Patent Application
20170356104
The invention relates to an industrial fabric, especially for transporting a web of a nonwoven during its production, with a product side that is in contact with the nonwoven and a machine side that is in contact with conveying devices of a machine for producing the nonwoven, wherein the fabric has MD threads oriented in the running direction of the web of the nonwoven and CMD threads that are oriented perpendicular to the MD threads and are interwoven with each other and there are at least two layers of MD threads that are arranged stacked in pairs one above and below each other, and form product contact MD threads and non-product contact MD threads, wherein at least the surface of the respective product contact MD threads facing the product side has a first material that has a contact angle, measured according to the Wilhelmy plate method, of at least 80°, preferably at least 90°.
In addition, the invention relates to a method for producing a nonwoven, especially in the form of an aerodynamically formed, chemically and/or thermally cured nonwoven, wherein a web of the nonwoven contacts a surface of a conveyor belt and is moved by this belt. The invention also relates to the use of an industrial fabric of the type specified above.
In the previously described industrial fabric, the specified contact angle of the material represents a measure for the free surface energy of the relevant surface of the MD threads. In the specified measurement method in the form of the “Wilhelmy plate method,” the contact angle between a fluid and a solid is determined. Here, the force acting in the vertical direction on a vertically submerged plate (test body) is measured. Typically, the plate is mounted on the force sensor of a so-called tensiometer. The contact angle here depends not only on the free surface energy of the material to be measured, but also on the surface tension of the fluid to be used. The previously specified values of the contact angle here relate to distilled water as a fluid. A contact angle of 0° means, in this case, that the fluid wets the material completely (spreading). For a contact angle between 0° and 90°, the material of the plate can be wetted; at a contact angle of greater than 90°, it cannot be wetted. For so-called ultrahydrophobic materials (usually using the so-called lotus effect principle), the contact angle approaches the theoretical limit of 180°. For pure PET, the contact angle is approx. 75°, for pure PPS it is approx. 90°, while PVDF has a contact angle of approx. 105°.
Nonwovens are textile fabrics that are produced from fibers of limited length or endless fibers (filaments) or cut threads of a wide range of different types, in that the fibers, filaments, or threads are joined to form a fiber layer and connected permanently to each other in some way. In particular, nonwovens produced from chemical fibers have increased enormously in importance in recent decades and used, for example, for hygienic products (e.g., baby diapers, etc.), for medical products, cleaning cloths, or as home textiles and clothing, and also for a plurality of technical applications (construction, filtration, automotive engineering, electrical engineering, packaging, agriculture, etc.). For example, the production of a nonwoven can be realized in that a nonwoven made from fibers is formed with the help of an airflow on an air-permeable backing (aerodynamic nonwoven formation). The nonwoven curing can be realized, e.g., in a chemical way by generating an adhesive bond. Here, additional materials, e.g., in the form of polymer dispersions (containing, e.g., latex) can be used and/or a thermal curing method can be used in which the fiber connection is also achieved by an adhesive bond that is achieved, e.g., with the help of thermoplastic fibers. For example, the nonwoven can have fibers made from two components, wherein a first, higher-melting-point component (e.g., polyester) forms a fiber core that is surrounded by a second component (polyethylene) melting at a lower temperature. The fiber composite is thus generated by melting the casing of the two-component fibers and/or curing the polymer dispersion in a furnace or a heated drying device.
During its run through the production system, the web of the nonwoven produced by the process is guided through the various treatment devices by means of a conveyor belt having a surface with which the web is in contact. A plurality of conveyor belts that are arranged one after the other in the passage direction are used in the different sections of the production system.
Industrial fabrics made from monofilament threads, like those known, for example, from US 2010/0291824 A1, are typically used as conveyor belts in systems for nonwoven production with aerodynamic nonwoven formation and chemical and/or thermal curing. To prevent the adhesion of fibers to the conveyor belt during the nonwoven production, especially during the heating of the web of the nonwoven produced during the process, in the previously mentioned US patent application, a surface roughness of those surfaces of the conveyor belt that are in contact with the nonwoven web is proposed, which should be between 5 μm and 100 μm. In this way, the tendency of fibers or other contaminating particles to adhere to the threads of the conveyor belt is reduced and the detachment of the cured nonwoven from the conveyor belt is made easier at transitions to subsequent conveyor belts or for other types of transport through the system.
