coating materials and application methods

INTRODUCTION TO

PAINT APPLICATION SYSTEMS

BY RON JOSEPH (1944-2011)

RON JOSEPH & ASSOCIATES INC., SAN JOSE, CALIF.

AND MICHAEL MURPHY

CONSULTANT, KILLIAN ENTERPRISES, VALLEY COTTAGE, N.Y.

A typical line for organic finishing of metals includes the following steps: precleaning,pretreatment, dry-off, primer application, flash-off and/or cure, andfinal cure. In some cases a primer or intermediate coating may not be used,and multiple applications of topcoat are not unusual. The combination of apigmented basecoat, often containing metallic flakes, and a final clear, lustroustopcoat is common today.

PRECLEANING

Procedures for preparation of metal parts for painting often include a precleaningstep. Removal of gross soils such as oil, grease, scale, weld spatter, lubricants,and other materials can be accomplished by mechanical means, with solvents,acids, or alkaline cleaning agents. Blasting, polishing, pickling, and solventdegreasing are described in the Metal Finishing Guidebook and Directory Issue. Whilethe purpose of precleaning is to remove the contaminants from the surface, thisprocess in and of itself does not provide any protection to the metals.

PRETREATMENT

The term pretreatment refers to a combination of chemical cleaning and conversioncoating. The purpose of a conversion coating is twofold. It improves corrosionresistance and provides a surface more accepting of the organic coating,subsequently enhancing adhesion. Further details will be found in the pretreatmentsection of this Guidebook.

DRY-OFF

After pretreatment the wet parts are immediately transferred into a high-temperaturedry-off oven. For lightweight metal parts the oven temperature can beas low as 250°F; however, if the parts are heavy and bulky, the oven temperaturemay be as high as 204°C (400°F). The sole purpose of this oven is to evaporateoff the water as quickly as possible to prevent flash rusting. In coating operationsin which the parts are immersed into a dip tank containing a waterbornecoating, the dry-off oven may sometimes be eliminated.

COATING APPLICATION METHODS

There is a wide choice of application methods available for both primers andtopcoats. In addition, consideration must be given as to whether manual orautomatic techniques should be used. Local regulations may also limit thechoice of equipment.Some states, notably California, regulate a minimum transfer efficiency ofcoating application. (Transfer efficiency refers to the percentage of solids appliedto the part.) In California the minimum acceptable transfer efficiency is 65%.Conventional air atomized, airless, and in some cases air-assisted airless sprayequipment are not approved for use in some parts of California. High volumelow pressure (HVLP), electrostatic, dip, flow, brush, and roller applications areconsidered to be “approved.” This does not imply that these methods necessarilymeet the 65% minimum; rather, they are considered to be more efficient thanthe nonapproved devices.The most extensively used coating techniques include spray, dip, electrodepositionor electrocoating, coil coating, and powder coating. Each of thesetechniques is described in one of the following sections of this Guidebook. Othermethods are described briefly below.

Brush

Brush techniques are well known to the homeowner and the public at large. Thisis a versatile method with high transfer efficiency, but is slow, labor intensive, andnot readily automated. It is commonly used for application of maintenance coatings,for touch-up, and in masking for a variety of finishing operations.

Flow Coating

In flow coating, the part is suspended and the coating is poured over it. Theexcess is collected for subsequent reuse. This technique is useful for large oroddly shaped parts, which may be difficult or impossible to dip.Nozzles may be directed over the part, but they are not of the atomizing typeused for spray application. This technique is also useful for paints without longtermstability, which could not be used for dipping. The same control factorsnoted under dipping also apply in this case. Similarly, as with dipping, thereis minimal control of film thickness, appearance, and film properties. Bothdip and flow coating generally require little space, are low in cost, and requireminimal operator skill.

Curtain Coating

Curtain coating consists of the rapid horizontal movement of flat or slightlycurved parts through a curtain of falling paint. Since the volume of paint can becarefully controlled by the slot width, it is possible to apply either thin or heavybuild films. The process is readily automated.

Direct Roll Coating

In this method the coating is applied by roller. It is limited to fixed shapes such assheets and is used for continuous coating of steel or aluminum in the containerindustry. The coatings can be patterned or embossed to add decorative effects.

CURING PROCESSES

Drying is the process by which the solvents and/or water in the coating evaporateto allow the film to achieve a “dry-to-touch” or “dry-to-recoat” state. Curing, onthe other hand, implies that the coating resin undergoes a chemical reaction,rendering the cured film hard, abrasion resistant, and relatively inert to theenvironment (chemicals, solvents, sunlight, etc.). Each resin type (alkyd, epoxy,polyurethane, etc.) undergoes its own type of curing mechanism. Some coatings,such as nitrocellulose lacquers, need only dry by solvent evaporation in order toachieve their final finish. Most other coatings require both drying and curing before they achieve their optimum properties.Plural-component paints include the curing agent as one of the components.These include two-part epoxies and polyurethanes. Some coatings cure whenthey are exposed to special high-energy ultraviolet light or electron beam sources.These coatings cure very quickly (1–5 sec) when exposed to the light of an ultravioletlamp. Screen inks for printed circuit boards are a typical application, asare other substrates, which are heat sensitive.Coatings can also be classified as air dry or bake. Air dry coatings will cureat room temperature. When heat is applied, usually less than194°F, they aretermed force air dried. Baked coatings require the use of an oven, and are usuallycured at temperatures in excess of 250°F. There are three major types of ovens.

1. Impingement ovens are used for forced-air drying of flat surfaces, or parts.Coated parts are passed along a conveyor belt and hot air is blown over them.

2. Forced convection ovens are versatile devices, which consist of an enclosurewith means to circulate heated air. They can handle a wide variety of shapesand sizes. The desired temperature for curing can be selected and a uniformtemperature maintained. Batch or continuous systems can be devised. Theseovens are described in a separate section of this Guidebook.

3. Radiant heat ovens use infrared lamps with reflectors arranged around thepieces. The primary feature of these lamps is that they provide rapid heating.Their main disadvantage is line of sight limitations. In other words, only coatedsurfaces that are exposed to the infrared rays benefit from the process. Coatedareas, such as in recesses shaded from the light, do not cure as rapidly. A separatesection of this Guidebook provides details on infrared ovens.

CONSIDERATION FOR AUTOMATION

In facilities that coat large volumes of metal, automated coating application iscommon. The application methods can vary depending on the size and shapeof the parts, the number of parts being coated per hour, whether or not thereare long runs of one part geometry, and other factors. When conditions favorautomation, or when the primer does not need to have a high appearance finish,any of the methods listed in Table I can be used for pri mer application.Automated spray guns can be in the fixed position or can bemounted on reciprocators or robots.Topcoat application is usually fairly demanding, particularly when the finalfinish is expected to have a high-quality appearance. Under such circumstancesdip and flow coating are less likely to be used, although electrodeposition, whichproduces a superior finish, remains a viable option. For the most part topcoatsare applied by spray.The decision to use manual or automated spray application depends largelyon the length of the runs and the consistency of part geometry .For instance, ifthe runs are short and part sizes vary significantly between runs, manual sprayapplication may be chosen. On the other hand, if the runs are long and the partgeometries are relatively simple, automated application may be preferred.