How are coatings applied? How can they be applied in the most efficient manner? In the past, those were the two questions most frequently asked by product finishers. Today there is a more important question: How can I apply coatings that meet the governmental regulations? The effect of the 1990 amendments to the Federal Clean Air Act of 1970 on the finishing industry was profound. The law was made more enforceable by the national permit program and the establishment of control technique guidelines which allow state and local governments to establish strict attainment rules. In addition to reduction of volatile organic compounds (VOCs), the reduction of hazardous air pollutants (HAPS) and waste minimization are also important. The coatings suppliers have reacted to the regulations by developing new materials and applications methods to help the product finisher comply, although costs will be higher. Often overlooked, when considering costs associated with compliance with the governmental regulations are cost savings through more efficient painting. These savings are an important attainment benefit.
Coatings are applied on industrial products by three basic methods: spraying, dipping and flow coating. For liquid coatings, the most frequently used application method is spraying.
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| Leaf springs are coated by autodeposition at Crown Enameling in Detroit. Process permits uniform coating of pre-assembled products. |
Spray coating equipment can be classi-fied by atomization method—air, hydraulic or centrifugal. These general types of spray equipment can be subdivided further into the following: conventional air atomize; airless; air-assisted-airless; air electrostatic; airless electrostatic; air-assisted-airless electrostatic; high-volume low-pressure; and rotating electrostatic discs and bells.
Liquid dip coating equipment ranges in complexity from a simple dip tank to a sophisticated electrocoating system.
Flow coaters can be simple or complex, manual or automated. Other less frequently used methods for applying paints include roller coating and centrifugal coating.
Powder coatings are applied using fluidized beds, electrostatic fluidized beds or electrostatic spray equipment. Most of the aforementioned application methods can be automated or operated manually.
Finishers have learned that, by applying compliance coatings with processes
offering high transfer efficiencies or high rates of coating material utilization (the percentage
of coating material used which actually coats the product and is not wasted),
they will not only meet the air and water quality standards but will also provide a safer
work place, decrease generation of hazardous wastes and decrease finishing costs. Table I
shows compliance coatings and their application methods.
Having answered the question about how coatings are applied, the next question is, How can they be applied in the most effective manner? To answer that question, the product's size,shape, ultimate appearance and intended use must be addressed. The transfer efficiency (TE) of the application equipment is also important. However, since these values are provided by various suppliers who may calculate them without benefit of a standard measurement method, the TE for the following application methods will not be listed.
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| Manual, air-atomize electrostatic painting operation |
Electrocoating. Developed during the 1960's, electrocoating is essentially a dipping method whereby coatings are deposited electrophoretically onto parts from an aqueous medium. This operation usually involves deposition of coatings on the anode from a slightly alkaline paint bath. But more recently developed materials deposit on the cathode from a slightly acidic bath.
Electrocoating has many advantages. Air pollution from solvents is decreased and corrosion protection of inaccessible areas is increased because of the outstanding covering power of the process. Less labor is required. Film thickness from top to bottom is uniform, thereby negating the wedge effect, while film thickness uniformity on the outside and inside areas is also improved. Recent advances in coating formulations and equipment have improved electrocoat properties, and have eliminated water pollution problems.
Electrocoating also has its disadvantages. Because it is a highly sophisticated and technical process, more competent operators are required. Capital equipment costs are higher, and energy costs may be higher. As a result of these facts and the higher costs for materials used, applications are usually limited to those requiring from 3 to 5 million sq ft per year. Despite these drawbacks, electrocoating has gained a significant share of the coatings market.
| Table I—Compliance Coatings and their Application Methods |
Application Method
Air Spraying
Airless Spraying
Air-Assisted-Airless Spraying
Air Electrostatic Spraying
Airless Electrostatic Spraying
Air-Assisted-Airless Electrostatic Spraying
High-Volume Low-Pressure Spraying
Rotating Electrostatic Discs and Bells
Dipping
Electrocoating (3)
Flow Coating |
Water Borne
X
X
X
X
X
X
X
X
X
X
X |
High Solids
X
X
X
X
X
X
X |
Coating Powders (1)
X
X
|
UV-Curable (2)
X
X
X
X
X
X
X |
| 1. Requires Powder Coating Equipment 2. Requires UV-Curing Equiment 3. Requires Electrocoating Equipment |
Autodeposition, a recent development in coating technology, is similar to electrocoating. But in this case, a water-borne organic coating material is deposited as the result of chemical reactions only. No externally applied electrical current is required.
