Thermosetting durable powder coating composition

A process for the preparation of an acid functional polyester resin having an acid value of at least 60 mg KOH/g, using hydroxyl functional components and acid functional components, the hydroxyl functional components comprising components containing 2 hydroxyl functional groups per molecule and/or components containing at least 3 hydroxyl functional groups per molecule, the acid functional components comprising components containing 2 acid functional groups per molecule and/or components containing at least 3 acid functional groups per molecule, wherein

• more than 90 mol. % of the total of hydroxyl functional components in the polyester resin originate from neopentyl glycol (NPG), and
• from 6 to 11 mol % of the total of hydroxyl functional components and acid functional components in the polyester resin originating from hydroxyl functional components and/or acid functional components have at least 3 functional groups per molecule;
• the process comprising:
• in a first stage of the process, adding all the hydroxyl functional components of the polymer to a reaction mixture as well as some acid functional components resulting in a hydroxyl functional pre-polymer;
• and in a second stage, adding the remaining acid functional components to the reaction mixture;
• and wherein in the first stage 15-60 mol. % of all components having at least 3 hydroxyl and/or acid functional groups are added to the reaction mixture, and in the second stage 40-85 mol % of the remaining components having at least 3 acid functional groups are added to the reaction mixture.