Improving Corrosion Resistance and Flexibility of Coil Coatings Using PVF Technology (2023)

Polyvinyl fluoride (PVF) is a highly stable and durable polymer used in a variety of demanding applications requiring the highest levels of chemical, weathering and hydrolytic stability. Originally commercialized by DuPont under the Tedlar® trade name, PVF has been used for over 60 years in a variety of applications requiring the highest level of surface protection in the harshest environments. Tedlar® PVF can be formulated as films and coatings. One of the most important applications of PVF films is in construction, where it provides durable aesthetics and barrier properties when laminated to the surface of materials used for building exteriors and interiors (figure 1). The film enables the user to ensure that the aesthetic appearance of the surface remains like new for decades, despite severe exposure to UV rays, rain, wind, humidity, graffiti, chemicals, cleaners and disinfectants, bird droppings, pollution and various other environmental conditions pressure source. The secret to top-notch durability is polyvinyl fluoride resin, a fluorinated polymer with a single repeating fluorine unit in the backbone. This single carbon-fluorine bond strengthens the chemical bonds throughout the polymer, making it extremely stable and resistant to attack from various chemical and energetic stressors. The polymer resin does not absorb UV light, is inert to acids and bases, and is insoluble in any known solvents at room temperature. It is a durable matrix that keeps pigments in a stable structure, imparting long-lasting color and appearance that changes little over time. It is also naturally flexible and can withstand a variety of hot and cold forming operations and other mechanical stresses without cracking or breaking and exposing the underlying substrate.

Coatings made from PVF bring the same benefits as films in a more versatile form that can be tailored to specific architectural design needs. Just like in the movies, the durable PVF polymer is the matrix that holds the pigments and other specialty ingredients, enabling a wide range of color and gloss aesthetics as well as custom features such as metallic looks or surface textures. PVF coatings are particularly suitable for coil or extrusion coating because the metal surface is suitable for the relatively high temperatures required to fully coalesce the PVF into a dense, durable film layer.

PVF coatings are unique in that they simultaneously offer superior chemical resistance, flexibility, hydrolysis and UV stability, which is unique to any current coil and extrusion coating resin such as PVDF, SMP, FEVE, PE or PU) cannot be achieved. Fluorinated backbone (figure 2) provides similar weatherability and UV stability to premium PVDF coatings, but differences in dielectric and crystallinity properties mean that it is more resistant to acids, bases and solvents than any other coating material, and the pure polymer can be extended and easily used It is more flexible without cracking or breaking.

Chemical resistance testing can be used to demonstrate the extreme durability of PVF coatings and their superior ability to protect the substrate. Immersing PVF-coated panels in strong acids, alkalis, oxidizers and disinfectants can accelerate degradation in real harsh environmental scenarios, such as exposure to acid rain, inside chemical plants or hospitals. life data (Table 1) shows that PVF-coated panels have higher chemical resistance compared to existing high-quality PVDF-coated panels (image 3）。

table1PVF and PVDF coatings provide long life in a variety of harsh chemicals

The simple nature of the semi-crystalline molecular structure makes PVF coatings flexible even when 100% pure. PVF can be molded without any additives or plasticizers. PVF coatings can be bent after application, even to the very harsh "0T" radius of curvature, without damaging or cracking the surface (Figure 4). This is important so that the coating retains its barrier properties and extreme chemical and environmental resistance on formed boards. However, PVDF typically must be blended with acrylic or other additives to achieve the desired flexibility, diluting the fluoropolymer resin often at the expense of its environmental performance.

Discussion of coating technologies also requires an examination of regulatory matters affecting this market. Many state, national and international agencies have and continue to enact legislation targeting perfluoroalkyl and polyfluoroalkyl substances (PFAS). Because of the large number of chemicals that could fall within the scope of any new legislation, it is important to understand how PFAS are regulated. Polyvinyl fluoride contains only one fluorine molecule attached to the carbon backbone of the polymer and does not contain any fully fluorinated methyl or methylene carbon atoms. Therefore, polyvinyl fluoride does not meet the chemical structure definitions of PFAS published by several US states, the Organization for Economic Co-operation and Development (OECD), the European Chemicals Agency (ECHA), and the US Environmental Protection Agency (EPA). Additionally, no PFAS are used in the manufacture of PVF. In fact, the European Chemicals Agency (ECHA) clearly identified polyvinyl fluoride as a viable alternative to other PFAS polymers in its PFAS restriction proposal and the University of Michigan's Graham Institute for Sustainability in a variety of industries, including Paints and Coatings Market.

