WO2022090245A1

WO2022090245A1 - Primer composition

Info

Publication number: WO2022090245A1
Application number: PCT/EP2021/079707
Authority: WO
Inventors: Komkrit SAJJAANANTAKUL; Arunwat PROMNIMIT
Original assignee: HAYDALE TECHNOLOGIES (Thailand) Company Limited; Haydale Graphene Industries Plc
Priority date: 2020-10-27
Filing date: 2021-10-26
Publication date: 2022-05-05
Also published as: GB2600786A; JP2023550845A; GB202103875D0

Abstract

The present invention relates to an aqueous conductive primer coating composition comprising: 5.0 - 10.0 wt.% carbon filler comprising a surface functionalised carbon filler; 10.0 -20.0 wt.% water miscible organic solvent; 5.0 -15.0 wt.% acrylic-based binder and 0.5 – 5.0 wt% cross-linking agent

Description

PRIMER COMPOSITION

FIELD OF THE INVENTION

The present invention relates to primer coating compositions suitable for coating plastic materials such as acrylonitrile butadiene styrene to prepare them for electrostatic spray coating. The invention also relates to a method of manufacturing the primer coating composition; a process for the formation of a coating film using this composition and a coated article.

BACKGROUND OF THE INVENTION

Plastic parts made of materials such as acrylonitrile butadiene styrene (ABS) and polyamide (PA) are used widely in automobiles and motorbikes e.g. in automotive bumpers and automotive body parts.

In modern automobile and motorcycle manufacturing, these parts are usually coated (painted) by electrostatic coating, which shows excellent deposition efficiency and thus emits only a small amount of environmentally harmful substances in comparison to alternative coating processes such as air spray coating.

To efficiently electrostatically spray a paint, the substrate must be conductive. Since plastic substrates generally have high electrical resistance (conductivity values usually about 10¹² to 10¹⁶ Ohm/Sq.) a method which involves applying a primer before coating a paint is often adopted in order to increase the conductivity of the plastic surface and ensure adhesion between the material and the paint.

Typically primer compositions are organic solvent based. For example US 4971727 describes a solvent based conductive primer for a plastic article (see the section titled “preferred embodiment” of US 4971727). PCT/US2004/003196 also describes an electrically conductive primer composition which contains solvent and pigments (see the abstract of PCT/US2004/003196).

However, as a result of environmental concerns and legislation such as the Environmental Protection Act in the United Kingdom, in recent years there has been increased interest in aqueous conductive primers which reduce the usage of volatile organic components (VOCs). Using water as the solvent can also have advantages in terms of improved thermoplastic qualities of the primer, and allows the equipment used for spraying the primer to be cleaned using water.

Aqueous primer compositions have previously been described, for example, in EP 1 354 923 A1 , which describes an aqueous primer coating composition comprising an acid-anhydride- modified chlorinated polyolefin emulsion resin, an aqueous urethane dispersion, an aqueous epoxy resin and an organic strong base and/or its salt (see claim 1 of EP 1 354 923 A1). US 2020/0010698 A1 also relates to an aqueous primer coating composition, the composition containing an aqueous polyolefin resin, an aqueous polyurethane resin, a curing agent and electrically conductive carbon. Similarly, Tanaka et al., SAE International Journal of Materials and Manufacturing (2013), 6, 1 , 113 - 123 describes an aqueous conductive primer comprising urethane resins with carboxylic acid groups and acrylic resins with amide groups, meaning that this primer is able to bind effectively to both ABS and PA substrates.

However, there remains a need in the art to develop improved aqueous primers for use as a substrate for electrostatic spray coating. In particular, there is a need to develop primers which demonstrate improved conductivity and/or use less or cheaper components. There is also a need to develop aqueous primers which are specific for ABS substrates and can be more evenly dispersed over the surface of a substrate. In addition, there is a need for primers that are universally compatible with a wide range of different types of paints and coating methods - for example, it is desirable that a primer is compatible with both acrylic and polyurethane paints and can be used in coating methods where only a single paint coat is applied to the substrate or in processes where multiple paint coats are applied to the substrate. Furthermore, there is a need for primers compositions with improved storage stability before use.

SUMMARY OF THE INVENTION

In view of the above problems in a first aspect the present invention provides an aqueous conductive primer coating composition comprising:

• 5.0 - 10.0 wt.% carbon filler comprising a surface functionalised carbon filler;

• 10.0 -20.0 wt.% water miscible organic solvent;

• 5.0 -15.0 wt.% acrylic-based binder; and

• 0.5 - 5.0 wt% cross-linking agent.

By “primer composition” we mean a preparatory coating applied to a substrate before painting. Generally, a primer is applied to ensure better adhesion of the paint to the surface, to increase paint durability and to provide additional protection for the material being painted.

The aqueous conductive primer coating composition according to the present invention has a number of advantageous features.

Firstly, the carbon filler displays high conductivity, meaning that the aqueous conductive primer coating composition can form suitable conductive layers on a substrate at relatively low loadings of carbon filler. These low loadings mean that the mechanical properties of the primer coating layer can be dominated by the binder. Aqueous conductive primer coating compositions according to the present invention have been demonstrated to produce coating films with sheet resistance values as low as 10³ Ohm/Sq. To our knowledge this is greater than any available water-based conductive primer. By comparison, known water-based primers often have sheet resistance values in the range of 10⁸-10⁹ Ohm/Sq (see the experimental section below).

Secondly, the primer according to the present invention can be used with many different types of paint and painting processes. In particular, primers according to the present invention can be used in coating methods which involve overcoating the primer layer with a single paint coat (so called “one layer” painting processes) to obtain a coated article or with multiple paint coats (so called “multi-layer” painting processes). Thirdly, surface functionalisation of at least a portion of the carbon filler in the aqueous conductive primer coating composition leads to increased dispersion and stability of the composition, meaning that an even coating of primer can be achieved on the substrate. This can lead to a smoother finish when the paint coat is applied. In addition, this surface functionalisation has also been found to reduce the amount of carbon filler required to achieve a desired level of conductivity, which can potentially lead to cost savings and allow the properties (in particular, the mechnical properties) of the binder to dominate. Moreover, the surface functionalisation helps the carbon filler to stay dispersed over longer periods (weeks, as opposed to the small number of days generally observed with an analogous non-functionalised carbon filler) with minimal sedimentation, increasing the period over which the composition can be stored before use.

