WO2014029769A1

WO2014029769A1 - Powder coatings

Info

Publication number: WO2014029769A1
Application number: PCT/EP2013/067306
Authority: WO
Inventors: Christer Lorentz ØPSTAD; Bjørn KARSLEN; Helge HOFF; Ulrik SELLMANS
Original assignee: Jotun Powder Coatings (N) As
Priority date: 2012-08-21
Filing date: 2013-08-20
Publication date: 2014-02-27

Abstract

This invention relates to powder coatings, in particular to a substrate coated with a primer layer and a top coat in which the primer layer is formed by blending the primer layer material with an amount of the top coat material.

Description

Powder Coatings

This invention relates to powder coatings, in particular to a substrate coated with a primer layer and a top coat in which the primer layer is formed by blending the primer layer material with an amount of the top coat material. In this way, we "tint" the primer coating making this have a colour more similar to the top coat thus enabling a thinner top coat to be applied to mask the colour of the primer underneath. Background of Invention

Powder coatings are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate is earthed. Alternative application methods include fluidised-bed and electrostatic fluidised-bed processes. After application, the powder is heated to melt and fuse the particles and to cure the coating.

Today, most methods for powder coating substrates such as MDF require a two-coat system. This is to ensure a proper sealing of the surface and to ensure a homogeneous and defect-free surface in the finished decorative coating. The requirement is especially important for smooth powder coating systems, where defects and fibre -raising are easily visible and impair appearance.

To achieve an even surface finish, the general process is as follows. First a layer of 50 -100 μιη primer is applied to seal the surface and ensure a homogeneous base for the top-coat. The primer is cured and then slightly sanded in order to remove defects and form a smooth surface. Subsequently, the decorative top-coat is applied in layers of 100-120 μιη and cured in order to achieve consistent flow and appearance of the final surface. This results in a total coating thickness of more than 150 μιη, and a laborious set-up with two coating booths and curing ovens.

Furthermore, the primer is generally supplied in one or two colours predefined by the producer. The top-coat needs to be applied in sufficient thickness to completely cover the colour of the primer to avoid colour deviations on the final surface. This is a challenge, especially on sharp edges and in crevices. It is often difficult to build sufficient coating layers in these areas. The requirement to mask the primer layer colour leads to the application of thick top coats which is both expensive and time consuming.

One method to reduce the thickness of the top-coat would be to prepare the primer in the same colour as the top-coat. However, this would require a significant primer stock to be developed adding significant costs to the product. It would also add to the complexity of the manufacture of the powder coatings as each primer would need to match the eventual top coat.

One known powder coating method is described in WO2001134986. In this document, a "dry-on-dry" application process is described which generally involves applying two coats with opposite "polarities". This can be achieved by applying a primer with an electrostatic, negative corona gun, and subsequently the top-coat with a friction, positive tribo gun. The opposite distribution of guns is also possible, as well as application with positive and negative electrostatic guns in sequence, or similar electrostatic guns for both coats. The point is that an electrostatic interaction between the two layers is possible.

US6032871 describes a similar process in which simultaneous or alternate application of charged powders is taught. Neither of these processes addresses however the problem of the colour of the primer layer showing through the top layer forcing the application of thick and expensive top layers.

The present inventors have realised that by adding an amount of the top coat material into the primer, the colour issue can be addressed. Moreover, the inventors have found that the addition of the top coat material to the primer layer does not detract from the function of the primer layer. The presence of the top coat in the primer layer does not impair interlayer adhesion which can be shown in cross-cut tests. Moreover, blending of the top coat material into the primer is preferably achieved using a recycle of the top layer overspray into the primer layer, ideally in a continuous process. Typically this overspray is recycled back to the top layer application meaning that the top layer coating includes recycled material which can actually be detrimental to the decorative properties of a surface. An important advantage with recycling the overspray top-coat to the primer is that only "virgin" powder (i.e. top coat powder which has not been mixed with recycled top coat powder) is used for the top-coat. This ensures surfaces without defects that may arise from powder recycling by removing risk of contamination from the recycling equipment and spray booth.

By making sure that the primer and top coat materials are miscible and compatible we achieve a new powder coating process with the potential to save significant costs for manufacturers without damaging properties of the coating. By mixing small amounts of the top-coat with the primer before application, the primer is, as a result, tinted with the colour of the top-coat. This results in a reduced demand for coating thickness of the top-coat, since the colour difference (ΔΕ) between the tinted primer and top-coat is much less than that of the separate components. We call this "tinting" of the primer herein. Summary of Invention

Thus, viewed from one aspect the invention provides a continuous process for applying a powder coating on a substrate comprising:

(I) applying to said substrate a first powder coating layer comprising a powder blend of a primer and 1 to 40 wt of top coat material of step (II); and thereafter

(II) applying to said first powder coating layer a powder top coat.

Viewed from another aspect the invention provides a process for applying a powder coating on a substrate comprising:

(II) applying to said first powder coating layer a powder top coat.

In one embodiment, layer (I) is cured before application of layer (II).

However, preferably the layers (I) and (II) are cured without having cured said primer layer (I) before application of the powder top coat layer. Viewed from another aspect the invention provides a substrate having thereon:

(I) a first powder coating layer comprising a powder blend of a primer and 1 to 40 wt of top coat material of component (II); and on top thereof

(II) a powder top coat.

Viewed from another aspect the invention provides a substrate having thereon:

(I) a first powder coating layer comprising a powder blend of a primer and 1 to 40 wt of a top coat material of component (II); and on top thereof

(II) a powder top coat;

which has been cured either after application of layer (I) and again after application of layer (II) or just after application of layer (II).

Viewed from another aspect the invention provides the use of the overspray of the top coat material during a powder coating process as an additive for the primer layer.

(I) applying to said substrate a first powder coating layer comprising a powder blend of a primer and 1 to 40 wt of a top coat material; and thereafter (II) applying to said first powder coating layer a powder top coat;

wherein at least a part of the overspray that occurs when the top coat material is applied onto the first powder coating layer is recycled to form at least a part of the top coat material in the powder blend in layer (I). Preferably, the top coat application is free of any recycle.

