Description

The invention relates to a process for producing superabsorbent polymer particles, comprising polymerization of a monomer solution, drying the resulting polymer gel and grinding the dried polymer gel with a roll mill, wherein the rolls of the roll mill are cleaned by reducing the feed rate to the roll mill, and if the deflection and/or the power consumption increases above a setpoint, operating the roll mill with reduced feed, and increasing the feed rate to the roll mill.

Superabsorbent polymer particles are used to produce diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in market gardening. The superabsorbent polymer particles are often also referred to as "absorbent resin",

"superabsorbent", "superabsorbent polymers", "absorbent polymers", "absorbent gelling materials", "hydrophilic polymers" or "hydrogels".

Commercial superabsorbent polymer particles are polymers of partially neutralized acrylic acid as described in the monograph "Modern Superabsorbent Polymer Technology", F.L. Buchholz and A.T. Graham, Wiley-VCH, 1998, pages 69 to 103.

It was an object of the present invention to provide an improved process for producing superabsorbent polymer particles, especially an improved cleaning of the roll mills that are used for grinding of the dried polymer gel.

The object was achieved by a process for producing superabsorbent polymer particles, comprising polymerization of a monomer solution, comprising a) partially neutralized acrylic acid,

b) at least one crosslinker, and

c) at least one initiator, drying the resulting polymer gel and grinding the dried polymer gel with a roll mill, wherein the rolls of the roll mill are cleaned by the following procedure i) determining the deflection and/or the power consumption of the roll mill, ii) reducing the feed rate to the roll mill or stopping the feed to the roll mill, if the deflection and/or the power consumption increases above a setpoint, iii) optionally increasing the gap width between the rolls of the roll mill,

iv) operating the roll mill with reduced feed rate or stopped feed,

v) optionally reducing the gap width between the rolls of the roll mill,

vi) optionally operating roll mill with reduced gap width until the deflection and/or the power consumption decreases below the setpoint, vii) optionally increasing the gap width between the rolls of the roll mill to the gap width prior to step ii), and

viii) increasing the feed rate to the roll mill to the feed rate prior to step ii) or re

starting the feed to the roll mill, wherein the gap width between the rolls of the roll mill must be reduced in step v) if the feed to the roll mill was stopped in step ii) and the gap width between the rolls of the roll mill was increased in step iii), and classifying the resulting polymer particles.

The setpoint of the deflection bases on vibrations of the rolls that indicates fouling on the rolls. The setpoint of the deflection is, for example, 5% higher than the deflection with clean rolls at the same feed rate.

The setpoint of the power consumption bases on a power consumption of the roll mill that indicates fouling on the rolls. The setpoint of the power consumption of the roll mill is, for example, 5% higher than the power consumption of the roll mill with clean rolls at the same feed rate.

The roll mill is a single- or multistage roll mill, preferably a two- or three-stage roll mill.

Each stage of a roll mill consists of a pair of rolls, wherein one roll is fixed and the other roll is non-fixed. The non-fixed roll can deflect in horizontal direction, if elastic material, i.e. incomplete dried polymer particles, sticks on a roll and the elastic material is larger than the gap width between the rolls. That deflection causes vibration of the non-fixed roll.

In a first embodiment of the present invention the rolls of the roll mill are cleaned by the following procedure i) determining the deflection of the rolls and/or the power consumption of the roll mill,

ii) reducing the feed rate to the roll mill, if the deflection and/or the power con sumption increases above a setpoint,

iv) operating the roll mill with reduced feed rate until the deflection and/or the

power consumption decreases below the setpoint,

viii) increasing the feed rate to the roll mill to the feed rate prior to step ii).

The feed rate to the roll mill is reduced in step ii), if the deflection and/or the power con sumption is above the setpoint preferably for at least 0.5 seconds, more preferably for at least 1 seconds, most preferably for at least 1.5 seconds.

The feed rate to the roll mill is increased in step viii), if the deflection and/or the power con sumption is below the setpoint preferably for at least 1.5 seconds, more preferably for at least 2.0 seconds, most preferably for at least 2.5 seconds. The feed rate to the roll mill in step ii) may be stepwise reduced and/or the feed rate to the roll mill in step viii) may be stepwise increased. A stepwise reduction of the feed rate means that the roll mill operates with the reduced feed rate for a pre-defined time. If the deflection and/or the power consumption are still above the setpoint, the feed rate is reduced again. A stepwise increase of the feed rate means that the roll mill operates with the increased feed rate for a pred-defined time. If the deflection and/or the power consumption are still below the setpoint, the feed rate is increased again.

