[0002] The present invention relates to polyester polyols having a low viscosity, a method for the preparation thereof, and the use thereof in an adhesive composition. The present invention also relates to an adhesive composition comprising the described polyester polyol, a polyurethane adhesive which can be obtained by reacting the polyester polyol with an isocyanate component, and a method for bonding using the polyurethane adhesive.

[0003] Polyester polyols are primarily linear polymers which are distinguished by their low production costs and wide range of possible uses. Polyester polyols are used, for example, to formulate reactive adhesive systems which can be widely used in the preparation of a large number of products, the product spectrum ranging from simple everyday objects such as food packaging to complex systems.

[0004] One class of polyester polyols which enjoys a particular economic success is polyol products which can be obtained by esterification of phthalic acid or phthalic acid anhydride with a polyhydric alcohol. Polyol products of this kind are generally present as viscous liquids and can be reacted with isocyanate compounds, for example, to form coatings, adhesives, foams, sealing agents and elastomers which have excellent properties such as tensile strength, adhesion and resistance to abrasion.

[0005] One problem that results from the use of polyester polyols of this kind is their high viscosity, which generally requires solvents or other measures which modify the viscosity to be used to achieve suitable handling, in particular when pumping and mixing compounds of this kind. The use of large amounts of solvents should be avoided for ecological and economic reasons. Other measures for reducing the viscosity, such as heating the compounds, are usually associated with a large technical and energy outlay, and can only be integrated into an industrial process with great difficulty.

[0006] There is therefore a need for polyester polyols which have a low viscosity and are easy to handle.

[0007] WO 99/42508 describes an aromatic polyester polyol of low viscosity that comprises an inter-esterification product consisting of 20 to 80 mol.% of a material based on phthalic acid with diethylene glycol, a higher functional polyol that has an average functionality of more than two and a long-chain alkyl acid, a long-chain ester or oil.

[0008] WO 2001/30878 discloses polyester ether polyols as reaction products from the reaction of phthalic acid with diethylene glycol and propylene oxide.

[0009] The proposed solutions described in the prior art are however disadvantageous in that they are very complicated to an extent and use starting materials which need to be handled using particular precautionary measures as a result of their potential to pose a risk to health or because they are easily flammable.

[0010] The problem addressed by the present invention is therefore that of providing a polyester polyol which has a low viscosity and can be industrially processed and used in adhesive compositions.

[001 1] It has surprisingly been found that the problem is solved by a polyester polyol which can be obtained by means of a simple transesterification reaction of an ester, formed from a material based on phthalic acid and at least one polyhydric alcohol, with a polyol, as is described in greater detail in the following.

[0012] The present invention first relates to a low-viscosity polyester polyol which is formed by transesterification of an ester (A) with a polyol (B), the ester (A) being formed by reacting at least one material based on phthalic acid with at least one polyhydric alcohol.

[0013] Within the meaning of the present invention, polyester polyol describes a polyol that contains ester bonds.

[0014] It has been shown, in particular for the use of polyester polyols in adhesive compositions, that the polyester polyol should have a viscosity which allows it to be mixed well with the other components of the adhesive composition and applied evenly to the substrate to be bonded, and which demonstrates good adherence to the substrate. It has surprisingly been shown that a good balance of these requirements can be achieved if the viscosity of the polyester polyol is no greater than 30,000 mPas at 25 °C. Therefore, in one embodiment of the present invention, it is preferable for the polyester polyol according to the invention to have a viscosity of 1000 to 30,000 mPas, preferably 1000 to 15,000 mPas, in each case measured at 25 °C.

[0015] Ester (A)

[0016] The polyester polyol according to the invention is obtained by transesterification of an ester (A) that consists of a material based on phthalic acid and at least one polyhydric alcohol with a polyol (B).

