EP2516496A2

EP2516496A2 - One-shot process for the production of thermoplastic polyurethane modified with styrenic block copolymers

Info

Publication number: EP2516496A2
Application number: EP10814693A
Authority: EP
Inventors: Andrea Martignoni; Andrea Correnti; Davide Brambillasca
Original assignee: Epaflex Polyurethanes Srl
Current assignee: Epaflex Polyurethanes Srl
Priority date: 2009-12-23
Filing date: 2010-12-21
Publication date: 2012-10-31
Also published as: WO2011077234A2; BR112012015378A2; WO2011077234A3

Abstract

A flexible modified thermoplastic polyurethanes having a Shore A hardness within the range of 55 to 98 Shore A is prepared by mixing and reacting together: a) 4,4'diisocyanatodiphenylmethane or a mixture of the same and 2,4-diisocianatodiphenylmethane, b) polyester polyols having an average molecular weight from 1000 to 4000, c) chain extenders, d) additives, e) a styrenic block copolymer selected from SBS, SEBS, SIS or mixtures thereof, the amount of such copolymer being within the range of 5 to 65% (w/w) of the total weight of the composition, and in heating and kneading the reaction mixture, preferably in an extruder, so as to carry out a one-shot process.

Description

"ONE-SHOT PROCESS FOR THE PRODUCTION OF THERMOPLASTIC

POLYURETHANE MODIFIED WITH STYRENIC BLOCK COPOLYMERS"

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FIELD OF THE INVENTION

The present invention relates to a one-shot process for the production of a thermoplastic elastomer based on polyurethane and styrenic block copolymers, and to the elastomer thus obtained. More in detail, the present invention relates to a process and a product obtained by reacting together the compounds forming a polyurethane "system" and styrenic block copolymers under kneading and heating.

The material object of this invention is flexible, resistant to hydrolysis, with a Shore hardness from 55 A to 98 A and can be processed in machines that are commonly used to process thermoplastic rubbers based on styrene block copolymers, without substantially modifying them.

BACKGROUND OF THE INVENTION

Thermoplastic polyurethane (TPU) is a versatile elastomer that is used in footwear products, automotive, electronics, and for industrial machinery. For all these applications thermoplastic polyurethanes show a good performance regarding resistance to chemicals and hydrolysis, tear and abrasion resistance, low temperature flexibility and tensile strength.

TPU is a block copolymer that owes its elastic properties to the phase separation of so-called "hard blocks" and "soft blocks".

Hard blocks are rigid structures that are physically cross-linked (i.e. by electrostatic forces, without chemical bonds) and give the polymer its firmness; soft blocks are stretchable chains that give the polymer its elasticity.

Thermoplastic polyurethane can be produced in several ways.

The most common production process is by reactive processing, also known as reactive extrusion, i.e. a process in which the first component of the "system" (polyol containing poly-hydroxyl compounds, chain extenders and additives) and the second one (isocyanate) are fed to the extruder in a precise ratio by a metering system. In some cases the chain extender is fed through a third line separated from the other polyol components.

Into the extruder, reaction and transport take place simultaneously and the reacting materials are transported from the feed zone to the outlet die. The polymer formed is cooled in water and granulated.

For reactive processing, a closely intermeshing co-rotating twin-screw extruder is often the preferred choice. In the twin-screw extruder the second screw wipes the first one, which prevents slippage and guarantees forward conveying. Due to the self-wiping action, the transport of material is largely independent of the viscosity of the material: this is an advantage for a reactive system, since the viscosity rises exponentially along the screw.

Another process provides for feeding the two components into a flat and large container where they are reacted, curing the reacted product and subsequently chop it to pieces and extrude the pieces into granules.

By adapting the composition and the ratio of the hard and soft blocks, TPU properties can be modified to try to achieve the properties needed in the final application; however this method is not flexible enough to obtain all the desirable properties.

It has been proposed to modify some of the properties of a TPU by mixing it with other polymers such as PA, PVC, ABS, PC, SEBS, SBS, SIS or mixtures thereof.

This is done by blending mechanically or by compounding with an extruder (co-etrusion) the selected TPU and the selected polymers, in order to achieve a more intimate mixing and obtain improved properties.

DE 10100225 disclose a compounding process.

US 6291587 discloses the melt blending of rigid TPUs with rubber-like materials such as SBR, EPDM, NBR, EPR, SBS, SIS, SEBS, EVA and blends of plastics and rubbers in general.

