PE-X is the standard term used for cross-linked Polyethylene. PE-X is based on the PE polyethylene polymer, but the structure is reinforced by the introduction of links tying the molecular chains together, termed 'cross-linking'. Today there are three principle types of PE-X, PE-Xa peroxide, PE-Xb silane, and PE-Xc radiation cross-linked (see 'What cross linking techniques are available?').

Cross-linking gives the material higher temperature resistance and superior toughness over a wider temperature range compared with PE.

The Molecule Structure of Polyethylene and Crosslinked Polyethylene

• No corrosion
• High maximum operating temperature, suitable for all classes of Hot & Cold water applications, including the UK Class S in accordance with BS 7291 which specifies a higher boiler malfunction temperature.
• Reduced deformation under load (ie lower rates of creep compared with thermoplastics)
• High chemical resistance
• High abrasion resistance
• Exceptional toughness giving impact resistance even at sub zero temperature
• Outstanding resistance to slow-crack growth
• Outstanding resistance to Rapid Crack Propagation (RCP) allowing high pressure gas applications at sub zero temperatures
• Ease of installation
• Pipe can be supplied in coils even up to pipe diameters of 180 mm, reducing the need for frequent jointing
• A variety of jointing systems can be used from full end load resistant electrofusion fittings to simple ‘push-fit’ systems.
• Pipe can be coloured allowing instant recognition of the application

PE-X materials have been used for piping applications for over 40 years now. Also the material has long been used for high performance wire and cable coating applications.

See ‘Industry News’ and ‘Library’ section for KWD Global Pipe edition 245 for a review of the history of the development of PE-X.

PEX is used in the many piping applications including the following:

• Domestic hot and cold water supply which includes all forms of central heating and under floor heating
  • Domestic and industrial natural gas supply outdoors and within buildings
  • Natural gas supply in extreme ambient conditions
  • Carrier pipe in RTP Reinforced Plastics Pipes for high pressure gas applications
• District heating systems
• Air-conditioning systems
• Solar energy systems
• Automotive and ship-building
• Transport of industrial gases, compressed air and fluids (Including fuel oil, gases, acids and alkalis and other chemicals)
• Process engineering and other specialised applications
• Buried pipe using ‘as-dug’ backfill, a significant cost saving compared with sand bedding

The following cross-linking techniques are available:

Peroxide cross-linking (PE-Xa)

• Cross-linking achieved by reacting compound in peroxides at elevated temperatures
• Cross-linking in an amorphous state ( > 136 degrees Celsius)
• Homogeneous cross-linking over the entire wall thickness
• Minimum degree of cross-linking required by Standards > = 70 %

Silane cross-linking (PE-Xb)

• Cross-linking via siloxane cross-linking groups
• Achieved by chemical reaction between hot water/steam and PE silane co-polymer.
• Cross-linking in a semi-crystalline state
• Minimum degree of cross-linking required> = 65 %

Irradiation cross-linking (PE-Xc)

• Cross-linking achieved by high-energy Beta or Gamma radiation
• Cross-linking in a semi-crystalline state
• Only wall thicknesses < 4 mm are economically cross-linkable.
• Degree of cross-linking required> = 60 %

Azo cross-linking (PE-Xd) (Currently little used)

• Cross-linking achieved by use of AIBN initiators (2, 2 ' - Azobisisobutyronitrile)
• Cross-linking in an amorphous state (> 136 degrees Celsius)
• Homogeneous cross-linking over the entire wall thickness
• Minimum degree of cross-linking required >= 70 %
  
  

The basis of performance assessment of a PE-X pipe and indeed any other plastics pipe intended for pressure applications is a series of pressure test to predict long term performance. Thermoplastics materials creep even at room temperature as do PE-X materials, but at lesser rate due to the molecular ties in the structure. This means that plastics pipes can withstand higher pressures for short time durations and lower in the long term. As most pipe systems are designed for 50 year or even 100 year lifetimes a means of predicting pressure rating for design purposes is required.

