# 1995 Edinburgh
This paper presents the utilization of the so-called Stepwise Isothermal Segregation (SIS) tech nique for the characterization of 5 batches of gas pipes from a currently used.so-called 2nd generation MDPE from 1987 to 1992. The methodology allows one to eva luate the crystallization parameters of the resin at the pipes midwall by means of a stepwise thermal program using Differential Scanning Calorimetry. The quantita tive evaluation of the crystallization exothermal peaks along with that of the related half crystallization times gives way to a fairly accurate determination of the kine tic parameters on the one hand and to the evaluation of the proportions of the crystallizable species at 2 diffe rent temperatures on the other hand. The data obtained enlight a sharp variation of the resin characteristics from ca. 1991 and confirm earlier trends on this mate rial independently of the pipes extrusion conditions. A comparison with data obtained on both a so-called 1st generation HDPE and a 3rd generation HDPE pipes from the same resin supplier brings strong evidence of the influence of specific crystallizable species regarding slow crack growth resistance and demonstrates the effi ciency of such methodology for the detection of batch to-batch variations at the most intimate level.