EP1702109A2 - Chemical improvement in paper making - Google Patents

Chemical improvement in paper making

Info

Publication number
EP1702109A2
EP1702109A2 EP04805226A EP04805226A EP1702109A2 EP 1702109 A2 EP1702109 A2 EP 1702109A2 EP 04805226 A EP04805226 A EP 04805226A EP 04805226 A EP04805226 A EP 04805226A EP 1702109 A2 EP1702109 A2 EP 1702109A2
Authority
EP
European Patent Office
Prior art keywords
paper
alkyl ketene
acid anhydride
maleic acid
styrene maleic
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP04805226A
Other languages
German (de)
French (fr)
Inventor
Anna-Liisa Tammi
Mika Laitila
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
BASF Schweiz AG
UPM Kymmene Oy
Original Assignee
Ciba Spezialitaetenchemie Holding AG
Ciba SC Holding AG
UPM Kymmene Oy
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ciba Spezialitaetenchemie Holding AG, Ciba SC Holding AG, UPM Kymmene Oy filed Critical Ciba Spezialitaetenchemie Holding AG
Publication of EP1702109A2 publication Critical patent/EP1702109A2/en
Withdrawn legal-status Critical Current

Links

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
    • D21H17/71—Mixtures of material ; Pulp or paper comprising several different materials not incorporated by special processes
    • D21H17/72—Mixtures of material ; Pulp or paper comprising several different materials not incorporated by special processes of organic material
    • 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
    • D21H21/16—Sizing or water-repelling 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
    • D21H11/00—Pulp or paper, comprising cellulose or lignocellulose fibres of natural origin only
    • D21H11/08—Mechanical or thermomechanical pulp
    • 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
    • D21H17/03—Non-macromolecular organic compounds
    • D21H17/05—Non-macromolecular organic compounds containing elements other than carbon and hydrogen only
    • D21H17/17—Ketenes, e.g. ketene dimers
    • 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
    • D21H17/37—Polymers of unsaturated acids or derivatives thereof, e.g. polyacrylates

