Coating degradation is a combination of both chemical and physical processes; however, physical processes have not received much attention. Physical aging has a non-negligible effect on coatings' mechanical properties and permeability etc. through the densification that continues as a polymer approaches its thermodynamic equilibrium below the glass transition temperature, T g . Observations in recent work showed that physical aging affects coatings' mechanical property response during accelerated weathering and is, itself, affected by the associated chemical degradation. Two crosslinked coating systems were studied in order to compare different chemical compositions, their T g , and their thermal response in accelerated weathering. During thermal cycling, physical aging measured by enthalpy recovery exhibited different trends in the two coatings. A ''rejuvenation'' mechanism was observed in the coating with a T g between the top and bottom limits of the exposure cycle; continued aging was observed for the coating with a high T g . Stress relaxation tests detected aging and ''memory'' behavior over periods comparable with accelerated weathering cycles. Both thermal and mechanical responses changed in complicated and different ways as the coatings degraded. Different degrees of coating thickness reduction were observed in both isothermal relaxation and degradation. When various coatings are evaluated, simply judging their performance under the same weathering environment is not reliable since polymer relaxation behavior depends on the relationship between the exposure temperatures and the T g of each polymer.

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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.

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