Patent Application
20220372701
The invention starts from a press cover, more particularly for a press apparatus for the treatment of a fibrous material web, for the purpose of smoothing or dewatering said web, for example, specifically in accordance with the independent claims. The invention also relates to a shoe press and the use of a press cover in such a press, and also to a machine comprising such a shoe press, specifically in accordance with the further independent claims.
Press apparatuses such as shoe presses have long been part of modern papermaking machines. They comprise essentially a shoe (also called press shoe), in a stationary arrangement, which extends in a cross-machine direction, and a press cover encircling the stationary shoe. This cover is deformable and takes on substantially a tubular shape in operation. The shape of the shoe is such that with an opposing roll it forms a press nip (press gap). The press nip is defined by the contact face of the opposing roll in the shoe. The shoe is designed to be movable and can be moved against the opposing roll.
The press cover is subject to very exacting requirements in terms of its stability, namely with regard to surface hardness and resistance to pressure, temperature, and hydrolysis. The press cover, moreover, is exposed to significant flexural cycling loads during operation. On entry at the edge of the shoe—upstream of the press nip as viewed in the direction of rotation of the press cover—there is initially a flexure with a relatively small radius. This immediately becomes an opposing flexure on passage through the press nip. On exit at the other edge of the shoe, in other words downstream of the press nip as viewed in the direction of rotation of the press cover, there is an opposing flexure again. This deformation of the press cover on entry and exit is also referred to as cycling nip. It is readily apparent that the tendency of the press cover to break particularly at this point is very high because of the high mechanical stress. Correspondingly there are numerous measures known from the prior art that are intended to increase the stability of the press cover.
The press cover must therefore have sufficient flexibility to allow it to be guided around the shoe; it must be stiff enough that it does not undergo excessive deformation or compression in the nip under the press load; and it must be sufficiently wear-resistant. Press covers therefore consist of a single-ply or multi-ply polymer layer, preferably of polyurethane, which may comprise embedded reinforcing threads in the form of scrims or woven fabrics.
The present invention relates to such articles of the generic type as specified at the outset.
Although press covers known from the prior art do have sufficient flexibility and at the same time sufficient stiffness, they are in need of improvement in their chemical resistance, particularly with respect to water and oil, their abrasion resistance, their resistance to cracking and crack propagation, and their swelling behavior.
An object of the invention, accordingly, is a press cover which avoids the disadvantages of the prior art.
The object is achieved with the features of the independent claims. Particularly preferred and advantageous embodiments of the invention are given in the dependent claims.
The inventor has recognized that the object of the invention is achieved if a thermoplastic elastomer (TPE) in the form of a copolymer is used for a polymer layer of the press cover.
Thermoplastic elastomers are—according to the definition of the so-titled Wikipedia article, most recently edited at 11.13 am on Jul. 22, 2019—materials where elastic polymer chains are incorporated in thermoplastic material. They can be processed in a purely physical process entailing combination of high shearing forces, heat exposure, and subsequent cooling. Conventional elastomers, conversely, are three-dimensional network molecules with wide-mesh chemical crosslinking. The crosslinks cannot be undone without the material decomposing. Although there is no need for chemical crosslinking, by vulcanization involving copious time and high temperature, for example, as in the case of the elastomers, the parts produced have a particular molecular structure and hence nevertheless have rubber-elastic properties. Renewed exposure to heat and shearing force leads again to the melting and deformation of the material.
Copolymers in the sense of the invention are polymers composed of two or more different kinds of monomer units. According to the afore-mentioned Wikipedia article, thermoplastic elastomers are differentiated as copolymers and elastomer alloys. Copolymers are employed either as random or as block copolymers. The former consist of a crystallizing (and therefore physically crosslinking) main polymer such as polyethylene, for example, whose degree of crystallation is reduced, by a comonomer incorporated randomly along the chain, such as vinyl acetate, for example, to an extent such that the crystallites (i.e., the hard phase) in the completed material (in the example, EVA) are no longer in direct contact. In conventional elastomers they then act as isolated crosslinking points. In block copolymers, there is a sharp separation between the hard and soft segments in a molecule (e.g., SBS, SIS). With TPEs, below a certain temperature, the material separates into a continuous phase and a discontinuous phase. As soon as the latter falls below its glass transition temperature Tg (the Tg of the continuous phase is well below the subsequent service temperature), it acts again as a crosslinking point. Elastomer alloys, on the other hand, are polyblends, these being commixtures (combines) of fully fledged polymers, with the plastic thus consisting of multiple varieties of molecule. Different mixing ratios and adjuvants are used to obtain customized materials (for example, polyolefin elastomer from polypropylene (PP) and natural rubber (NR)— they span a wide hardness range depending on quantitative proportion).
