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Effect of carbon black, compounding and pipe extrusion on the slow crack growth pent test result

 

Adriana Velasquez, Dr. Dane Chang, Dr. Siddharth Athreya

Papers # 2018 Las-Vegas

This paper studies the PENT results and how they vary based on sample preparation, equipment used, technique to place the sample as well as the number of heating cycles that the resin goes through after it leaves the manufacturing facility including those cycles needed to pre-compound virgin resin with a color masterbatch, specifically the one that was studied in this paper is with carbon black masterbatch.

The Pennsylvania Notch Test (PENT) as per ASTM F1473 is one of the tests recognized by the North America pipe industry as a method to measure the Slow Crack Growth (SCG) resistance of polyethylene resins used in pipe applications. ASTM F1473 specifies that the samples can be prepared from either pellets or from fabricated pipes. However, there is limited data quantifying the effect (if any) of the different sample preparation methods and effects of any additives such as carbon black on the PENT result.

This study investigates the sensitivity of the PENT result on the following: Black pipe extruded using (salt & pepper) blending of natural resin and carbon black master batch (Pipe BK S&P), and black pipe extruded using pre-compounded black resin (Pipe BK). For all the pipe samples evaluated, pipes were extruded at both high and low shear rates. All tests in this study have been done using the same lot of HDPE natural resin. The natural resin was used directly from production without additional processing. The compounded black material was prepared using a twin-screw extruder. Pipes made with both high and low shear rates were extruded using a standard groove feed pipe extrusion line. All pipe specimens were prepared by using pieces cut from the extruded pipes and compression molded. All PENT specimens were prepared following ASTM F1473 procedure. To eliminate potential experimental variability, all tests were conducted in the same pipe lab environment and the specimens were prepared and put on test by the same operator.

Results show that whether the carbon black is added to the pipe through an inline compound extrusion process or a twin screw pre-compounding process, the Slow Crack Growth performance of the pipe does not get affected and it is not different from one another.

Since the PENT test is very sensitive and has a high margin error of ±26%, the tests conducted in this study, tried to minimize the possibilities of any type of variation.

Please note that the whole article content is available on PPCA website only

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