HK40042832B - Method and device for comminuting a thermoplastic polymer and for producing a powdered material therefrom - Google Patents

Description

The invention relates to a process and device for the crushing of a thermoplastic polymer and the manufacture of a powdered material from it.

The aim is to achieve grain sizes of less than 500, and in particular less than 100 μm, e.g. particles in the range of 30 to 100 μm. The maximum upper limit may be 800 μm. The lower limit is in the range of fewer nanometres, preferably around 1 or 10 μm. The deviation from the spherical shape should be as small as possible so that the smallest cross-sectional dimension of a particle is not less than 20, preferably not less than 50% of the largest cross-sectional dimension of these particles.

The more spherical the individual particles, the more fluid the powder. Such powder is required by the market in the desired grain distributions specified for each application. To produce a concrete powder, a fragmented starting material in the form of a thermoplastic polymer is first crushed into a pulp by a separation step, the more spherical the individual particles, the more fluid the powder is. This final product is then melted down in a direction known as the flow direction. This process is known as the flow of a small amount of liquid and melted down into a pulp by a process called crystallization.

It is also known that the starting product plastic is first dissolved in a solvent and the particles are extracted from it, for example by spraying or spraying the resulting solution and keeping the resulting droplets isolated until the solvent is essentially evaporated.

In industrial applications, equipment and processes are used for grinding and for the production of powders. WO 2007/008480 A1 describes a process for grinding at cryothermal temperature and producing a powder, the description introduction describes in detail in several pages the state of the art for cryomining. EP 2 957 598 A1 reveals a process for producing polyamides which are transferred to powder by cryogenic grinding.

A cryogenic temperature is generally understood to be a temperature below minus 150 °C. For the purposes of this application, temperatures below -50 °C are to be included, including cooling by dry ice CO2.

It is also possible not to perform the shredding step at low temperatures. A shredding at low temperatures is usually necessary if the glass transition temperature of the polymer is below -20 °C, especially below -30 °C, otherwise the polymer concerned cannot be normally shredded.

The above methods are examples of crushing of plastic starting material into powdered plastic.

For example, a production process for the production of a water-absorbing resin is known from EP 1 754 725 A2, whereby a stationary polymerisation of an ethylene-unsaturated monomer containing acrylic acid and/or acrylate is carried out. The resulting aqueous, gelatinous polymer is cut and dried, with the resulting polymer consisting mainly of edge-like particles with 6 smooth layers. At least part of the surface of each sub-block can be coated with an adhesion block before, after and/or at the time of the cutting process. DE 10 2011 080 233 A1 is a known method for the production of dispersion pulverization by spraying of earth-like polymers and weathering agents.

The market dictates a size and distribution of the desired powdered materials, such as those needed for a special 3D printer. To obtain the correct grain distribution, the powdered plastics (the starting powder) obtained from the crushing step are screened at least once. Several screenings can be performed in succession. The grain distribution also depends on the type of crushing and how it is done.

Depending on the crushing process chosen, the resulting powder material has a characteristic shape of the individual particles or grains. The typical shape may differ more or less from the final desired spherical shape. The grains may be irregular, for example, have tails, be relatively flat (flat), be shaped or similar. This affects the sieving process and the result of the sieving. It is possible to round the grains after the crushing step and before the sieving step.For example, reference is made to the applicant's DE 10 100 2017 981.

In the practical implementation of the process, it has now been shown that problems arise during the sieving process due to the shredding step, especially if the starting powder was obtained from a thermoplastic elastomer, for example by grinding soft TPU, and shows a particular stickiness. The throughput of the device is often greatly reduced. The sieving device tends to clog. An economically reasonable sieving process can often not be carried out.

The purpose of the invention is to improve the methods and devices used to crush a thermoplastic polymer and to produce a powder from it in such a way as to facilitate and improve the sieving process.

This problem is solved by a process with the characteristics of claim 1 and a device with the characteristics of claim 10.

This method allowed a stable sieving process to be achieved, a constant powder distribution to be achieved, a significant increase in throughput, for example by ten times, and a minimum of 10% increase in the bulk weight of the final powder, usually at least 20%.

The process of crushing is performed using one of the common crushing processes known from the state of the art. Important examples are mentioned above. A mechanical crushing usually takes place in a mill, for example, using vortex mills, pen mills, ball mills or the like.

The specified grain size is generally determined by the market, usually by a specific customer. A typical example of a grain size is d90<125 μm, d50 between 60 and 80 μm, d10 between 20 and 30 μm. A polymer in the form of a piece is a granule, rod, block, commercial supply or other form of the starting material of a thermoplastic polymer. The pieces of the starting material have dimensions at least a thousand times larger than the starting powder, preferably in the range of centimetres and above.

