The present invention refers to a thermoplastic polyurethane obtainable from a composition comprising at least one linear crystalline polyester polyol and at least one polybutadiene polyol as well as to a solvent-free adhesive composition comprising the inventive thermoplastic polyurethane.

The amount of waste produced worldwide and the strain placed on the environment have become one of the main topics in today's society. In the course of the ongoing discussion on sustainability, linked to the catchword of "carbon footprint", the aim is to reduce the amount of waste material and to develop and improve eco-friendly methods of production.

A major part of the accumulating waste includes food packaging. Apart from the difficulties encountered by recycling the materials used in food packaging, the processes of production are very costly in terms of energy and resources. Although efforts are being made to replace petro-based materials by materials obtained from renewable sources, a lot of potential for improvement also lies within the optimization of the production processes.

Solvent-free adhesives on basis of polyurethane are widely used in the production of flexible packaging such as bags of chips and the like. However, solvent-free polyurethane adhesives have so far been found to be lacking in other fields, such as sealable packaging, due to poor adhesion. Although it is known that certain additives can improve the adhesive properties of polyurethane adhesives, most of them were found to be incompatible with solvent-free applications. Therefore, there still exists the need for solvent-free adhesive composition suitable as heat sealing lacquers.

EP 2 944 660 relates to a thermoplastic polyurethane wherein the thermoplastic polyurethane is obtainable by a method comprising the steps of a) reacting a mixture comprising at least one polyester polyol and at least one polyisocyanate and b) reacting the reaction product of step a) with at least one substituted or unsubstituted diol which contains at least a primary and a secondary OH group.

WO 2016/026807 discloses polyurethanes obtainable by reaction of a) polyisocyanates A selected from optionally modified methanediphenyl 4,4'-diisocyanate, homologues of methanediphenyl 4,4'- diisocyanate containing a larger number of rings, prepolymers based on methanediphenyl 4,4'- diisocyanate and containing isocyanate groups, and mixtures thereof, b) compounds having at least two hydrogen atoms reactive towards isocyanate groups, containing b1 ) block copolymers of a polybutadienol and a cyclic ester as component B, and b2) diols as low-molecular-weight chain extenders and optionally triols as crosslinking agents with molar mass from 62 to 500 g/mol as component B2, c) optionally other polymeric compounds C having at least two hydrogen atoms reactive towards isocyanates, d) optionally catalysts D, e) optionally water E, f) optionally physical blowing agents F, g) optionally other auxiliaries and/or additives G. WO 2016/124499 polyurethane laminating adhesives, methods for producing a multilayer laminate by laminating at least two films with a polyurethane laminating adhesive, and multilayer laminates obtainable by these methods wherein the polyurethane laminating adhesive comprises an NCO- terminated polyurethane prepolymer obtainable by reacting a polyol mixture comprising: 0.1 to 20.0 wt% relative to the total weight of the polyol mixture of at least one polybutadiene polyol; and 5.0 to 99.9 wt% relative to the total weight of the polyol mixture of at least one polyether polyol, wherein the at least one polyether polyol comprises at least one polyether polyol with a number average molecular weight M _n in the range of >1000 g/mol to 10000 g/mol; with at least one polyisocyanate, wherein the at least one polyisocyanate is used in an amount such that the isocyanate groups are present in molar excess relative to the hydroxyl groups of the polyol mixture.

In light of the efforts described in the prior art, the object of the present lies in the provision of a solvent-free adhesive composition which can be used as heat sealing lacquer and which shows good adhesion on non-treated substrates.

It was surprisingly found that the above object is solved by employing a thermoplastic polyurethane which is obtainable from a composition comprising at least one linear crystalline polyester polyol and at least one polybutadiene polyol in heat sealing applications.

A first object of the present invention is therefore a thermoplastic polyurethane obtainable from a composition (A) comprising a) 1 to 40 wt.% of at least one linear crystalline polyester polyol having an average hydroxyl value of 10 to 225 mg KOH/g; b) 1 to 60 wt.% of at least one polybutadiene polyol having an average hydroxyl value of 20 to 225 mg KOH/g; and c) 1 to 50 wt.% of at least one NCO-terminated compound, in which the weight percentages are based on the total weight of composition (A).

