Patent Application
20080006340
In the conventionally used press fabric, deterioration of the dewatering property occurs in about one month after its use is started. The dewatering performance gradually deteriorates while reducing the deterioration speed. The press fabric outlives its usefulness at the time when the water content of a pulp sheet exceeds a satisfactory level. The present inventors have therefore developed a press fabric capable of retarding the initial deterioration of its dewatering performance and maintaining the water content of a pulp sheet at a practically usable level for a long period of time.
In the press fabric of the present invention, monofilaments serving as warps are woven with wefts. This fabric is made endless by interweaving the end portions of warps each other and caused to travel continuously under a tension in the warp direction. Use of monofilaments having rigidity as warps makes it possible to prevent stretching or deformation. A weft has a multilayer structure in which at least an upper surface side weft and a lower surface side weft are arranged vertically and they are woven by a warp. Since the weft has a multilayer structure, water is absorbed from a pulp sheet in the upper side layer and is removed efficiently in the lower side layer.
Lower surface side wefts are formed of two yarns, that is, a monofilament and a yarn forming a fine water-sucking space. Two monofilaments and two yarns forming a fine water-sucking space are arranged alternately or two monofilaments and one yarn forming a fine water-sucking space are arranged. Use of the yarn forming a fine water-sucking space provides the fabric with sufficient water retention property and water sucking property, while use of the monofilament provides the fabric with rigidity and at the same time, washability and resistance to high-pressure washing. What is important in this invention is that two monofilaments serving as lower surface side wefts are arranged adjacent to each other. The yarn forming a fine water-sucking space is flattened and widened by pressing, while the monofilament is not flattened so much by pressing. Even if the yarn forming a fine water-sucking space is flattened and widened by pressing of the fabric, the dewatering channel between two monofilaments can be maintained without being flattened. The yarn arranged on the upper side layer and forming a fine water-sucking space is pressed while being brought into contact with a pulp, which is soft, so that even if the yarn forming a fine water-sucking space is flexible, severe flattening and deformation do not occur. On the other hand, the yarn forming a fine water-sucking space on the lower side layer is pressed while being brought into contact with a hard press roll made of, for example, iron so that it is flattened and widened inevitably.
The term “dewatering channel, as used herein means a space formed between the lower side surface wefts. It is a channel extending in a weft direction. Through this space, water is removed from the press fabric. This space is formed by two lower surface side wefts which are adjacent to each other but pass over and under respectively different warps. It is possible to confirm the presence of dewatering channels between wefts though their size is not uniform unless the fabric has only yarns forming a fine water-sucking space as wefts. When yarns which are easily flattened and widened are used, the dewatering channel has a relatively small size or sometimes cannot be observed visually owing to the properties of the yarns.
In order to maintain the dewatering property until the final stage of the life of the press fabric, it is important to maintain the structure of the dewatering channel. If the wefts are all made of monofilaments, the dewatering channel can be maintained until the final stage of the life of the press fabric but such a fabric does not have a sufficient water sucking property. It is very important to find yarns and arrangement design thereof suited for accomplishing both the dewatering property and water sucking property in order to efficiently discharge water from the fabric.
In the press fabric for a pulp machine, dewatering of a pulp sheet is carried out variously, for example, by gravity dewatering, centrifugal dewatering or pressing. In any dewatering method, dewatering by using a dewatering channel formed between two monofilaments on the lower surface side becomes necessary to efficiently remove water retained in the layer of the fabric. When the number of monofilaments serving as the lower surface side weft is one, a satisfactory dewatering channel cannot be formed because yarns forming a fine water-sucking space and present on both sides of the monofilament are flattened to fill the space therewith. When three monofilaments are used as the lower surface side weft, deterioration in water sucking property occurs because of a decrease in a ratio of the yarns forming a fine water-sucking space. It is therefore preferred that relative to two monofilaments, two yarns forming a fine water-sucking space may be arranged adjacent to each other or one yarn forming a fine water-sucking space may be arranged. In the conventional press fabric, it is also possible to have a dewatering channel between wefts by decreasing the density (the number) of wefts, but in this case, a reduction in the density of wefts of the whole press fabric leads to deterioration in rigidity. As a result, the press fabric fails to maintain its shape and changes its shape or it cannot apply a sufficient pressure onto the pulp sheet and fails to reduce the water content of a pulp sheet. The structure of the present invention in which two monofilaments are arranged each other makes it possible to ensure a dewatering channel while maintaining the density of wefts at a level comparable to that of the conventional press fabric.