Viewed from another aspect the invention provides a continuous process for applying a powder coating on a substrate comprising:

(I) applying to said substrate a first powder coating layer comprising a powder blend of a primer and 1 to 40 wt of a top coat material; and thereafter

(II) applying to said first powder coating layer a powder top coat;

wherein at least a part of the overspray that occurs when the top coat material is applied onto the first powder coating layer is continuously recycled to form at least a part of the top coat material in the powder blend in layer (I). Preferably, the top coat application is free of any recycle.

Detailed description of Invention

This invention relates to powder coating of substrates such as metal, wood, MDF etc in particular with a two layer coating structure involving a primer layer and top coat layer. In particular, the invention relies on the application of a primer which is mixed with an amount of the top coat material in order to achieve the benefits highlighted herein.

The process is a powder coating process so requires the application of a powder. Coating powders can be made by the extrusion of the components necessary to form the powder to obtain a homogeneous mixture. Grinding of the extrudate and screening the particles can then be used to obtain the desired particle sizes and particle size distribution.

In order to ensure homogeneity before extrusion, the components of each layer must be well mixed. Any type of high speed mixer can be used. All industrial extruders are suitable for powder preparation. It is preferred to keep the extrudate temperature below 140°C to prevent premature curing. More preferably, the temperature should be kept below 120°C during extrusion. This can be achieved by adjusting appropriate extruder settings.

The extruded granulates can be milled by all types of conventional mills and the particles thereafter classified by a method of choice, to a particle size found most suitable for powder application.. The particle size distribution of the powder coating composition may be in the range of from 0 to 120 microns with a mean particle size in the range of from 15 to 75 microns, preferably at least 20 or 25 microns, advantageously not exceeding 50 microns, more especially 20 to 45 microns.

Primer layer (i.e. the first powder coating layer)

The primer layer can be a conventional one in the art and can be made from, for example, an epoxy resin type material or polyester. This is termed the binder component. The inventors also envisage the use of a hybrid binder system however, based on the combination of a carboxy functional polyester type material and an epoxy resin. Suitable materials are described in detail below. In particular, the hybrid system can involve a polycarboxyl polymer and an epoxy resin.

The primer layer can be coloured. It is preferably a colour which is easy to cover such as white or grey.

The primer layer preferably comprises a binder based on an epoxy resin alone or a binder based on a blend of carboxy functionalised saturated polyester and epoxy resin.

Epoxy containing compound

The primer layer binder typically comprises an epoxy resin. The epoxy resin might be the only polymer component present or as noted below it can form part of a hybrid binder system. It is possible to use a mixture of epoxy containing

compounds.

The epoxy containing compound is preferably an epoxy resin. Ideally it is a solid resin containing one or more epoxy groups. Suitable resins are again well known in the art and well known commercial products. Epoxy resins include TGIC, Araldite PT 910, bisphenol A based resins, novolac resins, 4,4'- isopropylidenediphenol-epichlorohydrin resins (bisphenol F) based resins, and so on.

Most preferred are solid epoxy resins with an equivalent epoxy weight (EEW) of 300-2000. These resins are often described by their "type". Type 2, 2.5, 3, 4 and novalac type resins are all suitable here. Type 2 resins may have an EEW = 550-700, e.g. Epikote resin 1002, Epikote resin 3022-FCA. Type 2.5 resins may have a EEW = 600-750, e.g. Araldite GT 6450. Type 3 resins may have EEW = 700-850, e.g. Epikote resin 3003, Araldite GT 7004. Type 4 type resins may have EEW = 800-1000, e.g. Epikote resin 1055. Novalac type resins may include Epikote resin 2017 or Araldite GT 7255.

To allow curing of the binder a cross-linker is used as is well known in the art. In pure epoxy binder systems, different types of crosslinkers may be used. Typically, the crosslinker contains free hydroxyl- or amine-groups that react with the epoxide ring. Crosslinkers may be in the form of single molecules, oligomers, or polymers, e.g. dicyandiamide (DICY) or phenolic resins. The stoichiometric amount of crosslinker relative to epoxy containing compounds may be 20-98%, preferably 35-95%. Typically, the stoichiometric amount of epoxy containing compound based on number of epoxy groups will be around the same as or exceed that of the crosslinker.

In a preferred embodiment however, the epoxy binder is used alone as the only binder component or alternatively, epoxy binder is combined with a

polycarboxyl polymer to form a hybrid primer.

Polycarboxyl polymer

The primer layer binder therefore preferably additionally contains at least one polycarboxyl polymer. The term carboxyl is used here to define the presence of the group COOH or a salt thereof. Ideally, the carboxyl group will be in the form of a COOH group. It will be appreciated therefore that an ester group does not constitute a carboxyl group herein.

The polycarboxyl polymer therefore contains a plurality of carboxyl groups. These groups must be capable of reacting with the epoxy groups of the epoxy component and must therefore be available for reaction. That means carboxy groups must be pendant on the molecule and not exclusively in its backbone. Moreover, this component of the powder coating of the invention is a polymer, i.e. is formed from the polymerisation of monomers at least one of which is one containing a carboxyl group

It is possible to use a mixture of polycarboxyl polymers or use one polycarboxyl polymer.

The polycarboxyl polymer is preferably a solid resin containing a plurality of free carboxyl groups. Preferably the polycarboxyl polymer has a Tg above 30°C, more preferably above 40°C. It is preferred if the polycarboxyl polymer comprises at least 5 carboxyl groups, preferably at least 10 carboxyl groups, e.g. at least 20 carboxyl groups. Ideally, the carboxyl resin is characterised in terms of its acid number (AV). Most preferred are carboxyl resins with acid value (AV) between 10- 100 mg KOH/g, such as 20 to 90 mg, preferably 25-80 mg KOH/g.

More preferably, the polycarboxyl polymer is an acid functional polyester, especially one having the AV values above.