[0017] The at least one polyhydric alcohol is preferably selected from the group consisting of ethylene glycol, diethylene glycol, propylene glycol, glycerol, diglycerol and low-molecular-weight derivatives of glycerol. The low-molecular-weight derivatives of glycerol preferably have a molar mass of less than 600 g/Mol, more preferably less than 400 g/Mol. In a particularly preferred embodiment, the at least one polyhydric alcohol is selected from the group consisting of diethylene glycol, propylene glycol and glycerol.

[0018] In the context of the present invention, the material based on phthalic acid is understood to mean a material that has at least one phthalate functional group as a structural unit.

[0019] The material based on phthalic acid is in this case preferably selected from the group consisting of phthalic acid anhydride, phthalic acid, isophthalic acid, terephthalic acid, methyl esters of phthalic acid, isophthalic acid or terephthalic acid, dimethyl terephthalate, polyethylene terephthalate, trimellitic acid anhydride, pyromellitic acid anhydride and mixtures thereof.

[0020] In a particularly preferred embodiment of the present invention, the material based on phthalic acid is phthalic acid or phthalic acid anhydride. In one particularly preferred embodiment, the material based on phthalic acid is orf/70-phthalic acid. It has surprisingly been shown that particularly advantageous results can be achieved by using these compounds, in particular with respect to the viscosity of the product and the yield of the esterification.

[0021] The ester (A) consisting of the material based on phthalic acid and the at least one polyhydric alcohol is preferably prepared under conventional reaction conditions known to a person skilled in the art. In a preferred embodiment, the preparation takes place by reacting the material based on phthalic acid with at least one polyhydric alcohol in the presence of a catalyst. Tetraisopropyl titanate (T/PT) is a suitable catalyst, for example.

[0022] Polyol (B)

[0023] The polyester polyol according to the invention is obtained by transesterification of an ester (A) with a polyol (B). There should not be any particular requirements placed on the polyol (B) used. Instead, a number of polyols can be used which allow the properties of the polyester polyol according to the invention to be adapted individually to the respective requirements.

[0024] The polyol (B) is preferably selected from the group consisting of aliphatic polyols, polyester polyols and polyether polyols, in particular polycarbonate polyols, polycaprolactone polyols, polybutadiene polyols, polysulfide polyols and mixtures thereof. [0025] Suitable polyether polyols include linear and/or branched polyethers which have a plurality of ether bonds and at least two hydroxyl groups and preferably do not contain any further functional groups in addition to the hydroxyl groups. Examples of suitable polyether polyols of this kind are polyoxyalkylene polyols such as polyethylene glycol, polypropylene glycol, polytetramethyl glycol and polybutyl glycol. Moreover, homopolymers and copolymers of the mentioned polyoxyalkylene polyols and mixtures thereof can be used. Particularly suitable copolymers of polyoxyalkylene polyols are those which comprise an adduct of at least one compound from the group consisting of ethylene glycol, propylene glycol, diethylene glycol, propylene glycol, triethylene glycol, 2-ethylhexanediol-1 ,3-glycerol, 1 ,2,6-hexanetriol, trimethylolpropane, trimethylolethane, tris(hydroxyphenyl)propane, triethanolamine and triisopropylamine together with at least one compound selected from the group consisting of ethylene oxide, propylene oxide and butylene oxide.

[0026] In a particularly preferred embodiment, the polyol (B) is selected from the group consisting of polypropylene glycol, polytetramethyl glycol and statistical copolymers and/or block copolymers of ethylene oxide and propylene oxide.

[0027] In an alternative preferred embodiment, the polyol (B) is a polyester. Suitable polyester polyols can be formed, for example, by condensing one or more polyhydric alcohols having 2 to 15 carbon atoms with one or more polycarboxylic acids having 2 to 14 carbon atoms. Examples of suitable polyhydric alcohols include ethylene glycol, glycerol, propylene glycols such as 1 ,2- propylene glycol and 1 ,3-propylene glycol, pentaerythriol, trimethylolpropane, 1 ,4,6-octanetriol, butanediol, pentanediol, hexanediol, dodecanediol, octanediol, chloropentanediol, glycerol monoallylether, glycerol monoethylether, diethylene glycol, 2-ethylhexanediol, 1 ,4- cyclohexanediol, 1 ,2,6-hexanetriol, 1 ,3,5-hexanetriol and 1 ,3-bis-(2-hydroxyethoxy)propane.

