WO2006087323A1

WO2006087323A1 - Method for crystallising an amorphous plastic granulate

Info

Publication number: WO2006087323A1
Application number: PCT/EP2006/050918
Authority: WO
Inventors: Achim Becker; Milica Engel
Original assignee: Mann+Hummel Protec Gmbh
Priority date: 2005-02-21
Filing date: 2006-02-14
Publication date: 2006-08-24
Also published as: DE102005008115A1

Abstract

The invention relates to a method for crystallising amorphous plastic granulates. According to said method, a liquid, preferably water, or another liquid for crystallising granulates is poured into a mixing funnel. The temperature of the liquid is controlled for the respective material according to the crystallisation temperature requirements. Once crystallised, the material is removed from the container and dried. The advantages of the crystallisation process are that an agglomeration of the plastic granulate is prevented as a result of the fluid coating of the particle surface. In addition, the thermal transfer from the liquid to the particles is improved and can be carried out in a more targeted manner. There is also no risk of the particles overheating during crystallisation.

Description

Process for crystallizing an amorphous plastic granulate

Technical area

The invention relates to a method for crystallizing at least the outer skin of grains of an amorphous plastic granules according to the preamble of independent claim 1 and a device in particular for carrying out this method.

State of the art

From EP 0310968 B a method for crystallizing amorphous plastic granules is known. In this heating gas is supplied with a lying above the crystallization temperature of the granulate inlet temperature in a mixing funnel. The heating gas is passed in countercurrent through the granules, the granules are heated to crystallization temperature and then removed. The removal temperature of the granules is detected and the inlet temperature of the heating gas controlled such that it is above a predetermined value of the extraction temperature of the granules.

This method is characterized in that to avoid overheating of the granules, the temperature of the heating gas must be controlled very precisely. Overheating of the granules leads to an impermissible discoloration, which impairs the further processing of the granules.

It is also known from WO 0236255 a device for the thermal treatment or aftertreatment of plastic material, in particular of polyester material such as polyethylene terephthalate (PET) known. The fumigation of the granules takes place mainly in the conical outlet region of a shaft reactor. The disadvantage of this device is that only part of the granulate located in the outlet area is loaded. In addition, the leakage of the plastic granules through the shaft reactor and the movement within the shaft reactor causes abrasion of the plastic granules on the shaft resulting in abrasion and thus plastic dust, which must then be separated from the plastic granules. There is also the danger ² VSS 2289

the bonding in the glass transition area, d. H. the granules must be separated again in a complex manner.

The invention has for its object to avoid the disadvantages mentioned and to provide a method and apparatus for crystallizing an amorphous plastic granules, in which a gentle and temperature-accurate treatment of the granules is possible.

This object is achieved on the basis of the preamble of the independent claim 1 by its characterizing features.

Disclosure of the invention

The main advantage of the invention is that the heat transfer is not a gas, but a liquid, such as water, a liquid with water or a solvent is used.

In the heating of thermoplastics usually amorphous molecular structures are rearranged (glass transition). This can lead to a temporary softening of the plastic. The softening of the plastic often leads to bonding effects, which are strongly favored by the exothermic heat formed. Adhesions are agglomerations which significantly affect the flowability of the material or even bring it to a standstill. Especially in the crystallization with hot air or gas there is a risk that the granules stick together and cake due to the heating of the surfaces.

By the method according to the invention during this softening phase, the material against static loads, z. B. by the own bulk load, and thus maintained after the molecular reorientation or termination of the glass transition state upon solidification a Einzelkomschüttung, which allows the flowability of the material.

Thus, the inventive method achieves a decrease in load of the bed at the required temperature increase for crystallization and agglomeration prevention by changing the bonding conditions. The static material pressure of a bed is determined by the ³ VSS 2289

Use of the liquid medium changed. The material particles are surrounded by the liquid medium and float - due to the buoyancy and the difference in density - in the liquid. The weight of a bed is thus weakened, plus the buoyancy of the liquid. Ideally, the crystallization is therefore possible in a weightless state of the material to be crystallized. In this case, the particles to be crystallized float in the liquid. As the density of the plastic granules increases due to crystallization, e.g. B. from 1, 33 to 1, 38 kg / dm ³ , the already crystalline shares will fall faster. Each individual granule surface is coated with a liquid layer. An adhesion of several granules is therefore not expected by the intervening liquid layer and the statically eliminated load by bulk load in the crystallization of the particles. The individual material densities of plastics, such as polyester, polyethylene terephthalate, polylactide (polylactic acid), are in the range of the density of water, whereby preferably water can be used as the base medium.

Another advantage of the method is that no large amounts of air masses are required. Due to the higher density and better heat transfer from the liquid to the particles, depending on the crystallization parameters, energy savings can be expected. In addition, due to the lower volume and the higher density of water to air, the energy cost is much lower.