The use of carboxyl functional polyesters is preferred especially those designated 50/50 type resins to 80/20 type resins (i.e. where there is 80 wt% carboxy functional polyester is used to 20 wt% epoxy compound of the binders). The value of AV and EEW, discussed below should preferably complement each other. For example, the AV of a 50/50 type resin may be 60 to 80 mg KOH/g. Resins that are defined as 80/20 resins will have lower AV numbers, such as 20 to 40 mg KOH/g.

Other preferred polycarboxyl polymers are polyacids, carboxyl

functionalised dendrimers, or carboxyl functionalised acrylic resins. Most preferred are the polyester resins containing a plurality of free carboxyl groups.

The monomers used to form the polyesters of the invention may be based on terephthalic acid, isophthalic acid monomers together with, for example glycols such as neopentyl glycol.

The polycarboxyl polymer is preferably one with a Mw of at least 1000, more preferably at least 2000. The upper Mw value may be 10,000. Preferred Mw values are 2000 - 6000, preferably 2500 to 5000, such as about 3000. The molecular weights are determined by gel permeation chromatography (GPC) using a polystyrene standard.

Such resins are well known in the art and are sold under the trade names such as Crylcoat, e.g. Crylcoat E 37704, Crylcoat E 38051, Crylcoat E 04314, Crylcoat 1701- 1, Uralac P5071, Uralac P3270, Uralac P2450 and so on.

It will be appreciated that if both carboxy functionalised saturated polyester and epoxy resin are present that these react in order to cure the coating.

Accordingly, it is preferred if these components are mixed in such a ratio that the reactive carboxyl and epoxy groups are within +25% of stoichiometric ratio. A carboxyl and epoxy ratio within +10% of stoichiometric ratio is more preferred. A carboxyl and epoxy ratio within +5% of stoichiometric ratio is most preferred.

The skilled man will be aware that some of the additives discussed below may contain carboxyl groups. When calculating the EEW to AV ratio, account should be taken of the contribution made by any carboxyl groups in the standard additives used in the powder coating. Nevertheless, the presence of a carboxyl resin is essential as the amount of additives present is typically very much lower than the binder content.

The amount of epoxy containing compound relative to polycarboxyl polymer in the binder may be 10 to 90 wt , preferably 15 to 85 wt . Typically, the amount of polycarboxyl polymer will be around the same as or exceed that of the epoxy compound.

The total contribution of the binder to the primer layer material may be up to 99 wt%, e.g. up to 95 wt%, such as 50 to 90 wt% of the composition, e.g. 60 to 80 wt%.

The first coating layer of the invention might therefore contain 1 to 40 wt% of the top coat material and 60 to 99 wt% of the primer material.

The primer binder system may be accelerated by different types of catalysts, e.g. Lewis acids or Lewis bases, described later. It is preferred however if the primer is free of compounds comprising an imidazole ring.

Top Coat Material The top coat formulation typically comprises an epoxy resin or a polyester.

Obviously, the top coat material is different from the primer material. The primer is designed to be functional. It seals and protects the painted object from corrosion and the like. The top coat has a decorative function and also typically provides UV- protection.

The top coat therefore typically contains colour pigments giving shine or gloss/matting effects. In the primer, the only pigments that are present are usually pigments like Ti0₂ and carbon black. The person skilled in the art can determine whether a formulation is a primer or top coat formulation.

Polyester based powder coatings are powder coatings where the main component in the binder system is a polyester-based resin. These resins are generally carboxyl or hydroxyl functionalised. The polyester resins may be crosslinked by several different crosslinkers, such as epoxy functional compounds (e.g.triglycidyl isocyanate or PT-910), isocyanates or hydroxy-functionalised compounds (e.e. beta-hydroxy alkylamide). Example top coat formulations include carboxy functionalised saturated polyesters and/or epoxy resin materials mentioned above in connection with the primer layer. However, these components may be combined differently from the primer.

Ideally, however the top coat material comprises a hybrid binder modified via treatment with a Lewis acid or Lewis base as well as any additives. In a most preferred embodiment, the top coat is formed from a particulate coating composition comprising a blend of two components:

i) a first component comprising at least one epoxy containing compound, at least one polycarboxyl polymer and at least one organic Lewis base; and

ii) a second component comprising at least one epoxy containing compound, at least one polycarboxyl polymer and at least one organic Lewis acid.

Lewis Acid

Preferably a Lewis acid is present only in component (ii) of the preferred blend of the top coat. It is within the scope of the invention for a mixture of Lewis acids to be employed but preferably only one Lewis acid is present.

The term Lewis acid is used herein to define a compound (which cannot be water) which is capable of accepting a pair of electrons. Ideally the Lewis acid is one which is fully or partially soluble in the melted powder.

Lewis acids used in this invention are organic and therefore contain carbon. Ideally, the Lewis acid contains both carbon and phosphorus. Moreover, the Lewis acid is ideally a relatively small molecule, with a Mw of less than 1000.

Most preferred are organic phosphonium Lewis acids. Preferably the Lewis acid is of formula

R₄P⁺X^~ wherein X is a counter ion such as a halogen ion and each R is independently a C_1-12 hydrocarbyl group. Preferably each R is independently a C_1-6 alkyl group or an aryl group such as phenyl. Preferred R groups are methyl, ethyl, n-propyl, isopropyl, n-butyl and phenyl.

Specific examples of preferred Lewis acids include ethyl triphenyl phosphonium bromide, ethyl triphenyl phosphonium chloride, butyl triphenyl phosphonium bromide, and ethyl tributyl phosphonium bromide.

The amount of Lewis acid present in the powder coating of the invention may be 0.1 to 5 wt .

The amount of Lewis acid present with the second component (ii) of the preferred blend of the invention is 0.1 to 10 wt , preferably 0.2 to 5 wt .. The amount of Lewis acid within component (i) is preferably zero.

Lewis base

The Lewis base used in the invention is organic and therefore comprises carbon. The Lewis base (which cannot be water) is a compound which donates an electron lone-pair. It is preferred if that compound is fully or partially soluble in the melted powder.

