Reciprocity in weathering exposure and the kinetics of property degradation

Abstract

If the kinetics of degradation scaled in a simple way, accelerated testing could be correlated to the more moderate natural world and one could estimate service life. “Reciprocity” requires that degradation is simply proportional to the integral product of radiation intensity and time and is observed in spectroscopic studies but is difficult to observe in experimental studies of end-use properties. It is difficult to justify when there is more than one ingredient or more than one process contributing to the performance, e.g. corrosion protection, because not all the mechanisms involved depend in the same way on the environment. However, reciprocity remains a very appealing concept. Instead of using weathering data, this investigation explores simple chemical kinetics so algebraic equations show when reciprocity might be applicable. The approach also demonstrates how correlation of accelerated weathering and natural exposure will fail when the different exposures provoke different material characteristics, e.g. due to temperature differences. Assuming damage accumulation is stochastic, models generate realistic trends for performance properties without invoking complicated chemical kinetics. The trend in a property may retain reciprocity regardless of whether it depends on simultaneous degradation, the average degradation or its extremum value. While it is not predictive for all properties or circumstances, reciprocity is an intuitive and useful approximation for more than spectroscopy of degradation.