Metal Roof Coatings: What to Look for When Choosing a Durable Topcoat

When architects and building owners are looking for specific performance characteristics in choosing a metal roof, they are inevitably looking for its coating to act as a continuous barrier, adhere tightly to the substrate, and not fade, peel or chip over its life. They expect it to be hard enough to resist incidental scratches from transportation, installation, and roof traffic and impact. They do not expect it to be so brittle as to crack during metal forming. They want it to resist damage from atmospheric chemicals and salt and not chalk, fade, lose gloss, or stain. In other words, they want the coating system to be durable, enhancing the life of the metal roof underneath and extending the guarantee that the product will perform effectively for many years.

There are dozens of tests that paints can undergo to determine their performance characteristics for durability (See National Coil Coating Association Tool Kit #8). There are falling sand erosion tests, acid tests, adhesion after impact and bending tests and acid rain and salt spray simulations.  Fade, chalk and gloss are the most important characteristics measured in weathering tests.

Paints are usually identified by a resin type such as polyester, urethane, latex, etc. Tradeoffs in creating these systems are made to meet different price points for different needs. But, paints should also be considered as complete systems, rather than as a simple addition of a better or cheaper part of the product.Metal roof coating colors

Let’s dig a little bit more deeply to see what makes the difference in a superior metal roof coating. There are several coil coating topcoats available but only three are used primarily for coil coating exterior metal like that which is found on a standing seam metal roof system. Polyesters have traditionally occupied the low end of the market. They offer a hard, scratch-resistant finish and a wide range of gloss. But polyesters are prone to chalking when exposed to UV or sunlight. Since their introduction in the 1960’s, polyesters have been greatly improved with higher molecular weights and longer polymeric chains that create stronger, more UV-resistant bonds.

Silicone-modified polyesters (SMP), also known as silicone-protected and siliconized polyesters, use polymerized silicone to improve polyester chalk performance and gloss retention. But one disadvantage of silicone polyesters is lower flexibility due to the highly crosslinked and branched structure of these polymers. Polyvinylidene fluoride (PVDF)  is acknowledged as the premium resin for coil coatings and is especially effective on metal roof systems. PVDF resin has superior chalk resistance and gloss retention as well as stain and chemical resistance.  It is softer than polyester and SMP, making it highly formable without risk of cracking. However, it requires some special handling during transport or installation because the product can scratch a little more easily than polyesters and silicone-modified polyesters.

PVDF is manufactured as a powder. Paint makers mix it with acrylic copolymer resins for coil coating.  Years of testing on metal roofs and in test labs have shown PVDF to be most durable when it makes up 70% of the resin.  Higher concentrations do not coat well. Several paint companies have 50% PVDF formulations.  They are less costly than the 70% versions, but the performance drops off considerably.

Various colored metal roofing rollsPVDF-based systems are more expensive than many competing systems, however, when specifying metal roofing projects in which long-term weather resistance and aesthetic appeal are critical, PVDF-based systems represent the most cost-effective alternative.
Beyond polyesters, SMP’s, and PVDF’s, one other coil coating system shows up on metal roofs.  Plastisols are thick, soft coatings that have abrasion resistance generally superior to PVDF and comparable chemical resistance.  However, they have generally poor chalk and fade performance, particularly in high UV areas. Yet they are as expensive as PVDF.  In the United States, plastisols are primarily specified for highly corrosive environments such as chemical plants and animal confinement operations.

Coating technology is constantly evolving to provide better and more environmentally sound performance characteristics. To determine the best metal roof coating system for an individual project, contact your metal roofing expert at www.englertinc.com.

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