Process for producing ASA plastics

Abstract

The invention relates to a process for producing thermoplastic molding materials based on acrylonitrile-styrene-acrylate copolymers (ASA) having improved surface properties, in particular an improved stability of surface quality during storage in a hot and humid environment and a reduced content of residual monomers. The invention further relates to the use of a fluidized bed dryer and/or a flow dryer in the production of thermoplastic ASA molding materials for improving surface quality and the use of a fluidized bed dryer and/or a flow dryer in the production of thermoplastic ASA molding materials for reducing the content of residual monomers. The invention further provides ASA molding materials producible by means of the process according to the invention and also moldings.

Claims

1. A process for the production of a thermoplastic molding composition comprising: A: from 5 to 90% by weight of at least one thermoplastic copolymer A, produced from: A1: from 50 to 95% by weight, based on the copolymer A, of a monomer A1 selected from styrene, -methylstyrene, and mixtures of styrene with at least one other monomer selected from -methylstyrene, p-methylstyrene, and C.sub.1-C.sub.8-alkyl (meth)acrylate; and A2: from 5 to 50% by weight, based on the copolymer A, of a monomer A2 selected from acrylonitrile and mixtures of acrylonitrile with at least one other monomer selected from methacrylonitrile, acrylamide, vinyl methyl ether, anhydrides of unsaturated carboxylic acids, and imides of unsaturated carboxylic acids, B: from 5 to 70% by weight of at least one graft copolymer B comprising: B1: from 50 to 90% by weight, based on the graft copolymer B, of at least one graft base B1 which is obtained via emulsion polymerization of: B11: from 70 to 99.9% by weight, based on the graft base B1, of at least one C.sub.1-C.sub.8-alkyl (meth)acrylate, as monomer B11; B12: from 0.1 to 10% by weight, based on the graft base B1, of at least one polyfunctional crosslinking monomer B12; and B13: from 0 to 29.5% by weight, based on the graft base B1, of at least one other monomer B13 selected from styrene, -methylstyrene, C.sub.1-C.sub.4-alkylstyrene, acrylonitrile, methacrylonitrile, isoprene, butadiene, chloroprene, methyl methacrylate, alkylene glycol di(meth)acrylate, and vinyl methyl ether, where the entirety of B11+B12+B13 provides precisely 100% by weight; and B2: from 10 to 50% by weight, based on the graft copolymer B, of at least one graft shell B2 which is obtained via emulsion polymerization, in the presence of the at least one graft base B1, of: B21: from 50 to 100% by weight, based on the graft shell B2, of a monomer B21, selected from styrene, -methylstyrene, and mixtures of styrene with at least one other monomer selected from -methylstyrene, p-methylstyrene, and C.sub.1-C.sub.4-alkyl (meth)acrylate; and B22: from 0 to 50% by weight, based on the graft shell B2, of a monomer B22 selected from acrylonitrile and mixtures of acrylonitrile with at least one other monomer selected from methacrylonitrile, acrylamide, vinyl methyl ether, anhydrides of unsaturated carboxylic acids, and imides of unsaturated carboxylic acids, where the entirety of graft base B1 and graft shell B2 provides precisely 100% by weight; and C: from 0 to 90% by weight of at least one other polymeric component C; and K: from 0 to 40% by weight of at least one other component K selected from additives and auxiliaries; comprising the steps of: a) precipitation of the at least one graft copolymer B after the emulsion polymerization procedure via addition of a precipitation solution comprising at least one salt; b) mechanical dewatering of the precipitated graft copolymer B, where a graft copolymer B with water content smaller than or equal to 50% by weight is obtained; c) drying of the dewatered graft copolymer B with the use of a drying gas, where the graft copolymer B is moved within the drying gas and the temperature of the drying gas is in the range from 50 to 160 C.; and d) mixing of the thermoplastic copolymer A with the dried graft copolymer B and optionally the other polymeric component(s) C and optionally the other component(s) K, wherein the dried graft copolymer B obtained in step c) has a water content in the range from 0.05 to 0.6% by weight and a total content of residual monomers lower than 200 ppm, based on the dried graft copolymer B, wherein the drying of the graft copolymer in step c) is continued for a further 5 to 30 min starting at the juncture at which a water content of 2% by weight is achieved.

