TABLE OF CONTENTS The polyols for rigid foams (referred to as rigid polyols) discussed before (Chapters 13 and 14), are based on the alkoxylation of different polyols or polyamines, commercialised in a relatively high purity form. Some important starters for rigid polyols are obtained by the synthesis of the starter in situ, before the alkoxylation reaction, by the condensation reaction of some aromatic compounds (phenols, melamine and so on) with aldehydes (mainly formaldehyde), followed by the reaction of the resulting condensate with alkylene oxides. Some important rigid polyols based on the condensates mentioned are:
Mannich polyols,
Novolak-based polyols,
Melamine-based polyols.
Mannich polyols is a very important group of aromatic polyols obtained by the alkoxylation with propylene oxide (PO) [(and/or ethylene oxide (EO)] of the Mannich bases obtained by classical Mannich reaction between phenols (for example phenol, p-nonylphenol), formaldehyde and alkanolamines (diethanolamine, diisopropanolamine, monoethanolamine, monoisopropanolamine and so on). Synthesis of Mannich polyols is divided into two important steps:
Synthesis of Mannich base (Mannich reaction), and
Alkoxylation with PO (or with PO and EO) of the synthesised Mannich base.
a) Synthesis of Mannich Base [2 -16 , 22 , 23 ]
The Mannich bases are generally obtained by the stepwise addition of aqueous formaldehyde (25-37%) to a physical mixture of phenol - alkanolamine, at 50-70 C. The reaction (reaction 15.1) is slightly exothermic and needs cooling in order to maintain the reaction temperature. After the addition of formaldehyde,...
