Method and device for continuously modifying a polymer melt made of non-extracted polyamide 6 with one or more additives

Abstract

A method and a device for admixing additives into a polymer melt made of non-extracted polyamide 6 are disclosed. The polymer melt is combined in a highly concentrated form with an additional melt flow without additives and mixed therewith. Additionally, a part of the melt is branched off from a main melt flow (3), wherein the sub-melt flow (4) is transported into a dispersing device (5) and is supplied and mixed with one or more additives (12). The side-melt flow (4) with additives is then returned into the main melt flow (3), mixed with the main melt flow, and subsequently supplied for further processing.

Claims

1. A process for the continuous modification of a polymer melt of non-extracted polyamide 6 with one or more additives, wherein a part of the melt is branched off from a main melt flow, this melt side-flow is conveyed into a dispersing device, one or more additives are added and mixed, the additized melt side-flow is then returned back to the main melt flow where it is mixed and subsequently transported for further processing, characterized in that the melt supplied to the dispersing device is conveyed into a first region and from there is conveyed by conveying elements into a second region; in the second region one or more additives are supplied via a side-feeder and wetted with the melt; the additives are dispersed and mixed in a third region, wherein for optimal dispersion kneading blocks and conveying elements are used alternately, followed by partially permeable conveying elements which are designed to rotate forwards and/or backwards; in a fourth region equipped with a closable degassing zone a degassing is performed or the melt is only conveyed; in a fifth region, which has backward-rotating and/or forward-rotating and/or neutral mixing elements, the additives are further mixed with the melt and then the additive-laden melt is discharged from the dispersing device via conveying elements; and the melt which is discharged from the dispersing device is conducted back into the main melt flow, where it is mixed statically and diluted to a final concentration.

2. The method according to claim 1, characterized in that the dispersing device is a co-rotating twin-screw extruder.

3. The method according to claim 1, characterized in that the mixing elements in the fifth region are toothed discs and/or partially permeable conveying elements.

4. The method according to claim 1, characterized in that the additive is TiO.sub.2.

5. The method according to claim 1, characterized in that a rotational speed of screws of side feeder is adjustable independent of a rotational speed of the dispersing device.

6. The method according to claim 1, characterized in that an additive concentration in the melt side-flow prior to back-mixing with the main melt flow is 10-30 wt. %.

7. The method according to claim 1, characterized in that an additive concentration in the main melt flow after admixing the melt side-flow is 0.03 to 3%.

8. The method according to claim 1, characterized in that a shear rate of 150 to 1800 sec.sup.1 is used in the dispersing device.

9. The method according to claim 1, characterized in that a maximum size of the additive particles in a back-mixed end product is 5 pm.

10. The method according to claim 1, characterized in that a torque density of the dispersing device does not exceed 10 Nm/cm.sup.3.

11. An apparatus for continuously modifying a polymer melt of non-extracted polyamide 6 PA with one or more additives, wherein a part of the melt is branched off from a main melt flow, this melt side-flow is conveyed into a dispersing device with at least five regions, one or more additives are added and mixed, thereafter the additized melt is conveyed again into the main melt flow, where it is mixed and subsequently conveyed to further processing, characterized in that the melt is conveyed into a first region of the dispersing device equipped with conveying elements and from there is conveyed by conveying elements into a second region; on the second region equipped with the conveying elements a side feeder is arranged, which feeds one or more additives into the dispersing device and the introduced additives are wetted with the melt: in a third region equipped alternately with kneading blocks and conveying elements followed by partially permeable conveying elements which are designed to rotate forwards and/or backwards, the additives are dispersed; in a fourth region equipped with conveying elements or partially permeable a closable degassing device is arranged; in a fifth region, which has backward-rotating and/or forward-rotating and/or neutral mixing elements followed by conveying elements, the additives are further mixed with the melt and then the additive-laden melt is discharged from the dispersing device; and the melt which is discharged from the dispersing device is conducted back into the main melt flow, where it is mixed statically and diluted to a final concentration, wherein the discharged melt is either directly conducted back into the main melt flow for producing a single additive laden melt or is divided into multiple main melt flows which are respectively conducted into the main melt flows of the main melt flow which is de divided into multiple main melt flows for producing multiple melt flows having different additives.

12. Device according to claim 11, characterized in that the dispersing device is a co-rotating twin-screw extruder.

13. Device according to claim 11, characterized in that the mixing elements in the fifth region are toothed discs and/or partially permeable conveying elements.

14. Device according to claim 11, characterized in that a rotational speed of screws of side feeder is adjustable independent of a rotational speed of the dispersing device.

15. Device according to claim 11, characterized in that an additive concentration in the melt partial flow prior to back-mixing with the main melt flow is 10-30 wt. %.

16. Device according to claim 11, characterized in that an additive concentration in the main melt flow after admixing of the melt partial flow is 0.03 to 3%.

17. Device according to claim 11, characterized in that in the dispersing device a shear rate of 150 to 1800 sec.sup.1 is used.