The Lewis base is ideally a relatively small molecule, with a Mw of less than

1000.

More preferred are Lewis bases with the lone-pair on a nitrogen atom.

Suitable Lewis bases therefore include tertiary amines. More preferably, that nitrogen atom forms part of a ring system. A variety of N-heterocycles can therefore be used as the Lewis base.

Even more preferred are aromatic heterocycles, especially based on nitrogen. Especially preferred are Lewis bases based on imidazoles and pyridines. The use of imidazole based Lewis acids is preferred. Imidazoles of interest include imidazole itself as well as derivatives of imidazole in which one or more ring substituents are present selected from C 1 - 10 hydrocarbyl groups.

Ideally there is only one substituent. Ideally that one substituent is present on the 2-position of the ring. Special examples are 2-methylimidazole, 2- ethylimidazole, 2-isopropylimidazole, imidazole and 2-phenylimidazole, especially 2-methylimidazole and 2-isopropylimidazole.

The use of adducts of imidazoles with epoxy groups is also possible. This enhances the compatibility between the binder and the Lewis base.

The amount of Lewis base present in the top coat may be 0.05 to 8 wt , e.g.

0.1 to 3 wt%.

The amount of Lewis base present within the first component (i) of the preferred blend of the invention may be 0.05 to 8 wt , e.g. 0.1 to 3 wt . The Lewis base content in component (ii) is preferably zero.

As noted above, it is preferred if two separate components are made for blending in order to provide the top coat of the invention. The first contains at least a Lewis base, carboxyl and epoxy compounds. This should be free of phosphonium groups. Ideally it will be free of Lewis acids therefore.

The second component of the blend contains at least Lewis acid, carboxyl and epoxy compounds. This second component is preferably free of imidazole type Lewis bases. Ideally therefore this component is free of Lewis base.

The ingredients of each component of the blend can be mixed (separately) and extruded to form particles as it known in the art. These particles can then be milled to form powder. It is important that the extrusion of each component of the blend takes place separately. We have found that if the Lewis acid and Lewis base are present in the same composition before extrusion, the mechanical properties are less favourable. Nevertheless, the combined extrudate enables the formation of glossy coatings which can be cured at low temperatures.

To realise some of the improved properties discussed further below therefore, it is necessary to extrude the two components separately. When that occurs, it will be appreciated that the particulate which forms after extrusion and subsequent powder formed after milling is nominally made from individual particles which contain either Lewis acid or Lewis base.

The blending of the two components preferably takes place after extrusion and not before. Blending can take place before milling or after milling but each component of the blend should be extruded separately. A preferred mixing ratio of the two components (i and ii) is between 99: 1 and 1:99 % by weight, e.g. 10 :90 to 90: 10 wt . More preferred is a mixing ratio of the two components between 25:75 and 75:25 % by weight. Most preferred is a mixing ratio of the two components between 40:60 and 60:40 % by weight. Ideally, the mixing ratio is around 1 : 1 by wt.

In order to ensure homogeneity before extrusion, the components of each blend must be well mixed. Any type of high speed mixer can be used. All industrial extruders are suitable for powder preparation. It is preferred to keep the extrudate temperature below 140°C to prevent premature curing. More preferably, the temperature should be kept below 120°C during extrusion. This can be achieved by adjusting appropriate extruder settings.

The extruded granulates can be milled by all types of conventional mills and the particles thereafter classified by a method of choice, to a particle size found most suitable for powder application. The particle size distribution of the powder coating composition may be in the range of from 0 to 120 microns with a mean particle size in the range of from 15 to 75 microns, preferably at least 20 or 25 microns, advantageously not exceeding 50 microns, more especially 20 to 45 microns.

As the combination of Lewis acid and Lewis base provided via separate blend components provides, inter alia, lower gloss, it is clear that this solution offers a synergy, perhaps in terms of more efficient cross-binding.

Coating

The top coat is adjacent the primer layer and on top of the primer layer. It will be appreciated that it is applied once the primer layer is dry. The top coat layer can be any colour desired by the manufacturer.

The primer material and the top coat material are necessarily different and serve different functions.

The primer has a functional role to seal and protect the surface and form a good foundation for the top-coat, whereas the top-coat is decorative and provides durability and good appearance to the substrate. The chemistries of the systems may or may not be the same, but there are differences between the formulations of the two coats.

The primer will only keep its functional properties within a certain mixing- range. Too much top coat and the primer is so dilute that it ceases to function satisfactorily as a primer. The amount of top coat material added to the primer can therefore be up to 40 wt%, such as 1 to 30 wt%, preferably 5 to 25 wt%, especially 10 to 20 wt% top-coat in primer. These figures are based on the weight of the primer layer as a whole. It is stressed that the top coat material which forms the top coat layer and the top coat material added to the primer is exactly the same. The top coat added to the primer will therefore contain the colour pigments typically associated with a decorative top coat layer.

Thus, the first powder coating layer may comprise 1 to 40 wt% of the top coat material and 60 to 99 wt% of the primer material, preferably 1 to 30 wt% of the top coating material and 70 to 99 wt% of the primer material, preferably 5 to 25% of the top coat material and 75 to 95 wt% of the primer material, especially 10 to 20 wt% of the top coat material and 80 to 90 wt% of the primer material.

The more top-coat that is added in the primer, the smaller the colour difference between the "tinted" primer and the top-coat but the less well the primer layer acts as a primer.

When using decorative top-coats, a critical parameter is the "hiding power" of the top coat. This is basically defined as the critical thickness above which there is no longer a change in colour when increasing the coating thickness. The larger the colour difference between the base and the top-coat is, the higher thickness is necessary to achieve full hiding. (For example, it is more difficult to achieve clear white when painting on a black base, then when painting on a white base.)