The pre-defined time is preferably from 1 to 30 seconds, more preferably from 3 to 20 sec onds, most preferably from 5 to 15 seconds.

The feed rate to the roll mill in step ii) is stepwise reduced by preferably 2 to 20% per step, more preferably 5 to 15% per step, most preferably 7 to 10% per step, based on the feed rate prior to step ii).

The minimum feed rate to the roll mill in step iv) is preferably at least 25%, more preferably at least 30%, most preferably at least 35%, based on the feed rate prior to step ii).

The feed rate to the roll mill in step viii) is stepwise increased by preferably 0.5 to 10% per step, most preferably 1.0 to 5% per step, most preferably 1.5 to 5% per step, based on the feed rate prior to step ii).

The present invention is based on the finding that the rolls of the roll mill can be cleaned by reduction of the feed rate.

In a second embodiment of the present invention the rolls of the roll mill are cleaned by the following procedure i) determining the deflection of the rolls and/or the power consumption of the roll mill,

ii) stopping the feed to the roll mill, if the deflection and/or the power consumption increases above a setpoint,

iv) operating the roll mill with stopped feed,

v) reducing the gap width between the rolls of the roll mill,

vi) operating roll mill with reduced gap width until the deflection and/or the power consumption decreases below the setpoint,

vii) increasing the gap width between the rolls of the roll mill to the gap width prior to step ii), and

viii) re-starting the feed rate to the roll mill.

The feed rate to the roll mill is stopped in step ii), if the deflection and/or the power con sumption is above the setpoint preferably for at least 0.5 seconds, more preferably for at least 1 seconds, most preferably for at least 1.5 seconds. The roll mill operates with stopped feed in step iv) for preferably at least 0.5 minutes, more preferably at least 1.0 minutes, most preferably at least 2.0 minutes.

The gap width between the rolls of the roll mill in step v) is reduced to preferably less than 0.1 mm, more preferably less than 0.05 mm, most preferably less than 0.02 mm.

The feed rate to the roll mill is increased in step viii), if the deflection and/or the power con sumption is below the setpoint preferably for at least 1.5 seconds, more preferably for at least 2.0 seconds, most preferably for at least 2.5 seconds.

The present invention is based on the finding that the rolls of the roll mill can be cleaned by stopping the feed and reducing the gap width as low as possible. Under these conditions, incompletely dried polymer particles can be completely dried and easily be removed from the rolls.

In a third embodiment of the present invention the rolls of the roll mill are cleaned by the following procedure i) determining the deflection of the rolls and/or the power consumption of the roll mill,

ii) stopping the feed to the roll mill, if the deflection and/or the power consumption increases above a setpoint,

iii) increasing the gap width between the rolls of the roll mill,

iv) operating the roll mill with stopped feed,

v) reducing the gap width between the rolls of the roll mill,

vi) operating roll mill with reduced gap width until the deflection and/or the power consumption decreases below the setpoint,

vii) increasing the gap width between the rolls of the roll mill to the gap width prior to step ii), and

viii) re-starting the feed rate to the roll mill.

The feed rate to the roll mill is stopped in step ii), if the deflection and/or the power con sumption is above the setpoint preferably for at least 0.5 seconds, more preferably for at least 1 seconds, most preferably for at least 1.5 seconds.

The gap width between the rolls of the roll mill in step iii) is increased to preferably at least 1.0 mm, more preferably at least 1.5 mm, most preferably at least 2.0 mm.

The roll mill operates with stopped feed in step iv) for preferably at least 0.5 minutes, more preferably at least 1.0 minutes, most preferably at least 2.0 minutes.

The gap width between the rolls of the roll mill in step v) is reduced to preferably less than 0.1 mm, more preferably less than 0.05 mm, most preferably less than 0.02 mm. The feed rate to the rol l mil l is increased in step viii) , if the deflection and/or the power con su m ption is below the setpoint preferably for at least 1.5 seconds, more preferably for at least 2.0 seconds, most preferably for at least 2.5 seconds.

