PRESS COVER WITH REINFORCING THREADS FORMED AS TWISTED YARNS, SHOE PRESS AND METHOD OF PROCESSING A FIBROUS MATERIAL WEB

Abstract

A press cover has at least one polymer layer in which a reinforcing structure is embedded. The reinforcing structure is formed as a scrim with a radially inner layer of longitudinal threads radially outer layer of at least one circumferential thread. The threads are reinforcing threads formed with fibers or fiber bundles twisted in a first direction of rotation and at a first rate of rotation to form a roving. A number of rovings are twisted together in the opposite direction and at a second, higher rate of rotation. In the radial direction of the press cover, the longitudinal threads and the circumferential thread(s) are disposed to touch one another. There is also described a press roll and a shoe press with such a press cover for processing a web of fibrous material.

Claims

1. A press cover, comprising: at least one polymer layer; a reinforcing structure embedded in said at least one polymer layer, said reinforcing structure being a scrim with a first, radially inner layer made of multiple longitudinal threads extending in an axial direction of the press cover and a second, radially outer layer made of at least one circumferential thread extending substantially in a circumferential direction of the press cover; said longitudinal threads of said first layer, and optionally also the at least one circumferential thread of the second layer, being formed as a reinforcing thread that is formed as a twisted yarn, in that firstly several individual fibers or fiber bundles are twisted together in a first twist direction at a first turn rate so as to form a pre-twist, and then several such pre-twists are twisted together in a second twist direction, opposite the first twist direction, at a second turn rate; wherein the first turn rate is lower than the second turn rate and, viewed in the radial direction of the press cover, said longitudinal threads and said at least one circumferential thread are arranged to touch one another.

2. The press cover according to claim 1, wherein the first turn rate amounts to 70% to 90% of the second turn rate, and the first turn rate is between 70 and 90 turns per meter.

3. The press cover according to claim 2, wherein the first turn rate is between 75 and 85 turns per meter.

4. The press cover according to claim 2, wherein the first turn rate is 80 turns per meter.

5. The press cover according to claim 1, wherein the first twist direction is an S direction and the second twist direction is a Z direction.

6. The press cover according to claim 1, wherein each pre-twist is formed from two individual fibers or fiber bundles, and the final twisted yarn is formed from three pre-twists.

7. The press cover according to claim 1, wherein the longitudinal threads of the first layer in the press cover have a first pretension, whereas the at least one circumferential thread of the second layer has a second pretension, and the first pretension is greater than the second pretension.

8. The press cover according to claim 7, wherein the first pretension amounts to at least 7 times and/or at most 13 times the second pretension.

9. The press cover according to claim 1, wherein an entire said reinforcing structure of the press cover consists only of the first layer and the second layer.

10. The press cover according to claim 1, wherein the twisted yarn of said at least one reinforcing thread includes a coating.

11. The press cover according to claim 1, wherein the at least one reinforcing thread formed as a twisted yarn has a fineness between 800 dtex and 1500 dtex.

12. The press cover according to claim 11, wherein the at least one reinforcing thread has a fineness between 1000 dtex and 1200 dtex.

13. The press cover according to claim 11, wherein the at least one reinforcing thread has a fineness of 1100 dtex.

14. The press cover according to claim 1, wherein each of said pre-twists are formed from several fiber bundles, with each fiber bundle having between 180 and 230 individual filaments.

15. The press cover according to claim 1, wherein all threads of said reinforcing structure of the press cover correspond to said at least one reinforcing thread formed as a twisted yarn.

16. The press cover according to claim 1, wherein all threads of said reinforcing structure of the press cover are formed identically to one another.

17. A press roll for a shoe press for processing a fibrous material web, the press roll comprising at least one press cover according to claim 1.

18. A shoe press for processing a fibrous material web, the shoe press comprising: a press roll and a counter-roll which together form or delimit an extended pressing nip, wherein said press roll comprises a circumferential press cover configured as a press cover according to claim 1.

