Great expectations: Evaluating the performance of UV-durable coil coatings

by Catherine Howlett | March 1, 2013 3:53 pm

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by Scott W. Moffatt
Although silicone-modified polyester (SMP) and super-polyester coatings are well-suited for specific applications, they cannot provide the same proven long-term performance as 70 percent polyvinylidene fluoride (PVDF) products.

While many SMP and super-polyester coatings are excellent products well-suited to various architectural and building product applications, the practice of marketing them as comparable alternatives to 70 percent PVDF coatings may cause architects and specifiers to select the wrong product for their applications.

This article examines differences between 70 percent PVDF, SMP, and super-polyester coatings, and explains why the first type generally retain its color and gloss longer than coatings based on the other chemistry.

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The basics
Coil coatings are made from three basic ingredients: resins, pigments, and solvents.

Resins
Resins give coil coatings basic performance attributes such as resistance to abrasion, scratching, moisture, and ultraviolet (UV) light, as well as mechanical characteristics like adhesion, hardness, and flexibility during fabrication.

Coil coatings are formulated with several resin types, including acrylic, epoxy, polyester, and PVDF polymers. For applications demanding a highly durable coating surface (e.g. metal roofing, composite panels, building panels, and curtain walls), 70 percent PVDF coatings are regarded as superior because of their strong UV resistance. Due to their lower cost and harder finishes, SMP and super-polyester coatings are traditionally favored for applications such as warehouses, industrial, storage, and agricultural structures, and other non-monumental commercial buildings.

In recent years, some manufacturers have sought to position SMP and super-polyester coatings as viable, low-cost alternatives to 70 percent PVDF coatings. While it is true polyester coatings are more resistant to UV damage than early-generation coatings, they do not offer the same weatherability, color retention, and gloss retention as 70 percent PVDF coatings.

To understand why, it helps to know the chemical structure of various resins. PVDF resin molecules are composed of alternating carbon-fluorine and carbon-hydrogen bonds; the former are among the strongest in the chemical world. Consequently, they render PVDF resins chemically and photo-chemically inert, and, therefore, virtually immune to degradation from sunlight, moisture, acids, pollutants, and chemicals. That is the reason for their superior durability.

In contrast, molecules in SMP and super-polyester resins are based on carbon-hydrogen, carbon-oxygen, and carbon-silicone bonds. Since their molecular bonds are weaker, long-term exposure to UV light and environmental hazards eventually defeats the polyester coating’s structural chemical integrity, causing it to chalk or fade.

Pigments
Pigments are colorants made from fine powders. There are three types: organic, inorganic, and ceramic—their chemical structure determines their stability (i.e. ability to resist fading). Since ceramic pigments are made from metal oxides fused under high temperatures—instead of carbon-based pigments derived from chlorophyll or coal that break down more easily under environmental stresses such as UV light—they are the most chemically stable and fade-resistant. Consequently, they are the default choice for coatings warranted to satisfy the most demanding performance expectations, such as high-end architectural applications.

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Solvents
Solvents are carriers that make coatings easy to apply. They do so by solvating resin to a desired consistency and dispersing pigments evenly throughout the coating. Solvents have no effect on coating performance.

Ultimately, a coating is only as good as the sum of its parts. If a coating is formulated from strong, durable resins, but weak pigments—or weak resins and strong pigments—it has the potential to prematurely chalk or fade. Conversely, durable resins combined with strong pigment systems will deliver long-lasting  durability and performance.

In short, a coatings formulation cannot be strong if any of its individual components are weak. High-quality raw materials are essential to a coating’s long-term performanceres.

However, they cannot match the long-term performance of 70 percent PVDF coatings for two reasons:

1. SMP and super-polyester coatings do not have the chemical structure needed to sustain the long-term performance of 70 percent PVDF coatings.
2. SMP and super-polyester coatings do not have long-term weather exposure data equal to 70 percent PVDF coatings, which have a 45-year history of proven durability on buildings around the world.

Since 70 percent PVDF coil coatings debuted in the 1960s, they have been continuously subject to South Florida exposure testing. In this testing, coatings are applied to metal panels and exposed at a 45-degree angle to South Florida’s notoriously harsh humidity and UV light, then measured at five-year intervals for chalk, fade, and other signs of environmental degradation.

Since then, numerous SMP and super-polyester coatings were introduced to the market. While experience shows older, high-quality polyester resins can perform well for up to two decades before they experience a dramatic fall-off in color retention and fade, newer polyester-based coatings formulations have not been available long enough to provide the five- to 10- and 20-year exposure testing needed to assess their long-term performance. Despite the lack of requisite performance data, many newer SMP and super-polyester coatings are warranted at terms comparable to 70 percent PVDF coatings.

