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Durability Forecasting of Plastics Parts - The Engineer's Point of View

SpecialChem / Mar 2, 2011

The durability notion is highly versatile, depending on the opinion and requirements of purchasers and users. The durability limit is governed by degradations depending on evolutions of aspect, color, and transparency, mechanical, physical, electrical, chemical or biological properties. The degradation can be a bulk phenomenon or/and a spot one, weak points initiating the degradation.

The points of view of engineers and scientists are different but complementary. The engineer searches to estimate the lifetime of the studied part in more or less defined and more or less complex conditions when the scientist searches to determine the kinetics and the cause of the degradation in defined and simplified conditions. For long-term forecasts, engineers cannot wait a long time and must make the most of feedback from field performance, knowledge bases, and ‘reasonably fast’ trials taking advantage of modelling for a longer forecasting.

Several examples describe various paths and methods leading to imperfect but credible predictions: 50-Year service life for gas and water distribution pipes, 100-Year Service Life for Corrugated HDPE Drainage Pipe, Lifetime predictions for aramid ropes used as pre-stressing tendons in concrete structures, and Geomembrane lifetime prediction.

Main parameters responsible of polymer degradations are:

  • Stresses and deformations: immediate and at long term
  • Oxidation by oxygen of air
  • Photo-oxidation by UV and oxygen
  • Humidity and water
  • Cyclic stresses and deformations
  • Low and high temperatures
  • Chemicals: household cleaning products, fuels and acid for automotive parts...

Often several parameters can act simultaneously or successively with possible synergistic effects. All the properties have not the same kinetics of degradation leading to very different lifetimes according to the criteria under consideration.

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