PROCESS FOR CONVERTING ISOCYANATO GROUPS INTO CARBODIIMIDO AND/OR URETHONIMINO GROUPS

Abstract: A process for the partial or thorough conversion of isocyano groups, contained in organic compounds, into carbodiimido and/or urethonimino groups by heating com­ pounds containing free isocyano functional groups, in particular polyisocyanates, at temperatures in the range of from 140° to 300°C in the presence of solid, inorganic, acid catalysts, having surfaces of at least 2 m²/g, selected from amongst silica, alumina and the mixtures or derivatives wherein said two oxides are united and/or mixed with each other and/or with alkaline or alkaline-earth oxides.

Description: This invention relates to a process for partially or thoroughly converting isocyano groups, contained in organic compounds such as polyisocyanates, into carbodiimido and/or urethonimino groups. In particular, this invention relates to the preparation of liquid polyisocyanates stable to storing. It is known that the organic polyisocyanates, in particular the diisocyanates such as diphenylmethane diisocyanate, as well as the polymeric derivatives thereof, as compounds necessary in the preparation of polyurethanes, are relatively little stable to storing in fact, if kept in the liquid form, they lead to the formation of precipitates or even to solidification. A method of preparing stable polyisocyanates is that of modifying them by partially transforming their isocyano func tional groups into carbodiimido functional groups, with the production of a liquid modified polyisocyanate suitable for storing under ambient conditions. It is known that the organic isocyanates are capable of being converted in part or in whole into carbodimides in consequence of prolonged heating at high temperatures, generally exceeding 180 OC. According to said carbodiimidation redaction, a carbodiimi do groups (-N=C=N-) and a molecule of carbon dioxide are obtained from two isocyanate equivalentes. With a view to reducing both temparature and reaction times, i.e. to increasing the reaction rate, it w s suggested to employ a variety of catalysts, out of which we cite, by way of example: triethylphosphate, hexamethyl-phosphorami- de, tris(chloromethyl)-phosphine-oxide, 3-methyl-1-phenyl-2phospholene-l-oxide, n-butyltriphenylphosphonium bromide, tetraoctylzirconate, tris(isocyanate-hexyl)-biuret, tetraisopropyltitanate, triphenylarsine, ferropentacarbonyl, bis(/3 chloroethyl)-vinyl phosphonate and organic compounds containing isocyano groups and biuret, urethane, urea, amido, allophanate, isocyanurate, urethdione, urethonimine groups. In practice the aforesaid catalysts may be considered as belonging to two classes: the ones which are active also at temperatures below the room temperature, such as for example 3-methyl-1-phenyl-2-phospholene-1-oxide, and the ones which are active at high temperatures, generally above 150 C, with a minimum activity or without any activity at room temperature, such as for example triethylphosphate. To stop the carbodiimidation reaction at the desired conversion degree it would be necessary, therefore, in the case of the catalysts belonging to the former class, to desactivate or remove them, while for the ones belonging to the latter class it could be sufficient in principle to drastically lower the reaction temperature. However, the use of said catalysts exhibits several draw backs which are mainly due to the fact that it is difficult to remove them from the finished modified isocyano product. In fact, the presence of said catalysts in the finished modified isocyano product, in relation also to the storing, handling, transport conditions and to the successive utilization conditions, may cause, chiefly in the case of the high -activity catalysts, a further progress of the carbodiimida tion reaction. By consequence, variations in some characteristics of the product may occur, among which, for example, a variation in the NCO titer and in the viscosity, as well as an evolvement of gaseous carbon dioxide which can cause dangerous pressure increases in the storage tanks. To eliminate the aforesaid drawbacks it was proposed to use insoluble, solid heterophase catalysts,.which are separable by filtration or other techniques. Said heterophase catalysts, however, have the drawback that their preparation is difficult and expensive since it comprises, for example, the production of isocyanate insoluble polymers containing active groups of the previously listed catalysts, or the fixing of said active catalytic groups to an inert solid substratum, such as for example ion exchange resins or polystyrene. It has been now found that it is possible to partly or thoroughly convert the isocyano groups, contained in organic compounds, into carbodiimido and/or urethonimino