EP0131083A1 - Lining of fibrous material for glazing rolls - Google Patents

Abstract

Known fiber materials, generally in the form of paper, for the manufacture of covers for elastic calender rolls for the paper satin have the disadvantage that they tend to burn if the rolls overheat locally. The invention relates to a fiber material for covering such calender rolls which, in addition to the fiber materials conventionally used for this purpose, namely essentially cotton fibers, contains a proportion of carbon fibers. As a result, better heat dissipation within the roll is achieved without loss of the technological properties of the roll covers, and local burns can be avoided thereby. A certain amount of carbon black can also be added to the fiber material. The invention further relates to an elastic calender roll which is covered with such a fiber material.

Description

TECHNICAL AREA

The invention relates to a fiber material for the production of covers for elastic smoothing rollers, for example super calenders for paper satinizing, and elastic smoothing rollers, which are provided with a cover made of such a compressed fiber material.

For example, for calendering, i.e. the smoothing of high-quality printing papers, but also other special papers such as glassine, so-called super calenders are used, which consist of a set of successive rolls, each forming a press nip, which essentially alternate between hard steel rolls and rolls exist with a more elastic jacket that deforms under pressure in the press nip. Due to the resulting speed difference and the temperature generated by the flexing of the elastic rollers the paper that is moved back and forth through the individual press nips is smoothed.

UNDERLYING PRIOR ART

A special fiber material has become established as the material for the jacket or cover of the elastic rolls of supercalenders, which is pressed onto the roll cores under high pressures of about 500 to 600 bar and is then cylindrically turned and polished.

Cellulose fibers, namely cotton sinters, are primarily used as the fiber material. However, other fiber materials can also be added to these cellulose fibers. For example, the European standard cover for elastic calender rolls consists of 80% cotton and 20% wool. Roller covers with up to 50% asbestos fibers are also used for special purposes.

The fiber material used for covering elastic smoothing rolls or calender rolls, which for the most part consists mainly of cotton fibers with possibly a percentage of wool, is used in the form of paper made from this material, which is produced by conventional paper production processes on Fourdrinier paper machines. Octagonal or round disks with a central opening for the roller core are cut from the paper, which are then stacked on the roller core and pressed together with the pressures mentioned up to 600 bar in the axial direction. The rolls prepared in this way are then turned to an exact dimension and polished.

However, it is not absolutely necessary to provide the fiber material for the roll cover in the form of a paper. Manufacturing processes are also known in which the fiber material, for example carded cotton fiber, is pressed onto the roller core in a different manner. Nowadays, however, calender roll papers are used almost exclusively for re-covering the elastic smoothing rolls.

Although the cellulose fibers used to cover the elastic smoothing rollers, essentially cotton sintered, have first-class technological properties for smoothing the papers to be processed, which has led to their general use, they cause a number of possible and in general for the operation of the calenders cost-generating difficulties. Considerable temperatures are generated in the flexing work occurring in the circumferential area of the rolls under the line pressures of up to 300 daN / cm. Given the relatively poor thermal conductivity of the cellulose material of the cotton fibers, a build-up of heat occurs in the roll shells due to non-dissipated heat energy, which leads to the highest temperatures in a range of approximately 10 mm below the roll surface. In particular, temperature peaks occur in the area of surface damage to the rollers, which can easily occur when the satinized paper web is torn off or when foreign bodies pass through the nips. Such increases in temperature occur in particular at such points that the fiber material of the roller cover literally burns below the surface here. As a result, the roll cover loses its specific properties in these areas and is generally unusable for further use, which results in considerable costs for a new cover.

Various constructional measures in the calenders have been used to ensure that temperature peaks which lead to roller burns are avoided, for example by internal roller cooling. Given the poor thermal conductivity of the cellulose material, such measures have only a limited effect. The difficulties encountered and measures to try to eliminate them are described, for example, by E. Münch and W. Schmitz in the "Wochenblatt für Papierfabrikation" 1980, Issue 11/12. In this publication, the expert authors confirm that the technological possibilities of a supercalender have not yet been exploited because the aforementioned risk of burning the elastic rollers has not yet been mastered. For calenders for special papers, which, like glassine, require a very strong satin finish, calender roll covers with up to 50% asbestos fibers are used because these fibers are better able to withstand the high temperatures that occur. Such roller covers are not so favorable in their other technological properties. Therefore, further attempts have been made to find heat-resistant fiber material for calender roll covers, which has the same technological properties as the cotton covers. So far, however, these efforts have not been successful.

DISCLOSURE OF THE INVENTION

The aim of the present invention was to solve the problem by better heat dissipation from the roll cover without impairing the technological properties of the previously proven fiber materials for elastic smooth roll covers.

