Historical developments of PE pipe materials Choice of materials Material Classification PE-X Crosslinked Polyethylene

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Polyethylene was discovered almost b

1.  Historical developments of PE pipe materials 
Polyethylene was discovered almost by accident in by ICI in 1933.  Fortunately the commercial significance was recognised leading
to its widespread use today

Polyethylene was discovered almost by accident by ICI in 1933. Fortunately, the commercial significance was immediately recognised, leading to its widespread use today Consumer film packaging accounts for around 60% of the global consumption of around 50 Mte per annum of polyethylene today. However, it is used also for specialty products requiring the highest levels of performance like petrol tanks, cable insulation, chemical drums, and pressure pipes for gas and water distribution and other applications. The consequence of a failure in such products can lead to serious issues and therefore requires materials of the highest integrity. In the pipe application, the product must be installed under ground to give reliable service lifetimes in excess of 100 years without corrosion issues.

Following early applications for water distribution PE was seen to have many advantages for the transport of natural gas. The continuing introduction and development of polyethylene materials with significantly enhanced performance resulted in rapid growth of usage by both the water and gas industries from the early 1970s. Polyethylene is now a major material of choice for many gas and water utilities worldwide for new lines and renovation. Such a development has not come about by chance but principally because polyethylene has solved the corrosion and leakage issues of traditional iron, steel and concrete materials. Above all, the durability of the system has shown an impressive leak free track record.

Polyethylene started to be used for pipe applications in the early 1950s. Low density LDPE materials, promoted by ICI under the “Alkathene” trade name and other companies, were used for water pipes. The stress crack resistance was poor and thicker walled pipe was required owing to the low strength, equivalent to MRS of 3.2 MPa. Mechanical fittings were used for jointing. Significant use of LDPE for water service pipes ceased in the 1980s but improved blends meeting the PE 40 classification are still used today but mainly for irrigation applications.

At the same time in the 1950s, the HDPE family of polyethylene was developed in Germany to become the most common type of polyethylene used for pipe manufacture in Continental Europe. The materials are characterised by higher hoop strength and stiffness compared with the alternative low density materials. These materials were known as "Type 1 or 1st generation HDPE". However it was found that they were prone to develop slow crack growth. These materials were improved in the mid 1970s and materials termed "Type 2 or 2nd generation HDPE" were introduced. Although some of these materials are classified PE 80 in terms of stress rupture performance they are generally inferior in resistance to slow crack growth and rapid crack propagation (RCP) and are being phased out as they barely meet the current requirements of standards and specifications.

A new family of materials was developed in the UK and Europe in the late 1960s in response to the requirement of the gas distribution industry 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 unsurprisingly MDPE. Their introduction also heralded the use of colour pigments to aid identification of the pipeline, at the request of utilities such as the British Gas company requiring yellow pigmented product. 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 period from the early 1970s, a veritable revolution occurred in the use of plastics pipelines systems by the utilities created by properties of the polyethylene pipe products introduced. 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 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, getting away from reference to density, which 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 "1st generation" LDPE products are PE 32 (later ones PE 40) and "1st 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 has now new requirements. PE 100 with specific properties such as low sagging properties to allow very large diameter and heavy walls pipes production or resins with impressive resistance to stress crack propagation called “Superstress” resins developed for the use in sandless bedding applications or relining.

The table below summarises these developments

DATE	TYPE	CLASSIFICATION	APPLICATION	TODAY?
1950	1st Gen - LDPE	PE 32/40	Service/irrigation	Irrigation only
1950s	1st Gen - LDPE	PE 53/63	Large dia. water	Ducting/Non pressure
1969	2nd Gen - MDPE	PE80	Gas and water	Losing market share to PE100
1976	2nd Gen - HDPE	PE80	Gas and water	Behind withdrawn
1989	3rd Gen - HDPE	PE100	All	All
2004	4th Gen - HDPE	PE100 Superstress	Sandless bedding	External layer in multilayer pipe

2.  Choice of materials
Polyethylene is a whole family of materials, often categorized in terms of density. The application of PE in the pipe industry has resulted in a variety of materials being used which are low, medium, high density or even linear low density, abbreviated as LDPE, MDPE, HDPE, and LLDPE.

However a performance based classification and designation system was introduced in the mid 1990s for pipe, resulting in the publication of ISO 12162, see Material Classification. A material is given a designation number based on the predicted long-term hydrostatic strength at 50 years and 20 °C, eg PE 80 or PE 100.

For pressure applications, the choice is usually between a PE 80 or PE 100 compound. PE 40 and lower class LDPE materials have diminished in use for pressure pipe but are used for small diameter irrigation applications. The first generation HDPE materials would fall into the PE 50 or PE 63 class, but would only be used for non pressure or ducting applications nowadays. These have been long superseded by superior performance of second generation PE 80 and third generation PE 100 materials.

There is a choice of PE 80 compounds which can create some confusion. There are unimodal HDPE and MDPE materials meeting the PE 80 classification. However generally such types of HDPE materials can barely meet the requirements of current days PE pipe product specifications, and are being phased out by most suppliers.

The unimodal MDPE PE 80 materials offer excellent performance, but might be considered second choice to PE 100 nowadays and limited to service pipe. Some gas companies prefer to continue to use these materials for their low pressure systems for historical reasons and because of the flexibility of the pipe for installation. Following development of the bimodal PE 100 materials, not surprisingly most suppliers have used the same process to produce bimodal PE 80 materials. Although these offer notably improved RCP resistance over unimodal PE 80 materials, such materials have not really found their place in the market. PE 100 materials are the economic choice for large diameter pipes and the performance of unimodal MDPE PE 80 materials is adequate for smaller diameter service pipes.

Clearly, PE 100 has the advantage of a higher design stress allowing operation at higher pressures or even wall thinning, and the products have greater fracture resistance for large diameter pipe. PE 80 MDPE has more flexibility for ease of installation but this is not a serious advantage. PE 100 materials are suitable for all applications and even service pipes provided the extra stiffness is taken into account when handling coils during installation.

3.  Material Classification
The development of newer generation polyethylene pipe materials resulted primarily in the need to establish a classification system based on long term hydrostatic strength for all plastics materials used for pipe applications.

The introduction of bimodal HDPE products in 1989 gave rise to the need to clearly distinguish these higher performance materials from previous generation HDPEs and MDPEs. At the same time, the development of the ISO TR 9080 method of extrapolation of hydrostatic stress rupture data to determine long term hydrostatic strength and prediction of lower confidence limits was near to publication. To complete the classification of pipe materials, the standardization work resulted in the publication of ISO 12162 in 1995.

The method is quite simple but often misunderstood or considered complicated. The following steps are involved in order to classify a plastics material for pipe applications:

• Test pipe (usually 32 mm diameter) in accordance with ISO 1167 to obtain sufficient data in accordance with the requirements of ISO 9080 (normally 3 test temperatures and 30 data points at each temperature.
• Analyse data in accordance with ISO 9080 to give the Lower Prediction Limit (LPL, formerly LCL) at 20°C and 50 years (or 100 years).
• Round down this value in accordance with the Renard R10 series (values less than 10 MPa) or in accordance with the R20 series (values greater than 10 MPa).
• Compare with range of values in Table 1 of ISO 12162 to obtain the classification number, eg 80 or 100.
• The classification number determines the material designation and the MRS - Minimum Required Strength used in the calculation of the design stress to determine the pipe pressure rating or wall thickness.
• Materials are designated by using an abbreviation for the material type with the classification number, eg PE 80, PE 100, PE-X 100 etc