When the colour difference between the primer and the top-coat is reduced by blending in some of the top-coat, it is therefore not necessary to have as thick coatings to achieve uniform colour/full hiding (analogous to painting on a grey base instead of a black base.). We envisage a reduction of at least 20% in the thickness of the top coat to achieve full hiding relative to a system in which no primer tinting takes place. Typically, for a powder coating on a conventional primer, the primer layer will be 50-70 μιη in thickness carrying a top-coat of at least 100 μιη, giving a total film thickness of at least 150 μιη.

In contrast, for a "powder on powder" system according to the present invention, primer layers can be 40-50 μιη with a top-coat starting thickness of 60 μιη giving a total (minimum) film thickness of at least 100 μιη. Ideally therefore the thickness of the top coat/primer layer combination in the present invention after curing is less than 140 microns, such as less than 130 microns, especially less than 120 microns.

It will be appreciated that film thicknesses will vary depending on the nature of the layers, the substrate and the target end use of that substrate but the hiding power of our powder coating system is higher than that of the conventional art.

In the present invention there should be no incompatibility between the powders of the primer and the top-coat, resulting in good miscibility. This miscibility results in a surprising advantage for the coating process. Incompatibility can be determined by visual inspection of the cured film. Incompatibility is seen as craters, pinholes and fish-eyes.

Additives

It will also be appreciated that the powder coatings of the invention may contain a wide variety of standard industry additives. Additives of use include gloss modifiers, scratch resistors, pigments, fillers, degassing additives, flow improvers, waxes, antioxidants, optical brighteners and surface modifying agents. These additives in total can generally form up to about 60 wt of the powder coating, e.g. up to 40 wt%.

Preferred pigments and fillers are inorganic minerals such as titanium dioxide, talc, calcium carbonate, barium sulphate, organic pigments and so on.

These additives can be added to any top coat or primer layer material before extrusion. The additive package added to each layer can, of course, differ.

It will be appreciated that the powder coating of the invention must be dry and free of water. Application to substrates

The powder coating of the invention can be applied to a substrate by any conventional powder coating method such as spraying, e.g. electrostatically. The use of triboelectric guns can also be used. Coating techniques are well known in the art and will be familiar to the skilled man. In a corona charging system a high voltage generator is used to charge an electrode at the tip of the powder coating spray gun which creates an electrostatic field or ion cloud (corona) between the gun and the workpiece/substrate. The powder coating spray gun used in this type of process is called a Corona Gun. Compressed air is used to transport the powder through the gun, and also through the ion cloud. The powder particles pick up charge as they move through the cloud, and through a combination of pneumatic and electrostatic forces, travel towards and deposit upon the earthed target substrate. Most manufacturers of corona spray equipment utilize a negative corona voltage to impart a negative charge to the powder particles. It is possible, however, to use a positive corona voltage to apply a positive charge to a powder particle

In a tribo charging system use is made of the phenomenon that when two different insulating materials are rubbed together and then separated, they acquire opposite charges (+ and -). Instead of an electrode, tribo guns for the application of a powder coating rely on this friction charging to impart an electrostatic charge onto the powder particles. Compressed air is used to transport the powder particles through the gun. As they travel, the particles strike the walls of the gun, picking up a charge. The pneumatic force of the compressed air then carries the charged particles to the earthed substrate. It is known in the art that a positive charge can be applied to the powder particles by using a tribo gun made of a negative tribo material such as PTFE or similar material and that a negative charge can be applied to the particles by using a gun made of a positive tribo material such as nylon.

In a highly preferred embodiment the application of the primer layer and top coat layer occurs in a continuous process. In the present invention, this means that the top coat material is continuously added to the primer material whilst the process takes place. The person skilled in the art will appreciate that the coating of a substrate with primer and top coat layers is likely to take place in a production line situation which process is considered continuous herein.

Thus, a substrate to be coated is likely to be present on a conveyor, coated with primer layer tinted with top coat, passed to a drying (or curing step) before being coated with top coat followed by a curing step. The top coat used is continuously metered in to the primer (along with new primer material), ideally via a recycle of overspray from the top coat step on the production line to achieve the desired tint. Multiple objects can therefore be coated in succession.

In the process of the invention, the tinting of the primer with the top-coat material can ideally be achieved by recycling the over-spray powder from the topcoat to the primer powder-container. When powder coating occurs it is inevitable that some powder misses the substrate or fails to adhere to the substrate. This is the overspray and is typically collected and recycled to the stage in question, i.e. primer overspray is recycled to the start of primer application. Ideally, this recycle occurs continuously, i.e. there is a constant recycle from top coat application to primer application whilst the process takes place.

The present inventors have realised that it is this overspray that can ideally be used to provide the tinting top coat component of the primer layer powder blend. Moreover, this ensures that the top coat spray is always "virgin material". There is therefore no recycle of the top coat overspray to the top coat starting powder material and hence no contamination of the starting top coat material with powder which has already been sprayed. That improves the appearance of the top coat.

The overspray is instead gathered and added to the primer spray thus tinting it. This can occur continuously so there is a constant gathering and recycling of the overspray from the top layer application stage back to the primer starting material container.

It will be appreciated that it is not necessary to recycle all the overspray to the primer layer if that is not desired. Some overspray could be recycled to the top coat. It will also be appreciated that there may not be enough overspray to fulfil the requirements in the primer layer so top coat material might need to be added directly to the primer layer. It will also be obvious that to start the process for the first time requires the addition of fresh top coat material to the primer layer. Recycling can only occur once the layer (II) has been sprayed. The skilled person is able to devise suitable processes based on the principles herein.

Substrate

The substrate onto which the powder coatings of the invention should be applied can be any substrate such as a metal substrate but is preferably a substrate that is heat sensitive. Such substrates are those that cannot be exposed to temperatures greater than 160°C, preferably cannot be exposed to temperatures greater than 140°C without being damaged. Substrates of main interest therefore include wood, MDF, HDF, plywood, fibreboard, particleboard, plastic and so on. Substrates for internal or external use are envisaged.

The powder coating of the invention may therefore be used in coating glass, ceramics, and graphite-filled composites as well as metallic substrates such as steel and aluminium but has particular utility in the coating of heat sensitive substrates such as plastics, paper, cardboard and wood.

