CA2331528A1 - Method and material for producing model elements - Google Patents

Abstract

The invention relates to a method for producing model elements, according to which any desired three-dimensional structure can be obtained using plastic materials, especially selected bead polymers, by means of selective sintering with laser light. The invention also relates to a special material which is especially well suited for laser sintering. A bead polymer consisting of a homo- or copolymer of monoethylenically unsaturated compounds with an average particle diameter of 2 to 200 ~m is used as a plastic powder.

Description

Le A 32 996-FC
_1_ Process and material for the production of models This invention relates to a process for the production of models, in which., by using plastics in the form of selected bead polymers, any desired three-dimensional structure S may be built up by means of selective sintering using laser light. This invention furthermore relates to a special material which is particularly suitable for laser sintering.
This invention specifically relates to a process for the production of three-dimensional models from plastics in accordance with stored, geometric data by means of a computer-aided unit operating with laser beams for the direct production of prototypes and models (rapid prototyping unit).
The term rapid prototyping encompasses currently known computer-controlled, additive, automatic modelling processes. Laser sintering denotes a rapid p:rototyping process in which beds of certain pulverulent materials are heated at certain spatial coordinates under the action of a preferably software-controlled laser beam and fused or sintered.
Such a process is described, for example, in patent 1DE 19 701 078 C1. In s<~id patent, low-melting metals are sintered by means of a rapid prototyping unit in a process for the production of three-dimensional tools for shaping thermoplastics. Low-melting metals and/or metal alloys having a melting point of below 200°C are used in the form of metal powders or metal foils containing neither plastics binders nor metallic binders. The power of the laser radiation used is adjusted in accordance with the melting point of the metals and/or metal alloys used. This process cannot be used to produce plastics models, nor may metal models be made from high-melting metals.
It is also known to use plastics powders for laser sintering (A. Gebhardt, Rapid Prototyping, Carl Hanser Verlag, Munich, Vienna, 1996, pp. 115-116). The process may be used both for producing plastics models and. for making positive preforms for ceramic casting moulds.

Le A 32 996-Foreign

-2-One disadvantage of known plastics powders is tlhe poor flowability thereof, which may only partially be remedied by using flow auxiliiaries. Poor flowability complicates conveying within the laser sintering unit.
The following additional problems occur when producing positive preforms for ceramics. While ground polymer, for example polystyrene, may indeed be sintered to yield the preform, the surface quality of the preform is not entirely satisfactory. The polymer preform is then enclosed in ceramic m;~terial, which is fired at elevated temperature to solidify it. The polymer material is volatilised in this operation.
Complete volatilisation is desired. However, due to the use of flow auxili<rries, most polymer powders cannot be removed by firing without leaving residues.
The object of the invention was accordingly to provide a material suitable for the laser sintering process which forms a smooth to fine-grained surface after sintering and which may optionally virtually completely bc~ asked at conventional firing temperatures for firing ceramics, in particular at a temperature of greater than 1100°C.
This object is achieved by the use of special plastics powders in the form of bead polymers made from a homo- or copolymer of monoethylenically unsaturated compounds, preferably a copolymer having methyl methacrylate or styrene units, having an average particle diameter of 2 to 200 urn, as a sintering material for laser sintering.
The present invention provides a process for the production of three-dimensional models from plastics in accordance with stored, ~;eometric data by means of laser beams of a wavelength of 200 to 20000 nm, preferably of 500 to 10000 nm, controlled according to these data, wherein laser beams are directed in accordance with the geometric data onto certain spatial zones of a bed of a finely divided plastics powder and the material is fused or sintered, characterised in that the plastics powder used is a bead polymer made from a homo- or copolymer of monoethylenically unsaturated ' CA 02331528 2000-11-08 Le A 32 996-Forei~,n

