SE542075C2

SE542075C2 - Crack-resistant paperboard

Info

Publication number: SE542075C2
Application number: SE1751582A
Authority: SE
Inventors: Johan Magnusson; Ola K Karlsson
Original assignee: Billerudkorsnaes Ab
Priority date: 2017-12-20
Filing date: 2017-12-20
Publication date: 2020-02-18
Also published as: SE1751582A1

Abstract

There is provided a multi-layer paperboard produced in a full-scale paperboard machine, said multi-layer paperboard comprising a top layer, a bottom layer and a middle layer, which middle layer is arranged between the top layer and the bottom layer, wherein the top layer is formed from a top layer pulp comprising a chemical pulp and the middle layer is formed from a middle layer pulp comprising mechanical pulp, such as chemithermomechanical pulp (CTMP) or thermomechanical pulp (TMP), characterized in that the tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the top layer is above 900 J/kg, preferably above 950 J/kg, more preferably above 1050 J/kg, most preferably above 1150 J/kg.

Description

CRACK-RESISTANT PAPERBOARD TECHNICAL FIELD The present invention relates to the field of paperboard.

BACKGROUND Stiffness in the form of bending resistance is an important parameter for many paperboard applications. The prior art describes many ways of increasing this stiffness, preferably with minimal increase in fibre consumption. One way is to produce a paperboard of at least three layers, wherein the outer layers have a relatively high tensile stiffness and the middle layer is bulky and relatively weak. Such a bulky middle layer may for example be produced from a pulp comprising mechanical pulp, such as chemithermomechanical pulp (CTMP) or thermomechanical pulp (TMP).

Creasing is an operation which facilitates the folding of paperboard. During creasing the paperboard is weakened along well defined lines. The resulting lines of weakness, which are referred to as fold lines or crease lines, then act as hinges during folding. Without creasing, the surface layers typically crack and/or the edge formed by the folding becomes uneven. It is thus considered to be very difficult to fold paperboard with a good result without creasing.

SUMMARY The inventor has noted that paperboard having a chemical pulp-containing top layer and one or more bulky CTMP-containing middle layers often cracks when folded along fold lines, in particular when the fold lines extend in the cross direction (CD) of the paperboard. Further, the inventor has found that the cracking problem can be reduced by increasing the TEA index in the machine direction (MD) of the top layer of the paperboard.

The present disclosure thus provides a multi-layer paperboard produced in a full-scale paperboard machine. The multi-layer paperboard comprises a top layer, a bottom layer and a middle layer. The middle layer is arranged between the top layer and the bottom layer. The top layer is formed from a top layer pulp comprising a chemical pulp and the middle layer is formed from a middle layer pulp comprising mechanical pulp, such as chemithermomechanical pulp (CTMP) or thermomechanical pulp (TMP). The tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the top layer is above 900 J/kg.

There is also provided a method of producing the multi-layer paperboard.

DESCRIPTION As a first aspect of the present disclosure, there is thus provided a multi-layer paperboard. It is produced in a full-scale paperboard machine. It is thus not produced in a pilot machine or in a laboratory. The reason for excluding such produced paperboard is that it never exactly replicates the properties obtained by a full-scale paperboard machine. As an example, lab sheets typically have the same properties in all directions, whereas machine direction properties differ from cross direction properties in the paperboard produced in the full-scale paperboard machine.

The multi-layer paperboard comprises a top layer, a bottom layer and a middle layer, which middle layer is arranged between the top layer and the bottom layer. In one embodiment, the multi-layer paper comprises more than one middle layer, such as two or three middle layers.

The top layer is formed from a top layer pulp comprising a chemical pulp, such as kraft pulp. The chemical pulp may for example be a hardwood chemical pulp, a softwood chemical pulp or (preferably) a mixture of a hardwood chemical pulp and a softwood chemical pulp. The reason for using hardwood chemical pulp is e.g. improved surface properties. The reason for using softwood chemical pulp is e.g. improved surface properties. For example, the dry weight ratio of hardwood pulp to softwood pulp in the top layer may be between 10:90 and 100:0, such as between 30:70 and 90:10, such as between 40:60 and 85:15. Further, the top layer pulp may comprise a filler to improve surface and/or printing properties. If the filler is inorganic filler, such as calcined clay, it may be added in an amount of 1.5-10 % by dry weight of the top layer pulp. Accordingly, the amount of inorganic filler in the top layer may for example be 1.5-10 % by dry weight of the top layer.

In one embodiment, the top layer is coated, e.g. with a coating composition comprising pigment to improve printing properties. The bottom layer may also be coated.

