IE41439B1 - A biuret polyisocyanate mixture stabilised against monomer reversion - Google Patents

Abstract

1460685 Polyurethanes BAYER AG 29 July 1975 [1 Aug 1974] 31680/75 Heading C3R [Also in Division C2] Polyurethanes are obtained by the polyaddition reaction of at least one polyol and a polyisocyanate mixture stabilized against mono - mer reversion and based on tris-(isocyanatohexyl)-biuret and its higher homologues, containing from 1 to 36% by weight, based on the polyisocyanate mixture, of N-formyl urea groups of the formula

Description

Polyisocyanates containing biuret groups, especially those based on hexamethylene diisocyanate, have acquired universal commercial significance in the manufacture of light-stable and extremely weatherproof lacquers with high gloss retention. Polyisocyanates of this kind are commercially produced and marketed with as low a monomeric diisocyanate content as possible. Extensive toxicological investigations and long experience in the processing of these pi-oduets have shown that the upper limit of monomer (= hexamethylene diisocyanate) of 0.7 /, based on the solids content, which is maintained in polyisocyanates of this kind provides for safe processing of lacquers produced from them, providing the safety measures normally applied in the processing of lacquers are taken. The above-mentioned limit of 0.7 / is recorded in the literature, for example in u pamphlet entitled PURAnstriehstoffe issued by the ilauptverband der deutschen · gewerblichen Berufsgenossenschaft, and also in the Polyurethanes Report of the Faintmakers Association.

Fairly recent extensive investigations have shown that, in the aforementioned polymolecular polyisocyanate mixtures containing biuret groups, the aforementioned limit of 0.7 / of monomeric hexamethylene diisooyanate is exceeded during prolonged, especially uncontrolled, storage, for example during transportation by ship in southern elimes, etc,, as a result of wall catalysis in glass or metal containers attributable to as yet unknown catalytic effects and impurities which cannot be fully detected by,analysis, depending upon temperature, for example at temperatures in the range from 20 to 60°C, and that the monomer content can increase to more than 1 / over a period of 4 to 12 months. -l4 In the case oi low-viscosity biuret polyisocyanates having, viscosities in the range from 1000 to 4000 cP/20°C, which have been produced from a little biuretising agent and a lot of hexamethylene diisoeyanate (- molar ratio approximately 20 - 11 of hexamethylene diisocyanate and 1 mol of tert.-butanol oi' water or watc-tr-eliminating compounds), reversion rates of up to 1.5 by weight of monomeric hexamethylene diisocyanate are found after 2 to 3 weeks, especially at temperatures in the range from 40 to 50°C in undiluted form, but in many cases even at room temperature.

Since it is commercially possible to maintain the 0.7 £ monomer limit in the production of the above-mentioned polyisocyanates, and since products of this kind have already been safely processed for more than a decade, increasing stability and reducing monomer reversion are important requirements both from the technical and from the ecological point of view. It is also very important to reduce the viscosity of conventional biuret polyisocyanates based on aliphatic or cycloaliphatic diisocyanates, which is often in the range from 10,000 cP to 120,000 cP at 20°C. Such a reduction in viscosity would make it possible to produce solvent-free one-component and two-component polyurethane lacquers. In particular it is solvent-free biuret polyisocyanates stabilised against monomer reversion which are acquiring increasing interest especially in view of efforts to keep the environment free from pollution.

Accordingly, the object of the present invention is to stabilise biuret polyisoeyanates based on hexamethylene diisocyanate against monomer reversion and, at the same time, to reduce their viscosity so as to enable these polyisoeyanates to be more effectively diluted or to be applied without solvents. -2-41430 According to the invention, NCO-group-containing ureas and polyurens containing Nr-forrayl groups arc added to the polyisoeyanates containing biuret groups either in a separate operation or during the in situ production of biuret polyisoeyanates by using biuretising agents and, in addition, formic acid or compounds eliminating formic acid.

Accordingly, the invention relates to a biuret polyisoeyanate mixture based on tris-(isocyanatohexyl)-biuret and its higher homologues which is stabilised against monomer reversion and which is characterised by a content of organic compounds containing N-formyl urea groups II CH I -N-CO-NH, which corresponds to a concentration of from 1 to 36 ji by weight of these formyl urea groups in the polyisocyanate mixture.

The invention also relates to the use of this mixture as the isocyanate component in the production of polyurethane plastics by the isocyanate-polyaddition process.

The mixtures· according to the invention show considerable increased stability against monomer reversion during storage at room temperature compared with all standard commercial-grade biuret polyisoeyanates. Even at 5Q°C, the preferred mixtures according to the invention show increased stability in storage and their monomer content remains below the above-mentioned limit of 0.7 ¢. Even in the event of continuous heating for weeks at 80°C, the monomer content undergoes little change according to analysis by gas chromatography, thinlayer chromatography or gel chromatography, and does not exceed the above-mentioned critical limit (of. Example 2).

In addition, the mixtures according to the invention surprisingly show reduced viscosity levels in solvent-free form compared with standard commercial-grade biuret poly-3- , · isocyanates based on hexnmetliylene diisoeyannte. Accordingly, they are particularly suitable for solvent-free application in the production of lacquer coatings and other coatings.

The increased stability against monomer reversion found in the mixtures according to the invention is extremely surprising because it had been assumed that N-acylation with such strong acids as formic acid would lead to an increased tendency towards reversion of the N-formylated ureas through positivation of tho nitrogen. Surprisingly, however, not only are the N-formylated ureas and polyureas themselves very stable compounds without any appreciable tendency towards splitting of the urea groups, even at elevated temperatures of up to SO°C, hut also this surprising stabilising effect has been found to apply to biuret polyisocyanates known per se.

The N-formylated ureas and polyureas preferably used in accordance with the invention are reaction products of aliphatic and/or cycloaliphatic isocyanates, preferably hexamethylene diisocyanate, with formic acid or compounds which liberate formic acid at temperatures in the range from 80 to 190°C.

