WO2001098390A1

WO2001098390A1 - Aqueous coating composition

Info

Publication number: WO2001098390A1
Application number: PCT/GB2001/002420
Authority: WO
Inventors: David Robert Fenn; Andrew French
Original assignee: Ppg Industries Ohio, Inc.
Priority date: 2000-06-21
Filing date: 2001-05-31
Publication date: 2001-12-27
Also published as: BR0111550A; CA2409038C; CN100469809C; CN1432030A; US20040039112A1; CA2409038A1; BR0111550B1; EP1297042A1; GB0015027D0; EP1297042B1; AU2001260480A1; ES2321690T3; US20070100067A1; DE60138100D1

Abstract

There is disclosed a three component coating composition comprising: (i) a first component containing at least one acrylic polyol having a carboxyl group content of 0.035 to 2.0 moles/kg, said acrylic polymer being dissolved in volatile organic solvent, (ii) a second component containing a polyisocyanate, and (iii) a third component containing water, at least one of said components also including a base, the base being present in an amount capable of neutralising 0.035 to 0.2 moles of carboxyl groups per kilogram of polyol, the composition being essentially free from other hydrophilizing groups or components. There is also disclosed a method of preparing a coating composition, which comprises mixing the first component, second component and third component shortly before application. There is also disclosed a coated article obtainable by a process therefrom

Description

DESCRIPTION

AQUEOUS COATING COMPOSITION

This invention relates to a three component coating composition containing an

acrylic polyol having an acid value, a base, a crosslinker and water. It also relates to a

process for producing the coating composition, to a process of coating using the composition and to a coated substrate obtainable by the coating process.

Coating compositions that comprise hydroxy functional polymers and a

polyisocyanate compound have been known for some years. These traditionally comprise a hydroxyl functional acrylic copolymer together with a polyisocyanate compound in solution in an organic solvent. The isocyanate groups on the polyisocyanate react with the hydroxyl groups on the acrylic polymer so as to form a crosslinked film. Such reaction takes place at room temperature or at moderately

elevated temperatures.

Due to environmental considerations there is a general trend in the coatings

industry towards coating compositions with reduced organic solvent content. Coatings

with a lower organic solvent content emit lower levels of solvent when they are used and

so are less polluting of the atmosphere.

One way to achieve a lower solvent content is to use waterborne compositions. One method of incorporating acrylic addition polymers into water is to make them

carboxyl (-COOH) functional by the incorporation of some carboxyl functional

ethylenically unsaturated monomer such as acrylic acid during their manufacture, and

neutraUsing at least some of the carboxyl groups in the aqueous composition by adding

a base such as alkali metal hydroxide, ammonia or an amine. The resulting neutralised

carboxyl groups stabilise the polymer in dispersion in water.

US 5 075 370 describes a two component composition consisting of an aqueous solution or dispersion of an acrylic polyol and a polyisocyanate. The acrylic polyol contains chemically incorporated carboxylate or sulphonate groups. The polyisocyanate is emulsified in the aqueous solution or dispersion of the acrylic copolymer. The compositions contain relatively high levels of neutralised carboxyl groups.

EP 557 844 also describes a two component composition consisting of an aqueous dispersion of an acrylic polyol and a polyisocyanate. Once again the polyisocyanate is emulsified in the aqueous solution or dispersion of the acrylic copolymer. The acrylic copolymer has a low acid value in the range 0 to 7 mg KOH/g

and a total content of sulphonate and carboxylate groups of 0 to 4.5 milliequivalents per

100 g of solid resin. However the system can be dispersed into water primarily because

of the presence of anionic and/or non-ionic emulsifiers. Additionally the polyisocyanate

has an ethylene oxide content of 2 to 20% within polyether chains. An alternative approach is described in EP 663 413 which describes a coating

composition obtained by emulsifying a homogeneous mixture of a polyisocyanate and

an isocyanate reactive surface-active material in water. However isocyanate reactive

surface-active material has a relatively high carboxyl group content and the composition

contains relatively high levels of base.

A problem which we have discovered is that in practice dilution with water to

achieve suitable viscosities for application can result in a solids content that is too low

to achieve adequate film build per coat. For spray application a solids content of 35-60%

is preferred.

