WO2007051740A1

WO2007051740A1 - Process for preparing polyisocyanate mixtures

Info

Publication number: WO2007051740A1
Application number: PCT/EP2006/067742
Authority: WO
Inventors: Imbridt Murrar; Gerd MÜLLER-HAGEN; Ralf Fritz; Oliver Steffen Henze; Hans-Jürgen Reese; Michael Wind; Andres Cabrera
Original assignee: Basf Se
Priority date: 2005-11-04
Filing date: 2006-10-25
Publication date: 2007-05-10
Also published as: DE102005053065A1

Abstract

The invention relates to a process for preparing polyisocyanate mixtures containing at least 90% by weight, based on the polyisocyanate mixture, of a mixture a) of ai) diphenylmethane diisocyanate and aii) triphenylenedimethylene triisocyanate, where the mixture a) contains at least 2% by weight, based on the weight of the polyisocyanate mixture, of triphenylenedimethylene triisocyanate, which comprises the steps A) reaction of aniline and formaldehyde to form a mixture of diphenylmethanediamine and its higher homologues, B) reaction of the mixture from step a) with phosgene and separation of the volatile compounds from the reaction product, C) separation of diphenylmethane diisocyanate and triphenylenedimethylene triisocyanate from the product obtained in step B).

Description

Process for the preparation of polyisocyanate mixtures

description

The invention provides a process for the preparation of polyisocyanate mixtures, polyisocyanate mixtures preparable by this process, containing at least 90% by weight of diphenylmethane diisocyanate and triphenylendimethylene triisocyanate, and their use.

Polyisocyanates have long been known and described many times. Their application is mostly for the production of polyurethanes. The most widely used polyisocyanates include diphenylmethane diisocyanate, also referred to as MDI, MMDI, monomer MDI or 2-core MDI, and its higher homologs.

Of particular importance are the pure 2-core MDI and mixtures of 2-core MDI and its higher homologues, often referred to as crude MDI (PMDI, polymer MDI). Raw MDI is primarily used where high crosslinking is important, especially for the production of rigid polyurethane foams. Monomeric MDI is mostly used in the production of elastomers and flexible foams.

Pure 2-core MDI is difficult to process as a solid and is therefore often liquefied by chemical reactions. This can be done by reaction of the isocyanate groups with one another, for example with formation of carbodiimide, uretonimine, biuret, allophanate or isocyanurate groups. Frequently, the 2-core MDI is also reacted with alcohols to urethane-containing prepolymers.

For many fields of use in polyurethane production, in particular thermoplastic polyurethanes (TPU), a defined crosslinking of the reaction mixture is desired. This is often done by incorporating aliphatic triisocyanates into the reaction mixture. However, it is often desirable to have a uniform isocyanate base for formulations for the production of polyurethanes.

For many applications, it would further be desirable to provide MDI homologs that are readily processible and have a defined functionality.

The object could be achieved by a process for the preparation of polyisocyanate mixtures containing at least 90% by weight, based on the polyisocyanate mixture, of a mixture a) of ai) diphenylmethane diisocyanate and aii) triphenylene dimethyltriisocyanate, the mixture a) at least 2 Wt .-%, based on the weight of the polyisocyanate mixture, Triphenylendimethylentriisocyanat containing the steps A) conversion of aniline and formaldehyde to a mixture of diphenylmethanediamine and its higher homologs,

B) reaction of the mixture from step A) with phosgene and separation of the volatile compounds from the reaction product,

C) separation of diphenylmethane diisocyanate and triphenylendimethylene triisocyanate from the product obtained in step B).

The invention furthermore relates to polyisocyanate mixtures containing at least 90% by weight, based on the polyisocyanate mixture, of a mixture of ai) diphenylmethane diisocyanate and aii) triphenylendimethylene triisocyanate, where the mixture a) contains at least 2% by weight, based on the weight of the Polyisocyanatge- mixed, Triphenylendimethylentriisocyanat aii).

The invention furthermore relates to the use of polyisocyanate mixtures consisting of diphenylmethane diisocyanate and triphenylendimethylene triisocyanate in the preparation of polyurethanes.

