CA2451744A1

CA2451744A1 - Cyclic ketones as blocking agents

Info

Publication number: CA2451744A1
Application number: CA002451744A
Authority: CA
Inventors: Michael Schelhaas; Christoph Guertler; Beate Baumbach; Christian Fuessel
Original assignee: Individual
Current assignee: Bayer AG
Priority date: 2001-07-03
Filing date: 2002-07-03
Publication date: 2003-01-16
Also published as: HK1069181A1; HUP0401289A3; WO2003004545A1; HUP0401289A2; EP1404737A1; JP4118803B2; MXPA03011808A; CN100467511C; JP2004533526A; NZ530396A; KR100875808B1; KR20040015326A; HU226903B1; BR0210799A; CN1524099A; PL367450A1

Abstract

The invention relates to novel blocked polyisocyanates, from which high quality, crack-free coatings and paint lacquers with low yellowing index may be obtained, a method for production thereof and use thereof in single component polyurethane systems.

Description

' CA 02451744 2003-12-23 Le A 34 819-Foreign Countries PWE/ngb/NT

Cyclic ketones as blocking agents The present invention relates to new blocked polyisocyanates, to a method of producing them and to the use thereof in single-component polyurethane systems.
Blocking of polyisocyanates to effect temporary protection of the isocyanate groups thereof is a procedure which has long been known, and is described, for example, in Houben Weyl, Methoden der organischen Chemie XIV/2, pages 61-70. Hardenable compositions which contain blocked polyisocyanates are used in polyurethane lacquers, for example.
Single-component (1C) polyurethane systems are widely used in the field of industrial stoving lacquers such as mass-production automobile coating and coil coating, and are distinguished by their very good film properties, such as resistance to chemicals, scratch-resistance and resistance to weathering. These lacquer films are hardened by thermal activation (by a stoving operation) of the blocked poly isocyanates with polyols, optionally in the presence of a suitable catalyst. A
review of blocking agents which are suitable in principle here is given by Wicks et al. in Progress in Organic Coatings 1975, 3, pages 73-79, 1981, 9, pages 3-28 and 1999, 36, pages 148-172, for example.
For use in the field of automobile coating, the blocked polyisocyanates must be crosslinkable at maximum stoving temperatures of 140°C, and must only exhibit very slight yellowing, and preferably no yellowing, during the stoving operation. The stoving temperature is mainly controlled via the reactivity of the blocked poly-isocyanate.
Most stoving systems, such as melamine-formaldehyde and urea-formaldehyde resins, for example, are characterised by the release of volatile constituents during hardening, which increase the VOC value.

