Roll for a paper or board machine and method for manufacture of the roll

Abstract: The invention concerns a roll for use in the manufacture of paper and board, a press roll, in particular a centre roll in a press, a backup roll for an extended-nip press roll, a hot press roll, or equivalent, which is in direct contact with a wet paper web, or a calender roll, a method for manufacture of the roll. and a composition of coating for the roll. Onto the frame part (1) of the roll, first an adhesion layer (2) is applied, and then a ceramic layer (3) of a thickness of 100...2000 µm. which contains 50...95 % of Cr 2 O 3 and 3...50 % of TiO 2 , or Al 2 O 3 and ZrO 2 , or Al 2 TiO 5 , and the roughness Ra of the outer face of the roll is 0.4...2.0 µm, and the porosity of the outer face of the roll is 1...20 %. Besides, or in stead of, TiO 2 , the ceramic layer (3) may also contain oxides of aluminum, silicon, zirconium, magnesium, manganese, tin, or tungsten or mixtures of same.

Description: The invention concerns a roll for use in the manufacture of paper and board, in particular a press roll, a centre roll in a press, a backup roll for an extended-nip press roll, a hot press roll, or equivalent, which is in direct contact with a wet paper web, or a calender roll, a method for manufacture of the roll, and a composition of the coating for the roll. Press rolls and calender rolls are critical components in a paper machine both from the point of view of the runnability of the machine and from the point of view of the quality of the product, for in a press and in a calender, the roll face is in direct contact with the paper web. Direct contact with the face of a roll has a significant effect on the surface properties of paper, which imposes high requirements on the quality of the face of the roll. The surface properties of the roll are also critical from the point of view of the process. The wet or coated paper web must adhere to the roll in a suitable way, but, on the other hand, it must also be readily separable from the roll. It must be readily possible to doctor the roll, easy to keep it clean, and the roll must remain in good operating condition for a long period of time. Further, from the point of view of runnability, it is essential that the surface properties must not change during the process so that, for example, the separability of the web from the roll face cannot be controlled. Owing to the direct contact with the web, fibres and contaminations adhere to the roll face and block the surface layer, and, consequently, the surface properties of the roll are changed. In order to amend the effect of contamination, the roll is doctored, which again imposes its requirements on the mechanical surface properties of the roll. In a press, particularly demanding surface properties are required from centre rolls in presses, at which rolls the web is pressed against the roll face in two nips, and from backup rolls of extended-nip presses, in which the contact face between the web and the roll is longer and the nip load higher. In these cases, the tendency of sticking of the web and contamination of the roll in web contact are particularly intensive. In said positions, in particular in wide paper machines, variable-crown rolls are used. The roll face is further subjected to particularly demanding conditions when the pressing takes place at an elevated temperature either by heating the web before the centre roll (e.g., by means of a steam box) or by means of the roll when the web is on the roll face (heatable centre roll or so-called impulse drying). The granite roll, which has been used traditionally in the press section of a paper machine, has been abandoned in recent years in spite of the excellent surface properties of granite. The reason has been mainly the requirements of strength and durability increased along with higher running speeds as well as the need to profile and to heat the roll. Synthetic rock rolls, which have been coated with a mixture of ceramic powder added to a hard rubber and polyurethane or some other polymer, have proved poor because of their low mechanical strength and because of excessive adhesion of the paper web to the roll face. By means of calender rolls, the surface of paper or board can be given the desired gloss, smoothness, and finished face. Traditionally, calendering has been used mainly for compacting the surface of paper, which has taken place by means of rolls as smooth as possible. For this purpose, chilled rolls and hard-chromium plated rolls have been used commonly. Electrolytic hard-chromium plating has been highly laborious as a process for rolls of large size, and in respect of the quality of the face to be chromium-plated it has been highly demanding. From the point of view of the process, the high susceptibility of wear of the hard-chromium plated face has constituted a problem, which wear has been increased further by the micro-particles contained in the coating material, such as clay. Owing to the wear, the capability of holding of the face is deteriorated, and doctoring becomes more