METHOD FOR SEALING OF A COATING ON A ROLL

Abstract: The invention relates to a method for sealing the coating and particularly an intermediate layer of a roll, particularly of a press roll such as the centre roll of a press or the back-up roll of an extended-nip press roll, used in a pulp, paper, board or finishing machine, and, further, to a roll and a coating for a roll. The corrosion resistance and impermeability of the roll coating, particularly of such a coating that is made by thermal spraying, can be improved by way of sealing. The coating comprises a porous outermost surface layer (5) made out of a ceramic or ceramic-metal material by thermal spraying and at least one sealed layer (I, II) situated between said outermost surface layer (5) and the roll mantle (1).

Description: Method for sealing of a coating on a roll The invention relates to a method for sealing the coating of a roll and particularly an intermediate layer thereof, particularly of a press roll such as the centre roll of a press or the back-up roll of an extended-nip press roll, or of a calender roll used in a pulp, paper, board or finishing machine, and, further, to a roll and a coating for a roll. The coating of a roll, particularly a thermally sprayed coating, can be improved in terms of its corrosion resistance and impermeability by virtue of applying a sealing to the coating. In paper and board machines, rolls with ceramic and cermet coatings have by large replaced granite rolls used earlier in the art. With rolls with ceramic or cermet coatings, corrosion of various degrees occurs, in particular in more severe conditions, causing corroding of the roll, delamination of the coating and wear of the roll so that it gradually becomes unusable. A coating made on a roll by way of thermal spraying always remains somewhat porous, thus allowing gradual penetration of moisture through the porous surface toward the inner metallic layers thus causing corrosion and so the protection offered by the coating against corrosion is insufficient. Process conditions in the paper and board industry have continually become more and more severe and demanding on the equipment, among other reasons, due to the trend toward enhanced circulation of process water for environmental protection and change of the process environment in a more corrosive direction. There have been attempts to improve the corrosion resistance of rolls, e.g., by applying metal or cermet layers between the ceramic or cermet coating and the roll mantle. Such methods are described in FI Patent Nos. 86,566, 82,094 and 84,506. From FI Patent Application No. 971,541 is known the application of an impervious intermediary layer under the porous ceramic outer layer, whereby impermeability of the intermediate layer is attained by way of high-speed flame spraying. Attempts have also been made to seal the outer surface of a ceramic coating. In WO publication No. 97/15719, a technique is disclosed for sealing a ceramic coating by means of an inorganic solution. Therein, a separate adherence-improving layer is made underneath the ceramic coating layer, and the ceramic coating layer itself is formed by a single porous layer, reduction in the porosity of which has been sought by way of applying an inorganic solution on the roll surface after the coating step. In EP Patent Application No. 0,481,321, a roll for a paper machine is described having a synthetic resin or wax applied onto the ceramic coating in order to seal its pores. Conventional epoxy resins are mentioned as an example of suitable synthetic resins. In FI Patent No. 90,098, a method for coating a roll is described, in which noncontiguous areas of polymer are applied to the roll surface, whereby the ceramic coating of the roll outer surface is impregnated only by a partial length of the roll. Accordingly, all cited publications aim at sealing the pores of the outer surface of the ceramic coating by means of a sealing agent thus preventing corrosive substances from penetrating under the coating. The latter type of sealing systems performed according to the prior art are problematic in that the material used as the sealing agent changes the properties of the roll surface and, on the other hand, the sealing agent is rapidly lost from the surface thus again changing the surface properties. This makes the control of the runnability of the paper machine more difficult and increases the risk of web breaks. Obviously, there is an imminent need for an impervious roll coating and a method for sealing the surface of a roll so that corrosion can be prevented and, on the other hand, the original desirable properties of the ceramic coating can be kept unchanged. The object of the present invention is to provide an impervious coating for a roll used in a paper or board machine and a method for sealing the coating of a roll and, further, a roll having an impervious surface. The coating, method and roll according to the invention are characterized by what is stated in the claims. The shortcomings of conventional methods known in the art for sealing surface coatings in paper or board machines can be avoided or decisively reduced by virtue of the method described in the following . The invention is based on intermediate sealing of a thermally sprayed coating such as a ceramic coating, whereby the sealing step is carried out prior to the application of the outermost surface layer. Sealing of the coating is performed so that, during the surfacing of a roll, the application of the coating is stopped when a given thickness of coating has been reached and subsequently the sealing treatment is carried out, or alternatively the coating application may be continued and a plurality of such sealing treatments can be carried out. After the last sealing treatment, the coating process is continued up to