Le A 34 819-Foreign Countries Moreover, a certain proportion of the blocking agent remains in the lacquer film which is formed and has a disadvantageous effect on the properties thereof.
Due to the blocking agent which remains, properties such as the scratch-resistance and acid-resistance of single-component lacquer films are not comparable with those of what are termed two-component (2C) polyurethane lacquer coatings (e.g. T. Engbert, E.
Konig, E. Jiirgens, Farbe&Lack, Curt R. Vincentz Verlag, Hannover 10/1995).
Furthermore, separation of the blocking agent and the escape thereof in gaseous form from the lacquer film can also lead to bubble formation in the lacquer film.
Subsequent incineration of the emitted blocking agent can sometimes be necessary.
Isocyanates blocked with diethyl malonate have mainly been used recently for particularly low stoving temperatures within the range from 90 to 120°C
(e.g. EP-A
0947531). In contrast to blocking procedures which employ heterocyclic N
compounds, such as caprolactam or butanone oxime, for example, the blocking agent as a whole is not split off or separated here; rather, this blocking agent results in a transesterification reaction on the isocyanate which is blocked with diethyl malonate.
Ethanol is separated during this transesterification. This method can be employed at relatively low stoving temperatures, since the second, adjacent ester function is an activated ester. The disadvantage of this method is that systems such as these are extremely susceptible to the effect of acids, because the labile ester bond can be rapidly cleaved. The possibilities for the use of these products are thereby restricted.
The object of the present invention was to provide new blocked polyisocyanate systems which react without separation of the blocking agent, i.e. free from emissions, and which exhibit low crosslinking temperatures. The object was also that these blocked polyisocyanate systems should be stable on storage at ambient temperature, and that they should be suitable, particularly in combination with suitable polyol components, for the production of single-component stoving lacquers.
Surprisingly, it has now been found that acidic CH compounds which possess the basic structure of an activated cyclic ketone, particularly that of cyclopentanone-2-Le A 34 819-Foreign Countries carboxymethyl ester, are particularly suitable for blocking polyisocyanates in order to obtain emission-free coatings with a reduced tendency to exhibit yellowing.
The present invention relates to organic polyisocyanates comprising at least two isocyanate groups, the isocyanate groups of which are blocked with acidic CH
cyclic ketones of general formula (I), O
X
H
(I) R2 R' wherein X is an electron-attracting group, R' and RZ, independently of each other, represent the radicals H, a C,-C2o (cyclo)alkyl, a C6-C24 aryl, a C1-GZO (cyclo)alkyl ester or amide, a C6-C24 aryl ester or amide, or mixed aliphatic/aromatic radicals 1 S comprising 1 to 24 carbon atoms, which can also form part of a 4 to 8-membered ring, n is an integer from 0 to 5, and which have a content of blocked isocyanate groups (calculated as NCO) of 20 to 0 % by weight in total.
A content of blocked isocyanate groups (calculated as NCO) ranging from lS.Sb to 0% by weight is preferred. A content of blocked isocyanate groups (calculated as NCO) ranging from 14 to 0 % by weight is particularly preferred. Partial blocking of the polyisocyanate can optionally be effected; the non-blocked isocyanate groups can then be used for further reactions. Typically, all the isocyanate groups are blocked.