While chemical resistance and flexibility are the main advantages of PVF coatings, there are no compromises when it comes to weather and UV resistance. Just like PVDF and other fluoropolymer resins, the high electronegativity of the carbon-fluorine bond stabilizes the polymer chain, allowing it to withstand the photon energy imparted by sunlight. While PVF is transparent to UV rays in the solar spectrum, pigments and additives can be incorporated into PVF coatings to safely reflect or absorb and dissipate UV energy and protect the underlying material from UV damage.

Coatings made from PVF are formulated and applied in a similar manner to PVDF coatings. PVF resin is a key ingredient in paint formulations, accounting for about 25% by weight of the paint mixture. Various pigments typically account for up to 15% by weight of paint formulations and are used to add color and visual character to PVF paints. Proper selection of pigments and additives is critical to coating durability, including adhesion to the substrate as well as color and gloss retention. Organic pigments give PVF coatings more vibrant colors than inorganic pigments, but they may not be as stable under UV light as inorganic pigments. Inorganic pigments generally provide the highest level of stability. Solvents, typically about 55% by weight of the formulation, act as a vehicle to transport the coating solids to the substrate, allowing it to form a thin, uniform, and even wet film on the substrate. The choice of solvent is critical for PVF coating, because PVF resin is insoluble in any known solvents at room temperature and can only coalesce in certain solvents to form a uniform coating. Propylene carbonate is the most commonly used solvent for PVF coatings. Small amounts of additives are used to evenly disperse, regulate flow and smoothness, and enhance coating adhesion, color, gloss, and other performance characteristics.

PVF coatings can be applied to metal surfaces by coil coating or spraying. In coil coating, a liquid dispersion is applied to the flat metal of a coil using a roller coating process and cured and dried at a peak metal temperature of about 250 °C (480 °F). In spray coating, a liquid dispersion is sprayed onto the three-dimensional part and cured at about the same metal temperature. In both methods, the coating process usually involves cleaning and chemical pretreatment of the metal surface, applying a primer to impart adhesion and additional corrosion resistance, and finally applying a PVF topcoat.

Finished PVF coatings are currently being used in many different projects and applications around the world. Some examples include the very famous new site of the World Laureate Forum in Lingang, Shanghai. Scientists from around the world who have won the Nobel Peace Prize or the Wolf Prize are invited to attend the annual meeting held in this building. Tedlar® PVF coating was used on the aluminum at the building cornice and entrance. Due to the site's proximity to the sea, a PVF coating was chosen to ensure corrosion resistance from sea salt spray. The anti-fouling and UV-resistant properties of the PVF coating also mean that the building will not fade in the sun for decades and will look clean as new for the rest of its lifespan.

Another high-profile PVF coating project is the renovation of the Shanghai Stadium. It is the third largest stadium in China and hosted many important events including football and soccer during the 2008 Summer Olympics. The stadium was originally built in 1997 but has been remodeled ahead of the FIFA World Cup in 2021. Tedlar® PVF coating was chosen to protect new aluminum siding and cladding due to its durability and cleanability. Aluminum honeycomb panels are highly resistant to UV damage, and the paint's stain-resistant properties ensure clean, fresh looks for years to come.

The third project in the city of Zhongshan involves an aluminum curtain wall protected with PVF coatings, installed on a large office building in the city centre. Not only is the construction built to withstand the elements and resist fading, it also easily removes graffiti and quickly removes grime, smog buildup, or other damaging effects of the city.

PVF coatings have been used in commercial and public spaces (like these examples), but also in corrosive industrial facilities. These coatings often come with extended warranties of up to 50 years. This coating technology has been tried and tested throughout the long history of PVF films, providing users with the same trusted PVF performance, color and application flexibility. PVF coatings are available from stock and offer truly innovative solutions to the coil coating industry.