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
  • 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
  • D21H17/375—Poly(meth)acrylamide
• • 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
  • D21H17/14—Carboxylic acids; Derivatives thereof
  • D21H17/15—Polycarboxylic acids, e.g. maleic acid
  • D21H17/16—Addition products thereof with hydrocarbons
• • 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
  • D21H17/62—Rosin; Derivatives thereof
• • 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
  • D21H17/63—Inorganic compounds
  • D21H17/67—Water-insoluble compounds, e.g. fillers, pigments
• • 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
  • D21H17/63—Inorganic compounds
  • D21H17/67—Water-insoluble compounds, e.g. fillers, pigments
  • D21H17/68—Water-insoluble compounds, e.g. fillers, pigments siliceous, e.g. clays
• • 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
  • 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
  • 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
  • D21H23/04—Addition to the pulp; After-treatment of added substances in the pulp
  • D21H23/06—Controlling the addition
  • D21H23/14—Controlling the addition by selecting point of addition or time of contact between components

Definitions

• the present disclosure relates to the field of papermaking.
• An objective of the present disclosure is to provide a method of producing a recyclable paper having relatively high wet strength.
• the pulp preferably a mixture of hardwood pulp and softwood pulp. This embodiment is particularly relevant for bleached pulp.
• the dry weight ratio of hardwood pulp to softwood pulp may be between 70:30 and 10:90, such as between 60:40 and 25:75, preferably between 60:40 and 30:70, more preferably between 55:45 and 40:60.
• the pulp may be bleached or unbleached. When it is unbleached, it may be a 100% softwood pulp.
• the pulp is preferably a kraft pulp.
• it may be a mixture of a bleached hardwood sulphate pulp and a bleached softwood sulphate pulp.
• Broke pulp is typically added, e.g. in a proportion of 5-30% (w/w).
• the broke pulp is preferably obtained from the same method.
• the broke pulp is typically added downstream a refining step. However, the broke pulp may be added upstream the addition of the anionic polymer (discussed below). In one embodiment, broke pulp is added downstream a refining step, but upstream of all additions of papermaking chemicals.
• the method preferably comprises refining of the pulp, e.g. to such a degree that a Schopper-Riegler (SR) number of 20-30, preferably 22-30, such as 23-28. Refining can be carried out in different positions between the cook and the headbox. In any case, the SR number ranges given herein relate to the pulp in the head box. In the context of the present disclosure, SR numbers are measured according to ISO 5267-1:1999.
• the refining normally comprises low consistency LC refining, e.g. refining at a consistency of 2-5 %, such as 3-4 %. In some embodiments comprising LC refining, no high consistency (HC) refining is carried out.
• low consistency LC refining e.g. refining at a consistency of 2-5 %, such as 3-4 %.
• no high consistency (HC) refining is carried out.
• the pulp When the cationic G-PAM is added to the pulp, the pulp preferably has a pH in the range of 4.8-5.5, such as 5.0-5.5.
• the consistency of the pulp is preferably in the range of 1.5-3.0 % when the cationic G-PAM is added.
• refining is only carried out before the addition of the cationic G-PAM. Accordingly, in one embodiment of the method, the pulp is not subjected to any refining after the addition of the cationic G-PAM.
• the total amount of cationic G-PAM added to the pulp may be 1.5-3.0 kg/tonne dry fibre, preferably 2.0-3.0 kg/tonne dry fibre, such as 2.5-3.0 kg/tonne dry fibre.
• the cationic G-PAM may be added to the pulp in the piping leading from the machine chest to the wire pit of the paper machine.
• the pump is preferably arranged upstream a headbox pump (and, of course, downstream the point of addition of said cationic G-PAM).
• the pump is arranged upstream a wire pit, which in turn is arranged upstream the headbox pump.
• An embodiment of the method further comprises adding an anionic polymer to the pulp, i.a. to balance the charge of the pulp.
• the anionic polymer may for example be anionic polyacrylamide (A-PAM).
• A-PAM anionic polyacrylamide
• the total amount of A-PAM added to the pulp may be 0.20-1.00 kg/tonne dry fibre, preferably 0.25-0.75 kg/tonne dry fibre, more preferably 0.35-0.55 kg/tonne dry fibre.
• the anionic polymer is preferably added to the pulp before the cationic G-PAM is added is added to the pulp.
• the pH of the pulp may be in the range of 6.5-8.0 when the anionic polymer is added.
• LC refining is carried out before and after the addition of the anionic polymer. In such case, the amount of LC refining (measured as kWh/tonne dry fibre) may be higher before than after the addition of the anionic polymer.
