WO2009024556A1

WO2009024556A1 - Use of apolar-modified polyisocyanates

Info

Publication number: WO2009024556A1
Application number: PCT/EP2008/060805
Authority: WO
Inventors: Hendrik Jan Willem Van Den Haak; Antonius Hendrikus Gerardus Van Engelen
Original assignee: Akzo Nobel Coatings International B.V.
Priority date: 2007-08-21
Filing date: 2008-08-18
Publication date: 2009-02-26

Abstract

The invention relates to the use of an apolar-modified polyisocyanate for reducing the pinhole sensitivity of a coating prepared from a polyisocyanate-containing coating composition.The invention also relates to a coating composition comprising an apolar-modified polyisocyanate and to a kit of parts.

Description

USE OF APOLAR-MODIFIED POLYISOCYANATES

The invention relates to the use of an apolar-modified polyisocyanate for reducing the pinhole sensitivity of a coating prepared from a polyisocyanate- containing coating composition. The invention also relates to a coating composition comprising an apolar-modified polyisocyanate and to a kit of parts.

International patent application WO 2007/020269 A describes a coating composition having a satisfactory balance of low content of volatile organic solvent at application viscosity, fast drying rate, low susceptibility to foam stabilization in the drying coating, and having good appearance properties, in particular a low susceptibility to pinholes. The composition comprises a polyacrylate polyol obtainable by polymerization of olefinically unsaturated monomers wherein at least 40% by weight of the monomers comprises linear or branched alk(en)yl or alk(en)ylene groups having at least 4 carbon atoms; a polyester polyol obtainable by esterification of building blocks having ester- forming functional groups wherein at least 30% by weight of the building blocks comprises linear or branched alk(en)yl or alk(en)ylene groups with at least 4 carbon atoms per ester-forming functional group, the polyester polyol having a hydroxy value above 280 mg KOH/g and a hydroxy functionality of at least 2, and an isocyanate-functional crossl inker.

Many coating compositions comprising a high curing catalyst load to obtain a short drying time suffer from foam stabilization in the drying coating, leading to pinholes in the dried coating layer. Pinholes are a coating defect which is generally known by a skilled person and described in textbooks. Pinholes are described as a variant of craters, with the specific that the crater-like phenomenon protrudes into the inner part of the coating in the form of pores. The pore length is significantly higher than the diameter. The risk of pinhole formation increases with the layer thickness of the applied coating. Therefore, the minimum layer thickness at which pinholes are observed can be used as a measure for the pinhole sensitivity of a coating composition. Pinholes detract from the appearance and durability of coating layers. There is an ongoing need to further improve the balance of desirable properties of coating compositions and of the coatings prepared from these. More in particular, there is a need for further decreasing the pinhole sensitivity without compromising the curing rate and the VOC level. It is also desirable to find alternatives for the technology described in WO 2007/020269.

Furthermore, it has been found that some polyisocyanate crosslinkers exhibit insufficient compatibility with the specific polyols described in WO 2007/020269. The present invention seeks to alleviate the above-mentioned problems.

The present invention now provides a method for reducing the pinhole sensitivity of a coating prepared from a polyisocyanate-containing coating composition, which is characterized in that an apolar-modified polyisocyanate is used. The method of the invention provides for a decrease of the pinhole sensitivity of coatings without compromising the curing rate. Furthermore, it has been found that apolar-modified polyisocyanates exhibit very good compatibility with the specific polyols described in WO 2007/020269.

It should be noted that United States Patent US 5235018 describes the preparation of apolar-modified polyisocyanates and two-component coating compositions comprising them.

