Overview

In the previous chapters (Chapters 4-11) the chemistry and technology of oligo-polyols for elastic polyurethanes (PU; flexible and semiflexible PU foams, elastomers, adhesives, coatings, sealants, elastic fibres and microcellular elastomers) were presented. The elastic polyurethanes are the most important in commercial applications of polyurethanes, having around 72% of the global polyurethane market. Rigid polyurethanes, especially in the form of rigid polyurethane foams, wood substitutes, flotation and packaging materials represent, at this moment, around 28% of the global polyurethane market.

The oligo-polyols for rigid polyurethanes have two important characteristics: they are highly branched having a high functionality (around 3-8 hydroxyl groups/mol) and the chain derived from one hydroxyl group is short (the equivalent weight is low) [1-4]. As an immediate consequence of this special structure, reacting these polyols with aromatic diisocyanates [or polyisocyanates such as crude diphenylmethane diisocyanate (MDI) or polymeric MDI (PAPI)] gives a highly crosslinked and very rigid polyurethane structure. The high density of urethane groups, as a consequence of their low equivalent weight, leads to strong and intensive interchain forces, by hydrogen bonds, and an increased rigidity. Thus, the oligo-polyols for rigid polyurethanes (mainly for rigid polyurethane foams), have a much higher hydroxyl number than the oligo-polyols for flexible polyurethanes, for the majority of oligo-polyols in the average range of 300-600 mg KOH/g (some special polyols have a hydroxyl number outside this range, e.g., 200-300 mg KOH/g and 600-800 mg KOH/g). The high concentration of hydroxyl groups leads to a high density of hydrogen bonds formed between...