HK1063329A - Composition comprising a blocked polyisocynate - Google Patents

Description

Composition comprising blocked polyisocyanate

Cross reference to patent application

The present patent application claims priority from german patent application No. 10228730.9 filed on day 27/6/2002 under sections (a) - (d) of 35 u.s.c. 119.

Background

Technical Field

The present invention relates to the use of certain pyrazoles and 1, 2, 4-triazoles to stabilize blocked polyisocyanates against thermal yellowing. The invention further relates to compositions comprising blocked polyisocyanates and at least one pyrazole or 1, 2, 4-triazole, and to coatings comprising such compositions, coatings obtained from these coatings and substrates coated with such coatings.

Description of related documents

Blocked polyisocyanates are used, for example, in one-component polyurethane stoving lacquers (1K PU stoving lacquers), in particular for automotive clearcoat materials and for known coil coatings. In coil coating, a coil of thin steel sheet (called a coil) is unrolled and painted. Products in which coil coating is applied to steel sheets are used, for example, in manufacturing household appliances such as refrigerators (i.e., white goods). For end uses, especially under overbake conditions, the coatings are only allowed to yellow slightly on heating. Overbaking means exceeding the normal baking temperature of the coating. In the case of coil coatings, overbaking means exceeding the peak metal temperature. Yellowing by heat means that the coating becomes yellow at high temperatures. High temperatures occur especially during paint baking.

The preparation of blocked polyisocyanates is disclosed in Liebigs Annalen, volume 562, pages 205 to 229, 1949. Blocked polyisocyanates can be prepared by direct reaction of the polyisocyanate with a blocking agent. In the case of C-H-acidic blocking agents (e.g.malonates), it is possible to initiate the reaction of the polyisocyanate with the blocking agent using deprotonating agents.

DE-A19738497 discloses amine-blocked stable polyisocyanates to prevent thermal yellowing.

JP-A10-306254 discloses blocked polyisocyanates comprising benzotriazole.

One factor in the thermal yellowing of blocked polyisocyanates during coating is believed to be blocking agents. Farbe & rock, 7/96, Vol.102, pp.51 to 58, Engbert et al disclose that the blocking agent causes slight thermal yellowing due to the inclusion of 3, 5-dimethylpyrazole and 1, 2, 4-triazole. However, these substances have other disadvantages. 3, 5-dimethylpyrazole is expensive and some of the properties of the product 1, 2, 4-triazole are not suitable for widespread use. For example, blocked polyisocyanates based on 1, 6-diisocyanatohexane (HIDI) and 1, 2, 4-triazole yield highly crystalline products that are unsuitable for solvent-containing paints and coatings.

Other blocking agents, such as butanone oxime and diisopropylamine, for example, have the disadvantage of causing high thermal yellowing.

To reduce the abovementioned disadvantages, EP-A0654490 and DE-A4416750 disclose polyisocyanates which are blocked with mixtures of 1, 2, 4-triazole and/or 3, 5-dimethylpyrazole and other blocking agents. However, these substances, although of smaller extent, likewise have the disadvantages described above.

EP-A0829500 and DE-A19738497 disclose compound compositions as stabilizers for blocked polyisocyanates, one compound containing at least one 2, 2, 6, 6-tetramethylpiperidinyl group, known as HALS (hereinafter referred to as amine light stabilizer) group, and the other compound containing a hydrazide structure. However, the disadvantage is that the latter are sometimes not commercially available and have to be prepared, for example, by reaction of cyclic carbonates and hydrazines, requiring an additional operation. Furthermore, some compounds have two isocyanate-reactive groups, resulting in high viscosity and thus increased solvent content in the product. In some cases, gels are produced when these compounds are used.

Summary of The Invention

The invention is based on the object of stabilizing polyisocyanates against thermal yellowing.

This object is achieved by using one of the compounds of the formulae I to IIIWherein R is¹To R⁴Independently of one another, represents hydrogen, alkyl having 1 to 25 carbon atoms, or two, three or four R¹To R⁴The radicals being combined to form an aliphatic ring system having 1 to 25 carbon atoms, where R in the case of the formula I²And R³Combined or R³And R⁴In combination, it may also represent a substituted or unsubstituted (preferably unsubstituted) benzene ring fused to the pyrazole ring to stabilize the blocked polyisocyanate against thermal yellowing. In addition, the object is achieved by a composition comprising: A) at least one blocked polyisocyanate and B) at least one compound of one of the formulae I to IIIWherein R is¹To R⁴Independently of one another, represents hydrogen, alkyl having 1 to 25 carbon atoms or two, three or four R¹To R⁴The radicals being combined to form an aliphatic ring system having 1 to 25 carbon atoms, where R in the case of the formula I²And R³Combined or R³And R⁴In combination, may also represent a substituted or unsubstituted benzene ring fused to the pyrazole ring.