The reactants are preferably used in molar ratios which correspond to a molar ratio of diisocyanate to formic acid of 2:1 to 50:1, preferably 3:1 to l6;l and, with particular preference, from 5ί1 to 8:1. The N-formylated ureas can be produced largely by the process described in German Patent Specification 1, 174,760, although provision should be made to ensure that temperatures above 130°C are not exceeded during the primary reaction of formic acid with hexamethylene diisocyanate. As can he demonstrated for example (and in particular) by gel chromatography and by analytically checking the COg/CO-balances, the formation of N-formyl groups is governed to a very large extent by temperature. One very important factor in this respect is that preformed N-formyl -4 41439 groups in aliphatic and cycloaliphatic N-formyl urea diisocyanatos have a relatively high resistance to temperature and are relatively temperature-stable at temperatures of up to as high as 190°C, whereas aromatic urea diisocyanates containing X-formyl groups, for example with the formula give off CO to a much greater extent in the presence of excess quantities of polyisocyanates, even at temperatures as low as 100 to 150°C, and change into biuret polyisocyanates by way of the urea diisocyanate formed as intermediate. In order to obtain ureas and polyureas containing N-formyl groups, the formic acid should not be added to the hexamethylene diisocyancte at 150°C, in accordance with'Example 1 of German Patent Specifioation 1,174,760, because the water-removing effect of the isocyanate in accordance with UCOOn ___^.CO + HgO + NCO is too great at that temperature. If, by contrast, the mixtures according to the invention are prepared by a two-stage process described by way of example in the following, i.e. if temperatures in the range from 90 to 130°C are used for example in a first stage and temperatures in the range from 150 to 190°C used in a second stage, it is possible to convert in particular aliphatic, cycloaliphatic and araliphatic diisocyanetes with a variety of different constitutions into stable polyisocyanate mixtures containing N-formyl groups in relatively high yields. Accordingly, the outstanding role of a two-stage process was not recognised in German Patent -5- , Specification 1,174,760, and the teaching given in that Patent Specification is based mainly on the readily decomposable pnjyisoeyanates containing N-formyl groups together with aromatic urea and polyurea groups, which can be converted very easily into biuret polyisocyanates at temperatures as low as 90 to 120°C.

It has been found that urea diisocyanates of hexamethylene diisooyanate containing N-formyl groups are formed from formic ucid or compounds liberating formic acid at temperatures of, preferably, from 80 to 130aC, the molecular weights of the polyisocyanates obtained being governed by the rate of addition, by the molar ratio of hexamethylene diisocyanate to formic acid or compounds liberating formic acid and by the temperature used. The preferred temperature is in the range from 90 to 130°C. The question of whether this reaction at SO to 1jO°C, preferably at 90 to 150°C, should be followed by heating at 150 to 190°C, is governed primarily by the type of biuretising agent used in addition to the formic acid.

Heating at temperatures in this range is advisable in cases where tertiary alcohols, primary amines and ureas are used as biuretising agents. Even in cases where formic acid is used, it is also advisable to heat the reaction mixture in two stages at temperatures in the range from 60 to 130eC.

Basically, the proportion of II C=0 and polymer-homologue Ii H (ii) ocn-(ch2)6' x -641439 Mg = 508, = 16.45 £ NCO (χ = 1) .increases with inereasing quantities of hexamethylene . diisoeyanate at the beginning of the reaction, followed by removal of the unreacted excess, for example by thin-layer distillation, and amounts to between for example, about 36 and 58 7 by weight, in the reaction of 8 rools of hexamethylene diisoeyanate with 1 mol of formic acid for (i), to between about 65 and 68 jS by weight in the reaction of 24 ffiols of hexamethylene diisoeyanate with 1 mol of formic acid for (I), to between about 78 and 79 % by weight in the reaction of 32 mols of hexamethylene diisoeyanate with 1 mol of formic acid for (I), and to about 89 <5 by weight in the reaction of 40 mols of hexamethylene diisoeyanate with 1 mol of formic acid for (i).

Xt can be seen from the COg- and CO-balances measured and from the distribution of the components as determined by gel chromatography that the reaction of hexamethylene diisocyanate with formic acid or compounds which liberate formic acid is accompanied not only by the reactions 0CN-(CH2)g-NC0 + HCOOH - 50°C H -741439 or OCN~(CHg)6-NCO + 2 11C001I - 50°C £) C-O-C-NHV (CH2>6 -NH-C-O0 - 60°C H V1I C-N-(CH2)6-NH-C under the water-removing effect of the excess hexamethylene diisocyanate, but also by the reactions o) IICOOH _y CO + HgO or 0 0 II d) IICOOH + HCOOH _>n-C-O-C-H + IigO 0 , 11 11 e) HC-O-C-II_y 2 CO + HgO Where formic acid is used as the reaction component, the quantity of CO determined, as shown by a comparative examination of the component distribution in the reaction products by gel chromatography, is a measure of the biuretisation taking place during the reaction, because water disappears completely for biuretisation in a parallel reaction.

Reactions c), d), e) oan be suppressed at temperatures in the range from -20 to 20°C. At temperatures above 20°C reactions c), d), £), which always contribute towards biuretisation, take place to an increased extent, at 100 to 150°C approximately 30 to AO ¢, based on reaction £), namely the formation of an N-formyl hexyl monoisocyanate, which represents the preliminary stage in the formation of -841439 ί) ocn-(ch2)6-n HC •NH-(CH2)6-NCO i.e. the first member in the homologous series of N-polyformyl polyurea diisocyanates corresponding to the formula ocn-(ch2)60 II CH f II CH -N-C-NH-(CH2)g-NCO 0 X (n = 0 - 10, preferably 0-6).

It is very remarkable that, as a rule, the mixtures prepared from hexamethylene diisocyanate and formic acid * are substantially free from dimeric hexamethylene diisocyanate (= uret diones) and polyuret diones, It is also remarkable that, when formic acid is added dropwise to hexamethylene diisocyanate, the quotient of C02/C0 = 1.882, i.e. amounts to approximately 1.9, both on completion of mixed anhydride formation and on completion of the N-formylation reaction at temperatures upwards of about 100 to 130°C. In fact, the quotient of COg/CO should be exactly 1 according to the teaching of German Patent Specification 1,174,760 for the temperature conditions disclosed. In addition to analysis by gel chromatography, the COg/CO-quotient found of 1.882 itself clearly indicates that the polyisocyanate mixtures according to the invention contain large proportions of N-formyi groups.

