Ernest Lever
Posters # 2014 Chicago
The Popelar bidirectional shift factors have been now been used about a quarter century to great effect. The question has been raised as to whether the shift factors are applicable to modern materials. GTI has been using rheological principles and Dynamic Thermo Mechanical Analysis (DTMA) to routinely develop bi-directional shift factors for PE, PB, PEX and EPDM materials. These shift factors have been correlated to Rate Process Method (RPM) testing of several of these materials and have been shown to be consistent with the RPM method. This paper will discuss the results and the variability of the shift factors across generations of PE material. In addition the paper will present an interesting method of how to use the confidence levels provided by the DTMA method to properly quantify the results of limited validation testingof PE pipe and fittings.
Mavridis provides a methodology to develop bi-directional shift factors based on rheological principles. The test methodology employed is Dynamic Thermo-Mechanical Analysis. Horizontal activation energies are calculated by fitting an Arrhenius type model to the tangent of the loss modulus as a function of excitation frequency and test temperature. The vertical activation energy is extracted from a model fitting the tangent of the loss modulus to the complex modulus as a function of excitation frequency and temperature. This method is very convenient as results can be generated in a few hours. The simplicity and low cost of the method enables us to test a sufficient number of replicates to generate statistical confidence limits on thecalculated activation energies.
In this paper the bi-directional shift factors are compared to the Popelar shift factors as described in [2-4]. It is first shown that the Popelar shift factors are closely replicated for vintage Aldyl A materials. Several modern materials were evaluated using the Mavridis methodology and the implications of the difference between the Popelar bi-directional shift functions and the measured bi-directional shift functions are discussed.
