A new method to synthesize high solid content waterborne polyurethanes by strict control of bimodal particle size distribution

•A novel method was developed to synthesize WPUs with bimodal PSD.•The bimodal PSD of WPUs can be strictly controlled including the diameter ratio and volume ratio.•WPUs with bimodal PSD will effectively increase the packing efficiency of particles.•High solid content (55%) and low viscosity (489.1 mPa s−1) WPU was finally obtained.

A new method named two-step emulsification process was developed to synthesize high solid content waterborne polyurethanes by strict control of the bimodal particle size distribution. In the first step, a series of 40% solid content polyester-based (WPU-1) with low content of hydrophilic group and large particle size were firstly synthesized. In the second step, polyether-based prepolymers (WPU-2 prepolymers) with high content of hydrophilic group were firstly prepared and WPU-1 emulsions were used to emulsify WPU-2 prepolymers to obtain the final emulsions with high solid content (WPU-3). The particle size of WPU-3 present bimodal distribution and the diameter ratio and volume percentage of large particles to small particles in WPU-3 were able to be strictly controlled by this method. The viscosity of WPU-3 with 55% solid content was only 489.1 mPa s−1 when the diameter ratio of large particles to small particles was 9.2 and the volume percentage of large particles was 74%.

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