• the method may further comprise adding at least one hydrophobic size to the pulp, thereby reducing the water absorption of the paper and thus improving its durability in wet or humid conditions.
• the at least one hydrophobic size preferably comprises rosin size. Rosin size may be added to the pulp in a total amount of 1.0-4.0 kg/tonne dry fibre, preferably 1.5-3.0 kg/tonne dry fibre, more preferably 1.5-2.5 kg/tonne dry fibre.
• the at least one hydrophobic size may comprise Alkenylsuccinic anhydride (ASA) or Alkylketene dimer (AKD), preferably AKD.
• ASA Alkenylsuccinic anhydride
• ALD Alkylketene dimer
• AKD may be added to the pulp in a total amount of 0.3-2.0 kg/tonne dry fibre, preferably 0.4-1.5 kg/tonne dry fibre, more preferably 0.4-1.0 kg/tonne dry fibre.
• the at least one hydrophobic size comprises both rosin size and AKD.
• the pH of the pulp is preferably in the range of 4.8-5.5 and the consistency of the pulp is preferably in the range of 1.5-3.0 %.
• the pulp is not subjected to refining after the addition of the at least one hydrophobic size.
• the at least one hydrophobic size is preferably added after the addition anionic polymer discussed above.
• the at least one hydrophobic size may be added to the pulp in the piping leading from the machine chest to the wire pit of the paper machine.
• An embodiment of the method further comprises adding alum to the pulp, e.g. in a total amount of 2.0-8.5 kg/tonne dry fibre, preferably 2.5-5.5 kg/tonne dry fibre, more preferably 2.5-4.5 kg/tonne dry fibre.
• Alum is preferably added to the pulp before any hydrophobic size is added to the pulp.
• the headbox consistency of the pulp may be 0.20-0.60 %, such as 0.30-0.55 %, such as 0.35-0.50 %, in particular when the grammage is above 100 g/m 2 .
• the present inventors have managed to obtain good formation at this headbox consistency, also when the pulp is subjected to LC refining and papermaking chemicals are added.
• the forming section of the paper machine may comprise a wire shaker.
• the cross directional dry content of the web is controlled by a steam box arranged at the downstream end of the forming section.
• the pH of the pulp in the head box may be in the range of 4.8-5.5, preferably 5.0-5.5.
• inorganic filler such as clay
• Inorganic filler may be added in such an amount that the ash content of the paper is in the range of 2.0-5.5 %, such as 3.0-5.0 %.
• the addition of inorganic filler typically improves surface properties of the paper.
• a drawback of adding inorganic filler may however be lower strength properties.
• addition of inorganic filler typically increases the density of the paper. Still, the present investors have managed to keep the density of the paper below 915 kg/m 3 (in full scale trial 1 even below 890 kg/m 3 ) while obtaining relatively high wet strength properties at an ash content of 4% (see table 1).
• Clay is preferably added in the short circulation of the paper machine.
• An embodiment of the method further comprises adding a dry strength agent to the pulp.
• the dry strength agent may be starch, preferably cationic starch.
• the total amount of starch that is added to the pulp may be 2.0-7.0 kg/tonne dry fibre, preferably 3.5-6.0 kg/tonne dry fibre.
• the press section of the paper machine may comprise a shoe press, which may be operated at a line load of 650-950 kN/m, such as 750-900 kN/m. Upstream the shoe press, there may be at least one additional press nip, such as two additional press nips. The first of these may be operated at a line load of 40-90 kN/m, such as 50-70 kN/m, whereas the second may be operated at a line load of 50-100 kN/m, such as 60-90 kN/m. Hence the line load of the first press nip is preferably lower than the line load of the second press nip.
• the calendering of step f) is carried out when low surface roughness is desired.
• the calender of step f) is preferably a soft nip calender, which may be operated at a line load of 60-170 kN/m, preferably 80-140 kN/m, such as 100-140 kN/m.
• the hard roll of the soft nip calender may be heated, preferably to a temperature in the range of 100-200 °C, such as 100-150 °C or 120-180 °C.
• the moisture content of the web may be 7.0-8.5 % when entering the calender of step f).
• the method of the present disclosure facilitates the production of a paper having certain properties:
• the grammage of the paper is typically 50-200 g/m 2 , such as 70-160 g/m 2 .
• the method of the present disclosure is particularly beneficial in case of a grammage of at least 100 g/m 2 , such as at least 120 g/m 2 .