[0028] Examples of suitable polycarboxylic acids are phthalic acid, isophthalic acid, terephthalic acid, tetrachloroterephthalic acid, maleic acid, dodecylmaleic acid, octadecenylmaleic acid, fumaric acid, aconitic acid, trimellitic acid, 3,3'-thiodipropanoic acid, succinic acid, adipic acid, malonic acid, glutaric acid, pimelic acid, sebacic acid, cyclohexane-1 ,2-dicarboxylic acid, 1 ,4- cyclohexadiene-1 ,2-dicarboxylic acid, 3-methyl-3,5-cyclohexadiene-1 ,2-dicarboxylic acid and the corresponding anhydrides, acid chlorides and acid esters such as phthalic acid anhydride, phthaloyl chloride and dimethyl ester of phthalic acid.

[0029] The polyester polyol according to the invention is characterized by particular properties which, alongside the physical and chemical properties, also take into account further aspects such as health risks, in particular also in conjunction with the starting components used. [0030] For some time, phthalic acid and compounds derived therefrom, in particular low- molecular-weight phthalates, have been suspected of posing a potential health risk and causing infertility, obesity and diabetes, for example. The use of compounds of this kind, in particular in the production of everyday objects, should therefore be viewed critically. However, as of yet there are no sustainable alternatives, in particular for use in reactive adhesive compositions, and therefore it is desirable for the presence of structural units of this kind to be kept as low as possible, without however negatively influencing the properties of the adhesive system. It has surprisingly been shown that the amounts of phthalic acid or related compounds used in the prior art can be considerably reduced without any decrease in the performance of the polyester polyol or the later adhesive composition being detected.

[0031] Therefore, an embodiment of the polyester polyol according to the invention in which the proportion of phthalic acid residues in the polyester polyol is no more than 30 mol.% is preferred. The polyester polyol according to the invention preferably comprises 5 to 30 mol.%, particularly preferably 7 to 15 mol.%, of phthalic acid residues. As a result of it being possible to keep the proportion of phthalic acid residues in the polyester polyol according to the invention in the specified ranges, contact with these substances that are potentially hazardous to health can be limited without the properties of the polyester polyol deteriorating.

[0032] Within the meaning of the present invention, phthalic acid residues refer to the general group

in which the aromatic ring can have up to four substituents of the structure -COO- and optionally further substituents.

[0033] In a preferred embodiment, the polyester polyol according to the invention is characterized in that the number of OH groups in the polyester polyol is 40 to 100 mg KOH/g, preferably 45 to 80 mg KOH/g, determined in accordance with DIN 53240-2. It has surprisingly been found that a number of OH groups in the polyester polyol within the identified ranges allows the polyester polyol to be particularly well processed in adhesive compositions.

[0034] The polyester polyol according to the invention preferably has an acid number (SZ) of less than 2 mg KOH/g, more preferably of less than 1 mg KOH/g, determined in accordance with DIN EN ISO 21 14. In this way, a sufficient purity of the polyester polyol is ensured which allows reliable application in adhesive compositions. [0035] As stated above, methods described in the prior art for preparing low-viscosity polyester polyols based on phthalic acid are disadvantageous in that easily flammable and explosive starting materials, such as reactive epoxides, are used in the preparation thereof. In order to overcome this disadvantage, the present invention also relates to a method for preparing low- viscosity polyester polyols based on phthalic acid, in which the use of reactive epoxides can be omitted.

