EFFICIENT PRODUCTION OF A CONTAINERBOARD TO BE USED AS FLUTING

Abstract

There is provided a method of producing a containerboard having a geometric SCT index of 37.0-42.0 Nm/g when measured according to ISO 9895:2008, comprising the steps of: —providing a pulp having a Schopper-Riegler (SR) value of 15-19 when measured according to ISO 5267-1:1999, wherein at least 70% by dry weight of the pulp is NSSC pulp; —forming a web from the pulp; —pressing the web in a press section comprising a shoe press, wherein the shoe press comprises a shoe press belt having discontinuous grooves and the line load in the shoe press is in the range of 1400-2000 kN/m; —drying the web from the press section in a drying section to obtain said containerboard.

Claims

1. A method of producing a containerboard having a geometric SCT index of 37.0-42.0 Nm/g when measured according to ISO 9895:2008, comprising the steps of: providing a pulp having a Schopper-Riegler (SR) value of 15-19 when measured according to ISO 5267-1:1999, wherein at least 70% by dry weight of the pulp is NSSC pulp; forming a web from the pulp; pressing the web in a press section comprising a shoe press, wherein the shoe press comprises a shoe press belt having discontinuous grooves and the line load in the shoe press is in the range of 1400-2000 kN/m; drying the web from the press section in a drying section to obtain said containerboard.

2. The method according to claim 1, wherein the web is subjected to a press impulse of 105-190 kPa*s in the shoe press.

3. The method according to claim 1, wherein the speed of the web is in the range of 700-850 m/min.

4. The method according to claim 1, wherein grammage of the containerboard is in the range of 120-240 g/m.sup.2, when measured according to ISO 536:2012.

5. The method according to claim 1, wherein line load in the shoe press is in the range of 1500-2000 kN/m.

6. The method according to claim 1, wherein at least 80% by dry weight of the pulp is NSSC pulp.

7. The method according to claim 1, wherein at least 88% by dry weight of the pulp is NSSC pulp.

8. The method according to claim 1, wherein the yield of the NSSC pulp is 75-85%

9. The method according to claim 1, wherein the containerboard has a geometric SCT index of 38.0-42.0 Nm.

10. The method according to any claim 1, wherein at least 70% by dry weight of the NSSC pulp is hardwood NSSC pulp.

11. The method according to claim 1, wherein at least 80% by dry weight of the NSSC pulp is hardwood NSSC pulp

12. The method according to claim 1, wherein the steam consumption in the drying section is in the range of 1.20-1.35 ton steam per ton paper.

13. The method according to claim 1, wherein the press section comprises another press arranged upstream the shoe press.

14. The method according to claim 1, wherein the consistency of the pulp in a head box used for forming the web is 0.50%-1.20%.

15. The method according to claim 10, wherein the consistency of the pulp in a head box used for forming the web is 0.80%-1.20%.

16. The method of claim 10, wherein the hardwood NSSC pulp is birch NSSC pulp.

17. The method of claim 13, wherein the press arranged upstream of the shoe press is a jumbo press.

18. The method according to claim 5, wherein line load in the shoe press is in the range of 1600-2000 kN/m.

19. The method according to claim 8, wherein the yield of the NSSC pulp is 79-85%.

20. The method according to claim 9, wherein the containerboard has a geometric SCT index of 39.0-42.0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 is a graph showing the shoe press line load and the resulting specific steam consumption (medium pressure steam at about 8 bar) in the drying section during the inventive trial described below. The shoe press line load was 1300 kN/m during a first period (“1.sup.st”) and a third period (“3.sup.rd”). During a second period (“2.sup.nd”), the shoe press line load was 1700 kN/m.