Definitions

  • the invention relates to an improvement in the printability of uncoated mechanical pulp paper grades, especially to an improvement in the printability of supercalen- dered (SC) papers, particularly in rotogravure printing.
  • Objects of the invention are a method for improving the printability of uncoated mechanical pulp paper grades, the use of a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer in said method, and paper prepared by said method.
  • SC paper grades comprise various newsprint grades, SC paper grades, and coated mechanical paper grades.
  • Supercal- endered (SC) papers contain 70-90% mechanical and/or recycled pulp and 10-30% chemical pulp and do not have any surface coating. Instead, they can contain up to 35% minerals as fillers, relative to the amount of fibrous raw materials which is marked as 100. Paper quality of SC papers is largely based on the quality of fibrous raw materials and fillers and their treatment and processing.
  • SC paper printability properties have been affected by including in the pulp such chemicals which provide the paper with strength (such as starch) and hydrophobicity (such as alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), styrene maleic anhydride (SMA)).
  • the filler can be pretreated, for example to have a hy- drophobic character.
  • the web is passed through one or several nips for achieving the desired quality properties, especially as regards final thickness, gloss and smoothness.
  • Supercalenders have a plurality of rolls arranged one above the other and nips provided between the rolls through which the paper web travels.
  • a super- calender is usually arranged separately from the paper machine. It is also possible to calender the web 'on-line', which means that the calender is connected to the paper machine.
  • the web Before (super)calendering the web is usually wetted to a suitable calendering moisture by means of water and/or steam.
  • an arrangement for calendering a board web wherein a liquid containing a protein or a chemical that has properties similar to protein can be transferred on the board web via a moistening means before calendering.
  • the moistening means are arranged so that the board web can be surface-sized and moistened simultaneously, or a coating containing a pigment can be transferred on the board web.
  • US 6494988 discloses a process for improving the surface of offset paper wherein a binding agent is applied across a moving paper web in a drying section or in a calender.
  • a process for on-line manufacture of SC-A paper (a subgrade within SC papers) is disclosed wherein tensides are added to the moistening water in order to produce a spray mist water having a reduced surface tension.
  • US 6013359 discloses different polyacrylamides as surface treatment agents for improving surface strength and releasing property of printing papers.
  • Too strong and effective calendering can cause "blackening" (reduced brightness) of the paper which should be avoided.
  • Supercalendering under high pressure and temperature also takes off bulkiness of paper which is not a desirable effect.
  • the porosity of SC papers should preferably be decreased, thus making the paper surface more dense, in order to improve the paper quality for printing purposes.
  • the rotogravure printing process is generally most suited to those applications in which a very large number of copies is required to be printed, because the process by which the surface of a rotogravure printing cylinder is prepared is expensive relative to the cost of preparing surfaces of printing cylinders for use in other methods of printing.
  • a rotogravure printing cylinder has on its surface a matrix of cells or depressions which vary in depth according to the quantity of ink which is required to be transferred to the paper from each individual cell. Because of the large number of copies to be printed, it is generally advantageous to run the printing process at high speed.
  • Alkyl ketene dimer (AKD) and alkenyl succinic acid anhydride (ASA) are hydro- phobing sizing agents used in internal and surface sizing of paper and paperboard under neutral or alkaline conditions.
  • AKD is used as a minor component together with starch (for example below 5%, often 1-3% by weight of starch), polyvinyl alcohol (PNA), carboxy methyl cellulose (CMC) or other polymer in liquid form.
  • Structurally alkyl ketene dimers are unsaturated lactones.
  • Technical grade saturated fatty acids usually stearic acid, are used in the synthesis of AKD waxes.
  • the waxes Prior to use, the waxes need to be converted into tiny particles dispersed in water by melting and emulsifying the wax. AKD sizes are usually provided in the form of anionic or cationic starch-stabilized dispersions.
  • Styrene maleic acid anhydride (SMA) is a copolymer of styrene and maleic acid anhydride. SMA-based hydrophobic surface sizing agents are used as additives in surface sizes to improve the hydrophobic properties of fine papers (woodfree papers), which contain no or very little mechanical pulp.
  • the present invention is based on the finding that the printability of uncoated mechanical pulp paper is improved by applying a combination of alkyl ketene dimer dispersion and styrene maleic acid anhydride copolymer solution onto the paper web, particularly before supercalendering.
  • it is an object of the invention to provide a method for improving printability of uncoated mechanical pulp paper which method comprises applying a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer onto at least one side of the paper web, preferably before supercalendering, and calendering the paper web.
  • a further object of the invention is the use of a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer on paper surface for improving the printability of uncoated mechanical pulp paper grades.
  • Still another object of the invention is paper, preferably supercalendered paper or newsprint paper, prepared by a method wherein a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web in connection with, preferably before, calendering or supercalendering, and its use in a rotogravure printing process.
  • the above defined method is carried out by using any suitable application method, e.g. film transfer, blade, spray, moisturizing, or curtain application method.
  • a “paper” is meant paper or paperboard or any material, preferably cellulosic material, which is suitable for printing. All weight ratios are calculated as dry weight and all percentages are percentages by weight.
  • the weight ratio of alkyl ketene dimer to styrene maleic acid anhydride copolymer in the combination to be applied onto the paper web as a mixture or separately is pref- erably about 1:1, but it may vary within the range 1:5 - 2:1.
