JPS6030696B2 - Method for producing storage-stable ethylene-alpha-olefin-(diene)comb powder containing a separating agent - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a process for the production of storage-stable ethylene-Q-olefin rubber powders containing separating agents.

It is known to grind synthetic resins and rubbers into powders of various particle sizes, the degree of micronization that can be achieved depending on the type of grinding equipment used, the mill conditions and the defined properties of the polymers used. . Furthermore, it is known that rubber powders produced by grinding have a tendency, inter alia, to calcinate to a greater or lesser extent to stick together into large sticky agglomerates or lumps, inter alia under temperature conditions which may occur, for example, during transport or storage. It is being This is a nuisance for silo storage and also for the dry powder mixing technique known there from powdered synthetic resins and for use in processing called powder rubber techniques. In order to prevent undesired sintering, West German Patent Application No. 264830
According to the aspect of the specification No. 1, during the known powdering process, a filler is added to the rubber granules in the form of a powdering agent (Pudermit), so that the powdered rubber is powdered during subsequent processing. It is added in such an amount that it no longer has the properties, especially the mechanical properties, of undusted rubber.

On this basis, the specification of the above-mentioned West German patent application describes and claims a process for the production of filler particles/rubber powder according to a grinding process. It makes it possible to make do with dusting agent contents of less than 7% by weight. This multi-step process consists of first converting the rubber into particles with high surface area and high pores by mechanical pre-pulverization, and in which the entire amount of dusting agent is introduced/rhythmically into the pneumatically conveyed stream of rubber particles and is thus powdered. The adhesive is almost completely adsorbed on the rubber particles, and the powdered powder is distributed extremely uniformly among the powdered rubber particles and sent to the powdering process, and the particle density of the rubber particles is low. temperature, thus preventing the rubber from melting and preserving the structure of the large surface area and dense rubber powder particles formed by the pulverizing process; It is characterized in that the powdered or freshly metered dusting agent is adsorbed onto the rubber powder and that the dusted rubber powder obtained is cooled to such an extent that no warm rubber liquid appears. The subject of the present invention is to obtain a solution by pulverizing the corresponding grains or prepulverized grains and using as little separation agent (hennmittel) as possible.
The object of the present invention is to create a process which makes it possible in a simple and economical way to produce storage-stable ethylene-Q-olefin-(diene) rubber powders by the addition of s).

This problem was surprisingly solved by the claimed method, i.e. storage-stable, ethylene containing separating agent.
Q-Olefin-(Jen) Rubber powder is made into rubber granules (Kau
tsohukkromel) or by grinding the pre-cut rubber particles and adding a separating agent, before, during or after the grinding step, 5 to 2
■Rough hardness of MPa (Rohfesti rate eit) and 5
0~150 Mu (Moo fertilization) Viscosity ML (114) 1
Ethylene-Q-olefin (diene) with 00qo
To rubber, ethylene-Q-olefin-(gen)go 10
0.5 to 10 parts by weight of separating agent, i.e.,
Secondary particles of 5 to 50 particles selected from the group of polymers consisting of polyethylene, polypropylene, polypropylene and polyvinyl chloride.
lchen) has an average size. The problem was solved by a method characterized by adding and mixing a solid separating agent. As ethylene-Q-olefin(diene) rubbers, firstly saturated ethylene-Q-olefin rubbers (EPM) and secondly unsaturated ethylene-Q-olefin(diene) rubbers (EPDM) are used in the process according to the invention.

They are prepared from ethylene, one or more Q-olefins with 3 to 8 carbon atoms, in particular propylene and/or butene-'11, and optionally one or more non-conjugated hydrogens, so-called Ziegler-Natsuta catalysts ( This may additionally contain activators and modifiers) in solution or in dispersion at temperatures between -30 and 110,000 °C, for example as described in German Patent Application No. 1570352, 1595442. It is known that they can be made according to the methods disclosed in Japanese Patent No. 172,045 and Publication No. 2,427,343. 45-8 wt% ethylene, preferably 50-75 wt% ethylene and 55-2 wt%, preferably 50
Consisting of ~25% by weight of Q-olefins, preferably propylene and/or butene m'1), and optionally non-conjugated compounds ~0 per 100M solid carbon atoms of the rubber.
．． Preferred are ethylene-Q-olefin rubbers containing such amounts as to have 5 to 3 hard double bonds.