The present invention is based on the finding that the rol ls of the rol l mil l can be cleaned by stopping the feed and reducing the gap width as low as possible. U nder these conditions, incom pletely d ried polymer particles can be com pletely d ried and easily be removed from the rol ls. I ncreasing the gap width after stopping the feed removes quickly polymer particles from the rol l mil l so that the next steps can start earlier.

The production of the su perabsorbent polymer particles is described in detail hereinafter:

The superabsorbent polymer particles are produced by polymerizing a monomer solution and are typical ly water-insolu ble.

Acrylic acid typical ly comprises polymerization inhibitors, preferably hyd roquinone monoethers, as storage stabilizers.

The monomer solution com prises preferably u p to 250 ppm by weight, preferably at most 150 ppm by weight, more preferably at most 100 ppm by weight, and preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight and especial ly around 50 ppm by weight, of hydroquinone monoether, based in each case on acrylic acid prior to

neutralization. For exam ple, the monomer solution can be prepared by using acrylic acid with an appropriate content of hyd roquinone monoether.

Suitable crosslinkers b) are com pou nds having at least two grou ps suitable for crosslinking. Such grou ps are, for example, ethylen ica I ly u nsatu rated groups which can be polymerized f ree-rad ica I ly into the polymer chain, and fu nctional groups which can form covalent bonds with the acid grou ps of acrylic acid. I n addition, polyvalent metal salts which can form coordinate bonds with at least two acid grou ps of acrylic acid are also suitable as crosslin kers b) .

Crosslin kers b) are preferably com pou nds having at least two polymerizable grou ps which can be polymerized free-radical ly into the polymer network. Suitable crosslin kers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, al lyl methacrylate, trimethylol propane triacrylate, trial lylamine, tetraal lyl ammoniu m chloride, tetraal lyloxyethane, as described in EP 0 530 438 Al, di- and triacrylates, as described in EP 0 547 847 Al, EP 0 559 476 Al, EP 0 632 068 Al, WO 93/21237 Al, WO 2003/104299 Al, WO 2003/104300 Al, WO 2003/104301 Al and

DE 103 31 450 Al, mixed acrylates which, as wel l as acrylate grou ps, com prise further ethylenical ly u nsatu rated groups, as described in DE 103 31 456 Al and DE 103 55 401 Al, or crosslin ker mixtu res, as described, for example, in DE 195 43 368 Al, DE 196 46 484 Al, WO 90/15830 Al and WO 2002/032962 A2. The amou nt of crosslin ker b) is preferably 0.05 to 1.5% by weight, more preferably 0.1 to 1% by weight and most preferably 0.3 to 0.6% by weight, based in each case on acrylic acid prior to neutralization. With rising crosslinker content, the centrifuge retention capacity (CRC) fal ls and the absorption under a pressu re of 21.0 g/cm ² passes th rough a maximu m.

The initiators c) used may be al l com pou nds which generate free radicals under the polymerization conditions, for example thermal initiators, redox initiators, photoinitiators. Suitable redox initiators are sodium peroxodisu lfate/ascorbic acid, hydrogen

peroxide/ascorbic acid, sodiu m peroxodisulfate/sodiu m bisulfite and hydrogen

peroxide/sodiu m bisu lfite. Preference is given to using mixtu res of thermal initiators and redox initiators, such as sodium peroxodisu lfate/hyd rogen peroxide/ascorbic acid. However, the reducing component used is preferably disodium 2-hydroxy-2-su lfonatoacetate or a mixtu re of disodium 2-hydroxy-2-su lfinatoacetate, disodium 2-hydroxy-2-su lfonatoacetate and sodium bisulfite. Such mixtu res are obtainable as Bmggolite ^® FF6 and Bmggolite ^® FF7 (Bmggeman n Chemicals; Heil bron n; Germany).

Typical ly, an aqueous monomer solution is used. The water content of the monomer solution is preferably from 40 to 75% by weight, more preferably from 45 to 70% by weight and most preferably from 50 to 65% by weight. It is also possible to use monomer suspensions, i.e. monomer solutions with excess sodium acrylate. With rising water content, the energy requirement in the subsequent d rying rises, and, with fal ling water content, the heat of polymerization can on ly be removed inadequately.