difficult and causes streaks in the web. The production of dull-finish grades by means of the prior-art rolls has not been reasonable because of the rapid smoothing of the face. Owing to the circumstances mentioned above, alternative solutions for coatings of rolls have been developed both for press rolls and for calender rolls. Most commonly, at present, rolls with metal frames are used which have been coated with a metal, a ceramic, ceramic-metal, polymers, or elastomers and with various mixtures of same. Ceramic and ceramic-metal coatings and coating processes have been described, for example, in the following publications. In the FI Patent Application No. 853544, a roll coating is described which consists of a metal or of a mixture of a metal and a ceramic material. In the FI Patent No. 70,273, mixtures of metal powder and inorganic material are suggested as a coating for a press roll. On the other hand, in the FI Patent Application No. 861803, a construction of a press roll is described, in which a metallic adhesion layer has been applied onto the face of a metal frame, which adhesion layer has a thermal expansion coefficient lower than that of the metal frame, and onto the adhesion layer a ceramic surface layer has been prepared. In the FI Patent No. 84,506, a press roll is described whose metallic frame cylinder is coated with an intermediate layer consisting of a composite compound made of a ceramic material and a metal, so that the mixing ratio of the components in the composite compound is different in different parts of the intermediate layer in the direction of the radius of the roll, and with a ceramic surface layer. In the FI Patent No. 86,566, a centre roll for a press is described, which has been manufactured so that the mantle of the roll frame has been coated with a corrosion-protection layer which consists of chromium-containing stainless steel as well as of a surface layer which has been prepared by thermal spraying of a powder in which the metal and ceramic phases are contained in the same powder particle. In the patent WO 93/01326, a composition of coating for a press roll is described, which comprises a mixture of aluminum silicate and alkaline-earth metal oxide that has been plasma-sprayed onto the face of the roll. In the European Patent Application 0,657,237, thermal spraying of cobalt-containing tungsten carbide granules and nickel-containing chromium carbide granules onto the roll face is described. In the FI Patent No. 89,950, a press roll for a paper machine is described, in which the mantle of the metal core of the roll has been coated with a metal layer that contains molybdenum-based and nickel-based metal alloys. Onto the metal layer, a ceramic coating is prepared by means of thermal spraying. Suitable ceramic compounds are grey aluminum oxide (95 % Al 2 O 3 - 2.5 % TiO 2 ), white aluminum oxide (99 % Al 2 O 3 ), titanium dioxide (TiO 2 ), etc. and mixtures of same. In the European Patent Application 0,481,321, a press roll is described in which an intermediate layer made of a molybdenum-based or nickel-based alloy has been applied onto the metal core, for example, by plasma spraying, and onto this layer a ceramic layer has been applied by plasma spraying, which layer consists of metal oxides or of mixtures of same. Finally, the roll is coated with an organic polymer to fill the pores in the ceramic coating. The international patent application WO 96/41 918 is an example of a hot press roll, whose coating has been prepared by means of thermal spraying of a metal-ceramic and of a mixture of a ceramic and of a certain metal alloy. In the Finnish Patent No. 92,609 , a method is described in which, onto the face of a hot-glaze calender roll or of a calender roll for a machine stack, which rolls are supposed to increase the gloss of paper, a metal-ceramic face has been sprayed, which consists of tungsten carbide and of cobalt or of a nickel-chromium alloy, and after coating the face has still been finished by grinding. In the FI Patent No. 80,097 , a method is described in which the roll is coated with a mixture which consists partly of a metal and partly of a ceramic material, so that the outer surface is composed of carbide-rich areas and of matrix areas placed between said carbide-rich areas. Chemically, the coating on the roll is an alloy of tungsten, chromium and carbon, or an alloy of tungsten carbide, tungsten, cobalt, chromium, and carbon. A heatable calender roll with a ceramic coating is described in the Patent EP 598,737 , (corresponds to WO 9323617 wherein a face as smooth as possible is aimed at, and a press roll with a ceramic or cermet face is described in the Patent EP 597,814. EP A 0,369,968 discloses surface material compositions based on different oxides. It has, however, been noticed that the prior-art ceramic-coated rolls mentioned above and currently available on the market involve a number of problems, deficiencies or limitations, which have become ever more critical when the running speed of the paper machine becomes higher and when the basis weight of the paper becomes