Coating of a roll in a paper machine and a roll coating

Abstract: Method of a coating a roll in a paper machine with powder of thermo plastic and/or speciality plastic. The coating is carried out by spraying by using hypersonic plasma.

Description: The invention is concerned with a method of coating of a roll of a paper machine with powder of thermoplastic speciality plastic and the roll made with the method. Coated rolls are used for very different purposes in paper machines and in posthandling machines for paper. Among the applications can for example the following be mentioned: press rolls, suction rolls, soft rolls in calenders and super calenders and the like. Different quality requirements are set for the coating of the roll in different applications and in different processes. Conventional quality factors for the coating are for example the hardness in a given temperature, temperature resistance, press resistance, chemical resistance, surface smoothness, resistance against mechanical damages, elasticity, surface energy, releasing properties of the paper, conductivity, and non-ageing. US-A-5,023,985 discloses methods for applying a first plastic coating and a second metal/ceramic/cermet/plastic coating on a roll of a paper machine. One of the methods for applying the second coating of metal/ceramic/cermet is thermal spraying. Conventionally rolls of paper machines have been coated with rubber, polyurethane or epoxy. These polymeric materials are especially suitable for coating of big rolls of manufacturing technical reasons. One- or two- component polyurethane and epoxy are available in fluid form in which case the casting of those in a form or rotation casting is possible. It is also very easy to mix these polymeric materials with different fillers and additives to achieve new properties for the coating material. Suitable manufacturing technics (coating technics) for the polyurethane and epoxy are in addition to the form and rotation casting also extrusion, spraying, filament winding, tape winding, spun casting and different impregnated mats. Epoxy (a thermosetting plastic) and polyurethane (a thermosetting plastic or an elastomer) are materials for which the reasons for use as roll coating has in addition to manufacturing technical advantages been some good properties of these polymers. Polyurethane has good dynamic and abrasion properties and epoxy has been providing corrosion properties. The properties of the epoxy has retained also in higher temperatures. The use of thermoplastics as roll coatings has mainly been restricted by the lost of the properties with increasing temperature and by manufacturing problems (expressly with respect to coating of big rolls). A strong development has however occurred during the last 10 years with respect to thermoplastics. In figure 1 a classification of actual thermoplastics have been presented generally. In the following table 1 there is a list according to ISO 1043-1 of abbreviations and names for some polymers. CA Cullulose-acetate CAB Cellulose acetate butyrate CN Cellulose nitrate CP Cellulose propionate EP Epoxy or epoxide MF Melamine formaldehyde PA Polyamide (quality is expressed with numbers) PAI Polyamide-imide PAN Polyacrylnitrile PB Polybutene-1 PBT Polybutene terephtalate PC Polycarbonate PCTFE Polychlorotrifluorethene PDAP Polydiallyl phthalate PE Polyethene PEI Polyether-imide PEK Polyetherketone PEEK+derivatives Polyetheretherketone PES Polyethersulfon PET Polyethenterephtalate PF Phenol formaldehyde PFA Perfluoroalcoxyalkane PI Poly-imide PIB Polyisobutene PMI Polymetakryl-imide PMMA Polymethylmethacrylate PMP Poly-4-methylpentene-1 POM Polyoxymethene or polyacetal PP Polypropene PPE Polyphenylether, earlier polyphenylen oxide PPO PPS Polyphenylen sulfide PS Polystyrene PSU Polysulfone PTEE Polytetrafluoroethene PUR Polyurethane PVC Polyvinyl chloride PVDC Polyvinyliden chloride PVDF Polyvinyliden fluoride PVF Polyvinylfluoride SI Silicon UF Ureaformaldehyde UP Unsaturated polyester The group of speciality plastics are especially interesting. Typical properties for plastics belonging to this group are good temperature resistances (260°C), good mechanical properties, the retaining of the properties even in high temperatures, in spite of high tensile strengths and good hardness properties, retained elasticity and a low impregnation of water. In table 2 there has been presented properties of the speciality plastic PEEKK as a function of the temperature. Temperature property -40°C 23°C 80°C 120°C 150°C 220°C Unit Tensile strength 129 108 76 56 49 - N/mm 2 Ultimate elongation 4 6 6,5 9 10 - % Tear strength 109 86 69 55 48 35 N/mm 2 Tear elongation 30 28 100 124 128 142 % Tensile-E-Modulus 4150 4000 3490 3340 3100 230 N/mm 2 Bending stress 131 