Method for the coating of the centre roll in the press of a paper machine and a centre roll in the press of a paper machine

Abstract: The invention concerns a method for the coating of the centre roll (K) in the press of a paper machine and a centre roll (K) in the paper machine. The centre roll (K) in the paper machine is made of a ceramic or metal-ceramic basic material. In view of obtaining the desired adhesion properties of the face (K') of the centre roll (K), the desired portions of the centre roll (K) are coated or soaked with a polymer coating (X₂).

Description: The invention concerns a method for the coating of the centre roll in the press of a paper machine as defined in the preamble of claim 1 and a centre roll in the press of a paper machine as defined in the preamble of claim 10, such method and centre roll being known, for example, from EP-A-0 207 921. At present, centre rolls coated with a ceramic for press sections of paper machines are manufactured. When ceramic centre rolls are employed, it is difficult to control the properties of separation/adhesion of the paper web. In view of avoiding this problem, in the present patent application, a novel method is suggested for the coating of a roll, wherein the ceramic roll face is coated with a plastic only partly, being, for example, Teflon-coated so as to produce ready separation of the web from the roll face. The non-Teflon-coated portions of the roll guarantee an adequate adhesion of the roll face, so that the paper web does not follow, e.g., the pick-up felt after the second nip in the press. Impregnation of a ceramic with plastic is included in the scope of the prior art. This technique has, however, been applied so that the whole face of the roll has been treated, in which case the pores present in the face are fully or partly filled with plastic. In such a case, however, the whole of the surface properties of the roll are changed, and, for example, the desired properties of pure non-impregnated ceramic cannot be utilized. According to the present invention, the property of separation/adhesion of the web can be varied by varying the ratio of the area of coated face to the area of uncoated face. The coating can be carried out, for example, by covering the roll with perforated or grooved tape and by spraying the fluoro-plastic (PTFE = polytetrafluoroethylene, FEP = copolymer of tetrafluoroethylene and hexafluoropropylene, PCTFE = polychlorotrifluoroethylene, PFA = perfluoroalkoxy, or PVDF = polyvinylidenefluoride) into the roll face through the grooves and/or perforations in the tape. Hereupon the tape is removed, and the plastic is polymerized in an oven. The coating agent may also be applied by means of a raster-faced or grooved roll. By means of the method of the invention, it is possible to manufacture a ceramic centre roll of a novel type by treating the roll face in part only. Owing to the nature of the ceramic coating, a uniform surface structure is obtained as the coating agent is carried by absorption into the porous ceramic or metal-ceramic base material. Preferably, the base ceramic X₁ of the centre roll K may be an oxide ceramic, such as a Zr, Al, Si, Ti, Y, Cr oxide, or a carbide ceramic, such as a Cr, W, Ti, Nb carbide, or a boride, such as Fe, Cr, Ti boride, or a mixture or compound of same. Into the above ceramics, metals may have been alloyed, such as Al, Cr, Ni, Co. Under said ceramic or metal-ceramic coating, in contact with the roll body, it is further possible to employ one or several so-called adhesion layers to ensure reliable adhesion of the ceramic or metal-ceramic layer. The adhesion layer is preferably a thin, thermally sprayed metal layer with minimal porosity. The thickness of the ceramic or metal-ceramic coating is 0.05...4 mm, preferably 0.5...2 mm. The invention permits the manufacture of a roll whose face has an adhesion force that varies in the direction of the roll circumference, i.e. in the transverse direction of the paper. Thus, the roll face can be selected, e.g., in view of the desired doctoring properties. The wear of a roll in accordance with the invention is uniform, because the hardness of the coated roll area is substantially equal to the hardness of the uncoated roll area. The method in accordance with the invention is mainly characterized in that, in view of obtaining the desired adhesion properties of the face of the centre roll, the desired length portions of the centre roll are coated or soaked with the polymer The centre roll in accordance with the invention for a paper machine press is mainly characterized in that the ceramic or metal-ceramic basic material has been impregnated with the coating layer only over a part of the length of the roll. The invention will be described in the following with reference to the embodiments shown in the accompanying figures, the invention being not supposed to be confined to said embodiments alone. Figure 1 illustrates the problem areas occurring in the operation of a centre roll. Figure 2 illustrates a first preferred embodiment of the method of the invention for the coating of a centre roll. Figure 3 illustrates a second preferred embodiment