Method for producing expanded granulate

Abstract

A process for producing expanded pellets from a thermoplastic elastomer having an elongation at break of more than 100% measured to DIN EN ISO 527-2, comprising: (a) pressing a polymer melt comprising a blowing agent through a perforated disk (18) controlled to a temperature between 150 C. and 280 C. and into a pelletizing chamber (26), (b) using a cutting device (20) to comminute the polymer melt pressed through the perforated disk (18) into individual expanding pellets, (c) discharging the pellets from the pelletizing chamber (26) using a liquid stream (36),
wherein the blowing agent comprises CO.sub.2 and/or N.sub.2 and the amount of blowing agent in the polymer melt is from 0.5 to 2.5 wt %, the pelletizing chamber (26) is traversed by a stream of liquid at a temperature between 5 C. and 90 C. and the pressure of 0.1 bar to 20 bar above ambient pressure such that the pellets are expanded in the pressurized liquid by the blowing agent, producing expanded pellets having an uninterrupted skin.

Claims

1. A process for production of expanded pellets from a thermoplastic elastomer having an elongation at break of more than 100% measured to DIN EN ISO 527-2, said process comprising: (a) pressing a polymer melt comprising the thermoplastic elastomer having an elongation at break of more than 100% measured to DIN EN ISO 527-2 and a blowing agent through a perforated disk controlled to a temperature between 150 C. and 280 C., (b) using a cutting device to comminute the polymer melt pressed through the perforated disk into expanded pellets, the expended pellets disposed in a pelletizing chamber, (c) discharging the pellets from the pelletizing chamber using a liquid stream that traverses the pelletizing chamber, the liquid stream controlled to a temperature between 5 C. and 90 C. and to a pressure from 0.1 bar to 20 bar above ambient pressure, wherein the pressure and temperature of the liquid stream, and the temperature of the perforated disk being chosen such that the pellets expand in the pressurized liquid stream to produce expanded pellets having an uninterrupted skin, and the blowing agent comprises CO.sub.2 or N.sub.2, or a combination of CO.sub.2 and N.sub.2, and the amount of blowing agent in the polymer melt is in a range from 0.5 to 2.5 wt %.

2. The process according to claim 1 wherein the polymer melt comprises a thermoplastic elastomer selected from polyetheramides, polyetheresters, polyesteresters or styrene-butadiene-block copolymers.

3. The process according to claim 1 wherein the thermoplastic elastomer has an elongation at break of more than 200% measured to DIN EN ISO 527-2.

4. The process according to claim 1 wherein the thermoplastic elastomer has a Shore hardness measured to DIN 53505 in the range from A65 to A99.

5. The process according to claim 1 wherein the temperature of the liquid stream in the pelletizing chamber is lowered if the pellets undergo an uncontrolled expansion and do not form an uninterrupted skin, or the temperature of the liquid stream is raised if there is no or insufficient expansion of the pellets.

6. The process according to claim 1 wherein the temperature of the perforated disk is lowered if the pellets undergo an uncontrolled expansion and form an uninterrupted skin, or the temperature of the disk is raised if there is no or insufficient expansion of the pellets.

7. The process according to claim 1 wherein the polymer melt further comprises a nucleating agent.

8. The process according to claim 7 wherein the nucleating agent has a size between 0.01 m and 100 m and is selected from talc, calcium fluoride, sodium phenylphosphinate, aluminum oxide, carbon black, graphite, pigments, finely divided polytetrafluoroethylene or a mixture thereof.

9. The process according to claim 1 wherein the blowing agent comprises a co-blowing agent and wherein the co-blowing agent is selected from an alkane, an alcohol, a halogenated hydrocarbon or a mixture thereof.

10. The process according to claim 1 wherein a pressure and a temperature of the pelletizing chamber are chosen such that the bulk density of the expanded pellets is not more than 250 g/l.

11. The process according to claim 1 wherein the temperature of the liquid is between 25 C. and 45 C.

12. The process according to claim 1 wherein the discharged pellets and the liquid stream flow into a dryer, and the pellets are separated from the liquid and collected, and the liquid is filtered and recycled to the pelletizing chamber.