2. The process of claim 1, wherein the drying of the dewatered graft copolymer B in step c) uses a fluidized-bed dryer and/or a pneumatic dryer.

3. The process of claim 1, wherein the median particle diameter d.sub.50 of the graft copolymer B is in the range from 50 to 1000 nm.

4. The process of claim 1, wherein the graft copolymer B comprises: from 10 to 50% by weight, based on the graft copolymer B, of at least one graft shell B2 which is obtained via emulsion polymerization of: B21: from 50 to 95% by weight, based on the graft shell B2, of the monomer B21; and B22: from 5 to 50% by weight, based on the graft shell B2, of the monomer B22, where the entirety of B21 and B22 provides precisely 100% by weight.

5. The process of claim 1, wherein the at least one graft copolymer B comprises: B1: from 55 to 65% by weight, based on the graft copolymer B, of the at least one graft base B1; and B2: from 35 to 45% by weight, based on the graft copolymer B, of at least one graft shell B2 which is obtained via emulsion polymerization, in the presence of the at least one graft base B1, of: B21: from 65 to 80% by weight, based on the graft shell B2, of a monomer B21 selected from styrene and -methylstyrene; and B22: from 20 to 35% by weight, based on the graft shell B2, of a monomer B22 selected from acrylonitrile and mixtures of acrylonitrile with methacrylonitrile, where the entirety of graft base B1 and graft shell B2 provides precisely 100% by weight.

6. The process of claim 1, wherein the at least one graft copolymer B comprises: B1: from 50 to 70% by weight, based on the graft copolymer B, of the at least one graft base B1; B2: from 10 to 30% by weight, based on the graft copolymer B, of at least one graft shell B2 which is obtained via emulsion polymerization, in the presence of the at least one graft base B1, of: B21: 100% by weight, based on the graft shell B2, of a monomer B21 selected from styrene, -methylstyrene, and mixtures of styrene with at least one other monomer selected from -methylstyrene, p-methylstyrene, and C.sub.1-C.sub.4-alkyl (meth)acrylate; and B2: from 20 to 40% by weight, based on the graft copolymer B, of at least one graft shell B2 which is obtained via emulsion polymerization, in the presence of the graft base B1 grafted with B2, of: B21: from 50 to 95% by weight, based on the graft shell B2, of a monomer B21 selected from styrene, -methylstyrene, and mixtures of styrene with at least one other monomer selected from -methylstyrene, p-methylstyrene, and C.sub.1-C.sub.4-alkyl (meth)acrylate; and B22: from 5 to 50% by weight, based on the graft shell B2, of a monomer B22 selected from acrylonitrile and mixtures of acrylonitrile with at least one other monomer selected from methacrylonitrile, acrylamide, vinyl methyl ether, anhydrides of unsaturated carboxylic acids, and imides of unsaturated carboxylic acids.

7. The process of claim 1, wherein the drying in step c) is carried out a drying gas selected from air and/or nitrogen, where the drying is carried out with use of a fluidized-bed dryer, and the temperature of the drying gas is in the range from 50 to 160 C.; or where the drying is carried out with use of a pneumatic dryer, and the temperature of the drying gas is in the range from 100 to 160 C.

8. The process of claim 1, wherein the drying in step c) is carried out by a drying gas selected from air and/or nitrogen, where the drying is carried out with use of a fluidized-bed dryer, the temperature of the drying gas is in the range from 50 to 160 C., and the average residence time of the graft copolymer B in the fluidized-bed dryer is from 1 to 60 min; or where the drying is carried out with use of a pneumatic dryer, the temperature of the drying gas is in the range from 100 to 160 C., and the average residence time of the graft copolymer B in the pneumatic dryer is from 1 to 300 seconds.

