The following notes apply to liquid coating formulations, not to powder coatings, nor to textured materials. Legislation is moving steadily towards the elimination of solvents in coatings, to reduce the amount of volatile organic compounds (VOCs) released into the atmosphere, which can harm the environment.
Coatings formulators are reducing the VOC content of their products by either replacing them with water, or decreasing the proportion of VOCs by increasing the solids content. Industrial coatings are generally adopting the increased solids route, but this can lead to heavier materials that are more difficult to atomise. The trend is therefore to need higher atomisation pressures to provide the energy required to obtain a satisfactory spray.
Airspray guns are inefficient in transferring the material to the work, with the best conventional spray guns achieving no more than 30% transfer efficiency, because the air going through the gun, blows the paint particles away from the job. HVLP and compliant guns are a little more efficient, by using lower air pressures and higher volumes of air to help carry the paint particles on to the job, achieving up to 60% actual transfer efficiency, although the HVLP standard calls for at least 65% but this is under ideal conditions. Airspray guns are also limited in the viscosity of material that they can handle, which often has to be carefully thinned.
The simplest and most trasfer efficient method for spraying an industrial coating, is with airless spray, in which the atomisation is achieved from the pressure of the material alone. By eliminating air, the transfer efficiency can be very high, up to 95% depending on the viscosity of the material; lighter coatings tend to have a lower airless transfer efficiency because the particle size tends to be smaller. Airless equipment can provide very high material pressures, as high as 8,000 psi at the pump and more, which allows it to spray the most difficult coatings to atomise. The disadvantage of airless spray, is that it tends to produce a larger particle size than airspray, reducing the depth of gloss. For many industrial applications, such as steel structures and pipelines, this is not a problem and simple airless is used.
For applications requiring a better finish, the atomisation can be improved with the introduction of a pre-orifice between the gun and the spray tip, but this tends to be used in painting and decorating. In industrial applications the improved atomisation is usually achieved by injecting compressed air into the material stream, to further break up the particles; this is air-assisted airless (AA). The finer atomisation of the material with AA reduces the transfer efficiency, but this can be countered by applying an electrostatic charge between the gun and the work, which is electrostatic spraying. Whilst straight airless may require pressures at the spray gun of around 2,000 psi and often more, by using air-assisted airless, the material pressure can be reduced substantially. There are many AA systems in operation where the material pressure is less than 100 psi, however these may not be able to handle the higher solids coatings that are being launched.
The spray equipment needs to be matched to the material to be sprayed. In straight airless, the spray pressure required at the gun is the crucial factor, when spraying with the chosen spray tip (nozzle) for the material and the work. The spray pressure and tip size then define the airless equipment needed. For air-assisted airless, a similar estimation is needed, but the impact of the air pressure and its volume, makes it more difficult to predict, so more spray testing will be required.
The trend towards higher solids has lead to an increase in plural component formulations, containing little or no VOCs, in which 2 or more components are mixed together to form a coating that will harden in a known time at a particular temperature. If small volumes are to be sprayed, this is often done in a batch process, providing that there is sufficient time to apply the mixed coating and clean out the equipment before it hardens. The material manufacturer will advise on the pot life, which is the the time at which the material will start to cease to be pumpable and must have been cleaned out of the system.
Batch mixing of plural components is inefficient, due to material and solvent wastage, as well as loss of production time. For production work, plural component equipment is preferred, in which the different components are pumped separately to a special spray gun in the correct ratio, where they are mixed just before the spray tip. When spraying is stopped, the gun is purged of the small amount of mixed material. In older plural component equipment, the mixing is done in a manifold close to the pumps, but the paint line and gun need to be purged with a solvent when spraying is stopped, depending on the pot-life. Some formulations such as insulation foam, have very short cure times and require special knowledge and equipment to apply them.
In factory installations, the airless pumps will usually be driven by an air motor (pneumatic) as compressed air is usually readily available and this eliminates any fire hazard from spraks from electrical equipment, providing that the equipment is earthed.
The correct specification of plural component spray equipment requires a fair amount of experience and knowledge of the equipment and accessories available. Overspecification can lead to extra costs and excessive waste of material in equipment that is too large for the job.