Mannich polyols

Mannich polyols is a very important group of aromatic polyols obtained by the alkoxylation with propylene oxide (PO) [(and/or ethylene oxide (EO)] of the Mannich bases obtained by classical Mannich reaction between phenols (for example phenol, p-nonylphenol), formaldehyde and alkanolamines (diethanolamine, diisopropanolamine, monoethanolamine, monoisopropanolamine and so on). Synthesis of Mannich polyols is divided into two important steps  

The Mannich bases are generally obtained by the stepwise addition of aqueous formaldehyde (25-37%) to a physical mixture of phenol - alkanolamine, at 50-70 °C. The 

In the case of phenol, with the free para position, due to the interaction between the phenolic group (acidic) and the aminic nitrogen (basic) of the amino alcohol, the ortho position is occupied first [9]. After the synthesis of Mannich bases, the water resulting from the reaction and the water from the aqueous solution of formaldehyde is distilled under vacuum, at 90-125 °C (preferably in the range 90-100 °C). A low range of distillation temperatures is preferred in order to avoid the tendency of the Mannich base to condensate to superior oligomers (with 2-3 aromatic nuclei), which increase substantially the viscosity of Mannich base and, of course, of final Mannich polyol. The mechanism of the Mannich reaction is considered to be a two-step mechanism. In the first step the reaction between formaldehyde and the primary or secondary amine (reaction 15.2) takes place, with the formation of an immonium cation [7-9, 22, 23]. 

In the second step, the reactive immonium cation formed reacts with the tautomeric forms of the phenolate anions, having negative charges, in the ortho and para positions (reaction 15.3). Finally, by a tautomerisation reaction, the reformation of the aromatic 

A second mechanism is based on an SN-2 nucleophilic substitution [7, 8]. In the first step, a reaction between formaldehyde and the primary or secondary amine takes place  

---

Mannich polyols, aromatic polyester polyols, novolak-based polyols) lead, by the reaction with crude MDI, to very rigid polyurethane structures [2] (see Chapter 15). 

Addition of EO, together with PO (15-20% EO in the mixture with PO [16-18]), leads to Mannich polyols with lower viscosities than the polyols based exclusively on PO. By using a mixture of diethanolamine and diisopropanolamine (1 1 molar), Mannich polyols with lower viscosities than the Mannich polyols based exclusively on diethanolamine are obtained [11]. As mentioned previously, lower alkoxylation temperatures of 80-90 °C (maximum 95 °C), are preferred because polyols with lower final viscosities are obtained and the alkoxylation rate is higher at lower temperatures than at higher ones (see chapter 13). 

The last traces of alkylene oxides are removed by vacuum distillation at 100-110 °C. After the phenolic group alkoxylation, that is the first group which is alkoxylated, the resulting structure becomes much more stable and it is possible to develop degassing at higher temperature, without the risk of viscosity increase. The resulting Mannich polyols are used in polyurethane foam fabrication without any other supplementary purification. The reactions involved in the alkoxylation of Mannich bases to Mannich polyols are presented in reaction 15.10 [9]. 

The general technological flow for the synthesis of Mannich polyols is shown in Figure 15.1. 

For Mannich polyols from phenol, the alkoxylation reaction of the corresponding Mannich base is presented in reaction 15.11. Of course, the Mannich base can have one or two aminomethyl groups or a mixture of these structures. 

Figure 15.1 Flow chart for Mannich polyol synthesis...

A Mannich polyol is well characterised by the molar ratio between reactants ... 

Thus, one of the most popular Mannich polyols is based on the following molar ratios... 

The viscosity of the final Mannich polyol depends on the functionalities of the resulting Mannich bases (lower functionalities lead to lower viscosities) and on the molar ratio between the reacted PO/mol of the Mannich base. Figure 15.2 shows the variation of the Mannich polyol viscosities as a function of the molar ratio of the PO/Mannich base (Mannich base from 1 mol of nonyl-phenol, 2 mols of formaldehyde and 2 mols of diethanolamine). One observes that after the addition of one PO mol/mol of Mannich base a maximum of viscosity is obtained and by the addition of 2-5 mols of PO the viscosity decreases continuously. 