Suitable apolar-modified polyisocyanates used according to the invention comprise an average of at least two isocyanate groups. Generally, the apolar- modified polyisocyanates comprise on average from 2.3 to 6, preferably from 2.8 to 4, isocyanate groups. Apolar-modified polyisocyanates can suitably be prepared by reaction of existing polyisocyanates with apolar isocyanate-reactive compounds. Suitable isocyanate-reactive compounds have at least one isocyanate-reactive functional group and a hydrocarbon group having at least 6 carbon atoms. In further embodiments, the hydrocarbon group has at least 9 carbon atoms, or at least 12 carbon atoms. Generally, the hydrocarbon group contains at most 100 carbon atoms, or at most 48 carbon atoms. The hydrocarbon groups may be linear or branched. It is also possible for the hydrocarbon groups to contain aliphatic or aromatic cyclic moieties. Examples of suitable isocyanate-reactive functional groups are hydroxyl groups, primary and secondary amino groups, thiol groups, malonate groups, and acetoacetate groups. Specific examples of apolar isocyanate-reactive compounds are fatty alcohols and fatty amines having 12 to 22 carbon atoms. It is also possible to use synthetic branched alcohols having 12 to 32 carbon atoms. Such alcohols are also known as Guerbet alcohols and are commercially available from Sasol under the trademark Isofol^®. Also diols derived from dimehzed fatty acids can be used. Such diols are commercially available, for example Pripol 2033 ex Uniqema. In the context of the present invention apolar-modified polyisocyanate means polyisocyanates having at least 11 carbon atoms which are part of a hydrocarbon group per isocyanate group. Preferably, the apolar-modified polyisocyanate has at least 12, or at least 14 carbon atoms which are part of a hydrocarbon group per isocyanate group.

Suitable polyisocyanates which form the basis for modification with apolar isocyanate-reactive compounds are known, such as aliphatic, cycloaliphatic or aromatic di-, tri- or tetra-isocyanates. Examples of diisocyanates include 1 ,2- propylene diisocyanate, thmethylene diisocyanate, tetramethylene diisocyanate,

2,3-butylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 2,2,4-trimethyl hexamethylene diisocyanate, dodecamethylene diisocyanate, ω,ω'-dipropylether diisocyanate, 1 ,3-cyclopentane diisocyanate,

1 ,2-cyclohexane diisocyanate, 1 ,4-cyclohexane diisocyanate, isophorone diisocyanate, 4-methyl-1 ,3-diisocyanatocyclohexane, trans-vinylidene diisocyanate, dicyclohexyl methane-4,4'-diisocyanate (Desmodur^® W), toluene diisocyanate, 1 ,3-bis(isocyanatomethyl) benzene, xylylene diisocyanate, α,α,α',α'-tetramethyl xylylene diisocyanate (TMXDI^®), 1 ,5-dimethyl-2,4-bis(2- isocyanatoethyl) benzene, 1 ,3,5-triethyl-2,4-bis(isocyanatomethyl) benzene, 4,4'-diisocyanato-diphenyl, 3,3'-dichloro-4,4'-diisocyanato-diphenyl, 3,3'- diphenyl-4,4'-diisocyanato-diphenyl, 3,3'-dimethoxy-4,4'-diisocyanato-diphenyl, 4,4'-diisocyanato-diphenyl methane, 3,3'-dimethyl-4,4'-diisocyanato- diphenylmethane, and diisocyanatonaphthalene. Examples of triisocyanates include 1 ,3,5-thisocyanatobenzene, 2,4,6-triisocyanatotoluene, 1 ,8- diisocyanato-4-(isocyanatomethyl) octane, and lysine thisocyanate. Adducts and oligomers of polyisocyanates, for instance biurets, isocyanurates, allophanates, uretdiones, urethanes, and mixtures thereof are also included. Examples of such oligomers and adducts are the adduct of 2 molecules of a diisocyanate, for example hexamethylene diisocyanate or isophorone diisocyanate, to a diol such as ethylene glycol, the adduct of 3 molecules of hexamethylene diisocyanate to 1 molecule of water (available under the trademark Desmodur N of Bayer), the adduct of 1 molecule of trimethylol propane to 3 molecules of toluene diisocyanate (available under the trademark Desmodur L of Bayer), the adduct of 1 molecule of trimethylol propane to 3 molecules of isophorone diisocyanate, the adduct of 1 molecule of pentaerythritol to 4 molecules of toluene diisocyanate, the adduct of 3 moles of m-α,α,α',α'-tetramethyl xylene diisocyanate to 1 mole of trimethylol propane, the isocyanurate or imino-oxadiazinedione of 1 ,6-diisocyanatohexane (for example available under the trademarks Tolonate HDT, Desmodur N 3600, and Desmodur XP2410), the isocyanurate trimer of isophorone diisocyanate (available under the trademark Vestanat T 1890), the uretdion dimer of 1 ,6- diisocyanatohexane, the biuret of 1 ,6-diisocyanatohexane, the allophanate of 1 ,6-diisocyanatohexane, and mixtures thereof. Furthermore, (co)polymers of isocyanate-functional monomers such as α,α'-dimethyl-m-isopropenyl benzyl isocyanate are suitable for use.