13. The process according to claim 1 wherein the thermoplastic elastomer is a copolyamide, the pressure in the pelletizing chamber is from 5 to 20 bar above ambient pressure, and the blowing agent is present from 1.5 wt % to 2.5 wt %.

14. A process for production of expanded pellets from a thermoplastic elastomer having an elongation at break of more than 200% measured to DIN EN ISO 527-2, and a Shore hardness from A65 to A99 measured to DIN 53505, said process comprising: (a) pressing a polymer melt comprising the thermoplastic elastomer having an elongation at break of more than 200% measured to DIN EN ISO 527-2 and a blowing agent through a perforated disk controlled to a temperature between 150 C. and 280 C.; (b) using a cutting device to comminute the polymer melt pressed through the perforated disk into expanded pellets, the expended pellets disposed in a pelletizing chamber; (c) discharging the pellets from the pelletizing chamber using a liquid stream that traverses the pelletizing chamber, the liquid stream controlled to a temperature between 5 C. and 90 C. and to a pressure of from 0.1 bar to 20 bar above ambient pressure, wherein the pressure and temperature of the liquid stream, and the temperature of the perforated disk being chosen such that the pellets expand in the pressurized liquid stream to produce expanded pellets having an uninterrupted skin, and the blowing agent comprises CO.sub.2 or N.sub.2, or a combination of CO.sub.2 and N.sub.2, and the amount of blowing agent in the polymer melt is in a range from 0.5 to 2.5 wt %.

15. The process according to claim 14 wherein the temperature of the liquid stream in the pelletizing chamber is lowered, or the temperature of the perforated disk is lowered, if the pellets undergo an uncontrolled expansion and do not form an uninterrupted skin.

16. The process according to claim 14 wherein the temperature of the liquid stream is raised temperature, or the temperature of the disk is raised, if there is no or insufficient expansion of the pellets.

Description

(1) The invention will now be more particularly described with reference to drawings. The sole FIGURE shows a schematic depiction of an apparatus for pelletizing polymer melts.

(2) FIG. 1 shows a schematic depiction of an apparatus for producing expanded pellets from a polymer melt comprising a blowing agent. The starting polymer is introduced into an extruder 10 via a feed hopper 14. The extruder 10 is configured for example as a twin-screw extruder and is powered via a motor 12. The feed hopper 14 may further be used to add auxiliaries such as for example dyes or nucleating agents. The raw material introduced is melted and plasticated in the extruder. In the process, the material is transported in the direction of a perforated disk 18.

(3) In the embodiment depicted in FIG. 1 a melt pump 16 is disposed upstream of the perforated disk 18 to apply a pressure to the melt. The pressure is chosen as a function of the type and quantity of blowing agent used. The blowing agent is introduced into the polymer melt via an add point 40 on the extruder 10 between the feed hopper 14 and the melt pump 16. In the depicted embodiment the add point 40 for the blowing agent is disposed such that the blowing agent is only added after all the polymer has melted. The introduced blowing agent becomes incorporated into the melt during the remaining distance in the extruder. A mixture of carbon dioxide and nitrogen is an example of the suitable blowing agent.

(4) The melt pump 16 helps to force the melt through the perforated disk 18 and into a pelletizing chamber 26. The pelletizing chamber is traversed by a stream of liquid, the pressure of which is above ambient pressure. The direction of flow is indicated by the arrows 36. Inside the pelletizing chamber 26 a rotating blade 24 is disposed in a cutting device, for example a pelletizing means or device 20. The rotating blade 24 is driven by a motor 22. The polymer melt exits from the perforated disk 18 as a plurality of expanding polymeric strands which are chopped off by the rotating blade 24. Individual expanding pellets are produced in the process. The forcing pressure and also the speed of the cutting device are chosen such that the shape of the pellets is substantially spherical.

