Influence of Branching on Network Formation and the Thermomechanical Properties of UV-Curable Polyester Powder Coating Resins

A series of branched, UV-curable polyester oligomers were synthesized from neopentyl glycol, terephthalic acid, and variable amounts of trimellitic anhydride (TMA). 1H NMR spectroscopy and MALDI-ToF-MS confirmed that TMA was successfully incorporated into the oligomeric backbone. Differential scanning calorimetry experiments and zero-shear viscosity measurements were conducted to probe the thermal and rheological properties of the branched polyester resins. The Tg and the zero-shear melt viscosity were affected by the extent of branching as well as the functionality of the end group. UV-cured thin films were subsequently prepared and characterized via differential scanning calorimetry, thermal gravimetric analysis, dynamic mechanical analysis, tensile tests, and Soxhlet extractions. In general, the crosslink density of the films increased with the extent of branching. Linear and lightly branched samples (i.e., 2.5 and 5.0 mol % TMA) had higher % elongations at break, while the more highly branched samples (i.e., 7.5 mol % TMA) had higher Young's moduli and tensile strengths. However, further increases in the extent of branching (i.e., 10.0 mol % TMA) negatively affected the mechanical properties of the film due to poor crosslinked network development. The insights from this study highlight the importance of optimizing the extent of branching for UV-curable polyester coating resins.