120 107 91 84 8 N/mm 2 Bending-E-Modulus 3860 3640 3370 3120 3010 240 N/mm 2 Notch impact toughness (Charpy) 9 9 mJ/mm 2 The good properties of the speciality plastics in high temperatures are based on the substitution of the conventional aliphatic bond with an aromatic bond. The speciality plastics afford properties which are suitable for roll coatings for example in paper machines, board machines and paper refineries. They can be used either reinforced or not. The speciality plastics are however thermoplastics and their processing methods are typical for thermoplastics. Speciality plastics are available in granulates from which such fabricates as films, discs, tubes and bars are manufactured by injection moulding and extrusion. Thermoplastics are also available in powder form in which case possible manufacturing technics are dispersion spraying, electrostatic powder spraying, fluidized bed coating, flame spraying, plasma spraying and rotomolding. Filament winding and tape winding are typically suitable manufacturing technics for thermo setting plastics, but recently the use of these two technics have been more common also for thermoplastics. Thermoplastics and also speciality plastics can thus be achieved in powder form. Big rolls can be coated with plastic powder by: 1. Electrostatic spraying , but only relatively thin coatings. The porosity of the coatings is then high and in the case of speciality plastics the preheating and postheating temperatures of the roll body are high which is not advantageous with respect to the paper machine rolls (carton and paper ref.). 2. Fluidized bed coating , but as in the case of the electrostatic spraying, only thin coatings of a high porosity. The preheating/postheating temperatures of the roll bodies are high. Also manufacturing problems are associated with this method. 3. Dispersion spraying , in which technic the plastic powder is in the form of a dispersion in some suitable solvent. The dispersion is sprayed onto a surface of a body. The solvent evaporates/is evaporated away = > a very thin coating film is left on the surface of the working piece which often requires further temperature treating. Another possibility is to mix the plastic powder among some one-or two-component polymer. When the one- or two- component polymer reacts, a matrix is formed in which the plastic powder is left. 4. Rotormoulding technic , which is meant to coat interior surfaces, why it cannot be used for coating of outer surfaces of rolls. 5. Flame spraying , the problems of which is presented in the following. Only standard plastics (for example PE, EVA, PP) can in some extent be sprayed without preheating of the piece. These plastics do not however suit for technically requiring roll coatings. In connection with flame spraying with a speciality plastic, the working piece must be heated to a temperature as high as possible when thick coatings are wished. The temperature can however not exceed a given threshold in which the plastic bums. Also the roll construction can set a limit for the temperature. Working pieces with thin walls need a higher preheating temperature than compact pieces. It is especially difficult to flame spray pieces of different thicknesses. The plastic coating is sprayed in layers. The effect of the preheating decreases considerably after the first spraying layer. The piece has cooled down as the temperature has not been tried to keep. Even if the temperature would be tried to keep, the coating to be formed becomes an isolate when becoming thicker. Because of the differences in the cooling rates, the temperature differences have increased. The first plastic layer isolates the heat coming from the working piece which limits the coating thickness. In a too thick coating and in a plastic coating with lacking heat energy in the outer layer, the melt drops separate, whereat its construction becomes worse, the inner strength week and the crystallisation degree wrong. Similar difficulties appear also in connection with the conventional plasma spraying. In conventional plasma spraying the heat effect of the spraying is formed so that the electric energy forms an arc between the wolfram cathode and the annular copper anode. A gas or a gas mixture is led to the arc which is strongly heated up and the gas molecules are disintegrated to atoms and the atoms further to ions and electrons. The gas has converted to a plasma. Thus the electric energy has transmitted to the gas (to the plasma) and raised its inner energy. This inner energy is utilized in the melting of plastic powders so that the powder is fed to the out streaming plasma (figure 2) wherein it is plasticized. The plasma spray accelerates the melt drops with a high rate on the surface of the piece to be coated. The