PRESS ARRANGEMENT AND METHOD FOR PRESSING A FIBROUS WEB

Abstract

A press arrangement and method for pressing a fibrous web include a main press with extended press nip having a length of at least 150 mm. The main press is configured such that, when operated with a line load LL of at least 1,200 kN/m, a line-load ratio LLR of at least 0.69 and at most 1.52 is obtained. The line-load ratio LLR is defined as the quotient of a weighted line load WLL to the line load LL, the weighted line load WLL being the result of integration of the squared local pressure p(x).sup.2, weighted with a weighting factor A, over the nip length x, the weighting factor A being one divided by ten megapascal. The line load LL is the result of integration of the local pressure p(x) over the nip length x, such that the following formula for the line-load ratio LLR applies:

[00001]$\mathrm{LLR}=\frac{\mathrm{WLL}}{\mathrm{LL}}=\frac{A*{p\left(x\right)}^2\mathrm{dx}}{p\left(x\right)\mathrm{dx}}=\frac{\frac{1}{10\mathrm{MPa}}*{p\left(x\right)}^{2}dx}{p\left(x\right)dx}$

Claims

1. A press arrangement for pressing a fibrous web or a packaging paper web, the press arrangement comprising: a main press having an extended press nip, said press nip having a length of at least 150 mm; said main press configured to obtain a line-load ratio LLR of at least 0.69 and at most 1.52 when operated with a line load LL of at least 1,200 kN/m; said line-load ratio LLR being a quotient of a weighted line load WLL to said line load LL; said weighted line load WLL being a result of an integration of a squared local pressure p(x).sup.2, weighted with a weighting factor A over said nip length x, and said weighting factor A being one divided by ten megapascal; said line load LL being a result of an integration of a local pressure p(x) over said nip length x; and a formula being applied for said line-load ratio LLR as follows: $\mathrm{LLR}=\frac{\mathrm{WLL}}{\mathrm{LL}}=\frac{A*{p\left(x\right)}^2\mathrm{dx}}{p\left(x\right)\mathrm{dx}}=\frac{\frac{1}{10\mathrm{MPa}}*{p\left(x\right)}^{2}dx}{p\left(x\right)dx}$ with x being expressed in mm and p being expressed in MPa, and with a pressure in said press nip being constant in a cross-machine direction.

2. The press arrangement according to claim 1, wherein said press nip has a length of at least 190 mm.

3. The press arrangement according to claim 1, wherein said line-load ratio LLR is at least 0.71 and at most 1.35.

4. The press arrangement according to claim 1, wherein said line-load ratio LLR is at least 0.73 and at most 1.14.

5. The press arrangement according to claim 1, which further comprises a pre-press located upstream of said main press in a running direction of the fibrous web.

6. The press arrangement according to claim 5, wherein said pre-press is located directly upstream of said main press in said running direction of the fibrous web.

7. The press arrangement according to claim 5, wherein said pre-press has an extended press nip and is configured to obtain a line-load ratio LLR of said pre-press of less than 0.69.

8. The press arrangement according to claim 1, wherein said main press is configured to be operated with a line load LL of at least 1,300 kN/m.

9. The press arrangement according to claim 1, wherein said main press is a shoe press including a press shoe with a substantially concave surface and a press jacket or press belt rotatably supported about said press shoe.

10. The press arrangement according to claim 9, wherein said press jacket is formed at least partially of polyurethane formed by a reaction of a prepolymer and a crosslinker component, said prepolymer is a reaction product of 1,4-phenylene diisocyanate (PPDI) and a polyol component including at least one of at least one polyether polyol or at least one polycarbonate polyol, and said crosslinker component includes a C2-14 diol.

11. The press arrangement according to claim 10, wherein said polyol component of said prepolymer includes polytetramethylene ether glycol (PTMEG) and at least one polycarbonate polyol.

12. The press arrangement according to claim 10, wherein said crosslinker component includes at least one of polytetramethylene ether glycol (PTMEG) or at least one polycarbonate polyol.

13. A machine for producing a fibrous web or a packaging testliner, the machine comprising a press arrangement according to claim 1.

14. A method for pressing a fibrous web or a packaging paper web or a packaging testliner, the method comprising: guiding the fibrous web through a main press having an extended press nip of at least 150 mm in length; operating the main press with a line load LL of at least 1,200 kN/m, the main press configured to obtain a line-load ratio LLR of at least 0.69 and at most 1.52; providing the line-load ratio LLR as a quotient of a weighted line load WLL to the line load LL; the weighted line load WLL being a result of an integration of a squared local pressure p(x).sup.2, weighted with a weighting factor A, over the nip length x, and the weighting factor A being one divided by ten megapascal; the line load LL being a result of an integration of a local pressure p(x) over the nip length x; and applying a formula as follows for the line-load ratio LLR: $\mathrm{LLR}=\frac{\mathrm{WLL}}{\mathrm{LL}}=\frac{A*{p\left(x\right)}^2\mathrm{dx}}{p\left(x\right)\mathrm{dx}}=\frac{\frac{1}{10\mathrm{MPa}}*{p\left(x\right)}^{2}dx}{p\left(x\right)dx}$ with x being expressed in mm and p being expressed in MPa, and with a pressure in the press nip being constant in a cross-machine direction.

15. The method according to claim 14, which further comprises providing the length of the press nip as at least 190 mm, and operating the main press with the line load LL being at least 1,300 kN/m.