Fourthly, the aqueous conductive primer coating composition comprises low levels of organic solvents minimising the release of VOCs into the atmosphere as a result of the painting process.

Fifthly, the use of an acrylic-based binder (for example an acrylic binder or a urethane-acrylic binder) in the aqueous conductive primer coating composition means that this composition binds well to both underlying plastic substrates (such as ABS) and also to overlying acrylic and urethane based paints used in automotive manufacturing.

Carbon filler

The type of carbon filler used in the aqueous conductive primer coating composition of the present invention is not restricted, provided that it is conductive.

The carbon filler may be any type of carbon based material, such as carbon black, acetylene black (ACB), carbon nanotubes, carbon nanorods, or graphitic or graphene platelets, including graphene nanoplatelets. The carbon filler is generally a particulate carbon material.

Preferably, the carbon filler comprises carbon black, acetylene black (ACB - which is sometimes considered to be a specific type of carbon black) and/or graphene particles.

The aqueous conductive primer coating composition may comprise 5.0-10.0 wt.% carbon black; or 5.0-10.0 wt.% ACB; or 5.0-10.0 wt.% graphene particles; or mixtures of these fillers, wherein the total amount of carbon filler in all cases is in the range of 5.0-10.0 wt.%.

More preferably, the carbon filler consists of carbon black and/or ACB (acetylene black). The use of these materials as carbon fillers gives primers with high levels of conductivity at low costs. The use of compositions comprising carbon black is particularly preferred because it is relatively low cost in comparison to other forms of carbon filler.

The type of carbon black used in the present invention is not particularly limited. For example, the carbon black may be channel black, furnace black, lamp black or thermal black. Carbon black is generally obtained by the incomplete combustion of heavy petroleum products, for example FCC tar, coal tar or ethylene cracking tar. The carbon black may have a paracrystalline or amorphous structure. The carbon black may be acidic, neutral or basic.

Carbon black is commercially available, for example as CABOT BP 2000, Degussa Printex XE- 2B Mitsubishi MA-7 and Orion FW 200.

In addition, or alternatively, the carbon filler may comprise graphene particles. The graphene particles (which can be referred to as “graphene-material particles”, or “graphene-based particles”) may take the form of monolayer graphene (i.e. a single layer of carbon) or multilayer graphene (i.e. particles consisting of multiple stacked graphene layers). Multilayer graphene particles may have, for example, an average (mean) of 2 to 100 graphene layers per particle. When the graphene particles have 2 to 5 graphene layers per particle, they can be referred to as “few-layer graphene”.

Advantageously, graphene particles provide extremely high aspect ratio conductive particles. This high aspect ratio allows the formation of conductive paths at relatively low loading levels, decreasing the amount of carbon filer that must be added to the aqueous conductive primer coating composition to obtain the same levels of conductivity.

The graphene particles may take the form of plates/flakes/sheets/ribbons of multilayer graphene material, referred to herein as “graphene nanoplatelets” (the “nano” prefix indicating thinness, instead of the lateral dimensions).

The graphene nanoplatelets may have a platelet thickness less than 100 nm and a major dimension (length or width) perpendicular to the thickness. The platelet thickness is preferably less than 70 nm, preferably less than 50 nm, preferably less than 30 nm, preferably less than 20 nm, preferably less than 10 nm, preferably less than 5 nm. The major dimension is preferably at least 10 times, more preferably at least 100 times, more preferably at least 1 ,000 times, more preferably at least 10,000 times the thickness. The length may be at least 1 times, at least 2 times, at least 3 times, at least 5 times or at least 10 times the width.

The carbon filler is preferably at least 90 wt.% carbon (based on elemental analysis), more preferably at least 95 wt.% carbon.

Surface functionalised carbon filler

Preferably, the amount of surface functionalised carbon filler is at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, more preferably at least 70%, more preferably at least 80%, or even more preferably at least 90% of the total carbon filler in the primer composition. It is especially preferred that all of the carbon filler is surface functionalised carbon filler. All percentages are based on the weights of the carbon fillers.

The surface functionalised carbon filler is a carbon material which has had the surface chemistry of the functional groups on the surface of the material modified in order to achieve high conductivities and dispersal in the aqueous conductive primer coating composition. This can be achieved by adding, altering or removing selected chemical groups from the surface of the materials.

The surface functionalised carbon filler is preferably carbon black and/or ACB, since the present inventors have found that low levels of these carbon materials can achieve high conductivities. This means that the primer can be produced cheaply and that the matrix properties are dominated by the binder meaning that the primer coating retains its flexibility.

In cases where the aqueous conductive primer coating composition comprises both functionalised carbon filler and carbon filler that has not been functionalised, these materials can be the same type of carbon materials or a different type of carbon material. For example the carbon filler may consist of functionalised carbon black and unfunctionalised carbon black. Alternatively, the carbon filler may consist of functionalised carbon black and unfunctionalised ACB.

Preferably, the carbon filler is functionalised with hydrophilic functional groups. For example, the carbon filler may be oxygen-functionalised, hydroxy-functionalised, carboxy-functionalised, carbonyl-functionalised, amine-functionalised, amide-functionalised, halogen functionalised or silane functionalised or a hybrid of one or more of these types of functionalisation. Preferably the carbon filler is oxygen-functionalised, hydroxy-functionalised, carboxy-functionalised, carbonyl-functionalised, amine-functionalised, amide-functionalised or a hybrid of one or more of these types of functionalisation. These types of functionalisation are obtainable by plasma treatment of a carbon filler in a range of gases, liquids or monomers as outlined below. More preferably the carbon filler is oxygen and/or amine functionalised, most preferably oxygen and amine functionalised. This means that the particles have functionalities comprising oxygen (O) and nitrogen (N) on their surfaces. These types of functionalities are obtainable by plasma treatment of a carbon filler in oxygen (O2) or nitrogen (N₂) gas as outlined below.

The surface functionalised carbon filler may comprise at least 0.1 %, or at least 1%, or at least 5 % or at least 10 % elements other than carbon based on the total weight of carbon filler (based on elemental analysis). The surface functionalised carbon filler may be at least 0.1% oxygen, or at least 1% oxygen or at least 5 % oxgen based on the total weight of carbon filler (based on elemental analysis) or the surface functionalised carbon filler may be at least 0.1% nitrogen, or at least 1% nitrogen or at least 5 % nitrogen based on the total weight of carbon filler (based on elemental analysis).