In the context of the present invention, the hydroxyl value is defined as the number of milligrams of potassium hydroxide required to neutralize the acetic acid taken up on acetylation of one gram of the respective sample. The hydroxyl value can be calculated using the following equation. Note that a chemical substance may also have a measurable acid value affecting the measured end point of the titration. The acid value (AV) of the substance, determined in a separate experiment, enters into this equation as a correction factor in the calculation of the hydroxyl value (HV):

HV = [[(56.1)(N)(V _B -Vacet)]AA/acet] + A V wherein HV is the hydroxyl value; VB is the amount (ml) potassium hydroxide solution required for the titration of the blank; Vacet is the amount (ml) of potassium hydroxide solution required for the titration of the acetylated sample; Wacet is the weight of sample (in grams) used for acetylation; N is the normality of the titrant; 56.1 is the molecular weight of potassium hydroxide; AV is a separately determined acid value of the chemical substance. It was surprisingly found that adhesive compositions comprising the inventive polyurethane can be employed in a solvent-free state and show good adhesion to a variety of materials, in particular to materials with a low surface energy, especially non-treated surfaces. Non-treated surface as referred to in the present invention refers to surfaces which did not undergo any special treatment to enhance the adhesion on one of the surfaces or between the surfaces.

Surprisingly, the addition of a polybutadiene polyol having an average hydroxyl value of 20 to 225 mg KOH/g, led to a significant improvement of the adhesive properties, making the inventive thermoplastic polyurethane suitable for applications in solvent-free adhesives, in particular heatsealing lacquers.

The linear crystalline polyester polyol is a solid at 25°C and possesses a glass transition temperature (Tg) well below 0°C. The linear crystalline polyester polyol can comprise the reaction product of a linear aliphatic diol having from 2 to 10 methylene groups and a linear aliphatic diacid having from 2 to 20 methylene groups. Diols useful in forming the crystalline polyester polyol include ethylene glycol, 1 ,4butanediol, 1 ,5-pentanediol, 1 ,6-hexanediol, 1 ,8-octanediol, and 1 , 10-decanediol. Aliphatic diacids useful in preparing the crystalline polyester polyol include succinic acid, glutaric acid, adipic acid, sebacic acid, 1 ,12-dodecanedioic acid. Examples of suitable linear crystalline polyester polyols which can be utilized in the practice of this invention include poly(hexanediol adipate) polyol, poly(butanediol adipate) polyol, poly e-caprolactone polyol, polyhexanediol dodecanedioic acid) polyol, and the like. Such linear crystalline polyester polyols are commercially available under the trade names DYNACOLL series available from Evonik.

In a preferred embodiment, the at least one linear crystalline polyester polyol present in the composition (A) from which the inventive polyurethane is obtained has an average hydroxyl value of 15 to 125 mg KOH/g, in particular 5 to 65 mg KOH/g. Preferably, the composition (A) comprises the at least one linear crystalline polyester polyol in an amount of 1 to 40 wt.-%, preferably 5 to 35 wt.- %, based in the total weight of composition (A).

In a further preferred embodiment, the at least one polybutadiene polyol comprised in the reaction mixture has an average hydroxyl value of 25 to 150 mg KOH/g, in particular 35 to 1 15 mg KOH/g. The at least one polybutadiene polyol may be saturated or unsaturated. Especially good results with regard to stability of the inventive thermoplastic polyurethane were obtained when a hydrogenated polyol was employed. In a preferred embodiment, the at least one polybutadiene polyol is a hydrogenated polybutadiene polyol. Such polybutadiene polyols are commercially available under the trade name Krasol LBH and HLBH series available from Cray Valley. It was surprisingly found that especially the addition of the at least one polybutadiene polyol allows the use of the inventive polyurethane in solvent-free adhesive compositions. In a preferred embodiment, composition (A) therefore comprises the at least one polybutadiene polyol in an amount of up to 75 wt.-%, based on the total weight of composition (A). Preferably, composition (A) comprises the at least one polybutadiene polyol in an amount of 1 to 60 wt.-%, preferably 2 to 30 wt.-%, based on the total weight of composition (A).

The composition (A) from which the inventive polyurethane is obtainable further comprises at least one NCO-terminated compound. The at least one NCO-terminated compound is preferably present in composition (A) in an amount of 1 to 50 wt.-%, preferably 10 to 30 wt.-%, based on the total weight of composition (A). The at least one NCO-terminated compound is preferably selected from the group consisting of 1 ,5-naphthylene diisocyanate (NDI), 2,4'- and 4,4'-diphenylmethylene diisocyanate (MDI), isomers of toluylene diisocyanate (TDI), triphenylmethylene triisocyanate (MIT), hydrated triphenylmethylene triisocyanate (H12MDI), tetramethylenexylylene diisocyanate (TMXDI), isophoron diisocyanate (IPDI), xylylene diisocyante (XDI), hexane-1 , 6-diisocyanate (HDI), pentamethylene diisocyanate (PDI) and dicyclohexylmethane diisocyanate as well as mixtures thereof.

