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Dow Polyurethanes - Polyol Characterization

Answer ID 5693 | Updated 01/03/2011 06:32 PM 

How are polyols characterized?

With so many potential compositions, it is important to have adequate criteria for characterizing a polyol. Some of the more 

commonly used criteria will be discussed below.

Functionality - arises from the choice of initiator or blend of initiators. Defined simply as the average number of isocyanate 

reactive sites per molecule. For example, a four-functional initiator, such as pentaerythritol would give a nominal fourfunctional or tetrol polyol. In calculating functionality, two important variables must be considered: level of monol and level of 

diol. A net functionality is calculated by inserting those values into Equation 2.9.

Hydroxyl Number - a measure of the amount of reactive hydroxyl groups available for reaction. This value is determined in a 

wet analytical method and is reported as the number of milligrams of potassium hydroxide equivalent to the hydroxyl groups 

found in one gram of the sample. A review of wet chemical methods for this analysis is available.2.81 An infrared method has 

also been described.2.82 The most commonly used methods are available in ASTM D 4274-88.2.83

Equivalent Weight - classically defined as the molecular weight of a polyol divided by its functionality. Functionality of a 

polyol is complex because of the presence of monols from propylene oxide isomerization and diols (derived from water). In 

practice, the equivalent weight is calculated from the analyzed hydroxyl (OH) number. The equivalent weight is necessary for 

isocyanate requirement calculations and is derived from the following expression:

Primary Hydroxyl Content - The distribution of primary and secondary hydroxyl groups directly affects the reactivity of a 

polyol with isocyanates. In general, polyol reactivity increases as primary hydroxyl content increases (see example data in 

the figure below). The earliest measurements of primary and secondary hydroxyl contents were based on differences in 

solution based reaction rates.2.84-2.86 Nuclear magnetic resonance (NMR) is now the preferred analytical method. With NMR, 

the primary and secondary hydroxyl end groups can be distinguished from one another and from the ethylene oxide and 

propylene oxide units in the polyol chain.2.87-2.93 Detailsof the recommended test procedures can be found in ASTM D 4273-

88.2.94 A polarographic method has also been reported.2.95

Cloud Point - The cloud point of a polyol is defined as the temperature in degrees centigrade, at which a solution of the 

polyol in a water or water-alcohol mixture becomes turbid upon heating.2.96-2.98 Cloud point decreases with increasing polyol 

molecular weight and increases with the addition of ethylene oxide to the polyol. This analysis is taken to be a measure of the

water solubility, surfactant properties and reactivity of a polyol. Since polyols have inverse solubility in water, higher cloud 

points indicate increases in these important performance attributes.

CPR - the CPR (controlled polymerization rate) value defines quantitatively the weakly basic materials present in a polyol. 

These materials are not defined qualitatively but are determined as the total weak acid salts of the strong bases. These salts 

may act as catalysts for the reaction of polyols with isocyanates. The reported number is ten times the number of milliliters of 

0.01 N HCl necessary to neutralize 30 grams of polyol sample. More discussion of the CPR concept can be found in 

Reference 2.99.

Reactivity - a measure of the rate at which a polyol undergoes reaction to make a polyurethane polymer. A test measuring 

viscosity build in a model reaction with isocyanate has been in use since 1960.2.100-2.106 In recent years a refined Brookfield 

Viscosity Test (BVT) has proven useful for reactivity quantification of ethylene-oxide-capped polyols.2.107 Example data in the 

figure below show how polyol reactivity varies with increasing primary hydroxyl content (at a constant unsat).

The BVT has also been found useful for detecting polyol contamination. Other viscosity-based reactivity tests have been 

described.2.104, 2.108-2.110 Apparent polyol reactivity in this type of model reaction can also be strongly influenced by the 

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polymer-chain-terminating effects of monol (unsat). The contribution of monol to viscosity build for a wide range of primary 

hydroxyl levels in typical molded foam polyols is illustrated in the figure below.

Polyurethane Foam References

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