Method for the production of a thermoplastic polyurethane powder by means of precipitation polymerisation

Abstract

The invention relates to a method for the production of a thermoplastic polyurethane powder by means of precipitation polymerisation, comprising the steps of: i. providing A) a solvent mixture comprising at least one first aprotic solvent Al) with a relative permittivity of 3 to 20 and at least one second aprotic polar solvent A2) with a relative permittivity of at least 24; B) at least one polyol with a molar mass of between 60 g / mol and 250 g / mol; C) at least one diisocyanate; ii. reacting the polyol B) with the diisocyanate C) in the solvent mixture A) at a temperature of max. 150 °C to form the thermoplastic polyurethane, wherein the thermoplastic polyurethane precipitates as a solid in the solvent mixture A) and forms a dispersion; iii. separating from the solvent mixture; and iv. drying the thermoplastic polyurethane to form the thermoplastic polyurethane powder; wherein the thermoplastic polyurethane powder has a mass average of the molar mass Mw of > 35000 g / mol; an allophanate content of < 0.25 mol% in relation to the total thermoplastic polyurethane powder; and > 25.0 wt% of a particle fraction of < 0.500 mm, in relation to the total thermoplastic polyurethane powder and / or a ratio of z-average of the molar mass Mz to the mass average of the molar mass Mw of < 4.0.

Description

[0001] Process for producing a thermoplastic polyurethane powder by precipitation polymerization

[0002] The present invention relates to a process for producing a thermoplastic polyurethane powder by precipitation polymerization, the thermoplastic polyurethane powder obtained or obtainable by this process, and its use. Furthermore, the invention relates to a molded part obtained or obtainable by processing the thermoplastic polyurethane powder according to the invention.

[0003] State of the art

[0004] The production of polymer powders is of great interest for use in powder sintering processes, as additives for structured surfaces, or as abrasives and lubricants in cosmetic applications and in industry. Powder sintering processes are particularly processes in which a layer of powder is first applied, which is then sintered together or the powder particles are fused together in a subsequent step. Such processes are used, for example, to coat metals, polymers, wood, fibers, and other materials. The surface is impregnated with powder by dipping in a fluidized bed, using a spray gun, or via a dispersion, and then heated. In addition to epoxy and polyester resins, a wide variety of thermoplastic polymers such as polypropylene, polyamides, PVC, acrylates, and polyurethanes are also used for this purpose.

[0005] A process for the production of thermoplastic polyurethanes (“TPU”) was already described in 1937 in DE728981C. The TPUs, among which in particular an HDI-BDO adduct (HDI = 1,6-hexamethylene diisocyanate; BDO ​​= 1,4-butanediol) was later marketed for a time under the names Perlon U, Igamid U and Durethan U due to its combination of good properties such as a high melting point, high elastic modulus and good chemical resistance (O. Bayer Angew. Chem. 1947, 59, 9, 257-288), are produced by reaction in a solvent (such as chlorobenzene and dioxane). The solvent is usually removed under vacuum. In some examples the TPUs also precipitated as a gel or powder. However, it later became apparent that the molecular chain lengths achieved were sometimes insufficient to obtain good polymer properties. Higher molecular weight TPU powders based on, for example, BDO-HDI are therefore not accessible.

[0006] However, most polymers such as polypropylene, polyamide or thermoplastic polyurethanes, which can be used for powder coatings or powder sintering processes, are produced in the form of a melt. The polymer obtained in this way must be converted into powder form in a further process step, e.g. by grinding. In the melt production of aliphatic thermoplastic polyurethanes, for example, the pure monomers such as 1,4-butanediol (BDO) and 1,6-hexamethylene diisocyanate (HDI) are converted in a vessel to an OH-terminated prepolymer. The second stage of the conversion is then carried out in a heavy-duty mixer by adding HDI (in deficit; KZ approx. 0.98) to the prepolymer to produce the final product (BIOS Final Report No. 1472, ITEM No. 22, pp47-48).The disadvantages of the process include the high reaction temperature and the long and poor mixing process, which leads to an increase in undesirable side reactions. This melt process can also be carried out in a modified form. The monomers BDO and HDI are converted to the prepolymer in a loop reactor or alternatively in static mixers (Fluitec reactors). This prepolymer then reacts further with HDI in a second reaction step in reactive extrusion to form the final polymer. However, the disadvantages of a relatively high allophanate content and the occurrence of gel particles cannot be avoided even by limiting the conversion (deficient HDI, concentration n = 0.98) (WO2021122279). In the melt processes described here as examples, the polymer is obtained in the form of granules after reactive extrusion and must be converted into powder form in a subsequent process step.

[0007] For most polymers such as polyamides (shown, for example, in EP3491066A1), polypropylene, and the also frequently used thermoplastic polyurethanes (see EP3512687B1), grinding, particularly cryogenic grinding, is used to convert the granules into a powder. In cryogenic grinding, the polymer granules are cooled significantly using liquid nitrogen and then ground. The desired grain size is separated from the ground material by sieving. The disadvantage of grinding processes in general is that the particle shape is not spherical, but rather very irregular and angular. This negatively impacts the flow behavior and makes sieving the ground material more complex. In addition, the particle size distribution during grinding is very broad, so the yield is low or the grinding and sieving process must be repeated several times.In addition, cryogenic grinding is an expensive process due to the use of liquid nitrogen to sufficiently cool the polymers.

[0008] A special case is polyamide 12 (or polyamide 11), which is initially produced as PA 12 granules and dissolved in ethanol under pressure at elevated temperature, and then reprecipitated and dried under very precisely controlled conditions (see, for example, EP0911142B1). The advantage of this method is that the polymer powder is obtained as relatively spherical particles, which also have a particle size of < 100 pm (free-flowing) and do not require classification (sieving). Disadvantages of the process include additional processing steps and equipment, the associated costs, and no or only limited additives to the polymer.

[0009] The aforementioned processes are expensive due to their complexity and / or their disadvantages and are therefore not suitable for the production of thermoplastic polyurethane powders with long chain lengths in combination with low allophanate content. They therefore limit the use of thermoplastic polyurethane powders or the processes that rely on them (powder sintering processes; filler for surface structuring) in industry.

[0010] There is therefore a need for a process for producing thermoplastic polyurethane powders with high molecular weights and low allophanate contents.

[0011] Object of the invention

[0012] The object of the present invention was therefore to provide a process for producing thermoplastic polyurethane powders with high molecular weights and low allophanate contents. In particular, the process should lead to thermoplastic polyurethane powders with a mass-average molecular weight of > 35,000 g / mol and an allophanate content of < 0.25 mol%, based on the total thermoplastic polyurethane powder. Furthermore, the thermoplastic polyurethane powders should be capable of further processing into molded parts that are as free as possible from gel particles and / or continue to exhibit good mechanical properties, in particular, good elongation at break and a good modulus of elasticity.

[0013] Solution to the problem and detailed description of the invention

[0014] The object was achieved by a process for producing a thermoplastic polyurethane powder by precipitation polymerization, comprising the steps: i. Provision

[0015] A) a solvent mixture comprising at least a first aprotic solvent A1) with a relative permittivity s r from 3 to 20 and at least one second aprotic polar solvent A2) with a relative permittivity s r of at least 24, wherein the relative permittivity s r measured at 20 °C and 100 kHz;

[0016] B) at least one polyol having a molecular weight between 60 g / mol and 250 g / mol;

[0017] C) at least one diisocyanate;

[0018] D) optionally a catalyst;

[0019] E) optionally a chain regulator E1) and / or an additive E2); ii. Reaction of the polyol B) with the diisocyanate C) in the solvent mixture A) at a temperature of at most 150 °C, optionally in the presence of the catalyst D), the chain regulator E1) and / or the additive E2), to form the thermoplastic polyurethane, wherein the thermoplastic polyurethane precipitates as a solid in the solvent mixture A) and forms a dispersion; iii. Separation from the solvent mixture and optional washing of the thermoplastic polyurethane with a solvent; and iv. Drying the thermoplastic polyurethane to form the thermoplastic polyurethane powder; wherein the thermoplastic polyurethane powder

[0020] • a mass average molecular weight Mw of > 35000 g / mol;

[0021] • an allophanate content of < 0.25 mol%, based on the total thermoplastic polyurethane powder; and

[0022] • > 25.0 wt.% of a particle fraction of < 0.500 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < 4.0; wherein the mass average molecular weight Mw, the allophanate content, the particle fraction and the centrifuge average molecular weight Mz are each determined using the methods set out below.

[0023] In the course of the development work leading to the present invention, it was surprisingly found that the use of a special solvent mixture, namely comprising at least one first aprotic solvent A1) with a relative permittivity s r from 3 to 20 and at least one second aprotic polar solvent A2) with a relative permittivity s r of at least 24, wherein the relative permittivity s rmeasured at 20 °C and 100 kHz, leads to thermoplastic polyurethane powders with high molecular weights and low allophanate contents during precipitation polymerization.

[0024] The number-average molar mass (Mn), the mass-average molar mass (Mw), and the centrifugal force (Mz) of the thermoplastic polyurethane powder are determined using gel permeation chromatography (GPC). For this purpose, the sample to be measured is dissolved in a solution of 3 g of potassium trifluoroacetate in 400 cubic centimeters of hexafluoroisopropanol (sample concentration approximately 2 mg / cubic centimeter). The respective GPCs are measured with the following components at a flow rate of 1 cubic centimeter / minute:

[0025] Pump: HPLC pump 515 (Waters GmbH)

[0026] Detector: Smartline Rl-Detector 2300 (Knauer Scientific Equipment

[0027] GmbH)

[0028] Columns: 1 guard column, 1000 Ä PSS PFG 7pm, 300 Ä PSS PFG 7pm, 100 Ä

[0029] PSS PFG 7pm in this order (PSS Polymer Standards Service GmbH) Degassing: Degasser PSS (PSS Polymer Standards Service GmbH) Injection volume: 100 microliters Temperature: 23 °C - 25 °C

[0030] Molar mass standard: Polymethyl methacrylate standard kit (PSS Polymer Standards Service GmbH)

[0031] The number average molar mass (Mn or M n ) is calculated from the data obtained by gel permeation chromatography measurement using the following equation: where:

[0032] Mi is the molar mass of the polymers of fraction i, so that < M i+1 for all i, in g / mol, ni is the molar amount of polymer of fraction i, in mol.

