Degassing method, degassing device and use of screw elements

Abstract

The present invention relates to a process for devolatilising polymer-containing media such as, in particular, polymer melts, polymer solutions and dispersions and also devolatilisation apparatuses for carrying out the abovementioned process.

Claims

1. A process for removing volatile compounds from a polymer-containing medium which contains at least one polymer and at least one volatile compound, the process comprising: a) introducing the polymer-containing medium into a devolatilisation apparatus comprising: a barrel and n barrel holes B.sub.n having associated hole diameters D.sub.n where n is an integer, one or more screws W.sub.n arranged concentrically in each of the barrel holes B.sub.n and configured to be rotatable within the barrel holes, wherein each screw has an axis of rotation A.sub.n and is equipped with treatment elements, the treatment elements having a cross-sectional profile orthogonal to the axis of rotation, wherein the cross-sectional profile in a circumferential direction has: m relative maxima R.sup.m.sub.max n in respect of a radial dimension of the cross-sectional profile to the axis of rotation A.sub.n of the screw W.sub.n, where m is an integer from 1 to 8, a maximum value R.sub.max n in respect of the radial dimension of the cross-sectional profile to the axis of rotation A.sub.n of the screw W.sub.n, where R.sub.max n fulfils:
R.sub.max n<=(D.sub.n/2) at least one feed zone, one or more devolatilisation zones comprising in each case at least one devolatilisation opening which is suitable for discharge of volatile constituents from a polymer-containing medium from the extruder, and at least one discharge zone, wherein the treatment elements comprise screw elements having a pitch t, and the screw elements are configured so that each of the following three conditions are met: S1) the cross-sectional profile has at least one relative maximum R.sup.m.sub.max n based on the radial dimension of the profile curve, for which:
0.420 D.sub.n<R.sup.m.sub.max n<0.490 D.sub.n, S2) 1.38 D.sub.n<t<5.00 D.sub.n, and S3) the cross-sectional profile of the respective screw element has no tangential angle greater than 30, on the active flanks located at the front in the direction of rotation in the range of the radial dimension from 0.95 R.sub.max to R.sub.max, where the tangential angle is defined as a smaller of the two angles formed on drawing tangents at any point on the cross-sectional profile of the treatment element at which the cross-sectional profile cannot be continually differentiated, and is 0 at any point on the cross-sectional profile of the treatment element at which the cross-sectional profile can always be differentiated; and b) operating the devolatilisation apparatus to remove volatile compounds from the polymer-containing medium through the devolatilisation openings of the devolatilisation zones to deplete the polymer-containing medium of volatile compounds and isolate polymer as product P from the polymer-containing medium on discharge from the devolatilisation apparatus, wherein the product P has a lower proportion of volatile compounds than the polymer-containing medium introduced into the devolatilisation apparatus.

2. The process according to claim 1, wherein: the polymer-containing medium comprises at least one of natural or synthetic polymers, thermoplastic polymers, and elastomers; and the process comprise operating the devolatilisation apparatus to produce product P having a total concentration of volatile compounds of less than 1% by weight, based on the mass of the polymer.

3. The process according to claim 2, wherein: the thermoplastic polymers are selected from a group that includes polycarbonates, polyamides, polyesters, polylactides, polyethers, thermoplastic polyurethanes, polyacetals, fluoro polymers, polyether sulphones, polyolefins, polyimides, polyacrylates, polyphenylene oxide, polyphenylene sulphide, polyether ketone, polyaryl ether ketone, styrene polymers, styrene copolymers, acrylonitrile-butadiene-styrene block copolymers, and polyvinyl chloride; and the elastomers are selected from a group that includes styrene-butadiene rubbers, natural rubbers, butadiene rubbers, isoprene rubbers, ethylene-propylene-diene rubbers ethylene-propylene rubbers, butadiene-acrylonitrile rubbers, hydrogenated nitrile rubbers, butyl rubbers, halobutyt rubbers, chloroprene rubbers, ethylene-vinyl acetate rubbers, polyurethane rubbers, guttapercha, fluoro rubbers, silicone rubbers, sulphide rubbers, chlorosulphonyl-polyethylene rubbers.

