Molecular relaxation phenomena during accelerated weathering of a polyurethane coating

Results from a polyester-urethane (PU) coating system under accelerated weathering showed that crosslink density, obtained from high-temperature modulus data, diminished due to chain scission. However, T _g and room-temperature tensile modulus both increased with weathering. Molecular relaxation phenomena in polymers have long been investigated to explore changes occurring in a polymer at temperatures below its glass transition and were explored for an explanation to supplement chemical degradation for these observations. Relaxation was quantified using “enthalpy recovery” which first increased with exposure, then diminished. The concurrent physical and chemical aging effects were characterized by tracking nonexponentiality in the spectrum of relaxation times, and the size of “co-operatively relaxing regions” deduced from relaxation around the glass transition. Mechanical relaxation in this coating extended longer than cycle periods typical of accelerated weathering, suggesting that frequency effects might be important when comparing accelerated to natural weathering. This paper was awarded Third Place in the John A. Gordon Best Paper competition, presented as part of 2006 FutureCoat! Conference, sponsored by Federation of Societies for Coatings Technology, November 1–3, 2006, in New Orleans, LA.