lower. The process of detaching of the web and the doctoring quality are more difficult to control. These drawbacks are particularly problematic in the case of centre rolls in presses and of backup rolls in extended-nip presses. The Cr-oxide and Aloxide based coatings currently in use involve drawbacks in respect of the mechanical and chemical strength. Chromium oxides are hard, but their toughness is poor, and mechanical damage tends to arise. In mechanically highly abrading conditions, the roll face is worn and ground smooth, in particular in the case of Al 2 O 3 -based ceramics, and Al 2 O 3 -based ceramic coatings do not endure doctoring with a steel blade. The resistance of ceramic coatings to chemical strains is deficient, which results in damage to the surface layer of the roll, such as corrosion and delamination. In particular, Al 2 O 3 -based ceramic coatings do not endure washing with lye. Ceramic coatings have often been thermally sprayed onto the roll face, which unavoidably results in porosity of the face. Owing to the porosity, agents that produce corrosion have access to the boundary face between the roll and the adhesion coating unless the tightness of the adhesion and corrosion-protection layer is adequate. Flaws in the adhesion layer placed under the ceramic layer may result in corrosion in the roll under the coating and, thus, in destruction of the whole roll during a long period of time. Difficulties are further caused by the material gathering in the face of the roll, which tends to block the pores in the surface layer, in which connection the properties of adhesion of the roll are changed. The face of a roll must endure high linear loads, which strain the coating. Besides hardness, toughness and resistance to wear are also required from a coating. Also, the coating must be easy to repair, it must tolerate variations of temperature very well, and it must operate in a wide range of temperatures in a range of 10...250 °C and under a nip pressure of 5...50 MPa. The face of the roll must be sufficiently hard to endure the abrading effect of the filler agents in the paper, the abrading effect of a doctor, and the effect of a corroding environment. Moreover, a roll must have the necessary surface properties for keeping the roll clean, for adhesion and separation of the paper web, such as, among other things, suitable hydrophily. Also, the coating must retain its original roughness, i.e. the Ra value, as long as possible. The face of a roll must be capable of producing the desired properties, such as, for example, uniformity of quality, low gloss (so-called dull finish), and good smoothness, or good gloss (not dull) for the paper, in particular in calender applications. The object of the present invention is a more durable and improved ceramic roll, a method for manufacture of a roll in accordance with the invention, and a composition of the coating for the roll. The roll in accordance with the invention for a paper or board machine or for a finishing machine, the method for manufacture of the roll, and the composition of the coating for the roll are characterized in what is stated in the patent claims. This method for manufacture of the roll is suitable both for manufacture of new rolls and for coating of used rolls, provided that the old coating has been removed first. In view of achieving the objectives of the invention and solving the problems that have been encountered in the prior art, the roll of a paper/board machine or finishing machine is coated with a coating which gives the face hardness, toughness and hydrophily and with which coating the roughness that has been given to the face is retained in the desired Ra range unchanged for a long period of time. By means of a correctly chosen coating, attempts have been made to regulate the interaction between the solid matter, i.e. the roll face, and the liquid and the wet/coated paper face into an optimal range. Compositions of coating that meet these requirements include a mixture of chromium oxide and titanium dioxide, in which other metal oxides are possibly also included, a mixture aluminum oxide and zirconium oxide, as well as aluminum titanate. The preferred coating comprises chromium oxide and titanium dioxide. The proportions of the components are in the range Cr 2 O 3 50...95 %, 3...50 % TiO 2 . A more advantageous composition comprises Cr 2 O 3 55...80 % and TiO 2 20...45 %. The composition may possibly also include other metal oxide components, e.g. SnO 2 . With a chromium oxide content of 50...95 %, the coating can be given the necessary hardness and resistance to wear. The toughness of pure chromium oxide is poor, for which reason the content of chromium oxide cannot exceed 95 %. With an addition of 3...50 % of titanium oxide, the surface can be given toughness so as to endure impact-like strains, which also improves the resistance to wear in an impact-like situation of wear. Delamination of the coating is also prevented. Further, titanium oxide mixed with chromium oxide increases the hydrophily of the surface, which improves the separation of the