• the papers produced in the full-scale trials 1-3 described below all had a grammage above 120 g/m 2 (see table 1). According to the present disclosure, grammage is measured according to ISO:536:2019.
• the Bendtsen roughness of at least one side of the paper is preferably below 250 ml/min, preferably below 220 ml/min, more preferably below 160 ml/min. In one embodiment, the Bendtsen roughness of both sides of the paper is below 600 ml/min, such as below 500 ml/min, such as below 400 ml/min. In the context of the present disclosure, Bendtsen roughness is measured according to SS-ISO 8791-2:2013. To obtain low Bendtsen roughness values, step f) is carried out.
• the density (measured according to ISO 534:2011) of the paper is preferably below 950 kg/m 3 , more preferably below 920 kg/m 3 . At the same time, the density is normally above 600 kg/m 3 , more preferably below 650 kg/m 3 .
• the present method is not intended for tissue paper or similar lightweight paper.
• the specific formation number of the paper is preferably below 1.00 ⁇ g/m, such as below 0.90 ⁇ g/m, such as below 0.85 ⁇ g/m.
• the specific formation number is measured according to SCAN-P 92:09, preferably using an Ambertec Beta Formation Tester
• the Gurley value (measured according to ISO 5636-5:2013) is preferably above 50 s, more preferably above 60 s, such as above 65 s.
• a typical upper limit may be 150 s or 100 s.
• Higher Gurley values are obtained by a relatively high degree of refining, in particular LC refining, and/or calendering.
• the wet tensile strength index in the machine direction (MD) of the paper is at least 8.0 Nm/g, more preferably at least 9.5 Nm/g, such as above 11.0 Nm/g.
• the wet tensile strength index of the paper is preferably at least 5.5 Nm/g, more preferably at least 6.5 Nm/g, such as at least 7.5 Nm/g.
• Wet tensile strength index is sometimes referred to as only "wet tensile index”.
• Wet tensile strength index is measured according to the standard ISO 1924-3:2005, which specifies a wetting time of 10 min.
• the wet tensile stiffness index in the machine direction (MD) of the paper is at least 1000 Nm/g, more preferably at least 1045 Nm/g, such as at least 1200 Nm/g.
• the wet tensile stiffness index of the paper is preferably at least 400 Nm/g, more preferably at least 445 Nm/g, such as at least 545 Nm/g.
• Wet tensile stiffness index is measured according to the standard ISO 1924-3:2005, which specifies a wetting time of 10 min.
• a paper having a wet tensile strength index above 11.0 Nm/g in MD and above 7.5 Nm/g in CD as well as a wet tensile stiffness index above 1200 Nm/g in MD and above 545 Nm/g in CD was produced in full scale trial 1. This paper was also recyclable.
• the Cobb 60 s value of at least one side of the paper is below 30 g/m 2 , preferably below 25 g/m 2 , more preferably below 23.5 g/m 2 .
• the Cobb 60 s value of both sides of the paper is below 30 g/m 2 , such as below 25 g/m 2 , such as below 23.5 g/m 2 .
• the Cobb 60 s value is measured according to ISO 535:2014.
• A-PAM Franobond 85E
• was then added to the pulp in an amount of about 0.5 kg/tonne dry fibre. After the addition of A-PAM, the pulp was subjected to post LC refining (21 kWh/tonne dry fibre, consistency about 3.3%).
• the post LC-refined pulp was routed to the machine chest, in which alum was dosed in an amount of 3-4 kg/tonne dry fibre such that the pH of the pulp was adjusted to about 5.2.
• Dyes violet and blue
• cationic starch Rosin size (2.5 kg/tonne dry fibre), AKD (0.8 kg/tonne dry fibre) and cationic G-PAM (Fennobond 3150E, 2.8 kg/tonne dry fibre) were added to the pulp in the piping leading from the machine chest to the wire pit. The consistency of the pulp in this position was 2.2%. After the additions of rosin size, AKD and cationic G-PAM, but before the wire pit, the pulp was subjected to pumping.
• a paper web was formed in a forming section comprising a one-ply fourdrinier wire and a wire shaker. At the downstream end of the forming section, the web was steamed by means of a steam box to obtain an even moisture profile in the cross direction before the press section.
• the press section had three nips: first a single-felted nip operated at a line load of 60 kN/, then another single-felted nip operated at a line load of 70 kN/m and finally a shoe press nip operated at a line load of 850 kN/m.
• the web from the press section was dried in a drying section. After the drying section, at a moisture of 7.5-8.0 %, the web was subjected to soft nip calendering at a line load of 103 kN/m. The hard roll of the soft nip calender was heated to 150 °C.
