Colour polyether polyols

Tables 4.9-4.14 show some general characteristics of polyether polyol PO-EO block copolymers, such as MW, hydroxyl number, functionality, viscosity and colour, but also some other characteristics such as unsaturation, EO content, and potassium and sodium content which are specific for polyether polyols.

The colour of polyether polyols is an important characteristic, the maximum value of colour accepted being around 50 APHA. For some polyether polyols, the maximum accepted value for colour is 30 APHA, which is practically a colourless product. 

A low colour increases the commercial value and the polyether probably has an unaltered structure, without chain destruction and formation of new compounds, having labile groups such as aldehydes, ketones, esters or hydroperoxides. IR analysis of chromophoric groups in polyether polyols proved that colour is given mainly by carbonylic groups linked to double bonds [97]. Some important factors which have a strong influence on polyether polyol colour are discussed in Sections, 4.1.5.7.1, 4.1.5.7.2 and 4.1.5.7.3. 

The propionaldehyde content of PO strongly affects the colour of the resulting polyether polyols, explained by an uncontrolled condensation of the aldehyde in the presence of an alkaline medium. A maximum of 10 ppm of propionaldehyde in PO of is perfect for a very good colour in the resulting polyether polyols. A maximum content of 50 ppm propionaldehyde is acceptable, but the crude polyether has a yellow colour, which fortunately is removed in the propoxylation step. A propionaldehyde level higher than 100 ppm leads to serious colour problems in polyether synthesis. 

Effect of the Purification Step on the Polyether Polyol Colour... 

The purification step strongly improves the colour of final polyether polyols. Thus, by simple mixing of crude, alkaline polyether polyol with water, before neutralisation, in the presence of air, at 100 °C, a remarkable improvement in the colour takes place [42, 43]. In the absence of water, the colour deteriorates and the polyether becomes dark brown. Probably water participates chemically in complex reactions, by a mechanism which is not very clear. 

The treatment of crude, alkaline, polyether polyol with adsorbents, in the presence of water (silicates of aluminium or magnesium having a big adsorption surface), at 80-100 °C, leads to a remarkable improvement in polyether polyol colour. Probably the polar chromophoric groups are adsorbed preferentially on the solid surface of the adsorbents. 

Another experimental observation is that an acidic media always markedly improves the colour of polyether polyols. Thus by the addition to a crude polyether polyol, in the presence of water, of inorganic or organic acids, such as phosphoric, hydrochloric or adipic acids, a strong improvement in the colour takes place. 

The addition of very small quantities of oxalic acid to a purified polyether polyol, improves the colour [97] this is probably explained by the reducing capability of oxalic acid. 

Another practical observation to be noted is that the simple storage of a purified polyether polyol, under air and at room temperature for one to two weeks leads to an evident enhancement of the colour. 

The presence of antioxidants in polyethers has a negative influence on the colour. If the polyether is not very well purified and has a basic pH, the phenolic antioxidants are oxidised to quinonic chromophoric structures, which negatively affect the colour of polyether polyols. This is the reason why it is preferable to add antioxidants to the slightly acidic purified polyol and not to the alkaline polyether. 

It was proved experimentally that with polyether polyols having a high colour of 80-100 APHA, good flexible PU foams were obtained, with corresponding properties. 

The dark colour of polyether polyols obtained in the presence of imidazoles as catalysts (Gardner colour >18) can be improved substantially by the treatment with hydrogen peroxide (50% concentration) of around 0.1-0.3% against the polyol. The dark brown colour is the colour of the catalyst and it is not a consequence of polyether destruction. A better final colour is obtained using N-substituted imidazoles (such as N-methyl imidazole) [36]. 

A very interesting catalyst used in the synthesis of polyether polyols for rigid PU foams is urea [41]. Sucrose poly ether polyols obtained in the presence of urea as catalyst have a very light colour [41]. Unfortunately with urea it is possible to obtain lower molecular weight polyether polyols, with an hydroxyl number (OH ) higher than 500 mg KOH/g. 

The amidic polyol obtained by the propoxylation of bis (diethanolamide) of phthalic acid, with an hydroxyl number in the range of 380-400 mg KOH/g, is an alternative for ortho-toluene diamine (o-TDA) based polyether polyols. The amidic polyols do not have the problems of a dark colour as the o-TDA polyols do and have lower viscosities than o-TDA polyols. 

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