EP0351535A2 - Fabric for paper-making machines - Google Patents

Abstract

The invention relates to a fabric for use in paper machines as a forming fabric and base fabric for coated, impervious tapes and jackets for wide-nip presses and for forming roll covers. A fabric of this type is to be designed in such a way that its transverse stability is increased, its fibrillation resistance is increased even at high surface pressures, such as occur in the press nip of wide-nip presses, and its wear resistance is improved. This is achieved in that at least part of the longitudinal and / or transverse threads of the fabric consists of a copolymer, the main components of which are a polymer with a high molecular weight, corresponding to a viscosity of at least 0.80, the proportion of the copolymer in the copolymer Is 2 - 20%. Manufacture of fabrics for forming fabrics and forming roll covers in paper machines.

Description

The invention relates to a fabric for use in paper machines as a forming fabric and base fabric for coated, impervious tapes and jackets for wide-nip presses and for forming roll covers.

Extremely high demands are placed on paper machine fabrics, forming fabrics, fabrics for coated, impermeable belts or jackets for wide-nip presses and fabrics for forming roll covers, which is why only very specific materials that meet these requirements can be used for the production of such fabrics.

For example, fabrics for forming fabrics are made from polymer monofilaments, whereby only polyester monofilaments are used for the longitudinal threads, because only with these the required longitudinal stability can be achieved. Since longitudinal tensions of more than 200 N / cm screen width can certainly occur in the application, for example the essentially wear-resistant or abrasion-resistant polyamide cannot be used as material for the longitudinal threads, because it stretches too much, so that such screens do not would have required required stretch stability.

On the other hand, pure polyester fabrics, i.e. forming fabrics in which the transverse threads are made of polyester monofilaments, do not have a sufficient lifespan, because polyester has a significantly poorer abrasion resistance than polyamide, so that the running time of such screens is much too short to be able to use these screens as a useful alternative. This disadvantage is also not compensated for by the fact that, as tests have shown, pure polyester sieves in a known, conventional double-layer weave require approximately 30% less power than comparable so-called mixed sieves, which consist of both polyester and polyamide.

It has also been shown that sieves, the transverse threads of which consist exclusively of polyamide, are not sufficiently cross-stable because of the low dimensional stability and the high water absorption which leads to the swelling of the monofilament. This deficiency causes such screens to wrinkle on the paper machine. For this reason, DE-OS 25 02 466 proposes to use only polyamide monofilaments for some of the transverse threads, while the remaining transverse threads are to be made of polyester. With these so-called mix or interchangeable shot screens, the runtime, i.e. the operational usable time of use of the screen until it was replaced or repaired, could be increased significantly in many cases.

To date, base fabrics for coated and liquid-impermeable tapes or jackets for wide-nip presses are made from polyamide threads. Such tapes are guided over a concave shoe and pressed into the shoe by a roller arranged above them. At least one felt web and the paper web to be pressed and thus dewatered run between the shoe and roller. By on this way extended press nip (press nip) the degree of dryness of the paper web is significantly improved. Because of the high surface pressure in the press nip, very high demands are placed on the monofilaments of the base fabric of the belt or jacket with regard to their resistance to fibrillation. Although these requirements are met by polyamide monofilaments, the above-mentioned poor dimensional stability of polyamide monofilaments often leads to bubbles or circumferential changes, in particular elongations, occurring in the band or in the jacket if the pressure is distributed unevenly across the width. which lead to the fact that the tissue in this area is subject to increased flexing work and is therefore quickly destroyed.

Although these phenomena can be avoided with polyester monofilaments, these monofilaments are not sufficiently resistant to fibrillation for such a purpose.

As is known, the surface of the forming rolls of paper machines is designed as a honeycomb-shaped lattice structure. This surface serves as a contact surface, but is much too roughly structured for the fine sieve and would therefore destroy the sieve very quickly at the usual sieving speeds of approx. 1000 m / min and above. A hose-like cover made of a stable fabric, preferably in linen weave, with a mesh number of approximately 16 / cm 2 is therefore arranged between the wire and the surface of the forming roller. This so-called forming roll cover is shrunk onto the roll with the help of heat. This requires sufficient shrinkability at temperatures of approx. 100 ° to 130 ° C and a correspondingly high level Shrink force required to guarantee a tight fit of the coating on the surface of the forming roller. Considerable demands are placed on the wear and fibrillation resistance of such a coating. Until now, the firm fit could only be achieved with covers made of polyester monofilaments. However, as stated above, this material is not particularly resistant to wear and fibrillation, so that the service life or running times of such rolls or their coatings leave something to be desired.

The object of the invention is therefore to design a fabric of the type mentioned at the outset in such a way that the disadvantages explained which occur when using known monofilaments are eliminated and universal applicability is ensured within the scope of the stated application possibilities, which is particularly diverse in economic terms requirements.

This object is achieved in that at least part of the longitudinal and / or transverse threads of the fabric consists of a copolymer, the main components of which are a polymer with a high molecular weight, corresponding to a viscosity of at least 0.75, and a copolymer with a high molecular weight , corresponding to a viscosity of at least 0.80, the proportion of the copolymer in the copolymer being 2-20%.

In this way, the increase in power consumption caused by the use of polyamide transverse threads on the running side can be avoided in paper machine sieves, also called forming sieves, if, instead of the polyamide monofilaments, mixed polymer monofilaments are woven in on the running side of the sieve. However, reducing power consumption means energy saving. A substantial reduction in runtime or service life, as can be observed with pure polyester screens, does not occur in the screen according to the invention. Because of the chemical resistance to acids, sieves made from copolymers of the type according to the invention can be used successfully where polyamide can no longer be used due to its known sensitivity to acids. Since mixed polymers have a higher resistance to wear and fibrillation, considerable increases in runtime can even be expected. In the case of fine forming fabrics, especially those which contain additional fine filler weft, so-called floating threads, the use of mixed polymer monofilaments as floating threads is particularly advantageous since, up to now, the fine floating threads, the diameter of which is usually 0.13 mm and smaller , were very often destroyed when cleaning the sieves.