the final coating thickness. Herein, the sealing agent remains entrapped under the final ceramic coating layer proper, which acts as the functional coating, and thus prevents substances corrosive to the roll frame from reaching the metallic inner part of the roll. The sealing of the intermediate layers may also be performed on a metallic adhesion or corrosion protection layer applied to the outer surface of the metallic roll or, alternatively, to a selected area thereof, and/or said adhesion or corrosion protection layer is next covered with a ceramic layer of a given thickness that is sealed to form an intermediate layer in the above-described manner, whereupon the ceramic coating can be prepared by spraying to its final thickness. After each intermediate layer sealing step, the surface is subjected to an adhesion- improving complementary treatment prior to the spraying of the next coating layer. In the following, the invention is elucidated with the help of appended FIG. 1 which shows an embodiment of an impervious roll coating according to the invention, wherein the illustrated details are not limiting to the invention. FIG. 1 shows a coating consisting of several layers. On the outer surface of a roll mantle, an adhesion/corrosion protection layer 2 has been applied out of a metal or metal alloy. The layer 2 has been sealed by means of a sealing agent. The sealed layer is designated by reference symbol I. Subsequent to this first intermediate layer sealing, at least one ceramic layer 3 and 4 has been applied on the layer I. In the figure, the layer 4 is sealed (second sealed intermediate layer) and the sealed layer is designated by reference symbol II. On the layer II has been applied by spraying a ceramic layer 5 up to the desired final thickness. It should be noted that the number of sealed layers may be varied in the coating system, whereby sealed layers may be interleaved with unsealed layers, but advantageously all the layers are made sealed. It is essential that the outermost layer 5 is free from sealing agent. The ceramic layers 3-5 may comprise either similar or different ceramic material and, respectively, the sealed intermediate layers I and II may comprise either similar or different sealing agent. The thicknesses of the individual layers may be varied within comparatively large limits, a typical layer thickness being in the range of 50- 500 μm. Particularly suitable for use as an intermediate layer sealing agent are water-soluble compounds such as water-soluble polymer resins, of which preferred ones are acrylate esters, water-based epoxy resin systems and UV-curable acrylate esters and urethane acrylates as well as inorganic sealing agents, advantageously aluminium orthophosphate, or mixtures of the above-mentioned compounds. Also other synthetic organic polymer resins such as epoxy resins, phenolic resins, polyurethane resins and silicone resins, as well as fluorine-containing resins that are curable by conventional techniques may be used as sealing agents. Advantageous in use as a sealing agent are particularly water-soluble acrylate esters and UN-curable acrylate esters. Essential to the spirit of the invention is that the sealing agents used are featured by a low viscosity, low surface tension, a suitable hardening mechanism for the intended application, small curing shrinkage and sufficiently good corrosion protection capability. The sealing agent is applied at a desired step of the coating process to the surface by spreading, spraying or some other conventional means suitable for impregnating the surface layer. The sealing agent penetrates the coating layer pores up to a depth determined by the pore structure of the layer (which is affected by spraying velocity and material composition plus particle size) and the technique of sealing agent application. The layer to be sealed can be impregnated with the sealing agent down to the entire radial depth of the layer or by a partial depth of the layer. Excess sealing agent remaining on the layer surface can be removed, e.g., by wiping prior to launching the next process step. The sealing agent is cured if so required before the onset of the next step. To ensure sufficient adhesion after the sealing step, the treated surface is first subjected to a suitable further treatment and the sealed layer is cleaned free from organic and mechanical impurities before the coating process is continued. Advantageous further treatment techniques are flame treatment, mild shot blasting, solvent treatment, washing, abrasive grinding, carbon dioxide blasting, steaming and combinations thereof carried out prior to the spraying of the next or final coating layer. As a result of such a treatment, the metal or ceramic phase of the sealed surface can be sufficiently purged/activated for adhering the next sprayed coating layer. The most important benefits of the intermediate layer sealing method according to the invention and of the coating with the sealed intermediate layers, as well as of the roll made using the method according to the invention are a good impermeability and corrosion resistance of the coating. The sealing of the intermediate layers does not detrimentally affect the grinding of the coating or plug a grinding wheel as frequently occurs with rolls having their ceramic surface sealed with resins or waxes. Rolls having a surface coating made with sealed intermediate layers do not impair the runnability of a paper machine in regard to, e.g., detaching of the web, whereby the controlled porosity of the ceramic surface layer of the roll coating is an essentially contributing factor. Sealing the roll surface by its intermediate layer or layers leaves the roll outer surface intact, whereby sealing does not alter its properties as is the case with sealing the roll surface by waxes or resins.