Particularly suitable genes are cis- or trans-hexadiene-(1,4), dicyclobentagene, 5-methylene-5-ethylidene and 5-isopropylidene-2-norbornene. For the process according to the invention, saturated and unsaturated polyolefin rubbers are
Measured according to DIN 53504 at 5o○ with SI standard bar and 25mm feed.

It is important and essential to have a rough polymer hardness of ). This criterion is met by so-called continuous polymers (Seq Fertilizer NZPOIMmer). This word is
Ethylene-Q-olefin copolymers or ethylene-Q-olefin-gen ternaries in which one of the ethylene or Q-olefins is contained in the polymer chain in the form of chain fragments (continuous) of variable length independent of the statistical distribution of the monomers. means polymer. Along with the required rough polymer hardness, polymers that can be used have a Mooney hardness of 10,000.
) It is important to have a viscosity of 50 to 120, expressed in units of M ratio 14 (measured according to DIN 53523).

Among the group of separation agents used in the method according to the invention polyethylene is mentioned first. 0.910-0.97
Density of 50 to 330 blocks/day (DIN)
728) and 0.2 to 50 evening/10
Suitable are crystalline and partially crystalline polymers with a melt index (measured according to DIN 53735) of 100%.

The term polyethylene is used in the present invention to refer to homopolymers of ethylene as well as to ethylene and other Q-olefins having 3 to 8 carbon atoms, preferably propylene and/or
or copolymers with butene-1 which fall within the above-mentioned characteristic value limits. In addition to the polyethylene types described, from the polyolefin series, there are also densities of 0.90 to 0.910 m/m, viscosity numbers of 100 to 100 m/m (DIN 53728
crystalline and partially crystalline homopolymers and copolymers of propylene with a melt index (measured according to Mt. N53735) of 0.1 to 50 pm/10 min; 975 Tano ground density, 10
Crystalline and partially crystalline homopolymers of butene-1 with a viscosity number of 0 to 1000 min/min (measured according to DIN 53728) and a melt index of 0.1 to 100 min/min (measured according to Dm 53735) and Copolymers come into consideration as separating agents.

In the case of copolymers of propylene and butene-{1}, those made from propylene or butene-{1} and other Q-olefins having 8 and preferably up to 4 carbon atoms are of interest. Therefore, propylene or butene-{1
} The monomers which are preferably polymerized are ethylene, butene-'1} with respect to propylene, and propylene with respect to butene-'.

Its amount in the copolymer is determined by the characteristic values (density, viscosity,
The limits are set by a critical range of melt index). Then, in the method according to the invention, . 50-200 preferably < is a viscosity number (D
All emulsion- or suspension-polyvinyl chloride types with a polyvinyl chloride (measured according to N53726) can be used. All polymers which can be used as separating agents in the process according to the invention are products known in the art and are prepared by known processes (for example West German Patent No. 11
Polyethylene according to No. 17875, polypropylene according to No. 23 General No. 478, and No. Kainer''
Polyvinylchiorid and Viny
lchiorid-Mischpolymerisat
e '', Sprinhoko r 1 Verlag, &
mn/Heiderberg/New York (196
polyvinyl chloride as described in 5)).

When using this, at least 80% by weight of the secondary particles are <
It has a diameter of 100mm and the average diameter of the secondary particles is 5~
It is important that it be within the range of preferably 7 to 36 degrees of 50 Buddhas.

If the polymer does not fall or fall within this specification limit, it can be obtained by, for example, grinding granules with a relatively large particle size, or with an undesirable secondary particle diameter. It may be possible to obtain fractions by removing the fractions, for example by nodes. As is well known, the term "secondary particles" consists of minute parts.
It means a bag mass based on the cohesiveness of so-called primary particles (prim; mailchem).

The measurement of secondary particle size is carried out using the so-called “Microtrack” (
Microt pine c)” method, in which the dispersion of Caesar light is
Wei8,E utilized by a particle size analyzer made by nhWales, PE plate sylvania/USA)
L. and day. N. Frock: Rapip A female lysi
s of Particle size
tio order by Buddha ser LitSca ratio
ring, PowderTechnolo period 14 (19
76), pp. 287-293). . The analyzer operates according to the principle of particle size-dependent optical diffraction, producing a particle distribution based on the radial intensity distribution of the diffracted laser light.