(5) The pellets in the temperature-controlled liquid are expanded by the blowing agent they contain, while the temperature of the temperature-controlled liquid and of the temperature-controlled perforated disk and also the pressure of the temperature-controlled liquid have been chosen such that the expanded pellets have an uninterrupted foamed skin. The stream of temperature-controlled liquid discharges the resulting expanding/expanded pellets from the pelletizing chamber 26 and feeds them via the circuit line 38 into a dryer 30. In the dryer 30, the expanded pellets are separated from the temperature-controlled liquid and dried and sent, via the product discharge 32, into a collecting container 34. The temperature-controlled liquid from which the pellets have been removed continues to flow through the circuit line 38 into a circuit pump 28, where the temperature-controlled liquid is filtered, temperature controlled and pressurized. From the circuit pump 28 the temperature-controlled liquid flows back into the pelletizing chamber 26.

EXAMPLES

(6) A twin-screw extruder having a screw diameter of 18 mm and a length to diameter ratio of 40 is charged with 99.5 parts by weight of a thermoplastic elastomer (TPE) and 0.5 parts by weight of talc. The thermoplastic elastomer was melted in the melting zone of the twin-screw extruder and mixed with the talc. After the melting of the thermoplastic elastomer and the admixing of the talc, CO.sub.2 or in example 6 a mixture of CO.sub.2 and N.sub.2 was added as a blowing agent. The amounts of blowing agent added are each tabulated in the examples. In the course of traveling the remaining distance in the extruder, the blowing agent and the polymer melt became mixed with each other to form a homogeneous mixture. Total throughput through the extruder which contained the TPE, the talc and the blowing agent was 3.5 kg/h.

(7) In the examples 1 to 5, the following process parameters have been set: The temperature in the extruder in the melting zone and during the admixing of the talc into the TPU was, depending on the TPE used, between 230 C. and 220 C. The temperature at the extruder housing of the injection site was reduced to between 205 C. and 220 C. and the subsequent housing to between 200 C. and 220 C. All further housing parts up to the extruder end and also the melt pump were maintained at from 200 C. to 220 C. The melt pump produced a 90 bar pressure at the end of the extruder. The temperature of the startup valve was set to 210 C. or 220 C. and the perforated disk was heated to a target temperature of 250 C. by electric heating.

(8) In example 6, the following process parameters have been set: The temperature in the extruder been set uniformly to 180 C. up to the start-up valve and the perforated disk was heated to a target temperature of 250 C. by electric heating. The melt pump produced a 90 bar pressure at the end of the extruder.

(9) In all examples, the mixture of TPE, talc and blowing agent was pressed through the perforated disk having a hole with a diameter of 1 mm and chopped off in the downstream water-traversed pelletizing chamber by 10 rotating blades attached to a ring of blades. In the examples 1 to 5, the pressure in the pelletizing chamber was 1 bar and in example 6 the pressure in the pelletizing chamber has been set to 10 to 15 bar. The temperature-controlled medium was maintained at a constant 30 C. During its presence in the pelletizing chamber the mixture expands. Beads having an average size of about 2 mm and a weight of about 2 mg were produced in the process. To determine bulk density, a 100 ml vessel was filled with the expanded beads and weighed to an accuracy of 5 g/l. In all examples the produced pellets have an uninterrupted skin.

(10) The examples hereinbelow report the results.

Example 1

(11) The TPE used was a polyetherester based on polytetrahydrofuran (poly-THF) and polybutylene terephthalate (PBT) and having an elongation at break of more than 500%, a Shore hardness of 90 A and a melting range from 175 to 190 C. This TPE was processed by the method described above and the bulk density was determined as described above. The bulk densities corresponding to the particular proportions of blowing agent added are listed in table 1.

(12) TABLE-US-00001 TABLE 1 Example CO.sub.2 (wt %) Bulk density (g/l) 1.1 0.5 342 1.2 0.75 299 1.3 1.0 229 1.4 1.25 164 1.5 1.5 254 1.6 1.75 378 1.7 2 355 1.8 2.5 367

Example 2

(13) The TPE used was a polyesterester based on 1,4-benzdicarboxylic acid, dimethyl ester, 1,4-butanediol and -hydro--hydroxypoly(oxy-1,4-butanediyl) and having an elongation at break of more than 700%, a Shore hardness of 96 A and a melting range from 200 to 220 C., obtainable as Pelprene P-70B from Toyobo Co, Ltd., for example. This TPE was processed by the method described above and the bulk density was determined as described above. The bulk densities corresponding to the particular proportions of blowing agent added are listed in table 2.