The polyisocyanate mixtures according to the invention consist, as stated, of at least 90% by weight of diphenylmethane diisocyanate ai) and triphenylendimethylene triisocyanate aii). The remainder, also referred to below as aiii), may be reaction products of isocyanates with one another, secondary compounds derived from the preparation of the PMDI or higher homologs of the MDI with at least 4 aromatic nuclei.

In a preferred embodiment of the invention, the polyisocyanate mixture contains 97 to <100 wt .-%, based on the polyisocyanate mixture, Triphenylendimethy- lentriisocyanat aii).

In a further preferred embodiment of the invention, the polyisocyanate mixture contains 20 to 98 wt .-% ai), 2 to 80 wt .-% aii) and 0 to less than 10 wt .-% aiii).

In a further preferred embodiment of the invention, the polyisocyanate mixture contains 55 to 98 wt .-% ai), 2 to 35 wt .-% aii) and 0 to less than 10 wt .-% aiii).

In a further embodiment of the invention, the polyisocyanate mixture contains 35 to 65 wt .-% ai), 35 to 65 wt .-% aii) and 0 to less than 10 wt .-% aiii). The preparation of the polyisocyanate mixtures according to the invention comprises, as stated, the steps

A) conversion of aniline and formaldehyde to a mixture of diphenylmethanediamine and its higher homologs,

B) reaction of the mixture from step A) with phosgene to give a mixture of diphenylmethane diisocyanate and its higher homologues and separation of the volatile compounds from the reaction product,

The steps can be carried out directly one after the other or temporally and / or spatially separated.

In most cases, the steps A) and B) take place immediately after one another in a system. Step C) can take place at a different time in another plant,

The preparation of diphenylmethane diisocyanate and its higher homologs according to steps A) and B) of the process according to the invention is known and, for example, in Kunststoffhandbuch, volume 7 "Polyurethane", edited by Günter Oertel, Carl Hanser Verlag Munich Vienna, 1993, chapter 3.2, or EP 1073628.

As is known, in step A) aniline is reacted with formaldehyde in the presence of acidic catalysts to give a mixture of diphenylmethanediamine and its higher homologues, the acid is separated off and the resulting worked-up mixture in step B) is reacted with phosgene to give the corresponding polyisocyanate. The reaction in step B) is usually carried out in solution, with solvents used being inert organic solvents, preferably chlorobenzenes, in particular monochlorobenzene. It is also possible to use the polyisocyanate as a solvent.

After the reaction in step B) usually unreacted phosgene, the solvent, hydrogen chloride and volatile by-products are separated.

From the worked-up product from step B), in a further step, diphenylmethane diisocyanate ai) and triphenylendimethylene triisocyanate aii) are separated off.

A method for the separation of diphenylmethane diisocyanate from polymer MDI is described for example in EP 1518874. However, this document does not information on how the distillate should be made and how this can be achieved.

Also in DE 10 2004 005 320 a process for the preparation of MDI is described with a subsequent separation of the resulting homologues with different proportions of aromatic rings. In this document, the individual mixtures are reacted to adjust the reactivity with acidic compounds. This is not necessary in the method according to the invention. The polyisocyanate mixtures according to the invention can be used without further pretreatment for the preparation of polyurethanes.

The separation in step C) can be carried out in different ways, for example by means of distillation, filtration, decantation, extraction and / or crystallization.

The separation in step C) preferably takes place by means of distillation, the distillation taking place in particular using at least one short-path evaporator.

The distillation is preferably carried out at a pressure of 0.001 to 10 mbar, preferably 0.002 to 6 mbar, more preferably 0.002 to 4 mbar and an evaporator temperature of 70 to 200 ° C, preferably 85 to less than 160 ° C.

The distillate obtained in this step usually consists of 55 to 98% by weight of diphenylmethane diisocyanate ai), 2 to 35% by weight of triphenylendimethylene triisocyanate aii) and 0 to less than 10% by weight of compounds from the group aiii).

From the mixtures obtained in step C), further diphenylmethane diisocyanate ai) can be separated in a further step D) in order to set the ratio of 2-core to 3-core MDI in a targeted manner. The separated 2-core MDI can be used as usual for the production of polyurethanes.