Le A 34 819-Foreign Countries The electron-attracting group X can comprise any substituent which results in the a-terminal hydrogen exhibiting an acidic CH character. This substituent can comprise ester groups, sulphoxide groups, sulphone groups, nitro groups, phosphonate groups, nitrite groups, isonitrile groups, polyhatogen alkyl groups, fluorine, chlorine or carbonyl groups. Nitrite and ester groups are preferred, whilst methyl carboxylate and ethyl carboxylate groups are particularly preferred.
Compounds of general formula (I), the ring of which optionally contains hetero atoms such as oxygen, sulphur or nitrogen atoms, are also suitable.
The activated cyclic ketone of formula (I) preferably has a ring size of 5 (n = 1) or 6 (n = 2).
Preferred compounds of general formula (I) include cyclopentanone-2-carboxymethyl ester and -carboxyethyl ester, cyclopentanone-2-carboxylic acid nitrites, cyclohexanone-2-carboxymethyl ester and -carboxyethyl ester, or cyclopentanone-2-carbonylmethyl. Cyclopentanone-2-carboxymethyt ester and -carboxyethyl ester, as welt as cyclohexanone-2-carboxymethyl ester and -carboxyethyl ester, are particularly preferred. The cyclopentanone systems can readily be obtained industrially by the Dieckmann condensation of dimethyl adipate or of diethyl adipate. Cyctohexanone-2-carboxymethyl ester can by be obtained by the hydrogenation of methyl salicylate.
The polyisocyanate to be blocked can be any organic polyisocyanate which is suitable for the crosstinking of compounds comprising active hydrogen, i.e.
aliphatic polyisocyanates, including cycloaliphatic polyisocyanates, as well as aromatic and heterocyclic polyisocyanates comprising at least two isocyanate groups and mixtures thereof. Typical examples of potyisocyanates include aliphatic isocyanates such as di- or triisocyanates, e.g. butane diisocyanate (BDI), pentane diisocyanate, hexane diisocyanate (HDI), 4-isocyanatomethyl-1,8-octane diisocyanate (triisocyanato-Le A 34 819-Foreign Countries nonane, TII~, or cyclic systems such as 4,4'-methylene-bis(cyclohexyl isocyanate) (Desmodur W , Bayer AG, Leverkusen), 3,5,5-trimethyl-1-isocyanato-3-isocyanatomethyl-cyclohexane (IPDI), as well as w,w'-diisocyanato-1,3-dimethylcyclohexane (HMI). Examples of aromatic polyisocyanates include 1,5-S naphthalene diisocyanate, diisocyanato-diphenylmethane (MDI) or crude MDI, diisocyanatomethylbenzene (TDI), particularly the 2,4- and the 2,6-isomers thereof and industrial mixtures of the two isomers thereof, as well as 1,3-bis(isocyanato-methyl)benzene (XDI). Polyisocyanates which are also very suitable are those which can be obtained by the reaction of di- or triisocyanates with themselves via their isocyanate groups, such as uretdiones or carbodiimide compounds, or such as isocyanurates or iminooxadiazinediones which are formed by the reaction of three isocyanate groups. The polyisocyanates can also contain monomeric di- and/or triisocyanates and/or oligomeric polyisocyanates comprising biuret, allophanate and acylurea structural elements, triisocyanates which have a low monomer content or partially modified monomeric di- or triisocyanates, as well as any mixtures of the aforementioned polyisocyanates. Polyisocyanate prepolymers which on average contain more than one isocyanate group per molecule are also very suitable.
These are obtained by the preliminary reaction of a molar excess of one of the aforementioned polyisocyanates, for example, with an organic material which contains at least two active hydrogen atoms per molecule, e.g. in the form of hydroxy groups.
The preferred polyisocyanates are those which contain a uretdione, isocyanurate, iminooxadiazinedione, acylurea, biuret or allophanate structure, e.g. those which are based on butane diisocyanate (BDI), pentane diisocyanate, hexane diisocyanate (HDI), 4-isocyanatomethyl-1,8-octane diisocyanate (triisocyanatononane, TIN) or on cyclic systems such as 4,4'-methylene-bis(cyclohexyl isocyanate) (Desmodur W~', Bayer AG, Leverkusen), 3,5,5-trimethyl-1-isocyanato-3-isocyanatomethylcyclo-hexane (IPDI), as well as w,w'-diisocyanato-1,3-dimethylcyclohexane (H6XDI).
Examples of aromatic polyisocyanates include 1,5-naphthalene diisocyanate, diisocyanato-diphenylmethane (MDI) or crude MDI, diisocyanatomethylbenzene Le A 34 819-Foreign Countries (TDI), particularly the 2,4- and 2,6-isomers thereof and industrial mixtures of both isomers thereof, as well as 1,3-bis(isocyanato-methyl)benzene (XDI).
Polyisocyanates which are particularly preferred are those based on hexane diiso-cyanate (HDI), on 4,4'-methylene-bis(cyclohexyl isocyanate) or on 3,5,5-trimethyl-1-isocyanato-3-isocyanatomethylcyclohexane (IPDI).
The present invention further relates to a method of producing the blocked organic polyisocyanates according to the invention, characterised in that polyisocyanates are reacted with acidic CH cyclic ketones of general formula (I), O
X
)n H
(I) R2 F2~
wherein X is an electron-attracting group, RI and RZ, independently of each other, represent the radicals H, a C1-C2o (cyclo)alkyl, C6-C24 aryl, a CI-C2o (cyclo)alkyl ester or amide, a C6-C24 aryl ester or amide, or mixed aliphatic/aromatic radicals comprising 1 to 24 carbon atoms, which can also form part of a 4 to $-membered ring, and n is an integer from 0 to 5, in the presence of a catalyst, wherein 0.8 to 1.2 mol of the cyclic ketone of formula (I) are used per isocyanate group equivalent of the polyisocyanate to be blocked.

