Concurrent physical aging and degradation of crosslinked coating systems in accelerated weathering

Abstract

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.

Reduced indentation recovery temperature at the surface of a crosslinked epoxy coating in humid conditions

“…Mechanical modulus increases due to physical aging (structural relaxation) when a polymeric coating is kept at temperatures lower than its T g . 12 If the temperature is at, or above T g , there is no aging effect. Aging at 55°C was studied, since it is about 20°lower than bulk T g of the epoxy, and thus bulk mechanical properties would show obvious signs of aging.…”

Aging of amorphous raspberry powder: Enthalpy relaxation and fragility

“…If T g is measured without the interference of the physical aging peak (i.e., by the intersection of the extrapolated enthalpy-temperature for glassy and rubbery states), T g should decrease with aging time. Physical aging often results in densification, increase in relaxation time and decrease in free volume and molecular mobility (Shi et al, 2008;Surana et al, 2005). Therefore, the glass transition temperature of a food may increase with aging time.…”

Environmental and time dependence of moisture transportation in an epoxy coating and its significance for accelerated weathering

“…14 Surface modulus and hardness were tested with a Berkovich tip (radius 150 nm 3-sided pyramid) on a Hysitron TriboScope Ò NanoIndentor, at peak force 8000 lN, using 10 s (loading) À5 s (hold) À10 s (unloading) three segments mode. The reduced modulus was used as the coating modulus, as described in other literature.…”

“…A 2-h dry cycle at 35°C allows much less residual moisture than in a 1-h cycle, hence less degradation reaction (depending on the polymer) and thus less (possibly) corrosion. In addition, since a typical polymer's relaxation time is much longer than the exposure cycle length, 14 high frequency cycling will allow an accumulation of stress and accelerate brittleness, which may not be reflected in an appearance property, but may be important for materials' integrity. Figure 9 illustrates the possible mode of water molecule transport in wet and dry cycles of accelerated weathering.…”