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| Rotary atomizers on a short-stroke reciprocator electrostatically paint compressor tanks. |
Autodeposition is accomplished at room temperature. No pretreatment beyond thorough cleaning is required. A standard rinse and post-rinse with deionized water removes residual coating solution from the surfaces of parts that have been through the autodeposition tank. This rinse stops the reaction process. Autodeposition requires a low-temperature curing cycle (220-230F) after the coating is applied.
Used primarily on steel surfaces, auto-deposition is capable of coating any surface of a part that can be wet by the coating material. The thickness of the coating is self-limiting, assuring uniform film thickness.
Equipment is said to occupy 30 pct less floor space than that for an electrocoat system, and capital investment is said to be 25 to 30 pct less than that required for electrocoat.
Because autodeposition is a water-borne coating material, with no organic solvents, no air-pollution controls are required. Racks do not require stripping, since the cured coating prevents further deposition.
Powder Coating is another one of the newer finishing methods, developed in the 1950's, that has seen significant growth since the energy crisis and enactment of air quality standards. Simply stated, finely ground thermosetting and thermoplastic plastic powders are applied on products by fluidized bed or electrostatic spray methods, cured by heating to their melt temperatures and fused to become continuous coatings.
There are several variations of the powder coating method: fluidized bed, electrostatic fluidized bed and electrostatic powder spray. High material utilization is the norm with electrostatic powder spray due to elaborate reclaim systems that permit recycling and reusing oversprayed powder.
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| Flow coater floods paint onto parts in a tunnel. Paint is collected and reused. |
Fluidized Bed Powder Coating. A fluidized bed is an open-top container whose bottom is a porous plate that coversa plenum. The powder in the bed is fluidized (suspended and surrounded) by air that is introduced into the plenum and passes through the porous plate. The air/powder mixture resembles a boiling liquid.
To apply a coating in this equipment, the product is preheated to a temperature above the melting point of the powder. Then it is dipped into the fluidized powder, which melts and fuses. In the case of some thermosetting materials, the powder may also begin to cure. Next, the product is heated to fuse or cure the coating, depending on the nature of the powder.
This method is used to apply heavy coatings to a uniform and reproducible film thickness on complex-shaped parts by controlling the chemistry of the material, the preheat temperature of the parts and the residence time in the bed.
Electrostatic Fluidized Bed Powder Coating. The equipment used is essentially a fluidized bed with the addition of a grid that is charged to a high DC voltage. The grid is immersed in the powder or mounted below the porous plate. The charged particles are repelled by the charged grid and repel each other, forming a powder cloud over the bed. Powder is attracted electrostatically to grounded products placed in the cloud.
It is possible to obtain uniform film thicknesses varying from 1 1/2 to 5 mils on cold parts. Electrical and electronic components are coated by this method. Film thicknesses of 20 mils or greater can be applied on parts that have been slightly preheated.
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| Electrodeposition of paint takes place in this large tank filled with special water-borne paint. As parts ove through the tank, paint is "plated" onto the part. |
Electrostatic Powder Spray. Powder spray guns are usually served by a small fluidizer that mixes air with the powder, allowing it to be transported through hoses to the gun. A power pack supplies DC voltage to an electrode at or near the nozzle. This charged electrode charges the powder particles as they exit. In some equipment, the powder is charged by friction.
In all cases, the charged powder is sprayed onto grounded parts where it adheres by electrostatic attraction. Powder-coated parts are conveyed to an oven where the powder melts, fuses, and if it is thermosetting, cures.
These electrostatic powder guns can be either manual or automatic. In the automatic mode, multiple guns spray one or both sides of a grounded object traveling through a special booth having no operator.
Since electrostatic powder coating is self-limiting, uniform coatings of 1 1/2 to 5 mils are obtained on cold products. However, by heating the parts, thicker coatings of up to 20 mils are possible.
