Multi-color photochromic waterborne polyurethane with high color fastness from hydroxyl-functionalized trichromatic spiropyrans

With advancements in smart materials, photochromic polyurethane (PU) shows considerable promise for applications in anti-counterfeiting, optical storage, and colorimetric sensing. However, significant challenges remain, such as reliance on solvent-based PU, poor color fastness due to blending methods, and limited color options, hampering its broader application. In response, this study selected trichromatic spiropyrans (SPs, SP-R , SP-Y , and SP-B ) from preparing 14 hydroxyl-containing SPs. Under UV irradiation, SP-R , SP-Y , and SP-B can colorless transform to red, yellow, and blue, respectively. The photochromic waterborne polyurethanes (WPUs) were synthesized by incorporating trichromatic SPs as end-capping agents into WPU chains (denoted as CWPU-R/Y/B), exhibiting excellent photochromic performance. Unlike simple blending, the CWPU-R/Y/B process superior photochromic fatigue resistance (50 cycles), emulsion stability (6-month storage), solvent resistance, and color fastness (achieving the highest grade of 5), due to the covalent bonding effect. Furthermore, CWPU films with various colors were achieved through simple color mixing, providing a broader spectrum to meet diverse application needs. The potential application of CWPU in the anti-counterfeiting field was preliminarily explored, showing the gratifying results. This study presents a novel material applicable to the next generation of smart packaging, electronic information, and coatings.