To carry out step D), it is possible to distill the mixture. The distillation is preferably carried out at a pressure of 0.001 to 10 mbar, preferably 0.002 to 6 mbar, more preferably 0.002 to 4 mbar and an evaporator temperature of 70 to 200 ° C, preferably 85 to less than 160 ° C.

The polyisocyanate mixtures obtained in process step C) usually form a two-phase mixture with a liquid and a solid crystalline phase at room temperature. By simple decantation or filtration, the liquid phase, which is a polyisocyanate mixture with 20 to 98 wt .-% ai), 2 to 80 wt .-% aii) and 0 to less than 10 wt .-% aiii) is separated off. Polyisocyanate mixtures with this composition have an NCO content of 32.6 to 33.6 wt .-% and a dynamic viscosity of 5 to 200 mPa · s at 25 ° C. The polyols obtained in process step C) isocyanate mixtures can also be worked up by distillation in at least one further process step D), so that a polyisocyanate mixture with 20 to 98 wt .-% ai), 2 to 80 wt .-% aii) and 0 to less than 10 wt .-% aiii) is obtained. Preferred conditions for the distillation are a pressure of 0.001 to 10 mbar, preferably 0.002 to 6 mbar, more preferably 0.002 to 4 mbar and evaporator temperatures of 70 to 200 ° C, preferably 85 to less than 160 ° C.

In a particular embodiment, a polyisocyanate mixture having a content of 35 to 65 wt .-% ai), 35 to 65 wt .-% aii) and 0 to less than 10 wt .-% aiii) in process step D) by distillation in the process step C) obtained Polyiso- cyanatgemische. The workup is preferably carried out by distillation at a pressure of 0.001 to 10 mbar, preferably 0.002 to 6 mbar, more preferably 0.002 to 4 mbar and evaporator temperatures of 70 to 200 ° C, preferably 85 to less than 160 ° C. Particular preference is given to the distillation in at least one short-path evaporation stage. The polyisocyanate mixtures thus prepared have an NCO

Content of 32.6 to 33.6 wt .-% and a dynamic viscosity of 5 to 100 mPa · s at 25 ° C. Polyisocyanate mixtures with this composition are characterized in particular by good storage stability and can preferably be used as crosslinking agents for the production of thermoplastic polyurethanes.

In a further preferred embodiment, by reducing the feed stream in process step C) by a factor of at least three, a polyisocyanate mixture of the composition 35 to 65 wt .-% ai), 35 to 65 wt .-% aii) and 0 to less than 10 Wt .-% aiii) without additional workup can be obtained. That is, by reducing the plant throughput by a factor of at least 3 without additional process step D) polyisocyanate mixtures of the composition with 35 to 65 wt .-% ai), 35 to 65 wt .-% aii) and 0 to less than 10 wt .-% aiii). Depending on the market requirements, the process can be adjusted in a targeted manner, firstly at maximum system throughput, preferably 2-core MDI poor PMDI and a polyisocyanate mixture with 2 to 35 wt .-% Triphenylendimethy- lentriisocyanat or at a reduced system throughput 2-core MDI poor PMDI and preferably a polyisocyanate mixture with a high content of Triphenylendimethylentriisocyanat, d. H. be prepared with 35 to 65 wt .-% Triphenylendimethy- lentriisocyanat.

Another possibility for the separation of diphenylmethane diisocyanate in process step C) is the extraction. The extractants used are isocyanate-inert solvents, such as hydrocarbons, for example n-hexane. The extraction is used in particular for the preparation of polyisocyanate mixtures having a high content of triphenylendimethylene triisocyanate aii). The extraction is carried out according to the methods known and known in the art. As a result of said further separation of diphenylmethane diisocyanate ai), an increase in increase in the content of triphenylendimethylene triisocyanate aii) in the mixture. This is particularly important for applications where high functionality of the mixture is important.

The polyisocyanate mixtures obtained in process step C) can be worked up in at least one further process step D) so that a polyisocyanate mixture having 97 to <100% by weight of triphenylendimethylene triisocyanate, based on the polyisocyanate mixture, is obtained. The remainder is diphenylmethane diisocyanate ai) and compounds from the group aiii). The polyisocyanate mixture of this composition has an NCO content of between 32.2 and 33.2% by weight and a dynamic viscosity of from 5 to 100 mPa.s at 75 ° C.