DETAILED DESCRIPTION

[0024] There is thus provided a method of producing a containerboard having a geometric SCT index of 37.0-42.0 Nm/g. The containerboard is intended for use as fluting (i.e. corrugated medium) in corrugated board. As understood by the skilled person, the method is intended to be used on a full-scale paper machine, i.e. a machine adapted to produce at least 50,000 tons of containerboard per year, normally at least 100,000 tons of containerboard per year. The grammage of the containerboard may be in the range of 100-240 g/m.sup.2, such as 110-240 g/m.sup.2, such as 120-240 g/m.sup.2, such as 120-200 g/m.sup.2 or 140-230 g/m.sup.2, such as 150-230 g/m.sup.2. Grammage is measured according to ISO 536:2012.

[0025] Preferably, the geometric SCT index of the containerboard is 38.0-42.0 Nm/g, such as 38.0-41.0 Nm/g.

[0026] To obtain the geometric SCT index, the compressive strength in the machine direction (MD) and the cross direction (CD) of the containerboard is first measured using a short-span compressive tester (SCT) measured according to ISO 9895:2008. To calculate the compressive strength index, the compressive strength (N/m) is divided by the grammage. The unit of the SCT index is thus Nm/g. The geometric SCT index is calculated as the square root of the product of the SCT index in MD and CD:

geometric SCT index=√(SCT index (MD)*SCT index (CD)).

[0027] The compressive strength is considered to be more important in CD than in MD. The SCT index in the CD of the containerboard may for example be above 28 Nm/g, such as at least 29 Nm/g. An upper limit for the SCT index index may for example be 32 Nm/g.

[0028] The method comprises the step of: [0029] providing a pulp having a Schopper-Riegler (SR) value of 15-19 when measured according to ISO 5267-1:1999, wherein at least 70% by dry weight of the pulp is NSSC pulp.

[0030] In one embodiment, the SR value is 16-19. In another embodiment, the SR value is 15-18.

[0031] The SR value referred to above (and in the claims) is the SR value that the pulp has in the head box (i.e. the chamber from which the pulp is caused to flow onto the wire of the wire section). To obtain this SR value, the pulp of the present disclosure may be subjected to refining, such as LC refining, between the NSSC pulping process and the head box.

[0032] Preferably, at least 80% by dry weight of the pulp is NSSC pulp. In one example, at least 85% or 88% by dry weight of the pulp is NSSC pulp.

[0033] The NSSC pulp may comprise hardwood NSSC pulp. Preferably, at least 70% by dry weight of the NSSC pulp is hardwood NSSC pulp, such as birch NSSC pulp. In one example, at least 80% by dry weight of the NSSC pulp is hardwood NSSC pulp, such as birch NSSC pulp.

[0034] It has been reported in the prior art that a relatively low yield of the NSSC pulp is needed to obtain a large increase in strength. The results presented in the Examples section are however obtained using a NSSC pulp of a relatively high yield, more precisely about 82%. The yield of the NSSC pulp of the present disclosure may thus be 75%-85%, preferably 79%-85%, such as 80-84%.

[0035] In addition to the NSSC pulp, the pulp may comprise recycled fibers, reject pulp and/or clippings (typically corrugated box plant clippings). The above-mentioned LC refining may be carried out before or after mixing with such other fibres.

[0036] “NSSC pulp” is obtained from “NSSC pulping”, which in turn is defined in the background section. The NSSC pulp of the present disclosure may for example be sodium-based NSSC pulp, which means that the cooking liquor of the NSSC cook comprised Na.sub.2SO.sub.3.

[0037] The method further comprises the step of: [0038] forming a web from the pulp (that has a SR value of 15-19), typically in a wire section (as conventional in paper making).

[0039] In the method, the head box consistency may for example be 0.50%-1.20%, such as 0.80%-1.20%, such as 0.90%-1.15%. The higher consistencies are particularly relevant when a large proportion (e.g. at least 80%) of the NSSC pulp is derived from hardwood.

[0040] In one embodiment, the head box consistency is 1.05%-1.20% and the grammage of the containerboard is 140-240 g/m.sup.2, such as 140-240 g/m.sup.2.

[0041] The method further comprises the step of: [0042] pressing the web in a press section comprising a shoe press.

[0043] The nip length in the shoe press may for example be 200-330 mm, such as 250-300 mm. The shoe press is typically a double felted shoe press.