The maximum amount of elements other than carbon may be, for example, 20%, 30% or 40% based on the total weight of the filler (based on elemental analysis).

Preferably, the functional groups present on the surface of the surface functionalised carbon filler are hydroxyl, carboxyl, carbonyl, amine, amide or a mixture of these functionalities. Most preferably the functional groups present on the surface of the surface functionalised carbon filler are as follows:

Where represents the bond to the carbon surface and R is a C1-5 alkyl.

These functional groups are present at the surface of the filler and are generally not present in the bulk of the material. Without wanting to be bound by any theory it is believed that functionalisation is restricted to the outermost (e.g. 1 or 2) layers of the surface functionalised carbon filler. These groups may be added at any part of the surface, for example on the faces and/or edges of particles.

Any suitable type of functionalisation process can be used to achieve the desired functionalisation. However, preferably, the surface functionalised carbon filler is plasma- functionalised carbon particles (i.e. carbon particles which have been functionalised using a plasma-based process). Advantageously, plasma-functionalised carbon particles can display high levels of functionalisation, and uniform functionalisation. Using a plasma-based process to functionalise the carbon particles leads to a reduced level of damage to the structures of the particles compared to wet chemistry methods and allows bespoke functionalisation of the particles and avoids the presence of impurities. For example, the Hummers’ method used to generate graphene oxide can introduce metallic impurities (especially manganese from the catalyst used), as well as sulphur impurities (from the sulphuric acid used in the production process). Preferably, the carbon filler comprises less than 1% sulphur, less than 0.5% sulphur, less than 0.2% sulphur, or less than 0.1% sulphur, as assessed by elemental analysis. Likewise, the amount of metallic impurity may be less than 0.5%, less than 0.2%, or less than 0.1% on an elemental basis.

Plasma functionalisation of the carbon particles, may be achieved as follows: the starting carbon filler is subjected to a particle treatment method for disaggregating, de-agglomerating, exfoliating, cleaning or functionalising particles, in which the particles for treatment are subject to plasma treatment and agitation in a treatment chamber. Preferably the treatment chamber is a rotating container or drum. Preferably the treatment chamber contains or comprises multiple electrically-conductive solid contact bodies or contact formations, the particles being agitated with said contact bodies or contact formations and in contact with plasma in the treatment chamber.

Preferably the contact bodies are moveable in the treatment chamber. The treatment chamber may be a drum, preferably a rotatable drum, in which a plurality of the contact bodies are tumbled or agitated with the particles to be treated. The wall of the treatment vessel can be conductive and form a counter-electrode to an electrode that extends into an interior space of the treatment chamber.

During the treatment, desirably glow plasma forms on the surfaces of the contact bodies or contact formations. The pressure in the treatment vessel is usually less than 500 Pa. Desirably during the treatment, gas is fed to the treatment chamber and gas is removed from the treatment chamber through a filter. That is to say, it is fed through to maintain chemical composition if necessary and/or to avoid build-up of contamination.

The treated material, that is, the particles or disaggregated, deagglomerated or exfoliated components thereof resulting from the treatment, may be chemically functionalised by components of the plasma-forming gas, forming e.g. carboxy, carbonyl, hydroxyl, amine, amide or halogen functionalities on their surfaces. Plasma-forming gas in the treatment chamber may be or may comprise e.g. any of oxygen, water, hydrogen peroxide, alcohol, nitrogen, ammonia, amino-bearing organic compound, halogen such as fluorine, halohydrocarbon such as CF4, and noble gas (e.g. argon). Preferably the gases used are oxygen and ammonia, giving a carbon filler which is amine and oxygen functionalised.

Any other treatment conditions disclosed in WO2010/142953 and WO2012/076853 may be used, additionally or alternatively; or other means of functionalising (such as wet chemistry for example using the Hummers' method for providing oxygen functionalised carbon materials) and/or disaggregating carbon particles may be used for the present processes and materials.

Without wishing to be bound by any theory it is believed that using a surface functionalised carbon filler in the primer leads to increased conductivity levels and increased dispersion and stability of the primer.

Preferably the carbon filler particles (both surface functionalised and any non-functionalised filler) have a volume average mean particle size of less than 10 pm. Volume average mean particle size can be determined using any suitable method known to a skilled person in the art such as light scattering (mean size = mean hydrodynamic diameter of the particles) or laser distribution particle size. Preferably, light scattering is used. Light scattering can be measured using a light scattering analyser e.g., dynamic light scattering particle size distribution analyzer LB-550 (available from HORIBA).

The use of particles with a mean particle size of less than 10 pm, means that the particles are able to be better distributed in the aqueous conductive primer coating composition and are suitable for use in spray coating which requires a high level of smoothness allowing the primer to flow throught the filter used in the spraying apparatus easily. Optionally, the minimum mean particle size is 1 pm.

Optionally, the size distribution of the carbon filler is multimodal (has multiple peaks), for example, bimodal If the carbon filler consists of two different sizes of particle (i.e. a group of particles with a reletively larger volume average mean particle size and a group of particles with a reletively smaller volume average mean particle size). This may be achieved by using two different types of carbon particle e.g. ACB and carbon black. Without being bound by any theory it is believed that having two different sizes of particles in the carbon filler leads to better conductivity as the small particles are able to fill in the holes in the matrix formed by the larger particles.

Optionally, there are a limited number of particles having a size considerably larger than the volume average mean particles size. For example, the D₉₀ value (the value wherein the portion of particles with diameters below this value is 90%; determined using dynamic light scattering) may be less than 200%, less than 180%, or less than 160% of the mean particle size. In instances where the particles have a multimodal distribution, the D₉₀ value may be less than 200%, less than 180%, or less than 160% of the peak of the distribution occuring at the highest size value. The median particle size may be within 80% to 120% of the value of the mean particle size. Optionally, the D value (the value wherein the portion of particles with diameters smaller than this value is 10%; determined using dynamic light scattering) is at least 40 % of the mean particle size. In instances where the particles have a multimodal distribution, the D value may be within 80 to 120% of the peak of the distribution occuring at the lowest size value. For example, for a mean particle size of about 6.5 pm, the median is preferably from 5.2 - 7.8 pm. The D value is at least 2.6 pm and the D₉₀ value is less than 10.4 pm.

