Water-based lacquer drying article: Infrared Technology Dries Water-Based Lacquers Faster

Infrared waterbased lack

A consequence of the change over from solvent-based to water-based lacquers has been longer drying times in warm air circulation ovens. This is an opportunity for using infrared emitters, specially suited for the drying of water-based lacquers.

There is no question that the reduction of solvent emissions within VOC guidelines is a very important goal throughout industry. However, at the same time, competitive pressures in the coating sector demand that energy costs are reduced and finishing times are shortened. A conversion from solvent-based lacquers to powder coatings requires a completely new plant. A conversion from solvent-based to water-based lacquers is much simpler in terms of plant technology.

As water takes much longer to evaporate than solvents, conventional hot air ovens are no longer suitable. For example, if the drying length must remain the same because of space limitations, then the flow speed through the oven must be reduced to increase dwell time. This can lead to production bottlenecks which no coater wants. A practical solution to this problem is offered by infrared technology using infrared emitters and systems, which, because of their compact design, can be easily retrofitted into existing systems.

Infrared Boosters for Large Components

It has been shown that by retrofitting an infrared heating zone before the existing dryer when converting to water-based lacquers, it is possible to maintain product throughput and, often, even to increase it. A so-called infrared booster solves many of these conversion problems, as infrared radiation heat provides the required energy to evaporate the water in the lacquer. By using infrared, especially for large products, time savings of up to 50% can be achieved in reaching the stoving temperature compared with conventional hot air ovens.

By using an infrared module fitted before the existing dryer, the product is brought to the correct temperature and the existing dryer can then hold this temperature for as long as possible. It has also been shown in practice that, by retrofitting an infrared booster, the existing hot air plant can be used as a “cool-off” zone. Very large components, such as gearbox housings for example, which have to be coated with water-based lacquers, are not only dry at the end of the conveyor section but are also cooled off to the extent that they can be immediately packed.

water absorbation

Carbon Infrared Emitters Reduce Energy Consumption

The wavelength of the infrared radiation has a significant effect on drying. Water evaporates particularly quickly when irradiated with medium wave infrared. This is because the medium wave radiation is absorbed very efficiently by water and then directly converted into heat, in contrast to very short wave near infrared. Heraeus has developed the carbon emitter specifically for medium wave.

Infrared emitters incorporating carbon technology provide power densities up to 150 kW/m² and response times in terms of seconds. Carbon infrared emitters combine highly effective medium wave radiation with high power densities and accelerate the drying of water-based paints and lacquers at a high degree of efficiency.

Extensive tests have shown that carbon emitters dry water-based lacquers significantly more efficiently than short wave infrared emitters. A carbon infrared emitter requires up to 30% less energy than a conventional short wave emitter to carry out the same drying process.

water lack

Air Knife Module Reduces Drying Times

The so-called infrared air knife has already demonstrated significant benefits in the drying of water-based lacquer systems. It consists of a combination of effective infrared radiation and an intensive air flow, which ensures a rapid exchange of the air in the total drying zone.

The air knife module is mounted above the product being conveyed, transverse to the direction of travel; its length can be matched to the product width. Because of the combination of several factors the efficiency of such a module is very high. With its high energy density, the infrared radiation provides rapid heating of the lacquer layer and contributes directly to the water evaporation.

Coincident with the infrared radiation, a uniformly dispersed air flow of metered speed is blown into the drying zone. The dried air picks up the evaporated water and quickly transports it out of the irradiation zone to the exhaust hood. The exhaust hood removes the saturated air and the volume of the exhaust air should be 15% more than the volume of the feed air. This air change ensures significantly reduced drying times and a better utilisation of the thermal energy.

The dry air takes up water vapour and humidity and removes them from the infrared zone. The air is exhausted directly after the drying zone. As a result, the balance of the system is maintained and any solvents can be condensed out.

In this way, the atmospheric saturation in the drying zone is reduced, so that further evaporation can continue without hindrance and the infrared radiation is fully utilised.

The infrared emitters for an air knife module are so chosen that they are optimally matched to the absorption properties of the lacquer system and to the process. Consequently, efficient energy consumption can be achieved in a short drying zone.

Brilliant Surface Qualities

In the drying of coatings, infrared heat has already shown that infrared radiation can penetrate to a depth in a coating depending on the lacquer system and dries the lacquer film from the inside out. Consequently, skin- or blister-forming on the surface is prevented and lacquer drying is accelerated. The result is a brilliant surface quality.

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