Polyisocyanate mixtures according to the invention having a content of triphenylendimethylene triisocyanate aii) of at least 97% by weight are solid at room temperature. Their melting point is about 65 to 70 ° C, their NCO content between 32.2 to 33.2% by weight. In Chemistry and Technology of Isocyanates (Henri Ulrich, John Wiley & Sons, 1996; page 388), the melting ranges of the seven possible isomeric forms of triphenylenedimethylene triisocyanate aii) are published.

An advantage of the method according to the invention is that it manages without waste products. The residual product obtained in step C) can, as described in EP 1518874, also be used for the preparation of polyurethanes. This crude MDI mixture, which is substantially free of volatile diphenylmethane diisocyanate, can be used as starting material for highly crosslinked polyurethanes, in particular rigid polyurethane foams. Due to the almost complete freedom of diphenylmethane diisocyanate, there are no toxicological problems with this compound. As a result, such a 2-core MDI poor raw MDI mixture particularly advantageous for the production of can foams, but also for 1-component and 2-component polyurethane foams, coatings, adhesives, especially hot melt adhesives, such as hotmelts, packaging adhesives, paints , Thermoplastics, compact or cellular elastomers, or as an additive to prevent crystallization and highly functional crosslinkers.

The mixtures containing diphenylmethane diisocyanate obtained in step C) and / or D) can be used directly as starting material for the preparation of polyurethanes or, if they are not useful, added to the PMDI mixture (crude MDI) prepared in step B), without affecting the quality of these products.

The polyisocyanate mixtures a) prepared by the process according to the invention can be used in many ways. A field of application is the prevention of crystallization, for example of polyester alcohols based on adipic acid and an even C-atom diol, such as ethylene glycol, butanediol or hexanediol by proportionate urethanization. The amount of polyisocyanate mixtures added should be large enough to provide the requisite crystallization stability, but not so great as to interfere with the properties of the liquids.

Another possible use is the prevention of the crystallization of Polyisocyana- natmischungen based on diphenylmethane diisocyanate. Since pure diphenylmethane diisocyanate is difficult to process at room temperature as a solid, it is often liquefied by chemical reactions. These products often have a low storage stability. Surprisingly, the crystallization can be significantly reduced by the addition of the polyisocyanate mixtures according to the invention. Preferably, the polyisocyanate mixtures according to the invention should be added in such an amount that the content of 3-core MDI in the isocyanate mixture in the range between 0.1 to 49 wt .-% is.

Another possible use of the polyisocyanate mixtures a) according to the invention is as crosslinking agents in plastics, in particular in polyurethanes. In particular, those polyurethanes which are only weakly crosslinked or not at all crosslinked can be crosslinked with the polyisocyanate mixtures according to the invention. The polyisocyanate mixtures according to the invention are preferably used for crosslinking thermoplastic polyurethanes, also referred to as TPUs.

TPUs are essentially linear polyurethanes. To improve mechanical properties, such as hardness and modulus of elasticity, partial crosslinking of these products may be advantageous.

The conventional crosslinking agents, for example modified, in particular aliphatic triisocyanates, such as isocyanurate or biuret, have the disadvantage that groups are incorporated which differ from the other isocyanate structural components of the TPU.

Due to their similar structure, the polyisocyanate mixtures according to the invention are readily compatible with the other NCO-functional constituent components of the TPU. The amount of the polyisocyanate mixtures added according to the invention depends on the desired degree of crosslinking and is preferably in the range from 0.001 to 10% by weight of 3-core MDI aii), based on the weight of the TPU.

In principle, it is advantageous if the content of triphenylendimethylene triisocyanate in the added polyisocyanate mixture according to the invention is as large as possible. However, as described above, triphenylendimethylene triisocyanate with a purity is at least 97 wt .-% is fixed, it may be advantageous to use liquid mixtures with a lower content of Triphenylendimethylentriisocyanat, which are better processable. The amount of diphenylmethane diisocyanate in the polyisocyanate mixtures according to the invention must then be taken into account for the diisocyanate used. Preference is given to using polyisocyanate mixtures having a composition of from 35 to 65% by weight of ai), from 35 to 65% by weight of aii) and from 0 to less than 10% by weight aiii) as crosslinking agent.