The thermoplastic elastomer is preferably a segmented block copolymer. A copolymer of this kind may be a thermoplastic polyether-ester block copolymer (COPE or TPE-E) and/or a thermoplastic copolyamide elastomer (COPA or TPE-A). The two stated polymers may be alternatives to conventional polyurethanes or thermoplastic polyurethanes (TPU). The soft segments are formed with the polyol. For the hard segments of the COPE, use may be made of polyesters, polybutylene terephthalates, polyethylene terephthalates, methyl terephthalates, poly(butylene-2,6-naphthalene dicarboxylates), poly(butylene-co-isophthalates) or aromatic polycarbonates. For COPA use may be made of polyamides as hard segments. Polyols contemplated as being particularly suitable for the invention are polyether polyols, e.g., polytetramethylene glycol, polycarbonate polyols, polyether-polycarbonate polyols, and polycaprolactone polyols.
In principle it would be conceivable to use a polyol blend, i.e., an elastomer alloy between the stated thermoplastic copolymers and a thermoplastic elastomer based on urethane (TPU), for the at least one polymer layer of the press cover.
When it is said in accordance with the invention that something is made of a substance, this means that it is produced partly or completely from such a substance.
The press cover or the at least one polymer layer may be made partly or completely of a polymer. The polymer used in this case may be a castable, curable, preferably elastomeric polymer such as polyurethane. The polymer may therefore be formulated as a casting elastomer.
A polymer layer is a layer which comprises or is made completely of a castable, curable, preferably elastomeric polymer of this kind. The polymer layer may preferably be a cured layer made in one part by forming. Expressed alternatively, this layer is formed monolithically, in other words produced by casting, for example. The term “one part” also includes cases in which the one layer has been produced in turn during the casting of the polymer from multiple plies of the same material. This is only the case, however, in so far as these plies after curing are substantially no longer visible, instead forming a single, preferably uniform layer. The same applies correspondingly to the completed press cover.
Where two or more polymer layers are provided, they may be arranged one over the other as viewed in a radial direction—at least in sections over the width of the press cover. “At least in sections over the width of the press cover” means that the press cover, for example, at its axial ends, along the longitudinal axis of the press cover, is only single-layer, whereas between the axial ends it has a two-layer or multilayer formation. Alternatively the polymer layers may extend over the entire width of the press cover. In addition, the thickness of the press cover—and hence the thickness of the individual polymer layers—may vary, in a section through its longitudinal axis, in sections along the longitudinal axis. For example, in the region of the lateral edges of the press cover, the radially outermost polymer layer may be smaller than in the center of the press cover. Expressed alternatively, in the region of the lateral edges, the radially outermost polymer layer may be less thick than a radially inner or radially innermost polymer layer. Preferably there is or are exactly one, two or three polymer layers provided. These layers may be configured identically in terms of their polymer or may vary in terms of their hardness or stoichiometry of the prepolymer. An overall thickness of the completed press cover, in a section through the longitudinal axis thereof, measured in the radial direction, may be 5 to 10 mm, preferably 5 to 7, more preferably 5 to 6 mm. In accordance with the invention, when a single layer is provided, the press cover may be made from only one casting, i.e., monolithically, and so the single layer has the thickness just stated.
A completed press cover in the sense of the invention is a press cover in which at least one polymer layer has undergone curing and possible final working, thus being ready for use for the purpose stated at the outset, in a shoe press, for example. A completed polymer layer, similarly, is a layer which has undergone curing.
A reinforcing thread in the sense of the invention is a flexurally slack, textile linear structure which has a dominant extent and a uniformity in its longitudinal direction. Any reference to a fiber is to a single, continuous fiber in the manner of a monofilament. Where the text, conversely, refers to a fiber bundle in the sense of the invention, the fibers in question are not monofilaments but instead in turn comprise a single thread, such as a twist or yarn, i.e., a bundle of continuous fibers or monofilaments. The fiber bundles themselves may indeed be made of fibers twisted with one another.
