Viscosity Mannich bases

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]. 

The anhydrised Mannich base is heated under nitrogen at 80-125 °C (preferably at 80-90 °C to avoid viscosity increase) and PO (or a mixture of PO-EO or EO) is added stepwise within 4-6 hours. The reaction does not need a catalyst. The alkoxylation reaction is catalysed just by the tertiary amino nitrogen formed as a consequence of the Mannich reaction. 

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 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. 

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

Unfortunately, due to the presence of free phenolic groups, the viscosity of Mannich bases increases slowly in time. For example in one year, the viscosity of a nonyl-phenol Mannich base increases from 16,000 mPa-s, at 25 °C, to 90,000-100,000 mPa-s, at 25 °C. Fortunately, in polyols, when an unpropoxylated Mannich base is used, the viscosity remains practically unchanged. 

The simple mixing of the anhydrous oxazolidine with a phenol, at 80-90 °C, for 2-3 hours leads to the formation of Mannich bases. As a general observation, the Mannich bases made via the oxazolidine route have lower viscosities than the Mannich bases obtained by the classical reaction of phenol with formaldehyde and diethanolamine. This effect is explained by the absence of vacuum distillation in the presence of phenolic compounds which leads to polycondensation of Mannich bases to form viscous oligomers (with 2-3 aromatic nuclei). 

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 ( )...

Practically, the reaction takes place by the addition of aqueous formaldehyde to a mixture of melamine suspended in diethanolamine, at a molar ratio of [melamine] [formaldehyde] [diethanolamine] = 1 3 3, followed by water distillation under vacuum, at a lower temperature of 65-75 °C, in order to avoid the viscosity increase produced by polycondensation. The maximum water accepted in the Mannich base can be high, around 5%. 

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). 

---