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Isocyanate-containing thermoplastic polyurethane

Isocyanate-containing thermoplastic polyurethane description/claims

The invention relates to a thermoplastic polyurethane (i) comprising from 20% by weight to 70% by weight, preferably from 25% by weight to 70% by weight, particularly preferably from 30% by weight to 60% by weight, in particular from 35% by weight to 60% by weight, very particularly preferably from 40% by weight to 60% by weight, especially preferably from 45% by weight to 55% by weight, of isocyanate dissolved in the thermoplastic polyurethane, based on the total weight of the thermoplastic polyurethane (i) comprising the isocyanates, and also to processes for producing these thermoplastic polyurethanes (i) comprising isocyanate, in which preference is given to melting thermoplastic polyurethane and subsequently incorporating the isocyanate into the melt, preferably homogeneously. In addition, the invention relates to processes for producing polyurethanes, in which the production is carried out in the presence of the inventive thermoplastic polyurethanes (i) comprising the isocyanates. Furthermore, the invention relates to processes for reacting thermoplastic polyurethanes with isocyanate, for example in an extruder, in which the inventive thermoplastic polyurethanes (i) comprising isocyanates are used as isocyanate. In addition, the invention relates to processes for injection molding thermoplastic polyurethane, in which thermoplastic polyurethane is injection molded together with the inventive thermoplastic polyurethane (i) comprising isocyanates.

The production of thermoplastic polyurethanes, hereinafter referred to as TPUs for short, is generally known.

TPUs are partially crystalline materials and belong to the class of thermoplastic elastomers. A characteristic of polyurethane elastomers is the segmented structure of the macromolecules. Owing to the differing cohesion energy densities of these segments, a phase separation into crystalline “hard” and amorphous “soft” regions occurs in the ideal case. The resulting two-phase structure determines the property profile of TPUs. Thermoplastic polyurethanes are plastics having a wide range of applications. Thus, TPUs are used, for example, in the automobile industry, e.g. in dashboard skins, in films, in cable sheathing, in the leisure industry, as deposition areas, as functional and design elements in sports shoes, as flexible component in rigid-flexible combinations and in many further applications.

It is known from the literature that the property profile of TPU can be improved by introducing crosslinking into the TPU, leading to the strength being increased, the heat resistance being increased, the tensile and compressive sets being reduced, the resistance to media of all types, resilience and creep behavior being improved.

Known crosslinking methods are, inter alia, UV or electron beam crosslinking, crosslinking via siloxane groups and the formation of crosslinks by addition of isocyanates to the molten TPU. The reaction of a TPU, preferably in the molten state, with compounds bearing isocyanate groups is also referred to as prepolymer crosslinking and is generally known from U.S. Pat. No. 4,261,946, U.S. Pat. No. 4,347,338, DE-A 41 15 508, DE-A 4 412 329, EP-A 922 719, GB 2347933, U.S. Pat. No. 6,142,189, EP-A 1 158 011. Despite this general knowledge of the possible ways of achieving prepolymer crosslinking, this process has hitherto not been able to be implemented in industrial practice. A reason for this is, inter alia, the complicated apparatus. Very homogeneous mixing of the TPU, which is generally in the form of pellets, with the liquid or viscous compounds having isocyanate groups leads to considerable difficulties in practice. Secondly, the reaction of the TPU with the compounds having isocyanate groups also represents a difficult chemical task, since mixing of the molten TPU with diisocyanates can lead to a degradation of the molecular weight of the thermoplastic polyurethanes, while mixing with triisocyanates and polyisocyanates can cause an increase in the molecular weight as far as crosslinking of the thermoplastic polyurethanes in the extruder. In both cases, reliable processing of the polyurethane is made difficult or prevented. On the other hand, very pronounced crosslinking in the end product is sought.

It was an object of the present invention to optimize the chemical components in such a way that very good process reliability, e.g. melt stability, and very pronounced crosslinking can be achieved. In addition, the components should be able to be used, in particular, in injection molding and lead to articles which can be crosslinked.

These objects can be achieved by carrying out the introduction of the isocyanate by means of the concentrates described at the outset, i.e. the inventive TPU (i) comprising isocyanates in high concentration.

The present invention is distinguished from the prior art by, in particular, the substantially simplified handling of the isocyanates. While liquid isocyanates have to be handled in most of the documents cited above, according to the present invention it is possible to add solids in the form of the thermoplastic polyurethanes (i) comprising the isocyanates. The addition of a solid is of particular importance for injection molding. In addition, the adhesion of a thermoplastic polyurethane to other thermoplastic polymers in, in particular, 2-component injection molding has been able to be substantially improved by use of the concentrate (i), either alone or together with further thermoplastic polyurethane, presumably as a result of the free isocyanate groups.

