Mechanism of the thickening reaction of polyester resins: Study on models

The thickening reaction of polyester resins is investigated using model compounds, diesters or polyesters of 1,2‐propanediol and phthalic acid with dihydroxy or dicarboxy end groups. Soluble magnesium acetylacetonate was found to be an effective thickener, and for the first time it was possible to study a thickening reaction in homogeneous media (THF solutions). The results are compared with those obtained with MgO. The proposed mechanism supposes that the central step consists of the formation and the aggregation of magnesium carboxylates in ionic areas, with entanglement of attached polyester chains. This fits our experimental results better than the hypothesis of coordination of magnesium carboxylates by ester or hydroxyl groups of the chains2–6 or than the hypothesis of the formation of linear high molecular weight compounds.7–9