The definition that at least the longitudinal threads are produced as reinforcing threads of the invention means that only the longitudinal threads are configured in this way or, in addition, the longitudinal threads and at least one further circumferential thread are produced in this way. If preferably, for example, a laid scrim composed of circumferential and longitudinal threads is present, this means that at least the longitudinal threads are configured in accordance with the invention.
The term “reinforcing structure” in the sense of the invention means a reinforcement of the at least one layer comprising or consisting of the polymer—that is, of the polymer layer. This reinforcing structure may be completely embedded in the polymer layer, so that the reinforcing structure does not go beyond the confines of the polymer layer. Expressed alternatively, the polymer layer takes on the role of a matrix, which surrounds the reinforcing structure and binds it to the matrix as a result of adhesive or cohesive forces. A reinforcing structure of this kind may comprise textile linear structures—e.g., yarns or twists—and/or textile sheetlike structures—such as, for example, woven, knitted, braided or laid-scrim fabrics—and may be producible from a corresponding starting material, by winding, for example. Expressed alternatively, an individual reinforcing thread of the invention, considered per se, is a textile linear structure.
Multiple such reinforcing threads may be configured in such a way, as longitudinal and/or peripheral threads, for example, that together they may form a textile sheetlike structure. The at least one reinforcing thread which is embedded into the at least one polymer layer then constitutes the reinforcing structure of the press cover or polymer layer thereof.
A starting material is the material or precursor used for producing the reinforcing structure of the completed press cover of the invention—in the present case, accordingly, the at least one reinforcing thread.
The reinforcing thread or the reinforcing structure may be made of or comprise a polymer. Suitable polymers include polyesters, polyethylene naphthalate or polyamides, such as aramids. Accordingly the materials of the at least one polymer layer differ from those of the at least one reinforcing thread or reinforcing structure embedded therein.
In the sense of the invention, a press apparatus refers, for example, to a shoe press, for the dewatering or treatment, such as smoothing, of a fibrous material web, for example. The shoe press comprises a shoe press roll and an opposing roll, which together form or delimit a press nip. The shoe press roll further comprises a circulating press cover and a stationary press element, known as the press shoe. The latter is supported on a load-bearing yoke which is likewise stationary—support being via hydraulic press elements, for example—and is pressed against the circulating press cover. The press cover circulates relative to the fixed, stationary press shoe and yoke, and as a result is pressed against the opposing roll in the press nip. Press shoe and yoke are arranged radially within the press cover. The term “stationary” means that the press element does not circulate relative to the shoe press roll or the opposing roll, but is able to move translationally—toward the opposing roll and away from it, preferably in the radial direction of the roll—and hence relative to the opposing roll. In addition to the fibrous material web and the press cover, one or more press felts circulating continuously in the circumferential direction, and/or further continuously circulating press belts, may be guided through the press nip of the shoe press. A shoe press of this kind may of course comprise more than one press nip.
A fibrous material web in the sense of the invention is a scrim or entanglement of fibers, such as wood fibers, plastics fibers, glass fibers, carbon fibers, adjuvants, additives or the like. Hence the fibrous material web may take the form, for example, of a paper, cardboard or tissue web. It may substantially comprise wood fibers, in which case small amounts of other fibers or else adjuvants and additives may be present. This is left to the skilled person depending on the particular use.
Where, preferably, two or more reinforcing threads as longitudinal threads and at least one reinforcing thread as circumferential thread, surrounding the longitudinal threads in the circumferential direction, are embedded in the form of a scrim into the polymer layer, the advantages of the invention are achieved to particularly good effect. The reason is that a scrim is particularly well capable of accommodating local overload events.
The advantages according to the invention are achieved to particularly good effect if the press cover is constructed of preferably multiple polymer layers arranged one above another in the radial direction. Where two polymer layers are provided, the radially inner layer is the layer with the reinforcing structure of the invention. This means that the reinforcing structure is arranged only in the radially innermost polymer layer. Where three or more polymer layers are provided, the reinforcing structure is arranged preferably in the polymer layer second from bottom, in other words in the layer situated radially above the radially innermost polymer layer.
