Paper machine belt

Abstract

A paper machine belt that includes a cross-machine direction supporting layer providing strength in the cross-machine direction and at least two additional layers. The two additional layers include polymeric material and at least one of the additional layers that includes the polymeric material is positioned on one side of the cross-machine direction supporting layer and at least another of the additional layers that includes the polymeric material is positioned on an opposite side of the cross-machine direction supporting layer. The cross-machine direction supporting layer substantially includes cross-machine direction yarns loosely bound together with very fine machine-direction yarns. A ratio of mass of the cross-machine direction yarns to the machine direction yarns being at least 160:1, and mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional layer that is provided on the opposite side of the cross-machine direction supporting layer.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation in Part of International Application No. PCT/GB03/00830, filed Feb. 24, 2003, and claims priority of Great Britain Patent Application No. 0204310.7, filed on Feb. 23, 2002. Moreover, the disclosure of International Patent Application No. PCT/GB03/00830, filed Feb. 24, 2003, and International Patent Application No. PCT/GB03/00824 filed Feb. 24, 2003 are expressly incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to paper machine process belts and particularly, but not exclusively, to belts for transferring and/or smoothing the paper web within, to and/or from a press section of a paper machine.

2. Discussion of Background Information

Transfer belts are used for carrying a paper web through a portion of a paper machine so as to eliminate open draws in which the paper web is unsupported and is thus likely to break. When the web breaks the paper machine must be shut down and consequently this constitutes a serious problem to the papermaker. Such transfer belts tend to have a smooth surface which can aid smoothing of the paper sheet and provide an extremely uniform pressure distribution in a nip with no basecloth mark. The surface should nevertheless provide for eas sheet release. Furthermore, the belt in use should be water impermeable.

In EP 1127976, a transfer belt comprises a base support having a layer of thermoplastic material formed thereon. A batt of fibrous material is located on top of this thermoplastic material. The whole structure is then heated in order to allow the thermoplastic material to migrate to the surface. It is common for transfer belts, such as that described in EP 1127976, to suffer from the drawback of edge curl, i.e., inward curling of the edges of the belt. In severe cases this can lead to the belt folding over which would, at the very least, mark the roller and cause misalignment and damage to the paper sheet.

Edge curl is caused by differential contraction properties associated with different materials used within the belt structure, as well as various finishing processes. For example, polyurethane film has a much greater shrinkage than the yarns of a woven structure and so when a film layer attached to a woven base cloth is partially melted, on cooling, it exerts a contractive force on the base structure, resulting in curling.

The present invention has been made from a consideration of this.

SUMMARY OF THE INVENTION

According to the present invention there is provided a paper machine belt having paper machine a layer for providing strength in the cross-machine direction (hereinafter “the cd supporting layer”) and two or more additional layers, wherein at least two of the additional layers include a polymeric material and wherein at least one of the layers of polymeric material is provided on one side of the cd supporting layer and at least another of the layers of polymeric material is provided on the obverse side of the cd supporting layer.

For example, surprisingly, the provision of a similar mass of polymeric material on either side, i.e. face of the cd supporting layer, which gives the cross-machine strength and rigidity for the belt, results in a balancing effect insofar as the anticipated differential thermal shrinkage properties of the polymeric layers are concerned, so as to yield a belt with no edge curl. Different polymeric materials having different shrinkages may be used in combination, so that further to exposure to thermal energy, the shrinkage forces are balanced, eliminating edge curl.

Ideally at least one layer of polymeric material is provided on the paper facing surface of the belt. The polymeric material is ideally provided as a film, but may, for example, alternatively be provided as sinterable material or as a fibrous material. This layer, in the finished product, is ideally water-impermeable.

Preferably some, and ideally all, of the layers of polymeric material preferably have a Shore hardness in the range from 30A to 75D, and ideally have a hardness of substantially 90 Shore A. Ideally, the weight of each polymeric layer is approximately 400 g/m². The thickness of each layer of polymeric material is ideally in the range from 0.4 to 1.0 mm. The preferred polymeric material is a thermoplastic polymer, such as a polyether based polyurethane, ideally in film form.