Le A 34 819-Foreign Countries One isocyanate group equivalent of the polyisocyanate to be blocked is preferably reacted with 1 equivalent of the blocking agent.
Suitable catalysts include alkali metal and alkaline earth metal bases, such as powdered sodium carbonate (soda). Depending on the cyclic ketone used, trisodium phosphate or Dabco (1,4-diazabicyclo[2.2.2]octane) can also be used.
Carbonates of metals of subgroup II are also suitable. Sodium carbonate or potassium carbonate is preferably used. Alternatively, the reaction of the cyclic ketone with the isocyanate can also be conducted in the presence of zinc salts as catalysts. Reaction with zinc 2 ethylhexanoate is particularly preferred.
0.05 to 10 % by weight, preferably 0.1 to 3 % by weight of a catalyst, is added when conducting the method according to the invention. 0.2 to 1 % by weight of catalyst is most preferably used.
The reaction can be conducted at room temperature or at higher temperatures up to 140°C. A temperature range from 40 to 90°C is preferred.
Blocking can be effected free from solvents or in the presence of suitable solvents.
Suitable solvents comprise customary lacquer solvents such as butyl acetate, methoxypropyl acetate or the solvent naphtha supplied by Exxon-Chemie (Esso Deutschland GmbH, Hamburg) as solvents which contain aromatic compounds, as well as mixtures of the aforementioned solvents. Blocking is preferably effected in the aforementioned solvents, wherein the solids content should be adjusted so that it ranges between 10 and 90 %.
In addition to the cyclic ketones of general formula (I) which are used according to the invention, mixtures of any blocking agents can also be used in conjunction in the method according to the invention in order to achieve the lacquer properties which are required in each case, wherein the proportion of compounds of formula (I) is at least 30 % by weight, preferably 50 % by weight, most preferably 100 % by weight.

Le A 34 819-Foreign Countries _g_ Finally, the present invention also relates to a method of producing 1-C PUR
stowing lacquers, characterised in that organic polyisocyanates according to the invention are used as a crosslinking component for organic polyhydroxyl compounds.
The blocked polyisocyanates according to the invention are distinguished in that, in combination with a suitable organic polyhydroxyl compound and in the presence of suitable catalysts, they harden at stowing times of 15 to 30 minutes and at temperatures from 110 to 140°C, preferably from 120 to 140°C.
The stowing times depend in particular on the amount of catalyst used. Stowing is preferably conducted for a period of 30 minutes at a temperature of 120-140°C.
Examples of suitable catalysts for crosslinking include DBTL (dibutyltin dilaurate), zinc-2-ethylhexanoate and bismuth 2-ethylhexanoate. The preferred catalysts are zinc 2-ethylhexanoate and bismuth-2-ethylhexanoate.
Suitable polyhydroxyl compounds for this purpose of use, as well as further details with regard to the production and use of stowing lacquers of this type, can be taken from the literature. The most preferred field of application for the products according to the invention is the use thereof as crosslinking agents in automobile primer surfacers.
High-grade, emission-free coatings or lacquer coatings with reduced yellowing values can be obtained by using the blocked polyisocyanates according to the invention.
In addition, the blocked polyisocyanates according to the invention can be hardened with di- or polyamines. This reaction is preferably conducted at room temperature. It can be used for the production of lacquer coatings or workpieces.

