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Oxidation Induction Time

NEXOR Pipes R&D

July 8, 2006

Introduction 

Iron pipes are notorious for corrosion. While plastics are immune to corrosion, another chemical reaction phenomenon, which is widely unrecognized, happens in plastics. Polyethylene in particular is subject to oxidation which over time weakens the pipe and promotes failure. Polyethylene resin manufacturers normally add stabilizers to improve the resistance of polyethylene to oxidative degeneration. Further processing of polyethylene resin into useful products such as pipes may significantly reduce the ability of the material to withstand oxidative atmospheres.

Determination of oxidation induction time or OIT is the primary means of finding out the resistance of polyethylene pipes to oxidation. OIT is determined using a differential thermal analysis (DTA) instrument. The procedure essentially subjects the pipe to an accelerated oxidation environment, while monitoring for the occurrence of exothermic and endothermic reactions. Thus, the longer the OIT, the more oxidation resistant the pipe is. A useful comparison would be to find out the OIT of the unprocessed resin and the OIT of the processed plastic. The discrepancy between the OITís of the two basically reveals the extent of the impact of polymer processing on the reduction of oxidation resistance.

Below describes the general steps in determining the oxidation induction time performed at NEXOR PIPES ; the methodology closely follows ISO/TR 10837 Determination of the thermal Stability of Polyethylene for use in gas pipes and fittings.

Procedure

Prepare a plastic specimen with a weight of 15 +/- 5 mg.

Using a differential thermal analysis (DTA) instrument, the temperature of the plastic specimen starting at room temperature is increased at a constant ramp rate of 6 degrees Celsius per minute in a nitrogen environment. As the temperature increases, switchover to an oxidative environment is done after the completion of melting, as indicated by an endothermic curve in the thermogram.

The test concludes when an exothermic oxidation curve is observed in the thermogram. Measurement of the OIT commences after the completion of melting and ends at the start of oxidation.

References

ISO/TR 10837 Determination of the thermal stability of polyethylene (PE) for use in gas pipes and fittings

 

ISO 4427 Polyethylene Pipes for water supply - Specifications

 

Chasis, David. Plastic Piping Systems. 2nd Ed. Industrial Press Inc. USA. 1988

 

Skoog, Douglas et.al. Principles of Instrumental Analysis. Saunders College Publishing and Harcourt Brace College Publishers. USA 1998.

 

Willoughby, David et al. Plastic Piping Handbook. Mcgraw-Hill Handbooks. US