-3-compounds, preferably a copolymer of methyl m,ethacrylate or styrene, having an average particle diameter of 2 to 200 pin.
The determined particle diameter (particle size) is here stated as the weight average.
S
Bead polymers having an average particle diameter of 5 to 100 p,m are particularly suitable for the process according to the invention.
The bead polymers to be used according to the invf;ntion have much more favourable flow properties than other ground plastics and thus require no flow auxiliaries to improve the flow properties thereof.
A further substantial advantage of the bead polymers is that they leave no disruptive residues on ashing, for example as the core of a hollow ceramic mould. Ground plastics particles combined with flow auxiliaries have been found not to ash without leaving a residue.
This is of particular significance if the primary plastics models produced by laser sintering are to be further processed for precision casting. To this end, for example after coating the model produced using the process according to the invention with wax to further improve the surface of the model, the; model is dipped in a ceramic slip composition and the ceramic coated model is fired in a kiln. The model is intended to combust completely on firing to leave behind the empty ceramic hollow mould.
Since conventional ground plastics do not completely combust due to the flow auxiliaries, the metallic models subsequently cast in the ceramic mould frequently exhibit surface inaccuracies.
A further advantage of using bead polymers is achieved with regard to the surface accuracy and surface roughness of the models produced using the process according to the invention. By virtue of the round shape and food flow properties of the bead polymers, the models produced using the preferred bead polymers are smoother and thus also more accurate.

Le A 32 996-Forei~,n

-4-Bead polymers for the purposes of the present invention are polymer particles which are largely spherical. Various processes are known for the production of spherical particles, for example polymerisation processes such as suspension or bead polymerisation, dispersion polymerisation, seed/iEeed polymerisation, a;> well as atomisation methods and precipitation processes. Bead polymers of a particle size of approx. 10 to 200 pm may accordingly be obtained by suspension polymerisation or bead polymerisation. Suspension polymerisation is taken to mean a process iin which a monomer or a monomer-containing mixture which contains an initiator soluble in the monomers) is dispersed in droplet form, optionally mixed with small, solidl particles, in a phase which is substantially immiscible with the monomer(s), which phase contains a dispersant, and is cured by increasing; the temperature while stirring.
Further details of suspension polymerisation are described, for example, in the publication Polymer Processes, edited by C.E. Schildknecht, published 1956 by Interscience Publishers Inc., New York, in the chapter "Polymerization in suspension"
on pages 69 to 109.
Bead polymers having a particle size of 2 to 10 ~m may be produced by so-called dispersion polymerisation. A suitable process is described, for example, in published patent application EP-A-610 522. In dispersion polymerisation, a solvent its used in which the monomers used are soluble, but in which the resultant polymer is insoluble.
Dispersion polymerisation generally yields bead polymers having a narrow particle size distribution.
The bead polymers to be used according to the invention preferably consist of homo-or copolymers of monoethylenically unsaturated compounds (monomers). For the purposes of the invention, copolymers are taken to mean polymers synthesised from two or more different monomers. Suitable monomers are, for example., styrene, a,-methylstyrene, chlorostyrene, acrylic acid esters, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyi acrylate, methacrylic acid esters, such as methyl methacrylate, ethyl meth<~crylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethylhexyl mcth-Le A 32 996-Foreign

-5-acrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate, together with acrylonitrile, methacrylonitrile, methacrylamide and vinyl acetate.
Homo- and copolymers of methacrylic acid esters and/or acrylic acid esiters are in particular preferred. Polymethyl methacrylate and copolymers containing more than 60 wt.% of methyl methacrylate units are particularly preferred. Highly suitable copolymers are, for example, those comprising 60 t:o 98 wt.% of methyl mc;thacrylate units and 2 to 40 wt.% of units of acrylic acid esters and/or methacrylic acid esters having 4 to 18 C atoms in the alcohol portion, in particular copolymers of methyl methacrylate with one or more monomers from the group: n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethy:lhexyl methacrylate, decyl meth-acrylate, dodecyl methacrylate, stearyl methacrylate.
It has been found that the molecular weight of the bead polymers may be of significance to the suitability for the process according to the invention.
The molecular weight (weight average, MW) should in particular be from 10000 to 1000000, preferably from 10000 to 500000, particularly preferably from 20000 to 250000 g/mol. The desired molecular weight ma:y be established by using chain-transfer agents during the production of the bead polymers. Suitable chaiin-transfer agents are in particular sulfur compounds, for example n-butyl mercaptan, dodecyl mercaptan, thioglycolic acid ethyl ester and diisopropyl xanthogen disulfide.
The chain-transfer agents containing no sulfur stated i:n ICE 3 010 373 are also highly suitable for establishing molecular weight, for example the enol ethers of the formula I
Suitable types of lasers comprise any which cause t:he bead polymer to sinter, fuse or crosslink, in particular COZ lasers (10 pm), Nd-YA~G lasers (LOGO nm), He-Ne lasers (633 nm) or dye lasers (350-1000 nm). A COZ laser is preferably used.
The energy density in the bed on irradiation is preferably from 0.1 to 10 J/mnn3.

