Definition of the NCO value
The value of isocyanate group in chemical materials refers to the mass of the isocyanate (-NCO) group contained in a 100g specimen. The determination principle is: -NCO group and excess di-n-butylamine reaction to form urea, excess di-n-butylamine and then bromocresol green as an indicator, titrated with hydrochloric acid, so as to calculate the amount of di-n-n-butylamine consumed by the -NCO group, and then deduce the percentage content of the -NCO group in the analyte.
Instruments required for measuring the NCO value of coatings
Analytical balances, 250mL Erlenmeyer flasks, 50mL acid titration, 1000mL volumetric flasks, glass rods.
Drugs required for the measurement of NCO value of coatings Anhydrous toluene; isopropanol, 0.1mol/L di-n-butylamine-toluene solution (12.9g di-n-butylamine dissolved in toluene, transferred into a 1000mL volumetric flask, diluted with toluene to a scale, shaken well, stored in brown reagent bottles for use); 0.1% bromophenol blue indicator (0.1 g bromophenol blue dissolved in 100mL volume fraction of 20% ethanol); 0.1mol/L hydrochloric acid solution; deionized water.
· Measurement steps ·
1. Accurately weigh about 1g of the sample to be measured, and weigh the accuracy to 0.001g. Place the sample in a 250 mL Erlenmeyer flask.
2. Accurately transfer 40mL of di-n-butylamine-toluene solution with a concentration of 0.1mol/L with a pipette into the sample, shake it so that it mixes evenly, place it at room temperature for 20-30min, after the reaction is complete, add 40-50mL of isopropanol, wash the bottle mouth at the same time, terminate the reaction, and drop 2-3 drops of bromophenol blue ethanol solution indicator.
3. Titrate with 0.1mol/L hydrochloric acid standard solution, the solution changes from blue to yellow, and keeps the color unchanged for 1min, which is the end of the titration.
4. Write down the volume of the standard hydrochloric acid solution consumed, and do a blank test at the same time.
5. Calculation formula:NCO value = [(A-W)*4.2*C]/M formula, A: blank consumption of the volume mL; W: the volume of the sample consumption mL; C: hydrochloride ethanol solution concentration mol / L; M: the mass of the sample g; 42: NCO molecular weight; NCO weighing 1g.