Polyester resin offers many advantages, such as: low cost, adequate resistance to water and many chemicals, resistance to weathering and aging, reasonable temperature resistance (up to 80°C), good wetting to glass fibers, low shrinkage (4%-8%) during curing, and linear thermal expansion (100-200·10⁻⁶ K⁻¹). Depending on the starting materials, polyesters with a wide range of properties can be produced (Gubbels et al., 2018):

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High molecular mass linear polyesters (M_n > 10,000 g/mol) - produced from bifunctional alcohols and dicarboxylic acids (or derivatives) or from lactones - are usually thermally processed into molded materials and are often compounded with various additives.

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Low molecular mass polyesters (M_n < 10,000 g/mol) that are produced from saturated aliphatic or aromatic dicarboxylic acids and di-/trifunctional alcohols, are linear or slightly branched intermediates for polyurethanes and non-alkyd coating resins.

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Low molecular mass polyesters (M_n < 10,000 g/mol) that are produced from di-, tri-, and polyfunctional alcohols and polyfunctional (aromatic) carboxylic acids combined with (un)saturated fatty acids, are classified as alkyd resins.

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Unsaturated polyesters that can be copolymerized with unsaturated compounds, and are formed from polyfunctional alcohols and polyfunctional unsaturated carboxylic acids. After copolymerization with monomers (e.g., styrene) they can also be classified as thermo-sets.

The polyesterification reaction is reversible and thus it is influenced by the presence of water by-product in equilibrium with the reactants and the polymer formed. The polyesterification reaction proceeds at over 100°C leading to acid half-esters produced by the opening of the anhydride ring, but the reaction exothermicity raises the temperature over 150°C when the half-esters condense into polymers with formation of water by-product. As the viscosity of reaction mixture increases (limiting the water removal), the temperature is gradually increased to 220°C to keep a steady evolution of water condensate. Note that resins normally lose 8%-12% of initial charge weight as condensate (Nava, 2015). Since the water formed hinders the chemical equilibrium and limits the achievable conversion, the removal of water in the latter part (at higher conversion) is crucial for the development of the desired molecular weight (MW) that gives the structural performance of the polyester. In practice, water has to be continuously removed (e.g., by distillation) to drive the reaction to completion. If needed, polyesterification can be reversed by injecting steam into the reaction mixture in order to control the final MW attained by the polymer.

Polyesterification is usually carried out in the presence of an inert gas (e.g., nitrogen or CO₂) to prevent discoloration. The rate of inert gas is increased toward the final stage to enhance the removal of residual water. The removal of water can also be improved by azeotropic distillation (with aromatics) or by processing under vacuum but the latter is rarely used in large scale processes.

The reaction rate can be accelerated by acid catalysts, such as paratoluenesulfonic acid (PTSA) or tetrabutyl titanate, but tin salts (hydrated monobutyl tin oxide) are preferred to ensure product stability during storage (Nava, 2015). The viscosity of the polyester formed limits the progress of MW development, with a typical number-average molecular weight values (M_n) in the range of 1800-2500. Other side reactions (influenced by the choice of reactants) can also modify molecular weight growth, e.g., transesterification, formation of cyclic esters or addition products.

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