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New materials and a new test method open new horizons for polyethylenes in pressure pipes

World Pipelines,


In January 1995, a massive earthquake hit southern Japan causing substantial damage to the local infrastructure. The city worst hit was Japan’s fifth largest, Kobe, after which the earthquake is commonly named. Bridges collapsed, roads and railway lines broke up, and around one in five of all houses in the area were rendered uninhabitable. Underground, many pipes carrying gas and water were ruptured.

Fortunately, many pipes also remained undamaged, and one of the key reasons why they did is because they were made out of plastics, rather than iron or steel. Polymers such as PVC and, to an even greater extent, polyethylene, are more flexible than even ‘ductile’ iron, and can withstand noticeable movements in the ground without fracturing.

This may seem an extreme example to demonstrate the benefits of thermoplastics in pipes, but when we are talking about an application that many consider as ‘fit and forget’, it is a perfectly valid one. On a more mundane level, one might want to consider the case of some polyethylene pressure pipes that were laid in Germany in the 1960s and recently dug up. Close inspection found that, even after some 50 years in service, there was still very little wrong with them, and they could have been kept online for several more decades without the need for any kind of maintenance.

Scratch and crack resistance

The pipes in question were some of first ever constructed out of polyethylene put into commercial use. In the meantime, advances in polymer technology have enabled us to proceed through several generations to the point where so-called ‘PE 100’ pipes can be expected to last in the ground for a significant amount of time – quite possibly as long as an entire century. (PE 100 is a reference to the long-term minimum required strength of 10 MPa that pipes made in the materials need to achieve, as set down in ISO 12162.)

The developments go on: pipes made from high density polyethylene (HDPE) launched onto the market in recent months from companies such as SABIC can be produced more energy-efficiently, and they can be laid in more adverse conditions – using trenchless systems, for example, which cause less disruption to above-ground traffic and have less environmental impact, but which require pipes with improved resistance to slow crack growth.

The subject of scratch and crack resistance is a key one in the plastics pressure pipe world. With the wrong material, a tiny scratch made on the surface of the pipe during the laying process can grow over time and finally cause a failure. The phenomenon is unsurprisingly called slow crack growth (SCG). The process can be compared to fatigue cracking in airplanes.

Since this is a process that happens extremely slowly, the industry has needed to devise test methods that mimic it in a much shorter time frame, in order to assess materials’ resistance to SCG within an economically reasonable period.

The strain hardening method

The matter has seen increased urgency in recent years, as an increasing number of countries in Europe have begun adopting tougher requirements for pressure pipe, as laid down in the PAS 1075, a so-called Publicly Accessible Standard. This requires even higher resistance to SCG than is called for in the PE 100 specification. The move comes as a result of the gathering pace of adoption of new trenchless pipe installation methods such as guided boring and horizontal directional drilling.

The two most frequently used methods for assessing SCG resistance are the full notch creep test (FNCT) and notched pipe test. These produce results that correlate well with real-time phenomena, but even so they still take around a year to complete. In the absence of any alternative, they have gained widespread acceptance, but in the normal course of events they would be considered unacceptable for quality control purposes. Polymer makers find it extremely frustrating to have to wait so long to get feedback when they are developing what could well be game-changing materials.

This is the reason why SABIC has developed the ‘strain hardening method’ for testing the slow crack growth resistance of HDPE used for pressure pipes. In just a few hours, it produces repeatable results that can be correlated with those obtained with the FNCT over many months.

The strain hardening method requires a simple tensile test at 80 °C. Further advantages of this test method are very low measurement variation, absence of surfactants and notches, and the small amount of required testing material (< 50 g).

Apart from being faster and more cost-efficient than traditional test methods, this method is easy to implement in laboratories, can be suitable in the development of new grades by researchers, and is also very valuable as a batch release test for resin suppliers. SABIC is the first company to implement this method at its production site.

The company went back to basics to develop the new test. Strain hardening is based on fundamental postulates of crack propagation. Analysing the basic polymer properties that give rise to stress crack resistance is a fundamentally different way of obtaining the information needed to show pipes will perform to typical requirement for many decades. SABIC is strongly involved in the initiatives to use the strain hardening method as a standard test method for the pipe industry by test institutes in Europe and around the world.

The international quality authority Kiwa Technology, based in Apeldoorn, the Netherlands, was one of the first to adopt the method and used the test to quickly provide pipeline owners with relevant information about the long-term quality of their existing polyethylene gas and water distribution networks. Moreover, they provide resin manufacturers with the opportunity to compare the relative quality of development resins with other products.

New pipe materials with an enhanced SCG resistance, such as SABIC® Vestolen A RELY 5922R, could potentially be the right solution for the latest pipelaying installation methods.

The company’s Vestolen A RELY 5924R delivers good low sag performance for large diameter pipes and pressure pipes with a low standard dimension ratio (SDR); the first grade better helps customers to meet the stringent slow crack growth requirements for pressure pipe, enshrined in PAS 1075.

Apart from their good mechanical properties, SABIC® Vestolen A RELY 5924R and 5922R have a further convincing advantage. Their unique properties – created through a combination of proprietary catalysts and fine-tuned reactor and extrusion technology – could provide processors with considerable energy savings and help them reduce their carbon footprint.

Future outlook

A company already operating a line at full capacity could be able to increase output by as much as 16% (compared with commonly used bimodal HDPE) while reducing specific electricity consumption. On the other hand, a pipe producer that does not need to raise production can realise energy savings by running at the same speed. Pipe size will also play a part in determining exactly how big the savings are.

Polyethylene already has a large share of the market segment for pressure pipes with diameters under 300 mm. If the strain hardening test method is widely adopted, allowing new material developments such as SABIC® Vestolen A RELY to be brought to market much more quickly than before, it may be expected that that share will grow further. Given the conservative nature of the market, the change is unlikely to be fast – just faster than slow crack growth, one hopes.

Written by Ralph Handstanger, Technical Marketing Engineer HDPE pipe, and Jean Engels, Business Manager HDPE pipe, SABIC Europe, and Lada Kurelec, Technical Marketing Manager for HDPE, SABIC.

Read the article online at: https://www.worldpipelines.com/business-news/21052012/new_test_method_and_materials_for_polyethylene_pressure_pipelines/

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