Polymer Degradation and Performance - American Chemical Society

Chapter 14

Lifetime Prediction: Different Strategies by Example

Downloaded by CORNELL UNIV on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch014

U. W. Gedde and M. Ekelund Fibre and Polymer Technology, School of Chemical Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden

Two different approaches for lifetime prediction are presented. The underlying lifetime limiting processes have been identified in two cases. Mathematical expressions of chemical/physical relevance were used for the lifetime predictions for P E hotwater pipes and cables insulated with plasticized P V C . Accelerated testing, extrapolation and validation of the extrapolation by assessment of the remaining lifetime of objects aged during service conditions for 25 years were successfully applied to cables insulated with chlorosulfonated polyethylene. Polyolefin pipes exposed to chlorinated water showed a very complex deterioration scenario and it was only possible to find a method suitable for predicting the time for the depletion of the stabilizer system.

© 2009 American Chemical Society

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

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Introduction Lifetime predictions of polymeric products can be performed in at least two principally different ways. The preferred method is to reveal the underlying chemical and physical changes of the material in the real-life situation. Expected lifetimes are typically 10-100 years, which imply the use of accelerated testing to reveal the kinetics of the deterioration processes. Furthermore, the kinetics has to be expressed in a convenient mathematical language of physical/chemical relevance to permit extrapolation to the real-life conditions. In some instances, even though the basic mechanisms are known, the data available are not sufficient to express the results in equations with reliably determined physical/chemical parameters. In such cases, a semi-empirical approach may be very useful. The other approach, which may be referred to as empirical, uses data obtained by accelerated testing typically at several elevated temperatures and establishes a temperatures trend of the shift factor. The extrapolation to service conditions is based on the actual parameters in the shift function (e.g. the Arrhenius equation) obtained from the accelerated test data. The validity of such extrapolation needs to be checked by independent measurements. One possible method is to test objects that have been in service for many years and to assess their remaining lifetime. Before starting this analysis a clear formulation of a failure criterion for a given object is needed. In some cases, the criterion is self-evident, e.g. when failure occurs by fracture in a pipe whereas in other cases it requires significant experimental study and analysis. This paper presents a selection of data including their interpretation with the purpose of predicting lifetimes of four different cases: polyethylene hot-water pipes, polyolefin pipes distributing chlorinated water, chlorosulfonated polyethylene and plasticized polyvinylchloride as insulating materials used in cables in nuclear power plants. The failure criteria for these applications were brittle fracture failure preceded by thermal oxidation or chemical degradation for the pipes and loss of mechanical integrity and a certain minimum resistance of the insulating layers during a simulated nuclear power plant accident in the two latter cases. Different extrapolation methods were used. In the case of the hotwater pipes, a detailed picture of the deterioration scenario was obtained permitting extrapolation using relevant physical/chemical equations. The extrapolation method used in the case of the chlorosulfonated polyethylene cables were based on an empirical approach including data from accelerated testing and data of the remaining lifetime for objects aged during service for 25 years. Plasticized P V C deteriorates by dehydrochlorination and plasticizer migration. The latter is dominant at lower temperatures. Lifetime predictions for service-like conditions (20-50°C) were based on extrapolating migration rate data obtained under conditions with prevailing evaporation control.


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Polyethylene Hot Water Pipes


Polyethylene pipes exposed to hot water with access to oxygen may undergo the following stages of change (1-3): Equilibration of a supersaturated solution of the antioxidant in the polymer which include loss of antioxidant by internal precipitation or phase separation (Regime A ) ( 120°C: diffusion-control with concentration-dependent diffiisivity; (ii) Τ < 100°C; evaporation control with a constant evaporative loss rate. This loss rate was of the same order of magnitude as that from a free plasticizer film on a glass plate (22). Extrapolation of high temperature plasticizer evaporation data (60-100°C) yielded an accumulated plasticizer loss over 30 years of 0.3% for the jacketing, which is in accordance with data obtained for cables after 30 years of service (22).

Conclusions The lifetime-determining processes that prevail during service-like conditions were established for polyethylene hot water pipes and cables insulated with plasticized P V C . Lifetime predictions based on equations with physical/chemical relevance yielded reliable results. The lifetime of cables insulated with chlorosulfonated polyethylene aged in nitrogen were predicted with good precision by long-range extrapolation of accelerated testing data. A n efficient method for predicting the time for depletion of the stabilizer system in polyolefin pipes exposed to chlorinated water was presented.