  • the ratio of alkyl ketene dimer to styrene maleic acid anhydride copolymer is within the range of 1:1...2.
  • the styrene maleic acid anhydride copolymer is preferably provided as the ammo- niutn salt form of SMA, for example as 10% aqueous solution. Also other salt forms of SMA, such as sodium, potassium, etc. are possible.
  • the share of styrene in the styrene maleic acid anhydride copolymer is over 50%.
  • the ratio of maleic acid anhydride units to styrene units is generally in the range of from 10:90 to 45:55, preferably 20:80 - 40:60.
  • styrene maleic acid anhydride copoly- mers having a molecular weight from 5000 to 500000.
  • the styrene maleic acid anhydride copolymer is prepared in a known manner, for example by polymerizing the monomers in organic solution.
  • the alkyl ketene dimer can be supplied as an aqueous dispersion which usually con- tains starch or another polymer in liquid form as stabiliser.
  • the AKD content of said dispersion may be for example from 60 to 95%.
  • Alkyl ketene dimers with a melting point within the range of 15-70°C are suitable.
  • a mixture of low melting ( ⁇ 30°C) and high melting (>50°C) alkyl ketene dimers wherein for example ⁇ 30% of alkyl ketene dimers are low melting and >70% high melting alkyl ketene dimers.
  • Starch or other polymer in liquid form may be used as a protective colloid to stabilize the dispersion in an amount of 5-40% of the total dispersion, especially in an amount of 10-20%.
  • the combination of alkyl ketene dimer and styrene maleic acid anhydride can be applied onto the paper web as a mixture or as two separate dispersions.
  • the mixture of alkyl ketene dimer and styrene maleic acid anhydride copolymer is a dispersion with a dry matter content of 5-15%. If the two chemicals are applied separately, AKD may be applied as an AKD dispersion (for example as 18% dispersion) and SMA as a SMA solution (for example as 10% solution), one after the other.
  • the combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web before calendering.
  • Calendering may take place either "on-line" at the paper machine or at a separate calendering unit.
  • the combination of AKD and SMA can be applied onto the web at a separate (su- per)calender before or during calendering, or at the paper machine before or during on-line (super)calendering.
  • Another alternative is to apply the combination of AKD and SMA at the (dry end of) paper machine and then pass the web to a separate (su- ⁇ per)calender.
  • the web Before applying the combination of AKD and SMA, the web can be dried to a suitable moisture content, which is preferably ⁇ 20%, more preferably ⁇ 15%.
  • a further embodiment of the invention is to apply a mixture of alkyl ketene dimer and styrene maleic acid anhydride copolymer onto the paper web together with moistening water before or during calendering.
  • the mixture is applied onto the wet or dry web, the web is possibly dried to desired moisture, and then the web is passed to a calender, preferably to a supercalender. It is also possible to apply the mixture onto paper which has already been calendered, before an additional (super)calendering. In the recently developed new calendering units it may also be possible to apply the mixture in between two stacks of calender rolls.
  • AKD is preferably applied first, as SMA is a better film forming agent than AKD.
  • AKD and SMA improves the printability of uncoated mechanical pulp paper even if the paper web is not (superjcalendered after applying the combination of AKD and SMA to the web.
  • the paper web may, if desired, be calendered before the combination of AKD and SMA is applied on the web.
  • paper quality improving agents such as optical bright- eners, pH regulators, lubricating agents, etc. may be added together or separately with the combination of alkyl ketene dimer and styrene maleic acid anhydride.
  • alkyl ketene dimer and styrene maleic acid anhydride.
  • alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web in an amount which is less than or about 1 gm 2 , preferably about 0.5 g/m 2 , or from 0.05 to 0.8 g/m 2 .
  • a further advantage of the use of a combination of alkyl ketene dimer and styrene maleic anhydride is that there is no need for "over-effective" calendering, which means that no blackening effect appears and the bulkiness of paper is not decreased.
  • Uncalendered mechanical base paper was processed with different chemicals so that applied weight was about 0.5 g/m 2 .
  • the paper was calendered in a laboratory calender in paper moisture of 9.5% under pressure of 50 bar and at a temperature of 90°C.
  • the calendered paper was tested for printability in rotogravure printing.
  • the chemicals used were water, alkyl ketene dimer (AKD) dispersion, styrene maleic anhydride copolymer (SMA) solution, polyethylene glycol (PEG) and a mixture of AKD and SMA.
  • the SMA copolymer contained 28% maleic acid anhydride and 72% styrene.
  • the AKD dispersion was a mixture of low melting (20%) and high melting (80%) alkyl ketene dimers.
  • Commercial SC paper was used as a reference. Styrene acrylate latex was also tested but it stuck tightly on the calender roll and could not be analysed.
  • the number of missing dots is a traditional measure of rotogravure printability of paper.
  • the so-called Heliotest indicates the number of missing dots.
  • the Heliotest fitting developed by Centre Technique dumaschine is used here with an IGT AC2 laboratory printer.
  • the test figure is 110 mm long with an 8 mm wide raster where the tone changes from dark to light.
  • the size of the dots is constant over the entire printing area.
  • the num- ber of missing dots is defined as a distance from the dark end of the printed image within which 20 missing dots can be detected. The longer the distance the better the rotogravure printability of the paper.
  • a dot where more than half of the dot area is missing is considered as a missing dot.
  • SC paper The surface of usual SC paper was processed with different chemicals to about 0.5 g/m 2 .
  • the paper was calendered in a moisture of -5.5%.
  • the temperature was 90°C and a pressure of 40 bar was used.
  • the calendered paper was tested for printability in rotogravure printing. Reference was usual SC paper.
  • the mixture of AKD and SMA provides better print density and printed gloss than the reference or water processed SC paper. Heliotest is clearly better in missing dots appearance. Especially the printed gloss achieved by using a mixture of alkyl ketene dimer and styrene maleic acid anhydride as described in this invention is better than that achieved by any of its components alone. Polyethylene glycol (PEG) was not used because it adhered to calender roll.