Water miscible organic solvent

Within the meaning of this invention the term “water miscible organic solvent” or “miscible organic solvent” is defined as an organic solvent that is miscible with water. In some cases the water miscible organic solvent is a polar organic solvent.

The aqueous conductive primer coating composition comprises from 10.0 to 20.0 wt.% of a water miscible organic solvent. Preferably from 10.0 to 12.0 wt.%

Without being bound by any theory it is believed that the use of low levels of organic solvents in particular helps to better disperse the carbon filler in the primer. This is particularly important as it helps to increase the shelf life of the aqueous primer coating composition, which may be stored for a period of months. It is important that the carbon filler remains suspended in the composition during storage.

Preferably, the water miscible organic solvent is a straight or branched chain alcohol, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, 2-butanol, isobutanol, t-butanol, cyclohexanol. More preferably the water miscible organic solvent is a C1-5 alcohol. Most preferably the water miscible organic solvent is ethanol or isopropanol.

The aqueous conductive primer coating composition may further comprise dimethyl sulfoxide. In a certain prefered embodiment the water miscible organic solvent may consist of isopropanol and dimethyl sulfoxide. The water miscible organic solvent may comprise isopropanol and dimethyl sulfoxide in a 1 to 1 ratio.

Cross-linking agent

The aqueous conductive primer coating composition comprises 0.5 - 5.0 wt.% of a cross-linking agent, preferably from 0.5 to 2.0 wt.% more preferably from 0.5 to 1 .0 wt.%. The cross-linking agent enables curing of the composition at temperatures of 70 - 180 ^eC used in the drying step and helps to improve the film performance characteristics of the primer, such as water resistance. Any conventional cross-linking agent can be used. Preferably curing is conducted at a temperature of from 70 - 100 ^eC, more preferably at a temperature of from 70 - 85 ^eC, most preferably from 70 - 75 ^eC.

Usable cross-linking agents include melamine resins, epoxy resins, carbodiimide resins and oxazoline compounds etc.

Preferably the cross-linking agent is an epoxy silane.

Acrylic-based binder

The conductive primer coating composition of the present invention comprises 5.0 - 15.0 wt.% of an acrylic-based binder.

The binder may be, for example, an acrylic binder or a urethane-acrylic binder.

Examples of acrylic binders which may be used in the present invention are acrylic resins, which are copolymers of acrylic monomers and other ethylenic unsaturated monomers. Specific examples of acrylic monomers usable for the copolymer include acrylic or methacrylic ester such as methyl, ethyl, propyl, n-butyl, i-butyl, t-butyl, 2-ethylhexyl, lauryl, phenyl, benzyl, 2- hydroxyethyl, or 2-hydroxypropyl ester; amide group-containing acryl monomers such as acrylamide, methacrylamide, N,N-dimethylacrylamide, N,N-dimethylmethacrylamide, N,N- dibutylacrylamide, and N,N-dibutylmethacrylamide; caprolactone ring-opened adducts of 2- hydroxyethyl acrylate or methacrylate; (meth)acrylic esters of polyhydric alcohols; etc. Examples of the other ethylenic unsaturated monomers that can be copolymerized with the acrylic monomers include styrene, a-methylstyrene, itaconic acid, maleic acid, vinyl acetate, etc.

Commercial examples of acrylic binders are LR White, LR Gold, and the Lowicryl series (K4M, K11 M, HM20 and HM23).

Without being bound by any theory, it is believed that acrylic binders allow the aqueous conductive primer coating composition to effectively bind to an ABS sustrate as a result of dipolar interactions between the hydroxy groups of the acrylic binder and the nitrile groups in the ABS. Additionally, the use of an acrylic binder means that strong adhesion is demonstrated between the acrylic binder and polyurethane and acrylic based paints used in automotive coating applications.

Urethane-acrylic binders may be prepared by reacting an isocyanate functional compound with a hydroxyl-functional compound with a hydroxyl-functional acrylate.

The polyisocyanate that is reacted with the hydroxy functional acrylate can be any organic polyisocyanate. The polyisocyanate may be aromatic, aliphatic, cycloaliphatic, or heterocyclic and may be unsubstituted or substituted. Many such organic polyisocyanates are known, examples of which include: toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, and mixtures thereof; diphenylmethane-4,4'-diisocyanate, diphenylmethane-2,4'-diisocyanate and mixtures thereof; o-, m- and/or p-phenylene diisocyanate; biphenyl diisocyanate; 3,3'-dimethyl-4,4'- diphenylene diisocyanate; propane-1 ,2-diisocyanate and propane-1 ,3-diisocyanate; butane-1 ,4- diisocyanate; hexane-1 ,6-diisocyanate; 2,2,4-trimethylhexane-1 ,6-diisocyanate; lysine methyl ester diisocyanate; bis(isocyanatoethyl)fumarate; isophorone diisocyanate; ethylene diisocyanate; dodecane-1 ,12-diisocyanate; cyclobutane- 1 ,3-diisocyanate; cyclohexane-1 ,2- diisocyanate, cyclohexane-1 ,3-diisocyanate, cyclohexane- 1 ,4-diisocyanate and mixtures thereof; methylcyclohexyl diisocyanate; hexahydrotoluene-2,4-diisocyanate; hexahydrotoluene- 2,6-diisocyanate and mixtures thereof; hexahydrophenylene-1 ,3-diisocyanate; hexahydrophenylene-1 ,4-diisocyanate and mixtures thereof; perhydrodiphenylmethane-2,4'- diisocyanate, perhydrodiphenylmethane-4,4'-diisocyanate and mixtures thereof; 4,4'-methylene bis(isocyanato cyclohexane) available from Mobay Chemical Company as Desmodur W; 3,3'- dichloro-4,4'-diisocyanatobiphenyl, tris(4-isocyanatophenyl)methane; 1 ,5- diisocyanatonaphthalene, hydrogenated toluene diisocyanate; 1-isocyanatomethyl-5- isocyanato-1 ,3,3-trimethylcyclohexane and 1 ,3,5-tris(6-isocyanatohexyl)-biuret.

