EP0000232B1 - Process for the preparation of blocked polyisocyanates and blocked polyisocyanates obtainable therby - Google Patents

Description

Description and examples

The present invention relates to a process for the preparation of new compounds with a high content of latent isocyanate groups and the compounds obtainable by this process and their use.

The production of masked isocyanates, also called isocyanate releasers, is known and is described in Houben-Weyl, Methods of Organic Chemistry XIV / 2. Pp. 61-70. Tertiary alcohols, phenols, acetoacetates, malonic esters, acetylacetone, phthalimide, imidazole, hydrogen chloride, hydrogen cyanide and _E- caprolactam are known as blocking agents. In practice, the use of monophenols and e-caprolactam is preferred for this purpose.

These blocked isocyanates have the property of reacting like isocyanates at elevated temperature. The more acidic the H atom of the masking group is, the easier it is to split off. Such blocked isocyanates are described in DE-A-21 66 423. Terminally blocked isocyanates which additionally contain uretdione groups have also been described in DE-A-25 02 934.

The most significant disadvantage of using these blocking agents for a number of applications is the relatively high elimination temperature, which for most phenols is at least 190 ° C. for aliphatic polyisocyanates and approx. 30 ° C. lower for aromatic polyisocyanates. For this reason it has been proposed in the past to use thiophenols for which this temperature is much lower. The disadvantage of thiophenols, however, is their exceptionally unpleasant smell.

It has also been suggested to add 1-2 percent of a catalyst that could reduce the cleavage temperature down to 125-140 ° C. In this way, however, the service life of the uncured coating material is adversely affected. The splitting temperature of aliphatic e-caprolactam-blocked polyisocyanates is generally about 180 ° -190 ° C.

einsetzt, worin R gleiche oder verschiedene Substituenten aus der Gruppe Wasserstoff, Alkyl-, Cycloalkyl-, Aralkyl- und Arylrest sein können und die Umsetzung bei Temperaturen von 0-150°C, vorzugsweise 80-120 °C erfolgt.Surprisingly, it has now been found that it is quite possible to produce blocked polyisocyanates in a simple manner, which block at a temperature which is at least 20 ° -30 ° C lower than the isocyanates blocked with the conventional blocking agents, if the process for the preparation of blocked polyisocyanates is carried out by reacting polyisocyanates with N-containing cyclic compounds so that cyclic amidines of the general formula are used as N-containing cyclic compounds

uses, where R can be the same or different substituents from the group hydrogen, alkyl, cycloalkyl, aralkyl and aryl radical and the reaction takes place at temperatures of 0-150 ° C, preferably 80-120 ° C.

The polyisocyanates and the cyclic amidines are used in amounts such that there is 0.5-1.1, preferably 0.8-1.0 mol of cyclic amidine per isocyanate group.

The reaction can be carried out both in solvents, in the melt and in polyisocyanate presented in excess.

Suitable starting compounds which can be used for blocking with the cyclic amidines are, for example, polyisocyanates, in particular diisocyanates, such as aliphatic, cycloaliphatic, araliphatic, aryl-substituted aliphatic and / or aromatic diisocyanates, as described, for example, in Houben-Weyl, Methods of Organic Chemistry, Volume 14/2, pp. 61-70 and the article by W. Siefken in Justus Liebigs Annalen der Chemie 562, 75-136, describe how 1,2-ethyne diisocyanate, 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4- or 2,4,4-trimethyl-1,6-hexamethylene diisocyanate (TMDI), 1,12-dodecane diisocyanate, ω, ω'-diisocyanatodipropyl ether, cyclobutane-1,3-diisocyanate, cyclohexane-1, 3- and 1,4-diisocyanate, 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexyl isocyanate, which is also called isophorone diisocyanate and is abbreviated to IPDI, decahydro-8-methyl- (1,4-methano-naphthalene -2 (or 3) 5-ylenedimethylene diisocyanate, deca- hydro-4,7-methano-i ndan-1 (or 2) 5 (or 6) -ylenedimethylene diisocyanate, hexahydro-4-7-methaneindan-1- (or 2) 5 (or 6) -ylene-diisocyanate, hexahydro-1,3- or .- 1,4-phenylene diisocyanate, 2,4- and 2,6-hexahydro-toluenediisocyanate, perhydro-2,4'- and / or -4,4'-diphenylmethane diisocyanate, ω, ω'- Diisocyanato-1,4-diethylbenzene, 1,4-phenylene diisocyanate, 4,4'-diisocyanato-diphenyi, 4,4'-diisocyanato-3,3'-dichloro-diphenyl, 4,4'-diisocyanato-3,3 ' -dimethoxy-dipheny !, 4,4'-diisocyanato-3,3'-dimethyl-diphenyl, 4,4'-diisocyanato-3,3'-diphenyl-diphenyl, 4,4'-diisocyanato-diphenylmethane, naphthylene-1 , 5-diisocyanate, tolylene diisocyanates, tolylene-2,4- or 2,6-diisocyanate, N, N '- (4,4'-dimethyl-3,3'-diisocyanatodiphenyl) uretdione, m-xylylene diisocyanate, but also the triisocyanates such as 2,4,4'-triisocyanatodiphenyl ether, 4,4 ', 4 "triisocyanatotriphenyimethane, tris (4-isocyanatophenyl) thiophosphate, and any mixtures of these isomers. More ge Suitable isocyanates are described in the article mentioned in the annals on page 122 f. described.