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Abstract

The invention relates to an improvement in the printability of uncoated mechanical pulp paper grades, especially to an improvement in the printability of supercalendered (SC) papers, particularly in rotogravure printing. In the method according to the invention, a mixture of alkyl ketene dimer and styrene maleic acid anhydride co-polymer is applied onto the paper web in connection with supercalendering.

Description

Chemical improvement in paper making
The invention relates to an improvement in the printability of uncoated mechanical pulp paper grades, especially to an improvement in the printability of supercalen- dered (SC) papers, particularly in rotogravure printing. Objects of the invention are a method for improving the printability of uncoated mechanical pulp paper grades, the use of a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer in said method, and paper prepared by said method.
Background of the invention
Mechanical pulp paper grades, or mechanical printing papers, comprise various newsprint grades, SC paper grades, and coated mechanical paper grades. Supercal- endered (SC) papers contain 70-90% mechanical and/or recycled pulp and 10-30% chemical pulp and do not have any surface coating. Instead, they can contain up to 35% minerals as fillers, relative to the amount of fibrous raw materials which is marked as 100. Paper quality of SC papers is largely based on the quality of fibrous raw materials and fillers and their treatment and processing. Traditionally, printability properties of SC paper have been affected by including in the pulp such chemicals which provide the paper with strength (such as starch) and hydrophobicity (such as alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA), styrene maleic anhydride (SMA)). Alternatively, the filler can be pretreated, for example to have a hy- drophobic character.
In a calendering process, the web is passed through one or several nips for achieving the desired quality properties, especially as regards final thickness, gloss and smoothness. Supercalenders have a plurality of rolls arranged one above the other and nips provided between the rolls through which the paper web travels. A super- calender is usually arranged separately from the paper machine. It is also possible to calender the web 'on-line', which means that the calender is connected to the paper machine. Before (super)calendering the web is usually wetted to a suitable calendering moisture by means of water and/or steam.
In US 6200424, an arrangement for calendering a board web is disclosed wherein a liquid containing a protein or a chemical that has properties similar to protein can be transferred on the board web via a moistening means before calendering. Alternatively, the moistening means are arranged so that the board web can be surface-sized and moistened simultaneously, or a coating containing a pigment can be transferred on the board web.
US 6494988 discloses a process for improving the surface of offset paper wherein a binding agent is applied across a moving paper web in a drying section or in a calender. In US 6500305, a process for on-line manufacture of SC-A paper (a subgrade within SC papers) is disclosed wherein tensides are added to the moistening water in order to produce a spray mist water having a reduced surface tension.
US 6013359 discloses different polyacrylamides as surface treatment agents for improving surface strength and releasing property of printing papers.
However, as far as the inventor is aware, in the earlier attempts to improve the printability of uncoated mechanical pulp paper grades, particularly SC papers, the chemicals used in the present invention have not been applied to the surface of the paper in connection with supercalendering to improve printability.
Different printing processes place different requirements on the quality and properties of the paper being printed. High-quality printing using a rotogravure process requires the paper surface to be smooth and at the same time little elastic, in order for the printing ink to settle properly on the paper. In rotogravure printing, printing pressure is the key factor in transferring ink to paper and this requires elasticity from the paper. Approximately 80% of SC papers is used in rotogravure printing, while 20% is used in offset printing. However, as the surface of SC papers has tradition- ally not been treated with chemicals other than moistening water, high standards have been set for the supercalendering process in order to achieve the desired paper quality for printing, especially in case of rotogravure printing. Too strong and effective calendering can cause "blackening" (reduced brightness) of the paper which should be avoided. Supercalendering under high pressure and temperature also takes off bulkiness of paper which is not a desirable effect. Further, the porosity of SC papers should preferably be decreased, thus making the paper surface more dense, in order to improve the paper quality for printing purposes.
The rotogravure printing process is generally most suited to those applications in which a very large number of copies is required to be printed, because the process by which the surface of a rotogravure printing cylinder is prepared is expensive relative to the cost of preparing surfaces of printing cylinders for use in other methods of printing. A rotogravure printing cylinder has on its surface a matrix of cells or depressions which vary in depth according to the quantity of ink which is required to be transferred to the paper from each individual cell. Because of the large number of copies to be printed, it is generally advantageous to run the printing process at high speed.
