Synthesis of self‐matting waterborne polyurethane coatings with excellent transmittance

Abstract The effect of structural factors, including the type of diisocyanates and glycols, degree of crosslinking, hard‐segment content, crystallinity and hydrophilic group content, on optical properties of waterborne polyurethane (WPU) was investigated. Based on the relationship between structure and properties, a series of self‐matting WPUs with excellent transmittance were synthesized. Results demonstrated that refractive index of constituent groups was an important factor to influence the optical properties of coatings. For multiphase polyurethane, aromatic ring, ester bond, NHCOO and urea groups with high refractive index could decrease the matting effect and transmittance. Higher hard‐segment content resulting in an increase of the number of NHCOO or urea bonds also decreased the matting effect and transmittance. The aggregation structure of WPU was another factor to influence the optical properties of coatings. Crystalline soft and hard segment domains could decrease the transmittance and matting effect, whilst non‐crystalline segments, pendent groups and crosslinking destroyed the regularity or crystallinity of macromolecules, behaving as an auxiliary function to improve the transmittance. The particle size of WPU was of great importance for the matting effect. Smaller particle size (<5.0 µm) would endow the coating with a smoother surface and higher reflected light ratio, and thus the expected matting effect was hard to be achieved. A larger particle size could improve the matting effect but impart to the coating a coarse surface and hand‐feel. When the 2,2‐bis(hydroxymethyl)propionic acid content was 1.6 wt%, spherical particles with a size distribution of 7–8 µm were aggregated on the coating surface, giving the coating an excellent matting effect and hand‐feel. © 2017 Society of Chemical Industry