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EP-4739840-A1 - A METHOD FOR MANUFACTURING A MULTI-PLY PAPERBOARD

EP4739840A1EP 4739840 A1EP4739840 A1EP 4739840A1EP-4739840-A1

Abstract

The invention discloses a method for manufacturing a multiply paperboard comprising the steps of: - forming a web comprising at least a top ply, a back ply, and a middle ply arranged between the top ply and the back ply, wherein the middle ply is formed from a first furnish comprising at least 50 wt% of a HT-CTMP and 0.1 – 10 wt% of glue pulp based on the total dry weight of said first furnish, wherein said glue pulp has a Schopper Riegler (SR) value in the range of 55- 84 as measured according to standard ISO 5267-1 and a mass fraction of fines of less than 25% as measured according to standard ISO 10376:2011, - subjecting the thereof formed three-ply web to pressing in at least one shoe press, and - drying the web. It has been found that the combination of using the said furnish mixture including HT-CTMP and glue pulp with an SR value within said range enables efficient dewatering by use of a shoe press and gives rise to a paperboard with high bulk but maintained or improved strength properties.

Inventors

  • BACKFOLK, KAJ
  • MOBERG, ANDERS

Assignees

  • Stora Enso Oyj

Dates

Publication Date
20260513
Application Date
20240702

Claims (15)

  1. 1. A method for manufacturing a multiply paperboard comprising the steps of: - forming a web comprising at least a top ply, a back ply, and a middle ply arranged between the top ply and the back ply, wherein the middle ply is formed from a first furnish comprising at least 50 wt% of a HT-CTMP and 0.1 - 10 wt% of glue pulp based on the total dry weight of said first furnish, wherein said glue pulp has a Schopper Riegler (SR) value in the range of 55 - 84 as measured according to standard ISO 5267-1 and a mass fraction of fines of less than 25% as measured according to standard ISO 10376:2011 , - subjecting the thereof formed three-ply web to pressing in at least one shoe press, and - drying the web.
  2. 2. A method according to claim 1 , wherein the first furnish further comprises a strength additive selected from the group consisting of cationic starch, anionic polymers, microfibrillated cellulose, polyvinylamine, chitosan, primary and secondary amines, polyethylene amines and modified polyacrylamides and combinations thereof.
  3. 3. A method according to anyone of the preceding claims, wherein the HT- CTMP has a freeness (CSF) of at least 600 ml, preferably at least 650 ml, as measured according to ISO 5367-2.
  4. 4. A method according to anyone of the preceding claims, wherein the first furnish further comprises internal sizing agents, preferably chosen from the group of alkyl ketene dimer (AKD), alkyl succinic anhydride (ASA) and rosin resin, or a combination thereof.
  5. 5. A method according to claim 4, wherein the first furnish is formed by adding a pre-mixture of glue pulp and internal sizing agents to a cellulose containing furnish comprising HT-CTMP.
  6. 6. A method according to any one of the preceding claims, wherein the first furnish is formed by adding a pre-mixture of glue pulp and CMC having a degree of substitution of less than 0.4 to a cellulose containing furnish comprising HT-CTMP.
  7. 7. A method according to anyone of the preceding claims, wherein the SR value of the glue pulp is between 55 - 75, as measured according to standard ISO 5267-1.
  8. 8. A method according to anyone of the preceding claims, wherein the SR value of the glue pulp is between 55 - 60, as measured according to standard ISO 5267-1.
  9. 9. A method according to anyone of the preceding claims, wherein the glue pulp is fibrillated broke.
  10. 10. A method according to claim 9, wherein the broke at least partly comprises surface sized broke.
  11. 11 . A method according to anyone of the preceding claims, wherein the top ply is formed from a second furnish comprising 80 - 100 wt% of hardwood kraft pulp, preferably bleached hardwood kraft pulp, based on the total dry weight of said second furnish.
  12. 12. A method according to anyone of the preceding claims, wherein the back ply is formed from a third furnish comprising 80 - 100 wt% of unbleached kraft pulp from hardwood based on the total dry weight of said third furnish.
  13. 13. A method according to anyone of the preceding claims, wherein web comprising the top-ply, the middle-ply, and the back-ply is formed by use of a multi-layer headbox and the method further comprises applying a central layer in-between the top ply and the middle ply or in-between the middle ply and the back ply using the multilayer headbox, and wherein the central layer is formed from an aqueous suspension comprising 80 wt% of glue pulp having an SR in the range of 55 - 84, as measured according to standard ISO 5267-1 and based on the total dry weight of the aqueous suspension.
  14. 14. A method according to anyone of the preceding claims, wherein the at least one shoe press forms a shoe press nip and wherein the temperature of the web in the shoe press nip is in the range of 20 - 75 °C, preferably in the range of 35 - 75 °C and most preferably in the range of 40 - 75 °C.
  15. 15. A method according to anyone of the preceding claims, further comprising the step of reeling the web on a reel spool to form a paperboard roll, wherein the paperboard roll is loaded with two rider rolls during the reeling.