groups, in particular to prepare polyisocyanates containing carbodiimido and/or urethonimino groups, without the drawbacks men: tioned hereinbefore, by employing, as carbodiimidation reaction catalysts, solid insoluble inorganic. substances having a high specific surface and a high porosity, and containing, as main components, silica and/or alumina. Thus, object of the present invention is a process for the partial or total conversion of isocyano groups, contain ed in organic compounds, into carbodiimido and/or urethoni millo groups by heating compourds containing free isocyano functional groups, such as the polyisocyanates, at high tempe ratures in the presence of catalysts, the process being cha- racterized in that heating occurs at temperatures in the range of from 1400 to 300 OC in the presence of a catalyst composed of a solid insoluble inorganic substance, of acid nature, having a specific surface of at least 2 mê/g, select ed from the group consisting of silica, alumina and of mixtures or derivatives thereof in which these two oxides are united and/or mixed with each other and/or with alkaline or alkaline-earth oxides. The substances utilized as catalysts2 according to the present invention, are generally endowed with a high spe 2 cific surface, preferably higher than 10 m /g, and with a high porosity, they may be of natural or synthetic origin and may be used in the form of powders, granules, spherical or cylindrical particles and the like. Among such substances, besides silica and alumina, also crystalline silico-aluminates having zeolitic structure, also known as molecular sieves, may be cited. As is known, such substances are generally activated, prior to use, by heating them in anhydrous inert atmosphere or in the presence of a defined amount of water. The activity for each type of such catalysts depends also on the mode according to which the activation process is conducted. The catalysts of the present invention can be activated by heating at 100-600 OC, preferably at 140-350 OC, in a dry nitrogen or air stream; temperature can be kept constant or gradually increased starting from room temperature. Before activation the catalysts may be optionally subjected to a pretreatment with moist air at room temperature, or with heated air or nitrogen, having a certain humidity content. Said catalysts are easily available as commercial products, are moderately expensive and, being in the solid form, are easily removable from the isocyanic reaction product, for example by filtration, decantation, centrifuga tion, etc. Furthermore they can be reused for subsequent carbodiimidation cycles. The complete removal of the catalysts makes it possible to prevent any further forming of carbodiimide during the storage of the modified isocyanic products The organic compounds containg free isocyano functional groups, employable in the process according to this invention, may be monofunctional such as tolylisocyanate, cyclohexylisocyanate, or polyfunctional, such as the polyisocyanates of the aromatic, aliphatic and cycloaliphatic types, among which there are to be cited, for example, 4,4'-diphe nylmethane diisocyanate, 4,4'-diphenyl-dimethyl-methane diisocyanate, 2,4-toluene diisocyanate (either alone or in admixture with its isomer 2,6-),1,6-hexane-diisocyanate, 1; ;4-butane-diisocyanate, 4,4-dicyclo-hexyl-methane diisocyanate, isoforone diisocyanate, 2,4-methylcyclohexane diisocyanate, p-xylene diisocyanate, 3-phenyl-methane-4,4',4"- -triisocyanate, polymeric polyisocyanates and the products, containing free NC0, obtained by partial condensation between a polyisocyanate and compounds containing activated hydrogen atoms. In particular, by using, as an organic polyisocyanate, the diphenylmethane diisocyanate (MDI) or the polymeric diphenylmethane diisocyanate (crude-MDI) having a high content of diphenylmethane diisocyanate, it is possible to obtain liquid carbodiimide-modified polyisocyanates, which are stable to storing for long time at room temperature and do not. lead to the formation of precipitates or to solidification, as conversely happens in the case of the not modified start- I ing polyisocyanates. The process conforming to the present invention is conducted by contacting, according to any technique, the organic polyisocyanate with the catalyst and by maintaining the whole at temperature in the range.of from 1400 to 300 C, preferably from 1800 to 260 OC. The reaction can be started, or even thoroughly conduct ed in the presence of any organic solvent unreactive with isocyano groups, such as for example chlorobenzene, chloroform, diglyme, methylene chloride, N-N-dimethylacetamide. The reaction progress degree, i.e. the forming of car bodiimide at the expense of the isocyano functional groups, can be easily