PROCESS FOR THE MANUFACTURE OF THERMOPLASTIC POLYURETHANS

Abstract: The invention concerns a process for the manufacture of ther­ moplastic polyurethanes wherein an organic polyisocyanate is made to polymerize with a polyol (reactive with same isocyanate) in the presence of a chain extending agent having a molecular weight lower than 400, under continuous stirring, at a temperature higher than 150°C and for a time sufficient to obtain a product having a melt flow index lower than 25 g/10′.

Description: The present invention relates to a process for the manufacture of thermoplastic polyurethanes showing excellent physical features comparable to those of a technopolymer. Polyurethanes may be obatained by reacting long chain polyols, having a molecular weight of from 400 to 10,000, with organic polyisocyanates, preferably diisocyanates, in the presence of chain extending agents generally consisting of glycols, polyols or amines having a short chain and a molecular weight lower than 400. Polyurethanes are exploited for the manufacture of a wide variety of materials; in particular, elastomeric thermoplastic polyurethanes can be processed by injection molding, extrusion, calendering etc., to produce shaped articles useful in industry. The use of such products to prepare materials showing a structural resistance equal to that of the products known as "technopolymers" (for instance nylon) is, however, limited by the necessity of using reinforcing materials, such as glass or carbon fibres, in order to obtain the desired rigidity, impact resistance and so on. DE-A-1 964 834 relates to a process for the manufacture of elastomeric polyurethanes by extrusion of diisocyanates, polyhydroxy compounds having a moleular weight of 800 to 3500 and glycols having a molecular weight of up to 300 in a multiple-screw extruder in the presence of lubricants and at temperatures of from 90 to 220°C. DE-A-2 549 372 describes a process for the continuous production of polyurethane elastomers by reaction of polyisocyanates with compounds having Zerewitinoff-active hydrogen atoms in a double-screw extruder under particular extrusion conditions so as to obtain a homogeneous final product. DE-A-2 447 368 is directed to a continuous process for the production of thermoplastic polyurethanes, particularly linear polyurethanes soluble in organic solvents, from polyhydroxy compounds, diisocyanates and chain extenders by reacting the components in a double-screw extruder, the temperature within said extruder, from the feed zone to the die, decreasing from 240°C to 190°C. The features of polyurethanes, in particular hardness, can be changed within a wide range of values comprised between those of the very soft materials and those of hard materials having a limited flexibility, by changing the amount of the various components used for their preparation: polyols, polyisocyanates and chain extending agents. In particular, this change of features can be reached by changing, the -NCO group being the same, the weight ratio of long chain polyol to short chain polyol. In fact, the units of the polymeric chain derived from the low molecular weight extending agent and from the polyisocyanate give rise to hard segments having a high elastic modulus, whereas the chain units deriving from the high molecular weight polyol give rise to soft segments, having a low elastic modulus, due to the presence of the long chain of the polyol. In the case of an essentially linear polyurethane (prepared from a diisocyanate, a difunctional polyol and a difunctional extending agent), an increase of the percentage of the extending agent (in comparison with the polyol) is likely to incorporate into the chain a higher percentage of hard segments and this makes the polyurethanes harder and harder, but more brittle; i.e., the material, although thermoplastic, loses its elastomeric properties. The no more elastomeric material shows, due to the high percentage of hard segments, a hardness higher than 70 Shore D and a high flexural modulus. It is known from US-A-3,356,650 to prepare a thermoplastic polyurethane showing a hardness higher than 70 Shore D, using high ratios of hard to flexible segments. According to the process described in said patent, thermoplastic polyurethanes are prepared by reacting the organic polyisocyanate with a mixture of reagents consisting of at least one compound containing three