Plastic Pipes Conference Association # 2021 Amsterdam
Mike Troughton, Amir Khamsehnezhad, Changyi Yu
When butt fusion welding polyethylene (PE) pipes in the field, it is often specified that the pipes must undergo fusibility testing in all combinations of suppliers, diameters and thicknesses before being fused in production. This is a very onerous and expensive task, but must be carried out to ensure that the PE pipes are of sufficient quality to produce acceptable butt fusion joints. One of the reasons why PE pipes might not be of sufficient quality is insufficient homogenization of the resin, which during the extrusion of black pigmented PE pipes, may result in features called “windows”. These are local areas where there is a lack of carbon black pigmentation and normally occur in thicker- walled pipes, towards the mid-wall thickness. They can be seen, for example, in the shavings from the end of the pipes produced during the trimming stage of the butt fusion process. Two potential reasons why insufficient homogenization of pigmented PE resin might cause a reduction in the performance of butt fusion joints are: 1) if there are areas in thepipe where the carbon black content is zero, there are likely to be areas where the carbon black concentration is significantly higher than the average. These areas may act as either stress concentrations or cause lack of fusion in the butt fusion joints; 2) after the PE melt passes through the mandrel support plate in the pipe extruder, which contains hundreds of holes, the molten strands need to fuse back together to form a homogeneous melt stream. However, if there are areas in the melt that contain higher viscosity material, these melt strands may not fuse together perfectly and may subsequently result in areas of reduced fusion in the butt fusion joint. This paper describes an investigation into the effect of insufficient homogenization of pigmented resin in PE pipes on the integrity of the subsequent butt fusion joints and the development of a test to quantify the amount of windows in the pipe. It will describe the analysis carried out on a number of PE pipes, made using both pre-compounded and in- line compounded resin on the same pipe extruder but using different extrusion conditions in order to obtain different degrees of homogenization. The results of mechanical tests carried out on the butt fusion joints from these pipes were used to determine a critical value of windows below which the joint integrity is not affected.
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