Effect of acid groups in hydroxyl functional resins on the properties of polyester-urethane powder coatings

Hydroxyl-terminated polyester resins with different acid numbers were synthesized and used in powder coating formulations. Coatings were formulated with isophorone diisocyanate (IPDI)-based hardeners and tested for mechanical performance, appearance, and weathering. This study found that resins containing a high concentration of residual acid groups (also referred to as undercondensed resins) had to be formulated according to their total reactive number, the sum of acid and hydroxyl numbers, in order to achieve complete cure. High acid numbers also caused poor overbake and salt spray resistance. The addition of adipic or benzoic acids to the powder formulation accelerated cure.