A further problem is that these compositions posses inferior water and humidity resistance in comparison to their solvent borne counterparts. This is particularly the case

when they are used in conjunction with other waterborne compositions in multilayer

systems. One cause for the inferior water resistance may be the presence of hydrophilic

components necessary to allow the compositions to be carried in water. The use of

carboxyl functional monomers at relatively high levels, together with the associated

neutralising amine, may lead to poor water resistance when these polymers are made into

coating compositions. EP 557 844 uses acrylic polyols with low acid values but in

practice other emulsifiers are used during their preparation. The polyisocyanate is also

required to contain hydrophilic polyethylene oxide chains to assist with dispersion into

water. The presence of additional emulsifϊer and polyethylene oxide will adversely affect the humidity resistance of the coating composition.

The invention relates to the use of low levels of base neutralized acid

groups in the polyol and we have found that we can produce waterborne two pack

polyurethane coating compositions that contain low levels of base neutralised carboxylic

acid groups, and that these compositioris have high solids at application viscosity and

exhibit improved water and humidity resistance compared to prior art systems.

According to the present invention there is provided a three component coating

composition comprising;

(I) a first component containing at least one acrylic polyol having a carboxyl

group content of 0.035 to 2.0 moles/kg, said acrylic polyol being dissolved in a volatile organic solvent,

(ii) a second component containing at least one polyisocyanate, and

(iii) a third component containing water,

at least one of said components also including a base, the base being present in

an amount capable of neutralising 0.035 to 0.2 moles of carboxyl groups per kilogram

of polyol,

the composition being essentially free from other hydrophilizing groups or components.

Where reference is made to the base being present in an amount capable of

neutralising 0.035 to 0.2 moles of carboxyl groups per kilogram of polyol, it is to be

understood that the polyol includes the acrylic polyol referred to together with any other

polyol present in the composition.

By "essentially free from hydrophilizing groups or components" as used

throughout this specification (including the claims) we mean that the base neutralised

carboxy groups are primarily responsible for allowing the system to be dispersed in

water. Preferably the base neutralised carboxy groups are solely responsible for allowing

the system to be dispersed in water.

The acrylic polyol having a carboxyl group content of 0.035 to 2.0 moles/kg is

derived from polymerisable ethylenically unsaturated monomers such as alkyl esters of

(meth) acrylic acid and vinyl monomers.

When used herein, the term acrylic monomer refers to acrylic or methacrylic acid

or their esters. The term (meth)acrylate refers to both the acrylate and methacrylate

equally and the term (meth)acrylic acid refers to acrylic or methacrylic acid equally.

Examples of suitable alkyl esters of (meth)acrylic acid are C_j__j2 alkyl

esters such as methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (rneth)acrylate, n-propyl (meth)acrylate, 2-ethyl hexyl (meth)acrylate, isobornyl

(meth)acrylate and lauryl (meth)acrylate. Examples of vinyl monomers are styrene and

alpha-methyl styrene.

Optionally chain transfer agents can be utilised. Chain transfer agents are

compounds that are used in the manufacture of acrylic addition polymers to control their

molecular weight. Examples of known chain transfer agents include mercapto compounds. Examples of mercapto compounds that can be used include octyl mercaptan, dodecyl mercaptan and pentaerythritol tetra(3-mercaptopropionate).

The carboxyl group content can be derived from unsaturated monomers having carboxyl groups and/or initiators having carboxyl groups. Examples of monomers having carboxyl groups are acrylic acid, methacrylic acid and beta-carboxyethyl acrylate. Examples of chain transfer agents having carboxyl groups are mercaptoacetic acid, 3- mercaptopropionic acid and 2-mercaptopropionic acid. An example of an initiator having a carboxyl group is 4,4'-azobis(4-cyanoverlaric acid). It is also possible to

produce carboxyl groups by modifying the polymer. For example hydroxyl functional groups can be reacted with cyclic anhydrides such as phthalic anhydride or hexhhydraphthalic anhydride. Preferably the carboxyl groups are derived from

(rneth)acrylic acid and/or 3-mercaptopropionic acid.

The acrylic polyol has a carboxyl group content of 0.035 to 2.0 mol/kg. Preferably the carboxyl group content is 0.035 to 1.0 mol/kg, more preferably 0.035 to

0.20 mol kg, even more preferably about 0.09 mol/kg.