Accordingly, the reaction of hexamethylene diisocyanate with pure formic acid gives biuret? in addition to the N-foraylated ureas acting as stabilisers in accordance with the invention. Accordingly, the reaction of aliphatic, cycloN-C-NH-(CH2)6 •941439 aliphatic or araliphatic isocyanates with pure formic acid gives biurets in addition to the N-formylated ureas used as stabilisers in accordance with the invention. By removing the excess of unreacted starting isocyanate, for example by thinlayer distillation, it is thus possible to obtain mixtures containing from.1 to 36 9 by weight and preferably from 10 to # by weight of the following formylated urea groups P CH I -n-c-nhII These groups, which are of crucial significance to the invention, are present in the mixtures according to the invention, preferably in the form of'formylated di-(isoeyanatohexyl)-ureas or to ,υύ'-diisocyanatohexyl/and-polyhexamethylene polyureas, which may contain further isoeyanatohexyl groups, optionally through biuret branches. These mixtures, or their blends with conventional biuret polyisoeyanates based on hexamethylene diisocyanate, represent the mixtures according to the invention. As already mentioned, the mixtures may be added to conventional biuret isocyanates based on hexemethylene diisocyanate. The quantitative ratios are selected in such a way that the mixtures according to the invention contain from 1 to 36 % by weight and preferably from 1.6 to 18 % by weight of formylated urea groups II , CH ·' I -N-C-NHII .Λ The presence of the aforementioned N-formylated urea groups in the mixtures according to the invention alone is essential to the stabilising effect obtained in accordance with the invention. -10This meins in particular that the type of substituents on the N-formylated urea groups essential to the invention is of secondary importance. Thus in principle it would be entirely possible to stabilise biuret polyisoeyanates based on hexamethylene diisoeyanate using N-formylated ureas obtained ( in accordance with the principle described above by reacting any aliphatic, araliphatie or cycloaliphatic isocyanates with formic acid. In practice, however, the N-formylated ureas used as stabilisers will not be produced from monoisocyanates, because the N-formylated ureas thus obtained would not contain any more isocyanate groups and so it would not be possible for them to be incorporated in the polyurethane plastics ultimately obtained from the stabilised mixtures. N-formylated ureas based on the same diisoeyanate are preferably used in accordance with the invention for stabilising the biuret polyisoeyanates based on hexamethylene diisoeyanate.

Another possible method of preparing the stabilised mixtures according to the invention is a one-stage reaction in which the biuretising reaction known per so is carried out by reacting hexamethylene diisoeyanate with biuretising agents in the presence of formic acid, or compounds liberating formic acid. j In the context of the invention, biuretising agents are chemical compounds which react with organic isocyanates at elevated temperatures to form biurets. Monofunctional biuretising agents are compounds which? used in a quantity of one mole in a stoichiometric reaction, convert 3 mols of isocyanate groups into one biuret group. Typical examples of monofunctional biuretising agents are, for example, water or methyl amine.

For example, the monofunctional biuretising agent methyl amine, used in a quantity of one mol, reacts with 3 mols of monoisocyanate in accordance with the following equation to form the corresponding biuret: -11- ·' .

R ι J1H CO R-NCO + C1I--NHO-\ R-NU-CO-N-R + CH,-NCO A typical difunctionnl biurctising agent is, for example l,4-bis-(dimethyl hydroxy methyl)-benzene which, in a stoichiometric reaction, is sufficient, even in a half-molar concentration, for converting 3 isocyanate groups into one biuret group.

Preferred biuretising agents are water, water-liberating compounds, tertiary alcohols, more especially tert.-butanol, primary monoamines such as methyl amine, ethyl amine, propyl amine and the like. In this second embodiment for producing the mixtures according to the invention, these biuretising agents may be used in admixture with formic acid or even in a form in which they are chemically combined with formic acid. Examples of combined formylating and biuretising agents of this kind are, for example, salts of formic acid with the aforementioned primary amines or even oxalic acid dihydrate because it has been found that at 100 to 120°C polyisocyanates decompose oxalic acid to a large extent to form carbon dioxide and formic acid and, to a far lesser extent, to form water, carbon dioxide and carbon monoxide. It is of course also possible to use salts of oxalic acid with the aforementioned primary amines as combined formylating and biuretising agents.

In the second method of obtaining the stabilised mixtures according to the invention, hexamethylene diisocyanate is reacted with the combination of biuretising agent and (potential) formic acid, from 3 to 50 and preferably from 4 to IS mols of diisocyanate being used per mol of (potential) formic acid + biuretising agent (these figures are based on the total number of mols of biuretising agent + mols of formic acid). The molar ratio between (potential) -1241439 formic acid and biuretising agent is preferably from 14:1 to 1:14 and, with particular preference, from 2:1 to 1:2. All quantitative references to the biuretising agent are based on monofunctional biuretising agent.

In the second method for producing the claimed stabilised mixtures, the hexamethylene diisocyanate is preferably introduced first, followed by the gradual dropwise addition of the mixture of formic acid and biuretising agent at SO to 150°C, preferably at 90 to 130°C. On completion of the addition, the reaction mixture is preferably heated for 1 to 3 hours at 130 to 190°C, depending upon the biuretising agent. The unreacted hexamethylene diisocyanate is then removed, preferably by thin-layer distillation.

The mixtures according to the invention containing from 1 to 36 r> by weight, more especially from 1.6 to 18 $ by weight of the following N-formylated ureas (I CH I -N-C-NHII are directly formed in this way in a single operation.

The biuretising agents may of course be' added together 20 with formic acid in solution in inert organic solvents, for example during the preparation of the polyisocyanate mixtures according to the invention. It has been found that polar solvents, such as acetonitrile, propionitrile, dioxan, tetrahydrofuran, 1,3-dioxolane, and also acetone, methyl ethyl ketone, butyl acetate, are particularly suitable for this purpose.

In order to increase their NCO-functionality for example, the polyisocyanate mixtures according to the invention containing N-formyl groups may he modified in situ by using triols such as -1341439 glycerol or trimetbylol propane, or succinic acid or adipic acid in proportions of from 0.1 to 0.5 mols per mol of formic acid.

At least trifunctional to tetrafunctional polyisocyanates are proportionately formed in this way.

It way also be of advantiige to carry out all the abovementioned reactions in closed vessels under the natural pressure of the gases given off, namely COg and CO i.e. at 2 to 60 fitms, in order to avoid evaporation losses of hexamethylene diisooyanate and of formic acid and water and, hence, the formation of substantially insoluble polyureas in the discharge pipes of the apparatus used.

Another interesting variation of the process is to carry out the formic acid N-formylation reaction at about 100 to 130°C, in which case formic acid is drawn into hexamethylene diisocyanate in vapour form under reduced pressure, preferably under 200 to 20 Torr, One distinguishing feature of mixtures according to the invention prepared in this way is the fact that they ore almost crystal clear. Given a molar ratio of 10 mols of hexamethylene diisocyanate to 1 mol of gaseous formic acid, the diisocyanate corresponding to the formula II OCN- (CII2 ) g-N-C-NH- ( CHg ) g-NCO Cx W non be enriched by about 42 / by weight according to analysis by gel chromatography.

It can be of advantage to use catalysts which accelerate the biuretising reaction, such as JIC1, phosgene or catalysts according to the disclosure of German Offenlegungsschrift 1,951,055, especially in cases where formic acid and tertiary alcohols are used as the formylating and biuretising agents, i.veu in cases where the polyisocyanate mixtures according to -1441439 the invention are prepared in accordance with the teaching of this German Offenlegungsschrift, taking into account the ratios of the formic acid or formic acid donors used which are crucial factors in the process according to the invention, the improved volume-time yields of the process of German Offenlegungsschrift 1,951,055 are obtained without any change in viscosity, which is greatly reduced by comparison with conventional biuret polyisoeyanates and also the polyisocyanate mixtures containing N-formyl groups are additionally stabilised against monomer reversion.

