What is Equivalent Weight?

The equivalent weight (E) refers to the molecular weight ( Mn ) per unit of functionality (f) in a compound.

E = Mn / f  (1)

For example, if the number-average molecular weight of a polyether triol is 3000, then its equivalent weight is:

Ep = 3000 / 3 = 1000

MOCA, or 4,4′-Methylene-bis-(2-chloroaniline), has a molecular weight of 267. Although there are four active hydrogen atoms in the molecule, only two participate in the isocyanate reaction, so its functionality f = 2.

En = 267 / 2 = 133.5

In the product specifications for polyether or polyester polyols, the hydroxyl value (OH value) is usually provided. Therefore, calculating equivalent weight directly from the hydroxyl value is more practical.

Ep = 1000 × 56.1 / OH  (2)

Equation (2) is a simplified form of Equation (1). In general, the functionality (f) of polymers—especially polyesters or polyethers—is often not equal to that of the initiator. For example, a polyoxyalkylene polyol initiated by glycerol may have a hydroxyl value of 56 mgKOH/g, but thin-layer chromatography shows its actual functionality is not 3, but 2.8. This is mainly due to side reactions like disproportionation during polymerization. Therefore, bypassing functionality and using hydroxyl value directly to calculate equivalent weight offers better accuracy and practicality.

Using Equation (2):

Ep = 1000 × 56.1 / 56 = 1001.8

As another example, a polyether polyol used in rigid polyurethane foam production may have a hydroxyl value of 500 mgKOH/g. While its functionality typically ranges from 3 to 8, measuring it precisely is time-consuming. However, as long as the hydroxyl value is known, the equivalent weight can be calculated:

Ep = 1000 × 56.1 / 500 = 112.2