[0033] The mass average molar mass (Mw or M w ) is also calculated from the data obtained by gel permeation chromatography measurement using the following equation: where:

[0034] Mi is the molar mass of the polymers of fraction i, such that M t < M i+1 for all i, in g / mol, ni is the molar amount of polymer of fraction i, in mol.

[0035] The centrifuge agent of the molar mass (Mz or M z ) is calculated from the data obtained by gel permeation chromatography measurement using the following equation: where:

[0036] Mi is the molar mass of the polymers of fraction i, such that M t < M i+1 for all i, in g / mol, ni is the molar amount of polymer of fraction i, in mol.

[0037] The allophanate content of the thermoplastic polyurethane powder is determined by 1 H-NMR is determined. The measurements are performed using a Bruker AV III HD 600 spectrometer at 600 MHz in DMSO-D6 at 80 °C. The following peaks are analyzed:

[0038] U= CH2-NH 2. 98 ppm CH2 for urethane

[0039] 0= CH2-OH 3. 43 ppm CH2 with OH groups

[0040] N= NH 6. 6 ppm urethane

[0041] A= NH 8. 3-8.4 ppm allophanate

[0042] The allophanate concentration or the allophanate content in mol-% is calculated using the following formula:

[0043] Allophanate [mol%] = 100% * AZ (A+N)

[0044] The key figure for sales is calculated using the formula:

[0045] KZ = 1 / (1+O / U)

[0046] To determine the particle fraction, approximately 100 g of sample is sieved for 5 minutes using a Haver & Boecker laboratory sieve machine No. 7279 (built in 1978) and the appropriate sieve set (e.g., 0.100 mm, 0.250 mm, 0.500 mm), and the individual fractions are then weighed. The tensile test (to determine elongation at break and Young's modulus) is performed within the scope of the invention based on the DIN EN ISO 527 test method using Type 5A specimens (DIN EN ISO 527-2, thickness 2 mm). The specimens are stored under standard conditions for at least 24 hours prior to testing. The tensile tests are carried out at 22 °C and 50 % relative humidity using a Zwick Z010 universal testing machine at a speed of 10 mm / min. The modulus of elasticity is determined between 0.05 % and 0.25 % strain using a secant.

[0047] It is preferred that the first aprotic solvent A1) comprises or consists of halogenated aromatics, ketones, ethers, esters and carbonates or mixtures thereof, in particular chlorobenzene and / or ortho-dichlorobenzene; cyclopentanone; cyclohexanone; heptan-4-one; acetophenone or mixtures thereof, more preferably chlorobenzene and / or the second aprotic polar solvent A2) comprises or consists of dimethyl sulfoxide, dimethylformamide, / V-methyl-2-pyrrolidone, tetramethylurea (TMU), 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 1,3-dimethyl-2-imidazolidinone, / V-ethyl-2-pyrrolidone, benzonitrile, dimethylacetamide or mixtures thereof, preferably dimethyl sulfoxide.

[0048] The ratio of the first aprotic solvent A1) to the second aprotic polar solvent A2) is preferably 300:1 to 1:9, more preferably 200:1 to 1:1, even more preferably 100:1 to 8:2.

[0049] Furthermore, it is preferred that the polyol B) is ethylene glycol, 1,3-propanediol, 1,4-butanediol,

[0050] 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, 1,12-dodecanediol or mixtures thereof, wherein the polyol B) preferably contains at least 50.0 wt.% 1,4-butanediol, particularly preferably at least 90.0 wt.% 1,4-butanediol, based on the total weight of the polyol B).

[0051] The diisocyanate preferably comprises or consists of 1,4-butane diisocyanate,

[0052] 1,5-pentane diisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate, 1,1'-methylenebis(4-isocyanatocyclohexane), xylylene diisocyanate, in particular m-xylylene diisocyanate or mixtures of these, wherein the diisocyanate C) more preferably contains at least 50.0 wt.% 1,5-pentane diisocyanate or 1,6-hexane diisocyanate, based on the total weight of the diisocyanate C).

[0053] The optional catalyst D) can be selected, for example, from typical urethanization catalysts such as those specified, for example, in Becker / Braun, Kunststoffhandbuch Volume 7, Polyurethanes, Chapter 3.4, or mixtures thereof. The optional chain regulator E1) can be selected, for example, from the group comprising or consisting of monofunctional Zerewitinow H-acidic compounds, monofunctional isocyanates, or mixtures thereof. Examples of the optional chain regulator E1) include n-octanol, benzyl alcohol, n-octylamine, dioctylamine, s-caprolactam, propanone oxime, butane-1-thiol, acetylacetone, and diethyl malonate.

[0054] The optional additive E2) may, for example, be selected from the group comprising or consisting of stabilizers, dyes and markers or mixtures thereof.

[0055] Dyes are understood here to be organic compounds that absorb at least parts of the visible light between 380 nm and 790 nm. Particularly preferred are dyes that are dissolved under the reaction conditions mentioned in step ii. and react with a Zerewitinow H-acidic compound or with an isocyanate group, thus forming a covalent bond with the thermoplastic polyurethane.

[0056] Markers are understood here as compounds that are easy to detect and specific using analytical methods. These can be, for example, aromatic compounds that exhibit a specific UV absorption or UV absorption pattern, or can be detected by fluorescence spectroscopy. Further examples of markers are isotope-enriched compounds that can be easily detected by NMR spectroscopy or by their radioactive radiation. Particularly preferred are markers that are dissolved under the reaction conditions mentioned in step ii. and react with a Zerewitinow H-acidic compound or with an isocyanate group, thus becoming covalently bonded to the thermoplastic polyurethane.

[0057] Furthermore, it is preferred that

[0058] • 30.0 to 95.0 wt.%, preferably 60.0 to 90.0 wt.% of the solvent mixture A);

[0059] • 2.0 to 40.0 wt.%, preferably 3.0 to 20.0 wt.% of polyol B);

[0060] • 3.0 to 40.0 wt.%, preferably 5.0 to 25.0 wt.% of the diisocyanate C);

[0061] • 0 to 5.0 wt.%, preferably 0 to 0.1 wt.% of catalyst D);

[0062] • 0 to 10.0% by weight, preferably 0.001 to 1.5% by weight of the chain regulator E1);

[0063] • 0 to 20.0 wt.%, preferably 0.0001 to 3.0 wt.% of the additive E2); are provided, in each case based on the total amount of solvent mixture A), polyol B), diisocyanate C), catalyst D), chain regulator E1) and additive E2), which is standardized to 100 wt.%.

[0064] The reaction in step ii. is preferably carried out at a temperature of 50 °C to 150 °C, more preferably from 100 °C to 145 °C, even more preferably from 120 °C to 140 °C and / or at an isocyanate number of 0.95 to 1.1, more preferably 0.97 to 1.02, even more preferably 0.98 to 1.0. The dispersion formed in step ii. preferably has a solids content of 5.0 to 50.0 wt.%, more preferably from 15.0 to 45.0 wt.%, even more preferably from 20.0 to 40.0 wt.%, determined by gravimetric measurement with and without solvent.

[0065] The isocyanate index is calculated from the molar amount of reactive isocyanate groups divided by the molar amount of reactive Zerewitinow H-acidic groups.

[0066] KZ — niso / nH-acid

[0067] The reaction in step ii. can also be carried out at a lower or higher pressure than the ambient atmospheric pressure in the autoclave, for example, if solvent A1 and / or solvent A2 have a lower boiling point than the reaction temperature of approximately 1 bar. It is preferred to carry out the reaction in step ii. at ambient pressure.

[0068] Furthermore, it is preferred that the separation in step iii. is carried out by filtration, centrifugation, and / or evaporation of the solvents. The thermoplastic polyurethane can also be additionally washed with a solvent in step iii. (optional washing). This solvent preferably comprises or consists of a solvent with a boiling point between -30°C and +250°C (at 1 bar), preferably halogenated aromatics and alkanes, ketones, ethers, esters, alcohols, nitriles, water, and carbonates or mixtures thereof, in particular chlorobenzene, methyl ethyl ketone, acetone, a C1-C6 alcohol and their esters, dimethyl carbonate, diethyl carbonate, and mixtures thereof, particularly preferably chlorobenzene.

[0069] The drying in step iv. is further preferably carried out with movement or mixing of the dry material, particularly preferably in a paddle dryer.

[0070] It is also preferred that the thermoplastic polyurethane powder has • a mass average molecular weight Mw of 40,000 to 300,000 g / mol, preferably of 45,000 to 150,000 g / mol, particularly preferably of 55,000 to 100,000 g / mol;

[0071] • an allophanate content of 0 to 0.20 mol%, preferably 0.001 to 0.15 mol%, particularly preferably 0.01 to 0.10 mol%, based on the total thermoplastic polyurethane powder; and / or

[0072] • from 30.0 to 100% by weight, more preferably from 40.0 to 80.0% by weight of a particle fraction of < 0.500 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < 3.0, preferably from 1.5 to 2.2.

