Liquid polyether polyols

DMC catalysts are real heterogeneous coordinative catalysts [2, 51, 52]. At the end of polymerisation, the catalyst is dispersed in the liquid polyether polyols in the form of small solid particles of around 200 nm (0.2 im) diameter. By dilution with w-hexane and filtration, it is possible to achieve a quantitative removal of the DMC catalyst [51, 52]. 

The homopolymerisation and copolymerisation of vinylic monomers in liquid polyether polyols are typical chain reactions by radical mechanism and are characterised by initiation, propagation and termination steps [31] ... 

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). 

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 very unconventional way to obtain a polyurea dispersion is to react gaseous carbon dioxide, at higher pressures (> 20 MPa) and at around 50 °C, with diamines dissolved in liquid polyether polyols [74] ... 

A polymer polyol, consisting of an insoluble polyamide, finely dispersed in the liquid polyether polyols is obtained. The reaction is developed at 100-130 °C, under vacuum, for the elimination of the resulting ethanol. The stability of the resulting dispersions is probably... 

In all the previous variants for propoxylation of solid polyols one thing is very important the second substance added must solvate the solid polyol well. The reaction between gaseous monomer and solid polyol takes place at the surface, and partially, with the solvated polyol. Liquid adducts of PO to the solid polyol are formed. These adducts are solubilised into the liquid reaction medium and, step-by-step, all the solid is transformed in liquid polyether polyols. If the liquid reaction medium does not solvate the surface of the solid polyol well, a large quantity of unreacted polyol remains at the end of the propoxylation reaction. 

Unfortunately when KOH is used as catalyst, a suspension of sucrose in polyether polyol cannot be propoxylated totally, a substantial part of sucrose always remains unreacted. Sometimes, a very unpleasant phenomenon appears during propoxylation of solid sucrose suspended in a liquid polyether polyol, in the presence of KOH as catalyst. An aggregation of solid particles of sucrose into big particles takes place, which makes stirring impossible. This proves that polyether is a modest agent for sucrose solvation. 

Most moisture-curing liquid adhesives utilize poly(oxypropylene) (PPG) polyols, as shown above. These raw materials produce among the lowest-viscosity prepolymers but may not have sufficient modulus at higher temperatures for some applications. A certain percentage of polyester polyols may also be utilized to boost performance, but these may cause a large increase in viscosity, and so they are more often used in conjunction with polyether polyols to provide a high-performance adhesive with workable viscosities. Poly(butadiene) polyols may be utilized for specific adhesion characteristics. 

Polyether polyols are high molecular weight polymers that range from viscous liquids to waxy solids, depending on structure and molecular weight. Most commercial polyether polyols are based on the less expensive ethylene or propylene oxide oi on a combination of the two. Block copolymers are manufactured first by the reaction of propylene glycol with propylene oxide to form a homopolymer. 

A reactive liquid epoxide used as an organic solvent and surfactant intermediate its polymers can be used for polyester, polyurethane, and polyacrylic resins, polyether polyols, flame-retardants, etc. 

In practice, it is very important to obtain a high primary hydroxyl content with minimum EO quantity. A high EO content leads to turbid polyether polyols because longer poly[EO] chains are insoluble in liquid polypropylene oxide. The flexible PU foams made with highly ethoxylated polyols have poor humidity/ageing/degradation characteristics and a lower compression strength. 

The synthesis of polyether polyols by anionic polymerisation of gaseous monomers such as PO (bp 33.6 °C) and EO (bp 10.3 °C), at 100-125 °C, is a strong, diffusion dependent process. Of course the polymerisation reaction takes place in a liquid state, where the anionic catalyst is present. 

The gas-liquid contactor type reactors are extremely safe and may be considered the best reactors for the synthesis of polyether polyols by anionic polymerisation of alkylene oxides, initiated by various polyolic starters (Figure 4.31). 

The odour of polyether polyols is eliminated by several methods. One efficient method is to introduce into the liquid hot polyether polyol (at around 120 °C), under vacuum, fine drops of liquid water [142]. Water, of course, is transformed into a vapour, which is eliminated together with traces of substances that give the odour (some of these substances may give azeotropic mixtures with water, with a decrease in the boiling point). 

Polymer polyols are defined as very fine and stable dispersions of solid polymers (vinylic polymers and copolymers, polyurea, polyurethanes) in liquid polyethers. Currently polymer polyols represent one of the most important group of polyolic intermediates for elastic polyurethanes [1-10]. 

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]. 

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 practical technique to obtain polymer polyols by radical polymerisation is to add an homogeneous mixture of vinylic monomer, initiator, chain transfer agent and a part of polyether polyol, to the rest of polyether polyol containing the NAD (macromer or nonreactive NAD), at 115-125 °C. The mechanism of solid polymer particle formation during radical polymerisation of vinylic monomers in liquid polyethers, in the presence of a nonreactive NAD, in the form of very stable dispersions, is described next. 

