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Durability can take on a variety of meanings in different applications. For some, it may mean exceptional corrosion and chemical resistance in the environment where the coating will be used. For other applications, durability may mean exceptional flexibility and abrasion resistance. In still others, it also frequently means exceptional weathering resistance. The truth is, for most applications, “durability” is a combination of these properties.
Corrosion Resistance
Corrosion has a major impact on global economies. In the U.S., a combined NACE and NASA study found that the cost of corrosion in the United States was $279 billion/year.[1]
Powder coatings provide corrosion resistance in systems that comprise either two layers or one. Very high levels of corrosion resistance can be achieved with a two-layer system using a powder primer and a powder topcoat. Two-layer systems are generally used in highly aggressive environments such as the seacoast or above-ground petroleum piping; the primer provides the corrosion resistance, while the topcoat provides weathering resistance.
Alternatively, powder coatings can be a single-layer “direct-to-metal coating.” Single-layer systems are typically used in less aggressive environments and are more cost effective than their two-layer counterparts, but still can provide a significant amount of corrosion resistance.
As previously mentioned, coatings for many applications require chemical resistance and/or weathering resistance in addition to corrosion resistance. For a powder coating to impart both corrosion and chemical resistance, it must have exceptional adhesion and good barrier properties. In powder coating formulations, this is achieved mainly by using resins with high aromatic content to minimize moisture and chemical penetration.
| System | Corrosion Performance (ASTM B117) |
| Fusion-bonded epoxy | ≥ 3,500 hr |
| Epoxy | > 3,500 hr |
| Epoxy-polyester hybrid | 2,000 hr |
| Polyester TGIC/HAA | 1,000-1,500 hr |
| Polyurethane | 2,000 hr |
Fusion-Bonded Epoxy
These powder coatings use bisphenol-A (BPA)-based epoxies as the main resins and are crosslinked with dicyandiamide (DICY). They are generally cured above 200°C on preheated metals.
Fusion-bonded epoxy powders are used extensively in protecting concrete rebar, pipelines, water and sewage systems, and chemical processing plants. As these coatings are not weatherable, they require a weatherable topcoat for applications that will be exposed to sunlight for prolonged periods of time.
Epoxy-Polyester Hybrid Coatings
Epoxy-polyester hybrid coatings use a blend of BPA-based epoxy resins with acid functional polyester resins. These resins will react with each other during the curing cycle.
These coatings are generally used where weathering resistance is not required. Electrical enclosures, HVAC equipment, automotive interior parts, office furniture, lighting fixtures, and appliances are examples where hybrid powder coatings are used.
Polyester Coatings
Polyester powder coatings generally use carboxy functional polyester resins and are crosslinked with Primid(TM) (hydroxy alkyl amid) or TGIC (triglycidyl isocyanurate). These coatings provide good corrosion while having improved weathering resistance compared to fusion-bonded epoxies or hybrid coatings. Lawn furniture, lawn equipment, and bicycles are typical examples of applications where polyester coatings are used.
Polyurethane
Higher end polyurethane powder coatings provide good weathering along with good direct-to-metal corrosion resistance and chemical resistance. For improved weathering, predominately aliphatic polyester or acrylic polyol resins are cured with blocked isocyanate crosslinkers. These powder coatings can be used on applications such as window frames, automotive wheels and trim, agricultural and construction equipment, and premium outdoor furniture.
Flexibility and Abrasion Resistance
Durability can mean flexibility, as well as impact and chemical resistance. In automotive powder primer surfacers, for example, powder coatings are used to improve stone chip resistance at temperatures as low as -20°C. These are generally polyester-epoxy formulations cured with epoxy crosslinkers. Here, adhesion and impact resistance are critical. The epoxy provides good adhesion, while the polyester component provides the impact resistance.
For agricultural, construction, and earth moving equipment (ACE), resistance to impacts, abrasion, chemicals, weathering, and corrosion are all critical. These applications generally entail a two-coat primer and topcoat system, where the primer may be liquid or powder. In the case of powder, the primer will likely be a polyester-epoxy hybrid or an epoxy-based powder coating. Since the powder topcoat needs to provide weathering, chemical, and chip/abrasion resistance, it is usually a polyester-based coating cured with TGIC, hydroxy alkyl amide, or blocked isocyanate crosslinkers.
Powder coatings for appliance applications need to be resistant to corrosion, mar and scratch, and chemicals (especially to detergents, oils, alcohol, bleach, and other household chemicals). In appliance interiors (like washer drums), epoxy powders are generally used for good corrosion and abrasion resistance when exposed to water and detergents. For the appliance shells, polyester and polyester-epoxy hybrids are used to impart better UV resistance and exceptional chemical and abrasion resistance.
Weathering Resistance
Weathering resistance is a critical performance characteristic of powder coatings that must endure in outdoor exposure. The required amount of weathering depends on the application; many outdoor applications require certification.
For North America, the Fenestration & Glazing Industry Alliance (FGIA) is the governing body that sets the standards (referred to as American Architectural Manufacturers Association standards, or AAMA standards). AAMA 2603, 2604, and 2605 are the key standards for coatings (see Table 2).
| AAMA 2603 | AAMA 2604 | AAMA 2605 | |
| Intended use | Interior | Moderate exterior | Severe exterior |
| Coating type | Polyester or polyester hybrid | Super-durable polyester | Fluoropolymer |
| Weathering requirements | • 1 year Florida • Slight chalking • No checking, crazing, or loss of adhesion | • 5 years Florida • Gloss retention ≥ 30% • Color change ∆E≤5 | • 10 year Florida • Gloss retention ≥ 50% • Color change ∆E≤5 |
While not all applications require certifications to these specifications, they represent an efficient way to categorize weathering for powder coatings applications.
AAMA 2603
Coatings meeting this specification only require minimum weathering performance. Common applications include interior store fronts, stairway railings, computer cases, small appliance cases, electrical boxes, and others that do not encounter routine exterior exposure. Polyester coatings would meet this weathering performance capability.
AAMA 2604
This performance is for coatings that see substantial outdoor exposure, including those for higher end lawn and patio furniture, window and door frames, metal doors, lawn mowers, and garden equipment. These coatings are generally made from polyester resins with low aromatic content (often called “super-durable” resins). Crosslinkers can be epoxy, hydroxy alkyl amide (TGIC, PT910®, Primid®), or blocked isocyanate.
AAMA 2605
This is the highest performance standard and is dominated by coatings made with fluorinated resins. Fluoroethylene vinyl ether-based polymers (FEVE) cured with blocked isocyanate crosslinkers are the performance standard for this application. Polyvinylidene fluoride (PVDF)-based powder coatings are also used in applications where softer thermoplastic coatings are needed.
AAMA 2605 powder coatings are also commonly used in applications that are extremely expensive to repaint. Architectural aluminum extrusions (used to hold windows in tall commercial buildings), metal balconies, external rails, and lighting fixtures are some of the key applications. Additionally, public structures such as bus stops also tend to use this standard of coating.
Achieving the Ideal Durability Balance
Few powder coating applications require only one category of durability. Many AAMA 2605 coatings are direct to metal, for example, and therefore need to provide corrosion protection while also being resistant to cleaners. AAMA 2604-type coatings need to have good weathering, corrosion, and chemical resistance, especially those used in the ACE markets. Achieving the ideal balance of durability-related properties requires fine tuning the coating formulation for the target application, followed by rigorous testing for optimal coating performance.
To learn more, reach out to the author at gwebster@chemquest.com.
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Reference
1. NACE, “Corrosion costs and prevention strategies in the United States,” 2002.