METHOD FOR PRODUCING A THERMOPLASTIC POLYURETHANE WITH LOW MELT ENTHALPY

Abstract

The invention relates to a method for producing a thermoplastic polyurethane (G) using a reactive extrusion process, having the steps of: a) mixing a polyisocyanate stream (A) and a polyol stream (B) in a first mixing device (7) such that a mixture stream (C) is obtained, the mass flow rates of the polyisocyanate stream (A) and the polyol stream (B) being set such that the isocyanate index in the mixture stream (C) ranges from 55 to 85, b) introducing the mixture stream (C) into a circulating stream (D) which is conducted in a circular flow, the monomers of the polyisocyanates stream (A) and the polyol stream (B) in the circulating stream (D) being further reacted into OH-functional pre-polymers, c) separating a sub-stream from the circulating stream (D) as a prepolymer stream (E) and introducing same into an extruder (18), d) introducing an isocyanate feed stream (F) into the extruder (18) downstream of the introduction point of the prepolymer stream (E) in the extruder working direction, wherein the introduction process is carried out such that the OH-functional prepolymers contained in the prepolymer stream (E) and the polyisocyanate contained in the isocyanate feed stream (F) are in an isocyanate index of 85 to 120, and e) reacting the prepolymer stream (E) with the isocyanate feed stream (F) in the extruder (18), thereby obtaining the thermoplastic polyurethane (G) as the extrudate.

Claims

1. A process for preparing a thermoplastic polyurethane by means of reactive extrusion, comprising: a) mixing a polyisocyanate stream and a polyol stream in a first mixing device to obtain a mixed stream, wherein the mass flow rates of the polyisocyanate stream and of the polyol stream are adjusted such that the isocyanate index in the mixed stream is from 55 to 85, b) introducing the mixed stream into a circulation stream which is circulated, wherein the monomers of the polyisocyanate stream and of the polyol stream (B) react further in the circulation stream to give OH-functional prepolymers, c) separating a prepolymer stream from the circulation stream and introducing the prepolymer stream into an extruder, d) introducing an isocyanate feed stream into the extruder downstream of the introduction of the prepolymer stream in the working direction of the extruder, wherein the introduction is such that the OH-functional prepolymers present in the prepolymer stream and the polyisocyanate present in the isocyanate feed stream are in an isocyanate index of 85 to 120 with respect to one another, e) reacting the prepolymer stream with the isocyanate feed stream in the extruder to obtain the thermoplastic polyurethane as extrudate.

2. The process as claimed in claim 1, wherein the polyisocyanate stream and/or the isocyanate feed stream comprise hexamethylene 1,6-diisocyanate, wherein in particular the polyisocyanate stream (A) and the isocyanate feed stream (F) contain or consist of hexamethylene 1,6-diisocyanate.

3. The process as claimed in claim 1, wherein the polyol stream (B) comprises butane-1,4-diol.

4. The process as claimed in claim 1, wherein, prior to the introduction of the prepolymer stream into the extruder, gases and gaseous byproducts are removed from the prepolymer stream (E), preferably by passing the prepolymer stream (E) through a venting device (17) at a negative pressure of 0.1 mbar to 10 mbar below standard pressure, wherein the venting device (17) is preferably arranged on the extruder (18).

5. The process as claimed in claim 1, wherein the reaction in step e) is conducted at a temperature of 150° C. to 220° C., preferably of 180° C. to 200° C.

6. The process as claimed in claim 1, wherein gases and gaseous byproducts are removed from the thermoplastic polyurethane by applying a negative pressure of 50 mbar to 500 mbar below standard pressure at a devolatilization shaft which is preferably arranged in the last third of the extruder in the working direction of the extruder.

7. The process as claimed in claim 1, wherein the process further comprises: f) cooling the thermoplastic polyurethane below a melting point of the thermoplastic polyurethane in a cooling device, and g) comminuting the thermoplastic polyurethane in a comminution device.