[0044] The shoe press comprises a shoe press belt having discontinuous grooves. Such a shoe press belt is commercially available. One example is the BlackBelt G DG marketed by Valmet. Another example is Valmet Black Belt H DG marketed by Valmet. The hardness of the elastomer in the shoe press belt may for example be 93-95 Shore A.

[0045] The line load in the shoe press is in the range of 1400-2000 kN/m, preferably 1500-2000 kN/m, such as 1600-2000 kN/m, such as 1600-1800 kN/m.

[0046] In the shoe press, the web may be subjected to a press impulse of 105-280 kPa*s, such as 105-190 kPa*s, such as 105-135 kPa*s. The highest press impulses are obtained when the line load is relatively high and the web speed is relatively low (because of a high grammage of the produced containerboard).

[0047] The press section may comprise another press arranged upstream the shoe press. The other press is typically double felted and may for example be a jumbo press or a shoe press. The line load of the other press may be 100-300 kN/m, such as 150-250 kN/m, in particular in case of a jumbo press.

[0048] Finally, the method further comprises the step of: [0049] drying the web from the press section in a drying section to obtain said containerboard. As explained above, the method of the present disclosure facilitates a reduced steam consumption in the drying section and/or production at a relatively high speed, such as 700-850 m/min or even 750-850 m/min. When the method of the present disclosure is carried out, the steam consumption in the drying section may be as low as 1.20-1.35 tonne steam per tonne paper. This steam is normally “medium pressure steam”, which refers to steam having a pressure of 6-11 bar, such as 6-10 bar, typically about 8 bar.

EXAMPLES

First Reference Trial

[0050] Over a first period of about three months, NSSC containerboard (intended for fluting) of various grammages was produced in the Gruvön mill, Sweden. Data from the production is presented in table 1. The pulp was a mixture of about 90% NSSC pulp from birch and about 10% kraft pulp. The pulp was refined. The resulting SR numbers are presented in table 1. The press section had two double felted nips; first a jumbo press and then a shoe press with a shoe press belt having blind drilled holes. The line load of the jumbo press was 180 kN/m.

[0051] Over a second period of about three months, NSSC containerboard (intended for fluting) of the same grammages was produced in the Gruvön mill, Sweden. Data from the production is presented in table 2. Again, the pulp was a mixture of about 90% NSSC pulp from birch and about 10% kraft pulp and it was refined. The resulting SR numbers are presented in table 2. The press section was the same as during the first period except that another shoe press belt was used. This new shoe press belt had discontinuous grooves. The characteristics of the grooves were:

width: 1.30 mm;
land: 1.80 mm;
depth: 1.30 mm;
open area: 38%;
void volume: 460 ml/m.sup.2.

[0052] From the data of tables 1 and 2, the change of shoe press belt does not appear to have had any particular effect. It is notable that the specific steam consumption was not reduced during the second period despite that the average SR number was slightly lower than during the first period. It is also notable that the average geometric SCT index was almost the same during the second period as during the first period.

TABLE-US-00001 TABLE 1 First reference trial, first period (shoe press belt having blind drilled holes). Head box Line load SCT Grammage SR slice lip Head box shoe press SSC SCT Geo (kg/m.sup.2) number (mm) consistency (%) (kN/m) (ton/ton) Geo index 0.110 17.9 15.7 0.92 1 300 1.44 4.3 39.1 0.120 18.6 16 1.01 1 300 1.46 4.7 39.2 0.130 19.2 16.2 1.03 1 300 1.5 5.1 39.2 0.140 19.9 16.7 1.07 1 300 1.53 5.4 38.6 0.150 19.4 17.3 1.08 1 296 1.53 5.8 38.7 0.160 19.4 17.9 1.11 1 300 1.53 6.1 38.1 0.175 19.2 20.4 1.06 1 300 1.54 6.9 39.4 Average 19.1 1 299 1.504 38.9 “SSC” means specific steam consumption. The SR numbers were measured online.

