Technology Interchange: Choosing an Outdoor Durable Powder Coating

By: Kevin Biller

Selecting an exterior durable powder coating can either be a daunting task or possibly an overly simplistic exercise depending on your knowledge (or lack thereof) of the technology. Some powder pundits assign the descriptor “Outdoor Durable” on a product without a thorough understanding of what this really means The following addresses the differing levels of exterior durability and which powder chemistries meet which quality specification.

Labels are bandied about describing resistance to ultraviolet (UV) degradation and environmental conditions. The focus of this article is coating durability in typical outdoor conditions comprised of exposure to sunlight and moderate rain/moisture. Extremely corrosive environments involve another level of durability and is a topic of another article.

Powder coatings are built on a binder comprised of a resin and curing agent, colorant pigments, extenders (fillers) and additives. UV light combined with moisture attacks the chemical bonds in polymers and pigments which cause color fade, chalking and the degradation of barrier properties intended to protect a substrate. Polymers and pigments possess intrinsic resistance to this degradation based on their molecular composition.

Outdoor Durability Defined

Describing a powder coating merely as “outdoor durable” is inexact and does not provide sufficient information to select a product to meet a customer’s requirements. Coatings are delineated based on weathering metrics from actual locations that experience high levels of UV exposure. In the Western world, South Florida and to a lesser extent central Arizona are most commonly used whereas Japanese standards call for Okinawa exposure.

The most basic outdoor durable performance can be described as industrial or entry level architectural grade. These coatings maintain color and gloss for 12 to 18 months of exposure in South Florida. After two years these coatings exhibit significant color fade and gloss reduction of 50 percent or more. Rich and dark colors show the most visual change as degradation is typically expressed as a white, chalky phenomenon. Entry level architectural grade powders fit this performance profile and are used for items that experience only incidental UV exposure or are not expected to maintain appearance past a few outdoor seasons. Storm doors, consumer garden equipment, bicycles and lower grade lawn furniture are examples of products coated with industrial grade outdoor durable powder coatings.

High performance exterior durability describes a class of coatings expected to withstand up to 5 years exposure in South Florida before exhibiting visible evidence of degradation. These types of coatings are used for general purpose architectural applications such as consumer window frames, architectural hardware and high quality outdoor furniture. Some automotive trim products, wheels and agricultural implements use this technology as well.

Exterior automotive grade coatings require outstanding resistance to sunlight, moisture, air pollution (mainly acidic) and thermal shock. These coatings must maintain gloss and color for a service life of up to 10 years. Exterior automotive body coatings are typically comprised of a multi-layer system including electrodeposition primer, primersurfacer, basecoat and clear topcoat.

High end alloy wheels typically use a basecoat/clearcoat system and require similar durability. Powder coatings had been used as exterior body coats from 1999 through 2014 by BMW at their operations in Dingolfing, Germany. Powder coatings continue to be used for luxury auto alloy wheels. Superior performing, sometimes referred to as “hyperdurable” architectural grade powder coatings require 10 years durability without exhibiting significant film degradation. These coatings are specified for monumental end-uses such as commercial buildings, skyscrapers and the like.

Chemistries

A survey of powder coating chemistries provides a matrix of options for outdoor durable requirements. First of all, epoxy based and hybrid (epoxy polyester) powders are out. Any epoxy content in a powder coating will cause the finish to chalk and degrade in less than a couple months of outdoor exposure.

Entry level or industrial grade exterior durable powder coatings include polyester-TGIC, polyester- HAA and polyurethane chemistries. These are economical coatings available in a limitless variety of colors, gloss and special effects.

High performance architectural and general purpose powders are usually based on “superdurable” resin technology and are available in polyester-TGIC, polyester-HAA and polyurethane platforms. Color possibilities are wide but not as limitless as those available in an industrial grade product because high performance pigment options are fewer. Standard grade organic pigments fade in 18 to 24 months of outdoor exposure. High performance powder coatings must use either inorganic mixed-metal oxide pigments that are borrowed from the ceramics industry or high priced automotive grade organic pigments to meet color and durability requirements.

