Airless spraying

Inside coatings to protect both the metal and the contents are appUed to the can by an airless spray gun. After appUcation, the cans are baked in an oven to remove the solvent and cure the coating. 

Conventional nitrocellulose lacquer finishing leads to the emission of large quantities of solvents into the atmosphere. An ingeneous approach to reducing VOC emissions is the use of supercritical carbon dioxide as a component of the solvent mixture (172). The critical temperature and pressure of CO2 are 31.3°C and 7.4 MPa (72.9 atm), respectively. Below that temperature and above that pressure, CO2 is a supercritical fluid. It has been found that under these conditions, the solvency properties of CO2 ate similar to aromatic hydrocarbons (see Supercritical fluids). The coating is shipped in a concentrated form, then metered with supercritical CO2 into a proportioning airless spray gun system in such a ratio as to reduce the viscosity to the level needed for proper atomization. VOC emission reductions of 50% or more are projected. 

Airless spray uses hydrauHc pressure to deUver the paint. Paint is brought to the spray gun under 7—40 mPa (1000—6000 psi), where it is divided into small separate streams and forced through a very small orifice to produce the spray. Airless spray is faster, cleaner, and less wasteful than air atomization, but demands good technique because it deUvers paint very quickly. 

The principal factors affecting transfer efficiency are the size and shape of the object, the type of apphcation equipment, the air pressure to the spray gun, and the distance of the spray gun from the object. The transfer efficiency becomes lower as the object becomes smaller or more complex. The transfer efficiency increases when the spray gun is brought closer to the object and when the atomizing pressure is reduced. The transfer efficiency of different types of apphcation equipment in descending relative order is manual > electrostatic spray > airless spray > conventional atomized air spray. 

Acrylated rubber These are based on styrene butadiene and have become commercially available only relatively recently. They are manufactured in several grades but most have the advantage over other materials in this class of being based on white spirit solvent rather than the stronger and more obnoxious xylol. In other respects, they are similar to chlorinated rubber and cost approximately the same, although they are easier to airless spray and the dried film contains less pores. They are considered to have superior weather resistance to chlorinated rubber and vinyl. 

Suitable skilled labor may not be available, at a particular site, to use complicated equipment (e.g. hot airless spray). On a foreign site or in remote areas even conventional equipment may be unfamiliar or unobtainable. 

In considering design the first two can be reviewed together. Conventional and Airless Spray... 

In conventional spraying paint is forced under pressure to the spray gun, where it mixes with air and, forced through a small orifice, atomises. Airless spray is created by forcing paint at extremely high pressures through an accurately designed small hole. Rapid expansion as it leaves the gun produces an extremely fine and very even spray pattern. No air is mixed with the paint before it leaves the gun, so avoiding dry spray . A wetter, heavier... 

In the industrial world of manufacturing plants, power stations and the like, more than 80% of painting is concerned with the maintenance of existing structures and plant. Although airless spraying can be more than 20 times faster than brush application and conventional spray some four times as fast, there are many occasions where access difficulties and safety considerations prevent their use. Rollers and brushes therefore continue to be widely used. 

Neither roller nor brush is capable of attaining the same film thickness as airless spray. A paint which consistently gives 100/im dry-film thickness when applied by the latter means is unlikely to produce more than 75 m d.f.t. comfortably by roller or brush the more complex the geometry of a structure the more unlikely it is that consistent results will be obtained. 

Airless spray In this process, a high pressure (12-35 MN/m ) is applied to the paint to force it through a fine orifice in the spray gun. This process allows rapid transfer with reduced overspray. 

Air assisted airless spray This concept is a combination of air spray and airless methods. Paint can be atomised with full spray patterns at low pressures. Turbulence is reduced significantly and overspray is minimised. 

Airless spray Suitable for most components Superior penetration in awkward areas to normal spraying. 

Topcoats over primer are often applied by airless spray. Trends to higher standards of exterior durability have encouraged the use of methacrylated alkyds and two-component urethane finishes. 

AntifouJing composition Airless spray, brush or roller 1 50-80... 

Conventional non-bituminous system Tung oil/phenolic medium Airless spray. 2-3 150-200... 

Antifouling composition Airless spray brush or roller 1 80-100... 

High build chlorinated rubber Airless spray 2 175-225... 

Red lead primer in quick-drying Airless spray. 2 100-125... 

Gloss finish, alkyd medium Airless spray. 2 50-80... 

High build epoxy (2-pack) Airless spray 2 200-250... 

Gloss finish, chlorinated rubber Airless spray. 1 50... 

Zinc phosphate primer in quick- Airless spray. 2 80-100... 

Semi-gloss undercoat, alkyd Airless spray. 1 40-60... 

As stated above, high performance coatings based on epoxies, vinyls or chlorinated rubbers are used almost exclusively on all large ships. A general development in these materials has been the introduction of highly thixotropic types that can be airless sprayed at wet film thicknesses of 300 im or more, that do not run or sag on vertical surfaces. This enables the requisite film thickness to be applied in fewer coats, saving time and reducing application costs. 

The conventional bituminous or oleoresinous paints previously described are still used on the bottoms of smaller ships, the chief difference being that they are applied mainly by airless spraying. The formulations may be adjusted to permit application of thicker coats than by brush or roller, although the coats must not be too thick because oleoresinous paints require... 

Various thermosetting and thermoplastic resins BS 3900 Powder/liquid system in factory. Airless spray/trowel in field Superior chemicai and abrasion resistance compared with enamels. Comparatively expensive. Simultaneous coating internally/externally possible. Various resins available to suit particular requirements. 

Airless Spraying the process of atomisation of paint by forcing it through an orifice at high pressure. This effect is often aided by the vaporisation of the solvents especially if the paint has been previously heated. The term is not generally applied to those electrostatic spraying processes which do not use air for atomisation. 

---