4. The process according to any of claims 1, characterized in that the polymer-containing medium contains butyl rubber and/or halogenated butyl rubbers.

5. The process according to claim 1, wherein the polymer-containing medium is in the form of suspensions, pastes, melts, solutions, particulate solid compositions or mixed forms of the abovementioned forms.

6. The process according to claim 1, wherein the polymer-containing medium contains from 3 to 98% by weight of a polymer and from 2 to 97% by weight of volatile compounds where the polymer and volatile components are 90-100% by weight-of the total mass of the polymer-containing medium.

7. The process according to claim 6, wherein: the volatile compounds include at least one of an organic solvent and water; and the polymer and volatile components comprise 95 to 100% by weight, of the total mass of the polymer-containing medium.

8. The process according to claim 1, further comprising operating the devolatilization apparatus at a pressure of 1 hPa to 2000 hPa in the devolatilisation openings and devolatilisation domes.

9. The process according to claim 1, wherein the devolatilisation apparatus further comprises a pre-extruder or a pre-kneader located upstream of the extruder, and the process further comprises introducing stripping agents into the extruder or the pre-extruder or the pre-kneader.

10. The process according to claim 1, wherein the process comprises operating the devolatilisation apparatus to produce product P having a total concentration of volatile compounds of less than 0.5% by weight, based on the mass of the polymer.

11. A method for removing volatile compounds from a polymer-containing medium which contains at least one polymer and at least one volatile compound, the method comprising devolatilizing the polymer-containing medium during extrusion in an extruder comprising devolatilization zones, wherein the extruder comprises: a barrel having n barrel holes with associated hole diameters D.sub.n where n is an integer, and screw elements within the barrel, wherein the screw elements have a pitch t and are configured so that each of the following three conditions are met: S1) the cross-sectional profile has at least one relative maximum R.sup.m.sub.max n based on the radial dimension of the profile curve, for which:
0.420 D.sub.n<R.sup.m.sub.max n<0.490 D.sub.n, S2) 1.38 D.sub.n<t<5.00 D.sub.n, and S3) the cross-sectional profile of the respective screw element has no tangential angle greater than 30, on the active flanks located at the front in the direction of rotation in the range of the radial dimension from 0.95 R.sub.max to R.sub.max, where the tangential angle is defined as a smaller of the two angles formed on drawing tangents at any point on the cross-sectional profile of the treatment element at which the cross-sectional profile cannot be continually differentiated, and is 0 at any point on the cross-sectional profile of the treatment element at which the cross-sectional profile can always be differentiated.

Description

(1) The invention will be illustrated by way of example below with the aid of the figures, but without being restricted thereto.

(2) FIG. 1 shows a conventional two-flighted Erdmenger screw profile having the geometric test criteria as described in detail at the outset.

(3) FIGS. 2a, 2b, 2c and 2d show continually integratable screw profiles which satisfy at least the feature S3).

(4) The following conventions apply to FIGS. 2a, 2b, 2c and 2d: the coordinates x and y have their origin at the point of rotation of one of the screws. All angles indicated are in radians. All other measurements indicated are normalised to the spacing of the axes and are represented by upper case letters: A=a/a; Rj=r/a; RA=ra/a; etc.

(5) Furthermore: RG=normalised barrel radius, RA=normalised outer radius of the profile, RF=normalised outer radius of the screw to be manufactured, S=normalised play between the screws (gap), D=normalised play of the screw to the barrel.

(6) FIGS. 2a, 2b, 2c and 2d show examples of profiles of screw elements used according to the invention with plays which are according to the invention. In FIG. 2a, the gap S in the mutual cleaning of the screws was selected so as to be the same as the gap D in the cleaning of the barrel. In FIG. 2b, the gap S is smaller than D and in FIGS. 2c and 2d, D is, conversely, smaller than S.