[0073] It is further preferred that the thermoplastic polyurethane powder

[0074] • a mass average molecular weight Mw of 40,000 to 300,000 g / mol, preferably of 45,000 to 150,000 g / mol, particularly preferably of 55,000 to 100,000 g / mol;

[0075] • an allophanate content of 0 to 0.20 mol%, preferably 0.001 to 0.15 mol%, particularly preferably 0.01 to 0.10 mol%, based on the total thermoplastic polyurethane powder; and / or

[0076] • > 25.0 wt.%, preferably from 40.0 to 100 wt.%, more preferably from 50.0 to 95.0 wt.% of a particle fraction of < 0.250 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < 3.0, preferably from 1.5 to 2.2.

[0077] It is further preferred that the thermoplastic polyurethane powder has a mass average molecular weight Mw of 40,000 to 300,000 g / mol, preferably of 45,000 to 150,000 g / mol, particularly preferably of 55,000 to 100,000 g / mol; an allophanate content of 0 to 0.20 mol%, preferably of 0.001 to 0.15 mol%, particularly preferably of 0.01 to 0.10 mol%, based on the total thermoplastic polyurethane powder; and / or

[0078] • > 5.0 wt.%, preferably from 15.0 to 100 wt.%, more preferably from 25.0 to 95.0 wt.% of a particle fraction of < 0.100 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < 3.0, preferably from 1.5 to 2.2.

[0079] Depending on the choice of powder sintering process and application specifications for the polyurethane powder, powders with different grain sizes are used, but the grain sizes usually range up to 0.500 mm and smaller.

[0080] Another important aspect of particle size is the formation of a dispersion that is as stable as possible during the reaction and for processing. After being scaled up to industrial scale, the dispersion must be transported by pumps, must not settle prematurely, and should also exhibit good filtration properties. If the particle size is too large, the particles settle too quickly, leading to deposits and increased wear on the pumps.

[0081] The thermoplastic polyurethane according to step iii. and / or the thermoplastic polyurethane powder according to step iv. can further be impregnated with a stabilizer from a stabilizer solution. For this purpose, the thermoplastic polyurethane according to step iii. and / or the thermoplastic polyurethane powder according to step iv. is preferably dispersed in the stabilizer solution and then separated and dried. The stabilizer solution preferably comprises or consists of

[0082] • a solvent which is selected from the group comprising or consisting of solvents from the group consisting of chlorinated aromatics; aliphatics; esters; ethers; alcohols; water; ketones; nitriles; carbonates; or mixtures thereof, or preferably the solvent(s) A1) from step i., wherein the solvent(s) preferably have a boiling point of < 250 °C at 1 bar; and • a stabilizer dissolved therein which is selected from the group comprising or consisting of sterically hindered phenol derivatives, organic phosphine, phosphite and / or phosphonate derivatives (generally also called phosphorus-based stabilizers), sulfur-containing antioxidants, 2,2,6,6-tetramethylpiperidine derivatives, benzotriazole derivatives, triazine derivatives,

[0083] Hydroxybenzophenone derivatives, cyanoacrylate derivatives, oxalinide derivatives, or mixtures thereof; wherein the weight fraction of the stabilizer is preferably from 0.001 to 10.0 wt.%, more preferably from 0.05 to 5.0 wt.%, based on the total weight of the stabilizer solution.

[0084] The stabilizer solution preferably contains phosphorus(III) compounds as phosphorus-based stabilizers, which are oxidized to phosphorus(V) compounds when heated in the presence of oxygen. Organic phosphites, phosphine compounds, and phosphonates, especially organic phosphites and phosphonates, are further preferred.

[0085] Particularly preferably, the phosphorus-based stabilizer is selected from at least one compound according to the structures (1), (2), (3), (4), (5) and (6).

[0086]

[0087] The compound (1) is classified as CAS: 31570-04-4 and is commercially available under the name Irgafos™ 168 from BASF (Germany).

[0088] The compound (2) is classified as CAS: 237-249-1 and is commercially available under the name Brüggolen™ H10 from Brüggeman (Germany).

[0089] Compound (3) is classified as CAS: 26741-53-7 and is commercially available under the name Irgafos™ 126 from BASF (Germany). Compound (4) is classified as CAS: 603-35-0 and is commercially available under the name triphenylphosphine from BASF (Germany). Compound (5) is classified as CAS: 80693-00-1 and is commercially available under the name ADK Stab™ PEP 36 from Adeka (Japan).

[0090] The compound (6) is classified as CAS: 126050-54-2 and is commercially available under the name ADK Stab™ HP-10 from Adeka (Japan).

[0091] Another suitable stabilizer containing phosphorus(III) is AddWorks™ LXR 568 MP from Clariant (Switzerland).

[0092] It is preferred that the phosphorus-based stabilizer is selected from the group comprising or consisting of compounds according to structures (1), (2), (4), (6), or mixtures thereof. The compound according to structure (1) and / or (2) is particularly preferably used as the phosphorus-based stabilizer. These phosphorus-based stabilizers achieve a particularly high tensile strength of the produced molded article.

[0093] The stabilizer solution may also contain a sterically hindered phenol. Mixtures of several such components and different stabilizers can also be used.

[0094] Preferably, the sterically hindered phenol is a compound of the general structure (7a) or (7b) where n is 1, 2, 3 or 4,

[0095] R1, R2, and R3 each independently represent C1- to C4-alkyl or hydrogen. X represents a direct bond or an organic radical with C1- to C6o- and the organic radical may contain oxygen and / or nitrogen. R4 represents a direct bond, carbon, C1-C8-alkyl, aryl, or a structure according to formula (8a), (8b), or (8c). Particularly preferably, the sterically hindered phenol is selected from at least one compound according to the structures (9), (10), (11) and (12).

[0096] The compound (9) is classified as CAS: 6683-19-8 and is commercially available under the name ADK Stab™ AO-60 from Adeka (Japan)

[0097] Compound (10) is classified as CAS: 85-60-9 and is commercially available under the name Songnox™ 4425 from Songwon Industrial Group (South Korea). Compound (11) is classified as CAS: 23128-74 and is commercially available under the name Songnox™ 1098 from Songwon Industrial Group (South Korea).

[0098] The compound (12) is classified as CAS: 36443-68-2 and is commercially available under the name Irganox™ 245 from BASF (Germany).

[0099] Furthermore, the stabilizer solution can optionally contain a sulfur-containing antioxidant. Mixtures of several such components can also be used.

[0100] For example, a sulfur-containing antioxidant can have a structure according to RI-CH2-(S) X- CH2-R2, where x = 1 or 2 and where R1 and R2 may be the same or different and represent aromatic or aliphatic groups. R1 and R2 are preferably aliphatic groups, which may be linear or branched and may contain functional groups. Examples of commercially available sulfur-containing antioxidants are dilauryl-3,3-thiodipropionate (CAS 123-28-4), distearyl-3,3'-thiodipropionate (CAS 693-36-7), ditridecylthiodipropionate (CAS 10595-72-9); Pentaerythritol tetrakis ([ß-laurylthiopropionate (CAS 29598-76-3), 2,2'-thiodiethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] (CAS 41484-35-9), dimyristylthiodipropionate (CAS 16545-54-3) and distearyl disulfide (CAS 2500-88-1) as well as mixtures of these substances.

[0101] Alternatively, the sulfur-containing antioxidant may also comprise one or more diphenyl thioesters such as 4,4'-thiobis(2-t-butyl-5-methylphenol) (CAS 96-69-5) and 2,2'-thiobis(6-t-butyl-4-methylphenol) (CAS 90-66-4).

[0102] Particularly preferred as a sulfur-containing antioxidant is pentaerythritol tetrakis ([ß-laurylthiopropionate] (CAS 29598-76-3) (structure (13).

[0103] Furthermore, it is preferred that the thermoplastic polyurethane powder after the aforementioned impregnation contains 0.005 to 2.0 wt.% of the stabilizer from the stabilizer solution, based on the total weight of the thermoplastic polyurethane powder. The thermoplastic polyurethane powder obtained after impregnation preferably has a b-value of <4.0, more preferably 0.1 to 2.5, determined in the CIE Lab color space using a spectrophotometer with illuminant D 65 at 8°C according to DIN EN ISO 11664-4.Furthermore, it is preferred that the thermoplastic polyurethane powder has a change in the b-value (Ab) after exposure to a temperature of 120 °C for a duration of 96 h and / or of 155 °C for a duration of 6 h, in each case under an air atmosphere, of < 3.5, preferably from 0.1 to 2.8, more preferably from 0.2 to 2.0, wherein the b-value in the CIE Lab color space is determined by means of a spectrophotometer with the illuminant D 65 at 8° in accordance with DIN EN ISO 11664-4 and the change in the b-value is obtained by subtracting the b-value before exposure to temperature from the b-value after exposure to temperature.It has surprisingly been found that the aforementioned impregnation according to the invention reduces yellowing of the thermoplastic polyurethane powder, even after exposure to heat, which can otherwise be very pronounced in thermoplastic polyurethane powders produced by solvent processes, particularly aliphatic thermoplastic polyurethane powders, due to the drying step (German Plastics Practice, 1946, p. 304). The b value in the CIE Lab color space, as set out above, is used within the scope of the invention as a measure of the yellowing or "white impression" of the polyurethane powder. The CIE Lab color space is composed of the L, the a, and the aforementioned b values. "L" defines the brightness, "a" the red / green value, and "b" the yellow / blue value.To determine the b-value, a color measurement of the respective sample (approximately 3 mm powder layer between two coverslips for microscopy) is performed using a portable spectrophotometer (CM5 from Konica Minolta) with illuminant D 65 at 8° (observer) and diffuse illumination according to DIN EN ISO 11664-4. The color of the powder is measured in reflection and expressed in the CIE Lab color space with the L, a, and b values. The b-value is calculated from the measured spectral reflection curve using the device software.

[0104] The invention further relates to a thermoplastic polyurethane powder obtained or obtainable by the process according to the invention. As already explained above, these thermoplastic polyurethane powders have a high mass-average molecular weight combined with a low allophanate content. If they are additionally impregnated according to the invention, they also exhibit less yellowing upon exposure to heat.