In one embodiment of the invention, the blocked polyisocyanates are blocked with compounds other than those of the formulae I to III to a degree of blocking of more than 90 mol%, preferably all without the use of compounds specified by the formulae I to III.

In one embodiment of the invention, at least 95 mol% of the isocyanate groups of the blocked polyisocyanate are in blocked form.

In one embodiment of the invention, the blocked polyisocyanates contain a total of 5 to 27% by weight of unblocked and blocked isocyanate groups (calculated as NCO, molecular weight 42).

In one embodiment of the invention, B) specifies compounds in an amount of from 0.1 to 10% by weight, based on the amount of blocked polyisocyanate in the composition according to the invention.

In one embodiment of the invention, the composition further comprises C) other auxiliaries or additives.

In one embodiment of the invention, C) is present in the composition in an amount of up to 5% by weight, based on the amount of blocked polyisocyanate.

The invention discloses a composition and a use thereof.

The invention further provides a coating comprising the composition of the invention.

The invention further comprises coatings obtainable from the coating.

The present invention further provides a coated substrate obtained by coating a substrate with the coating material.

Description of the preferred embodiments

Blocked polyisocyanates are obtained by reacting polyisocyanates (a1) with blocking agents (a2) and, if appropriate, further compounds (a3) which are reactive toward isocyanates (known as isocyanate-reactive compounds).

Polyisocyanates (a1), in particular aliphatic, cycloaliphatic, araliphatic and/or aromatic diisocyanates and any desired polyisocyanates prepared from modified these diisocyanates, are synthesized from at least two isocyanates and have uretdione (uretdione), isocyanurate, allophanate, biuret, iminooxaziridinedione and/or oxaziridinone structures, as disclosed, for example, in "Polyurethane fur rock und Beschichhtung", pages 18 to 35 (M Bock, Vincentz Verlag, Hannover, 1999), or mixtures of these compounds.

Suitable diisocyanates are, in particular, any desired diisocyanates having a molecular weight in the range from 140 to 400g/mol, which are obtainable by phosgenation or by phosgene-free processes, for example by thermal urethane cleavage, and which contain aliphatically, cycloaliphatically, araliphatically and/or aromatically bonded isocyanate groups, such as1, 4-diisocyanatobutane, 1, 6-diisocyanatohexane (HDI), 2-methyl-1, 5-diisocyanatopentane, 1, 5-diisocyanato-2, 2-dimethylpentane, 2, 4-and 2, 4, 4-trimethyl-1, 6-diisocyanatohexane, 1, 10-diisocyanatodecane, 1, 3-and 1, 4-diisocyanatocyclohexane, 1, 3-and 1, 4-bis (isocyanatomethyl) cyclohexane, 1-isocyanato-3, 3, 5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), 4, 4-diisocyanatodicyclohexylmethane, 1-isocyanato-1-methyl-4 (3) -isocyanatomethylcyclohexane, bis (isocyanatomethyl) norbornane, 1, 3-and 1, 4-bis (2-isocyanatoprop-2-yl) benzene (TMXDI), 2, 4-and 2, 6-diisocyanatotoluene (TDI), 2, 4' -and 4, 4, -diisocyanatodiphenylmethane, 1, 5-diisocyanatonaphthalene or any desired mixtures of these diisocyanates.

Preferably the polyisocyanate is a polyisocyanate or a mixture of polyisocyanates of the above mentioned type containing only aliphatic and/or cycloaliphatic isocyanate groups.

More preferably the polyisocyanate is a polyisocyanate or a mixture of polyisocyanates having an isocyanurate structure and/or a biuret structure based on HDI, IPDI and/or 4, 4-diisocyanatodicyclohexylmethane or mixtures thereof.