In general, particularly preferred are the technically easily accessible aliphatic, cycloaliphatic or aromatic diisocyanates and especially 3-isocyanatomethyl-3,5,5-trimethyl-cyclohexyl isocyanate and 2,4-tolylene diisocyanate and their isomeric mixtures.

In addition to the monomeric polyisocyanates, the dimeric and trimeric forms of the polyisocyanates, such as uretdiones and isocyanurates, which can be prepared by known methods, can of course also be used as starting materials for blocking with the imidazolines described in detail below.

For the purposes of the present invention, polyisocyanates are also understood to mean those which, prior to blocking with the imidazolines, have been subjected to a reaction for molecular enlargement with the so-called chain extenders customary in isocyanate chemistry, such as water, polyols, polyamines and others, the bifunctional or trifunctional chain extender , that is to say compounds having groups which are reactive toward isocyanate groups, such as hydroxyl and / or amino groups, are used in such amounts that the resulting new isocyanate bears at least 2 isocyanate groups on average. When water is used as a chain extender, polyisocyanates with one or more urea groups result.

Suitable polyols include diols and triols, e.g. Ethylene glycol, propylene glycols such as 1,2- and 1,3-propanediol, 2,2-dimethylpropanediol- (1,3), butanediols such as butanediol- (1,4), hexanediols e.g. Hexanediol- (1,6), 2,2,4-trimethylhexanediol- (1,6), 2,4,4-trimethylhexanediol- (1,6), heptanediol- (1,7), octadecene-9,10- diol- (1.12), thiodiglycol, octadecanediol- (1.18), 2,4-dimethyl-2-propyl-heptanediol- (1.3) butene- or butynediol- (1.4), diethylene glycol, triethylene glycol, trans- and cis-1,4-cyclohexanedimethanol, 1,4-cyclohexanediols, glycerol, hexanetriol- (1,2,6), 1,1,1-trimethylolpropane, 1,1,1-trimethylolethane and others Mixtures of the aforementioned compounds can also be used.

Of the polyamines suitable for chain extension or molecular enlargement, for example, ethylenediamine 1,2, propylenediamine-1,2 and -1,3, butylenediamine-1,2, -1,3 and -1,4 as well as hexamethylenediamines should be mentioned .

The reaction of the polyisocyanates before blocking with the chain extenders mentioned in the stated proportions can be carried out at temperatures in the range from 0-150 ° C., preferably 80-120 ° C.

Suitable imidazoline derivatives for the purposes of the present invention, which correspond to the general formula described earlier, are, for example, those with optionally aryl-substituted alkyl radicals, with optionally alkyl-substituted aryl radicals, such as 2-methylimidazoline, 2,4-dimethylimidazoline, 2-methyl-4- (n -butyl) -imidazoline, 2-ethylimidazoline, 2-ethyl-4-methylimidazoline, 2-benzylimidazoline, 2-phenylimidazoline, 2-phenyl-4- (N-morpholinylmethyl) imidazoline, 2- (o -Tolyl) imidazoline, 2- (p-tolyl) imidazoline, etc. Mixtures of the imidazoline derivatives can also be used according to the invention. This is particularly useful if blocked isocyanates with low melting points or ranges are required.

The imidazoline derivatives which can be used according to the invention can be prepared by known processes from optionally substituted geminal diamines and aliphatic or aromatic mononitriles in the presence of elemental sulfur or sulfuryl chloride as a catalyst.