[0105] Furthermore, the invention relates to the use of the thermoplastic polyurethane powder according to the invention in an extrusion process, injection molding process, powder sintering process, solvent and / or melt process, in particular for producing molded parts and / or coatings.

[0106] Furthermore, the invention relates to a molded part obtained or obtainable by processing the thermoplastic polyurethane powder according to the invention. The molded part is preferably free of gel particles. The gel particles are determined using a so-called gel determination. The thermoplastic polyurethane powder is injection-molded into a standard rod (dimensions 80 mm x 10 mm x 4 mm). The rod is placed with its flat side on a light table. The gel particles are visible to the naked eye in transmitted light as bright spots in the otherwise homogeneous sample. In addition, the circular depressions on the smooth surface of the test specimens, which are created by gel particles close to the surface, are counted.In order to take minor errors in injection molding into account, if the total number of gel particles counted is X < 5, the sample is considered free (“free of gel particles”), if 5 < X < 15, it is considered little, 15 < X < 30, it is considered a lot, and X > 30, it is considered a lot.

[0107] Embodiments:

[0108] The invention particularly relates to the following embodiments:

[0109] According to a first embodiment, the invention relates to a process for producing a thermoplastic polyurethane powder by precipitation polymerization, comprising the steps: i. Providing

[0110] A) a solvent mixture comprising at least a first aprotic solvent A1) with a relative permittivity s r from 3 to 20 and at least one second aprotic polar solvent A2) with a relative permittivity s r of at least 24, wherein the relative permittivity s rmeasured at 20 °C and 100 kHz;

[0111] B) at least one polyol having a molecular weight between 60 g / mol and 250 g / mol;

[0112] C) at least one diisocyanate;

[0113] D) optionally a catalyst;

[0114] E) optionally a chain regulator E1) and / or an additive E2); ii. Reaction of the polyol B) with the diisocyanate C) in the solvent mixture A) at a temperature of at most 150 °C, optionally in the presence of the catalyst D), the chain regulator E1) and / or the additive E2), to form the thermoplastic polyurethane, wherein the thermoplastic polyurethane precipitates as a solid in the solvent mixture A) and forms a dispersion; iii. Separation from the solvent mixture and optional washing of the thermoplastic polyurethane with a solvent; and iv. Drying the thermoplastic polyurethane to form the thermoplastic polyurethane powder; wherein the thermoplastic polyurethane powder

[0115] • a mass average molecular weight Mw of > 35000 g / mol;

[0116] • an allophanate content of < 0.25 mol%, based on the total thermoplastic polyurethane powder; and

[0117] • > 25.0 wt.% of a particle fraction of < 0.500 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < 4.0; wherein the mass average molecular weight Mw, the allophanate content, the particle fraction and the centrifuge average molecular weight Mz are each determined using the methods set out in the description.

[0118] According to a second embodiment, the invention relates to a process according to the first embodiment, characterized in that the first aprotic solvent A1) halogenated aromatics, ketones, ethers, esters and carbonates or mixtures thereof, in particular chlorobenzene and / or ortho-dichlorobenzene; cyclopentanone; cyclohexanone; heptan-4-one; acetophenone or mixtures thereof, preferably chlorobenzene and / or the second aprotic polar solvent A2) dimethyl sulfoxide, dimethylformamide, / V-methyl-2-pyrrolidone, tetramethylurea, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinion,

[0119] 1 .3-Dimethyl-2-imidazolidinone, / V-ethyl-2-pyrrolidone, benzonitrile, dimethylacetamide or mixtures thereof, preferably dimethyl sulfoxide.

[0120] According to a third embodiment, the invention relates to a process according to the first or second embodiment, characterized in that the ratio of the first aprotic solvent A1) to the second aprotic polar solvent A2) is 300:1 to 1:9, preferably 200:1 to 1:1, more preferably 100:1 to 8:2.

[0121] According to a fourth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the polyol B) comprises or consists of ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, 1,12-dodecanediol or mixtures thereof, wherein the polyol B) preferably contains at least 50.0 wt.%

[0122] 1,4-butanediol, particularly preferably at least 90.0 wt.% 1,4-butanediol, based on the total weight of the polyol B). According to a fifth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the diisocyanate C) comprises or consists of 1,4-butane diisocyanate, 1,5-pentane diisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate, 1,1'-methylenebis(4-isocyanatocyclohexane), xylylene diisocyanate, in particular m-xylylene diisocyanate or mixtures thereof, wherein the diisocyanate C) preferably contains at least 50.0 wt.% 1,5-pentane diisocyanate or 1,6-hexane diisocyanate, based on the total weight of the diisocyanate C).

[0123] According to a sixth embodiment, the invention relates to a method according to one of the preceding embodiments, characterized in that

[0124] • the catalyst D) is selected from the group comprising or consisting of typical urethanization catalysts as specified, for example, in Becker / Braun, Kunststoffhandbuch Volume 7, Polyurethanes, Chapter 3.4 or mixtures thereof;

[0125] • the chain regulator E1) is selected from the group comprising or consisting of monofunctional Zerewitinow H-acidic compounds, monofunctional isocyanates or mixtures thereof; and / or

[0126] • the additive E2) is selected from the group comprising or consisting of stabilizers, dyes, markers or mixtures thereof.

[0127] According to a seventh embodiment, the invention relates to a method according to one of the preceding embodiments, characterized in that

[0128] • 30.0 to 95.0 wt.%, preferably 60.0 to 90.0 wt.% of the solvent mixture A);

[0129] • 2.0 to 40.0 wt.%, preferably 3.0 to 20.0 wt.% of polyol B);

[0130] • 3.0 to 40.0 wt.%, preferably 5.0 to 25.0 wt.% of the diisocyanate C);

[0131] • 0 to 5.0 wt.%, preferably 0 to 0.1 wt.% of catalyst D);

[0132] • 0 to 10.0% by weight, preferably 0.001 to 1.5% by weight of the chain regulator E1);

[0133] • 0 to 20.0% by weight, preferably 0.0001 to 3.0% by weight of the additive E2); are provided, in each case based on the total amount of solvent mixture A), polyol B), diisocyanate C), catalyst D), chain regulator E1) and additive E2), which is standardized to 100% by weight. According to an eighth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the reaction in step ii. takes place at a temperature of 50 °C to 150 °C, preferably from 100 °C to 145 °C, more preferably from 120 °C to 140 °C.

[0134] According to a ninth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the reaction in step ii. takes place at an isocyanate number of 0.95 to 1.1, preferably 0.97 to 1.02, more preferably 0.98 to 1.0.

[0135] According to a tenth embodiment, the invention relates to a process according to any one of the preceding embodiments, characterized in that the dispersion formed in step ii. has a solids content of 5.0 to 50.0 wt.%, preferably 15.0 to 45.0 wt.%, more preferably 20.0 to 40.0 wt.%, determined by gravimetric measurement with and without solvent.

[0136] According to an eleventh embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the separation in step iii. is carried out by means of filtration, centrifugation and / or evaporation of the solvents.

[0137] According to a twelfth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the drying in step iv. takes place in a paddle dryer.

[0138] According to a thirteenth embodiment, the invention relates to a method according to one of the preceding embodiments, characterized in that the thermoplastic polyurethane powder

[0139] • a mass average molecular weight Mw of 40,000 to 300,000 g / mol, preferably of 45,000 to 150,000 g / mol, particularly preferably of 55,000 to 100,000 g / mol;

[0140] • an allophanate content of 0 to 0.20 mol%, preferably 0.001 to 0.15 mol%, particularly preferably 0.01 to 0.10 mol%, based on the total thermoplastic polyurethane powder; and / or

[0141] • from 30.0 to 100.0 wt.%, more preferably from 40.0 to 80.0 wt.% of a particle fraction of < 0.500 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < 3.0, preferably from 1.5 to 2.2.

[0142] According to a fourteenth embodiment, the invention relates to a process according to one of the preceding embodiments, characterized in that the thermoplastic polyurethane after step iii. and / or the thermoplastic polyurethane powder after step iv. is impregnated with a stabilizer from a stabilizer solution, wherein for this purpose the thermoplastic polyurethane after step iii. and / or the thermoplastic polyurethane powder after step iv. is dispersed in the stabilizer solution and then separated and dried.

[0143] According to a fifteenth embodiment, the invention relates to a method according to the fourteenth embodiment, characterized in that the stabilizer solution

[0144] • a solvent selected from the group comprising or consisting of solvents from the group consisting of chlorinated aromatics; aliphatics; esters; ethers; alcohols; water; ketones; nitriles; carbonates; or mixtures thereof, or preferably the solvent(s) A1) from step i., wherein the solvent(s) preferably have a boiling point of < 250 °C at 1 bar; and

[0145] • a stabilizer dissolved therein, which is selected from the group comprising or consisting of sterically hindered phenol derivatives, organic phosphine, phosphite and / or phosphonate derivatives, sulfur-containing antioxidants, 2,2,6,6-tetramethylpiperidine derivatives, benzotriazole derivatives, triazine derivatives, hydroxybenzophenone derivatives, cyanoacrylate derivatives, oxalinide derivatives or mixtures thereof; wherein the weight fraction of the stabilizer is preferably from 0.001 to 10.0 wt.%, more preferably from 0.05 to 5.0 wt.%, based on the total weight of the stabilizer solution.

[0146] According to a sixteenth embodiment, the invention relates to a process according to the fourteenth or fifteenth embodiment, characterized in that the thermoplastic polyurethane powder comprises 0.005 to 2.0 wt.% of the stabilizer from the stabilizer solution, based on the total weight of the thermoplastic polyurethane powder.

