MXPA01007554A - Foamed thermoplastic polyurethanes - Google Patents

Abstract

Process for the preparation of foamed thermoplastic polyurethanes characterised in that the foaming of the thermoplastic polyurethane is carried out in the presence of thermally expandable microspheres.

Description

PROCESS FOR THE PREPARATION OF FOAMED THERMOPLASTIC POLYURETHANES FIELD OF THE INVENTION The present invention relates to a process for the preparation of foamed thermoplastic polyurethanes, with new foamed thermoplastic polyurethanes and with reaction systems for preparing the foamed thermoplastic polyurethanes.

BACKGROUND OF THE INVENTION Thermoplastic polyurethanes, hereinafter referred to as TPUs, are well-known thermoplastic elastomers. In particular, they exhibit a very high tensile and tear strength, high flexibility at low temperatures and an extremely good resistance to abrasion and grating. They also exhibit high stability against oil, grease and many solvents, as well as stability against UV radiation and are being used in a number of end-use applications such as in the automotive and footwear industry.

As a result of the increasing demand for lighter materials, it is necessary to develop a low density TPU that, in turn, represents a large technical challenge to provide, at a minimum, physical properties equal to those exhibited by the low density PU conventional. The production of shoe soles and other polyurethane parts is already known by a polyaddition reaction of liquid reagents which results in a solid elastic molded body. Up to now, the reagents used were polyisocyanates and polyesters or polyethers containing OH groups. Foaming was effected by the addition of a low-boiling liquid or by means of C02 to thereby obtain a foam comprising at least partially open cells. The reduction of the weight of the materials by foaming the TPU has not provided satisfactory results to date. Attempts to foam TPU using well-known blowing agents, such as azodicarbonamide (exothermic) or sodium bicarbonate (endothermic) based products, were unsuccessful in the molding of molded articles with lower densities, below 800 kg / cm3.

With the endothermic blowing agents, a good surface finish can be obtained but the lowest density that can be achieved is around 800 kg / cm3. Also, the processing is not very consistent and translates into long demoulding times. Very little or no foaming is obtained on the surface of the mold as a consequence of a relatively low mold temperature, which results in a fairly thick compact skin and a coarse cell nucleus. By using exothermic blowing agents, a lower density foam can be achieved (with values that drop to 750 kg / cm3 with a very thin cell structure, but the surface finish is not acceptable in most applications and time However, from the above, it is evident that there is a continuous demand for low density TPUs that have an improved skin quality and that can be produced with shorter demoulding times. surprisingly, the foaming of the TPUs in the presence of thermally expandable microspheres allows the above objectives to be met.The demolding times are significantly reduced and the process can be performed at lower temperatures, resulting in better stability in the cylinder On the other hand, the use of microspheres allows even to reduce the density even more, maintaining at the same time or improving the quality of the skin and the time of demolding. Thus, the present invention relates to a process for the preparation of foamed thermoplastic polyurethanes wherein the foaming of the thermoplastic polyurethane is carried out in the presence of thermally expandable microspheres and the presence of an additional blowing agent, the same in the present contain a hydrocarbon. The low density thermoplastic polyurethanes thus obtained (density not exceeding 800 kg / cm3) have a fine cellular structure, a very good surface appearance, a relatively thin skin and exhibit physical properties comparable to those exhibited by a conventional PU, which makes that they are suitable for a wide variety of applications.

The invention provides TPU products that have excellent properties of low temperature dynamic bending and green strength at the time of demolding, at a density of 800 kg / cm 3 and below. The term "green strength", as is known in the art, represents the basic integrity and strength of the TPU in the demolding. The polymer skin of a molded article, eg a shoe sole and other molded articles, should possess sufficient values of tensile strength and elongation and tear strength, to overcome bending of 90 to 180 degrees without receiving surface cracks . The processes of the state of the art usually require a demoulding time of at least 5 minutes to achieve this characteristic. On the other hand, the present invention therefore provides a significant improvement in the minimum demolding time. That is, a demolding time of 2 to 3 minutes can be achieved. The use of microspheres in a polyurethane foam has been described in EP-A-29021 and US-A 5418257.