9. The process of claim 1, wherein the drying of the graft copolymer B in step c) is carried out using a fluidized-bed dryer and/or a pneumatic dryer, wherein the water content of the graft copolymer B is lower than or equal to 50% by weight, after emulsion polymerization, precipitation, and mechanical dewatering of the graft copolymer B, in order to improve the surface quality of the thermoplastic molding composition.

10. The process of claim 9, wherein the drying of the graft copolymer B is carried out in a fluidized-bed dryer and/or a pneumatic dryer by a drying gas selected from air and/or nitrogen, and the temperature of the drying gas is in the range from 50 to 160 C.

11. The process of claim 9, wherein the improvement of the surface quality of the thermoplastic molding composition comprises reduction of the number of surface defects after storage of the molding composition at a temperature in the range from 20 to 100 C. and at a relative humidity in the range from 65 to 100% and/or direct action of liquid water at a temperature in the range from 30 to 100 C. on the surface of the molding composition.

12. The process of claim 1, wherein the drying of the graft copolymer B in step c) is carried out using a fluidized-bed dryer and/or of a pneumatic dryer, wherein the water content of the graft copolymer B is lower than or equal to 50% by weight, after emulsion polymerization, precipitation, and mechanical dewatering of the graft copolymer B, in order to reduce the total content of residual monomers in the graft copolymer B.

13. A thermoplastic molding composition obtained by the process of claim 1, wherein the thermoplastic molding composition comprises salt inclusions and where the size of at least 80% of the salt inclusions, based on the total number of salt inclusions, is smaller than 0.3 mm.

14. A molding produced from the thermoplastic molding composition of claim 13.

Description

(1) FIG. 1 describes the content of styrene (S) in ppm (squares .square-solid.) and the residual moisture content (RMC) (diamond .diamond-solid.) in the graft copolymer B-I as in example 1 as a function of drying time (t [min]).

(2) FIG. 2 describes the content of styrene (S) in ppm (squares .square-solid.) and the residual moisture content (RMC) (diamond .diamond-solid.) in the graft copolymer B-II as in example 1 as a function of drying time (t [min]).

(3) The invention is explained in more detail via the following examples and claims.

EXAMPLES

Example 1: Production of Components

(4) Thermoplastic Copolymer A

(5) Thermoplastic copolymers produced by free-radical solution polymerization with peroxidic initiation were various random styrene/acrylonitrile copolymers and alpha-methylstyrene/acrylonitrile copolymers. The ratio of styrene or alpha-methylstyrene to acrylonitrile was varied.

(6) The viscosity number VN was determined in accordance with DIN EN ISO 1628-1 and -2 in 0.5% solution in N,N-dimethylformamide (DMF) at 23 C.

(7) The following copolymers A were used: A-I: Styrene/acrylonitrile in ratio by weight 65:35, viscosity number VN=80 cm.sup.3/g A-II: Styrene/acrylonitrile in ratio by weight 65:35, viscosity number VN=60 cm.sup.3/g A-III: Styrene/acrylonitrile in ratio by weight 81:19, viscosity number VN=70 cm.sup.3/g A-IV: alpha-Methylstyrene/acrylonitrile in ratio by weight 70:30, viscosity number VN=57 cm.sup.3/g.

(8) Other polymeric component C used was Makrolon 2800 as polycarbonate C-I.

(9) Graft Copolymer B

(10) Two graft copolymers B-I and B-II were produced, differing in the size of the latex particles.

(11) Graft Copolymer B-I (Small-Particle ASA Graft Copolymer)

(12) The graft bases B1 were produced by a method based on EP-A 0450485 (graft copolymer A, page 7).