As wefts constituting the upper side layer, yarns forming a fine water-sucking space or both the yarns forming a fine water-sucking space and monofilaments may be used. The yarns forming a fine water-sucking space as upper surface side wefts receive water directly from a pulp sheet so that they must have a fine water-sucking space. In a fabric having a double weft structure, yarns forming a fine water-sucking space may be placed as both of the upper surface side weft and lower surface side weft. In a fabric having a triple weft structure, yarns forming a fine water-sucking space may be placed as at least a portion of upper surface side wefts, middle wefts and lower surface side wefts. Such a structure makes it possible to form a fine water drainage space connected by yarns forming a fine water-sucking space from the upper side layer to the lower side layer, thereby transferring water efficiently from the upper side layer to the lower side layer by means of the capillary phenomenon.
For efficient water sucking, it is important that a pressure fluid gradient passing through a pulp sheet and press fabric is high under a nip pressure, in other words, a force of water running from the pulp sheet toward the press fabric is large. It is apparent that water is smoothly squeezed when the resistance against running water is small. When the press fabric has an adequately dense surface, the re-absorption of water by the pulp sheet at the outlet side of the press nip can be reduced. The water sucking property and cushioning property have a close relationship. Yarns obtained by bundling raw yarns of a small diameter and having a fine water-sucking space therebetween have adequate cushioning property so that they can exhibit a water sucking effect at the time when they restore to a state before compression. They can also buffer the press nip action by adequate compressive elasticity, whereby breakage or the like of the pulp sheet does not occur easily.
The press fabric woven using, as wefts, yarns obtained by bundling raw yarns of a small diameter and having a fine water-sucking space therebetween is, similar to a batt of a needle felt, an aggregate of fine fibers but has a woven mesh structure as a whole so that wefts are woven by warps or warps are woven by wefts and they are constrained strongly each other on a short cycle so that the fabric has excellent rigidity and does not have a thinning tendency compared with a felt. As a result, the water-sucking spaces are not flattened completely or the yarns are neither broken nor lost by the impact of shower water. The press fabric of the present invention does not adopt a needle-felt like structure dense with fine fibers in an entire direction of Z axis but adopts a fabric structure in which an aggregate of fine fibers is constrained strongly so that the fabric has openings by which dirt does not accumulate on the fabric. In addition, yarns obtained by bundling raw yarns of a small diameter and having a fine water-sucking space therebetween have good shower resistance so that it can bear high pressure shower.
It is recommended to use, as upper surface side wefts on the upper side layer, many yarns forming a fine water-sucking space in consideration of water sucking property. For example, all the upper surface side wefts may be yarns forming a fine water-sucking space. When rigidity and an endless weaving and connecting property of end portions of warps are taken into consideration, arrangement of monofilaments for interweaving facilitates endless weaving. The monofilaments and the yarns forming a fine water-sucking space are arranged at any ratio insofar as the fabric has the latter one.
The upper surface side wefts and lower surface side wefts are arranged preferably at a ratio of from 1:1 to 2:1. When the number of the lower surface side wefts is made smaller than that of the upper surface side wefts, the lower surface bide wefts have a wider space therebetween, making it possible to secure a greater dewatering space. This however decreases the absolute number of yarns forming a fine water-sucking space, leading to deterioration in water retention and water sucking property. It is therefore necessary to determine an arrangement ratio while taking various conditions such as the number of warps to be set, the number of wefts inserted, diameter of yarn and properties of yarn into consideration.
The structure of the fabric is, for example, a single warp double weft structure, single warp triple weft structure, double warp double weft structure and double warp and triple weft structure. It is not limited insofar as wefts of it are each composed of at least two layers.
No particular limitation is imposed also on the fabric design. A design in which an upper surface side weft passes over a plurality of warps to form a long crimp on the upper side surface facilitates transfer of water from the pulp to the fabric. When the fabric has, on the lower side layer, a design in which a lower surface side weft passes under a plurality of warps to form a long crimp on the lower side surface, yarns obtained by bundling raw yarns of a small diameter and forming a fine water-sucking space between the raw yarns, which yarns are present on the running surface side, form a wide contact surface with a roll so that transfer of water to the roll occurs fully and the fabric can have a good water sucking property. In addition, the resulting fabric has abrasion resistance superior to that of the conventional fabric because of the design with a weft long crimp.