(7) FIG. 3 depicts a screw element which satisfies the features S3) and S1).

(8) The construction of the profile as such is disclosed in WO2011/039016, especially in FIG. 26 and the associated description. However, the radial maxima R.sup.1.sub.max, R.sup.2.sub.max, and R.sup.3.sub.max relative to the diameter of the barrel holes D1 and D2 were reduced so that the radial plays of the feature S1) were obtained. The radius R.sub.max in this is 0.96 of the barrel diameter.

(9) FIG. 4 depicts a screw element which satisfies the features S3) and S1).

(10) The construction of the two-flighted profile as such is likewise disclosed in WO2011/039016, especially in FIG. 22 and the associated description. However, the radial maxima R.sup.1.sub.max and R.sup.2.sub.max relative to the diameter of the barrel holes D1 and D2 were reduced so that the radial plays of the feature S1) were obtained. The radius R.sub.max is in this case 0.96 of the barrel diameter.

(11) Both in FIG. 3 and in FIG. 4, the relative maximum forming an edge, which at the same time in each case represents an absolute maximum, has degenerated to a relative point maximum, so that the tangential angle b obtained for these points remains outside consideration since the points by definition are not assigned to the active flank F.sub.akt.

(12) FIG. 5 depicts the extruder of a devolatilisation apparatus according to the invention in longitudinal section and the upstream PR extruder in cross section.

(13) FIG. 6 shows the pre-extruder located upstream of the extruder in longitudinal section.

(14) The following examples are illustrated with the aid of FIGS. 5 and 6.

EXAMPLES

(15) Analytical Methods

(16) Water content of polymer-containing media PM: The sample was introduced into a centrifuge and centrifuged at 4000 rpm at room temperature for 5 minutes. The water was then collected at the bottom of the ampoule and weighed.

(17) Total concentration of volatile compounds: A sample of the product (P) was cut into small pieces having a size of 22 mm. About 30 g of the product were introduced into an aluminium pot. The weight of the pot and of the product were determined exactly. The pot with the sample of the product was then placed in a vacuum oven at a vacuum of 130 hPa at a temperature of 105 C. for 60 minutes. After drying, the pot was placed in a desiccator and allowed to cool for 30 minutes. The pot was then weighed again. The weight loss was determined.

(18) Residual solvent content in product P: The residual solvent content in the product P was determined by head-space gas chromatography. 0.50.005 g of the sample was weighed out and introduced into a head-space ampoule and a measured amount of solvent (1,2-dichlorobenzene, ODCB) was added. The ampoule was closed and shaken until the product had dissolved. The ampoule was heated until the volatile organic compounds had equilibrated between the sample and the gas phase in the ampoule (head-space). Part of the head-space gas was injected into a stream of carrier gas which conveyed the sample along a chromatography column. Standards of known composition were used for calibrating the GC. Toluene was added to the solvent for use as internal standard.

(19) Residual water content in the product P: The total amount of volatile compounds is the sum of water, solvents and other volatile compounds. Since the proportion of other volatile compounds such as monomers was usually less than 0.0005% by weight in the cases examined, the residual water content could be determined by subtracting the solvent content from the total content of volatile compounds.

(20) The solvent content in the polymer-containing media PM was measured by means of gas chromatography. The internal standard was isooctane. The sample was diluted with toluene and then injected into the gas chromatograph. The gas chromatography was carried out on an HP 6890 chromatograph having the following specifications: column type DB-5 from J&W, length 60 m, diameter 0.23 mm, film thickness 1.0 m Injector temp.: 250 C. Detector temp.: 350 C. Carrier gas: helium Column pressure: 96 kPa Detector: FID

(21) The following polymer-containing media PM were used for the examples below:

(22) Preparation of PM-I

(23) A crude bromobutyl rubber solution was obtained from a commercial production plant and the organic phase was separated from the aqueous phase volume. The separation of the aqueous phase from the organic phase is known from WO2010/031823 A, in particular FIG. 7 and the associated description. The organic phase was then used as PM-I for carrying out the experiment. PM-I contained about 23% by weight of bromobutyl rubber, about 74% by weight of hexane isomers and 3% by weight of water, calculated on the basis of 100% by weight of these three components. (The concentration of the additives based on the bromobutyl rubber fraction was:

(24) ESBO: from 1 to 1.6 phr, calcium stearate: from 1.3 to 1.7 phr and Irganox: from 0.03 to 0.1 phr

(25) The bromobutyl rubber obtained from PM-I had the following properties after extrusion:

(26) Mooney (ML 1+8, 125 C.) from 28 to 36, content of bound bromine from 1.6 to 2.0% by weight.

Example 1

Preconcentration

(27) The Concentrator Unit

(28) The concentrator unit used for the examples was similar to that depicted in WO2010/031823 A, in particular FIG. 1. A gear pump was used for pumping the polymer-containing medium PM-I which had been prepared as described above to the healing device. The heating device was a shell-and-tube heat exchanger. A plurality of tubes which can be heated internally by means of steam are in this case accommodated in an outer tube serving as the shell, which at the same time accommodates the product. In addition, mixing elements are provided on the outside of the internal tubes which are in contact with the product in order to ensure good heat transfer. The beating medium was steam, the flow of which could be regulated according to the temperature set for the medium. A pressure release valve was installed upstream of the concentrator unit, and the pressure upstream of the valve was controlled automatically to a set value. This set value was selected so that boiling of the heated polymer-containing medium PM-I in the heating device was prevented. The healed polymer-containing medium PM-I was passed from the top into the devolatilisation tank. The conical output of the devolatilisation tank was equipped with a gear pump. The gear pump had the advantage that it was able to handle high viscosities and build up high pressures. Samples were taken from the concentrated polymer-containing medium PM-Ul in order to examine the concentration after the concentration stage.

Example 1

(29) The heating medium of the heating device was set to 160 C., so that the polymer-containing medium PM-I was heated to a temperature of 135 C. The pressure in the devolatilisation tank was atmospheric. For the present purposes, atmospheric pressure means that the vaporised volatile constituents from the devolatilisation tank were conveyed through a condenser. The condenser was cooled by means of water and the condensed liquid constituents flowed into a collection vessel which was connected directly to the environment. As a result, virtually ambient pressure was established in the devolatilisation tank. The concentrated polymer-containing medium PM-II at the output of the devolatilisation tank could be conveyed from the concentrator unit by means of the extraction pump as described above. The concentrated polymer-containing medium PM-II had a hexane concentration of about 43% by weight.

(30) The Devolatilisation Apparatus (1)

(31) The preconcentrated PM-II was conveyed via a heating device into the devolatilisation apparatus (1). The heating device was a heat exchanger of the same construction as was also used in the concentrator unit. The devolatilisation apparatus comprised a pre-extruder (2), a contra-rotating twin-screw extruder having a hole diameter of D1=D2=57 mm and an effective length of 720 mm, and a main extruder (3), a co-rotating twin-screw extruder having a hole diameter of D1=D2=58.3 mm and an effective length of 3225 mm. Effective length in this case means the length over which contact with the product takes place.

(32) Both extruders of the devolatilisation apparatus comprised a regulating valve (5 or 5.1) as a pressure control device upstream of the respective feed zones (4 and 4.1) of the extruder or of the pre-extruder.

(33) The pre-extruder had a devolatilisation zone (7.1) downstream of the feed zone (4.1) of the pre-extruder (6) and a devolatilisation zone (7.R) upstream of the feed zone (4.1) of the pre-extruder (6). The devolatilisation zone (7.R) had a devolatilisation opening (8.R) with devolatilisation dome (9.R) which was connected to a gas discharge line, and the devolatilisation zone (7.1) had a devolatilisation opening (8.1) with devolatilisation dome (9.1) which was connected to a gas discharge line. Downstream of the devolatilisation zone (7.1) of the pre-extruder (6) there was a pressure buildup zone (10.1) and a banking-up element (II). Downstream of the banking-up element (11), a transfer zone (12) led to the main extruder (3). The transfer zone (12) comprised a heatable tube which opened into the inlet of the regulating valve (5) which in turn marked the beginning of the feed zone (4) of the main extruder (3).