The top layer pulp is preferably bleached. Accordingly, the top layer may have a brightness according to ISO 2470-1 of at least 70 %, such as 70-100 %.

Preferably, the brightness according to ISO 2470-1 is at least 75 %, such as 75-100 %. However, the top layer may also be unbleached.

As discussed below, it may be advantageous to add carboxymethyl cellulose (CMC) to the top layer. Accordingly, the top layer comprises CMC in one embodiment. The amount of CMC in the top layer may for example be 0.20-0.90 % by dry weight of the top layer, preferably 0.35-0.70 % by dry weight of the top layer.

The middle layer is formed from a middle layer pulp comprising mechanical pulp, such as chemi-thermomechanical pulp (CTMP) or thermomechanical pulp (TMP). CTMP is the preferred type of mechanical pulp.

Further, the middle layer pulp may comprise chemical pulp and/or broke pulp. The proportion of mechanical pulp in the middle layer pulp is preferably 15-70 % by dry weight, such as 20-70 % by dry weight, such as 25-70 % by dry weight. The proportion of chemical pulp in the middle layer pulp may be 10-40 % by dry weight, such as 15-30 % by dry weight. The proportion of broke pulp in the middle layer pulp may be 10-60 % by dry weight, such as 20-50 % by dry weight.

In one embodiment, the middle layer pulp comprises (by dry weight): - 15-70 % mechanical pulp, such as chemi-thermomechanical pulp (CTMP) or thermomechanical pulp (TMP); - 0-40 % chemical pulp; and - 10-60 % broke pulp.

The botom layer may be formed from a botom layer pulp comprising chemical pulp, such as kraft pulp. The proportion of chemical pulp in the botom layer pulp is preferably at least 50 % by dry weight, such as at least 75 % by dry weight.

At least one, such as all of the layers of the multi-layer paperboard may be hydrophobized from a sizing agent treatment, such as treatment with alkyl ketene dimer (AKD), alkenyl succinic anhydride (ASA) or a combination thereof. Another example of a hydrophobic sizing agent is rosin size. When all the layers have been hydrophobized, the multi-layer paperboard may be a liquid packaging board.

At least one of the layers of the multi-layer paperboard may comprise a dry strength agent, such as a polyacrylamide, a polyvinylamine, a starch, a microfibrillated cellulose (MFC), CMC or a combination thereof. In one embodiment, the middle layer comprises starch.

The tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the top layer of the multi-layer paperboard is above 900 J/kg. It is preferably above 950 J/kg and more preferably above 1050 J/kg. In a particularly preferred embodiment, it is above 1150 J/kg. The effects of such top layer TEA index values are shown in the Examples below. A typical upper limit may be 1800 J/kg.

To separate the top layer from the multi-layer paperboard and thus facilitate the TEA measurement, a FORTUNA Bandknife-Spliting Machine (Type AB 320 E/P) can be used. Such a machine that has been customized for paperboard spliting is commercially available and is used by several major companies in the paperboard field.

Alternatively, a surface grinding technique can be used to remove all layers but the top layer of the multi-layer paperboard. Such a surface grinding is one of the services that are commercially available at RISE Bioeconomy (formely Innventia) in Stockholm, Sweden. TEA measurements are also offered by RISE Bioeconomy. A sample of a multi-layer paperboard can thus be sent to RISE Bioeconomy for a measurement of the MD TEA index of its top layer.

Preferably, the MD TEA index of the top layer is neither the effect of an extremely high tensile index nor of an extremely high stretchability. Instead, the tensile index in the MD of the top layer is typically between 60 and too kNm/kg and preferably between 70 and 95 kNm/kg. Further, the stretchability in the MD of the top layer is typically between 1.7 and 2.7 % and preferably between 1.9 and 2.6 %. The tensile index as well as the stretchability is measured according to ISO 1924-3.

It is further shown in the Examples below that the MD TEA index of the whole multi-layer paperboard may also have an impact on the cracking resistance. Accordingly, the tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the multi-layer paperboard may be above 875 J/kg, preferably above 1000 J/kg and more preferably above 1050 J/kg.

The Examples below described various modifications that can be made to a process of producing a multi-layer paperboard in order to improve the MD TEA index of the top layer. The modifications that are considered by the inventor to be most effective in this regard are the following: - increasing the moisture content of the final paperboard; - decreasing the draw in the drying section; - decreasing the amount of filler in the top layer; - setting the jet/wire ratio to < 1 in the formation of the top layer; - decreasing the ratio of hardwood pulp to softwood pulp in the top layer; - setting the jet/wire ratio to < 1 in the formation of each middle layer; - adding no glue pulp to the middle layer pulp; - increasing the degree of post refining of CTMP used in the middle layer pulp; and - reducing or omitting the refining of chemical pulp used in the middle layer pulp.