When a space is formed between monofilaments serving as lower surface side wefts, a design in which two monofilaments adjacent to each other pass over and under the same warp is not preferred because of difficulty in forming a space between the monofilaments.
Examples of the above-described yarn obtained by bundling raw yarns of a small diameter and having a fine water-sucking space therebetween include a spun yarn, multifilament, a raised yarn, a monofilament twisted yarn, a chenille yarn, a filament-processed yarn, a yarn obtained by winding a spun yarn on a core of monofilament, a yarn obtained by winding a multifilament on a core of monofilament, and a yarn obtained by twisting two or more of these yarns together. The term “spun yarn” as used herein means a yarn formed by gathering and bundling short fibers, and it includes a yarn manufactured by spinning, or the like. The term “multifilament” means a yarn formed by gathering and bundling fine short fibers; the term “raised yarn” means a yarn formed by scratching the surface of a multifilament with a needle like material to cause nap; the term “filament-processed yarn” means a yarn formed by subjecting a filament yarn to expansion and contraction processing, bulking processing, crimp processing, or the like, and it includes yarns generally called as textured yarn, bulky yarn, stretcher yarn, and Taslan processed yarn, as well as wooly nylon and the like; the term “chenille yarn” means a yarn formed by arranging short fibers radially with a core yarn such as multifilament as an inner core and it includes a yarn obtained by subjecting short fibers arranged radially to crimp processing or the like.
The monofilament may be a monofilament-twisted yarn or a twisted yarn having a monofilament as a core. The monofilament used in such a yarn is effective for improving the rigidity and size stability.
No particular limitation is imposed on the material of the yarn and various materials such as synthetic fibers including polyesters, polyamides and polyphenylene sulfide, chemical fibers including rayon, and natural fibers including cotton can be used. Use of a polyamide for yarns is preferred because the resulting fabric can have satisfactory nip resistance and fibrillation resistance which will otherwise occur upon pressing. Even strong pressing does not cause cracks of it. When polyester is used, the resulting fabric has increased rigidity and becomes resistant to stretching and deformation. Therefore, it is preferable to select the material of the yarn depending on its using purpose. In particular, use of a polyamide monofilament as the yarn forming a fine water-sucking space and monofilament serving as wefts is suited to avoid the problem of cracks.
A fabric for a pulp machine according to the present invention and a fabric for a pulp machine according to a conventional example were used practically in a pulp manufacturing machine and the state of the back surface after use and water content of the pulp sheet at that time were analyzed to compare these two fabrics.
In Example of the present invention, a polyamide monofilament having a diameter of 0.62 mm was arranged as a warp; a polyamide monofilament having a diameter of 0.50 mm and a polyamide multifilament having a diameter of 1.30 mm were arranged alternately at a ratio of 1:1 as upper surface side wefts; and a polyamide monofilament having a diameter of 0.50 mm and a polyamide multifilament having a diameter of 1.30 mm were arranged at a ratio of 2:2 as lower surface side wefts, whereby a fabric having a single warp-double weft structure was obtained.
In a similar manner to Example except that the monofilament and multifilament of the lower surface side weft were arranged at a ratio of 1:1, a press fabric of the conventional example was obtained.
The press fabrics obtained in Example and Conventional Example were used on a practical pulp machine. For comparing these two fabrics, the water content of the pulp sheet was measured and the state of the back surface after use was analyzed.
It has accordingly been elucidated that in the press fabric for a pulp machine, maintenance of a dewatering space on the back surface side is a very important factor for carrying out efficient dewatering of the pulp sheet. It is apparent that arrangement of a multifilament which is one of yarns forming a fine water-sucking space and a monofilament under the most suited condition influences whether the resulting press fabric can keep desired physical properties for a long period of time or not.
Although only some exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciated that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.
The disclosure of Japanese Patent Application No. 2006-187789 filed Jul. 7, 2006 including specification, drawings and claims is incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-187789 | Jul 2006 | JP | national |