The hydroxyl groups can be derived from vinyl and/or acrylic monomers having

hydroxyl groups and from chain transfer agents having hydroxyl groups. An example

of a vinyl monomer having hydroxyl groups is vinyl alcohol. Examples of acrylic

monomers having hydroxyl groups are hydroxyethyl (meth)acrylate, hydroxypropyl

(meth)acrylate and 4-hydroxybutyl (meth)acrylate. An example of a chain transfer agent

having hydroxyl groups is mercaptoethanol.

Other examples of suitable acrylic monomers having hydroxyl groups are the reaction products of glycidyl (meth)acrylate with mono-carboxylic acids, such as versatic acid and the reaction product of (meth)acrylic acid with monoepoxy compounds such as Cardura E (the glycidyl ester of versatic acid; trade mark of Shell).

Preferably the acrylic polyol has a hydroxyl value of 5 to 500 mgKOH/g of acrylic polyol, more preferably 50 to 250.

The acrylic polyol can contain other functional groups that may take part

in chemical reactions during the application and cure of the coating composition. Such

functional units can be derived from monomers which carry reactive groups other than

hydroxyl groups or carboxyl groups, such as acetoacetate groups and epoxy groups. An

example of a monomer carrying an acetoacetate group is aceotacetoxyethyl (meth) acrylate. An example of a monomer carrying an epoxy group is glycidyl (meth) acrylate.

Preferred acrylic polyols having carboxyl group content have a number

average molecular weight as measured by gel permeation chromatography of 700 to 10

000, more preferably 1 000 to 6000, most preferably 1 500 to 5 000.

Preferred acrylic polyols have a theoretical glass transition temperature (Fox Tg)

of -30 to 100 °C, more preferably -10 to 70 °C.

The acrylic polyol can be produced by conventional means. For example, in general it can be produced by contacting a mixture of the appropriate monomers including the chain transfer agent with a polymerisation initiator at a temperature at which polymerisation occurs.

The process for preparing the acrylic polyol can be carried out in volatile organic solvent. For example, the initiator can be fed into the solvent at the polymerisation

temperature simultaneously with the monomer mixture. The volatile organic solvent can be any solvent which will dissolve the acrylic polyol. It can be an aliphatic or aromatic

hydrocarbon such as Solvesso 100 (trademark), toluene or xylene, an alcohol such as butanol or isoproponal, an ester such as butyl acetate or hexyl acetate, a ketone such as

methyl isobutyl ketone, methyl ethyl ketone or methyl amyl ketone, an ether, an ether-

alcohol or an ether-ester or a mixture of any of these. Preferred solvents are esters and

ketones. Particularly preferred solvents are butyl acetate, methyl n-amyl ketone and methyl iso-amyl ketone.

Typical polymerisation temperatures are 50 to 150°C. Initiators can include for

example typical free radical types such as hydrogen peroxide, t-butyl hydroperoxide, di-

t-butyl peroxide, di-t-amyl peroxide, butylperoxy-2-ethyl hexanoate, benzoyl peroxide,

2,4-dichlorbenzoyl peroxide, t-butylperacetate and 2,2' azobis (2-methylbutyronitrile).

Polymerisation initiators are usually added in amounts between about 0.1 and 6% by

weight of the monomers polymerised, preferably between 0.5 and 5%.

The compositions of the invention also comprise a base which at least partially

neutralises the carboxyl groups on the addition polymer. Ammonia or an amine or

mixtures thereof are the preferred bases, while alkali metal hydroxide bases are useful

but less preferred. Examples of suitable amines are dimethylethanol amine, 2-amino-2-

methyl-1-propanol and triethylamine. The amount of base present is such as to be

capable of neutralising 0.035 to 0.2 moles of carboxyl groups per kilogram of polyol,

preferably 0.035 to 0.15 moles, more preferably 0.06 to 0.09 moles.

The compositions are essentially free from other hydrophilizing groups or

compounds. Hydrophilizing groups and compounds are well known and are used to

disperse otherwise hydrophobic compositions in aqueous media.

