UV-Durable Coil Coating Performance Expectations

by Jonathan McGaha | 30 September 2015 12:00 am

Looking at differences and choices of coil coatings

Photo courtesy of Bryan Wayne Photography

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 (PVDF) coatings, due to the latter’s superior chemistry.

This article examines the differences between 70 percent PVDF, SMP and super-polyester coatings, and explains why the former is still the superior choice for long-term performance despite significant advances in SMP and super-polyester coating technologies. Coil coatings are made from three basic ingredients: resins, pigments and solvents.

 

Resins

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

Coil coatings are formulated with several resin types, including acrylic, epoxy, polyester and PVDF polymers. For applications that demand a highly durable coating surface, such as metal roofing and building panels, 70 percent PVDF coatings are considered superior due to their strong UV resistance. Because of their lower cost and harder finishes, SMP and super-polyester coatings are traditionally favored for utilitarian applications such as industrial, storage and agricultural buildings.

While today’s polyester coatings are generally more resistant to UV damage than early-generation polyester 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. Carbon-fluorine bonds are among the strongest in the chemical world. Consequently, they render PVDF resins chemically and photochemically inert, and, therefore, virtually immune to degradation from sunlight, moisture, acids, pollutants and chemicals.

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

 

Pigments

Pigments are colorants made from fine powders. There are three types⎯organic, inorganic and ceramic⎯and their chemical structure determines their stability (ability to resist fading). Because ceramic pigments are made from metal oxides fused under high temperatures, they are the most chemically stable and fade-resistant. Consequently, they are the default choice for coatings systems warranted to satisfy the most demanding performance expectations for high-end architectural applications.

 

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.

 

Be Wary of Performance Claims

In recent years, some manufacturers have sought to imply that SMP and super-polyester coatings can offer the same long-term performance benefits as 70 percent PVDF coatings. Such claims are misleading for two reasons.

First, as explained earlier, SMP and super-polyester coatings do not have the chemical structure needed to sustain the long-term performance of 70 percent PVDF coatings.

Second, and perhaps even more critically, SMP and super-polyester coatings do not have long-term weather exposure data equal to that of 70 percent PVDF coatings, which have been subject to South Florida testing since the 1960s and also have delivered 50 years of proven durability on buildings around the world.

Because new SMP and super-polyester coatings are introduced every few years, these products typically have no more than 10 to 15 years of performance data in South Florida exposure testing or actual building performance.

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

 

Comparing the Coatings

The chart illustrates the relative strengths of four commonly specified polyester and PVDF coatings:

 

	Standard Polyester	Super Polyester/ SMP Coatings	70% PVDF KYNAR/HYLAR
Film Integrity	Excellent	Excellent	Excellent
Color Retention	Good	Very Good	Excellent
Chalk Resistance	Good	Very Good	Excellent
Dirt Resistance	Poor	Very Good	Very Good
Gloss Retention	Good	Very Good	Excellent

 

Summary

• Coil coatings formulated with polyester resins, including the latest SMP and super-polyester resins, do not offer the same long-term performance as 70 percent PVDF coatings due to the superior molecular strength of 70 percent PVDF resins.

• Warranties for coil coatings are not a reliable indicator of coatings performance.

• When comparing warranties for SMP, superpolyester and 70 percent PVDF coatings, scrutinize their length and terms, particularly for performance factors such as chalk and fade.

• South Florida exposure data should cover a reasonable amount of time. 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 Moffatt is market manager for building products, PPG Industries Inc., Pittsburgh, and has 36 years of experience in the coatings industry. For more information, visit www.ppgideascapes.com[1].

Source URL: https://www.metalconstructionnews.com/articles/uv-durable-coil-coating-performance-expectations/

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