Odour polyether polyols

The polyether polyols for flexible PU foams frequently have a distinct, unpleasant smell. In case of flexible slabstock foams where, in the centre of the PU bun, there are high temperatures (150-170 °C), an unpleasant odour appears during the foaming process which remains for a long time in the resulting PU foams. 

The explanation for the existence of a polyether polyol odour is given by the presence of some low MW volatile impurities which in small concentrations give an intense and unpleasant odour. 

Another group of substances which give an unpleasant odour to polyether polyols is the allyl alcohol (formed by the rearrangement of PO) and its hydroxypropyl ethers formed... 

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 odour of polyether polyols is eliminated by several methods. One efficient method is to introduce into the liquid hot polyether polyol (at around 120 °C), under vacuum, fine drops of liquid water [142]. Water, of course, is transformed into a vapour, which is eliminated together with traces of substances that give the odour (some of these substances may give azeotropic mixtures with water, with a decrease in the boiling point). 

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 ... 

All these polyols, based on N-methyl alkylated polyamines such as N-methylpropylenediamine, N,N-dimethyldipropylenediamine, N,N dimethyl trimethylolethane and other similar polyamines, have an improved self-catalytic activity and need only a low concentration of volatile tertiary amine catalysts in the foaming process. The resulting flexible PU foams have practically none of the unpleasant odours characteristic of the conventional flexible PU foams. The low amine emission in the foaming process diminishes the risk of workers exposed to the amines, and these polyether polyols may be considered to be more environmentally friendly. 

The formation of relatively high yield (15-25%) cylic oligomers, means that the cationic polymerisation of alkylene oxides cannot be used for high MW polyether polyol synthesis on an industrial scale [38, 56]. The cationic polymerisation process is only used industrially for producing PTHF- and THF-alkylene oxide copolymers [2, 3, 7, 35, 36, 54, 57, 58]. The cyclic oligomers are totally inert in the chemistry of PU formation because they do not have hydroxyl groups (are simple diluents) and confer a very unpleasant odour to the synthesised polyether polyols. 

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