Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
  • D21H17/21—Macromolecular organic compounds of natural origin; Derivatives thereof
  • D21H17/24—Polysaccharides
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
  • D21H17/33—Synthetic macromolecular compounds
  • D21H17/34—Synthetic macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds
  • D21H17/41—Synthetic macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds containing ionic groups
  • D21H17/42—Synthetic macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds containing ionic groups anionic
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H21/00—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties
  • D21H21/14—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties characterised by function or properties in or on the paper
  • D21H21/18—Reinforcing agents
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
  • D21H17/21—Macromolecular organic compounds of natural origin; Derivatives thereof
  • D21H17/24—Polysaccharides
  • D21H17/25—Cellulose
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
  • D21H17/21—Macromolecular organic compounds of natural origin; Derivatives thereof
  • D21H17/24—Polysaccharides
  • D21H17/31—Gums
  • D21H17/32—Guar or other polygalactomannan gum
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H17/00—Non-fibrous material added to the pulp, characterised by its constitution; Paper-impregnating material characterised by its constitution
  • D21H17/20—Macromolecular organic compounds
  • D21H17/33—Synthetic macromolecular compounds
  • D21H17/34—Synthetic macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds
  • D21H17/41—Synthetic macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds containing ionic groups
  • D21H17/44—Synthetic macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds containing ionic groups cationic
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H23/00—Processes or apparatus for adding material to the pulp or to the paper
  • D21H23/02—Processes or apparatus for adding material to the pulp or to the paper characterised by the manner in which substances are added
  • D21H23/22—Addition to the formed paper
  • D21H23/50—Spraying or projecting

Definitions

• the present invention relates to a method for improving papermaking or board making process, use of a polysaccharide and a paper according to the preambles of the enclosed claims.
• Strength of wet paper web is one of the important factors in making of paper or board. 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.
• Common dry strength agents do not improve strength of the wet paper web.
• One example of such dry strength agents is starch, which has a 1 ,4-a-anomeric structure. Typical starches include amylose, which is a linear 1 ,4-a-glucan polymer and amylopectin, which has branched structure. The amylopectin backbone is 1 ,4- a-glucan polymer and the branches are linked to the backbone with 1 ,6-a- glycosidic bonds.
• Fillers such as clay, calcium carbonate, calcium sulphate or talc are used in paper and board making to reduce costs and to improve optical properties of paper or board. Fillers are added to the stock before the headbox of the 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 reduced strength properties of the paper and reduced web runnability.
• 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 tensile strength of a paper web or the like.
• An object of the present invention is to increase filler content of paper in order to reduce papermaking costs.
• Typical method according to the present invention for improving papermaking or board making process comprises
• Typical paper according to the present invention is produced by using the method according to the invention.
• Typical use according to the present invention of a polysaccharide which has 1 ,4- ⁇ -anomeric configuration in linkages between saccharide units of the polysaccharide backbone or the main polysaccharide backbone is for increasing filler content of the paper or board, for reducing basis weight, and/or improving runnability of a wet paper or board web.
• the tensile strength of the wet paper or board is clearly improved when a polysaccharide having 1 ,4- -anomeric configuration in the linkages between saccharide units is brought into a contact with fibres in the stock or with fibres in a wet paper web.
• the improved tensile strength of the wet web, as well as the improved dry tensile strength of the paper that may be achieved with the present invention enables an increase in the filler content of paper.
• a high filler content in the base paper may be used, corresponding to ash content e.g. over 25 % both for uncoated fine paper and for coated fine paper base paper made from softwood and hardwood mixture.
• 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 tensile strength may enable an ash content increase also for other paper and board grades, such as ash content increase to over 15 % for newsprint grades, or ash content increase over 12 % coated mechanical base paper, or ash content increase over 34 % for SC paper. Improvement in tensile strength also may be utilised by changing to a 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.
• OCC corrugated container
• the ash content of recycled fibre based fluting or test liner board may be increased over 1 5%.
• Fillers which are used in making or paper or board, which are suitable for use in the present invention, and the content of which may be increased, are preferably clay, calcium carbonate, calcium sulphate, titanium dioxide or talc, or their mixtures. Often the used filler has an anionic net charge.
• the polysaccharide having 1 ,4- ⁇ - anomeric configuration in the linkages between saccharide units of the polysaccharide backbone or the main polysaccharide backbone is selected from the group comprising water soluble cellulose derivatives; galactomannans, such as guar gum or locust bean gum; galactoglucomannans; carboxymethyl cellulose; xylan and substituted glucans, such as xyloglucans; other suitable hydrocolloids, such as tamarind gum; chitosan; chitin; or their derivatives.