The addition of blowing agents during the processing of TPUs is already widely known, see, for example, WO-A 94/20568, which describes the production of foamed TPUs, in particular TPUs in expanded particles; EP-A 516024, which discloses the production of foamed sheets from TPU by mixing with a blowing agent and thermal processing in a form by extrusion; and DE-A 4015714, which relates to TPUs reinforced with glass fiber, prepared by injection molding of TPU mixed with a blowing agent. However, none of the documents of the prior art disclose the use of thermally expandable microspheres to improve the skin quality of low density foamed TPU (density 800 kg / cm ° and even lower) nor do these documents suggest the benefits associated with the present invention.

Detailed Description Thermoplastic polyurethanes can be obtained by reacting a difunctional isocyanate composition with at least one functional polyhydroxydi compound and optionally, a chain extender, in amounts such that the isocyanate index is between 90 and 110, preferably between 95 and 110. 105, and more preferably between 98 and 102. The term "di functional" as used herein means that the average functionality of the polyisocyanate composition and the polyhydroxy compound is about 2. The term "isocyanate index" as such used here is the ratio of isocyanate groups to the isocyanate-reactive hydrogen atoms present in a formulation, expressed as a percentage. In other words, the isocyanate index expresses the percentage of isocyanate actually used in a formulation with respect to the amount of isocyanate theoretically required to react with the amount of isocyanate-reactive hydrogen used in a formulation. It should be noted that the isocyanate index, as used herein, is considered from the point of view of the actual polymer formation process which involves the isocyanate ingredient and the isocyanate-reactive ingredients. Any isocyanate groups consumed in a preliminary step to produce modified polyisocyanates (including those derived from isocyanates referred to in the art as quasi-or semi-prepolymers) or any active hydrogens reacted with isocyanate to produce modified polyols or polyamines, are not taken into account in the calculation of the isocyanate index. Only the free isocyanate groups and the free isocyanate-reactive hydrogens present in the actual elastomer formation step are taken into account. The difunctional isocyanate composition may comprise any aliphatic, cycloaliphatic or aromatic isocyanates. Preferred are isocyanate compositions comprising aromatic diisocyanates and more preferably diphenylmethane diisocyanates. The polyisocyanate composition used in the process of the present invention can consist essentially of, 4 '-di-phenyl-amethane-diisocyanate or in mixtures of that diisocyanate with one or more other organic polyisocyanates, in particular other diphenylmethane diisocyanates, for example, the 2,4'-isomer optionally in combination with the 2, 2' - isomer The polyisocyanate component can also be a variant of MDI derived from a polyisocyanate composition containing at least 95% by weight of 4,4'-diphenylmethane diisocyanate. MDI variants are well known in the art and, to be used according to the invention, include particularly liquid products obtained by introduction of carbodiimide groups in said polyisocyanate composition and / or by reaction with one or more polyols. Preferred polyisocyanate compositions are those containing at least 80% by weight of 4,4'-diphenylmethane diisocyanate. More preferably, the content of 4,4'-di-phenemethane diisocyanate is at least 90% by weight and more particularly at least 95% by weight. The difunctional polyhydroxy compound used has a molecular weight between 500 and 20,000 and can be chosen from polyester amides, poly thioethers, polycarbonates, polyacetals, polyolefins, polysiloxanes, polybutadienes and, especially, polyesters or polyethers, or mixtures thereof. Other hydroxy compounds such as hydroxyl terminated styrene block copolymers, of the SBS, SIS, SEBS or SIBS type can also be used. As the difunctional polyhydroxy compound, mixtures of two or more compounds of these or other functionalities can also be used and in ratios such that the average functionality of the total composition is about 2. For the polyhydroxy compounds, the actual functionality can be, for example, somewhat lower than the average functionality of the initiator due to some terminal unsaturation. Therefore small amounts of trifunctional polyhydroxy compounds can also be present in order to achieve the desired average functionality of the composition. Polyether diols which may be used include products obtained by polymerization of a cyclic oxide, for example, ethylene oxide, propylene oxide, butylene oxide or tetrahydrofuran, in the presence, when necessary, of di functional initiators. Suitable initiator compounds contain 2 active hydrogen atoms and include water, butanediol, ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, 1,3-propanediol, neopentyl glycol, 1,4-anodiol, 1,5-pentanediol, , 6-pentanediol and the like. Mixtures of initiators and / or cyclic oxides can be used. Especially useful polyether diols include polyoxypropylene diols and poly (oxyethylene glycol) diols obtained by the simultaneous or sequential addition of ethylene or propylene oxides or di functional initiators as widely described in the state of the art. Mention may be made of orange blossom copolymers having 10-80% oxyethylene contents, block copolymers having oxyethylene contents of up to 25% and orange / block copolymers having oxyethylene contents of up to 50%, based on the total weight of the oxyalkylene units, in particular those having at least part of the oxyethylene groups at the end of the polymer chain. Other useful polyether diols include polytetramethylene diols obtained by polymerization of tetrahydrofuran. Polyether diols containing low levels of unsaturation (ie, less than 0.1 milliequivalents per gram of diol) are also suitable. Other diols that can be used comprise dispersions or solutions of addition or condensation polymers in diols of the types described above. Said modified diols, often referred to as "polymeric" diols, have been widely described in the state of the art and include products obtained by the in situ polymerization of one or more vinyl monomers, for example, styrene and acrylonitrile, in polymeric diols, for example polyether diols, or by the in situ reaction between a polyisocyanate and an amino and / or hydroxy functional compound, such as triethanolamine, in a polymeric diol. Polyoxyalkylene diols containing from 5 to 50% dispersed polymer are also useful. Dispersed polymer particle sizes of less than 50 microns are preferred. Polyester diols that can be used include hydroxylated reaction products of dihydric alcohols such as ethylene glycol, propylene glycol, diethylene glycol, 1,4-but-anodiol, neopent-il-glycol, 2-methyl-1-propanediol, 3-methyl-1 -pentane-1,5. -diol, 1, 6-hexanediol or cyclohexanedimethanol or mixtures of such dihydric alcohols, with dicarboxylic acids or their ester-forming derivatives, for example, succinic, glutaric and adipic acids or their dimethyl esters, sebacic acid, phthalic anhydride, anhydride tetrachloro to ico or dimethyl terephthalate or mixtures of the above. Polyester teramides can be obtained by the inclusion of aminoalcohols, such as ethanol amine, in polyesterification mixtures.

