Polyurethane Terminology and Calculation Methods

Functionality: Refers to the number of special groups (and reactivity) in the structure of an organic compound. For polyether or polyester polyols, functionality refers to the number of atoms containing active hydrogen.

Hydroxyl Value (OHV): From an analytical perspective, the hydroxyl value is defined as the milligrams of potassium hydroxide equivalent to the hydroxyl value in 1 gram of the sample. The weight percentage of hydroxyl value is represented as OH%. OHV = Weight percentage of hydroxyl value * 33.

Molecular Weight: The molecular weight refers to the relative weight of a single substance or compound molecule, equal to the sum of the atomic weights of the atoms in the molecule.

Molecular Weight = 56.1*Functionality *1000 / Hydroxyl Value

56.1 is the molecular weight of potassium hydroxide. For simple compounds, the molecular weight is the sum of the atomic weights of the atoms in the molecule.

Isocyanate Group Percentage Content: The isocyanate group percentage content is typically expressed as NCO%. For pure TDI/MDI, it can be calculated using the molecular formula:

NCO% for TDI = 42 * 2 / 174 = 48%

NCO% for MDI = 42 * 2 / 250 = 33.6%

42 is the molecular weight of NCO.

Sometimes NCO% is also represented using amine equivalents: the number of grams of isocyanate ester consumed per gram of molecule amine to form the corresponding urea.

Amine Equivalent and NCO% are related as follows:

Amine Equivalent = 4200 / NCO%

Equivalent Value and Equivalent Number: Equivalent value refers to the molecular weight corresponding to one unit functionality in each compound molecule.

Equivalent Value = Number average molecular weight / Functionality

For polyester and polyether products, the equivalent value is calculated directly from the hydroxyl value data.

Equivalent Value = 56100 / Hydroxyl Value

TDI Equivalent = Number average molecular weight / Functionality

                           = 174 / 2 = 87

                           = 42.02 * 100 / NCO%