Examples of hydroxyl-functional acrylates which can be reacted with the polyisocyanate polyurethanes to form urethane-acrylic binders include: 2-hydroxyethyl(meth)acrylate; glycerol di(meth)acrylate; the (meth)acrylates of the glycidyl ethers of butanol, bisphenol-A, butanediol, diethylene glycol, trimethylolpropane and other mono-, di-, tri- and polyhydric alcohols; the (meth)acrylates of epoxides such as styrene oxide, 1 -hexane oxide, 1 -decene oxide, 1 -butene oxide; the (meth)acrylates of epoxidized fatty acids such as linoleic and linolenic acid; the (meth)acrylates of epoxidized linseed and soya oils; 2- and 3-hydroxypropyl(meth)acrylate, 4- hydroxybutyl(meth)acrylate; and halogenated hydroxyalkyl acrylates such as 3-chloro-2- hydroxypropyl(meth)acrylate; 3-bromo-2-hydroxypropyl(meth)acrylate; 2-chloro-1 - (hydroxymethyl)ethyl(meth acrylate, and 2-bromo-1 -(hydroxymethyl)ethyl(meth)acrylate.

Preferably, the aqueous conductive primer coating composition has a viscosity value of not more than 600 mPa s^-1 or not more than 500 mPa s^-1 measured at 20 °C using a dynamic sheer rheometer (Kinexus DSR from Nesus analytics) Preferably the aqueous conductive primer coating composition has a viscosity value of more than 5 mPa s^-1 or more than 10 mPa s^-1 or more than 20 mPa s^-1.

A viscosity modifying agent may be added to the aqueous conductive primer coating composition in order to control the viscosity and smoothness of the primer. This is because the primer may be difficult to use if it is too viscous. Additionally, if the primer is not viscous enough it may not form a smooth film on the substrate being coated.

The viscosity modifying agent may be a diol. For example, the viscosity modifying agent may be ethylene glycol, propylene glycol, butane diol, pentane diol, hexane diol, pentaerythritol or glycidol. Most preferably, the viscosity modifying agent is ethylene glycol. The viscosity modifying agent may also comprise glycol ethers. The amount of viscosity modifying agent added to the aqueous conductive primer coating composition may be, for example, 10.0 - 20.0 wt.%, for example 16.0 to 20.0 wt.%.

Water

The aqueous conductive primer coating composition according to the present invention comprises water. The amount of water present in the composition is not particularly limited and can be adjusted in order to give primer compositions with a particular viscosity value, which may be selected dependening on the particular application. Preferably, the conductive primer composition comprises 40.0 - 50.0 wt% water. Most preferably, about 45 wt% water.

In the aqueous conductive primer coating composition accoridng to the present invention the balance of components is generally made up to 100 wt.% with water. pH

The pH of the present invention aqueous primer coating composition is preferably in the range of 6.5 to 9.5, more preferably 7.0 to 8.0. In the case where the pH of the aqueous primer coating composition is less than 6.5, the dispersion stability tends to be deteriorated. On the other hand, in the case where the pH of the aqueous primer coating composition is more than 9.5, there is a tendency for the composition to have such a high viscosity as to be difficult to use.

In the case that the pH values is below 6.5 it can be neutralised using a base, such as an organic base, e.g. an organic amine or ammonia. Preferably, the neutralising agent is selected from the group of ammonia, dimethylethanolamine (DMAE), ethylamine, diethylamine and triethylamine.

Generally, ammonia and DMAE are added to the composition as aqueous compositions of from 25% to 50% concentration (w/w) in water. Addition of 0.1 wt.% aqueous ammonia (50 wt.% concentration) is therefore the molar equivalent of the addition of 0.05 wt.% anhydrous ammonia. Similarly, the addition of 0.1 wt.% aqueous DMAE (25 wt.% concentration) is the molar equivalent of addition of 0.025 wt.% anhydrous DMAE.

Additional components

The aqueous primer coating composition of the present invention can also comprise low levels of additional components (i.e. less than 5%), which are known in the art to be used as part of primer compositions. These components can include for example wetting agents, such as polyoxyethylene glycol octylphenol ether, dioctyl sodium sulfosuccinate and zwitterionic surfactants.

Use

In a further embodiment, the present invention relates to the use of an aqueous conductive primer coating composition according to the present invention in a coating process. The use of the aqueous conductive primer coating composition may simply involve the application of the aqueous conductive primer coating composition to a substrate or may also involve subsquent steps of overcoating the primer layer with one or more paint coats.

Method of preparation of the aqueous conductive primer coating composition

In a further embodiment, the present invention relates to a method of preparation of an aqueous conductive primer coating composition comprising 5.0 - 10.0 wt.% carbon filler comprising surface functionalised carbon filler; 10.0 -20.0 wt.% water miscible organic solvent; 5.0 -15.0 wt.% acrylic-based binder and 0.5 - 5.0 wt% cross-linking agent. The method comprising the steps of: providing a carbon filler comprising surface functionalised carbon filler, mixing the carbon filler with water (water is provided to make the primer composition up to 100 wt.%), the water miscible organic solvent, the acrylic-based binder and the cross-linking agent and optionally adjusting the pH of the mixture.

Method of preparation of a coating film

In a further embodiment, the present invention relates to a method for the formation of a coating film comprising the steps of: coating a substrate with the aqueous conductive primer coating composition according to the present invention; and thereafter drying the aqueous conductive primer coating composition on to the substrate to form a dry primer layer. Generally, the primer layer is applied directly to a substrate (such as a plastic substrate) and there is no intermediate layer between the primer and the substrate.

By “substrate” we mean the base material onto which the aqueous conductive primer coating composition is applied.

Though not especially limited, examples of the substrate, which is to be coated with the aqueous conductive primer composition according to the present invention, include plastic materials such as polyolefins (e.g. polypropylene (PP) and polyethylene (PE)), acrylonitrilestyrene (AS), acrylonitrile-butadiene-styrene (ABS), polyphenylene oxide (PPO), polyvinyl chloride (PVC), polyurethane (PU), and polycarbonate (PC). Preferably, the substrate is acrylonitrile-butadiene-styrene (ABS). Preferably, the substrate is a component for use in a motorcycle or automobile.

There is no particular limitation on the method for coating the substrate with the aqueous primer coating composition, but the coating can be carried out either by air spray coating or by airless spray coating. After the step of coating the substrate with the coating composition, the step of drying the resultant primer coating film is carried out.

Preferably, the aqueous conductive primer coating composition is stirred continuously during the spraying process.

