Powder coating compositions are solid compositions that generally comprise a solid film-forming resin or mixtures of different kind of resins, usually with one or more pigments and, optionally, one or more performance additives such as plasticizers, stabilizers, flow aids and extenders. The resins are usually thermosetting, incorporating, for example, a binder resin and a corresponding crosslinking agent (which may itself be another binder resin). Generally, the resins have a T _g , softening point or melting point above 30°C.

Conventionally, the manufacture of a powder coating comprises melt-mixing the components of the composition. Melt-mixing involves the high speed, high intensity mixing of dry ingredients and then the heating of the mixture to an elevated temperature - above the softening temperature of the resin but below the curing temperature - in a continuous compounder to form a molten mixture. The compounder preferably comprises a single or twin screw extruder as these serve to improve the dispersion of the other ingredients in the resin as the resin melts. The molten mixture is extruded, typically rolled in the form of a sheet, cooled to solidify the mixture and subsequently crushed to flakes and subsequently pulverised to a fine powder

Such processing is then generally followed by a sequence of particle sizing and separation operations - such as grinding, classifying, sifting, screening, cyclone separation, sieving and filtering - that precede the application of the powder to a substrate and the heating of that powder to melt and fuse the particles and to cure the coating. The main methods by which powder coatings are applied include fluidized-bed, wherein a substrate is preheated and dipped in a fluidised bed of the powder resulting in the powder fusing on contact with hot surface and adhering to the substrate, and electrostatic fluidized- bed processes and electrostatic spray processes in which the powder coating particles are electrostatically charged by electrodes within a fluid bed or by an electrostatic spray gun and directed to be deposited onto an earthed substrate. Powder coatings can be used in a wide range of applications. One area where powder coatings are used is in the architectural area where powder coated metal panels that are used to protect and decorate the outside of buildings.

In addition to corrosion protection, one of the requirements for such powder coatings for architectural use is their durability. Once the powder coated metal panels are mounted to a building, they should keep their physical appearance (amongst others color and gloss) for a large number of years.

Coatings that show a > 50% gloss retention after 36 months open exposure at 5° south facing in Florida can be classified as a superdurable product. These products have a (guaranteed) service life of more than 10 years in an outdoor environment.

US 6635721 discloses a thermosetting powder coating composition which comprises a mixture of polyesters containing carboxyl groups, and a crosslinking agent with functional groups capable of reacting with carboxyl groups. The polyesters include semi-crystalline polyester (a) and amorphous polyester (b). Coatings made with said composition have very good weatherability, and good mechanical properties. A variety of mineral fillers such as sulphates may be added to the thermosetting composition. WO 99/55754 discloses a thermosetting powder coating composition which comprises an organic resin component and a silane. Also silane treated pigments and fillers are mentioned. For the powder coating compositions disclosed in this reference, these silane compounds are useful as modifiers of film properties such as environmental resistance. This improved environmental resistance is in particular visible as improved gloss retention of the cured compositions.

WO 06/125545 discloses a thermosetting powder coating composition which comprises a mixture of amorphous polyesters containing carboxyl groups, and a crosslinking agent with functional groups capable of reacting with carboxyl groups. Coatings made with said composition have very good weatherability, and good mechanical properties (see column 1 , lines 16-18). The good weatherability is reflected in the gloss retention properties of cured powder coatings.

US 2004/143073 relates to a method for determining gloss in a polyester resin based powder coating composition. It discloses a thermosetting powder composition which comprises a mixture of polyesters containing carboxyl groups, and a crosslinking agent with functional groups capable of reacting with carboxyl groups. A variety of mineral fillers may be added to the thermosetting composition. US 6660398 discloses a low gloss thermosetting powder coating composition which comprises a mixture of polyesters containing carboxyl groups, and a crosslinking agent with functional groups capable of reacting with carboxyl groups. The polyesters include semi-crystalline polyester (a) and amorphous polyester (b). Coatings made with said composition have very good weatherability, and good mechanical properties. A variety of mineral fillers such as sulphates may be added to the thermosetting composition.

It was found that superdurable powder coating products available on the market with good weatherability properties, i.e., good gloss retention properties, show so-called "blanching", in particular when exposed to humid outdoor conditions often present in a tropical climate. Blanching is a phenomenon observed as whitish patches on a cured coating film, when the coating is exposed to high temperatures and high humidity, typical conditions that are found in a tropical climate. It was found by the inventors of the present invention that this is caused by absorption of moisture in the coatings which creates a thin layer below the surface and causes total internal reflection of transmitted light. This phenomenon is more visible on black or dark colours and less visible in white or lighter colours.

Blanching can be simulated by keeping coated panels in hot water (55°C) for 48 hours. These whitish patches are not permanent. When the coated panels are heated for some time the absorbed moisture evaporates and the whitish patches completely disappear. Blanching should be avoided in particular for powder coatings that are used outdoor in a tropical climate.