[0147] According to a seventeenth embodiment, the invention relates to a method according to the fourteenth to sixteenth embodiment, characterized in that the thermoplastic polyurethane powder has a b-value of < 4.0, preferably from 0.1 to 2.5 and / or the thermoplastic polyurethane powder has a change in the b-value after exposure to a temperature of 120 °C for a duration of 96 h and / or of 155 °C for a duration of 6 h, in each case under an air atmosphere, of < 3.5, preferably from 0.1 to 2.8, more preferably from 0.2 to 2.0, wherein the b-value is determined in the CIE Lab color space by means of a spectrophotometer with illuminant D 65 at 8°C in accordance with DIN EN ISO 11664-4 and the change in the b-value is obtained by subtracting the b-value before exposure to temperature from the b-value after exposure to temperature.

[0148] According to an eighteenth embodiment, the invention relates to a thermoplastic polyurethane powder obtained or obtainable by a process according to any one of embodiments 1 to 17.

[0149] According to a nineteenth embodiment, the invention relates to the use of a thermoplastic polyurethane powder according to the eighteenth embodiment in an extrusion process, injection molding process, powder sintering process, solvent and / or melt process, in particular for producing molded parts and / or coatings.

[0150] According to a twentieth embodiment, the invention relates to a molded part obtained or obtainable by processing a thermoplastic polyurethane powder according to the eighteenth embodiment.

[0151] According to a twenty-first embodiment, the invention relates to a molded part according to the twentieth embodiment, characterized in that the molded part is free of gel particles, wherein "free of gel particles" is defined and determined as set out in the description.

[0152] Examples

[0153] The present invention is explained in more detail with reference to the following examples.

[0154] Measurement methods:

[0155] The following measurement methods were used:

[0156] GPC method for determining number average molecular weight Mn, mass average molecular weight Mw and centrifuge average molecular weight Mz:

[0157] Determination by gel permeation chromatography (GPC). For this purpose, the sample to be measured was dissolved in a solution of 3 g of potassium trifluoroacetate in 400 cubic centimeters of hexafluoroisopropanol (sample concentration approximately 2 mg / cubic centimeter). The respective GPCs were measured with the following components at a flow rate of 1 cubic centimeter / minute:

[0158] Pump: HPLC pump 515 (Waters GmbH);

[0159] Detector: Smartline Rl-Detector 2300 (Knauer Scientific Equipment

[0160] GmbH);

[0161] Columns: 1 guard column, 1000 Ä PSS PFG 7pm, 300 Ä PSS PFG 7pm, 100 Ä

[0162] PSS; PFG 7pm in this order (PSS Polymer Standards Service GmbH);

[0163] Degassing: Degasser PSS (PSS Polymer Standards Service GmbH);

[0164] Injection volume: 100 microliters;

[0165] Temperature: 23 °C - 25 °C;

[0166] Molar mass standard: Polymethyl methacrylate standard kit (PSS Polymer Standards Service GmbH).

[0167] The number average molar mass ( M n ) was calculated from the data obtained by gel permeation chromatography measurement using the following equation: where:

[0168] Mi is the molar mass of the polymers of fraction i, so that < M i+1 for all i, in g / mol, ni is the amount of polymer of fraction i, in mol. The mass average molar mass (M w ) was also calculated from the data obtained by gel permeation chromatography measurement using the following equation:

[0169] 77 E; n; M; 2 . , 1

[0170] M W = in G / mo1 where:

[0171] Mi is the molar mass of the polymers of fraction i, so that < M i+1for all i, in g / mol, ni is the molar amount of polymer of fraction i, in mol.

[0172] The centrifuge agent of the molar mass (M z ) was calculated from the data obtained by gel permeation chromatography measurement using the following equation: ing / mo 1where:

[0173] Mi is the molar mass of the polymers of fraction i, such that M t < M i+1 for all i, in g / mol, ni is the molar amount of polymer of fraction i, in mol.

[0174] Allophanate content:

[0175] The allophanate content was determined using 1 H-NMR. The measurements were carried out with a

[0176] Bruker AV III HD 600 spectrometer at 600 MHz in DMSO-D6 at 80 °C.

[0177] The following peaks were evaluated: u= CH2-NH 2.98 ppm CH2at urethane

[0178] 0= CH2-OH 3.43 ppm CH2 at OH groups

[0179] N= NH 6.6 ppm urethane

[0180] A= NH 8.3-8.4 ppm allophanate

[0181] The aliphatic allophanate concentration in mol-% was calculated using the following formula:

[0182] Allophanate [mol%] = 100% * fij (A+N)

[0183] The key figure for sales is calculated using the formula:

[0184] KZ = 1 / (1+O / U) Color values:

[0185] The color values ​​in the CIE Lab color space were determined using a Konica Minolta CM5 spectrophotometer under diffuse illumination (d / 8 geometry) according to DIN EN ISO 11664-4.

[0186] Differential Calorimetry (DSC):

[0187] The melting point was determined using DSC (Differential Scanning Calorimetry) with a DSC Q2000 V24.11 from TA Instruments in accordance with DIN EN 61006 (November 2004). Calibration was performed using the melting onset temperature of indium and lead. Approximately 10 mg of substance were weighed into standard capsules. The measurement was performed by heating twice from -20 °C to +220 °C at a heating rate of 20 K / min, with intermediate cooling at a cooling rate of 20 K / min. Cooling was performed using liquid nitrogen. Nitrogen was used as the purge gas. The first heating erases the thermal history of the sample. The stated values ​​are based on the evaluation of the first cooling curve and the second heating curve.

[0188] Determination of the modulus of elasticity and elongation at break:

[0189] The tensile test was conducted according to the DIN EN ISO 527 test method using type 5A specimens (DIN EN ISO 527-2, thickness 2 mm). The specimens were stored under standard conditions for at least 24 hours prior to testing. The tensile tests were conducted at 22 °C and 50% relative humidity using a Zwick Z010 universal testing machine at a speed of 10 mm / min. The Young's modulus was determined between 0.05% and 0.25% strain using a secant curve.

[0190] Isocyanate titration:

[0191] By back titration of butylamine with 0.1 N hydrochloric acid after addition of an excess of amine to an isocyanate solution using a Metrohm, 751 GPD titrino 685 Dosimat and 728 stirrer.

[0192] Production of test specimens for tensile tests

[0193] The polymer was dried at 80 °C in a drying cabinet for 4 h and then processed on an Xplore MC 15 HT microextruder (15 ml_) and molded into the required shape using the associated Micromoulder IM 12 injection molding machine. Yellowness determination:

[0194] The respective polymer powder was injection-molded into a standard rod (dimensions 80 mm x 10 mm x 4 mm). The rod was placed flat-side down on a light table. The gel particles were visible to the naked eye in transmitted light as bright spots in the otherwise homogeneous sample. In addition, the circular depressions on the smooth surface of the test specimens, which were created by gel particles close to the surface, were counted. To account for minor injection-molding errors, the sample was considered free of gel particles if the total number of gel particles counted was X < 5; 5 < X < 15 was considered low; 15 < X < 30 was considered high; and X > 30 was considered high.

[0195] Sieving:

[0196] A sample of 100 g was sieved for 5 min using a Haver & Boecker laboratory sieve machine No. 7279 (built in 1978) and the corresponding sieve sets (0.100 mm, 0.250 mm and 0.500 mm) and the individual fractions were then weighed.

[0197] Materials:

[0198] The following materials were used for components A) to E):

[0199] First aprotic solvent A1):

[0200] • Chlorobenzene in analytical quality, purchased from Azelis Deutschland GmbH, relative permittivity: s r = 5.6 (see Literature 1 below)

[0201] • o-Dichlorobenzene (oDCB) 99%, supplied by Acros Organics, relative permittivity: s r = 9.9 (see Literature 1 below)

[0202] • Acetone in analytical quality, purchased from Merck, relative permittivity: s r = 21 ,5 (see Literature 1 .) below)

[0203] • Butyl acetate, purchased from Azelis Deutschland GmbH, relative permittivity: s r = 5.0 (see Literature 1 below)

[0204] • Iso-amyl acetate, isomer mixture of 2- and 3-methylbutyl acetate, 99% purchased from Thermo Fischer, relative permittivity: s r = 4.8 (see Literature 1 below)

[0205] • 4-Heptanone 98%, purchased from Thermo Fischer, relative permittivity: s r = 12.6 (see Literature 1 below)

[0206] • Cyclohexanone 99.8%, purchased from Merck, relative permittivity: s r = 18.2 (see Literature 9.) below)

[0207] • Methoxypropyl acetate, purchased from Azelis Deutschland GmbH • Xylene, purchased from Azelis Deutschland GmbH, relative permittivity: s r =

[0208] 2,3 (see Literature 1 below)

[0209] Second

[0210] • Dimethylformamide (DMF) > 99%, purchased from Merck, relative permittivity: s r = 37.6 (see Literature 1 below)

[0211] • Dimethyl sulfoxide (DMSO) 99.9%, purchased from Merck, relative permittivity: s r = 47.3 (see Literature 2.) below)

[0212] • Dimethylacetamide (DMA) > 99%, purchased from Merck, relative permittivity: s r = 38.9 (see Literature 8.) below)

[0213] • Benzonitrile (PhCN) 99%, purchased from Merck, relative permittivity: s r = 25.6 (see Literature 1 below)

[0214] • N-Methylpyrrolidone (NMP), purchased from Azelis Deutschland GmbH, relative permittivity: s r = 32.5 (see Literature 3.) below)

[0215] • Tetramethylurea (TMU) 99%, purchased from Merck, relative permittivity: s r = 24.5 (see Literature 6.) below)

[0216] • 1,3-Dimethyl-2-imidazolidinone > 99.5%, purchased from Merck, relative permittivity: s r = 37.6 (see Literature 7.) below)

[0217] • Propylene carbonate 99%, purchased from Merck, relative permittivity: s r = 65 (see Literature 5.) below)

