Development of Sustainable Thermoplastic Zein by Partial Substitution of Glycerol with Olive Oil or Oleic Acid: a New Route for Injection-Molded Biopolymers

Thermoplastic zein formulations containing 25 wt% total modifier were processed by extrusion and injection molding. Glycerol was progressively replaced with oleic acid (OA) or olive oil (OO) at substitution levels ranging from 2.5 to 10 wt%, while maintaining the zein and total modifier contents at 75 and 25 wt%, respectively. The formulation containing 10 wt% OO could not be successfully processed. Partial substitution with OO increased tensile stiffness and strength, reaching 1294 MPa and 25.5 MPa, respectively, for ZEI-GLY-7.5OO, while maintaining elongation at break close to, but below, that of the glycerol-only control. Conversely, substitution with OA generally reduced tensile strength and elongation at break. All modified formulations exhibited lower impact strength than the control. Differential scanning calorimetry revealed low-temperature calorimetric transitions at 2.0–6.3 °C for the OA-containing formulations and − 17.4 to -14.2 °C for the OO-containing formulations, whereas DMTA showed a principal thermomechanical relaxation around 50–60 °C. Thermogravimetric analysis showed onset degradation temperatures of approximately 198 °C for the OO-containing materials, compared with 173 °C for the control and 154–172 °C for the OA-containing formulations. Water contact angles increased from 26° for the control to approximately 30–50° after OA or OO incorporation, indicating reduced surface wettability. Most formulations achieved complete disintegration within four weeks under simulated composting conditions, and all materials were completely disintegrated by week 5. Overall, among the compositions investigated, partial substitution of glycerol with OO produced a favorable tensile-property profile while retaining rapid disintegration. However, these responses reflect the combined effects of OO incorporation and the simultaneous reduction in glycerol content.