Description Adhesive aqueous compositions. Technical field [0001] This invention is related to polyurethane aqueous compositions suitable as thermo-activable adhesives and to the procedure for their preparation. The adhesive aqueous compositions of the invention have low activation temperature and high open time and are particularly useful as adhesives for leather, textiles, rubber, PVC, ABS and plastic materials in general. Background Art [0002] The known polyurethane aqueous compositions suitable as thermo- activable adhesives are mainly of the anionic kind, the polyurethane being anionically modified by means of the introduction of sulfonate groups and/or carboxylic groups and the heat activation characteristics being mainly imparted by polyester diol chains chemically incorporated in the polyurethane itself. [0003] GB 1336050 (BAYER AG) discloses aqueous dispersions of anionic polyurethanes based on N-omega-amino-alkane-omega-aminoalkane sulphonic acid salts, diisocyanates and polyester polyols. [0004] The polyurethane dispersions described in GB 1336050 (BAYER AG) are said to be also useful as glues and adhesives, but not as thermo- activable adhesives. [0005] In EP 3521 A (BAYER AG) aqueous solutions or dispersions of polyisocyanate-polyaddition products characterised by containing from 0.01 to 200 meq (milliequivalents) of sulfonate ions -SO3" per 100g and from 0.01 to 200 meq of carboxyl groups -COOH per 10Og, based on dissolved or dispersed polyisocyanate-polyaddition products are disclosed, together with their use as adhesives; nothing is said about their use as thermo-activable adhesives. [0006] US 4870129 (BAYER AG) relates to thermo-activable adhesives based on an aqueous solution or dispersion of an anionically modified polyurethane and to the use of such a solution or dispersion for the formation of bonds on any substrates. [0007] Although these thermo-activable adhesives possess good activation temperature, medium heat resistance and form homogeneous films, their open time is rather low; this fact causes problems particularly in manual or partially automated application, because the activated surfaces remain workable for a too short time. [0008] Moreover the hydrolytic resistance of the films obtained from these thermo-activable adhesives is poor, impairing the duration of bond strength especially in the presence of dampness. [0009] US 5608000 (H. B. FULLER LIC. FIN.) discloses aqueous dispersion adhesives of anionic polyurethanes for use in thermoforming; the polyurethane is the reaction product of an isocyanate terminated polyurethane prepolyrner, and a diamine chain extender. The polyurethane prepolymer is the reaction product of a polyol component and a diisocyanate component, the polyoi component including: a sulfonated polyester polyol, a hydroxy carboxylic acid, and a low molecular weight aliphatic diol having a molecular weight of from 60 to 400. Although it is said that the heat resistance of the aqueous polyurethane dispersions is significantly improved and the activation temperature is maintained low, they are not solvent free and their solids content is only about 30-35%; moreover, they lack electrolytic and pH resistance, due to the fact that the polyurethane incorporates carboxylic groups. [0010] It is an object of the present invention to provide a thermo-activable adhesive having high solids content, high open time, low activation temperature and high stability to hydrolysis and electrolytes. Disclosure of Invention [0011] We have now found that such thermo-activable adhesive having the advantages mentioned above is an aqueous dispersion of an anionic semi-crystalline polyurethane containing from 2 to 50 meq of sulfonate groups per 100 g of solids content, the polyurethane being the reaction product of an isocyanate terminated pre-polymer containing from 0.01 to 20 meq of sulfonate groups per 100 g, and an aliphatic diaminosulfonate chain extender. [0012] In the present text with the term "semi-crystalline polyurethane" we mean semi-crystalline polyurethane-polyureas, i.e. high molecular weight compounds containing urea groups in addition to urethane groups for which a melting and crystallisation