Formation Mechanism of the Eigen-line in Electrofusion Joints of Polyethylene Pipes

Cold welding is the most dangerous defect in the electrofusion joint of polyethylene (PE) pipes. A proprietary method was developed to detect the degree of cold welding by using an Eigen-line, which was discovered in our previous study. To understand when, where, and how the Eigen-line would appear, the forming mechanism was investigated in this paper. Three factors, i.e., micro-air-bubbles, difference of acoustic impendence of PE in the melted and unmelted region, and small-crowded crystals in the region of the interface of melted and unmelted region, may cause the appearance of the Eigen-line. It was found that the number of Eigen-lines was the same as that of welding times when rewelded with decreasing welding time, and only one Eigen-line could be observed when rewelded with increasing welding time. The result showed that small-crowded crystals in the region of the interface of melted and unmelted region may be the dominating factor. This was then verified by the temperature analysis and differential DSC tests, and the forming process of the small-crowded crystals was discussed in detail.