Hydrophilizing compounds include emulsifiers commonly used, for example, in

emulsion polymerisations. Emulsifiers include anionic emulsifiers and nonionic emulsifiers. Examples of anionic emulsifiers include sodium lauryl sulphate, sodium

dioctyl sulpohsuccinate, disodium octadecyl sulphosuccinamate and the ammonium salt

of a sulphate ester of a condensate of nonyl phenol and ethylene oxide.. Examples of

nonionic emulsifiers include the reaction products of ethylene oxide with long chain

alcohols such as stearyl alcohol or lauryl alcohol, the reaction products of ethylene oxide

with fatty acids and the poly(ethylene glycol) ether of nonyl phenol.

Hydrophilizing groups include groups capable of generating anions upon neutralisation, such as phosphoric groups, groups capable of generating cations upon neutralisation, such as amino groups, and hydrophilic non-ionic groups such as polyether chains formed from ethylene oxide.

When the acrylic polyol has been prepared in organic solvent, some or all of this can be removed, for example by distillation, before or after the polyol is combined with any other constituents of the first component. However, it is preferably that the type and level of organic solvents used during the preparation of the acrylic polyol are chosen such that they can remain as a constituent of the first component. The first component

can contain additional volatile organic solvent. This can be any solvent which will

dissolve the acrylic polyol. It can be an aliphatic or aromatic hydrocarbon such as

Solvesso 100 (trademark), toluene or xylene, an alcohol such as butanol or isoproponal,

an ester such as butyl acetate or hexyl acetate, a ketone such as methyl isobutyl ketone,

methyl ethyl ketone or methyl amyl ketone, an ether, an ether-alcohol or an ether-ester or a mixture of any of these. Particularly preferred solvents are butyl acetate, methyl n-

amyl ketone and methyl iso-amyl ketone.

The first component can contain water but the water must remain in solution or

must be dispersed in the form of a water in oil emulsion. The first component must not

be in the form of an oil in water emulsion. Preferably the first component contains

substantially no water.

So far as the second component is concerned, polyisocyanates are also well

known in the coatings art. Polyisocyanates are compounds having two or more

isocyanate groups per molecule. Suitable polyisocyanates are aliphatic or aromatic

polyisocyanates. Examples of suitable aliphatic diisocyanates are hexamethylene

diisocyanate and isophorone diisocyanate. Examples of suitable aromatic diisocyanates

are toluene diisocyanate and 4,4'-diphenylmethane diisocyanate.

Other suitable polyisocyanates include the isocyanurate trimers, allophanates and

uretdiones of diisocyanates such as those described above as well as the reaction

products of these diisocyanates with polyols. Polyols are compounds having three or

more hydroxyl groups. Suitable polyols include trimethylol propane, glycerol and

pentaerythritol.

Preferably the polyisocyanate contains more than 2 isocyanate groups per

molocule. Many such polyisocyanates are commercially available, for example under the Desmodur trade mark from Bayer and under the Tolonate trade mark from Rhone

Poulenc. Preferred polyisocyanates are isocyanurate trimers of hexamethylene

diisocyanate and the reaction product of trimethylol propane and 4,4'-diphenylmethane

diisocyanate available as Cythane 3174 from Cytec Industries.

Polyisocyanate crosslinkers are preferably used in an amount such that the ratio

of isocyanate groups to hydroxyl groups is in the range 0.8:1 to 2:1.

In addition to the polyisocyanate, the second component can contain volatile

organic solvent. This can be any solvent free from functional groups that will react with

the isocyanate groups and which will dissolve the polyisocyanate. It can be an aliphatic

or aromatic hydrocarbon such as Solvesso 100 (trademark), toluene or xylene, an ester

such as butyl acetate or hexyl acetate, a ketone such as methyl isobutyl ketone, methyl

ethyl ketone or methyl amyl ketone, an ether, an ether-ester or a mixture of any of these.

Particularly preferred solvents are butyl acetate, methyl n-amyl ketone and methyl iso-

amyl ketone.

Preferably the third component contains at least 60% by weight water, more

preferably at least 80% even more preferably at least 90%.

The compositions can also contain catalysts for the isocyanate-hydroxyl reaction.

Suitable catalysts include tin catalysts such as dibutyl tin dilaurate. The compositions can also contain other conventional paint additives such as

reactive diluents, pigments, fillers, UN absorbers, rheology control agents and flow aids.