active hydrogen atoms and other organic compounds containing two active hydrogen atoms, determined according to the Zerewitinoff method, said active hydrogen atoms being reactive with the isocyanate group. The main characteristics of the process described in said US-A are as follows: 1) The reactant mixture must have an average molecular weight lower than 500, whereas each single component of the mixture can have a higher molecular weight, up to 5,000 or higher; 2) the mixture must contain an amount of components having a functionality (for each molecule) higher than 2, in such a manner that from 0.01 to 20% of the groups containing the active hydrogen atoms are supplied by the compounds containing more than 2 functional groups per molecule; some of the properties, such as thermoplasticity, are negatively affected by the values outside of these limits; 3) the reactants are used in such an amount as to keep the equivalent NCO/OH ratio between 0.7 and 1.3. The conditions above are critical to produce hard polyurethanes having a high heat distorsion temperature and a high hardness (even higher than 80 Shore D). However, when particular polyisocyanates and/or diols and/or particular process conditions are used, the polyurethanes can be brittle and therefore not suitable for application in those fields where engineering type properties are required. The Applicant has now succeeded in overcoming these drawbacks and, therefore, in producing polyurethanes characterized by high impact-resistance, high flexural modulus and high heat distortion temperature, by carrying out the reaction between the organic polyisocyanate, the long chain polyol and the chain extending agent under particular process conditions. In its broadest aspect the present process is characterized in that the physical properties of polyurethanes, and in particular the impact resistance,are improved by carrying out the polymerization of an organic polyisocyanate with a polyol or polyamine reactive with said polyisocyanate and having an average functionality of at least 2 and a molecular weight higher than 400 in the presence of a chain extending agent having a molecular weight lower than 400, under continuous stirring at a temperature, at least in the final step of the process, higher than 150°C and for a time sufficient to obtain a product having a melt flow index (MFI) of from 0 to 25 g/10' (at 200°C and under a 2.16 kg load; determined according to ASTM D1238/74). The above mixture of reactants containing hydrogen reactive with the isocyanate (polyols + chain extending agent), according to the Zerewitinoff method, must have an average molecular weight lower than 300. Even values of the melt flow index (MFI) equal to 0 g/10', at 200°C, are suitable to obtain polyurethanes showing improved physical characteristics. The reaction temperature, according to the present invention, is at least equal to 150°C, preferably from 200°C to 250°C, either to increase the reaction rate or to assure a low viscosity of the reacting mixture so that it can be stirred as long as possible and then transferred, as soon as the desired MFI value is reached, to the subsequent process steps for the preparation and curing of the product. By a proper selection of the temperature, of the reaction time (under stirring), and of the other operative conditions (depending on the type of raw materials and/or on the composition of the reaction mass), it is possible to reach a molecular weight bestowing on the final product an excellent quality. It is possible to verify that the final reaction point (and therefore the desired quality of the product) is reached, by recording the viscosity of the molten product. The materials obtained according to the invention show MFI values corresponding to viscosity levels much higher than those of the materials obtained according to the known techniques and therefore have significantly better physical features. In fact, there was observed the existence of a correlation between the mechanical properties, such as resilience, of polyurethanes and their viscosity in the molten state, verified by MFI measurements; particularly, it was observed that the higher values of the viscosity of the polymer in the molten state and, consequently, its improved mechanical properties are due to better technological synthesis conditions. According to the used formulations, it is possible to