As a rule, only one polymer is processed in each case according to the procedure, but it is also possible to process two or more polymers simultaneously in the same device.

The improvement of the sieving process minimizes the fine particulate matter of less than 10 μm in the final powder. This can eliminate an additional step of de-dusting. It improves health protection during processing and also during further processing of the final powder, for example at an end customer. The ripping capacity of the final powder is significantly improved compared to the final powder according to the state of the art. Since less aggregate formation takes place, which facilitates the sieving process, less coarse particulate matter also needs to be returned to the shredding step.

The method is suitable for thermoplastics which tend to stick and form aggregates after the shredding step. It is also suitable for thermoplastic polymers such as PP, PA, PPS, ABS, PBT, PE, PS, PET, PMMA, PC, PEEK, PEKK. The method is particularly suitable for TPU, ester and ether-based thermoplastics such as TPEE.

The separator is preferably in a very fine powder form. It is intended to envelop the particles of the starting powder in a dusty form and thus avoid direct contact between two neighboring particles of the starting powder. This increases the ripple capacity of the starting powder. The separator can also be called an additive, ripple aid or anti-agglomerate agent. The separator improves the flow capacity of the starting powder. It also improves the flow capacity in the process plant, i.e. in its pipes, cell locks, etc.

The most preferred separators are metal stearates and amide waxes, the melting point of which should preferably be between 50 and 160 °C.

It is possible to heat the starting powder after the crushing step, which makes it possible to carry out the screening step more efficiently on a case-by-case basis.

The present invention does not require such a step, since the additive is already added at the shredder step or another additional additive, which is intended to be added to the separator, can be added with it.

It is also possible to carry out the shredding step separately from the screening step, preferably using long-mesh screens, such as the Allgaier AVTM1600 board screening machine with a 300x110 μm long-mesh.

The crushing device includes all the components in front of the screening device, including, for example, the crankcase locks, the conveyor pumps, etc.

Examples:

Example 1: In the milling circuit of a vortex mill, a piece of TPU is inserted and crushed. 1 wt.% of Baerlocher's amide wax Baerolub L-AS (based on TPU inserted into the mill) is introduced together with the TPU. The mill distributes the separator in the raw powder in an excellent manner during the milling process.

Example 2: The same procedure as in example 1 is followed, but 0.13% by weight of aluminium oxide (AlC) is added as the separating agent, resulting in a bulk weight of 372 g/l for grain distribution d90<125 μm, d50 between 60 and 80 μm, d10 between 20 and 30 μm.

Example 3: The procedure is the same as in Example 2, but now 2% by weight of Ca-stearate (114-36 L3 of Valtris Speciality Chemicals) is added as the separating agent, resulting in a bulk weight of 462 g/l for the specified grain distribution.

Further advantages and features of the invention are shown by the other claims and by the following description of two non-limiting embodiments of the invention, which are explained in more detail by reference to the following drawing. Figure 1 a basic representation of a device for the crushing of a thermoplastic polymer, which is done by melting and melting, and Figure 2 a basic representation similar to Figure 1, but now for a device where the crushing is done mechanically by a mill. The device described in Figure 1 shall first be described, the device described in Figure 2 shall be described only in so far as its individual parts differ from the device described in Figure 1.

The TPU is melted in a melting tank 20 and fed from the melting tank 20 through a conveyor line 22 to a pump 24 and a spray tower in a tank 26 at the top of the tank 26 with a nozzle 28 to which the melted material is fed. The nozzle openings release liquid material, for example, in the form of thin threads that separate into droplets further down.

In tank 26 a line 30 for cryogas, especially liquid nitrogen, protrudes from the top next to the conveyor 22 and is connected to a supply unit 32 which is shown here as a ring above and outside the nozzle arrangement 28.

The polymer material exits from the nozzle arrangement 28 within a cone 34; the cryogas exits from the feed unit 32 in the form of a cone shell 36; the cone 34 is located within the cone shell 36; the cone 34 is so arranged that it does not, as far as possible, hit the side walls of the container 26.

In the lower part of the container 26 there are side nozzles 38 or a similar device for introducing, which introduce a separator, a metal stearate, into the interior of the container 26 in such a way, see arrows, that as far as possible a layer 40 of the separator forms in the lower part of the container 26. Through this layer 40 the droplets fall through and are thereby more or less enveloped by the separator. Depending on the process, part of the separator falls from layer 40 downwards, this part falls on the area of the top of the wire which has a slant. On these slanted slopes the particles meet before they can slowly reach the lowest slope of the particle 42. The layer is also filled with the separator when they are lying on the walls and can also be filled with the particles while they are lying on the walls and during the slope they can also be filled with the particle 42

In the lower part of the container 26, this has a conical outlet 42. There the coated droplets are cooled to the point where they are essentially deformable, forming the outlet powder. This now enters a sieve device 44. In a variation, a cell wheel gate may be intermediate, as shown in Figure 2. It is part of the shrinkage device. In a variation, it is possible to feed the separator into this cell wheel gate.