Measurements are carried out in aqueous suspension with a measurement time of 50 seconds and a thickness of the light-transmitting layer of 1.8 pigs. The separating agent is applied to the ethylene-Q-olefin-(diene) rubber before, during or after the grinding step, in an amount of 0.5 to 7, preferably 3 parts by weight, per 10 parts by weight of the ethylene-Q-olefin-(diene) rubber. Added in an amount of ~7 parts by weight.

Optimal amounts are easily ascertained by a few experiments. At that time, it cannot be said that all of the added separating agent is adsorbed by the rubber powder. A part thereof can be present in completely free, unadsorbed form and is optionally separated therefrom at a relatively late point, for example before use of the rubber powder, for example by knotting. In the method according to the invention,
The ethylene-Q-olefin rubber, generally in the form of grains or pre-reduced particles with a diameter of up to a few grains, preferably less than five grains, is crushed in a commercially available crusher, e.g. Machine (11ermu on Prall)
hle) (Fi plate a Pallma skin Maschi rudder nnebrikGmbH-&CO.KG, 6600Zwei
brdcken) to an average particle diameter of about 1 cape to the desired particle size, and one or more of the aforementioned separation agents are added before, during or after grinding.

During the grinding process, care must be taken to ensure that the temperature of the ground product immediately after the grinding process (SI) is not higher than 4.0°C.

This can be achieved in a simple way by cooling the crusher during the grinding process. It may be appropriate to cool the ethylene-Q-olefin-(gen) rubber in powder form containing a separating agent to, for example, room temperature (25° C.) before its storage or processing.

The storage-stable ethylene-Q-olefin rubber containing a separating agent made by the method according to the invention is
First of all, it is used for the production of moldings according to the method of powder rubber technology.

Furthermore, it is also advantageous for applications where the incorporation of ethylene-propylene-(diene) rubber into a liquid medium, such as bitumen, is required. As already mentioned, pre-removal of unadsorbed separating agent may be necessary for certain applications. The method according to the invention is explained in more detail in the following examples.

In this case, the ease of flow of the product and, if necessary, the formation of aggregates, measured after 2 hours under a load of 185 mA at 45° C., are indexed as follows: Indicator Status 1 No aggregate formation , a free-flowing powder 2 A very sticky agglomerate in a free-flowing powder (which breaks down again into a powder after a single application of a small mechanical force) 3 A loose and sticky agglomerate partially in the form of a storage container Agglomerates (which break down into powder and possibly smaller agglomerates under light abrasion or pressure, e.g. when dropped from a small height) Agglomerated objects in the form of storage containers (which can be broken down by repeated rubbing or pressure) 5. Formation of a tightly cohesive agglomerate body in the form of a storage container (which is difficult to disintegrate) Example 1 Propylene content in % by weight of 2nd base An ethylene-propylene rubber present in the form of irregular grains (particle diameter of 3 mm), with a polymer hardness of 1 mm and a viscosity of 83 ML (14 mm) at 100 °C, was prepared by Finna Pallman.
n Maschinen Sasabrik GmbH &
Co. Grind to an average particle size of 0.5 ribs in a water-cooled impact plate crusher manufactured by KG, 6600 Zweibrdcken.

The temperature of the ground product immediately after the end of the grinding process (about 10 minutes) is 3.
It is 830. Water-cooled 10 capacity bread mixer (He
The EPM powder thus made was melted at 200 pm for 2 minutes in a low-pressure polymerization method, with a density of 0.962 min at room temperature and a melt temperature of 20 pm/10 min. 5% by weight of polyethylene powder with a secondary particle average size of 25 mm and index (according to DIN Ball 73 1190/5) is ground.

From the E-PM powder loaded with this separation agent, 250 ml was placed in an 80° capacity beaker and stored at 45° C. for 24 hours in a heating cabinet to determine flowability or agglomerate formation.

At this time, a load of 185 cape a is applied by the metal disk.
The index is 2-3. This shows that even after storage at elevated temperatures, a pulverulent product was obtained in which the tendency to form agglomerates could be reversed by a small amount of mechanical force. Example 2 The rubber, separating agent and grinding step of this example correspond to Example 1, except that the addition of polyethylene powder in an amount of 5% by weight to the granular EPM was tromel mixed before the grinding step. Do it by machine.

