Development of clean performance-tunable waterborne polyurethane using acetyl tributyl citrate for transferable holographic films

Scalable and transparent polyether-based waterborne polyurethane (WPU) thin films that adhere firmly to Al nanofilm but not to PET film without release agents have been designed and prepared. In order to realize cleaner production, eco-friendly acetyl tributyl citrate (ATBC) instead of toxic additives like N-methylpyrrolidone was introduced into polymerization system as both an effective dispersant of dimethylolpropionic acid crystal as hydrophilic chain extender and a viscosity thinner of the synthesized precursors to promote homogeneous hydrophilic extension reaction in the absence of toxic catalysts. The appropriate ATBC and dimethylolpropionic acid contents are critical to formation of stable translucent dispersion of the WPU with total hard segments as high as 56 wt% at –NCO/-OH molar ratio of 1.05. Molecular structure and ATBC existence of the WPU film have been characterized by IR, NMR, gas chromatography, and thermogravimetry, suggesting that ATBC will not release into the environment even if at 140 °C because of its high boiling point and ultralow volatility. The reserved ATBC in the WPU film also acts as an effective plasticizer and lubricant so that the WPU film can be quickly peeled off from PET. The lowest T-peel strength of 1.3 N m−1 on PET is achieved for the WPU film with a high tensile strength of 15.8 MPa and a medium elongation at break of 237%. The WPU films have tunable stickiness onto PET film from sticky to completely nonsticky. Importantly, the WPU films offer simultaneously strong adhesion to Al nanofilm and high thermal stability but high peelability from the PET, allowing to fabricate a stable holographic/photolithographic composite film by Al deposition at >1000 °C and ensure high flatness and luster of the composite film. Optimal formulations of WPU materials with high comprehensive performance including good film-processability, ultralow VOC emission, high mechanical and thermal properties, controllable surface tension, and tunable adhesion to Al nanofilm have been proposed for transferable holographic films.