The invention relates to a novel use of powder coating material, such as waste powder coatings. 5 A general description of "powder coating", which may also be identified as "coating powder", is found in Kirk-Othmer, Encyclo¬ pedia of Chemical Technology, 3rd Edition, Vol. 19. pp. 1-27.

The present invention in particular relates to the use of thermosetting powder coatings which melt, for example, between 100

10 and 150°C and set, for example, between 150 and 200°C. During the application, said powders are charged electrostatically and are sprayed onto the object to be coated. The powder which does not land on the object is recovered with the aid of a recovery device, for example a cyclone. Sometimes this powder cannot be reused, for

15 example towards the end of the treatment of a batch of objects.

Waste powder is also produced when the colour is changed, because powder is formed with a contaminated colour, which is no longer usable as a coating. Furthermore, the manufacturer of powder coatings also has to face the powder coating waste problem because

20 certain batches can no longer or must no longer be distributed commercially.

Hitherto, waste powder from the powder coating industry has been destroyed in special combustion plants, causing considerable environmental problems. 25 The invention is designed to overcome this problem, and it makes efficient use of the waste powder from the powder-coating production and processing industry. Attempts to find a solution for a useful application of this waste powder by mixing this powder with other (plastic) materials and melting it failed as it appeared that 30 a uniform, homogeneous melt could not be obtained.

The present invention is based on the surprising finding that a particulate carrier material may be coated first with a thermo- i setting powder coating and, subsequently, the individual particles are fused together. Although one would expect that such a technique * 35 would be suitable for thermoplastic powders, it is surprising that thermosetting powder coatings may find a useful application by such a technique.

Accordingly, it was surprisingly found that coating powder can be used for producing composite material.

The invention therefore relates to a process for producing a composite material, in which a particulate carrier material is coated with uncured, possibly molten, thermosetting coating powder, and the coated particles of the carrier material are heated in a mould to a temperature at which the powder coating coats of the individual particles melt and fuse together forming a plastic matrix with the particulate carrier material distributed therein, whereupon thermal curing takes place with the formation of the composite material.

The coating powder, for example, is a thermosetting waste powder coating, as described above.

In the process according to the invention, the particulate carrier material is preferably a material in the form of grains, beads or fibres. In general, a carrier material will be used which will not melt at the temperatures used for melting the powder coating or for subsequent hardening thereof.

In the process according to the invention, the carrier material is preferably chosen from ceramic materials such as baked clay, (blast furnace) slags, lava (pumice), glass, naturally occurring materials such as gravel, sand and wood or other products such as paper, plastics, (sintered) fly ash or metal.

It is found that, for example, baked clay grains used for hydrocultures or glass beads are particularly satisfactory.

The dimensions of the carrier material particles may vary within wide ranges and, for example, be in the range of 0.01 mm - 10 cm, preferably 0.1 mm - 5 cm, particularly preferably 1 mm - 2 cm.

The process according to the invention is suitable for processing various types of powder coatings which may be applied by melting around the particulate carrier material and which can be thermally cured subsequently. There is, however, a particularly wide range of powder coatings on a polyester basis, for example trigly- cidyl isocyanurate (TGIC) , so that there is a preference for using products of this type.

As the abovementioned article in Kirk-Othmer describes, the melting point and the curing temperature of powder coatings can vary

over relatively wide ranges. In the process according to the invention, it is preferred that the melting point range and the curing temperature range partially overlap each other, but in such a way that the centre of the melting range is below that of the curing • 5 range, the melting point of the powder coating in particular being between 130 and 150 ^* C, for example around l40°C, and the curing t. temperature between 170 and 190°C, for example around 180 ^* C.

The application of the powder coating on the carrier material can be carried out in various ways. Preference is given to a method 10 in which the carrier material is coated by heating the particles of the carrier material to a temperature above the melting point of the powder coating and dipping the heated carrier material particles into the powder coating.

On the other hand, a method is preferred in which the coating 15 of the carrier material is carried out by passing the carrier material, which has been heated to above the melting point of the powder coating, through a fluidised bed of the powder coating.

