Transfer Belt Description The invention relates to a transfer belt of a wet press with extended press gap for drying a paper web. In wet presses of paper machines, a substantial portion of fluid contained in a fresh paper web is squeezed out between pressure rollers which form a press gap or, in the case of a so-called shoe press, between a press shoe and a counter roller. The paper web is usually guided by means of a continuous felt belt through the press gap in which the felt belt picks up fluid from the paper web and discharges it. In a so-called tandem shoe press, dewatering is as a rule carried out between two press felts. In a new development, one of these press felts is replaced by a transfer belt, thus allowing dewatering output in the press gap (nip) to be improved and the gap between the last web press and the dry section of the paper machine (the so called "dry party") to be closed. A transfer belt must ensure even pressure transfer within the press gap, offer good sheet delivery and must not cause a substantial rehumidification of the paper web when running out of the press gap. Transfer belts used in practice are composed substantially of polyurethane and have a smooth, as a rule ground surface. It has been found that the paper delivery properties of these polyurethane transfer belts are not entirely satisfactory; furthermore, a paper web guided on them through the press gap has surfaces of unequal smoothness ("two sidedness"); this two-sidedness is to be seen as a lack in quality in the case of graphic papers. It is, therefore, an object of the invention to offer a transfer belt with improved application properties which in particular offers advantages in sheet delivery and surface texture of a produced paper web. This object is achieved by a transfer belt with the features of Claim 1. The invention encompasses the basic idea of replacing a conventional transfer belt with a smooth surface facing towards the paper web with one having a fibrous structured surface which substantially matches the surface texture of the press felt located on the other side of the paper web. This substantially matches the surface texture of both surfaces of the paper web and substantially disposes of the disadvantageous two-sidedness. It further incorporates the idea of realising this feltlike, fibrous surface texture of the transfer belt by a two or more component coating of a support belt, which is in particular woven or machined. The aforementioned multiple component coating is needled thereto and so firmly joined by thermo-mechanical means, i.e. by a suitably chosen pressure and temperature application regime, that the composite matches the high mechanical loads a transfer belt is subjected in the web press section of a paper machine. The aforementioned two or more component coating of the support which delivers the fibrous structured surface incorporates a relatively thick water repellent or only slightly water permeable polymer coating which is firmly joined to the support and a thin fibrous surface coating of non-melting (respectively melting at higher temperatures) and non-adhesive fibres which are themselves firmly joined to the polymer coating. Preferably the transfer belt is insulated on both sides with a polymer layer in order to prevent the "carrying along" of water on the underside of the belt. Preferably low melting polyolefins, polyamides, polyesters, polyacrylate or polyvinyls are used for the polymer layer. The low melting polymer layer is formed after the needling process or during the fusing process by melting the corresponding fibres at temperatures in the range between 1000 and 2200 C, preferably between 1200 and 1500 C. Through the application of pressure in the range between 5 kg/cm 2 and 70 kg/cm 2 the softening temperature of the polymers can be reduced and the compression of the belt increased. The polymer layer is formed in that meltable or melt-adhesive components within a feltlike structure which is applied to the support belt are molten under pressure. This method of forming a polymer layer ensures a predetermined compressibility which substantially contributes to an even pressure transfer in the press gap and excellent delivery properties during transfer of the paper web into the dry part. The low thickness of the fibrous surface layer, which is arranged on the water repellent polymer layer, ensures excellent delivery properties at the transfer of the paper web in the dry part and minimum rehumidification which disposes of a grave disadvantage of press felts relative to the suggested transfer belt. The transfer belt has in particular a multi-layered or laminated structure of fine twists in all layers, both in the longitudinal and transverse direction. The fine fibres include preferably monofilaments with a wire thickness between 0.1 and 0.3 mm which can be multiply twisted, filament twists or mixed twists out of monofilaments and multifilaments. With the application of a support fabric with a seam, coarser monofilaments with a wire thickness between 0.3 and 0.8 mm are used. In an advantageous embodiment, a fleece layer is provided at the side of the support belt facing away from the polymer layer which is firmly joined to the support and further improves the pressure transfer properties in the press gap. When applying a plurality of fabric layers for the support web, then these are preferably jointly needled, which produces in particular the fibre structure for forming the polymer layer simultaneously with joining the fabric layers. The area of the transfer belt which following the thermo-mechanical finish forms the polymer layer, is structured in particular of fibres with a proportion of melting or melt adhesive fibres of at least 10%, though preferably in the region of between 25 and 100%. The polymer layer is in a particularly lasting design additionally reinforced by longitudinal and/or transverse threads additionally molten into said (needled) fibre structure. The thickness of the polymer layer (respectively, both polymer layers together) lies in the region between 20 and 90%, in particular between 60 and 90%, of the total thickness of the transfer belt. In contrast thereto, the mean thickness of the fibrous surface layer lies only in the region between 1 and 10% of the total thickness of the transfer belt. For practical reasons, the fibrous surface layer is essentially composed of high temperature and friction resistant fibres, thus ensuring long life of both this surface layer and the transfer belt as a whole with constant texturing properties relative to the paper web. The fibrous surface layer is formed preferably out of fibrous material with a melting point that lies 50* to 100*C higher than the melting point of the polymer layer out of e.g., polyester, polyamide or polycarbonate fibres, and/or with non-melting fibres such as PAC, aramid, Teflon or carbon fibres. Due to high rigidity of a transfer belt, a design as a seamed belt or as a seamed felt fabric which is fully sealed when pulled in is particularly preferred.