Exterior automotive grade powder coatings are most often high gloss clearcoats, silver metallics and black trim coatings. Color is limited as formulators must use high performance pigments that are expensive. This product space is dominated by acrylic resin technology and has an impressive track record of performance on BMW cars.

Specialty polymers based on fluoropolymer chemistry are required to meet superior performing architectural grade powder coating specifications. These coatings resist fade and film degradation a minimum of 10 years in South Florida climate however studies have shown this chemistry to withstand 20 to 30 years in the field. Color is limited to muted hues generated by inorganic mixed metal oxides and gloss levels typically below 70 GU (gloss unit).

Specifications

Each industry and often each manufacturer have their own specifications that delineate the performance requirements for coatings used on their products. The architectural industry has a very good set of standards established by the American Architectural Manufacturers Association (AAMA) in the US and Qualicoat and GSB in Western Europe and Australia. Descriptions of these are summarized in Table 2.

Automotive body topcoat specifications are established by each car manufacturer but all have the expectation that the coating must last 10 years without evidence of significant fade, discoloration, cracking or erosion of the finish. Most automotive companies use a combination of accelerated exposure test methods and natural outdoor testing. The most common exterior automotive use of powder coatings is as a clear coat for alloy wheels. General Motors’ 9985586 specification requires 3 years Florida durability of cosmetic qualities. For trim parts their 9984047 specification calls for an acrylic powder topcoat which requires 5 years durability.

Test Methods

Scientists have worked for decades trying to simulate outdoor conditions in the laboratory in an attempt to accelerate the degradation process. The goal is to establish a rapid predictor of weathering performance to correlate long term exposure.

Historically the Q-Lab QUV® Accelerated Weather Tester has been the prevalent technique in outdoor durability assessment. ASTM D4587 describes QUV® testing which entails alternating cycles of humidity and UV energy generated from fluorescent lamps. These lamps are categorized as UVA 340, UVA 351, FS-40 and UVB- 313EL. (See Table 3.)

It is crucial to understand the effect various lamps have on powder coating chemistries and how valid the results may be. UVA lamps more closely approximate sunlight than UVB lamps and take longer to degrade powder coating polymers. The UVB lamps emit unnatural short wavelength energy (below 295 nm) that more aggressively degrades powder resins especially polyester types. Hence, the use of accelerated UVB test conditions generates evidence of coating failure significantly faster than UVA testing. Typically polyesters fail within 250 to 300 hours. UVB exposure, whereas the same products last over 1000 hours. UVA testing.

A highly accurate but more expensive test protocol uses the Xenon Arc Weatherometer (ASTM D7869). Xenon arc testing closely approximates the wavelength of sunlight and combines it with a humidity cycle. Exposure is measured in MJ/m2 (megaJoules per meter squared) of energy and can be correlated to actual outdoor exposure. Some weathering experts use 275 MJ/m2 as a measure of a typical year of South Florida exposure. Xenon arc test cabinets emit about 0.55 W/m2 light energy per hour which equates one year South Florida exposure to about 2,360 hours in the cabinet using a 2-hours light to 1-hour dark humidity cycle.

Besides real-time natural exposure in South Florida, the most realistic measure of UV durability uses a solar concentrating technique described in ASTM D4141 and D4364. This technique provides an acceleration of natural UV degradation by reflecting sunlight onto a powder coated surface with an arrangement of 10 mirrors. Test racks are stationed in Central Arizona and include a water spray feature to accelerate degradation. Using this method approximately 1400 MJ/m2 of solar energy are delivered to the coating surface in one year. This equates to 10 to 12 years of natural South Florida exposure. Solar concentration test methodology is highly accurate but very expensive to conduct.

Selecting an outdoor durable grade powder coating to meet the needs of a customer takes careful analysis of options, costs and specifications. The powder coating industry offers a wide array of chemistries to provide the perfect match to the expectations of the manufacturer. Understanding which products meet industry standards for weatherability is a good foundation in making the right choice for your customer.

Kevin Biller is technical editor of Powder Coated Tough magazine and president of The Powder Coating Research Group. He can be reached via email at kevinbiller@yahoo.com.