In order to obtain sufficient outdoor durability, aliphatic polyisocyanates are preferred over aromatic polyisocyanates, in particular when the coating composition is applied as a top coat in a multi-layer lacquer coating. The aliphatic groups may be acyclic or cyclic. Reaction of the above-mentioned polyisocyanates with apolar isocyanate- reactive compounds to form the apolar-modified polyisocyanates used according to the invention is carried out by known methods for forming urethanes, allophanates, thiourethanes, or ureas, depending on the functional groups of the isocyanate-reactive compound and the reaction conditions. The modification of polyisocyanates with apolar isocyanate-reactive compounds can be carried out during or after preparation of the polyisocyanate. The modification can for example be carried out in a specific reaction step in a factory. Alternatively, modification can also be carried out in situ by mixing an existing polyisocyanate with an apolar isocyanate-reactive compound, for example prior to packaging of the polyisocyanate or prior to application of the coating composition. In the latter case, the mixing can also be carried out by the applicator. The reaction can be catalyzed by basic catalysts, such as described in US 5235018, which is concerned with polyisocyanates containing allophanates and isocyanurate groups. Specific reference is made to Examples 1 to 3 of US 5235018.

Alternatively, it is also possible to use metal based catalysts for the modification reaction, such as described in US 7038003. Zinc, tin, and zirconium based catalysts are suitable. Good results have been obtained with tin octoate. Usually the ratio of apolar isocyanate-reactive compound to polyisocyanates is chosen such that on average there is approximately at least 0.1 to 0.5 apolar group per molecule of apolar-modified product, and at most 1.0 to 2.2 apolar groups per molecule of apolar-modified product. Particularly good results were obtained with apolar-modified polyisocyanates having an average isocyanate functionality between 2.8 and 3.5. Generally, at least 15% by weight, preferably at least 20% by weight, based on weight of the apolar-modified polyisocyanate, originates from the apolar isocyanate-reactive compound. The weight fraction originating from the apolar isocyanate-reactive compound suitably does not exceed 45%, or 30%. In order to achieve a good curing rate of the applied coating, the coating composition suitably contains a curing catalyst which catalyzes the crosslinking reactions of isocyanate groups and isocyanate-reactive groups. Suitable catalysts include metal based catalysts. Suitable metals include zinc, cobalt, manganese, zirconium, bismuth, and tin. It is preferred that the coating composition comprises a tin based catalyst. Well-known examples of tin based catalysts are dimethyl tin dilaurate, dimethyl tin diversatate, dimethyl tin dioleate, dibutyl tin dilaurate, dioctyl tin dilaurate, and tin octoate, dibutyl tin diacetate, dibutyl tin di-2-ethlyhexanoate, dibutyl tin dichloride, dibutyl tin disulfide, dibutyl tin dimercaptide, and dibenzyl tin di-2-ethlyhexanoate. Other organotin catalysts and metal based catalysts which catalyze the reaction of isocyanate groups and moisture can also be used. Examples of further catalysts include alkaline compounds, such as tertiary amines, alkylated guanidines, and inorganic bases. Also usable, but less preferred, are acidic catalysts, for example dibutyl phosphate. The curing catalyst is usually present in the composition as a specific catalyst component. However, it is also possible for one or more of the polymeric binders present in the coating composition to have catalytically active groups. The catalytically active groups may be linked to the polymeric binder by covalent bonds or by ionic bonds. Examples of catalytically active groups which can be linked to a polymeric binder by ionic groups are ammonium carboxylates. Examples of catalytically active groups linked to the binder by covalent bonds are amine groups or acidic groups. The curing catalyst is generally present in the composition in an amount of 0.01 % to 4% by weight, calculated on the non-volatile content of the composition. The curing rate is generally increased by a higher proportion of catalyst, for example at least 0.1 % by weight, at least 0.2% by weight, or at least 0.4% by weight, calculated on the non-volatile content of the composition. A particularly good balance of cure rate and low pinhole sensitivity is obtained when the amount of curing catalyst is at least 0.2% by weight, preferably at least 0.3% by weight, calculated on the non-volatile content of the coating composition. A too high proportion of catalyst may cause the storage stability of single-pack compositions or the pot life of multi-pack compositions to deteriorate undesirably. Therefore, in order to safeguard a superior storage stability or pot life of the composition, the proportion of catalyst suitably does not exceed 5% by weight, preferably 2.5% by weight, calculated on the non-volatile content of the composition. In individual cases, the optimum proportion of catalyst may depend on the specific type of catalyst and on the desired balance of properties. The method for reducing the pinhole sensitivity according to the invention can be applied in non-aqueous coating compositions, such as solvent-free and organic solvent based liquid coating compositions. The non-aqueous coating composition can be a single-pack composition curable under the influence of atmospheric moisture. Alternatively, the coating composition can be a multi- pack coating composition comprising an isocyanate-reactive binder module and an apolar-modified polyisocyanate-containing crosslinker module. The multi- pack composition may be a non-aqueous composition or an aqueous composition.