16. The method according to claim 14, which further comprises providing the line-load ratio LLR as at least 0.71 and at most 1.35.

17. The method according to claim 14, which further comprises providing the line-load ratio LLR as at least 0.73 and at most 1.14.

18. The method according to claim 14, which further comprises further guiding the fibrous web through a pre-press located upstream or directly upstream of the main press in a running direction of the fibrous web.

19. The method according to claim 14, which further comprises carrying out the method at a speed of at least 1,000 m/min.

20. The method according to claim 14, which further comprises carrying out the method at a speed of at least 1,200 m/min.

21. The method according to claim 14, which further comprises carrying out the method at a speed of more than 1,400 m/min.

22. The method according to claim 14, which further comprises forming the fibrous web of at least 20 wt. % of OCC fibers.

23. The method according to claim 14, which further comprises forming the fibrous web of at least 50 wt. % of OCC fibers.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0045] FIG. 1 is a diagrammatic, cross-sectional view of a press arrangement according to the invention, which includes a main press and a pre-press;

[0046] FIG. 2 is an enlarged and detailed cross-sectional view of the main press of the press arrangement shown in FIG. 1; and

[0047] FIG. 3 is a diagram showing an exemplary pressure curve in the extended press nip of the main press.

DETAILED DESCRIPTION OF THE INVENTION

[0048] Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a diagrammatic illustration of a press arrangement 10 according to the invention including a main press 1 and a pre-press 11 disposed immediately upstream in the running direction BR of a fibrous web 8. In this embodiment, both the main press 1 and the pre-press 11 are configured as shoe presses and therefore each have an extended press nip. Alternatively, however, the pre-press 11 may have no extended press nip and/or the pre-press 11 and the main press 1 may share a common central roll. In that case, the central roll would be a press element forming both the extended press nip of the pre-press 11 and the extended press nip of the main press 1.

[0049] FIG. 2 shows an enlarged and detailed view of the main press 1, which is of particular importance in the invention. The main press 1 is configured to be operated with a line load LL of at least 1,200 kN/m, preferably at least 1,300 kN/m. The extended press nip 7 of the main press 1 is formed by two press elements, namely a shoe press roll 2 and a backing roll 3. The shoe press roll 2 includes a press shoe 5, which is supported on a stationary yoke 4, and a press jacket 6 (press belt), which is disposed to rotate about the press shoe 5. The fibrous web 8 is preferably guided sandwich-like between two press felts 9 through the extended press nip 7. The press shoe 5 has a substantially concave surface over which the press jacket 6 runs, while the press shoe 5 presses it with a high pressing force F toward the backing roll 3. The geometric configuration of the press elements, in particular the press shoe 5, is selected such that a line-load ratio LLR of at least 0.69 and at most 1.52 is obtained.

[0050] The pressing force F is preferably selected such that the peak pressure acting on the fibrous web 8 in the extended press nip 7 is at least 10 MPa. The length of the extended press nip of the main press 1 is at least 150 mm, preferably at least 190 mm.

[0051] At such peak pressures, it has proven particularly advantageous if the press jacket 6 is made at least in part of polyurethane formed by reaction of a prepolymer and a crosslinker component. The prepolymer is a reaction product of 1,4-phenylene diisocyanate (PPDI) and a polyol component containing at least one polyether polyol and/or at least one polycarbonate polyol, and the crosslinker component contains a C2-14 diol. For example, the press jacket 6 may include a reinforcement structure of threads embedded in the polyurethane layer, the prepolymer of the polyurethane layer being formed of 50 wt. % of a mixture of 1,4-phenylene diisocyanate (PPDI) and C5-6 polycarbonate diol and 50 wt. % of a mixture of 1,4-phenylene diisocyanate (PPDI) and polytetramethylene ether glycol (PTMEG), and the crosslinker including, or preferably predominantly being formed of, polytetramethylene ether glycol (PTMEG) and 1,6-hexanediol.

[0052] The pre-press 11 shown diagrammatically in FIG. 1 may, and preferably does, differ in configuration from the main press 1. In particular, unlike the main press 1, it may be configured such that the line-load ratio LLR of the pre-press 11 is less than 0.69. In the press arrangement, the pre-press 11 serves in particular to preconsolidate the fibrous web 8 sufficiently for passage through the main press 1, so that despite the relatively high peak pressure in the second press 1 the web is not excessively crushed.

[0053] The fibrous web 8 is preferably intended for the production of a packaging paper web or is such a packaging paper web. Furthermore, the fibrous web preferably is formed of at least 20 wt. %, more preferably at least 50 wt. %, of OCC fibers, which are characterized by particularly high resistance even to large peak pressures.

[0054] The inventive press arrangement 1 could theoretically include more presses than just the pre-press 11 and the main press 1. Preferably, however, the main press 1 is the last press of the press arrangement, i.e. the last press before the fibrous web 8 is transferred to a dryer section downstream of the press arrangement.

[0055] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: [0056] 1 Main press [0057] 2 Shoe press roll [0058] 3 Backing roll [0059] 4 Stationary yoke [0060] 5 Press shoe [0061] 6 Press jacket [0062] 7 Extended press nip [0063] 8 Fibrous web [0064] 9 Press felt [0065] 10 Press arrangement [0066] 11 Pre-press [0067] BR Running direction (machine direction, MD) [0068] F Pressing force