Method for sealing of a wearing face
The invention relates to a method for sealing a wearing surface in a pulp/paper/ board/finishing machine and/or for repairing damage in such a surface, as well as to the use of a UV curable sealing agent for sealing and/or for repairing wearing surfaces and, further, to articles manufactured by way of the method. Specific applications of the method are rolls surfaced with a ceramic/cermet or polymer coating. Further objects of the invention relate to ceram coated dewatering blades and doctor blades, among others.
In a paper and board machine, rolls with ceramic and cermet coatings have by large replaced granite rolls used earlier in the art. With a roll with a ceramic or cermet coating, corrosion of various degrees occurs, particularly under harsh conditions causing corroding of the roll, delamination of the coating and thus the roll gradually wears unusable, further occurs mechanical wear and damage due to different reasons. A roll coating made by way of thermal spraying unavoidably always remains somewhat porous, thus allowing moisture penetration through the porous surface toward the inner metallic layers of the roll thus causing corrosion. 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 process environment in a more corrosive direction.
Various methods have been attempted to improve the corrosion resistance of rolls. As examples of these, reference can be made to FI Patent Nos. 86,566, 82,094 and 84,506 teaching the improvement of corrosion resistance in rolls by way of adding metal or cermet layers between the ceramic/cermet roll surface coating and the roll mantle. From FI Patent Application No. 971,541 is known the application of an impervious intermediary layer deposited by high-speed flame spraying under a
porous ceramic surface coating. Attempts have also been made to seal the outer surface of ceramic coatings. In WO publication No. 97/15719, a technique is disclosed for sealing a ceramic coating by means of an inorganic solution, in which technique, in order to reduce the porosity of the ceramic layer, an inorganic compound is applied to the surface of the roll after surfacing the same. In EP Patent Application No. 0,481,321, a roll for a paper machine is described having a synthetic resin or wax applied to the ceramic coating in order to seal its pores, wherein conventional heat-curable epoxy resins are mentioned as suitable synthetic resins. Damage caused by mechanical stresses and wear are repaired most typically with the help of a heat-curable epoxy resin, whereby the paper machine must be shut down for at least 1 to 3 days in order to allow the repaired or sealed area dry sufficiently. Attempts have been made to speed up the curing process by infrared radiation, but nevertheless the method still is slow. It is obvious from the above-discussed background that there is an imminent need for finding a quick and easy-to-use technique for sealing the surfaces of rolls used in a paper and board machine and for repairing damage and wear occurring in the same.
The object of the present invention is to provide a method for sealing surfaces subject to wear, particularly those of a roll in a paper or board machine such as a press roll or calender roll or ceramic-surfaced dewatering blades, and for repairing surface damage in the same and, further, to disclose the use of a UN curable sealing agent in the sealing and repair of surfaces subject to wear, particularly those of a roll in a paper or board machine, and components made by virtue of said method for a paper machine.
The method, use and components according to the invention are characterized by what is stated in the claims.
The shortcomings of conventional methods known in the art for sealing and repairing of surface coatings in paper or board machines can be avoided or decisively reduced by virtue of the method described in the following. It has now been found that sealing and repair of rolls can be made faster and easier than in the prior art by means
of using a sealing and repair agent that is curable by electromagnetic radiation. In the method according to the invention, wearing surfaces that are in contact with a paper web and/or pulp and/or a moving fabric are treated by applying on the surface to be treated a synthetic polymer compound curable by electromagnetic radiation, advantageously UV radiation.