For optimal action, the preferred polymerization in hibitors require dissolved oxygen. The monomer solution can therefore be freed of dissolved oxygen before the polymerization by inertization, i.e. flowing an inert gas th rough, preferably nitrogen or carbon dioxide. The oxygen content of the monomer solution is preferably lowered before the polymerization to less than 1 ppm by weight, more preferably to less than 0.5 ppm by weight, most preferably to less than 0.1 ppm by weight.

For better control of the polymerization reaction, it is optional ly possible to add al l known chelating agents to the monomer solution or suspension or to the raw materials thereof. Suitable chelating agents are, for exam ple, phosphoric acid, diphosphoric acid,

triphosphoric acid, polyphosphoric acid, citric acid, tartaric acid, or salts thereof.

The monomer solution is polymerized. Suitable reactors are, for example, kneading reactors or belt reactors. I n the kneader, the polymer gel formed in the polymerization of an aqueous monomer solution or suspension is com minuted continuously by, for example,

contrarotatory stirrer shafts, as described in WO 2001/038402 Al. Polymerization on the belt is described, for example, in DE 38 25 366 Al and US 6,241,928. Polymerization in a belt reactor forms a polymer gel which must be comminuted in a further process step, for example in an extruder or kneader. To improve the drying properties, the comminuted polymer gel obtained by means of a kneader can additional ly be extruded.

The acid groups of the resu lting polymer gels have typical ly been partial ly neutralized.

Neutralization is carried out at the monomer stage. This is typical ly accomplished by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid. The degree of neutralization is preferably from 50 to 85 mol%, more preferably from 60 to 80 mol% and most preferably from 65 to 75 mol%, for which the customary neutralizing agents can be used, preferably al kali metal hydroxides, al kali metal oxides, al kali metal carbonates or al kali metal hyd rogen carbonates and mixtu res thereof. I nstead of al kali metal salts, it is also possible to use am monium salts. Particu larly preferred al kali metals are sodiu m and potassium, but very particu lar preference is given to sodiu m hydroxide, potassiu m hyd roxide and mixtu res thereof.

The resulting polymer gel is d ried. The d riers are not su bject to any restriction. However, the d rying of the polymer gel is preferably performed with a belt d rier u ntil the residual moistu re content is preferably 0.5 to 10% by weight, more preferably 1 to 7% by weight and most preferably 2 to 5% by weight, the residual moistu re content being determined by EDANA recom mended test method No. WSP 230.2 (05) "Mass Loss U pon Heating". I n the case of too high a residual moistu re content, the dried polymer gel has too low a glass transition temperatu re T _g and can be processed fu rther on ly with difficu lty. I n the case of too low a residual moisture content, the d ried polymer gel is too brittle and, in the su bsequent grinding steps, undesirably large amounts of polymer particles with an excessively low particle size are obtained (“fines”). The solids content of the gel before the d rying is preferably from 25 to 90% by weight, more preferably from 35 to 70% by weight and most preferably from 40 to 60% by weight. However, a fluidized bed d rier or a paddle d rier may optional ly also be used for d rying pu rposes.

Subsequently, the d ried polymer gel is ground and classified.

The mean particle size of the polymer particles removed as the product fraction is preferably at least 200 pm, more preferably from 250 to 600 pm and very particu larly from 300 to 500 pm. The mean particle size of the product fraction may be determined by means of EDANA recom mended test method No. WSP 220.2 (05) "Particle Size Distribution", where the proportions by mass of the screen fractions are plotted in cu mu lated form and the mean particle size is determined graphical ly. The mean particle size here is the value of the mesh size which gives rise to a cu mu lative 50% by weight.

To improve the properties, the polymer particles may su bsequently be thermal ly su rface post-crosslinked. Suitable su rface post-crosslinkers are com pou nds which com prise groups which can form covalent bonds with at least two acid grou ps of the polymer particles.

Suitable compounds are, for example, polyfunctional amines, polyfu nctional amido amines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfu nctional alcohols, as described in DE 33 14 019 Al, DE 35 23 617 A1 d

and EP 0 450 922 A2, or b -hyd roxyal kyl amides, as described in DE 102 04 938 A1 and US 6,239,230.

The amou nt of su rface post-crosslinker is preferably 0.001 to 2% by weight, more preferably 0.02 to 1% by weight and most preferably 0.05 to 0.2% by weight, based in each case on the polymer particles.

I n a preferred em bodiment of the present invention, polyvalent cations are applied to the particle surface in addition to the su rface post-crosslin kers before, du ring or after the su rface post-crosslinking.