temperature can be observed and possessing a fusion enthalpy (ΔHf) of at least 15 J/g, measured according to ASTM Standard D3417. [0013] The isocyanate terminated pre-polymer containing sulfonate groups is the reaction product of: i) a polyisocyanate; ii) a hydroxy terminated sulfonated polyester polyol having a molecular weight from 400 to 4000 and containing from 0.05 to 50 meq of sulfonate groups per 100 g; iii) a hydroxy terminated non-ionic polyester polyol having a molecular weight from 400 to 4000 obtained from the reaction of one or more non- sulfonated aliphatic dicarboxylic acids and one or more non-sulfonated aliphatic diols, or a polylactone polyol having a molecular weight from 400 to 4000 started on non-sulfonated aliphatic diols, [0014] the molar ratio between the sulfonated polyester polyol and the non-ionic polyester polyol or the polylactone polyol being comprised betweeni :0.3 and 0.1 :1. [0015] The sulfonated polyester polyol may incorporate sulfonate groups via sulfonated dicarboxylic acids or sulfonated diols. [0016] The sulfonate functional groups may be in acid or salt form; suitable salt forms are alkali metal salts, or tertiary amine salts. [0017] Examples of the sulfonated diols include 1 ,4 dihydroxybutane sulfonic acid and succinaldehyde disodium bisulfite. [0018] The preferred sulfonated dicarboxylic acids are aromatic sulfonated dicarboxylic acids, that is dicarboxylic acids in which the carboxyl groups and the sulfonate groups are directly Ii nked to an aromatic ring; 5-sulfoisophthalic acid monosodium salt is particularly preferred. [0019] According to a preferred aspect of the invention, the sulfonated polyester polyol is obtained from the reaction of one or more aromatic sulfonated dicarboxylic acids, one or more non-sulfonated aliphatic dicarboxylic acids selected from the group consisting of succinic, glutaric, adipic, azelaic, suberic, pimelic, phthalic acids and one or more non-sulfonated aliphatic diols selected from the group consisting of ethylene glycol, diethyleneglycol, 1 ,4-butanediol, 1 ,3-p ropanediol, neopentylglycol, 1 ,6- hexanediol, 1 ,4-cyclohexane dimethanol, 1 ,2-propylene glycol and 2- methyl-1 ,3 propanediol. [0020] Particularly preferred are sulfonated polyester polyol obtained from the reaction of 5-sulfoisophthalic acid monosodium salt, adipic and iso- phthalic acid, neopentylglycol and diethyleneglycol. [0021] The sulfonated polyester polyols useful for the realisation of the present invention preferably have a molecular weight from 700 to 3000. Molecular weights referred to herein are number average molecular weights. [0022] The non-ionic polyester polyols useful for the realisation of the invention are preferably obtained from the reaction of one or more non-sulfonated aliphatic dicarboxylic acids selected from the group consisting of succinic, glutaric, adipic, azelaic, suberic, pϊmelic, phthalic acids and of 1 ,4- butanediol, 1 ,6-hexanediol or mixture thereof. [0023] The preferred polylactone polyol is ε-caprolactone polyol. [0024] According to a preferred feature of the invention the molar ratio between the sulfonated polyester polyol and the non-ionic polyester polyol or the polylactone polyol is comprised between 0.8:1 and 1 :0.8. [0025] The polyisocyanates suitable for the realisation of the present invention are either of the aromatic, or of the aliphatic or of the cycloaliphatic type, such as 4,4'-dicyclohexyl-methanediisocyanate, 1 -isocyanate-3- isocyanate-methyl-S.δ.δ-trimethyl-cyclohexane (or isophorone diisocyanate), hexamethylenediisocyanate, 2,4- toluenediisocyanate either alone or in admixture with the 2,6 -toluenediisocyanate isomer, and 4,4'- diphenyl-methanediisocyanate, m-tetramethylxylylene diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, or mixtures thereof; particularly preferred are isophorone diisocyanate, hexamethylenediisocyanate and mixture thereof. [0026] The isocyanate terminated pre-polymer containing from 0.01 to 20 meq of sulfonate groups per 100 g is prepared by adding a polyisocyanate to a mixture of: [0027] - a hydroxy terminated non-ionic polyester polyol having a molecular weight from 400 to 4000 and obtained