Apart from the type of formylating and biuretising agents used in the preparation of the stabilised polyisocyanate mixtures according to the invention, the mixtures are distinguished from conventional biuret polyisoeyanates by two major odvantages: 1. The tendency towards monomer reversion is greatly reduced. In general, the free hexamethylene diisocyanate content of the freshly prepared mixtures according to the invention following removal of the excess hexamethylene diisocyanate by thin-layer distillation or even by extraction, for example with n-hexane, amounts at most to 0.5 % by weight. The critical limit of 0.7 % by weight of free hexamethylene diisocyanate is not exceeded, even after storage for 6 months at 25 to 35°C, whereas similar, freshly prepared conventional biuret polyisoeyanates .generally have a free hexamethylene diisocyanate content of 1.2 to 1.5 £ by weight after only 5 weeks' storage nt 25°C. 2. The mixtures according to the invention have a greatly reduced viscosity by comparison with corresponding , conventional biuret polyisoeyanates. In general, the mixtures according to the invention have a viscosity in the range from 300 to 6000 cP at 20°C-1541439 By virtue of their low viscosity, the mixtures according to the invention are eminently suitable for use os isocyanate components, especially in the production of solvent-free two-component polyurethane lacquers. Basically, however, the mixtures according to the invention are suitable for use as reactive diluents for conventional polyisoeyanates. Thus, the viscosity of the often highly viscous polyisoeyanates commonly encountered in polyurethane chemistry may be considerably reduced by adding the mixtures according to the invention. On the other hand, it would of course also be possible to apply the formylating reaction described here to other aliphatic, cycloaliphatic or araliphatie polyisoeyanates in order to obtain modified polyisoeyanates having greatly reduced viscosity in exactly the same way as described herein with reference to the example of hexamethylene diisoeyanate.

The mixtures according to the invention are not only valuable isocyanate components for solvent-free two-coinponcnt polyurethane lacquers, they can also be hardened to form surprisingly elastic, highly light stable lacquei's in the presence of catalysts such as, for example, the known organic zinc or tin salts with atmospheric moisture without any need for polyhydroxyl compounds or solvents to be added. -1641439 EXAMPLE 1 (Comparison) 100 parts by weight of hexamethylene diisocyanate (5.952 rnols) and 50 parts by weight of tert.-butyl alcohol (0.676 mol) are mixed at room temperature. The molar ratio corresponds to 8.8 mols of di isocyanate to 1 mol of tert.-butanol The temperature of the reaction mixture is increased to l6o°C over a period of about 30 minutes. A vigorous evolution of carbon dioxide and isobutylene begins at that temperature. The temperature is then slowly increased to 185°C over a period of another 30 minutes. After a total time of about 3 hours at 185°C, the evolution of CO,, and isobutylene gas has ceased, indicating the end of the reaction. The reaction product is then freed from monomeric hexamethylene diisocyanate in a thin-layer evaporator under a pressure of 0.2 Torr and at a temperature of 175°C. Approximately 284 parts by weight of a viscous biuret polyisocyanate are obtained. NCO-content: 21.3 # by weight.

The product has a viscosity of 5200 cP at 20°C.

According to analysis by gel chromatography and separation into individual fractions using tetrahydrofuran as eluent, the biuret polyisocyanate thus prepared shows the following monomer content and the following molecular composition: Hexamethylene diisocyanate: Dimeric hexamethylene diisocyanate, molecular weight 336: approximately Triisooyanatohexyl biuret, molecular weight 478: approximately Polyisocyanato poly biuret, molecular weight 789: approximately Polyisocyanato poly biuret, molecular weight 1099: approximately Polyisocyanato poly biuret, molecular weight approximately 1410: approximately 0.5 % by weight 2.5 % by weight 37-9 % by weight 23.9 % by weight 11.9 f by weight 6.8 % by weight ίο Polyisocyanato poly biurets and polyisocyanates of relatively high molecular weight, i.e.> 1600: approximately 16.6 by weight When this biuret polyisocyanate mixture is tempered for 4 weeks at 50°C, the monomer content of hexamethylene diisocyanate increases from 0.5 # to 1.1 i by weight. This and all the following analytical values were determined by thin-layer chromatography, gas chromatography or gel chromatography. Accordingly, the established limit of 0.7 of hexamethylene diisocyanate was exceeded byA+ = 0.4 i.

EXAMPLE 2 1.5 3u Molar ratio of hexamethylene diisocyanate to formic acid = approximately 8.1: 2688 g (16 mols) of hexamethylene diisocyanate are first heated under nitrogen to 95 - 98°C. Thereafter 92 g (approximately 2 mols) of 98 i formic acid are added dropwise with thorough stirring over a period of 2.5 hours during which time the flow of nitrogen is shut off. The evolution of COg and CO volume ratio of approximately 3:1,(subsequently approximately 2:1) begins immediately. After the formio acid has been added, a released gas volume of 54.4 litres is found. After stirring for 1 hour at 100°C, the quantity of gas released amounts to 64.4 litres of COg + CO. Average composition: approx. 2 parts by volume of COg + 0.8 parts by volume of CO.

The reaction mixture is then stirred for 2 hours at an internal temperature of 130°C. The residual evolution of gas is minimal and only amounts to about 3 litres. Total of C02 + CO = 67.7 litres.

£ NCO- of the solution: 42.0 Visocosity 6,2 oP Although the solution is completely clear at room temperature, it is filtered before thin-layering in order to remove dust particles, etc.

Thin-layering of the crude solution was carried out at.. 175°C/appr,ox. 0,1 Torr.

Yield: 710 g (based on formic acid used = substantially 100 $ of the theoretical) End product; 23.1 $ ^θ'^Ο'Ό “ 827 cP Residual monomer content: approx. 0.3 $ by weight of hexamethylene diisocyanate.

Continuous gas-analytical determination of the carbon monoxide and carbon dioxide ratio shows that only about 15 litres of CO are released out of the theoretical C0content of 44.4 litres for pure biuretising reactions. The degree of biuretisation by the formic acid used, essentially due to HCOOH _X.C0 + HgO = biuretising agent only amounts to approximately 34 ¢, i.e. approximately 66 JS by weight of the formic acid used for the reaction is combined in the form of N-formyl groups. The degree of formylation determined from gas analyses is fairly consistent with that determined the much more accurate separation of the individual components of the reaction mixture by gel chromatography.

