Oligo-polyols

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. 

Prepolymers are formed by the reaction of a diisocyanate with an oligo-polyol, at the molar ratio [diisocyanate]/[OH group] of 1/1, in fact only one group of diisocyanate reacts with one hydroxyl group of the polyol. A structure with free terminal -NCO groups called prepolymer is produced (see Equation 2.4) ... 

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. 

Quasiprepolymers are obtained in a similar way to the prepolymers, with the difference that the reaction between oligo-polyol and the isocyanates is developed in the presence of a large excess of isocyanate. Quasiprepolymers are a mixture of prepolymers and free isocyanates (around 16-32% free isocyanates) ... 

In order to simplify the procedure of using too many components, a masterbatch , that is a mixture of the components that do not react with each other, (e.g., oligo-polyol, water, chain extender, catalysts, etc.), is made before foaming. Then it is possible to use only two components one is the polyolic component (called component A or formulated polyol, containing a mixture of all raw materials except for the isocyanate, in the proportions needed) and the second component is the isocyanate (called component B or isocyanate component). The polyurethane that results is a consequence of the very efficient contact between the isocyanate component and the polyolic component. Usually, in rigid PU foams only two components are used. In flexible foams, the polyolic component is divided into two components, especially in order to avoid the contact of some hydrolysable component with water, (e.g., stannous octoate). The gravimetric ratio between the components is verified before the foaming process and if necessary, it is corrected. 

All the previous information regarding the general chemistry of polyurethanes and the structure of isocyanates have a role in the better understanding of how the oligo-polyols get chemically inserted in the high MW polyurethane structure and to understand the role played by the polyol structure in the properties of the resulting polyurethanes. 

As mentioned previously, the synthesis of polyurethanes, by the reaction of a diisocyanate (or polyisocyanate) with oligo-diols (or oligo-polyols), is a polyaddition reaction (or step-addition polymerisation), a particular type of polycondensation reaction. There is a great difference between the polycondensation and the polyaddition reactions and the classical radical polymerisation or ionic (living) polymerisation reactions. In radical polymerisations (typical chain reactions), the high MW polymer is formed at the beginning of polymerisation. The reaction system is constituted from monomer and high... 

The general formula of an oligo polyol for polyurethane is shown in Figure 3.1 ... 

Figure 3.1 The general formula of oligo-polyols for polyurethanes...

An oligo-polyol for polyurethanes, may have two, three, four, five, six, seven or a maximum of eight hydroxyl groups/mol. Polyols with a higher number of hydroxyl groups/mol are rarely used (for example dendritic polyols). Oligo-polyols with only one hydroxyl groups/mol are present in all the polyether polyols based on propylene oxide (see Chapters 4.1.1-4.1.4). 

Irrespective of the chemical structure of the oligomeric chain, the oligo-polyols have general and common characteristics and these characteristics are determined by the same analytical methods. These first common elements permit an unitary and general point of view on all oligo-polyols for polyurethanes. 

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. 

Hydroxyl percentage is defined as the gravimetric percentage of all the hydroxyl groups in an oligo-polyol molecule ... 

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 ... 

Figure 3.2 General structure of a monol (oligo polyol with only one hydroxyl group/mol)...

Figure 3.8 General structure of an octol (oligo-polyol with eight hydroxyl groups/mol)...

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. 

A broad MWD represents a large number of macromolecules which have a wide distribution of molecular weights. The MWD of oligo-polyols is currently determined by GPC using tetrahydrofuran as solvent [21]. 

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

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