The main disadvantage of this method is that the compounding process, since it is done a relatively high temperatures, has a detrimental effect on the physical properties of the modified TPU produced that way, since it will undergo a thermal degradation and a partial de-polymerisation; another disadvantage is that the production is long and costly. A further disadvantage is that a compatibilizing agent is required to prevent separation of the styrenic block copolymer from the polyurethane during the processing of the materials.

US 2005/0261427 relates to a TPU composition containing styrene-based block copolymers containing hydroxyl groups. The process to manufacture this composition provides for reacting the polyol and the isocyanate compounds in the feed zone of an extruder and in adding the functionalized styrene copolymer in the compression zone of the said extruder. In the compression zone, the reaction process of polyurethane formation has already progressed and the obtained polyurethane is hard enough to be compressed. According to this document, hydroxyl functional groups on the styrene block copolymer are required in order to react with the polyurethane product and favour mutual compatibilization of the TPU and block copolymers.

US 2004/0176525 carries out a process similar to the one previously disclosed with reference to US 2005/0261427, i.e. the styrene block copolymers are provided with an hydroxyl functional group.

In US 2004/0171751 , too, styrene block polymers functionalized with hydroxyl groups are added to an already formed TPU; additionally, a vinyl chloride polymer is added to the mixture of the previous two polymers.

EP 0611806 discloses a composition resulting from TPU and a styrene block copolymer that have been blended together at 200°C. The TPU is obtained from a poly(3-methyl-1 ,5-pentane adipate)diol, that is said to be critical to obtain the final result i.e. a compatibilization of the TPU and styrene copolymer.

Summarizing, known processes provide for a mechanical blending of already reacted polyol and isocyanate (i.e. solid TPU) with a styrene copolymer (solid); it was proposed to improve compatibilization of the block polymers by adding hydroxyl groups to the styrene polymer. The resulting process is costly and it does not solve the problems of polymer degradation during blending.

SUMMARY OF THE INVENTION

It is an aim of the present invention to solve the above mentioned problems and to provide an efficient reliable and easy method of producing flexible thermoplastic elastomers based on TPU and styrenic block copolymers as SBS, SEBS, SIS or mixtures thereof and that are suitable to be used in production of shoe soles or other items like wheels (e.g. for industrial carriages), profiles, household items or similar products.

The above aim is reached by means of the present invention, that provides a process to obtain this elastomer using a one-shot process by polymerizing components usually used for TPU production (a diisocyanate, a polyol containing relatively high molecular weight polyol, chain-extender, antioxidants and catalyst) in the presence of styrenic block copolymers used in quantities from 10 to 45 weight percent of the total mixture of isocyanate, high molecular weight polyol, additives and chain extender.

It is therefore an object of the invention a process according to claim 1. As above mentioned, in the invention process, the polymerization reaction of the starting compounds for producing the TPU is carried out in the presence of the styrenic block copolymers. In a preferred embodiment, the above mentioned starting compounds for the TPU forming reaction and the styrenic block copolymers are all fed together to the feeding zone (or feeding area) of an extruder so that also the styrenic polymers are mixed together with the liquid reaction components for the TPU substantially from the very first steps of the TPU polymerization reaction, i.e. the polymerization reaction is started and occurs in the presence of the styrenic block copolymers.

A further object of the invention is a composition according to claim 9.

Another object of the invention is an extruder according to claim 11.

Concerning the scope of protection of the products of the present invention, styrenic block copolymers carrying hydroxyl (-OH) groups are excluded; as a matter of fact, it is believed that the presence of hydroxyl groups on the styrenic (co)polymers could interfere with the TPU polymerization reaction. According to a preferred embodiment of the invention, the diisocyanate is 4,4'diisocyanatediphenylmethane or a mixture of 4,4'diisocyanatediphenylmethane and 2,4-diisocyanatodiphenylmethane wherein the amount of 2,4 is up to 5% by weight of the total amount of diisocyanate, i.e. the amount of the 2,4 isomer is within the range of 0 to 5 % in weight.

Polyols suitable for the present invention have an average molecular weight from 1000 to 4000 g/mol, preferably from 1750 to 3250 g/mol. Poyois that can be used are polyester polyols, most preferably polyester polyols based on adipic acid or on caprolactone derivatives or mixtures thereof. Poly(3- methyl-1 ,5-pentane adipate) diol is excluded from the scope of the products of the present invention.

