A Practical Approach to Determine the Excess of Amine Based Chain Extender in the Aqueous Polyurethane Medium

摘要

Aqueous polyurethane dispersion (PUD) have a widespread application in coating industry. These materials have been synthesized by various method for the industrial purposes. Over the years, research have focused on the synthesis of polyurethane-urea dispersion however there has been relatively little published research on the monitoring of polymerization process after chain extension. It is more particularly critical point at industrial scale. pH value measurement, NCO determination with dibutylamine back titration and FTIR measurements are known and widely used quick methods. They are limited techniques to determine remaining amine content to see chain extension efficiency because of the complex structure. In this work, the detection and estimation of remaining amine content is accomplished by reacting it with ninhydrin. Previous works on ninhydrin reactions with amino acids had no experimental evidence but after the ninhydrin reactions with amine and amides had been published , this mechanism have been used widely by biochemists. Nowadays, ninhydrin test is a common application of amino acids visualization. Chain extended polyurethane dispersions with one type of primary amine react with ninhydrin. In two steps, the ketohydrindylidene-diketohydrindamine (DYDA), Ruhemann's reddish-purple is appeared. Qualitatively, purple coloured product can be seen while the reference materials are not purple-reddish coloured. Quantitatively, UV absorbency of reddish-purple substance helps the calculation of intensity and determine the consumption of ninhydrin. By using this method, with changing the percentage of extension step, pH and film properties of PUDs are observed. As a consequence, it is useful to determine chain extension step efficiency and practical tool for avoiding excess amine yellowing problem/high pH. Keywords: excess amine, aqueous polyurethane dispersion.