Polymeric materials having different colors may be used. For example, if a laminate of two polymeric materials were to be used in which the individual lamina had different colors then as the outer layer wears away the lower layer (having a different color) would become exposed. This would act as wear indicator. Alternatively, the use of differently colored polymeric materials might be useful for providing a guiding bar mark across the belt width. Pigments and additives may be included in the polymeric material as desired, such as photochromic pigments and/or ultra-violet fluorescent material.

The cd supporting layer providing the cross-machine strength is ideally a structure made up of a plurality of cross-machine direction yarns, preferably multi-strand (e.g. multifilament or cabled) yarns, which are laid in close proximity to one another. However, in order to give ease of handling, the cross-machine direction yarns may preferably be loosely bound together with very fine machine-direction yarns. For example, the machine-direction yarn diameter may be in the order of 0.1 mm and selected for pliabilty, compared to the stiff cross-machine direction yarn with a diameter of approximately 0.5 mm. The ratio of the mass of the cross-machine direction yarns to machine direction yarns is ideally substantially at least 160:1. The whole layer providing cross-machine stability has a weight of approximately 200 g/m². Ideally, this layer includes approximately 9 to 15 cross-machine direction yarns/cm, preferably 12 cross-machine direction yarns/cm.

It was found that this quasi-nonwoven cd supporting layer embedded between the layers of thermoplastic polymer, further to heat treatment and calendering, results in a laminated product with much improved macro-level pressure uniformity, due to the fact that there was far less chance of sheet marking, as is typical of substrates containing woven substrates with pronounced warp knuckles.

In addition, the belt preferably includes a machine direction (md) supporting layer to be needled on the roll side of the structure, to provide strength and md stability. This md supporting layer may be in the form of a woven, knitted or molded perforated membrane, for example of the type described in EP 0285376. However, this supporting fabric ideally includes an array of strong, stable, spirally wound, machine direction yarns. Layers of fibrous batt can also be needled in order to hold the yarns in position and to provide a coherent structure. The provision of additional batt on the roll side can also offer better wear resistance. In a preferred embodiment, the machine roll side layer is made up of spirally wound, machine direction, 0.2. mm/2 ply/3 cabled, polyamide yarns, with approximately 7 to 12 yarns/cm. There is approximately 50-800 g/m² of polyamide batt fiber in the range of 3 to 67 dtex needled thereon. This whole layer preferably has a weight in-the range from 450 to 480 g/m².

The spirally wound layer of md yarns with batt needled thereon is the preferred supporting substrate for a number of reasons. Firstly, marking due to cd/md yarn cross-over knuckles, as exist in woven substrates, is substantially eliminated.

Secondly, shrinkage upon heat-setting of a fabric generally takes place mainly in the cross machine direction because the machine direction length is held constant between the two rollers on the stretcher. The spirally wound layer is more yielding than a woven structure in the cd, due to there being only batt between the md yarns, which does not prevent the yarns from bunching together in order to conform to the shrinkage requirements of the thermoplastic film located thereon.

It is possible to use, in place of the cd and md supporting layers, a single base structure, which acts as both a cd and md supporting layer. This may be, for example, a woven material or possibly a nonwoven or a film. A preferred example is a double layer woven fabric with polymeric material on both faces.

Examples include laminates and integrally woven multiple layer bases. Yarn sizes would typically be from 0.2 mm to 0.6 mm in diameter.

A layer of batt fiber, preferably in the range from 3.1 to 44 dtex, would ideally be needled to this woven base structure.

The structure as a whole may additionally include separate layers of batt fibers. The batt is needled to any other layers so as to mechanically inter-lock them together, as well as providing a higher level of pressure distribution. The batt used preferably has a weight in the range from 50 to 800 g/m² and ideally in the order of 300 g/m². The total belt thickness is normally betveen 2.4 and 3.2 mm, with an average weight of between 2600 and 3300 g/m².