Preferably the coating composition has a total volatile organic solvent content

(NOC) of 420 g/1 or less, more preferably about 250 g/1. The NOC can be calculated

using the following equation

VOC = 1000 x (100 - W_ΝVM - W_w) x D_c /((100-(W_W x D /D_w))

where

W_ΝVM is the mass in grams of non volatile material present in 100 g of coating

composition.

W_w is the mass in grams of water present in 100 g of coating composition.

D_c is the density in g/cm³of the coating composition at 23 °C

D is the density in g/cm³of the water at 23 °C

The compositions can also comprise one or more additional polymers such as

film forming polymers having hydroxyl groups. Examples of additional film forming

polymers having hydroxyl groups are polyester polyols and polyurethane polyols.

Preferably any additional polymers contain carboxyl groups. Preferably any additional

polymer or polymers have a carboxyl group content of 0.035 to 2.0 mol/kg, more preferably the 0.035 to 1.0 mol/kg, even more preferably 0.035 to 0.20 mol/kg, most

preferably about 0.09 mol/kg.

Preferably any additional polymeric components are incorporated in component

1 or component 3, more preferably in component 1.

Preferably the polymeric components of the composition, excluding any

crosslinkers, are made up of at least 60% by weight of acrylic polyol having carboxyl

group content of 0.035 to 0.02 mol/kg.

The coating composition of the invention can be applied as a layer to the surface

of a substrate and then allowed or caused to dry and cure. According to the present

invention there is provided a process for coating a substrate which comprises the steps

of applying a layer of a coating composition according to the present invention to a

surface of the substrate and thereafter causing or allowing the layer to cure.

The compositions are particularly useful as vehicle refmish primers or topcoats.

Primers are somewhat heavily pigmented compositions which are applied over the bare

substrate or over the pre-existing coating before the new topcoat is applied. Topcoats are

the final coating applied to give the vehicle its colour and gloss as well as providing

protection from the elements and physical damage.

The coating compositions are prepared by mixing the first component, second component and third component shortly before application. Preferably the first

component is mixed with the second component, creating a homogeneous solution of

polyol and polyisocyanate, before mixing with the third component to create an aqueous

dispersion. Mixing can be carried out by simple stirring, for example with a pallet knife,

or by mechanical means.

The coating composition can be applied by conventional means such as brushing,

rollercoating or spraying, preferably by spraying.

The applied layer can be allowed to cure at ambient temperature. Alternatively

the layer can be baked at elevated temperatures, for example 50-130°C.

According to the present invention there is also provided a coated article

obtainable by the process.

The invention will now be illustrated by means of the following examples.

In the examples the following abbreviations are used:-

AA acrylic acid

AMS alpha-methylstyrene

AN acid value (mg KOH/gΝN) BA butyl acrylate

Cythane 33174 adduct of 1,3-bis isocyanate 1-methylethyl) benzene (TMXDI) and 2-

ethyl-2- (hydroxymethyl)-l,3-propanediol (trimethyl propane) available from Cytec.

DMAE N,N-dimethylethanolamine

HBA 4-hydroxybutyl acrylate

HTD LV isocyanurate trimer of 1 ,6-diisocyanatohexane available from Rhodia

MiAK methyl isoamyl ketone

Mma methyl methacrylate

MPA 3-mercaptopropionic acid

POM 1-octanethiol

TBA t-butyl acrylate

Nazo 67 polymerisation initiator available from DuPont

St Styrene NN non-volatiles

Byk 346 a polyether modified polymethylsiloxane available from Byk Chemie

HeMa hydroxyethyl methacrylate

IboMa isobornyl methacrylate

Lma lauryl methacrylate

Synthesis of acrylic polyols 1-7

The formulations of the acrylic polyols used in this work are given in Table 1. They were all synthesised using the following procedure.

The charge was heated to reflux (approximately 140°C) in a reaction vessel fitted

with stirrer, heating mantle, water condenser and nitrogen blanket. The charge was held

at reflux and stirred whilst the feed mixture was fed into the reaction vessel at a uniform

rate over 180 minutes. The mixture was stirred for a further 15 minutes. Initiator shot

1 was added over 10 minutes, the mixture stirred for one hour. Initiator shot 2 was added

over 10 minutes, the mixture was stirred for a further one hour and then allowed to cool.

