*2.2. Preparation of WPU-SS Emulsion*

WPU-SS was synthesized by three steps, in which acetone was replaced by DMAc to reduce the viscosity of polyurethane prepolymer. Table 1 lists the experimental formulation of different WPU samples. The molar ratio of the isocyanate group to hydroxyl group was kept at 1.4, and the ratio of PTMG to HEDS was 1/2 (WPU1), 1/1(WPU2) and 2/1(WPU3).

**Table 1.** Molar ratio formulations of different WPU samples.


According to the experimental formulation listed in Table 1, the synthesis process of WPU2 was illustrated. As shown in Figure 1, firstly, PTMG and DMPA were added in a 500-mL four-necked round-bottomed flask with a mechanical agitator, nitrogen inlet and drying condenser tube. DMAc was used as a solvent, and DBTDL was used as a catalyst. IPDI was slowly added after thoroughly stirring. Under mechanical stirring, the reaction temperature was 80 ◦C, when the content of −NCO reached the theoretical value, the polyurethane prepolymer terminated with −NCO was generated. Then, the temperature was reduced to 60 ◦C, HEDS containing −OH was added and reacted with isocyanate

under nitrogen protection until the content of −NCO in the system reached the theoretical value again.

**Figure 1.** The synthesis process of WPU2 (R1 is the repeating chain segment).

TEA with DMPA and other substances was added to neutralize −COOH in DMPA, and the system temperature was further reduced to 6~10 ◦C. The synthesized mixture was added to deionized water for high-speed dispersion for 1 h to obtain the final WPU2 emulsion. In the emulsification process, the residual −NCO of the prepolymer reacted with deionized water to form urea and biuret derivatives. The final WPU2 emulsion was light blue and the solid content was about 30 wt%. The WPU samples with PTMG and HEDS ratios of 1/2 and 2/1 were synthesized by the same method.