The polyvalent cations usable in the process according to the invention are, for exam ple, divalent cations such as the cations of zinc, magnesiu m, calciu m, iron and strontiu m, trivalent cations such as the cations of aluminu m, iron, chromiu m, rare earths and manganese, tetravalent cations such as the cations of titaniu m and zirconium. Possible cou nterions are ch loride, bromide, hyd roxide, su lfate, hydrogen su lfate, carbonate, hyd rogen carbonate, nitrate, phosphate, hyd rogen phosphate, dihydrogen phosphate and carboxylate, such as acetate and lactate. Aluminu m hyd roxide, aluminu m su lfate and alu minu m lactate are preferred. Apart from metal salts, it is also possible to use polyamines as polyvalent cations.

The amou nt of polyvalent cation used is, for example, 0.001 to 1.5% by weight, preferably 0.005 to 1% by weight and more preferably 0.02 to 0.8% by weight, based in each case on the polymer particles.

The surface post-crosslinking is typical ly performed in such a way that a solution of the su rface post-crosslinker is sprayed onto the dried polymer particles. After the spray application, the polymer particles coated with su rface post-crosslinker are d ried thermal ly, and the su rface post-crosslinking reaction can take place either before or du ring the drying.

The spray application of a solution of the su rface post-crosslinker is preferably performed in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers. Particu lar preference is given to horizontal mixers such as paddle mixers, very particu lar preference to vertical mixers. The distinction between horizontal mixers and vertical mixers is made by the position of the mixing shaft, i.e. horizontal mixers have a horizontal ly mou nted mixing shaft and vertical mixers a vertical ly mou nted mixing shaft. Suitable mixers are, for example, horizontal Pflugschar ^® plowshare mixers (Gebr. Lodige Maschinenbau Gm bH ; Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV; Doetinchem; the Netherlands) , Processal l Mixmil l mixers (Processal l I ncorporated;

Cincinnati; USA) and Schugi Flexomix ^® (Hosokawa Micron BV; Doetinchem; the

Netherlands) . However, it is also possible to spray on the su rface post-crosslinker solution in a fluidized bed. The surface post-crosslinkers are typical ly used in the form of an aqueous solution. The penetration depth of the su rface post-crosslinker into the polymer particles can be adjusted via the content of non-aqueous solvent and total amount of solvent.

The thermal su rface post-crosslinking is preferably performed in contact d riers, more preferably paddle driers, most preferably disk driers. Suitable driers are, for example, Hosokawa Bepex ^® Horizontal Padd le Dryer (Hosokawa Micron Gm bH; Leingarten;

Germany) , Hosokawa Bepex ^® Disc Dryer (Hosokawa Micron G mbH; Leingarten; Germany) and Nara Padd le Dryer (NARA Machinery Europe; Frechen; Germany). Moreover, fluidized bed driers may also be used.

The thermal su rface post-crosslinking can be affected in the mixer itself, by heating the jacket or blowing in warm air. Equal ly suitable is a downstream drier, for exam ple a shelf d rier, a rotary tube oven or a heatable screw. It is particularly advantageous to effect mixing and drying in a fluidized bed d rier.

Preferred su rface post-crosslinking temperatu res are in the range of 100 to 250° C, preferably 110 to 230° C, more preferably 120 to 210° C and most preferably 130 to 190° C. The preferred residence time at this temperatu re in the reaction mixer or d rier is preferably at least 10 minutes, more preferably at least 20 minutes, most preferably at least 30 minutes, and typical ly at most 60 minutes.

Su bsequently, the su rface post-crosslinked polymer particles can be classified again, excessively smal l and/or excessively large polymer particles being removed and recycled into the process.

To fu rther improve the properties, the su rface post-crosslin ked polymer particles can be coated or remoistu rized.

The remoisturizing is preferably performed at 30 to 80° C, more preferably at 35 to 70° C, most preferably at 40 to 60° C. At excessively low temperatu res, the superabsorbent polymer particles tend to form lum ps, and, at higher temperatu res, water al ready evaporates to a noticeable degree. The amount of water used for remoistu rizing is preferably from 1 to 10% by weight, more preferably from 2 to 8% by weight and most preferably from 3 to 5% by weight. The remoistu rizing increases the mechanical stability of the polymer particles and reduces their tendency to static charging.

