Main types of polyethylene used are the so-called PE80 and PE100 grades classified according to EN and ISO standards for transport of drinking water and gas under pressure.
More recently, new challenges are appearing in this market. New installation techniques are used, and engineering, as well as gas and water companies, are increasing their requirements. INEOS O&P Europe eases the market pressure on their customers by proposing new materials with outstanding properties such as low sagging during extrusion or stress-cracking resistance to meet these new needs.
Historical developments of PE pipe materials
A new family of materials was developed in the UK and Europe in the late 1960s. Its development came in response to the gas distribution industry's requirement for improved resistance to slow crack growth, and flexibility to facilitate installation compared with some of the HDPE materials. These compounds using a base polymer of density between that of LDPE and HDPE are termed MDPE. Their introduction also heralded the use of colour pigments to aid identification of the pipeline, at the request of utility providers, such as British Gas, which required yellow pigmented products. In the early 1980s the blue pigmented MDPE material for potable water systems was developed for the UK water industry by BP Chemicals. Compared to traditional HDPEs available at that time, the MDPEs were considerably more tolerant of site abuse, allowing the development of a range of "No Dig" techniques to allow rapid installation of long lengths of pipes, and for insertion of PE liners for renovation of corroded iron and steel pipelines. The outstanding reliability of this type of material has contributed to the success and growth of the use of polyethylene for distribution systems for both the gas and water utilities.
In the 20 years following the early 1970s, a true revolution occurred in the use of plastics pipelines systems by the utilities - created by the properties of polyethylene pipe products that had become available. PE became the material of choice for low pressure gas and water service pipelines. However, for gas and some larger diameter water applications, the allowable operating pressure was limited by the need to eliminate any risk of rapid crack propagation (RCP), a design philosophy proven in practice.
In the early 1990s, a new type of HDPE material was introduced in Europe, having a higher hoop strength but also with a much higher tolerance to RCP, whilst maintaining the excellent resistance to slow crack growth offered by the MDPE materials. Effectively, these materials are termed "3rd generation HDPE", and sometimes referred to as bimodal as a result of the two-stage polymerisation process used to produce them. Not only have these materials removed the risk of RCP, allowing gas applications up to 10 bar and water up to 16 bar in SDR 11 pipe, but they have made PE more competitive against other materials by allowing wall thinning for lower pressure rated pipe.
At the same time, a material classification system for plastics pipes was developed in ISO standardization committees, resulting in the publication of ISO 12162 in 1996, Ref.1. This allowed the PE 80 and PE 100 classifications, based on long-term hydrostatic strength, to become established, replacing references to density that had become confusing. The 2nd generation HDPE and MDPE materials fall into the PE 80 class and the bimodal 3rd generation HDPEs are generally PE 100. The original early "First generation" LDPE products are PE 32 (later ones PE 40) and "First generation" HDPE are PE 63. The introduction of PE 100 was a major step forward, and going beyond PE 100 with similar practical processing, welding and pipe installation properties seems less likely. However PE 100 materials continue to improve as the market now has new requirements. PE 100 with specific properties such as low sagging properties allow very large diameter and heavy wall pipe production. Also, resins with impressive resistance to stress crack propagation, called “Superstress” resins, have been developed for use in sandless bedding applications or relining.
The table below summarises these developments:
|1st Gen -LDPE
|1st Gen -LDPE
|Large diameter water
|Ducting/ Non pressure
|2nd Gen -MDPE
|Gas and water
|Losing market share to PE100
|2nd Gen -MDPE
|Gas and water
|3rd Gen -HDPE
|4th Gen -HDPE
|External layer in multilayer pipe