However, despite the previously mentioned microstructuring of the thread surfaces, previously known conveyor belts still have too great a tendency toward adhesion and soiling. With advancing time of use, the known industrial fabric therefore loses part of its air permeability, so that the volume flow through the conveyor belt and the nonwoven produced in the process decreases to impermissible values. This leads, in turn, to inadequate heating of the fibers forming the nonwoven web, so that the melting of the bicomponent fibers and/or the wetting of the polymer dispersion having the bonding properties is inadequate. This results in insufficient bonding of the fibers of the nonwoven, so that the strength of the end product is not satisfactory. For the operator of a system for producing nonwoven, this means that the conveyor belts must be replaced when the air permeability falls below a certain limit value. Replacing the conveyor belt causes not only costs due to the necessary purchase of a new conveyor belt, but also due to the stoppage of the production system during the belt replacement.
As is also described in US 2014/0127959 A1, a high surface energy above the specified value (realized by corresponding material selection) can also effectively prevent bonding of fibers or other contaminating particles during the nonwoven production on a conveyor belt produced from the fabric according to the invention and used in a production system. The service life of the conveyor belts produced from the fabric according to the invention can be increased significantly in this way and the production costs can be reduced accordingly.
In this context, it is particularly important that the known fabric has two separate fabric layers whose MD threads are in a stacked arranged relative to each other. In this way, the product contact MD threads that are located in the upper of the two layers can be optimized to the prescribed effect of reducing the adhesion tendency, while the non-product contact MD threads located in the lower layer can be optimized with respect to a different requirement, namely a high strength for receiving the necessary tensile stress in the direction of the MD threads. The effect of adhesion tendency is not significant with respect to the non-product contact MD threads, because these do not contact the nonwoven web to be formed and the fibers used for this purpose. The non-product contact MD threads can also have particularly high wear resistance, in order not to exhibit excessive wear phenomena due to contact with the deflection devices (rollers) for continuing circulation in the system. It is obviously understood that all MD threads, but also the CMD threads, must have a sufficient thermal stability, in order to be able to withstand the temperatures in the furnaces or drying devices, which reach up to 200° C., wherein the required mechanical properties must also be guaranteed even at these temperatures.
One disadvantage of the two-layer fabric known from US 2014/0127959 A1, however, is the high costs of its production and the large fabric thickness. The roughness of the fabric surface due to the bonding is also large and the contact surface with a web assumed to be flat and made from nonwoven material to be transported is small accordingly.
WO 2009/030033 A1 discloses a fabric that is used as a conveyor belt for the production of a nonwoven, wherein the web construction of this previously known fabric has no stacked MD threads. The previously known fabric construction also has a plurality of CMD thread layers. An essential feature of this previously known fabric that can also be constructed as a spiral fabric is a deliberately large surface roughness of the threads exposed on the product side of the fabric in the range between 5 μm and 100 μm. This arrangement should, on one hand, reduce the soiling of the industrial fabric during wetting as the conveyor belt and simultaneously make the detachment of the nonwoven web formed on the conveyor belt easier.
U.S. Pat. No. 7,121,306 B2 discloses a technical fabric with MD threads in a stacked arrangement. Some of the shown embodiments disclose fabric with a single layer of CMD threads. The previously known fabric should be used, in particular, as paper machine fabric or as filter fabric. For this application, a fabric is to be created whose opposing fabric surfaces can be different, in particular, can have different physical properties. In addition, the seam being used should have a lower tendency to leave undesired marks on the paper web, while simultaneously, however, having high strength. The problem of fibers adhering to the fabric according to U.S. Pat. No. 7,121,306 B2 is not discussed in that publication.
Finally, from US 2003/0175514 A1, filaments are also known, from which are produced threads, textile fabric produced from these two elements, and associated production methods. The previously known filaments have a two-component structure with a filament core made from a material with a high tensile strength and a filament casing with a material that has a contact angle of greater than 90° and is typically made from halogenated hydrocarbons, e.g., PTFE. The known filaments and the threads or fabric produced from these filaments should be water-repellent, so that the textile fabric produced from these products will be impermeable to water. In contrast, the textile fabric should be breathable, i.e., permeable for water vapor and other gases. Preferably, the previously known filaments should be spun as stacked fibers into thread and then into fabric for use as clothing, tents, or camping products. A use of the fibers for technical fabric, such as special bond types for fabrics, is also provided for to a lesser degree.
The invention is based on the objective of providing an industrial fabric, for a use as a conveyor belt in a system for producing nonwoven, that has a very low tendency to adhere to fibers during the processing step of nonwoven curing, and therefore has a long service life, but is also characterized by a small fabric thickness, low surface roughness, and reduced production costs.