[0218] • Gamma-butyrolactone > 99%, purchased from TCI, relative permittivity: s r = 42.3 (see Literature 4.) below)

[0219] Literature: 1.) DK Handbook, Endress+Hauser Messtechnik GmbH&Co. (1999); 2.) Hunger et al J. Chem. Eng. Data 2010, 55, 5, 2055-2065; 3.) Granzhan et al, Zh. Prikl. Khim. 43 (1970) 1875-1877; 4.) MouMouzias et al J. Chem. Eng. Data 1999, 44, 6, 1273-1278; 5.) Barthel et al J. Chem. Eng. Data 2000, 45, 6, 1007-1011 ; 6.) Data from NIST Standard Reference Database 69: N / ST Chemistry Web Book 7.) Volume 17 'Static Dielectric Constants of Pure Liquids and Binary Liquid Mixtures (Supplement to IV / 6)' of Landolt- Börnstein Group IV 'Physical Chemistry'; 8.) Dielectric Constant of Common solvents.xls (washington.edu); 9.) Reference values ​​for dielectric constants of Magtech products | Emerson DE Polyol B):

[0220] • Ethylene glycol (EG)

[0221] • 1,4-Butanediol (BDO) (company: Ashland), purity: > 99% wt.%;

[0222] Diisocyanate C):

[0223] • 1,6-Hexamethylene diisocyanate (HDI) (company: Covestro AG), purity: > 99% wt.%;

[0224] • 1,5-pentamethylene diisocyanate (PDI) (company: Covestro AG), purity: > 99% wt.%;

[0225] • 4,4'-Methylenediphenyl isocyanate (MDI) (Company: Covestro AG, Purity: > 99% wt.%).

[0226] Chain regulator E1):

[0227] • n-Octanol of analytical quality, obtained from Arcos Organics, purity: > 99% w / w;.

[0228] Additive E2):

[0229] Irganox™ 245 from BASF

[0230] Irgafos™ 168 from BASF

[0231] Experimental procedure and results:

[0232] Experiment 1 : HDI and BDO in chlorobenzene (solvent A1) and various solvents A2 (hereinafter also called co-solvents)

[0233] 1200 mL of a mixture of chlorobenzene and co-solvent (10:1), 100.9 g (1.12 mol) of BDO, 1.43 g (11 mmol) of n-octanol, and 94.9 g (0.565 mol) of HDI were charged to a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to reflux on an oil bath, and an additional 94.9 g (0.565 mol) of HDI was added via a dropping funnel with stirring until the mixture gently boiled at reflux. After the addition was complete, the mixture was heated at reflux for a further 5 hours. The polymer precipitated as a white powder. The mixture was cooled to 20 °C and filtered (suction filter), and the residue was washed with two 150 mL portions of chlorobenzene and two 150 mL portions of acetone. The white solid was dried first in air and then in vacuo at 120 °C until constant weight was reached. No allophanate was detected in the NMR spectrum of the inventive examples. The sprayed particles showed no gel particles.The product was then separated into four particle fractions using vibratory sieving with sieve inserts of 0.100 mm, 0.250 mm, and 0.500 mm. The results are summarized in Table 1.

[0234] Table 1

[0235] Experiment 2: Variation of the ratio of solvent A1 (chlorobenzene) to solvent A2 (DMSO) (according to the invention)

[0236] 1200 mL of a mixture of chlorobenzene and DMSO, 198.0 g (2.2 mol) of BDO, 2.86 g (22 mmol) of n-octanol, and 189.8 g (1.13 mol) of HDI were placed in a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to reflux on an oil bath, and an additional 189.8 g (1.13 mol) of HDI was added via a dropping funnel while stirring until the mixture gently boiled at reflux. After the addition was complete, the mixture was heated at reflux for a further 5 hours. The polymer precipitated as a white powder.

[0237] The mixture was cooled to 20 °C and filtered (Nutsche filter), and the residue was washed with two 150 mL portions of chlorobenzene and two 150 mL portions of acetone. The white solid was dried first in air and then in vacuo at 120 °C until constant weight was reached. No allophanate was detected in the NMR spectrum. The syringes showed no gel particles. The product was then vibrated using sieves with sieve inserts of 0.100 mm, 0.250 mm, and 0.500 mm, divided into 4

[0238] Particle fractions were separated. The results are summarized in Table 2:

[0239] Table 2 Experiment 3: PDI+BDO in chlorobenzene / DMSO (according to the invention)

[0240] 1100 mL of chlorobenzene, 100 mL of DMSO, 198.0 g (2.2 mol) of BDO, 2.86 g (22 mmol) of n-octanol, and 170.94 g (1.11 mol) of PDI were placed in a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to reflux on an oil bath, and a further 169.4 g (1.10 mol) of PDI was added via a dropping funnel while stirring until the mixture gently boiled at reflux. After the addition was complete, the mixture was heated at reflux for a further 5 hours. The polymer precipitated as a white powder. The mixture was cooled to 20 °C and filtered (suction filter), and the residue was washed first with 800 mL and then twice with 100 mL of acetone. The white solid was first dried in air and then under vacuum at 120 °C to constant weight. The yield was 495 g (91%). Sieving revealed a proportion of 39 wt.% with particle sizes smaller than 0.100 mm, 45 wt.% with < 0.250 mm, and 49 wt.% with < 0.500 mm.No allophanate was detected in the NMR spectrum. GPC yielded Mw: 77491 g / mol; Mn: 12739 g / mol; Mz: 169409 g / mol. DSC revealed a melting temperature Tm of 160 °C. Tensile testing revealed a Young's modulus of 993 ± 33 MPa and an elongation at break of 340 ± 11%. The injection molded parts showed no gel particles.

[0241] Experiment 4: HDI / BDO / MDI in chlorobenzene / DMSO (according to the invention)

[0242] 1000 mL of chlorobenzene, 100 mL of DMSO, 100.9 g (1.12 mol) of BDO, 0.28 g (2.2 mmol) of n-octanol, and 94.9 g (0.565 mol) of HDI were placed in a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to reflux on an oil bath, and a further 85.4 g (0.508 mol) of HDI was added with stirring from a dropping funnel until the mixture gently boiled at reflux. After the HDI addition was complete, the mixture was heated at reflux for a further 2 hours. Then, 16.95 g (0.068 mol) of 4,4'-MDI dissolved in 100 mL of chlorobenzene was added dropwise, and the solution was heated at reflux for a further 2 hours. The mixture was cooled to room temperature with constant stirring. The polymer precipitated as a white powder. The mixture was filtered off (suction filter), and the residue was washed three times with 250 mL of acetone. The white solid was dried to constant weight under vacuum at approximately 120 °C. The yield was 280 g (93%).Sieving revealed a proportion of 31 wt.% with particle sizes smaller than 0.100 mm, 36 wt.% with < 0.250 mm, and 38 wt.% with < 0.500 mm. No allophanate was detected in the NMR spectrum. GPC yielded Mw: 155,479 g / mol; Mn: 14,144 g / mol; Mz: 476,293 g / mol. DSC revealed a melting temperature Tm of 172 °C. Tensile testing revealed a Young's modulus of 1,425 ± 77 MPa and an elongation at break of 131 ± 28%. The injection molded bodies showed no gel particles.

[0243] Experiment 5: HDI / EG in chlorobenzene / DMSO (according to the invention)

[0244] 1140 mL of chlorobenzene, 60 mL of DMSO, 94.79 g (1.53 mol) of ethylene glycol, and 1.95 g (15 mmol) of n-octanol were charged to a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C, and the mixture was heated to reflux in an oil bath. 260.71 g (1.55 mol) of HDI were then added using a dropping funnel while stirring until the mixture gently boiled at reflux. After the HDI addition was complete, the mixture was heated at reflux for a further 5 hours and then cooled to room temperature. The polymer precipitated as a white powder. The mixture was filtered off (with a suction filter), and the residue was washed three times with 200 mL of chlorobenzene. The white solid was dried in a convection oven at approximately 80 °C for 24 hours. The yield was 301 g (84%). Sieving revealed 0 wt.% with particle sizes smaller than 0.100 mm, 4 wt.% with < 0.250 mm, and 48 wt.% with < 0.500 mm. No allophanate was detected in the NMR spectrum.GPC yielded Mw: 165224 g / mol; Mn: 22951 g / mol; Mz: 425648 g / mol. DSC revealed a melting temperature Tm of 168 °C. The sprayed particles showed no gel particles.

[0245] Experiment 6: HDI and BDO at different T in solvent A1 (o-dichlorobenzene; not according to the invention) a. At 110 °C

[0246] 100 mL of ortho-dichlorobenzene, 10.03 g (0.11 mol) of BDO, 0.16 g (1.25 mmol) of n-octanol, and 9.49 g (0.056 mol) of HDI were placed in a 250 mL reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to 110 °C on an oil bath. Then, a further 9.49 g (0.056 mol) of HDI were added via a dropping funnel while stirring, ensuring the temperature of the mixture did not exceed 110 °C. After the addition was complete, the mixture was heated for a further 5 hours at 110 °C. The mixture was cooled to 20 °C with continuous stirring. The polymer precipitated as a white powder. The mixture was filtered (Nutsche filter) and the residue was washed three times with 100 mL of acetone each time. The white solid was then dried to constant weight in vacuo at 80 °C. The yield was 25.6 g (88%). No allophanate was detected in the NMR spectrum. GPC yielded Mw: 16337 g / mol; Mn: 5737 g / mol; Mz: 32101 g / mol. b. At 130 °C

[0247] The experiment was conducted as described under a.) but at 130 °C. The yield was 25.6 g (88%). No allophanate was detected in the NMR spectrum. GPC yielded Mw: 30846 g / mol; Mn: 9721 g / mol; Mz: 59423 g / mol. The tensile test yielded a Young's modulus of 1985 ± 92 MPa and an elongation at break of 16 ± 2%. The injection molded parts showed no gel particles. c. At 150 °C

[0248] The experiment was carried out as described under a.) but at 150 °C. The yield was 26.2 g (90%). The NMR spectrum revealed 0.5 mol% allophanate. GPC yielded Mw: 254,174 g / mol; Mn: 14,363 g / mol; Mz: 3,243,653 g / mol. The tensile test yielded a Young's modulus of 1,815 ± 129 MPa and an elongation at break of 134 ± 9%. The injection molded parts showed numerous (»30) gel particles. d. At 180 °C

[0249] The experiment was conducted as described under a.), but at 180 °C. Visible gels initially formed in the reaction mixture, and after the addition of the HDI was complete, a large gel clump formed on the stirrer. The product was not further processed.

