Limits on the slow crack growth resistance evaluation for the PE100 and PE100RC polyethylene resins


Papers # 2016 Berlin

With the last advances and improvements in the polyethylene resins used for pipe application, new methods for evaluating long-term mechanical properties need to be introduced. Standard PENT and FNCT tests needs more than one year for characterizing the new PE100RC grades with the consequent thermal ageing problems. Strain Hardening modulus (SH) determination at 80ºC seems to be one of the best alternatives. Correlation between SH method and classic SCG tests are reported on this work. It is proved that the method guarantees a rapid and accurate evaluation.

Slow Crack Growth (SCG) resistance is the main property that must be controlled for assessing the long-term performance on polyethylene resins used for pipe application. Fourth generation PE resins have shown very high resistance to slow crack growth, with failure times even higher than one year at the standard temperature of 80ºC used in conventional tests like Full Notch Creep Test (FNCT), Pennsylvania Notched Tensile Test (PENT), Notch Pipe Test (NPT) and Point Loading Test (PLT). More severe conditions increasing temperature and load for PENT and FNCT tests have been proposed in recent years with significant reduction on the failure time. These stricter test conditions must be rigorously controlled due to the possibility that the SCG was not the main process that regulates the failure mechanism of the resin. For that purpose, the failure morphology inspection and the crack opening displacement (COD) curve have been evaluated to assess such failure mechanism. 

On the other hand, among the different alternatives to conventional methods that are being developed to accelerate the resin evaluation, the Strain Hardening (SH) modulus determination at 80ºC is becoming as the best alternative for the evaluation of the SCG process. Thus, the SH modulus has been determined for a wide range of PE resins grades. The results show that the method guarantees a rapid and accurate evaluation. Additionally, the method unexpectedly displayed remarkable differences in resins with the same SCG resistance measured by conventional methods but with different comonomer type used in the resin synthesis. To the best of our knowledge, this is the first report where these differences are depicted.

Related keywords : Slow Crack Growth (SCG), Long-term mechanical properties, Strain Hardening modulus, PENT test, FNCT test.
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