Styrenic block copolymers (SBC) suitable for the present invention are SBS (styrene-butadiene-styrene), SEBS (styrene-ethylene-butadiene-styrene) and

SIS (styrene-isoprene-styrene) copolymers, linear or branched or grafted.

According to the preferred embodiment, the rubber (SBC) is selected from

SEBS and SBS copolymers and mixtures thereof, the total amount of rubber being within the range of 10 to 45% (w/w) of the total weight of isocyanates, polyols additives and chain extenders. Preferably, a grafted SEBS and/or

SBS copolymer is used, in this case, the polymer is grafted with maleic anhydride or with methyl-acrylate or a mixture thereof.

As previously mentioned, SBC rubbers carrying hydroxyl groups are excluded from the scope of the products of the present invention.

According to a further preferred embodiment, the styrenic copolymers contain fillers and/or are oil extended, i.e. they contain paraffinic or naphtenic oils.

More particularly, the amount of oil can reach 35-40% (w/w) and the amount of fillers (generally inorganic fillers) can reach 50-60% (w/w).

Chain extenders suitable for the invention can be selected from the following ones: 1 ,4 butanediol, 1 ,6 hexanediol, monoethylene glycol, diethylene glyocol, hydroquinone di-p-hydroxyethyl ether and mixtures thereof. The common additives and ancillary materials known to those skilled in the art of production of thermoplastic polyurethane via reactive extrusion can be used, including, for example, antioxidants, catalysts, fillers, pigments, anti- hydrolysis agents, lubricants, UV-protectors, plasticizers and flame retardants.

The process subject of the present invention provides several advantages over the prior art. First of all, it was surprisingly found that the process provides such a high degree of compatibility between the styrenic polymers and the TPU that it is possible to obtain transparent products. In the present invention, transparency, or total luminous transmission is measured according to ASTM D-1008, method B. The measures were taken with a BYK Gardner Spectrophotomer. The higher the transparency value, the more transparent the final product is. Transparency is also an indicator of the degree of compatibilization between the polyurethane portion and the rubber portion as obtained by the reaction.

The products of the invention have a transparency (measured as above disclosed) of at least 40, preferably of at least 70, more preferably of at least 80, and can reach very high values, e.g. 99 as disclosed in example 3

The modified TPU obtained with the current process thus has better mechanical properties than the modified TPU obtained with the conventional two-step process.

Secondly, the process is less energy intensive, since it is a single step process and it is possible to carry out at the same time the PU reaction (extrusion-reaction) and the blending/mixing with the rubber from the very beginning of the extruder, if an extruder is used.

A further advantage is that by changing the amount of rubber, without exiting the claimed range 10-65% (w/w), and/or by modifying the ratio of the claimed diisocyanates and polyester polyols, the product properties can be effectively controlled in a simple and reliable way, as disclosed by the following examples 1-4. Moreover, the process is carried out without using compatibilizing agents, i.e. the composition is free from compatibilizing agents.

Further advantages are that it is no longer required to dry the granules before using them in the moulding process that will result in the final moulded product and that the invention TPU product can be moulded by the machines actually used for rubber moulding.

The invention will now be disclosed in greater detail with reference to the enclosed non-limiting drawings and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

- Fig. 1 is a scheme of a twin screw extruder suitable for the invention process.

DISCLOSURE OF THE INVENTION

As mentioned, according to the claimed process, TPU that are flexible and have a Shore-A hardness (as measured by ISO 868) within the range of 55 to 98 shore A, are obtained by reacting 4,4'diisocyanatodiphenylmethane or a mixture of 4,4'diisocyanatodiphenylmethane and 2,4- diisocianatodiphenylmethane wherein the amount of the 2,4 isomer is within the range of 0 to 5% in weight of the total diisocyanate fraction, with polyols having an average molecular weight from 1000 to 4000 in the presence of chain extenders, additives, a styrenic block copolymer selected from SBS, SEBS, SIS and mixtures thereof. The amount of the rubber fraction is within the range of 5 to 65% (w/w) of the total weight of the final composition, preferably 10 to 55% (w/w), most preferably 20 to 45% (w/w) on the total weight of the composition.

In the invention process, the reaction of the starting compounds for producing the TPU is carried out in the presence of the styrenic block copolymers or, to put it into different words, a solid styrenic block copolymer is added to liquid polyols and isocyanates. In a preferred embodiment, the above mentioned starting compounds for the TPU forming reaction and the styrenic block copolymers are fed to the feeding zone (or feeding area) of an extruder so as to start and carry out at least part of the polymerization reaction in the feeding area together with, i.e. in the presence of, the styrenic block copolymers.