Acknowledgements This research is based on the financial support from several sources: the Swedish Research Council (grant #621-2001-2321), Studsvik Polymer A B



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U Cl

0.01

L

0.0001 200

Figure 4. Lifetime (logarithmic scale; time to reach 2χ the initial indenter modulus = failure criterion according to LOCA tests) as a function of temperature (T) for the Hypalon core insulation aged in nitrogen. The hightemperature data (open circles) were obtainedfrom experimental data. The low-temperature data (filled circles) were obtained by extrapolation of the hightemperature data according to the Arrhenius equation. The remaining lifetime of a Hypalon cable core insulation in service for 24.8 years at 54°C in nitrogen was obtained by subsequent ageing at 155°C and 170°C and the data were used to estimate the total lifetime at 54°C (indicated by crosses). From Sandelin and Gedde (13) and with permission from Elsevier, UK.


169 (now Bodycote Polymer A B ) , Ringhals A B and Forsmarks Kraftgrupp A B (both owned by Vattenfall A B ) . The following co-authors of the papers which this report is based on are gratefully acknowledged: the late M r . Mats Ifwarson (Bodycote Polymer A B ) , Prof. Grant D. Smith (University of Utah), Dr. Jens Viebke (GE Healthcare), Mr. Jarno Hassinen (Bodycote Polymer A B ) , Prof. Mikael Hedenqvist (KTH), Dr. Marie Lundbàck, and Mr. Mikael Sandelin.


References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

Karlsson, K.; Smith, G. D.; Gedde U . W. Polym. Eng. Sci. 1992, 32, 649. Smith, G. D.; Karlsson, K.; Gedde, U . W. Polym. Eng. Sci. 1992, 32, 658. Gedde, U . W.; Viebke, J.; Leijström, H . ; Ifwarson, M . Polym. Eng. Sci. 1994, 34, 1773. Viebke, J.; Elble, E.; Ifwarson, M.; Gedde, U . W. Polym. Eng. Sci. 1994, 34, 1354. Viebke, J.; Elble, E.; Gedde, U . W. Polym. Eng. Sci. 1996, 36, 458. Viebke, J.; Hedenqvist, M.; Gedde, U . W. Polym. Eng. Sci. 1996, 36, 2896. Viebke, J.; Gedde, U . W. Polym. Eng. Sci. 1997, 37, 896. Lundbäck, M.; Strandberg, C.; Albertsson, A . -C.; Hedenqvist, M . S.; Gedde, U . W. Polym. Degrad. Stab. 2006, 91, 1071. Lundbäck, M.; Hedenqvist, M . S.; Mattozzi, Α.; Gedde, U . W. Polym. Degrad. Stab. 2006, 91, 1571. Viebke, J.; Gedde, U . W. Polym. Eng. Sci. 1998, 38, 1244. Hassinen, J.; Lundbäck, M.; Ifwarson, M.; Gedde, U . W. Polym. Degrad. Stab. 2004, 84, 261. Lundbäck, M.; Hassinen, J.; Andersson, U . ; Fujiwara, T.; Gedde, U . W. Polym. Degrad. Stab. 2006, 91, 842. Sandelin, M.; Gedde, U . W. Polym. Degrad. Stab. 2004, 86, 331. Miner, M . A . J. Appl. Mech. 1945, A159. Gillen, K . T.; Celina, M.; Clough, R. L . ; Wise, J. Trends Polym. Sci. 1997, 5, 250. Celina, M.; Wise, J.; Ottesen, D. K.; Gillen, K . T.; Clough, R. L . Polym. Degrad. Stab. 2000, 68, 171. Celina, M.; Gillen, K . T.; Graham, A . C.; Assink, R. Α.; Minier, L . M . Rubber Chem. Technol. 2000, 73, 678. Gillen, K . T.; Celina, M.; Bernstein, R. Polym. Degrad. Stab. 2003, 82, 25. Gillen, K . T.; Bernstein, R.; Derzon, D. K . Polym. Degrad. Stab. 2005, 87, 57. Gillen, K . T.; Bernstein, R.; Celina, M. Polym. Degrad. Stab. 2005, 87, 335. Ekelund,M.;Edin, H.; Gedde, U . W. Polym. Degrad. Stab. 2007, 92, 617. Ekelund, M.; Azhdar, B.; Hedenqvist, M. S., Gedde, U . W. Polym. Degrad. Stab. submitted.


Polymer Degradation and Performance - American Chemical Society

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