Rigid polyether polyols

Due to the complications generated by the catalysis with tertiary amines, at this moment the most widely used catalyst to obtain rigid polyether polyols is KOH, but some polyethers, especially of very high functionality, are obtained by tertiary amine catalysis. 

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. 

Generally, the polymerisation conditions for polyfunctional polyol alkoxylation, with KOH as catalyst, to rigid polyether polyols are ... 

At this moment the discontinuous batch processes are the most important processes used worldwide for the fabrication of rigid polyether polyols. 

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 ... 

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. 

A variant of the previously mentioned application is to make the neutralisation of the crude alkaline, rigid polyether polyol with a cyclic anhydride (for example with succinic or maleic anhydride). A structure is obtained in which the potassium salt is chemically linked to a rigid polyol structure (reaction 13.18) and which enters the polyurethane network during the foaming process. Unfortunately, phthalic anhydride, an easily available and cheap cyclic anhydride, leads to partially insoluble potassium salts [36]. 

The technological flow for the rigid polyether polyols fabrication with KOH as catalyst and with tertiary amines as catalysts are presented in Figures 13.5 and 13.6, respectively. 

Figure 13.5 Flow diagram for rigid polyether polyol fabrication (catalyst KOH)...

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). 

Kinetic Considerations Concerning the Alkoxylation of Polyols to Rigid Polyether Polyols... 

In case of rigid polyether polyols, where the polymerisation degree is low (maximum 6-16 PO units/mol of polyol), it is not possible to neglect the initiation step. 

Unfortunately, it is a difference between a starter, such as nonylphenol, or a fatty alcohol, which have only one type of hydroxyl group and polyols. Some polyols used as starters for rigid polyether polyols have in the same molecule various types of hydroxyl groups (for example, primary hydroxyls and secondary hydroxyls) which do not have equivalent reactivities in the alkoxylation reactions. For example, sorbitol has two primary hydroxyls and four secondary hydroxyls, sucrose has three primary hydroxyls and five secondary hydroxyls. In both polyols, the secondary hydroxyls have different substituents and they are not totally equivalent. TMP, pentaeriythritol and dipentaerythritol have only one type of equivalent primary hydroxyl group. Thus, the initiation reaction (reaction of PO with hydroxyl groups of starter) is in fact the sum of the reactions of PO with each type of hydroxyl group of the starter ... 

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 ... 

Table 13.4 The characteristics of some sorbitol-glycerol based rigid polyether polyols for rigid PU foams ...

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 ... 

DEG is a very interesting copolyol for making sucrose-based rigid polyether polyols. Due to the high polarity of ethylene oxide units, DEG-sucrose mixtures [51] show an excellent stirrability of the initial reaction mass, but only for medium functionalities of around 4-5 OH groups/mol. 

Table 13.5 shows the characteristics of some rigid polyether polyols, based on sucrose... 

Table 13.6 The characteristics of a representative sucrose-triethanolamine-based rigid polyether polyol for rigid PU foams (structure I) ...

Utilisation of sucrose as a starter polyol for rigid polyether polyols is extremely advantageous from the economic point of view. Sucrose is in fact a renewable raw material, commercialised on a large scale and available in a high purity form. The cycloaliphatic structure and the high functionality lead to high performance rigid polyether polyols. 

The removal of the catalytic effect of the remnant tertiary amines in the rigid polyether polyols synthesised in amine catalysis [36]. 

Resol resins, having very reactive methylol groups (obtained by the condensation of phenol with formaldehyde in basic media), are rarely used as starters for rigid polyether polyols. One reason is the impossibility of melting these resins at the propoxylation temperature, because upon heating they rapidly polycondensate and crosslink. An interesting representative of this group of resin is trimethylol phenol (reaction 15.26). 

Sucrose, the most important starter for rigid polyether polyols is produced exclusively by extraction from naturally resources (Figure 17.1) [1]. 

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. 

By the hydrogenation of D-glucose, a hexafunctional polyol, sorbitol is obtained, one of the most important starters to initiate the polymerisation of propylene oxide (PO) to hexafunctional rigid polyether polyols. 

The diamine (for example diphenylmethane diamine) is transformed into a rigid polyether polyol by alkoxylation with PO and EO (reaction 20.18). 

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