Polyester Polyol Building Blocks
Polyesters are a class of polymers allowing for an enormous amount of structural and property design. Polyesters are the products of condensation polymerization between polyhydroxy and polyacid monomers. In some cases, a polyester or anhydride monomer is substituted for the polyacid, usually dependent on processing parameters related to difficulties associated with insoluble acids, inconvenience associated with relative cost, methanol toxicity, or incompatibility of manufacturing materials with the condensation water product (Fig. 2.22). Polyester chemistry is relatively straightforward, the monomers are usually not expensive, and the polymer properties are reasonably easy to anticipate . There are a large number of building blocks and a large number of commercial polyesters to choose from when designing a specific polyurethane. Table 2.3 is an abbreviated but representative list of common building blocks for construction of polyester soft segments for polyurethane synthesis.
Although the process of combining polyacids and polyalcohols to form polyesters is generically similar in every case, the individual monomers are obtained from vastly different building blocks and sources. In fact, each building block can be obtained by multiple routes, each one having advantages, disadvantages, and economic sensitivities. For instance, adipic acid can be obtained from cyclohexane or from butadiene . Butanediol can be obtained from propanediol (to allyl alcohol followed by hydroformylation and hydrogénation), acetylene, and formaldehyde or from fermentation of
TABLE 2.3 Structures of acids and alcohols common to polyester polyol synthesis for making polyurethanes
sugar to succinic acid followed by reduction . Terephthalic acid is produced by oxidation of xylene . Thus, each of these monomers will vary in price independently of all the others, their manufacturers will be independent of one another, and the macro-economic pressures may vary independently. In contrast to polyether manufacturers who generally are back integrated into the building blocks (EO, PO, THF), polyester polyol manufacturers purchase building blocks such as those in Table 2.3 from several sources and will qualify multiple sources as primary suppliers using contracts and futures strategies. This also results in polyester manufacturers being "middleman" manufacturers, primarily suppliers to other businesses. Thus, the three largest polyester manufacturers in the world are, in order, Stepan Corporation, Coim Corporation, and Huafeng Group, all certainly well-regarded companies, but not generally well known or in the popular consciousness. The lone exception in this regard would be BASF, which is a major chemical manufacturer with extensive back integration into polyester building blocks and forward integration into products that use polyester polyols. On the other hand, BASF produces only about Vi the volume of either of the two largest polyester polyol producers .