According to one embodiment of the invention salts can be added to the water to increase the density and / or increase of the boiling point or low boiling components to lower the boiling point or components to reduce the density.

Alternatively or additionally, it is also possible to crystallize the material to be crystallized in a container under pressure. This also makes it possible to influence the crystallization temperature.

As already mentioned, the density of the liquid medium can be adapted to the density of the material to be crystallized. The advantage ⁴ VSS 2289

is that during the process, the amorphous Kunststoffgranulatbestandteile initially have a lower density and thus swim in the upper region of the liquid. During crystallization, these condense and sink down, so that a continuous, continuous conveying process of the granules takes place. This also means that the granules can be removed continuously crystallized in the lower part of the container, while in the upper part of the container, the material to be crystallized is filled.

In a further embodiment of the invention, a mixing or turbulence of the plastic granules takes place within the container. This can be supported for example by compressed air.

It is also possible to keep the granules in motion by means of appropriate stirring devices.

A device, in particular for carrying out the method has a funnel-shaped container having a Granulateinfülleinrichtung above and at the bottom of a granule removal device and connections for the supply and discharge of the heat transfer medium or the heated water.

Further, a stirring device is provided in the container. For tempering the heat transfer medium, d. H. the liquid, a heater is provided within the container - conveniently in the lower region or outside the container.

The stirring device may be a vertical stirring rod in one embodiment of the invention. It is also possible to arrange one or more horizontal stirring rods in the container, which are driven by a motor. For discharging the crystallized plastic granules in a further embodiment of the invention, a pneumatic slide or a rotary valve is provided.

If a pressure to be built up in the container, the pressure-stable container compressed air inlet nozzles for supplying compressed air or gases.

[0021] A particularly advantageous form of the container for optimal crystallization is a lower funnel-shaped outlet part and an upper cylinder. ⁵ VSS 2289

drischer part, which has the largest possible volume for a high throughput. ] Another advantage of this process is that recycled material can be crystallized using this process. For example, such recy- cles can be found in the recycling of PET bottles that are being reprocessed. Such material may also be polylactide (PLA) as a raw material that crystallizes

Lactic acid from glucose, to which in a second step a poly-

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lent the use in different, also technically demanding applications in the medical technology, as technical plastic and as

A working method for crystallizing amorphous plastic granules preferably consists of the following process steps: The granules are introduced into the container and crystallized continuously or discontinuously according to the container design. The liquid temperature adjustment is adjusted slightly above the glass transition temperature of the material. Within the container, a liquid and granular movement is generated, wherein the plastic granules sink during crystallization down and discharged through a discharge lock. Subsequently, a drying of the plastic granules in a drying device and the return of moisture or liquid in the container. This means that the discharged water is returned directly to the crystallization process and this is also associated with an energetic advantage. These and other features of preferred embodiments of the invention will become apparent from the claims and from the description and the drawings, the individual features may be realized alone or in each case in the form of sub-combinations in the embodiment of the invention and in other fields ⁶ VSS 2289

and may represent advantageous and protectable versions for which protection is claimed here.

Brief description of the drawings

The invention will be explained in more detail with reference to embodiments. 1 shows a flow chart for the production of plastic granulate according to the prior art, FIG. 2 shows a flow chart for the production of plastic granules according to the invention,

Figure 3 shows the structure of a device for the crystallization of plastic in a schematic representation.

Embodiment (s) of the invention

FIG. 1 shows the process chain of a granulation. First, a plastic melt is fed to an extruder 10. At the outlet of the extruder, underwater granulation is carried out with a suitable granulating device 11. After granulation, drying takes place in a Flash Dryer 12. The dried material is transported via containers or silos, for example pneumatically without any intermediate stop, to the processing plant. There, the crystallization takes place in a crystallization tank 13 before the use of the plastic. This process is generally state of the art. In the method according to the invention, the combination of underwater granulation with drying via the flash dryer can be dispensed with.

FIG. 2 shows the reduced expenditure on equipment. Here, the plastic material supplied from the extruder is granulated in a water crystallization vessel 14 and at the same time transferred from an amorphous state to a crystalline state. The drying of the material then takes place in a plastic dryer 15 except for a residual moisture content required for processing or transport.