The biuret polyisocyanate mixture containing'N-formyl urea and N-polyformyl urea polyisocyanate shows the following component distribution in a gel chromatogram: Component distribution Hexamethylene diisocyanate: 0.4 $ by weight N-formyl diisocyanatohexyl urea, molecular weight 338: 36 $ by weight Triisocyanatohexyl biuret, molecular weight 478: 34,5 $ by weight -19 41439 Polyi'ormyl-polyurea di isocyanates, molecular weight 678: Polyisocyanatopoly biuret, molecular weight 796: Polyformyl-polyurea polyisocyanates, molecular weight 1018: Polyisocyanatopoly biurets, molecular weight 1114: 11.8 $ by weight 8.8 J® by weight 3.6 Ji by weight 2.5 'a by weight Higher molecular weight polyi soeyanates of polyformyl-polyurea and poly biuret structure, molecular weight > 1600: 2.9 $ by weight Total -N(ClIO)-CO-NH-content: approximately 15-1 i® by weight Whereas, in Example 1, the monomer content of the product increased to 1.3 $ oi hexamethylene diisoeyanate after storage for 4 weeks at 50°C, so that the established monomer limit of 0.7 # by weight of hexamethylene diisoeyanate was exceeded by A = + 0.4 # by weight and the absolute hexamethylene diisoeyanate reversion amounted to + 0.6 ¢, tempering of the mixture according to the invention under exactly the same conditions shows that, after storage for 4 weeks at 50°C, the original value of hexamethylene diisoeyanate has risen from 0.4 % to only 0.5 J®. Accordingly, the polyisocyanate mixture according to the invention shows remarkable stability in storage and remains below the aforementioned monomer limit of 0.7 J® by Δ = - 0.2 ¢, even after storage for 1 month at 50°C. Despite tempering, the viscosity of the mixture remains substantially at the aforementioned remarkably low value of approximately 850 cP at 50°C. The 0.7 % monomer limit is not exceeded even when the polyisoeyanate mixture according to the invention is stored for 3 weeks.

EXAMPLE 5 The following Example demonstrates the excellent reproducibility of Example 2, the after-reaction at 130°C being shortened by half an hour in relation to Example 2.

A polyisocyanate mixture having an NCO-value of 22.9 and a viscosity of 867 cP at 20°C is obtained after thin-layering.

Analysis by gel chromatography shows the following component distribution: Hexamethylene diisocyanate: 0.4 % by weight N-formyl diisocyanatohexyl urea, molecular weight 338: approximately 35.8 % by weight Triisocyanatohexyl biuret, molecular weight 478: 35.5 % by weight Polyformyl-polyurea diisocyanates, molecular weight approx. 678 . approximately 11.9 $ by weight Polyisocyanatopoly biuret, possibly containing some N-formyl groups, molecular weight 796: Polyformyl-polyurea polyi socyanates, molecular weight 1018: Polyisocyanatopoly biurets molecular weight 1114: approximately 8.1 JS by weight approximately 3.5 % by weight approximately 2.1 % by weight Higher molecular weight polyisocyanates of polyformyl urea and poly biuret structure, molecular weight > 1600: approximately 2.7 % by weight -N(CH0)-C0-NH-: . approximately 14.8 % by weight Whereas, in Example 1, the monomer content of the product increased from its original level of 0.5 % by weight to 1.1 ji by weight of monomeric hoxamethylene .diisooyanate after storage for 4 weeks, so that the established monomer limit of 0.7 % by weight of hexamethylene diisooyanate was exceeded by 4 = ψ 0.4 % by weight, and the absolute hexamethylene diisocyanate reversion amounted to Δ = * 0.6 ¢, tempering of the mixture according to the invention carried out in exactly the same way at 50°C shows that the original level of monomeric hexamethylene diisooyanate of 0,4 Ji by weight has risen to only 0.5 $ by weight, in other words the «d + increase found is withift the limits of error, Accordingly, - 21 41439 JO the mixture according to the invention shows remarkable stability in storage and remains below the aforementioned monomer limit of 0.7 % by weight of hexane i by 1 eno diisocyanate by A- 0.2 % by weight, even after storage for 1 month at -50°G, The monomer content only increases to 0.6 / by weight after storage for 3 weeks at 80°C and only amounts to 0.7 9 by weight, of monomeric hexamethylene diisocyanate after storage for 4 weeks at 80°C.

EXAMPLE 4 100 parts by weight of a 100 % biuret polyisoeyanate having the gel-chromatography component distribution specified in Example 1, are mixed with 100 parts by weight of the mixture according to the invention as described in Example 3. The effect, of mixing is that the original viscosity of the 100 '!» commercial-grade product of approximately 5200 cP at 2(kG, fulls to i3Oft cP, Accordingly, the viscositylowering effect of the mixture according to the invention upon highly viscous biuret polyisoeyanates is exremely intense. The mixture has a monomer content of 0.4 % by weight of hexamethylene diisocyanate. The distribution of components in the l:l-inixture calculated from the component distributions of Examples 1 and 3 now amounts to: Hexamethylene diisocyanate: Dimeric hexamethylene. dii seeynuat.e, molecular 0.4 % by weight weight 356: approximately 1.2 % by weight Tri isoeyanatohexyl biuret, molecular weight 476:· approximately 57.2 % by weight, X-formyldi isoeyanatohexyl urea, molecular weight 536approximately 19.5 't> by weight l’olyformyl-polyurea dii sneyanutes molecularweight 678: approximately 5.9 % by weight l’olyi soeyanatopoly biurets, molecular weight, 796: approximately 34.2 # by weight Polyisocyanatopoly biurets, molecular weight 1099: approximately 7.1% 1 by weight Polyisocyanatopoly biurets, molecular weight 1410: approximately 4.7 % by weight 5 Polyformyl urea polyisocyanates, molecular weight 1018: approximately 1.7 % by weight 1() Higher molecular weight polyisocyanates with polyformyl urea and poly biuret structures, molecular weight 1500: approximately 1.4 % by weight 15 High molecular weight polyisocyanatopoly biurets, molecular weight 1500: approximately 7.6 {S by weight -n(cho)-co-kii-: approximately 7.4 % by weight Whereas the biuret polyisocyanate mixture used for preparing the mixture according to the invention undergoes an increase in its monomeric hexamethylene diisocyanate content from 0.4 % by weight of 1.2 % by weight after storage for 4 weeks at 50°C, so that the established monomer limit of 0.7 % by weight is exceeded by 21+ 0.5 % and the absolute hexamethylene diisocyanate reversion amounted to Δ = + 0.8 % by weight, storage of the mixture according to the invention, i.e. the mixture of this example, for 4 weeks at 50°C causes an increase in the monomer content to only 0.7 % by weight of monomeric hexamethylene diisocyanate, i.e. within the aforementioned limit of 0.7 % by weight of hexamethylene diisocyanate, 30 EXAMPLE 5 The mixtures according to the invention are prepared having a similar component distribution to Example 4, the difference being that the individual components are not mixed but instead biuretisation and formation of the urea and polyurea polyisocyanates containing N-formyl groups are carried out in situ. i.er in one operation, as follows: a) 74 parts by weight (l mol) of tert.-butanol and 46 parts - 23 41439 by weight (i mol) of approximately 98 / formic acid arc heated with 2668 ports by weight of hexamethylene diisocyunate (]6 mols) first at 98°C, subsequently for 2 hours at 14O°C and then for 2 hours at 185OC, and the '5 polyisocyanate mixture purified in a thin-layer evaporator to a residual content of 0.4 / of hexamethylene diisocyanate.