For the purposes of this invention, wood is defined as any lignocellulosic material whether it comes from trees or other plants and whether it be in its natural forms, shaped in a saw mill, separated into sheets and made into plywood, or chipped and made into particleboard, or its fibres have been separated, felted, and compressed. It is exemplified by lumber, panels, moulding, siding, oriented strand board, hardboard, medium density fiberboard, and the like.

Curing

Once a substrate is coated with the powder coating, the coating must be cured. The primer may be cured before application of the top coat (and then again after application of the top coat) but ideally, there is no curing step between the application of the primer layer and the top coat. Thus, both primer layer and top coat are cured simultaneously. The coated substrate may be cured in a conventional convection oven or an IR/convection combination oven. It is also possible to use inductive heating. The use of a convection/induction oven or even convection/induction/IR oven is also contemplated. It is preferred if the curing schedule at least partially includes an IR-curing step. The use of a combination of heat and IR curing is preferred.

Where heating is used during cure, the temperature should preferably be in the range of 100 to 160°C, e.g. 110 to 140°C.

It has also been surprisingly found that the powder coatings can be cured by IR radiation optionally in the absence of conventional heating. The fact that the substrate of the invention can be cured by irradiation opens up enormous

possibilities in terms of the substrates to be coated. As we do not need to employ an heating oven, substrates such as coils become available here. The use of the powder coating of the invention to powder coat a coil forms a further aspect of the invention. Coils coated with our powder coating form a still yet further aspect. The IR curing process is fast making this an attractive commercial process. Moreover, any type of finish can be developed using IR curing.

It is preferred if the IR-curing process involves IR-irradiation of wavelengths between 1-20 μιη.

A preferred curing schedule involves an initial boost by IR-heating which results in melting of the powder and flow of the film at elevated temperature.

Secondary heating in a convection- oven ensures the full curing of the film, and allows lower oven temperatures (110- 140°C). The short, high temperature IR-boost does not result in elevated object temperatures in non-conducting materials and materials with low thermal conductivity, and thus allows the use of alternative substrates as e.g. wood, plastics, particleboard, etc.

A further benefit of the invention is short curing cycles. The powder coating of the invention can be cured using short curing cycles, e.g. of 15 minutes or less.

This is despite lower heating temperatures. Where IR radiation is used alone, curing times can be less than 10 minutes.

The powder coating is preferably free flowing during the curing operation and therefore this leads to smooth, even finishes.

The formulations used in the layers of the invention should be designed to ensure proper degassing of the melted film and at the same time ensuring good flow and a uniform surface of the cured film. During curing, the powder is heated above its melting point, thereby flowing out and coalescing into a continuous film, before curing to the solid coating. When both the primer and the top coat melt and flow out simultaneously, the total thickness of the liquid layer is thicker than any of the two layers separately. This means that the levelling resulting from melting and flow will be comparatively better. It is a well known fact that the flow of powder coatings improve on increasing film thickness. This therefore is an important advantage of curing both coats together

The advantage of flow/levelling is obtained by having the primer and top- coat melting/flowing simultaneously. Thus the total thickness of melted coating is thicker than any of the two coats melting/flowing separately, and the flatness of the cured surface is comparatively better.

It is preferred if the top coat is applied without the primer layer having been cured. By applying both coats without intermediate curing, the complexity of the continuous coating line is significantly reduced because it is only necessary to have one curing oven. Spraying booths may also be installed closer and at a different configuration, resulting in less space requirement for the line. Since overspray powder is preferably recycled to the primer container, this also results in a simpler set-up of the recycling equipment.

An important reason for using a primer is to seal the surface quickly and make a good foundation for the top-coat, without fibre-popping or pinholes from bubbling or pores in the film. Whilst the top coat is preferably applied to the primer before curing takes place, it is advantageous if the primer layer cures rapidly. By having a rapid curing primer with a top-coat curing on top at a slower rate due to the different properties of the formulations, the top coat essentially cures on a cured primer therefore mimicking the conventional two cure solution.

The curing rates of the coatings can be adjusted by varying the make up of the layers. The skilled person is able to design layers with different curing speeds by employing various binder components and catalysts.

It is also a feature of the invention that the cured powder coating of the invention is smooth. The smoothness of the coating film is judged visually to 4 and above on the PCI- scale (Powder Coating Institute). Values of 6 and above are preferred.

It is also very important that the tinting of the primer layer reduces the colour difference between primer layer and top coat as compared to the case where tinting of the primer layer is not used. This difference is called delta E herein. Delta E values below 14 are preferred although it will be appreciated that these values are heavily dependent on the colour of the layers. That means that the colour difference between the primer and the top-coat is as small as possible, facilitating thinner layers (e.g. the difference between light green and green is smaller than between grey and green).

The process of the invention therefore enables in-line tinting of the primer by, for example, recycling overspray top-coat into the primer and thereby reducing the colour difference between the coats. This has several advantages. The solution allows a lower total film thickness while maintaining superior flow and surface appearance. The line set-up is simpler, reducing the space and equipment demand. Also, if the curing of the primer layer is not carried out before top layer application, only one curing oven is needed and this reduces the energy demand for the application process, and the overall processing time is shorter.

The invention will now be described with reference to the following non limiting examples and figures. Figures 1 to 3 show the tinting effect with varying amounts of top coat added to the primer layer. Figure 1 relates to experiments 1 to 7. Figure 2 to experiments 8 to 14 and figure 3 to experiments 15 to 21.

Analytical methods

Film thickness: Measured according to ISO 2178 for metallic substrates, and ASTM D4138-A for non-metallic substrates.

Gloss: Measured according to DIN 67530. Flow: Is determined by visual comparison to standard samples supplied by the Powder Coating Institute (PCI). The flow is rated on a scale from 1-10, where 10 is a smooth mirror-like surface and 1 is semi-texture. Colour difference: Colour is measured with a reflective spectrometer according to the "L*a*b" colour-space (CieLab) system - ISO 7724-3. Here, L is the lightness and a and b are the colour-opponent dimensions. The colour difference between a sample (s) and a reference (r), is given by ΔΕ (dE):

ΔΕ² = Ah² + Aa² + Ab² = (L_r - L_s)² + (a_r - a_s)² + (b_r - b_s)²

and defines the absolute difference between points (colours) in the colour space.

