Graft polyether polyols

Graft polyether polyols are synthesised by in situ radical polymerisation of vinylic monomers in liquid polyethers, by batch, semi-continuous or continuous processes. The solid fraction varies between 10-50%, more frequently being between 10-40% [1-10]. 

The selected monomers are acrylonitrile (ACN), styrene, a-methyl styrene methylmethacrylate, hydroxyalkyl acrylates and methacrylates, vinyl chloride and others [1-10, 13-18]. The most favoured monomers for industrial production of graft polyether polyols are ACN and styrene [1-10, 18-29]. The resulting products from the radical polymerisation of vinylic monomers in polyethers are opaque, generally white dispersions (except those derived from ACN, which are yellow dispersions). A graft polyether polyol has three polymeric components  

The third component, the graft species, acts as a nonaqueous dispersant (NAD), a compound having, in the same structure, polyetheric chains and vinylic polymer chains. This compound assures the stability of the resulting polymer dispersion and prevents sedimentation and coalescence of the vinylic polymer particles [1-5]. The mechanism of this dispersion stabilisation will be discussed later. The median diameter of solid particles for a performance polymer polyol is generally less than 1 [am, usually 0.2-0.5 im [30]. 

Of course, a fundamental role in the formation of very fine particles plays the stabilising efficiency of the nonaqueous dispersant. 

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Modified polyether polyols have appeared in recent years, i.e., graft polyether polyols (polymer polyols, copolymer polyols) which were first developed by Union Carbide Corp. in the mid-1960 s. 

The utilisation of the high MW aminic polyether polyols in the synthesis of polymer polyols [graft polyether polyols and polyisocyanate polyaddition (PIPA) polyols] is presented in Chapter 6 [148, 151]. 

As mentioned before, the most important monomers for the industrial production of graft polyether polyols are ACN and styrene. 

Thus, the most important commercial graft polyether polyols are based on ACN - styrene copo ymers styrene content 50-80%). The graft polyethers, having a high content of styrene, do not have a tendency to scorch . 

The graft polyether polyols, based exclusively on ACN or with a high ACN content (more than 50% ACN in the monomer mixture), do not need a special NAD due to the formation of graft species, the inevitable dispersion stabilisers formed in situ, the resulting dispersions being perfectly stable. 

The macromers used in the stabilisation of polymer dispersions are in fact polyether polyols with terminal double bonds, able to copolymerise with vinylic monomers (ACN, styrene) and to form graft species during the radical copolymerisation. The resulting graft polyether polyol, formed in situ by the copolymerisation process, is in fact a NAD ... 

Generally, the nonreactive NAD is used in higher concentrations, than the macromers in graft polyether polyols synthesis, of around 10-15% compared to the final polymer polyol. 

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 ... 

Usually the viscosity of PHD polymer polyols is higher than the viscosity of graft polyether polyols, at the same solids content. For example a graft polyether polyol, with a 20% solid fraction (copoly[ACN - styrene]), has a viscosity of 2000-3000 MPa-s at 25 °C, but a PHD polyol, with the same solids concentration has a viscosity of 3000-3500 MPa-s at 25 °C [10, 67-69]. This high viscosity is direct evidence of the intensive interaction, by secondary forces, between the polyurea filler and the continuous liquid polyether phase. 

Usually an equimolecular mixture diamine, TDI is used (a very small excess of TDI is frequently used to compensate for the consumption of TDI in reaction with hydroxyl groups), which normally leads to maximum molecular weight in the resultant polyurea. As a general observation, the molecular weight of solid polyurea from PHD polyols has a lower molecular weight than the copoly [ACN - styrene] obtained as filler in graft polyether polyols. 

Generally, PHD polyols are obtained with 5-20% solid content (maximum 28%) and have a similar aspect with graft polyether polyols, i.e., white opaque dispersions, the median diameter of particles being < 1 im. 

In the practice, the most important polymer polyols are graft polyether polyols, PHD and PIPA polyols, but other good quality polymer dispersions in liquid polyethers have been created, which at this moment are not industrially important, such as ... 

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