checked volumetrically by measuring the amount of evolved carbon dioxide. As soon as the. conversion of the organic polyisocyanate to carbodiimide reaches the value desired, the reaction can be stopped by removing the catalyst from the modified iso cyanic product or by suitably lowering the temperature of the reacted mass. In the latter case, the catalyst can be successively removed according to any separation technique, such as, for example, filtration, decantation, centrifugation. The so separated catalyst may be subsequently utilized again. The reaction progress can be determined also by means of other techniques, for instance by checking the refractive index, the infrared spectrum or the NC0 titer. When operating discontinuously it is preferable to suspend the catalyst, by means of intense stirring, in the liquid isocyanate mass. The process may be conducted also continuously, using the solid catalyst' as a fixed bed in a column, to which polyisocyanate is continuously fed. By properly chosing the type and caking degree of the catalyst, its particle size and filling degree in the column, as well as the temperature, pressure and flow rate of the polyisocyanate in the column, it is possible to achieve: the desired conversion of the isocyano functional groups into carbodiimide. It is possible to use more columns arranged in series, and it is also possible to recycle the isocyanic product till attaining the desired conversion. The continuous process with catalyst fixed bed is particularly acvantageous because the final modified isocyanic product results already separated from the catalyst which remains in the reaction column. According to this process it is possible to directly obtain a modified polyisocyanate at the desired conversion degree, namely with the carbodiimide content which is useful for its application, or to reach a higher conversion of the isocyano functional groups into carbodiimide than the necessary one, and then to dilute the resulting modified isocyanic product with the unmodified polyisocyanate or with another type of polyisocyanate, up to the carbodiimide content to be obtained. The advantage resulting therefrom consists in the fact that not only remarkably lower amounts of polyisocyanate are made to react, but also that the obtained isocyanic product, having a high carbodiimide content, is capable of providing, by dilution, a wide range of modified polyisocyanates with a carbodiimide content varying according to the different requirements. Dilution may be performed directly in the reaction vessel or column by using unreacted polyisocyanate maintained at low temperature, with the further advantage of a sudden cooling of the reacting mass which stops the reaction and reduces as much as possible the formation of undesired urethdionic by-products. It is to be pointed out that the polyisocyanates containing carbodiimide can form an adduct (urethonimine), by interaction between an isocyano group and a carbodiimido group according to the following scheme: EMI9.1 Such reaction tends to occur spontaneously at room temperature and is easily reversible; for example by heating, the adduct is decomposed while restoring the isocyanate and carbodiimide groups. Therefore such an adduct can be considered a "masked" isocyanate. The modified polyisocyanates obtained according to the process of the present invention may therefore contain, generally after a more or less prolonged storing period, urethonimino groups. The titer of modified polyisocyanates which takes the masked.isocyanate into account is defined as "total NC0", while the one which does not take it into account is de fined as "apparent NC0" and it will be of course always lower than the "total NC0". The following examples are given to better illustrate this invention, without being however a limitation thereof. In such examples, the carbon dioxide evolved during the reaction was measured volumetrically or by titrating, with ffydrochloric acid, the caustic soda aqueous solution on which it was collected. The percentage of apparent NC0 was determined according to method ASTM D-1638 or by infrared spectrophotometry. The percentage of total NC0 was determined according to method ASTM D-1638 which, however, was modified by heating at reflux the mass being titred, before adding isopr alcohol, in order to be able to titrate also the isocyanate masked as urethonimine. The viscosity was measured at 25 OC by using a Brookfield torsion viscometer. Besides the presence of little amounts of urethdione (isocyanate dimer),. no significartpresence of other by-products, such as urea derivatives and isocyanurates, was detected, what proves that the catalyst employed according to the invention is highly selective. Some of the infrared bands typical of the