Stabilisation polyether polyols

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. 

For an improved stabilisation efficiency, the NAD which have longer polyetheric chains than the polyetheric chains of the polyether polyols are preferred for use as continuous phase [4, 6]. 

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 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 ... 

The aromatic nuclei of the bisphenol A segment have a high affinity for the aromatic nuclei of styrene - ACN copolymer styrene units, the polyether chains having a strong interaction with the liquid poly ether medium. As an immediate consequence, the structure 6.15 assures a good steric stabilisation of polymeric dispersions in liquid polyether polyols (see the structure in Figure 6.6). 

This kind of NAD, resulting from the reaction of crude or pure MDI with high molecular weight polyether triols, is very efficient in stabilising copoly (ACN - styrene) dispersions or even polystyrene dispersions in polyether polyols. 

The resultant polymer dispersion of cured epoxy resin is stabilised by a NAD resulting in situ from the reaction of a small part of the hydroxyl groups in the polyether polyol, with the epoxy group of the epoxy resin (reaction 6.27) [89]. 

Shell developed a special line of polymer polyols based exclusively on styrene [11], the stabilisation of the resulting dispersion of polystyrene in liquid polyether being given by special NAD [8, 9, 32, 33] and of course not to the graft species, which in the case of polystyrene are practically absent. 

The rates of thinning of vertically-supported, thin liquid films of polyol solutions of various silicone surfactants have been measured [37]. It was found that the rate for films stabilised by a trimethylsilyl-capped polysilicate (TCP a highly branched silicone not containing polyethers), was much lower than that for the films stabilised by common silicone polyether copolymer surfactants. The retardation of drainage rate was correlated with an increase in surface viscosity. Furthermore, it was noted that PU foams prepared using TCP were significantly more stable than those containing the commercial surfactants. 

As reported in the literature, interference fringes in many vertical aqueous films are horizontal with little curvature. These films also displayed mobile surfaces, rapid and extensive surface flows and rapid rates of drainage [22, 28-30]. These features were all linked to the presence of a low surface viscosity. Based on our experimental observations, the PU films stabilised by DABCO DC 198 have low surface viscosities. This conclusion is supported by reports of measurements of low surface viscosities in polyol solutions containing silicone polyether surfactants [12, 15, 38]. 

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