The invention also relates, furthermore, to a press roll, such as shoe press roll, for a shoe press for treating a fibrous material web, characterized in that the press roll has at least one press cover as claimed in any of the preceding claims.
The invention also relates to a shoe press for treating a fibrous material web, preferably a paper, cardboard, tissue or pulp web, comprising a press roll and an opposing roll, which together form or delimit a nip, the press roll comprising a circulating press cover, characterized in that the press cover is configured in accordance with the invention.
The invention relates, furthermore, to the use of a press cover of the invention for a press, such as shoe press, for treating a fibrous material web, preferably of paper, cardboard, tissue or pulp web.
The invention also relates to the use of a thermoplastic elastomer in the form of a copolymer, preferably a segmented block copolymer, which is preferably a thermoplastic copolyester elastomer (COPE) and/or a thermoplastic copolyamide elastomer (COPA), for at least one polymer layer of a press cover for a shoe press for treating a fibrous material web.
Without limitation of its general nature, the invention is elucidated in more detail below with reference to the drawings. In the drawings
While the opposing roll 14 here consists of a cylindrically configured roll rotating about its longitudinal axis, the shoe press roll 12 is assembled from a shoe 16, a stationary yoke 18 supporting said shoe, and a press cover 20. Shoe 16 and yoke 18 are in a fixed, stationary arrangement relative to the opposing roll 14 and the press cover 20, respectively. This means that they do not rotate. The shoe 16 is supported by the yoke 18 and is pressed onto the radially innermost surface of the press cover 20, circulating relative to it, by way of hydraulic press elements, which are not shown. The press cover 20, which surrounds shoe 16 and yoke 18 in the circumferential direction, rotates about its longitudinal axis in the opposite direction of rotation to the opposing roll 14. Because of the concave configuration of the shoe 16 on its side facing the opposing roll 14, a comparatively long nip 22 is obtained.
The shoe press 10 is suitable more particularly for the dewatering of fibrous material webs 24. In the operation of the shoe press, a fibrous material web 24 is guided with one or two press felts 26, 26′ through the press gap 22. In the present case there are exactly two press felts 26, 26′, which accommodate the fibrous material web 24 between them in the manner of a sandwich. On passage through the nip 22, a pressure is exerted indirectly in the nip 22 on the fibrous material web 24 by the press felts 26, 26′. This takes place through the radially outermost surface of the opposing roll 14, on the one hand, and the radially outermost surface of the press cover 20 coming into direct contact with the corresponding press felts 26, 26′. The liquid emerging from the fibrous material web 24 is taken up temporarily by the press felt or felts 26, 26′ and by any depressions (not shown) provided in the press cover surface. Following departure from the nip 22, the liquid taken up by the depressions in the press cover 20 is thrown off, before the press cover 20 enters the press gap 22 again. Moreover, following departure from the press gap 22, the water taken up by the press felt 26, 26′ can be removed using suction elements.
In a further embodiment of the invention, not shown in the figures, it is possible to omit the press felts 26, 26′. In such a case, the fibrous material web 24 is in direct contact with the press cover 20 on the one side and with the opposing roll 14 on the other side, these components together forming a press nip. The latter roll may in this case be configured as a heated drying cylinder.
The press cover shown in
In accordance with
As shown, there may be a reinforcing structure 20″ in the second polymer layer 20.2. This structure presently is completely embedded in the polymer layer 20.2. This is indicated by the shaded circles, which may be textile sheetlike or linear structures such as fibers. This means that the reinforcing structure 20″ does not extend beyond the confines of the polymer layer 20.2.
The reinforcing structure 20″ here comprises a plurality of reinforcing threads 21 serving as longitudinal thread 21.1. These threads are arranged running with distance in parallel to one another in the longitudinal direction of the press cover 20 over its circumference. Additionally here there is at least one further reinforcing thread 21 provided as a circumferential thread 21.2, which preferably runs in the form of a helical line in the circumferential direction of the press cover within the same polymer layer 20.1, 20.2, 20.3 in which the longitudinal threads 21.1 are also arranged. The longitudinal threads 21.1 and the circumferential thread 21.2 form with one another a scrim, in such a way that the longitudinal threads 21.1 are arranged radially within the at least one circumferential thread 21.2—as viewed in relation to the longitudinal axis 20′ of the press cover 20.