Examples I to XVII The formulations given in table 3 were prepared and tested as follows :-

The polymer solution and DMAE were mixed thoroughly in the proportions

shown in Table 3. The polymer numbers correspond to the polymers of Table 1. The

iscocyanate was then added whilst stirring until homogeneous. Demineralised water was

then slowly added whist the mixture was stirred with a spatula. The mass of water

required to reduce the viscosity of the mixture to between 25 and 30 seconds in a DIN

4 flow cup was recorded. The results are shown in Table 4.

Synthesis of acrylic polyol 8

Butyl acetate (998.8g) was heated to reflux (approximately 130°C) in a reaction vessel fitted with stirrer, heating mantle, water condenser and nitrogen blanket. Reflux

was maintained whilst a homogeneous mixture of styrene (932Jg), lauryl methacrylate

(711.7g), 4-hydroxybutyl acrylate (669.6g), acrylic acid (14.9g), 1-octanethiol (69.9g)

and Vazo 67 (102.5g) was fed into the reaction vessel at a uniform rate over 180 minutes.

The mixture was stirred for a further 15 minutes. Triganox 21 S (5.1g) was added over

10 minutes and the mixture stirred for one hour. Further Triganox 21 S (5.1 g) was added

over 10 minutes, the mixture was stirred for a further one hour and then allowed to cool. Table 1 polyols - formulations

impositions and molecular weights of the acrylic polyols are shown in Table 2

Table 2 c polyols - composition and molecular weight

iber of moles of carboxyl groups per kilogram of acrlyic polyol

Table 3

formulations

* ISO is a mixture of 80% by weight Cythane 3174 and 20% by weight HDT LN

** ISO is HDT LN

*** Number of moles of acid groups per kilogram of polyol that could be neturalised

by the base Table 4 - results

Example NIII

A cold rolled steel test panel was sanded with PI 80 sandpaper and

solvent wiped with white spirit. The panel was then coated with 20-25 microns of

chromate free etch primer P565-713 (available from ICI Autocolor, used as instructed

on the product data sheet), followed by 100 microns of Hidur Rapide undercoat

P565-693 (available from ICI Autocolor, used as instructed on the product data

sheet). 100 parts by weight of Aquabase medium coarse aluminium basic P968-9987

was mixed with 10.3 parts by weight of Aquabase activator P935-2018 and 3 parts

by weight of Aquabase thinner P275-366 (all available from ICI Autocolor). This

was spray applied to the panel to give a film build of 12-15 microns. The panel was then coated with the clearcoat composition given below.

Acrylic polyol solution 8 70g

DBTDL solution* 0.6g

Byk 346** 0.49g

RM8*** 2.0g

DMAE 0.38g

Polyisocyanate **** 37.4g

* solution of DBTDL (2% by weight) in butyl acetate

** a polyether modified polymethylsiloxane available from Byk Chemie

*** 11% by weight Acrysol TM-8W (a rheology modifier available from

Rohm and Haas) in demineralised water.

* * * * a mixture of 80% by weight Cythane 3174 and 20% by weight HDT LN

The clearcoat composition was prepared by first mixing all of the

ingredients except the polyisocyanate until homogeneous. The polyisocyanate was

added with stirring to give a homogeneous mixture. Demineralised water was then

added slowly to the mixture whilst it was stirred with a spatula. The mass of water flow cup was recorded. 25g of water was required giving a solids content of 58%.

The clearcoat was spray applied to give a film build of about 60 microns. After

application the coated panel was left at room temperature for thirty minutes before

being low-baked at 60° C for thirty minutes.

One week after application, the panel was immersed in demineralised water

at a temperature of 38 °C for ten days. The panel was inspected for signals of blistering or other defects every two days. No blisters or defects were evident.

Claims

1. A three component coating composition comprising;

(i) a first component containing at least one acrylic polyol having a carboxyl group content of 0.035 to 2.0 moles/kg, said acrylic polymer being dissolved in volatile organic solvent,

(ii) a second component containing a polyisocyanate, and

(iii) a third component containing water,

at least one of said components also including a base, the base being present in an amount capable of neutralising 0.035 to 0.2 moles of carboxyl groups per kilogram of polyol,

2. A composition as claimed in claim 1, wherein the amount of base present is such as to be capable of neutralising between 0.035 and 0.15 moles of carboxy groups per kilogram of polyol.