18. Device according to claim 11, characterized in that a torque density of the dispersing device does not exceed 10 Nm/cm.sup.3.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a schematic illustration of a process sequence according to the invention; and

(2) FIG. 2 shows a schematic illustration of the construction of a dispersing device according to the invention

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(3) The process sequence will be described below with reference to FIG. 1. According to the invention, non-extracted polyamide 6 melt is conveyed from a one- or two-stage polymerization plant (in FIG. 1, by way of example, a two-stage plant having a first stage (1) and a second stage (2)) is conveyed through a main melt line (3), from which a partial melt flow (4) is branched off and metered via a melt pump (not shown in FIG. 1) into a dispersing device (5). The dispersing device (5) is preferably a twin-screw dispersing extruder or a compounder. Downstream, one or more additives, shown in FIG. 1 exemplarily for TiO.sub.2, are metered via a side feeder to the dispersing device (5), where they are first wetted with melt and subsequently dispersed. An example of a side feeder can be found in The twin-screw extruder: Basics and applications/Publisher: Association of German engineers, VDI Society Plastics Technology4th revised editionDusseldorf, VDI-Verlag, 1998, page 63. According to the invention, the additives are dyes and pigments, antistatics, antiblock additives, slip additives and/or flame-retardant additives, most preferably TiO.sub.2, carbon black, inorganic color pigments or silica/siliceous earths. Shortly before the melt discharge, it is optionally possible to degas before the melt is conveyed out of the dispersing device (5) after further mixing.

(4) Subsequently, the mixture is fed to a further melt pump (not shown in FIG. 1) behind the dispersing device (5), metered back into the main melt line (3) where it is again statically mixed once more. In order to achieve different degrees of matting, the main melt line (3) can advantageously be divided into, for example, main melt partial flows (6, 7, 8) for a Bright, Semi Dull and Full Dull product, and then, corresponding quantities of the melt partial flow (4) for the Semi Dull and Full Dull product are fed or further transported for further processing for the Bright product without addition from the melt partial-flow (4).

(5) In contrast to the configuration described in DE 40 39 857 A1, the additive is not introduced into the first extruder housing, but downstream via a side feeder. The advantage of the use according to the invention of a side feeder is that the rotational speed of the side feeder screws can be set separately from the speed of the compounder. In the process described in DE 40 39 857 A1, this is not possible. Bonding, as described for the process of Chemiefaser/Textilindustrie 1 (1986) 24 et seq. does not occur. A supply of the additive in the first housing of the extruder, as described in DE 40 39 857 A1, is thus not required.

(6) In addition, it has surprisingly been found that it is also possible to dispense with a side-feeder in an embodiment as described in DE 10 2007 060 338 A1. An elaborate grinding of screw elements, as found necessary in DE 40 39 857 A1, or the use of a conveying gap to prevent compacting effects during feeding, as disclosed in DE 10 2007 060 338 A1, is surprisingly not required. Thus standard feed equipment can be used, which further increases the efficiency of the process.

(7) Since the new method uses a melt charge, the torque density in the disperser (5) is small. It is preferably less than 10 Nm/cm.sup.3. The mean shear rates, calculated according to the above-mentioned approximation formula from EP 0 852 533 B2, are significantly greater than 150 s.sup.1 at a ratio D.sub.a/D.sub.i of 1.48 present in the examples below. Preferably, in the dispersing device (5) an average shear rate calculated from the approximation formula of 150 to 1800 sec.sup.1, more preferably from 250 to 1500 sec.sup.1, most preferably from 350 to 1200 sec.sup.1 is used.

(8) The additive concentration in the melt side-flow (4) prior to back-mixing with the main melt flow (3) is preferably 10-30 wt. %, more preferably 14-25 wt. %, most preferably 17-22 wt. %.

(9) Preferably, the additive concentration in the main melt flow (3) or the main melt side-flows (7, 8) after admixing the melt partial flow (4) is 0.03 to 3 wt. %, particularly preferably 1.5 to 2.3 wt. %, most preferably 1.7 to 2.0 wt. %. The produced product is preferably a Full Dull product (in the case of PA 6 corresponding to 1.7-1.8 wt. % of TiO.sub.2).

(10) The maximum size of the additive particles, in particular for the case of TiO.sub.2 particles, in the back-mixed end product is preferably 5 m, more preferably 3 m.

(11) The construction of a dispersing device (5) according to the invention and its melt supply and discharge is shown by way of example in FIG. 2. The extruder used in the examples described below is a compounder of the ZE25A-48D-UTX type from KraussMaffei Berstorff GmbH. The screw structure of the twin screw (11) of the compounder driven by the extruder drive (10) has the following five regions with different treatment zones: a first region (13) with a zone (a) for feeding the non-extracted PA 6 by means of the melt pump (9), thereafter downstream optionally alternating kneading blocks and in any case conveying elements (b). The kneading blocks can be designed to be conveying, neutral or backward-conveying in the entire process, depending on the process requirement. This is followed downstream in a second region (14) by a zone for introducing the additive (c) via a side feeder device connected to the connection for the side feeder (12), with subsequent wetting. Following thereafter, in a third region (15), are again alternately kneading blocks and conveying elements for optimal dispersion, followed by partially permeable conveying elements (zone (d)), which can be designed to rotate forward or backward. This is followed in a fourth region (16) by a closable degassing zone (e) equipped with conveying elements or partially permeable conveyor elements, followed in a fifth region (17) by mixing elements such as pulleys, which may also be configured for backward or forward rotation, in zone (f). Finally, subsequent thereto in zone (g) the highly concentrated melt is discharged from the compounder via conveying elements. After the dispersing device (5), the additized melt is further transported by means of the melt pump (18).