TABLE-US-00002 TABLE 2 First reference trial, second period (shoe press belt having discontinuous grooves). Head box Line load SCT Grammage SR slice lip Head box shoe press SSC SCT Geo (kg/m.sup.2) number (mm) consistency (%) (kN/m) (ton/ton) Geo index 0.110 19 15.8 0.92 1 300 1.5 4.3 39.1 0.120 18.8 16.1 1.00 1 275 1.49 4.7 39.2 0.130 19 16.4 1.02 1 289 1.49 5 38.5 0.140 18.3 16.6 1.08 1 341 1.53 5.4 38.6 0.150 18 17.1 1.09 1 309 1.56 5.7 38.0 0.160 18 17.9 1.11 1 325 1.54 6.2 38.8 0.175 16.8 19.5 1.11 1 300 1.48 6.9 39.4 Average 18.3 1 306 1.513 38.8 “SSC” means specific steam consumption. The SR numbers were measured online.

Second Reference Trial

[0053] NSSC containerboard (intended for fluting) having a grammage of 140 g/m.sup.2 was produced in the Gruvön mill, Sweden. The pulp was a mixture of about 90% NSSC pulp from birch and about 10% kraft pulp. The pulp was refined.

[0054] The resulting SR numbers are presented in table 3 along with other data from the production. The press section had two double felted nips; first a jumbo press (line load=180 kN/m) and then a shoe press with a shoe press belt having blind drilled holes. During a first period of about 28 hours, the shoe press line load was 1500 kN/m. During a second period of about 34 hours, the shoe press line load was again 1300 kN/m. Data from the production in presented in table 3.

[0055] The data in table 3 show that the increased line load of 1500 kN/m did not reduce the specific steam consumption.

TABLE-US-00003 TABLE 3 Second reference trial (grammage = 140 g/m.sup.2, shoe press belt having blind drilled holes). Average Head box Head box Line load Average SR slice lip consis- shoe press SSC Period number (mm) tency (%) (kN/m) (ton/ton) First 15.9 17.0 1.05 1500 1.54 Second 16.1 17.1 1.05 1300 1.52 “SSC” means specific steam consumption. The SR numbers were measured online.

Inventive Trial

[0056] NSSC containerboard (intended for fluting) having a grammage of 160 g/m.sup.2 was produced in the Gruvön mill, Sweden. The pulp was a mixture of about 90% NSSC pulp from birch and about 10% kraft pulp. The pulp was refined. The resulting SR numbers are presented in table 4 along with other data from the production. The press section had two double felted nips; first a jumbo press (line load=180 kN/m) and then a shoe press having a shoe press belt having discontinuous grooves with the characteristics described above under the first reference trial. During a first period of about 2.5 hours, the shoe press line load was 1300 kN/m. The shoe press line load was then gradually increased and during a second period of about two hours, the shoe press line load was 1700 kN/m. The shoe press line load was then gradually decreased and during a third period of about one hour, the shoe press line load was again 1300 kN/m. Data from the production is presented in table 4 (see also FIG. 1).

[0057] The data in table 4 and FIG. 1 show that in case of a shoe press belt having discontinuous grooves, increasing the line load above 1300 kN/m (e.g. to 1700 kN/m) reduced the specific steam consumption considerably. On average, the specific steam consumption was 5% lower at 1700 kN/m than at 1300 kN/m.

[0058] Further, the average geometric SCT index of the containerboard produced during the second period was 38.3 Nm/g. During the same period, the SCT index in the cross direction was 29.4 Nm/g.

TABLE-US-00004 TABLE 4 Inventive trial (grammage = 160 g/m.sup.2, shoe press belt having discontinuous grooves). Average Head box Head box Average SR slice lip consis- Line load shoe SSC Period number (mm) tency (%) press (kN/m) (ton/ton) First 17.2 18.1 1.10 1300 1.38 Second 17.3 18.0 1.11 1700 1.33 Third 17.3 18.1 1.10 1300 1.41 “SSC” means specific steam consumption. The SR numbers were measured online.