functional groups usually present in the isocyanates obtained according to this invention are indicated hereinbelow for illustrative purposes: -l isocyano group 2270 cm -I carbodiimido group 2120, 2140 cm urethonimino group 1720, 1730, 1370 cm -l urethdione group 1780 cm Example 1 (comparative test) A comparative test was carried out without employing cata lysts2 using a 2-liter glass flask equipped with ,a thermometer, a blade stirrer, a reflux cooler and a feeding inlet. The flask is heated to 240 OC by means an oil bath. 600 g of pure MDI (4,4'-diphenylmethane diisocyanate contain ing less than 3-45 of its 2,4'-isomer) in the molten state were introduced into the flask. Charging and reaction were conducted whole stirring under a dry nitrogen or carbon dioxide atmosphere. The reaction occurred slowly. As a matter of fact, on the basis of the evolved carbon dioxide volume, the following values of apparent NCO d and of conversion into carbodiimide at different reaction times were calculated reaction time apparent NC0 do conversion 75 minutes 32.7 1.85% 180 " 32 3.46% 289 " 31.4 4.9Q 300 " 31.3 5.15 420 " 30.5 6.85 Example 2 3 It was operated according to example 1, but using a 250 cm flask and charging 240 g of pure MDI. When the diisocyanate temperature reached, after about 15 minutes, the reaction temperature of 240 CC, 30 g (12.5 referred to MDI) of Molecular Sieves type 3 A, in the form of small balls, produced by Grace-Davison, were added; such catalyst had been previously subjected to a heat treatment at 300-400 CC in a dry nitrogen stream. Reaction was stopped after 6 hours by cooling. The infrared spectroscopy of the reacted product revealed peaks relating to carbodiimides, urethonimine, urethdione and residual isocyanate. After a 6-day ageing at 40-42 OC, no traces of carbodiimides were detected any longer on infrared analysis. On the basis of the infrared bands relating to the NCO groups present in the product, there were calculated 1.85% of urethdione and 21.25 of apparent NCO, corresponding to a conversion of the starting isocyanate' into, carbodiimide of 25.8. Example 3 It was operated according to example 2, but employing 24 g (10 < in respect of MDI) of Molecular Sieves type AW 500, in the form of 1/8" pellets, having a specific surface of 340 mê/g, produced by Carlo Erba/Union Carbide. Said molecular sieves had been previously activated by means of heating in a dry nitrogen stream at 300-400 OC. After a 3-hour reaction a sample was drawn and reaction was stopped by cooling. The sample, after 13 days ageing at 25 OC, appeared still liquid. The infrared analysis revealed the presence of urethonimines, of urethdione (0.9 < 0), of residual NCO and the absence of carbodiimides. By titration there were determined 28.6% of apparent NCO and 30.3 of total NCO, which a conversion into carbodiimide of ii% was corresponding to. Another sample after a 5-hour reaction and a 25 days ageing at 25 OC, exhibited a I.R. titer of apparent NCO equal to 23.3% and of urethdione equal to 1.3%, corresponding to a conversion of 21.8% into carbodiimide. Example 4 It was operated according to example 1. When the temperature of the diisocyanate reached, after about 15-30 minutes, a temperature of 240 OC, 60 g (10% referred to MDI) of catalyst were added. The catalyst was powdered alumina (alkali-free Awl203, produced by Carlo Erba) having a specific 2 surface of 140 m /g, previously activated by heating at 250-340 OC in a dry nitrogen stream. After 170 minutes, when a conversion of 29.2% was calculated on the basis of the evolved carbon dioxide, 1196 g of fresh MDI at 40-45 OC were added, whereupon the entire mass was cooled down to room temperature. The product so obtained was liquid at room temperature. Such product, separated from the catalyst simply by decantation and stored in a closed tank, remained liquid and clear for a long time without any gas evolution (meaning that carbodiimide formation was stopped). After ageing for a few days at room temperature, the infrared analysis of the product -showed that functional groups such as isocyanurates, ureas and urethanes were absent. The product so obtained was therefore a MDI, the isocyano groups thereof were partially converted into carbodiimide and successively into the urethonimino adduct, and therefore it was a modified MDI, liquid and stable at room tempera ture for a long time, which did not originate overpressures inside the tanks wherein it was stored, that being indicative of the fact that the carbodiimidation reaction was stopped. The sample, after a 17-day storing, revealed a refractive index nD of 1.6179, a viscosity R 25 of 70 cps and an infrared spectroscopic titer of apparent NCO equal to 28.9% and of urethdione equal to 0.6, which a conversion of 10% into