The preferred structure of the invention includes at least five main layers, which working from bottom to top include:

• • 1) an md supporting structure,
  • 2) a thermoplastic film or films,
  • 3) a cd supporting structure,
  • 4) a fibrous batt, and
  • 5) further thermoplastic film or films, ideally wherein the mass of thermoplastic material of layer (2) is substantially the same as that in layer (5).

The belt is preferably made endless, but could feasibly include a seam.

The whole structure is consolidated through needling at various stages during the manufacturing process. As a result of the needling stage, the surface is not entirely smooth as there are around 1 to 200 batt fibers per square cm, and preferably 10-100 batt fibers per square cm protruding through to, and in some instances standing proud of, the polymer surface. These provide for good sheet release. The fibers might take the form of loops, that is the middle of the fiber has been pushed through with both ends still remaining locked within the structure. If desired, the protruding fibers may be removed.

Further to the needling process, the entire structure is then exposed to sufficient thermal energy to cause the lower melt point, thermoplastic, polymeric film to melt. This melted polymer bonds the structure together, embedding the cd supporting layer and part of the batt in a matrix of molten polymer and forms a very smooth and well defined impermeable surface., which is resistant to de-lamination. The belt is then smoothed with a cold polished cylinder. There is no need to grind the finished product, which is advantageous because this is extremely difficult to achieve in the case of low melt polymeric elastomers.

In an alternative preferred structure the order of layers “1” and “2” hereinbefore described is swapped around, such that the structure includes at least five main layers, which working from bottom to top include:

• • 1) a thermoplastic film or films,
  • 2) a supporting structure providing machine-direction stability,
  • 3) a structure providing cross-machine stability,
  • 4) a fibrous batt, and
  • 5) further thermoplastic film or films, wherein the mass of thermoplastic material of layer (1) is preferably substantially the same as that in layer (5) to minimize edge-curl. Such an arrangement helps prevent batt loss and assists with ease of cleaning.

Another aspect of the invention includes a paper machine belt that includes a cross-machine direction supporting layer providing strength in the cross-machine direction and at least two additional layers. The two additional layers include polymeric material and at least one of the additional layers that includes the polymeric material is positioned on one side of the cross-machine direction supporting layer and at least another of the additional layers that includes the polymeric material is positioned on an opposite side of the cross-machine direction supporting layer. The cross-machine direction supporting layer substantially includes cross-machine direction yarns looselv bound together with very fine machine-direction yarns. A ratio of mass of the cross-machine direction yarns to the machine direction yarns being at least 160:1, and mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional layer that is provided on the opposite side of the cross-machine direction supporting layer.

Further aspects of the present invention include at least two additional layers that can include the polymeric material with different polymers. At least one of the additional layers can include the polymeric material provided on a surface of the belt and structured and arranged to support a paper web. Moreover, the additional layer that includes the polymeric material, provided on the surface of the belt, can be water-impermeable. Additionally, at least one of the additional layers that includes the polymeric material can have a Shore hardness of from 30A to 75D. A thickness of the additional layer that includes the polymeric material can be from 0.4 to 1.0 mm. At least one of the additional layers of polymeric material can comprise a thermoplastic material.

Further aspects of the present invention include the thermoplastic material can be a polyether-based polyurethane. The cross-machine direction supporting layer can exhibit different shrinkage to the additional layers of polymeric material. Additionally, at least two of the additional layers can include the polymeric material with different colors. Moreover, the cross-machine direction supporting layer can include a plurality of multi-strand cross-machine direction yarns. Additionally, the belt further can include a machine direction supporting layer. The machine direction supporting layer can include an array of spirally wound machine direction yarns. Moreover, the belt further can include at least one layer of batt.