web decisively and also clearly reduces the tendency of contamination. An increase of the content of titanium oxide beyond 50 %, however, reduces the values of hardness and lowers the resistance to wear. The titanium oxide can also be partly substituted for by oxides of aluminum, silicon, zirconium, magnesium, manganese, tin and tungsten, or by mixtures of same. A tight and strong ceramic layer in accordance with the invention also improves the effect of protection from corrosion provided by a possible adhesion coating on the roll. The resistance to corrosion of chromium oxide is excellent, so that, for example, alkaline or acid chemicals used for washing of the roll do not cause corroding of the roll face. Mixing of titanium oxide does not reduce the wider pH-range that has been achieved, in which range it is possible to operate in the way indicated by Table 1. Solubility of coatings (ppm) in acid (pH 1) and alkaline (pH 13) conditions. Test period 1 week. Coating pH 1 pH 13 Al 2 O 3 (97 %) 4.9 4.6 Cr 2 O 3 (75 %) + TiO 2 (25 %) 0.1 0.6 Cr 2 O 3 (92 % ) 0.1 0.6 The porosity of a thermally sprayed ceramic coating in particular for press rolls is typically in a range 1...20 %. Penetration of contaminations into the coating is reduced when the porosity becomes lower. Thus, the effect of titanium oxide of tightening the Cr 2 O 3 coating also promotes the keeping of the face clean, besides favourable toughness and surface properties. The roughness value Ra of a ceramic surface in accordance with the invention is in the range 0.2...2.0 µm, preferably Ra is 0.4...1.5 µm. Based on practical experiments, it has been noticed that an increase in the roughness up to a certain limit facilitates the detaching of the web, but, on the other hand, an excessively high roughness deteriorates the hold of the doctor and increases the wear of the blade. For the face of a calender roll, a surface profile suitable for dull-finish operation can be finished by brushing with silicon carbide. This roll endures doctoring considerably better than hard-chromium plated rolls do. In spite of variations in temperature, the face neither is delaminated nor cracks. In the following table, properties of a roll with ceramic coating in accordance with the invention are compared with prior-art rolls. PROPERTIES OF CALENDER ROLL COATINGS CHILLED CARBIDE CHR.PL. CERAMIC HARDNESS (HV) 600 1100 900 1100 THICKNESS (mm) 10 0.1 0.1 0.3 ROUGHNESS Ra 0.1 0.2 0.1 0.4 Some preferred solutions in accordance with the invention will be illustrated by means of the accompanying Figure 1. In Figs. 2 to 5, the properties of a roll in accordance with the present invention are compared, among other things, with the properties of prior-art rolls. List of illustrations: 1. A solution in accordance with the invention. 2. Hydrophily of roll surface. 3. Separation of paper web from the roll face. 4. Effect of roughness of the roll on separation of the web. 5. Resistance to wear of roll coatings. Fig. 1 illustrates a preferred solution in accordance with the invention in the form of a cross-section of the face of a roll. Onto the roll core, i.e. the frame part 1, a nickel-chromium adhesion face/corrosion-protection layer 2 is applied, and onto said face/layer a ceramic coating 3 of a thickness of 100...2000 µm, in a calender application 300...500 µm, is applied out of a composition of coating in accordance with the invention. The thickness of the layer 2 is 50...400 µm, in a press application preferably 100...400 µm, in a calender roll preferably 50...200 µm. Fig. 2 illustrates the hydrophily, i.e. the property of attracting water, of the roll face by means of the contact angle of water. In the figure, faces consisting of granite, Al 2 O 3 , Cr 2 O 3 , Cr 2 O 3 75 % + TiO 2 25 %, and Cr 2 O 3 50 % + TiO 2 50 % are compared. The smaller the contact angle, the higher is the hydrophily. Based on this figure, it can be noticed that a face that contains 75 % Cr 2 O 3 + 25 % TiO 2 is clearly the most hydrophilic face of all. The water film formed on the hydrophilic face prevents sticky agents from adhering to the roll face. At the same time, a sufficient film of water facilitates detaching of the web. Fig. 3 illustrates the web separation work as a function of the web separation angle. The ease of separation of the web is illustrated best by the separation work (J/m 2 ). Under comparable conditions, the values of separation work illustrated in the figure are given as a function of the separation angle while a granite roll is compared with a press roll with a Cr 2 O 3 75 % + 25 % TiO 2 coating and with a press roll whose coating contained Cr 2 O 3 92 % + 3 % TiO 2 + 5 % SiO 2 . The best values were obtained with a roll whose coating consisted of Cr 2 O 3 75 % + 25 % TiO 2 . Fig. 4 illustrates the angle of separation as a function of the roughness of the surface of the press roll. The separation of the web can be measured by means of the angle of separation when all the other factors, for example the properties of the web etc., are kept invariable. A small angle of separation correlates with