A FLAT STRUCTURE PERMEABLE TO LIQUID, AND A METHOD FOR MANUFACTURING SUCH A STRUCTURE

Abstract: A flat structure permeable to liquid, especially to a press felt for use in a papermaking machine, the structure being made up for a body (1) made of a castable polymer, the body having holes (2) formed during the casting process and possibly cavities (6), the diameter of the holes increasing substantially progressively from one surface to the other in such a way that the web-side surface is microporous and the other surface has pores substantially more open. The invention also relates to a method for the manufacture of such a structure.

Description: A flat structure permeable to liquid, and a method for manufacturing such a structure The present invention relates to a flat structure permeable to liquid, in particular to a press felt used in papermaking machines, and to a method for manufacturing such a structure. The structure according to the invention is used primarily as a press felt, hut it can also be used as a press wire, for example. In a papermaking machine, water is extracted in three different sections, i.e. in the wire section by filtering, in the press section by wet pressing, and in the dryer section by evaporation. For economy in papermaking it is important that the web is dewatered as thoroughly as possible before it arrives in the dryer section. In the wire section, i.e. the forming unit, of the papermaking machine as much water as possible is extracted from the stock by using a wire. By a wire is meant an endless fabric through which water is filtered from the stock. The wires are in general made of phosphor-bronze or a synthetic fiber. * The fiber web which has been formed in the wire section - * -•* of the papermaking machine still has a high moisture content, which is reduced in the wet press of the papermaking machine by pressing the web, together with one " or several press felts, between rolls. The main function of the wet pressing is thus to dewater the web, but its functions also include compacting the web and strengthening it in order to achieve sufficient wet strength. Sufficient wet strength of the web is a prerequisite for the web being capable of being moved, without breaking, from the press section to the dryer section and further from one drying drum to another. In the wet press it is possible to use rolls of different types, such as plain, perforated, grooved or covered rolls. It is also possible to use a press wire between the press felt and the roll surface in order to improve the dewatering ability and to eliminate marking of the paper by the roll. The primary function of the press felt is to receive the water extracted from the web. It is also a function of the felt to support the web in the press and to protect -it from being damaged in the press. The structure of the press felt plays a crucial role in the final quality of the paper. An effective press felt is required to have a good dewatering ability, mechanical strength, a long usage life.and an ability to contribute to producing unmarked paper of an even quality. In general, needled press felts having a high synthetic fiber content are used. Typically these synthetic fibers are polyamide, polyester and polyacrylic nitrile fibers. In most felts there is a wad layer attached to the base fabric. The properties of a completed felt can be adjusted by using fibers of different fineness and of different raw materials in the wad needled to the felt. It is possible to give the felt the desired properties by combining different types of wad layers. In felts of this type, achieving sufficient smoothness of the felt is a problem; endeavors have been made to improve the smoothness of felts by, for example, needling a thin layer of finer fibers on the web-side surface of the felt. The press felt has a crucial effect on dewatering, and therefore its quality is a very important factor. In general the aim is to use uncαπpressible, stiff felts with an open structure. However, a felt which is too open may produce detrimental patterns in the web and an uneven distribution of pressure on the fibers of the web. Present-day felts made totally or in part of synthetic fibers have a relatively poor ability to resist compression and, consequently, they lose their elasticity and compact, and this decreases their dewatering ability. Also clogging substances such as fibers promote the compaction of the structure and thereby shorten the economical useful life of the felt. Because of the disadvantages presented above, particular attention must be paid to the maintenance of the felts, and they must be replaced relatively often. In present-day papermaking machines, continuous-working felt-cleaning systems are used; these systems include various felt-maintenance devices such as pressure sprays and suction slits extending across the wire. From Finnish Patent Application 810884 there _.s known a structure used as a wire or a felt, a structure which comprises a film part having, passing through the film, ducts produced by laser beams, and a reinforcement part combined with the film part, consisting of a fabric comprising staple fibers. In a structure of this type the clogging of the ducts and fabrics may also constitute a problem. The object of the present invention is to provide a flat structure permeable to liquid, for use especially as a press felt, a structure which has a good dewatering ability and a high mechanical strength, and by means of which an even distribution of pressure is achieved