from the reaction of one or more non-sulfonated aliphatic dicarboxylic acids and one or more non- sulfonated aliphatic diols, or a polylactone polyol having a molecular weight from 400 to 4000 started on non-sulfonated aliphatic diols; [0028] - a hydroxy terminated sulfonated polyester polyol having a molecular weight from 400 to 4000 and containing from 0.05 to 50 meq of sulfonate groups per 100 g, [0029] and stirring the reaction mixture, preferably without any solvent, at 40°- 1000C for 0.5-3 hours until the isocyanate value remains constant; the molar ratio between the polyisocyanate and the sum of the polyols shall be between 1.2:1 and 2.5:1 , preferably between 1.5:1 and 2:1 , and the molar ratio between the sulfonated polyester polyol and the non-ionic polyester polyol or polylactone polyol shall be between 1 :0.3 and 0.1 :1 , preferably between 0.8:1 and 1:0.8. [0030] The isocyanate terminated pre-polymer containing sulfonate groups is then dissolved in a suitable solvent, preferably acetone or methyl ethyl ketone, and chain-lengthened with an aqueous solution of an aliphatic diaminosulfonate. [0031] The aliphatic diaminosulfonate chain extender is preferably an aliphatic diaminosulfonate represented by the formula H2N-A-NH-B-SO3" M+ in which A and B represent an aliphatic hydrocarbon group having from 2 to 6 carbon atoms, and M+ represents a sodium or potassium cation or a substituted or unsubstituted ammonium cation. [0032] More preferably, the aliphatic diaminosulfonate chain extender is sodium- N-(2-aminoethyl)-2-aminoethane sulfonate (AAS salt). [0033] In combination with the aliphatic diaminosulfonate chain extender, other normally used non-sulfonated chain extender may be employed, such as other hydrazine or amine functionalised chain extenders. [0034] The molar ratio between the isocyanate terminated pre-polymer and the aliphatic diaminosulfonate is comprised between 1 :0.05 and 1 :1 , preferably between 1:0.2 and 1 :0.6. [0035] The resulting polyurethane solution is then mixed with water under vigorous stirring until a fine dispersion is obtained; the solvent is then distilled off and the amount of water is adjusted to obtain a solids content from 30 to 60% by weight. [0036] According to a preferred advantageous aspect of the present invention the solids content of the aqueous dispersion of polyurethane is from 40 to 50% by weight and Brookfield viscosity is in the range of 20 to 2000 mPa*s measured at 230C and 50 rpm, according to EM ISO 2555. [0037] In industrial application a high solids content is desired to speed up the drying step which precedes the heat activation, and the low viscosity enables easy application of the product. [0038] The polyurethane of the aqueous dispersion of the invention contains from 2 to 50 meq, preferably from 10 to 25 meq, of sulfonate groups per 100 g of solids content. [0039] Thermo-activable adhesives comprising the polyurethane dispersions described above and, optionally, the auxiliary agents normally used in the field of adhesives, such as water dispersible polyisocyanate cross-linking agents, are another fundamental aspect of the present invention. [0040] The aqueous dispersions of anionic polyuretha ne of the invention and the thermo-activable adhesives comprising them are suitable for bonding any substrates, particularly thermolabile substrates , e.g. paper, textiles, non woven textiles, cardboard, wood, metal or leather, rubber materials or plastics, and are particularly suitable for bonding footwear materials. [0041] The thermo-activable adhesives of the invention can be used according to the normal procedure: the substrates coated with the adhesive are dried to obtain a tack-free film, heat-activated and bonded under pressure. [0042] The thermo-activable adhesives of the invention exhibit low activation temperature and high open time, as it is shown from the data reported in Table 2. [0043] Other advantages of the thermo-activable adhesives of the invention are their resistance to hydrolysis and the homogeneity of the film obtained therewith. [0044] The examples given below serve to further illustrate the invention. [0045] In the following examples all parts and