Particularly suitable for an embodiment of the support web are in particular polyamide, polyester, aramid as well as other fibres of high expansion resistance and great strength and flexibility. Polyamide, polyester, polypropylene and PVC fibres as well as selected copolymers thereof, but also other readily available polymer fibres can be used as needled fibres as well as for structuring the rear fleece. The aforementioned fibres have thermo-plastic properties and are thus suitable for forming a dense, water repellent polymer layer below the surface of the transfer belt which is facing the paper web. Precise setting of desired properties is carried out in a conventional manner by selection of base polymers of suitable structure, in particular chain length and degree of bonding, and, if appropriate, by an addition of softeners and other additives. When forming the surface layer of thermoplastic fibres, it has to be observed in the production of the transfer belt that the thermo-mechanical treatment is matched to the physico-chemical properties of the surface fibre material in such a manner that no fusing (with accompanying loss of texture) of the fibres takes place at the surface. Advantages and practicalities of the invention are contained in the subclaims and the following description of preferred exemplary embodiments based on the figures. Shown are, in Figure 1 a diagrammatical (not true to scale) cross-sectional illustration of a transfer belt according to a first form of embodiment; Figure 2 a diagrammatical cross-sectional illustration of a second form of embodiment; Figure 3 a diagrammatical cross-sectional illustration of a third form of embodiment; Figure 4 a diagrammatical cross-sectional illustration of a fourth form of embodiment, and Figure 5 a diagrammatic illustration of a web press into which the inventive transfer belt is inserted. Figure 1 illustrates the structure of a transfer belt 10 with single-layered support fabric 11 in cross-section.
To a surface of support fabric 11 is applied a polymer fibre structure 12, and on the surface facing away from the latter a short-fibre fleece 13. The polymer fibre structure 12 is needled onto fabric 11 and thus subjected to pressure and heat treatment, so that it is molten into a widely water repellent and compressible polymer layer 14 (not shown in the figure) whilst forming cavities of different size and shape. The water permeability should lie in the range of 0 to 50 1/dM 2 .min, preferably below 30 /dm 2 .min under the conventional pressures in the press gap. A thin fibrous or feltlike surface layer (flocking) 15 of high temperature resistant and friction resistant fibres is arranged on the polymer fibre structure 12 or a therefrom produced polymer layer 14. Support fabric 11 is made of high tensile but relatively fine twists, for example of polyamide or p-aramide, ensuring tensile strength and running properties of the transfer belt which are virtually of the same value as those of conventional polyurethane belts. Thermo-mechanical treatment of the needled fibre structure 12 with a proportion of more than 50% melt-adhesive fibres ensures a development of a polymer water barrier layer of approximately 60% of the belt thickness with predetermined compressibility, which simultaneously fulfils a plurality of important functions. Firstly, it acts in the press gap as pressure distributing medium and ensures advantageous pressure transfer properties from the roller onto the paper web. Furthermore, it ensures that the humidity of the paper web can enter only to a limited extent into the depth of the transfer belt, so that only minimal rehumidification of the paper web takes place. Finally, it superposes the fibrous primary texture of flock 15 with a coarser but also feltlike random and pressure resistant secondary texture, thus ensuring together with the surface layer that the paper web which has been dried by means of the suggested transfer belt is practically free of two sidedness. A further embodiment of a transfer belt 20 illustrated in Fig. 2 substantially corresponds in its design with the first embodiment shown in Figure 1 and described above, so that the reference number are based on those of Fig. 1, and components denoted by corresponding reference numbers are not described again. The main difference of this second embodiment from the first embodiment lies in the provision of a two-layered support fabric 21 of which one fabric layer is linked to a fleece 23 whilst a polymer fibre structure 22, which is additionally fixed by pressure fusion to the upper fabric layer of support fabric 21, is needled through both fabric layers. Polymer layer 