The invention also relates to a coating composition comprising an apolar- modified polyisocyanate, wherein the coating composition additionally comprises a curing catalyst in an amount which causes at least the same curing rate acceleration as 0.2% by weight, preferably at least 0.3% by weight, of dibutyl tin dilaurate, calculated on the non-volatile content of the coating composition. It is to be understood that the curing rate in the coating composition of the invention is to be measured at the same temperature as the curing rate caused by dibutyl tin dilaurate. Suitable temperatures are for example room temperature, i.e. between 18°C and 25°C, or elevated temperature, for example 60⁰C. The curing rate can suitably be measured by monitoring the decrease of the isocyanate signal in an infrared spectrum of the coating composition in time. In a specific embodiment, the coating composition comprises at least 0.2% by weight of a curing catalyst, calculated on the non-volatile content of the coating composition. In one embodiment, the coating composition is a non-aqueous coating composition.

The non-aqueous coating composition can be a single-pack composition curable under the influence of atmospheric moisture. The single-pack coating composition generally contains an organic solvent.

A solvent-borne isocyanate based moisture-curing coating composition is known from United States Patent US 6245877. This document describes a moisture-cure urethane composition comprising an isocyanate-terminated polymer and a solvent. The composition optionally includes a crosslinking catalyst, which allows curing to occur quickly. The composition can be used as primer and base coat to protect metal, wood, and concrete surfaces such as water tanks, pipes, bridges, and decks.

Hitherto it was believed that such moisture-curing isocyanate based compositions were not suitable for refinishing of automobiles due the possible formation of carbon dioxide bubbles generated in the reaction of isocyanate groups with atmospheric moisture. Such bubbles would detract from the performance and appearance requirements for automobile coatings. Furthermore, it was believed that the curing speed of such moisture-curing isocyanate based compositions would depend largely on the atmospheric moisture content, making the curing properties too unreliable for refinishing of automobiles.

The single-pack coating composition according to the invention has a practically unlimited pot life and does not require metering and mixing of its components prior to application. It has surprisingly been found that the applied coating has an acceptable curing speed under various conditions of atmospheric moisture. The cured coating exhibits the properties required for automobile coatings, such as good hardness, scratch resistance, elasticity, durability, resistance to water and solvents, and a good appearance, e.g. gloss. The cured coatings have also been found to exhibit a reduced number of pinholes, as compared to similar coatings based on polyisocyanates without apolar modification. In addition to the above-described apolar-modified polyisocyanate, the single- pack coating composition according to the invention may optionally comprise other polymeric and/or oligomeric binders and resins, provided that these materials are substantially inert towards the isocyanate groups of the polyisocyanate. This means that the polymers and/or oligomers do not comprise active hydrogen-functional groups capable of reacting with isocyanate groups, such as hydroxyl groups, amino groups or thiol groups. Examples of suitable materials are vinyl polymers, i.e. polymers which are obtainable by the polymerization of olefinically unsaturated monomers, polyesters, polyamides, polycarbonates, polyurethanes, and modified cellulose based materials.

