Diisocyanate weight calculations

Polyurethane networks were prepared from polyoxypropylene (POP) triols(Union Carbide Niax Polyols) after removal of water by azeotropic distillation with benzene. For Niax LHT 240, the number-average molecular weight determined by VPO was 710 and the number-average functionality fn, calculated from Mjj and the content of OH groupSj determined by using excess phenyl isocyanate and titration of unreacted phenyl isocyanate with dibutylamine, was 2.78 the content of residual water was 0.02 wt.-%. For the Niax LG-56, 1 =2630, fn=2.78, and the content of H2O was 0.02wt.-%. The triols were reacted with recrystallized 4,4"-diphenylmethane diisocyanate in the presence of 0.002 wt.-% dibutyltin dilaurate under exclusion of moisture at 80 C for 7 days. The molar ratio r0H = [OH]/ [NCO] varied between 1.0 and 1.8. For dry samples, the stress-strain dependences were measured at 60 C in nitrogen atmosphere. The relaxation was sufficiently fast and no extrapolation to infinite time was necessary. 

The desired weight of diisocyanate is added to the flask plus any ingredients to either adjust the acidity or catalyze the reactions. See Appendix 5 for the required calculations. The diisocyanate is heated to the start temperature with gentle agitation. It is best to provide a complete blanket of nitrogen in the reaction setup from this point on. 

There are two main methods to calculate the weight of the polyol and diisocyanate to use in a reaction ... 

Hexamethylene diisocyanate was supplied as pure by W. Hopkins of this department (71), having a density of 1.045, and an average equivalent weight of 83.7 0.5 (theoretical equivalent 84.09). The weights of isocyanate used were calculated on the analysis of the particular sample. 

Figure 9. Plot of heat of fusion vs. weight fraction of hard segment for each random copolymer, assuming k > 1 or k > 6, where k is the number of diisocyanate (hard-segment) units betwen two consecutive macrodiol (soft-segment) units. (Peebles (30) calculation of hard, block-length distribution in segmented polyurethane block copolymer is applied.)...

Equation 3.8 for EW is very convenient for practical use because it does not depend on the functionality, which is very difficult to determine. The EW of an oligo-polyol is very useful for the required isocyanate quantity calculation. One equivalent weight of an oligo-polyol reacts with one equivalent weight of the diisocyanate (the MW of the isocyanate divided by the number of -NCO groups). The EW of polyols with the same OH are identical, irrespective of the functionality. Thus a diol with an OH of 56.1 mg KOH/g (MW = 2000) and a triol of the same OH (MW = 3000), have the same EW of 1000. 

For theoretical purposes a chemical reaction is only considered to occur between Class A and Class B and when materials of Class C are present they are included in the calculation as a chemically inert series of components. In practice these inert components usually contain absorbed moisture or small traces of other active components which may react to a limited extent with the diisocyanate. Hence some additional diisocyanate may have to be included in a practical formulation to counteract this diluent effect on ideal stoichiometry. Determining the equivalent weights for chain extenders (crosslinker and diisocyanates) is performed as follows ... 

---