Polyethyle diols which can be used include products obtained by thiodiglycol condensation either alone or with other glycols, alkylene oxides, dicarboxylic acids, formaldehyde, aminoalcohols or aminocarboxylic acids. Polycarbonate diols that can be used include those prepared by reaction of glycols such as diethylene glycol, triethylene glycol or hexanediol, with formaldehyde. Suitable polyacetals can also be prepared by polymerization of cyclic acetals. Suitable polyolefin diols include hydroxy-terminated butadiene polymers and copolymers, and suitable polysiloxane diols include polymethyl ioxan diols. Suitable di-functional chain extenders include aliphatic diols, such as ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-propanediol, 2-methylpropanediol, 1, 3-butanediol, 2,3-butanediol, 1,3-pent-anodiol, 1,2-hexanediol, 3-methopentane-1,5-diol, diethylene glycol, dipropylene glycol and tripropylene glycol, and amino alcohols such as ethanolamine, N- met i Idiethanolamine and the like. 1,4-Butanediol is preferred. The TPUs suitable for the processing according to the invention can be produced according to the known methods of reaction in a single container of the semi-prepolymer or of the prepolymer, by molding, extrusion or any other process known to the person skilled in the art and in General are supplied as granules or pellets. Optionally, small amounts, ie up to 30, preferably 20, and more particularly 10% by weight, based on the total mixture, can be mixed with the TPU of other conventional thermoplastic elastomers, such as PVC, EVA or TR. Any thermally expandable microspheres can be used in the present invention. However, microspheres containing hydrocarbons, in particular aliphatic or cycloaliphatic hydrocarbons, are preferred. The term "hydrocarbon" as used herein is intended to include non-halogenated hydrocarbons and partially or fully halogenated hydrocarbons. Commercially available thermally expandable microspheres containing a (cyclo) aliphatic hydrocarbon, which are particularly preferred in the present invention. Said microspheres are generally dry, unexpanded or partially unexpanded microspheres, consisting of small spherical particles with an average diameter of usually 10 to 15 micrometers. The sphere is constituted by a polymer shell impervious to gases (consisting, for example, of acrylonitrile or PVDC), which in capsule is a tiny drop of an aliphatic (cyclo) hydrocarbon, for example, liquid isobutane. When these microspheres are subjected to heat at a high temperature level (for example, 150 to 200 ° C) sufficient to soften the thermoplastic shell and volatilize the (cyclo) aliphatic hydrocarbon encapsulated therein, the resulting gas expands to the shell and increases the volume of the microspheres. When they expand, the microspheres have a diameter of 3.5 to 4 times their original diameter as a result of which their expanded volume is about 50 to 60 times greater than their initial volume in the unexpanded state. An example of such microspheres are the EXPANCEL-DU microspheres marketed by AKZO Nobel Industries of Sweden ('EXPANCEL' is a registered trademark of AKZO Nobel Industries). A blowing agent is added to the system, which may be a well-exothermic or endothermic blowing agent, or a combination of both. However, more preferably an endothermic blowing agent is added In the present invention, any of the known blowing agents used in the preparation of foamed thermoplastics can be used as blowing agents Examples of suitable chemical blowing agents include compounds gaseous substances such as nitrogen or carbon dioxide, gas-forming compounds (for example C02), such as azodicarbonamides, carbonates, bicarbonates, citrates, nitrates, borohydrides, carbides - such as carbonates and bicarbonates of alkaline earth metals and alkali metals, for example, sodium bicarbonate and sodium carbonate, ammonium carbonate, diaminodi phenylsulphone, hydrazides, acid malonic, citric acid, sodium monocyte, ureas, azodicarbonic acid methyl ester, diazabicyclooctane and mixtures of acids / carbonates.