According to FR-A 14 66 282, it is already known to produce 0-1,2-imidazoline-N-carboxyalkyl- or carboxycycloalkylamides by reacting Δ-1,2-imidazolines and monoisocyanates for therapeutic purposes. The blocking of diisocyanates with 0-1,2-imidazolines is not yet known.

As already mentioned, the blocking can also be carried out in solvents. Suitable solvents for this reaction are only those which do not react with the polyisocyanates, for example ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, cyclohexanone and others; Aromatics such as benzene, toluene, xylenes, chlorobenzene, nitrobenzene and others; cyclic ethers such as tetrahydrofuran, dioxane and others; Esters such as methyl acetate, n-butyl acetate and others; aliphatic chlorinated hydrocarbons such as chloroform, carbon tetrachloride and others; and aprotic solvents such as dimethylformamide, dimethylacetamide, dimethyl sulfoxide, etc.

If the blocking agent is used in a ratio of 1 1 to the number of isocyanate groups, the reaction mixtures are kept at the stated temperatures until the NCO content of the reaction mixture has dropped to values below 0.2% NCO, otherwise until a constant is reached NCO value.

Another object of the present invention is the subsequent implementation of a group of blocked polyisocyanates, namely those in which cyclic amidines have been used in substoichiometric amounts, i.e. the ratio of cyclic amidine to isocyanate groups was <1: 1 with the same chain extenders that have previously been described as molecular enlargement agents. The reaction also takes place at temperatures in the range from 0-150 ° C., preferably 80-120 ° C., but below the deblocking temperature of the blocked polyisocyanate. This process variant allows the product range of the blocked polyisocyanates to be adapted to practical requirements within very wide limits. This process variant is of particular interest for polyisocyanates with differently reactive NCO groups, i.e. asymmetric polyisocyanates.

By changing the order of adduct formation / blocking, blocked polyisocyanates with different reactivity, melting range and structure can be obtained.

The invention also relates to the adducts obtainable by the processes described. These are generally compounds of the molecular weight range from 300-2,500, preferably from 300-1,000. The process products have a melting range in the temperature range from 30-220 ° C., preferably 80-160 ° C. The polyisocyanates blocked with the cyclic amidines are further characterized by a content of terminal isocyanate groups present in blocked form (calculated as NCO) of 4-25% by weight, preferably 10-21% by weight.

in der

und

worin R gleiche oder verschiedene Substituenten aus der Gruppe Wasserstoff, Alkyl, Cycloalkyl, Aralkyl bedeutet und R' der zweiwertige organische Rest einer Verbindung mit 2 OH-Gruppen ist.They are preferably blocked polyisocyanates of the following formulas:

in the

and

wherein R is the same or different substituents from the group hydrogen, alkyl, cycloalkyl, aralkyl and R 'is the divalent organic radical of a compound with 2 OH groups.

The compounds of the invention described above are particularly suitable because of their melting points and at the same time higher molecular weights as catalysts for the anionic polymerization of _E- caprolactam. The compounds can also be used to produce wire enamels.

The process of the present invention is illustrated by the following examples:

example 1

To a mixture of 222 parts by weight. Isophorone diisocyanate (IPDI) and 300 parts by weight anhydrous acetone were slowly 292 parts by weight at room temperature. 2-phenylimidazoline, which in 500 parts by weight. anhydrous acetone were dissolved, added dropwise. After the 2-phenylimidazoline addition was complete, the mixture was heated at 50 ° C. for one hour. The acetone was distilled off. The last residues of acetone were removed by drying the reaction product at 60 ° C. in a vacuum drying cabinet. The IPDI blocked with 2-phenylimidazoline is a white powder with a melting range of 98-106 ° C, a glass transition temperature (DTA) of 63-80 ° C and a splitting temperature of approx. 120 ° C. The IR spectrum of this blocked polyisocyanate is shown in Fig. 1.

Example 2

To a melt of 320 parts by weight. 2-phenyl-4-methylimidazoline were 222 parts by weight. IPDI added dropwise so that the temperature in the reaction flask did not exceed 120 ° C. To complete the reaction, the reaction mixture was kept at 120 ° C for 3 hours. These conditions were sufficient for an almost complete conversion (NCO content of the reaction product 0.2% by weight). The reaction product is a white crystalline powder with a melting range of 95-98 ° C, a glass transition temperature (DTA) of 65-85 ° C and a splitting temperature of approx. 140 ° C. The IR spectrum of the blocked polyisocyanate is shown in Fig. 2.

