After the coating has been applied in the form of a solution, dispersion or as powder, lacquered or otherwise coated objects are subjected to a stoving step. "Stoving"is understood to mean the technique known to a skilled person of heating the surface of a coated object to a high temperature. Depending on the type of coating material which is used, during stoving, evaporation takes place of solvents and/or diluents, insofar as present. In addition, for many coating materials, the desired lacquer properties are obtained by chemical reactions (including polymerization reactions, in particular cross-linking reactions). These reactions take place as a result of the heat supplied during stoving. For so-called powder coatings, in particular, the polymerization reactions, cross-linking reactions and/or sinter processes are important, while, for other coatings, in particular, the evaporation of the solvents and/or diluents is important.

Usually, the formation of the definitive coating involves a combination of evaporation and chemical conversions.

According to the state of the art, stoving is carried out in stoving ovens, in which hot air supplied ensures the supply of heat and the discharge of the solvent and/or diluent vapors and the reaction products formed.

However, the known processes for stoving have a number of drawbacks. For instance, the use of air as a heat transfer medium in the evaporation of solvents and diluents entails an explosion risk. In order to remain below the (lower) explosion limits, in stoving practice, larger

amounts of air need to be supplied than is required for the heat transfer.

This extra air also needs to be heated up, which involves extra energy costs.

In addition, heat recovery from the large amount of air is often not economically attractive due to the relatively low temperature (usually approx. 100 to 200°C) and the low condensation temperature of the vapors.

In practice, the recovery of solvents and/or diluents is often not economically attractive either due to the low concentration of these agents. The evaporated solvents and/or diluents are often emitted to the outside air without further measures, which results in a impact on the environment.

Optionally, an off-gas treatment installation may be deployed to purify the gases before emission and possibly even recover the solvents and/or diluents therefrom. Due to the large gas flows, such off-gas treatment installations usually involve high investment costs and operational costs.

A further drawback of conventional stoving techniques is that the reaction products formed in stoving, including oxidation products resulting from the presence of oxygen in combination with the elevated temperature, are difficult to remove and can cause odor nuisance. Examples of such products are acrolein and/or formaldehyde. Non-volatile compounds, such as tar-like cracking products, can also be formed.

A further drawback of the conventional stoving techniques is that, due to the presence of oxygen, the stoving temperature is limited to a maximum of approximately 250°C. At higher temperatures, the oxidation reactions can result in discoloring or other deteriorations of the surface.

In addition, the use of air as a medium for stoving is characterized by a limited heat transfer, which further limits the efficiency of the known stoving techniques.

EP-A-0 310 708 describes the curing of resins with a specific chemical composition by contacting them with water.

GB-A-1 596 827 relates to the UV curing of sp. ecific polymerizable materials with an ethylenic unsaturation in the presence of superheated steam.

JP-A-2002-48473 describes an apparatus for heating painted objects using a gas containing 30 to 100% of superheated steam.

It is the object of the present invention to provide a process for stoving which at least partly obviates the above-mentioned drawbacks. It has been found that this object can be achieved by carrying out the stoving in an inert atmosphere comprising superheated steam. The superheated steam typically contains a maximum of 10 vol. % of other gases. The present invention thus relates to a process for stoving a coating on an object, in which this object is introduced into an atmosphere, which atmosphere comprises superheated steam.

By using superheated steam in the stoving process according to the invention, the risk of explosion is removed. In addition, this reduces the formation of undesired compounds, in particular of oxidation products. The oxidation products in particular are considered to be responsible for odor nuisance in conventional stoving processes. The present invention thus provides the possibility to reduce odor nuisance in stoving processes.

Further, the heat content of superheated steam is considerably larger than that of air, so that a more efficient process is obtained and, in principle, more compact equipment is sufficient.

Because, basically, one can work in the absence of oxygen, it is, moreover, possible to use higher temperatures of, for instance, 400°C or more, without undesired oxidation reactions occurring.

A further advantage of the use of superheated steam is that the heat transfer is considerably improved compared to the heat transfer of air. This is particularly important if metal or ceramic objects are stoved, because such objects are usually characterized by a considerable heat capacity. By

using steam according to the invention, the required amount of heat can thus be rapidly transferred.

It has surprisingly been found that the use of superheated steam need not have any negative effects on the quality of the stoved coating.