Yet another aspect of the invention includes a paper machine belt that includes a cross-machine direction supporting layer providing strength in the cross-machine direction and at least two additional layers. The two additional layers include polymeric material and at least one of the additional layers that includes the polymeric material is positioned on one side of the cross-machine direction supporting layer and at least another of the additional layers that includes the polymeric material is positioned on an opposite side of the cross-machine direction supporting layer. The cross-machine direction supporting layer substantially includes cross-machine direction yarns bound together with machine-direction yarns. A ratio of mass of the cross-machine direction yarns to the machine direction yarns being at least 160:1, and mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional layer that is provided on the opposite side of the cross-machine direction supporting layer.

Another aspect of the invention includes a method of making a paper machine belt. The method including providing a cross-machine direction supporting layer having strength in the cross-machine direction, positioning one of at least two additional layers that include a polymeric material on one side of the cross-machine direction-supporting layer, positioning at least another of the additional layers that include the polymeric material on an opposite side of the cross-machine direction supporting layer, and heating the at least two additional layers and the cross-machine direction supporting layer. Additionally, a mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional layer that is provided on the opposite side of the cross-machine direction supporting layer.

Further aspects of the method can further include calendering the at least two additional layers and the cross-machine direction supporting layer. Moreover. at least two of the additional layers can include the polymeric material include different polymers. Additionally, the additional layer that includes the polymeric material, provided on the surface of the belt, can be water-impermeable. Also, at least one of the additional layers can include the polymeric material which has a Shore hardness in the range from 30A to 75D. Additionally a paper machine belt can be manufactured according to the above-noted method.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood, a specific embodiment thereof will now be described by way of example only with reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic cross-section of a transfer smoothing belt in accordance with the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring to FIG. 1, a transfer and smoothing belt 17 for use in the press section of a paper machine includes an endless loop having five layers 11-15.

The supporting layer 11, includes spirally wound machine direction yarns 16 into which batt has been needled to hold the yarns 16 in position. In this embodiment the machine direction yarns include three pairs of yarns twisted together.

The second layer 12, located on layer 11, itself includes two individual layers of thermoplastic polyurethane each having a weight of 400 g/m² and being 0.4 mm thick. On heating these two polyurethane layers, a single homogeneous layer is formed which bonds and partially impregnates the md supporting fabric 11 and the adjacent upper cd supporting layer 13.

Layer 13 includes a quasi non-woven structure made up of multifilament, cross-machine direction yarns and extremely fine machine direction yarns, for loosely holding the cross-machine direction yarns in position. This layer has a weight of approximately 195 g/m². The mass of material ratio of cross-machine direction yarns to machine direction yarns is approximately 160:1. This layer provides cross-machine direction strength and rigidity.

A layer 14 of batt is located above the cross-machine direction supporting structure 13 to facilitate inter-locking of the various layers by needling. The batt material preferably has a weight in the order of 300 g/m².

The final layer 15 of thermoplastic material is ideally identical to the inner thermoplastic material layer 12. On heating, the constituent polyurethane layers flow and bond the structure together, embedding the top cd supporting layer and part of the batt 14, in a polymeric matrix, to form a single homogenous layer. The belt is cured at a surface temperature of approximately 200° C. with a dwell time of 5 minutes. It is then calendered at 1 to 40 KN/m at a temperature of less than 200° C.

Surprisingly, by embedding a cd supporting structure between layers of melted thermoplastic polymeric material, a balance of contractive forces is achieved. That is, the relatively stiff, high bending modulus cross-machine direction orientated yarns, placed in a position relatively near to the top plane of the fabric, between the melted polymeric material, can successfully balance the contractive forces of the melted polymeric material, so as to yield a belt with no edge curl.

From recent in-house trials on a pilot machine it has been found that the belt described above gives excellent transfer at 2000 m/min and the surface has been found, using Martindale Abrasion testing methods, to exhibit good abrasion resistance. In particular, the belt was found not to suffer from edge curl.

It is to be understood that the above-described embodiment is by way of illustration only. Many modifications and variations are possible.

