PROCESS FOR THE PROCESSING OF AN UNDRIED, PARTICULATE POLYMER OR POLYMER MIXTURE BY MEANS OF A SINGLE- OR MULTISCREW EXTRUDER

Abstract

A process for the processing of an undried particulate polymer or polymer mixture by means of a single- or multiscrew extruder comprising a barrel comprising one or more screws, a feed section intended for the polymer and provided on the barrel and, provided on the barrel, a melting region in which the polymer or polymer mixture melts, where the undried polymer or polymer mixture introduced by way of the feed section into the barrel is devolatilized in at least one vacuum devolatilization region downstream of the feed section and upstream of the melting region in the barrel.

Claims

1. A process for the processing of an undried particulate polymer or polymer mixture by means of a single- or multiscrew extruder comprising a barrel comprising one or more screws, a feed section intended for the polymer and provided on the barrel and, provided on the barrel, a melting region in which the polymer or polymer mixture melts, wherein the undried polymer or polymer mixture introduced by way of the feed section into the barrel is devolatilized in at least one vacuum devolatilization region downstream of the feed section and upstream of the melting region in the barrel.

2. The process according to claim 1, wherein the prevailing pressure in the vacuum devolatilization region is in the range from 500 to 10-1 mbar.

3. The process according to claim 1, wherein the polymer or polymer mixture is introduced into the feed section by way of a valve.

4. The process according to claim 3, wherein the polymer or polymer mixture is introduced by way of one or more rotary valves.

5. The process according to claim 1, wherein air is introduced and flows from the barrel as far as the vacuum devolatilization region.

6. The process according to claim 5, wherein the air flows into the barrel by way of the valve, by way of introduction equipment provided on the barrel or by way of the region where the one or more screws enter(s) the barrel.

7. An extruder, in particular for the conduct of the process according to claim 1, comprising a barrel comprising one or more screws, a feed section intended for the particulate polymer or polymer mixture and provided on the barrel and, provided on the barrel, a melting region in which the polymer or polymer mixture melts, wherein there is, provided downstream of the feed section and upstream of the melting region in the barrel, at least one vacuum devolatilization region with assigned vacuum-generation equipment.

8. The extruder according to claim 7, wherein by way of the vacuum-generation equipment it is possible to generate a pressure in the range from 500 to 0.1 mbar in the vacuum devolatilization region.

9. The extruder according to claim 7, wherein arranged between metering equipment that introduces the polymer to be processed and the feed section, there is a valve arranged permitting generation of vacuum in the vacuum devolatilization region.

10. The extruder according to claim 9, wherein the valve is a rotary valve or a combination of a plurality of rotary valves.

11. The extruder according to claim 9, wherein the design of the valve is such that by way of the same air flows into the system and passes through the barrel as far as the vacuum devolatilization region.

12. The extruder according to claim 7, wherein, provided on the barrel, there is introduction equipment for air, by way of which air flows into the system and passes through the barrel as far as the vacuum devolatilization region.

13. The extruder according to claim 7, wherein in the region where the one or more screws enter the barrel they have been sealed by way of one or more sealing elements.

14. The extruder according to claim 13, wherein the design of the sealing elements is such that air flows into the system and passes through the barrel as far as the vacuum devolatilization region.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0027] In the drawing:

[0028] The single FIGURE is a schematic representation of the extruder of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] The FIGURE shows an extruder 1 of the invention, suitable for carrying out the process of the invention. The extruder comprises a barrel 2 composed of a series of separate barrel segments 3 arranged in series behind one another. The barrel 2 comprises one or more screws 4 coupled to drive equipment 5. The one or more screws 5 rotate in the barrel 2 and thus transport a polymer, which has been input and which is to be melted and/or processed, as far as a discharge point 6 at the end of the barrel 2, which is optionally followed by other equipment in which the melted polymer is further processed to form an intermediate product, as depicted by the arrow at the right-hand end of the barrel 2.

[0030] The barrel 2 comprises a feed section 7, formed by the first barrel segment shown on the left-hand side of the FIGURE. The feed section 7 comprises a feed hopper 8. Arranged thereon, or upstream thereof, there is a valve 9 which in the example shown is a rotary valve 10 (or optionally a plurality of rotary valves 10, arranged in series). Upstream of the rotary valve 10 there is in turn metering equipment 11 into which the particulate polymer 12 to be processed is introduced and by way of which the said polymer is introduced with precise metering into the valve 9.