Surprisingly, it was found a superdurable powder coated substrate can be obtained which shows no blanching in a tropical climate when the substrate is coated with a polyester powder coating according to one embodiment of the present invention, viz. a powder coating composition comprising

a polyester resin A and a curing agent for said polyester resin;

a polyester resin B and a curing agent for said polyester resin, and - a hydrophobic filler

wherein polyester resin A and polyester resin B are not the same.

The powder coating compositions that are used in the various embodiments of the present invention comprise a solid polymeric binder system comprising a carboxy- functional polyester film-forming resin. Such carboxy-functional polyester systems are currently the most widely used powder coatings materials. The polyester generally has an acid value in the range 10-100, a number average molecular weight Mn of 1 ,500 to 10,000 and a glass transition temperature Tg of from 30°C. to 85°C, preferably at least 40°C. Suitable polyester resins can be prepared by reacting an OH- functional monomer (e.g. glycol, neopentyl glycol, trimethylolpropane) with an acid-functional monomer (e.g. terephthalic acid, isophthalic acid, phthalic acid). Examples of commercial carboxy-functional polyesters are: Uralac (Registered trademark) P3560 (DSM Resins) and Crylcoat (Registered trademark) 314 or (UCB Chemicals).

At least one of the polyester resins A or B should be an amorphous polyester resin. The other polyester resin can either be amorphous or semi crystalline.

In one embodiment the present invention relates to a polyester powder coating composition wherein polyester resin A has an acid value in the range of 45 - 70 mg KOH/g and polyester resin B has an acid value in the range of 20 - 45 mg KOH/g. In another embodiment the present invention relates to a polyester powder coating composition wherein polyester resin A has a softening point Tg in the range of 50 - 70 °C and polyester resin B has a softening point Tg in the range of 30 - 50 °C.

It is well known for powder coating compositions to use carboxy-functional polyester film-forming resin in combination with an epoxy curing agent or crosslinker. Commonly, triglycidylisocyanurate is used as a crosslinker for carboxy-functional polyesters.

However, in one embodiment the present invention relates to a polyester powder coating composition wherein the curing agent for polyester resin A and the curing agent for polyester resin B are substantially free of triglycidylisocyanurate (TGIC). Substantially free means that the total composition contains less than 5 wt.% (based on the total weight of the composition) of TGIC, preferably less than 1 wt.%, more preferably less than 0,1 wt.%.

It was found that powder coating compositions with a very good durability and anti- blanching properties can be obtained if in the powder coating composition according to the present invention a hydroxyalkylamide compound is used as a curing agent. Examples of suitable curing agents include bis(beta-hydroxyalkylamide) curing agents such as tetrakis(2-hydroxyethyl) adipamide (Primid (Registered trademark) XL-552).

Alternatively, polyepoxy compounds which are solid at room temperature and contain at least two epoxy groups per molecule such as, for example, diglycidylterephthalate and, triglycidyltrimellitate or mixtures thereof, can also be used as curing agent. In accordance with the present invention, a hydrophobic filler is present in the powder coating composition according to the present invention. Hydrophobic fillers are fillers that lack affinity for water, tend to repel and not absorb water, tend not to dissolve in or mix with water and also tend not to be wetted by water. The hydrophobicity of materials can be determined, amongst others, by measuring the contact angle with water. A hydrophobic filler normally shows a contact angle with water which is larger than 90 degrees, preferably larger than 100 degrees. The contact angle can be measured in accordance with ASTM D7334-08

Suitable hydrophobic fillers that can be used in accordance with the present invention include organic compounds and inorganic compounds, e.g. surface treated inorganic compounds. It was found that surface treated BaSO ₄ , or surface treated AI2O3 performs very well in the powder coating compositions according to the present invention. Surface treatment can be done by coating the filler particles with a thin wax layer or by treatment with a polysiloxane liquid, resulting in an inorganic particle with siloxane groups present at the outside of the particle. For example, surface-treated barium sulfate can be made by mixing a linear and non-crossl inked silicon-hydride containing polysiloxane, in neat or in aqueous emulsion form, with a quantity of barium sulfate particles and then drying the resultant mixture.

Alumina trihydrate fillers, particularly Haltex 310, also shows similar hydrophobic performance and antiblanching properties in this system In addition to a polyester film forming resin, a curing agent, and a hydrophobic filler, the powder coating compositions according to the present invention can also comprise one or more pigments. Examples of pigments which can be used are inorganic pigments such as titanium dioxide, red and yellow iron oxides, chrome pigments and carbon black and organic pigments such as, for example, phthalocyanine, azo, anthraquinone, thioindigo, isodibenzanthrone, triphendioxane, and quinacridone pigments, vat dye pigments and lakes of acid, basic and mordant dyestuffs. Dyes can be used instead of or as well as pigments.