Suitable blocking agents (a2) are known from the prior art, for example alcohols, ketones, oximes, malonates, alkyl acetoacetates, phenols, and amines, for example butanone oxime, diisopropylamine, diethyl malonate, ethyl acetoacetate, acetoxime, epsilon-caprolactam or any desired mixtures of these blocking agents. Preferred blocking agents are butanone oxime, diethyl malonate, diisopropyl amine and epsilon-caprolactam or mixtures of these compounds. More preferred blocking agents are butanone oxime and/or diisopropylamine.

Isocyanate-reactive compounds (a3) are, for example, compounds which contain one or more isocyanate-reactive amino and/or hydroxyl groups, such as monoalcohols, polyols, monoamines, polyamines and aminoalcohols, or mixtures of these compounds. These substances serve to pre-augment or accelerate the reaction in order to adapt the properties of the blocked polyisocyanates according to the invention to the particular requirements. The crosslinking density can thus be increased, for example, by increasing the average NCO functionality in the final curing agent. The components may affect the elasticity, resulting in greater hardness or softness. May affect the tendency to crystallize.

Preference is given to using 1, 6-hexanediol, 2-ethylhexane-1, 3-diol, 1, 3-butanediol, 1, 4-butanediol, 2, 4-trimethyl-1, 3-pentanediol, 2, 4, 4-trimethyl-1, 3-pentanediol, trimethylolpropane, polyester polyols, polyacrylate polyols, polycarbonate polyols and/or polyurethane polyols, or mixtures of these compounds.

The molar ratio of the isocyanate groups of component al) to the sum of the isocyanate-reactive groups a2) and a3) is preferably from 0.80 to 1.05, particularly preferably from 0.95 to 1.00; more preferably the ratio is 1.00. When compounds containing isocyanate-reactive groups are used as component B), the ratio is preferably from 0.80 to 1.00, in particular from 0.95 to 1.00; a particularly preferred ratio is 1.00.

The proportion of isocyanate-reactive groups of component a3) to the sum of the isocyanate-reactive groups of a2) and a3) is preferably from 1 to 50 mol%, preferably from 3 to 30 mol%, particularly preferably from 4 to 8 mol%.

Blocked polyisocyanates can be prepared according to methods known from the prior art by reaction of components a1), a2) and optionally a 3). This is disclosed, for example, in Liebigs Annalen, volume 562, pages 205 to 229, 1949. Blocked polyisocyanates can be prepared by direct reaction of a polyisocyanate and a blocking agent. In the case of C-H-acidic blocking agents (e.g.malonates), it is possible to initiate the reaction of the polyisocyanate with the blocking agent using deprotonating agents.

Component B) of the compositions according to the invention comprises one compound of the formulae I to III or any desired mixtures of these compoundsWherein R is¹To R⁴Independently of one another, represents hydrogen or alkyl having 1 to 25 carbon atoms, or R¹To R⁴Two, three or four of the radicals being combined to form an aliphatic ring system having from 1 to 25 carbon atoms, where in the case of the formula I R²And R³Combined or R³And R⁴In combination may also represent a substituted or unsubstituted (preferably unsubstituted) benzene ring fused to the pyrazole ring.

Preference is given to compounds of the formulae I to III in which R¹Represents a hydrogen atom, and R²、R³And R³Independently of one another, represents a hydrogen atom, or an alkyl radical having 1 to 6 carbon atoms, or R²To R⁴Two or three of the radicals being combined to form an aliphatic ring system having from 1 to 25 carbon atoms, or any desired mixtures of these compounds.

More preferably R in the compounds of the formulae I to III¹Represents a hydrogen atom, and R²、R³And R⁴Independently of one another, hydrogen or alkyl having 1 to 6 carbon atoms, or any desired mixtures of these compounds.

Particularly preferred compounds are 3, 5-dimethylpyrazole, 3, 5-dimethyl-1, 2, 4-triazole and 1, 2, 4-triazole, or any desired mixtures of these compounds.

The compounds of one of the formulae I to III (component B of the compositions according to the invention) are commercially available or can be prepared by known methods. Corresponding preparation methods are known to the person skilled in the art. Conventional heterocyclic synthesis methods can be used. For example, hydrazine can be condensed with dicarbonyl compounds.

non-B) auxiliaries and additives C) which may suitably be used are, for example, antioxidants such as 2, 6-di-tert-butylUV absorbers of the 4-cresol, 2-hydroxyphenyl-benzotriazole type, or light stabilizers of the HALS compound type substituted at the nitrogen atom, e.g. Tinuvin^292(Ciba Spezialitaten GmbH, Lambertheim, DE), or other commercially customary stabilizers, as disclosed, for example, in "Lichtschuttzmittel fur Lacke" (A. Valet, Vincentz Verlag, Hannover, 1996) and "Stabilization of Polymeric Materials" (H.Zweifel, Springer Verlag, Berlin, 1997, appendix 3, page 181-.