Example 3

To 222 parts by weight IPDI were 196 parts by weight at 80 ° C. 2,4-Dimethylimidazoline added dropwise so that the temperature did not rise above 90 ° C. After the 2,4-dimethylimidazoline addition had ended, the mixture was heated at 100 ° C. for a further hour. The reaction product is a colorless powder with a melting range of 104-110 ° C, a glass transition temperature (DTA) of 75-90 ° C and a splitting temperature of approx. 160 ° C. The IR spectrum of the blocked polyisocyanate is shown in Fig. 3.

Example 4

• a) To 444 parts by weight IPDI were 106 parts by weight at 80 ° C with good stirring. Diethylene glycol slowly added. After diethylene glycol had been added, the mixture was heated at 80 ° C. for a further 2 h. The NCO content of the IPDI / diethylene glycol mixture was then 15.1% by weight.
• b) To 556 parts by weight. of the adduct prepared in 4 a from 2 moles of IPDI and 1 mole of diethylene glycol was 292 parts by weight at 120 ° C in portions. 2-Phenylimidazoline added so that the temperature did not rise above 120 ° C. After the 2-phenylimidazoline addition was complete, the reaction mixture was heated at 120 ° C. for one hour. The reaction product is a pale yellow powder with a melting range of 100-106 ° C, a glass transition temperature (DTA) of 70-90 ° C and a splitting temperature of approx. 140 ° C.

Fig. 4 shows the IR spectrum of the blocked polyisocyanate.

Example 5

To 556 parts by weight of the IPDI diethylene glycol adduct described in Example 4 a are 320 parts by weight at 100 ° C. 2-Phenyl-4-methylimidazoline added so that the temperature of the reaction mixture did not rise above 110 ° C. To complete the reaction, the reaction mixture was heated at 110 ° for a further 2 hours. The reaction product is a white powder with a melting range of 95-100 ° C, a glass transition temperature (DTA) of 60-85 ° C and a splitting temperature of approx. 150 ° C. NCO could no longer be detected in the reaction product.

Fig. 5 shows the IR spectrum of the blocked polyisocyanate.

Example 6

To 556 parts by weight of the adduct of 2 moles of IPDI and 1 mole of diethylene glycol described in Example 4 a was 196 parts by weight at 100 ° C. 2,4-Dimethylimidazoline added dropwise so that the temperature did not rise above 110 ° C. After 2,4-dimethylimidazoline had been added, the reaction mixture was heated at 110 ° C. for a further 2 hours. NCO could no longer be detected in the reaction product thus produced. The reaction product is a colorless product with a melting range of 100-107 ° C, a glass transition temperature (DTA) of 60-95 ° C and a splitting temperature of approx. 170 ° C.

Fig. 6 shows the IR spectrum of the blocked polyisocyanate.

Example 7

To a melt of 168 parts by weight. 2-methylimidazoline were 222 parts by weight. IPDI added dropwise so that the temperature in the reaction flask did not rise above 130 ° C. To complete the reaction, the reaction mixture was kept at 130 ° C for 3 hours.

These conditions were sufficient for an almost complete reaction (NCO content of the reaction product 0.2% by weight). The reaction product is a white crystalline powder with a melting range of 107-110 ° C, a glass transition point (DTA) of 82-89 ° C and a splitting temperature of approx. 150 ° C.

Example 8

To 556 parts by weight of the adduct of 2 moles of IPDI and 1 mole of diethylene glycol described in Example 4 a was 168 parts by weight at 120 ° C. 2-Methylimidazoline added so that the temperature of the reaction mixture did not rise above 110 ° C. To complete the reaction, the reaction mixture was heated at 110 ° C. for a further 2 h. The reaction mixture is a white powder with a melting range of 109-112 ° C, a glass transition point (DTA) of 83-91 ° C and a splitting temperature of 160 ° C.

Example 9

To a melt of 210 parts by weight. 2,4,4- (2,2,4) -trimethylhexamethylene diisocyanate (1: 1) was 292 parts by weight. 2-Phenylimidazoline was added dropwise so that the temperature in the reaction flask did not exceed 120 ° C. To complete the reaction, the reaction mixture was kept at 120 ° C for 3 hours. These conditions were sufficient for an almost complete conversion (NCO content of the reaction product 0.1% by weight). The reaction product is a white crystalline powder with a melting range of 64-76 ° C, a glass transition temperature (DTA) of 57-72 ° C and a splitting temperature of approx. 140 ° C.

