WO2008086896A1

WO2008086896A1 - Paper machine roll with ceramic cover

Info

Publication number: WO2008086896A1
Application number: PCT/EP2007/050545
Authority: WO
Inventors: Kari Niemi; Ville Eronen; Samppa Ahmaniemi
Original assignee: Metso Paper, Inc.
Priority date: 2007-01-19
Filing date: 2007-01-19
Publication date: 2008-07-24

Abstract

The present invention is concerned with a roll of a papermaking machine, which has an outermost surface layer which is made of ceramic and has an average surface roughness Ra of 0.01 to 0.30 µm. A method for producing the roll comprises the steps of forming an outermost surface layer made of ceramic on a roll support and finishing the outermost surface layer to an average surface roughness Ra of 0.01 to 0.30 µm.

Description

PAPER MACHINE ROLL WITH CERAMIC COVER

Description

Field of the Invention

The present invention is directed to a roll which is especially used in the field of paper manufacturing and processing as well as to a method for producing such a roll. The roll of the present invention is especially suitable as a calender roll, particularly as a calender roll in a paper production line.

Background of the invention

In paper manufacturing, different types of rolls are used, including calender rolls by which the treated paper is given particular characteristics. The covers used for such rolls, which actually come into contact with the paper, are typically ceramic-metallic (cermet) covers, for example made of WC/CoCr.

Due to their manufacturing process, such covers usually require a grinding finishing so as to achieve the desired surface characteristics. The grinding of the present covers is time-consuming and expensive. Further, on the micro- scale, the structure of the cermet covers is non-homogenic and consists of carbides and a metal matrix binding them together. It has been discovered that the gloss giving properties of the cover deteriorate very quickly as a consequence of even the smallest wear. In this course, the matrix metal between the carbides wears off and leaves a very irregular surface leading to unsatisfactory results in the paper treatment. Another problem encountered in the art is that the thicknesses of the cermet covers cannot be arbitrarily increased from those used at present because of internal stresses so that the risk of crack formation becomes significant .

Several rolls for meeting all requirements of a paper manufacturing process have been proposed so far. For example, U.S. 4,951,392 describes the use of a particular powder mixture for coating the surface of a press roll in order to control the detachment of adhering paper from the press roll. A similar objective is addressed in U.S. 5,167,068 in which a coating of the roll described therein has an outer face formed of carbide-rich areas and of matrix areas formed between the carbide-rich areas. This coating also allows a controlled detachment of adhering webs like paper. In U.S. 5,235,747, a roll construction consisting of a roll mantle, an intermediate layer made of stainless steel and an outer layer formed of a particle mixture comprising metal and ceramic phases is disclosed. The described construction serves to achieve a suitable wear protection and detachment properties. Further, manufacturing processes for rolls to be used in paper treatment are described in WO 01/21850, WO 91/13204, EP-A-O 924 337, WO 97/15719 and EP-A-O 870 867. However, it is desirable that some of the properties of the rolls known in the prior art are further improved and that a better balance between the different characteristics of these rolls is achieved so as to simultaneously meet all requirements on an improved level.

In fact, there is an ongoing demand for a further improvement in the different properties of the used rolls in the art, especially with regard to the gloss imparted to a treated paper, cost efficiency of the roll manufacture, cleanliness of the cover as well as resistance against high temperatures and thermal shocks.

Summary of the invention

In view of the foregoing circumstances, it is an object of the present invention to provide an improved roll of a papermaking machine, especially an improved calender roll. It is a further object of the present invention to provide an improved method for producing such a roll.

Specific objects to be achieved by the present invention are inter alia that the gloss giving properties of a calender roll should be better retained in a calendering process .

Further, the total costs of manufacturing the cover should be less than for ceramic-metal covers.

Also, the cleanliness of the cover should be better.

Another object preferably achieved is that the cover should better resist high temperatures and thermal shocks.

In order to achieve the foregoing object, the present invention provides a roll of a papermaking machine, which has an outermost surface layer which is made of ceramic. The surface layer further has an average surface roughness Ra of 0.01 to 0.30 μm and preferably of 0.05 to 0.15 μm.

