Method for producing thermoplastic molding compounds, and thermoplastic molding compounds produced according thereto

Abstract

A method for extruding thermoplastic molding compounds, the production of thermoplastic molding compounds taking place in a screw machine with mechanical dewatering, and impact-modified molding compounds or polymer blends which contain impact-modified thermoplastic materials that were produced by means of the method according to the invention.

Claims

1. A process for the production of thermoplastic molding compositions, where an extruder is used which consists essentially of the following, in the direction of conveying (in the direction of flow): a) at least one feed zone (DA) where a thermoplastic molding composition comprising water is introduced by means of feed equipment into the extruder, where said feed zone (DA) optionally comprises at least one dewatering aperture equipped with a metal-wire-mesh composite sheet (MV), a finely perforated metal sheet, or a slit diaphragm, b) at least one squeeze zone (Q) which serves to dewater the thermoplastic molding composition, which comprises at least one baffle, and which also comprises in each case at least one associated dewatering aperture, said at least one dewatering aperture being equipped with a metal-wire-mesh composite sheet (MV), a finely perforated metal sheet, or a slit diaphragm, c) at least one input zone (Z) where other components of the thermoplastic molding composition in the form of melt are introduced into the extruder, d) at least one plastifying zone (P) provided with mixing and/or kneading elements, e) at least one vent zone (E) which has at least one vent and in which further water or liquids are removed from the thermoplastic molding composition as vapor, where at least one of the vents is preferably equipped with a metal-wire-mesh composite sheet (MV), a finely perforated metal sheet, or a slit diaphragm, and f) a metering zone (AT) at which the thermoplastic molding composition is discharged from the extruder, where the squeeze zone (Q) in the process uses a screw-based extruder (S) in which the diameter (Ds) of at least one screw is from 30 to 230 mm, and where the rotation rate (DZ) of the at least one screw of the extruder is from 60 to 270 rpm, and where the screw-based extruder (S) used has at least one vent and/or dewatering aperture where there is, secured in the dewatering aperture, at least one metal-wire-mesh composite sheet (MV), with two or more layers, there being at least one smaller-mesh layer present on a coarsely woven, large-mesh but mechanically stable backing layer (first layer), where the location of the layer having the smallest mesh is on a side facing toward the product, and where the thermoplastic molding composition comprises at least one rubber-modified styrene-acrylonitrile (SAN) copolymer, with at least one acrylate-styrene-acrylonitrile (ASA) rubber with bimodal particle size distribution and an average particle size from 80 nm to 600 nm, and also with an SAN matrix with AN content from 25% by weight to 35% by weight.

2. The process as claimed in claim 1, characterized in that the thermoplastic molding composition comprises at least one rubber-modified styrene-acrylonitrile copolymer, where a rubber component is based on an acrylate-styrene-acrylonitrile copolymer or on a polybutadiene.

3. The process as claimed in claim 1, characterized in that the thermoplastic molding composition comprises at least one impact-modified copolymer or one impact-modified copolymer blend, and also optionally other components, and the thermoplastic molding composition is produced from a component comprising water, comprising up to 90% by weight of residual water, and said component is optionally mixed with the other components with devolatilization and/or dewatering, and then the thermoplastic molding composition is discharged from the extruder.

4. The process as claimed in claim 1, characterized in that the process comprises, as at least one step, devolatilization and/or mechanical dewatering.

5. The process as claimed in claim 1, characterized in that the diameter (Ds) of at least one screw of the screw-based extruder (S) is from 80 mm to 180 mm.

6. The process as claimed in claim 1, characterized in that the rotation rate (DZ) of the at least one screw of the screw-based extruder (S) is from 100 rpm to 200 rpm.

7. The process as claimed in claim 1, characterized in that the pressure in the squeeze zone (Q) of the screw-based extruder is from 10 bar to 55 bar.

8. The process as claimed in claim 1, characterized in that the screw-based extruder (S) is equipped with at least two co- or contrarotating screws with diameter (Ds) from 30 mm to 230 mm.