The IR spectrum of the blocked polyisocyanate is shown in Fig. 7.

Example 10

To 210 parts by weight 2,4,4- (2,2,4) -trimethylhexamethylene diisocyanate (1: 1) were 196 parts by weight at 80 ° C. 2,4-Dimethylimidazoline added dropwise so that the temperature did not rise above 90 ° C. After the 2,4-dimethylimidazoline addition had ended, the mixture was heated at 100 ° C. for a further hour. (NCO content of the reaction product 0.1% by weight). The reaction product is a colorless powder with a melting range of 85-105 ° C, a glass transition temperature (DTA) of 70-91 ° C and a splitting temperature of approx. 150 ° C.

Fig. 8 shows the IR spectrum of the blocked polyisocyanate.

Example 11

To a melt of 210 parts by weight. 2,4,4- (2,2,4) -trimethylhexamethylene diisocyanate (1: 1) were 164 parts by weight. 2-Methylimidazoline was added dropwise so that the temperature in the reaction flask did not rise above 130 ° C. To complete the reaction, the reaction mixture was kept at 130 ° C for 3 hours. These conditions were sufficient for an almost complete reaction (NCO content of the reaction product 0.1% by weight). The reaction product is a white crystalline powder with a melting range of 65-78 ° C, a glass transition point (DTA) of 59-76 ° C and a splitting temperature of approx. 150 ° C.

Example 12

To a melt of 174 parts by weight. Toluene-2,4- (2,6) -diisocyanate (from 80% 2,4- and 20% 2,6-) were 292 parts by weight. 2-Phenylimidazoline added dropwise so that the temperature in the reaction flask did not exceed 140 ° C. To complete the reaction, the reaction mixture was kept at 140 ° C for 3 hours. These conditions were sufficient for an almost complete reaction (NCO content of the reaction product 0.2% by weight). The reaction product is a white crystalline powder with a melting range of 90-103 ° C, a glass transition temperature (DTA) of 78-90 ° C and a splitting temperature of approx. 130 ° C.

The IR spectrum of the blocked polyisocyanate is shown in Fig. 9.

Example 13

To a mixture of 174 parts by weight. Toluene-2,4- (2,6) -diisocyanate (from 80% 2,4- and 20% 2,6-) and 300 parts by weight. anhydrous acetone were slowly 196 parts by weight at room temperature. 2,4-dimethylimidazoline, which in 500 parts by weight. anhydrous acetone were dissolved, added dropwise. After the 2,4-dimethylimidazoline addition was complete, the mixture was heated at 50 ° C. for one hour. The acetone was distilled off. The last residues of acetone were removed by drying the reaction product at 60 ° C. in a vacuum drying cabinet (NCO content of the reaction product 0.1% by weight). The diisocyanate blocked with 2,4-dimethylimidazoline is a white powder with a melting range of 85-105 ° C, a glass transition temperature (DTA) of 70-91 ° C and a splitting temperature of approx. 150 ° C.

The IR spectrum of this blocked polyisocyanate is shown in Fig. 10.

Example 14

To a melt of 174 parts by weight. Toluylene-2,4- (2,6) -diisocyanate (from 80% 2,4- and 20% 2,6-) were 320 parts by weight. 2-Phenyl-4-methylimidazoline was added dropwise so that the temperature in the reaction flask did not exceed 140.degree. To complete the reaction, the reaction mixture was kept at 140 ° C for 3 hours. These conditions were sufficient for an almost complete conversion (NCO content of the reaction product 0.15% by weight). The reaction product is a white crystalline powder with a melting range of 82-99 ° C, a glass transition temperature (DTA) of 68-89 ° C and a splitting temperature of approx. 130 ° C.

Example 15

• a) To 420 parts by weight 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate (1: 1) were 106 parts by weight at 80 ° C with good stirring. Diethylene glycol slowly added. After diethylene glycol had been added, the mixture was heated at 80 ° C. for a further 2 h. The NCO content of the 2,4,4- (2,2,4) trimethylhexamethylene diisocyanate / diethylene glycol adduct was then 15.8% by weight.
• b) To 532 parts by weight. of the adduct prepared in 15 a from 2 mols of 2,4,4- (2,2,4) -trimethylhexamethylene diisocyanate and 1 mol of diethylene glycol were 292 parts by weight at 120 ° C in portions. 2-Phenylimidazoline added so that the temperature did not rise above 120 ° C. After the 2-phenylimidazoline addition was complete, the reaction mixture was heated at 120 ° C. for one hour. The reaction product is a pale yellow powder with a melting range of 66-83 ° C, a glass transition temperature (DTA) of 59-75 ° C and a splitting temperature of approx. 150 ° C.

