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Polymeric materials used in earth structures and waste containment sites have minimum performance requirements to be maintained over a service life ranging from 1 to 100 years. The ability to predict the material performance in these applications with time has great appeal. However, it is necessary to know the mode, mechanism, and profile of behavior of these materials over an anticipated lifetime. Today, there is a proliferation in the use of rate expressions supposedly representing a material's response with time to a specific environmental condition. The objective is to review the impact of current protocol and simplifying assumptions on the prediction of long-term behavior of polymeric materials.
The problems with the current methodology are multiple. For a specific environmental condition associated and polymeric material, literature shows that the rate expression and its order vary with the experimenter and his or her parameters of choice in measuring the material's behavior. Thus, the integrated expressions predict different behaviors with time. Elevated temperatures are commonly used to accelerate a deteriorative mode sufficiently to collect data within a reasonable time frame. Typical range of elevated temperatures commonly traverse at least one polymeric phase transition. Deteriorative mechanisms change with physical state thus affecting the development of rate parameters representative of another physical state. Within the Arrhenius equation, the rate constant (k) is to be a function solely of temperature, and the activation energy (E) is to remain constant over the temperature range of evaluation and extrapolation and with time in long-term predictions. Literature has established that a ±1.0 kcal/mol (4.186 kJ/mol) error in E will cause an error of ±100% at a 95% confidence level in the extrapolation of kinetic rate predictions from 225 to 25°C.
The engineer and designer will see the dangers and the consequences associated with using rate expressions supposedly representing a deteriorative mode, and the impact of methodology and simplifying assumptions commonly employed to predict long-term behavior.
life prediction, Arrhenius equation, geosynthetics
Manager, Technology Development, The Tensar Corporation, Morrow, GA