Oligo-polyols for rigid polyurethanes

As was mentioned previously, for practical reasons, the oligo-polyols are divided in the present book into two important groups oligo-polyols for elastic polyurethanes and oligo-polyols for rigid polyurethanes. 

Oligo-polyols for elastic polyurethanes Oligo-polyols for rigid polyurethanes... 

In Chapter 12, the oligo-polyols for rigid polyurethanes, having low MW (less than 1000 daltons) and high functionality (of around 3-8 hydroxyl groups/mol) will be discussed. 

The most important oligo-polyols for rigid polyurethanes are polyether polyols and aromatic polyester polyols [1-4, 6]. The aromatic polyether polyols, based on condensates of aromatic compounds with aldehydes, become very important polyols, especially after the introduction of new blowing agents (see Chapter 21). 

After this general presentation of oligo-polyols for rigid polyurethanes, each group of polyols will be presented in detail in the next few chapters, in order of importance, the most important being the group of polyether polyols, followed by the polyester polyols. 

In order to simplify the procedure of using too many components, a masterbatch , that is a mixture of the components that do not react with each other, (e.g., oligo-polyol, water, chain extender, catalysts, etc.), is made before foaming. Then it is possible to use only two components one is the polyolic component (called component A or formulated polyol, containing a mixture of all raw materials except for the isocyanate, in the proportions needed) and the second component is the isocyanate (called component B or isocyanate component). The polyurethane that results is a consequence of the very efficient contact between the isocyanate component and the polyolic component. Usually, in rigid PU foams only two components are used. In flexible foams, the polyolic component is divided into two components, especially in order to avoid the contact of some hydrolysable component with water, (e.g., stannous octoate). The gravimetric ratio between the components is verified before the foaming process and if necessary, it is corrected. 

For crosslinked polymers (in this category they are the majority of polyurethanes, for example flexible, semiflexible and rigid PU foams, etc.), which have a MW that is practically infinite [12], the molecular weight between the branching points (Mc) is considered. The value of Mc depends strongly on the oligo-polyol structure. 

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