Archive: Jun 2021

The basics of liquid and powder coatings

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New to liquid and powder coating? Consider this.

According to the Fabricator:

“Almost all metal fabricators consider on-time part delivery a key metric. In today’s just-in-time manufacturing environment, the parts need to be there so the customers don’t miss a beat in their own manufacturing facilities.

“To maintain their ability to get parts where they need to be and when they need to be there, more fabricators are investigating in-house finishing capabilities. They know that once metal parts leave the shop to receive a liquid or a powder coating, they have lost their ability to guarantee a delivery date. It is quite literally out of their hands.

“Now, a finishing contractor may be a good supply chain partner, but a metal fabricator is one of its many customers—perhaps one of the smaller ones. If the custom coater needs to clear the production schedule for one of its largest customers, guess where that leaves the small batch from the metal fabricator?

“Metal fabricators looking to take on finishing should know about two of the most common finishing alternatives—liquid and powder coating—and the requirements involved for a company hoping to apply one or both.

What Makes up the Coating?

“Liquid. The basic raw materials comprising a liquid coating are additives, carriers, pigments, and resins. Additives make up the smallest portion of any liquid coating composition, but they impart special characteristics on the overall finish; for example, they might assist with rust prevention or UV protection. The carrier is the main liquid used to formulate the paint. The carrier can be water, solvent, or a combination of the two. Liquid coatings heavy in solvents traditionally have been the dominant form of liquid finish applied to metal parts over the years, but more interest has been directed to waterborne and high-solids coatings, which release a minimal amount of volatile organic compounds (VOCs) during application when compared to traditional solventborne coatings.

“Pigments play a role in final appearance and performance to some extent. As a rule, the volume of pigments influences the gloss of the film. The more pigment present, the lower the coating’s gloss. Resins act as the base of the liquid coating. They primarily govern the overall performance of the coating, helping the paint to excel for particular applications.

“Powder. Powder coatings don’t require a carrier. The additives, pigments, and resins are formulated in a powder form. To apply the material to parts, the powder is electrostatically charged and conveyed via compressed air. The charged powder is attracted to a grounded part. The part is then taken through an oven, where the heat changes the powder from a solid to a liquid and then to a solid coating. Generally, powders do not have any or extremely low VOCs.

How Does the Coating Affect the Environment?

“Liquid. Solventborne coatings are often specified for a finish because of their dependable performance and ability to air-dry in a matter of minutes. Unfortunately, most of the traditional liquid paint formulations from yesteryear no longer are around because of the need to reduce VOCs emitted during application.

“That has led to the development of more environmentally friendly coatings, such as new waterborne formulations and high-solids coatings, that emit low VOCs. The waterbornes, which have come a long way from the early versions used 15 years ago, are slowly growing in acceptance. However, some manufacturers still have reservations about applying a water-based product directly to metal. High-solids are liquid coatings that have a solids content of at least 65 percent, which means minimal solvents are present. But fewer solvents also means that the coating is more viscous. That has led to the development of multipart application systems (referred to as 2K systems if two parts are mixed, 3K if three parts are mixed, etc.) that are formulated to be mixed only seconds before application.

“All finishers that apply liquid coatings spray to waste. The overspray can’t be reclaimed. If filters are used to capture the overspray, the finisher has to dispose of the filters according to regulations established by local authorities.

“Powder. The powder booth does not require an exhaust. As stated previously, if any VOCs are emitted during the powder coating process, they are typically very low.

“Powder coatings can be recycled with the right reclamation equipment. Companies looking for Class A finishes have reclamation systems that depend on thorough cleaning and maintenance because any cross-contamination of reclaimed material ruins the original material’s ability to deliver a specific color.

“Again, disposal of unreclaimed powder coating material is governed by local regulations. In some instances, local law may require disposal in sealed containers or require that the powder coating material be baked into the form of a brick for disposal in a local landfill.

What Are the Characteristics of the Final Coating?

“Liquid. When someone talks about a Class A finish, typically used to define the coating quality on a new automobile, most people think of liquid coatings. Specialty finishes, such as those incorporating metallic flakes, also are possible in liquid coatings.

“Durability has improved over the years as well, especially with the emergence of 2K coatings. These are typically used on large metal parts that can’t be powder-coated because they can’t fit into a typical booth and oven setup.

“Liquid coating finishes can be applied in various thicknesses. Obviously, the more mils applied, the better the protection. In many instances, a manufacturer, such as an automaker, will seek to balance maximum protection with the minimum amount of paint mil thickness.