(13) a1) Production of the Graft Base B1-I

(14) 16 parts by weight of butyl acrylate (BA) and 0.4 part by weight of dicyclopentadienyl acrylate (DCPA) were heated to 60 C., with stirring, in 150 parts by weight of water with addition of one part of the sodium salt of a C12-C18-paraffinsulfonic acid, 0.3 part by weight of potassium peroxodisulfate and 0.38 part by weight of sodium bicarbonate. 10 minutes after the start of the polymerization reaction, a mixture of 82 parts by weight of butyl acrylate with 1.6 parts by weight of DCPA was added within 3 hours. After monomer addition had ended, reaction was allowed to continue for one further hour. The solids content of the resultant crosslinked butyl acrylate polymer rubber was 40% by weight. The particle size distribution was narrow (quotient Q=0.20).

(15) a2) Production of the Graft Copolymer B-I

(16) 4200 g of the latex emulsion produced as specified in (a1) were mixed with 2300 g of water and 5.4 g of potassium peroxodisulfate, and heated to 65 C., with stirring. Once the reaction temperature had been reached, a mixture of 840 g of styrene with 280 g of acrylonitrile was added over the course of 3 hours. Once the addition had ended, the emulsion was kept at 65 C. for a further 2 hours. The median particle size of the resultant graft copolymer latex was 95 nm. Work-up was achieved as described below.

(17) Graft Copolymer B-II (Large-Particle ASA Graft Copolymer)

(18) b1) Production of the Graft Base B1-II

(19) The following were added to an initial charge made of 2.5 parts by weight of the rubber produced as described in a1), after addition of 50 parts by weight of water and 0.1 part by weight of potassium peroxodisulfate, over the course of 3 hours: firstly a mixture of 49 parts by weight of butyl acrylate with 1 part by weight of DCPA and secondly a solution of 0.5 part by weight of the sodium salt of a C.sub.12- to C.sub.18-paraffinsulfonic acid in 25 parts by weight of water. The temperature of the initial charge here was 60 C. Once the feed procedure had ended, polymerization was continued for two hours. The solids content of the resultant rubber was 40%. The median particle size (weight average) of the rubber was determined as 480 nm.

(20) b2) Production of the Graft Copolymer B-II

(21) 150 parts by weight of the rubber obtained as in b1) were mixed with 15 parts by weight of styrene and 60 parts by weight of water, and heated to 65 C. for 3 hours, with stirring, after addition of a further 0.03 part by weight of potassium peroxodisulfate and 0.05 part by weight of lauroyl peroxide. The resultant dispersion was polymerized for a further 4 hours with 25 parts by weight of a mixture of styrene with acrylonitrile in the ration 75:25. The median particle size of the resultant graft copolymer latex was 530 nm. Work-up was achieved as described below.

(22) Precipitation of the Graft Copolymers B

(23) The graft copolymers B-I and B-II as described above were separately coagulated continuously by a magnesium sulfate solution.

(24) For this, the respective graft copolymer B-I or B-II and a magnesium sulfate solution (18% by weight) were metered continuously into a first stirred precipitation container. Steam was injected into the system to maintain the precipitation container at 60 C. in the case of graft copolymer B-I and at 88 C. in the case of graft copolymer B-II. The concentrations maintained here in precipitation container I were as follows: 0.8% by weight of magnesium sulfate, based on the entire aqueous phase, and 18% by weight of graft copolymer B-I or B-II, calculated as solid, based on all of the substances added in the precipitation container.

(25) The average residence time in the first precipitation container was 15 minutes. In order to complete the precipitation procedure, the contents from the first precipitation container were metered continuously into a second precipitation container. The average residence time in the second precipitation container was 15 minutes, and the temperature was kept at about 92 to 94 C. The precipitated graft copolymer from the second precipitation container was cooled to 70 C. and then separated from the aqueous phase by centrifugation to give a water-moist graft copolymer B with residual moisture content 30% by weight.