In particular, the solid concentrates (i) offer the advantage that the volatility of the isocyanates is significantly reduced. It has surprisingly been found that a TPU comprising 50% by weight of a prepolymer based on MDI and having an NCO content of 23% and a viscosity of 650 mPas determined in accordance with DIN 53018 was, firstly, free-flowing and, secondly, no volatile MDI could be detected. In addition, the isocyanates are stable in the inventive TPUs (i), i.e. they barely react or do not react at all and are therefore, contrary to expectations, sufficiently storage-stable.

The inventive thermoplastic polyurethanes (i) comprising the isocyanates can thus be used and processed like concentrates. While in the prior art the addition of the isocyanate to the thermoplastic polyurethane is carried out immediately before processing to the end product and crosslinking, according to the invention it is possible to produce a stable concentrate (i) which can be processed only at a significantly later point in time together with further thermoplastic polyurethane to form the end product. A distinction is therefore made in the present text between the concentrates of the invention, i.e. the thermoplastic polyurethane (i) comprising the isocyanate, and the “normal” thermoplastic polyurethanes which do not comprise isocyanates in the amounts according to the invention. The concentrates are denoted by (i) in the present text.

In the thermoplastic polyurethanes (i) of the invention, the isocyanates are present as a solution in the TPU, in particular in the soft phase of the thermoplastic polyurethane. Reaction of the isocyanate with the TPU and thus degradation or crosslinking of the TPU can be avoided, in particular, by a sufficiently low temperature being selected during incorporation. The molecular weight of the TPU usually does not change or changes only very slightly during the incorporation according to the invention of the isocyanates. On the other hand, it is preferred that the thermoplastic polyurethane is molten during the incorporation of the isocyanate in order to be able to reach a very high concentration of isocyanates in the TPU very quickly. The inventive thermoplastic polyurethane (i) comprising isocyanate is preferably stored at a temperature below 40° C. until it is processed.

The concentrate according to the invention, i.e. the TPU (i) comprising the isocyanates, has the additional advantage that no foreign polymer is introduced on addition to the TPU to be crosslinked. Thus, the TPU to be crosslinked can be admixed with the same TPU (i) comprising the isocyanates. Mixtures can thus be avoided, as can substantial adaptations by means of formulation changes, i.e. by addition of a foreign polymer.

The TPUs of the invention, i.e. the thermoplastic polyurethanes (i) comprising the isocyanates, particularly preferably have an NCO content of greater than 5%, preferably greater than 8%, particularly preferably from 10% to 40%.

Here, the NCO content is determined as the sum of isocyanate and allophanate. The sample is for this purpose dissolved in dimethylformamide comprising the amine and maintained at 80° C. for 4 hours. The unreacted excess of amine is backtitrated with acid.

Specifically, the following procedure is employed:

A sample to be tested for isocyanate content is weighed out. The amount weighed out depends on the expected content of isocyanate groups and is weighed to a precision of ±0.001 g. The analysis is carried out as a duplicate determination.

For each analysis, blank determinations without a sample but otherwise completely identical are carried out as a triplicate determination.

20.00 ml of a di-n-hexylamine solution (8.8(±0.01)g of di-n-hexylamine are made up with DMF to give 2000 ml of solution) are metered by means of a Dosimat 665 (Metrohm Dosimat 665 with a 20 ml burette attachment) into a wide-neck bottle (Schott 250 ml laboratory bottles with screw closure made of PP (blue), DIN thread GL45). 100 ml of DMF are then added by means of a Dispensette and the weighed-out sample is subsequently added. The wide-neck bottle is closed firmly and the solid sample comprised therein is then dissolved by means of a magnetic stirrer bar (magnetic stirrer bar, triangular, I=55 mm) in the firmly closed bottle on an oil bath at 80° on a magnetic stirrer.

In all three cases, the samples are then cooled to room temperature and can then be titrated.

Three drops of indicator solution (bromophenol blue, 1% in DMF) are added to the cooled solutions. The solution is then backtitrated with 0.1 N hydrochloric acid (prepared by making up the content of an ampoule of Titrisol 0.1 mol/l hydrochloric acid to 1 l with 1-butanol) in 1-butanol using a Dosimat 665 (Metrohm Dosimat 665 with 5 ml burette attachment) The end point is reached when the color changes from light green to yellowish greenish yellow. The acid consumption is denoted by “A” in the calculation.

The mean of the amount of acid consumed in the three blank determinations is denoted by “B” in the calculation.

Both analytical values are calculated separately.

Calculation of the total isocyanate content in % of NCO (sum of isocyanate groups and allophanate groups, calculated as NCO):

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