The preparation of the TPU is carried out according to the usual method by reacting diisocyanates with compounds having at least two isocyanate-reactive hydrogen atoms, preferably difunctional alcohols.

Suitable diisocyanates are customary aromatic, aliphatic and / or cycloaliphatic diisocyanates, for example diphenylmethane diisocyanate (MDI), tolylene diisocyanate (TDI), tri-, tetra-, penta-, hexa-, hepta- and / or octamethylene diisocyanate, 2-methylpentamethylene diisocyanate 1, 5, 2-ethyl-butylene-diisocyanate-1, 4, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl-cyclohexane (isophorone diisocyanate, IPDI), 1, 4 and / or 1 , 3-bis (isocyanatomethyl) cyclohexane (HXDI), 1,4-cyclohexane diisocyanate, 1-methyl-2,4- and / or -2,6-cyclohexane diisocyanate, 4,4'-, 2,4 ' - And / or 2, 2'-dicyclohexyl methane diisocyanate are used. Preferably, MDI is used as the diisocyanate in the process according to the invention.

As isocyanate-reactive compounds, generally known polyhydroxy compounds having molecular weights of 500 to 8,000, preferably 600 to 6,000, especially 800 to 4,000, and preferably an average functionality of 1, 8 to 2.6, preferably 1, 9 to 2.2 , in particular 2, for example polyesterols, polyetherols and / or polycarbonate diols. Preference is given to using polyesterdiols obtainable by reacting butanediol and hexanediol as diol with adipic acid as dicarboxylic acid, the weight ratio of butanediol to hexanediol preferably being 2: 1. Also preferred is polytetrahydrofuran having a molecular weight of 750 to 2500 g / mol, preferably 750 to 1200 g / mol.

As chain extenders it is possible to use generally known compounds, for example diamines and / or alkanediols having 2 to 10 C atoms in the alkylene radical, in particular ethylene glycol and / or butanediol-1, 4, and / or hexanediol and / or di- and / or Tri-oxyalkylenglykole having 3 to 8 carbon atoms in the oxyalkylene lenrest, preferably corresponding oligo-polyoxypropylene glycols, mixtures of the chain extenders can be used. As chain extenders it is also possible to use 1,4-bis (hydroxymethyl) benzene (1,4-BHMB), 1,4-bis (hydroxyethyl) benzene (1,4-BHEB) or 1,4-bis (2 -hydroxyethoxy) -benzene (1, 4-HQEE) are used. Preferred chain extenders are ethylene glycol and hexanediol, particularly preferably ethylene glycol. Usually, catalysts are used which accelerate the reaction between the NCO groups of the diisocyanates and the hydroxyl groups of the synthesis components, for example tertiary amines, such as triethylamine, dimethylcyclohexylamine, N-methylmorpholine, N, N'-dimethylpiperazine, 2- (dimethylaminoethoxy) - ethanol, diazabicyclo- (2,2,2) -octane and the like and in particular organic metal compounds such as titanic acid esters, iron compounds such as iron (Ml) - acetylacetonate, tin compounds such as tin diacetate, tin dilaurate or Zinndialkylsalze aliphatic carboxylic acids such as dibutyltin diacetate, dibutyltin dilaurate or similar. The catalysts are usually used in amounts of from 0.0001 to 0.1 parts by weight per 100 parts by weight of polyhydroxyl compound.

In addition to catalysts, it is also possible to add conventional auxiliaries to the structural components. Mention may be made, for example, of surface-active substances, flame retardants, nucleating agents, lubricants and mold release agents, dyes and pigments, inhibitors, stabilizers against hydrolysis, light, heat, oxidation or discoloration, protective agents against microbial degradation, inorganic and / or organic fillers, reinforcing agents and plasticizers ,

To adjust the molecular weight, it is possible to use isocyanate-reactive monofunctional compounds, preferably monoalcohols.