carbodiimmide was corresponding to. After a three-month storing at 20-30 OC, a total NCO percentage of 30.3 was determined by titration. After a four-month storing the product was still liquid and perfectly limpid without any traces of precipitate. ExamPle 5 It was operated as in example 2, using as a catalyst 24 g (10at in respect of MDI) of Molecular Sieves type 5 A, in the form of 1/16" pellets, having a specific surface of 390 mê/g, produced by Carlo Erba/Union Carbide and previously activated by heating to 330-450 C in a dry nitrogen stream. After a 3-hour reaction, a conversion of 12.1, corresponding to 28.1 of apparent NCO, was calculated on the basis of the carbon dioxide evolved. The infrared spectrum was similar to the ones of the preceding examples. After separation of the catalyst, the product appeared liquid and limpid. After a three-month storing at 25-30 OC its appearance was unchanged. Example 6 Example 5 was -repeated, but the catalyst was activated by heating it to 160-220 OC in a nitrogen stream after a pretreatment at- 90-130 OC with highly moist nitrogen, obtained by bubbling it through water at 90 OC. Reaction was stopped by cooling after 21 minutes. A conversion of 16.2% and an apparent NCO content of 26.3% were calculated on the basis of the volume of carbon dioxide evolved. The infrared spectrum was similar to the ones of the preced ing examples. Example 7 It was operated as in example 4; the catalyst (60 g, 10 referred to MDI) was silica gel in granules, produced by Carlo Erba, having a specific surface of 800 gê/g and activated by heating at 250-340 OC in a dry nitrogen stream. On the basis of the gaseous carbon dioxide evolved it was possible to determine that a 105 conversion was attained after a'reaction time of 269 minutes. After a 277-minute reaction time it was cooled down, thus obtaining a product which was liquid and clear at room temperature. The catalyst was easily and rapidly removed simply by decantation. The infrared spectrum was similar to the ones of ,the preceding examples. After a 50-day storing at room temperature a refractive 25 index nD of 1.6162 was determined. After a 83-day storing at room temperature a viscosity '25 of 54 cps was determined. Example 8 Example 7 was repeated, but the catalyst was activated by heating to 140-180 O.C in a dry nitrogen stream... A 10% conversion, calculated on the basis of CO2 evolved, was attained after 162 minutes. Reaction was stopped after 165 minutes. The infrared spectrum was quite like the ones of the preceding examples. After a 20-day storing at room temperature, a total NCO content of 30.4do was determined. Example 9 Example 7 was repeated, but the catalyst was activated by heating first at 120-160 OC in an air stream made to bub ble it through water heated at 80 OC, and successively at 150-180 OC in a dry air stream. A 10% conversion, calculated on the basis of the evolved CO2, was reached in 30 minutes. The product was then diluted with 1138 g of fresh MDI heated at 40-45 OC, thus obtaining, after separation of the catalyst, a liquid, limpid and fluid product. An apparent NCO content of 28.9% was determined bn the basis of the carbon dioxide evolved. The infrared spectrum was similar to the ones of the preceding examples. The product, stored at room temperature, exhibited after 25 14 days a refractive index nD of 1.6177 and after 48 days a viscosity /k25 of 74 cps. After two months the product.was still liquid and perfectly limpid, without any precipitates. Example 10 Example 7 was repeated, but the catalyst was activated first by heating at 100-130 OC in an air stream, which was previously made to bubble through water heated at 65 OC, and then at 250-340 OC in a dry nitrogen stream. A 10% conversion, calculated on the basis of the evolved C02, was attained after 220 minutes. Reaction was stopped after 333 minutes, when the conversion had reached 18.5. The product was then diluted with 463 g of fresh MDI heated at 40-450C. The final product, after removal of the catalyst, was a fluid and limpid liquid. The infrared spectrum was similar to the ones of the preced ing examples. After a 9-day storing at room temperature the product exhibited a refractive index nD of 1.6165. After 42 days the product was liquid and still limpid, free from any precipitate and has a viscosity *25 of 56 cps. Example 11 It was operated as in example 2, but using as a catalyst, 24 g (10 in respect of MDI) of Molecular Sieves type 5 A, in the powder form, having a specific surface of 475 mê/g, produced by Carlo Erba/Union Carbide and previously activated by heating it at 330-450 C in a nitrogen stream. After a 82-minute reaction, 10 of conversion was attained. Reaction was stopped after 180 minutes, when the conversion, calculated on the basis