The compositions according to the invention can be prepared by mixing components A), B) and optionally C) in any order, each of which can be in the form of a solution in a solvent, it being possible for further solvents to be additionally added. The mixing may be carried out at a temperature of 0 to 100 deg.C, preferably at a temperature in the range of 20 to 80 deg.C, more preferably at a temperature in the range of 20 to 50 deg.C.

In addition, a further preferred embodiment of the above-disclosed process for preparing polyisocyanates according to the invention is characterized in that the mixing of component B) with component A) is carried out during the preparation of the component A) and/or immediately after the preparation of the component A). When compounds containing isocyanate-reactive groups are used as component B), component B) is not added until the NCO groups of a1) > 99% and the isocyanate-reactive groups of a2) and a3) components have reacted. If appropriate, further auxiliaries and additives C) can be added during and/or after the preparation of component A). To prepare component A), a1), a2) and a3) can be dissolved in a solvent, respectively, or a solvent can be added. The reaction of components a1), a2) and a3) can be carried out at temperatures in the range from 0 to 200 ℃, preferably from 20 to 130 ℃, particularly preferably from 20 to 90 ℃. The capping reaction as well as the promotion reaction can be accelerated by using catalysts known in polyurethane chemistry. This may be advantageous, for example, when a2) and/or a3) contain hydroxyl groups.

Suitable solvents are the customary lacquer solvents, such as ethyl acetate, butyl acetate, 1-methoxy-2-propyl acetate, N-butyl 3-methoxyacetate, acetone, methyl ethyl ketone, 4-methyl-2-pentanone, cyclohexanone, toluene, xylene, N-methylpyrrolidone, chlorobenzene or white spirit. Also suitable are, in particular, mixtures containing aromatics with a higher degree of substitution, for example those sold under the names Solvent Naphtha, Solvesso  (Exxon Chemicals, Houston, USA), Cypar  (Shell Chemicals, Eschborn, DE), Cyclo Sol  (Shell Chemicals, Eschborn, DE), ToIn Sol  (Shell Chemicals, Eschborn, DE), Shell Sol  (Shell Chemicals, Eschborn, DE). However, it is also possible to use alcohols, for example isobutanol, for example once the NCO groups of component a1) have been completely consumed by reaction with the isocyanate-reactive groups of components a2) and a 3). Preferred solvents are acetone, butyl acetate, methyl ethyl ketone, 1-methoxypropyl acetate, xylene, toluene, especially mixtures containing aromatic compounds with a higher degree of substitution, such as are sold under the names Solvent Naphtha, Solvent sso  (Exxon Chemicals, Houston, USA), cyclop  (Shell Chemicals, Eschborn, DE), Cyclo Sol  (Shell Chemicals, Eschborn, DE), ToIu Sol  (Shell Chemicals, Eschborn, DE), Shelisol  (Shell Chemicals, Eschbom, DE).

The blocked polyisocyanates according to the invention preferably have at least 95% of the isocyanate groups, particularly preferably at least 98%, more preferably at least 99.5%, in blocked form, and contain 5 to 27% by weight, preferably 5 to 22% by weight, particularly preferably 7 to 19% by weight of unblocked and blocked isocyanate groups, calculated as NCO.

The blocked polyisocyanates according to the invention preferably contain from 0.1 to 10% by weight, particularly preferably from 0.5 to 7% by weight, more preferably from 1 to 4% by weight, of component B), preferably in chemically unbound form, and up to 5% by weight of component C), the amounts of components A), B) and C) preferably adding up to 100% by weight.

Component B) is preferably in chemically unbound form and functions preferably as a stabilizing additive. The compounds containing isocyanate-reactive groups are used as polyisocyanates according to the invention of component B) and therefore differ from the systems disclosed in EP-A0654490 and DE-A4416750 which are blocked by hybridization of five-membered ring heteroatoms.

The present invention is based on the surprising observation that, after the addition of a specific compound consisting of substituted and unsubstituted five-membered ring heteroatom groups having two or three ring structure nitrogen atoms, the blocked polyisocyanates show a marked reduction in thermal yellowing when applied in a 1K coating, compared to the comparative blocked polyisocyanates without the addition of this compound.

