Polyvinylidene fluoride (PVDF)-based topcoats have long been the choice of architects and specifiers who are seeking superior weatherability.1,2,3 Traditionally, these solventborne coatings have typically been used as factory-applied topcoats for metal (coil-coatings) and can pass AAMA 2605 specifications, the highest standard available for organic coatings on architectural aluminum.4 The PVDF resin is dispersed in a latent solvent (VOCs ranging from 500-700 g/L) along with an acrylic copolymer, selected for miscibility, that ultimately leads to an excellent film upon baking above the PVDF melting point (170 °C).5 Panels prepared in this way have exhibited excellent color retention for over 50 years in South Florida.
Outdoor weathering provides exposure to ultraviolet (UV) radiation, which prompts photooxidative degradation, breaking the chemical bonds in a polymer resin backbone. The polymer resin, which initially encapsulates the pigment, will deteriorate, creating a rough surface, manifesting in chalking and gloss loss. With the pigment now exposed and no longer protected, it can undergo deterioration, resulting in color fade. There are multiple tools to photostabilize the polymers such as UV absorbers or HALS. However, these small molecules can lose effectiveness or leach out of the coating over time. An alternate method is to select polymers that are less sensitive to photo-degradation, such as those that contain stronger atomic bonds, like the C–F bond. The innate structure of PVDF, repeating CH2–CF2 units, produces a photochemically inert resin.6 Because of this, the typical oxidative photodegradation pathway for other organic coatings are not entirely applicable to PVDF-based coatings. Typical degradation of coatings begin on the surface and then penetrate as the initial surface erodes away.7 The PVDF-based coatings are a homogenous blend of PVDF and miscible acrylic. Degradation of these coatings will occur almost exclusively on the acrylic portion. Over time, the acrylic will erode (by evidence of physical chalking), leading to an enriched PVDF surface (Figure 1).