temperature of the plasma spray is very high; 7000 - 15000°C. Due to the high temperature the thermal radiation of the plasma is very high. There is obtained some advantages from this radiation energy in the melting of plastic powders as it increases the temperature of the working piece which is advantageous with respect to the polymerization and thus with respect to the forming of the coating. The drawback with the conventional plasma spraying is that the temperature of the plasma flame is too high with respect to the plastic, and the plastic tends to oxidize. Further disadvantages with the conventional plasma spray is the low flowing rate of the gas and that the heat effect of the flame is too low to keep the compact pieces warm. Generally the plastics of table 3 is sprayed with conventional plasma; in other words not speciality plastics. In this context mention is made of US-3,962,486, which discloses applying cured thermoset resinous coatings to substrates, in particular thin metal substrates, by using a plasma arc generated flame. Liquified resin particles have preferred velocities from about 152,4 m/s (500 ft/sec) to about 304,8 m/s (1000 ft/sec), but also excess of sonic velocities is mentioned (cf. column 6, lines 15,16). A COMPARISON OF USUAL POWDERY COAT TYPES OF COATINGS THERMO SETTING PLASTICS THERMO PLASTICS Epoxy Polyester urethane Polyester TGIC Hybride Acryl Nylon PVC Application/curing temperature °C 120-122 150-200 140-200 140-220 140-200 180-320 170-290 Thickness of the film <1-12 <1-3,0 <1-4,0 <1-4,0 <1-3,0 4-12 10-20 Hardness HB-5H HB-5H HB-5H H-2H 2H-5H Outer strength - + + - + + 0 Weather strength - + + - + + - QUV-strength + 0 0 - + 0 0 Solvent strength + 0 0 0 0 + - Chemical strength + + + + + + + Impact strength + + + + 0 + + The meanings of the signs: + Generally preferable/acceptable 0 Sometimes preferable/acceptable - Generally not preferable/acceptable The primary object of the invention is to prepare more resistant coatings having the desired property or properties on the same time. More in detail, the object of the invention is a method that overcomes the drawbacks of prior art so that a coating that is thick enough can be prepared also of speciality plastics. According to the method of the invention the coating is carried out by spraying by using hypersonic plasma with a velocity of about 2000 m/s or more, preferably 2000-3000 m/s. The preferable embodiments of the invention has the characteristics of the subclaims. The difference between the hypersonic plasma device (figures 3 and 4) and a conventional gas plasma apparatus affords some advantages which can be utilized in accordance with the invention in spraying plastic powders. Thus hypersonic plasma is used according to the invention in the spraying of powders of speciality plastics, whereat the high effect of the plasma device of for example figure 3 is utilized in its different forms (200 kW) (plasma flame, radiation heat, convection). The preheating temperature of the working piece is tried to keep so low that the coating plastic does not burn (depends on the plastic) but in spite of that thick layers of 200 µm - 100 µm can be sprayed. Even thick coatings can get the right crystallisation degree in the invention ,whereat optimal properties of the plastic are achieved even in thick coatings. The granule sizes of the powders to be sprayed are in the range of 20 µm - 1000 µm. The rolls to be coated can be variable crown compensated rolls, suction rolls, center rolls and rolls of super calenders and soft calenders. The melt particles of the hypersonic plasma spray produce coatings of good quality with high rate having a high density, good adhesion, a smooth and sprayed surface wherein very little disintegration occurs. The particles that are moving with an oversonic rate produce very dense and non-porous coatings, partly also in a non-melt state. A given procedure must be followed to produce a hypersonic plasma spray. Plasma sprays can in some extent be achieved with a high rate with a conventional spray by increasing the gas stream and by using a smaller diameter in the nozzle. However, if the rate of the plasma is increased, it should be noted that the retention time of the powder is shortened at the same time and the heat content shall also be increased to melt the powder. Then a higher effect must be used, mainly by increasing the arc flow, as a very high potential, over 100 V, cannot be achieved with a conventional plasma spray. Ca 80 kW is the threshold of the high effect to be used in a conventional plasma apparatus. Hypersonic plasma must be used for a higher effect. Very high gas streams (even 30 m 3 ) are used in high effect plasma sprays of the invention used in figure 3, whereat the rate of the out streaming