Starting from an industrial fabric of the type described above, the previously mentioned objective is achieved according to the invention in that the fabric has a single layer of CMD threads and a respective cross section of the product contact MD threads has at least two areas of which a first area is made from the first material and a second area is made from the second material, wherein a significant, preferably predominant, portion of a tensile force acting on the respective product contact MD thread can be transferred by the second area and the cross section of the product contact MD threads has a second area in the form of a core and a first area in the form of a casing surrounding the core, wherein the product contact MD threads are preferably coextruded or extruded in two successive steps and the MD threads have a flattened, preferably rectangular cross section, wherein a ratio of a height of the cross section to a width of the cross section is preferably between 1:1.2 and 1:10, preferably between 1:1.5 and 1:4.
The fabric construction according to the invention is very special because it has only a single CMD layer despite the stacked MD thread layers, i.e., the fabric involves a conventional one-layer fabric (plain weave). The thickness of such a fabric is significantly reduced compared with fabrics with multiple layers of CMD threads, which produces, e.g., increased flexibility and the ability to realize smaller radii on deflection rollers. The material use is significantly reduced in contrast with known fabrics with multiple layers of CMD threads, so that the fabric according to the invention can be produced economically. Contributing to this result is also that, according to the invention, there are not multiple, independent, i.e., standalone, fabric layers that must be bound to each other by binding CMD threads. Despite this minimalist design with respect to the number of CMD thread layers, due to the two stacked layers of MD threads, it is possible to simply differentiate the fabric properties between the product side and machine side. In particular, the anti-stick properties required on the product side for reducing the tendency to become soiled can be combined in a very simple way with the strength and durability properties required on the machine side.
In addition, a respective cross section of the product contact MD threads has at least two areas on which a first area is made from the first material and a second area is made from the second material, wherein a significant, preferably predominant part of a tensile force acting on the respective product contact MD thread can be transferred by the second area.
Because there are only a limited number of materials with a high surface energy, as is required in the present case, and the relevant materials are often relatively expensive, the use of this special “anti-stick material” should be limited to the amount necessary for achieving the desired effect. Therefore, there is the possibility of being able to form a “coating” on a base material (second material) to form the product contact MD threads or their surface facing the product side. The cross section of the product contact MD threads not needed by the anti-stick material can be made from a second material (base material) that has good mechanical properties and simultaneously a low price. The coating can here make up only a small portion of the total cross section of the thread (less than 20%).
The “coating” of the product contact MD threads according to the invention can be realized using a so-called wet coating method or alternatively a plasma coating method (in a vacuum or under atmospheric conditions), or in principle using any other conceivable coating method. While the binding of the coating material to the base material takes place by means of adhesion during a wet coating method, cohesive bonds that are more durable in comparison with adhesive bonds are formed in a plasma coating method.
The two-component construction of a product contact MD thread according to the invention is provided in that the cross section of the product contact MD threads has a second area in the form of a core and a first area in the form of a casing surrounding the core, wherein such product contact MD threads are preferably produced using a coextrusion method or a 2-step extrusion method (1st step=core, 2nd step=casing). As an alternative to “coatings,” such coextruded or 2-step extruded threads have a casing thickness in the range between 0.02 mm and 0.07 mm, preferably between 0.03 mm and 0.06 mm. The risk that the anti-stick material is mechanically worn away in these two first areas over time so that the anti-stick properties of the fabric according to the invention are lost is prevented by the material thickness selected to be sufficiently large without additional means during the coextrusion or 2-step extrusion method.
The cross section of the MD threads or a part of it and/or the cross section of the CMD threads or a part of it has a flattened, especially rectangular, in particular, flat rectangular shape. For rectangular cross section, the height-width ratios are between 1:1.2 and 1:10 (height:width), preferably between 1:1.5 and 1:4.
Preferably, the MD threads or a part of them and/or the CMD threads or a part of them are constructed as monofilaments. The cross section of the CMD threads or a part of it can have a round, elliptical, or oval or polygonal and/or flattened, in particular, rectangular, shape.
Preferably, the first area, i.e., the anti-stick material, is made from a fluorine-containing polymer, for example, a PVDF, an ETFE, or a PTFE or copolymers of polyethylene with the previously mentioned fluorine-containing polymers, and the second area, i.e., the core material is made from polyester, polyamide, polyphenylene sulfide, polyether ether ketone, polypropylene, aramid, polyethacetone, or polyethylene naphthalate.
The tendency of the fibers forming the nonwoven to stick to the fabric according to the invention can be further reduced if at least the material of a surface facing the product side at least with the CMD threads that can be in contact with the web of the nonwoven to be formed or that could receive fibers from this web has a contact angle, measured according to the Wilhelmy plate method, of greater than 80°, preferably greater than 90°, even more preferred greater than 100°. Especially for binding types in which a not insignificant part of the contact surface with the nonwoven web is also formed by the CMD threads, the anti-stick properties are very advantageous.