Description

A METHOD FOR MANUFACTURING A MULTI-PLY PAPERBOARD Technical field The present disclosure relates to a method for manufacturing a multiply paperboard. Background Paperboard intended for conversion into packages using fast-running automatic machines must possess the requisite strength to withstand the strain and stress associated with converting processes. Additionally, it must exhibit high bending resistance, not only to facilitate smooth converting operations but also to ensure optimal package performance. For products with extended shelf life, the package must also possess excellent barrier properties against light, oxygen, and moisture. The bulk of paperboard (inverse of density) is a significant property contributing to its thickness. Increased thickness enhances the bending stiffness of the board and enables the papermaker to reduce the amount of fibers used, resulting in cost savings. However, higher bulk often leads to a decrease in internal strength. One of the challenges faced by papermakers is to increase the bulk of the paperboard while maintaining its strength and to ensure for the convertability (folding and creasing) of the paperboard. Typically, paperboard consists of 1-5 plies (layers). Paperboard intended for conversion usually comprises multiple plies, exhibiting higher bending resistance index compared to single-ply paperboard. Multi-ply paperboard generally consists of top and back plies, along with one or more middle plies. The middle plies provide bulk to the paperboard. Optionally, one or several layers of bonding agents are added between the plies to improve ply bond strength To provide strength and excellent printing properties, chemical pulp is commonly used in the top and back plies of the board. The middle ply may contain both mechanical pulp and/or chemical pulp. Mechanical pulp or semimechanical pulp, such as bleached or unbleached CTMP (chemi-thermomechanical pulp), is oftentimes preferred due to its lower cost compared to chemical pulp and its bulk inducing properties. Mechanical or semi-mechanical pulp also offers higher raw material efficiency and yield. Softwood CTMP is frequently employed in the middle ply or bulk layer in high-quality boards, as it provides high bulk and comprises a low content of shives. Chemical pulp is typically used in conjunction with mechanical pulp in the middle ply to enhance strength. In recent years, high-temperature chemi-thermomechanical pulp (HT-CTMP) has been preferred in certain applications when forming the bulk layer. HT-CTMP refers to CTMP that has undergone pre-heating before the refining step in the manufacturing process, resulting in a pulp with a higher opacity and/or higher content of longer fibers, which further improves the bulk and the stiffness of a paperboard made thereof. In addition, less refining energy is needed. On the other hand, a higher content of longer fibers or bulky fibers may affect formation and the flocculation behavior of the fibers negatively. Moreover, high yield pulp based on for example CTMP and HT-CTMP will require dry strength bonding agents, especially when targeting higher bulk. Dry bond strength can be improved with natural polysaccharides such as starch as well as synthetic polymers. Recently, nanocellulose or fine microfibrillated cellulose has been used to improve strength properties such as tensile strength, burst strength and compression strength of the paperboard. Unfortunately, many of the available strength enhancing additives increases the density of the paperboard, which in turn may impact e.g. stiffness and mechanical behavior. Moreover, the use of a high content of water soluble polysaccharides or/and nanocellulose increases the risk of low (self-) retention and may further consume a higher content of retention and drainage aids. It is a challenge to improve the retention of strength chemicals when using a high content of nanocellulose. The patent publication WO15087293 discloses a paperboard comprising HT- CTMP and strength additives, such as MFC and starch. The addition of MFC to paperboard may however increase the density of the paperboard, whereby the bulk is reduced. To control sheet density and especially the density profile (z- direction) and to increase or maintain high bulk, it is common to optimize dewatering and wet pressing by using extended nips, such as shoe-presses, in the pressing section in combination with low nip loads. However, the utilization of shoe presses for preserving bulk in MFC-containing paperboard poses challenges due to the difficulty in dewatering the board effectively, resulting in the requirement of high nip loads. This might lead to uneven (z-)distribution of strength additives, which may result in delamination, reduced strength and problems with dimensional stability. There thus remains a need for a method for manufacturing a paperboard with high bulk and yet excellent strength properties, which method further enables high retention and an even distrib