The preparation of the TPU is usually carried out by conventional methods, such as by belt systems or reaction extruder.

The invention will be explained in more detail in the following examples.

Example 1 :

2.24 kg / h of polymer-MDI, prepared by reacting aniline and formaldehyde and subsequent reaction of the resulting amine mixture with phosgene, containing 55.2 wt .-% of diphenylmethane diisocyanate, 31, 8 wt .-% free NCO and a Viscosity at 25 ° C of 62 mPa-s were in two Kurzwegverdampferstufen with 0.06 in ² evaporator per short-path evaporator continuously to a monomer poor polyphenylenepolymethylene polyisocyanate with a residual content of diphenylmethane diisocyanate 1690 ppm, 29.6 wt .-% free NCO and a Viscosity at 25 ° C of 24480 mPa-s demonomerized. At 1, 19 kg / h, the distillate fraction of the first evaporator stage was removed. The analysis revealed pure 2-core MDI. At 0.17 kg / h, the distillate fraction of the second evaporator stage was obtained, which consisted of 74.0 wt .-% diphenylmethane diisocyanate and 24.1 wt .-% Triphenylendimethylentrii- socyanat. The remaining 1.9% by weight of the second distillate fraction were car- bamoyl chloride and higher functional homologs of MDI with at least 4 aromatic nuclei.

Example 2:

2.15 kg / h of a Triphenylendimethylentriisocyanat containing Polyphenylenpoly- methylene polyisocyanate mixture, obtained in the second evaporator stage of Example 1, were in two evaporator stages (1st evaporator stage: thin-film evaporator with 0.06 m ² evaporator surface: 0.13 mbar and 195 ° C. 2. evaporator stage: short-path evaporator with 0.06 m ² evaporator surface: 0.007 mbar and 152 ° C) to a Polyphenylenpolymethylenpolyisocyanat mixture with a content of Triphenylendi- methylentriisocyanat of 97.8%, a content of free NCO of 32.5 Wt .-% and a melting range of 65 to 67 ° C concentrated. The remaining 2.2% by weight were 2-core MDI.

Example 3:

A polyphenylenepolymethylene polyisocyanate mixture containing 16.6% by weight of triphenylendimethylene triisocyanate and 79.4% by weight of diphenylmethane diisocyanate was added at 4.88 kg / h by means of short-path evaporator (0.06 m ² evaporator surface) at 0.13 mbar and 145 ° C evaporator temperature to a 37.6 wt .-% Triphenylendimethylentriiso- cyanate containing Polyphenylenpolymethylenpolyisocyanat mixture having a viscosity at 25 ° C of 40 mPa-s concentrated.

Example 4:

Production of Thermoplastic Polyurethane Elastomer (TPU)

The following isocyanate group-containing compounds have been used: Isocyanate 1: Lupranat® MP 102: modified diphenylmethane diisocyanate Isocyanate 2: Basonat® HI 100: triisocyanate based on isocyanurated hexamethylene diisocyanate Isocyanate 3: Basonat® HB 6500: Triisocyanate based on biuretized hexamethylene diisocyanate Isocyanate 4: Triphenylendimethylene triisocyanate-containing polyphenylene polymethylene polyisocyanate mixture consisting of 2-core MDI and 3-core MDI with a 2-core to 3-core ratio of 52:46.

Crosslinker mixtures A, B and C were obtained by mixing the respectively difunctional and trifunctional isocyanates (Table 1).

Table 1

In each case 2.5% by weight of a crosslinker mixture according to Table 1 were added to an Elastollan® 1 195A by a process described in [1]. Different TPU samples were obtained (Table 2).

[1] ... "Increasing the End-Use Temperature of TPU Products"; ANTEC 2005, Proceedings of the 63 ^rd Annual Technical Conference & Exhibition, Boston, MA, May 1- 5, Society of Plastics Engineers, pp. 3620-3624; Oliver Henze and Armando Sardanopoli.

Table 2:

The gel contents of the obtained samples were determined. For this purpose, in each case 2 g of a comminuted sample in 50 ml of DMF were stirred for 14 hours at room temperature. Subsequently, the insoluble fraction was determined for the samples by mass balancing. The TPU samples, which additionally contained a crosslinker, were no longer completely soluble like the comparative sample AO. The largest insoluble fraction of more than 90% was determined for the TPU sample E 2.5 (Table 3).