[0031] The barrel 2 moreover comprises a melting region 13 (indicated by A), which is formed by way of a barrel segment 3 in the example shown. However, it can also, if required by its length, be formed by way of two or more barrel segments 3 arranged in sequence. The polymer melts mainly as a result of kneading by the screws, i.e. as a result of power dissipated from the drive. The barrel segment, or the respective barrel segments, can moreover be heatable, so that it is also possible to introduce an appropriate quantity of heat into the polymer by way of the barrel segment(s), so that the said polymer melts in this melting region 13.

[0032] The melted polymer is then transported onwards by way of the one or more screws 4. Downstream of the melting region 13 there are two vacuum devolatilization regions 14, in each case formed by an appropriate barrel segment 3 to which appropriate vacuum-generation equipment 15 has been assigned, usually appropriate pumps which in the example shown are attached by way of appropriate hose connections 16 at appropriate connection flanges 17 of the barrel segments 3. By way of these vacuum devolatilization regions 14 it is possible to achieve vacuum devolatilization, i.e. removal, from the barrel 2 of volatile constituents that form as a result of melting of the polymer 12, and of residual moisture that is present, with the result that the polymer discharged at the end of the barrel is very substantially or almost completely devolatilized.

[0033] According to the invention, there is a vacuum devolatilization region 18 provided between the feed section 7 and the melting region 13, i.e. downstream of the feed region 7 and upstream of the melting region 13. Its location is preferably directly before the melting region 13, i.e. before the, or the first, barrel segment whose temperature is appropriately controlled for melting. The vacuum devolatilization region 18 is formed by way of a barrel segment 3 or a plurality of barrel segments 3 connected by way of an appropriate hoseline or pipeline 20, attached to a connection flange 21 of the barrel segment 3, to appropriate vacuum-generation equipment 19, which in turn takes the form of, or comprises, an appropriate pump. By way of the vacuum-generation equipment it is possible to generate a pressure in a range that is preferably from 500 to 5 mbar; this means that an appropriate pressure reduced in comparison with atmospheric pressure is generated in the vacuum devolatilization region 18, in particular of course in the direction of the feed section 7, while in the other direction, i.e. towards the melting region 13, the barrel is sealed by the melted polymer.

[0034] By way of this vacuum devolatilization and the vacuum devolatilization region 18 it is possible to remove evaporating substances, in particular water, residual moisture, to a large extent from the barrel; this therefore means that the polymer 12, which was input in undried form, is dried during actual processing, before melting. This method therefore permits in-situ drying of the polymer 12, which was introduced in undried form. Because evaporating substances and residual moisture are removed, hydrolytic degradation of the polymer which can otherwise take place in the melting region 13 is reduced or occurs only to a negligible extent which has no effect of any kind on the quality of the intermediate product to be produced.

[0035] Generation of the desired reduced pressure in the vacuum devolatilization region 18 or in the barrel section between the feed section 7 or the valve 9 and of the melting region 13 requires appropriate sealing of the one or more screws 4 in the region of entering into the barrel 2, and this is achieved by way of appropriate sealing elements which seal the screw(s) with respect to the barrel segment. There is no need for absolute gas-impermeability here, because the pressure reduction to be generated is relatively small. The valve 9 and, respectively, rotary valve 10 should also be sufficiently vacuum-tight to permit generation of the desired reduced pressure.

[0036] It is nevertheless advantageous that a small quantity of air flows into the barrel 2 by way of the valve 9 or by way of the shaft seal, or optionally by way of both. This flow of air into the system serves, as it were, as transport medium to entrain the evaporating substances, i.e. mainly water, along the barrel and to transport the same to the vacuum devolatilization region 18, where the air flowing into the system is discharged together with the volatile substances transported. The nature of the air flow and, respectively, of the design of the valve 9 and of the shaft seal should be such that the air flow does not impair generation of the reduced pressure, i.e. that the level of reduced pressure to be achieved can be generated without difficulty, and that air flow velocity is not excessive, the aim here being that input polymer, in particular if its bulk density is very low, is not entrained by the air flow and discharged at the vacuum devolatilization region. Because discharge of a few polymer particles by way of the vacuum devolatilization region cannot be entirely excluded by virtue of the vacuum applied, it is advantageous to use vacuum-generation equipment 19 with assigned solids separator which can by way of example be arranged in the connection hose 20.

[0037] While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.