Hiding power: Is determined according to ASTM D6441 on a black and white panel. Full hiding is achieved when the colour difference (dE) between the black and the white side is less than 1.

Impact resistance: Measured according to ASTM D 2794 at 70 μιη film thickness.

MEK test: This is a test of solvent resistance, used to determine the degree of curing of the film. A cotton stick is wetted with methyl ethyl ketone (MEK) and rubbed across the cured film with a slight pressure (rub-length is approximately 10 cm). After 30 double rubs, the surface is checked by visual inspection, and change in hardness is assessed by scratching the surface with a finger nail. Results are reported in the following way;

AO = No change of gloss. No softening (no scratch from finger nail).

Al = Slight loss of gloss. No softening.

A2 = Slight loss of gloss. Some softening (can be scratched by finger nail).

A3 = Strong reduction in gloss. Softening (easily scratched by finger nail).

A4 = Strong reduction in gloss. Strong softening (coating partially removed).

A5 = Coating removed down to substrate.

Gel time: Measured according to ISO8130. Adhesion: Measured according to ISO 2409 EEW: Measured according to ASTM D-1652 Acid Value (AV): Measured according to ASTM D974

Preparation of powder:

The weighed out components are added to a high speed mixer in order to ensure sufficient dispersion of the powder pre-mix. The pre-mix is then added to an Theysohn TSK 20-24 twin-screw extruder and extruded under the following conditions: 30°C in the feed zone, 50°C in the middle, 100°C at the head at 500 rpm.

The extruded material was fed to a chilled roll and passed through a crusher, reducing the chilled material to flakes. The crushed flakes were then fed to a mill. The extruded chips are milled in a mill and sieved in order to ensure a particle size distribution (PSD) within defined specifications. Mixing of any two component powders was performed either as flakes (prior to milling) or by mixing of milled powder.

Mill:

- Hosokawa Micropul ACM 30 mill. Sieved through a 125 μιη rotation sieve.

Particle size distribution: powders were produced to ensure a mean particle size (d₅o) of 25-50 μιη. Application of powder to substrate

The powder was applied to the substrates using standard corona or tribo air-guns. Substrates were either hanging in metal wires or suspended on a suitable stand. The substrates used are cold rolled steel unless otherwise specified.

Curing of films Substrates coated with powder were cured in an oven. Curing temperatures were varied between 130-180°C object temperature, with curing times of 3- 15 minutes at object temperature, dependent on the precise powder formulation.

Oven: Heraeus conventional benchtop oven.

Example 1 - polyester/epoxy hybrid primer (white) and top-coat (white)

A polyester/epoxy hybrid primer was developed to cure rapidly and ensure a proper sealing of the surface. The primer was prepared in grey-white colour, based on the following formulation:

Ingredients Amount [kg]

50/50 type PE* 0,377

Type 2 epoxy 0,317

EtPBr 0,085

Acrylic anti-

0,010

cratering agent

Ti0₂ pigment 0,030

BaS0₄ 0,185

* carboxy functional polyester

A white polyester/epoxy hybrid top-coat with smooth surface and low gloss prepared based on the following formulation:

Ingredients Amount [kg]

A B

70/30 type PE 0,330

Carboxyl

functional acrylic 0,091

polymer

2- 0,00425

isopropylimidazole Ingredients Amount [kg]

A B

Acrylic anti-

0,010 0,010

cratering agent

Degassing agent 0,002 0,002

Ti0₂ pigment 0,250 0,300

Polypropylene wax 0,013 0,014

50/50 type PE 0,320 —

Type 2 epoxy 0,240 0,279

EtPBr 0,0073

The powders were prepared and tested according to the general procedures stated above. The two components of the top-coat were mixed before milling.

Miscibility studies were performed by mixing primer and top-coat at different ratios, applying them on a sheet of 0,8 mm cold rolled steel, and subsequently curing for 8 minutes at 140 °C object temperature. Tests were performed on the panel after 16 hours conditioning at room temperature.

All tests were performed on the cured mixes of primer (1-7) alone. Colour difference was determined with a spectrometer, with the pure top-coat applied and cured on steel for 8 minutes at 140 °C object temperature as a standard.

Table 1

1 2 3 4 5 6 ₇*

White top-coat — 0,01 0,05 0,10 0,20 0,30 0,50

White-grey primer 1 0,99 0,95 0,90 0,80 0,70 0,50

Gloss (angle 60°) 86 90 90 88 85 80 78

Flow (PCI) 5 5 5 5 5 5 5

Impact resistance (5/8")

Front @70μ 160 160 160 160 160 160 160 (inch. lbs.)

Reverse @70μ 160 160 160 160 160 160 160 (inch. lbs.) 1 2 3 4 5 6 7*

MEK test (30 double A2 A2 A2 A2 A2 A2 A2 rubs)

Gel time (140 °C, 80/7 85/7 89/8 94/9 97/9 99/10 110/12 Coesfeld, s)

Hiding power — — — — >250 μ — —

Colour difference, to topcoat

AL -13,81 -14,18 -12,74 -12,54 -11,49 -10,60 -7,67

Aa -0,25 -0,27 -0,25 -0,26 -0,28 -0,30 -0,34

Ab -5,43 -5,62 -5,30 -5,22 -4,94 -4,71 -3,82

AE 18,84 15,26 13,80 13,59 12,51 11,60 8,57 comparative example

The total colour difference (ΔΕ) between the top-coat and the primer with different mix-ratios of the top coat is significantly reduced by increasing the mix -ratio. This is shown in figure 1. There is however so much top coat added in experiment 7 that it does not act satisfactorily as a primer layer. It has poor mechanical properties for example.