Alkyl ketene dimer (AKD) and alkenyl succinic acid anhydride (ASA) are hydro- phobing sizing agents used in internal and surface sizing of paper and paperboard under neutral or alkaline conditions. In surface sizing, AKD is used as a minor component together with starch (for example below 5%, often 1-3% by weight of starch), polyvinyl alcohol (PNA), carboxy methyl cellulose (CMC) or other polymer in liquid form. Structurally alkyl ketene dimers are unsaturated lactones. Technical grade saturated fatty acids, usually stearic acid, are used in the synthesis of AKD waxes. Prior to use, the waxes need to be converted into tiny particles dispersed in water by melting and emulsifying the wax. AKD sizes are usually provided in the form of anionic or cationic starch-stabilized dispersions. Styrene maleic acid anhydride (SMA) is a copolymer of styrene and maleic acid anhydride. SMA-based hydrophobic surface sizing agents are used as additives in surface sizes to improve the hydrophobic properties of fine papers (woodfree papers), which contain no or very little mechanical pulp.
Summary of the invention
The present invention is based on the finding that the printability of uncoated mechanical pulp paper is improved by applying a combination of alkyl ketene dimer dispersion and styrene maleic acid anhydride copolymer solution onto the paper web, particularly before supercalendering.
Consequently, it is an object of the invention to provide a method for improving printability of uncoated mechanical pulp paper which method comprises applying a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer onto at least one side of the paper web, preferably before supercalendering, and calendering the paper web.
A further object of the invention is the use of a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer on paper surface for improving the printability of uncoated mechanical pulp paper grades.
Still another object of the invention is paper, preferably supercalendered paper or newsprint paper, prepared by a method wherein a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web in connection with, preferably before, calendering or supercalendering, and its use in a rotogravure printing process.
La a further, preferred embodiment of the invention, the above defined method is carried out by using any suitable application method, e.g. film transfer, blade, spray, moisturizing, or curtain application method. ithin this disclosure, by a "paper" is meant paper or paperboard or any material, preferably cellulosic material, which is suitable for printing. All weight ratios are calculated as dry weight and all percentages are percentages by weight.
Detailed description of the invention
The weight ratio of alkyl ketene dimer to styrene maleic acid anhydride copolymer in the combination to be applied onto the paper web as a mixture or separately is pref- erably about 1:1, but it may vary within the range 1:5 - 2:1. Usually the ratio of alkyl ketene dimer to styrene maleic acid anhydride copolymer is within the range of 1:1...2.
The styrene maleic acid anhydride copolymer is preferably provided as the ammo- niutn salt form of SMA, for example as 10% aqueous solution. Also other salt forms of SMA, such as sodium, potassium, etc. are possible. The share of styrene in the styrene maleic acid anhydride copolymer is over 50%. The ratio of maleic acid anhydride units to styrene units is generally in the range of from 10:90 to 45:55, preferably 20:80 - 40:60. Especially suitable are styrene maleic acid anhydride copoly- mers having a molecular weight from 5000 to 500000. The styrene maleic acid anhydride copolymer is prepared in a known manner, for example by polymerizing the monomers in organic solution.
The alkyl ketene dimer can be supplied as an aqueous dispersion which usually con- tains starch or another polymer in liquid form as stabiliser. The AKD content of said dispersion may be for example from 60 to 95%. Alkyl ketene dimers with a melting point within the range of 15-70°C are suitable. Especially preferred is a mixture of low melting (<30°C) and high melting (>50°C) alkyl ketene dimers, wherein for example <30% of alkyl ketene dimers are low melting and >70% high melting alkyl ketene dimers. Starch or other polymer in liquid form may be used as a protective colloid to stabilize the dispersion in an amount of 5-40% of the total dispersion, especially in an amount of 10-20%.
The combination of alkyl ketene dimer and styrene maleic acid anhydride can be applied onto the paper web as a mixture or as two separate dispersions. The mixture of alkyl ketene dimer and styrene maleic acid anhydride copolymer is a dispersion with a dry matter content of 5-15%. If the two chemicals are applied separately, AKD may be applied as an AKD dispersion (for example as 18% dispersion) and SMA as a SMA solution (for example as 10% solution), one after the other.
Preferably the combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web before calendering. Calendering may take place either "on-line" at the paper machine or at a separate calendering unit. The combination of AKD and SMA can be applied onto the web at a separate (su- per)calender before or during calendering, or at the paper machine before or during on-line (super)calendering. Another alternative is to apply the combination of AKD and SMA at the (dry end of) paper machine and then pass the web to a separate (su- ■per)calender. Before applying the combination of AKD and SMA, the web can be dried to a suitable moisture content, which is preferably <20%, more preferably <15%.