Surprisingly, it has been found that a certain addition of carbon fibers to the fiber materials of the material for the roll cover almost completely eliminates the temperature build-up below the roll surface, and at the same time the technological properties, in particular the elasticity of the fiber material, can even be improved.

When one speaks of "fiber material" in this context, it means the entire material for the roll cover, which is generally made available in the form of a paper. By contrast, "fibrous material" means the actual fibrous substances in the fiber material which, together with possibly other additives, form the fiber material as a material for the roll cover.

According to the invention, proportions of carbon fiber of the total fibrous material in the range from about 1.5 to 15% by weight are possible, preferably from about 3 to 12% by weight. With a proportion of 2% by weight, a noticeable effect can be expected depending on the other conditions and additives. To ^g abemengen about 10 wt .-% are indeed possible, but no longer lead to a significant improvement of the effect as it is necessary under operating conditions to eliminate the temperature-staues under the roll surface. Since carbon fiber is relatively expensive, higher additions would prove to be at least disadvantageous in terms of costs. When selecting the carbon fiber, care must be taken to ensure that it mixes sufficiently with the other fiber materials in the material suspension. Carbon fibers which float in aqueous suspension or which are essentially hydrophobic are less suitable in so far as the fiber material is previously produced in the form of a paper in a normal paper production process. For example, a carbon fiber based on polyacrylonitrile has been used proven to be processable. The fiber lengths of the carbon fibers should expediently be of the order of magnitude of the prevailing fiber lengths of the other fiber materials in order to be able to produce a suspension which is as homogeneous as possible. The fiber thickness should also be of the same order of magnitude as the rest of the fibrous material, so that a mutual felting of the fibers can occur during paper production. For example, carbon fibers with a length of 3 mm and a diameter of 5 to 10 µm could be processed successfully with cotton sinters which have a length of 2 to 3 mm and a diameter of 17 to 27 µm.

The thermal conductivity properties of the fiber material according to the invention can also be improved by adding an electrically conductive carbon black to the fiber material. Additions of 0.5 to 10% by weight, based on the total fiber material, are possible. However, the effect of the addition of carbon black based on an equal weight fraction is significantly less compared to the carbon fibers. The use of carbon black in papermaking also has the disadvantage that this non-fibrous material is less retained on the paper machine screen and therefore puts a strain on the water cycle. The carbon fiber also contributes to the strength and elasticity of the paper. A possible addition of carbon black with appropriate adjustment of the carbon fiber content is to be determined for the individual case from a technological and cost point of view.

In addition to the proportion of carbon fibers, preferred papers according to the invention consist of a fibrous material consisting essentially of exclusively cotton fibers or of cotton sinter and wool in weight ratios of 7 to 3 to 9 to 1.

The invention also relates to the use of the new fiber material for the production of covers for elastic Smoothing rolls, in particular calender rolls, as well as elastic smoothing rolls, which are provided with a cover made of a compressed fiber material, which contains a proportion of carbon fibers, preferably in the amounts as previously described for the paper. The reference of the rollers according to the invention does not necessarily have to be derived from a paper. The addition of soot is also possible here.

EXAMPLES

The exemplary embodiments described below are intended to explain the invention in more detail without, however, representing a limitation in the scope of the invention:

example 1

Laboratory sheets with a basis weight of approx. 150 g / m were placed on a laboratory sheet former of the "Rapid-Köthen" type (see Zellcheming leaflet V / 8/57: standard method for the strength test of cellulose, sheet production with the aid of the Rapid-Köthen device) ^l made from a fibrous material made from 90% cotton and 10% carbon fiber. A carbon fiber based on polyacrylonitrile with the type designation Sigrafil SFC 3 from SIGRI Elektrographit GmbH in Meitingen was used as the carbon fiber. This fiber has a fiber length of 3 mm and fiber diameter between 5 and 10 pm. The diameter of the carbon fiber was therefore about half the diameter of the cotton sinter used, which is usually between 17 and 27 μm. The fiber length of second-cut cotton sinters is approximately between 2 and 3 mm. The length of the carbon fibers therefore essentially corresponded to the length of the cellulose fibers used.

These papers were tested for their suitability in a laboratory test procedure, which essentially simulates the load on elastic calender rolls. This test method is mentioned in the publication by E. Münch and E. Schmitz from the Wochenblatt für Papierfabrikation, 1980, No. 11/12, mentioned at the beginning. In this test method, a cube with an edge length of 40 mm is pressed from a stack of sheets of test paper, as is also used in the manufacture of the calender roll covers. A plunger is then placed on this test cube, which is placed under alternating loads by means of a jackhammer. Below the load point, temperature sensors are embedded in the test cube, specifically a first temperature sensor 10 mm below the surface and a second temperature sensor 20 mm below the surface. The alternating loading of the test cube is carried out until the area under the plunger burns, a so-called "burn-out" occurs. For conventional calender roll papers made of approximately 80% cotton fiber and 20% wool fiber, the ram test conditions are at a load of 50 kp and a frequency of 50 Hz, corresponding to an alternating pressure of 5.0 bar.