[0036] According to the invention, the method for preparing the polyester polyol is characterized in that, in a first step, a material based on phthalic acid is esterified with at least one polyhydric alcohol to obtain an ester (A), and in a second step this ester (A) is subjected to transesterification with a polyol (B), as a result of which the polyester polyol is obtained. The polyester polyol is thus prepared by transesterification of the ester (A) consisting of the material based on phthalic acid and at least one polyhydric alcohol, which ester was prepared in the first step. During this reaction, the residues introduced in the first step of the method according to the invention are replaced by the polyol (B), and this comprises the splitting of at least one of the introduced residues.

[0037] The ester (A) can be prepared in the way described above and by using the material based on phthalic acid described above. The aforementioned compounds are preferably used as polyol (B).

[0038] The material based on phthalic acid and the at least one polyhydric alcohol are preferably reacted in a weight ratio of 3: 1 to 0.9: 1 , particularly preferably 2:1 to 1.1 : 1. The material based on phthalic acid is particularly preferably added in a slight excess.

[0039] The ester (A) obtained in the first step is reacted in a second step with the polyol (B), the weight ratio of polyol (B) to ester (A) preferably being 12: 1 to 2: 1 , particularly preferably 9:1 to 3:1 .

[0040] In a preferred embodiment, the method according to the invention does not include the use of alkoxylation agents, in particular those selected from the group consisting of ethylene oxide, propylene oxide, butylene oxide and mixtures thereof.

[0041] The ester (A) consisting of the material based on phthalic acid and at least one polyhydric alcohol is preferably transesterified under conventional reaction conditions known to a person skilled in the art. In a preferred embodiment, the transesterification takes place in the presence of a catalyst. Tetraisopropyl titanate (T/PT) is a suitable catalyst, for example.

[0042] The present invention also relates to a polyester polyol that can be obtained by means of the method according to the invention.

[0043] The polyester polyol according to the invention is particularly suitable for use in adhesive compositions. The present invention therefore also relates to an adhesive composition which comprises the low-viscosity polyester polyol according to the invention.

[0044] In a preferred embodiment, the adhesive composition also comprises an isocyanate component in addition to the polyester polyol according to the invention.

[0045] Within the meaning of the present invention, isocyanate component is understood to mean compounds which have at least one free isocyanate group (NCO group). The isocyanate component is preferably selected from the group consisting of polyisocyanates and NCO- containing prepolymers.

[0046] Within the meaning of the present invention, polyisocyanates are compounds which contain at least two active NCO groups, preferably diisocyanate or triisocyanate.

[0047] Suitable NCO-containing prepolymers are reaction products from compounds carrying OH groups and/or NH groups with an excess of polyisocyanates, there being no particular requirements placed on the polymers and polyisocyanates used. The polyisocyanates can be the aforementioned isocyanates, for example. Polyether amines can be used as compounds carrying NH groups, for example. The aforementioned polyols can be used as compounds carrying OH groups, it then being possible to further react the obtained prepolymer with another or the same polyol.