Le A 34 819-Foreign Countries Examples The polyisocyanate used was an HDI polyisocyanate with an isocyanurate structure, an NCO content of 21.8 %, and a viscosity of 3200 mPa.s (Desmodur N3300, Bayer AG, Leverkusen).
The cyclopentanone-2-carboxymethyl ester and cyclohexanone-2-carboxymethyl ester which were used as blocking agents were ordered from the Fluka company and were used without further purification.
Preparation of~olyisoc~anates blocked with acidic a cyclic ketones Example 1 A solution of 58.5 g (0.3 equivalent) Desmodur~ N3300 in 81 ml butyl acetate was added, slowly and with intensive stirring, to a solution of cyclopentanone-2 carboxyrnethyl ester (42.7 g, 0.3 equivalent) dissolved in 20 ml butyl acetate. 1.02 g zinc 2-ethylhexanoate was added as a catalyst. The batch was heated to a temperature of 50°C, (for about 8 hours) until a determination of the NCO value gave a value of about 0.2 %. The theoretical blocked NCO content was 6.2 %.
Example 2 A solution of 42.6 g (0.25 equivalent) Desmodur~ N3300 in 71.4 ml butyl acetate was added, slowly and with intensive stirring, to a solution of cyclohexanone-carboxymethyl ester (42.6 g, 0.25 equivalent) dissolved in 20 ml butyl acetate. 0.9 g zinc 2-ethylhexanoate was added as a catalyst. The batch was heated to a temperature of 80°C, until a determination of the NCO value gave a value of about 0.3 % (after about 6 hours). The theoretical blocked NCO content was 5.75 %.

Le A 34 819-Foreign Countries Production of polyurethane lacquers according to the invention The polyisocyanates listed in the following Table were processed in stoichiometric amounts with polyols to form clear lacquers according to the formulations listed below, and with the addition of the customary additives Baysilone~ OL 17 (Bayer AG, Leverkusen (flow enhancer), 0.1 % solid with respect to solid binder vehicle) and Modaflovv~ (Monsanto Corp., Solutia Inc., USA; 0.01 % solid with respect to solid binder vehicle).
Example 3 Lacquer formulation A
Desmodur N3300 (Bayer AG, Leverkusen), blocked with cyclopentanone-2 carboxymethyl ester (supplied as an approximately 50 % solution in butyl acetate;
blocked NCO content: 6.2 %) (SN = solvent naphtha):
by weight Desmophen~ A 870 (Bayer AG, Leverkusen), 70 % in BA 35.94 Desmodur~ N3300. blocked with cyclopentanone 2-carboxymethyl ester (SO % in BA from Example 1) 34.82 Baysilone OL 17, 10 % in xylene 0.48 Modaflow , 1 % in xylene 0.48 Tinuvin~ 292 (Ciba AG, Basle, Switzerland), 10 % in xylene 4.78 Tinuvin- 1130 (Ciba AG, Basle, Switzerland), 10 % in xylene 9.56 bismuth 2-ethylhexanoate, 10 % in MPA 7.17 MPA/SN 100(1:1) 6.77 Total 100.00 Le A 34 819-Foreign Countries ratio of blocked NCO/OH: 1.0, solids content: about 45 %, catalyst content: 1.5 % (solid with respect to solid binder vehicle).
The system exhibited only very slight yellowing. This system could also be used successfully when the NCO/OH ratio was 1:1.5.
Lacquer formulation B (comparison) % by weight Desmopheri A 870, 70 % in BA 37.15 mixed trimer of hexamethylene diisocyanate and IPDI, blocked with diisopropylamine (50 % in BA) 33.88 Baysilone OL 17, 10 % in xylene 0.48 Modaflow , 1 % in xylene 0.48 Tinuviri 292 (Ciba AG, Basle, Switzerland), 10 % in xylene 4.80 Tinuvin~ 1130 (Ciba AG, Basle, Switzerland), 10 % in xylene 9.61 DBTL, 10 % in xylene 4.80 MPA/SN 100(1:1) 8.80 Total 100.00 Stoving conditions: 30 minutes at 140°C.
In solvent-containing lacquers, even at relatively low stoving temperatures, this system exhibited a clear yellow coloration. The delta b value from 140 to 160°C (30 minutes) was 3.2, and was thus about four times higher than that of a system which exhibited only slight yellowing (e.g. dimethylpyrazole), when applied over a base lacquer which contained a white solvent in each case.