Pressure tests are normally carried out in accordance with ISO 1167. There is a European EN 921 method but this is being phased out, and in future EN product standards will reference the ISO method. A series of short term and long term tests of up to a year are carried out. The data is analysed in accordance with ISO 9080 Standard Extrapolation Method to give a predicted stress at 50 years and 20C. A design factor is applied according to the application to give a design stress for the material used. Typical a factor of 1.25 is used for PE and PE-X for water application but a higher factor of a minimum of 2 is used for gas applications. Material suppliers are normally responsible for generating this data.

In the case of Hot & Cold water applications a different approach is taken. Generic stress versus time curves are published over a range of temperatures for each material used, ISO 10146 for PE-X. These curves are referenced by the H&C product standards, ISO 15875 for PE-X, to facilitate design of the system.

Although this may appear to be complicated it enables manufacturers of Hot & Cold water systems to arrive at the correct pipe pressure ratings for the intended application, eg under floor heating or hot water radiators.

During installation pipe systems are pressure tested to ensure integrity of the joints. A different approach is used for plastics systems taking into account creep. If a system is subject to a static pressure test, the pressure will decay, not due to a leak but due to the slight creep expansion of the pipe. Methods are available taking this into account.

The highest practical short term application temperature for PE-X is 95 C.
However the material is capable of a continuous operation at 10 bars at 70 C.

Maximum temperature
The maximum specified temperature of water in a Hot & Cold piping system during operation is for boiler malfunction. In accordance with EN ISO 15875 a boiler malfunction temperature of 100C is specified and the system is designed for operation up to 100 h operation at this temperature. However for the UK and other countries referencing BS 7291 a higher boiler malfunction temperature of 110C is specified. Of the plastics material used for H&C applications only PE-X and one other thermoplastic are suitable for this higher temperature.

Minimum Temperature
All materials will eventually reach a transition to brittle behaviour at low temperatures. Some plastics reach this around 0C limiting use. However PE-X does not reach such a transition until of the order of -70C. This allows a PE-X piping systems being used for transport of gaseous fuels to be installed in operating environments as low as - 50C.

Use of standard thermoplastics pipes requires care in the installation of the pipe to avoid excessive external point loads and notches, which may reduce the lifetime of the pipes. PE-X is absolutely stress-crack-resistant. The lifetime of PE-X pipes is not influenced by point loading or scratches and notches. Pipes made out of PE-X can be used without sand-embedding without risk. This allows the use of ‘as-dug’ backfill for buried pipes reducing the cost of installation significantly.

The PE product standards for gas and water set a bench mark ISO 13479 Notched Pipe Test requirement of 500 h for slow crack growth resistance. However PE-X materials easily exceed this requirement by 10 or even 20 fold.

The dimensions of the pipes are given in the following tables. The plastics industry generally follows the recommended external diameters specified in ISO 161. However there are some exceptions such as pipe produced to BS 7291 and in Eire to IAB standards. Most manufacturers will supply a pipe system complete with appropriate fittings or recommend fittings to be used.

Although the majority of PE-X pipes are produced in the smaller diameter range of 12 mm to 63 mm for Hot & Cold water applications, large diameter pipes of up to around 450 mm have been produced. Developing technology will no doubt lead to an increase in the diameter range if the demand arises.

For the determination of the dimensions, the methods specified in ISO 3126 are usually followed.

Yes, it can be welded using electrofusion fittings which gives a fully end load resistant piping system.
It is also possible to butt-fusion weld PE-X pipes. However for practical reasons this is essentially restricted to PEXb pipes which are only partially cross-linked.

Electrofusion welding	Buttfusion welding

The following joint techniques are available for PEX pipes:

• Fusion welding using electrofusion fittings and butt-fusion welding in certain circumstances
• Mechanical systems similar to those used with other pipe systems
• Push-fit, compression fittings, flanges, screwed joints

Mechanical fittings	Electrofusion
Buttfusion	Push fit

Yes, PEX is used for potable water supply pipe conforming to the latest national drinking water regulations globally including UK DWI, German DWVG, US NSF etc.