As regards the composition and the structure of the outermost surface layer, it is preferable that the outermost surface layer is homogenous in this regard. - A -

In a preferred embodiment, the ceramic of the outermost surface layer of the aforementioned roll includes an oxidic portion .

More specifically, it is preferred that the ceramic includes at least one selected from chromium oxide, aluminium oxide, titanium oxide, zirconium oxide and a mixture thereof.

Additionally, the ceramic may further includes tungsten carbide .

As regards the thickness of the outermost surface layer, a thickness within the range of 0.01 to 2 mm is preferable.

Further, it is also preferable that the ceramic is obtainable by thermal spraying, especially by a thermal spraying which is selected from plasma spraying, flame spraying and high velocity flame spraying.

The above roll of a papermaking machine is preferably a calender roll.

The present invention further provides a method for producing a roll as defined above, comprising the steps of forming an outermost surface layer made of ceramic on a roll support, and finishing the outermost surface layer to an average surface roughness Ra of 0.01 to 0.30 μm.

In the inventive method, it is preferable that the outermost surface layer is formed by thermal spraying, especially by a thermal spraying method which is selected from plasma spraying, flame spraying and high velocity flame spraying. Brief description of the drawing

Figure 1 illustrates how the average surface roughness Ra is defined.

Figure 2 shows the results of a hazing test according to one embodiment of the present invention.

Detailed description of the invention

The roll of a papermaking machine of the present invention has an outermost surface layer made of ceramic and having an average surface roughness Ra of 0.01 to 0.30 μm.

The roll according to the present invention leads to several improvements. Particularly, the surface layer having the afore-mentioned features enhances the gloss generating properties, i.e. the gloss imparted to a treated material, for example a web and especially paper, is improved by this surface layer. Also, this effect is retained longer in a calendaring process so that the need for service (e.g. a grinding process as a maintenance measure) of the roll surface is reduced.

Further, the cover of the present invention (i.e. the outermost surface layer) is less expensive to manufacture and can be made thicker when needed (e.g. if more allowance for grinding is desired) .

Additionally, the resistances against heat, thermal shock and also corrosive environments are improved over those of a comparable cermet cover.

In comparison to cermet covers or rougher covers (those having an Ra of larger than 0.30 μm) , the cleanliness of the cover is better and the cover can be also used in calendering mat grades, when needed, by adjusting the surface roughness within the ranges defined in the present invention .

In the present invention, a ceramic is defined as an inorganic, nonmetallic material which is suitable to be processed by thermal spraying methods.

The average surface roughness Ra is a standard measure for the surface roughness of a substrate and is well-known to the person skilled in the art. Actually, Ra defines an arithmetical mean deviation, namely the average roughness or deviation of all points z (x) from a plane fit to the surface of the tested specimen. The points z (x) can either refer to profile data or to areal data. The average surface roughness Ra is calculated according to the following equation :

Figure 1 explains the average surface roughness Ra as the mean deviation from the center line, i.e. from the center line which corresponds to the plane fit to the surface of the tested specimen.

According to the present invention, the ceramic cover for the roll, that is the outermost surface layer of the roll, is made of ceramic, more preferably of full-ceramic, wherein full-ceramic means that the ceramic portion is made of at least 95 wt . % ceramic, preferably 98-100 wt . % ceramic. Thus, in contrast to e.g. a cermet cover (a cover made of a ceramic-metal-composite) , the outermost surface layer wears uniformly which enables a better retention of the gloss properties which are imparted by the roll when a material is treated therewith. Specifically, the uniform wearing allows to constantly maintain the gloss properties of a paper calendered with the inventive roll on an improved level even during long-term use. Also, the need for grinding as a finishing treatment in the manufacture of the roll and in its maintenance, respectively, is reduced, as well.

Additionally, the very low average surface roughness Ra of 0.01 to 0.30 μm of the outermost roll surface, i.e. of the surface finishing, leads to an improved gloss which is imparted to the material which is processed by the roll, especially the gloss which is imparted to paper. These effects become even more prominent when the average surface roughness Ra of the outermost roll surface is in the range of 0.05 to 0.15 μm. Also, such an average surface roughness of 0.05 to 0.15 μm is readily and reliably achieved during the manufacture of the roll, thereby also improving the cost efficiency.

