Section 5 Common Urethane Calculations
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Equivalent Weight
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 OH (OH).

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

Molecular Weight of a Polyol
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 

Total Weight of MDI Required for Reaction
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

)


Calculation for Formulating a Prepolymer to a Specific Percent NCO
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 
)


Terms and Formulas Used in Urethane Polymer Preparations
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.
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Reference: Technical Center: Guide to urethane calculations. Adhesives Coatings & Elastomers. Huntsman International LLC, 2001.
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