19. The shoe press according to claim 18 configured for processing a paper web, a cardboard web, or a tissue web,

20. A method of processing a fibrous material web which comprises providing a shoe press with a press cover according to claim 1, and processing a fibrous material web, being a paper web, a cardboard web, or a tissue web in the shoe press.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0040] FIG. 1 shows a sketch to illustrate the general correlation between yarn diameter and twist angle;

[0041] FIG. 2 shows an example of a typical twisted yarn such as a sewing thread, in which three pre-twists twisted in the S direction are twisted together in the Z direction;

[0042] FIG. 3 shows a sketch to illustrate how the radial hardness of the reinforcing thread is determined;

[0043] FIG. 4 shows a comparison of the radial hardnesses of different reinforcing threads at different pretensions;

[0044] FIG. 5 shows a reinforcing thread for a press cover according to the present invention;

[0045] FIG. 6 shows a shoe press with a press cover according to the invention; and

[0046] FIG. 7 shows a sketch to illustrate a production process for the press cover according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0047] FIG. 5 shows an exemplary reinforcing thread 10 which is configured according to the present invention in order to be installed as part of the reinforcing structure 100 in a press cover 200 according to the invention (see FIG. 6). The reinforcing thread 10 is formed as a twisted yarn, wherein firstly two fiber bundles 30 are twisted in the S twist direction at a first turn rate into a pre-twist 20, and then three such identically produced pre-twists 20 are twisted in the Z direction at a second turn rate to form the final yarn or reinforcing thread 10. The reinforcing thread 10 may then also be coated.

[0048] According to the present invention, the first turn rate is lower than the second turn rate. The first turn rate in this exemplary embodiment is 80 and the second turn rate 100 turns per meter. The thread has a fineness of 1100 dtex. Thus the reinforcing thread 10 according to the present invention can be characterized in brief as follows:

[00002]$\mathrm{dtex}1100?2?3S80/Z100.$

[0049] This exemplary embodiment of a reinforcing thread 10 for a press cover according to the invention, which exemplary embodiment is designated below as AB-1, was now tested with respect to its radial hardness in the test structure described above with reference to FIG. 3, and compared with two exemplary embodiments AB-2 and AB-3 of a respective reinforcing thread from the prior art. The reinforcing threads according to AB-2 and AB-3 indeed have the same basic structure (as shown in FIG. 5) as the exemplary embodiment AB-1, but in these reinforcing threads the first turn rate is higher than the second turn rate. In brief, the reinforcing thread according to AB-2 may be characterized as follows:

[00003]$\mathrm{dtex}1100?2?3S165/Z150$

and the reinforcing thread according to AB-3 as follows:

[00004]$\mathrm{dtex}1100?2?3S100/Z80.$

[0050] In AB-2, the first turn rate is therefore 165 turns per meter, and the second turn rate 150 turns per meter, whereas in AB-3 the first turn rate is 100 turns per meter and the second turn rate 80 turns per meter.

[0051] FIG. 4 shows the result of this comparison, wherein the X-axis shows the pretension in Newtons (N) to which the reinforcing threads were exposed during the test. The Y axis shows the hardness in Pussey & Jones (P&J). It should be noted that a lower P&J value indicates a greater hardness than a higher P&J value.

[0052] It is evident from FIG. 4 that under a low pretension of 4N, the P&J hardness of the exemplary embodiment AB-1 according to the present invention is 34, which is significantly higher than for AB-2 (only 21) and slightly higher than for AB-3 (32). In other words, under a low pretension, the reinforcing thread 10 according to the invention is relatively soft in the radial direction in comparison with reinforcing threads of the prior art. At a significantly higher pretension of 50N, the P&J hardness of the exemplary embodiment AB-1 according to the present invention is however only 20. It is therefore lower than the P&J hardness of AB-3 (24) and only slightly higher the P&J hardness of AB-2 (18). In other words, under a higher pretension, the reinforcing thread 10 according to the invention is harder, or at least as hard, in the radial direction as the reinforcing threads from the prior art.