percentages are by weight unless otherwise specified. [0046] Examples. [0047] In the examples the following compounds are used: [0048] Polyester 1 = a sulfonated polyester polyol from adipic acid, isophtalic acid, 5-sulfoisophthalic acid monosodium salt dietrryleneglycol and neopentyl glycol, having 6% by weight of 5-sulfoisophthalic acid monosodium salt and molecular weight 1000 g/mol ; [0049] Polyester 2 = adipic acid-tetramethylenediol polyester, having molecular weight 1000 g/mol; [0050] Polyester 3 = adipic acid-tetramethylenediol polyester, having molecular weight 2000 g/mol; [0051] Polyester 4 = adipic acid-tetramethylenediol polyester, having molecular weight 3000 g/mol; [0052] Polyisocyanate 1 = 1,6-diisocyanatohexane; [0053] Polyisocyanate 2 = isophorone diisocyanate; [0054] AAS salt = sodium salt of N-(2-aminoethyl)-2-aminoethane sulphonic acid (35.30% in water); [0055] EDA = ethylene diamine. [0056] Example 1 [0057] A reaction vessel, equipped with internal thermometer, stirrer and cooler, is filled, under nitrogen atmosphere and at room temperature, with 105.042 g (210.084 meq) of Polyester 1 , 315.126 g (210.084 meq) of Polyester 4 and 0.105 g of benzoyl chloride. The mixture is heated to 50° C, then 40.000 g (476.190 meq) of Polyisocyanate 1 and 26.464 g (238.095 meq) of Polyisocyanate 2 are added under stirring. The reaction temperature is brought to 90° C and maintained for approximately 2 hours, until the titrimetric determination of the free NCO-groups still present gives a value of 2.54% by weight (value determined in this example as well as in the other examples according to the standard method ASTM D2572). [0058] The reaction mixture is then cooled and diluted with 973 g of acetone while the temperature is maintained at 50° C. Subsequently a solution of 1.326 g (44.118 meq) of EDA and 28.517 g (102.941 meq) of AAS salt in 27.119 g of water is stirred into the homogeneous acetonic solution at 40° C. After 5 minutes 452.455 g of demineralised water are added at a high stirring speed. [0059] The organic solvent (acetone) is distilled off in vacuum. [0060] A stable finely divided dispersion is obtained, with solids content of 50% by weight, pH 7.77 and Brookfield viscosity of 225 mPa*s (at 23°C, 50 rpm, spindle No. 2). [0061] Example 2 (comparison example). [0062] A reaction vessel, equipped with internal thermometer, stirrer and cooler, is filled, under nitrogen atmosphere and at room temperature, with 105.042 g (210.084 meq) of Polyester 2, 315.126 g (210.08-4 meq) of Polyester 4 and 0.105 g of benzoyl chloride. The mixture is heated to 50° C, then 40.000 g (476.190 meq) of Polyisocyanate 1 and 26.464 g (238.095 meq) of Polyisocyanate 2 are added under stirring. The reaction temperature is brought to 90° C and maintained for approximately 2 hours, until the titrimetric determination of the free NCO-groups still present gives a value of 2.54% by weight. [0063] The reaction mixture is then cooled and diluted with 973 g of acetone while the temperature is maintained at 50° C. Subsequently a solution of 1.326 g (44.118 meq) of EDA and 32.591 g (117.647 meq) of AAS salt in 30.235 g of water is stirred into the homogeneous acetonic solution at 40° C. After 5 minutes 448.141 g of demineralized water are added at a high stirring speed. [0064] The organic solvent (acetone) is distilled off in vacuum. [0065] A stable finely divided dispersion is obtained, with solids content of 50% by weight, pH 6.85 and Brookfield viscosity of 250 mPa*s (at 23°C, 50 rpm, spindle No. 2). [0066] Example 3 (comparison example) [0067] A reaction vessel, equipped with internal thermometer, stirrer and cooler, is filled, under nitrogen atmosphere and at room temperature with, 420.168 g (420.168 meq) of Polyester 3 and 0.105 g of benzoyl chloride. The mixture is heated to 50° C1 then 40.000 g (476.190 meq) of Polyisocyanate 1 and 26.464 g (238.095 meq) of Polyisocyanate 2 are added under stirring. The reaction temperature is brought to 90° C and maintained for approximately 2 hours, until the titrimetric determination of the free NCO-groups still present gives a value of 2.54% by weight. [0068] The reaction mixture is then cooled and