Crosslinking through acyl hydrazone formation by reacting water soluble polyurethanes derived from ketone diol comonomers and those containing hydrazide pendant groups

Fully water-based formulations are gaining traction with the sustainability initiative for various applications. In this regard, producing the components required for making these formulations by eliminating toxic reagents is highly desirable. Hydrazone formation is widely used as a non-hazardous curing process in coating applications. Generally, carbonyl compounds that are used in the hydrazone forming reaction are nearly exclusively derived from vinyl polymers. This article describes a facile process for making carbonyl group containing diols and keto group containing water-soluble polymers therefrom by step growth or condensation polymerization. A water-soluble polymeric hydrazide was prepared by DCC coupling reaction between hydrazine hydrate and a polyurethane bearing pendant carboxylic acid group. The aqueous solutions of keto group containing, and hydrazide pendant polymers were mixed and coated on glass slides. Subsequently, the acyl hydrazone formation was studied under static heating in a hot air oven and a dynamic condition provided by blowing air under ambient conditions with a hair dryer. The dynamic nature of the latter process more than compensated the absence of direct heating to induce drying and hydrazone formation sequentially as studied by FT-IR spectroscopy. The cured slides were stable to water and were sensitive to weakly acidic aqueous solutions (about pH 3) formed by dissolving citric acid in water. The keto group containing diols enabled the hydrazone formation in polymer pairs entirely derived from step growth polymerization. As step growth or condensation polymers potentially offer both degradability and recycling through the cleavage of labile bonds prevailing in the polymer backbone, these keto group containing diols are preferable alternatives to keto group containing vinyl monomers commonly used for making hydrazones.