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| Isocyanate titration |
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The free isocyanate content can easily be measured by titration of
the free isocyanate with an amine and back-titration of the
unreacted amine.
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R-NCO +R'-NH2 ® R-NHC=ONH-R' urea formation
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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.
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Run a blank and use the calculation below. In the blank all the
materials are charged as shown above, but no sample is charged.
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%NCO = 4.2 x [(ml HCl blank) - (ml HCl sample)]/sample weight
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Bromophenol indicator: 0.1 g bromophenol blue
add 1.5 ml of 1 N sodium hydroxide and dilute to 1000 ml with
distilled water.
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