Le A 32 996-Foreign

-6-Depending upon the application, the effective diameter of the laser beam is preferably from 0.01 to 0.5 nm, preferably 0.1 to 0.5 nm.
Pulsed lasers are preferably used, wherein an elevated pulse frequency, in particular of S 1 to 100 kHz, has proved particularly suitable.
The preferred method of performing the process ma;y be described as follows:
The laser beam hits the uppermost layer of the bed of the material to be used according to the invention, so sintering the material to a specific layer thickness. This layer thickness may be from 0.01 to 1 mm, preferably from 0.05 to 0.5 mm. In this manner, the first layer of the desired component is produced. The working chamber is then lowered by an amount which is less than the thickness of the sintered layer. The working chamber is filled to the original depth with additional polymer material.
Renewed irradiation with the laser sinters the second layer of the component and bonds it to the preceding layer. The remaining layers are produced by repeating the sequence until the component is complete.
The speed of exposure during scanning of the laser is preferably 1 to 1000 mm/s. A
speed of approx. 100 mm/s is typically used.
The present invention also provides the models obtainable using the process according to the invention.
The present invention furthermore provides the use of the models produced using the process according to the invention for the production of preforn~s, in particular of ceramics, for the precision casting of metals.
Figure 1 illustrates the invention in greater detail. below but without linniting the invention to these specific details.
Figure 1 shows a simplified schematic diagram of a rapid prototyping unit.

Le A 32 996-Foreign Examples Examples 1 to 6 below demonstrate the production of a finely divided plastics material suitable for laser sintering.
S
The rapid prototyping unit used to produce the models is of the following fundamental structure (Figure 1).
The beam from an IR laser 1 is directed by means of a deflection mirror 2 in accordance with the instructions from a scanner unit (not shown) onto the surface of the bed 4 of a bead polymer, which is held in a rolmd mould S with a mobile bottom ram 6.
Layers 3a, 3b of sintered plastics material are formed by exposure to laser light. After 1S each exposure and production of a layer (for example 3b), the ram 6 is lowered by a layer thickness and the bed 4 is topped up with new plastics material, which is exposed to laser light in the next step, so producing the next layer 3a of the model.
Example 1 Production of a bead polymer having a particlf; size of S.3 ~m by dispersion polymerisation S60 g of polyvinylpyrrolidone, 80 g of methyltrica.prylammonium chlorides, 6.4 g of 2S azodiisobutyronitrile and 0.64 g of dodecyl merca.ptan were dissolved in a solvent mixture comprising 14 1 of methanol and 2 1 of ethanol in a reactor equipped with a reflux condenser, stirrer and thermometer. 9S0 g of methyl methacrylate and SO
g of n-butyl acrylate were added to the solution. The resultant mixture was reflvxed with stirring for S hours and then cooled to 2S°C. The bead polymer formed was isolated by centrifugation, washed with methanol and dried at 50°C. 780 g of a bead polymer were obtained having an average particle size of S.3 Vim. The average molecular weight M", was 110000 g/mol.

