Modeling of Processes for Unsaturated Polyester Production

Description of the branched structure of the resins is less problematic than in the case of alkyds, because the conversion of double bonds by Ordelt reaction is in the range 10 to 20%, so the molecules are almost comb-like and the approximations used by Yang and Pascault [140] should be reasonable. 

Modeling of alkyd resin production is a rather formidable task because of the high number of distinguishable chemical groups, the branched structure of the polymer, a nonnegligible amount of intramolecular reaction, and side reactions of the double bonds in fatty acids. The usual problems found in previously discussed polyesterifications, namely lack of data for liquid-vapor and Uquid-sohd equilibria and associated mass transfers, are also present. 

Industrial processes [141-143] use batch stirred reactors, connected to partial condensers in order to recover volatile glycols. Azeotropic distillation with xylene (for instance) is often used. Monomers may be added in several steps to overcome solubility problems. Vacuum and inert gas sparging is also used in different stages. Catalysts (mostly alkyltin salts) are used to convert most of the carboxyl groups, as specifications often require less than 1 mg g KOH acid value. Long reaction times are sometimes avoided through different techniques for eliminating residual carboxyls. 

The same plant usually produces different varieties of resins, according, for instance, to fatty acid content. New compositions are often sought in order to improve end use properties or to compensate for fluctuations of raw material prices. Therefore, it is often necessary to look for initial amounts of monomers which satisfy stoichiometric constraints (such as mole ratio and fatty acid content), will not lead to gelation, and meanwhile keep an acceptably high M (for instance). 

An early review of the foundations of the macromolecular chemistry of alkyds has been presented by Kienle [143] and simple methods for predicting gelation conversion have been reviewed by Jonason [144]. These predictions are not very accurate, but good data on intramolecular reactions and differences of reactivity of functional groups will be needed if better control of physico-chemical properties is sought. 

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