of the method of the invention. Figure 4 is a sectional view of a surface of a centre roll, into which a coating agent has been absorbed. Fig. 1 is a schematic illustration of the nips N₁ and N₂ placed in connection with the centre roll K of the press of a paper machine. The web R is passed from the nip N₁ onto the face of the centre roll K and further into the nip N₂ and, from the nip N₂, further along the roll face K' of the centre roll K a certain distance, until the web R is separated from the roll face. In the area a₁ of the centre roll K it is essential that the paper web R should adhere to the roll face, in which case an adhesion is required from the face of the centre roll K in order to hold the paper web R on the face of the centre roll K, and, in a corresponding way, in the area a₂, in which the web is guided away from the face of the centre roll K, a lower adhesion force is required from the face in order that the paper web should be detached from the roll faces as well as possible. Thus, in the area a₁ and a₂, the properties required from the face K' of the centre roll K are contradictory to each other. Thus, with respect to the centre-roll operations, the roll face must be selected separately from case to case. The method of the invention permits selection of the coating X₂ for the centre roll K variably. X₂ is preferably a polymer. According to the invention, into the roll face, a plastic coating 21, is soaked, preferably a Teflon coating PTFE (polytetrafluoroethylene). The coating may also be some other fluoro-plastic, such as PCTFE (polychlorotrifluoroethylene), PFA (perfluoroalkoxy), PVDF (polyvinylidenefluoride), or FEB (copolymer of tetrafluoroethylene and hexafluoropropylene). As is shown in Fig. 2, the coating agent X₂ is spread by spraying. Fig. 2 shows the stage in which the tapes 11 are removed after the spraying of the coating agent. The portions of the roll that are not to be coated are covered with a tape 11, which is grooved and/or perforated. By varying the perforations 12a / grooves 12b in the tape, the roll face can be provided with the desired pattern form and, thus, with the desired adhesion force. If it is desirable to vary the adhesion force of the roll, for example, across the length of the roll, the ratio of uncoated roll face to coated roll face is varied, e.g., by varying the size of the perforations 12a in the tape 11 and/or by varying the spacing of the perforations in the tape 11. It is also possible to vary the groove width (L) and/or the groove spacing of the grooves 12b in the tape 11. After the coating, the tape 11 is removed and the coating is polymerized by heating in an oven. Fig. 3 illustrates a second preferred embodiment of the coating in accordance with the invention. The coating agent is applied to the face K' of the centre roll K by means of a rasterized and/or grooved roll 13,14. The centre roll K is rotated in contact with the raster rolls 13,14. The raster rolls 13,14 are placed partly in a basin C that contains coating agent X₂. The coating agent X₂ adheres to the face of the rasterized roll 13,14 , and from said face, in the nip N between the raster roll and the centre roll K, as the rolls K,13,14 revolve, said coating agent is transferred to the application area on the face K' of the centre roll K. By means of the method of the invention, it is possible to manufacture rolls whose adhesion force is higher in the middle area of the roll face than at the edges of the roll. It is also possible to manufacture rolls whose face has a higher adhesion in the lateral areas of the roll as compared with the middle area of the roll. The coating form and the coating area can be chosen in accordance with the requirements imposed by the operation of the roll in each particular case. Fig. 4 is a sectional view of the face of a centre roll K in accordance with the invention. In Fig. 4, the material X₁ of the area D₁ consists of a ceramic basic material, into which the coating agent X₂ has been impregnated or soaked in the area D₂. The base-material ceramic X₁ may be an oxide ceramic, such as a Zr, Al, Si, Ti, Y, Cr oxide, or a carbide ceramic, such as a Cr, W, Ti, Nb carbide, or a boride, such as Fe, Cr, Ti boride, or a mixture or compound of same. Into the above ceramics, metals may be alloyed, such as Al, Cr, Ni, Co. The coating agent X₂ absorbed into the ceramic roll face is preferably a fluoro-plastic, such as PTFE (polytetrafluoroethylene), FEP (copolymer of tetrafluoroethylene and hexafluoropropylene), PCTFE (polychlorotrifluoroethylene), PFA (perfluoroalkoxy), or PVDF (polyvinylidenefluoride). The use of a coating method in accordance with the invention permits variation of the surface properties on a very small scale. By means of the method, it is possible to provide the roll face with the desired pattern form, in which the roll face and the coating-agent face alternate in the desired way. The wear of the centre roll K in accordance with the invention is uniform, because the hardness of the material X₁ + X₂ in the coated area D₂ of the roll is substantially equal to the hardness of the basic material X₁ in the uncoated area D₁ of the roll.