/ NCO of the polyisocyanate mixture: 22.8 / ]j) Λ mixture of 46 g of anhydrous formic acid (l mol) and JO 18 parts by weight of water (l mol) is reacted at 97aC with 2688 parts by weight of hexamethylene di isocyanate (lb mols), the reaction mixture kept at 150°C for 5 hours ;md subsequently freed from monomeric hexamethylene diisocyanate in a thin-layer evaporator. Residual monomeric iiexemethylene di isocyanate content: 0.5 / by weight.

/ NCO of tiie polyisocyanate mixture: 23.1 /. c) 75.6 parts by weight of 86 / aqueous formic acid, containing 65 parts by weight of formic acid (1.41 mols) and .6 parts by weight of water (0.59 mol) and 2688 parts by weight of hexamethylene diisocyanate (16 mols) are reacted as described in b) and freed from excess hexamethylene di isocyanate in a thin-layer evaporator.

Residual hexamethylene diisocyanate content: 0.4 / by weight.

/ NCO-eontent of the polyisocyanate mixture: 25.4 / ti) 05 parts by weight of ammonium formate (l mol) and 2688 parts by weight of hexamethylene diisooyanate (lb mols) are reacted and purified as described in b).

Residua] hexamethylene diisocyanate content: 0.5 / by wei gilt / NCO-eontent of the polyisocyanate mixture: 21.8 / - 24 41439 r) 172 ports by weight of N,N*-bis-formyl hexaniethylene diamine and 672 parts by weight (4 mols) of hexamethylcne diisocyanate are reacted and purified as in b)., Residual hexamethylene diisocyannte content: 0.4 i by weight i NCO-content of the polyformyl polyurea polyisocyanate: 21.4 $.

The resulting polyisocyanate mixtures containing N-formyl groups all show increased stability after storage for 4 weeks at 50°C compared to Example 1, and have the following monomer contents of hexamethylene diisocyanate which1are greatly reduced compared to those of Example 1, as determined analytically by thin-layer chromatography: a) 0.6 i b) 0.7 i £) 0.6 i d) 0.6 i e) 0.5 i According to gel chromatography, all the polyisocyanates a) to e) have biuret polyisoeyanate contents of from about ,29 to 34 i by weight. The -N(CHO)-CO-NH-content is between 7 and 9 1° by weight in every case.

EXAMPLE 6 A polyisocyanate mixture containing biuret groups, prepared from 1 mol of water and 16 mols of hexamethylene diisocyanate and purified by thin-layer distillation, has a viscosity of 5049, an NCO-content of 23.5 and a residual hexamethylene diisocyanate content of 0.4 i by weight.

According to a gel chromatogram, it has the following component distribution: Eexamethylene diisocyanate: o.4 i by weight Dimeric hexamethylene diisocyanate (uret dione) molecular weight 336: approximately 4.5 $ by weight - 25 41439 Tri isocyanatolicxyl biuret, molecular weigh! 478: approximately Polyi sopyannto biuret, molecular weight 78'): approximately Polvi socyanalopoly biuret, molecular weight 1099: approximately Polvisocyanalopoly biuret, molecular weight 1410: approximately .1.5 λ by weight 17.4 7 by weight 9.9 Jo by weight 7.0 $ by weight lu Polyisocyanatopoly biuret, molecular weight 1720: approximately Higher molecular weight poiyi socyanatopoly biurets with possibly uret dione and isoeyanurate groups, molecular weight >3700: approximately 3.0 $ by weight 7.0 $ by weight This biuret polyisocyanate mixture has a monomeric hexamctliylenc di isocyanate content of 0.9 ΐ by weight after storage for only 4 weeks at 50uC, and hence exceeds the established monomeric hexamethylene diisocyanate limit of 0.7 by weight by + 0.2 ;ί by weight. However, when 100 parts by weight of this poiyisocyanate mixture are mixed with 100 parts by weight of the poiyisocyanate mixture according to the invention containing N-formyl groups described in Example 2, the mixture obtained remains below the 0.7 # limit after storage for 4 weeks nt 50°C, and according to a thin-layer chromatogram has a hex'amethylene diisocyanate content of 0.7 $· EXAMPLE 7 Particularly interesting polyisocyanates containing K-formyl croups which greatly reduce viscosity in accordance wiili the invention are obtained by the procedure of Example 2, except 1h,-it 20 mols of hexamethylene diisocyanate are reacted 1/)1)1 1 mol of formic acid. Purification is also carried out as in Example 2. Residual monomer content: 0.4 $ by weight of hexamethylene diisocyanate. The poiyisocyanate mixture according to the invention surprisingly has a viscosity of - 41439 only about 350 cP at. 20°C. chromatography, it consists Component distribution Hexamethylene diisoeyanate: N-f ormyldiisocyanatoliex-yl urea, molecular weight. 358: Triisocyanatohexyl biuret, molecular weight 478: Polyformyl polyurea diisocyanates, molecular weight 678: According to analysi s by gel of the following components: approximately approximately approximately approximately 0.4 '4 by weight 50.8 % by weight 38.5 # by weight 4.5 £ by weight Polyformyl-polyurea-polyisoeyanates + polyisocyanatopoly biurets, molecular weight 1114: approximately Higher molecular weight polyisoeyanates of polyformyl polyurea and poly biuret structure, molecular weight > 1300: approximately -N (OHO )-C0-NII-: approximately 3.9 7 by weight % by weight 12.8 £ by weight This low-viscosity polyisoeyanate mixture represents an outstanding, reactive viscosity-reducing diluent and a liquefier having reactive incorporable NCO-groups for a variety of different high-viscosity diisoeyanate polyaddition products.

The aforementioned, extremely low viscosity polyisoeyanate mixture according to the invention may also be used as a reactive thinner for all the various high-viscosity biuret polyisoeyanates mentioned hereinafter and for polyisoeyanates containing urethane groups of trimethylol propane or glycerol, in which case solvent-free polyisoeyanates of greatly reduced viscosity are obtained, for example in 1:1 admixture: ji) Λ biuret polyisoeyanate of hexamethylene diisoeyanate and water in a molar ratio of 6:1, obtained in accordance with German Patent Specification 1,101,394, has a viscosity of 10,500 cP at 20°C. When 100 parts by weight - 27 41439 of this polyisocyanate are mixed with 100 parts by weight of the mixture according to the invention of N-formyl urea and N-polyformyl urea polyisocyanates described in Example 7, a viscosity of only 2500 cP is found. _b) A polyurethane polyisocyanate mixture obtained from trimethylol propane and hexamethylene diisocyanate in a molar ratio of 1:8, and purified, has a viscosity of approximately 120,000 cP at 20°C after removal of the monomers by thin-layer distillation. When this polyisocyanate is mixed with the mixture according to the invention of Example 7, as in a), a reduced viscosity of about 12,500 cP at 20°C is found.