Powder can also be applied by coating support material with an adhesive such as a fatty or waxy material which has a boiling 20 point in the curing range of the powder coating. The carrier material is then dipped into the powder, as a result of which a powder layer adheres to the surface, the material being coated in such a way that the outermost layer comprises loose powder, as a result of which the grains do not adhere to each other and a bulk- 25 pourable (free-flowing) material is produced. During curing, the binder will boil, as a result of which the coating will foam. This produces a composite of the carrier material with a foam-like or porous structure.

According to a preferred embodiment of the process of the

30 invention the porosity of the composite material is controlled by adjusting the starting temperature of the particulate carrier material. For instance, it is possible to adjust the porosity from

^ 0 % up to e.g. 40 % by adjusting the temperature of the used particulate carrier material to a value of e.g. 30θ"C (porosity 0 %)

% 35 to a value of the temperature in a melting range of the powder coating (porosity e.g. 40 %) . If a particulate carrier material such as fly ash having a particle size in the range of 4-8 mm and a pow-

der coating having a melting range between 130-150 ^β C (e.g. TGIC) are used, a porosity of 33 % results if the starting temperature of the particulate carrier material is about 200°C. Temperatures of 230°C, 250 ^β C, and 270°C yield a porosity of 22 %, 10 %, and 0 % respectively. For carrier materials of another type, which have other specific heat properties, other temperatures are required for obtaining corresponding porosity values. E.g. carrier material comprising gravel or pebbles require temperatures which may be about 50-70°C lower for obtaining similar porosity results. In the process of the invention the particles of the carrier material will generally be heated to a temperature above the melting point as well as above the curing point of the powder coating before the carrier material particles are coated with the powder coating. Consequently, the temperature on which the carrier particles are heated will generally be above 170 or 190°C. Although the curing temperature is exceeded, the individual coated particles fuse together, which seem to be contradictory to the thermosetting property. As noted in the above, the heating temperature of the (un- coated) particles may be in the range of 170-400°C, preferably 190-350°C, in particular 200-300 ^β C.

The application of the powder coating coat on the carrier material must take place in such a way that the thickness of the uncured powder coat is sufficient to ensure that, after mutual fusing of the separate coats of the coated particles and the curing step, a coherent composite material is produced. It was found that according to the invention the thickness of the uncured powder coating coat of the carrier material is expediently 0.1-5 mm, preferably 1-3 mm.

For the purpose of producing a foamed composite material, the powder coating material used as the coat can be admixed with a blowing agent which boils or is decomposed at a temperature above the melting point of the powder coating. In this way it is possible to obtain very light yet strong composite materials.

It is also possible to incorporate, in the composite material according to the invention, reinforcing means such as glass fibres, carbon fibres and/or aramid fibres.

It will be evident that the density of the composite material

produced according to the invention will depend strongly on the type of the carrier materials used. Beads composed entirely of glass will produce a heavier material than hydroculture grains which can be used as a carrier material. There are innumerable application possibilities for the composite material produced according to the invention, for example separating walls for use indoors and outdoors, floors, sound panels, flower pots, fishponds, swimming pools, ceilings and other construction and decorative materials in the construction industry, shipping, masts, pillars and furniture.

The present invention also relates to composite material obtainable according to the process described in the above. Also shaped articles form part of the invention. The porous composite material according to the invention has many interesting appli- cations in view of the permeable structure. It is e.g. possible to produce a composite material having a predetermined water permeability. This property is useful in applications such as sheet pilings or covering materials for dykes but also as drain pipes or drain sheets. A further possibility is the use as acoustic sound absorbing material.

To produce, for example, plates, pillars or sections, a con¬ tinuous process can be used.

The invention is explained in more detail in the following examples.

Example I

Sintered ceramic grains having an average size of 2-40 mm are heated to a temperature of 180°C. The heated grains are dipped into the powder coating. The powder melts on the grain surface, but then resolidifies owing to the falling temperature of the grain. The grains thus coated form the new base material, which can be used to make products.

Said grains are poured into a mould having the negative form of the desired product. The mould is heated up to the curing temperature. During heating, the coating will flow again, as a result of which the grains fuse together. Thereafter, the coating

will cure, to form a connecting element between the grains, thus producing a firm structure.

Example II

Grains having a particle size between 4 and 20 mm (ARGEX or LYTAG) are dipped into a liquid, so that the surface is wetted. These grains are dipped or rolled in the powder, so that a crust of moist powder is formed. The subsequent process is the same as in Example I, except that the liquid contains a foaming agent.