Flexible foams from polyester polyols

Commonly used isocyanates are toluene dhsocyanate, methylene diphenyl isocyanate, and polymeric isocyanates. Polyols used are macroglycols based on either polyester or polyether. The former [poly(ethylene phthalate) or poly(ethylene 1,6-hexanedioate)] have hydroxyl groups that are free to react with the isocyanate. Most flexible foam is made from 80/20 toluene dhsocyanate (which refers to the ratio of 2,4-toluene dhsocyanate to 2,6-toluene dhsocyanate). High-resilience foam contains about 80% 80/20 toluene dhsocyanate and 20% poly(methylene diphenyl isocyanate), while semi-flexible foam is almost always 100% poly(methylene diphenyl isocyanate). Much of the latter reacts by trimerization to form isocyanurate rings. 

Polyurethane foams may be rigid, semi-rigid or flexible. They may be made from polyesters, polyethers or natural polyols such as castor oil (which contains approximately three hydroxyl groups in each molecule). Three general processes are available known as one-shot, prepolymer or quasi-prepolymer processes. These variations lead to 27 basic types of product or process, all of which have been used commercially. This section deals only with flexible foams (which are made only from polyesters and polyethers). Since prepolymers and... 

Products intermediate to the flexible and rigid foams may be obtained from castor oil (a trihydroxyl molecule), synthetic triols of moderate molecular weight and polyesters with a moderate amount of trifunctional hydroxyl compound in the strueture. Current practice, however, is to use tipped polyols of the type used for flexible foams with MDI. Semi-rigid foams are used for such purposes as crash pads, car steering wheels and packaging equipment. 

Much work has been done on the incorporation of castor oil into polyurethane formulations, including flexible foams [64], rigid foams [65], and elastomers [66]. Castor oil derivatives have also been investigated, by the isolation of methyl ricinoleate from castor oil, in a fashion similar to that used for the preparation of biodiesel. The methyl ricinoleate is then transesterified to a synthetic triol, and the chain simultaneously extended by homo-polymerization to provide polyols of 1,000, 000 molecular weight. Polyurethane elastomers were then prepared by reaction with MDl. It was determined that lower hardness and tensile/elongation properties could be related to the formation of cyclization products that are common to polyester polyols, or could be due to monomer dehydration, which is a known side reaction of ricinoleic acid [67]. Both side reactions limit the growth of polyol molecular weight. 

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]. 

This relative order explains that numerous physico-mechanical properties of the polyurethanes based on polyester polyols are superior to the polyurethanes derived from polyether polyols or from polyhydrocarbon polyols (this relative order is valuable for PU elastomers, flexible and rigid PU foams). 

From high MW triols or low branched oligo-polyols (MW = 3000-6500 daltons) polyethers, polyesters, filled polyols (polymer polyols), are obtained elastic PU with a low degree of crosslinking (flexible and semiflexible foams, coatings etc). 

Basically, flexible polyurethane foams are formed from polyols and isocyanates. The polyols have hydroxyl functionalities of at least 2 if the foam is to be made by a prepolymer process and of 3 or more if the foam is to be made by a "oneshot" process. The prepolymer process proceeds in two distinct steps while in the one-shot process, all reactants are intensively mixed together at the begiiming of the foaming process. Early in the development of urethane foams, the polyols were polyester diols now, however, by far the largest volume of polyols for urethane foams are poly(propylene oxide) polyols or poly(propylene oxide) polyols containing up to 25... 

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