Biobased Ketal-Diester Additives Derived from Levulinic Acid: Synthesis and Effect on the Thermal Stability and Thermo-Mechanical Properties of Poly(vinyl chloride)

A new class of biobased plasticizers is herein proposed as a replacement of the currently used additives. The molecules were synthesized by a selective protecting group -free route, which was developed in order to produce valuable asymmetrical ketal-diester derivatives from the renewable levulinic acid, overcoming the typical drawbacks of these types of reactions. Five molecules with different side chains were successfully obtained and fully characterized by means of NMR and FT-IR spectroscopies. These ketal-diester additives were then tested in poly(vinyl chloride) (PVC) as the model polymer. Their compatibility and plasticizing effect were evaluated by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic-mechanical thermal analysis (DMTA), and thermogravimetric analysis (TGA). The emerged trend showed a clear correlation between the structural features of the ketal-ester and the thermal and mechanical properties of the material. In particular, the best results in terms of glass transition temperature reduction were achieved when the isobutyl- and benzyl-terminated additives were used. Leaching tests in both hydrophilic and hydrophobic environments were also performed. No significant aqueous leaching was found; despite the aggressive treatment in hexane, the extraction for all additives was remarkably low. Importantly, when compared to commercial plasticizers such as di-2-ethylhexyl phthalate (DEHP), 1,2-cyclohexane dicarboxylic acid diisononyl ester (Hexamoll DINCH), acetyl tributyl citrate (ATBC), and diethylhexyl adipate (DEHA), the proposed ketal-diester additives performed comparably and, in some cases, even better. In particular, it was observed that the plasticizing effect already starts at lower concentrations, permitting one to use significantly less additive to obtain a similar effect to the one achieved with the nowadays available plasticizers.