“Powder. Powder coatings offer the same characteristics that come with liquid coatings.

“Properly cured powder coatings can offer superior protection against chipping, scratching, UV rays, and corrosive elements. This is why powder coatings are often specified to coat metal products destined for outdoor use.

“Material development has progressed to the point where powder coatings can deliver a Class A finish. In fact, a major European automaker and a U.S. motorcycle manufacturer are using powder coatings for their clear coating. However, many manufacturers, including automakers, still prefer liquid coatings for that topnotch finish.

“Standard powder coating finishes are applied in the thickness range of 2 to 4 mils. Specialty finishes like a hammertone or a texture are usually 3 to 5 mils thick. Functional coatings can be 10 to 40 mils thick.

What Type of Cleaning Is Necessary Before Parts Are Coated?

“Liquid. A lot of fabricators simply wipe the part clean with a rag soaked in solvent. Others rely on a wash of some kind with pretreatment chemicals.

“Solvent helps to prepare the metal surface because it has aggressive cleaning action and actually prepares the surface for adhering to the paint. New coatings that have less solvent content may require much more formalized pretreatment processes to achieve a quality finish.

“Powder. Pretreatment is critical when it comes to powder coating. If a powder coating is going to last, the part needs to be thoroughly cleaned.

“Pretreatment can range from a simple abrasive media blasting chamber to a multistage pretreatment system with several chemical application and rinse stations and an oven. (Parts have to be dried and cooled before any application of powder takes place.) Some of the pretreatment chemicals, such as zinc phosphate, have to be treated before disposal, but newer, environmentally friendly pretreatment chemicals have emerged in recent years to ease the disposal hassles. The Environmental Protection Agency, however, likely will seek changes in the near future that will call for treatment of all wastewater prior to discharge.

What Are the Basic Booth Requirements?

“Liquid. Booths or stations used in liquid coating are typically made of metal. Whether in a manual or an automated setup, the paint is often sprayed to waste.

“Fumes are exhausted outside during the painting process to keep the work environment in and around the paint booth free from the strong odors.

“Powder. In some instances, particularly if batch finishing is occurring, a shop might use the same booth for both liquid and powder coating. But fabricators need to keep in mind that if they choose to powder coat in a liquid application booth and exhaust overspray outside the building, employees could be walking out to cars with all new finishes at the end of the shift on a very hot day—depending on where that powder overspray landed. For the most part, however, filters and correct powder coating technique should prevent most powder overspray from going outside.

“If a company is doing any kind of high-volume powder coating, it should consider a reclamation system. Single-color reclamation systems are cartridge-based and typically made of stainless steel. Multicolor systems, which are designed for fast color changeout, are plastic, making them easier to clean. Sophisticated fan setups keep the overspray in the booth, and the polymer-based interior walls prevent the powder from adhering to them. The overspray is collected and recycled for another application.

How Do the Application Guns Work?

“Liquid. Conventional spray guns for liquid coatings rely on highly pressurized compressed air (2 to 3.5 bar) to propel atomized coating material to the surface. These guns have a low transfer efficiency when compared to more modern paint application guns and may not be approved for use in some areas. However, they are relatively inexpensive and easy to maintain.

“High-volume, low-pressure (HVLP) spray guns also atomize the paint like a conventional spray gun, but use low-pressure air, usually less than 0.7 bar, to propel the paint onto the object. The lower velocity of the air results in less paint jetting through the spray gun’s air nozzle and allows for a more controlled application. Higher application rates are then possible.

“Other spray gun technologies are airless spray guns that force the paint through a smaller nozzle and electrostatic-based guns that rely on paint being “attracted” to the object to be coated. (In electrostatic application, a charge is applied to the liquid coating while it is being atomized. In turn, the coating is attracted to any surface that is grounded, which happens to be the workpiece. Obviously, this approach works very well with metal workpieces.) For new users of paint technology, conventional or HVLP spray guns are typically the choice for paint application.

“Powder. Powder coatings rely on the charging of material for application as well. Today’s market primarily uses corona guns to do this.

“These guns impart a strong electrostatic charge on the powder material as it leaves the spray gun via compressed air. As the powder coating is discharged, it is attracted to the grounded metal part hanging from a metal rack. It is necessary for the rack to have some area of exposed metal to ensure a solid grounding for good electrostatic powder application.