[0250] Experiment 7: HDI+BDO in various solvents A1 (not according to the invention)

[0251] 1100 mL of solvent A1, 202.5 g (2.25 mol) of BDO, and 190.2 g (1.13 mol) of HDI were charged to a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to reflux, or 135 °C, on an oil bath. A further 190.2 g (1.13 mol) of HDI were then added via a dropping funnel while stirring until the mixture gently boiled at reflux (or the temperature did not exceed 135 °C). After the addition was complete, the mixture was heated at reflux (or at 135 °C) for a further 2 hours. The polymer precipitated as a white powder. The mixture was filtered off (with a suction filter), and the residue was washed twice with 150 mL of acetone each time. The white solid was then dried in vacuum at approximately 80 °C to constant weight. The results are summarized in Table 3: Table 3

[0252] Experiment 8: HDI+BDO in solvent A2 Propylene carbonate (not according to the invention as it is not a powder) 1000 mL of propylene carbonate, 178.2 g (1.98 mol) of BDO, 2.60 g (20 mmol) of n-octanol and 168.0 g (1.00 mol) of HDI were placed in a 2L reaction vessel equipped with a thermometer, reflux condenser and mechanical stirrer at 20 °C and the mixture was heated to 135 °C on an oil bath. A further 168.0 g (1.00 mol) of HDI were added dropwise using a dropping funnel while stirring so that the internal temperature of the mixture did not exceed 135 °C. After the addition was complete, the mixture was heated at 135 °C for a further 5 hours, during which time the polymer began to precipitate. The reaction mixture was cooled, the solid was filtered off (Nutsche filter), and the residue was washed three times with 250 mL of propylene carbonate. The filter cake was then dried under vacuum at 120 °C. The yield was 480 g (92%) as a large, white lump. Sieving revealed a fine fraction with a particle size of <0.500 mm, less than 5 wt.-%. The NMR spectrum revealed 0.2 mol% allophanate. GPC yielded Mw: 85606 g / mol; Mn: 17210 g / mol; Mz: 189611 g / mol.

[0253] Experiment 9: HDI+BDO in solvent A2 y-butyrolactone (not according to the invention as it is not a powder)

[0254] 1000 mL of gamma-butyrolactone, 178.2 g (1.98 mol) of BDO, 2.60 g (20 mmol) of n-octanol, and 168.0 g (1.00 mol) of HDI were placed in a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was then heated to 135 °C in an oil bath. A further 168.0 g (1.00 mol) of HDI were then added rapidly using a dropping funnel while stirring to keep the mixture at approximately 135 °C. After the addition was complete, the mixture was heated at 135 °C for a further 5 hours. The mixture was cooled, during which the polymer precipitated as a large lump hanging from the stirrer. A small portion of the polymer lump was separated, washed with acetone, and dried for GPC and NMR measurements. The NMR spectrum revealed 0.1 mol% allophanate. GPC yielded Mw: 137,707 g / mol; Mn: 23,203 g / mol; Mz: 295,338 g / mol.

[0255] Experiment 10: Thermostabilization of TPU powder (according to the invention)

[0256] 950 mL of chlorobenzene, 47.5 mL of DMSO, 191.71 g (2.13 mol) of BDO, 2.72 g (21 mmol) of n-octanol, and 180.3 g (1.07 mol) of HDI were placed in a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to reflux on an oil bath, and a further 180.32 g (1.07 mol) of HDI was slowly added using a dropping funnel while stirring until the mixture gently boiled at reflux. After the addition was complete, the mixture was heated at reflux for a further 5 hours. The mixture was cooled to room temperature with constant stirring, during which the polymer precipitated as a white powder. The polymer was filtered off, and the residue was washed twice on a suction filter with 250 mL of chlorobenzene each time. The solid was then suspended in a solution of 1.52 g Irgafos 168 and 1.52 g Irganox 245 in 250 mL chlorobenzene for 10 minutes and filtered.The moist filter cake was first pre-dried at room temperature for 24 hours, then for 3 hours at 50 °C, and then dried according to the times shown in Table 4. Experiment 11: Comparative example of non-thermostabilized TPU powder (not according to the invention).

[0257] The experiment was carried out analogously to experiment 10. However, no Irgafos 168 or Irganox 245 was added in the final washing step (see Table 4).

[0258] Experiment 12: Stabilizer added during the reaction (not according to the invention)

[0259] The experiment was carried out analogously to experiment 10, but Irgafos 168 and Irganox 245 were added at the beginning of the reaction and not in the last washing step (see Table 4).

[0260] Experiment 13: Stabilizer mixed dry (not according to the invention)

[0261] 950 ml of chlorobenzene, 47.5 ml of DMSO, 191.71 g (2.13 mol) of BDO, 2.72 g (21 mmol) of n-octanol, and 180.3 g (1.07 mol) of HDI were placed in a 2 L reaction vessel equipped with a thermometer, reflux condenser, and mechanical stirrer at 20 °C. The mixture was slowly heated to reflux on an oil bath, and a further 180.32 g (1.07 mol) of HDI was slowly added using a dropping funnel while stirring until the mixture gently boiled at reflux. After the addition was complete, the mixture was heated at reflux for a further 5 hours. The mixture was cooled to room temperature with constant stirring, during which the polymer precipitated as a white powder. It was filtered off, and the residue was washed twice on a suction filter with 250 ml of chlorobenzene each time. The moist filter cake was first pre-dried at room temperature and then for 3 hours at 50 °C.The white polymer powder was dry mixed with powdered 1.52 g Irgafos 168 and 1.52 g Irganox 245 and then dried according to the times in Table 4.

[0262] Experiment 14: Comparison example PA12 (not according to the invention)

[0263] Commercially available PA12 powder (Farsoon, FS3300PA) was stored in a convection oven at 155 °C for 2 hours and 5 hours, respectively. The yellowness index b of the samples and the untreated virgin material was then measured. The results are summarized in Table 4.

[0264] Experiment 15: Comparative example PA12 with stabilizer (not according to the invention)

[0265] 0.06 g of Irganox 245 and 0.06 g of Irgafos 168 were added to 20 g of powdered PA12 (Farsoon, FS330PA). The mixture was then treated with 150 ml of acetone and stirred for 30 minutes. The solvent was then removed using a rotary evaporator at 40 °C and approximately 150 mbar, and the powder was then stored for 2 hours and 5 hours, respectively, in a convection oven at 155 °C. The yellowness index (b) of the samples and the untreated virgin material was then measured. The results are summarized in Table 4.

[0266] Table 4

[0267] Discussion:

[0268] The patent replications of experiments 1a) (DE728981C) and 1b) (German Plastics Practice 1947, 289) using chlorobenzene and a mixture of chlorobenzene / o-dichlorobenzene, respectively, as well as the further experiments 7a) - 7f) using a solvent from group A1, yielded thermoplastic polyurethanes, which, however, did not achieve the necessary molecular weight Mw > 35,000 g / mol to exhibit practically usable mechanical properties of the polymer. Thus, molded articles made from these materials were very brittle. This was confirmed by testing the fracture behavior (experiments 1a and b). Furthermore, some of the resulting polymers did not precipitate as powders, but formed sticky or gel-like lumps (experiments 7b) and 7d)), which could not be easily processed and are not producible on an industrial scale.Among the solvents tested in this group A1, chlorobenzene, o-dichlorobenzene, butyl acetate and amyl acetate proved to be the most suitable, as the polyurethanes precipitated as white powders, with the polymers prepared in chlorobenzene or dichlorobenzene showing the highest molecular weight in the group.

[0269] Attempts to improve the solubility of the polyurethane in these solvents by increasing the temperature, thus achieving higher molecular weights (Mw) and the desired polyurethane powders according to the invention (Experiments 6a) to 6d)), failed. Although the molecular weight could be increased slightly, the temperature increase above 140 °C led to an increase in the allophanate content and thus to branching in the polymer. At 150 °C, the allophanate content became so high that numerous gel particles were observed, and at 180 °C, the polymer precipitated as a single large lump.

[0270] Even the sole use of solvent A2 did not produce the polyurethane powders according to the invention. Both experiment 8) with propylene carbonate and experiment 9) with γ-butyrolactone yielded polymers with a high molecular weight Mw, which also suggested good mechanical properties. However, in both cases, the polymer precipitated as a large lump upon cooling, which is difficult to process, especially on an industrial scale, and does not constitute a powder within the meaning of the invention.

[0271] Only the use of a combination of solvents A1) and A2) resulted in the desired thermoplastic polyurethanes as powders and with the molecular weight of over 35,000 g / mol required for good elongation and toughness. Chlorobenzene was chosen as solvent A1) because it has a boiling point within the desired temperature range, allowing heat dissipation for the highly exothermic urethane reaction to be achieved easily and pressure-free through evaporative cooling. Furthermore, chlorobenzene exhibits high reactive stability and compatibility, as well as miscibility with solvents A2). Experiments 1c) - 1i) demonstrate further possible variations for solvent A2), with the use of DMSO in particular achieving very good results. Furthermore, DMSO is preferable to the other solvents used in group A2) from the point of view of hazard classifications (see safety data sheet).

