Oligo-triols

In Figure 1.9 one can see an hypothetical crosslinked structure of a flexible polyurethane foam resulting from an oligo-triol of MW of 3000-6500 daltons and a diisocyanate [3-16, 20]. 

If a prepolymer derived from an oligo-triol or an oligo-polyol, having three or more terminal -NCO groups is used, if it is in contact with atmospheric humidity, crosslinked polyurethanes are obtained. 

In practice mixtures are frequently obtained between oligo-polyols of different functionalities, for example octol with triol or hexol with triol. For example an equimolecular mixture between an oligo-octol and an oligo-triol (see Figure 3.9) has the equivalent functionality of 5.5 OH groups/mol (calculated easily with formula 3.7) ... 

Figure 3.9 General structure of a mixture of two different oligo-polyols (octol + triol, having an equivalent functionality (fe) of 3 < fe < 8)...

The MW of any oligo-polyol is calculated with formula 3.4 if the functionality (f) and the hydroxyl number (OH ) are known, in fact it is a particular case of MW determination by the quantitative analysis of the terminal functional groups, in our case the hydroxyl groups. Thus, a triol with an OH of 27 mg KOH/g has a calculated MW of 6,233 daltons, but a tetraol having the same OH, has a calculated MW of 8,311 daltons. Table 3.1 gives the values of the MW for oligo-polyols of different functionalities. 

Equation 3.8 for EW is very convenient for practical use because it does not depend on the functionality, which is very difficult to determine. The EW of an oligo-polyol is very useful for the required isocyanate quantity calculation. One equivalent weight of an oligo-polyol reacts with one equivalent weight of the diisocyanate (the MW of the isocyanate divided by the number of -NCO groups). The EW of polyols with the same OH are identical, irrespective of the functionality. Thus a diol with an OH of 56.1 mg KOH/g (MW = 2000) and a triol of the same OH (MW = 3000), have the same EW of 1000. 

All the oligo-polyols are used to build the polyurethane high MW structure in a reactive process, as a consequence of the oligo-polyols terminal hydroxyl group reaction with polyisocyanates. The reactivity of oligo-polyols in polyurethane fabrication is a very important practical characteristic. Reactivity is a measure of the reaction rate of an oligo-polyol with an isocyanate in order to make the final polyurethane polymer. One practical method is the measurement of viscosity, in time, by Brookfield Viscosity Test (BVT), especially used to determine the reactivity of ethylene oxide capped polyether polyols. Figure 3.12 shows the effect of the primary hydroxyl content upon the reactivity of ethylene-oxide capped polyether triols of MW of 5,000 daltons. 

The high MW polyether triols, copolymers of PO-EO are the most important oligo-polyols for PU, having the biggest volume of industrial production. This is the reason why the synthesis of polyether triols, by polymerisation of PO and/or EO and initiated by glycerol, will be presented in detail. 

From high MW triols or low branched oligo-polyols (MW = 3000-6500 daltons) polyethers, polyesters, filled polyols (polymer polyols), are obtained elastic PU with a low degree of crosslinking (flexible and semiflexible foams, coatings etc). 

Investigations currently in progress are directed towards the synthesis of iron-substituted oligo-and polysiloxanes produced via self-condensation of the silanedi- and -triols 4b,c. 

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