This drying step may be carried out either by air drying or by forced drying. As to the forced drying, the drying step may be carried out by any of warm-wind drying, near-infrared-ray drying, and electromagnetic-wave drying. Preferably, the drying step will lead to a hardened layer of electrically conductive material with a VOC content of less than 3 %, preferably less than 1 %, more preferably less than 0.5 %, more preferably less than 0.1 %, more preferably less than 0.01 %, more preferably less than 100 parts per million (ppm), most preferably less than 10 parts per million (ppm). In certain embodiments, the VOC content will be undetectable.

Preferably, the drying step involves drying the aqueous conductive primer coating composition on to the substrate at a temperature between 70-180 ^eC, for 1 - 30 minutes. Preferably, the temperature is from 70 - 100 ^eC, more preferably from 70 - 85 ^eC, most preferably from 70 - 75 ^eC. Preferably, the drying is conducted for a period of from 15 to 30 minutes, most preferably for a period of from 20 to 25 minutes. The drying time usually depends upon the drying temperature and may be set in consideration of energy efficiency.

The film thickness of the dry primer layer is preferably in the range of 2 to 30 pm, more preferably 5 to 20 pm. In the case where the dried-film thickness is less than 2 pm, there is a tendency for the film to be too thin to obtain a continuous uniform film. On the other hand, in the case where the dried-film thickness is more than 30 pm, the water resistance and the weather resistance tend to be deteriorated. Film thicknesses of more than 30 pm are also associated with high unit costs.

The method for the formation of a coating film may further comprise the step of overcoating the dry primer layer with one or more coats of paint, so as to obtain a painted article. The process of applying the one or more coats of paint is not particularly limited and may be a so called “one layer” painting processes and “double layer” painting processes. That is, when the method involves overcoating the dry primer layer with only one paint coat in order to obtain a painted article, or wherein the method involves overcoating the dry primer layer with a base coat of paint and then subsequently with a second coat of paint to obtain a painted article. Subsequently, a clear top coat may be overcoated onto the paint layer(s) in order to obtain a three-layered (primer/paint/top-coat) or four layered (primer/base coat of paint/secondary coat of paint/top- coat) coating film.

The aqueous conductive primer coating composition has the advantages that it is compatible with “one layer” painting processes, “double layer” painting processes and other “multi-layer” painting processes.

The primer layer may have a media thickness of from 180 g/m² to 220 g/m². Without wanting to be bound by any theory it is believed that paint coats generally have a lower media thickness such as less than 170 g/m².

For conventional water based primers when a paint coat is sprayed onto the primer layer, the solvent in the paint will merge into the primer layer which can affect the appearance of the paint layer, leading to a layer that is not glossy. Therefore, with conventional water based primers two layers of paint are required with the first layer covering the first paint coating, which is not glossy. However, surprisingly, the present inventors have discovered that the aqueous conductive primer coating composition of the present invention can be used to obtain a paint coat with a one layer painting process that has a glossy finish.

There is no particular limitation on the method for applying the paint coats and the clear paint; although, generally these methods are electrostatic methods, but they can be coated by the same method as used for the aqueous primer coating composition.

The paint may be a solvent-based one- or two- component curing type paint including at least one pigment. The paint may be a polyurethane or acrylic type paint. Without wanting to be bound by any theory it is believed that the composition of the aqueous conductive primer coating composition allows it to bind effectively to a wide range of different types of paints, and in particular to both polyurethane and acrylic based paints.

Examples of pigments, which may be included in the paint are inorganic pigments (e.g. titanium oxide, carbon black, iron oxide, chromium oxide, and Prussian blue) and organic pigments (e.g. azo pigments, anthracene pigments, perylene pigments, quinacridone pigments, indigo pigments, and phthalocyanine pigments); brilliant pigments such as aluminium flakes; and mica pigments. These may be used either alone respectively or in combinations with each other.

The paint coat layers may be baked onto the substrate. There is no particular limitation on the method for baking the coating films formed onto the substrate. The baking temperature may be in the range of 70 to 100 °C, more favourably 70 to 90 °C. The baking time usually depends upon the baking temperature and is preferably in the range of 15 to 60 minutes, more preferably 20 to 30 minutes, most preferably 20 to 25 minutes.

Coated article

In a further embodiment, the present invention relates to a coated article which is obtainable by coating a substrate according to the method described in the section above.

The substrate to be coated is not particularly limited and may comprise any of the materials listed for the substrate in the method section above. Preferably, the substrate is made of acrylonitrile-butadiene-styrene (ABS).

The article is preferably a component for use in an automotive application. For example, for use in an automobile or a motorcycle. Examples of such components include automobile bumpers, automotive body parts, automotive trim components and housings for motors and electronics on automobiles or motorbikes. In a further aspect, the present invention relates to a dry primer layer. The dry primer layer is obtainable by drying the aqueous conductive primer coating composition according to the present invention in accordance with the method described above.

"Dry primer layer” refers to a solid layer of material with a residual water content of less than about 1% or less than about 0.1%. The residual water content is determined in accordance with the Karl Fisher Titration method.

Preferably, the dry primer layer is obtainable by preparing an aqueous conductive primer coating composition according to the present invention; applying the aqueous conductive primer coating composition to a substrate; drying the coating composition.

The dry primer layer may comprise 10.0 - 33.0 wt.% surface functionalised carbon filler in a cross-linked acrylic-based binder matrix. Preferably, the dry primer layer comprises: 15.0 - 25.0 wt.% surface functionalised carbon filler in a cross-linked acrylic-based binder matrix, more preferably wherein the binder is a crosslinked arcylic binder matrix or urethane-acrylic binder matrix.

Preferred embodiments

Particularly preferred embodiments include:

An aqueous conductive primer coating composition according to the present invention comprising or consisting of:

- 5.0 - 10.0 wt.% surface functionalised carbon filler;

- 10.0 - 20.0 wt.% C1-5 alcohol;

- 10.0 - 20.0 wt.% glycol;

- 5.0 -15.0 wt.% acrylic-based binder;

- 0.5 - 5.0 wt.% cross-linking agent;

- 0.1 - 1 .0 wt.% aqueous ammonia (50 wt.% concentration) or aqueous amine;

- 40.0 - 50.0 wt.% water.

Preferably, the C1-5 alcohol is ethanol or isopropanol.

Preferably, the surface functionalised carbon filler is surface functionalised carbon black (carbon black which has had the surface chemistry of the functional groups on the surface of the material modified in order to achieved high conductivities and dispersal in the aqueous conductive primer coating composition). This may be referred to as “surface adjusted carbon black”.