• Table 1 The properties of the resulting paper are presented in table 1 below.
• Table 1 The properties of the paper produced in trial 3 are presented in table 1 below. Table 1. Properties of the papers produced in full scale trials 1-3. PS means printing side. RS means reverse side. MD means machine direction. CD means cross direction. The formation number and the specific formation number were measured using an Ambertec Beta Formation Tester according to the standard SCAN-P 92:09.
• Table 1 shows that recyclable papers having relatively high wet strength properties were successfully produced in trial 1-3.
• the produced papers exhibited satisfactory specific formation numbers, low porosity (i.e. high Gurley values) and low surface roughness (in particular on the printing side (PS)), while the density was kept at an acceptable level.
• That beneficial formation numbers were obtained despite the additions of various papermaking chemicals is, at least in part, attributable to the order in which the papermaking chemicals were added as well as their concentrations and the pH and/or consistency of the pulp at their points of addition.
• a never-dried bleached softwood kraft pulp was provided.
• HC high consistency
• LC low consistency
• Broke pulp was added to the refined pulp and the resulting pulp was subjected to thick stock screening.
• the proportion of broke pulp was about 15% (w/w).
• A-PAM Framobond 85E
• the post LC-refined pulp was routed to the machine chest, in which alum was dosed in an amount of 3-4 kg/tonne dry fibre such that the pH of the pulp was adjusted to about 5.2.
• Dyes violet and blue
• cationic starch Rosin size (about 2 kg/tonne dry fibre), AKD (0.5 kg/tonne dry fibre) and cationic G-PAM (Fennobond 3150E, 2.5 kg/tonne dry fibre) were added to the pulp in the piping leading from the machine chest to the wire pit. The consistency of the pulp in this position was 2.2%. After the additions of rosin size, AKD and cationic G-PAM, but before the wire pit, the pulp was subjected to pumping.
• a paper web was formed in a forming section comprising a one-ply fourdrinier wire and a wire shaker. At the downstream end of the forming section, the web was steamed by means of a steam box to obtain an even moisture profile in the cross direction before the press section.
• the press section had three nips; first a single-felted nip operated at a line load of 60 kN/, then another single-felted nip operated at a line load of 70 kN/m and finally a shoe press nip operated at a line load of 850 kN/m.
• the web from the press section was dried in a drying section having seven dryer groups and a Clupak unit (used to compact/microcrêpe the web) arranged in series.
• the Clupak unit was arranged between dryer groups four and three, which means that the paper web was dried in the drying section both before and after the Clupak unit. No calendering was carried out.
• a never-dried unbleached softwood kraft pulp was provided.
• HC high consistency
• LC low consistency
• Broke pulp was added to the refined pulp and the resulting pulp was subjected to thick stock screening.
• the proportion of broke pulp was about 15% (w/w).
• the post LC-refined pulp was routed to the machine chest, in which alum was dosed in an amount of 7 kg/tonne dry fibre such that the pH of the pulp was adjusted to about 5.3.
• Cationic starch (Raisamyl 50021, 5 kg/tonne dry fibre) was also added to the pulp in the machine chest. Rosin size (about 1.5 kg/tonne dry fibre), AKD (0.4 kg/tonne dry fibre) and cationic G-PAM (Fennobond 3150E, 2.8 kg/tonne dry fibre) were added to the pulp in the piping leading from the machine chest to the wire pit. The consistency of the pulp in this position was 2.2%. After the additions of rosin size, AKD and cationic G-PAM, but before the wire pit, the pulp was subjected to pumping.
• cationic retention aid Fennopol K 7526P, 0.2 kg/tonne dry fibre
• anionic retention aid/silica FennoSil 5000, 0.2 kg/tonne dry fibre
• a paper web was formed in a forming section comprising a one-ply fourdrinier wire and a wire shaker. At the downstream end of the forming section, the web was steamed by means of a steam box to obtain an even moisture profile in the cross direction before the press section.
• the press section had three nips; first a single-felted nip operated at a line load of 60 kN/, then another single-felted nip operated at a line load of 70 kN/m and finally a shoe press nip operated at a line load of 850 kN/m.
• the web from the press section was dried in a drying section having seven dryer groups and a Clupak unit (used to compact/microcrêpe the web) arranged in series.
• the Clupak unit was arranged between dryer groups four and three, which means that the paper web was dried in the drying section both before and after the Clupak unit. No calendering was carried out.

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