*2.3. Preparation of WPU Films*

The predetermined amount of WPU-SS emulsion was poured into the horizontally placed PTFE mold with the size of 10 cm × 10 cm × 1.5 cm. After drying at room temperature for 48 h, most of the water evaporated. Then, the film was stripped from the mold, and the water and solvent were completely removed within 24 h in a vacuum drying oven at 60 ◦C to obtain a constant weight WPU-SS film. The prepared WPU-SS films were stored in the dryer containing silica gel before further characterization.

#### *2.4. Measurement of Isocyanate Content by Dibutyl-Amine Method*

	- a. Preparation of 0.1 mol/L hydrochloric acid standard solution and calibration with anhydrous sodium carbonate.
	- b. Configure 0.1-mol/L di-n-butylamine-acetone solution: 12.9-g di-n-butylamine was placed in a 1000-mL volumetric flask, diluted with acetone and shaken.

$$-\text{NCO}\,\%=[(V\_0 - V)\times\text{c}\times\text{4.202}]/\text{m}\times100\% \tag{1}$$

Thereinto, c is HCL concentration (mol/L), *V* is the sample consumed HCL volume (mL), *V*<sup>0</sup> is blank consumed HCL volume (mL) and m is Sample mass (g).