Le A 34 819-Foreign Countries Measurement of yellowing due to overstoving: after stoving the lacquers for 30 minutes at 140°C, a first colour measurement was made using what is termed the CIELAB method. The higher the positive b value which is determined in this manner, the more yellow the clear lacquer has become. This was followed by overstoving for 30 minutes at I60 C. The increase in yellow coloration was subsequently measured, namely what is termed the Ob value according to the CIELAB colour system (DIN 6174, "Colorimetric determination of colour separations for body colours according to the CIELAB formula" (Edition 01.79).
For non-yellowing clear lacquers, this value should be as close as possible to 0.

Claims

Claims 1. Organic polyisocyanates comprising at least two isocyanate groups, the isocyanate groups of which are blocked with acidic CH cyclic ketones of general formula (I), wherein X is an electron-attracting group, R1 and R2, independently of each other, represent the radicals H, a C1-C20 (cyclo)alkyl, a C6-C24 aryl, a C1-C20 (cyclo)alkyl ester or amide, a C6-C24 aryl ester or amide, or mixed aliphatic/-aromatic radicals comprising 1 to 24 carbon atoms, which can also form part of a 4 to 8-membered ring, n is an integer from 0 to 5, and which have a content of blocked isocyanate groups (calculated as NCO) of 20 to 0 % by weight in total.
2. Organic polyisocyanates according to claim 1, characterised in that the electron-attracting group X is selected from the group comprising ester, sulphoxide, sulphone, nitro, phosphonate, nitrile, isonitrile, polyhalogen alkyl groups, fluorine, chlorine or carbonyl groups.

3. Organic polyisocyanates according to claim 1, characterised in that the acidic CH cyclic ketones of general formula (I) are cyclopentanone-2-carboxymethyl ester and -carboxyethyl ester, cyclopentanone-2-carboxylic acid nitriles, cyclohexanone-2-carboxymethyl ester and -carboxyethyl ester, or cyclopentanone-2-carbonylmethyl.

4. Organic polyisocyanates according to claim 1, characterised in that the acidic CH cyclic ketones of general formula (I) are cyclopentanone-2-carboxymethyl ester and carboxyethyl ester, or cyclohexanone-2-carboxy-methyl ester and -carboxyethyl ester.

5. A method of producing organic polyisocyanates according to claim 1, characterised in that polyisocyanates are reacted with acidic CH cyclic ketones of general formula (I), wherein X is an electron-attracting group, R1 and R2, independently of each other, represent the radicals H, a C1-C20 (cyclo)alkyl, a C6-C24 aryl, a C1-C20 (cyclo)alkyl ester or amide, a C6-C24 aryl ester or amide, or mixed aliphatic/aromatic radicals comprising 1 to 24 carbon atoms, which can also form part of a 4 to 8-membered ring, and n is an integer from 0 to 5, in the presence of a catalyst, wherein 0.8 to 1.2 mol of the cyclic ketone of formula (I) are used per isocyanate group equivalent of the polyisocyanate to be blocked.

5. A method according to claim 4, characterised in that the organic poly-isocyanate contains a uretdione, isocyanurate, iminooxadiazinedione, acyl-urea, biuret or allophanate structure.

6. A method according to claim 4, characterised in that alkali metal and alkaline earth metal bases or zinc salts are used as catalysts.

7. A method of producing 1-C PUR stoving lacquers, characterised in that organic polyisocyanates according to claim 1 are used as a crosslinking component for organic polyhydroxyl compounds.

8. A method according to claim 7, characterised in that organic polyisocyanates according to claim 1 and organic polyhydroxyl compounds are hardened at stoving times from 15 to 30 minutes and at temperatures from 1 IO to 140°C.

9. Use of the organic polyisocyanates according to claim 1 as crosslinking agents in automobile primer surfacers.

10. Coatings or lacquers containing blocked organic polyisocyanates according to

claim 1.