Examples of suitable organic solvents for the coating composition according to the invention are aromatic or aliphatic hydrocarbons, such as toluene, xylene, Solvesso 100; ketones, such as acetone, 2-butanone, methyl amyl ketone, and methyl iso-amyl ketone; terpenes, such as dipentene or pine oil; halogenated hydrocarbons, such as dichloromethane or para-chlorobenzotrifluoride; ethers, such as ethylene glycol dimethyl ether, dipropyl ether, dibutyl ether, dipentyl ether, dioctyl ether; esters, such as ethyl acetate, ethyl propionate, n-butyl formate, n-butyl acetate, n-butyl propionate, n-butyl butyrate, the corresponding tert. -butyl, sec-butyl, and iso-butyl esters, esters of linear or branched pentanol, hexanol, or octanol, such as 2-ethyl-hexanol; or ether esters, such as methoxypropyl acetate or ethoxyethyl propionate. Also mixtures of these compounds can be used. In view of current and future legislation it is preferred that the composition according to the invention has a low content of volatile organic compounds (VOC). Examples of suitable VOC values are 500 g/l or less, 420 g/l or less, or 250 g/l or less.

In a further embodiment, the coating composition of the invention is a multi-pack coating composition comprising an isocyanate-reactive binder module and an apolar-modified polyisocyanate-containing crosslinker module. The crosslinker module contains as essential component the apolar-modified polyisocyanate as described above.

The binder module, which is to be combined and mixed with the crosslinker module prior to application, comprises an isocyanate-reactive binder. Suitable isocyanate-reactive binders are generally known to the skilled person. Isocyanate-reactive binders include hydroxyl-functional oligomers and polymers. Examples include polyether polyols, polyacrylate polyols, polyurethane polyols, cellulose acetobutyrate, hydroxy-functional epoxy resins, alkyds, and dendrimehc polyols such as described in International patent application WO 93/17060. Also, hydroxy-functional oligomers and monomers, such as castor oil, trimethylol propane, and diols may be present. Branched diols such as described in WO 98/053013, e.g. 2-butyl-ethyl-1 ,3-propanediol, may be particularly mentioned. The coating composition can also comprise latent hydroxy-functional compounds such as compounds comprising bicyclic orthoester, spiro- orthoester, or spiro-ortho silicate groups. These compounds and their use are described in WO 97/31073 and WO 2004/031256.

The apolar-modified polyisocyanates are particularly suitable for combination with hydroxyl-functional binders as described in WO 2007/020269.

Latent or non-latent amino-functional compounds such as oxazolidines, ketimines, aldimines, diimines, secondary amines, and polyamines can also be used as isocyanate-reactive binders. These and other compounds are known to the skilled person and are mentioned, int. al., in US 5214086.

Also thiol-functional binders can be used. In the case of thiol-functional binders, it is preferred to use them in combination with a basic curing catalyst. Preferably, the basic curing catalyst is a latent base. Thiol-functional binders and suitable catalysts are described in WO 01/92362. The one or more component coating compositions may further comprise other ingredients, additives or auxiliaries commonly used in coating compositions, such as pigments, dyes, surfactants, pigment dispersion aids, levelling agents, wetting agents, anti-cratering agents, antifoaming agents, antisagging agents, heat stabilizers, light stabilizers, UV absorbers, antioxidants, and fillers.

The multi-pack coating composition may be a non-aqueous liquid coating composition, as described above for the single-pack composition. In that case, the composition will suitably comprise an organic solvent in order to achieve the required application viscosity. Suitable organic solvents have been described above for the single-pack composition.

Alternatively, the multi-pack coating composition can be an aqueous composition, wherein water is the primary liquid diluent. Aqueous multi-pack coating compositions are known per se and they are described, int. al., in WO 2000/39181 , WO 01/081441 , WO 01/90265, and WO 2006/064035.

