymer to be added to the pulp may be any cationic polymer suitable for use in the pulp. The cationic polymer may be chitosan, a cationized polysaccharide such as cationic starch or cationic guar gum, or a cationic synthetic polymer such as cationic acrylamide copolymers, vinylamine copolymers or polyamide amine. Glyoxylated cationic polyacrylamides may also be used as the cationic polymer in the present invention. The cationic polymer may also be a graft polymer of cationic starch as disclosed, for example, in US 7,786,238 B2. Preferably, the cationic polymer to be added to the pulp is cationic starch.

When the cationic polymer to be added to the pulp is a cationic acrylamide polymer, it may be prepared from water-soluble ethylenically unsaturated monomers or a mixture of monomers containing cationic monomers. Preferably, the cationic acrylamide polymer has an apparent intrinsic viscosity value of at least 1.0 dl / g, more preferably at least 1.5 dl / g.

According to one embodiment of the invention, the cationic polymer may be an amphoteric polymer provided that its net charge at the pH of the papermaking process is cationic.

In an embodiment of the invention where the synthetic cationic polymers are added to the pulp, the addition is preferably made to a thick pulp having a consistency of> 2%, preferably 3-5%. Synthetic cationic polymers are typically added to the thick pulp in an amount of 0.5-5 kg / t. Preferably, the synthetic cationic copolymers added to the pulp have a low charge density, for example, cationic acrylamide copolymers are typically low charge cationic acrylamide copolymers having a charge of <1.7 meq / g, preferably <1.2 meq / g at pH 4.

According to one embodiment of the invention, the surface charge of the fibers is modified by adding a cationized polysaccharide, such as cationic starch or cationic guar gum, to the fiber mass. The cationized polysaccharide, such as cationic starch or cationic guar gum, is preferably added to a thick pulp having a consistency between 2 and 5% between the pulp storage tower and the short rotation. The cationic starch may be any cationic starch suitable for use in papermaking, such as potato, rice, corn, waxy corn, wheat, barley or tapioca starch. Starches having an amylopectin content of> 75% are preferred. Typically, the cationic starch comprises cationic groups, such as quaternized ammonium groups, and the degree of substitution (DS), which indicates the average amount of cationic groups in the starch per unit glucose, is typically 0.01-0.20, preferably 0.01-0.06. Typically, the cationic starch has a charge of 0.06 to 1.04 meq / g, preferably 0.06 to 0.35 meq / g. The starch may be used in an amount of 2 to 20 kg / tonne, typically 7 to 12 kg / tonne.

The anionic polymer solution can be added to the wet paper web with a dryness of the web of <50%, typically <40%, more typically <30%, most typically 8-15%. When the pulp suspension enters the headbox, and thus the paper machine, it typically has a dryness level of greater than or equal to 0.3% and less than 2%. The first dewatering from the web is by gravity as the web comes from the headbox to the wire section. As the paper moves forward through the wire section, various suction units are involved in dewatering. After the wire section, the paper typically has a dryness of 20%. Paper dryness increases to 40-50% during wet pressing. The addition of the anionic polymer solution is preferably carried out before the last suction area of the wire section, preferably by injection.

According to one embodiment of the invention, the addition of the anionic polymer solution to the wet paper web is performed before the press section of the paper machine.

The present invention is advantageous for improving wet paper web strength in the manufacture of wood-free uncoated and coated paper grades. The present invention is also suitable for improving wet paper web strength in the manufacture of paper grades including, but not limited to, super-calendered (SC) printing paper, ultra-lightweight (ULWC), lightweight (LWC) paper and newsprint. Typically, coated magazine paper such as LWC comprises mechanical pulp of about 40-60% by weight, bleached softwood pulp of about 25-40% by weight and fillers and / or coating materials of about 20-35% by weight. SC paper comprises about 70-90% by weight mechanical pulp and about 10-30% by weight long-fiber cellulosic pulp. In particular, the paper webs that will be used to make the recording substrates for inkjet printing are suitable for processing according to the method of the present invention. The paper web may comprise fibers of hardwoods or conifers, or a combination of both. The fibers may be obtained by any suitable pulping or milling technique commonly used in papermaking, such as hot-pulp fiber (TMP), chemimechanical (CMP), chemical-pulp fiber (CTMP), ground fiber, alkaline sulfate (kraft) fiber, and acidic sulfite fiber. The paper web may comprise only virgin or recycled fibers or a combination of both. The weight of the finished paper web is 30-800 g / m 2, typically 30-600 g / m 2, more typically 50-500 g / m 2, preferably 60-300 g / m 2, more preferably 60-120 g / m 2, more preferably 70-100 g / m 2. m2.

In some embodiments, the paper web may comprise fibers derived from non-wood material, such as bamboo, sugarcane shredder waste, hemp, wheat or rice straw.

According to another embodiment, the addition of the anionic polymer solution to the wet paper web is preceded or followed by the addition of a cationic or amphoteric polymer solution. Such sequential addition of polymers to the wet paper web, preferably by spraying, can significantly improve the dry and wet strength of the paper web. The anionic and cationic polymer solutions may also be premixed before being added to the wet paper web, preferably by injection. In this embodiment, the addition of a cationic polymer to the pulp is not mandatory but can be performed.