Some coatings manufacturers seek to dismiss this concern by insisting performance data for new SMP and super-polyester coatings supersedes that of older products. However, new polyester technologies may in fact be superior to the earlier polyester formulations, but they cannot achieve the long-term performance of 70 percent PVDF coatings because ultimately, the molecular structure of the base polyester resins cause them to fail.

Coatings specifiers should also examine if and how their warranty distinguishes between vertical and non-vertical surfaces. Many warranties cover vertical and non-vertical surfaces differently because the latter (e.g. roofs) is more susceptible to failure. Seventy percent PVDF coatings purchased from proven, reputable coil coatings manufacturers should provide equal chalk and fade coverage for both vertical and non-vertical surfaces.

Chalk and fade
As explained earlier, chalking or fading directly results from the chemical breakdown of a coating’s base resins and pigments, which ultimately appear as a visible loss of color and/or gloss.

Chalk is the appearance of a powdery substance on a coating’s surface. In accordance with ASTM D4214-07, Standard Test Methods for Evaluating the Degree of Chalking of Exterior Paint Films, it is measured by rubbing the coated surface with a soft fabric and calculating the amount of powder picked up on a scale from one (extremely poor) to 10 (perfect), as depicted in Figure 1.

Fade is the loss of color calculated in Hunter ΔE units in accordance with ASTM D2244-11, Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates. (Hunter ΔE units are an internationally recognized system for measuring fade or color loss on siding and building panels.) One ΔE Hunter unit denotes the smallest degree of color change visible to the naked eye (Figure 2).

While it may not appear so on paper, a difference of one or two points in a chalk rating—or more than five ΔE Hunter units in a fade measurement—can mean the difference between a coating that maintains its original appearance for 50 years from one that looks old much sooner.

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PVDF versus polyester
After 40 years of UV exposure, PVDF and polyester coatings react differently in exterior environments. As Figure 3 indicates, both coatings will have a slight shift in color fade and chalk values in the initial five years of exposure; over time, the differences in their composition will become clearer.

While PVDF coatings have a very gradual decline in color fade and chalk over a 40-year period, polyester coatings match PVDF coatings for a time, then abruptly lose performance. On average, durable polyesters may have half the life expectancy of PVDF coatings, which severely limits their long-term performance.

The benefits of adding a clear coat
PVDF coatings are available as two- or three-coat systems, with the third coat representing the application of a clear topcoat. A clear coat is required for coatings with metallic flake to prevent tarnishing and discoloration from UV exposure, but they also are recommended to protect and enhance coatings formulated with solid color and micas, or to provide longevity to coatings made with weaker pigments.

Another oft-overlooked benefit of clear coats is they minimize dirt accumulation and make metal building surfaces easier to clean. This is critical in severe industrial environments or coastal areas where buildings can be exposed to chemical fallout or salt spray (Figure 4).

Resistance to UV exposure is also enhanced with a clear coat, as demonstrated in Figure 5, allowing manufacturers to provide improved warranties for chalk and fade.

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Conclusion
In recent years, some manufacturers have sought to position SMP and super-polyester coatings as viable, lower-cost alternatives to 70 percent PVDF coatings; however, the latter will always provide better long-term performance because of the inherently superior chemical and molecular structure. Nevertheless, there are applications for which SMP and super-polyester coatings can be favored over 70 percent PVDF coatings, particularly when long-term color and gloss retention are less of a priority than cost control and basic durability.

Warranties for coil coatings are not a reliable indicator of coatings performance. When comparing warranties for SMP, super-polyester, and 70 percent PVDF coil coatings, architects and specifiers should scrutinize their length and terms, particularly for performance factors such as chalk and fade. They should also be wary of test data that does not extend beyond five to 10 years because it does not effectively represent the long-term performance expectations associated with most high-end commercial and residential architectural applications.

Scott W. Moffatt is PPG Industries’ marketing director for coil and extrusion coatings. He is a 34-year veteran of the company and has held global positions covering sales, sales management, and marketing for multiple technologies and market segments in the industrial coatings market. Moffatt is a member of Metal Construction Association (MCA), The Metal Initiative (TMI), Metal Roofing Alliance (MRA), Metal Building Manufacturers Association (MBMA), National Coil Coating Association (NCCA), and Cool Roof Rating Council (CRRC). He can be reached at moffatt@ppg.com.

Source URL: https://www.constructionspecifier.com/great-expectations-evaluating-the-performance-of-uv-durable-coil-coatings/

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