Turn to powders for more effective coatings

Feb. 26, 1999
Powder coatings can dramatically increase manufacturing productivity and enhance product performance

Scott Dennis
Industrial Coatings Div.
Ferro Corp.
Cleveland, Ohio

Ever since powder coatings were introduced to replace liquid finishing systems, their use has steadily increased in a variety of applications. That’s because powder coatings offer easier compliance with environmental regulations, improve aesthetics and finish, virtually eliminate solid wastes, and lower application costs.

Three areas of advanced technology are also responsible for this increasing preference for powder among design engineers and product finishers. The first is the emergence of powder coatings as engineered materials. Newer powder coatings can be uniquely formulated to satisfy specific application and performance requirements. Formulation technology has progressed to the point where many powder coatings can be considered as polymer alloys.

Another area of rapid development is the pretreating and powder coating of flat metal blanks prior to forming. Coated blanks offer greater design flexibility, harder and more durable films, greater control of film thickness, and a smoother appearance with thin films. Production benefits include cut-edge protection, less scrap, less in-process inventory, faster cycles, and greater storage and handling efficiencies.

Both advanced polymer formulations and blank coating offer important environmental advantages. For example, newer polymer powder systems have greatly reduced, or eliminated, the visible emissions common with conventional coatings and the precoating of blanks significantly reduces powder waste.

A third area promising to have a significant impact on product finishing is the development of low-temperature curing powders. This will open the door for coating of materials that previously could not be powder coated such as plastics, wood, glass, or heat-sensitive metals. Low curing temperatures also make possible the coating of parts that are already assembled with heat-sensitive materials such as rubber. In addition, low-temperature curing has fostered developments in die transfer technology, where decorative patterns or label text are created in the coating itself. This permits the design of attractive, permanent control panels or instructions without the need for silk screening or separate panels or name plates.

Advanced polymer systems
Powder coatings were commonly characterized according to their basic resin type, each with its own unique properties and characteristics. But, with the advent of increasingly sophisticated powder technology, traditional distinctions based on resin type are no longer meaningful.

Newer powder formulations are modifications or blends of resins and additives formulated and manufactured to process well in a variety of finishing systems. Although finished-coating quality and performance are still important, manufacturers now place greater emphasis on formulations that cost less to apply.

Other formulating parameters must also be considered. For example, rheology (flow characteristics) of the final powder is critical for textured metal substrates or when characteristics such as sharp-edge film build, weld seam coverage, thin-film uniformity, and appearance are important. Special characteristics such as hardness and scratch resistance might be built into a powder coating if finished parts must endure rough in-plant handling and transportation. Storage stability of inventoried powder may also be a concern.

Depending on the part, line speed, application equipment, and booth design, various formulation modifications may be required. These modifications may significantly influence transfer efficiency, reclaim-to-virgin ratios, film uniformity, Faraday penetration, system losses, and other parameters affecting operating costs. As a result, select a coating based on the total cost of materials, including pretreatment systems and substrate options, not solely on product cost.

Because powders are generally easier to apply, the systems often require fewer operators and less maintenance, as compared with liquid top coats or electrocoat systems. Powder allows for a one-coat, direct-to-metal application, replacing two-coat systems in many applications. New powders not only have exceptional thin-film capabilities but also can build thicker films for special applications requiring high corrosion or abrasion resistance.

Powder performance demands vary by application. For example, consumer laundry equipment requires detergent resistance, hardness, scratch, mar and chip resistance, and flexibility. Performance over galvanized and CRS substrate (primed and nonprimed) are also important issues. Newer powder formulations replace porcelain on washing machine lids and tops and on a variety of dryer parts. For refrigerators, gasket stain resistance, salt spray and filiform corrosion performance, and foam release/foam adhesion properties can be critical. With cooking products, grease, food stain, and chemical resistance — especially at elevated temperatures — are major considerations. And particularly in North America, white goods manufacturers are becoming increasingly concerned with UV resistance.

Application considerations
Powder manufacturers and formulators continue to look for incremental improvements in coating quality, but the emphasis is on production efficiencies and lower costs. Although quality cannot be sacrificed, the focus has shifted away from improvements in salt spray or detergent resistance to thinner films and enhanced consistency.

Transfer efficiency. Advanced coating systems, such as Ferro’s newest Polymer Alloy systems — a unique blend of proprietary resins — have been specifically formulated to improve first-pass transfer efficiency. This results in less waste and better particle-size consistency and distribution which, in turn, means a more uniform film thickness. As a result, many OEM users have been able to reduce powder flow rate and losses through the system while maintaining coating quality and effectiveness. Increases in transfer efficiency of up to 35% have been realized reducing powder delivery rate by as much as 50%.

Fluidization. Good fluidization is necessary for efficient and uniform feeding of powder to the application guns. Although several factors affect powder fluidization, this characteristic primarily depends on uniform particle size.