diluted with 973 g of acetone while the temperature is maintained at 50° C. Subsequently a sol ution of 1.326 g (44.118 meq) of EDA and 32.591 g (117.647 meq) of AAS salt in 30.235 g of water is stirred into the homogeneous acetonic solution at 40° C. After 5 minutes 448.141 g of demineralised water are added at a high stirring speed. [0069] The organic solvent (acetone) is distilled off in vacuum. [0070] A stable finely divided dispersion is obtained, with solids content of 50% by weight, pH 6.70 and Brookfield viscosity of 265 mPa*s (at 23°C, 50 rpm, spindle No. 2). [0071] Example 4 [0072] A reaction vessel, equipped with internal thermometer, stirrer and cooler, is filled, under nitrogen atmosphere and at room temperature, with 105.042 g (210.084 meq) of Polyester 1 , 210.084 g (210.084 meq) of Polyester 3 and 0.105 g of benzoyl chloride. The mixture is heated to 50° C, then 40.000 g (476.190 meq) of Polyisocyanate 1 and 26.464 (238.095 meq) of Polyisocyanate 2 are added under stirring. The reaction temperature is brought to 90° C and maintained for approximately 2 hours, until the titrimetric determination of the free NCO-groups still present gives a value of 3.24% by weight. The reaction mixture is then cooled and diluted with 763.181 g of acetone while the temperature is maintained at 50° C. Subsequently a solution of 1.326 g (44.118 meq) of EDA and 28.517 g (102.941 meq) of AAS salt in 27.119 g of water is stirred into the homogeneous acetonic solution at 40° C. After 5 minutes 347.413 g of demineralised water are added at a high stirring speed. [0073] The organic solvent (acetone) is distilled off in vacuum. [0074] A stable finely divided dispersion is obtained, with solids content of 50% by weight, pH 6.73 and Brookfield viscosity of 310 mPa*s (at 230C, 50 rpm, spindle No. 2). [0075] Example 5 (comparison example) [0076] A reaction vessel, equipped with internal thermometer, stirrer and cooler, is filled, under nitrogen atmosphere and at room temperature, with 105.042 g (210.084 meq) of Polyester 2, 210.084 g (210.084 meq) of Polyester 3 and 0.105 g of benzoyl chloride. The mixture is heated to 50° C, then 40.000 g (476.190 meq) of Polyisocyanate 1 and 26.464 g (238.095 meq) of Polyisocyanate 2 are added under stirring. The reaction temperature is brought to 90° C and maintained for approximately 2 ho urs, until the titrimetric determination of the free NCO-groups still present gives a value of 3.24% by weight. The reaction mixture is then cooled and diluted with 763.181 g of acetone while the temperature is maintained at 50° C. Subsequently a solution of 1.326 g (44.118 meq) of EDA and 32.591 g (117.647 meq) of AAS salt in 30.235 g of water is stirred into the homogeneous acetonic solution at 40° C. After 5 minutes 343.099 g of demineralised water are added at a high stirring speed. [0077] The organic solvent (acetone) is distilled off in vacuum. [0078] A stable finely divided dispersion is obtained, with solids content of 50% by weight, pH 6.60 and Brookfield viscosity of 285 mPa-s (at 23°C, 50 rprn, spindle No. 2). [0079] [0080] In Table 1 , the characteristics of the dispersions obtained in Ex. 1-5 are reported. Table 1 [0081] comparison example [0082] Rm NCO/OH = molar ratio of NCO/OH in the preparation of the prepolymer [0083] Rm NCO/ AAS = molar ratio of NCO/ AAS salt in the preparation of the polyurethane [0084] Rm NCO/NH = molar ratio of NCO/ NH in the preparation of the polyurethane [0085] meq SO3" in pre-poly. = milliequivalents of SO3" in 100 g of the pre-polymer [0086] meq SO3" tot. = milliequivalents of SO3" in 100 g of the polyurethane [0087] [0088] [0089] Evaluation tests. [0090] a) Determination of activation temperature and open time. [0091] Bonds were produced by means of the dispersions prepared as described in Ex. 1-5. [0092] The material to be bonded was a SBR material. All tests were carried out in accordance with UNI EN 1392. [0093] The activation temperatures were measured according to UNI EN 12Q61 (optimum activation temperatures) and are reported in Table 2. [0094] The open time (the period of time during which the activated surfaces remain workable and components can be bonded by the application of pressure) was measured after activation at the optimum activation temperature; open times are reported in Table 2. [0095] The open time is related to the rate of re-crystallisation of the heat activated polymer, the slower the polymer re-crystallises the longer is the open time. [0096] Table 2