8. A thermoplastic polyurethane obtained by a process as claimed in claim 1.

9. The thermoplastic polyurethane as claimed in claim 8, wherein the thermoplastic polyurethane has: a mass-average molecular weight M.sub.w of 50 000 g/mol to 70 000 g/mol determined by gel permeation chromatography, in which the sample was dissolved in a solution of potassium trifluoroacetate in hexafluoroisopropanol at a concentration of 2 mg/cm.sup.3 and a polymethylmethacrylate standard was used, and an enthalpy of fusion ΔH.sub.fus of 60 J/g to 100 J/g determined by differential scanning calorimetry in accordance with DIN EN ISO 11357-1:2017-02 at a heating rate of 10 K/min in the range from 20° C. to 250° C., wherein M.sub.w=ΔH.sub.fus*f, where f is a number from 600 to 900, preferably from 625 to 900, more preferably from 650 to 850, more preferably still from 700 to 850.

10. The use of a thermoplastic polyurethane as claimed in claim 8 in a shaping process involving melting of the thermoplastic polyurethane, in particular for the production of vehicle components.

11. An apparatus for performing a process as claimed in claim 1, comprising: an isocyanate reservoir vessel from which an isocyanate conduit for conveying a polyisocyanate stream departs, which isocyanate conduit opens into a first mixing device; a polyol reservoir vessel from which a polyol conduit for conveying a polyol stream departs, which polyol conduit opens into the first mixing device, where the polyol conduit is especially merged with the isocyanate conduit upstream of the first mixing device; a circulation feed conduit for conveying a mixed stream which exits from the first mixing device, which circulation feed conduit opens into a circulation conduit for conveying the circulation stream and chemically reacting the components of the circulation stream with the components of the mixed stream; wherein the circulation conduit preferably comprises, in flow direction, a second mixing device, a temperature-controllable mixing device and a temperature-controllable conveying device; a prepolymer feed conduit for conveying a prepolymer stream, which departs from the circulation conduit and opens into an extruder at the inlet side; a pressure control valve provided in the prepolymer feed conduit for regulating the pressure of the prepolymer stream; a three-way valve which is arranged in the prepolymer feed conduit and especially downstream of the pressure control valve, and from which a waste conduit which opens into a waste vessel departs, via which waste conduit the prepolymer stream can be guided wholly or partly into the waste vessel, especially in the event of startup, shutdown or a fault in the apparatus; a venting device which is preferably arranged at the opening of the prepolymer feed conduit into the extruder for removal of gases and gaseous byproducts from the prepolymer stream; an isocyanate feed conduit which departs from the isocyanate reservoir vessel or isocyanate conduit and opens into the extruder, preferably downstream of the prepolymer feed conduit in the working direction of the extruder, for conveying an isocyanate feed stream; wherein the extruder is suitable for reaction of the components of the prepolymer stream with the components of the isocyanate feed stream to give a thermoplastic polyurethane, and this has an assigned devolatilization shaft for removal of gases and gaseous byproducts by means of reduced pressure from this reaction, wherein the devolatilization shaft is preferably arranged in the last third of the extruder in the working direction of the extruder; optionally a cooling device arranged beyond the outlet from the extruder, preferably a water bath, for cooling of the thermoplastic polyurethane to a temperature below its melting point; optionally a comminution device that adjoins the cooling device, for comminution of the cooled thermoplastic polyurethane.

12. The apparatus as claimed in claim 11, wherein, as first conveying device and/or as second conveying device, independently of one another, an annular gear pump is used and/or, as temperature-controllable conveying device, a gear pump is used.

13. The apparatus as claimed in claim 11, wherein, as first and/or second mixing device and/or as temperature-controllable mixing device, independently of one another, a static mixer is used.

14. The apparatus as claimed in claim 11, wherein the circulation conduit consists of jacketed conduits heatable with a heating medium, wherein preferably the second mixing device, the temperature-controllable mixing device and the temperature-controllable conveying device are also heatable with a heating medium, wherein the heating medium is preferably suitable for a heating temperature of 160° C. to 220° C.

15. The apparatus as claimed in claim 11, wherein the extruder is a planetary roller extruder or a screw extruder, wherein the extruder is preferably a co-rotating twin-screw extruder.