A very simple method to obtain polymeric dispersions in liquid polyethers is to make a mixture between a polyether polyol and a polymeric latex, such as the azeotropic copolymer styrene - ACN (StACN copolymer), obtained by emulsion copolymerisation, having around 20-40% solid content. The water is eliminated step-by-step by vacuum... 

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. 

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 ... 

A relatively new generation of filled polyols was obtained by the reaction of an epoxy resin with an epoxy hardener in situ, in liquid polyether media. Cured epoxy resins, finely dispersed in the liquid polyether (with around 20% solid content), are obtained. 

Other polycondensation reactions which lead to finely dispersed polymers in liquid polyethers are the polycondensation reactions of urea and melamine with aqueous formaldehyde [92-95]. The reaction medium is usually polyether polyols, PO homopolymers or PO-EO copolymers (random or block copolymers), with MW of 3000-5000 daltons. During the polycondensation reaction, the aminoplast polymer precipitates, being insoluble in polyether and water (water from formaldehyde solution and reaction water), is eliminated by vacuum distillation. A variant of this reaction is to develop the polycondensation in water, and water containing the aminoplast polymer (as a viscous solution) is added to a polyether polyol, under vacuum, and at high temperature (100-130 °C), water being continuously eliminated from the reaction medium. The aminoplast insoluble polymer precipitates in the form of fine particles. 

Polyether polyols for rigid PU foams are obtained in the same type of polymerisation reactors as those used for high molecular weight polyether polyols, i.e., in stainless steel loop reactors, with an external heat exchanger, preferably with the possibility of generating a large surface of the liquid reaction mass, by a spray technique or by an ejector technique... 

Reactors used for the synthesis of rigid polyether polyols need an internal stirrer, because frequently high melting point polyols (such as pentaerythritol or sucrose) are used as starters and the initial reaction mass is a suspension of solid polyols in liquid. 

In order to decrease the total reaction time, a small reactor, with a stirrer, is linked to the polymerisation reactor, for the preparation of the initial starters - catalyst mixture. In this reactor, there are 1-3 polyols used as starters, the catalyst (KOH, NaOH or a tertiary amine) and sometimes, for solid polyols, an initial liquid medium (for example a part of an intermediary or final polyether polyol called heel , or an inert solvent). Generally, in the synthesis of polyether polyols for rigid foams it is preferred to avoid the utilisation of inert solvents, which need recycling and a more complicated installation. 

The synthesis of rigid polyether polyols, by polymerisation of PO or EO, initiated by polyols which are liquid under the conditions of the polymerisation temperature, is simple, and similar to the synthesis of the prepolyether by propoxylation of glycerol (see Chapters 4.1.1 and 4.1.5). 

The single polyol from this group that needs special attention is sorbitol, which is delivered in the form of an aqueous solution of around 70%. It is possible to use solid sorbitol, which is delivered in the form of crystalline monohydrate, but it is much more expensive than liquid sorbitol (calculated as a dry substance) and more difficult to handle and melt. The polyols delivered as aqueous solutions need water distillation under vacuum, in order to limit the formation of polyether diols during the reaction with PO, which decreases the functionality of the resulting polyether polyols. There are two possibilities to distill water until a relatively low level (0.1-0.5%) is reached or to make a controlled distillation of water, by stopping the distillation at a level of water which, together with sorbitol should lead to a functionality of 4.5-5 hydroxyl groups/mol. 

The polymerisation of PO and EO, initiated by polyfunctional starters, to make short chain polyether polyols is a reaction that is strongly dependent on diffusion. The consumption rate of PO or EO is given by two simultaneous factors the rate of the chemical reaction in the liquid phase and the efficiency of the monomer mass transfer from the gaseous phase to liquid phase (see details in section 4.1.5). The PO (or EO) consumption rate, considering the mass transfer, is described by equation 13.27 [45-50] ... 

The polyether polyols for rigid PU foams based on polyols which are liquid under the conditions of alkylene oxides polymerisation are glycerol and TMP polyether triols, of various molecular weights, sorbitol-based polyols (based on a mixture of sorbitol - glycerol, sorbitol - dipropyleneglycol, sorbitol - dithylene glycol) and xylitol-based polyether pentaols. 

In Table 13.4 the characteristics of some rigid polyether polyols based on a sorbitol -glycerol mixture are presented. The initial starter mixture is solution of sorbitol (70%) and glycerol. After water vacuum distillation, the mixture of sorbitol - glycerol containing 0.1-0.5% water, is propoxylated in the presence of a KOH catalyst, followed by the usual purification. These polyether polyols are transparent viscous liquids, which are colourless or slightly yellow polyols ... 

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