In order to further enhance the gloss-imparting properties of the roll of the present invention, it is preferred that the roll has a gloss-retention ratio of at least 85% (of the original gloss) and more preferably 90% after 1000 seconds. For the same reason, the gloss-retention ratio is preferably at least 80% and more preferably at least 85% after 2000 seconds and/or after 3000 seconds and/or after 3500 seconds.

The gloss-retention ratio can be measured in a so-called hazing test. The test is conducted with a common laboratory polisher, in the present case an "Abramin" by Struers. The polishing machine is equipped with an aluminium specimen holder which can hold six cylindrical specimens of 30 mm in diameter at the time. The specimen holder is equipped with pneumatic force control that makes it possible to adjust the force with which the specimens are pressed against the polishing cloth. The machine has a 0300mm anticlockwise rotating wheel for attaching grinding papers or polishing cloths. The specimen holder is also rotating anticlockwise. Testing is made with an abrasive slurry on a common polishing cloth, in the present invention a Buehler Microcloth 12" diameter cloth with an adhesive backing. A 20 wt .% water-based TiC^-slurry is used as an abrasive medium. Tiθ2 has a spherical morphology and a particle size of 0.3 μm (micrometers). Other abrasives, such as CaCU3, kaolin or talc can also be used.

Specimens for the hazing test are made by cutting a 4 mm thick piece from the coated end of a cylindrical specimen with a diameter of 25 mm and a height of 25 mm. These pieces are then mounted in polymer resin using a Struers hot mounting press. Mounted specimens are plane ground, fine ground and polished according to the data in the following Table 1.

Table 1. Specimen grinding and polishing parameters

In the following test procedure, the polished specimens are first inspected with a scanning electron microscope (SEM) to measure reference surface images before the hazing test. Subsequently, surface as-finished gloss is measured with an Erichsen Picogloss Model 560 glossmeter. A total of six specular gloss readings are measured for each specimen with the glossmeter in different directions. These starting values are written to sample inspection reports for each specimen .

The specimens are placed into the specimen holder of Struers Abramin polisher. The polisher disc and the specimen holder head are set to a rotating speed of 150 rpm. The abrasive Tiθ2 slurry is fed from a dispenser bottle to the polishing cloth wherein the slurry feed rate is controlled so that fresh abrasive is fed constantly on the cloth.

The test is divided into cycles of different lengths and stopped at fixed time intervals to examine the specimen characteristics with a glossmeter. The surface gloss is measured at these points of time, namely at 5, 15, 30, 60, 120, 180, 300, 600, 900, 1800, 2700 and 3600 seconds from the start of the test. The pressing force for six specimens is 125 N, i.e. approximately 21 N per sample. Before each cycle the polishing cloth is saturated with abrasive slurry. A final examination of the worn specimens is made by SEM observation after the test.

Based on the results as e.g. obtained by the above described testing procedure, the gloss-retention ratio for an outermost surface layer of a roll can be determined. This gloss-retention ratio is actually very useful to compare the surface characteristics of a roll coating according to the present invention with those of coatings known from the prior art. Additionally, in order to support the uniform wearing of the roll cover, thereby further enhancing the reliability and stability of the imparted gloss properties, it is preferred that the composition and the structure, respectively, of the outermost surface layer is homogenous. Homogeneous means that there are basically no gradients in the chemical composition of the ceramic (s) forming the outermost surface layer. Similarly, there are preferably also no gradients in the configuration and texture of the outermost surface layer.

In principle, the ceramic forming the outermost surface layer in the present invention can be based on compounds including non-metallic elements from groups 13 to 17 in the periodic table of elements. However, from the viewpoints of stability of the compounds, suitable hardness and availability, oxide, sulfide, nitride and boride compounds are preferable.

In a preferred embodiment, the ceramic of the outermost surface layer of the aforementioned roll includes an oxidic portion since such an oxidic portion can be readily deposited on a roll substrate by e.g. a thermal spraying method and is also resistant to even very severe manufacturing environments and is widely available with reasonable cost.