16. A thermoplastic polyurethane having: a mass-average molecular weight M.sub.w of 50 000 g/mol to 70 000 g/mol determined by gel permeation chromatography, in which the sample was dissolved in a solution of potassium trifluoroacetate in hexafluoroisopropanol at a concentration of 2 mg/cm.sup.3 and a polymethylmethacrylate standard was used, and an enthalpy of fusion ΔH.sub.fus of 60 J/g to 100 J/g determined by differential thermal analysis in accordance with DIN EN ISO 11357-1:2017-02 at a heating rate of 10 K/min in the range from 20° C. to 250° C., wherein M.sub.w=ΔH.sub.fus*f, where f is a number from 600 to 900, preferably from 625 to 900, more preferably from 650 to 850, more preferably still from 700 to 850.

Description

[0073] The present invention is more particularly elucidated hereinbelow with reference to FIGS. 1 and 2. These figures show:

[0074] FIG. 1 shows an apparatus for performing the process according to the invention, and

[0075] FIG. 2 shows a thermogram from a differential thermal analysis of a polyurethane that had been prepared by the process according to the invention and of a polyurethane that had been prepared by a different process.

[0076] FIG. 1 depicts an apparatus 32 for preparing thermoplastic polyurethane G by the process according to the invention. The apparatus comprises an isocyanate reservoir vessel 1, from which an isocyanate conduit 22 for conveying a polyisocyanate stream departs, the latter being divided into a polyisocyanate stream A and a polyisocyanate stream F at a first junction 23. The polyisocyanate stream A is fed, downstream of the first junction 23, by means of a first conveying device 2 to a first mixing device 7, in the present case a static mixer, the mass flow rate of the polyisocyanate stream A being monitored by means of a first mass flow meter 3.

[0077] The apparatus 32 further comprises a polyol reservoir vessel 4, from which a polyol conduit 25 departs, the latter serving to feed a polyol stream B to the first mixing device 7. Polyol conduit 25 has a second conveying device 5 and is connected to a second mass flow meter 6.

[0078] Upstream of the mixing device 7, the isocyanate conduit 22 and the polyol conduit 25 are merged at a second junction 26, with the result that the polyisocyanate stream A and the polyol stream B are fed to the mixing device 7 together.

[0079] From the first mixing device 7 departs a circulation feed conduit 27, through which a mixed stream C is fed to a third junction 28, from where it is circulated as a circulation stream D in a circulation conduit 29, the components of the circulation stream D chemically reacting with the components of mixed stream C in the process. The mass flow rates of the polyisocyanate stream A and the polyol stream B are adjusted such that there are stoichiometrically more OH groups than isocyanate groups, with the result that an OH-functional polyurethane prepolymer is formed in the circuit line 29. Circulation conduit 29 comprises, in flow direction, a second mixing device 8, in the present case a static mixer, a temperature-controllable mixing device 9 and a temperature-controllable conveying device 10. The temperature-controllable mixing device 9 is preferably suitable for removing heat of reaction. The temperature rise which is caused by the thermal energy released during the reaction of the components of the circulation stream D and of the mixed stream C can be controlled via the circulation regime.

[0080] From the circulation conduit 29, at a fourth junction 11 positioned between the temperature-controllable mixing device 9 and the temperature-controllable conveying device 10, a prepolymer stream E is separated as a substream from the circulation stream D and is fed via a prepolymer feed conduit 30 to the inlet side of an extruder 18 which in the present case is designed as a twin-screw extruder. The pressure prevailing in the prepolymer feed conduit 30 can be controlled by means of a pressure control valve 12. Downstream of the pressure control valve 12 in the prepolymer feed conduit 30 is positioned a three-way valve 13, from which a waste conduit 31 departs, which opens into a waste vessel 14. Via waste conduit 31, prepolymer stream E can be guided wholly or partly into the waste vessel 14 in the event of startup, shutdown or a fault in the apparatus 32.