As is well known, the feeding area of an extruder used in a reaction extrusion process is the first area of the extruder, where the polymerization reaction occurs, while the compression zone is the area or part of the extruder where the formed TPU is solid enough to be compressed and plasticized.

Fig. 1 shows a scheme of a twin screw extruder 1 and the localization of the feeding zone 2, where the TPU polymerization reaction is carried out, and a compression zone 7; the starting material, i.e. isocyanates, polyols and additives and the SBC rubbers are fed to the same feeding means 6 provided at the extruder's feeding zone 2, from respective containers 3, 4 and 5.

It is to be noticed that a further advantage of the invention is that the SBC rubber is fed in an area of the extruder where feeding the rubber into the extruder is much easier than feeding the same rubber to an area of the extruder where the pressure is high, as is the case in the compression area. The reaction is preferably carried out in a double screw co-rotating extruder having a L/D from 25 to 60 of a type well known to those skilled in the art. The temperatures used in the barrels of such extruder are within the range from 170 to 220 °C.

The diisocyanate employed in the process is 4,4'diisocyanatodiphenylmethane 4,4'diisocyanatodiphenylmethane or a mixture of 4,4'diisocyanatodiphenylmethane and 2,4- diisocianatodiphenylmethane, the amount of 2,4 diisocianatodiphenylmethane being up to 5% by weight, preferably from 0 to 2,5% in weight. Most preferably, the stoichiometric ratio diisocyanate/polyol is greater than 1.0, i.e. there is fed to the extruder a greater amount (moles) of diisocyanate than of polyols. This ratio will provide a very good transparency of the final product. The preferred range is 1.0 to 1.2.

Suitable relatively high molecular weight polyols are those based on adipic acid/(1 ,4 butanediol) polyesters, or adipic acid/(1 ,6 hexanediol) polyesters, or adipic acid /(1 ,4-butanediol and 1 ,2-ethanediol) polyesters. In this last family of polyester the ratio (1 ,4-butanediol / 1 ,2-ethanediol) can vary from 1 ,5 to 0,5. Also suitable are relatively high molecular weight polyesters based on polycaprolactones. Preferably, the polyester polyol is at least 90% (w/w) of the polyol fraction of the TPU. Suitable polyesters can be used alone or in mixtures. Suitable polyesters contain terminal hydroxyl groups and a number average molecular weight from 1000 g/mol to 4000 g/mol, preferably from 1750 g/mol to 3250 g/mol. From the scope of the products of the present invention poly(3-methyl-1 ,5-pentane adipate) diol is excluded.

Chain extenders employed in the process can be 1 ,4 butanediol, 1 ,2 ethanediol, diethylene glycol, hexanediol and hydroquinone di-3-hydroxyethyl ether or mixtures thereof. Suitable antioxidants are those based on sterically hindered phenols, such as, for example octadecyl 3,5-di-tert-butyl-4- hydroxycinnamate or based on mixtures of sterically hindered phenols and alkyl phosphates, such as, for example, di-isodecyl phenyl phosphate. Antioxydants are used in quantities from 0,05 to 2,5 weight %.

Suitable catalysts are those based on metal-organic compounds, particularly organic tin derivatives, organic titanium derivatives, organic bismuth derivatives and organic zirconium derivatives, preferably stannous octoate, alkyl titanates, titanium acetilacetonate, bismuth tris neo-decanoate and zirconium chelates.

Additives routinely used in the production of thermoplastic polyurethane via reactive extrusion can be used also in the present invention. These additives include anti-hydrolysis agents (based on carbodiimide chemistry), anti-UV agents (based on the so-called HALS chemistry or on similar chemistries), inorganic fillers or pigments, organic pigments or reactive dyes, lubricants based on polyethylene waxes or montanic acid waxes, plasticizers based on phthalate esters or benzoate esters or adipate esters or citrate esters or mixtures thereof.

The so-called thermoplastic rubbers that can be used in the present invention are styrenic block copolymers like SBS, SEBS, SIS. Suitable polymers include linear, branched or grafted copolymers based on styrene and butadiene, or based on styrene -ethylene-butadiene, or based on styrene and isoprene, or based on a mixture thereof. Suitable polymers have a polystyrene content ranging from 10 to 30 % in weight, based on the total polymer weight. The use of grafted polymers (where grafting is obtained with a reaction of maleic anhydride or methyl acrylate or a mixture of them onto the rubber mid-block) improves the compatibility of these polymers with thermoplastic polyurethane. As previously mentioned, styrenic copolymers carrying an hydroxyl group are excluded from the scope of present invention and do not fall within the definition of SBS, SEBS, SIS (either grafted and non grafted) previously given.

