PVDF versus SMP
A look at performance in UV-durable coil coatings.
October 22, 2012 By Scott Moffatt PPG Industries
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 per cent polyvinylidene fluoride
(PVDF) coatings, due to the latter’s superior chemistry.
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 per cent polyvinylidene fluoride (PVDF) coatings, due to the latter’s superior chemistry.
|All steel-coating technologies are not created equal. When it comes to resisting chalk and fade over time, nothing beats 70 per cent PVDF.
In today’s competitive environment, many companies make claims that SMP and super-polyester coatings equal or exceed the performance capabilities of 70 per cent PVDF coatings. This article examines the differences between 70 per cent PVDF coatings, SMP and super-polyester coatings, and evaluates the true long-term per- formance of each coating technology.
First, the basics. Coil coatings are made from three basic ingredients: resins, pigments and solvents.
Resins give coil coatings basic performance characteristics such as resistance to abrasion, scratching, moisture and ultraviolet light, as well as mechanical characteristics such as adhesion, hardness and flexibility during fabrication.
Poorest chalk rating: 1
| Chalk rating: 5
| Best chalk rating: 10
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, composite panels, building panels and curtainwalls, 70 per cent PVDF coatings are regarded as superior due to their strong UV resistance. Because of their lower cost and harder finishes, SMP and super-polyester coatings are traditionally favoured 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 per cent PVDF coatings. While it is true that polyester coatings are more resistant to UV damage than early generation coatings, they do not offer the same weatherability, colour retention
and gloss retention as 70 per cent 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. That is the reason for their superior durability.
By contrast, molecules in SMP and super-polyester resins are based on carbon-hydrogen, carbon-oxygen and carbon-silicone bonds.
Because their molecular bonds are weaker, long-term exposure to ultraviolet light and environmental hazards eventually defeats the structural chemical integrity of the polyester coating, causing it to chalk or fade.
Pigments are colourants made from fine powders. There are three types of pigments – 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, such as high-end architectural applications.
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.
|Two rows of coloured, coated metal panels depict colour changes (fade) of eight E Hunter Units and five E Hunter Units. One E Hunter unit denotes the smallest degree of colour change visible to the naked eye.|
|PDF and various polyeser coatings – 20 years south Florida Exposure. Comparing the performance of a PVDF-coated metal panel (left) and four metal panels with different polyester coatings after 20 years of South Florida exposure. Panel photo courtesy of Arkema Corp.|
|Duranar 2-coat vs XL – 5 yeras industrial environment. The differences in fade after five years. The left side of each panel has been treated with a clear topcoat as part of a three-coat system.|
|Three coat PVDF: 20 years south florida exposure. while the right side of each panel has been left unprotected with a conventional two-coat system and no clear coat.|
|Two coat PVDF: 20 years south florida exposure. Demonstrating the long-term performance of PVDF coatings treated with a three-coat system with a clear coat after 20 years of South Florida exposure testing.|
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, it has the potential to chalk or fade prematurely. Conversely, durable resins combined with strong pigment systems will deliver long-lasting performance. In short, a coatings formulation cannot be strong if any of its individual components are weak. High-quality raw materials are essential to the long-term durability and performance of any coating.
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 per cent PVDF coatings. This practice is misleading for two reasons. First, as demonstrated in the previous section, SMP and super-polyester coatings do not have the chemical structure needed to sustain the long-term performance of 70 per cent PVDF coatings.
Second, and perhaps even more critical, SMP and super-polyester coatings do not have long- term weather exposure data equal to 70 per cent PVDF coatings, which have a 45-year history of proven durability on buildings throughout the world. Since 70 per cent 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. New SMP and super-polyester coatings are introduced every four to five years. As a result, these products have no more than 10 to 15 years of performance data in South Florida exposure testing or actual building performance. Despite the lack of requisite testing, many of these coatings are warranteed at terms comparable to 70 per cent PVDF coatings.
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. New polyester technologies may, in fact, be superior to the earlier polyester formulations, but they cannot achieve the long-term performance of 70 per cent PVDF coatings because, ultimately, the molecular structure of the base polyester resins will cause them to fail.
Coatings specifiers also should examine if and how their warranty distinguishes between vertical and non-vertical surfaces. Many warranties cover vertical and non-vertical surfaces differently because non-vertical surfaces such as roofing are much more susceptible to failure than vertical exposures. Seventy-per cent PVDF coatings purchased from proven, reputable coil coatings manufacturers will provide equal chalk and fade coverage for vertical and non-vertical surfaces.
Chalk and fade
As explained earlier, chalk and fade result directly from the chemical breakdown of a coating’s base resins and pigments, which ultimately appear as a visible loss of colour 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 one (extremely poor) to 10 (perfect), as depicted in the sidebar on page 15.
Fade is the loss of colour calculated in Hunter E units in accordance with ASTM D2244-02 procedures. One E unit denotes the smallest degree of colour 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 E Hunter units in a fade measurement, can mean the difference between a coating that maintains its original appearance for 30, 40 or 50 years and one that looks old much sooner.
PVDF versus polyester
After 40 years of UV exposure, PVDF coatings and polyester coatings will react differently in exterior environments. As the pictures below indicate, in the initial five years of exposure, both types of coatings will have a slight shift in colour fade and chalk values, yet, over time, the differences in their composition will become clearer.
Although PVDF-quality coatings will have a very gradual decline in colour fade and chalk over a 40-year period, polyester coatings will 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 chart illustrates the relative strengths of four commonly specified types of architectural coatings.
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. In most circumstances, the clear coat is applied to protect the metallic flake from tarnishing and changing colour 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.
Resistance to UV exposure also is enhanced with a clear coat, as demonstrated by the two photos below, allowing manufacturers to provide improved warranties for chalk and fade.
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