of the evolved C02, was of 14.3%. The infrared spectrum was qualitatively similar with the ones of the preceding examples. Example 12 It was operated as in example 2, but using as a catalyst, 24 g of granular pumice stone produced by Carlo Erba (a complex silicate of aluminium, sodium and potassium), hav 2 ing a specific surface of 2 m /g and previously heated at 250-340 C in a dry nitrogen stream. After a 180-minute reaction, a conversion of 7.6, determin ed on the basis of the CO2 evolved, was attained. The infrared spectrum of the product was qualitatively similar to the ones of the preceding examples. Example 13 (comparative test) It was operated as in example 2, but using as a catalyst, 24 g (10% referred to MDI) of Alundum, a product of Nortons consisting for at least 90% of Awl 203 and for the remaining part of SiO2 with 0..1-0.24 ó of other impurities. The catalyst was in the form of macroporous balls of 5-7 mm 2 diameter, having a specific surface of less than 1 m /g; it had been previously activated by heating at 250-340 OC in a dry nitrogen stream. After a 76-minute reaction, a conversion of 1.3% was determined on the basis of the C02 evolved. By comparing the results with the ones of example 1 without catalyst, where after a 75-minute reaction a cpnuerSion of 1.85% was attained, one may infer that Alundum, compact alu mina aggregated to form a macroporous mass, has no catalytic activity. Example 14 It was operated as in example 1, but using as isocyanate, a mixture of isomers 2,4- and 2,6- (in the ratio 80/20) of toluene-diisocyanate. 60 g of catalyst (10% in respect of toluene-diisocyanate) were used, consisting of Molecular Sieves type 5A, in the form of 1/16" pellets, having a specific surface of 390 m2 /g produced by Carlo Erba/Union Carbide and previously activated by heating at 100-130 OC in an air stream - which had been bubbled through water heated at 65 OC and successively heated at 250-340 OC in a dry nitrogen stream A 10% conversion, calculated on the basis of the C02 evolved, was attained after a 27-minute reaction, a 20% conversion after 56 minutes and a 30d conversion after 8'5 minutes. Reaction was stopped after 150 minutes, when the conversion had reached 50g0. The product so obtained was diluted with an equal amount of fresh toluene diisocyanate. By titration a percentage of total NCO of 3 & 76, corresponding to a conversion of 24.7f, was determined. The infrared analysis revealed peaks typical of an isocyanate-carbodiimide-urethonimine system. Example 15 Example 14 was repeated, starting from pure MDI. A 10% conversion, calculated on the basis of the C02 evolved, was attained after 28 minutes of reaction, a 205 conversion after 58 minutes and a 305 conversion after 86 minutes. Reaction was stopped after 91 minutes when the conversion was of 31.8%. The product was then diluted with a double amount of fresh MDI. After removal of the catalyst, a liquid, limpid product was 25 obtained, which had a refractive index nD of 1.6180 and a viscosity /025 of 77 cps.

Claims: CLAIMS 1) A process for partially or thoroughly converting iso cyano groups, contained in organic compounds, into car bodiimido and/or urethonimino groups by heating organic compounds containing free isocyano functional groups at high temperatures in the presence of catalysts, cha racterized in that heating is effected at temperatures in the range of from 1400 to 300 OC in the presence of a solid, inorganic catalyst of acid nature, having a specific surface of at least 2 m2 /g, selected from the group consisting of silica, alumina and mixtures and derivatives thereof wherein said two oxides are united and/or mixed with each other and/or with alkaline or alkaline-earth oxides. 2) The process according to claim 1, in which polyisocya nates are used as organic compounds containing free iso cyano functional groups. 3) The process according to claim 2, in which 4,4'-diphe nylmethane diisocyanate either alone or in admixture with its isomers and 2,4-toluene diisocyanate either alone or in admixture with its isomer 2,6- are employed as polyisocyanates. 4) The process according to claim 1, in which the cata lyst consists of molecular sieves. 5) The process according to claim 1, in which heating is conducted in the presence of organic solvents not con taining groups reactive towards the isocyano groups. 6) Isocyanic compounds and polyisocyanates containing carbo diimido and/or urethonimino groups obtained according to the process as claimed in claims 1 and 2. 7) Carbodiimido compounds obtained by total conversion of the isocyano groups, contained in organic compounds, according to claim 1.

Inventors: FELICE, RIVA MARIO, DR. CHEM.
Applicants: MONTEDISON S.P.A.
Application year: 1981
Main classification: C07D229
IPC codes: C07D229