Preferred endothermic blowing agents comprise bicarbonates or citrates. Examples of suitable physical blowing agents include volatile liquids such as chlorofluorocarbons, partially halogenated hydrocarbons or non-halogenated hydrocarbons, such as propane, n-butane, isobutane, n-pentane, isopentane and / or neopentane. Preferred endothermic blowing agents are the blowing agents known as 'HYDROCEROL' as described inter alia, in EP-A 158212 and EP-A 211250, which are known as such and commercially available ('HYDROCEROL' is a registered trademark of Clariant). Azodicarbonamide blowing agents are preferred as exothermic blowing agents. The microspheres are normally used in an amount of 0.1 to 5 parts by weight per 100 parts by weight of thermoplastic polyurethane. The amounts of microspheres in the order of 0.5 to 4 parts by weight per 100 parts by weight of thermoplastic polyurethane are preferred. More preferably, the microspheres are added in amounts of 1 to 3 parts by weight per 100 parts by weight of thermoplastic polyurethane. The total added amount of blowing agents is usually from 0.1 to 5 parts by weight per 100 parts by weight of thermoplastic polyurethane. Preferably 0.5 to 4 parts by weight of blowing agents are added per 100 parts by weight of thermoplastic polyurethane. More preferably, the blowing agent is added in amounts of 1 to 3 parts by weight per 100 parts by weight of thermoplastic polyurethane. In the process of the present invention, additives traditionally used in the processing of thermoplastics can also be used. Such additives include catalysts, for example tertiary amines and tin compounds, surface active agents and foam stabilizers, for example siloxane-oxyalkylene copolymers, flame retardants, antistatic agents, plasticizers ,. organic and inorganic fillers, pigments and internal mold release agents. The foamed thermoplastic polyurethanes of the present invention can be prepared according to a variety of processing techniques, such as extrusion, calendering, thermoforming, flow molding or injection molding. However, injection molding is the preferred production method. The presence of thermally expandable microspheres allows to reduce the processing temperatures. Typically, the process of the present invention is carried out at temperatures between 150 and 175 ° C. Conveniently, the mold is pressurized, preferably with air, and the pressure is released during foaming. Although said process is known and customarily available by various machine producers, it has surprisingly been found that carrying out the process of the present invention in a pressurized mold results in TPU articles having an excellent surface finish as well as excellent properties. physical, at the same time that they have an even more reduced density (values that descend up to 350 kg / m3). By the method of this invention, thermoplastic polyurethanes of any density of between 100 and 1200 kg / m 3 can be prepared, but mainly foamed thermoplastic polyurethanes having densities lower than 800 kg / m 3, more preferably less than 700 kg / m 3 and very particularly less than 600 kg / m3. The thermoplastic polyurethane is usually prepared in the form of pellets for further processing to the desired article. The term 'pellets' is understood and used herein to encompass various geometric shapes, such as squares, trapezoids, cylinders, lenticular shapes, cylinders with diagonal faces, chunks and substantially spherical shapes including a powder particle or a larger-sized sphere . Although thermoplastic polyurethanes are usually marketed as pellets, the polyurethane could be in any shape or size that is suitable for use in the facility used to form the final article. According to another embodiment of the present invention, the thermoplastic polyurethane pellet of the present invention comprises a thermoplastic polyurethane body, thermally expandable microspheres and a binder that binds the body and the microspheres. The binder comprises a polymer component having a start temperature for its melt processing less than the start temperature of the melt processing range of the TPU. The pellets may also include blowing agents and / or additive components such as dyes or pigments. The binder covers at least part of the body of thermoplastic polyurethane. In a preferred embodiment, the thermoplastic polyurethane body and the microspheres are substantially encapsulated by the binder. By the term "substantially encapsulated" is meant that at least three quarters of the surface of the thermoplastic polyurethane body is coated and preferably are coated at least about nine tenths of the resin body. It is particularly preferable that the binder substantially covers the entire polyurethane body and the microspheres. The amount of binding agent with respect to the thermoplastic polyurethane can normally range between at least 0.1% by weight and up to approximately 10% by weight, based on the weight of the thermoplastic polyurethane pellet. Preferably, the amount of binder is at least 0.5% by weight and up to about 5% by weight, based on the weight of the thermoplastic polyurethane pellet.