Claims: Method for the coating of the centre roll (K) in the press of a paper machine, the face of said centre roll (K) in the paper machine being made of a ceramic or metal-ceramic basic material, the face (K') of the centre roll being coated or impregnated with a polymer (X₂), characterized in that, in view of obtaining the desired adhesion properties of the face (K') of the centre roll (K), the face of the centre roll (K) is coated or impregnated with a polymer (X₂) only over a part of the length of the roll. Method as claimed in claim 1, characterized in that the coating agent (X₂) that is used is a fluoro-plastic. Method as claimed in claim 1, characterized in that the coating agent (X₂) that is used is Teflon (PTFE) (polytetrafluoroethylene). Method as claimed in claim 2, characterized in that the coating agent is FEP, PCTFE, PFA, or PVDF. Method as claimed in any of the preceding claims, characterized in that the coating agent is applied to the face (K') of the centre roll (K) by spraying. Method as claimed in any of the preceding claims, characterized in that, in the method, before the coating agent (X₂) is applied to the roll face, the roll face is covered with a perforated and/or grooved tape (11) or equivalent, and hereupon the coating agent (X₂) is introduced through the perforations and/or grooves of the tape onto the roll face (K'). Method as claimed in any of the preceding claims, characterized in that, in the method, the adhesion force of the roll face is adjusted as desired by varying the proportion of the area of the perforations (12a) and/or of the grooves (12b) in the tape or by varying the groove width (L) and/or the groove spacing of the grooves (12b) in the tape (11) and/or the size of the perforations (12a) in the tape (11) and/or the spacing of the perforations (12a) in the tape (11). Method as claimed in any of the preceding claims, characterized in that the coating agent (X₂) is applied onto a rasterized and/or grooved roll (13,14) and, from said roll (13,14), onto the face (K') of the centre roll (K), said raster rolls (13,14) being placed in a basin (C) that contains the coating agent (X₂), and, along with the rotation of the raster rolls (13,14), the coating agent (X₂) being transferred onto the face (K') of the centre roll (K) in the nip (N) between the raster roll (13,14) and the centre roll (K). Method as claimed in any of the preceding claims, characterized in that, after the coating process, the coating (X₂) is polymerized by means of heat treatment preferably in an oven. Centre roll (K) manufactured by a method as claimed in any of the preceding claims in the press of a paper machine, the face of said centre roll (K) being made of a ceramic or metal-ceramic basic material, and said roll (K) comprising a coating layer (X₂) which consists of a polymer and which has been absorbed into the ceramic or metal-ceramic basic material, characterized in that the ceramic or metal-ceramic basic material has been impregnated with the coating layer (X₂) only over a part of the length of the roll. Centre roll as claimed in claim 10 for the press of a paper machine, characterized in that the coating agent (X₂) is a fluoro-plastic. Centre roll as claimed in the preceding claim 11 for the press of a paper machine, characterized in that the coating (X₂) that bas been soaked into the basic material (X₁) is Teflon (polytetrafluoroethylene) (PTFE). Centre roll as claimed in claim 11, characterized in that the coating agent (X₂) is FEP, PCTFE, PFA, or PVDF. Centre roll as claimed in any of the preceding claims 10 to 13 for the press of a paper machine, characterized in that, on its face (K'), the ceramic centre roll (K) comprises a pattern form made by means of the coating agent (X₂), in which pattern the ceramic roll face proper and the coating-agent face alternate. Centre roll as claimed in any of the preceding claims 10 to 14 for the press of a paper machine, characterized in that there is a coating pattern which has been produced by covering the roll face with a tape and by spraying the coating agent onto the portions of the roll face that are not covered by the tape (11). Centre roll as claimed in any of the preceding claims 10 to 15 for the press of a paper machine, characterized in that the ceramic face (K') of the centre roll (K) comprises a coating (X₂), whose amount per area of the roll varies over the length of the roll, the adhesion force of the roll face being affected by the ratio of the area of coated roll face to the area of uncoated roll face. Centre roll as claimed in any of the preceding claims 10,11,12,13,14, or 16 for the press of a paper machine, characterized in that, on its face (K'), the centre roll (K) comprises a coating agent (X₂) which has been applied to the face of the centre roll (K) by means of a raster roll (13,14).

Inventors: SNELLMAN JORMA, SNELLMAN, JORMA
Applicants: VALMET PAPER MACHINERY INC, VALMET PAPER MACHINERY INC.
Application year: 1993
Main classification: D21F3
IPC codes: D21F3