Each finishing method has limitations and electrostatic powder coating is not without its share. Color changes are difficult because of the extensive reclaim system that must be cleared of the previous color. Pinhole-free coatings of less than one mil thickness are difficult to obtain.
Even with these limitations, electrostatic powder spray coating is replacing liquid coatings for a growing number of applications.
Dip Coating. Dip coating is a fast and efficient finishing method, which typically
provides coverage in recessed areas. It is used where product finish appearance is not critical. In its simplest form, this finishing method involves immersing products in a tank of coating material, draining off the excess in a solvent saturated atmosphere and then drying or curing. Owing to its simplicity, this method lends itself to automation. Film thickness is controlled by coating viscosity and rate of withdrawal from the tank. Dip tanks come in all shapes and are sized to accommodate the largest object to be coated.
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| Pre-heated wire baskets being fluidized bed coated in nylon powder. |
Dip coating has its drawbacks, including: light parts tend to float and fall from the conveyor; film thickness can vary from top to bottom ("wedge effect"); a fire hazard is created by the large volume of coating in the tank and the solvent-laden atmosphere of the drain tunnel; fatty edges develop on the bottom of parts as excess coating drains; and refluxing by the solvent vapors above the tank removes some of the coating. Fire hazards and reflux problems encountered with solvent-thinned dip coatings have been eliminated by many finishers who have converted to water-borne paints.
Centrifugal Coating. A centrifugal coater, a self-contained unit, is actually a modified dip tank. After small parts are loaded into an inner basket, the tank and basket are filled with sufficient coating material to cover the parts and then it is emptied. The basket is then spun rapidly to remove excess coating from the parts by centrifugal force. The surplus coating drains down the inside surface of the tank and into a sump to be recycled. Film thickness is controlled by coating viscosity, rotational speed and pct solids.
This method, sometimes called "dip-spin" coating, is used extensively for finishing fasteners and other small parts in large volumes where appearance is not critical.
Flow Coating. The flow coating method overcomes many of the limitations of conventional dip coating. In this method, the coating material is pumped from a reservoir through hoses and nozzles onto the upper surfaces of the product, where it flows over and down its sides, finally draining into a shallow reservoir to be recycled. Although similar to dipping in its film building and control parameters, this coating method has the following advantages: the volume of coating in the system is lower; parts will not "float" off the conveyor; and extremely large, complex products, too large to dip, can be painted. Flow coating can be fully automatic in totally enclosed conveyorized units or manual in exhausted booths.
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| Triboelectric-type powder spray gun uses a frictional charging method. |
Flow coating has most of the dip coating disadvantages such as the wedge effect and solvent reflux. When using solvent-thinned paints, the process requires careful control of evaporation to obtain uniform coatings. When using water-borne paints to avoid reflux problems and fire hazards, bubbling and foaming control is required. In both the dipping and flow coating methods, hanging of the product is important to its final appearance.
A continuous coater is similar in operation to a flow coater, but is more advanced, has better control and applies a more even coating. It consists of airless spray nozzles mounted on rotating arms in a special cabinet that reclaims and reuses the coating material with high efficiency. Continuous coaters can be sized to coat products ranging from pipe to automobile engines.
Curtain Coating. Curtain coaters are modified flow coaters used mainly on high-speed, conveyorized production lines for coating flat substrates. In these units, the coating material is pumped to and flows from a slotted pipe or over a weir, from which it falls by gravity in an unbroken stream, or "curtain," to coat objects passing under it. Low profile, three-dimensional objects can be coated on all six sides in two passes. Excess coating material is collected in a gutter and returns to the reservoir to be reused. Film thickness in a curtain coater is controlled by flow rates and conveyor speeds.
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| Manual electrostatic powder spraying operation. |
Roller Coating. While roller coating is not used extensively for finishing fabricated products, it is still an important method for finishing flat sheets and coiled metals. In this method, coatings are rolled onto surfaces by resilient rollers. When the substrate and rolls travel on the same direction and at the same speed, the method is called direct roller coating. On the other hand, when the roller motion is contrary to the surface movement, the method is called reverse roller coating. When applied on a continuous strip it is called coil coating, which is used to produce a wide range of products including roof decking, siding, containers, appliances and signs. Coil coating, which requires a major investment in capital equipment, is done by basic metal producers and private concerns called toll coaters.