Table 3:

The mechanical properties of the four TPU samples were analyzed. All crosslinker-containing TPU samples had a lower tensile set, lower elongation at break and an increased tensile strength compared to the uncrosslinked standard material AO. th modulus of elasticity. The trend and thus the degree of crosslinking was most pronounced for the material E 2.5 (Table 4).

Table 4

The various TPU samples were tested for short-term heat resistance by TMA (Thermo-Mechanical Analysis: Measurement Conditions: heating rate 20 K / min, sample geometry: thickness about 2 mm, diameter 8 mm, load: 0.5 N with 6 mm quartz flakes). For the material E2.5, the highest softening temperature was found, which was improved by more than 25% compared to the standard material AO (Table 5).

Table 5

Claims

claims

1. A process for the preparation of polyisocyanate mixtures containing at least 90 wt .-%, based on the polyisocyanate mixture, of a mixture a) of ai) diphenylmethane diisocyanate and aii) triphenylendimethylene triisocyanate, wherein the

Mixture a) contains at least 2 wt .-%, based on the weight of the polyisocyanate mixture, Triphenylendimethylentriisocyanat, comprising the steps

2. The method according to claim 1, characterized in that the separation in step C) takes place by means of distillation.

3. The method according to claim 1, characterized in that the separation in step C) takes place by means of distillation using at least one short-path evaporator.

4. The method according to claim 1, characterized in that the distillate from step C) is subjected to at least one further separation D) of diphenylmethane diisocyanate.

5. The method according to claim 1, characterized in that the further separation D) by distillation, decantation, filtration, extraction and / or crystallization takes place.

6. Polyisocyanate mixture containing at least 90 wt .-%, based on the polyisocyanate mixture, of a mixture of ai) diphenylmethane diisocyanate and aii) Triphenylendimethylentriisocyanat, wherein the mixture at least 2 wt .-%, based on the weight of the polyisocyanate mixture, Triphenylendimethylentrii- socyanat aii) which can be produced by a method according to one of claims 1 to 5.

7. polyisocyanate mixture according to claim 6, characterized in that it contains 97 to <100 wt .-%, based on the polyisocyanate mixture, triphenylendimethyl lentriisocyanat aii).

8. polyisocyanate mixture according to claim 6, characterized in that it is less than 10 wt .-%, based on the polyisocyanate mixture, homologues of diphenylmethane diisocyanate with at least 4 aromatic nuclei, reaction products of isocyanates with one another and / or from the production of

PMDI-derived secondary compounds aiii) contains.

9. polyisocyanate mixture according to claim 6, characterized in that it contains 20 to 98 wt .-% ai), 2 to 80 wt .-% aii) and 0 to less than 10 wt .-% aiii).

10. polyisocyanate mixture according to claim 6, characterized in that it contains 55 to 98 wt .-% ai), 2 to 35 wt .-% aii) and 0 to less than 10 wt .-% aiii).

1 1. polyisocyanate mixture according to claim 6, characterized in that it contains 35 to 65 wt .-% ai), 35 to 65 wt .-% aii) and 0 to less than 10 wt .-% aiii).

12. Use of polyisocyanate mixtures according to claim 6 for the suppression of crystallization.

13. Use of polyisocyanate mixtures according to claim 6 for suppressing the crystallization of diphenylmethane diisocyanate at room temperature.

14. Use of polyisocyanate mixtures according to claim 6 for the preparation of polyurethanes.

15. Use of polyisocyanate mixtures according to claim 6 as crosslinkers in the preparation of polyurethanes.

16. Use of polyisocyanate mixtures according to claim 6 as a crosslinker in the preparation of thermoplastic polyurethanes.

17. A process for the preparation of thermoplastic polyurethanes by reacting polyisocyanates with compounds having at least two isocyanate-reactive hydrogen atoms, characterized in that the polyisocyanate 0.001 to 10 wt .-%, based on the weight of the polyurethane,

Triphenylendimethylentriisocyanat aii).