[0081] A venting device 17 is provided at the opening of the prepolymer feed conduit 30 into the extruder 18 for removal of gases and gaseous byproducts from the prepolymer stream E. Downstream of the prepolymer feed conduit 30 in the working direction of the extruder, the extruder 18 has a junction for an isocyanate feed conduit 24 which, proceeding from a first junction 23 located on the isocyanate conduit 22 upstream of the first conveying device 2, feeds the isocyanate feed stream F from the isocyanate reservoir vessel 1 to the extruder 18 by means of the third conveying device 15. The mass flow in the isocyanate feed conduit 24 is monitored by means of a third mass flow meter 16. In the extruder 18, the prepolymer stream E is chemically reacted with the isocyanate feed stream F to give the thermoplastic polyurethane G. Extruder 18 has an assigned devolatilization shaft 19 for removal of gases and gaseous byproducts by means of a negative pressure from this reaction, which is arranged in the last third of extruder 18 in the working direction of the extruder. Beyond the outlet of extruder 18 is provided a cooling device 20 for cooling of thermoplastic polyurethane G to a temperature below its melting point. Cooling device 20 is adjoined by a comminution device 21 for comminution of the cooled thermoplastic polyurethane G.

EXAMPLES

[0082] The present invention is elucidated further by the examples that follow, but without being restricted thereto.

[0083] Two thermoplastic polyurethanes, which were produced by different synthesis methods, were compared with one another, during which the enthalpy of fusion and the molecular weight of these two polyurethanes were analyzed.

Determination of the Enthalpy of Fusion

[0084] The enthalpy of fusion is determined by means of differential scanning calorimetry (DSC) on the basis of DIN EN ISO 11357-1:2017-02. The measurement was effected on a Q1000 instrument (TA Instruments). Two heatings and a cooling in the range from 20° C. to 250° C. with a heating/cooling rate of 10 K/min were performed. The sample mass was about 6 mg. The purge gas flow (nitrogen) was 50 ml/min.

[0085] The enthalpy of fusion was determined by integrating the area above the glass transition temperature up to about 10° C. above the end of the melting peak.

Determination of the Molecular Weight

[0086] GPC method for the determination of M.sub.n and M.sub.w:

[0087] The number-average and the mass-average molar mass were determined using gel permeation chromatography (GPC), in which the sample to be analyzed was dissolved in a solution of potassium trifluoroacetate in hexafluoroisopropanol at a concentration of 2 mg/cm.sup.3 and a polymethylmethacrylate standard was used. [0088] Pump: 515 HPLC pump (Waters GmbH) [0089] Detector: Smartline 2300 RI detector (Knauer Wissenschaftliche Geräte GmbH) [0090] Columns: 1 precolumn, 1000 Å PSS PFG 7 μm, 300 Å PSS PFG 7 μm, 100 Å PSS PFG 7 μm in this sequence (PSS Polymer Standards Service GmbH) [0091] Degassing: PSS Degasser (PSS Polymer Standards Service GmbH) [0092] Injection volume: 100 microliters [0093] Temperature: 23° C.-25° C. [0094] Molar mass standard: Polymethylmethacrylate standard kit (PSS Polymer Standards Service GmbH)

[0095] Unless explicitly stated otherwise, in the present invention, the centrifuge-average molar mass M.sub.z was determined by means of gel permeation chromatography (GPC) using polymethylmethacrylate as standard. The sample to be analyzed was dissolved in a solution of 3 g of potassium trifluoroacetate in 400 cubic centimeters of hexafluoroisopropanol (sample concentration about 2 mg/cubic centimeter), and then applied via a pre-column at a flow rate of 1 cubic centimeter/minute and then separated by means of three series-connected chromatography columns, first by means of a 1000 Å PSS PFG 7 μm chromatography column, then by means of a 300 Å PSS PFG 7 μm chromatography column and lastly by means of a 100 Å PSS PFG 7 μm chromatography column The detector used was a refractive index detector (RI detector). The mass-average molecular weight was calculated from the data obtained by the gel permeation chromatography measurement with the aid of the following equation:

[00001]${\overline{M}}_w=\frac{{.Math.}_i{n}_i{M}_i^2}{{.Math.}_i{n}_i{M}_i}\mathrm{in}g/\mathrm{mol}$

[0096] in which:

[0097] M.sub.i is the molar mass of the polymers of the fraction i, such that M.sub.i<M.sub.i+1 for all i, in g/mol, n.sub.i is the molar amount of the polymer of the fraction i, in mol.

