PHD Polymer Polyols Polyurea Dispersions

PHD polymer polyols are a special class of filled polyols developed successfully by Bayer, PHD being the abbreviation of the German name polyharnstoff dispersion or polyurea dispersions [67-69]. PHD polyols contain organic urea, oligomeric or polymeric polyurea, finely dispersed in liquid polyether polyols [67-73]. The difference between PHD polyols and graft polyether polyols is the different nature of the solid polymer dispersed (it is a heterocatenary polymer - polyurea - instead of carbocatenary polymer) which is obtained by another synthetic procedure (polyaddition reaction between a diisocyanate and a diamine instead of radical polymerisation). The reaction between the diisocyanate and the diamine, takes place in situ (reaction 6.19), in liquid poly ether. The resultant polyurea being insoluble in polyether, precipitates in the form of very fine particles  

A small part of the -NCO groups of diisocyanate reacts with the terminal hydroxyl groups of polyethers and forms a block copolymer structure, having a polyurea segment chemically linked to a polyether segment, which plays the role of a nonaqueous dispersant and assures the excellent stability of the resulting polyurea dispersion (reaction 6.20) [67, 68]. 

The concentration of this hybrid structure (polyether block linked to a polyurea block) is low, but it is enough to assure a good stability of the dispersion. This very reactive system is based on the big difference in reactivity between primary amines and hydroxyl groups and the -NCO groups of the diisocyanate. Thus the -NCO group reacts 3300 times more rapidly with primary amines than with secondary hydroxyl groups and around 1000 times more rapidly than with the primary hydroxyl groups. 

This big difference in the reactivity of isocyanates, with primary amines and with hydroxyl groups results in the polyurea being formed preferentially - only a small part of the hydroxyl groups being reacted. 

The most favoured diamine used in practice is hydrazine [8-10, 69-73], as hydrazine hydrate (around 65% hydrazine and 35% water). By the reaction of hydrazine with TDI in liquid polyethers, a polyhydrazodicarbonamide is formed in situ, having a solid fraction of 5-40% (the preferred level is 10-20%), the median diameter of the solid particles being less than 1 im. 

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