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Comparison of in-line compounding vs. Pre-compounded resin on polyethylene pipe performance

 

Vivek Rohatgi, Pamela L. Maeger, Ashish M. Sukhadia

Papers # 2016 Berlin

This study is related to two common approaches used for pipe fabrication, viz. in-line compounding and extrusion of a pre-compound. Pipes were extruded using both approaches and properties were evaluated per the relevant ASTM and ISO standards for water and gas pressure pipe applications to compare the performance of extruded pipes. The results show that when high quality pipes are produced, the performance of pipes made via either in-line compounding or a pre-compound, is equivalent.

For pressure piping applications, the ASTM standard allows the use of in-line compounding wherein the natural resin pellets are physically mixed with the color masterbatch pellets just prior to pipe extrusion. The ISO standard, however, requires the use of a pre-compound, wherein the same two components are melt blended prior to pipe extrusion. Both these approaches are being used successfully for commercial pipe production globally. However, questions still arise as to how these two methods compare, in addition to ongoing questions about the implications of the carbon black particle size and screw design in influencing the overall processability (throughput) and quality of the extruded pipes. These factors were investigated in this study in which pipes were extruded on a 45 mm pipe extrusion line with 36 L/D grooved feed screw design using 1) in-line compounding of a bimodal high density polyethylene PE4710/PE100 resin and a commercial carbon black masterbatch, and 2) a precompound of the same two components. For blends prepared using in-line compounding, color masterbatches based on N550 (~ 42 nm) and P type (~25 nm) carbon blacks were used. For the pre-compounded resin, the N550 type carbon black based masterbatch was the same as for in-line compounding. Extruded pipes were evaluated for processability, carbon black content and dispersion, oxidation induction time (OIT) and thermal stability temperature, numerous physical properties common to pressure piping grades, quality of fusion joints, slow crack growth (SCG) via PENT, notched pipe test via ISO 13479 test method, rapid crack propagation (RCP) performance using the S4 test, and short term hydrostatic pressure testing via the 3- point ISO 1167 method and ASTM F714 standard. Results of the study are discussed in detail relating to both pipe processing and property performance and conclusions provided.

Related keywords : Pipe extrusion, in-line compounding, pre-compound, screw design, polyethylene pipe properties, ASTM International, ISO, water and gas piping.
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