Further, the inventor has found that addition of CMC to the top layer pulp increases the MD TEA of the top layer, which is further discussed below.

The present disclosure further provides a sheet of the multi-layer paperboard of the first aspect or a laminate that comprises the multi-layer paperboard of the first aspect, which sheet comprises at least one crease lines. Preferably, the sheet comprises crease lines that extend in the cross direction of the sheet. In one embodiment, the sheet comprises at least one crease line that extends in a first direction and at least one crease line that extends in a second direction, wherein the second direction is perpendicular to the first direction.

There is also provided a container, such as a package or box, comprising at least two walls, such as at least three walls, which walls are formed by folding a sheet according to the above. One embodiment of such a container comprises two pairs of opposing side walls and a bottom wall.

As a second aspect of the present disclosure, there is provided a method of producing a multi-layer paperboard in a full-scale paperboard machine, wherein said multi-layer paperboard comprising a top layer, a bottom layer and a middle layer, which middle layer is arranged between the top layer and the bottom layer. The top layer is formed from a top layer pulp comprising a chemical pulp and the middle layer is formed from a middle layer pulp comprising mechanical pulp, such as chemi-thermomechanical pulp (CTMP) or thermomechanical pulp (TMP). The tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the top layer is above 900 J/kg.

The embodiments of the first aspect apply to the second aspect mutatis mutandis. However, some embodiments of the second aspects are still discussed below.

In one embodiment of the second aspect, the jet/wire ratio in the formation of the top layer is below 1. The effect of such an embodiment is discussed above in connection with the first aspect. Further, it was found in a machine trial carried out by BillerudKorsnäs that changing the jet/wire ratio from 1.016 to 0.972 in the formation of the top layer increased the MD TEA index of the top layer by about 8 %. Accordingly, the jet/wire ratio in the formation of the top layer may be between 0.950 and 0.990, such as between 0.960 and 0.985.

In an embodiment of the second aspect, the jet/wire ratio in the formation of the middle layer is below 1. The effect of such an embodiment is discussed above in connection with the first aspect.

As well known to the skilled person, the “jet/wire ratio” is the ratio of the speed of the jet from the headbox to the speed of the wire.

In an embodiment of the second aspect, the middle layer pulp comprises unrefined chemical pulp, such as unrefined unbleached kraft pulp. The Schopper-Riegler (SR) number measured according to ISO 5267-1:1999 of unrefined chemical pulp is typically between 8 and 12.

In an embodiment of the second aspect, the middle layer pulp comprises post-refined CTMP.

In a machine trial carried out by BillerudKorsnäs, the effect of adding CMC on the MD TEA index of the top layer was tested. The top layer was formed from a mixture of a bleached hardwood (HW) kraft pulp and a bleached softwood (SW) kraft pulp. The paperboard of the machine trial further had two middle layers formed from CTMP-containing pulp and a bottom layer formed from chemical pulp. The results of the machine trial, which are presented in table 1 below, show that a CMC addition increases the MD TEA index of the top layer. The results further show that a higher amount of CMC results in a larger increase and that a CMC addition is particularly effective when it is made to the HW pulp.

Image available on "Original document" Image available on "Original document" Accordingly, the top layer pulp of the second aspect may comprise CMC, e.g. in an amount of 0.20-0.90 % dry weight of the top layer pulp, preferably 0.35-0.70 % dry weight of the top layer pulp.

In one embodiment of the second aspect, the method comprises a preparation of the top layer pulp, which preparation comprises providing a hardwood pulp and a softwood pulp, adding CMC to the hardwood pulp and then mixing the softwood pulp with the hardwood pulp.

The method may comprise a step of coating the multi-layer paperboard by applying a coating composition, such as a pigment coating composition, onto the top layer, e.g. further improve the printing properties.

The method may also comprise a step of coating the multi-layer paperboard by applying a coating composition to the bottom layer, e.g. to control curl and/or prevent dusting.

EXAMPLES The inventor hypothesized that the cracking problem during folding of multilayer paperboard having at least one bulky CTMP-containing middle layer and relatively dense and strong outer layers could be reduced by increasing the TEA index in the machine direction (MD) of the top layer.

Therefore, machine trials were carried out in which multi-layer paperboard comprising a top layer, a bottom layer and two middle layers was produced in a full-scale paperboard machine. The paperboard was first produced according to an ordinary process to obtain reference samples. Then, various modifications of the process were made, primarily to increase the TEA index in the machine direction (MD) of the top layer.