“Tribo guns also are used for powder application. With this method, the powder material picks up a positive charge while rubbing against the gun’s interior Teflon® walls.

“An operator needs less experience to apply powder coatings in an efficient and effective manner than someone applying liquid coatings.

What Are the General Oven Requirements?

“Liquid. Because most liquid coatings can air-day, ovens aren’t necessarily needed. However, if a manufacturer wants to speed up drying times, it needs an oven that is capable of heating between 130 and 170 degrees F.

“Powder. Powder coatings need a much hotter oven to melt the particles so that they can flow and react chemically to form a smooth finish on the workpiece. Most powder coatings reach this stage in an oven heated to 350 to 400 degrees F. For a proper curing, the substrate must be at this temperature for at least 10 minutes.

“On some occasions a manufacturer can use the same oven for drying liquid coatings and curing powder coatings (as long as both coating chemistries are compatible with each other). The key is scheduling parts headed through the oven so that the temperature can be adjusted accordingly for specific coating jobs.

Why Use a Liquid Coating?

“This coating technology is prevalent for many reasons:

“It can be cured quickly, resulting in faster production cycles.

“It is cost-effective in the sense that the initial investment for the equipment is much less than for powder coating equipment (although liquid material is more expensive than powder and can’t be reclaimed).

“The coating can be used to finish parts containing sensitive materials, such as a metal cylinder with a rubber seal, because it does not require dramatically high temperatures to dry.

“It can be used to finish very large parts that are not able to fit into an oven or can’t be moved easily.

“A thin coating is achievable. It can routinely be applied as thin as 0.5 mil.

“It provides an automotive-quality finish (although this performance advantage over powder coatings has been narrowed greatly over the years).

Why Use a Powder Coating?

“Users of powder coatings turn to this technology for a few specific reasons:

“As soon as the workpiece has cooled after curing, the part doesn’t require overly protective handling and immediately can be sent to downstream processes such as assembly or packaging.

“It is very durable. It is commonly used for outdoor applications ranging from outdoor furniture to agricultural.

“The process does not emit VOCs, which means local air quality regulations aren’t likely to be an issue for the manufacturer undertaking powder coating. Additionally, the material can be reclaimed, if the right equipment is installed, which limits the amount of waste material that has to be placed into the waste stream.

“Although it requires a large upfront investment, the long-term costs of applying powder are less than those for a liquid system. In some instances, the price of liquid material may be four times more.”

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“CARC” (Chemical Agent Resistant Coating)

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New to CARC? Consider this.

According to Military Trader:

“Chemical Agent Resistant Coating or CARC is, in simple terms, a low gloss military version of the polyurethane paints that were developed for use in commercial industry. CARC has a low porosity that prevents chemical warfare agents from “soaking in” to the finish and makes decontamination easy to perform. TB 43-0242 states, “[Chemical agents] just bead up on the surface like water on a newly waxed car.” More importantly, the CARC finish is not affected by the solvents used in the decontamination process.

“The finish is also much more durable and resistant to fading, lasting up to four times longer than the alkyd paint previously used by the Army. This durability promised to keep fielded vehicles looking better for a longer period of time and to reduce the number of times a vehicle would need to be repainted in its life cycle, thus reducing maintenance costs. The resistance to solvents allows regular washing of vehicles without fear of damaging the finish.

“To provide some idea about the durability of the CARC finish, TB 43-0242 gives the following as a test to determine if a vehicle is painted with CARC, “wet a cloth with acetone and rub hard on the painted surface for 10 seconds. Wet a clean corner of the cloth with acetone and rub another 10 seconds if no paint comes off the second time, it’s CARC.”

BACKGROUND OF CARC
“The first chemical agent resistant coatings were developed as early as 1974, and by 1983 the Army was ready to make CARC the required coating for all combat, combat support, tactical wheeled vehicles, aircraft and tactical ground support equipment. The US Army officially adopted CARC in May of 1983.

“Besides its chemical resistance and durability, CARC has some other unique properties. For example, the base green color, “Green 383.” uses pigments that mimic the reflective properties of chlorophyll which is found in living plants, making the vehicle harder to detect using infrared detectors. During the Gulf War, “Tan 686” was reformulated to reduce the amount of solar heat absorption and keep vehicles cooler in the desert environment. The new color became “Tan 686A” and was standardized in the latter stages of the conflict.