[0097] comparison example [0098] [0099] b) Determination of melting and crystallisation temperature and of fusion enthalpy by DSC. [00100] DSC ( Perkin Elmer, DSC7) was used to study non isothermal crystallisation behaviour of the films obtained from the dispersions of Examples 1-5 and from Dispercoll ® U54, a thermo-activable polyur&thane from Bayer AG, Germany. The data obtained are reported in Table 3_ [00101] The melting peak temperature ( Tpm ) and the crystallisation peak temperature (Tpc ) were measured according to standard test method ASTM D3418-99. [00102] The fusion enthalpy (ΔHf) was measured according to standard test method ASTM D3417-99. [00103] All samples from Ex. 1-5 were dried in a vacuum oven prior to measuring. [00104] A sample (about 10 mg) was weighed in an aluminium DSC sample pan. The calorimeter was operated with a stream of oxygen-free, dry nitrogen flowing through the chamber throughout the experiment. A preliminary thermal cycle was performed and recorded by heating the sample at a rate of 10°C/min from 500C below to 30 0C above the melting point. The temperature is held for 10 min. The sample is then cooled to 50 0C below the peak crystallisation temperature at a rate of 10 °C/min. The sample is re-heated at a rate of 10 °C/min. The desired temperatures are measured. [00105] Table 3

[00106] comparison example [00107] [00108] c) Determination of isothermal crystallisation rate by DSC. [00109] DSC was also used to study isothermal crystallisation behaviour of the films obtained from the dispersions of Ex. 1-5 and from Dispercoll ® U54. [00110] All samples were dried in a vacuum oven prior to measuring. [00111] A sample (about 10 mg) was weighed in an aluminium DSC sample pan. The calorimeter was operated with a stream of oxygen-free, dry nitrogen flowing through the chamber throughout the experiment. The sample was heated at a rate of 10 °C/min up to 70 0C and held for 10 mi nutes. The isothermal crystallisation temperature was reached by rapidly cooling the sample (quenching) at 100 °C/min. The sample was then held at that temperature for 45 minutes to reach thermal equilibrium and ensure that the crystallisation was complete. The sample was then heated again at 10 °C/min to 70 0C and held for an additional 10 minutes, then quenched to another isothermal crystallisation temperature for 45 minutes to complete the crystallising process. The entire procedure was repeated for a series of isothermal crystallisation temperatures and crystallisation peaks at the different crystallisation temperatures were collected. After obtaining the DSC curves of the crystallisation peaks, the enthalpy of crystallisation (ΔH) was calculated. Once this information was obtained, the time needed to complete half time of crystallisation (tv2) was determined using a partial area software program. By comparing t1/2 , reported in Tab Ie 4, it is possible to compare the relative crystallisation rates of the films. [00112] Table 4

[00113] comparison example [00114] [00115] Both isothermal and non-isothermal DSC crystallisation studies showed that the introduction of sulfonate groups in the backbone of the polyurethane according to the invention doesn't influence significantly the melting temperature but decreases the crystallisation rate of polymers, so providing a longer open time. [00116] d) Evaluation of hydrolytic resistance. [00117] The hydrolytic resistance of the films obtained from the dispersions of Ex. 1-5 and from Dispercoll ® U54 was evaluated as follows. [00118] The films were immersed in water for 24 hours at room temperature, then dried for 24 hours at 25°C and 60 wt% r.h. [00119] The film obtained from the dispersions according to the invention (Ex. 1 and 4) remained flexible and strong; the films obtained from the dispersions of the comparison examples and from Dispercoll ® U54 became very weak and brittle.