easy separation. In tests, under comparable conditions, the interdependence illustrated in Fig. 4 was obtained, on whose basis the roughness of the surface of the press roll must be kept in a certain range in order to minimize the risk of passing through of the web and, on the other hand, to maximize the ease of separation. Fig. 5 illustrates the resistance to wear of different coatings in a rubber wheel abrasion test. The losses of weight of granite, an Al 2 O 3 97 % coating, a Cr 2 O 3 92 % coating, and of a Cr 2 O 3 75 % + TiO 2 25 % coating were measured after quartz sand abrasion. Mixing of titanium oxide (25 %) improves the abrasive wear resistance, because the toughness of the coating is better than with a purer (92 %) chromium oxide. On the other hand, the hardness of the coating higher than that of granite provides a better resistance to wear. It can be ascertained that the loss of weight of the Cr 2 O 3 75 % + TiO 2 25 % coating was lowest, i.e. its resistance to wear was best. The method in accordance with the invention for coating of rolls as well as the coating in accordance with the invention are suitable for coating of a roll used in the manufacture of paper and board, in particular of a centre roll in a press, of a backup roll for an extended-nip press roll, or of a hot press roll or equivalent, which rolls are, as a rule, variable-crown rolls, or of a calender roll, in particular a thermo roll for a calender, or of a roll used for impulse drying. The following examples illustrate the invention in more detail. The mantle of the roll frame, which can be made of iron, steel, or equivalent, is coated with an adhesion face, which is made of a metal or an alloy of metals, preferably nickel-chromium alloy, and whose thickness is 100...400 µm, by means of a thermal spraying method. Onto the adhesion face, a ceramic surface layer is applied, which contains Cr 2 O 3 50...95 % and TiO 2 3...50 %, by means of the high-velocity flame spraying method (HVOF) or the plasma spraying method (APS), which produces the necessary melting of the particles and results in a tight and strong face. The coating has very high hardness, toughness and resistance to corrosion. In the face, no effect of delamination occurs at all, and the wear and the smoothing of the face as a result of mechanical strain are very little. This is why, in connection with the coating, it is possible to use a steel doctor, which does not scratch the face. A wet paper web adheres to the roll in accordance with the invention appropriately but is separated from the roll readily so that the separation angle is in an optimal range even at high running speeds. The roll can be doctored readily and is easy to keep clean. Also, the roll face is appropriately hydrophilic and resists to contamination and provides the paper web with optimal properties of adhesion and separation. Further, the porosity and the roughness of the roll face are in a range in which the properties of separation and adhesion of the paper web are optimal and in which, on the other hand, the properties of doctoring of the roll remain good. A coating in particular suitable for dull-finish calendering is achieved when a roll 1 which is provided with a conventional roll frame, such as, for example, a chilled roll frame, and which has a hard face is coated with an adhesion/corrosion-protection layer 2 of a thickness of about 50...200 µm, preferably 100...150 µm, which layer 2 consists of an alloy of nickel and chromium. Onto the adhesion layer 2, a ceramic coating layer of a thickness of 300...500 µm, preferably 350...400 µm, is applied by means of plasma spraying or high-velocity flame spraying (HVOF). By means of the choice of the coating, the wear resistance and the toughness of the roll face can be affected to a substantial extent. The face is roughened to the desired roughness, and the surface profile is finished. When a coating is employed that has been prepared by means of the plasma spraying method, the most advantageous combinations in respect of wear resistance and toughness are obtained with a combination of Cr 2 O 3 - TiO 2 . When a calender roll in accordance with the invention is used for manufacture of paper/board, the roughness of the roll coating is copied onto the paper that is produced. As a result of this, the roughness measured from the paper is lowered, but the gloss is not increased, whereas it is increased when smooth rolls are used. An elevated temperature of 60...250 °C and a higher nip pressure 5...50 MPa often enhance the operation of the roll, but the method is also well suitable for operating at lower temperatures of 10...50 °C. The desired quality of the paper/board that is produced and the non-calendered roughness of the paper determine the roughness of the roll to be used. A preferable range is Ra = 0.2...2.0 µm. In this way a face is provided that is considerably less dependent on the filler and coating agents of paper, as compared with the earlier methods, so that it is possible to choose the coating and filler agents suitable for printing or equivalent more freely.