Preferably, the binder has a start temperature for its melt processing range that is lower than the start temperature of the melt processing range of the thermoplastic polyurethane body. In this way, the binder can be applied as a molten mixture to the thermoplastic polyurethane body composition while the latter is in a solid or substantially solid state. The starting temperature of the in-melt processing range of the binder is preferably above about 20 ° C and more preferably above 60 ° C and even more preferably at least 80 ° C. start of the melt processing range of the polymeric component of the coating is preferably at least 20 ° C and even more preferably at least 40 ° C below the starting temperature of the melt processing range of the body of thermoplastic polyurethane. If the sequestering thermoplastic polyurethane pellets are to be dried using a dryer, then the range of the molten processing of the binding agent is preferably above the temperature of the dryer. In a preferred embodiment, the binder is chosen to prevent or decrease the absorption of water so that it is unnecessary to perform a drying step before forming the desired article. The binder can then be added to the TPU pellets by several different methods. According to one method, the pellts are placed in a container with the coating composition while the pellets are still at a temperature above the starting temperature of the melt processing range of the binder. In this case, the binder may already be melted or melted by the heat of the pellets or by the heat applied outside the container. For example, without this entailing any limitation, the binder can be introduced into the container as a powder when it has to melt in the container. The binder can be any substance capable of binding the thermoplastic polyurethane body and the microspheres. Preferably, the binder comprises a polymer component. Examples of suitable polymeric components include polyisocyanates and / or their prepolymers. The foamed thermoplastic polyurethanes obtainable by the process of the present invention are particularly suitable for use in any application of thermoplastic rubbers including, for example, footwear or integral skin applications such as in steering steering of vehicles. Custom thermoplastic polyurethanes can be produced more efficiently using the process of the present invention. Custom thermoplastic polyurethanes can be formed into any of the articles usually prepared with thermoplastic resins. Examples of articles are interior and exterior parts of automobiles, such as interior panels, bumpers, housings for electrical devices such as televisions, personal computers, telephones, video cameras, watches, diaries; packaging materials; leisure items; sporting goods and toys. In another embodiment, the present invention relates to a reaction system comprising (a) a TPU and (b) thermally expandable microspheres.