In Europe harmonised water quality regulations are being developed under the guise of the EAS European Acceptance Scheme. The aim of this scheme is to subject all materials in contact with drinking water into a ‘level playing field’ of test requirements. However in the meantime national regulations continue to be applied. The plastics industry has implemented taste and odour and migration assessment testing for many years, but metals and cementitious products are not tested currently.

Over and over again, cases of the so-called legionella disease have been well publicised to the public. In heating and distribution systems, there is danger of a longer stagnation of water within the pipe system which could create conditions for growth of legionella bacteria. Cooling tower have been a source if not subject to the correct treatment. The temperature range in which legionella growth appears is often between 30 and 45 degrees Celsius.

Measures Regulating

Legionella Growth and Origins of Growth dominate. According to reports, they are able to survive at temperatures lying between below within the pipe system and a regular and complete water exchange takes place within this freezing up to plus 60 C. In domestic houses in which the water runs only short distances Legionnella growth can occur where stagnated water and favourable temperatures (25C to 50C) system, there is only an extremely low risk of legionella growth. However this can be different in larger buildings equipped with more complex equipment.

Potential contamination sources within drinking water systems:-

• Ventilation pipes
• Less or non-used system areas (especially 'dead pipes')
• Drinking water heater
• Membrane expansion container
• Evacuation pipes of great importance.

In most cases mistakes with planning, installation, assembly (design, hydraulic comparison etc.) hospitals, row showers or indoor swimming pools etc.) and the type of use (e.g. whirlpools) are Therefore, operating conditions, building size including a possibly complex network (e.g. and treatment are responsible for the legionella growth within a pipe system.

The material from which pipes are made is rather insignificant. Legionellas can settle on all kind of materials such as iron, steel, copper, plastics, ceramic or glass. Preferentially, they grow in imperfections of the inner pipe surfaces.

See www.teppfa.com for further information.

PE-X is based on the PE polymer but the structure is reinforced by the introduction of links tying the molecular chains together termed cross linking (see ‘What cross linking techniques are available?’).
This gives PE-X higher temperature resistance and superior toughness over a wide temperature range.

PE-X pipes have enjoyed strong growth over the last 10 years for Hot and Cold water plumbing applications alongside other suitable plastics materials, at the expense of copper. The high temperature resistance and exceptional toughness makes PE-X a leading material. Availability of complete systems, ease of installation, a competitive price and above all corrosion resistance are other factors that have contributed to this growth. Another reason has been the development of strong standards for this application. EN ISO 15875 PE-X for Hot and Cold water applications was published in 2003. National standards for the application have been very prominent also, in particular BS 7291-1/3 and DIN 16892/3.

However PE-X has also come into prominence for gas applications because of its toughness and fracture resistance, which gives it the ability to meet stringent requirements at both sub zero and high ambient temperatures. The scope of the ISO 14531 PE-X gas standards allows operation at temperatures from – 50 °C to +60 °C and it can be installed using as dug backfill, a significant cost saving compared with normal use of sand backfill. It is also an option for the carrier pipe for products manufactured to the ISO TS 18226 Reinforced Thermoplastic Pipe standard for high pressure gas for use in more severe conditions.

PE-X standards are being developed for other applications including district heating pipe systems and industrial applications.

In recent years standards have been developed for multilayer pipe in recognition of the growing interest for this type of product. Multilayer pipes can be tailored to give ideal performance characteristics for particular applications. The PE-X – Aluminium – PE-X combination is the leading product suitable for all common applications. This type of pipe is popular with plumbers and installers because small diameter pipes can be bent like a copper pipe. Multilayer pipe standards have been developed for Hot & Cold water, water supply, and for both indoor and outdoor gas applications.

Drinking water is becoming a scarce commodity globally, and is coming under increased regulation particularly in Europe . The requirements for products in contact with water are being scrutinised in Europe with the development of harmonised water quality regulations. The development of the EAS European Approval Scheme for products in contact with drinking water is being closely followed by the Association. Plastics have long been tested for taste and odour and by migration assessment, but metallic and cement products have not. The EAS will bring in a ‘level playing field’ for all materials.