Claims

1. A paper machine belt comprising:

a cross-machine direction supporting layer providing strength in the cross-machine direction; and

at least two additional layers that include polymeric material and at least one of said additional layers that includes the polymeric material is positioned on one side of the cross-machine direction supporting layer and at least another of said additional layers that includes the polymeric material is positioned on an opposite side of the cross-machine direction supporting layer,

wherein the cross-machine direction supporting layer substantially comprises cross-machine direction yarns loosely bound together with very fine machine-direction yarns, a ratio of mass of the cross-machine direction yarns to the machine direction yarns being at least 160:1, and mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional layer that is provided on the opposite side of the cross-machine direction supporting layer.

2. The paper machine belt according to claim 1, wherein at least two of said additional layers that include the polymeric material comprise different polymers.

3. The paper machine belt according to claim 1 wherein at least one of the additional layers that include the polymeric material is provided on a surface of the belt and is structured and arranged to support a paper web.

4. The paper machine belt according to claim 3 wherein the additional layer that includes the polymeric material, provided on the surface of the belt, is water-impermeable.

5. The paper machine belt according to claim 1 wherein at least one of the additional layers that includes the polymeric material has a Shore hardness of from 30A to 75D.

6. The paper machine belt according to claim 1 wherein a thickness of the additional layer that includes the polymeric material is from 0.4 to 1.0 mm.

7. The paper machine belt according to claim 1 wherein at least one of the additional layers of polymeric material comprises a thermoplastic material.

8. The paper machine belt according to claim 7, wherein the thermoplastic material is a polyether-based polyurethane.

9. The paper machine belt according to claim 1 wherein the cross-machine direction supporting layer exhibits different shrinkage to said additional layers of polymeric material.

10. The paper machine belt according to claim 1 wherein at least two of the additional layers that include the polymeric material have different colors.

11. The paper machine belt as claimed according to claim 1 wherein the cross-machine direction supporting layer comprises a plurality of multi-strand cross-machine direction yarns.

12. The paper machine belt according to claim 1 wherein the belt further comprises a machine direction supporting layer.

13. The paper machine belt according to claim 12, wherein the machine direction supporting layer comprises an array of spirally wound machine direction yarns.

14. The paper machine belt according to claim 1 wherein the belt further comprises at least one layer of batt.

15. A method of making a paper machine belt comprising:

providing a cross-machine direction supporting layer having strength in the cross-machine direction;

positioning one of at least two additional layers that include a polymeric material on one side of the cross-machine direction supporting layer;

positioning at least another of said additional layers that include the polymeric material on an opposite side of the cross-machine direction supporting layer; and

heating the at least two additional layers and the cross-machine direction supporting layer,

wherein a mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional layer that is provided on the opposite side of the cross-machine direction supporting layer.

16. The method according to claim 15 further comprising:

calendering the at least two additional layers and the cross-machine direction supporting layer.

17. The method according to claim 15, wherein at least two of said additional layers that include the polymeric material comprise different polymers.

18. The method according to claim 17, wherein the additional layer that includes the polymeric material, provided on the surface of the belt, is water-impermeable.

19. The method according to claim 15 wherein at least one of the additional layers that includes the polymeric material has a Shore hardness in the range from 30A to 75D.

20. A paper machine belt that is manufactured according to the method of claim 15.

21. A paper machine belt comprising:

a cross-machine direction supporting layer; and

at least two additional layers that include a polymeric material and at least one of said additional layers that includes the polymeric material is positioned on one side of the cross-machine direction supporting layer and at least another of said additional layers that includes the polymeric material is positioned on an opposite side of the cross-machine direction supporting layer,

wherein the cross-machine direction supporting layer substantially comprises cross-machine direction yarns bound together with machine-direction yarns, the ratio of a mass of the cross-machine direction yarns to the machine direction yarns being at least 160:1, and a mass of the polymeric material of the additional layer on the one side of the cross-machine direction supporting layer is substantially the same as a mass of the polymeric material of the additional laver that is provided on the opposite side of the cross-machine direction supporting layer.