The invention is illustrated, but not limited, by the following examples in which all parts, percentages and ratios are by weight.

EXAMPLES Example 1 (Comparative) TPU pellets were mixed dry (Avalon 62AEP, 'Avalon' is a registered trademark of Imperial Chemical Industries Ltd.) with an endothermic blowing agent (1% NCI 75 or 2% INC71 75ACR powder (which is an equivalent master mix), both supplied by Tramaco GmbH). The dry mix was then processed in an injection molding machine (Desma SPE 231) to form a test molding article of dimensions 19.5 * 12.0 * 1 cm. The processing temperatures for all the samples can be seen in Table 1. The physical properties obtained for all the samples can be seen in Table 2. The abrasion was measured in accordance with DIN53516.

Example 2 (Comparative) The TPU of Example 1 was mixed dry with an exothermic blowing agent (Celogen AZNP130, from Uniroyal) and then processed in the same manner as in Example 1. The minimum density attainable to avoid severe marking. on the surface it is 1000 kg / m3 with an addition level of 0.3%.

Example 3 (Comparative) The TPU-of Example 1 was mixed dry with a mixture of an exothermic blowing agent and an endothermic blowing agent (0.3% Celogen AZNP130 and 0.7% NC175) and processed in the same manner as in the Example 1.

Example 4 (Comparative) The TPU of Example 1 was mixed dry with 4% thermally expandable microspheres (Expancel 092 MB120, from Akzo Nobel). The mixture was processed in the same manner as in Example 1.

Example 5 The TPU of Example 1 was mixed dry with 2% of thermally expandable microspheres (Expancel 092 MB120) and with an endothermic blowing agent (1% NC175 or 2% INC7175ACR) and processed as in Example 1. Example 6 The TPU of Example 1 was mixed dry with 2% thermally expandable microspheres (Expancel 092 MB120) and 1% of an exothermic blowing agent (Celogen AZNP130). Again it is processed as in Example 1.

Example 7 The TPU of Example 1 was mixed dry with 2% thermally expandable microspheres (Expancel 092 MB120), 0.7% endothermic blowing agent (NC175) and 0.3% exothermic blowing agent (Celogen). AZNP130). It is then processed again as in Example 1.

Example 8 The TPU of Example 1 was mixed dry with 2% thermally expandable microspheres (Expancel 092 MB 120) and an endothermic blowing agent (1% NC175 or 2% INC7175ACR). It was then processed in a Main injection molding machine Group.

Example 9 The TPU of Example 1 was mixed dry with 2% thermally expandable microspheres (Expancel 092 MB120) and 2% exothermic blowing agent (IM7200, commercially available from Tramaco GmbH). This dry mix was then processed in a Main Group machine with an air injection system (Simplex S16).

Example; The TPU of Example 1 was mixed dry with 2. 5% thermally expandable microspheres (EXP 092 MB120) and 2% of an exothermic blowing agent (IM7200). This dry mix was processed in a Main Group machine with an air injection system (Simplex S16).