(34) The gas discharge lines of the pre-extruder (2) were connected to an extraction and condenser unit. The gases were extracted by means of a vacuum pump from where the compressed gases were fed into a water-cooled condenser. The barrel (13) of the pre-extruder was configured so as to be able to be heated by means of steam.

(35) The main extruder had three devolatilisation zones (15.1, 15.2 and 15.3) located downstream of the feed zone (4) of the extruder (14) and one devolatilisation zone (15.R) located upstream of the feed zone (4) of the extruder (14). The devolatilisation zone (15.R) had a devolatilisation opening (16.R) with devolatilisation dome (17.R) which was connected to a gas discharge line, and the devolatilisation zones (15.1, 15.2 and 15.3) each had a devolatilisation opening (16.1, 16.2 and 16.3) with devolatilisation dome (17.1, 17.2 and 17.3) which was in each case connected to a gas discharge line. The gas discharge lines were connected to a condenser unit comprising a mechanical vacuum pump and a downstream water-cooled condenser. The gas discharge lines were connected to a condenser unit comprising two mechanical vacuum pumps connected in series and a downstream water-cooled condenser.

(36) Downstream of the devolatilisation zone (15.1) of the extruder (14) there was a pressure buildup zone (18.1) and downstream of this there was a first dispersing zone (19.1).

(37) Downstream of the devolatilisation zones (15.2 and 15.3) of the extruder (14) there was in each case likewise a pressure buildup zone (18.2 and 18.3). Downstream of the pressure buildup zones (18.2 and 18.3) there was in each case a dispersion zone (19.2 and 19.3). Between the pressure buildup zones (18.1, 18.2 and 18.3) and the dispersion zones (19.1, 19.2 and 19.3), there was in each case a banking-up element (20.1, 20.2 and 20.3), and downstream of the dispersion zones (19.1 and 19.2) of the extruder (14) there was in each case a divided sieve disc pair (22.1 and 22.2) removably fastened in the barrel (21).

(38) Downstream of the last pressure buildup zone (18.3) of the extruder (14) there was the discharge zone (23) from the extruder. This discharge was formed by a fixed die plate which opens into an underwater pelletization device (24). Between the pressure buildup zone of the extruder (18.3) and the die plate of the pelletizer (23), there was a start-up valve which allows the product to be extruded via a bypass into a collection vessel provided instead of the product being conveyed through the die plate to the underwater pelletizer. This bypass is used, in particular, for starting-up and shutting-down the extrusion apparatus.

(39) In the region of the dispersion zones (19.1, 19.2 and 19.3), the extruder had inlet openings (25.1, 25.2 and 25.3) for the introduction of stripping agents.

(40) The barrel was made up of a plurality of parts and configured so that it could be divided into three independently heated or cooled zones in order to at least partially control the temperature profile in the extruder. Heating and cooling were effected by means of steam and cooling of water, respectively.

(41) The treatment elements used for the devolatilisation, pressure buildup and dispersion zones and their specification are indicated in the following examples.

Example 2

(42) The preconcentrated polymer containing medium PM-II obtained from Example I was fed via a heating device into the devolatilisation apparatus at a rate of 180 kg/h, resulting in about 80 kg/h of devolatilised dry product at the discharge zone (24) of the devolatilisation apparatus. The steam supply to the heating device was set so that the temperature of PM-II at the regulating valve (5.1) was about 110 C. The pressure at the regulating valve was set to 13 MPa. The pressure in the two zones of the pre-extruder was set to 400 mbar absolute. The heating temperature in the heatable parts of the barrel (13) of the pre-extruder was about 160 C. At the beginning of the transfer zone (4), the rubber content of the further-concentrated polymer-containing medium PM-III was about 80% by weight. PM-III was then introduced at a temperature of 100 C. and a pressure of about 2.0 MPa into the main extruder (3) in the feed zone (4). The pressure at the transfer zone was obtained with the pressure control device at the feed zone of the main extruder completely opened.

