Characteristics of dual-curable blocked isocyanate with thermal radical initiator for low-temperature curing of automotive coatings

Abstract

Dual-curable blocked isocyanate with a thermal radical initiator (BL-Tri-cHD) was designed and synthesized to perform low-temperature curing of automotive coatings. BL-Tri-cHD can simultaneously react with both unsaturated C=C double bonds for free-radical polymerization and hydroxyl groups for the urethane reaction in hydroxyl-functionalized urethane methacrylate oligomers (HFUMO) under mild thermal curing conditions. The dual crosslinking behavior and mechanical properties of HFUMO-based coatings using BL-Tri-cHD were compared with those of a commercial blocked isocyanate thermal crosslinker (Desmodur (R) BL-PL350). The onset temperature for initiating cured by BL-Tri-cHD was about 68 degrees C, confirming faster crosslinking at relatively lower temperature than BL-PL350, which initiated curing at 140 degrees C. To quantitatively evaluate the properties of cured coatings, a nano-indentation was made in the surfaces of clearcoat films cured at 150 degrees C, 130 degrees C, and 120 degrees C. Films cured using BL-Tri-cHD and BL-PL350 at 150 degrees C exhibited similar indentation patterns. The indentation curves for the films with BL-Tri-cHD cured at lower curing temperatures such as 130 degrees C and 120 degrees C showed favorable penetration depth profiles similar to the sample cured at 150 degrees C, whereas the films with BL-PL350 cured below 150 degrees C had a less dense network structure, resulting in a deeper penetration depth than the sample cured at 150 degrees C. Therefore, we report that dual-curable BL-Tri-cHD demonstrates promising performance in automotive coating applications and possesses the ability to form highly crosslinked networks, even at low curing temperatures.