where n is the average number of repeat units. In the simplest terms, if n=0, the resulting structure is that of phenyl isocyanate and, if n=1, the structure of pure MDI is obtained. The formula in figure 1 leads eventually to a general expression for the number average molecular weight.
Furthermore, the corresponding isocyanate equivalent (IE) can be expressed by the following formula:
Similarly, the NCO content can also be easily calculated by the formula:
For phenyl isocyanate, where n=0, it follows that MW = 119, IE= 119, %NCO = 35.29 In the case of MDI, where n=1, the calculations lead to the following values MW = 250, IE = 125, %NCO = 33.60
IE and %NCO of polymericsWhenever the polyisocyanates are the result of a polymerisation process, n assumes an average value. The equivalent weight of species with an n value approaching infinity can be calculated as follows.
Therefore, any polyisocyanate with the structure given in figure 1, cannot have anisocyanate equivalent value higher than 131. Any experimental number, higher than 131 will suggest the absence of free NCO groups (dimerisation, carbodiimide formation, prepolymerisation, uretonimine formation, etc.) or the presence of unreactive chemicals contributing to an equivalent weight increase. In a similar way it can be shown that %NCO reaches a limiting value, when n tends to infinity.