The Mannich polyols described are aromatic aminic polyols, the aromatic rings have a real contribution in improving the physico-mechanical, thermal and fire proofing properties of the resulting rigid polyurethane (PU) foams. The Mannich bases, for example the Mannich base resulting from one mol of nonyl-phenol, 2 mols of formaldehyde and 2 mols of... 

Figure 15.2 Variation of Mannich polyol s viscosity as function of molar ratio PO/Mannich base, [nonyl-phenol] [formaldehyde] [diethanolamine] = 1 2 2...

The aromaticity of a Mannich polyol is calculated with the following formula ... 

The molecular weight of Mannich polyol can be calculated with formula 15.12 or with formula 15.13. 

M = molecular weight of Mannich polyol Mp = molecular weight of phenol ... 

A variant of Mannich polyol synthesis is based on the reaction between phenols and an oxazolidine (OXA), a heterocyclic compound resulting from the reaction of an alkanolamine (primary or secondary, not tertiary) and an aldehyde or a ketone (reaction 15.14). 

In the particular case of Mannich polyol synthesis, the main alkanolamine used is diethanolamine (and to a lesser extent, diisopropanolamine [11]) while the carbonyl compound is formaldehyde [5, 6, 9]. 

The synthesis of Mannich polyols based on oxazolidine chemistry has the following three steps ... 

The alkoxylation of the Mannich base with PO (or PO-EO mixtures), takes place by the stepwise addition of the oxiranic monomers, at 80-95 °C, in an inert nitrogen atmosphere [5, 9]. Figure 15.4 shows that the Mannich polyols obtained by the oxazolidine technology have lower viscosities than the corresponding Mannich polyols obtained by classical Mannich reactions. This effect is explained by the low viscosity of the intermediate Mannich bases used as starters. 

Figure 15.4 Variation of Mannich polyol s viscosity as function of the molar ratio of PO/Mannich base made by classic Mannich reaction (O) and by oxazolidine route ( ) [nonyl-phenol] [formaldehyde] [diethanolamine] = 1 2 2 (O) [nonylphenol] [oxazolidine] = 1 2 ( )...

The functionality of Mannich polyols obtained by oxazolidine technology is given by the formula 15.22 ... 

Figure 15.5 Synthesis of Mannich polyols by the oxazolidine route...

Mannich polyols are aromatic polyols, which confer excellent physico-mechanical, thermal and fire proofing properties to rigid PU foams. Mannich polyols, especially those based on p-nonyl phenol, have a very good compatibility with pentanes used as blowing agents (for example sucrose polyether polyols have a poor compatibility with pentanes, giving emulsions at normal concentrations for foaming, but not real solutions). 

A very interesting Mannich polyol, of low functionality (f = 3.5 OH groups/mol) and low hydroxyl number (OH = 325 mg KOH/g), derived from nonyl phenol was developed successfully for all water blown rigid PU foams [19]. 

The total time needed for synthesis of a Mannich polyol via the oxazolidine route is shorter than the time needed for normal Mannich technology. 

The quality of Mannich polyols obtained by the oxazolidine route is good, comparable with that of normal Mannich polyols. 

Table 15.1. The characteristics of two representative Mannich polyols based on nonyl-phenol (NP) obtained by oxazolidine technology ...

Reacting melamine with ethylene carbonate or propylene carbonate, at 150-200 °C, in liquid medium (for example a Mannich polyol derived from phenol), 2-hydroxyalkyl carbamates of melamine are obtained [4] (reaction 15.42). The reaction developed in the absence of this liquid polyol takes place with difficulty and with decomposition. Ethylene carbonate is reactive, but propylene carbonate has a much lower reactivity. 

Dimethyl phthalate, a very convenient liquid raw material, is a by-product in the fabrication of dimethylterephthalate. The amidic polyols can be used to replace the Mannich polyols. The self catalytic effect in PU formation is much less important in the case of amidic polyols, as compared with Mannich polyols. The rigid PU foams derived from amidic polyols are much more thermoresistant than the rigid PU foams derived from Mannich polyols [2]. 

---