(14) TABLE-US-00002 TABLE 2 Example CO.sub.2 (wt %) Bulk density (g/l) 2.1 0.5 340 2.2 0.75 267 2.3 1.0 202 2.4 1.25 153 2.5 1.5 257 2.6 1.75 393 2.7 2 372 2.8 2.5 379

Example 3

(15) The TPE used was a styrene-butadiene block copolymer (SBC) having the properties of a thermoplastic elastomer (S-TPE, elongation at break greater than 300%, Shore hardness 84 A, an MVR (melt volume rate) (200 C./5 kg)=14 cm.sup.3/10 min), obtainable as Styroflex 2G66 from Styrolution for example. This TPE was processed by the method described above and the bulk density was determined as described above. The bulk densities corresponding to the particular proportions of blowing agent added are listed in table 3.

(16) TABLE-US-00003 TABLE 3 Example CO.sub.2 (wt %) Bulk density (g/l) 3.1 0.5 256 3.2 0.75 181 3.3 1.0 244 3.4 1.25 349 3.5 1.5 389 3.6 1.75 392 3.7 2 401 3.8 2.5 387

Example 4

(17) The TPE used was a polyetherester having a polyether soft segment having an elongation at break of more than 450%, a Shore hardness of 38 D and an MVR (190 C./2.16 kg) of 28 cm.sup.3/10 min, obtainable as Arnitel PL380 from DSM for example. This TPE was processed by the method described above and the bulk density was determined as described above. The bulk densities corresponding to the particular proportions of blowing agent added are listed in table 4.

(18) TABLE-US-00004 TABLE 4 Example CO.sub.2 (wt %) Buik density (g/l) 4.1 0.5 289 4.2 0.75 192 4.3 1.0 183 4.4 1.25 169 4.5 1.5 257 4.6 1.75 278 4.7 2 275 4.8 2.5 281

Example 5

(19) The TPE used was a polyetherester based on hard (crystalline) polybutylene terephthalate segments and soft (amorphous) units derived from long-chain polyether glycols having an elongation at break of more than 700%, a Shore hardness of 30 D and an MFR mass flow rate (190 C./2.16 kg) of 5 g/10 min, obtainable as Hytrel 3078 from DuPont for example. This TPE was processed by the method described above and the bulk density was determined as described above. The bulk densities corresponding to the particular proportions of blowing agent added are listed in table 5.

(20) TABLE-US-00005 TABLE 5 Example CO.sub.2 (wt %) Bulk density (g/l) 5.1 0.5 273 5.2 0.75 204 5.3 1.0 175 5.4 1.25 214 5.5 1.5 265 5.6 1.75 299 5.7 2 287 5.8 2.5 284

Example 6

(21) The TPE used was a polyether copolyamide based on elastic polyether units and crystalline polyamide units having an elongation at break of more than 750%, a Shore hardness of 27D and a melting point of 134 C. to ISO 11357, obtainable as Pebax 2533 SD 02 from Arkema. This TPE was processed by the method described above and the bulk density was determined as described above. The bulk density corresponding to the particular proportions of blowing agent and the different pressures of the temperature-controlled liquid flowing through the pelletizing chamber are listed in table 6.

(22) TABLE-US-00006 TABLE 6 pressure in the pelletizing Example CO.sub.2 (wt %) N.sub.2 (wt %) chamber (bar) bulk density (g/l) 6.1 1.75 0.3 10 213 6.2 1.75 0.3 15 167

LIST OF REFERENCE NUMERALS

(23) 10 Extruder 12 Motor 14 Feed hopper 16 Melt pump 18 Perforated disk 20 cutting device, for example a pelletizing means or device 22 Motor 24 Blade 26 Pelletizing chamber 28 Circuit pump 30 Dryer 32 Product discharge 34 Collecting container 36 Flow direction 38 Circuit line 40 Add point for blowing agent