During the grinding process, the separating agent collected by the knotting device is fed back into the grinding process. After storage at elevated temperatures, the flowability or agglomerate formation of the product exhibits an index of 2. Example 3 In addition to ethylene, 2% by weight of zuropyrene and 5% by weight of 5
- Ethylidene - Ethylene-propylene present in spherical form, containing 2 norbornene, having a polymer rough hardness of 1 koso a and a ML (14) 100 qo Mooney viscosity of 85.
(JEN) The rubber is first pre-reduced into chips with an average diameter of about 1 go.

To this chip is then added 5 weights of the polyethylene used in Examples 1 and 2 in a tromel mixer before the subsequent grinding step as described in Example 1. The temperature of the ground product approximately 8 minutes after the end of the grinding process is 37°C. After storage at elevated temperatures, the flowability or agglomerate formation of the product with an average particle size of 0.5 ribs shows an index of 2-3. Example 4 An EPM powder containing a separating agent is made with the EPM rubber and procedure according to Example 1.

For this purpose, a polypropylene powder produced by a low-pressure polymerization process and having an average secondary particle diameter of 30 mm is used. This polypropylene has a density of 0.907 min/lump at room temperature and a melt index of 20 min/10 min (DIN $ 739 MFI
I90/5). The temperature of the ground material after about 1 piece after the end of the grinding process is 385'0. Product flowability or agglomerate formation shows an index of 3. Example 5 An EPM powder containing a separating agent is made by the EPM rubber and procedure according to Example 1.

For this purpose, a polyvinyl chloride powder made by the emulsion method and having an average secondary particle diameter of 9 mm is used. The temperature of the ground product about 12 minutes after the end of the grinding process is 37
It is 0. Product flowability or aggregate formation is indicator 2
shows. Example 6 Example 5 is repeated using the procedure of Example 2.

The ease of flow of the product or the formation of agglomerates shows an index of 1-2. Comparative Examples A to C The process described in Examples 1, 4 and 5 is repeated without addition of the separating agent according to the invention.

The resulting product has a flowability or aggregate formation index of 4-5, 5 and 4. That is, after warm storage, the product condenses and sticks to an agglomerated body in the form of a storage container, which can practically no longer be broken up. Next, the subject matter of the present invention is to have a primary particle average size of 0.01 to 0.2 mm and a particle size of 5 to 3 mm.
Synthetically made amorphous silicic acid (Si02) with an average secondary particle size of 5 calyxes and at least 65% by weight of calyxes with an average secondary particle size of 5 calyxes and 7 to 50 calyxes. Naturally derived silicic acid with an average size of 0.5~
The problem was solved by the method described above, which is characterized in that a silicic acid-containing mineral with an average primary particle size of 2.5A skin and an average secondary particle size of 10-35r skin is added as a separating agent.

Among the separating agents used in the method according to the invention, mention may first be made of synthetically produced amorphous silicic acid (Si02).

For example, it is obtained by fuming hydrolysis of silicon halides (SiQ content >99.8%) or by precipitation with acids of alkali metal silicates in a wet process. Silicic acid obtained by precipitation method is relatively pure (SiQ content >99.8%)
and has a constant calcium silicate proportion (Si〇2 content>85%). Furthermore, sodium aluminum silicate is also used in the present invention. Analysis of a typical product, for example 75% Si02, 8% A203, 7% N, gives 0.5% incandescence loss and 5% drying loss (wt%). In addition to the synthetic amorphous silicic acids mentioned above, naturally occurring silicic acids as well as silicic acid-containing minerals can be used in the process of the invention.

Chalcedony-type silicic anhydride (Si02) as natural silicic acid
So-called silica, which is a selected fraction of kaolitoite (AZ4(OH)8Si40,o), is suitable. In this case, the proportion of kaolinite can be between 15 and 95% by weight. Typical analytical values for silica are:
82%Si〇2, 12%A〆2〇3, 0-6%Fe2〇3, 0-2%Ti02, 0.5%Ca○/Mg○, 0.
6% Na20/K20 and 4.1% incandescent loss. Aluminum silicates as silicic acid minerals such as kaolin or kaolin/S with a kaolinite proportion of more than 45%
I02 mixtures or shale flour as well as magnesium silicates such as talc (Mg3(Si40,o)(OH)2) come into consideration. All of the separating agents mentioned above can be made by known methods or are of natural origin (e.g. Gm
elinS HambuCh der anOrgan
iSChenChemie, 8th edition, “Silicim
”, Kirk-05mer, EnCyCIopedia
of Chemical Tech plateology, Vol.
, 18 (1969) and UI1manm EMykl
opadie der chnischenchemi
e, 19 Lid (1964)). These are commonly used as fillers in the rubber industry. The average size (average diameter) of the primary and secondary particles is important for the action of the individual separating agent in the process according to the invention.