fix a value of said MFI above which better physical properties are obtained, such as a flexural modulus at 23°C of at least 1,000 MPa, a Izod (notch) resilience at 23°C of at least 150J/m and a heat distortion temperature, at 1.82 MPa, of at least 50°C. Such superior physical features are the proof of the surprising results which can be obtained by applying the above operative technological conditions. Depending on the reactivity of the used materials, it is possible to select the so-called "one -shot"technique or, alternatively, the "pre-polymer" technique. According to the one-shot technique, the diisocyanate, the polyol and the chain extending agent are brought into contact in one single operation, optionally by pre-mixing the chain extending agent; the prepolymerization technique, on the contrary, first provides for the preparation of a prepolymer of the polyisocyanate and the high molecular weight polyol and then for the reaction between the thus formed prepolymer and the low molecular weight polyol (chain extending agent). It is preferable, when possible, to employ the "prepolymer" technique in the synthesis of the thermoplastic polyurethane because: 1) it allows the handling of materials which are generally less sensitive,as to chemical and physical stability; 2) it allows the formation of more regular polymeric chains (more homogeneous as to the size and to the statistical distribution of the hard/soft segments); 3) it makes it easier to reach high conversions and molecular weights, expecially when one has to deal with polyhydric intermediates reactive with isocyanates which differ from each other as to structure, molecular weight and reactivity. Another advantage of the prepolymer technique is that it allows, when particularly reactive materials are used, a longer "pot-life" of the reaction mixture at high temperatures and therefore makes possible, in practice, synthesis conditions which otherwise cannot be used but are advantageous for the properties of the final product. The process according to the present invention can be performed using any apparatus suitable for this purpose, such as, for instance, reaction vessels of the "batch" type, connected to a curing mold, to a heated conveyor belt or to an extruder-reactor. It is thus possible to work according to a continuous process in an extruder-reactor, which directly supplies the product pellets. Said extruder-reactor is to ensure the temperature conditions, the shear stress (in the mixing) and the residence time required by the process. Such extruders may be provided with one or two screws, or provided with screws or other devices suitable for mixing and extrusion, which move in a reciprocating way along their axes; in particular, twin-screw extruders having co-rotating and self-cleaning screws (endowed with a high percentage of homogenizing elements) are preferred. These extruders are generally coupled with a usual casting machine; examples of twin-screw extruders are the fast assemblable extruders of the Werner-Pfleiderer ZSK 53 type of the Company Werner, or of the Maris 55V type of the Company Maris. Thereby it was possible to obtain polymers having high molecular weights, generally higher than 30,000, which allow the material to reach high physical characteristics. For the preparation of a thermoplastic polyurethane, according to the present invention, a catalyst can be used, although this is not absolutely necessary; one can use all the catalysts known in the art which catalyze the reaction between isocyanate groups and compounds containing reactive hydrogen atoms. A detailed list of catalysts is given, for instance, in US-A-2,620,516; 2,621,166 and 2,729,618. Also the raw materials to be used for the preparation of thermoplastic polyurethanes are already known; see for instance US-A-3,356,650. According to a preferred embodiment of the present invention the organic poly-isocyanates have the general formula: wherein X is 0 or 1 R₁ can be arylene, preferably phenylene or naphthylene, optionally substituted, e.g. with one or more chloro-, amino-and/or methyl groups; alkylene having from 1 to 6 carbon atoms; an alicyclic group having from 1 to 6 carbon atoms, preferably saturated or having one double bond, optionally substituted e.g. with one or more methyl- and/or carbonyl groups, if X is 0 and R₁ and R₂ can be phenylene or cyclohexylene, if X is 1. Examples of