Electrostatic Spraying Equipment. This equipment which was developed in the 1940's is the most versatile for applying all types of primers, topcoats and one-coat enamels. It is available in three basic types classified by its atomization method: air, airless, and rotating discs and bells.
In these units, coating droplets are charged as they pass or contact an electrode; and they are attracted electrostatically to the surface of the products to be finished, which are usually at ground potential. Not only are the charged coating droplets attracted electrostatically to the front surfaces of these products, they also wrap around and, in some cases, completely coat their back surfaces as well. Because of this wraparound effect, electrostatic applicators are especially suitable for coating tubular products.
There are, however, two distinct disadvantages associated with this equipment. First
is the greater film build on outside corners, edges and around cut-outs due to increased electrostatic attraction in these areas. Second is the lower film build on inside corners and recesses due to limited electrostatic attraction in these areas, commonly described as the "Faraday Cage" effect.
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| Aluminum extrusions being powder coated by automatic spraying equipment. |
Rotating Electrostatic Discs and Bells. Unlike the others, this spray method takes advantage of the phenomenon where centrifugal and electrostatic forces atomize a coating which is charged by the high voltage concentrated on the machined, razor-edged, outer rim. High-viscosity, high-solids coatings (65 pct volume solids and higher) can be atomized and applied successfully using high-rotational-speed discs and bells. Electrostatic rotating discs are always used in the automatic mode with a conveyor looping around them in a horseshoe shape. Rotating electrostatic bells are used in either fixed or reciprocating modes. Rotating electrostatic bells also have been developed for attaching to spray painting robot arms.
Airless Electrostatic Spraying. These units are essentially high-pressure hydraulic spray guns with a power pack added to charge the atomized droplets. The low velocity imparted to these droplets reduces blow back and overspray. Airless electrostatic spray guns are used manually or in automatic modes where they are used singly or in multiple head setups, either as fixed units or in conjunction with reciprocators to coat products on a conveyor line.
Air-Assisted-Airless Electrostatic Spraying. These units are really a modification of airless electrostatic spray units. The increased use of high viscosity and high-solids coatings presented problems for airless electrostatic spraying equipment. When heating and higher fluid pressures were used to atomize many of these newer materials, other problems arose. With the introduction of air-assisted-airless spray-ing equipment, which uses air to provide additional atomization and pattern control, many of these application problems were solved.
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| Air-Assisted-Airless hand spray guns provide good transfer efficiency. |
Air Electrostatic Spraying. In order to improve transfer efficiency, electrostatic capability was added to air-atomizing spray units by the addition of power packs supplying high voltage to charge liquid droplets. These units are also used either manually or automatically, singly or as multiple units in fixed or reciprocating mountings. Since they impart a higher velocity to coating droplets, their transfer efficiency is lower than the
other electrostatic methods.
Airless Spraying. These hydraulic atomizing spray guns force fluid under pressure through a small orifice in the same manner as water through a garden hose nozzle. Upon emerging from the orifice, the tremendous internal pressures cause the paint stream to blow apart into atomized particles. The viscosity of the paint, the size of the orifice and hydraulic pressure determine the speed at which the fluid emerges from the gun. Unlike air-atomized spraying where the compressed air imparts a greater velocity to coating droplets, causing overspray and blow back from flat and recessed areas, airless spraying causes less overspray and blow back because the droplets are propelled only by their own momentum. This accounts in part for their transfer efficiency, which is higher than air-atomizing sprayers.
Heating the coatings using in-line heaters has the same effect as adding solvents. Hot airless spraying reduces the amount of fluid pressure required, the coating viscosity, the amount of solvent required and the amount of overspray. Higher viscosity paints using less solvent can also be sprayed using heated airless equipment. Consequently, coating with hot airless spray equipment causes less air pollution and decreases the amount of coating residue for disposal. It also compensates for seasonal changes in ambient temperatures.