Examples 3 to 6

(43) The further-concentrated product PM-III obtained as described in Example I and Example 2 was introduced into the main extruder (3) in which different screw elements were used in the devolatilisation zones and dispersing zones.

(44) The devolatilisation zone (15.R) and the devolatilisation zone (15.1) were operated at an absolute pressure of about 100-180 mbar. The pressure in the devolatilisation zones (15.2 and 15.3) was in each case set to about 50 mbar absolute. From an engineering point of view, it is difficult to keep a reduced pressure exactly constant in such a process, and there are therefore fluctuations which even out over the course of the experiment.

(45) Nitrogen was introduced as stripping agent into the dispersing zone (19.1) downstream of the devolatilisation zone (15.1) at a rate of 0.5-0.6 kg/h.

(46) A dispersion composed of water and calcium stearate (45% by weight of calcium stearate) was introduced at a rate of 3.6 kg/h into the dispersing zone (19.2) downstream of the devolatilisation zone (15.2).

(47) A dispersion composed of water and calcium stearate (45% by weight or calcium stearate) was introduced at a rate of 3.6 kg/h into the dispersing zone (19.3) downstream of the devolatilisation zone (15.3).

(48) The speed of rotation of the extruder screws of the main extruder was in the range from 60 min.sup.1 to 90 min.sup.1.

(49) The screw elements used in the respective examples are summarised in Table 2a).

(50) TABLE-US-00001 TABLE 2a Screw elements used Devolatilisation zones Dispersion zones Example 15.1, 15.2 and 15.3 19.1, 19.2 and 19.3 3 two-flighted standard Kneading blocks & Erdmenger profile eccentric discs analogous to FIG. 1 4 two-flighted standard Kneading blocks & Erdmenger profile eccentric discs analogous to FIG. 1 5 two-flighted screw element two-flighted screw element with a profile which can be with a profile which can be continually differentiated continually differentiated analogous to FIG. 2b analogous to FIG. 2b 6 two-flighted standard two-flighted standard Erdmenger profile Erdmenger profile analogous to FIG. 1 analogous to FIG. 1

(51) TABLE-US-00002 TABLE 2b Pitch t and gap measurement Devolatilisation zones Example 15.1, 15.2 and 15.3 3 R.sup.1.sub.max n = R.sup.2.sub.max n = 0.496 D.sub.n, t = 1.37 D.sub.n 4 R.sup.1.sub.max n = R.sup.2.sub.max n = 0.474 Dn, t = 1.37 D.sub.n 5 R.sup.1.sub.max n = R.sup.2.sub.max n = 0.479 Dn, t = 2.06 D.sub.n 6 R.sup.1.sub.max n = R.sup.2.sub.max n = 0.474 Dn, t = 2.06 D.sub.n

(52) TABLE-US-00003 TABLE 2c Results Hexane content Total volatile in the product P substances incl. water Example [ppm by weight] [% by weight] 3 (for comparison) 2900 <0.30 4 (for comparison) 3000 <0.30 5 450 <0.30 6 500 <0.30

(53) The following can be seen from the examples:

(54) In Example 3, none of the conditions S1), S2) or S3 is satisfied and the devolatilisation result is unsatisfactory.

(55) In Example 4, only the condition S1) is satisfied and the devolatilisation result is unsatisfactory.

(56) In Example 5, all conditions S1), S2) and S3) are satisfied and the devolatilisation result is very good.

(57) In Example 6, the conditions S1) and S2) are satisfied and the devolatilising result is likewise very good.