First reference process The paperboard was first produced according to the ordinary process, in which: - the top layer was formed from a mixture of bleached softwood chemical pulp and bleached hardwood chemical pulp; - the bottom layer was formed from a mixture of bleached softwood chemical pulp, bleached hardwood chemical pulp and unbleached softwood chemical pulp; and - the middle layers were formed from a mixture of broke pulp, CTMP, glue pulp and chemical pulp.

Samples (“Ref 1”) were obtained from the paperboard produced according to the ordinary process.

Modified processes The process was then modified according to the following: - the moisture content of the final paperboard was increased; - the draw in the press section was decreased; - the draw in the drying section was decreased; - the basis weight of the top layer was increased; - the amount of filler added to the top layer was decreased; - the jet/wire ratio in the formation of the top layer was changed from > 1 to < 1; - the ratio of hardwood pulp to softwood pulp in the top layer was decreased; - the degree of refining of the softwood pulp for the top layer was decreased; and - the degree of refining of the hardwood pulp for the top layer was increased.

These modifications are collectively referred to as the “Pi modifications”.

Samples (“P1”) were obtained from the paperboard produced according to the modified process.

The process was then further modified according to the following: - in the formation of each middle layer, the jet/wire ratio used was changed from > 1 to < 1; and - a breast roll shaker used in the formation of one of the middle layers was turned off.

These modifications are collectively referred to as the “P2 modifications”.

Samples (“P2”) were obtained from the paperboard produced according to the further modified process (i.e. the process including the P1 modifications and the P2 modifications).

In addition to the P1 and the P2 modifications, the following modifications of the process were then made: - the amount of CTMP in the middle layers was reduced; - the addition of glue pulp to the middle layer was excluded; - the degree of post refining of the CTMP was increased; - the refining of the chemical pulp used for the middle layer was omitted; - the degree of refining of the broke pulp was increased (to increase the strength in the Z direction); and - the amount of starch in the middle layer was increased.

These modifications are collectively referred to as the “P3 modifications”.

Samples (“P3”) were obtained from the paperboard produced according to the additionally modified process (i.e. the process including the P1 modifications, the P2 modifications and the P3 modifications).

The P2 modifications were then reversed and samples (“P4”) were obtained from the paperboard produced according to the resulting process (i.e. the process including the P1 modifications and the P3 modifications, but not the P2 modifications).

Paperboard was then produced according to a process in which the P2 modifications were made and the P1 modifications and the P3 modifications were carried out to about 50 % (the modification of the jet/wire ratio in the formation of the top layer was however carried out to 100 %). Samples (“P5”) were then obtained from this paperboard.

Second reference process Finally, all the modifications were reversed such that paperboard was produced according to the ordinary process again. Samples (“Ref 2”) were obtained from this paperboard.

Measurement of TEA index in the MD From paperboard samples obtained as described above, the top layer was separated using a FORTUNA Bandknife-Splitting Machine Type AB 320 E/P (Germany) that had been customized by the supplier for paperboard splitting.

The TEA in the machine direction (MD) of the separated top layers was then measured according to ISO 1924-3. To obtain a TEA index of a top layer, the TEA value was divided by the cube of the grammage of the top layer. The TEA index values are presented in table 3 below.

TEA values in the MD were also measured on non-split samples, i.e. complete board samples, and divided by the cube of the grammage of the board to obtain TEA index values that are also presented in table 3.

Testing of cracking To facilitate folding, paperboard is provided by crease lines (also referred to as fold lines). To form the crease lines, a crease wheel is pressed against the paperboard. The depth of the crease lines is determined by the height position of the crease wheel; a higher position results in a more shallow crease line, while a lower position results in a deeper crease line. If a crease line is too shallow, the board will crack during folding. If the crease line is too deep, the paperboard is crushed.

To quantify the cracking resistance of a paperboard, the “crease depth window” in which the paperboard can be folded without unacceptable cracking was determined according to the following protocol: Samples obtained according to the above were crease lined at various crease wheel depths in the range of -0.79 to -2.14. The crease lines extended in the cross direction (CD). For each type of paperboard, four samples were crease lined at each crease wheel depth. The samples were then folded along the crease line. The folded samples were graded according to the following scale: - No cracks: o: - A few surface cracks (invisible to the naked eye): 1; - Evenly distributed surface cracks (invisible to the naked eye): 2; - A few top layer cracks: 3; - A few significant top layer cracks: 4; - Evenly distributed top layer cracks: 5; - Crushed paperboard: 6.