A service life of a sample cube made of conventional material of 20 minutes is rated as good, that of only 10 minutes as poor. The temperature difference between sensors 1 and 2 is around 90 ° C. at the end of the test in conventional calender roll papers. Since the temperature gradient between the two sensors is a measure of the heat dissipation of the sample, this value already indicates the poor heat dissipation of cellulose-based calender roll papers, which leads to the temperature build-up mentioned and ultimately to the burn-out below the sample surface.

With the test paper according to the invention, no burn-out occurred even after a test duration of 40 minutes. The _T em peraturdifferenz between the sensors 1 and 2 set to 30 ° C after a time and did not change more, from which it could be concluded that a state of equilibrium had been established in the heat dissipation, so that, under a burnout of the sample the pestle was no longer to be expected.

Example 2

A test cube was again produced under the same conditions as described in Example 1, but the load on the plunger was doubled. With conventional calender roll papers, a burn-out occurs within a few minutes under this load. In the test paper according to the invention, no burnout occurred even under these more severe conditions. A burn-out could only be achieved after a standing time of 55 minutes after an additional increase in the exercise frequency. The temperatures measured on the sensors were 216 ° C (sensor 1) and 152 ° C (sensor 2).

Example 3

On the basis of these extraordinarily favorable test results, a calender roll paper made of 90% by weight of cotton sinter and 10% by weight of carbon fibers of the type mentioned in Example 1 was used on a commercial paper machine at a machine speed of about 80 to 90 m / min with a basis weight of about 160 to 170 g / m ² produced. A calender roll was covered with this paper, which was inserted into a calender for the satinizing of glassine papers, which was under extremely high satinizing loads works and therefore normally only elastic calender rolls are used, whose roll covers have a high proportion of asbestos fibers. Previous tests with elastic roller covers made of cotton resulted in service lives for the rollers of less than 2 hours. The roller with the cover according to the invention could be run over a production time of 526 hours. A matt surface then appeared and when the cover was subsequently split off, it was found that the roller was completely burned. Contrary to this phenomenon, most conventional rollers have to be replaced due to local burns. Use until the material is completely burned is never achieved. This indicates that the surface damage that can never be avoided during operation of a calender, which leads to local heating and local burnout, is hardly of any influence in the roller according to the invention, because the local temperature increases are apparently better derived there and distributed over the entire roller.

COMMERCIAL APPLICABILITY

The exemplary embodiments prove that covers for elastic calender rolls can be produced with the fiber material according to the invention, which are considerably superior to the previously known roll covers with regard to their stability, the addition of the carbon fibers also having a technologically favorable effect on the roll work. These very favorable results have enabled machine manufacturers to consider changes and new applications in calender technology for some time that could not be realized due to the risk of burnout of conventional calender roll covers.

Claims (10)

1. fiber material in the form of a paper for the manufacture of covers for elastic smoothing rollers, for example super calenders for paper satin, characterized in that the actual fiber material of the fiber material contains a proportion of carbon fibers. 2. Fiber material according to claim 1, characterized in that the proportion of carbon fibers, based on the total fiber material, is between 1.5 and 15 wt .-%, preferably between 3 and 12 wt .-%. 3. Fiber material according to claim 1 or 2, characterized in that it contains, in addition to the fiber material, a proportion of 0.5 to 10% by weight of an electrically conductive carbon black, based on the total fiber material. 4. Fiber material according to one of claims 1 to 3, characterized in that the non-carbon fiber material consists essentially of cotton fibers, in particular cotton sinters. 5. Fiber material according to one of claims 1 to 3, characterized in that the non-carbon fiber material consists of cotton sinter and wool in a weight ratio of 7 to 3 to 9 to 1. 6. Fiber material according to one of claims 1 to 3, characterized in that the fibrous material contains up to 50% asbestos fibers. 7. Use of the fiber material according to one of claims 1 to 6 for the manufacture of covers for elastic smoothing rollers, in particular calenders. 8. Elastic smoothing roller, in particular for paper satin with a cover made of compressed fiber material, characterized in that the cover contains a proportion of carbon fibers. 9. Elastic smoothing roller according to claim 8, characterized in that the proportion of the cover of carbon fibers is between 2 and 15 wt .-%, based on the total fiber material. 10. Elastic smoothing roller according to claim 8 or 9, characterized in that the cover additionally contains a proportion of 0.5 to 10 wt .-% of an electrically conductive carbon black, based on the total fiber material.