Le A 32 996-Foreign _g_ Example 2 Production of a bead polymer having a particle size of 45 pm by bead polym.er-isation 450 g of methyl methacrylate, 50 g of ethylhexyl acrylate, S g of dibenzoyl peroxide and 1 g of enol ether of the formula I were mixed together to yield a homogeneous solution. The solution was transferred into a tubular reactor, which had previously been filled with 1.5 litres of a 1 wt.%, aqueous alkaline solution, adjusted to a pH
value of 8 with sodium hydroxide solution, of a copolymer comprising 50 wt.%
methacrylic acid and 50 wt.% methyl methacrylate. The stirring speed was set at 420 revolutions per minute. The temperature was maintained at 78°C for 8 hours and then at 85°C for 1 hour. The mixture was then cooled t:o room temperature, the resultant bead polymer was isolated by decanting, washed repeatedly with water and dried at 60°C. 465 g of a bead polymer were obtained having an average particle size of 45 p.m. The average molecular weight MW was 125000 g/mol.
Example 3 Production of a bead polymer having a particle size of SO pm by bead polymerisation 600 g of methyl methacrylate, 9 g of dibenzoyl peroxide (75% in water) and 12 g of enol ether of the formula I were mixed together to yield a homogeneous solution. The solution was transferred into a tubular reactor, which had previously been filled with a solution consisting of 2.0 litres of water and 8 g of polyvinylpyrrolidone K90. The speed of the paddle stirrer was set at 600 revolution:; per minute. The temperature was maintained at SS°C for one hour, then at 75°C for 12 hours and then at 9~0°C for 4 hours. The mixture was then cooled to room temperature, the resultant bead polymer was isolated by filtration through a 32 um filter cloth, washed repeatedly with water and dried at 35°C under a vacuum of 20 mbar. After screening through a 125 ~m screen, a bead polymer was obtained having an intrinsic viscosity [r)) (Lrbbelohde Le A 32 996-Foreign capillary viscosimeter in chloroform at 25°C) of t).50 dl/g, which corresponds to a molecular weight MW of 110000 g/mol.
Example 4 S
Production of a bead polymer having a particle size of 40 pm by bead polymerisation 510 g of methyl methacrylate, 90 g of n-butyl methacrylate, 9 g of dibenzoyl peroxide (75% in water) and 12 g of enol ether of the formula I were mixed together to yield a homogeneous solution. The solution was transferred into a tubular reactor, which had previously been filled with a solution consisting .of 2.0 litres of water and 8 g of polyvinylpyrrolidone K90. The speed of the paddle: stirrer was set at 600 revolutions per minute. The temperature was maintained at SS°C for one hour, then at 7'.i°C for 12 hours and then at 90°C for 4 hours. The mixture was then cooled to room temperature, the resultant bead polymer was isolated by filtration through a 32 pm filter cloth, washed repeatedly with water and dried at 35°C under a vacuum of 20 mbar. After screening through a 125 ~m screen, a bead polymer was obtained having a~n intrinsic viscosity [rl] (Ubbelohde capillary viscosimeter in chloroform at 25°C) of 0.48 dl/g, which corresponds to a molecular weight MW of 100000 g/mol.
Example 5 Production of a bead polymer having a particle size of 40 pm by bead polymerisation 600 g of methyl methacrylate, 6 g of dibenzoyl peroxide (75% in water) and 3 g of enol ether of the formula I were mixed together to yield a homogeneous solution. The solution was transferred into a tubular reactor, which had previously been filled with a solution consisting of 2.0 litres of water and 20 g of polyvinylpyrrolidone K90. The speed of the paddle stirrer was set at 600 revolutions. per minute. The temperature was maintained at 55°C for one hour, then at 75°C for 12 hours and then at 9~0°C for 4 hours. The mixture was then cooled to room temperature, the resultant bead polymer Le A 32 996-Foreign was isolated by filtration through a 32 pm filter cloth, washed repeatedly with water and dried at 35°C under a vacuum of 20 mbar. After screening through a 125 p,m screen, a bead polymer was obtained having an intrinsic viscosity [r)]
(ITbbelohde capillary viscosimeter in chloroform at 25°C) of 1.41 dl/g, which corresponds to a molecular weight MW of 250000 g/mol.
Example 6 Production of a bead polymer having a particle size of 40 p,m by bead polymerisation 510 g of methyl methacrylate, 90 g of n-butyl aciylate, 6 g of dibenzoyl peroxide (75% in water) and 3 g of enol ether of the formula I were mixed together to yield a homogeneous solution. The solution was transferred. into a tubular reactor, which had previously been filled with a solution consisting of 2.0 litres of water and 20 g of polyvinylpyrrolidone K90. The speed of the paddle stirrer was set at 600 revolutions per minute. The temperature was maintained at 55°C'. for one hour, then at 75°C for 12 hours and then at 90°C for 4 hours. The mixture was then cooled to room temperature, the resultant bead polymer was isolated by filtration through a 32 pm filter cloth, washed repeatedly with water and dried at 35°C under a vacuum of 20 mbar. After screening through a 125 pm screen, a bead polymer was obtained having are intrinsic viscosity [rl] (Llbbelohde capillary viscosimeter in chloroform at 25°C) of 0.96 dl/g, which corresponds to a molecular weight M~,, of 230000 g/mol.
Example 7 The flowability of some polymers was quantified by determining the angle of repose and angle of flow. The angle of repose is here taken to mean the angle between the side of a cone of the pulverulent polymer and the line normal to the base of the cone.
The cone is produced by discharging the bulk material from a hopper with a narrow outlet onto a level surface.