• the polysaccharide having 1 ,4- -anomeric configuration in the linkages between saccharide units is selected from the group comprising water soluble cellulose derivatives; galactomannans, such as guar gum or locust bean gum; galactoglucomannans; carboxymethyl cellulose; xylan and substituted glucans, such as xyloglucans; other suitable hydrocolloids, such as tamarind gum; or their derivatives.
• the polysaccharide, which has 1 ,4- -anomeric configuration in the linkages between saccharide units of the polysaccharide backbone or the main polysaccharide backbone is guar gum.
• guar gum is understood as a carbohydrate polymer containing galactose and mannose structural building blocks, especially containing one galactose unit for every two mannose units.
• the backbone is a linear chain of ⁇ 1 ,4-linked mannose residues to which galactose residues are 1 ,6- linked at every second mannose, forming short side-branches.
• Guar gum is typically obtained as an extract of guar bean. It may be used in native form or it may be used in cationised or anionised form.
• anionised guar gum is applied to the fibre stock or on the wet fibre web after application of cationic strength agent to the fibre stock and/or on the wet fibre web.
• the cationic strength agent may be cationic or amphoteric polyacrylamide, polyvinylamide, polyamidoamine, epichlorohydrin, starch, cationic guar gum or derivative of these.
• cationic wet strength agent may be applied on the wet fibre web by spraying, after which anionised guar gum is applied by spraying. More typically, cationic wet strength agent is applied into the fibre stock, after which anionised guar gum is applied by spraying on the wet fibre web.
• Anionised guar gum has typically a charge density ⁇ 2 meq/g.
• the polysaccharide having 1 ,4- ⁇ -anomeric configuration in the linkages between saccharide units of the polysaccharide backbone or the main polysaccharide backbone is carboxymethyl cellulose, CMC.
• Carboxymethyl cellulose is understood here as an anionic polymer, which is produced by introducing carboxyl methyl groups to the cellulose chain, the degree of substitution and the chain length of the cellulose backbone affecting the properties of CMC, such as water solubility. When the degree of substitution exceeds 0.3, carboxymethyl cellulose becomes water soluble.
• the polysaccharide having 1 ,4- ⁇ -anomeric configuration in the linkages between saccharide units of the polysaccharide backbone or the main polysaccharide backbone is a polysaccharide with high degree of polymerisation (DP).
• DP polysaccharide with high degree of polymerisation
• SEC Size exclusion chromatography
• Polysaccharide having 1 ,4- -anomeric configuration in the linkages between saccharide units of the polysaccharide backbone or the main polysaccharide backbone and used in the present invention is water soluble. In case a derivative of the polysaccharide is used, the derivative is also water soluble.
• the viscosity (Brookfield) of the polysaccharide solution is ⁇ 5000 mPas, preferably ⁇ 2000 mPas. Solution may be diluted in order to achieve the desired concentration.
• the polysaccharide having 1 ,4- -anomeric configuration in the linkages between saccharide units is applied as a solution to the wet fibre web in any suitable manner.
• the solution is obtained by dissolving the polysaccharide in powder form into a solvent, typically water.
• the polysaccharide solution is free from discrete polysaccharide particles.
• the polysaccharide solution may comprise one polysaccharide or it may comprise a mixture of different polysaccharides, for example a mixture of two or three polysaccharides.
• a mixture of different polysaccharides may be applied to the fibre stock after machine chest or on the wet fibrous web.
• the concentration of the polysaccharide(s) in the polysaccharide solution is ⁇ 60 weight-%, more typically 0.02 - 5 weight-%, preferably 0.05 - 3 weight-%, more preferably 0.05 - 2 weight-%.
• Concentration of polysaccharides with high degree of polymerisation (DP) in the solution may be even ⁇ 1 weight-%, more typically 0.05 - 1 weight-%, even more typically 0.2 - 0.6 weight-%.
• the polysaccharide is applied to the fibre stock between the last pump preceding the paper or board machine headbox and the outlet of the paper or board machine headbox.
• the polysaccharide is added to the stock as near the headbox as possible, or the polysaccharide may be added directly to the headbox, if adequate mixing to the stock can be secured. Addition of the polysaccharide near the headbox improves the bonding of the fibres together with the polysaccharide, as the polysaccharide remains in outstretched form due to short residence time in the stock and the adsorption of the polysaccharide over the fibre surface is reduced.
• the polysaccharide when added to the stock after the last pump, the risk for breaking the flocks generated by the polysaccharide and fragmentation of the polysaccharide backbone due to shear forces is minimised.
