Toxicity Reduction in Powder Coatings: Synthesis an Environmentally Friendly Trimellitic Anhydride-Free Solid Polyester Resin

Abstract

Hypothesis: Trimellitic anhydride (TMA) monomer, a toxic substance, is widely used in powder coating formulations, posing severe respiratory risks to workers in this industry. This study aims to eliminate TMA monomer in
saturated polyester resin formulations for hybrid coating systems while maintaining physical-mechanical resistance comparable to conventional systems
Methods: To achieve this, trimethylolpropane (TMP) was employed for branching and adipic acid was used to carboxylate the branches. The degree of branching (functionality) was varied by adjusting the TMP content from 0 to 2 moles per polyester chain with a molecular weight of 4000 (g/mol). The synthesized resin was characterized using Fourier-transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (¹H-NMR) spectroscopy. Its properties were evaluated through differential scanning calorimetry (DSC), gel permeation chromatography (GPC), acid value, and viscosity measurements.
Findings: The resulting polyester resins exhibited molecular weights of 3900–4400 g/mol, glass transition temperatures (Tg) of 44–63°C, acid values of 32–56 mg_KOH/g and viscosities of 15–38 poise. Powder coatings with thicknesses of 50–200 μm were prepared (curing conditions: 200°C for 10 min), and their performance was assessed.
Findings revealed that increasing TMP content led to reduced gloss, an initial increase followed by a decrease in hardness, direct/indirect impact resistance, and gel fraction However, all samples demonstrated excellent adhesion (5 B) and mandrel bending resistance. Consequently, the optimal formulation contained 1.5 moles of TMP yielding a carboxylated solid polyester resin with a molecular weight of 4210 g/mol viscosity of 34.5 poise, acid value of 52 mgKOH/g, and Tg of 56.5 °C. The resulting coating exhibited superior properties: gloss of 94%, pendulum hardness of 132 s direct and indirect impact resistance of 160 μm and 95 μm, gel fraction of 98%, crack free mandrel bend resistance, and 5B metal adhesion