i Le A 32 996-Foreign The angle of flow denotes the angle assumed by an inclined surface relative to the horizontal at which the bulk material spread out on the surface begins to flow under the effect of gravity.
As is evident from Table 1, a bead polymer accorf.ing to Example 4 has advantages over amorphous ground materials such as polycarbonate and polystyrene.
By virtue of the round shape, it is possible to dispense with the addition of flow auxiliaries, as are added to the ground materials. Angle of repose measurements confirm good processability in sintering plant. It has thus been found that, by virtue of the shape thereof, bead polymers have greater flowability than ground polymer pellets with added flow auxiliaries.
Table 1 Determination of flowability of sintering materials Material Shape Angle Angle Diameter of of of repose flow cone (mm) () () Example Bead polymer18.75 20.8 64.4 Invention (40 p.m) PC*) Ground + 41 46.2 44 Comparative flow auxiliary Example PS**) Ground + 33.75 38.4 45.8 Comparative flow auxiliary Exarr~ple *) Polycarbonate having an average particle size of 40 pm **) Polystyrene having an average particle size of 40 pm Le A 32 996-Foreign The smaller are the angle of flow and angle of repose, the greater is the flowability of the material. Flowability increases with the increasing diameter of the cone.
Example 8 S
Ashing of some polymers was also investigated (see; Table 2 for results).
Ashing of the materials revealed that no residues remain when the bead polymer from Example 4 is used. As could be demonstrated in. the case of the ground polymer materials provided with additives, the plastics provided with flow auxiliaries do not ash completely without residues.
Table 2 Ashing with modelling materials Material Shape Ashing ResidualColour of range (C) ash residual ash (wt.%) Example Bead 3SS.6-411.30.02 Not discernibleInvention polymer (40 pm) PC Ground 471.1-457.30.80 Black/metallicComiparative + flow Example auxiliary PS Ground 394.2-475.40.10 White Comparative + flow Example auxiliary 1 S Example 9 Production of models. Models of dimensions S0x 10x4 rpm' are produced by sintering the plastics powder in a modelling unit. To this end, the plastics powder is exposed to light in layers at a speed of 1 mm/s under an infra-red laser of a wavelength of Le A 32 996-Foreign 10000 nm (COZ laser) at a maximum temperature of 500°C. The plastics;
powders tested in Examples 7 and 8 were sintered.
Example 10 The quality of the models from Example 9 was additionally tested.
The polycarbonate and polystyrene, which are suitable in principle, do indeed yield satisfactory results (cf. Table 3).
However, using bead polymers generates a particular advantage with regard i;o surface accuracy and roughness. By virtue of the round shape, the surfaces of the model are smoother, as is shown by the following Perthemeter results. This not only has the advantage that more precise plastics models (preforrr~s) may be produced, but also that the precision castings made from the preform are more accurate.
Table 3 Surface roughness of sintered specimens Material Shape RmaX (gym)RZ (inn)Ra (~.m) Example Bead polymer62.31 48.59 9.24 Invention (40 Vim) PC Ground + 97.88 82.34 i 7.13 Comparative flow auxiliary Example PS Ground + 93.71 71.80 14.51 Comparative flow auxiliary Example