Therefore, synthetically produced amorphous silicic acid is 0.01-0.
Naturally derived silicic acid having an average primary particle size of 2 mounds preferably up to 0.15 french and a secondary particle average size of 5 to 35 mounds preferably up to 7 to 25 mf
The weight percent has a primary particle average size of <5 degrees, preferably 0.7 to 3.5 degrees, and a secondary particle average size of 7 to 50 degrees, preferably 10 to 40 degrees, and the silicic acid-containing mineral is 0. ．．
It has an average primary particle size of 5 to 2.5 mm, preferably 0.8 to 2.0 mm, and an average secondary particle size of 10 to 35 mm, preferably 15 to 30 mm. If the separating agent does not fall within this specification range, it can be obtained, for example, by grinding relatively large particles or by removing fractions with undesirable average primary and secondary particle sizes.

The above-mentioned primary particle sizes are measured by electron microscopy (W.
scope and fts application
to rub sting and resea
reh'', Vanderbilt Rubber, New York (19 spots), see Mitsugu et al.). Example 7 Ethylene-propylene rubber is ground in the same manner as in the previous step of Example 1.

The EPM powder thus obtained was heated at 200 pm for 2 minutes in a water-type, 10-capacity bread mixer (Heschel type), with a primary particle average size of 0.012 mounds and a secondary particle size of 10 mm. Grind 5% by weight of amorphous silicic acid made at high heat with an average particle size.

When the EPM powder provided with this separating agent is measured for the index in the same manner as in Example 1, the index for flowability or agglomerate formation is 1. This shows that, even after storage at elevated temperatures, a powdered product in a free-flowing, free-flowing state and without agglomerate formation is permanently obtained.
Example 8 The rubber, separating agent and grinding step in this example are the same as in Example 7, except that the addition of silicic acid is carried out in a tromel mixer before the grinding step.

During the grinding process, the separation agent collected in the muscle device is integrated into the grinding process again. The flowability index of the product after storage at elevated temperature is 1. Example 9 Same as Example 3, but using Examples 7 and 8 as separating agents.
This is done using the separating agent used in .

The temperature of the ground material approximately 8 minutes after the end of the grinding process is 370°C. After storage at elevated temperature, the index for a product with an average particle size on the side of 0.5 is 2. Example 10 EPM rubber according to Example 7 and in the same manner as E with separating agent
Create a PM costume.

For this purpose, 0. Average primary particle size of female 5 Buddha skin and 11
Wet-precipitated amorphous silicic acid is used, having an average secondary particle size of 100,000 yen. The temperature of the ground product approximately 10 minutes after the end of the grinding process is 38.5°C. Product index is 2-3. Example 11 Example 10 using the method of Example 8
Repeat.

Product index is 1-2. Example 12 An EPM powder containing a separating agent is made with EPM and procedure according to Example 7.

For this purpose, a compound consisting of 4% by weight of silica-containing silicic anhydride and 6% by weight of kaolinite has an average primary particle size of 0.8 mounds and a secondary particle average size of 20 mounds.
5% by weight of a naturally derived mixture selected by veins is used. The temperature of the ground product approximately 12 minutes after grinding is 37°C.

The product index is 2. Example 13 Example 7 is followed, except that it has a primary particle average size of 1.8 buds and a secondary particle average size of 28 mounds.
We use naturally derived tributary rock powder (main component is kaolinite) that is sorted by age.

The temperature of the ground material about 10 minutes after grinding is YO.

Product index is 3. Comparative Examples D to G The methods described in Examples 7, 9, 12 and 13 are carried out without the addition of separating agents.

