Interview: Coil coatings in architecture

Comparing roof coatings with façade coatings, what are the differences?

Per-Erik Sundell: The same types of coil coatings are generally used for both roofs and façades and do not differ greatly in their basic formulation. Polyester/melamines, polyurethanes and polyvinyideneflouride coatings are the categories to choose from. Depending on geographical region and climate zones, however, different key properties are required and the highest demands are put on roof coatings. For instance, intense UV-radiation and harsh snow/ice conditions inflict considerably stresses on the coating. Mechanical toughness, UV-resistance and hydrolytic stability are the most critical properties that secure coating integrity and prevent gloss loss and color deviation as well as corrosion of the substrate. Aesthetically, the ability of staying clean is also of significance, even more so for façade coatings. Due to the excellent weatherability combined with the dirt-shedding, hydrophobic surface, PVDF is generally the best choice when there are high demands on appearance. So called, self-cleaning coatings where full wetting of a hydrophilic surface allows for rain to wash the whole surface clean is also on the market.

How do architectural trends drop into coatings development?

Per-Erik Sundell: Coil coated metal sheet is a product which provide architects with a multitude of design possibilities, e.g. a variety of shapes, colors, effects and textures is available. Meanwhile, a steady stream of newly developed functional coatings, such as heat reflecting, heat absorbing, self-cleaning, dirt-shedding is finding its way into exterior building applications. All these new functional coatings are the result of environmental aspects being an important and strong driver for coating development.

Research in the field of “green coating chemistry” has resulted in development and introduction of bio based coil coatings for exterior use, having same or even better performance profile than present coatings. A considerable part of the conventional solvents are replaced by bio renewable reactive diluents, i.e. alkyl esters of natural oils. The reactive diluent serves as a solvent during formulating, shipping and application of the paint until it enters the curing oven. Once there, the alkyl ester reacts with the polyester binder resin instead of being evaporated as conventional solvents, thus becoming a part of the final coating. Further research is focused on using bio based building blocks in polyester resins which further will increase the bio renewable content in coil coatings. Coatings, having biological matter as high as 30 wt% of dry film are presently evaluated on roofs in harsh and demanding climate.