Effect of Acid Groups in Hydroxyl Functional Resins on the Properties of Polyester-Urethane Powder Coatings

By Bin Wu , Sanjay Padak, Daniel Maetens

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.