The multi-pack coating composition, in particular when it is a non-aqueous composition, can also comprise a pot life extending agent. Pot life extending agents increase the pot life of the coating composition, i.e. the time between the mixing of all components and the moment when the viscosity becomes too high for the composition to be applied. Pot life extending agents can suitably be present in similar amounts to those of the curing catalysts mentioned above. Preferred pot life extending agents have only a limited or no negative impact on the drying speed of the coating composition. Thus, these pot life extending agents improve the balance of pot life and drying speed. Examples of suitable pot life extending agents are carboxylic acid group-containing compounds, such as acetic acid, propionic acid or pentanoic acid. Aromatic carboxylic acid group- containing compounds are preferred, in particular benzoic acid. Other suitable pot life extending agents are dicarbonyl compounds, such as 2,4- pentanedione, phenolic compounds, tertiary alcohols such as tertiary butanol and tertiary amyl alcohol, and thiol group-containing compounds. It is also possible to use a combination of the above-mentioned pot life extending agents, such as a combination of an aromatic carboxylic acid group- containing compound and a thiol group-containing compound.

As is customary with multi-pack coating compositions comprising an isocyanate-functional crosslinker, the multi-pack composition according to the invention has a limited pot life. Therefore, the composition is suitably provided as a multi-component composition, for example as a two-component composition or as a three-component composition. Therefore, the invention also relates to a kit of parts for preparation of the coating composition comprising a) a crosslinker module comprising an apolar-modified polyiso- cyanate and b) a binder module comprising an isocyanate-reactive binder, wherein the binder module b) or an additional module c) comprises a curing catalyst in an amount sufficient to provide at least 0.2% by weight of a curing catalyst in the coating composition, calculated on the non-volatile content of the coating composition.

If the coating composition also comprises a pot life extending agent, it is preferred that the pot life extending agent is comprised in the binder component or in an optional reducer component.

An optional volatile diluent can be comprised in either or both of the components of the kit of parts. Alternatively, it is possible to provide a third reducer component comprising a volatile diluent. Alternatively, if the reducer component is used, either or both of the curing catalyst and the pot life extending agent may be comprised in the reducer component. The coating composition of the invention can be prepared by mixing the components of the kit of parts.

Application of the coating composition onto a substrate can be via any method known to the skilled person, e.g., via rolling, spraying, brushing, flow coating, dipping, and roller coating. Preferably, a coating composition such as described is applied by spraying.

The coating composition of the present invention can be applied to any substrate. The substrate may be, for example, metal, e.g., iron, steel, and aluminium, plastic, wood, glass, synthetic material, paper, leather, or another coating layer. The other coating layer can be comprised of the coating composition of the current invention or it can be a different coating composition. The coating compositions of the current invention show particular utility as clear coats, base coats, pigmented top coats, primers, and fillers. When the coating composition of the invention is a clear coat, it is preferably applied over a colour- and/or effect-imparting base coat. In that case, the clear coat forms the top layer of a multi-layer lacquer coating such as typically applied on the exterior of automobiles. The base coat may be a water borne base coat or a solvent borne base coat.

The coating compositions are suitable for coating objects such as bridges, pipelines, industrial plants or buildings, oil and gas installations, or ships. The compositions are particularly suitable for finishing and refinishing automobiles and large transportation vehicles, such as trains, trucks, buses, and airplanes.

The applied coating composition can be cured very effectively at a temperature of, e.g., 0-60°C. If so desired, the coating composition may be oven cured, e.g. at a temperature in the range of 60-120°C. Alternatively, curing may be supported by (near) infrared radiation. Before curing at elevated temperature the applied coating composition may optionally be subjected to a flash-off phase. It is to be understood that the term coating composition as used herein also includes its use as adhesive composition.