Example 2: Drying of the Graft Copolymers B

(26) The graft copolymer B-I or B-II obtained after precipitation and dewatering was dried in various ways. In each case, one of the following drying steps described at a later stage below was carried out: 2A Drying in a fluidized-bed dryer 2B Drying in a pneumatic dryer 2C Drying in an extruder

Example 2A: Drying of the Graft Copolymer B by a Fluidized-Bed Dryer

(27) The water-moist graft copolymer B (precipitated graft copolymer B-I or B-II, produced as described above) with residual moisture content 30% by weight was dried in a batch fluidized-bed dryer with air as carrier gas. Residence time was in each case 25 minutes. The temperature of the air was about 110 C., and the product temperature was in the range from 39 to 81 C.; table 1 below shows how the product temperature and the drying gas temperature varied as a function of time.

(28) TABLE-US-00001 TABLE 1 Temperature variation during batch fluidized-bed drying Time [min] 0 5 10 15 20 25 Temperature of drying gas 109.5 109.4 109.6 109.6 108.0 96.6 TG [ C.] Product temperature 38.9 38.1 38.0 44.4 80.8 TP [ C.]

(29) Residual moisture content after the drying procedure was 0.5% by weight. The material was obtained in the form of a fine-particle powder with median grain size d.sub.50 800 m (determined by sieve analysis in accordance with ISO 3310-1).

Example 2B: Drying of the Graft Copolymers B by a Pneumatic Dryer

(30) The water-moist graft copolymer (precipitated graft copolymer B-I or B-II as described above) with residual moisture content 30% by weight was dried in a pneumatic dryer with use of a nitrogen/air mixture with oxygen contents lower than 1% by volume, as carrier gas. The residence time of the graft copolymer B required to reach its final residual moisture content was typically from 5 to 10 seconds. The temperature of the carrier gas was 145 C., and the product temperature was 85 C. Residual moisture content after drying was 0.7% by weight. The material was obtained in the form of a fine-particle powder with median grain size d.sub.50 600 m (determined by sieve analysis in accordance with ISO 3310-1).

Example 2C: Drying of the Graft Copolymer B in an Extruder with Simultaneous Processing to Give Molded Bodies

(31) The water-moist graft copolymer B (precipitated graft copolymer B-I or B-II as described above) with residual moisture content 30% by weight was dried as in DE-B 4402394. For this, the graft copolymer B was mechanically dewatered in a twin-screw extruder. The thermoplastic copolymer A was introduced in the form of melt into the twin-screw extruder and mixed with the graft copolymer B, whereupon the ASA molding compositions were obtained in the form of granulates after extrusion by way of a die plate and granulation. The molding composition was obtained in the form of granulate in one step, and then processed to give molded bodies. Table 2 below states the compositions of the ASA molding composition after drying in the extruder.

Example 3: Production of ASA Molding Compositions and ASA Molded Bodies

(32) The graft copolymers B-I and B-II described above, which were worked up and dried as in examples 2A and 2B, were mixed and granulated in a ZSK 25 extruder (manufacturer: Coperion) with in each case a thermoplastic copolymer A and optionally polycarbonate component C1. Table 2 below states the proportions.

Example 4: Surface Homogeneity Testing

(33) Plaques (75502.5 mm) were produced from the granulates described above in an injection mold with polished surface at a melt temperature of 240 C. and a mold temperature of 70 C.

(34) In order to simulate moist, warm environmental conditions, the plaques (75502.5 mm) were stored at 80 C. in deionized water for 8 hours. After drying of the plaques, visible surface defects were counted by the naked eye at a viewing distance of about 30 to 40 cm, to give a number of surface defects (specks).

(35) Four plaques per molding composition were tested. The test evaluated only the polished area. The number of surface defects on a test area totaling 150 cm.sup.2 was therefore determined and stated in table 2 below.

(36) The molding compositions F1 to F9 are molding compositions of the invention, i.e. molding compositions comprising graft copolymers B-I and/or B-II dried in the invention. The molding compositions F10 to F13 are comparative examples comprising dried graft copolymers B-I and/or B-II not of the invention.

