Rigid polyether polyols synthesis

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. 

Reactors used for the synthesis of rigid polyether polyols need an internal stirrer, because frequently high melting point polyols (such as pentaerythritol or sucrose) are used as starters and the initial reaction mass is a suspension of solid polyols in liquid. 

Continuous processes for the synthesis of rigid polyether polyols are discussed [42], Generally a synthesis of a polyether polyol for rigid PU foams has the following steps ... 

The synthesis of rigid polyether polyols, by polymerisation of PO or EO, initiated by polyols which are liquid under the conditions of the polymerisation temperature, is simple, and similar to the synthesis of the prepolyether by propoxylation of glycerol (see Chapters 4.1.1 and 4.1.5). 

Many important polyols used as starters for synthesis of rigid polyether polyols are solid in the conditions used for PO (or/and EO) poly addition, having a melting point higher than 130 °C. Such polyols are sucrose (mp = 179-180 °C), pentaerythritol (mp = 253 °C), dipentaerythritol (mp = 222 °C), a-methyl glucoside (mp = 164-165 °C) and other polyols. As mentioned previously, the main technical problem is to react a solid polyol with a gaseous monomer. This problem was solved by several practical solutions ... 

Xylitol (Figure 17.2), a polyol starter for rigid polyether synthesis, having five hydroxyl groups, is produced by the hydrogenation of the same pentosans used for THF synthesis [8]. By propoxylation of xylitol excellent rigid polyether polyols (see Chapter 4.1) are obtained. 

The general synthesis reaction of polyether polyols for rigid PU foams by polymerisation of alkylene oxides (PO, EO) initiated by polyolic starters is presented in reaction 13.1. 

The most important low molecular weight polyols used as starters for polyether polyols destined for rigid PU foams synthesis are glycerol, trimethylolpropane (TMP), triethanolamine, pentaerythritol, dipentaerythritol, a-methyl glucoside, xylitol, sorbitol and sucrose [1-27]. The main properties of these starter polyols, which are of interest for polyurethane chemistry, are presented in Table 13.1. 

A second important group of starters used in the synthesis of polyether polyols for rigid PU foams is the group of polyamines, aliphatic or aromatic, having 2-3 amino groups/mol (primary or secondary amino groups) such as ethylenediamine (EDA), diethylenetriamine (DETA), ortho-toluene diamine (o-TDA) and diphenylmethanediamine (MDA) [1,2] (see Chapter 4.2). The main properties of these polyamines which are of interest in polyurethane chemistry are presented in Table 13.2. 

As a general observation, the main reactions involved in the synthesis of polyether polyols for rigid polyurethane foams are ... 

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. 

In order to decrease the total reaction time, a small reactor, with a stirrer, is linked to the polymerisation reactor, for the preparation of the initial starters - catalyst mixture. In this reactor, there are 1-3 polyols used as starters, the catalyst (KOH, NaOH or a tertiary amine) and sometimes, for solid polyols, an initial liquid medium (for example a part of an intermediary or final polyether polyol called heel , or an inert solvent). Generally, in the synthesis of polyether polyols for rigid foams it is preferred to avoid the utilisation of inert solvents, which need recycling and a more complicated installation. 

Thus, glycerol, the most important starter for the synthesis of polyether polyols for flexible PU foams and for polyether for rigid foams is produced by the hydrolysis of natural triglycerides (esters of glycerol with fatty acids with C6 to C22 carbon atoms), from vegetable or animal resources (reaction 17.1) [1]. Large quantities of glycerol appear in bio-diesel production, by transesterification of natural oils with methanol. 

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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