Suitable coatings for im proving the free swel l rate and the saline flow conductivity (SFC) are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers and di- or polyvalent metal cations. Suitable coatings for dust binding are, for exam ple, polyols. Suitable coatings for cou nteracting the u ndesired caking tendency of the polymer particles are, for example, fu med silica, such as Aerosil ^® 200, or precipitated silica, such as Sipernat ^® D17, and su rfactants, such as Span ^® 20. The present invention fu rther provides hygiene articles, comprising superabsorbent polymer particles prepared according to the inventive process.

Examples

Preparation of the su perabsorbent polymer particles Example 1

By continuously mixing deionized water, 48% by weight sodiu m hyd roxide solution and acrylic acid, an acrylic acid/sodium acrylate solution was prepared, such that the degree of neutralization corresponds to 72,4 mol%. The solids content of the monomer solution was 40.0% by weight.

The monomer solution was further cooled. Next, 3-tu ply ethoxylated glycerol triacrylate was added as crosslinker to the monomer solution. The amount of crosslin ker was 1.43 kg per t of monomer solution.

The free-radical polymerization was initiated by adding 1.31 kg of a 0.25% by weight aque ous hydrogen peroxide solution, 3.00 kg of a 30% by weight aqueous sodiu m peroxodisu lfate solution, and 0.98 kg of a 1% by weight aqueous ascorbic acid solution, each based per t of monomer solution. The peroxides were added to the monomer solution.

The th rough put of the monomer solution was 21 t/h. The monomer solution had a tempera tu re of 26° C at the feed.

The com ponents (monomer solution and aqueous ascorbic acid solution) were metered continuously into a continuous kneader reactor with a capacity of 6.3 m ³ (LIST AG, Arisdorf, Switzerland) .

Between the addition point for the crosslin ker and the addition points of the peroxides, the monomer solution was inertized with nitrogen.

After approx. 50% of the residence time in the polymerization reactor, a metered addition of fines (1270 kg/h) , which were obtained from the production process by grinding and screening, to the reactor additional ly took place. The residence time of the reaction mixtu re in the reactor was 15 minutes.

The resulting polymer gel was placed onto a belt dryer. On the belt d ryer, an air/gas mixtu re flowed continuously around the polymer gel and d ried it.

The d ried polymer gel was grou nd by means of a two-stage rol l mil l (model WMC152; Neu- haus Neotec Maschinen u nd An lagen bau G mbH; Ganderkesee; Germany). The rol ls have a length of 1,500 m m and a diameter of 250 m m. The gap of the upper rol ls was in the range from 0.3 to 0.5 m m. The tip speed of the upper rol ls was in the range from 5.8 to 8.1 m/s. The gap of the lower rol ls was in the range from 0.1 to 0.2 m m. The tip speed of the lower rol ls was in the range from 7.0 to 10.0 m/s. The feed had a tem perature of 40 to 60° C and moisture content of 1 to 3% by weight. The feed rate was 23.9 % (the reading of the DCS corresponds to approximately 2,500 kg/h) .

The deflection of the rol ls was approx. 19.1% (100% corresponds to the maximum possible deflection of the non-fixed rol l) and the power consu m ption of the rol ls was approx. 28.8 A.

The grou nd polymer was screened off to a particle size fraction of 150 to 850 pm.

Cleaning of the rol l mil l

Example 2

The rol l mil l in Exam ple 1 was cleaned by the first embodiment of the present invention. The setpoint for the deflection was 21.0%. The setpoint for the power consu mption was 31.0 A. Both conditions must be fu lfil led for starting the cleaning procedu re.

The feed rate was stepwise reduced, if deflection and power consu mption were above the setpoint for 2.0 seconds. Then, the feed rate was reduced by 2.0 % (8.3% based on the feed rate prior to the cleaning of the rol l mil l) per step. The time between two steps was 10.0 seconds.

The minimu m feed rate was 10% (41.8% based on the feed rate prior to the cleaning of the rol l mil l).

The feed rate was stepwise increased to a feed rate of 23.9%, if deflection and power con su m ption were below the setpoint for 3.0 seconds. Then, the feed rate was increased by 0.5% (2.1% based on the feed rate prior to the cleaning of the rol l mil l) per step. The time between to steps was 10.0 seconds.