[0053] This great deviation in radial hardness of the reinforcing thread 10 according to the invention is advantageously utilized by the press cover 200 of the present invention. Several of these reinforcing threads 10, which extend as longitudinal threads 220 parallel to the axis 1 of the press cover 200, form a first layer of a reinforcing structure 100. Furthermore, at least one of these reinforcing threads 10, which is wound as a circumferential thread 230 in a helix about the axis A (see FIG. 7) radially on the outside of the first layer, forms a second layer of the reinforcing structure 100. Preferably, the entire reinforcing structure 100 of the press cover 200 according to the invention consists only of these two layers.

[0054] The press cover 200 may be produced as illustrated schematically in FIG. 7. In a highly schematic side view, FIG. 7 shows a device for production of the press cover 200 according to the invention. The device in this case has precisely one cylindrical winding mandrel. On the periphery are a plurality of reinforcing threads 10 formed as longitudinal threads 220 spaced apart from one another. A polymer is applied on the radially outermost casing surface of the winding mandrel in order to create a polymer coating 240. In addition, for example, a circumferential thread 230 is introduced in a helical pattern into the polymer of the polymer layer 240. After embedding in the polymer, the circumferential thread 230 together with the longitudinal threads 220 forms the reinforcing structure 100 of the final press cover 200 according to the invention. According to the invention, the circumferential thread 230 touches the longitudinal threads 220, i.e. viewed in the radial direction of the press cover 200, there is no spacing between them.

[0055] The winding mandrel is mounted rotatably about its longitudinal axis, which corresponds to the longitudinal axis A of the later press cover 200. The longitudinal axis here runs orthogonally into the drawing plane. Via a line 300, the casting material, in the form of a castable, hardenable elastomer polymer, e.g. polyurethane, is applied from above through a casting nozzle 310 onto the radially outermost casing surface of the winding mandrel or onto the longitudinal threads 220. Such a casting material may be selected, e.g. with respect to its pot life and viscosity, such that it does not drip from the winding mandrel during casting. During this process, the winding mandrel is rotated about its longitudinal axis in the direction of the arrow. At the same time as this rotation, the casting nozzle 310 is guided parallel to and along the longitudinal axis A relative to the winding mandrel via a suitable guide (not shown in detail in FIG. 7). Simultaneously with the casting of the casting material, the at least one circumferential thread 230 is unrolled and wound in a helix pattern to form turns about the rotating winding mandrel. The casting material can penetrate onto the winding mandrel through the longitudinal threads 220. In this example, after the step of hardening, the polymer forms a radially innermost and preferably elastomer polymer layer such as e.g. the polymer layer 240. In addition, if necessary further polymer layers may be applied radially on the outside. Preferably however, the entire reinforcing structure 100 is completely embedded in the radially innermost polymer layer 240.

[0056] The casting material emerging from the casting nozzle 6 is a mixture of the pre-polymer and a cross-linking agent. The former is provided from a prepolymer container (not shown) in which it is stored or agitated. The prepolymer may comprise an isocyanate according to the invention and a polyol. It may be present in the prepolymer container for example in the form of a prepolymer of the above-mentioned substances. The cross-linking agent may be provided in a cross-linking agent container. The prepolymer container and cross-linking agent container are assigned to the device for production of the press cover 200. They are connected fluid-conductively via lines (not shown) to a mixing chamber (also not shown) arranged upstream of the casting nozzle 310 in the flow direction. The mixture of prepolymer and cross-linking agent is thus produced upstream and outside the casting nozzle 310, i.e. mixed in the mixing chamber. Irrespective of production of the mixture, this is then applied to the surface of the winding mandrel in order to form the at least one polymer layer of the press cover 200.

[0057] By means of such a continuous casting process, which is also known as rotational casting, gradually over the width of the winding mandrel, an endless cylindrical press cover 200 is produced which is closed in itself about its longitudinal axis a and the inner circumference of which corresponds substantially to the outer circumference of the winding mandrel 4.

