Purification polyether polyols

Polymerization Solvent. Sulfolane can be used alone or in combination with a cosolvent as a polymerization solvent for polyureas, polysulfones, polysUoxanes, polyether polyols, polybenzimidazoles, polyphenylene ethers, poly(l,4-benzamide) (poly(imino-l,4-phenylenecarbonyl)), sUylated poly(amides), poly(arylene ether ketones), polythioamides, and poly(vinylnaphthalene/fumaronitrile) initiated by laser (134—144). Advantages of using sulfolane as a polymerization solvent include increased polymerization rate, ease of polymer purification, better solubilizing characteristics, and improved thermal stabUity. The increased polymerization rate has been attributed not only to an increase in the reaction temperature because of the higher boiling point of sulfolane, but also to a decrease in the activation energy of polymerization as a result of the contribution from the sulfonic group of the solvent. 

Figure 4.30 Scheme for polyether polyol fabrication by anionic polymerisation of alkylene oxides, initiated by glycerol or diols (variant). 1 - Reactor for potassium glycerolate synthesis 2 - Reactor for prepolyether synthesis 3 - Reactor for polyether synthesis 4 - Reactor for purification 5 - Filter press 6 - Storage tank for final purified poly ether 7 - Heat exchangers for removal of the reaction heat 8 - Condensers 9 - Vacuum pumps 10 - Vessels for distilled water 11 - Recirculation pumps 12 - Gear pump or screw (or double screw) pump... 

The resulting crude, alkaline polyether contains around 0.2-0.3% of the KOH used as the catalyst. The idea of polyether polyol purification is to remove as much as possible of the alkaline ions until a very low level of 5-10 ppm (the requirements for industrial polyether polyols are < 5 ppm) is left. 

Purification of crude , alkaline polyether polyols is by several methods, applied industrially,... 

The principle of polyether polyol purification by neutralisation with acids is simple the alkaline polyether is neutralised with an aqueous solution of an acid (inorganic or organic)... 

The treatment of alkaline, crude polyether polyols with strongly acidic cation exchange resins (copolymer of styrene - divinylbenzene with sulfonic acid groups) is a very efficient purification method. The treatment is performed at moderate temperatures (for example 50-70 °C) in the presence of water or, better still, in the presence of a solvent such as methanol or a methanol-water mixture. The treatment may be static (by mixing the crude polyether with cation exchange resin in a reactor, followed by filtration) or, much better, in a dynamic system, in columns with cation exchange resins. The removal of alkaline cations is very efficient, sometimes less than 1 ppm of potassium ions being obtained ... 

The most important processes for polyether polyol purification, used frequently on an industrial scale, are the first two processes the neutralisation with acids followed by crystallisation of potassium salts, then filtration and the treatment with adsorbents. 

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. 

This compound proved to be the most powerful odorant agent in polyether polyols. Even in trace amounts, this substance confers an unpleasant odour to polyether polyols. The formation of this compound takes place in the purification step, in acidic media, involving the terminal propenyl-ether groups (in fact vinyl ether groups are very sensitive to acidic attack). The probable mechanism is the following cationic cyclisation ... 

The most interesting method of polyether polyol odour elimination is based on the acidic hydrolysis of the purified polyether [90]. Thus a polyether polyol is treated with 10% water in the presence of an acid (H2S04 or HC1) at 90-100 °C, for one to two hours. The propenyl ether is hydrolysed to propionaldehyde and, instead of a double bond, a hydroxyl group is generated (reaction 4.23). At the same time the cyclic compound in Figure 4.35, formed during the purification step, is hydrolysed with the formation of propionaldehyde and dipropyleneglycol ... 

Obviously, the extension of the polyetheric chain using KOH as catalyst needs a purification step. The advantage of using tertiary amines or the catalytic effect of the amino polyol is very important from the technological point of view because the purification step is eliminated, the production cycle is short and the yield in polyether polyol is very high. 

Of course for rigid polyether polyol synthesis in the presence of tertiary amines as catalysts, the purification step and sometimes filtration are eliminated, the fabrication process being shorter and simpler. 

In Table 13.4 the characteristics of some rigid polyether polyols based on a sorbitol -glycerol mixture are presented. The initial starter mixture is solution of sorbitol (70%) and glycerol. After water vacuum distillation, the mixture of sorbitol - glycerol containing 0.1-0.5% water, is propoxylated in the presence of a KOH catalyst, followed by the usual purification. These polyether polyols are transparent viscous liquids, which are colourless or slightly yellow polyols ... 

By the hydrolysis of a flexible foam based on toluene diisocyanate (TDI) one obtains toluene diamine (2,4 and 2,6 isomers), the polyether triol and, of course, carbon dioxide. The difficulty of the process is the separation of the amine. The amine may be used for TDI synthesis (after a previous purification), or be transformed into a valuable rigid polyol (aminic polyol) by alkoxylation with PO and EO. 

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