The above mentioned thermoplastic rubbers are generally used in amounts within the range of 5 to 65% (w/w) of the total weight of the composition, preferably 10 to 55% (w/w), most preferably 20 to 45% (w/w) on the total weight of the composition.

EXAMPLES

In the next examples it will be shown that the physical and mechanical properties of the final elastomer can be modified by:

changing the quantity of the styrenic block copolymer (examples n. 1 and 2);

changing the ratio between the components of the polyurethane part (1.A and 1.B) fixing styrenic block copolymer quantity (examples n. 3 and 4). For the following examples the NCO:OH-ratio was in the range 0.900:1.200 and the rotational speed of the screws was set from 300 to 500 rpm.

A thermoplastic elastomer polyurethane-based was produced introducing in the feed zone of a twin-screw extruder the following starting materials:

- component 1.A (polyol, chain extender and additive);

- component 1.B (isocyanate);

- component n. 2 (linear tri-block copolymer based on styrene and ethylene/butylenes with a polystyrene content of 30% and with maleic anhydride (MA) grafted onto the rubber mid block);

in the quantities (parts by weight) indicated herein below.

Components 1.A and 1.B were introduced in the feed zone: the first one from a storage vessel kept at 100 °C, the second one from the storage vessel kept at 55 °C.

Component n. 2, too, was introduced in the first zone, i.e. the feed zone, of extruder by means of metering screws at about 25 °C in several quantities wt %. The temperature of the feed zone of the extruder is generally of about 195-200°C. The extruder is comprising a total of 11 sections as per the following tables.

After curing (thermal treatment at 90 °C), the elastomer polyurethane-based was processed in conventional injection-moulding machine and the specimens after post-curing for 12 h at 70 °C were tested to verify:

melt flow index (ATS FAAR instrument);

density (DIN 53479);

hardness (ISO 868);

modulus @ 100%, modulus @ 300%, tensile strength, breaking elongation (EN 12803), tear strenght (ISO 34-2) (Instron Dynamometer);

loss of abrasion (UNI EN 12770).

EXAMPLE N. 1 and N. 2

Components n. 1

63.770 parts of an adipic acid/butane diol polyester (MW 2000);

7.430 parts of 1 ,4 butane diol;

1.A 0.129 parts of octadecyl 3,5-Di-(tert)-butyl-4-hydroxyhydrocinnamate;

0.001 parts of titanium-tert-butyl-titanate;

1.B 28.670 parts of 4,4'-diisocyanatodiphenylmethane containing 0.5 wt % of 2,4'-isomer

In the example n. 1 80 parts of component 1 and 20 parts of component 2 are used, i.e. the rubber portion is 20% (w/w) of the composition, and the barrel temperatures were as follows:

Barrel 1 2 4 6 7 9 10 11 Head zone

TfC/ 195 200 200 195 195 195 195 195 200

In the example n. 2 we used 60 parts of component 1 and 40 parts of component 2, i.e. the rubber portion is 40% (w/w) of the composition, and the following barrel temperatures:

Barrel 1 3 5 6 7 9 10 11 Head zone

TTC] 195 200 195 185 180 180 180 175 200 After post-curing the specimens showed the tensile strength (correlated to the MFI) set out in next Tables:

Example n. 1

EXAMPLE N. 3

Component n. 1

55.570 parts of an adipic acid/butane diol polyester (MW 2000);

9.960 parts of 1 ,4 butane diol;

1.A 0.089 parts of octadecyl 3,5-Di-(tert)-butyl-4-hydroxyhydrocinnamate;

0.001 parts of titanium-tert-butyl-titanate;

1.B 34.380 parts of 4,4'-diisocyanatodiphenylmethane containing 0.5 wt % of 2,4'-isomer.