Claims: A roll for a paper/board machine or for a finishing machine, comprising a ceramic layer (3) of a thickness of 100... 2000 µm that has been applied onto the frame part (1) of the roll, and the roughness Ra of the outer face of the roll being 0.2...2.0 µm, preferably 0.4...1.5 µm, characterized in that the ceramic layer (3) contains 50...95 %, preferably 55...80 %, of Cr 2 O 3 and 3...50 %, preferably 20...45 %, of TiO 2 and possibly other metal oxides. A roll as claimed in claim 1, characterized in that the possible other metal oxides in the ceramic coating (3) are oxides of aluminum, silicon, zirconium, magnesium, manganese, tin, or tungsten, or mixtures of same. A roll as claimed in claim 1 or 2, characterized in that , between the frame part (1) and the ceramic layer (3), at least one adhesion/corrosion-protection layer (2) of a thickness of 50...400 µm, preferably 100...150 µm, and consisting of a metal or a metal alloy has been applied. A roll as claimed in any of the claims 1 to 3, characterized in that the roll is a press roll, in particular a variable-crown roll and in particular a centre roll for a press or a backup roll for an extended-nip press or a hot press roll; or a calender roll, in particular a thermo roll for a calender. A roll as claimed in any of the claims 1 to 4, characterized in that the roll is heatable. A roll as claimed in any of the claims 1 to 5, characterized in that the roll is a press roll, and the porosity of the roll face is 1...20 %. A method for manufacture of a roll for a paper/board machine or for a finishing machine, wherein, onto the frame part (1) of the roll, a ceramic layer (3) of a thickness of 100...2000 µm, is applied, which contains 50...95 %, preferably 55...80 %, of Cr 2 O 3 and 3...50 %, preferably 20...45 %, of TiO 2 and possibly other metal oxides, and the outer face of the roll is finished to a roughness of Ra = 0.2...2.0 µm, preferably 0.4...1.5 µm. A method as claimed in claim 7, characterized in that the possible other metal oxides in the ceramic coating (3) are oxides of aluminum, silicon, zirconium, magnesium, manganese, tin, or tungsten, or mixtures of same. A method as claimed in claim 7 or 8, characterized in that , between the frame part (1) and the ceramic layer (3), at least one adhesion/corrosion-protection layer (2) of a thickness of 50...400 µm, preferably 100...150 µm, is applied by means of thermal spraying. A method as claimed in any of the claims 7 to 9, characterized in that the surface layer (3) is prepared by means of the high-velocity flame spraying technique or by means of plasma spraying. A method as claimed in any of the claims 7 to 10, characterized in that the roll is a press roll, a variable-crown roll, in particular a centre roll for a press or a backup roll for an extended-nip press or a hot press roll; or a calender roll, in particular a thermo roll for a calender. A method as claimed in any of the claims 7 to 11, characterized in that the roll is a calender roll, and after coating the face is ground to a roughness of Ra = 0.2...2.0 µm, preferably 0.4...1.0 µm, and the surface profile is finished to make it technically suitable for dull-finish operation.

Inventors: KUOSA HARRI, LEHTONEN PENTTI, MAJAVA JORMA, NIEMI KARI, TUOMELA JUSSI, VESTOLA JUHANI, WAHLROOS JUHA, KUOSA, HARRI, LEHTONEN, PENTTI, MAJAVA, JORMA, NIEMI, KARI, TUOMELA, JUSSI, VESTOLA, JUHANI, WAHLROOS, JUHA
Applicants: METSO PAPER INC, METSO PAPER, INC.
Application year: 1998
Main classification: D21F3
IPC codes: D21G1, D21F3, F16C13