[0272] In experiments 2a) to 2e), the ratio of solvent A1) (chlorobenzene) and solvent A2) (DMSO) was varied over a wide range. It was shown that the polyurethane from HDI and BDO always resulted in the powder according to the invention, and that different molecular weights (Mw) could be controlled depending on the mixing ratio. This makes it possible to specifically produce polymers with different molecular weights, i.e., with different melt viscosities, for a wide variety of processing methods.

[0273] In experiments 3), 4) and 5), variations of the process according to the invention are shown in which different isocyanates and polyols or combinations were used and show that a wide range of polyurethane powders is accessible with the process.

[0274] As already described by J.M. DeBell (German Plastics Practice 1947, p. 304), TPU powders produced by precipitation processes tend to yellow slightly at elevated temperatures. This required a lengthy drying process at low temperatures and also resulted in a higher residual solvent content. Furthermore, the yellowing partially prevents the powders from being used in sintering processes, where the powder is exposed to high temperatures for extended periods.

[0275] The tendency to yellowing was confirmed in experiments 11b), 11c), and 11d). The tendency to yellowing increased significantly, particularly at temperatures above 100 °C. Drying or storage in a vacuum (100 mbar) also led to yellowing, presumably due to oxidation processes, since traces of oxygen cannot be completely excluded over extended periods. The high surface area of ​​the precipitated powders may be responsible for this, as this behavior was not observed in the melt even at much higher temperatures (O. Bayer Angew. Chem. 1947, 59, 9, 257-288). The addition of known thermal stabilizers (WO2022128170, WO2022128172) in situ during the synthesis of the polyurethane powder did not result in any significant improvement in the yellow color (experiments 12b) and 12c)). A significant improvement was achieved by dry blending the TPU powder with the powdered thermal stabilizers (tests 13b) and 13c)).However, the improvement is not yet sufficient to ensure subsequent application in all manufacturing processes. Furthermore, dry blending does not solve the problem of the lengthy drying step of the TPU powder. Only the addition of the thermal stabilizers as a solution in the final washing step resulted in a TPU powder that could be dried and stored at elevated temperatures for extended periods (Experiments 10b) - 10d)). The mechanism of action is not fully understood, especially since a comparison with PA 12 powder, which is also used, for example, for powder coating processes and is prone to yellowing (Experiments 14a) - 14c)), showed no improvement after a similar addition of thermal stabilizers (Experiments 15)).

Claims

Patent claims 1. A process for producing a thermoplastic polyurethane powder by precipitation polymerization, comprising the steps of: i. Providing A) a solvent mixture comprising at least a first aprotic solvent A1) with a relative permittivity s r from 3 to 20 and at least one second aprotic polar solvent A2) with a relative permittivity s r of at least 24, wherein the relative permittivity s r measured at 20 °C and 100 kHz; B) at least one polyol having a molecular weight between 60 g / mol and 250 g / mol; C) at least one diisocyanate; D) optionally a catalyst; E) optionally a chain regulator E1) and / or an additive E2); ii. Reaction of the polyol B) with the diisocyanate C) in the solvent mixture A) at a temperature of at most 150 °C, optionally in the presence of the catalyst D), the chain regulator E1) and / or the additive E2), to form the thermoplastic polyurethane, wherein the thermoplastic polyurethane precipitates as a solid in the solvent mixture A) and forms a dispersion; iii. Separation from the solvent mixture and optional washing of the thermoplastic polyurethane with a solvent; and iv. Drying the thermoplastic polyurethane to the thermoplastic polyurethane powder; wherein the thermoplastic polyurethane powder • a mass average molecular weight Mw of > 35000 g / mol; • an allophanate content of < 0.25 mol-%, based on the total thermoplastic polyurethane powder; and • > 25.0 wt.% of a particle fraction of < 0.500 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < wherein the mass average molecular weight Mw, the allophanate content, the particle fraction and the centrifuge average molecular weight Mz are each determined using the methods set out in the description.

2. The process according to claim 1, characterized in that the first aprotic solvent A1) comprises or consists of halogenated aromatics, ketones, ethers, esters and carbonates or mixtures thereof, in particular chlorobenzene and / or ortho-dichlorobenzene; cyclopentanone, cyclohexanone; heptan-4-one; acetophenone or mixtures thereof, preferably chlorobenzene and / or the second aprotic polar solvent A2) comprises or consists of dimethyl sulfoxide, dimethylformamide, / V-methyl-2-pyrrolidone, tetramethylurea, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone, 1,3-dimethyl-2-imidazolidinone, / V-ethyl-2-pyrrolidone, benzonitrile, dimethylacetamide or mixtures thereof, preferably dimethyl sulfoxide.

3. Process according to claim 1 or 2, characterized in that the ratio of the first aprotic solvent A1) to the second aprotic polar solvent A2) is 300:1 to 1:9, preferably 200:1 to 1:1, more preferably 100:1 to 8:

2.

4. Process according to one of the preceding claims, characterized in that the polyol B) comprises or consists of ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol, 1,12-dodecanediol or mixtures thereof, wherein the polyol B) preferably contains at least 50.0 wt.% 1,4-butanediol, particularly preferably at least 90.0 wt.% 1,4-butanediol, based on the total weight of the polyol B) and / or the diisocyanate C) 1,4-butane diisocyanate, 1,5-pentane diisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate, 1,1'-methylenebis(4-isocyanatocyclohexane), Xylylene diisocyanate, in particular m-xylylene diisocyanate or mixtures thereof, wherein the diisocyanate C) preferably contains at least 50.0 wt.% 1,5-pentane diisocyanate or 1,6-hexane diisocyanate, based on the total weight of the diisocyanate C).

5. Process according to one of the preceding claims, characterized in that the reaction in step ii. takes place at a temperature of 50 °C to 150 °C, preferably from 100 °C to 145 °C, more preferably from 120 °C to 140 °C and / or at an isocyanate number of 0.95 to 1.1, preferably 0.97 to 1.02, more preferably 0.98 to 1.

0.

6. Process according to one of the preceding claims, characterized in that the dispersion formed in step ii. has a solids content of 5.0 to 50.0 wt.%, preferably 15.0 to 45.0 wt.%, more preferably 20.0 to 40.0 wt.%, determined by gravimetric measurement with and without solvent.

7. Process according to one of the preceding claims, characterized in that the thermoplastic polyurethane powder • a mass average molecular weight Mw of 40,000 to 300,000 g / mol, preferably of 45,000 to 150,000 g / mol, particularly preferably of 55,000 to 100,000 g / mol; • an allophanate content of 0 to 0.20 mol%, preferably 0.001 to 0.15 mol%, particularly preferably 0.01 to 0.10 mol%, based on the total thermoplastic polyurethane powder; and / or • from 30.0 to 100.0 wt.%, more preferably from 40.0 to 80.0 wt.% of a particle fraction of < 0.500 mm, based on the total thermoplastic polyurethane powder and / or a ratio of centrifuge average molecular weight Mz to mass average molecular weight Mw of < 3.0, preferably from 1.5 to 2.

2.

8. Process according to one of the preceding claims, characterized in that the thermoplastic polyurethane after step iii. and / or the thermoplastic polyurethane powder after step iv. is impregnated with a stabilizer from a stabilizer solution, wherein for this purpose the thermoplastic polyurethane after step iii. and / or the thermoplastic polyurethane powder after step iv. is dispersed in the stabilizer solution and then separated and dried.

9. A method according to claim 8, characterized in that the stabilizer solution • a solvent selected from the group comprising or consisting of solvents from the group consisting of chlorinated aromatics; aliphatics; esters; ethers; alcohols; water; ketones; nitriles; carbonates; or mixtures thereof, or preferably the solvent(s) A1) from step i., wherein the solvent(s) preferably have a boiling point of < 250 °C at 1 bar; and • a stabilizer dissolved therein, which is selected from the group comprising or consisting of sterically hindered phenol derivatives, organic phosphine, phosphite and / or phosphonate derivatives, sulfur-containing antioxidants, 2,2,6,6-tetramethylpiperidine derivatives, benzotriazole derivatives, triazine derivatives, hydroxybenzophenone derivatives, cyanoacrylate derivatives, oxalinide derivatives or mixtures thereof; wherein the weight fraction of the stabilizer is preferably from 0.001 to 10.0 wt.%, more preferably from 0.05 to 5.0 wt.%, based on the total weight of the stabilizer solution.

10. The method according to claim 8 or 9, characterized in that the thermoplastic polyurethane powder comprises 0.005 to 2.0 wt.% of the stabilizer from the stabilizer solution, based on the total weight of the thermoplastic polyurethane powder.

11. The method according to any one of claims 8 to 10, characterized in that the thermoplastic polyurethane powder has a change in the b-value after exposure to a temperature of 120 °C for a period of 96 h and / or of 155 °C for a period of 6 h, in each case under an air atmosphere, of < 3.5, preferably from 0.1 to 2.8, more preferably from 0.2 to 2.0, wherein the b-value is determined in the CIE Lab color space by means of a spectrophotometer with illuminant D 65 at 8° in accordance with DIN EN ISO 11664-4 and the change in the b-value is obtained by subtracting the b-value before exposure to temperature from the b-value after exposure to temperature.

12. Thermoplastic polyurethane powder obtained or obtainable by a process according to any one of claims 1 to 11.

13. Use of a thermoplastic polyurethane powder according to claim 12 in an extrusion process, injection molding process, powder sintering process, solvent and / or melt process, in particular for producing molded parts and / or coatings.

14. A molded part obtained or obtainable by processing a thermoplastic polyurethane powder according to claim 12.

15. A molded part according to claim 14, characterized in that the molded part is free of gel particles, wherein “free of gel particles” is defined and determined as set forth in the description.

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