such organic poly-isocyanates are methylene-diphenyl-diisocyanate (2,4'-and 4,4'-isomers and mixtures thereof), m- and p-phenylene-diisocyanates, chlorophenylene-diisocyanates, alpha, alpha'-xylylene-diisocyanate, 2,4- and 2,6-toluene-diisocyanate (and mixtures thereof), toluidene-diisocyanate, hexamethylene-diisocyanate, 1,5-naphthalene-diisocyanate, isophorone-diisocyanate, methylene-bis-(cyclohexyl-isocyanate), its 4,4'-and 2, 4'-isomers and the mixtures thereof. Preferred diisocyanates are 2,4-and 2,6-toluene-diisocyanate, in the 80/20 ratio, and the 4,4'-methylene-diphenyl-diisocyanate. As to the high molecular weight polyols or polyamines, examples thereof are polyol-polyethers, polyol-polyesters, polyethers with amine termination, polycarbonates ending with hydroxy groups, polybutadienes ending with hydroxy groups, polybutadiene-acrylonitrile copolymers with hydroxy termination, polybutadiene-acrylonitrile copolymers with amine termination, copolymers of dialkyl-siloxanes and alkylene oxides (hydroxy ended) and so on. Examples of polyol-polyethers are polyoxyethylene glycols, polyoxypropylene glycols, optionally capped with ethylene oxide, block or random copolymers of ethylene- and propylene-oxide, propoxylated tri- and tetrahydroxy alcohols, such as glycerine, trimethylolpropane, pentaerythritol and the like, optionally capped with ethylene oxide, polytetramethylene glycol, block or random copolymers of tetrahydrofuran and of ethylene oxide and/or propylene oxide, and the products derived from the reaction of the above mentioned polyol-polyethers with acids and/or esters of polyfunctional polycarboxylic acids. Illustrative examples of polyol-polyesters are those prepared by polymerizing epsilon-caprolactone using as an initiator ethylene glycol, ethanolamine and the like, and those prepared by esterifying polycarboxylic acids, such as phthalic acid,terephthalic acid,succinic acid, glutaric acid, adipic acid, azelaic acid and the like, with polyhydroxy alcohols, such as ethylene glycol, butanediol, glycerine, trimethylolpropane, 1,2,6-hexanetriol, cyclohexanedimethanol and like. Examples of polyethers having aminic endings are di- and tri-primary aliphatic amines, structurally derived from glycols or polyoxypropylene triols, which are known on the market under the trade name JEFFAMINE® and manufactured by the Jefferson Chemical Company-U.S.A. Examples of polycarbonates containing hydroxy groups are those prepared by the reaction of diols, such as propane-1,3-diol, butane-1,4-diol, hexane-1,6-diol, diethylene glycol, triethylene glycol, dipropylene glycol and the like, with diaryl-carbonates, such as diphenyl-carbonate, or phosgene. Examples of polyethers containing silicon are the copolymers of alkylene oxides with dialkylsiloxanes and an example of hydroxy ended poly-butadiene copolymers are compounds which are available under the trade name "Poly BD Liquid Resins" (Arco Chemical Co.). Examples of butadiene/acrylonitrile copolymers,having hydroxy and amino termination, are the materials available under the trade name "HYCAR®" with hydroxy ending (HT) or with amino termination (AT). Preferred polyols are the difunctional or trifunctional polyol-polyethers, and mixtures thereof, having a molecular weight comprised between 500 and 10,000 and preferably between 1,000 and 6,000. The chain extending agents consist of linear or branched chain aliphatic diols, and also cycloaliphatic diols, preferably containing from 2 to 8 carbon atoms and having a molecular weight lower than 400. Examples of said diols are: ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-propanediol, 1,3-butanediol, 2,3-butanediol, 1,3-pentanediol, 1,2-hexanediol, 3-methylpentane-1,5-diol, 1,4-cyclohexane-dimethanol and the like and mixtures thereof. Diethyleneglycol,dipropyleneglycol, tripropyleneglycol, ethanolamine, N-methyl-diethanol-amine, N-ethyl-diethanolamine and the like can also be used as diol chain extending agents; diol esters obtained by esterfying the above mentioned diols with acids, such as adipic acid, azelaic acid, glutaric acid and the like or the adducts with ε-caprolactone can also be used.Preferred compounds are: 1,4-butanediol, ethylene glycol, diethylene-glycol and 1,4-cyclohexane-dimethanol; all these compounds can be used either alone or as a mixture. The mixture of the organic compounds (polyols + chain extending agent) containing