[0048] The isocyanate component is preferably selected from the group consisting of 1 ,5- naphthylene diisocyanate, 4,4'-diphenylmethane diisocyanate (MDI), 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate, hydrogenated or partially hydrogenated MDI (H 12MDI, H6MDI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate (TMXDI), di- and tetraalkylene diphenylmethyl diisocyanate, 4,4'-dibenzyl diisocyanate, 1 ,3-phenyl diisocyanate, 1 ,4-phenyl diisocyanate, the isomers of toluene diisocyanate (TDI), 1-methyl-2,4- diisocyanatocyclohexane, 1 ,6-diisocyanato-2,2,4-trimethylhexane, 1 ,6-diisocyanato-2,4,4- trimethylhexane, 1-isocyanatomethyl-3-isocyanato-1 ,5,5-trimethylcyclohexane (IPDI), chlorinated and brominated diisocyanates, phosphorous-containing diisocyanates, tetramethoxybutane-1 ,4- diisocyanate, naphthalene-1 ,4-diisocyanate (NDI), butane-1 ,4-diisocyanate, hexane-1 ,6- diisocyanate (HDI), dicyclohexylmethane diisocyanate, tetramethylene diisocyanate, hexamethylene diisocyanate, undecamethylene diisocyanate, dodecamethylene diisocyanate, 2,2,4-trimethyl-hexane-2,3,3-trimethyl-hexamethylene diisocyanate, cyclohexane-1 ,4- diisocyanate, ethylene diisocyanate, methylenetriphenyl triisocyanate (MIT), phthalic acid-bis- isocyanato-ethylester, diisocyanates having reactive halogen atoms such as 1- chloromethylphenyl-2,4-diisocyanate, 1-bromomethyl-phenyl-2,6-diisocyanate and 3,3-bis- chloromethylether-4,4'-diphenyl diisocyanate. Further diisocyanates which can be used are trimethyl-hexamethylene diisocyanate, 1 ,4-diisocyanatobutane, 1 ,12-diisocyanatododecane and dimer fatty acid diisocyanate, lysine diisocyanate, 4,4-dicyclohexamethane diisocyanate, 1 ,3- cyclohexane diisocyanate and 1 ,4-cyclohexane diisocyanate.

[0049] In a particularly preferred embodiment, the isocyanate component is selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4'diphenylmethane diisocyanate and 2,2'diphenylmethane diisocyanate and oligomers and polymers thereof.

[0050] In a further preferred embodiment, the content of isocyanate component in the adhesive composition is 5 to 30 wt.%, preferably 15 to 25 wt.%, in each case based on the total weight of the composition.

[0051] In a further preferred embodiment, the molar ratio of polyester polyol to isocyanate component in the adhesive composition is 5:1 to 1 : 1 , preferably 3:1 to 1.5: 1.

[0052] The adhesive composition according to the invention preferably additionally comprises a catalyst. All known compounds which can catalyze the reaction of isocyanates with alcohols can be used as the catalyst. The catalyst is preferably selected from the group consisting of titanates such as tetrabutyl titanate and tetrapropyl titanate, tin carboxylates such as dibutyl tin dilaurate (DBTL), dibutyl tin diacetate, tin octoate, tin oxides such as dibutyl tin oxide and dioctyl tin oxide, organoaluminum compounds such as aluminum trisacetylacetonate, aluminum trisethylacetonate, chelate compounds such as titantetraacetylacetonate, amine compounds such as triethylene diamine, guanidine, diphenyl guanidine, 2,4,6-tris(dimethylaminomethyl)phenol, morpholine, N- methylmorpholine, 2-ethyl-4-methylimidazole, 1 ,8-diazabicyclo-(5,4,0)-undecene-7 (DBU), 1 ,4- diazabicyclooctane (DABCO), 1 ,4-diazabicyclo[2,2,2]octane, N,N-dimethylpiperazine, 1 ,8- diazabicyclo[5,4,0]undec-7-ene, dimorpholinodimethylether, dimorpholinodiethylether (DMDEE) and mixtures thereof.

[0053] The amount of catalyst in this case is preferably 0.01 to 5 wt.%, particularly preferably 0.05 to 2 wt.%, based on the total weight of the composition.

[0054] The present invention also relates to a polyurethane adhesive which can be obtained by reacting the polyester polyol according to the invention with an isocyanate component. The isocyanate component is preferably one of the aforementioned compounds, in particular however 4,4'-diphenylmethane diisocyanate, 2,4'diphenylmethane diisocyanate, 2,2'diphenylmethane diisocyanate or mixtures thereof and/or oligomers. In a further preferred embodiment, the polyester polyol according to the invention is reacted with the isocyanate component in the presence of a catalyst, such as described above.

[0055] The reaction can in this case take place under conventional conditions which are well known to a person skilled in the art.

[0056] The polyurethane adhesive according to the invention preferably has a viscosity before curing of 1000 to 30,000 mPas, more preferably 1200 to 25,000 mPas, measured at 75 °C.

