The Market Top
Diversity of Systems
Many different systems exist in today's industrial coatings market place. Choosing the correct one will be dependent upon such considerations as end user needs, application equipment, environmental requirements, and economics. The following products are representative of the main generic types of industrial finishes available.
The Market Top
Application Information
Standard Air Atomization
The range of types available includes air feed, high volume low pressure, low volume low pressure, and turbo guns. Each can be used with gravity, suction and/or pressure feed, and as external auto-mixing guns for two part catalyst cured paint systems. These are mainly used by trade applicators but also on some OEM items.
Application Techniques Top
Airless Spray
Paint is delivered to the gun by high-pressure hydraulic pump and atomisation occurs by pressure release at the fluid tip of the spray gun. Airless spray is mainly used for marine, protective and heavy-duty maintenance coatings where high build is required and aesthetic appeal is less important. Advances in pump technology now allow lower pressure to be used, improving surface finish. With air assisted units, the transfer efficiency can be greatly improved.
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Electrostatic Spray
The paint particles are electrostatically charged at the gun nozzle and atomisation may be by spinning disc, air atomisation or a combination of the two. This technique is used exclusively for coating more complex articles such as tubular steel and white goods where overspray losses must be minimised. Spray guns may be hand held or incorporated into an automated production line.
Where distinctness of image, haze free gloss and showroom appeal are of prime importance, electrostatic spraying is usually by the High Speed Bell. The best example is in automotive OEM applications. Atomisation and paint particle charging occur at the periphery of the high speed bell giving a very high degree of atomisation and paint utilisation on automated production lines.
Application Techniques Top
Hot Spray
Hot Spray techniques are those that use heat applied to the feed pot in one of the above application methods in order to achieve application viscosity, without excessive use of solvents. Uses include wood finishing with traditional non-convertible coatings such as nitrocellulose.
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Dipping
Dipping is used extensively for the coating of smaller production line items. Film thickness can be controlled by the rate of withdrawal from the dipping tank. Electrostatic de-tearing may be used to reduce defects.
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Flow Coating
Flow coating is a similar application to dipping but paint is pumped from a reservoir onto articles through pressure jets. Paint drainage is returned to the reservoir.
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Curtain Coating
This method is suitable for coating flat articles such as doors and ceiling panels. The articles are transported on a conveyor belt through a curtain of paint falling from a reservoir into a recirculating tank. The flow rate and/or conveyor speeds are adjusted to control the thickness of the applied film.
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Tumbling
Tumbling is suitable for coating very small items such as screws, buttons and small electrical components. The articles are tumbled in a hexagonal barrel containing just sufficient paint to coat them to the correct film thickness.
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Centrifugal Coating
This is a variation of tumbling where the barrel can be spun at high speed to discharge excess paint.
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Electrocoating
Electrocoating anodic and cathodic methods are used for applications such as the deposition of primers in the automotive and similar industries and for the coating of radiators. The items are immersed in a paint bath and the insoluble coating is deposited by the passage of electric current. On removal from the bath, excess soluble material is normally rinsed from the insoluble electrically deposited coating.
Application Techniques Top
Systems Information
Air Drying Systems
Non-convertible, air-drying coatings are materials that dry by solvent evaporation with no change in the chemical nature of the binder. The coating remains soluble in the original solvent after drying. Typical examples are
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Nitrocellulose
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Chlorinated rubber
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Solution vinyl
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Thermoplastic acrylic
Convertible coatings are those that undergo a chemical change, for example, an oxidation reaction, during drying. This change makes the coating insoluble in the original carrier solvent. Typical air-drying systems are
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Oil modified alkyds
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Styrenated alkyds
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Vinyl toluenated alkyds
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Silicone alkyds
Diversity of Systems Top
Two-Pack Systems
Two pack systems are coatings that cure by a chemical reaction when two reactive components are mixed together. To allow sufficient pot life and working time, the level of reactivity of the components must be carefully selected. Equipment is available which will mix the two components just prior to the spray gun, so removing pot life problems when spraying. Typical examples of two pack products are:
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Two pack polyurethanes
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Isocyanate cured acrylics
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Polyamide cured epoxies
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Two component wash primers
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Peroxide cured unsaturated polyesters
Diversity of Systems Top
Stoving Systems
These are coatings cured by cross-linking at elevated temperature, usually in an oven. Infrared lamps are sometimes used to provide localised heating for repair work. Most production line industrial finishing uses stoving systems. There is an increasing trend towards energy saving methods, for example, lower bake resins and/or more reactive resins, both of which reduce retention time in the oven. However, these produce further challenges for the formulator. Typical examples of stoving systems are:
Diversity of Systems Top
Plastisols and Organosols
These two types of coating are usually non-reactive polymers dispersed (not dissolved) in plasticisers or mixtures of plasticisers and organic non-solvents. The dispersed polymeric particles fuse together to form a continuous film when heated. This type of product allows a system to be applied at much higher solids than is possible when the polymer is dissolved in solvents. Vinyl based plastisols are possibly the most common example.
Diversity of Systems Top
Other Curing Techniques
Ultraviolet cured, Infrared initiated and electron beam cured systems are increasing in use. These are particularly popular with non-metallic substrates such as wood, paper and plastics.
Diversity of Systems Top