In the example n. 3 there were used 65 parts of component 1 and 35 parts of component 2 and the following barrel temperatures:

Barrel 1 2 4 6 7 9 10 11 Head zone

TfC/ 200 205 205 205 195 190 190 190 200

After post-curing the specimens showed the tensile strength (correlated to the MFI) set out in next Table:

Example n. 3

INJECTION MACHINE TEMPERATURE PROFILE: 205,210,205 [°C] EXAMPLE N. 4

Component n. 1

66.577 parts of an adipic acid/butane diol polyester (MW 2000);

5 6.658 parts of 1 ,4 butane diol;

1.A 0.133 parts of octadecyl 3,5-Di-(tert)-butyl-4-hydroxyhydrocinnamate;

0.001 parts of titanium-tert-butyl-titanate;

1.B 26.631 parts of 4,4'-diisocyanatodiphenylmethane containing 0.5 wt % of 2,4'-isomer.

In the example n. 4 there were used 65 parts of component 1 and 35 10 parts of component 2 and the following barrel temperatures:

Barrel 1 2 4 6 7 9 10 11 Head zone

T[°C] 195 200 200 195 180 180 180 180 200

15 Example n. 4

INJECTION MACHINE TEMPERATURE PROFILE: 190,195,190 [°C]

In the tables the values reflect the results obtained by modifying the ratio (w/w) of components 1a and 1 b from a ratio that is below the stechiometric ratio and a ratio that is above the stoichiometric. This shows the advantage of carrying out the invention process: it is possible to modify in real time the ratio of 1a and 1 b of component 1 so as to reach the maximum degree of compatibilization in the final product, that is measurable by means of the transparency of the final product.

Claims

1. A process for preparing flexible modified thermoplastic polyurethanes having a Shore A hardness within the range of 55 to 98 Shore A (ISO 868) characterised in that it provides for mixing and reacting together:

a) 4,4'diisocyanatodiphenylmethane or a mixture of 4,4'diisocyanatodiphenylmethane and 2,4-diisocianatodiphenylmethane where the amount of 2,4 isomer is within the range of 0 to 5% by weight of the total amount of isocyanates,

b) polyester polyols having an average molecular weight from 1000 to 4000, c) chain extenders,

d) additives,

e) a styrenic block copolymer selected from SBS, SEBS, SIS or mixtures thereof, the amount of such copolymer being within the range of 5 to 65% (w/w) of the total weight of the composition,

and in heating and kneading the reaction mixture.

2. A process according to claim 1 , wherein all said compounds a) to e) are fed to the feeding zone of an extruder.

3. A process according to claim 1 , wherein said polyester polyols are selected from those based on adipic acid/1 ,4 butanediol polyester, adipic acid/(1 ,4 butanediol and 1 ,2 ethanediol) polyester, adipic acid (1 ,6 butanediol) polyester and polycaprolactone polyester or mixtures thereof.

4. A process according to any claim 1 to 3, wherein said styrenic block copolymers are grafted with maleic anhydride or with methyl-acrylate or a mixture thereof.

5. A process according to any claim 1 to 4, wherein said styrenic block copolymers are free from hydroxyl groups.

6. A process according to any claim 1 to 5, wherein said chain extenders are selected from 1 ,4-butane diol, 1 ,6-hexanediol, monoethylene glycol, diethylene glycol and hydroquinone di- -hydroxyethyl ether or mixtures thereof.

7. A process according to any previous claim, wherein the stoichiometric ratio diisocyanate/polyol is greater than 1.0.

8. A process according to any previous claim, wherein the polyester polyol is at least 90% (w/w) of the polyol fraction of the TPU.

9. A polymer composition having a Shore A hardness within the range of 55 to 98 Shore A (ISO 868), said composition containing TPU and styrenic copolymer blocks selected from SBS, SEBS and SIS, as obtainable through a process according to any claim 1 to 7, wherein said TPU is obtained from polyols having an average molecular weight from 1000 to 4000 that are selected from those based on adipic acid/(1 ,4 butanediol) polyesters, adipic acid/(1 ,6 hexanediol) polyesters, adipic acid /(1 ,4-butanediol and 1 ,2- ethanediol) polyesters, wherein the ratio (1 ,4-butanediol / 1 ,2-ethanediol) is in the range from 1.5 to 0.5, polycaprolactone polyester or mixtures thereof.

10. A composition according to claim 9, characterized in that it has transparency of at least 40 as measured by ASTM D-1008.

11. A composition according to claim 9 or 10, wherein said styrenic block copolymers are free from hydroxyl groups.

12. An extruder for TPU reactive extrusion, provided with means for feeding isocyanates and polyols to the feed zone of said extruder, characterized in that it further comprises means to feed a styrenic block copolymer selected from SBS, SEBS, SIS or mixtures thereof, to the said feed zone of the said extruder.