Claims: Claims 1. Coating for a mantle (1) of a roll used in a pulp, paper, board or finishing machine, comprising a ceramic or cermet material, characterized in that the coating comprises an outer porous surface layer (5), made by thermal spraying out of a ceramic or cermet material, and at least one sealed layer (1,11) situated intermediate to the outermost surface layer (5) and the roll mantle (1). 2. Coating according to claim 1, characterized in that the sealed layer (1,11) is a thermally sprayed ceramic or metallic or ceramic-metal material layer having its pores sealed with an organic or inorganic sealing agent or a mixture thereof. 3. Coating according to claim 1 or 2, characterized in that the sealed layer (1,11) is situated immediately under the porous outermost surface layer (5). 4. Coating according to any one of claims 1 - 3, characterized in that the sealed layer (1,11) is the first layer deposited on the surface of the roll mantle (1). 5. Coating according to any one of claims 1 - 4, characterized in that there are a number of the sealed layers (1,11) in a superimposed succession or alternately with unsealed layers (3,5). 6. Coating according to any one of claims 1 - 5, characterized in that each layer of the coating with the exception of the outermost surface layer (5) is sealed. 7. Coating according to any one of claims 1 - 6, characterized in that there are several sealed layers (1,11) and different sealing agents have been used in them. 8. Coating according to any one of claims 1 - 7, characterized in that the organic sealing agent is an organic compound, advantageously a water-soluble synthetic polymer such as an acrylate ester or a water-based epoxy resin system or a UV- curable synthetic resin such as an acrylate ester or urethane acrylate. 9. Coating according to any one of claims 1 - 8, characterized in that the inorganic sealing agent is an aluminium-containing compound, advantageously aluminium orthophosphate. 10. Method for producing a coating comprising a ceramic or cermet material on a roll used in a pulp, paper, board or finishing machine, characterized in that on the roll mantle is made at least one porous layer, the pores of which are sealed with an organic or inorganic sealing agent or a mixture thereof, and on the thus sealed layer is deposited an outer surface layer of a ceramic or cermet material by thermal spraying. 11. Method according to claim 10, characterized in that, during the coating process, after a given layer thickness is attained, the coating process is intermitted, the sealing agent is applied to the surface in one or more treatment step and, optionally, the coating/sealing sequences are repeated at least once, whereupon the coating process is continued up to the desired final thickness of the coating. 12. Method according to claim 10 or 11, characterized in that after the sealing step the sealed surface is subjected to an adhesion-improving treatment such as a flame treatment, shot blasting, solvent treatment, washing, abrasive grinding, carbon dioxide blasting, steaming or any combinations thereof, whereupon the coating process is continued. 13. Method according to any one of claims 10-12, characterized in that the layer to be sealed is a ceramic or metallic or ceramic-metal material layer. 14. Method according to any one of claims 10-13, characterized in that each layer of the coating is sealed with the exception of the outermost surface layer. 15. Method according to any one of claims 10-14, characterized in that the sealing agent is an organic polymer which is cured prior to starting the next process step. 16. Ceramic-coated roll of a paper/board or finishing machine having a coating sealed by an organic or inorganic sealing agent, characterized in that the sealed layer is situated radially interleaved in the coating between its outermost surface layer and the roll mantle. 17. Roll according to claim 16, characterized in that said roll is a press roll, advantageously a centre roll thereof, or a back-up roll of an extended-nip press roll. 18. Roll according to claim 16 or 17, characterized in that said roll is a calender roll.

Inventors: HELLMAN BJARNE, LEHTONEN PENTTI, NIEMI KARI, WAHLROOS JUHA, HELLMAN, BJARNE, LEHTONEN, PENTTI, NIEMI, KARI, WAHLROOS, JUHA
Applicants: HELLMAN BJARNE, LEHTONEN PENTTI, NIEMI KARI, VALMET CORP, WAHLROOS JUHA, HELLMAN, BJARNE, LEHTONEN, PENTTI, NIEMI, KARI, VALMET CORPORATION, WAHLROOS, JUHA
Application year: 2000
Main classification: F16C13
IPC codes: D21F3, D21G1, F16C13