Isocyanate titration
The free isocyanate content can easily be measured by titration of the free isocyanate with an amine and back-titration of the unreacted amine.
R-NCO +R'-NH     R-NHC=ONH-R'   urea formation
Weigh approximately 5 g of prepolymer accurately weight into a dried flask. Add with a pipet 20 ml of dibutylamine solution (338 g of dibutylamine diluted with dry toluene to 1000 ml.). Dilute with 50 ml of dry tetrahydrofuran (THF). Stir until homogenously and add 100 ml of anhydrous isopropanol. (IPA). Add 0.5 ml of a bromophenol blue indicator and titrate with 1.0 N hydrochloric acid (HCl). The endpoint is yellow.
Run a blank and use the calculation below. In the blank all the materials are charged as shown above, but no sample is charged.
%NCO = 4.2 x [(ml HCl blank) - (ml HCl sample)]/sample weight
Bromophenol indicator: 0.1 g bromophenol blue add 1.5 ml of 1 N sodium hydroxide and dilute to 1000 ml with distilled water.

Last edited on:

June 20, 2006

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