The compositions of the present invention may be used as ingredients in coatings. In particular, they can be used as crosslinkers for organic polyols in polyurethane one-component stoving lacquers (polyurethane 1K stoving lacquers), in particular as automotive clearcoat materials or coil coatings.

These coatings may include conventional other ingredients, solvents, and other adjuvants and additives.

These coatings can be used to coat a variety of substrates, particularly metals, particularly steel. The metal may have been coated with other coatings, so that by applying a coating comprising the composition of the invention is another coating applied.

The advantages achieved by the polyisocyanates according to the invention are a marked improvement in the yellowing resistance on overbaking (for example 232 ℃ above the normal peak metal temperature in the case of coil coatings or 140 ℃ in the case of baking of automotive clearcoat materials) and on thermal treatment (for example storage at 120 ℃ for 120 hours, as required for "white goods").

The blocked polyisocyanates of the invention can thus be readily prepared using existing blocking agents, such as malonic esters, diisopropylamine and butanone oxime, to achieve relatively low thermal yellowing, which would otherwise only be achieved by using expensive or not widely available blocking agents, such as 3, 5-dimethylpyrazole and 1, 2, 4-triazole.

A further advantage of the polyisocyanates according to the invention, in comparison with polyisocyanates and hybrid blocked polyisocyanates which are disclosed in DE-A19856968 and DE-A04416750 and are completely blocked at both ends with 3, 5-dimethylpyrazole and 1, 2, 4-triazole, is that blocking agents which have a blocking unblocking temperature which is lower than that of other yellowing-stabilizing blocking agents, for example blocking agents for 1, 2, 4-triazole and 3, 5-dimethylpyrazole, for example diisopropylamine or diethyl malonate, can be used without impairing the yellowing stability and can therefore have economic advantages or lead to better film properties, for example in the absence of baking.

The advantages of the compounds of improved yellowing stability, reduced tack and/or better usability are effective with a maintained stability compared to the polyisocyanates disclosed in EP-A0829500 and DE-A19738497.

Examples

All percentages in the following examples are in% by weight, unless otherwise specified.

The product solids content is the calculated value corresponding to the content of components which are not used as solvent.Example 1 (comparative example, without using five-membered ring heteroatom B)

According to DE-A19738497, example 1 an isocyanurate-containing coating polyisocyanate mixture based on HDI and IPDI, a labile diisopropylamine-blocked polyisocyanate was prepared. The blocked NCO group content was 8.5%. The solids content was calculated to be 65%. 140.0g (0.7eq) of HDI trimer having an NCO content of about 21% and the monomer 1, 6-diisocyanato

Hexane, content about 0.2%, viscosity at 23 ℃ of about 3000mPas105.0g (0.3eq) of IPDI trimer, NCO content about 12%, solution in solvent naphtha

A concentration of 70% 106.0g (1.05eq) of diisopropylamine 70.0g of methoxypropyl acetate 70.5g of isobutanol 491.5g (1.0eq) of a blocked polyisocyanate

65% solids (calculated)

Blocked NCO content: 8.5% (calculation) step:

the two polyisocyanates and methoxypropyl acetate were charged and the initial charge was heated to 50 ℃. Diisopropylamine was added portionwise with stirring and a slight exotherm was observed. After the addition was complete, stirring was continued at 70 ℃ for 30 minutes. During this time, the disappearance of NCO groups can be detected by infrared spectroscopy. Once no NCO groups could be detected by infrared spectroscopy, the mixture was diluted with isobutanol and cooled.

The product was diluted with 1-methoxypropyl 2-acetate to a solids content of 60% and a viscosity of 1500mPas at 23 ℃.Example 2 (comparative example, stabilization by a composition of hydrazide and HALS Compound)

According to DE-A19738497, example 2 an isocyanate-containing polyisocyanate coating mixture based on HDI and IPDI, a diisopropylamine-blocked polyisocyanate stabilized with a hydrazide and a HALS compound was prepared. The blocked NCO group content was 8.5%. The solids content was calculated to be 65%. The viscosity was 1300mPas at 23 ℃. 1-methoxypropyl 2-acetate was used to dilute it to 60% solids and a viscosity of 4000mPas at 23 ℃.Example 3 (inventive)