Preferably, the acrylic-based binder is an acrylic binder or a urethane acrylic binder.

Preferably, the glycol is diethylene glycol. In a preferred implementation, the aqueous conductive primer coating composition according to the present invention comprises or consists of:

- 5.0 - 10.0 wt.% surface adjusted carbon black;

- 10.0 - 20.0 wt.% ethanol;

- 10.0 - 20.0 wt.% glycol;

- 5.0 -15.0 wt.% acrylic binder;

- 0.5 - 5.0 wt.% cross-linking agent;

- 0.1 - 1 .0 wt.% aqueous ammonia (50 wt.% concentration);

- 40.0 - 50.0 wt.% water.

In a further particularly preferred embodiment the aqueous conductive primer coating composition according to the present invention comprises:

- 5.0 - 10.0 wt.% surface adjusted carbon black;

- 10.0 - 20.0 wt.% ethanol or isopropanol;

- 10.0 - 20.0 wt.% diethylene glycol;

- 5.0 -15.0 wt.% urethane-acrylic binder;

- 0.5 - 5.0 wt.% cross-linking agent;

- 0.1 - 1 .0 wt.% aqueous amine;

- 40.0 - 50.0 wt.% water.

BRIEF DESCRIPTION OF THE FIGURES

The present proposals are now explained further with reference to the accompanying figures in which:

Fig. 1 is a graph showing the surface resistance of a primer coating against the wt.% of carbon black in the primer coating.

Fig. 2 is a plot showing the average particle size of a representative batch of carbon particles used in the aqueous conductive primer coating compositions according to the present invention.

Fig. 3 is a diagram showing the layers of paint coating on a plastic substrate;

Fig. 4 is an example of a plastic substrate with low conductivity;

Fig. 5 is an example of a plastic substrate after spray coating with the aqueous conductive primer coating composition according to the present invention

Fig. 6 shows the steps in a paint adhesion test (cross cut test) performed in accordance with ASTM D3359 (test method B).

Fig. 7 is an example of the plastic substrate after spray coating showing result 5B in the paint adhesion test.

DETAILED DESCRIPTION

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains.

Although, any methods and materials similar or equivalent to those described herein can be used in practice for testing of the present invention, the preferred materials and methods are described herein. In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. Unless clearly indicated otherwise, use of the terms "a," "an," and the like refers to one or more.

Figure 1 shows the results of the tests carried out in example 1 . Aqueous primer compositions were prepared as described in example 1 with varying amount of functionalised carbon black (%). The data in figure 1 demonstrates that aqueous conductive primer coating compositions according to the present invention demonstrate surface resistance values from 10⁵ - 10³ Ohm/Sq and that above a critical content of carbon black (5-7 wt.%), surface resistance values of 10³ Ohm/Sq were achieved, this could be designated as conductive.

Figure 2 is a plot showing the volume average particle size of a representative batch of carbon particles used in the aqueous conductive primer coating compositions in example 1 determined using dynamic light scattering (using a light scattering particle size distribution analyzer LB-550, available from HORIBA).

Figure 3 is a diagram showing the different layers of a coated article. The black layer is a plastic substrate, the dark grey layer is a conductive primer layer (aqueous conductive primer coating composition according to the present invention) and the light grey layer is a layer of topcoat (paint layer).

Figure 4 is an example of a plastic workpiece, which could be used as a substrate for application of the aqueous conductive primer coating composition according to the present invention. The plastic substrate has high sheet resistance, which is too high to be shown on the resistivity meter however, this is usually about 10¹² Ohm/Sq.

Figure 5 is an example of a plastic substrate after spray coating with the aqueous conductive primer coating composition according to the present invention. This coated article has a lower sheet resistance value, as shown on the resistivity meter. Sheet resistance values for plastic substrates coated with the aqueous conductive primer coating composition according to the present invention are usually about 10³-10⁴ Ohm/Sq.

Figure 6 shows the steps in an adhesion test (cross cut test), as described in the test method section below.

Figure 7 is an example showing an adhesion test with the result 5B. The plastic substrate in figure 7 has been spray coated with the aqueous conductive primer coating composition according to the present invention and subsequently electrostatically coated with a paint. EXPERIMENTAL

Test methods

Adhesion test (cross cut test)

Adhesion of the primer to the substrate and the paint to the primer was measured using an adhesion or cross cut test. This was conducted in accordance with ASTM D3359 (test method B). The steps of the adhesion test may be summarised as follows:

1) Prepare a 1 cm square of crosshatch pattern on an article coated with the film using a knife;

2) Apply Scotch tape to the surface of the crosshatched pattern;

3) Remove the scotch tape at an angle of 45 degrees to the test sheet;

4) Rate the adhesions on a scale of 0 to 5

• OB- Greater than 65% area removed;

• 1 B 35-65% area removed;

• 2B 15 - 35 % area removed;

• 3B 5 - 15 % area removed;

• 4B less than 5% area removed and

• 5B is 0% area removed.

Conductivity test

The conductive properties of the primer and the binding strength between the primer and paint is measured as follows: a) Spray coat the primer composition onto the surface of the plastic substrate using gravity feed type spray coating until a certain level of film thickness is achieved. b) Bake the workpiece from a) at a temperature of 70-75 °C for 20-25 minutes. c) Measure the sheet resistance of the workpiece from b) using a surface resistance meter (model: TR1380); and test the binding of the primer coating and the plastic surface using the cross cut test (ASTM D3359) as shown in figure 6 and described above. d) Spray the solvent based paint onto the workpiece b) until a certain level of thickness can be observed from the shininess of the coating which will be used in testing the binding between the primer and the solvent based paint. e) Test the binding of the coating surface between the inductive primer and the solvent based paint using the cross cut test. Experimental Examples

An aqueous conductive primer coating composition was prepared comprising:

• 10.0 -20.0 wt.% ethanol,

• 10.0 - 20.0 wt.% glycol;

• 5.0 -15.0 wt.% acrylic binder;

• 0.5 - 5.0 wt% cross-linking agent;

• 0.1 - 1 .0 wt.% aqueous ammonia (50 wt.% ammonia) and

• 40.0 - 50.0 wt.% water.

The composition was prepared by mixing the above components with various amounts of surface functionalised carbon black (wt.%) from 1 - 10 wt.%.

The surface resistance was then determined using the method described in the test methods section above, with the results as shown in Figure 1 .