Example 2 MDF panels were preheated to a surface temperature of 50-60 °C - 30-35 °C surface temperature during powder application. The primers of experiments 1 to 7 were applied with a corona, negative electrostatic gun. The coated panel was then cured in an infrared (IR)/convection combination oven for 8 minutes at 140 °C.

Table 2

1 2 3 4 5 6 7*

Gloss (angle 60°) 90 88 84 80 70 69 59

Adhesion (Gt) 0 0 0 0 0 0 0

MEK test (30 double rubs) A2 A2 A2 A2 A2 A2 A2

Flow (PCI) 4-5 4-5 4-5 4-5 4-5 4-5 4-5 Example 3 - polyester/epoxy hybrid primer (white) and top-coat (purple)

The polyester/epoxy hybrid primer was prepared in grey- white colour as per example 1.

A polyester/epoxy hybrid top-coat with smooth surface and low gloss was prepared as per example 1 but the top-coat was prepared in purple by adding extra pigments to the formulation.

Table 3

The total colour difference (ΔΕ) between the top-coat and the primer with different mix-ratios of the top coat is significantly reduced by increasing the mix -ratio and is shown in figure 2. There is however so much top coat added in experiment 14 that it does not act satisfactorily as a primer layer. It has poor mechanical properties for example. Example 4

MDF panels were preheated to a surface temperature of 50-60 °C - 30-35 °C surface temperature during powder application. The primer of experiments 8 to 14 were applied with a corona. The coated panel was then cured in a infrared (IR)/convection combination oven for 8 minutes at 140 °C.

Table 4

Example 5 - epoxy primer (grey) and top-coat (white)

A pure epoxy primer was developed to cure rapidly and ensure a proper sealing of the surface. The primer was prepared in grey.

Ingredients Amount [kg]

Novolac epoxy 0,277

Type 4 epoxy 0,277

Phenolic

0,147

crosslinker

2-methyl imidazole 0,0105

Acrylic anti-

0,001

cratering agent

Degassing agent 0,010

Ti0₂ pigment 0,010 Ingredients Amount [kg]

BaS0₄ 0,200

CaC0₃ 0,200

Carbon black 0,0007

A polyester/epoxy hybrid top-coat as per example 1 was used. The top-coat was prepared in white.

Miscibility studies were performed by mixing primer and top-coat at different ratios, applying them on a sheet of 0,8 mm cold rolled steel, and

subsequently curing for 8 minutes at 140 °C object temperature. Tests were performed on the panel after 16 hours conditioning at room temperature.

Table 5

The total colour difference (ΔΕ) between the top-coat and the primer with different mix-ratios of the top coat is significantly reduced by increasing the mix -ratio and is shown in figure 3. There is however so much top coat added in experiment 21 that it does not act satisfactorily as a primer layer. It has poor mechanical properties for example.

Example 6

MDF panels were preheated to a surface temperature of 50-60 °C - 30-35 °C surface temperature during powder application. The primer of experiments 15, 18 and 21 were applied with a corona, negative electrostatic gun. The coated panel was then cured in a infrared (IR)/convection combination oven for 8 minutes at 140 °C. Table 6

Example 7

MDF panels were preheated to a surface temperature of 50-60 °C - 30-35 °C surface temperature during powder application. The primer of experiment 18 was applied with a corona, negative electrostatic gun, and the top-coat of experiment 18 was directly thereafter applied with a tribo friction gun. Four panels were prepared, with varying thicknesses of primer and top-coat.

In parallel primer and top-coat was similarly applied with intermediate curing at varying thicknesses. Table 7

The top coat can therefore be applied successfully onto a primer layer "tinted" with the top coat material.

Claims

1. A process for applying a powder coating on a substrate comprising:

(II) applying to said first powder coating layer a powder top coat.

2. A continuous process for applying a powder coating on a substrate comprising:

(II) applying to said first powder coating layer a powder top coat;

wherein the top coat material is continuously added to the primer material.

3. A process for applying a powder coating on a substrate comprising:

(II) applying to said first powder coating layer a powder top coat;

wherein at least a part of the overspray that occurs when the top coat material is applied onto the first powder coating layer is recycled to form at least a part of the top coat material in the powder blend in layer (I).

4. A process for applying a powder coating on a substrate comprising:

(II) applying to said first powder coating layer a powder top coat;

wherein at least a part of the overspray that occurs when the top coat material is applied onto the first powder coating layer is continuously recycled to form at least a part of the top coat material in the powder blend in layer (I).

5. A process as claimed in claim 1 to 4 wherein layer (I) and (II) are cured together after application of both layers.

6. A process as claimed in claim 3 to 4 wherein the top coat is free from any recycled over spray material.

7. A substrate having thereon:

(II) a powder top coat.

8. A substrate as claimed in claim 7 which has then been cured either after application of layer (I) and again after application of layer (II) or just after application of layer (II).

9. A substrate as claimed in claim 7 or 8 wherein the primer comprises an epoxy resin or mixture of epoxy resins.

10. A substrate as claimed in claim 7 to 9 wherein the primer comprises a binder which forms 50 to 95 wt of said primer.

11. A substrate as claimed in claims 7 to 10 wherein the primer comprises a hybrid binder of an epoxy resin and a polycarboxylic polymer.

12. A substrate as claimed in claims 7 to 11 wherein the first coating layer comprises 1 to 30 wt of the top coat material and 70 to 99 wt of the primer.

13. A substrate as claimed in claims 7 to 12 wherein the top coat comprises a blend of two components: i) a first component comprising at least one epoxy containing compound, at least one polycarboxyl polymer and at least one organic Lewis base; and

14. A substrate as claimed in claims 7 to 13 wherein the top coat material forms 10 to 20 wt of the first powder coating layer (I).

15. A substrate as claimed in claims 7 to 14 wherein the primer layer is 50 to 70 microns thick and the top layer is less than 100 microns thick.

16. A substrate as claimed in claims 7 to 15 wherein the primer layer and the top layer thickness combined is less than 120 microns.

17. Use of the overspray of the top coat material during a powder coating process as an additive for the primer layer.