The products exhibit indices of 4, 4-5, 4-5 and 5. That is, after flooding, the product condenses and sticks to an agglomerate in the form of a storage container, which can practically no longer be broken up.
Furthermore, the object of the invention was likewise achieved by the method described above, which is characterized in that carbon black having an average primary particle size of 0.3 W is added as a separating agent. The carbon black that can be used in the method according to the invention is commonly used as a filler in the rubber industry, preferably 0.3-1, preferably 0.01-0.25, particularly preferably 0.0
All those with a primary particle average size of 2 to 0.2 r kites are suitable.

Such carbon blacks can be obtained, for example, by a furnace, arc furnace or channel method (e.g. encyclopasine).
die der ni chnischenchemie, 3
Edition, 1 o'clock lid (19 lines), page 793 onwards, S. Bosoom
, KauGch Blood-Handbuch, 2 volumes (1960
), 259 pages, ASTM, Desi bait atjon:○1
765-Spot and Kayama schuk-Buddha x ikonvon
K. F. Heinisch, A.W. Centner-V
(1977), pp. 434 et seq.). If the carbon black does not fall within the specification ranges mentioned above, it can be obtained by grinding relatively large particles or by eliminating fractions with undesirable particle diameters, for example by knotting.

Example 14 The same ethylene-propylene rubber as in the previous step of Example 1 was ground.

In the same manner as in Example 1, carbon black produced by the electric arc furnace method and having a primary particle size of 0.02 grains is ground. Example 1 EPM powder provided with this separating agent
When the index is measured in the same way as , it is 2.

This shows that, even after storage at elevated temperatures, a powdery product is obtained in which the tendency to form agglomerates is broken again by a small amount of mechanical force. Example 15 The rubber, carbon black and grinding steps were the same as in Example 14, except that the addition of arc furnace carbon black was
This is done in a trommel mixer before the grinding process.

During the grinding process, the separating agent collected in the knotting device is fed back into the grinding process. The product index is 1. Example 16 Same as Example 3, but using Examples 14 and 1 as separating agent.
This is carried out using the arc furnace carbon black used in 5.

The temperature of the ground product approximately 8 minutes after grinding is 37°C. 0.
The index of products with an average size of 5 ribs is 2-3.

Example 17 EPM rubber and procedure according to Example 14 make an EPM powder containing a separating agent.

For this purpose, carbon black produced by the furnace method is used, having an average primary particle size of 0.025 mm. The temperature of the ground product about 12 minutes after grinding is 370.

Product index is 1-2. Example 18 Example 17 is repeated using a furnace produced carbon black with a primary particle size of 0.2 French m.

Product index is 3. Comparative Examples H-J The procedure described in Examples 14, 16 and 17 is carried out without the addition of separating agents.

Claims (1)

Claims: 1. Ethylene-α-olefin-(dienes) which are fluid and retain this property in storage and transportation under conditions of practical temperature, pressure and time, i.e. are storage stable. A method of making a rubber powder comprising grinding rubber granules or rubber particles obtained by mechanical pre-pulverization of spheres and adding a separating agent, comprising: a. 1 Mooney viscosity ML (1+
4) Having a temperature of 100°C; a. 2. The rubber is a continuous polymer and has a rough hardness of 5 to 17 MPa; b.
The separating agent is b. 1 Polyethylene, polypropylene, polybutene-(1), polyvinyl chloride with a secondary particle average size of 5 to 50 μm; b. 2.1 Synthetically produced amorphous silicic acid (SiO_2) with an average primary particle size of 0.01-0.2 μm and an average secondary particle size of 5-35 μm; b. 2.2 Naturally derived silicic acid, at least 65% by weight of which has an average secondary particle size of <5 μm and an average secondary particle size of 7 to 50 μm; b. 2.3 silicic acid-containing minerals having an average primary particle size of 0.5 to 2.5 μm and an average secondary particle size of 10 to 35 μm; or b. 3 Carbon black with an average primary particle size of <0.3 μm, c 0 per 100 parts by weight of rubber before, during or after the grinding process.
．． A method characterized in that 5 to 7 parts by weight of the separating agent described above is added. 2 Ethylene-α-olefin-(diene) rubber 100
2. The method according to claim 1, wherein 3 to 7 parts by weight of the separating agent is added and mixed to the weight of the separating agent. 3 Polymer added and mixed as a separating agent has a thickness of 7 to 35μ
3. The method according to claim 1, wherein the secondary particles have an average size of m. 4. The method according to claim 1, 2 or 3, wherein the temperature of the ground product immediately after the grinding step is not higher than 40°C.