What specific changes occur in the surface properties of aromatic polyurea coatings under long-term UV exposure?
Publish Time: 2026-03-03
Aromatic polyurea coatings, with their superior mechanical strength, exceptional abrasion resistance, and 100% solids content, hold a significant position in industrial corrosion protection, automotive chassis protection, and battery pack protection. However, as clearly stated in product descriptions, these materials are "suitable for applications where anti-yellowing properties are not required." The core reason for this limitation lies in the high sensitivity of the aromatic isocyanate components in their molecular structure to UV light. When aromatic polyurea coatings are exposed to direct sunlight for extended periods, their surface properties undergo a complex evolutionary process, from the breaking of microscopic chemical bonds to macroscopic physical deterioration. Understanding this process is crucial for proper material selection and maintenance.1. Color Transformation: The Photo-oxidation Process from Amber to Dark BrownThe most intuitive and significant change in aromatic polyurea coatings under long-term UV exposure is a dramatic change in color. Initially, the coating typically appears light yellow or amber. Upon contact with UV light, photon energy excites electrons on the aromatic rings, triggering a photo-oxidation reaction. Aromatic isocyanate groups readily undergo molecular rearrangement under ultraviolet (UV) radiation, forming quinone imine structures. These structures possess strong chromophore properties, causing the coating color to deepen rapidly. This color change does not linearly stop over time but accumulates. The coating surface gradually changes from an initial light yellow to a deep reddish-brown, eventually approaching blackish-brown.2. Chalking and Loss of Gloss: The Collapse of Surface MicrostructureBesides the dramatic color change, prolonged UV exposure also leads to severe chalking and loss of gloss on the coating surface. Under the continuous action of photo-oxidation, the urethane and urea bonds in the polyurea molecular chain break, and the long polymer chains degrade into low-molecular-weight fragments. These fragments lose their original cohesive force and can no longer adhere tightly to the coating surface, forming a fine powdery layer. Touching an aromatic polyurea surface exposed to UV radiation for an extended period reveals a noticeable roughness and powderiness. The originally smooth, dense, and highly glossy surface gradually becomes dull and lacks shine, exhibiting a matte texture similar to weathered rock.
