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

Relaxation of surface defects shows that the surface of a crosslinked epoxy system, under normal laboratory conditions, apparently has a softening transition temperature that is approximately 20°C lower than calorimetric measures of T _g in the bulk. Physical aging data here confirms that this transition does resemble a glass transition. This observation is significant for properties that are determined by the surface of a coating. When ambient humidity is such that an epoxy may absorb significant quantities of moisture, it is plasticized, reducing its T _g. When the relaxation of nanoindentations was measured in a humid environment, the value of this softening surface transition temperature was further reduced by 10°C. Thus, the surface properties of a polymer coating, such as wear, durability, friction, will depend not only on the ambient temperature, but also on whether the environment is humid. This is very important when such a coating responds to an environment that changes, e.g., natural weather.