Of course, the water crystallization technique can also be used in the prior art according to FIG. In this case, the crystallization tank 13 is replaced by the crystallization process or container according to the invention. ⁷ VSS 2289

Figure 3 shows a schematic representation of a stirrer tank 16 in a schematic representation in which, according to the arrow 17, the amorphous plastic granules to be crystallized are introduced. The agitator container is cylindrical in the upper region, in the lower region it has a conical part 18. Below the conical part 18, a discharge valve 19 connects. Within the container there is an agitator 20, consisting of a vertical shaft 21 and arranged thereon Rührarmen 22. On the container inner wall a plurality of webs 23 are arranged, which provide in conjunction with the Rührarmen 22 for a good mixing. The agitator 20 is driven via a motor 24 arranged on the container. In the lower region of the container is a liquid outlet 25. The discharged liquid is led to a heating device 26 and supplied from the heating device via the line 27 to the container in the upper area again. At the conical portion 18 is a temperature sensor 28 and an inlet 29 for possibly required compressed air. As an alternative to the heating device 26, a heater 30 can also be arranged on the container below the conical part 18. The material to be crystallized, which is preferably in granular or flake form, is introduced from above in accordance with arrow 17 in the mixing funnel. The material falls by gravity into the container filled with liquid. The material particles are surrounded by the liquid medium and move due to the density ratios of gravity following through the crystallization vessel, wherein the pressure load of the individual particles is reduced by the buoyancy of the liquid to air. Each individual particle surface is covered by a liquid layer. A bonding of several particles is not to be expected by the intermediate liquid layer and the eliminated static load of the bulk load.

The material is heated by the liquid to crystallization temperature in the mixing funnel and then removed, wherein the extraction temperature of the granules is detected by the temperature sensor 28. Thus, the amount of heat introduced into the agitating funnel can ⁸ VSS 2289

be controlled. A plastic granulate, such as thermoplastic polyester, may be in amorphous or partially crystalline form. To transform the granulate from amorphous or partially crystalline to the crystalline state, the granules are heated by the liquid to a temperature of 90 ° to 110 °. The crystallized material sinks down and is discharged via the discharge valve 19. Subsequently, the liquid is separated from the material and returned to the process via a suitable pump and the material dried in a downstream dryer.

Claims

⁹ VSS 2289

claims

1. A process for crystallizing at least the outer skin of grains of an amorphous plastic granules, wherein in a hopper a heated medium with an in particular above the crystallization temperature of the granules lying inlet temperature is supplied and the granules are passed through the hopper, wherein the granules heated to crystallization temperature and is subsequently removed, characterized in that for heat transfer, a liquid, in particular a liquid with water or water fractions or a solvent is used.

2. The method according to claim 1, characterized in that the liquid medium has salts for increasing the boiling point or low-boiling components for lowering the boiling point.

3. The method according to claim 1 or 2, characterized in that the material to be crystallized is crystallized in a container under pressure.

4. The method according to any one of the preceding claims, characterized in that the density of the liquid medium corresponds to the density of the material to be crystallized or smaller than the density of the crystallized or partially crystallized material.

5. The method according to any one of the preceding claims, characterized in that the inlet temperature of the supplied liquid medium is maintained in a range of 60 ° to 120 ° C.

6. The method according to any one of the preceding claims, characterized in that the liquid medium compressed air is supplied to support the mixing or turbulence of the plastic granules. one of the previous ones

8. Device, in particular for carrying out the method, according to one of claims 1 to 6, having a preferably funnel-shaped container having a Granulateinfülleinrichtung above and an outlet for the heat transfer medium and in the lower part of a granule removal device and a connection for supplying the heat transfer medium and ¹⁰ VSS 2289

wherein in the container at least one stirring device is provided, characterized in that the heat transfer medium is a liquid medium, in particular water, or a medium with water fractions.

9. Apparatus according to claim 7, characterized in that inside or outside of the container in a liquid line, a heating device for controlling the temperature of the liquid medium is provided.

10. The device according to claim 7, characterized in that the stirring device consists of at least one vertically or horizontally arranged stirring rod with stirring blades.

11. The device according to claim 7, characterized in that a plurality of horizontally or vertically arranged stirring devices are provided.

12. The device according to claim 7, characterized in that the arrangement for discharging the crystallized plastic granules is a pneumatic slide or a rotary valve.

13. The device according to claim 7, characterized in that in the lower region of the container compressed air inlet nozzles are provided for supplying compressed air or gases.

14. The device according to claim 7, characterized in that the funnel-shaped container has a cylindrical upper part, which is fastened on a conically tapered to the granulate receiving subset and wherein a shaft extends axially through the container, which axially spaced stirrer blades and on the inner wall the container having the wings cooperating, extending into the container interior webs.

15. The device according to claim 7, characterized in that at least one temperature sensor is provided within the container, by which the temperature of the liquid is controlled to a variable setpoint and wherein an exothermic energy is taken into account in the product to be crystallized in the temperature control of the liquid.

16. A method of crystallizing amorphous plastic granules, the method comprising the steps of:

Filling a liquid in a container and simultaneously tempering the liquid to a range of 60 ° to 120 ° C, ¹¹ VSS 2289

Filling of amorphous plastic granulate into the container,

Generating a liquid and granulate movement within the container, the plastic granules remaining within the container until crystallization of at least the outer skin of the granulate takes place,

Discharging the crystallized portion of the plastic granules via a discharge lock,

- Subsequent drying of the plastic granules in a drying device and

- Returning the moisture or the liquid into the container.