A further embodiment of the invention is to apply a mixture of alkyl ketene dimer and styrene maleic acid anhydride copolymer onto the paper web together with moistening water before or during calendering. In a still further embodiment the mixture is applied onto the wet or dry web, the web is possibly dried to desired moisture, and then the web is passed to a calender, preferably to a supercalender. It is also possible to apply the mixture onto paper which has already been calendered, before an additional (super)calendering. In the recently developed new calendering units it may also be possible to apply the mixture in between two stacks of calender rolls. The application methods such as curtain application and spray application which have recently become more general, make it also possible to apply AKD and SMA one after another. In this embodiment according to the invention, AKD is preferably applied first, as SMA is a better film forming agent than AKD.
A person skilled in the art understands that a combination of AKD and SMA, especially their mixture, improves the printability of uncoated mechanical pulp paper even if the paper web is not (superjcalendered after applying the combination of AKD and SMA to the web. However, the paper web may, if desired, be calendered before the combination of AKD and SMA is applied on the web.
Further, if desired, known paper quality improving agents, such as optical bright- eners, pH regulators, lubricating agents, etc. may be added together or separately with the combination of alkyl ketene dimer and styrene maleic acid anhydride. How- ever, in the method according to the invention no pigments, latexes, nor polymers like starch, PNA or CMC (except the polymers possibly included in AKD dispersion) are applied onto the paper web.
The combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web in an amount which is less than or about 1 gm2, preferably about 0.5 g/m2, or from 0.05 to 0.8 g/m2.
Applying the combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer onto the paper web in connection with calendering or supercalendering improves the printability properties of the (super)calendered paper. The paper surface becomes more hydrophobic, smoother, and has improved gloss properties compared to usual SC papers. The porosity of the paper surface is reduced and the surface is more dense, as the application of AKD and SMA forms a thin film on the paper surface. The high-quality paper obtained by the method of the invention is particularly suitable for rotogravure printing, being fully competitive with pigment- coated papers. A further advantage of the use of a combination of alkyl ketene dimer and styrene maleic anhydride is that there is no need for "over-effective" calendering, which means that no blackening effect appears and the bulkiness of paper is not decreased.
The following examples further illustrate the invention.
Example 1
Uncalendered mechanical base paper was processed with different chemicals so that applied weight was about 0.5 g/m2. The paper was calendered in a laboratory calender in paper moisture of 9.5% under pressure of 50 bar and at a temperature of 90°C. The calendered paper was tested for printability in rotogravure printing.
The chemicals used were water, alkyl ketene dimer (AKD) dispersion, styrene maleic anhydride copolymer (SMA) solution, polyethylene glycol (PEG) and a mixture of AKD and SMA. The SMA copolymer contained 28% maleic acid anhydride and 72% styrene. The AKD dispersion was a mixture of low melting (20%) and high melting (80%) alkyl ketene dimers. Commercial SC paper was used as a reference. Styrene acrylate latex was also tested but it stuck tightly on the calender roll and could not be analysed.
Table 1.
In the Heliotest, missing dots are inevitable at 5% halftone but disastrous when occurring at 20% and 30% halftones. The number of missing dots is a traditional measure of rotogravure printability of paper. In laboratory printing, the so-called Heliotest indicates the number of missing dots. The Heliotest fitting developed by Centre Technique du Papier is used here with an IGT AC2 laboratory printer. The test figure is 110 mm long with an 8 mm wide raster where the tone changes from dark to light. The size of the dots is constant over the entire printing area. The num- ber of missing dots is defined as a distance from the dark end of the printed image within which 20 missing dots can be detected. The longer the distance the better the rotogravure printability of the paper. A dot where more than half of the dot area is missing is considered as a missing dot.
From the results it can be seen mat the mixture of alkyl ketene dimer and styrene maleic anhydride copolymer gives better density than other chemicals tested. Printed gloss is also better and in Heliotest there is a big difference in relation to missing dots.
Example 2
The surface of usual SC paper was processed with different chemicals to about 0.5 g/m2. The paper was calendered in a moisture of -5.5%. The temperature was 90°C and a pressure of 40 bar was used. The calendered paper was tested for printability in rotogravure printing. Reference was usual SC paper.
Table 2.
The mixture of AKD and SMA provides better print density and printed gloss than the reference or water processed SC paper. Heliotest is clearly better in missing dots appearance. Especially the printed gloss achieved by using a mixture of alkyl ketene dimer and styrene maleic acid anhydride as described in this invention is better than that achieved by any of its components alone. Polyethylene glycol (PEG) was not used because it adhered to calender roll.