The unstabilized diisopropylamine-blocked polyisocyanate of example 1 was prepared according to DE-A19738497, based on an isocyanate-containing polyisocyanate mixture of HDI and IPDI, and mixed with 1, 2, 4-triazole as starting compound B) having a solids content of 3% in 1-methoxypropyl 2-acetate of the general formula,

the blocked NCO group content was 7.6%. The solids content was calculated to be 60%. The viscosity was 1300mPas at 23 ℃.Example 4 (inventive)

The unstabilized diisopropylamine-blocked polyisocyanate of example 1 was prepared according to DE-A19738497, based on an isocyanate-containing polyisocyanate mixture of HDI and IPDI, and reacted with 3% solids of 3, 5-bis-3-dimethylpropyl 2-acetate solution of the general formulaMethylpyrazole as starting compound B) are mixed,

the blocked NCO group content was 7.6%. The solids content was calculated to be 60%. The viscosity was 1300mPas at 23 ℃.

By comparing the inventive examples with the comparative examples, it is clear that the stabilizers influence the stability of the compositions disclosed in DE-A19738497, with example 2 having a significantly increased viscosity compared with the unstabilized polyisocyanate of example 1, which is not the case for inventive examples 3 and 4.Example 5 (for coil coating, inventive and comparative)

Actual coil coatings were made based on the polyisocyanate of example 1 and the hydroxy-functional polyester polyol Alkynol  1665 from Bayer AG, levikusen Germany. Titanium dioxide Tronox  R-KB-4 (from Kerr-McGee, Krefeld-Uerdingen, Germany) was also used, as well as the additive cellulose acetobutyrate CAB 531-1 (from Krahn Chemie GmbH, Hamburg, Germany), dibutyltin dilaurate (from Brenntag, Muhlheim/Ruhr, Germany), a levelling assistant based on n-butyl acrylate polymers (Acronal  4F, from BASF AG, Ludwigshafen, Germany), and as solvent a high-boiling aromatic hydrocarbon mixture (Solvesso  200S, from Deutsche Exxon, Cologne, Germany).

The coating was prepared such that the molar ratio of the hydroxyl groups of the polyester to the blocked NCO groups of the polyisocyanate was 1: 1 and the weight ratio of the non-volatile constituents of the polyisocyanate and polyester to the pigment was 1: 1. Based on the solids content of the polyisocyanate and polyester, the coating contained 0.3% by weight of dibutyltin dilaurate, 1.2% by weight of CAB 531-1 and 0.3% of Acronal  4F. The application viscosity was adjusted to about 100S (DIN ENISO 2431, 5mm orifice cup/23 ℃) by dilution with Solvesso  200S. Coating 1 was prepared based on the polyisocyanate of example 1. (comparative) coating 2 was prepared based on the polyisocyanate of example 2. (comparative) coating 3 was prepared based on the polyisocyanate of example 3. (invention) coating 4 was prepared based on the polyisocyanate of example 4. (present invention)

The coating was applied to a chromate aluminum plate and baked at either a Peak Metal Temperature (PMT) of 232 ℃ (bake) or 254 ℃ (overbake). The thickness of the film is between 20 and 22 mu m.

The Berger (Berger) whiteness of the coated aluminum panels was thus measured. The Burger whiteness can be determined by the CIELAB method of 1976(DIN 6174).

Rx, Ry and Rz are derived from measurements L, a and b. For berger whiteness, W ═ Ry +3(Rz-Rx) is the case. The results are summarized in the following table:

	comparative example	Comparative example	The invention	The invention
					Coating material	1	2	3	4
Berger whiteness of 232 deg.C	93.7	93.4	93.4	93.8
					Berger whiteness of 254 deg.C	93.0	92.8	94.4	93.3
Delta whiteness	0.7	0.6	-1.0	0.5

In comparison with the prior art polyisocyanates disclosed in DE-A19738497, it is evident that the unstable polyisocyanates of examples 2 and 1 according to the invention lead to coatings having a lower thermal yellowing (. DELTA.whiteness), sometimes even lower when overbaked. The metal plate baked at 232 ℃ PMT was stored at 120 ℃ for 120 hours, and the difference in whiteness before and after storage was measured. It is apparent from the table below that the difference in whiteness resulting from the heat treatment of the coating according to the invention is much lower than for the unstable coatings.