• the activity of the polysaccharide remains in a high level, and it dosage may be reduced or better tensile strength values may be obtained by using the same dosage.
• the polysaccharide is applied into the fibre stock together with a retention or drainage agent.
• the polysaccharide and the retention agent are added to the fibre stock typically near the headbox, for example by dosing at the machine filter.
• the retention or drainage agent may be any suitable retention agent.
• the retention agent may be selected from a group comprising anionic or cationic polyacrylamide, polyvinylamine, polyethyleneimine, cationic starch, bentonite or silica. Especially the retention agent may be anionic or cationic polyacrylamide, polyvinylamine or polyethyleneimine.
• the retention agent and the polysaccharide may be added as separate solutions, or they may be added as single solution, comprising both the retention agent and the polysaccharide.
• Polymeric retention agent dosage may be 50 - 1000 g/t, preferably 100 - 600 g/t, given as dry polymer, and the polysaccharide dosage may preferably be 200 - 4000 g/t, preferably 500 - 2500 g/t, given as dry polymer.
• the polysaccharide is applied into the fibre stock together with an anionic, cationic or amphoteric dry strength agent.
• the dry strength agent is selected from the group comprising polyacrylamides, glyoxylated polyacrylamides, polyvinylamines, polyamine epichlorohydrine co-polymers (PAAE), starch derivatives, and carboxymethyl cellulose.
• the dry strength agent may be applied in amount of 0.1 - 4 kg/t paper, typically in amount 0.2 - 2 kg/t, given as active substance.
• the polysaccharide is applied on the wet fibre web between the headbox and the last nip of a press section.
• the polysaccharide is applied on the wet fibre web by spraying, by coating, by film transfer or by foam layer application. It may be applied by using film transfer to a press belt, or by feeding of polysaccharide solution from a separate headbox.
• the application of the polysaccharide solution is performed by spraying. It has been found out that the spraying of the polysaccharide solution onto the fibre web provides many surprising advantages. Spraying of the polysaccharide solution does not influence the formation of the paper web, whereby there is no negative effects to be noticed in the final paper properties.
• the polysaccharide is applied by spraying onto the wet paper web. It has been observed that the polysaccharide amount, which is applied, may be reduced when the application is done by spraying, and still the improved tensile strength characteristics of the paper web are obtained.
• a polysaccharide solution suitable for use in the spraying may be obtained, for example, by dissolving a polysaccharide in powder form into water in order to form a 0.2 - 20 weight-%, preferably 0.3 - 3 weight-% solution.
• the polysaccharide is applied by foam layer application or foam coating.
• the polysaccharide may be applied by foam coating, whereby the polysaccharide is applied as a foam, which has an air content of 60 - 95 %, onto the wet paper web.
• the polysaccharide is applied in amount ⁇ 0.1 - 10 kg/(ton paper), preferably 0.3 - 3 kg/(ton paper).
• the polysaccharide When the polysaccharide is applied by spraying, it may be applied in amount ⁇ 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 on the wet paper web.
• the polysaccharide solution is applied on the wet paper web when the dryness of the web is ⁇ 50 %, typically ⁇ 40 %, more typically ⁇ 30 %, preferably 8 - 15 %.
• 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 14 - 22 %. The dryness of paper increases to 40 - 55 % during wet pressing.
• the applying of the polysaccharide solution is preferably conducted before the last vacuum zone of the wire section, preferably by spraying.
• two or more polysaccharides may be applied on the wet fibre web after each other by spraying.
• layers of different polysaccharides may easily be applied on top of each other in order to obtain desired properties.
• an anionic or cationic polymer solution may be applied to the wet paper web before or after the addition of the polysaccharide.
• the application of the anionic polymer is performed to the wet paper web before press section of a paper machine.
• the application of the polysaccharide to the wet paper web may be preceded or followed by application of cationic or anionic polymer solution.
• This kind of sequential application of polysaccharide and one or more 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 pre-mixed together before their application, preferably by spraying, to the wet paper web.
• 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 runnability of a wet paper or board web by improving strength of the wet paper web when producing paper grades including 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, but not limited to these.
• 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.
• TMP thermomechanical pulping
• CMP chemimechanical
• CMP chemithermomechanical pulping
• 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/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 , even more preferably 70 - 100 g/m
• the paper web may comprise fibres originating from non- wood material, such as bamboo, sugar cane bagasse, hemp, wheat or rice straw.
• the filler content of the paper or board is increased, whereby the ash content in the wet paper or board web is >25 % for wood-free uncoated paper, >25 % for wood-free coated paper base paper, >34 % for super calendered (SC) paper, >13 % for coated mechanical base paper, >15 % for newsprint paper, fluting board or testliner board, the ash content being measured by burning the stock sample completely in 525 °C.
