By Scott Moffatt, PPG
In recent years, some manufacturers have sought to imply that silicon-modified polyester (SMP) and ultra-durable polyester coatings can offer the same long-term performance benefits as 70% PVDF or FEVE fluoropolymer coatings. Such assertions are misleading for two reasons.
First, SMP and ultra-durable polyester coatings do not have the chemical structure needed to sustain the long-term performance of fluoropolymer coatings. Second, and perhaps even more critically, SMP and ultra-durable polyester coatings do not have long-term weather exposure data equal to 70% PVDF coatings, which have a 50-year history of proven durability on buildings throughout the world.
Since 70% PVDF coil and aluminum extrusion 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 UV light; then measured at five-year intervals for chalk, fade and other signs of environmental degradation.
The same cannot be said of newer SMP and ultra-durable polyester coatings. Variations of these products have historically been introduced in four- to five-year cycles since they debuted in the market, which means they have had limited time to prove their performance in South Florida exposure testing or in actual building environments.
The American Architectural Manufacturer Association (AAMA) requires ten years of Florida exposure data before a manufacturer can claim that an aluminum extrusion or composite panel coating meets AAMA 2605 performance specifications. Despite the lack of requisite 10-year testing, many SMP and ultra-durable polyester coatings are warranted at longer terms comparable to fluoropolymer coatings, which can mislead unsuspecting buyers about their potential weathering performance.
Some coatings manufacturers seek to dismiss such concerns by insisting that performance data for new SMP and ultra-durable polyester coatings supersedes that of older products. New polyester technologies may, in fact, be superior to the earlier polyester formulations, but they cannot achieve the long-term performance of fluoropolymer coatings because, ultimately, the molecular structure of the base polyester resins will cause them to fail.
Before specifying a coating, customers also should examine if the warranty for the product they are considering distinguishes between vertical and non-vertical surfaces. Most polyester coatings warranties cover vertical and non-vertical surfaces differently because non-vertical surfaces such as roofing are much more susceptible to failure than vertical exposures due to their direct angle to the sun.
Fluoropolymer coatings purchased from proven, reputable coatings manufacturers will provide equal chalk and fade coverage for vertical and non-vertical surfaces. That means buyers should be wary of warranties that inflate the number of years covered for film integrity while diminishing the number of years provide for chalk and fade performance. Buyers should be equally wary of warranties that establish “proration” terms, which is another tool that is commonly used to compensate for inflated years of coverage.
Chalk and Fade
Chalk or fade result directly 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 the surface of a coating. In accordance with ASTM D4214-98 test procedures, it is measured by rubbing the coated surface with a soft fabric and calculating the amount of powder that is picked up on a scale from 1 (extremely poor) to 10 (perfect).
Fade is the loss of color calculated in Hunter ΔE units in accordance with ASTM D2244-02 procedures. One ΔE unit denotes the smallest degree of color change visible to the naked eye.
While it may not appear so on paper, a difference of one or two points in a chalk rating, or of more than five ΔE Hunter units in a fade measurement, can mean the difference between a coating that maintains its original appearance for 30, 40 or 50 years from one that looks old much sooner.
Fluoropolymers vs. Polyester
After 50 years of UV exposure, fluoropolymer coatings and polyester coatings will react differently in exterior environments. In the initial five to 10 years of exposure, both types of coatings will have a slight shift in color fade and chalk values; yet, over time, the differences in their composition will become clearer.
quality coatings will have a very gradual decline in color fade and chalk over a 50-year period, polyester coatings will match fluoropolymer coatings for a time then abruptly lose performance. On average, durable polyesters may have half the life expectancy of fluoropolymer coatings, which severely limits their long-term performance.
The Benefits of Adding a Clear Coat
Fluoropolymer coatings are available as one-, two- or three-coat systems. Liquid coatings offer a third coat representing the application of a clear topcoat. In most circumstances, the clear coat is applied to protect the metallic flake from tarnishing and changing color during UV exposure.
Another benefit of clear coats that is often overlooked is that they minimize accumulation of dirt 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.
Clear coats also provide more resistance to color fade, add an extra barrier against moisture and feature graffiti-resistant properties. Together, these attributes make three-layer liquid coatings the highest performing systems for exterior exposure applications.