The road to a new detergent for the Accelerated Notched Pipe Test (ANPT)

The Accelerated Notched Pipe Test (ANPT) plays a crucial role in assessing the resistance of polyethylene (PE) pipes to slow crack growth, because it mimics real-world installation damage. However, recent restrictions on the detergent Arkopal N100, traditionally used in the test, have posed challenges for compliance with international standards. This article explores the search for a more sustainable alternative detergent, with Kiwa Technology at the forefront of these developments.

During trenchless installation, scratches may occur on the outside of a PE pipe. These scratches can slowly grow through the entire pipe wall. The Notched Pipe Test (NPT) specified in ISO 13479 simulates this process. However, failure times of the pipes in this test will be over one year for PE 100-RC. Therefore, a correlated accelerated method, the Accelerated Notched Pipe Test (ANPT), was developed. This method is described in an annex to ISO 13479 first published in 2022.

The ANPT is very similar to the standard NPT, but with one important difference: the notches are exposed to an external detergent solution instead of water to reduce the time to failure. To keep the test eco-friendly, Kiwa Technology uses smaller containers for each pipe rather than a large bath to minimize the required detergent quantities. The current annex to ISO 13479 specifies that the ANPT should be carried out with 2% nonylphenol ethoxylate (NPE). This is better known by its trade name, Arkopal N100.

Use of Arkopol restricted

A revised version of the standard series EN 1555 (PE for gas) was published in 2021 in which the requirements for PE 100-RC materials were specified. A test period of ≥ 300 hours using the ANPT with Arkopal N100 was introduced as one of the requirements of PE 100-RC materials.

However, it quickly became apparent that the use of nonylphenol ethoxylate was meanwhile restricted in Europe by the REACH regulation published by the European Chemicals Agency because it’s toxic to aquatic life. It has long-lasting effects and is endocrine disrupting. This means it can’t be manufactured, imported, distributed and sold on the market in Europe for a wide variety of purposes. While there is an exception for laboratory use, in practice it means that there is no importer in Europe, which makes it impossible for European test labs to obtain Arkopal N100.

Dehyton as possible alternative

A new detergent for the ANPT had to be found to be able to determine whether the PE 100-RC pipes indeed would meet the minimum requirements specified in the EN 1555 series. One alternative detergent, which is also used for the accelerated full notch creep test (AFNCT) specified in ISO 16770, is lauramine oxide. This detergent is commercially available as Dehyton PL. The PE100+ Association has kindly taken the lead to find this alternative and support the standardisation groups. In 2022 they asked one laboratory to test PE 100-RC materials of different colours with the detergent lauramine oxide in the ANPT.

The results were distributed in the standardisation working group ISO/TC138/SC5/WG20 early 2023. An acceleration factor of 2 for six different PE 100-RC grades was found when comparing Arkopal N100 and Dehyton PL. Kiwa Technology has independently also investigated the acceleration factor between Arkopal N100 and Dehyton PL. These results were published on the Plastic Pipes XXI conference. A factor of 2 was confirmed as conservative acceleration factor.

A round robin is still needed

The experts in the standardisation working groups agreed that a round robin would be necessary to check the comparability between different laboratories systematically and the take lab-to-lab variations into consideration. Therefore, the EN 12201 series (PE for water) and ISO 4437 series (PE for gas), which were both published early 2024, still contain the requirement of ≥ 300 hours using Arkopal N100. A footnote states that this detergent will be replaced by lauramine oxide, but the requirement is still under development.

The PE100+ Association is planning a round robin, but meanwhile more and more laboratories run out of Arkopal N100 supply, making it impossible to test the requirements of the standards. The PE100+ Association therefore published a position letter on the ANPT where they recommend for certification purposes to consider 1 year NPT testing in water for raw material suppliers and to consider/accept the strain hardening test (SHT) for pipe producers.