Oligo-polyols functionality

Dimensional stability of rigid PU foams (a very important characteristic in the thermoinsulation of refrigerators, especially at lower temperatures) is strongly improved by using high functionality oligo-polyols. 

Fortunately, after the gel point, the reaction between unreacted -NCO groups and unreacted OH groups continues slowly, over time, and the properties are improved, especially when very high functionality oligo-polyols are used which is when the best dimensional stabilities are obtained. 

A polyol of low functionality, having around 2-3 hydroxyl groups/mol and with a high MW of 2000-10000 daltons, leads to an elastic polyurethane and on the contrary, a low MW oligo-polyol of 300-1000 daltons, with a high functionality of around 3-8 hydroxyl groups/mol leads to a rigid crosslinked polyurethane. 

Equation 3.3 is a fundamental equation in oligo-polyol chemistry, having very important practical applications. The number average molecular weight, of an oligo-polyol is easily calculated with this formula, if the functionality and the OH are known ... 

OH = hydroxyl number of oligo-polyol in mg KOH/g, Mn = number average molecular weight (g/mol), f = functionality, the number of OH groups/mol, and 56,100 = equivalent weight of KOH, in milligrams. 

Functionality is the second important characteristic of an oligo-polyol and is defined as the number of hydroxyl groups/molecule of oligo-polyol. The functionality of an oligo-polyol is not very easy to determine. An old method is based on the determination of the conversion... 

For poly ether polyols an NMR method was elaborated for functionality determination [20]. The most usual and practical method for functionality determination is based on the assessment of the MW of an oligo-polyol, by a method applicable to low MW compounds (MW < 10,000 daltons), such as vapour pressure osmometry (VPO) or gel permeation chromatography (GPC), together with hydroxyl number determination. The functionality is calculated by using the fundamental equation 3.4 ... 

In the case of a mixture of two oligo-polyols with different functionalities (fj and f2), the equivalent functionality, fe, of the oligo-polyol mixture is calculated using the general formula ... 

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)...

Xj = 0.5 and x2 = 0.5. As an immediate consequence the equivalent functionality of this oligo-polyol mixture is ... 

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. 

Table 3.1 The oligo-polyols MW values function of the functionality (f) ...

The equivalent weight (EW) [1, 22] of an oligo-polyol is defined as the oligo-polyol MW divided by its functionality ... 

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 oligo-polyols have a functionality, a number of hydroxyl groups/mol, in the range 2-8 OH groups/mol. 

Because the functionality of an oligo-polyol for elastic polyurethane is 2-3 hydroxyl groups/mol, the general formula shown in Figure 4.1 becomes the formula shown in Figure 4.2. 

The advantage of CPL-based polyester polyols is that the final functionality of the resulting oligo-polyol is identical to the functionality of the starter used and, generally, no side reactions were observed to markedly affect the functionality. 

In Chapter 3, the chemistry and technology of the most important oligo-polyols used for elastic polyurethanes fabrication, in fact high MW oligomers (2000-12000 daltons) with terminal hydroxyl groups and low functionality (2-4 hydroxyl groups/mol) were discussed. Polyalkylene oxide polyols (homopolymers of PO or copolymers PO - EO, random or block copolymers), polytetrahydrofuran polyols, filled polyols (graft poly ether polyols, poly Harnstoff dispersion - polyurea dispersions (PHD) and polyisocyanate poly addition (PIPA) polyols), polybutadiene polyols and polysiloxane polyols were all discussed. The elastic polyurethanes represent around 72% of the total polyurethanes produced worldwide. 

In Chapter 12, the oligo-polyols for rigid polyurethanes, having low MW (less than 1000 daltons) and high functionality (of around 3-8 hydroxyl groups/mol) will be discussed. 

Table 21.1 shows the molar cohesive energies of functional groups in oligo-polyols and inPU. 

The degree of crosslinking depends firstly on the functionality of oligo-polyols and on the MW between the branch points (in fact on the MW of the oligo-polyol). 

This behaviour concerning the cumulative effect of functionality and molecular weight of oligo-polyols is observed in the case of PU foams (flexible, semiflexible and rigid) based on stress-strain relationship [2, 12]. 

Table 21.4 Conversion at gel point as function of oligo-polyol functionality and isocyanate functionality ...

A very subtle effect of the oligo-polyols functionality is seen in the case of propoxylated polyether polyols, due to the presence in oligo-polyol composition of a polyether monol as a consequence of the rearrangement of PO to allyl alcohol during anionic PO polymerisation (see section 4.1), with this structure ... 

The hydrolytic resistance of a polyurethane depends strongly on the nature of the oligo-polyol chain. As a general rule, very hydrophobic chains and water repellent polyols give polyurethanes with excellent hydrolytic stability. The relative order of the hydrolysis resistance of polyurethanes function of oligo-polyol nature is ... 

Based on the values of cohesive energies and dissociation energies of the bonds involved in the polyurethane structure, it may be possible to establish the following relative order regarding the thermal stability of polyurethanes function of the oligo-polyol structure ... 

All oligo-polyols have functionality (a definite number of hydroxyl grouyps/mol) ... 

---