Accelerated clearcoat weathering studied by dynamic mechanical analysis

Automotive basecoat/clearcoat (BC/CC) systems of the acrylic polyol/melamine formaldehyde (ACR/MF) type were analyzed by dynamic mechanical analysis (DMA) before and after exposure to QUV. DMA changes after 500 h exposure were compared to gloss retention after five year Florida exposure or 2500 h QUV exposure. DMA readily distinguished between ‘poorer’ and ‘better’ BC/CC films received in a blind study. Crosslink density and T_g both increased during QUV. DMA changes depended strongly on the presence or absence of hindered amine light stabilizer (HALS). Variations in the basecoats had only minor effects on DMA results. Poly(urethane) clearcoats (without basecoats) were also studied. Differences in weathering behavior of films prepared with three widely used poly(isocyanates) are reported. Crosslink density decreased and T_g increased during QUV exposure. Stabilizers were very effective in the poly(urethane) clearcoats. DMA results are discussed in relation to FT-IR spectra obtained before and after QUV.