at the nip and the above-mentioned disadvantages are avoided. It is also an object of the invention to provide a simple structure which is easy to service, requires little maintenance, and has a long useful life. These objectives can be achieved by means of the structure according to the invention, which is characterized in that it is made up of a body made of a castable polymer, the body having holes and possibly cavities formed during the casting process, the diameter of these holes increasing substantially progressively from one surface to the other in such a way that, the surface facing the web is microporous and the other surface has substantially more open pores, and possibly of a reinforcement material. The body can be made from castable polymers suitable for this purpose, preferably elastomers, such as casting urethane. Casting urethane is a rubber-like material which is also called urethane rubber. In addition to urethane rubber if is also .possible to use other liquid rubbers, such as nitrile rubbers and silicone rubbers. The castable polymers, such as casting urethane, can easily be blended with polymeric additives such as epoxy, to form a uniform castable basic material. " The properties of the structure can, when so desired, be modified in various ways. Thus, the mechanical properties, such as stiffness, hardness and resistance to wear and flex, can be adjusted by selecting a suitable polymer. The strength properties can be improved by means of various reinforcement materials such as glass fiber, chopped fiber, monofilaments, multifilaments, and/or metal netting. It is also possible to use both woven and non-woven reinforcement materials. Synthetic fibers such as polya ide, aramide ("Kevlar") and polyester fibers are typical raw materials. The reinforcement materials are preferably fitted inside the body. Casting urethane, being a rubber-like material, has good mechanical-dynamic properties as the body material of the structure, and, in addition, at the casting stage it is highly reactive, so that it is easy to achieve a chemical bond between it and most reinforcement materials. The dewatering ability of the structure can be improved and its elasticity can be adjusted by forming in the structure cavities and/or pores of different shapes and sizes; these cavities and pores may be open or closed. The open porousness may also be such that the pores are progressively more open through the entire structure. It is also possible to form in the structure cavities which interconnect holes, whereby a possibility for internal equalization of hydraulic pressure is produced in the horizontal direction. Also the surface properties of the structure, as regards friction and release, can be adjusted to be those desired. The properties of the structure can also be modified by forming a sandwich structure in which the different layers behave mechanically in different ways. The web-side surface of the structure according to the invention is very smooth and microporous, and the dewatering ability of the structure is good. By means of the structure according to the invention, a very even pressure is also achieved at the nip, and this even pressure contributes to the formation of paper of even and good quality. In addition, when the structure according to the invention is used, elastic covering of the rolls is not necessary. The invention also relates to a method for the manufacture of a structure permeable to water, especially a press felt for use in papermaking machines, which method is characterized in that the raw materials of a polymeric material are applied to an endless base which has conical members projecting stabstantially perpendicularly from the surface of the base; this procedure is, when necessary, followed by curing, whereafter the structure is detached, from the base. The applying'of the raw materials to the base is preferably done by casting, spreading or spraying. The base must in general be treated,with a lubricant so that the completed structure can be easily detached from the base. According to one preferred embodiment the endless base is fitted between two rolls. According to another embodiment the endless base is fitted around a large-diameter roll. The cicrumfarential surface of a large-diameter roll can also constitute the endless base. The invention is described in greater detail below with reference to the accompanying drawing, in which Figure 1 depicts a longitudinal section of the structure according to one embodiment of the invention. Figure 2 depicts a top view of the same structure as Figure 1. Figure 3 depicts a longitudinal section of the structure according to another embodiment, Figure 4 depicts a longitudinal section of the structure according to one further embodiment, Figure 5 depicts a longitudinal section of the structure according to one further embodiment, and Figure 6 depicts a longitudinal section of an apparatus used for the manufacture of the structure according to the invention. In Figure 1 the flat body made of castable polymer is indicated by reference numeral 1. The body 1 is preferably made of urethane rubber. The body 1 has conical holes 2. The web-side surface 3 (Figures 1 and 2) is microporous and very smooth. The other surface, for its part, has pores which are clearly more open. Owing to this structure, the structure according to the invention, when used as a press felt or press wire, has a good dewatering ability and, furthermore, the holes 2 do not get easily clogged. Figure 3 depicts a structure