UV Ageing Behavior of Chinese Lacquer Coatings on 3D-Printed PLA Substrates

Chinese lacquerware is valued for its distinctive gloss, hardness, and durability. In this study, three layers of natural lacquer were applied to 3D-printed PLA substrates and exposed to UVA-340 accelerated aging for 25 days. The lacquer film gradually became lighter in color, with the lightness value increasing from 30.69 to 44.69. At the same time, gloss decreased from 59.37 to 48.28 GU, while surface roughness increased significantly, with Ra rising from 2.11 to 10.07 μm. Pencil hardness declined from H to 5B, indicating a reduction in surface strength. FTIR results showed partial oxidation of phenolic hydroxyl groups, whereas the aromatic backbone and aliphatic side chains remained largely unchanged. These results suggest that UV aging mainly causes surface photo-oxidation, leading to fading, gloss loss, roughening, and reduced durability of the lacquer coating. SEM images showed that the lacquer surface changed gradually during UV exposure. In the first few days of aging, small cracks started to appear on the surface, along with a bit of powdering. As UV exposure continued, the cracks gradually became larger and began to spread. By the final stage, many of them linked up into a network, but the overall damage slowed down compared to earlier stages. Overall, the process moved through a quick initial change, then a period of crack growth, and finally a more stable phase. These results help make it clearer how UV light affects lacquer coatings on polymer-based materials.