« Go Back to Search Results 

## 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 init iators. Defined simply as the average number of isocyanate reactive sites per molecule. For example, a four-functional in itiator, such as pentaerythritol would give a nominal fourfunctional or tetrol polyol. In calculating functionality, two im portant variables must be consid ered: level of monol and level of diol. A net functionality is calculat ed 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 po lyol divided by its functi onality. Functionality of a polyol is complex because of the presence of monols from propyl ene oxide isomerization and diols (derived from water). In practice, the equivalent weight is calculat ed 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 hydro xyl groups directly affe cts 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 measurement s 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 th e preferred analytical method. With NMR, the primary and secondary hydroxyl end groups can be distingu ished 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 poly ol. Since polyols have inverse solubility in water, higher clou d points indicate increases in these important performance attributes. CPR - the CPR (controlled polymerization rate) value defines quant itatively the weakly basic materials present in a polyol. These materials are not defined qualitativ ely 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 o f 0.01 N HCl necessary to neutralize 30 grams of polyol samp le. More discussion of the CPR concept can be found in Reference 2.99. Reactivity- a measure of the rate at which a polyol undergoes reac tion 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 reactivi ty quantification of ethy lene-oxide-capped polyols. 2.107Example 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 cont amination. 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 

# Pa g e 1 of 2 Dow Perf Materials & Basic Chem Answer Center

7/8/2013 mhtml:file://C:\Doc uments and Settin g s\HTon\Deskto p \OPPT-2011-0489\Pol y urethanes an

...polymer-chain-terminating ef fects 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 

# Pa g e 2 of 2 Dow Perf Materials & Basic Chem Answer Center

7/8/2013 mhtml:file://C:\Doc uments and Settin g s\HTon\Deskto p \OPPT-2011-0489\Pol y urethanes an

...