The reason for this is the cleaning water directed to the surface of the sieve with high-pressure pointed pipes, which not only damages the sensitive floating threads due to its impact energy. The damage caused by fibrillation in the known polyester monofilaments does not occur in the mixed polymer monofilaments or only occurs at greatly increased injection pressures. In the case of screens according to the invention, the spray water pressures, which were previously around 30 bar, can therefore be increased in favor of more effective screen cleaning.

In the case of coated, liquid-impermeable tapes for wide-nip presses in paper machines, the use of mixed polymer monofilaments and mixed polymer multifilaments in the base fabric of these tapes or jackets brings very considerable advantages, since the material according to the invention achieves the stability and dimensional stability of polyester with regard to wear resistance Polyester is superior and, in terms of fibrillation resistance, even surpasses polyamide. This material is therefore ideal for base fabrics of press belts, with the possibility of to replace previously necessary polyamide threads with the material according to the invention.

Experiments have shown that the fabric or material according to the invention meets the highest requirements in terms of shape and fibrillation resistance. The elongation of a jacket for wide-nip presses made of the material according to the invention in the axial direction is at least 50% less than that of known jackets whose base fabric has the same weave but is woven from polyamide monofilaments. Since the paper machine has to be shut down in order to retighten the jackets in the axial direction, the lower axial expansion means a reduction in the tensioning downtimes, that is to say a better operational use of the machine. In the case of press belts or press sleeves which have a water storage volume which is formed by the fabric protruding from the coating or the fabric parts protruding from the coating, the dimensional stability against high pressure loads is particularly important. Here too, the fabric according to the invention is far superior to a polyamide fabric as a base fabric.

Forming roll coverings, which are made from the fabric according to the invention, develop shrinkage forces at temperatures of approximately 100 ° to 130 ° C., which are about twice or even higher than with a corresponding polyamide fabric, with sufficient shrinkage. In contrast to the polyamide fabric, this ensures a secure, firm fit of the forming roll covers. The fibrillation resistance of the fabric according to the invention not only exceeds that of a polyamide cover, but is even several times higher than that of a polyester cover of the quality previously used.

Another advantage of the fabric according to the invention can be seen in the fact that the mixed polymer threads swell by absorbing water in contrast to polyamide threads is excluded. According to an advantageous embodiment of the invention, the proportion of the copolymer can be between 5% and 10%, polyethylene terephthalate being used as the polymer and dimethyl isophthalate or polybutylene terephthalate being used as the copolymer. The viscosity of the polymer can expediently be set to at least 0.85 and that of the copolymer to at least 1.0.

In the cases in which the fabric according to the invention is used as a paper machine screen, it has proven particularly useful as a single-layer fabric or double-layer fabric, the transverse threads then advantageously consisting of the said mixed polymer.

In the event that the fabric according to the invention is used as a coated, impermeable tape or corresponding jacket for the wide-nip press of a paper machine, the fabric is used as a base or reinforcing fabric in one layer, at least the longitudinal threads being made of said copolymer. The latter also applies in the event that the fabric according to the invention is designed as a double-layer fabric for the application mentioned.

In the event that the fabric according to the invention is used for forming roll coverings, it has proven useful to form the fabric in one layer, both the longitudinal and the transverse threads being made of the said copolymer.

Claims (12)

1. Fabric for use in paper machines as a forming fabric and base fabric for coated, impervious tapes and jackets for wide-nip presses and for forming roll covers, characterized in that at least part of the longitudinal and / or transverse threads of the fabric consists of a copolymer, the main components of which are a polymer with a high molecular weight, corresponding to a viscosity of at least 0.75, and a copolymer with a high molecular weight, corresponding to a viscosity of at least 0.80, the proportion of the copolymer in the copolymer being 2- Is 20%. 2. Fabric according to claim 1, characterized in that the proportion of the copolymer is between 5 and 10%. 3. Fabric according to claim 1 or 2, characterized in that the polymer is polyethylene terephthalate and the copolymer is dimethyl isophthalate. 4. Fabric according to claim 1 or 2, characterized in that the polymer is polyethylene terephthalate and the copolymer is polybutylene terephthalate. 5. Fabric according to claim 3 or 4, characterized in that the viscosity of the polymer is at least 0.85. 6. Fabric according to claim 3 or 4, characterized in that the viscosity of the copolymer is at least 1.0. 7. Fabric according to one of claims 1-6 for use as a paper machine screen, characterized in that the fabric is single-ply and its transverse threads consist of a mixed polymer. 8. Fabric according to one of claims 1-6 for use as a paper machine screen, characterized in that it is double-layered and its transverse threads consist of the copolymer. 9. Fabric according to one of claims 1-6 for use as a coated, impervious tape or jacket for the wide-nip press of a paper machine, characterized in that the fabric is the base or forms reinforcement fabric of the band or jacket, is single-ply and its longitudinal threads consist of the copolymer. 10. Fabric according to one of claims 1 to 6 for use as a coated, impervious tape or jacket for the wide-nip press of a paper machine, characterized in that the fabric serves as a base or reinforcing fabric of the tape or jacket, is double-layered, and that its longitudinal threads consist of the copolymer. 11. A fabric according to claim 9 or 10, characterized in that the transverse threads consist of the copolymer. 12. Fabric according to one of claims 1 to 6 for use as a forming roll cover, characterized in that the fabric is single-ply, both the longitudinal and the transverse threads consist of the copolymer.