For example, polymer solutions of carboxylmethylcellulose (CMC), polyvinyl alcohol (PVA), modified polyvinyl alcohol, guar gum and / or chitosan are added to the wet web before the press portion. Carboxylmethylcellulose, polyvinyl alcohol, modified polyvinyl alcohol, guar gum or chitosan may be added one at a time, or two or three may be added sequentially. CMC and chitosan improve the wet strength of the web with a web dryness of more than about 55%, and PVA also improves the wet strength of the web at lower web dryness. The polyelectrolyte multilayers of anionic and cationic polymers increase the molecular contact surface at the fiber-to-fiber junctions, thereby increasing the strength of the dry paper. It has now been found that the deposition of polymers, for example two layers, can also greatly improve the strength of a wet paper web. This indicates that the deposition of polymers also increases the fiber-to-fiber interaction in the wet state. It is also likely that injection of the anionic polymer into the outermost layer may reduce adhesion between the wet web and the anionic center roll of the papermaking machine. When forming a polymeric bilayer or multilayer by injection molding, the amount of polymers sprayed on a paper machine is preferably minimized.

In yet another embodiment, one or more layers of chemical solutions are applied to the wet paper web before the press section or dryer section. The addition of a cationic polymer to the pulp is not mandatory, but can be performed. Cationic polymers are as previously defined herein. The chemical solutions are preferably added to the wet paper web by spraying as described in the application, but may be added by coating, film transfer, foam layer addition or feed from a separate headbox. The chemical solution that is added to the web, e.g. by injection, may be a solution of carboxylmethylcellulose (CMC), polyvinyl alcohol (PVA), chitosan or guar gum. Guar gum refers here to galactomannan. It is a polysaccharide comprising galactose and mannose. The Guarkum backbone is a linear chain of β 1,4-linked mannose residues to which galactose residues are 1.6-linked to every other mannose, forming short side branches. Guar gum may be added to the web in the form of native guar gum, anionic guar gum or cationic guar gum. For example, native, cationic or anionic guar gum may be added to a wet paper web formed without the addition of a cationic polymer to the pulp. In another example, native or anionic guar gum may be added to a wet paper web formed from a pulp to which a cationic polymer such as cationic guar gum has been added.

EXPERIMENTAL PART

Example 1

The raw material used is commercially dried unbleached softwood pulp from a Finnish paper mill. The cellulose leachability value (Canadian Standard Freeness, CSF) is 500 ml as measured by SCAN-C 21:65.

Wet handmade test sheets having a basis weight of 60 g / m 2 are prepared using SCAN-CM 64:00 to prepare laboratory sheets with recycled zero water. The cationic starch, which has been boiled for 30 minutes at T = 97 ° C, is added to the stock in each sheet separately. The amount of cationic starch added is 10 kg / h (Raisamyl 135 starch DS 0.035, BASF). The control is prepared without cationic starch.

The anionic polymer solution is injected onto formed test sheets adhered to the wire underneath the vacuum. Vacuum strengthens the penetration of the anionic polymer solution into the wet sheet of paper during injection. The concentration of the sprayed anionic polyacrylamide (Fennopol A 8500 R, Kemira Oyj) is 0.3% w / w in consistency and stirred at room temperature overnight before spraying. Control sheets are sprayed with water.

After injection, the test sheets are wet pressed. Wet pressing is performed at two different pressure levels, 50 kPa and 350 kPa, to provide two different degrees of dryness for wet test sheets. Wet specimens are cut to 20 mm width with a sample length of 100 mm. Wet samples are stored in an airtight container in a plastic bag at 7 ° C to maintain the degree of drought.

The mechanical properties of dry and wet paper samples are determined with an impactor operating at 1.0 m / s (VTT Finland, FAST TENSILE TEST RIG IMPACT). The impactor and test method are disclosed in the following references: Kurki, M., Kouko, J., Kekko, R., Saari, T., Laboratory scale measurement procedure of wet web runnability: Part 1, Paper and Wood, 86 (2004) 4; and Kouko, J., Salminen, K., and Kurki, M., Laboratory Scale Measurement Procedures for Wet Web Runnability: Part 2, Paper and Wood, 89 (2007) 7-8.

The results can be seen in Figure 1 which shows the effect of the addition method considered on the initial wet strength of the paper. The following abbreviations are used in Figure 1:

Cs = cationic starch A-PAM = anionic polyacrylamide

Although the invention has been described with reference to embodiments which at present appear to be most practical and preferred, it should be understood that the invention is not to be limited to the above embodiments, but is intended to encompass various modifications and related technical solutions within the scope of the appended claims.

Claims (10)

A process for improving the runnability of a wet paper web or the like comprising: - adding a cationic polymer to the pulp, - forming a wet paper web from the pulp, characterized in that an anionic polymer solution is added to the wet paper web. Method according to Claim 1, characterized in that a synthetic anionic polymer solution is added to the wet paper web as an anionic polymer solution. Process according to claim 1 or 2, characterized in that the anionic polymer is added to the wet paper web in an amount of <2 g / m2. Process according to Claim 1, characterized in that a cationized polysaccharide, such as a cationic starch, is added to its pulp. Process according to Claim 1, characterized in that the addition of the anionic polymer solution is carried out by injection. Process according to Claim 1, characterized in that the anionic polymer solution is added to the wet paper web at a dryness of the web <50%, preferably 8-15%. A method according to claim 1, characterized in that the anionic polymer solution is added to the wet paper web before the press section of the paper machine. A method according to claim 1, characterized in that the anionic polymer solution is added to the wet paper web before the last suction area of the wire section. 9. A method according to claim 1, characterized in that anionic polymer solution is added to the wet paper web to improve the strength of the wet paper web in the manufacture of wood-free uncoated paper, wood-free coated paper, supercalendered paper (SC), ultra lightweight paper (ULWC) or lightweight. Paper made by the process of any one of claims 1-9.