gas increases up to 2000 m/s. The temperature of the plasma flame decreases to ca 6000°C due to the higher flow rate of the gas. Thus, as the exposure temperature and exposure time are lower, less damaging oxidation of the plastic particles occur in the high effect plasma spray than in an conventional plasma spray. Due to the higher gas flow rate, the cathode and the anode are at a bigger distance from each other, whereat the potential between the cathode and the anode increases to ca 300-450 volt (when it is in a conventional plasma spray is some 10 volts). Due to a higher potential, the heat energy of the flame can be increased up to 250 kW (when it in a conventional spray is some tens kW). This high heat energy can effectively be used to heat up massive pieces. The heat from the plasma flame radiates in all directions but the radiation can be lead onto the surface of the working piece by different cooled mirrors to be placed beyond and at the side of the flame in the same way as in the situation in which the light is reflected by a cup in lamps. Furthermore, the heat effect of the flame can be regulated by means of gases used so that the increase of the flowing rate can raise the heat effect. The heat effect can be further raised by use of hydrogen and helium. The heat effect can be decreased in a corresponding way by means of argon. In the method the body can be preheated, if desired, but this is not often so necessary or desirable. It is also possible to use a new plasma spraying system that uses atmospheric plasma to produce hypersonic plasma which has double anodes for example according to figure 4. The driving costs can be decreased with this system to less than 50% of those which are caused by conventional systems, even if conventionally used materials are in question. Thin films of materials with a high melting point can also be made, as ZrO 2 , with this system that sprays atmospheric plasma as with a conventional system that sprays plasma of low pressure. When it is question of cermet as WC-CU, a very abrasion resistant film can be made which is as good as that made with the above mentioned hypersonic plasma device. The double anodes of the device can be heated by effectively feeding the materials to be sprayed directly in the flame centre of the plasma arc and the spraying pattern can be made more narrow. Therefore the efficiency of the plasma spraying can be improved so that it is better than in conventional systems. Thus the invention can be used for preparing also thick coatings by using speciality plastics and so to achieve optimal properties for the coating. Especially the properties of the coating can be regulated in the thickness direction of the coating or in the direction of the roll axle. For example the elasticity modulus can be regulated by regulating the porosity of the coating between the layers. If a smaller elasticity modulus is wished the heat introduction is decreased. The module of elasticity of the coating can be regulated also in the direction of the roll axle, for example, in the ends of the roll there can be a different module of elasticity compared with the central region. The regulation possibilities of the heat introduction - preheating of the roll - regulation of the flame by regulation of the electric effect by regulation of the amount of the gas by regulation of gas proportions by reflection of the flame by using outer extra heaters (for example IR and induction) For example in the journal KONEPAJAMIES number 3, 1991 usable speciality plastics for the invention have been presented (see figure 1, page 2). For example the following kinds of rolls of board and paper machines and paper finishing machines are coated with a coating of the invention: guide rolls, suction rolls, press rolls, center rolls, cylinders, calender rolls, cutting machine rolls and so on. The usability of the method of the invention is improved in that coatings of the method of the preparation can be modified by commonly known methods of consolidation of engineering plastics for example a so-called Whiskers fibre reinforcing (the Whiskers fibre is a very little individual crystal fibre) or winding of a continuous fibre (Filament Winding). Especially the use of the filament winding method enables an effective raise of the peripherential strength of the coating which has special importance when the intention is to achieve higher nip loads. Further advantages of the method of the invention are that simultaneously with the speciality plastic for example metal, ceramic or cermet particles can be sprayed. Herewith the properties of the coating can be influed on for example, the abrasion strength. Then the feeding place of the particles in question to the plasma must be chosen so that they are coming to