Example 1 according to the Invention

[0098] An annular gear pump 2 (HNP, MZR 7255) was used to convey a polyisocyanate stream A consisting of hexamethylene 1,6-diisocyanate from a 250 liter reservoir 1 for hexamethylene 1,6-diisocyanate to a static mixer 7. The throughput of the polyisocyanate stream A was measured using a mass flow meter 3 (Bronkhorst, Mini Cori-Flow MIX, max. flow rate 12 kg/h). The temperature of the hexamethylene 1,6-diisocyanate was room temperature.

[0099] An annular gear pump 5 (HNP, MZR 7205) was used to convey a polyol stream B consisting of butane-1,4-diol from a 250 liter reservoir 4 for butane-1,4-diol to the static mixer 7. The throughput of the polyol stream B was measured using a mass flow meter 6 (Bronkhorst, Mini Con-Flow M1X, max. flow rate 8 kg/h). The temperature of the butane-1,4-diol was 40° C.

[0100] In the static mixer 7 (Sulzer SMX, diameter 6 mm, ratio of length to diameter L/D=10), polyisocyanate stream A and polyol stream B were mixed with one another so as to obtain a mixed stream C. The mass flow rates of the polyisocyanate stream A and of the polyol stream B were adjusted such that the isocyanate index in the mixed stream C was 78.

[0101] Mixed stream C was fed via a junction 28 into the circulation conduit 29 in which a circulation stream D was circulated. Downstream of the junction 28, the circulation stream D was guided into a static mixer 8 (static mixer equivalent to Sulzer SMX, internal diameter 34 mm, L/D=20). The temperature of prepolymer stream D was 182° C.

[0102] Downstream of static mixer 8, circulation stream D was guided into a temperature-controllable static mixer 9. The oligomerization of the circulation stream D with the mixed stream C took place there for the most part and the heat of reaction formed was removed. The temperature-controllable static mixer 9 was of similar construction to a Sulzer SMR reactor with internal crossed tubes. It had an internal volume of 1.9 liters and a heat exchange surface area of 0.44 square meters. It was heated/cooled with heat-transfer oil. The heating medium temperature at the inlet was 180° C.

[0103] The circulation stream D exited the temperature-controllable static mixer 9 at a temperature of 183° C. Downstream of the temperature-controllable static mixer 9, a prepolymer stream E was separated from circulation stream D at a junction 11, and the circulation stream D was guided onward to a gear pump 10. The prepolymer stream E was guided into an extruder 18.

[0104] The pressure of circulation stream D was increased in a gear pump 10. The gear pump 10 (Witte Chem 25,6-3) had a volume per revolution of 25.6 cubic centimeters and a speed of 50 revolutions per minute. Circulation stream D was combined with mixed stream C downstream of the pump at junction 28, as already described.

[0105] Circulation conduit 29 consisted of jacketed pipe conduits heated with thermal oil. The heating medium temperature was 182° C. The static mixer 8, the temperature-controllable static mixer 9 and the gear pump 10 consisted of apparatuses heated with thermal oil. The heating medium temperature was 182° C.

[0106] The reactive extrusion was conducted in an extruder 18 at a temperature of 200° C. and a speed of 66 revolutions per minute. The extruder 18 was a ZSK 26 MC from Coperion, with a screw diameter of 26 mm and a length to diameter ratio of 36 to 40.

[0107] The extruder 18 had a venting means 17 that was operated at a negative pressure of about 1 mbar relative to standard pressure, and in which prepolymer stream E was freed of any inert gases entrained with the polyisocyanate stream A and the polyol stream B, and possible gaseous reaction products.

[0108] A micro annular gear pump 15 (MZR 6355 from HNP) was used to withdraw an isocyanate feed stream F consisting of hexamethylene 1,6-diisocyanate from reservoir 1. The throughput of the isocyanate feed stream F was measured by means of a mass flow meter 16 (Bronkhorst, Mini Cori-Flow MIX, maximum flow rate 2 kg/h). The temperature of the hexamethylene 1,6-diisocyanate was room temperature. Isocyanate feed stream F was guided into the extruder 18 downstream of prepolymer stream E. In the extruder 18, the prepolymer stream E was reacted with the isocyanate feed stream F at an isocyanate index of 99 to give a thermoplastic polyurethane G.