“To create these special pigments, CARC is a two-part coating that is mixed before application. The components are not interchangeable, it is not possible to mix component A of one color with component B of another color, and intermixing components from different manufacturers is also not feasible. Once mixed, unused CARC will not keep and must be disposed of as a hazardous material. CARC is also highly flammable and it is recommended that the cans and mixing equipment be grounded when mixing the paint. A one part CARC was also made available for brush or roller touchup at the unit level.

CARC HEALTH CONCERNS
“One problem of CARC is the toxicity of its components. Polyurethane paints were in use in the commercial and automotive industries for some years before the Army adopted CARC, so the health risks associated with them were well documented. All polyurethane paints contain isocyanates, and this alone poses a significant health risk. Add to this the array of volatile solvents and cleaners needed and you have a recipe for serious health risks.

“Hexamethylene diisocyanate (HDI) is the isocyanate found in CARC. It can be released when CARC is being sprayed, and it is also released when CARC burns making the smoke from welding and vehicle fires a greater potential health hazard. HDI is also a known sensitizer for asthmatics; soldiers with asthma were not to be involved with the application of CARC as “a severe life threatening allergic reaction may occur.”

“CARC poses no known health risks once dry unless disturbed by sanding or grinding. As a result of the hazards involved with applying CARC, individual units and crews were no longer responsible for the painting of vehicles. Complete repaints had to be done at the direct support, general support or depot level, provided the facility had an OSHA approved paint booth.

“This rule also applied to complete repaints of non-CARC painted equipment as the Army strove to become compliant with ever more stringent environmental regulations. Moving the responsibility for major painting tasks to the maintenance level also served to improve the quality of the paint finish and help maintain greater uniformity in camouflage patterns.

“The October 1990 version of TB 43-209 is the source for CARC patterns for several military vehicles.

“Soldiers at the unit level were limited to touching up their vehicles using a brush or roller; they were not supposed to spray CARC for any reason. Touch ups for cosmetic reasons were also officially prohibited; only blemishes in the paint that went to bare metal were to be retouched, to inhibit corrosion. Unit level touch up was only supposed to be performed using the one part CARC. Rules governing CARC application were spelled out in AR 750-1, paragraph 4-41.

“It was determined that brush or roller application of CARC could be done outdoors without needing a respirator, as long as the individual was exposed to no more than one quart each day. It was, however, required that the individual wear chemical goggles, NBC gloves, a hat, and long sleeves. Use of the M40 NBC mask was permitted at the commander’s discretion.

“As with any army, rules are not always followed. The U.S. Army provides a good barometer in its PS Magazine. According to PS Magazines online article index, from 1990-2000, there were 22 articles on CARC, 8 of which were about CARC touch up and what painting was allowed at the unit level. It would appear that many soldiers were unfamiliar with AR 750-1!

THE CHANGEOVER TO CARC

“When the Army adopted CARC in 1983, it issued a set of guidelines on how the transition from alkyd to CARC was to transpire. The transition began in October 1985 (start of Fiscal Year 1986) for systems already in the field. The guidelines were as follows:

1. At the unit level, crews were to continue touching up the existing paint jobs until such a time as the whole vehicle was in need of a repaint, complete repaints were to be done only as needed.
2. Any vehicle in a depot level repair program would be repainted using CARC and the 3-color scheme.
3. If an approved 3-color CARC pattern was not available for the vehicle, the Green 383 base color was to be applied.
4. Vehicles painted in CARC were to be marked near the data plate with “CARC mo/yr.”

“It was spelled out that any new systems in procurement as of the May 1983 message would be procured with CARC in the 3-color patterns. Systems already in production would convert to CARC as soon as possible, but no later than October 1985.

“Some fielded systems that were near the end of their life cycles were left in the old 4-color alkyd patterns until they left service. By the onset of the Gulf War in 1991, most everything was CARC painted, but the occasional piece of support equipment could be found in the old pattern into the 1990’s. Once a piece of equipment was painted in CARC, all markings were to be applied using CARC or lusterless black pressure sensitive decals.

CARC OR NO CARC
“Certain items were not to be coated with CARC, they were spelled out in AR750-1, paragraph 4-41, line 10:

a.) Painted items that attain surface temperatures of 400 degrees Fahrenheit serve a heat-conducting function or serve a function of expanding and contracting during operation. Examples are manifolds, turbo chargers, cooling fins, and rubber hoses.
b.) Displacement watercraft that will be subject to prolonged salt-water immersion. Examples are the logistical support vessel and the landing craft utility.
c.) Non-deployable equipment and fixed installation systems. Examples are railroad rolling stock and fixed power generation systems.
d.) Installation / TDA equipment such as military police cars, non-tactical fire trucks and buses.
e.) Aluminum transmissions that are enclosed in combat vehicle powerpack compartments. However any ferrous components of the transmission must be protected with CARC or other rust-preventative agents.