the right place on the basis of their melting temperature. The problem with the polymer materials is in some cases that the humidity tends to diffuse due to the thermal diffusion from the warmer roll surface to the colder body. This means that special requirements are set for the body with respect to the corrosion resistance. The roll body can be effectively taken care of with the method of the invention so that a metallic corrosion resistant layer is sprayed with the same spray as also the polymeric coating before the polymeric layer. In this respect a hypersonic spraying affords a superior advantage compared with conventional methods as the coating becomes very compact and corrosion resistant due to the high rate of the flame. Naturally some other layer, an epoxy adhesion layer, can be used as substrate layer. Coating materials of the invention have been presented in figure 1, page 2 and the thickness of the coating is preferably in the range of 200 µm - 10 mm. In the following the method of the invention is presented by means of figures which are not meant to restrict the invention. Figure 2 presents a conventional plasma spray. Figure 3 presents a function principle of a high effect plasma spray usable in the method of the invention. Figure 4 presents the principle of a spraying system that uses an atmospheric plasma to be used in the method of the invention which contains a double anode. In figure 2 that presents a conventional plasma spray, the feeding of the powder takes place at 1 and the feeding of the gas at position 2. The wolfram cathode is marked with the reference number 3 and the copper anode with the reference number 4. The part that has been marked with the reference number 5 is an intermediate isolation and number 6 are electrical and valve connections. The plasma spray comes out from position 7 and is sprayed in form of melt particles 8 over the substrate 9. The construction of the high effect plasma spray has been presented in figure 3. The arc is transferred from the electrode (-) far into the cylindrical nozzle ( + ), but the gas stream forces it to the centre of the nozzle and it proceeds out of the nozzle and returns to the surface of the output. When the arc extends over 125 mm it uses a very high potential 500 volt and produces an oversonic high energy plasma spray. An extended plasma arc is well parallellized and retains in a concentrated form to long distances from the nozzle. The theory of the extensive plasma arc is the following. The high stream 2' of the plasma arc, mainly nitrogen, is fed from the electrode through the gas distributer far to the cylindrical nozzle that makes a very strong vortex. A very high DC-potential, 600 volt, of the open circuit is used between the nozzle (-) and the electrode (+). The high frequency ignites the spray and the arc transfers from the electrode to the nozzle but a strong gas stream forces it to its centre and it extends far out from the nozzle and returns to its outer surface because there are no other passages. A very long arc, over 100 mm, raises the potential very high, up to 400 volt, and effectively heats the plasma gas to produce a very hot hypersonic plasma spray. As a very high potential is easily achieved for the arc with these sprays that produce a very extensive plasma arc, the stream of the arc can be set low to be able to use a very high effect in the spray. The hypersonic plasma device designed by Jim Browning consists of only five components which are a water-cooled electrode (-) with gas distribution holes, a water-cooled cylindrical nozzle ( + ) and an isolated space, a front frame for the spray and an isolated back frame. Cooling water is led in from position 11 and out from position 12. The plasma spray is marked with the reference number 7' and the extended arc with number 13 and the impact diamond with number 14. The plasma spray is very controlled and centred even at a long distance from the surface of the nozzle. The plasma spray, for example of wolfram carbide particles, proceeds straight more than one meter and is very concentrated at this distance. It looks like a plasma flame in low pressure. More than 70 % of the fed electric effect is given to the high gas stream and the rate of the plasma spray becomes oversonic at valves over 3000 m/sek and is observed through protection glasses with impact diamonds 14. A powder 1' is fed from the output of the nozzle directly to the very hot and extended arc. An addition of hydrogen to the plasma gas further raises the heat energy. Typically values of the energy used are electric effect 200 kW (400 V x 500 A) gas stream ca 230 SLM (500 SCFH) output enthalpy 35 x 10 6 J/kg /15.000 BTU/Lb) plasma temperature 6000°C spray rate 3000 m/sek For the details of the device