Example 16

To 532 parts by weight of the adduct prepared in 15 a from 2 moles of 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate and 1 mole of diethylene glycol was added in portions at 120 ° C 196 parts by weight. 2,4-Dimethylimidazoline added so that the temperature did not rise above 120 ° C. After the 2,4-dimethylimidazoline addition had ended, the reaction mixture was heated at 120 ° C. for a further hour. The reaction product is a pale yellow powder with a melting range of 71- 82 ° C a glass transition temperature (DTA) of 61-81 ° C, and has a splitting temperature of approx. 170 ° C.

Example 17

• a) At 348 parts by weight Toluylene-2,4- (2,6) -diisocyanate (from 80% 2,4- and 20% 2,6-) was 106 parts by weight at 80 ° C with good stirring. Diethylene glycol slowly added. After diethylene glycol had been added, the mixture was heated at 80 ° C. for a further 2 h. The NCO content of the tolylene-2,4- (2,6) -diisocyanate / diethylene glycol mixture was then 18.3% by weight.
• b) 459 parts by weight of the adduct prepared in 17 a from 2 moles of toluene-2,4- (2,6) -diisocyanate (from 80% 2,4- and 20% 2,6-) and 1 mole of diethylene glycol, 292 wt .-T. 2-Phenylimidazoline added so that the temperature did not rise above 160 ° C '. After the 2-phenylimidazoline addition had ended, the reaction mixture was heated at 160 ° C. for one hour (NCO content of the reaction product 0.15% by weight). The reaction product is a pale yellow powder with a melting range of 92-105 ° C, a glass transition temperature (DTA) of 79-92 ° C and a splitting temperature of approx. 140 ° C.

Example 18

To 459 parts by weight of the adduct prepared in 17 a from 2 moles of toluene-2,4- (2,6) -diisocyanate (from 80% 2,4- and 20% 2,6-) and 1 mole of diethylene glycol were added in portions at 150 ° C 196 wt .-T. 2,4-Dimethylimidazoline added so that the temperature did not rise above 150 ° C. After the 2,4-dimethylimidazoline addition had ended, the reaction mixture was heated for a further hour at 150 ° C. (NCO content of the reaction product 0.1% by weight). The reaction product is a pale yellow powder with a melting range of 90-107 ° C, a glass transition temperature (DTA) of 73-95 ° C and a splitting temperature of approx. 160 ° C.

Claims (11)

1. A method of producing blocked polyisocyanates by reacting polyisocyanates with cyclic nitrogenous compounds which comprises using cyclic amidines of the general formula

as the cyclic nitrogenous compounds, wherein each R may be the same or different substitute from the hydrogen, alkyl-, cycloalkyl-, aralkyl- and arylradical group and the reaction is carried out at temperatures of 0-150 °C, preferably 80-120 °C.

2. A method according to Claim 1, wherein the polyisocyanates and cyclic amidines are introduced in such quantities that 0,5-1,1, preferably 0,8-1,0 mole of cyclic amidine is combined with an isocyanate group.

3. A method according to Claim 2, wherein the blocked polyisocyanates to which cyclic amidines have been added in sub-stoichiometric quantities, are then reacted with polyvalent chain-lengthening agents at temperatures in the range of 0-150 °C, but below the deblocking temperature and any excess chain-lengthening agent is removed in a known manner either directly or with a solvent, if necessary.

4. A method according to Claim 3, wherein the reaction is carried out at temperatures in the range of 80-120 °C.

5. A method according to Claims 3 and 4, wherein the clain-lengthening agent is water.

6. A method according to Claims 3 and 4, wherein the chain-lengthening agents are polyols.

7. A method according to Claims 3 and 4, wherein the chain-lengthening agents are polyamines.

8. Blocked polyisocyanates according to Claims 1-7.

9. A blocked polyisocyanate of the formula

wherein R has the same meaning as before.

10. A blocked polyisocyanate of the formula

wherein R has the same meaning as before and R' is the bivalent organic radical of a compound with 2 OH-groups.

11. Blocked polyisocyanates of the formula

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