Enhancing the colour depth of polyester fabrics via a low‐refractive‐index surface film: A synergistic two‐bath process using cationic fluorosilicone and polyurethane

Achieving high colour depth while reducing dye consumption poses a significant challenge in textile finishing. This study introduces a synergistic strategy to enhance the apparent colour depth of polyester fabrics by creating a low‐refractive‐index surface film, thereby improving the K/S value of black polyester fabrics. Two distinct dual‐bath application sequences were compared: Process II, which involves a polyurethane (PU) pre‐coating with 10 wt% PU followed by a 20 wt% cationic fluorosilicone (COFS) finish, outperformed both Process I (reverse sequence) and the single‐bath COFS method. Specifically, Process II achieved a 34.8% increase in K/S value, surpassing the 29.7% maximum enhancement observed in the single‐bath control. The treated fabrics demonstrated excellent colour fastness, rated at Grade 4 for washing (GB/T 3921‐2008) and Grade 4 for rubbing (GB/T 3920‐2008). Scanning electron microscope (SEM) analysis confirmed the formation of a smooth, uniform coating on the fibre surface, suggesting enhanced light‐trapping capability. Notably, the PU/COFS bilayer architecture enabled a significant reduction in dye usage compared to conventional methods, while maintaining a soft handle and superior durability. This work presents a viable approach towards achieving ultra‐deep‐coloured polyester textiles.