Free isocyanate groups in the polyurethane molecule may cause potential health risk, especially in applications in adhesive composition which are employed in food packaging. The amount of free isocyanate groups (NCO groups) in the inventive polyurethane is therefore preferably less than 0.1 mol-%, especially less than 0.05 mol-%, in particular less than 0.02 mol-%, based on the polyurethane molecule.

In a preferred embodiment, composition (A) from which the inventive polyurethane is obtainable may comprise one or more polyether polyols. The polyether polyol is preferably present in composition (A) in an amount of 0.5 to 20 wt.-%, preferably 1 to 10 wt.-%, based on the total weight of composition (A). Polyether polyols which are to be used as the polyol are preferably obtained by reaction of low molecular weight polyols with alkylene oxides. The alkylene oxides preferably contain 2 to about 4 carbon atoms. Suitable polyether polyols are, for example, the reaction products of water, ethylene glycol, propylene glycol, the isomeric butanediols or hexanediols, as mentioned above, or mixtures of two or more thereof with ethylene oxide, propylene oxide or butylene oxide or mixtures of two or more thereof. Other suitable polyether polyols are products of the reaction of polyhydric alcohols, such as glycerol, trimethylol ethane or trimethylol propane, pentaerythritol or sugar alcohols or mixtures of two or more thereof, with the alkylene oxides mentioned to form polyether polyols. Alkoxylation of amines such as ammonia, methyl amine, ethylenediamine, tetra- or hexamethylenediamine, triethanolamine, aniline, phenylenediamine, 2,4- and 2,6-diaminotoluene and polyphenyl polymethylene polyamines may also be practiced to form suitable polyether polyols. Suitable polyether polyols may also be formed by the ring-opening polymerization of cyclic ethers such as tetrahydrofuran. Polyether polyols with a molecular weight (Mn) of about 100 to about 3,000 g/mol and preferably in the range from about 200 to about 2,000 g/mol obtainable from the reactions mentioned are particularly suitable. Amorphous or crystalline polyether polyols may be used in the composition (A) of the present invention. In a more preferred embodiment, the mixture of at least one amorphous polyether polyol and at least one crystalline polyester polyol is used in the composition (A) of the present invention.

In a preferred embodiment, composition (A) from which the inventive polyurethane is obtainable may comprise at least one amorphous polyester polyol. The amorphous polyester polyols which can be utilized in the practice of this invention are liquid at 25°C and have a Tg above 0° C. Such amorphous polyester polyols are the reaction product of diol and diacid. Diols useful in forming the amorphous polyester polyol include hexanediol, butanediol, neopentyl glycol, ethylene glycol, diethylene glycol, propylene glycol, 2-methylpropanediol, and the like. Diacids useful in forming the amorphous polyester polyol include adipic acid, isophthalic acid, terephthalic acid, and the like. Such amorphous polyester polyols are commercially available under the trade name of Loctite Liofol LA series from Henkel. More preferably, the amorphous polyester polyol is an amorphous linear polyester polyol prepared by one or more linear diols and one or more linear diacids.

If present, composition (A) comprises the at least one amorphous polyester polyol in an amount of up to 60 wt.-%, based on the total weight of composition (A). Preferably, composition (A) comprises the at least one amorphous polyester polyol in an amount of 1 to 60 wt.-%, preferably 5 to 55 wt.-%, and more preferably 30 to 55 wt.-%, based on the total weight of composition (A).

The composition (A) from which the inventive polyurethane is obtainable may also comprise a chain extender. The chain extender may be used to adapt the mechanical and thermal properties of the inventive polyurethane. In a preferred embodiment, the chain extender is a diol compound, preferably selected from the group consisting of 1 ,2-propanediol, 1 ,2-butanediol, 1 ,3-butanediol, 1 ,2- pentanediol, 1 ,3-pentanediol, 1 ,4-pentanediol, 2-methyl-1 , 3-propanediol, 2,2,4-trimethyl-1 ,3- pentanediol, 1 ,2-hexanediol, 1 ,3-hexanediol, 1 ,4-hexanediol, 1 ,5-hexanediol, 2-ethyl-1 ,3- hexanediol, 1 ,2-heptanediol, 1 ,3-heptanediol, 1 ,4-heptanediol, 1 ,5-heptanediol, 1 ,6-heptanediol, 1 ,2-octanediol, 1 ,3-octanediol, 1 ,4-octanediol, 1 ,5-octanediol, 1 ,6-octanediol, 1 ,7-octanediol, and combinations thereof.