Examples

Materials and abbreviations used:

Tolonate^® XFD 9OB Hexamethylene diisocyanate based polyisocyanate ex Rhodia Tolonate^® HDT LV Hexamethylene diisocyanate based polyisocyanate ex Rhodia lsofol 18T and lsofol Branched monoalcohols with 16 - 20 carbon atoms

18E ex Sasol

Solvesso 150 Aromatic solvent mixture ex ExxonMobil

BYK 331 Silicone additive ex BYK Chemie

BYK 355 Acrylic levelling agent ex BYK Chemie

10% DBTDL solution A mixture consisting of 10 parts by weight of dibutyl tin dilaurate and 90 parts by weight of butyl acetate

Autobase^® Plus Modular solvent borne base coat system ex Akzo

Nobel Car Refinishes Autowave^® Modular water borne base coat system ex Akzo

Nobel Car Refinishes Autosurfacer^® 940HS Primer-surfacer ex Akzo Nobel Car Refinishes

Example 1 and comparative Example A

In these Examples a standard moisture cure single-pack isocyanate composition and an apolar-modified isocyanate single-pack composition were compared with each other.

The compositions were prepared one day in advance of application. The spraying viscosity and the VOC are approximately on the same level. The formulations were sprayed on base coat (Autobase Plus) applied to coil coated steel panels.

Directly after spraying the panels were cured at 6O⁰C.

The drying time and the foaming sensitivity were evaluated after curing. The components of the compositions, in parts by weight, and the results are summarized in Table 1.

Table 1

Comparative

Components Example A Example 1

Tolonate XFD 9OB 200.00 200.00 lsofol 18T 36.00

Solvent* 155.00 207.70

Byk 331 1.80 1.80

10% DBTDL solution 4.50 4.50

Properties

Viscosity after 1 day storage (DIN C4 cup, seconds) 14.1 14.5

VOC (g/l) 505 518

Drying time at 6O⁰C 20 min. 20 min

Foam density at layer thickness 40 μm few none

Foam density at layer thickness 50 μm medium none

^* Butylacetate/Solvessoi δO^-ethylhexylacetate 70/15/15.

The results indicate that there is a clear decrease in foaming sensitivity (pinholes) with the apolar-modified polyisocyanate of Example 1 over comparative Example A. All other properties are essentially on the same level.

Example 2 Preparation of an apolar-modified polyisocyanate for use according to the invention:

In a 1 -litre reactor equipped with a stirrer, a heating mantle with thermocouple, a nitrogen inlet, and a cooler, 500 grams of 1 ,6-hexanediisocyanate (HDI) and 100 grams of lsofol 18E were heated to 80⁰C under stirring under a nitrogen atmosphere. After 1 hour at 80⁰C the mixture was heated to 90°C and 2.09 grams of the catalyst solution (see below) were added in 90 minutes while keeping the temperature between 80 and 90⁰C. After 1 further hour at 90⁰C the reaction was stopped by addition of 0.75 grams of a 25% solution of bis(2- ethylhexyl)hydrogen phosphate in HDI. The excess of HDI was then removed by falling-film vacuum distillation at 0.1 mbar using toluene as heating liquid. 350 grams of product with an NCO-content of 15.5 % were obtained.

Catalyst solution: 12.5 grams of a 40% solution of benzyl thmethyl ammonium hydroxide in methanol were mixed with 100 g of n-butanol. About 20 grams of the solvent were removed by rotary evaporation and the solution was replenished with butanol to obtain 100 grams of solution.

Example 3

Preparation of an apolar-modified polyisocyanate for use according to the invention: Tolonate HDT-LV (162.88 grams) and lsofol 18E (66.33 grams) were reacted in a 500-ml 4-necked flask with nitrogen inlet, stirrer cooler, thermocouple, and heating mantle at 80°C until an NCO-content of 11.76 % was obtained.

Examples 4 and 5 and comparative Example B

A binder solution for the preparation of two-component clear coats was prepared by mixing the following components:

Compound g

Acrylic polyol¹⁾ 56.2

Polyester polyol²⁾ 21.0

10 % DBTDL solution 2.2

12 % Benzoic acid solution 12.7

50 % BYK 331 solution 1.0

Byk 355 0.5

Butyl acetate 19.5

Butylglycol acetate 3.5 1 ) Acrylic polyol resin prepared by polymerization of hydroxyethyl acrylate, butyl acrylate, tert. -butyl methacrylate, and methacrylic acid. The acrylic polyol is defined by the following parameters:

Tg = 345K, OH value = 140 mg KOH/gram, Acid value = 8.0 mg KOH/gram, Mn = 2,090, Mw = 4,470, non-volatile content = 67% in n- butyl acetate

2) Polyester polyol prepared from 44 parts by weight of trimethylol propane, 17 parts by weight of hexahydrophthalic anhydride, and 39 parts by weight of Edenor V85 (mixture of linear Cs and Cio fatty acids ex Cognis). The polyester polyol had a hydroxyl value of 306 mg KOH/g and an average hydroxyl functionality of 2.3.

