Viscosity polymer polyols

In the 1990s this approach became more common in order to ensure sufficient compressive strength with the trend to lower bulk densities. Furthermore the proportion of SAN to polyol has been increased to about 40%. This may lead to serious stability problems and care must be taken to control the size and distribution of the particles and prevent agglomeration. Polymer polyols using polystyrene as the polymer component have recently become available (Postech-Shell) and are claimed to exhibit good stability, low viscosity and less discolouration as well as providing price advantages. 

Dow also developed polyurethane foams from polyols via hydroformylation of fatty acids. The foams have properties which are comparable to foams from petrochemicals in terms of density and flexibility. The advantages of using sustainable feedstocks in viscoelastic foams are increased load bearings and tensile and tear properties [39, 40]. The hydroformylation and consecutive hydrogenation of fatty acids derived from seed oil can also be used to form low viscosity polyester polyols. Therefore, fatty acid methyl esters are transesterified with diols, e.g., glycol (Scheme 12). The polymer contains chemically active hydroxy groups which can be used for polyurethanes in coating applications [41]. 

The structure (6.8) is another type of NAD formed in situ by transfer reaction with the tertiary amine type polyethers. Addition of a high molecular weight polyether initiated by an alkanolamine, ethylene diamine, N-methyl substituted propylene diamine, or N,N dimethyl dipropylene diamines in the polyether polyol used for grafting leads to the formation of very stable polymeric dispersions [37]. The solid fraction has particles of low median diameter (<1.5 pm). The resulting polymer polyols have low viscosities which give good stabilisation of the polymeric dispersion. 

The concentration of these kinds of macromers, used in polymer polyol synthesis, is around 2-5% compared to the final polymer polyol. Higher concentrations lead to an undesired, substantial viscosity increase together with a decrease in median diameter of the resulting solid polymer particles. A lower concentration of macromer leads to poor stabilisation of the resulting polymer dispersion. 

It is possible to obtain a bimodal distribution of final particles median diameter (distribution with two maxima), by seeded radical polymerisation. For example, if a polymer polyol having particle diameter of 0.3-0.6 pm is added to the poly ether before the radical polymerisation, after the normal polymerisation of the vinylic monomers, polymer polyols with a bimodal distribution of particles are obtained. As a general rule, a bimodal distribution leads to lower viscosities than an unimodal distribution of particle diameters, at the same solid content [32]. This is a method to obtain high, solid polymer... 

The polymer polyols of low solid content are obtained by dilution of high solid content polyether polyols with ungrafted polyether polyol (the same polyol used for radical polymerisation). At the same solid content, the polymer polyols obtained by dilution of high solid content polymer polyols with ungrafted polyethers, always have a lower viscosity than the polymer polyols obtained by direct synthesis. Thus, a polymer polyol having 20% copoly (ACN - styrene) obtained by direct synthesis, has a higher viscosity... 

MPa-s at 25 °C) than a polymer polyol having the same solid content, but obtained by the dilution of a polymer polyol with 40% solid content (viscosity after dilution around 1800-2100 MPa-s). 

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. 

As is the case for all polymer polyols, the main problem is to obtain high solid contents at lower viscosities. Two general phenomena linked to the viscosity of PIPA polyols are observed ... 

These "polymer-polyols" are made by the in situ polymerization of vinyl monomers such as acrylonitrile (although grafting with other monomers has also been reported in a liquid polyol solution, e.g., polyether triol of molecular weight 3000) to give stable dispersions of the polymeric portion in the liquid polyol. Grafting is carried out with azobis(isobutyronitrile) or dibenzoyl peroxide as initiators at 80-90 °C. A polymer-polyol containing about 20% acrylonitrile appeared to be the best compromise between polyol viscosity and urethane foam properties (108). 

Low viscosity urethane polymers have been prepared from castor od and polymeric isocyanates (82). These low mix viscosity systems are extremely usehd for potting electrical components where fast penetration without air voids, and fast dispensing cycles are desirable. Very low viscosity urethane systems containing castor polyols have been prepared for use in reclaiming water-logged buried telephone cable and for encapsulating telephone cable sphces (83—86). 

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