The inventive thermoplastic polyurethane is preferably obtainable by a method comprising the following steps:

i) providing a mixture comprising the at least one linear crystalline polyester polyol and the at least one polybutadiene polyol;

ii) optionally adding at least one amorphous polyester polyol, at least one polyether polyol, and at least one chain extender to the mixture of step i); and

iii) adding the at least one NCO-terminated compound to the mixture of step i) or ii). The thermoplastic polyurethane according to the invention is especially suited for the employment in solvent-free adhesive composition. A further object of the present invention is therefore a solvent- free adhesive composition comprising the thermoplastic polyurethane according to the invention. It was surprisingly found that employment of the inventive thermoplastic polyurethane makes it possible to provide a solvent-free adhesive composition which shows good adhesion properties even when used on non-treated substrates with low surface energy. Apart from saving valuable resources, forgoing of any solvents in the use and application of adhesive composition also allows for a more time and money efficient production process as time and energy consuming evaporation and drying steps can be omitted.

Since the inventive solvent-free adhesive composition is to be employed in the production of food packaging, any potential health risks should be avoided. Therefore, the amount of free NCO-groups in the inventive solvent-free adhesive composition is preferably less than 1 wt.-%, especially 0.005 to 0.05 wt.-%, based on the total weight of the adhesive composition.

The inventive solvent-free adhesive composition may be employed in the form of a 1 K-system or a 2K-system. Therefore, in a preferred embodiment, the inventive solvent-free adhesive composition is a 1 K-system or a 2K-system.

The inventive solvent-free adhesive composition is especially useful in sealing applications, in particular in the field of packaging applications. A further object of the present invention is therefore a method for the production of an article, the method comprising the steps of applying the inventive solvent-free adhesive composition to the surface of a first substrate and connecting the surface of the first substrate to the surface of a second substrate. It was surprisingly found that the inventive adhesive composition can also be applied to non-treated surfaces without deterioration of the adhesion between the two surfaces. Therefore, in a preferred embodiment of the inventive method, at least one of the two surfaces is a non-treated surface, in particular both surfaces.

Another object of the present invention is an object obtainable by the method according to the invention.

A further object is the use of the thermoplastic polyurethane in adhesive compositions, in particular solvent-free adhesive compositions.

Another object of the present invention is the use of a solvent-free composition according to the invention as heat-sealing lacquer, especially in food packaging, pharma and lidding applications.

The present invention will be explained in more detail with references to the following examples which are not to be understood as limiting the spirit of the invention.

Examples:

The materials used in the examples are as follows: NCO-terminated compound: 4,4'-diphenylmethylene diisocyanate (MDI) from Covestro under the trade name of Desmodur 44 M flakes;

Chain extender: 1 ,2-Propane diol from Dow;

Amorphous polyester polyol: an amorphous linear polyester polyol obtained from Henkel under the trade name of Loctite Liofol LA6707.

Polyether polyol a liquid polypropylene glycol having an average molecular weight of 400 g/mol from Dow under the trade name of Voranol P 400.

Linear crystalline polyester polyol: a linear crystalline polyester polyol obtained from Evonik under the name of Dynacoll having an average molecular weight M _n of 3700 g/mol and a hydroxyl value of 30 mg KOH/g.

Polybutadiene polyol: hydrogenated polybutadiene polyol having an average molecular weight M _n of 2100 g/mol and a hydroxyl value of 47 mg KOH/g, obtained from Total Cray Valley under the name of Krasol HLBHP-2000.

Thermoplastic polyurethanes according to the invention were prepared according to Table 1 as follows, the average molecular weight Mn and the hydroxyl value being determined according to the respective methods described above.

The components except MDI were heated under nitrogen atmosphere. As soon as the polymers were completely melted, MDI as NCO-terminated compound was added. The reaction was terminated when the NCO-content was less than 0.05%, determined according to the method of Spiegelberger (EN ISO 1 1909).

Different compositions having varying contents of the polybutadiene polyol were prepared and the adhesion properties evaluated by applying the obtained thermoplastic polyurethane in an amount of 5 g/m ² to an aluminum substrate. The adhesion properties of the thermoplastic polyurethane were determined 24 hours after sealing against an untreated polypropylene substrate. The compositions as well as the results of the adhesion tests are summarized in Table 1 :

Table 1 : Formulations of thermoplastic polyurethanes in the examples (by weight%) and test results

‘Sealing conditions: Top bar (aluminum film) 180 °C / lower bar 25 °C (PP film)/1s/650N, with sealing apparatus FA. Brugger

As can be seen from the results summarized in Table 1 , the inventive thermoplastic polyurethane showed improved adhesion properties when employed as heat-sealing lacquer in comparison to common polyurethanes which do not comprise a polybutadiene polyol having an average hydroxyl value within the claimed range.