Thermal-driven self-healing and green recyclable waterborne polyurethane films based on double reversible covalent bonds

Recently, the self-healing and recyclability of waterborne polyurethane (WPU) has attracted significant research attention. Herein, different weight ratios of Diels-Alder (DA) diol and diacetyl oxime (DMG) were used as the reactive monomer with polytetramethylene ether glycol (PTMG), isophorone diisocyanate (IPDI) and dimethylolpropionic acid (DMPA). A series of thermal-driven self-healing and green recyclable waterborne polyurethane films (WPU-DMGx-DAy) were prepared based on synergistic dynamic reactions of oxime-carbamate and DA bonds. We investigated the properties of WPU-DMGx-DAy emulsions by particle size and zeta potential tests, and the results show that these emulsions have excellent storage stability. The self-healing properties of WPU-DMGx-DAy films were identified by dynamic mechanical analysis. It was revealed that the WPU-DMG4-DA4 film has the best self-healing capability, and the self-healing efficiency can reach 92. ± 0.1 % after self-healing at 120 °C for 4 h. The mechanism of the self-healing behavior was studied by Fourier transform infrared spectroscopy, and the effective reversibility of the oxime-carbamate and DA bonds were verified. In addition, thermal gravimetric analysis and tensile test results showed that the thermal stability and mechanical properties of the WPU-DMGx-DAy films were significantly improved. More importantly, the WPU-DMG4-DA4 film can be dissolved in a mixed acetone/ethanol solvent at room temperature. The recycled polyurethane solution could be dispersed in water to obtain a WPU emulsion again. After being recycled three times, the recovery efficiency of the mechanical properties of the WPU-DMG4-DA4 film was higher than 90.1 ± 0.1 %, and the recycled film maintained its thermal stability and self-healing behavior. Moreover, the synthetic leather obtained by compounding WPU-DMG4-DA4 and non-woven fabrics could be easily recycled through this process. The basic properties of the recycled non-woven fabric remained unchanged and the recycled WPU-DMG4-DA4 resin could be reused. Therefore, the WPU-DMG4-DA4 resin has broad application prospects in the tanning industry.