By Letitia Luu, Evonik Corporation, USA
The mounting pressure to reduce VOC emissions in industrial coating systems has led to increased demand and new developments of high-solids coatings within the cool-roof market. These coatings allow for greater solar reflectance, keeping residential and commercial buildings from overheating. Organofunctional alkoxysilanes are widely used additives in industrial coatings as they form a bridge between organic coatings and inorganic substrates. While most alkoxysilane monomers liberate VOCs in the range of 450–600 g/L, new alkoxysilane oligomers offer even better adhesion properties for coatings while emitting lower VOCs (<350 g/L). This work will demonstrate the significant improvement to adhesion for difficult roofing membranes such as EPDM, PVDF, and SPF, and to mechanical properties of high-solids alkoxy-cured silicone roof coatings with the addition of low-VOC silane oligomers. As the market for these coatings continues to grow, alkoxysilane additives will play an important role in achieving better performance and increasing their longevity.
The cool-roof market has been rapidly expanding alongside advancements in sustainable roof coatings technology. In addition to functionally cool roofs that provide greater solar reflectance, these roof coatings need to maintain low volatile organic compounds (VOCs) and excellent performance properties. Although the current market utilizes oxime-cured silicone roof coatings, high-solids alkoxy-cured silicone roof coatings can provide equally good performance while eliminating toxic methyl ethyl ketoxime as a byproduct. The work covered in this article will demonstrate that with the use of organofunctional silane additives, several crucial performance characteristics of alkoxy-cured silicone roof coatings can be improved.
The mechanism behind an organofunctional silane adhering to a roofing membrane surface is an important process to understand. Organofunctional silanes contain an alkoxy functional group (Si-OR) that can be hydrolyzed to bond with inorganic surfaces and an organofunctional group that can react with organic systems such as silicone resins. The reactions of the alkoxy functional groups and organofunctional groups allow these silanes to act as an adhesion promoter between inorganic and organic materials. These reactions are moisture driven and the alkoxy sites must first undergo hydrolysis to form silanol groups. The silanol groups then react with the hydroxyl groups on the inorganic surface of the roofing membrane to create siloxane bonds, which provide strong adhesion properties (Figure 1). As well as bonding with the substrate’s inorganic surface, the silanol groups can self-condense to create additional siloxane bonds, increasing crosslink density and improving mechanical properties of the coating (Figure 2).
Several key coating properties will be investigated in accordance with ASTM D6694, the standard for liquid-applied silicone roof coatings used by roofing manufacturers and contractors. These properties include surface hydrophobicity, water-ponding resistance, elongation, tensile strength, and dry and wet adhesion to various roofing membranes.