[0057] The polyurethane adhesive according to the invention is characterized by excellent properties, in particular high tensile strength and elongation at break after curing. In a preferred embodiment, the polyurethane adhesive according to the invention has, after curing, a tensile strength of from 1 to 60 N, preferably 2 to 45 N, determined in accordance with ISO 527-1 . After complete curing, the elongation at break is preferably 7.0 to 650%, particularly preferably 7.5 to 600%, determined in accordance with ISO 6922. The polyurethane adhesive is in this case preferably cured by means of moisture crosslinking.

[0058] The polyurethane adhesive according to the invention can be used in a number of manufacturing processes. The present invention therefore also relates to the use of a polyurethane adhesive according to the invention in the manufacture of component parts. The adhesive according to the invention is preferably used as a hot-melt adhesive. The use of the adhesive according to the invention for manufacturing smartphones and tablets is particularly preferred.

[0059] The present invention further relates to the use of the polyester polyol according to the invention in an adhesive composition, in particular in an adhesive composition as described within the scope of the present invention.

[0060] The polyester polyol according to the invention is particularly suitable for use in adhesive compositions. The present invention therefore also relates to a method for bonding comprising the following steps:

a) providing a mixture comprising the polyester polyol according to the invention and an isocyanate component;

b) applying the mixture from step a) to at least one substrate surface to be bonded, c) joining the substrate surface to be bonded within the open time, and

d) curing the applied mixture.

[0061] The mixture in step a) is preferably an adhesive composition according to the present invention. [0062] The present invention is intended to be described in greater detail with reference to the following examples, these not being understood as limiting the concept of the invention.

[0063] The acid number was determined in accordance with DIN EN ISO 21 14.

[0064] The number of OH groups was determined in accordance with DIN 53240-2.

[0065] The viscosity was determined by means of Brookfield (Thermosel), spindle 27 at 10 revolutions/minute.

[0066] The tensile strength was determined in accordance with ISO 527-1. [0067] The elongation at break was determined in accordance with ISO 6922. [0068] Raw materials used:

Diethylene glycol (BASF SE, Germany), Mw 106.12 g/mol

Propylene glycol (BASF SE, Germany), Mw 76.1 1 g/mol

Phthalic acid (Sigma Aldrich, Germany), Mw 166.13 g/mol

Capa 2067A (Croda, Germany), OHZ 168-177

Capa 2205 (Croda, Germany), OHZ 54-58 (ASTM D4274-94d)

Pluronic PE 3100 (BASF SE, Germany), Mw 1000 g/mol

P-THF 1000 (BASF SE, Germany), OHZ 107-1 18 (DIN 53240)

Desmodur 44 MC flakes (Covestro, Germany), NCO content (theoretic): 33.6 wt.%

Example 1 :

[0069] A 5I flask is loaded with 1300 g of phthalic acid and 1250 g of propylene glycol. The reaction mixture is heated under a nitrogen stream to 190 °C. When the temperature has been reached, 0.5 g of tetraisopropyl titanate (T/PT) is added. After a total of 8 hours, an acid number of 0.5 mg KOH/g was reached. The number of OH groups was determined using 426 mg KOH/g. The viscosity was 2500 mPas, measured at 25 °C.

[0070] 1358 g of ethylene oxide-propylene oxide block copolymer ("Pluronic PE 3100") was added to 215 g of this polyester. The reaction mixture was heated under a nitrogen stream to 220 °C. When the temperature had been reached, 0.5 g of T/PT was added. After a total of 8 hours of transesterification, the following characteristics were determined: Acid number: 0.8 mg KOH/g

Number of OH groups: 62 mg KOH/g

Viscosity: 2500 mPas at 25 °C.