[0109] A devolatilizer 19 arranged in the last third of the extruder 18 in flow direction was used to free the thermoplastic polyurethane G of volatile constituents at 200 mbar below standard pressure with the aid of a vacuum dome arranged on top of a devolatilization shaft of the extruder. The thermoplastic polyurethane G after exiting from the extruder 18 through two nozzles was cooled in a water bath 20 filled with deionized water (DM water) and chopped into pellets by means of a pelletizer 21.

TABLE-US-00001 TABLE 1 Material streams in the preparation of the TPU in example 1 [kg/h] Stream A (HDI) 2.911 Stream B (BDO) 2.000 Stream J (HDI) 0.784 Stream E (prepolymer) 120

Reference Example 2

[0110] In a stirred tank (250 ml), 24.59 g of butane-1,4-diol was heated to 90° C. while stirring (170 revolutions per minute (rpm)) and with introduction of nitrogen, for 30 minutes. Subsequently, 45.39 g of HDI was metered continuously into the butanediol over a period of 45 minutes. In the course of this, the temperature of the reaction mixture was increased constantly by 4° C. per minute until a temperature of 190° C. had been attained (25 minutes). As soon as a product temperature of 190° C. had been attained, the stirrer speed was increased to 300 rpm. The temperature in the stirred tank was kept constant between 190° C. and 200° C.

[0111] After the metered addition of HDI had ended, the melt was stirred for a further 5 minutes. Subsequently, it was poured into an aluminum mold in the hot state.

Results

[0112] The enthalpy of fusion and the molecular weights of the thermoplastic polyurethanes obtained from examples 1 and 2 were analyzed by the methods described above. The results are compiled in table 2:

TABLE-US-00002 TABLE 2 Enthalpy of fusion and molecular weight of the polyurethanes obtained Example 1 according Comparative to the invention example 2 Enthalpy of fusion ΔH.sub.fus [J/g] 88.1 106.4 Melting temperature [C°] 183.4 185.3 M.sub.w [g/mol] 58510 60020 M.sub.w/M.sub.n 4.97 2.55 M.sub.z/M.sub.w 2.71 2.05 f 664 564

[0113] f=M.sub.w/ΔH.sub.fus

[0114] The DSC thermograms of the two materials analyzed are shown in FIG. 2. The thermoplastic polyurethanes from examples 1 and 2 each have a mass-average molecular weight in a similar range of 58 000 g/mol to approx. 60 000 g/mol and also have similar melting temperatures in the range from 183° C. to 186° C. However, the TPU according to the invention has an enthalpy of fusion that is 17.2% lower compared to the TPU of the comparative example. Due to the lower enthalpy of fusion of the TPU according to the invention, in the further processing by melting and subsequent shaping less energy needs to be supplied for the melting than for the further processing of the TPU from the comparative example. If the same amount of energy is used to melt the thermoplastic polyurethanes from the examples, the TPU according to the invention melts quicker.

LIST OF REFERENCE SYMBOLS

[0115] (A) polyisocyanate stream

[0116] (B) polyol stream

[0117] (C) mixed stream

[0118] (D) circulation stream

[0119] (E) prepolymer stream

[0120] (F) isocyanate feed stream

[0121] (G) thermoplastic polyurethane

[0122] (1) isocyanate reservoir vessel

[0123] (2) first conveying device

[0124] (3) first mass flow meter

[0125] (4) polyol reservoir vessel

[0126] (5) second conveying device

[0127] (6) second mass flow meter

[0128] (7) first mixing device

[0129] (8) second mixing device

[0130] (9) temperature-controllable mixing device

[0131] (10) temperature-controllable conveying device

[0132] (11) fourth junction

[0133] (12) pressure control valve

[0134] (13) three-way valve

[0135] (14) waste vessel

[0136] (15) third conveying device

[0137] (16) third mass flow meter

[0138] (17) venting device

[0139] (18) extruder

[0140] (19) devolatilization shaft

[0141] (20) cooling device

[0142] (21) comminution device

[0143] (22) isocyanate conduit

[0144] (23) first junction

[0145] (24) isocyanate feed conduit

[0146] (25) polyol conduit

[0147] (26) second junction

[0148] (27) circulation feed conduit

[0149] (28) third junction

[0150] (29) circulation conduit

[0151] (30) prepolymer feed conduit

[0152] (31) waste conduit

[0153] (32) apparatus