reference is furthermore made to the article "Coatings by 250 kW Plasma Jet Spray System" T. MORISHITA, Plazjet Ltd, Tokyo, Japan. (Source: Proceedings of 2nd Plasma Tec. Symphosium, June 5-7, 1991, Vol. 1p-137). The construction of the device spraying atmospheric plasma that comprises a double anode is presented in figure 4. To stabilize the anode place of the arc the device is foreseen with one cathode jet 15 and two anode jets 16 so that the anode jets are symmetrically arranged as is presented in figure 4. The cathode place and the anode place are protected with inert gas as Ar 17 or N 2 . In this system the arc is not instable in any way which could lead to abrasion of the anode place or migration of the anode place or abrasion of the electrodes, whereas such an instability is a problem in conventional systems. Thus the spraying conditions can be retained stable for a long time. The accelerating nozzle 18 can be loosened and its diameter and length are set in forehand to be appropriate for the plasma spraying. In other words the rate and temperature of the plasma can be regulated by varying the diameter length and effect. This nozzle corresponds to the wearing part of conventional jets. But it does not touch the arc directly and generally there is no need to change it. As is presented in figure 4, the plasma arc 19 consists of a cathode arc on the axle of the cathode jet and anode arc on the axle of the anode jet. A strong cold housing is formed around each arc flame and it increases the direction of the arc and the concentration of the heat. Such a stable condition is retained even if the main arc exceeds the sonic speed. The plasma gas that forms the main arc is fed from a place outside the chamber wherein the cathode is protected with inert gas 17 as is presented in figure 4 and with air 20. The rate and enthalpy of the plasma gas can as a result of this be extensively regulated with the effect of 10-100 kW. The plasma spray produced is presented with the reference number 7'' that is sprayed as particles 8'' on a substrate 9'' and coating 21. The device is preferably also foreseen with a plasma cleaning device 22 to maintain a good quality. The effect is fed in from place 1''. The direct current circuits of the device have also been marked in the figure (D.C.). The main feed of the effect takes place in a bigger circuit. For the part of the device reference is furthermore made to the article A. BUNYA etc. "New Plasma Spraying System Twin Torch α" (Source NTSC 91/Pittsburg).

Claims: Method of coating a roll in a paper machine with powder of thermoplastic speciality plastic, wherein the coating is carried out by spraying by using hypersonic plasma with a velocity of 2000 m/s or more, preferably 2000-3000 m/s. Method of claim 1 characterized in that the heat energy of the plasma flame is 50-250 kW. Method of claim 1 or 2 characterized in that there is used hypersonic plasma with a heat energy of ca 200 kW. Method of claim 1 or 2 characterized in that atmospheric hypersonic plasma is used with a heat energy of ca 100 kW. Method of any of claim 1-4 characterized in that an amorphous or crystalline component of speciality plastic is used in the powder to be sprayed. Method of claim 5 characterized in that the plastic component is any of the following speciality plastics: polyamide-imide PAI, polyether-imide PEI, polyetherketone PEK, polyetheretherketone PEEK, polyethersulphone PES, polyimide PI, polymethacryl-imide PMI, polyphenylensulfide PPS, polysulphone PSU. Method of any of claims 1-6 characterized in that the preheating temperature of the working piece is 20°C-300°C. Method of any of claims 1-7 characterized in that the particle size of the powder to be sprayed is 20 µm - 100 µm. Method of any of claims 1-8 characterized in that the coating is sprayed to a thickness of 200 µm - 10 mm. Roll made with a method of any of claims 1-9 characterized in that its coating is made of powder of thermoplastic speciality plastics. Roll according to claim 10 characterized in that the coating consists of some of the following plastics: polyamide-imide PAI, polyether-imide PEI, polyetherketone PEK, polyetheretherketone PEEK, polyethersulphone PES, polyimide PI, polymethacrylimide PMI, polyphenylensulfide PPS, polysphone PSU. Roll of claim 10 characterized in that the thickness of the coating is 200 µm - 10 mm. Roll of claim 10 or 11 characterized in that the crystallization degree of the coating is 0-100 %. Roll of any of claims 10-13 characterized in that it is a variable crown roll, suction roll, center roll or a roll in a super or soft calender.

Inventors: LEHTONEN PENTTI, LEHTONEN, PENTTI
Applicants: VALMET CORP, VALMET CORPORATION
Application year: 1993
Main classification: B05D1
IPC codes: D21F3, B05D1, D21G1