similar to that in Figure 1, except that it has been reinforced with a reinforcement material 4. Various synthetic fibers, threads and fabrics, glass fibers, and metal wires and nettings are among suitable reinforcement materials. Figure 4 depicts a structure similar to that in Figure 1, except that the structure has pores 5. The pores may be open or closed pores. Open pores make the structure more "sponge-like", thereby improving its dewatering ability. Closed pores, for their part, improve the elasticity properties of the structure. Figure 5 depicts a structure similar to that in Figure 4, except that the structure has cavities 6 which interconnect the holes, and cavities which open into the holes. Only a few embodiments according to the invention are described above, but it is evident that these embodiments can be combined and modified within the scope of the claims. Thus, for example, the structure according to the invention may both be reinforced and contain pores. With reference to Figure 6, two methods are described below for the manufacture of a structure according to the invention. A belt-like-mold 7 is stretched, between two rolls 9, the surface of the mold having conical spikes 8 the shape of which corresponds to the shape of the holes desired in the structure. The raw materials of a polymeric material are applied to this "mat of nails" to produce a structure of desired thickness. The raw material can be applied by casting, spreading , or spraying. For the final curing of the material, a heat treatment is usually necessary. The completed structure is detached from the "mat of nails" . If it is desired to reinforce the structure, the applying of the raw material is carried out in layers, the reinforcement material being placed between layers. If it is desired to increase the porosity of the structure, suitable chemicals, such as volatile substances or water-soluble substances, are added to the polymeric material during the manufacturing process, these substances forming, under the effect of heat or water, pores in the structure. One example of such water-soluble substances is common salt, the grain size of which determines the size of the pores.

Claims: Claims 1. A flat structure permeable to liquid, in particular a press felt for use in papermaking machines, c h a r a c t e r i z e d i n that it is made up of a body (1) made of a castable polymer, the body containing holes (2) and possibly cavities (6) formed during the casting process, the diameter of the holes increasing substantially progressively from one surface to the other in such a way that the web-side surface (3) is microporous and the other surface has pores substantially more open, and possibly of a reinforcement material (4) . 2. A structure according to Claim 1, c h a r a c t e r i z e d i n that the body (1) is made of a castable elastomer, such as casting urethane. ι 3. A structure according to Claim 1 or 2, c h a r a c t e r i z e d i n that the body (1) has cavities (6) which interconnect the holes (2) in order to equalize the internal hydraulic pressure in the structure in the horizontal direction. 4. A structure according to any of the above claims, c h a r a c t e r i z e d i n that the body (1) additionally has closed pores (5) in order to adjust the elasticity properties of the structure to the desired level and/or open pores in order to adjust the water volume in the structure to the desired level. 5. A structure according to any of the above claims, c h a r a c t e r i z e d i n that the reinforcement material (4) is fitted inside the body (1) . 6. A structure according to any of the above claims, c h a r a c t e r i z e d i n that it is a sandwich structure. 7. A method for the manufacture of a structure permeable to liquid, in particular a press felt for use in papermaking machines, σ h a r a c t e r i z e d i n that the raw materials of a polymeric material are applied to an endless base (7) which has conical members (8) projecting substantially perpendicularly from the surface of the base, which procedure is, when necessary, followed by curing, whereafter the structure is detached from the base. 8. A method according to Claim 7, c h a r a c t e r i z e d i n that the endless base (7) is fitted between two rolls (9) . 9.- A method according to Claim 7 or 8, c h a r a c t e r i z e d i n that the polymeric material is an elastomer such as casting urethane. 10. A method according to any of Claims 7-9, c h a r a c t e r i z e d i n that the application is carried out by casting, s p reading . or spraying. 11. A method according to any of Claims 7-10, c h a r a c t e r i z e d i n that the application is carried out in layers. 12. A method according to Claim 11, c h a r a c t e r i z e d i n that a reinforcement material (4) is placed between the layers. 13. A method according to any of the above claims, c h a r a c t e r i z e d i n that the porositv of the polymeric material is increased by using volatile substances and/or water-soluble substances, which substances during the manufacturing process under the effect of heat and/or water form the desired pores and cavities, either open-cell or closed-cell ones, in the desired places. 14. A method according to Claim 13, c h a r a c t e r i z e d i n that the water-soluble substance used is salt, the grain size of which determines the size of the pores and cavities.

Inventors: SORSA SEPPO, SORSA, SEPPO
Applicants: NOKIA OY AB, OY NOKIA AB
Application year: 1986
Main classification: D21F1
IPC codes: D21F1, D21F7