Understanding the coatings best suited for metal panel projects involves considering critical factors related to 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 help inform decision-making when selecting coatings for upcoming metal panel projects.

SMP-based coatings

SMP is a silicone-modified or siliconized polyester coating system for application in metal building products. SMP resin systems are typically blended with high-quality pigments, UV additives, and a proprietary resin blend to achieve excellent colour and gloss retention, as well as excellent weather resistance. With extensive research and performance testing, SMP-based coatings have been formulated and refined over decades, and today’s SMP coatings are much closer to the durability of PVDF. SMP offers more gloss options and textured finishes than PVDF coating systems, and it shows slightly different colour fade on medium- and dark-coloured surfaces. The coating can begin to chalk and fade slightly more quickly than PVDF coatings, especially when bright or dark colours are used in extreme conditions. However, lighter colours are inherently better at deflecting UV rays, making SMP a viable option. Once cured, SMP is tougher than PVDF and offers excellent scratch resistance, helping reduce corrosion of exposed metal over the life of the building.

PVDF-based coatings

PVDF is a very stable fluoropolymer used in high-performance, highly durable exterior paint finishes. It was discovered more than 60 years ago by DuPont scientists, who created Teflon, 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 polymers, and the molecule’s spiral shape is structurally very stable. These thermoplastic polymers soften upon heating and harden upon cooling. This process is repeatable, making them flexible during coil and extrusion manufacturing. The unique molecular structure of these polymers accounts for 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 for any given environment, it is worth considering the Fenestration and Glazing Industry Alliance (FGIA) performance ratings, which use three AAMA specification codes. These specifications, AAMA 2603, 2604, and 2605, use a combination of accelerated and real-world environmental testing to evaluate coating performance. While tested in the United States, these standards apply across North America.

AAMA 2603, Voluntary Specification, Performance Requirements and Test Procedures for Pigmented Organic Coatings on Aluminum Extrusions and Panels, is primarily used for interior facades, retail point-of-sale areas, or commercial storefronts in shopping malls. Typically, high-solid polyester and acrylic coatings meet the requirements of AAMA 2603. They are not exposed to harsh weather conditions, so the need for exceptional performance in outdoor weather is unnecessary.

The AAMA 2604, Voluntary Specification, Performance Requirements and Test Procedures for High Performance Organic Coatings on Aluminum Extrusions and Panels (with Coil Coating Appendix), 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 is equivalent to that of ordinary polyesters over one year. SMP coatings are often applied to storefronts, windows, doors, and low-rise curtain walls. Although the specification is for five years, they are usually accompanied by a 10-year guarantee.

The toughest specification, AAMA 2605, Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels (with Coil Coating Appendix), requires 10-year South Florida testing for colour retention, chalk resistance, and gloss retention. AAMA 2605 is most commonly associated with the performance properties achieved with 70 per cent PVDF coatings. This makes PVDF coatings ideal for use in any environmental conditions to protect exterior curtain walls, facades, windows, and doors of buildings, stadiums, and high-value residences. These polymers provide long-lasting protection for buildings’ structural and esthetic qualities and often come with 20-year warranty coverage.

Selecting the right coating system

PVDF is particularly effective at providing long-lasting protection for vivid and dark colours. A choice of bright colours is restricted, though, if a two-coat rather than a three-coat system is used. The PVDF coating’s flexibility supports the construction 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; or climates with extreme heat, UV sunlight, and moisture or humidity.

Coating selection can vary by project. SMPs are a good substitute in environments such as parts of Canada where PVDF may not be needed. With recent advancements in technology, SMP coatings will perform well in these areas and other extreme environments, including coastal and mountainous regions with temperature swings, heavy rainfall, hail, and snow. Although these environments might be considered extreme, SMP’s toughness will withstand them well.

Both PVDF and SMP coatings are well-suited for metal roofing and metal cladding applications. Due to their lower prices and harder final finish, SMP coatings are often the preferred alternative for warehouses, industrial storage, agricultural structures, and other non-monumental commercial buildings. SMP coatings work well in all but the most extreme environments. PVDF coatings are often still primarily selected for coastal environments and areas with intense sunlight, as well as for heavy industrial areas with chemical pollutants.

Warranty coverage

A good warranty will cover the following areas:

• Film integrity—A quality coating system will maintain film integrity throughout the structure’s life. The film integrity aspect of a paint warranty ensures that the paint maintains adhesion to the substrate. The coating will not peel, flake, or otherwise lose adhesion. This is measured in time (years).
• Colour retention—A quality coating system will retain its original colour over the life of the structure. Colour fade results from chemical breakdown of a coating’s base resins and pigments, which appear as a visible loss of colour. Reported as a Delta E in Hunter units, colour retention signifies the maximum colour change (fade) over a specified number of years. In this case, a lower number is best.
• Washed versus unwashed panels—Apparent discolouration 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 throughout the structure’s life. Decomposition of the coating film caused by UV 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, Standard Test Methods for Evaluating the Degree of Chalking of Exterior Paint Films.

What is not covered

It is important to understand what is outside the scope of common coating warranties. Some common non-warrantable situations include, but are not limited to:

• Coating film breaches caused by installation scratches, abrasions, or hard impacts.
• Damage caused by moisture entrapment, including moisture trapped between bundles or during transit.
• Substrate corrosion, including failure or damage resulting from under-film corrosion or edge corrosion.
• Coatings that are not applied in accordance with specified application procedures and film thickness requirements.
• Mixing or combining different coating systems.
• Other situations that fall outside the conditions and requirements outlined in the coating warranty.

Author

Brent Fletcher is the regional product marketing manager for coil and extrusion coatings Americas at AkzoNobel.