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Effect of Insufficient Homogenisation of Pigmented Resin During the Extrusion of Polyethylene Pipes on Mechanical Performance

Project Code: TBC

Objectives

  • Determine whether pipes containing windows produce butt fusion welds with lower mechanical integrity than pipes that do not contain windows
  • Determine whether the root cause of reduced mechanical integrity of butt fusion welds lies in areas of high carbon black concentration or areas of incompletely fused PE within the extruded pipe
  • Determine whether there is a relationship between the extent of windows seen in the shavings from the ends of PE pipes during the trimming stage of the butt fusion welding process and the mechanical performance of the welded joints
  • Determine whether there is a level of windows which would not affect the mechanical performance of the butt fusion welded joint, and could hence form the basis of an acceptance criterion

Project Outline

The concept of this project is to undertake a detailed investigation into whether poor homogenisation of pigmented resin in PE pipes, which results in windows does reduce the integrity of the subsequent butt fusion welds, and if there is a clear link, to create a cost effective test to determine an unacceptable amount of inhomogeneity in the pipe.

A number of PE pipes containing varying amounts of windows will be obtained and analysed using optical microscopy, scanning electron microscopy (SEM), X-ray microscopy, Fourier transform infrared spectroscopy (FTIR), local reheating and nanoindentation to map the distribution of windows, the concentrations of carbon black and areas of incomplete fusion of the parent material at the ends of the pipe. The pipes will then be butt fusion welded using standard welding procedures, and the welded joints will be mechanically tested using both short-term (tensile, bend) and long-term (whole pipe tensile creep rupture) tests, as well as analysed using X-ray microscopy. The fracture surfaces will be examined using optical microscopy, SEM and FTIR to determine and analyse the positions of failure initiation.

A number of pipes, made from the same grade of PE, that do not exhibit windows will also be analysed, butt fusion welded and mechanically tested using the above methods. The results will be compared to determine whether there is an acceptable amount of windows that does not affect the mechanical performance of the joint, what happens to the windows and concentrations of carbon black that are at the joint interface during the welding process, and whether it is areas of high concentrations of carbon black or areas of incompletely fused PE that causes a reduction in joint integrity.

Relevant Industry Sectors

Technical and Economic Benefits

The new mandatory Appendix to Section III of the ASME code for HDPE Class 3 piping states that all PE 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 welded joints. Variable homogenisation of pigmented resin is considered the primary quality issue when joining PE pipes. 

Insufficient homogenisation of pigmented resin during the extrusion of polyethylene (PE) pipes may result in features called “windows”. These are mainly seen in black pigmented pipes that have been produced using a process where the natural PE resin pellets are dry-mixed with carbon black masterbatch pellets before going into the pipe extruder rather than being pre-compounded. The windows are 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 in the shavings from the end of the pipes, which are produced during the trimming stage of the butt fusion welding process.

A number of published reports, as well as some internal work at TWI, show that insufficient homogenisation of pigmented PE resin, resulting in windows in the pipe has a detrimental effect on the performance of both the pipe and its welded joints However, no research has been published that determines the exact reason for the reduction in performance or to define an acceptable amount of windows for welding.

For more information please email:


contactus@twi.co.uk