The sieve 44 is made according to the state of the art, and for the sake of simplicity it has only one sieve 46. Of this sieve 46 the specified grain distribution is omitted, see arrow 48, the rest is derived, see arrow 50.

In the apparatus shown in Figure 2, TPU is contained in a container of granules in the form of a container for the application of the product 52. The granules are fed through a cell wheel slot 54 to an inlet 56 of a mill 58, which is shown here as a pin mill. In addition, a line 60 for separating agent, in this case an amide wax, is fed into the inlet 56 of the mill 58 through a cell wheel slot 54 and is mixed in the mill 58 during the milling process.

The initial situation is similar to that shown in Figure 1. The output 42 of the mill 58 is also directly connected to the sieving device 44 in this case. Terms such as essentially, preferably and similar and possibly misunderstood data are to be understood as meaning that a deviation of plus-minus 5%, preferably plus-minus 2%, and in particular plus-minus one percent of the normal value is possible.

Reference mark

20melting tank 22conveyor 24pump 26container 28pump arrangement 30conductor 32feed unit 34cone 36cone coat 38pump 40layer 42outlet 44sewing device 46sewing 48 arrow 50 arrow 52task container 54cells wheel lock 56inlet 58mill 60conductor

Claims (10)

1. Method for comminuting a thermoplastic polymer, and for producing a powdery substance with a predefined grain distribution therefrom, having the following method steps:

   - predefining a desired grain distribution of the powdery substance,

   - comminuting the thermoplastic polymer, provided in lump form, in a comminution apparatus to produce a starting powder comprising grains,

   - sieving said starting powder at least once until the powdery substance with the predefined grain distribution is obtained,

   wherein a release agent is introduced into the comminution apparatus in the comminution step, wherein the release agent has the form of a fine powder and coats the grains of the starting powder in the manner of dust, thereby reducing the starting powder's tack and agglomerate formation capability,

   wherein a) the comminution step is carried out in a mill that is part of the comminution apparatus and during this process a coolant is fed into the comminution apparatus at a temperature < -50°C, or

   b) the comminution step uses a spray tower, in which the thermoplastic polymer is comminuted by melting or dissolving and then atomising by means of at least one spray nozzle, wherein an coolant is fed into the comminution apparatus at a temperature < -50°C.
2. Method according to Claim 1, characterized in that the release agent is selected from at least one from the following group: surfactants, waxes, metallic soaps.
3. Method according to Claim 1 or 2, characterized in that the release agent is hydrophobic.
4. Method according to any one of the preceding claims, characterized in that the release agent has a melting point temperature differing from the melting point temperature of the polymer by not more than 30°C.
5. Method according to any one of the preceding claims, characterized in that the release agent is supplied before the production of the starting powder is completed.
6. Method according to any one of the preceding claims, characterized in that at least one mechanically moved sieve (46), in particular a vibrating sieve (46), a sieve (46) that performs a tumbling motion, or a sieve (46) which is excited by ultrasound, is used in the sieving step.
7. Method according to any one of the preceding claims, characterized in that the release agent is supplied in a percentage by weight not exceeding 5% relative to the weight of the thermoplastic polymer.
8. Method according to any one of the preceding claims, characterized in that the release agent is supplied in a percentage by weight not less than 0.1 percent by weight relative to the weight of the thermoplastic polymer.
9. Method according to any one of the preceding claims, characterized in that the thermoplastic polymer has a Shore A hardness less than 90 at normal temperature.
10. Apparatus for carrying out the method according to any one of Claims 1 to 9, which is configured for comminuting a thermoplastic polymer and producing a powdery substance with a predefined grain distribution, having as characterizing features

    - a comminution apparatus for thermoplastic polymers provided in lump form, which includes a mill or a spray tower, and which has an inlet for the thermoplastic polymer, an outlet (42) for starting powder, and a feed line (30) for a coolant with a temperature < -50°C,

    - and a sieving apparatus (44) which is disposed downstream of the comminution apparatus and connected thereto via pipelines in which the starting powder is conveyed, wherein the sieving apparatus (44) has at least one sieve (46) which is designed for the predefined grain distribution,

    wherein the comminution apparatus has a means for introducing a release agent into the comminution apparatus.