In the present case, one of the polymer layers 20.1, 20.2 or 20.3 shown in
The illustration shows an initial stage of the production process. For this purpose, in the present case, one end of the starting material 20′″ is secured on a polymer which is arranged on the outer periphery of the winding mandrel 4. Aside from the schematic illustration shown, the end of the starting material 20′″ could also lie or be applied directly, in other words without mediation, on the winding mandrel 4, without a polymer being provided initially between starting material 20′″ and winding mandrel 4. The starting material 20′″ in this case may be a textile sheetlike structure or linear structure.
The winding mandrel 4 is mounted so as to be rotatable about its longitudinal axis 20′, which corresponds to the longitudinal axis of the press cover being produced. Longitudinal axis 20′ here runs perpendicularly into the plane of the drawing. Via a conduit 5, a casting material, such as castable, curable elastomeric polymer, here polyurethane for example, is applied through a casting nozzle 6 from above onto the radially outermost circumferential surface of the winding mandrel 4 or onto the starting material 20′″. A casting material of this kind may be selected in terms of its pot life and viscosity, for example, such that it does not drip down from the winding mandrel 4 during casting. During this process, the winding mandrel 4 is rotated about its longitudinal axis in the direction of the arrow. Concurrently with this rotation, the casting nozzle 6 is guided via a suitable guide, not shown further in
The casting material emerging from the casting nozzle 6 is presently a mixture of a prepolymer and a crosslinker. The former is provided from a prepolymer container, not shown, in which it is stored or prepared by stirring. The prepolymer is the reaction product of an isocyanate and a polyol. In the prepolymer container it may be present, for example, in the form of a prepolymer of the substances just stated.
The crosslinker may be provided in a crosslinker container.
Prepolymer container and crosslinker container are assigned to the apparatus for producing a press cover 20. They have a flow-conducting connection, via conduits which are likewise not shown, to a mixing chamber (not shown) which is upstream of the casting nozzle 6 in the flow direction. The prepolymer-crosslinker mixture is therefore produced upstream and outside of the casting nozzle 6, i.e. it is mixed in the mixing chamber. Irrespective of the production of the mixture, it is then applied to the surface of the winding mandrel 4 to form the at least one polymer layer of the press cover 20.
In principle it would be conceivable for two or more casting nozzles 6 to be provided. These nozzles could be connected via corresponding conduits to separate prepolymer containers and crosslinker containers, in order independently of one another for different polymers as well to be supplied to the plurality of casting nozzles 6. In that case the casting nozzles 6 could be arranged with distance from one another along the longitudinal axis of the press cover 20, to produce multiple polymer layers 20.1, 20.2, 20.3 simultaneously by concurrent delivery of the polymer from the casting nozzles 6, in one casting.
By means of a continuous casting operation of this kind, also known as rotational casting, therefore, a continuous cylindrical-tubular press cover 20 which is intrinsically closed about its longitudinal axis 20′ is gradually produced over the width of the winding mandrel 4, the internal circumference of said cover 20 corresponding substantially to the outer circumference of the winding mandrel 4.
In principle it would be conceivable to wind the starting material 20′″ onto more than the one winding mandrel 4 shown in
Although not illustrated in the figures, the reinforcing structure 20″ of the at least one polymer layer 20.1, 20.2 could also be constructed of a plurality of starting materials 20′″ placed one above another in the radial direction and each running in the longitudinal axis direction and in the circumferential direction of the press cover 20.
The structure of a corresponding polymer layer, which contains or comprises a thermoplastic elastomer in the form of a copolymer, may be produced analogously with such an apparatus. In this case the thermoplastic elastomer is applied over the circumference of the winding mandrel 4, if this is the radially innermost polymer layer of the press cover. Alternatively or additionally, the elastomer may be applied to a polymer layer already produced from polyurethane, in order then to constitute the radially outermost polymer layer of the press cover. The thermoplastic elastomer of the invention may be applied as a polymer layer by means, for example, of an extrusion process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2019 125 908.8 | Sep 2019 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/070406 | 7/20/2020 | WO |