Claims (13)

Claims

1. Process for the production of three-dimensional models from plastics in accordance with stored, geometric data by means of laser beams of a wavelength of 200 to 10000 nm controlled according to these data, wherein one or more laser beams are directed in accordance with the geometric data onto certain spatial zones of a bed of a finely divided plastics powder and the material is fused or sintered, characterised in that the plastics powder used is a bead polymer made from a homo- or copolymer of monoethylenically unsaturated compounds having an average particle diameter of 2 to 200 µm.

2. Process according to claim 1, characterised in that the plastics material used is a bead polymer having an average particle diameter of 5 to 100 µm.

3. Process according to claim 2, characterised in that the bead polymer used is a homo- or copolymer of monoethylenically unsaturated compounds, in particular styrene, .alpha.-methylstyrene, chlorostyrene, acrylic acid esters, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, methacrylic acid esters, such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate, together with acrylonitrile, methacrylonitrile, methacrylamide and vinyl acetate.

4. Process according to claim 3, characterised in that the bead polymer is a homo- or copolymer of methacrylic acid esters and/or acrylic acid esters, preferably polymethyl methacrylate or copolymers containing more than 60 wt.% of methyl methacrylate units.

5. Process according to claim 3, characterised in that the bead polymer is a copolymer which comprises 60 to 98 wt.% of methyl methacrylate units and 2 to 40 wt.% of units of acrylic acid esters and/or methacrylic acid esters having 4 to 18 C atoms in the alcohol portion, in particular copolymers of methyl methacrylate with one or more monomers from the group: n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate.

6. Process according to one of claims 2 to 5, characterised in that the molecular weight (weight average, M W) of the bead polymer is from 10000 to 1000000, preferably from 10000 to 500000, particularly preferably from 20000 to 250000 g/mol.

7. Three-dimensional models obtainable using the process according to the invention according to one of claims 1 to 6.

8. Use of the models according to claim 7 for the production of preforms, in particular of ceramics, for the precision casting of metals.

9. Use of bead polymers having an average particle diameter of 2 to 200 µm, preferably of 5 to 100 µm, as a material for laser sintering.

10. Use according to claim 9, characterised in that the bead polymer used is a homo- or copolymer of monoethylenically unsaturated compounds, in particular styrene, .alpha.-methylstyrene, chlorostyrene, acrylic acid esters, such as ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, methacrylic acid esters, such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate, together with acrylonitrile, methacrylonitrile, methacrylamide and vinyl acetate.

11. Use according to claim 10, characterised in that the bead polymer is a homo- or copolyer of methacrylic acid esters and/or acrylic acid esters, preferably polymethyl methacrylate or copolymers containing more than 60 wt.% of methyl methacrylate units.

12. Use according to claim 10, characterised in that the bead polymer is a copolymer which comprises 60 to 98 wt.% of methyl methacrylate units and 2 to 40 wt.% of units of acrylic acid esters and/or methacrylic acid esters having 4 to 18 C atoms in the alcohol portion, in particular copolymers of methyl methacrylate with one or more monomers from the group: n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, dodecyl acrylate, n-butyl methacrylate, isobutyl methacrylate, hexyl methacrylate, 2-ethylhexyl methacrylate, decyl methacrylate, dodecyl methacrylate, stearyl methacrylate.

13. Use according to one of claims 9 to 12, characterised in that the molecular weight (weight average, M W) of the bead polymer is from 10000 to 1000000, preferably from 10000 to 500000, particularly preferably from 20000 to 250000 g/mol.