Understanding the materials best suited for metal roofing projects involves considering critical elements of the coating’s performance and warranty. This article delves into the differences between silicone-modified polyester (SMP) and Polyvinylidene fluoride (PVDF) coatings, providing insights to make informed decisions.
SMP-based coatings
SMP is a silicone-modified or siliconized polyester coating system that is applied to metal building products. SMP resin systems are typically blended with high-quality pigments and UV additives to achieve excellent color and gloss retention and enhanced weather resistance. Higher silicone levels can improve the gloss retention characteristics of these coating systems. With extensive research and performance testing, SMP-based coatings have been formulated and improved over decades, and today’s coatings are much closer to the durability of PVDF. SMP offers more gloss options and textured finishes compared with PVDF coating systems, but differs slightly when comparing color fade in medium and dark shades. The coating can begin to chalk and fade slightly quicker than PVDF coatings, especially when bright or dark colors are used in extreme conditions. However, lighter colors are inherently better at deflecting UV rays, making SMP a viable option. Once cured, SMP is tougher than PVDF and has excellent scratch resistance, which can support reduced corrosion of exposed metal during the life of the building.
PVDF-based coatings
PVDF is a stable fluoropolymer used in high-performance, highly durable exterior paint finishes. It was discovered more than 60 years ago when DuPont scientists created Teflon, which is most commonly used in non-stick cookware. Fluoropolymers are the most chemically inert of all polymers. PVDF’s strength comes at the molecular level. Its carbon-fluorine bonds produce one of the strongest chemical bonds in chemistry, and the molecule’s spiral shape is structurally very stable. These thermoplastic polymers soften upon heating and are rigid again when cooled. This repeatable process makes them flexible during the coil and extrusion manufacturing process. The unique molecular structure of these polymers provides their well-known performance. PVDF remains stable in most chemical environments at high temperatures and is highly durable in extreme weather conditions.
AAMA specifications in extrusion coatings
To help decide which coating is suitable in any given environment, it is worth looking at the performance rating of the Fenestration and Glazing Industry Alliance (FGIA), which uses three AAMA specification codes. These specifications, AAMA 2603, 2604, and 2605, use a mix of accelerated and real-world testing in the harsh South Florida environment to evaluate coatings. While there are other local standards across the world, the AAMA specifications remain the leading one even outside of the United States.
AAMA 2603 is mainly used for interior façades, retail points of sale, or commercial storefronts in shopping malls. Typically, high-solid polyester and acrylic coatings will meet the needs of AAMA 2603 specifications. They are not exposed to harsh weather conditions, so the need for exceptional performance in outdoor weather is unnecessary. The AAMA 2604 performance specification aligns well with systems designed with SMP resins. These coatings can cost more than standard polyesters yet are five times more protective. Their weathering performance over five years equals one year for ordinary polyesters. SMP coatings are often applied to storefronts, windows, doors, and low-rise curtain walls. Although the ratings last for five years, they are usually supplied with a 10-year guarantee. The toughest specification (AAMA 2605) requires 10 years of South Florida testing for color retention, chalk resistance, and gloss retention. AAMA 2605 is most associated with the performance properties achieved with 70 percent PVDF coatings. This makes PVDF coatings ideal for use in any environmental conditions to protect exterior curtain walls, façades, windows, and doors of buildings, stadiums, and high-value residences. These polymers provide extended lasting protection for buildings’ structural and aesthetic qualities with warranties of 20 years for color and gloss on aluminum.
Selecting the right coating system
PVDF is perfect for long-lasting protection of vivid and dark colors. However, a choice of bright colors is restricted if a two-coat rather than a three-coat system is used. The PVDF coating’s flexibility supports the manufacturing of metal building components. However, this flexible coating can be relatively soft and more susceptible to scratching and marring than other systems. With white coatings, more metal marking occurs than with other, tougher coating systems. Careful handling in production helps avoid these issues. PVDFs are particularly suited for severe weather environments such as industrial sites with higher concentrations of acid rain and chemical pollutants; coastal areas with saltwater spray from the ocean; desert climates with the risk of wind and sand erosion; or climates with extreme heat, UV sunlight, and moisture or humidity (e.g., South Florida).