Specific gravity. Powder specific gravity is an important factor in lowering application cost. Reducing specific gravity permits a given weight of powder to cover a greater area, for a given film thickness. Low specific-gravity powders cost more than conventional powders, but their greater coverage can lower overall application cost.

Material usage. The powder coating process generally produces substantially fewer rejects since powder does not run and drip like liquid paints. When a problem occurs, the powder can simply be blown off the part and run back through the system. Even if the powder has been cured, parts can often be recoated without rework, reducing labor and scrap.

Coating characteristics.
Although most formulators are concentrating on lower application and process costs, powder coating performance, durability, and quality continue to foster coating innovations.

Hardness/flexibility. High hardness is often desirable for scratch resistance and overall product durability. However, hardness may have an effect on coating chip resistance. This characteristic is especially important in applications where powder coatings have replaced hard porcelain enamels, such as in some home laundry products. Hard coatings are more brittle and less flexible than softer coatings.

Flexibility is a key issue with coatings applied to substrates prior to final forming operations. In such applications, the coating must maintain its integrity without cracking or pulling away from the substrate.

Coating hardness is primarily a function of raw material, but is also influenced by substrate type and gage, pretreatment, film thickness, and curing. Thinner films are generally more flexible than thicker ones.

Color retention. Powder formulation, application methods, and curing operations can affect color control during production. For example, temperature gradients within a curing oven can result in nonuniform color. Also, coated units produced in separate plants but used together — such as laundry washers and dryers — or units composed of components produced and coated in separate plants may vary in color.

Control of curing time and temperature, film thickness, and type and nature of the substrate all help maintain uniform color. Typically, a powder supplier issues what is called a “window of operation” specifying the limits of curing time, temperature, and film thickness that will maintain color uniformity within specifications for a particular formulation. Appliance manufacturers will usually issue a color uniformity specification for a specific over-bake period that a coating must meet without exhibiting an out-of-specification color change.

Some powder chemistries can emit cure volatiles, such as e-caprolactum (e-cap), that adversely affect color stability. Excessive build-up of oven contaminants can be detrimental to color control. An advantage of some of the newer polymer systems, such as Ferro’s Polymer Alloy system, is that for all practical purposes, no cure volatiles are emitted.

In some cases, adjusting oven settings may bring a color back into tolerance. Or, changes in powder formulation can adjust color, to a point, to temporarily correct for changes due to oven contaminants until the system can be corrected.

Rheological characteristics. Newer powder systems control rheological characteristics to produce smooth, thin films with good hiding characteristics and the ability to uniformly coat textured substrates, such as those on refrigerator surfaces. Without close control of flow characteristics, a coating may tend to fill valleys in the substrate, resulting in a loss of texture definition.

Concurrently, substrate peaks may become exposed, causing a change in color and loss of coating protection. Formulators control rheological characteristics that include resin and additive selection, cure speed, melt viscosity of the particle components, and particle size.

Blank coating
Manufacturers are realizing the benefits of applying powder coatings as early as possible in the manufacturing process. Earlier application results in greater control of coating characteristics and increased production.

As a result, use of coil-coated and blank-coated materials in North America is on the rise. Significant investment in powder-coating blank technology has been made in Japan, South Korea, Mexico, Germany and, more recently, India and Asia. Powder coating of precut blanks leads to increased line speeds – up to 100 fpm (several times the speed of a conventional powder coating line), more precise powder distribution, and more uniform, thinner films. Another advantage is that there are no unfinished cut edges following finishing. One challenge with blank coating is formulating powders that cure quickly yet maintain color quality during extremely fast baking.

However, converting to blank coating requires changes in the way products are fabricated, so manufacturers have to deal with more than just the finishing system. Integrating a powder-coated blank line in a manufacturing facility can significantly change the rest of the operation.

Some manufacturers have eliminated their finishing lines and have third parties coat blanks. As this trend continues, more custom coaters will offer blank coating.

Low-temperature curing
Newer formulations with dramatically lower curing temperatures are paving the way for powder coating of temperature-sensitive substrates, such as plastics, which may distort at temperatures lower than the bake cycles necessary to cure thermosetting powder coatings. Also, some substrates emit volatiles at high temperatures. With plastics, these may be low molecular weight fractions such as unreacted monomers or moisture. With metal alloys, these could be entrapped air and/or water. Previously painted surfaces may emit by-products of cure or residual solvents. Emission of these volatiles can damage the final coating.

Lower cure temperature powders provide the opportunity to increase line speed. For instance, with lower cure temperatures, oven settings may remain the same as with higher-cure temperature materials, but line speed can be increased. In some applications this may increase production capacity without additional capital expenditures.

Formulators are developing new powders that will have greater batch-to-batch consistency and dramatically lower curing temperatures. This will enhance the capability of powder coating heat-sensitive substrates, such as plastics.

Exciting developments will continue in the application of powder coatings to molded plastic parts. This allows intricate molded shapes to be coated with powder coatings that become an integral part of the finished piece.

© 2010 Penton Media, Inc.

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