The CMD threads preferably have a round cross section, which improves the ability to form a web. Typically, the MD threads are the warp threads in the web production of the fabric according to the invention, while the CMD threads are the weft threads of the fabric.
To achieve a large contact surface, especially on the product side of the fabric, a CMD thread with a larger diameter and a CMD thread with a smaller diameter can be arranged alternately one after the other, wherein the CMD threads with the smaller diameter bind with the MD threads and are preferably made from a material that has a contact angle of greater than 80°, preferably greater than 90°, even more preferred greater than 100°, at least on a surface facing the product side.
In addition, a part of the MD threads and/or the CMD threads can be electrically conductive. This can be achieved especially in that, on an outer casing of the cross section of the relevant threads, there is carbon that forms a conductive layer. The carbon coating can be produced, for example, with the help of the typical coating method, especially by a plasma coating method. If the fabric has an electrically conductive construction, at least in the form of distributed individual threads (for example, every fifth or eighth MD thread or CMD thread could have an electrically conductive construction), an electric field could be generated around the industrial fabric used as a circulating belt, which can further suppress the adhesion of fibers during the production of the nonwoven. The introduction of the voltage necessary for generating the electric field is realized by the deflection devices (rollers), which are typically metallic and come in contact with the machine side of the fabric.
A seam with especially good load bearing properties for an endless belt is produced if a seam connecting two fabric ends is closed to form an endless conveyor belt, wherein the seam is a spiral seam that has two seam spirals that extend over the entire width of the conveyor belt and are each turned in or hooked into loops of MD threads of opposing fabric ends and both are coupled with each other by a closing wire extending over the entire width of the conveyor belt.
The excellent anti-stick properties of the actual product side of the fabric are also produced in the area of the spiral seam if the seam spirals are each made from a thread whose cross section has at least two areas, namely one in the form of a core and the other in the form of a casing surrounding the core, wherein the casing is made from a material that has a contact angle, measured according to the Wilhelmy plate method, of at least 80°, preferably at least 90°, even more preferred at least 100°.
The objective specified above is also achieved according to the invention by a method for producing a nonwoven, especially an aerodynamically formed and chemically and/or thermally cured nonwoven, in which a web of the nonwoven is moved onto a surface of the conveyor belt in a production system in which, according to the invention, the conveyor belt is made from an industrial fabric having one or more of the features of the invention.
Finally, the objective forming the basis of the invention is also achieved by the use of an industrial fabric having one or more features of the invention as a conveyor belt for transporting a web of a nonwoven during its production, especially during its aerodynamic formation and chemical and/or thermal curing in a furnace or a heating device.
The invention will be explained in more detail below with reference to an embodiment of a system for producing a nonwoven, as well as several embodiments of industrial fabric from which a conveyor belt for use in a production system can be made.
Shown are:
A system 1 shown in
Downstream of the spraying device 6, the nonwoven web is fed into a first drying device 8 (furnace) where the web is transported by a conveyor belt 9. In the drying device 8, the casings of the two component fibers are melted and the polymer dispersion sprayed in the spraying device 6 is hardened. This produces the cohesion of the fibers of the nonwoven.
Downstream of the first drying device 8, the nonwoven web is fed by a belt 10 through a second spraying device 11 before it is guided by a second conveyor belt 12 through a second drying device 13. A final curing of the nonwoven web is performed in a curing device 14 where the nonwoven web is transported with the help of a third conveyor belt 15. Finally, the final nonwoven web is guided with the help of a discharge belt 16 to the output (position 2) of the system 1.
One problem of known systems is that the spaces in conveyor belts 9, 12 become clogged with non-bound fibers, so that the permeability of the conveyor belts 9, 12 decreases and sufficient air can no longer be guided into the drying devices 8, 13 through the nonwoven web. The heat transfer to the nonwoven web is then inadequate, which leads to inadequate cohesion of the fibers and thus inadequate strength of the nonwoven web, because the necessary temperatures can no longer be achieved. A remedy is now created according to the invention by a fabric that is shown in
An industrial fabric 40 shown in
From
Due to the surface properties of the threads 75 in the seam area, the risk is prevented that undesired adhesion is produced in this area, which could have occurred if the threads 75 forming the seam spirals 74 were made from a material with a lower contact angle. The core 76 present in this thread 75 makes it possible, through the selection of a material with a high tensile strength, to ensure the necessary stability and tensile load bearing capacity of the seam.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2015 101 449.1 | Feb 2015 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/052027 | 2/1/2016 | WO | 00 |