Coating selection can vary by project. From our experience, SMPs are a good substitute in more environments than most people would think. The Central and Northern U.S. and Canada are all examples of areas where PVDF may not be needed. With advancements in SMP coatings technology, they will perform well in these areas and other extreme areas, including coastal areas or mountainous regions with swings in temperature, heavy rainfall, hail, and snow. Although these environments might be considered extreme, SMP’s toughness will weather these conditions well.
Given exposure conditions in the United States, PVDF and SMP coatings are well-suited for metal roofing and metal sidewall applications. Because of more competitive prices and a harder final finish, SMP coatings are often the preferred alternative for warehouses, industrial storage, agricultural structures, and other commercial buildings. SMP coatings work well in all but the most extreme environments. PVDF coatings are often still primarily selected when sourcing materials for coastal environments and areas with intense sunlight, such as deserts or heavy industrial areas with chemical pollutants.
Selecting the correct finish for the building envelope
Selecting the correct type of finish for the building envelope requires careful evaluation of several factors, including whether it is vertical or horizontal, the kind of structure, and specific building components such as siding, window cladding, fascia, and roofing. Horizontal surfaces, including roofing and fascia, are typically more exposed to direct UV radiation and water accumulation, which places greater demands on the coating system. In these instances, PVDF coatings are often the preferred choice due to their superior performance in resisting color fade, chalking, and chemical degradation. On the other hand, vertical surfaces such as siding and wall cladding experience less environmental stress, and SMP coatings generally provide sufficient durability and aesthetics in these areas, especially in non-extreme climates. The structure type also plays a significant role in determining the appropriate coating system. Monumental buildings, including high-value commercial or institutional projects, often require the long-term durability, color stability, and premium finish characteristics that PVDF coatings provide. In contrast, residential, agricultural, and many light commercial buildings can be well served by SMP coatings, which offer a cost-effective alternative without significantly compromising performance in moderate conditions. SMP is particularly well-suited for inland or temperate climates with minimal exposure to salt spray, intense UV, or industrial pollutants.
Costs are another important consideration, with SMP coatings typically offering a significant advantage over PVDF systems. While PVDF may have a higher upfront investment, the long-term maintenance and aesthetic preservation savings may justify the cost in harsher climates or high-profile structures. Ultimately, coating selection should be based on a balance of performance requirements, environmental conditions, structural importance, and budget constraints. With proper application and handling, SMP and PVDF can provide long-lasting, attractive finishes for metal roofing and cladding systems across various building types.
Warranty coverage
A good warranty will cover the following areas:
- Film integrity: A quality coating system will maintain film integrity over the structure’s life. The film integrity aspect of a paint warranty ensures that the paint will maintain adhesion to the substrate. The coating will not peel, flake, or otherwise lose adhesion. This is reported in time (years).
- Color retention: A quality coating system will retain its original color over the structure’s life. Color fade results from the chemical breakdown of a coating’s base resins and pigments, which appear as a visible loss of color. Reported as a Delta E in Hunter units, color retention signifies the maximum color change (fade) over a specified number of years. In this case, a lower number is best.
- Washed vs. unwashed panels: Apparent discoloration of the coating may occur when it has been exposed to dirt-laden atmospheres for long periods. A good cleaning will generally restore the appearance of these coatings.
- Chalk resistance: A quality coating system will maintain chalk resistance over the structure’s life. Decomposition of the coating film caused by ultraviolet rays, moisture, or heat (or any combination of the three) results in chalk-like residue on the building’s surface. The residue is transferred to a soft fabric by rubbing the coated surface and assessed visually against photographic standards in accordance with ASTM D4214.
What is not covered
It is important to understand what is outside the scope of common coating warranties. Some everyday non-warrantable situations include, but are not limited to, breaches in the coating film caused by installation scratches, abrasions, or hard impact. Another example is caused by moisture entrapment (between bundles or during transit). Other situations include substrate corrosion, failure, or damage from under-film or edge corrosion. Coatings not applied per application and film thickness requirements and mixing of different coating systems are also not typically covered under coating warranties. Due to boundaries around coating warranties, it is critical to understand and select appropriate substrates and ensure coaters follow best practices for surface preparation, pretreatment, and coatings application.
Brent Fletcher is the product marketing manager, Americas, for AkzoNobel.