24, which is established by thermo-mechanical treatment, below the belt surface, which is here again formed by flocks 25, extends here over a somewhat lesser proportion of the total thickness of the transfer belt as support fabric 21, the two-layers of which serve to attain increased rigidity and tensile strength, itself claims a larger proportion of the total belt thickness. In this embodiment, both fabric layers of support fabric 21 are essentially joined together by the needled polymer fibre structure 22 with a specified elastic displaceability in the movement direction of the belt, which ensures high long term stability of the laminated fabric structure. The other advantages correspond with those referred to above with reference to the embodiment of Fig. 1. Fig. 3 shows an embodiment which is simplified relative to Fig. 1, and again matching components are given reference numbers based on Fig. 1. The main difference relative to the embodiment of Fig. 1 lies in dispensing with the rearsided fleece coating, as a result of which transfer belt 30 with fabric support 31 runs directly on the rollers of the wet press (also see Fig. 4 and the description further down). Of all here described embodiments, transfer belt 30 is the most flexible and structurally simplest and thus, when using identical fibre materials, also the most cost-effective one. Dispensing with the fleece layer, which in the two other embodiments protects the support fabric from friction, then requires a selection of a sufficiently wear resistant material for the support fabric, for example polyamide. The aforementioned materials are suitable as materials for polymer fibre structure 32, of which polymer layer 34 is formed, and for surface coating 35. Surface layer 35 is, as in the other designs, in particular itself needled to polymer layer 34, and its thermo mechanical treatment takes care of highly wear resistant fixing of the surface layer. For transfer belt 30 illustrated in Fig. 3 is provided an arrangement of longitudinal threads 36 in polymer layer 34 in order to increase the tensile strength and for optimisation of distribution of tensile loads over the height of the belt. Fig. 4 shows a modified transfer belt 10' with respect to Fig. 1 in which a polymer fibrous structure 12 is formed extending on both sides of the support fabric 11 and from which a first polymer layer 14a lying above the support fabric 11 and a second polymer layer 14b lying underneath the support fabric are formed. This embodiment is advantageous in that the transfer belt here is insulated on both sides with a polymer layer and the entrainment of water on the underside of the transfer belt is thus prevented. Fig. 5 is a basic diagram of the functions of a web press 100 which includes the inventive transfer belt. Fig. 4 is a basic diagram of a wet press 100 as part of a paper machine (not illustrated in its entirety). A paper web 101 runs between a first roller arrangement 102 (at the top of the illustration) and a second roller arrangement 103 (at the bottom of the illustration), of which the first roller arrangement 102 transports a press felt 104 and the second roller arrangement 103 transports a transfer belt 105, for example of one of the aforedescribed designs. Between press rollers 102A of the first group of rollers and 103A of the second group of rollers is set a press gap 106 wherein paper web 101 is substantially dewatered between press felt 104 and transfer belt 105. Paper web 101 then runs into a dry party 107 (of which only a portion is shown in the figures), where it is guided by drying belt 108. After dewatering, press felt 104 and transfer belt 105 then return to the input of web press 100. The inventive design is not restricted to the aforedescribed exemplary embodiments but is also possible in a plurality of modifications. In particular, the multi-component structure of the transfer belt is variable in many ways in dependence of concrete conditions of application, for example of a paper quality to be produced and operational parameters of the web press. Materials applied are not restricted to aforementioned plastic materials, but they can be replaced by other fibre materials of properties known to the expert.
List of Reference Numbers 10, 10'; 20; 30 - Transfer Belt 11,21,31 - Support Fabric 12, 22, 32- Polymer Fibre Structure 13,23 - Fleece 14, 14a; 14b, 24; 34 - Polymer Layer 15; 25; 35- Surface Layer 36 - Reinforcement Thread 100 - Wet Press 101 - Paper Web 102 - First Group of Rollers 102A - Press Roller 103 - Second Group of Rollers 103A - Press Roller 104 - Press Felt 105 - Transfer Belt 106 - Press Gap 107 - Dry Party 108 - Transport Belt