In one embodiment the present invention relates to a polyester powder coating composition which comprises

- from 30 - 40 wt.% of polyester resin A,

- from 30 - 40 wt.% of polyester resin B,

- from 2 - 10 wt.% of curing agents for polyester resins A and polyester resin B, and

- from 10 - 40 wt.% of a hydrophobic filler or a mixture of hydrophobic fillers. wherein the wt.% is taken on the total weight of the powder coating composition. The composition of the invention may also include one or more performance additives, for example, a flow-control agent, a dispersing agent, a plasticiser, a stabilizer against UV degradation (an anti-oxidant), or an de-gassing agent, such as benzoin, or two or more such additives may be used. Normally, a coating composition of the invention comprises between 0 - 5 wt.% of one or more performance additives. As with pigments, these other additives can be included during or after dispersing the binder components, but for optimum distribution it is preferred that they are mixed with the binder components before both are dispersed.

The powder coating composition of the current invention can be manufactured in a process, comprising the steps of:

- Preparation of a polyester powder coating composition CA, comprising a polyester resin A and a curing agent for said polyester resin;

- Preparation of a polyester powder coating composition CB, comprising a polyester resin B and a curing agent for said polyester resin; and

dry blending said polyester powder coating composition CA and polyester powder coating composition CB, wherein polyester resin A and polyester resin B are not the same, and wherein polyester powder coating composition CA and/or polyester powder coating composition CB comprises a hydrophobic filler. In a conventional process for the manufacture of a powder coating composition, all ingredients of the powder coating are pre-mixed and thereafter melt-mixed in an extruder. The molten mixture is then cooled and granulated and grinded into a powder coatings.

In one embodiment of this invention, polyester powder coating composition CA, comprising a polyester resin A and a curing agent for said polyester resin is prepared by melt-mixing in an extruder as described above from a pre-mix containing all components of powder coating composition CA. In a separate step, polyester powder coating composition CB, comprising a polyester resin B and a curing agent for said polyester resin is also prepared by melt-mixing in an extruder as described above from a pre-mix containing all components of powder coating composition CB. Both compositions CA and CB are then dry-blended to obtain the powder coating composition according to the present invention, provided that either powder coating composition CA or powder coating composition CB, or both compositions, comprise an hydrophobic filler.

In one embodiment the present invention relates to a polyester powder coating composition wherein the hydrophobic filler is present in the powder particles mainly containing polyester resin A and/or the powder particles mainly containing polyester resin B. By using the above dry-blending step of powder coating compositions CA and CB, a polyester powder coating composition is obtained that comprises some powder particles mainly containing polyester resin A and other powder particles mainly containing polyester resin B.

A powder coating composition according to the invention may in principle be applied to a substrate by any of the processes of powder coating technology, for example, by electrostatic spray coating (corona-charging or tribo-charging) or by fluidized-bed or electrostatic fluidized-bed processes.

After application of the powder coating composition to a substrate, conversion of the resulting adherent particles into a continuous coating (including, where appropriate, curing of the applied composition) may be effected by heat treatment and/or by radiant energy, notably infra-red, ultra-violet, or electron beam radiation.

The powder is usually cured on the substrate by the application of heat (the process of stoving); the powder particles melt and flow, and a film is formed. The curing times and temperatures are interdependent in accordance with the composition formulation that is used

The substrate may comprise a metal, a heat-stable plastics material, wood, glass, or a ceramic or textile material. Advantageously, a metal substrate is chemically or mechanically cleaned prior to application of the composition, and is preferably subjected to chemical pre-treatment, for example, with iron phosphate, zinc phosphate or chromate. Substrates other than metallic are in general preheated prior to application or, in the case of electrostatic spray application, are pre-treated with a material that will aid such application. In one embodiment the present invention relates to the use of the superdurable polyester powder coating composition for the coating of panels for architectural use.

The invention will be elucidated with reference to the following examples. These are intended to illustrate the invention but are not to be construed as limiting in any manner the scope thereof. Examples

Two powder coating compositions A and B were prepared from the following ingredients:

All ingredients of composition A were mixed and melt extruded. After extrusion the particles were crushed, milled, and classified, and a powder coating composition PA was obtained.

All ingredients of composition B were mixed and melt extruded. After extrusion the particles were crushed, milled, and classified, and a powder coating composition PB was obtained. Powder coating composition PA and PB were dry blended and a final powder coating composition PC was obtained.

Powder coating composition PC was sprayed on a metal panel, the panel was heated to allow the powder coating to melt, flow, and cure.

The obtained powder coated panels were immersed in hot water (55°C) for 48 hours and then dried an examined for whitish patches (water blanching).

The following results were found

*) Comparative example