• SC super calendered
• One or more layers of chemical solutions may be 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.
• 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 ⁇ 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.
• 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.
• 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.
• the experimental unit comprised a vacuum box, moving sample sledge with wire, and spraying unit.
• the amount of the chemical sprayed was adjusted by the speed of the moving sample sledge, while the spray remained constant and was immobilized.
• the samples were wet pressed with 350 kPa and 50 kPa for 5+2 minutes after spraying. The higher pressure gives higher dryness for test sheet. Chemical consistency during spray tests was 0.5%.
• Figure 1 shows results of laboratory tests (Example 1 ) for guar gum, which was added to thick stock pulp or sprayed on wet web. It can be seen that it is advantageous to add guar gum for wet web strength later at paper machine process rather than to thick stock pulp.
• Figure 2 shows results for laboratory tests (Example 1 ) for CMC, chitosan and guar gum, each of which was sprayed on wet web. The effect of different polysaccharides on wet web strength can be seen. Guar gum is most effective. Chitosan and carboxymethyl cellulose (CMC) improved also wet web strength. As reference were used a spraying of water or handsheet without any spraying.
• Pulp containing 70% hardwood with SR-value 24 and 30 % softwood with SR- value 28 was acquired from a Finnish pulp mill. SR-value was measured according to ISO 5267-1 . Precipitated calcium carbonate was used as filler. Filler was added to the pulp and target level for addition was 20 % filler content in the final web.
• Retention chemical was Fennopol K3400R (Kemira Oyj) with dosage 200 g/t and it was added to the headbox feed flow. Tests were made with a small fourdrinier type of wire section. Grammage of formed web was 70 g/m 2 . Chemicals were sprayed on the wet web at the wire. The samples were wet pressed with 350 kPa and 50 kPa for 5+2 minutes after spraying.
• Tensile strength was measured according to ISO 1924-2:2008. The dryness of the paper samples was determined by using a Mettler Toledo HR73 infra-red dryer.
• FIG 3 shows results for tensile strength in semipilot test (Example 2) for sprayed guar gum and Figure 4 shows tensile energy adsorption in semipilot test (Example 2) for sprayed guar gum.
• the effect of guar gum wet web spraying dosage levels on wet web strength can be observed in the Figures. Spraying dosages were 0.1 g/m 2 (1 .4 kg/t), 0.3 g/m 2 (4.2 kg/t) and 0.5 g/m 2 (7.1 kg/t). Dosages of 0.1 g/m 2 and 0.3 g/m 2 improved both wet web tensile and solids content after wet pressing.
• 0.5 g/m 2 improves strength further, but solids content in wet pressing is reduced. Therefore optimum dosage may be between 0.1 g/m 2 and 0.5 g/m 2 , at least under these experimental conditions.
• Tensile strength is needed for web to keep enough tension at paper machine dryer section to allow high operation speed. If the tension of the web is not high enough, the sheet does not follow dryer fabric and sheet fluttering may cause web break due to wind effect caused by high speed.
• Tensile energy adsorption T.E.A. improvement helps to avoid web break, if web has fault such as a hole, slime spot, sticky particle or locally lower basis weight, because higher strength reduces risk that web tears apart beging from the fault position.
• Test furnish was a mixture of softwood and hardwood kraft pulp for fine paper, containing 40 weigth-% scalenohedric precipitated calcium carbonate (PCC) filler. In some tests additional PCC was applied. Chemicals were added to furnish under stirring with magnetic stirrer before sheet preparation. Dosing time of cationic polyacrylamide (C-PAM) retention aid and guar gum was typical for paper machine retention system, see Table 1 . Guar gum and C-PAM were premixed as powder in proportion 1 :1 , and then dissolved to 0.5 weight-% concentration with water, giving a final concentration of 0.25 weight-% guar gum and 0.25 weight-% C-PAM.
• C-PAM cationic polyacrylamide
• Chemicals used in the tests were: cationic potato starch (DS 0.035), guar gum (Sigma G4129) and cationic polyacrylamide, C-PAM, Fennopol K 3400 R (Kemira Oyj). All were dissolved to 0.5 weight-% solution except starch was cooked to 1 weight-% solution.
• Handsheets were prepared with Rapid Kotchen semi-automatic sheet former to 80 g/m2 basis weight according to ISO 5269-2:2004. Ash content was measured according to ISO 1762:2001 .
• Table 1 The wet web tensile index and dry tensile index result for different handsheets.

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