the hydrogen reactive with isocyanate (according to the Zerewitinoff method) has an average molecular weight lower than 300. The NCO/OH ratio is from 0.7 to 1.3 and preferably from 0.9 to 1.1. By the process of the present invention polyurethanes having high modulus, high resilience and heat distortion temperature can be obtained both in expanded and non-expanded form; the latter form can be obtained by applying methods well known in the art. A foaming agent, for instance, can be incorporated into the reaction mixture when polyurethanes are prepared. Preferably, it may consist of a volatile organic liquid, which evaporates during the exothermal reaction which takes place during the formation of the polyurethane. These liquids must be inert towards the reactants which take part in the reaction, must not interfere in any way with the reaction course, and should have a boiling point from 20°C to about 110°C. Examples thereof are: butane, hexane, heptane, methylene chloride. chloroform, monofluorotrichloromethane, chlorodifluoromethane, dichlorodifluoromethane and the like. Finally, the cellular form can be obtained during the extrusion molding of the corresponding flakes or "pellets" etc., of thermoplastic polyurethane, using an inert blowing gas during the operation, such as nitrogen, air, CO₂ and so on. The following examples are reported to better define the present invention and are not in any case limitative. EXAMPLE 1 (comparison example) Into a 400 ml reaction vessel, insulated and provided with stirrer, there were introduced 23.0 g of polyoxypropyleneglycol (M.W. 2,000), 21.7 g of a trifunctional polyol (M.W. 3,000) obtained by reacting glycerine and propylene oxide and 65.3 g of dipropyleneglycol. The polyol mixture was deaerated and dried under vacuum and slight agitation for one hour at 60°C. Thereafter, when the temperature was lowered down to 50°C, there were added 90.0 g of a mixture consisting of 2,4-and 2,6-toluenediisocyanate in a 80/20 ratio, kept at room temperature; the reaction mixture was then stirred under vacuum. The temperature was raised up to 90°C within 10 minutes and kept at this level by means of an electric resistor (applied out of the reaction vessel), for a further 20 minutes; the reaction product was then poured into a frame mold, placed into a laboratory press kept at 135°C and finally compressed and quickly cooled down to room temperature. Thereby a transparent polyurethane plate (135 x 135 x 3.2 mm) was obtained, which after testing as to its physical properties gave the results recorded in Table 1. EXAMPLE 2 (comparative) Example 1 was repeated and the reaction product was poured into the frame mold and transferred in a laboratory press kept at 135°C. The material was maintained in the mold at this temperature for 75 minutes and then was cooled down to room temperature; the thus obtained product was transparent and gave the results recorded in table 1. EXAMPLE 3 (comparative ) Example 2 was repeated up to the keeping of the polyurethane in the mold for 75 minutes at 135°C. Thereafter, a post-curing was carried out for 22 hours at 75°C and for 2 hours at 130°C. The obtained material was transparent and had the properties reported in table 1. EXAMPLE 4 Example 1 was repeated under the following working conditions: The mixture of polyols was dried for 1 hour at 60°C and the temperature was then raised up to 80°C, whereafter the isocyanate, kept at room temperature, was added. Under continuous stirring (for 9 minutes) the temperature spontaneously rose to 121°C and then (by supplying heat for 8 minutes) up to 200°C; this temperature was maintained in the reaction vessel for a further 8 minutes. The polymer was transferred into a frame mold and placed into a press at a temperature of 200°C for 10 minutes. After having discontinued the heating, the material was allowed to reach room temperature within about 2 hours; the thus obtained product was dull and showed the properties recorded in table 1. EXAMPLE 5 Using the same apparatus as in example 1, there were introduced: 23.0 g of the polyoxypropyleneglycol of example 1; 21.7 g of the trifunctional polyol of example 1; 50.4 g of the dipropylene-glycol of example 1; 10 g of 1,4-butanediol. Under stirring and under vacuum the mixture was dried and deaerated for 1 hour at 60°C. To the resulting polyol mixture, kept at 60°C, there were added 90 g of the