[0058] Preferably, the longitudinal threads 220 are pretensioned with a greater pretension, for example a pretension of 50N, than the at least one circumferential thread 230 which may be pretensioned with a pretension of just 4N, when the reinforcing structure 100 is embedded in the polymer layer 240. As a result, the reinforcing threads 10 according to the invention, which as longitudinal threads 220 form the first layer of the reinforcing structure 100, are substantially harder than the at least one reinforcing thread 10 according to the invention, which as a circumferential thread 230 forms the second layer of the reinforcing structure 100. This has an advantageous effect on the resistance of the press cover 200 according to the invention on passage of a knot.

[0059] FIG. 6 shows a partially sectioned, schematic side view of a shoe press 500 which in the present case comprises a press roll 400 according to the invention, namely a shoe press roll, and a counter-roll 450. The longitudinal axes of the shoe press roll 400 and the counter-roll 450 are arranged parallel to one another. Together these rolls form or delimit an extended pressing gap 510.

[0060] Whereas the counter-roll 450 here consists of a cylindrical roll configured to rotate about its longitudinal axis, the shoe press roll 400 is composed of a shoe 410, an upright yoke carrying this, and the press cover 200 according to the invention. The shoe 410 and the yoke are arranged stationarily with respect to the counter-roll 450 or the press cover 200. This means that they do not rotate. The shoe 410 is supported by the yoke and not pressed via the hydraulic press elements (not shown) against the radially innermost surface of the press cover 200 which rotates relative thereto. The press cover 200, which surrounds the shoe 410 and the yoke in the circumferential direction, rotates about its longitudinal axis A in the opposite rotational direction to the counter-roll 450. Because of the concave design of the shoe 410 on its side facing the counter-roll 450, the pressing gap 510 is comparatively long.

[0061] The shoe press 500 is particularly suitable for dewatering fibrous material webs FB. During operation of the shoe press 500, a fibrous material web FB with one or two press felts 520 is guided through the pressing gap 510. In the present case there are precisely two press felts 520 which receive the fibrous material web FB between them in sandwich fashion. On passage through the extended pressing gap 510, the press felt 520 exerts an indirect pressure on the fibrous material web FB in the extended pressing gap 510. This is because the radially outermost surface of the counter-roll 450 on one side, and the radially outermost surface of the press cover 200 on the other, come into direct contact with the corresponding press felt 520. The liquid emerging from the fibrous material web FB is temporarily received by the press felt(s) 520 and any depressions, in particular grooves (not shown), provided in the surface of the press cover. After leaving the extended pressing gap 510, the liquid received by the depressions of the press cover 200 is expelled before the press cover 200 again enters the pressing gap 510. Also the water received by the press felt 520 may be removed with suction elements after leaving the pressing gap 510.

[0062] In a further embodiment of the invention (not shown in the figures), the press felt 520 may be omitted. In such a case, the fibrous material web FB is in direct contact on one side with the press cover 200 and on the other with the counter-roll 450, which together form a pressing gap 510. The latter roll may then be configured as a heated dry cylinder.

[0063] The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: [0064] 10 Reinforcing thread [0065] 20 Pre-twist [0066] 30 Fiber bundle [0067] 100 Reinforcing structure [0068] 200 Press cover [0069] 220 Longitudinal thread [0070] 230 Circumferential thread [0071] 240 Polymer layer [0072] 300 Line [0073] 310 Casting nozzle [0074] 400 (Shoe) press roll [0075] 410 Shoe [0076] 450 Counter roll [0077] 500 Shoe press [0078] 510 (Extended) pressing gap [0079] 520 Press felt [0080] A Axis (of press cover) [0081] AB-1 Exemplary embodiment 1 (according to the present invention) [0082] AB-2 Exemplary embodiment 2 (according to the prior art) [0083] AB-3 Exemplary embodiment 3 (according to the prior art) [0084] FB Fibrous material web