£) After thin-layering, a biuret polyisocyanate of 20 mols of 3,3,5-trimethylol-5-isocyanatomethyl cyclohexyl isocyanate (= isophoronediisocyanate) and 1 mol of tert.-butanol has a viscosity of 135,500, cP at 20°C. When this polyisocyanate is mixed as in a) with the mixture of Example 7 according to the invention, a viscosity of 12,800 cP at 20°C is obtained.

Viscosities reduced by about 1 power of 10 are found as in c) in biuret polyisocyanates of l-methyl-2,4dii socyanato-cyclohexane, 4,4'-dii socyanatodicyclohexyl methane, m-xylylene diisooyanate, whose viscosities are greater than 130,000 cP at 20°C in the absence of the reactive thinner according to the invention.

EXAMPLE 8 (Application Example) mg of a zinc salt of'2-ethyl caproic acid are added to 10 parts by wight of the low viscosity mixture a) of Example 7. The low viscosity polyisooyanate can readily be cast onto substrates of any kind. When applied to glass substrates, excellent levelling is obtained without any need - 28 41439 to add a solvent. The lacquer crosslinks over a period of 4H hours to form an elastic, light-stable film through urea groups formed by reaction with atmospheric moisture. EXAMPLE 9 When 200 parts by weight of the extremely low viscosity polyisocyanate mixture described in Example 7 containing 0.2 parts by weight of water (dissolved in 2 parts by weight of ethyl glycol acetate) are left to react, through urea bonding and biuretisation accompanied by the evolution of COg, the following gel chromatographic component distribution is found after about 3 weeks at room temperature, the initial viscosity of the product being increased from 350 cP to approximately 1723 cP. at 20°C. The polyisocyanate mixture obtained remains completely clear and there is no formation, even in traces, of insoluble polyureas. In a gel chromatogram, the end product shows the following composition and increase in molecular weight ranges: Component distribution Hexamethylene diisocyanate: approximately 0.5 ί» by weight 20 N-formyl diisocyanatohexyl urea, molecular weight 338: approximately 30.6 / by weight Triisocyanatohexyl biuret, molecular weight 478: approximat ely 15.5 $ by weight 25 Polyformyl polyurea diisocyanates, molecular weight 678: approximately 15.1 / by weight Polyformyl polyurea polyisocyanates, molecular weight 1016: approximately 7.7 / by weight 30 Polyformyl polyurea polyisocyanates + polyisocyanatopoly biurets, molecular weight 1114: approximately 5.4 / by weight 35 higher molecular weight polyisocyanates with polyformyl polyurea and poly biuret structures, molecular weight >1700: approximately 25.4 / by weight Comparison with tho component distribution of Example 7 shows that the spectrum of the high molecular weight polyformyl polyurea polyisocyanates and polyisocyanatopoly biurets is displaced by about 22.5 % by weight towards the higher molecular weight, ranges by the water reaction. EXAMPLE 10 This Example shows another advantageous modification of the process for preparing the polyisocyanate mixtures according to the invention, the polyisocyanate mixtures obtained in this case being almost crystal clear: 100B parts by weight of hexamethylene diisocyanate (6 mols) are heated to 60°C with continuous removal of the dissolved atmospheric oxygen by a stream ol' dry nitrogen. 25 parts by weight of anhydrous formic acid (0.5 mol) are then drawn in over a period of 2 'hours at 14 Torr from a small flask connected to iiio stirrer-equipped apparatus. The contents of the reaction apparatus are then stirred for 1 hour, during which all the carbon dioxide is released from the reaction mixture. Tho temperature is then increased to 85°C, approximately 6.5 litres of carbon monoxide being released in the second stage. The evolution of gas is completed by stirring at 120°C.

Monomeric diisocyanate is removed from the polyisocyanate mixture by repeated extraction of the reaction mixture with n-hexane (4 x extraction). Following removal ol' the extractant, a polyisocyanato mixture having an extremely reduced viscosity is obtained containing approximately 42 % by weight of the \'-formy] urea diisocyanate n Ii lie I ocn-(ch2 )6-x-g-xh-(cj;2 )6-nco o so 4143S and approximately 15 i by weight of -N-(C1IO)-CO-NH-.

Yield: 576 parts by weight; NCO-eontent: 24.8 Viscosity at 20°C; 750 cP.

EXAMPLE 11 It is interesting to use anhydrous oxalic acid or oxalic acid diliydrate both as biuretising and, at the same time, as a formylating agent. Thus, the reaction of 1 mol of oxalic acid with 52 mols of hexamethylene diisocyanate in accordance with Example 2, after purification, gives polyisocyanate mixtures containing N-formyl groups according to the invention with a viscosity of approximately 1500 cP at 20°C. Residual monomer content: 0.4 % by weight of hexamethylene diisocyanate. The polyisoeyanate mixture with a % NCO content of 22.8 % contains approximately 25 % by weight of H C=0 OCN-(CIig )6-N-C-NH- ( CI12 )6-nco an 11 $ by weight of -N(CH0)-C0-NH, EXAMPLE 12 To prepare the polyisoeyanate mixtures containing N-formyl groups according to the invention, it is also possible to use other formic acid donors, as shown herein after.

The procedure is as in Example 2 using 6 mols of hexamethylene diisocyanate and, as substances acting as biuretising and formylating agents, the following compounds each in a quantity of 0.5 mol in other words a molar ratio of 12:1 is used for formylation and biuretisation: a) 30.5 parts by weight of ammonium formate h) 38 parts by weight of methyl ammonium formate £) 22 parts by weight of Ν,Ν’-dimethyl urea and 11.5 parts - 31 41439 by weight of 98 i formic acid To begin with, the reaction is carried out at 120°C under nitrogen as in inert and propellent gas, after which the temperature is increased to 145°C and, in tests b) and c) methyi isocyanate is condensed in cold traps at -70°C/40 mm Hg.

The mixtures are then heated for another 1.5 hours to 150cC.

The solutions are then freed from monomeric hexamethylene diisocyanate at 0.15 Torr in a thin-layer evaporator.