[0071] The polyester according to the invention was reacted with 4,4'-MDI in a ratio of 2.2:1 at 130 °C in a vacuum for 1.5 hours to form an NCO-terminal polyurethane prepolymer. The viscosity was 1250 mPas, measured at 75 °C.

[0072] After a 100 μιη thick film of this adhesive had been completely moisture crosslinked, the following data was determined:

Tensile strength: 2.5 N

Elongation at break: 7.5%

Example 2:

[0073] 3000 g of PPG 1000 was added to 322 g of the polyester consisting of phthalic acid and propylene glycol. The reaction mixture was heated under a nitrogen stream to 230 °C. When the temperature had been reached, 0.5 g of T/ ^' PT was added. After a total of 8 hours of transesterification, the following characteristics were determined:

Acid number: 0.3 mg KOH/g

Number of OH groups: 72 mg KOH/g

Viscosity: 3000 mPas at 25 °C.

[0074] The polyester according to the invention was reacted with 4,4'-MDI in a ratio of 2.2:1 at 130 °C in a vacuum for 1.5 hours to form an NCO-terminal polyurethane prepolymer. The viscosity was 3500 mPas, measured at 75 °C.

[0075] After a 100 μιη thick film of this adhesive had been completely moisture crosslinked, the following data was determined:

Tensile strength: 6.9 N

Elongation at break: 250%

Example 3:

[0076] A 5I flask was loaded with 1992 g of phthalic acid (PA), 840 g of diethylene glycol (DEG) and 840 g of propylene glycol (PG). The reaction mixture was heated under a nitrogen stream to 190 °C. When the temperature had been reached, 0.5 g of (tetraisopropyl titanate) T/PT was added. After a total of 8 hours, an acid number of 0.7 mg KOH/g was reached. The number of OH groups was determined using 242 mg KOH/g.

[0077] 3000 g of P-THF 1000 was added to 322 g of this polyester consisting of PA, DEG and PG. The reaction mixture was heated under a nitrogen stream to 230 °C. When the temperature had been reached, 0.5 g of T/PT was added. After a total of 8 hours of transesterification, the following characteristics were determined:

Acid number: 0.2 mg KOH/g

Number of OH groups: 49 mg KOH/g

Viscosity: 1 1 ,500 mPas at 30 °C.

[0078] The polyester according to the invention was reacted with 4,4'-MDI in a ratio of 2.2:1 at 130 °C in a vacuum for 1.5 hours to form an NCO-terminal polyurethane prepolymer. The viscosity was 1 1 ,500 mPas, measured at 75 °C.

[0079] After a 100 μιη thick film of this adhesive had been completely moisture crosslinked, the following data was determined:

Tensile strength: 30 N

Elongation at break: 600%

Example 4:

[0080] 874 g of Capa 2067A and 950.1 g of Capa 2205 was added to 489.6 g of this polyester consisting of PA and PG from example 1. The reaction mixture was heated under a nitrogen stream to 230 °C. When the temperature had been reached, 0.5 g of T/PT was added. After a total of 8 hours of transesterification, the following characteristics were determined:

Acid number: 0.1 mg KOH/g

Number of OH groups: 55 mg KOH/g

Viscosity: 14,000 mPas at 25 °C.

[0081] The polyester according to the invention was reacted with 4,4'-MDI in a ratio of 2.2:1 at 130 °C in a vacuum for 1.5 hours to form an NCO-terminal polyurethane prepolymer. The viscosity was 21 ,500 mPas, measured at 75 °C.

[0082] After a 100 μιη thick film of this adhesive had been completely moisture crosslinked, the following data was determined:

Tensile strength: 40 N

Elongation at break: 550%

[0083] The results are shown once again in Table 1.

[0084] As the examples show, the polyester polyol according to the invention can be obtained by means of a simple reaction, which renders the use of easily flammable substances superfluous. The polyester polyol has a viscosity which allows the product to be handled easily. Furthermore, adhesives which are prepared from the polyester polyol according to the invention demonstrate good tensile strength and high elongation at break.