Claims

Claims
1. A method for improving printabilily of uncoated mechanical pulp paper which method comprises - applying a combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer onto at least one side of the paper web, and - calendering the paper web.
2. The method according to claim 1 wherein the combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied before (super)calendering.
3. The method according to claim 1 or 2 wherein the combination of alkyl ketene dimer (AKD) and styrene maleic acid anhydride copolymer (SMA) is a mixture of AKD and SMA.
4. The method according to claim 3 wherein the mixture of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied together with moistening water.
5. The method according to any one of claims 1 to 4 wherein the paper to be treated is uncalendered paper.
6. The method according to any one of claims 1 to 5 wherein the weight ratio of alkyl ketene dimer to styrene maleic acid anhydride copolymer is 1 :5 — 2: 1 , prefera- bly l:1...2.
7. The method according to any one of claims 1 to 6 wherein the combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web in an amount of 0.05 - 0.8 g/m2.
8. The method according to any one of claims 1 to 7 wherein the combination of alkyl ketene dimer and styrene maleic acid anhydride copolymer is applied onto the paper web - at a separate (super)calender before or during calendering, or - at the paper machine before or during on-line (super)calendering, or - at the paper machine before the paper web is passed to a separate (super)calender.
9. The method according to any one of claims 1 to 8 wherein before applying the combination of AKD and SMA the paper web is dried to a suitable moisture content, preferably to a moisture content <15%.
10. Use of a combination of alkyl ketene dimer and styrene maleic anhydride copolymer on paper surface for improving the printability of uncoated mechanical pulp papers.
11. The use according to claim 10 wherein the paper is SC paper or newsprint paper.
12. Paper prepared according to the method of any one of claims 1 to 9.
13. Use of the paper according to claim 12 in a rotogravure printing process.
EP04805226A 2003-12-23 2004-12-22 Chemical improvement in paper making Withdrawn EP1702109A2 (en)

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US20120138249A1 (en) * 2010-12-02 2012-06-07 Patrick Sundholm Method for improving paper and board's resistance to the penetration of liquids
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