The binder solution was mixed with a crosslinker solution prepared from 40.8 grams of either Tolonate HDT LV (comparative Example B) or the modified polyisocyanate from Examples 2 and 3 (Examples 4 and 5, respectively), 8.75 grams of butyl acetate and 8.75 grams of ethyl 3-ethoxypropionate. The mixtures of binder and crosslinker were sprayed over panels pre-coated with

Autosurfacer^® 940HS and a blue Autowave^® base coat. The coatings were sprayed in two layers with an increasing film thickness in order to judge the sensitivity to pinhole formation and foam building. The results are given below.

Again the results indicate that there is a clear decrease in foaming sensitivity (pinholes) with the apolar-modified polyisocyanates used in Examples 4 and 5 over comparative Example B. All other properties are essentially on the same level.

Claims

1. Use of an apolar-modified polyisocyanate for reducing the pinhole sensitivity of a coating prepared from a polyisocyanate-containing coating composition.

2. Use according to claim 1 , wherein the apolar-modified polyisocyanate has at least 11 carbon atoms which are part of a hydrocarbon group per isocyanate group.

3. Use according to claim 1 or 2, wherein the apolar-modified polyisocyanate is obtained by reaction of a polyisocyanate with an isocyanate-reactive compound having a hydrocarbon group having at least 6 carbon atoms.

4. Use according to any one of the preceding claims, wherein the isocyanate- reactive compound has at least one hydroxyl group and a hydrocarbon group having at least 9 carbon atoms.

5. Use according to any one of the preceding claims, wherein the coating composition is a non-aqueous coating composition.

6. Use according to claim 5, wherein the coating composition is a single-pack composition curable under the influence of atmospheric moisture.

7. Use according to claim 5, wherein the coating composition is a multi-pack coating composition comprising an isocyanate-reactive binder module and an apolar-modified polyisocyanate-containing crosslinker module.

8. Use according to any one of claims 1 to 4, wherein the coating composition is an aqueous coating composition.

9. Use according to any one of the preceding claims, wherein the apolar- modified polyisocyanate is the reaction product of a polyisocyanate and a diol derived from dimerized fatty acids.

10. Use according to any one of the preceding claims, wherein the isocyanate- containing coating composition comprises at least 0.2% by weight of a curing catalyst, calculated on the non-volatile content of the coating composition.

11. A coating composition comprising an apolar-modified polyisocyanate, wherein the coating composition additionally comprises a curing catalyst in an amount which causes at least the same curing rate acceleration as 0.2% by weight of dibutyl tin dilaurate, calculated on the non-volatile content of the coating composition, wherein the curing rate in the coating composition of the invention is measured at the same temperature as the curing rate with dibutyl tin dilaurate.

12. A coating composition according to claim 11 , wherein the coating composition is a non-aqueous composition.

13. A coating composition according to claim 12, wherein the coating composition is a single-pack composition curable under the influence of atmospheric moisture.

14. A coating composition according to claim 11 , wherein the coating composition is an aqueous coating composition.

15. A coating composition according to claim 11 , wherein the coating composition is a multi-pack coating composition comprising an isocyanate- reactive binder module and an apolar-modified polyisocyanate-containing crosslinker module.

16. A kit of parts for preparation of the coating composition according to claim 15 comprising a. a crosslinker module comprising an apolar-modified polyisocyanate and b. a binder module comprising an isocyanate-reactive binder, wherein the binder module b) or an additional module c) comprises a curing catalyst in an amount sufficient to provide at least 0.2% by weight of a curing catalyst in the coating composition, calculated on the non-volatile content of the coating composition.

17. An apolar-modified polyisocyanate which is the reaction product of a polyisocyanate and a diol derived from dimerized fatty acids.