9. The process as claimed in claim 1, characterized in that the metal-wire-mesh composite sheet has from 2 to 30 layers.

10. The process as claimed in claim 1, characterized in that the average mesh width of the smallest-mesh layer of the metal-wire-mesh composite sheet (MV) is from 1 m to 500 m.

11. A thermoplastic molding composition produced in an extruder by a process as claimed in claim 1, where the thermoplastic molding composition comprises at least one rubber-modified styrene-acrylonitrile (SAN) copolymer, with at least one acrylate-styrene-acrylonitrile (ASA) rubber with bimodal particle size distribution and an average particle size from 80 nm to 600 nm, and also with an SAN matrix with AN content from 25% by weight to 35% by weight.

12. The thermoplastic molding composition as claimed in claim 11, comprising a butyl acrylate-styrene-acrylonitrile copolymer, and also an SAN copolymer, and optionally other components, characterized by less than 20 ppm content of 1-butene.

13. The thermoplastic molding composition as claimed in claim 11, characterized by less than 50 ppm content of 1-butene, based on the total mass of the thermoplastic molding composition.

Description

(1) The figure FIG. 1 is a diagram of the extruder. FIG. 1 shows the arrangement of the feed zone (FZ), the squeeze zone (SZ), the input zone (IZ), the plastifying zone (PZ), the vent zone (VZ), and the metering zone (MZ) in the extruder which is used for the mechanical dewatering of the elastomer A and for mixing with the thermoplastic B. A is added in (FZ), whereas B is introduced in (IZ). The squeeze zone (SZ) of the screw-based extruder has the screw diameter (Ds), and also the screw rotation rate (RR). There is moreover a metal-wire-mesh composite sheet (MC) incorporated in the squeeze zone (SZ).

(2) The examples and the claims provide further explanation of the invention.

COMPARATIVE EXAMPLE AND INVENTIVE EXAMPLE

(3) The comparative example and inventive example compare the resultant content of degradation products in the product at high and low rotation rates (RR) of the extruder screw and also with and without use of a metal-wire-mesh composite sheet (MC) in the squeeze zone (SZ).

(4) TABLE-US-00001 Comparative example Inventive example Extruder ZSK133 with Ds = ZSK133 with Ds = 133 mm 133 mm Zone FZ, Conveying elements Conveying elements Feed zone Zone SZ1, Baffles/conveying Baffles/conveying Squeeze zone element + retention elements + retention screw screw Zone SZ2, Baffles/conveying Baffles/conveying Squeeze zone element + retention element + metal-wire- screw mesh composite sheet Zone IZ, Mixing elements Mixing elements SAN input rpm in SZ2 270 160 Pressure in SZ2 57 bar 32 bar Throughput of 1.1 t/h 1.1 t/h rubber* Throughput of 1.1 t/h 1.1 t/h SAN polymer* Degradation products in compounded ASA polymer in ppm of 1-butene 51 6 ppm of residual 39 16 styrene *ASA rubber and SAN polymer as described in EP-A 1 400 337; Quantity of rubber based on dry rubber

(5) The mesh width of the metal-wire-mesh composite sheet (MC) used in squeeze zone SZ2 in the inventive example is 75 m. 1-Butene content in the compounded ASA polymer was determined by weighing 0.5 g of a sample into a 22 ml headspace ampoule. The sample is heated at 80 C. for 3 hours. The volatile constituents are then quantified by headspace GC analysis.

(6) The specific use of a metal-wire-mesh composite sheet (MC), and the reduction of the screw rotation rate (RR) from 270 to 160 rpm permitted processing of the molding compositions under less aggressive conditions. This can be discerned inter alia from the content of degradation products of the thermoplastic molding composition. In the inventive example the undesired content of the degradation product of 1-butene is reduced from 51 ppm to 6 ppm, and the content of residual monomer styrene is reduced from 39 ppm to 16 ppm.

(7) The (thermoplastic molding) compositions obtained can be processed with better results and lead to higher-specification moldings.