In this regard, it is especially preferred that the ceramic includes at least one selected from chromium oxide, aluminium oxide, titanium oxide and zirconium oxide or a mixture thereof, e.g. mixtures of two, three or four of the aforementioned oxides, namely chromium oxide/aluminium oxide, chromium oxide/titanium oxide, chromium oxide/zirconium oxide, aluminium oxide/titanium oxide, aluminium oxide/zirconium oxide, titanium oxide/zirconium oxide, chromium oxide/aluminium oxide/titanium oxide, chromium oxide/aluminium oxide/zirconium oxide, aluminium oxide/titanium oxide/zirconium oxide, and chromium oxide/aluminium oxide/titanium oxide/zirconium oxide.

By using one of the above listed oxides or oxide mixtures as a portion of the outermost surface layer, preferably as the entire outermost surface layer, the manufacturing costs for the roll cover can be reduced and it is easier to grow a comparably thick uniform ceramic layer which allows more frequent grinding in maintenance, thereby lengthening the life time of the roll cover.

It is, however, to be understood that the ceramic materials forming the outermost surface layer in the present invention are not limited to the above listed materials. In order to control particular characteristics of the roll cover, it can be advantageous to use other ceramic materials or to incorporate additional ceramic materials. Such other or additional ceramic materials can be alkaline earth oxides, silicon oxide, tin oxide, silicon carbide, tungsten carbide, chromium carbide, niobium carbide, vanadium carbide, silicon nitride, titanium nitride, boron nitride, and titanium boride. The most preferred is still oxide ceramics with a purity of at least 95%, preferably 98-100%, the rest being other ceramic materials.

Improved resistances against heat, thermal shock and corrosive environments can be achieved in some cases when one or more of the aforementioned ceramic materials are contained in the outermost surface layer of the roll of the present invention. These effects are particularly significant when the ceramic includes tungsten carbide. The thickness of the outermost surface layer lies preferably in the range of 0.01 to 2 mm, more preferably in the range of 0.01 to 0.5 mm. Below 0.01 mm, the advantageous gloss-imparting properties may be insufficiently exhibited and it may be difficult to manufacture the surface layer evenly enough. On the other hand, a too large thickness increases the costs and tends to reduce the smoothness and cleanliness of the surface, so that an upper limit of 2 mm is preferred, which an upper limit of 0.5 mm being more preferred for the above mentioned reasons.

It is further to be noted that the exact chemical composition and the structure of the ceramic coating forming the roll cover of the present invention depend on its formation method. That is, a compositional and/or structural fingerprint of the outermost surface layer results from the different applicable methods. This means that ceramic covers obtainable by coating, sintering, cladding, plating, spraying and so on are differentiable from each other.

From the viewpoint of achieving a sufficiently hard surface layer at low costs, which imparts improved glossiness characteristics to the material treated with the roll, particularly a to paper, it is preferred in the present invention that the ceramic is obtainable by thermal spraying. In a thermal spraying process, a continuous coating is formed by melting the consumable material (target) into droplets and impinging these droplets on the substrate .

An easy realization of such a thermal spraying method, which allows to control the desired surface properties more reliably, particularly the roughness, is especially achieved plasma spraying, flame spraying or high velocity flame spraying.

In a typical flame spraying process, an oxyacetylene flame (around 2760⁰C) is used to melt the targets which are usually powders.

Plasma spraying (or plasma arc spraying) is similar to flame spraying. However, in plasma spraying temperatures of up to 16650⁰C are applied for melting powders and yet the surface temperature of the substrate rarely exceeds 150⁰C (300⁰F). Plasma spraying is thus more suitable for spraying ceramics on metals and thermoset plastics for building up dimensions or wear resistance. The coatings are usually denser, contain less porosity, and have better adhesion than in flame spraying.

High-velocity flame spraying gives higher particle velocity which results in an improved density of the coating.

According to the present invention, it is also possible to use a so-called detonation gun (D-Gun) , which melts target powders in a gun by spark ignition of explosive gas.

All of the aforementioned methods can be employed for forming the advantageous outermost surface layer of the present invention made of ceramic which can easily be provided with a Ra within the afore-mentioned range. A suitable grinding treatment is used in addition for increasing the homogeneity of the roughness across the entire surface.

