Waterborne polyurethanes with novel chain extenders bearing multiple sulfonate groups

The anionic chain extenders used in waterborne polyurethane (WPU) generally contain one anionic ion per extender. Increasing its content in WPU makes the WPU emulsion more stable, however also makes the WPU film more water sensitive, which is a dilemma needed to be solved. Herein, a novel class of anionic chain extenders bearing two or four sulfonate groups is rationally designed. The ion strength of chain extender is thus enhanced and the distribution of ionic charges on the resulting WPU is also altered. The relationship between the structure of the chain extenders and the properties of WPU is studied systematically. The results show that the critical percentage of the anionic extender needed to for stable WPU emulsion is reduced in a nonproportional way with the increase of the number of anionic groups per extender. For example, while the critical percentage of the chain extender for stable emulsion formation for aliphatic-type chain extender containing two sulfonates is 6 mol%, that for chain extender containing four sulfonates is only 2 mol%. Hence the overall hydrophilic segments on the WPU backbone is greatly decreased, which make the WPU more water resistant without sacrificing the emulsification capability of WPU. Among them, the threshold to form stable emulsion for WPU made from the ether-type chain extender containing four sulfonates is as low as 1.2 mol%, and the water absorption of the resultant film is only 3.5 %. The resulting WPU films show strong adhesion to various surfances and excellent resistance toward abrasion. This provides a new strategy to balance the emulsification capability and water-resistence property of WPU.