The equivalent weight (eq. wt.) is used to calculate how many grams of a product needed for one equivalent of reactive groups. For an **isocyanate**, the reactive group is -N=C=O (NCO. Its concerntration is measured by weight percent NCO).

Isocyanate equivalent weight = |
4,202
NCO |
g/eq |

The reactive group for a **polyol** is -O-H (OH).

Equivalent weight of a polyol = |
56,100
OH Value |
g/eq |

The molecular weight (mol. wt.) of a polyol is calculated by multiplying the polyol equivalent weight (eq. qt.) by the nominal funcationality (fn).

Polyol mol. wt. = (eq. wt.)(fn) = |
( | 56,100
OH value |
) | (fn) g/mole |

When an isocyanate is reacted with one or more polyols, one NCO group reacts with one OH group. When the number of NCO groups is equal is to the number of OH groups, the result is a stoichioetric NCO : OH ratio of 1.0. A ratio of 1.0 is commonly known as the index.

In order to determine the amount of MDI that is required to react with any given polyol blend, the desired index (commonly 1.0) must be known, as well as the MDI equivalent weight (MDI eq. wt.), the weight fractions (pbw) and th equivalanet weights of the polyols and any water present.

Total weight of MDI required =

(index)(MDI eq. wt.)(fn) = |
( | pbw polyal A
eq. wt. polyol A |
+ | pbw polyal B
eq. wt. polyol B |
+ | ..... | + | pbw polyal E
eq. wt. polyol E |
+ | pbw H2O
eq. wt. H2O |
) |

MDI Prepolymer calculation:

Where N = Desired NCO

Where X = Equivalent weight of the diisocyanate

Where Y = Equivalent weight of the polyol

MDI = |
X + | ( | N (X+Y)
(42/X) - N |
) |

Introduction:

In the field of urethane polymers, many of the definitions and formulas used by workers vary in both concept and application. Assumptions for simplifying the calculations are also not consistent.

Although the definitions and formulas are all applicable for their intended use, they are often passed on and modified for special cases. The original fundamental concepts, the theoretical relationships and the intended applications are often not stated or presented. This inevitably has created confusion and misuse. It has also hampered proper field work evaluations and comparisons.

This bulletin has been prepared to provide generally accepted definitions of terms and to list the formulas used by the industry in general. The concepts, assumptions, and modifications are given. Sample calculations are also shown, for clarification.

*Reference:* Technical Center: Guide to urethane calculations*. Adhesives Coatings & Elastomers. *Huntsman International LLC, 2001.