It is to be noted that the outermost surface layer has a different composition and/or structure than the roll mantle (roll support) on which it is arranged. Typically, roll mantles can be made of metals, steels, cast iron and synthetic materials. One or more intermediate layers can also be present between the roll mantle and the outermost surface layer. A preferred embodiment of such an intermediate layer is a steel layer, preferably made of stainless steel having a chromium content, specifically a chromium content in the range of 5 to 45% and more preferably 10 to 30%. Such an intermediate layer protects the roll mantle from corrosion and can enhance the bonding strength of the outermost surface layer to the roll.

In addition to the above described roll according to the present invention, a method for manufacturing such a roll is provided by the present invention, as well. Namely, the present invention provides a method for producing the above described roll having the steps of forming an outermost surface layer made of ceramic on a roll support by thermal spraying, and finishing the outermost surface layer to an average surface roughness Ra of 0.01 to 0.30 μm.

For the same reasons as given above the thermal spraying is used for forming the ceramic on the roll support. More preferably, one of plasma spraying, flame spraying and high velocity flame spraying is used for this formation because of the above explained advantages.

Example

A cast iron cylindrical rod with a height of 25 mm and a diameter of 25 mm was used as a test specimen substrate. Standard thermal spraying methods and equipment were used to form a 0.2 mm coating on the planar end of the rod. For cermets, high velocity oxy fuel spraying method (HVOF) was used and for ceramic air plasma method was used to achieve the best quality. Three different specimens were made:

1) Ceramic: a ceramic coating according to the invention was made of 100% Amdry 6415 CrO₂ fine powder by standard air plasma spraying.

2) Cermet 1: a comparative example of a typical calender roll coating was made of Amperit 551 powder by HVOF.

3) Cermet 2: a comparative example of a typical calender roll coating was made of Amperit 559 powder by HVOF.

Details of the production of the three specimen are set forth in the Tables below. In the case of the ceramic cover, a current of 630 A and a voltage of 70 V were applied, respectively.

Note: slpm means standard liter per minute

All coatings were ground to a surface roughness of not more that 0.1 μm. Next, all covered specimens were subjected to the above described hazing test in order to determine the retention ratio of the original gloss. Figure 2 shows the obtained results. As can be seen from Figure 2, a full- ceramic cover according to the present invention maintains a very high level of gloss-imparting properties even in a long-term test, while the comparative cermet covers both show a remarkable decrease in the percentage of the original gloss already shortly after the initial stage. It is therefore understood that the rolls according to the present invention can be advantageously used in paper manufacturing when the gloss of the produced paper is of specific importance.

Claims

1. Roll of a papermaking machine, said roll having an outermost surface layer which is made of ceramic and has an average surface roughness Ra of 0.01 to 0.30 μm.

2. Roll according to claim 1, wherein said surface roughness Ra is 0.05 to 0.15 μm.

3. Roll according to claim 1 or 2, wherein said outermost surface layer is homogenous.

4. Roll according to any one of claims 1 to 3, wherein said ceramic includes an oxidic portion.

5. Roll according to any one of claims 1 to 3, wherein said ceramic includes at least one selected from chromium oxide, aluminium oxide, titanium oxide, zirconium oxide and a mixture thereof.

6. Roll according to any one of claims 1 to 5, wherein said ceramic further includes tungsten carbide.

7. Roll according to any one of claims 1 to 6, wherein said outermost surface layer has a thickness of 0.01 to 2 mm.

8. Roll according to any one of claims 1 to 7, wherein said ceramic is obtainable by thermal spraying.

9. Roll according to claim 8, wherein said thermal spraying is selected from plasma spraying, flame spraying and high velocity flame spraying.

10. Roll according to any one of claims 1 to 9, wherein said roll is a calender roll.

11. Method for producing a roll according to any one of claims 1 to 10, comprising the steps: forming an outermost surface layer made of ceramic on a roll support, and finishing the outermost surface layer to an average surface roughness Ra of 0.01 to 0.30 μm.

12. Method according to claim 11, wherein the outermost surface layer is formed by thermal spraying.

13. Method according to claim 12, wherein said thermal spraying is selected from plasma spraying, flame spraying and high velocity flame spraying.