	Comparative example	The invention	The invention
				Coating material	1	3	4
Delta whiteness	2.3	2.0	1.4

Example 6 (varnish materials for automobile, invention and comparison)

Based on the polyisocyanates of examples 1, 3 and 4 and hydroxyl-functional polyacrylate polyols (Desmophen  A870 from Bayer AG, Leverkusen, Germany), automotive clearcoat materials were prepared. By mixing the polyacrylate polyol and the corresponding polyisocyanate in an equivalent ratio (N CO: OH ═ 1). The application viscosity was adjusted to about 25-30s (DIN EN ISO 2431, 5mm orifice cup/23 ℃) by dilution with 1-methoxypropyl 2-acetate. The coating contains 1.0% by weight of dibutyltin dilaurate, based on the content of non-volatile polyisocyanate and polyester components.

Coating 1 was prepared based on the polyisocyanate of example 1. (comparative example)

Coating 2 was prepared based on the polyisocyanate of example 3. (present invention)

Coating 3 was prepared based on the polyisocyanate of example 4. (present invention)

The coating is applied to aluminium panels made using a solvent-containing, commercially customary white primer material, for example from Spiess/Hecker, colongen, Germany, dried at room temperature and baked at 140 ℃ (baking) for 30 minutes. The thickness of the resulting film was between 30 and 40 μm. The panels were then baked again at 160 ℃ (overbake) for 30 minutes and the difference in yellowing (total Δ b) after baking and overbake was measured. The total Δ b value can be determined by CIELAB (1976 CIELAB method according to DIN 6174; b directly determined therefrom). The results are summarized in the following table:

coating material	1	2	3
					Comparative example	The invention	The invention
Thermal yellowing under overbaking (Total Δ b)	2.3	1.3	0.9

It is evident that the two polyisocyanates of the invention give coatings which yellow on overbaking (total Δ b) with less heat than the unstable polyisocyanate of example 1.

While the invention has been described in detail in the foregoing illustrative embodiments, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.

Claims (11)

1. A method of stabilizing a blocked polyisocyanate against thermal yellowing comprising mixing a blocked polyisocyanate and at least one compound of one of the formulae I to III

Wherein

R¹To R⁴Independently of one another, represents hydrogen, or an alkyl radical having 1 to 25 carbon atoms, or two, three or four R¹To R⁴The bonding of the groups is an aliphatic ring having 1 to 25 carbon atoms,

wherein

In the case of formula I, R²And R³Combined or R³And R⁴In combination, may also represent a substituted or unsubstituted benzene ring fused to the pyrazole ring.

2. A composition comprises

A) At least one blocked polyisocyanate and

B) at least one compound of one of the formulae I to III

Wherein

R¹To R⁴Independently of one another, represents hydrogen, or an alkyl radical having 1 to 25 carbon atoms, or two, three or four R¹To R⁴The bonding of the groups is an aliphatic ring system having 1 to 25 carbon atoms,

wherein

In the case of formula I, R²And R³Combined or R³And R⁴In combination, may also represent a substituted or unsubstituted benzene ring fused to the pyrazole ring.

3. The composition of claim 2, wherein the blocked polyisocyanate is blocked using only a compound of one of the formulae I to III

Wherein

R¹To R⁴Independently of one another, represents hydrogen, or an alkyl radical having 1 to 25 carbon atoms, or two, three or four R¹To R⁴The bonding of the groups is an aliphatic ring system having 1 to 25 carbon atoms,

wherein

In the case of formula I, R²And R³Combined or R³And R⁴In combination, may also represent a substituted or unsubstituted benzene ring fused to the pyrazole ring.

4. The composition of claim 2 wherein at least 95 mole% of the isocyanate groups of the blocked polyisocyanate are in blocked form.

5. The composition of claim 2 wherein the blocked polyisocyanate contains a total of 5 to 27 weight percent of unblocked and blocked isocyanate groups (calculated as NCO, molecular weight 42).

6. The composition of claim 2, wherein the at least one compound of one of the formulae I to III is present in an amount of from 0.1 to 10% by weight, based on the amount of blocked polyisocyanate.

7. The composition of claim 2, further comprising other adjuvants or additives.

8. A composition according to claim 7, wherein the other auxiliaries or additives are present in the composition in an amount of up to 5% by weight, based on the amount of blocked polyisocyanate.

9. A coating comprising the composition of claim 2.

10. A coating obtained from the coating of claim 9.

11. A coated substrate obtained by coating a substrate with the coating material of claim 9.