Extremely low viscosity mixtures according to the invention having the following NCO-contents and the following viscosities at 20°C arc obtained: a) NCO-content: 23-5 ii 850 cP/20°C b) NCO-content: 22.4 ii 956 cP/20°C c) NCO-content: 22.9 ii 1080 cP/20°C 15 According to nnalysi s by j »el chromatography, all the polyisoeyanate mixtures have N-formyl urea diisocyanate contents of from 30 to 35 i by weight and an -N(CHO)-CO-NH-content of approximately 5.5 to 7 i by weight.

EXAMPLE 13 This Example unexpectedly demonstrates that polyisocyanates containing N-formyl groups according to the invention having only moderately increased viscosity levels can also be obtained by reacting hexamethylene diisocyanate and approximately 98 i formic acid in a molar ratio of 4:3 (=- a) to 3:1 ( = J)) by a two-stage process in accordance with Example 2, and purifying the resulting product: a) ;ί NCO = 22.8; viscosity 1200 cP/20°C b) i NCO = 18.4; viscosity 2100 eP/20°C The polyisoeyanate mixtures according to the invention contain approximately 15 ί by weight of monomeric diisocyanato-hexyl-N-formyl urea and only about 15 £ by weight of biuret polyisocyanates. The remainder consists of Npolyformyl polvurea polyisoeyanates. ~N(CHO)-C-NH-contenl.

II approximately 32 to 36 % by weight. For a residual monomer content of 0.4 $ by weight in a), an increase in the monomer content of only 0.2 $ is found'after storage for 4 weeks at 50cC. Accordingly, this Example shows that, even with a highly variable component distribution in the polyisoeyanate mixtures according to the invention, extremely effective stabilisation against monomer reversion is obtained in relation to the biuret polyisoeyanates of Example 1 and their tempering for 4 weeks at 50°C.

EXAMPLE 14 (Application Example) The procedure is exactly the same as in Example 5a), except that a mixture of 0.3 parts by weight of dimethyl ammonium chloride, 0.5 parts by weight of N,N-dimethyl hydrazinium chloride and 1.5 parts by weight of dimethyl carhamic acid chloride is used as catalyst and the reaction carried out at a temperature of 165°C.

Compared with the uncatalysed reaction, a greatly accelerated tert.-butyl urethane dissociation is found, a polyisoeyanate mixture containing N-formyl groups according to the invention of greatly reduced viscosity and high colour stability under heat again being obtained. When stoved with polyhydroxyl compounds, it gives substantially colourless lacquers of extremely high light stability and gloss retention ^20°C: cP; NCO-content: 26.15 ¢5 yield; 234.5 parts by weight.

Non-pigmented stoving lacquers of the polyisoeyanate mixture of this Example remain colourless for prolonged periods at temperatures of up to 200°C, whereas polyisocyanates produced in the absence of a catalyst show streaks of brown after stoving with polyhydroxyl compounds. - 33 41439 EXAMPLE 15 (Application Example) 100 parts by weight of a polyester of 3 mols of phthalic acid and 4 mols of trimethylol propane (10.1 % Oil) are processed into a paste with 100 parts by weight of a mixture of equal parts of toluene, ethyl acetate, butyl acetate and glycol monomethyl ether acetate and with 105 parts by weight of titanium dioxide (= rutile type). Another 179 parts by weight of the solvent mixture and 2 parts by weight of polyvinyl methyl ether are added t.o the resulting paste. The mixture of Example 2 according to the invention (106 parts by weight) is added as a crosslinker to the pigment-containing mixture without the addition of any further solvent is such a quantity that the polyestercontaining hydroxyl, groups is crosslinked in an NC0:0H ratio of 1:1. The mixtures have a sufficiently long pot life. When the lacquer solutions are applied to any substrates, extremely resistant, elastic lacquers are obtained after thorough drying. When 0.1 part by weight of a tin(ll)salt of 2-ethyl caproic acid or zinc (il)salts of 2-ethyl caproic acid are added to the lacquer mixtures, the lacquers obtained are hard, scratchproof and resistant to solvents, for example toluene, after only 24 hours. They do not show any signs of yellowing on exposure both to artificial and to natural light and exhibit outstanding gloss retention.

EXAMPLE 16 (Application Example) % by weight solutions of a polyester of phthalic acid and trimethylol propane having a hydroxyl group content of 8.5 % in ethyl glycol ether acetate and methyl ethyl ketone (l:l) are diluted with the polyisocyanate mixture according to the invention prepared in accordance with Example 3(22.9 % NCOeontent, 80 parts by weight) to form a two-component clear lacquer solution which levels very effectively without hazing. - 34 - 41439 · fi.()50 part by weight of zinc salt of 2-ethyl caproic acid is added as catalyst to this mixture. When the mixture is applied in thin layers to surfaces, hard, high-gloss nonyellowing clear lacquer films are formed over a period of 24 hours at room temperature.

EXAMPLE 17 (Application Example) a) 200 parts by weight of a hexane diol polycarbonate (0Hnumber 56) and 168 parts by weight of a polyester of adipic acid, neopentyl glycol and hexane diol and 1,4-butylene glycol (ratio of the diols 1:1:1) and 200 parts by weight of a polyester of adipic acid and 1,4-hutane diol having an OH-number of 54, are mixed without preraaturo crosslinking ut 75°C with 300 parts by weight of the low-viscosity pdlyisocyanate cont15 aining N-formyl groups described in Example 2 (NCOcontent 23.3 ?). Mixtures df NCO-prepolymers are · formed in the absence of any premature crosslinking reactions by reaction of the linear hydroxyl polyesters with the polyisocyanate mixtures according to the invention used in excess. h) Highly elastic coatings on any substrates can be produced from melts of these NCO-prepolyroer mixtures, crosslinking under the effect of atmospheric moisture. They are elastomeric in character and show outstanding light stability.

£) When e-caprolactain, methyl ethyl ketoxime, phenol are added to the aforementioned NCO-prepolymer mixtures in tlie absence of solvents, the corresponding isocyanate donors ere formed in a slow reaction. Elastic jo rubber-like coatings are obtained by applying the readily spreadable donors melting at 90°C and crosslinking them with molten trimethylol propane.

Claims (4)

1. A biuret polyisocyanate mixture, stabilised against monomer reversion, based on tris-(isocyanutohexyl)-biuret and its higher homologues, characterised by a content of organic 5 compounds with N-formyl urea groups II CII I -N-CO-MI- , which corresponds to a concentration of from 1 to 36 / by weight of these formyl urea groups in the polyisocyanate mixture. 10

2. The use of the polyisocyanate mixture claimed in Claim 1 as isocyanate component in the production of polyurethane plastics by the isocyanate polyaddition process.

3. Process for the preparation of a biuret polyisocyanate mixture stabilised against monomer reversion, substantially 15 as hereinbefore described and exemplified.

4. A biuret polyisocyanate mixture whenever prepared by a process claimed in claim 3·