Suitable synthetic polymer resins curable by radiation are epoxy acrylates, polyester acrylates, urethane acrylates, silicone acrylates, polyether acrylates and acrylate esters and mixtures thereof having a reactive thinner, a photoinitiator and a secondary curing catalyst added thereto. Advantageous polymer resins curable by radiation are urethane acrylates and acrylate esters. When a polymer resin such as urethane acrylate is cured using UV light, an exothermic reaction is triggered in which the primary curing is launched by the UV light but the hardening process is continued as a secondary, heat-supported curing reaction taking place via a catalytic or anaerobic curing mechanism that secures curing deeper at the bottom of the pores. Polymers suitable for use as sealing compounds are characterized by both a primary UV-catalyzed curing mechanism and a secondary heat-supported catalytic curing mechanism or, alternatively, an anaerobic curing mechanism that needs the absence of oxygen. An organic polymer resin may also be blended with thixotropic agents and fillers. Suitable thixotropic agents are hydrophobic or hydrophilic compounds such as silicon dioxide, whereby the particle size of the thixotropic agent is advantageously in the range of 10 - 2000 nm. Suitable filler materials are silicon dioxide, ceramic fillers such as aluminium oxide and chromium oxide, metals, polymeric filler particles and fibers. The filler is advantageously in the form of a pulverized or microspherical or fibrous material and, advantageously, composed of the same material as that used in the original coating of the surface to be sealed or repaired. The particle size of the filler material is advantageously in the range of 2 - 45 μm. Sealing a surface does not necessarily require the use of a filler or thixotropic agent. The repair of worn or damaged areas is advantageously made using a composition comprising not more than 75 vol.-% of a filler, not more than 10 vol.-% and, advantageously, 1 - 5 % of a thixotropic agent such as silicon dioxide, and not less than 15 vol.-% and advantageously 50 - 95 vol-% of an organic
polymer such as urethane acrylate or acrylate ester.
The sealing or repair agent is applied to the desired surface area by spreading, dipping, spraying or using some other mechanical means depending on the point of use. When the surface of a roll is being sealed, the filler may also be applied to the roll surface separately after the radiation-curable organic polymer is first applied thereto, whereby the filler covering the roll surface can improve the wear resistance of the surface. The sealing or repair agent is cured on a porous coating by means of electromagnetic radiation, advantageously using UV light emitted by conventional UV lamps at a suitable wavelength, whereby the radiation intensity maxima advantageously fall at wavelength bands about 270 nm and 365 nm. The maxima of radiation intensity needed are determined by the type of photoinitiator used. The electromagnetic radiation is imposed at an intensity of 1 W/cm2 - 100,000 W/cm2 and the power output of the UV lamp is advantageously in the range of 1 - 10,000 W/cm2. The radiation is allowed to irradiate the desired surface from 0.1 s to 10 h when the surface is to be repaired or sealed. Typically, the curing time is less than one hour and most typically 5 to 10 minutes. Curing can be carried out either by irradiating the surface only once after the entire amount of the sealing or repair agent has been applied or, alternatively, by alternate steps of applying a layer of the sealing or repair agent at the desired area, curing the same and then applying a new layer and curing the same again.
If the surface to be treated is pure from metal ions, it is advantageous to pretreat the surface with a solution containing metal ions, advantageously using copper chloride solution in which the copper chloride is dissolved in an organic solvent such as hexane, whereby the applied copper ions act as activator. The use of an activator is particularly advantageous in the case that the application area is totally free from metal ions or the compound contains a lot of filler that hinders the penetration of UV light through the material.
The sealing and repair method according to the invention can be exploited in paper and board machines on their rolls, dewatering blades, doctor blades, ceram-coated
blades and, generally, on wear-resistant surfaces coming into contact with a paper web or pulp. Advantageously, the invention is used on rolls having a ceramic or cermet coating. The invention is also applicable to polymer-coated rolls, especially for repairing local damages of their outer coverings. The technique according to the invention can be utilized for improving the corrosion resistance of the combination of a surface coating and underlayer and the mechanical qualities of the coating. As compared to heat-catalyzed sealing materials such as conventional epoxy sealant systems, the UV-curable sealing agent excels particularly by its low curing temperature, fast curing to a high degree of cross-linking, low exothermic heat release during curing, small curing shrinkage, low viscosity at room temperature, low surface tension and low partial vapour pressure at room temperature.
By virtue of the method according to the invention, it is possible to produce quickly on a desired surface, such as that of a roll, a protective surface layer of good corrosion resistance and to seal the pores of a surface, especially those of a ceramic surface, in an efficient way. Simultaneously, it is possible to affect the surface qualities of a roll by way of selecting a suitable polymer and, optionally, a suitable filler.
In conjunction with rolls and dewatering blades, a particular benefit is that entire workpiece need not be taken to an elevated temperature, which generally is awkward to arrange and costly.
In repair operations, the speed of the method is a particular advantage, because the repair can be carried out rapidly and without dismounting a roll from the machine, thus achieving significant savings through lower shutdown costs.