An average number (based on the four samples) above 1.75 was considered unacceptable. The crease depth window for a paperboard was calculated by subtracting the deepest crease wheel depth giving an acceptable number from the shallowest crease wheel depth giving an acceptable number. If the average number was acceptable already at the shallowest depth tested (-0.79), the “shallowest crease wheel depth giving an acceptable number” was set to -0.75. The results are shown in table 2 below.

Image available on "Original document" Image available on "Original document" Image available on "Original document" Table 3 shows that the P1 modifications increased the MD TEA index of the whole paperboard from 743 J/kg to 943 J/kg, i.e. by 27 %. The MD TEA index of the top layer was increased even more, by 73 %. The P2 modifications further increased the MD TEA index of the top layer (+ 8 %). There was also a slight increase of the MD TEA index of the whole paperboard (+ 3 %). The P3 modifications increased the MD TEA index of both the top layer (+ 8 %) and the whole board (+ 15 %). When the P2 modifications were then reversed (see P4), the MD TEA index of both the whole paperboard (- 11 %) and the top layer (- 16 %) was reduced Table 3 further shows that the crease depth window was significantly wider when the MD TEA index of the top layer was above 900 J/kg than when it was below 900 J/kg. When, at the same time, the MD TEA index of the whole board was above 1000 J/kg, the crease depth window was even wider. A particularly wide crease depth window was obtained when the MD TEA index of both the whole board and the top layer was above 1050 J/kg. The widest crease depth window was obtained when the MD TEA index of the top layer was above 1150 J/kg.

Another conclusion from table 3 is that the correlation between MD TEA index of the top layer and the crease depth window is stronger than the correlation between MD TEA index of the whole board and the crease depth window.

Claims

1. Multi-layer paperboard produced in a full-scale paperboard machine, said multi-layer paperboard comprising a top layer, a bottom layer and a middle layer, which middle layer is arranged between the top layer and the bottom layer, wherein the top layer is formed from a top layer pulp comprising a chemical pulp and the middle layer is formed from a middle layer pulp comprising mechanical pulp, such as chemi-thermomechanical pulp (CTMP) or thermomechanical pulp (TMP), characterized in that the tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the top layer is above 900 J/kg, preferably above 950 J/kg, more preferably above 1050 J/kg, most preferably above 1150 J/kg.

2. Multi-layer paperboard according to claim 1, wherein the tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the multi-layer paperboard is above 875 J/kg, preferably above 1000 J/kg, more preferably above 1050 J/kg.

3. Multi-layer paperboard according to claim 1 or 2, wherein top layer is bleached.

4. Multi-layer paperboard according to any one of the preceding claims, wherein the top layer pulp comprises a chemical hardwood pulp.

5. Multi-layer paperboard according to claim 4, wherein the top layer pulp further comprises a chemical softwood pulp.

6. Multi-layer paperboard according to any one of the preceding claims, wherein the top layer comprises at least one filler.

7. Multi-layer paperboard according to any one of the preceding claims, wherein the top layer comprises carboxymethyl cellulose (CMC).

8. Multi-layer paperboard according to any one of the preceding claims, which is a liquid packaging board.

9. multi-layer paperboard according to any one of the preceding claims, wherein the middle layer pulp further comprises broke pulp and/or chemical pulp.

10. A method of producing a multi-layer paperboard in a full-scale paperboard machine, wherein: said multi-layer paperboard comprising a top layer, a bottom layer and a middle layer, which middle layer is arranged between the top layer and the bottom layer; and the top layer is formed from a top layer pulp comprising a chemical pulp and the middle layer is formed from a middle layer pulp comprising mechanical pulp, such as chemi-thermomechanical pulp (CTMP) or thermomechanical pulp (TMP), characterized in that the tensile energy absorption (TEA) index according to ISO 1924-3 in the machine direction (MD) of the top layer is above 900 J/kg, preferably above 950 J/kg, more preferably above 1050 J/kg, most preferably above 1150 J/kg.

11. Method according to claim 10, wherein the jet/wire ratio in the formation of the top layer is below 1.

12. Method according to claim 10 or 11, wherein the jet/wire ratio in the formation of the middle layer is below 1.

13. Method according to any one of claims 10-12, wherein the middle layer pulp comprises unrefined chemical pulp, such as unrefined unbleached kraft pulp.

14. Method according to any one of claims 10-13, wherein the top layer pulp comprises carboxymethyl cellulose (CMC).

15. Method according to claim 14, wherein the amount of CMC in the top layer pulp is 0.20-0.90 % by dry weight of the top layer, preferably 0.35-0.70 % by dry weight of the top layer.