(37) From the test results it can be seen that the molding compositions produced by the process of the invention, and moldings produced therefrom, have a noticeably small number of surface defects (specks), and therefore better surface homogeneity after storage in a moist, warm environment, whereas a noticeably larger number of surface defects occurs in the case of the molding compositions not of the invention.

(38) TABLE-US-00002 TABLE 2 Compositions and test data for molding compositions F1 to F13 B-I B-II A-I A-II A-III A-IV C-I Surface Molding % by % by % by % by % by % by % by defects composition Drying of B wt. wt. wt. wt. wt. wt. wt. Number F1 2A 50 50 4 F2 Fluidized- 48 52 3 F3 bed dryer 20 10 30 40 0 F4 30 70 1 F5 25 10 10 55 1 F6 16 17 33 34 0 F7 6 6 13 75 0 F8 2B 50 50 2 F9 Pneumatic 48 52 0 dryer F10 2C 50 50 >25 F11 Extruder 48 52 >25 F12 20 10 30 40 21 F13 16 17 33 34 24

(39) It is believed that drying in the fluidized-bed dryer or in a pneumatic dryer is particularly rapid and uniform, leading to small salt inclusions (e.g. salt crystals) which bring about few surface defects visible to the naked eye. When a fluidized-bed dryer is used, an average residence time of about 20 to 30 min has proven to be particularly advantageous. When a pneumatic dryer is used, an average residence time of about 1 to 10 seconds has proven to be particularly advantageous.

Example 5: Testing of Content of Residual Monomers

(40) Each of the graft copolymers B-I and B-II described above (in each case produced and precipitated as in example 1) was dried for 50 min in a fluidized-bed dryer as described in example 2A; samples of the graft copolymer were taken at defined time intervals here. Headspace gas chromatography was used to determine content of acrylonitrile, styrene and ethylbenzene, with external calibration and use of para-xylene as internal standard. For this, in each case 1 g of the graft copolymer powder was dispersed by shaking in 5 g of dimethyl sulfoxide with added internal standard, and subjected to measurement by headspace gas chromatography at a sample temperature of 94 C.

(41) The temperature of the injection needle was 98 C., and the temperature of the transfer line between the headspace autosampler and the gas chromatograph was 135 C. The headspace autosampler used was a Perkin-Elmer HS-40, and the gas chromatograph used was a Hewlett Packard 5890 Series II. The detection limit is about 10 ppm for styrene, about 3 ppm for acrylonitrile and about 6 ppm for ethylbenzene.

(42) Table 3 below collates residual moisture content (water content) and content of residual monomers as a function of drying time. It was apparent that in particular styrene content was noticeably further reduced when drying was continued for a further 5 to 10 min after a residual moisture level of about 0.4 to 0.5% had been reached. Residual moisture content (RMC) and content of styrene (S) are moreover depicted as a function of drying time in FIGS. 1 (graft copolymer B-I) and 2 (graft copolymer B-II).

(43) TABLE-US-00003 TABLE 3 Content of residual monomers Residual Drying moisture Graft time content Acrylonitrile Styrene Ethylbenzene copolymer [min] [%] [ppm] [ppm] [ppm] B-I 0 31.5 108 598 102 B-I 15 7.8 <3 260 26 B-I 20 0.4 <3 180 12 B-I 25 0.3 <3 <10 <6 B-I 30 0.3 <3 <10 <6 B-I 35 0.1 <3 <10 <6 B-I 40 0.4 <3 <10 <6 B-I 49 0.2 <3 <10 <6 B-I 50 0.1 <3 <10 <6 B-II 0 35.7 62 622 101 B-II 15 8.9 <3 252 22 B-II 20 0.5 <3 201 18 B-II 25 0.5 <3 181 14 B-II 30 0.1 <3 28 <6 B-II 35 0.2 <3 <10 <6 B-II 40 0.2 <3 <10 <6 B-II 49 0.2 <3 <10 <6 B-II 50 0.2 <3 <10 <6