Table 1: Processing temperatures of injection molding comparative example comparative example Example 11 Example 11 provides TPU pellets comprising microspheres formulated with binder. The TPU pellets were previously heated in a hot air oven at 100 ° C. Subsequently, an isocyanate prepolymer based on Daltorez® P321 and Suprasec® MPR is prepared at 80 [deg.] C. as binder. The binder (1-2% by weight) is then mixed in the TPU pellets until the TPU surface is completely wetted. The additives are then added and the mixing is continued until a homogeneous distribution of the additives on the surface of the TPU pellets is achieved. This mixture is then discharged into a polyethylene container and cooled to 10 ° C to allow the coating to solidify. This "cake" is then deagglomerated manually and is ready to be used in the injection molding machine. These coated pellets were processed in the injection molding machine and blown successfully at densities of 0.73 g / cc. Daltorez® P321 is a polyester polyol based on adipic acid and 1,6-hexanediol. Suprasec® MPR is pure MDI.

Claims (21)

1. CLAIMS 1. Process for the preparation of foamed thermoplastic polyurethanes, characterized in that the foaming of the thermoplastic polyurethane is carried out in the presence of thermally expandable microspheres and in the presence of an additional blowing agent, such microspheres contain a hydrocarbon.
2. 2. Process according to claim 1, wherein the hydrocarbon is an aliphatic or cycloaliphatic hydrocarbon.
3. 3. Process according to any of the above indications, where an endothermic blowing agent is present.
4. 4. Process according to any of the preceding claims, wherein an exothermic blowing agent is present.
5. Process according to claim 3 or 4, wherein the endothermic blowing agents comprise bicarbonates or citrates.
6. 6. Process according to any of claims 4-6, wherein the exothermic blowing agents comprise compounds of the zodi-carbonamide type.
7. 7. Process according to any of the preceding claims, which is effected by injection molding.
8. Process according to any of the preceding claims, which is carried out in a pressurized mold.
9. Process according to any of the preceding claims, wherein the starting thermoplastic polyurethane is prepared using a difunctional isocyanate composition comprising a difunctional aromatic isocyanate.
10. 10. Process according to the claim 9, wherein the difunctional aromatic isocyanate comprises diphenylmethane diisocyanate.
11. 11. Process according to the claim 10, wherein the diphenylmethane diisocyanate comprises at least 80% by weight of 4,4-di-phenylmethane diisocyanate.
12. 12. Process according to claims 9-11, wherein the difunctional polyhydroxy compound comprises a polyoxyalkylene diol or polyester diol.
13. 13. Process according to claim 12, wherein the polyoxyalkylene diol comprises oxyethylene groups.
14. 14. Process according to claim 13, wherein the polyoxyalkylene diol is a poly (oxyethylene-oxypropylene) diol.
15. Process according to any one of the preceding claims, wherein the amount of microspheres is between 0.5 and 4.0 parts by weight per 100 parts by weight of thermoplastic polyurethane.
16. 16. Process according to claim 15, wherein the amount of microspheres is between 1 and 3 parts by weight per 100 parts by weight of thermoplastic polyurethane.
17. Process according to any one of the preceding claims, wherein the amount of blowing agents is between 0.5 and 4.0 parts by weight per 100 parts by weight of thermoplastic polyurethane.
18. 18. Process according to claim 17, wherein the amount of blowing agents is between 1.0 and 3.0 parts by weight per 100 parts by weight of thermoplastic polyurethane.
19. 19. Foamed thermoplastic polyurethane obtainable by the reaction of a difunctional isocyanate composition with at least one difunctional polyhydroxy compound, in the presence of hydrocarbon-containing thermally expandable microspheres, and in the presence of an additional blowing agent, such a polyurethane has a non-specific density. greater than 700 kg / m3.
20. 20. Foamed thermoplastic polyurethane according to claim 19, having a density not greater than 600 kg / m3.
21. 21. Reaction system comprising TPU and thermally expandable microspheres containing a hydrocarbon, such a reaction system comprising an additional blowing agent.