Primer compositions were prepared by mixing the components specified in table 1 for each of the examples. The conductivity was determined according to the method described in the test methods section above.

Table 1 : Conductivity of the example compositions

a Honda C.W.C. contains an amine salt and glycol ether. b A solvent based primer based on soft xylene type solvents or solvesso 100, 150 from ExxonMobil manufactured by BASF or Kansai. The results in table 1 demonstrate that the conductivity of the water-based primers in the inventive examples are 10⁴ -10⁵ Ohm/Sq, this is the same level as the conductivity of solvent based primers. Water based primers known in the art demonstrate lower conductivities (e.g. comparative example 1 has a conductivity of 10⁸ - 10⁹ Ohm/Sq,).

It is possible to reduce the carbon content from 6% to 5% by using functionalised carbon materials, when maintaining the same thickness of primer layer.

Claims

1 . An aqueous conductive primer coating composition comprising

- 5.0 - 10.0 wt.% carbon filler comprising a surface functionalised carbon filler;

- 10.0 -20.0 wt.% water miscible organic solvent;

- 5.0 -15.0 wt.% acrylic-based binder; and

- 0.5 - 5.0 wt% cross-linking agent

2. The aqueous conductive primer coating composition of claim 1 , wherein the surface functionalised carbon filler is oxygen-functionalised, hydroxyfunctionalised, carboxy-functionalised, carbonyl-functionalised, amine- functionalised or amide-functionalised.

3. The aqueous conductive primer coating composition of claim 2, wherein the surface functionalised carbon filler is oxygen-functionalised and/or amine- functionalised.

4. The aqueous conductive primer coating composition according to any one of the preceding claims, wherein the surface functionalised carbon filler has hydroxyl, carbonyl, carboxyl, amine, amide or a mixture of these functionalities present on the surface of the functionalised carbon filler.

5. The aqueous conductive primer coating composition according to any one of the preceding claims, wherein the carbon filler comprises carbon black, ACB (acetylene black) and/or graphene particles.

6. The aqueous conductive primer coating composition according to any one of the preceding claims, wherein the surface functionalised carbon filler is at least 50%, preferably at least 60%, more preferably at least 70%, more preferably at least 80%, more preferably at least 90% of the total carbon filler in the primer composition.

7. The aqueous conductive primer coating composition according to claim 6, wherein the carbon filler consists of surface functionalised carbon filler.

8. The aqueous conductive primer coating composition according to any one of the preceding claims, wherein the carbon filler is a particulate material with a volume average mean particle size of less than 10 pm.

9. The aqueous conductive primer coating composition according to any one of the preceding claims, wherein the size distribution of the carbon filler is bimodal or multimodal.

10. The aqueous conductive primer coating composition according to any one of the preceding claims, wherein the water miscible organic solvent comprises a straight or branched chain alcohol preferably wherein the water miscible organic solvent comprises a C1-5 alcohol.

11 . The aqueous conductive primer coating composition of claim 10, wherein the water miscible organic solvent is ethanol or isopropanol.

12. The aqueous conductive primer coating composition according to any one of the preceding claims, wherein the crosslinking agent is epoxy silane.

13. The aqueous conductive primer coating composition according to any one of the preceding claims, further comprising 10.0 - 20.0 wt.% of a viscosity modifying agent, preferably wherein the viscosity modifying agent is ethylene glycol.

14. The aqueous conductive primer coating composition of any one of the preceding claims, comprising 40.0 - 50.0 wt% water.

15. The aqueous conductive primer coating composition according to any one of the preceding claims consisting of:

- 5.0 - 10.0 wt.% surface functionalised carbon black;

- 10.0 - 20.0 wt.% ethanol;

- 10.0 - 20.0 wt.% glycol;

- 5.0 -15.0 wt.% acrylic-based binder;

- 0.5 - 5.0 wt.% cross-linking agent;

- 0.1 - 1 .0 wt.% aqueous ammonia (50 wt.% concentration);

- 40.0 - 50.0 wt.% water.

16. Use of an aqueous conductive primer coating composition according to any one of claims 1 -15 in a coating process.

17. A method for preparing an aqueous conductive primer coating composition comprising

- 5.0 - 10.0 wt.% carbon filler comprising a surface functionalised carbon filler;;

- 10.0 -20.0 wt.% a water miscible organic solvent;

- 5.0 -15.0 wt.% acrylic-based binder;

- 0.5 - 5.0 wt% cross-linking agent the method comprising the following steps

Step 1 : Providing the carbon filler;

Step 2: Mixing the carbon filler with water, the water miscible solvent, the acrylic-based binder and the cross-linking agent;

Step 3: Optionally adjusting the pH of the mixture.

18. A method for the formation of a coating film, which comprises the steps of a) Coating a substrate with an aqueous conductive primer coating composition according to any one of claims 1 -15; b) Drying the aqueous conductive primer coating composition on to the substrate to form a dry primer layer.

19. The method of claim 18, wherein step b) involves drying the aqueous conductive primer coating composition on to the substrate at a temperature between 70 - 180 ^eC, for 1 - 30 minutes, preferably wherein step b) involves drying the aqueous conductive primer coating composition on to the substrate at a temperature between 70 - 100 ^eC for 20 to 30 minutes.

20. The method of claim 18 or 19, further comprising c) Overcoating the dry primer layer with one or more paint coats.

21 . The method of claim 20, wherein step c) consists of overcoating the dry primer layer with a single paint coat and optionally overcoating the paint coat with a clear top coat to obtain a painted article.

22. The method of claim 20, wherein step c) comprises step c1 : overcoating the dry primer layer with a base coat of paint; step c2: overcoating the base coat layer with a secondary coat of paint and optionally overcoating the secondary coat of paint with a clear top coat to obtain a painted article.

23. A coated article, which is obtainable by coating a substrate by the method according to any one of claims 18 to 22.

24. The article of claim 23, wherein the substrate includes a plastic material, preferably wherein the substrate is acrylonitrile butadiene styrene.

25. The article of claim 23 or 24, wherein the coating comprises

10.0 - 33.0 wt.% carbon filler comprising a surface functionalised carbon filler; in a cross-linked acrylic based binder matrix; preferably, wherein the dry primer coating composition comprises:

15.0 - 25.0 wt.% carbon filler comprising a surface functionalised carbon filler; in a cross-linked acrylic based binder matrix.