diisocyanate mixture of example 1 and the evolved heat led to a temperature increase up to 115°C. Thereafter external heat was supplied and the temperature rose up to 205°C within 10 minutes; this temperature was kept constant for a further 2 minutes. At the end the material was transferred into a frame mold and placed into a laboratory press at a temperature of 200°C for 10 minutes; the material was allowed to cool to room temperature within 2 hours. The resulting plate was dull and, when tested, gave the results recorded in table 1. EXAMPLE 6 Into the reaction vessel of example 1 there were charged the follwing polyols: 23.0 g of the polyoxypropylene glycol of example 1 and 21.8 g of the trifunctional polyol of example 1. Under stirring and under vacuum the materials were dried for 1 hour at 80°C. To the thus obtained mixture there were added 90.0 g of the diisocyanate mixture of example 1 and the reaction between the polyols and the isocyanate was carried out under vacuum, while stirring for 1 hour at 80°C. To the resulting pre-polymer there were added 44.0 g of 1,4-butanediol and the mixture was vigorously stirred under vacuum for 60 seconds, whereafter the material was transferred into a frame mold and placed into a laboratory press at a temperature of 200°C for 20 minutes. The thus obtained polymer was then allowed to cure for 22 hours at 75°C and for 2 hours at 150°C. EXAMPLE 7 Into a fast twin-screw extruder of the Werner ZS53 type (36 mm diameter) with such a screw profile that the homogenizing zone was brought up to 55% of the total length of the extruder, there were fed from a casting machine: 100 parts of pre-polymer obtained from 13.1 parts of a trifunctional polyol having a molecular weight of 6,000 (obtained in its turn from glycerine and propylene oxide and capped with ethylene oxide) and from 86.9 parts of 4,4'-methylene-bis (phenylisocyanate), said prepolymer being kept at 80°C; 29.8 parts of butanediol containing 0.013 g of butyl-tin-di-laurate. The temperature profile along the extruder was allowed to vary between 200 and 220°C; the screw rotated at 150 rpm and the residence time of the reactants was 90 seconds. The strands leaving the outlet of the extruder were cooled and pelletized; the obtained material was dull and showed the properties reported in table 1.

Claims: Process for the manufacture of thermoplastic polyurethanes showing improved physical properties, wherein an organic polyisocyanate is reacted with a polyol or polyamine reactive with said polyisocyanate and having an average functionality of at least 2 and a molecular weight higher than 400 in the presence of a chain extending agent having a molecular weight lower than 400, the mixture of reactants containing hydrogen reactive with the isocyanate (polyols + chain extending agent), according to the Zerewitinoff method, having an average molecular weight lower than 300, and wherein the reaction is carried out under continuous stirring at a temperature, at least in the final step of the process, higher than 150°C and for a time sufficient to obtain a product having a melt flow index (MFI) of from 0 to 25 g/10' (at 200°C and 2.16 kg). Process according to claim 1, wherein the reaction temperature is from 200°C to about 250°C. Process according to any of the preceding claims, which is performed according to the prepolymer technique. Process according to any of the preceding claim, which is performed continuously, e.g., in an extruder. Process according to any of the preceding claims, wherein the organic polyisocyanate is 2,4- and 2,6-toluene-diisocyanate in a 80/20 ratio or 4,4'-methylene-diphenyl-diisocyanate. Process according to any of the preceding claims, wherein the polyol reactive with the polyisocyanate is a difunctional and/or trifunctional polyether polyol having an average molecular weight of from 500 to 10,000, preferably of from 1,000 to 6,000. Process according to any of the preceding claims, wherein the chain extending agent is a linear or branched aliphatic diol or a cycloaliphatic diol, containing from 2 to 8 carbon atoms. Process according to any of the preceding claims, wherein the NCO/OH ratio is from 0.7 to 1.3 and preferably from 0.9 to 1.1.

Inventors: BENETTI, ERMANNO, CREDALI, LINO, DR., LUNARDON, GIANFLAVIO, DR., MULAS, CARLO, DR.
Applicants: AUSIMONT S.P.A.
Application year: 1987
Main classification: C08G18
IPC codes: C08G18