Nevertheless, under some circumstances, it may be desired to first preheat the object to be stoved before subjecting it to the stoving process according to the invention. In particular, this holds true if the temperature of the object is so low that there would be a risk of undesired condensation of the superheated steam upon contact with the coated surface. Condensation of the superheated steam can lead to the formation of water drops on the coated surface, which could lead to undesired stains or damages on the coated surface.

It has been found that preheating to a temperature of 50 to 120°C, preferably of 75 to 100°C, can completely, or virtually completely, prevent condensation in virtually all practical processes according to the invention.

In most practical cases, a preheating temperature of less than 5Q°C leads to condensation and, consequently, to a final product with an unacceptable quality of the coating. A preheating temperature above 120°C is possible, but usually not desired, because this inter alia causes too large a part of the solvent or diluent to evaporate already in the preheating step, so that this cannot be recovered anymore in the subsequent steam stoving step. In addition, the above-described drawbacks of conventional stoving techniques can occur if the preheating temperature is chosen too high. The preheating is preferably carried out by using hot air, for instance heated ambient air under atmospheric pressure. The preheating may also be carried out by means of infrared radiators, induction heating or heating by microwaves. It is also possible first to preheat the substrate and subsequently to apply the coating on the preheated substrate, prior to the stoving process.

The atmosphere comprising superheated steam is a gas, which, for a considerable part, for instance, 90 vol. % or more, consists of water (vapor)

and further contains, for instance, air, if present. Preferably, the gas consists entirely, or virtually entirely, of superheated steam.

By stoving with superheated steam, solvents and/or diluents will evaporate from the coating and mix with the superheated steam. This offers a further advantage because, in this manner, the solvents and/or diluents can be recovered in a simple manner by condensing the steam after is has been discharged from the stoving installation. In particular, if the solvents and/or diluents are not or poorly water-soluble, which is the case for many organic solvents/diluents, these can simply be separated from the water phase.

The invention can very suitably be used for stoving lacquer or paint layers obtained by applying water-based lacquers or paints, i. e. in which water forms an important component in the solvent, or the diluent.

Further, the present invention can very suitably be used for stoving ceramic layers (enamel). For this purpose, the dispersion for the glaze is applied in a conventional manner and sintered using the stoving process according to the invention. For the stoving of ceramic layers, the temperature of the superheated steam is typically 500-700°C or more.

According to a preferred embodiment of the present invention, a sub-flow of the steam is cooled down, after it has been discharged from the stoving installation, until condensation of solvent/diluent and steam occurs.

The condensation heat released therein can be energetically utilized, which results in an energy saving which may amount to 50% or more.

Preferably, the atmosphere in which the stoving according to the invention takes place substantially consists of superheated steam, in which, during the process, an increasing concentration of solvents/diluents and/or reaction products can be present. Preferably, superheated steam of atmospheric pressure is used because this is easiest to realize in practice.

Other pressures are not excluded.

Preferably, the superheated steam according to the invention has a temperature of 120 to 200°C. The suitable temperature depends on inter alia the type of lacquer and the contact time. Good results can be achieved at, for instance, temperatures higher than 180°C. With particular preference, the temperature of the superheated steam is less than 200°C, which is in keeping with the current practice of stoving in hot air.

Temperatures of more than 200°C, for instance to 400°C or more, are also possible, but are not preferred for most stoving applications.

Suitable contact times (i. e. time periods during which the coated objects are present in the atmosphere of superheated steam) preferably vary from a few seconds to tens of minutes, for instance 10 seconds to approx.

60 minutes. With shorter contact times, insufficient curing and/or evaporation of the solvent and/or diluent is/are achieved.

The choice for combinations of contact times and temperatures depends on inter alia the type of cross-linking or polymerization reactions which occurs; on the lacquer composition, particularly the amount of solvent/diluent present; and on the desired properties of the final coating, including layer thickness, hardness, flexibility and gloss.

If desired, the stoving process according to the invention may be used in combination with other curing and/or cross-linking processes, such as, for instance, additional treatment using radiation (for instance IR radiation and/or UV radiation), induction heating and/or heating by microwaves.

An apparatus according to the invention preferably comprises steam-tight inlet and outlet provisions, which may, for instance, make use of so-called stratification. In order to condense the steam to be discharged, which contains solvent/diluent and other components, preferably, a heat exchanger is included in the discharge flow. By means of the heat exchanger, then, also, the energy recovery can be realized.

If the object needs to be preheated, which is preferred, the apparatus further comprises a preheating room, for instance a closed room through which hot air can be blown or a room provided with infrared radiators or other heat sources.