(12) The mentioned sequence of the five areas with the treatment zones is predetermined according to the invention, but the exact screw structure can be adapted in detail to the respective polymer and additive properties as well as the desired additive loading by means of the abovementioned options.

(13) In the examples, a TiO.sub.2 product with the name Hombitan LO-CR-S-M W/O Si from the company Sachtleben was used.

(14) The comparative example is a commercially available Full Dull PA 6 product from LiHeng (Changle) Polyamide Technology Co., Ltd., which is used by LiHeng as a raw material for various yarns. It is a finished, extracted PA 6 polymer in which the low molecular weight fractions were removed by extraction prior to the additizing. This example serves for comparing the TiO.sub.2 particle size distribution.

(15) Table 1 summarizes the experimental data for the preparation of the examples in the compounder.

(16) TABLE-US-00001 TABLE 1 Comparative Example 1 Example 2 Example Melt-entry temperture, [ C.] 265 265 Mass temperature (Exit), [ C.] 269 269 Mass pressure, [bar] 11 14 Rotational speed, [min.sup.1] 1200 1200 Throughput ZE25, [kg/h] 25.5 12.75 TiO.sub.2-concentration in the melt 10 20 partial flow, [weight.-%] Total throughput, [kg/h] 150 150 TiO.sub.2-final concentration, 1.7 1.7 1.7 [weight.-%]

(17) The samples of the examples and the comparative example were characterized by Scanning Electron Microscopy (SEM) and viscometry.

(18) In the SEM investigations, the samples were measured by scanning electron microscopy and evaluated by means of the statistical evaluation method x.sub.50. A description of the statistical evaluation x.sub.50 can be found in The Science and Engineering of Granulation Processes, Jim Litster, Bryan Ennis, Springer Science+Business Media Dordrecht, 2004, ISBN 978-1-4020-1877-0, page 17. For the present investigations, an evaluation based on the volumetric distribution was used, which accordingly generates a higher weighting of the larger particles relevant for the clogging of melt filters than a number-based distribution. The values contained in the following Table 2 correspond to particles that are passed through in a cumulative volume-based analysis of 50% of the particles.

(19) The actual electron microscopic measurement was carried out as follows:

(20) For sample preparation, the chips were fixed in a sample holder and subjected to the plasma etching process. To then capture the chips, four chips were fixed in the sample holder and three images per chip or twelve images per sample were made.

(21) The SEM measurement detects particles larger than 80 nm on a viewing area of at least 0.0074 mm.sup.2. This results in a total particle number of about 500-1000 particles. The measured particles are then divided into parts >0.3 m/>0.6 m/>1.0 m and evaluated as described above by means of the static evaluation x.sub.50.

(22) The relative viscosities were measured according to DIN EN ISO 307. The measurement results are summarized in Table 2.

(23) TABLE-US-00002 TABLE 2 Comparative Example 1 Example 2 Example SEM (cumulative, x.sub.50, 0.48 0.45 0.47 TiO.sub.2), [m] 3 largest TiO.sub.2-Partikel 2.37/1.25/1.22 1.3/1.12/1.1 1.12/1.03/1.0 (SEM), [m] Relative Viscosity, [] 2.29 2.30 2.5

(24) As shown in Table 2, the novel process provided an excellent particle size distribution as compared to the commercial product. Thus, the product produced by the new process is absolutely comparable to the commercial product from a conventional production.

(25) The relative viscosities of the samples of Example 1 and Example 2 were 2.29 and 2.30. In addition to the comparative example of Table 2, which represents the commercial product, a blank sample was taken from the non-extracted, i.e., non-extruded starting material examined. This non-extruded and non-extracted starting material had a relative viscosity of 2.25. That is, no polymeric chain degradation has occurred during the compounding process.

(26) The measured maximum TiO.sub.2 particle sizes are usually less than 2 m after back-mixing for the Full-Dull end product. Considering that a much coarser filtration is used in a PA 6 production process, e.g. 10 m, this is an excellent and surprising result. The use of an enhanced filtration, for example of the type that is usually essential for a Full Dull end product to obtain an acceptable end product is no longer necessary according to the new method. That is, the otherwise commonly occurring harmful re-agglomeration behind the compounder that results in particle sizes capable of clogging production filters of fineness customary in production does not occur.

(27) The scanning electron micrographs show comparable to better values than the commercial Full Dull product, which was produced by the conventional process with TiO.sub.2 addition in the polymerization process.