“AR 750-1, paragraph 14-4, also spelled out, “If items do not require painting, do not paint them. For example, items made of fabric or which have anodized or parkerized surfaces are not painted.”

“The old practice of continuing camouflage pattern painting onto vehicle soft-tops was also ended, as there was no flex agent available to mix with CARC paints.

CARC ON WOOD
“It was found that CARC did not last well on wood. Wood expands and contracts with weather changes and CARC lacked the flexibility needed move with the wood, so cracks would form and the finish would begin to peel. It was recommended in TM 43-0139, “Properly seasoned wood shall be sealed prior to application of CARC with a polyurethane sealer …wood shall be treated…dried to a moisture content no greater than 20% and pressure treated in accordance with [the] American Wood Preservers Bureau…” At some point in the 1990s, it was decided that CARC was not to be used on wood at all because it simply did not last.

THE FUTURE OF CARC
“Given the volatile and toxic nature of the CARC coatings, research to develop more environmentally friendly coatings with the same properties as CARC were underway even before CARC became standardized. CARC has a Volatile Organic Compound (VOC) content of 420 g/L. This was pushing the limits of federal and local regulations stemming from the Clean Air Act of 1990.

“The Army, as well as the private sector, began experimenting with waterborne coatings in an effort to meet the requirements of new clean air regulations. By the close the century the US Army Research Laboratory had developed a coating that had a VOC content of 220g/L while exhibiting many of the same characteristics of CARC. Development and testing continued as the century drew to a close.”

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Top 5 Myths about Powder Coating

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When it comes to powder coating, be wary of these myths.

According to ICI Online:

“Many people have their preconceived notions about powder coating which are not all very accurate. Before discussing the top 5 myths about powder coating, let’s go over the process and how it works.

What is powder coat?

“Powder coating is a type of coating that is applied as a free-flowing, dry powder. The main difference between conventional paint and powder coating is that powder coating does not require a solvent to keep the binder and filler parts in a liquid suspension form. The result is a paint-like finish.

“Once a metal is prepared for powder coat, depending on the application process, a powder is applied to the metal with an electrostatic gun, this creates a positive electric charge and when sprayed on the metal part, adheres to the metal. Once applied, the metal part goes into a sizeable hi-temp oven where it melts down and creates a film.

Myth #1. Powder coat doesn’t crack or chip.

“Although powder coat creates a film over the part, it, unfortunately, is not damage proof. Like many materials, under certain conditions powder coat can crack or chip and then peel away like an eggshell. Even when the powder coating is applied correctly, the environment can play a role in the effectiveness of the coating. Extreme heat, cold, or rough grainy substances, such as dirt, or salt may corrode the coating faster than you intended.

Myth #2. Powder coat can be applied to the same materials as wet paint.

“Although used as an alternative to wet paint, this is not the case for all materials. Earlier, when I talked about the powder coat process, you can see why it would not work on materials such as plaster, wood, rubber and other non-compatible electrostatic materials. The primary utilization for powder coat is on metals.

Myth #3. Powder Coat is great for covering imperfections.

“This one is a little tricky because the powder coat applies to the top of a surface, it seems logical that it would hide flaws. If there is discoloration on the surface, the powder coat can usually cover that depending on the color of the blemish. However, if the surface is scratched, dented, or has other imperfections the powder coat will not hide these. Powder Coat can end up drawing attention to such flaws.

Myth #4. Powder coat has a quicker dry and cure time than wet paint.

“Powder coat does not have a dry time, only a cure time. The powder remains dry during the entire application process, so there is no “dry-time” as opposed to liquid paint. For the curing process, the object is placed in a large “oven” to bake the powder into a “skin.” For example, at ICI for a standard bumper, we first prime the bumper with powder coating, cure it for 15-30 min and then use the black powder coat and cure it at 420 degrees Fahrenheit for 45 min.

Myth #5. Powder coat prevents rust and corrosion.

“This one is one of the most common misconceptions amid powder coating. Although the powder coat creates a skin, it does not prevent rust or corrosion, especially for exterior parts. Primer and proper application are essential to the longevity of your powder coated components.”

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