3. A composition as claimed in claim 2, wherein the amount of base present is such as to be capable of neutralising between from 0.06 and 0.09 moles of carboxy groups per kilogram of polyol.

4. A composition as claimed in any one of the preceding claims, wherein the carboxyl groups are derived from (meth)acrylic acid and/or 3-mercaptopropionic acid.

5. A composition as claimed in any one of the preceding claims, wherein the carboxyl groups are present at a level sufficient to give the acrylic polyol a carboxyl group content of 0.035 to l.Omol/kg.

6. A composition as claimed in claim 5, wherein the carboxyl groups are present at a level sufficient to give the acrylic polyol a carboxyl group content of 0.035 to 0.20 mol/kg.

7. A composition as claimed in claim 3, wherein the carboxyl groups are present at a level sufficient to give the acrylic polyol a carboxyl group content of 0.06 to 0.09 mol/kg.

8. A composition as claimed in any one of the preceding claims, wherein the polyisocyanate is the isocyanate trimer of hexamethylene diisocyanate and/or the reaction product of trimethylol propane and 4,4'-diphenylmethane diisocyanate.

9. A composition as claimed in any one of the preceding claims, wherein the polyisocyanate is used in an amount such that the ratio of isocyanate groups to the number of hydroxyl groups is in the range of 0.8 to 2.

10. A composition as claimed in any one of the preceding claims, wherein the acrylic polyol has a hydroxyl value of 5 to 500 mgKOH/g of polymer.

11. A composition as claimed in claim 1 wherein the ar.rvlϊr nr»1vnl hoc α hydroxyl value of 50 to 250mgKOH/g of polymer.

12. A composition as claimed in any one of the preceding claims, wherein the acrylic polyol has a number average molecular weight as measured by gel permeation chromatography of 700 to 10 000.

13. A composition as claimed in claim 12, wherein the acrylic polyol has a number average molecular weight as measured by gel germeation chromatography of 1000 to 6000.

14. A composition as claimed in claim 13 wherein the acrylic polyol has a number average molecular weight as measured by gel germeation chromatography of 1500 to 5000.

15. A composition as claimed in any one of the preceding claims, wherein the acrylic polyol has a theoretical glass transition temperature (Fox Tg) of -30 to 100 °C.

16. A composition as claimed in claim 15, wherein the acrylic polyol has a theoretical glass transition temperature of -10 to 70° C.

17. A composition as claimed in any one of the preceding claims, wherein the base is ammonia or an amine or mixtures thereof.

18. A composition as claimed in any one of the preceding claims, wherein the third component contains at least 60% by weight water. includes one or more of the following:-

(i) catalysts for the isocyanate-hydroxyl reaction,

(ii) reactive diluents,

(iii) pigments,

(iv) fillers,

(v) UN absorbers,

(vi) rheology control agents, and

(vii) flow aids.

20. A composition as claimed in any one of the preceding claims, which also comprises one or more additional polymers.

21. A composition as claimed in claim 20, wherein the additional polymers are selected from polyester polyols and polyurethane polyols.

22. A composition as claimed in claim 20 or 21 wherein any additional polymers have a carboxyl group content of 0.035 to 2.0 mol/kg.

23. A composition as claimed in any one of the preceding claims, wherein the polymeric components of the composition, excluding any crosslinkers, are made up of at least 60% by weight of acrylic polyol having a carboxyl group content of 0.035 to 0.02 mol/kg.

24. A process for coating a substrate which comprises the steps of applying a layer of a coating composition as claimed in any one of the preceding claims, to a surface of the substrate and thereafter causing or allowing the layer to cure.

25. A process as claimed in claim 21, in which the coating is a vehicle refinish primer or topcoat.

26. A method of preparing a coating composition as claimed in any one of claims 1 to 23, which comprises mixing the first component, second component and third component shortly before application.

27. A method as claimed in claim 26, wherein the first component is mixed with the second component, creating a homogeneous solution of polyol and polyisocyanate, before mixing with the third component to create an aqueous dispersion.

28. A coated article obtainable by a process as claimed in claim 24 or 25.

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20 JUNE 2000