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| Air-atomize spray is used here to coat automobile valve covers. |
High-Volume Low-Pressure Spraying. Older units used centrifugal pumps, while newer versions use turbines to supply high volumes of low-pressure, heated air to single or multiple spray guns. The use of heated air eliminates the cooling associated with air and airless atomization and not only reduces the tendency to condense atmospheric moisture but also stabilizes the evaporation of solvent from the coating droplets. The latest development is specially designed HVLP spray guns, using compressed air from conventional sources, that can directly replace air atomizing guns. Owing to the low atomizing air pressures, coating droplet velocities are low enough to eliminate blow back. Furthermore, this lower pressure reduces overspray and eliminates the cloud of vapor often associated with spray painting.
Air-Assisted-Airless Spraying. These units add compressed air to airless spray units in which paints are forced through an orifice under hydraulic pressure. Owing to the increased use of high viscosity and high solids coatings, which require higher temperatures and higher fluid pressures to atomize, other problems arose. In air-assisted-airless spraying equipment, compressed air provides additional atomization and pattern control to viscous, high solids coating, thereby allowing lower fluid pressures and temperatures.
Air-Atomized Spraying. These are the conventional spray guns that have been used for 75 years to apply attractive finishes on industrial products. In these guns, a stream of liquid coating material mixes with a stream of compressed air either internally or externally to cause the liquid stream to atomize. Most internal mix guns have controls to regulate fluid flow, atomizing air and spray patterns. Since these adjustments allow them to meet the finishing requirements of a wide variety of sizes and shapes, they are used for coating a large number of high-quality items. Conventional spray guns can be used to apply catalyzed, high-solids and water-borne coatings.
The greatest asset of conventional air spray equipment is versatility in the hands of a skillful sprayer who, by careful manipulation, can coat practically any object using coatings whose viscosity, flow and drying rate are controlled by solvent blending. These same solvents, which are costly to buy and which evaporate into the atmosphere to become photochemical smog, are effecting a reduction in the use of these spray guns. Furthermore, their low transfer efficiency causes a large amount of overspray that results in the use of great volumes of make-up air and high exhaust rates. Despite all these problems, it is difficult to match the versatility of these manually operated air-atomizing spray guns.
Multi-Component Spraying. These multi-component spray units come in many sizes and configurations. Their function is to meter and apply multi-component coating materials in one operation. They can be either hydraulic or air atomizing, internal or external mixing. Aside from having multiple supply and metering pumps feeding a common applicator, they have the same components as other spray equipment.
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| Automated fluidized bed coating machine includes three-stage washer, blow-off, primer dip, preheat, fluidized bed dip, post heat and water spray cooling. |
Finishing costs can be reduced by using a coating method having a higher transfer efficiency. Consider, for example, a product requiring one gallon of 100 pct solids coating for complete coverage. When using spray equipment whose transfer efficiency is 30 pct, 3.33 gallons must be sprayed to apply one gallon on the product. At a more realistic volume solids of 50 pct, 6.66 gallons must be sprayed. By switching to a spraying system whose transfer efficiency is 70 pct, only 2.85 gallons of 50 pct solids coating must be sprayed. Going a step farther, if the coating is applied at 75 pct solids, only 1.9 must be sprayed. Savings in solvent costs can be calculated by subtracting 25 pct of 1.9 gallons from 50 pct of 6.6 gallons, which leaves 2.85 gallons. If the solvent costs $2.50 per gallon, the savings is $7.13.
Coating cost savings can be calculated using the same example. In the first case, 3.33 gallons of coating solids are sprayed and in the third case, 1.43 gallons of coating solids are sprayed. By subtracting 1.43 from 3.33, we show a saving of 1.9 gallons of coating solids. At $15.00 per gallon, $28.50 in coating costs can be saved per unit of product coated. The total savings in coating and solvent costs is $35.63 per unit. Furthermore as much as 2.33 gallons of coating overspray (a waste product) can be avoided.
Since these calculations can be applied to most coating methods, material costs can be reduced in most finishing operations by using more transfer-efficient application equipment. To achieve these higher efficiencies, coatings are applied today by the newer, more efficient methods as well as by conventional equipment using automated systems involving robots, program controlled reciprocators and automatic conveyorized lines.
By a careful consideration of the governmental regulations, the product's size, shape, ultimate appearance and intended use, as well as a review of the coating methods described above, all three questions; How are coatings applied?, How can they be applied in the most effective manner? and How can I apply coatings that meet the governmental regulations? can be answered. PFD