The introduction of REACH for the use and handling of chemicals is just beginning in Europe . This will have wide ranging implications for all industries and even for every day life. This is another regulatory subject that the Association are following closely.

Registration, Evaluation, and Authorisation of Chemicals

The new Reach regulation on industrial chemical control entered into force on 1st June after being agreed by Council and European Parliament on the 18 th December 2006 and came into force on 1 st June 2007. As a regulation it will have direct effect in member states, cutting out any need for lengthy and uncertain national transposition procedures. It will replace over forty existing EU chemical policy laws and introduce new registration requirements covering all substances supplied above 1 tonne per year, and new authorisation requirements covering substances of very high concern, eg carcinogens.

Producers of chemicals will have to plug gaps in safety data for thousands of "existing" chemicals produced above 1 tonne annual volume. The most dangerous chemicals will be gradually squeezed out of the market through an authorisation process designed to foster substitution by safer alternatives, providing this is technically and economically feasible. Around 1500 substances fall into the area of high concern and will require authorisation. A registration document is to be prepared by industry to provide core hazard data, plus risk assessments. Registration will be phased in over 11 years to 2018 starting with high tonnage substances. 20,000 substances are supplied in volumes between 1 to 10 tonnes per year and have the full 11 years to become registered. Most of the provisions cover manufacturers and importers of chemicals, but downstream users have rights and obligations. Their supplier’s chemical safety assessment must cover their use, but the downstream user is obliged to implement risk reduction measures recommended by the supplier.

The entry-into-force date has triggered a cascade of deadlines affecting businesses, member states and the creation of the European Chemicals Agency to be based in Helsinki . Most of Reach’s substantive provisions will apply a year later, ie from 1st June 2008. The first business relevant deadline is 1st December 2008, when a six-month window for chemical suppliers to pre-register substances closes.

The first substance registration deadline falls on 1st December 2010. This will apply to manufacturers or importers of category 1 and 2 carcinogens, mutagens and repro-toxins produced above 1 tonne, to substances classed as very toxic to aquatic organisms supplied in volumes above 100 tonnes, and to all other substances above 1000 tonnes. 1 st June 2013 is the deadline for manufacturers/importers of substances above 100 tonnes per year. Firms that fail to register these will be denied access to the EU market.

By 1st June 2008 the European commission must adopt a regulation setting the fees payable by firms under various parts of Reach. By 1st December the commission must review criteria for identifying persistent and bio-accumulative substances, a class of dangerous chemicals targeted by the authorisation procedure. By the same date the commission must define the criteria that will allow some firms to omit some testing requirements under Reach.

On 1st January 2009 the ECA will publish the list of pre-registered substances and by 1st June the same year it will make its first recommendations on priority substances to be included under the authorisation requirement.

It is expected that competent authority bodies in each country will be assigned to raise awareness and help with administration of REACH. For instance in the UK the Health and Safety Executive will act as the competent authority for REACH and liaise with the new European Chemicals Agency in Helsinki .

For the plastics industry converters EuPC have set-up a REACH helpdesk on reach@eupc.org . Other sources of information are TEPPFA, www.teppfa.com and the Chemical Industries Association, www.cia.org .  

What does this mean for producers of PE-X products ?

Product manufacturers are ‘down stream converters’ taking polymers, additives and other chemicals to manufacture compounds or end products from these. It is the manufacturers and suppliers of these raw materials who will be required to register these and be responsible for obtaining and providing the necessary information. However the ‘downstream converters’ will need to ensure that they have the information from their suppliers and be obliged to take the required safety precautions for their use. Likewise compound suppliers will need to pass the information down the chain to product manufacturers.

There is a preconception that PE-X cannot be recycled. Clean thermoplastics materials can be chopped or ground and re-pelletised for reprocessing into products by extrusion or moulding. However PE-X acts more like a thermoset with no distinct melting point. The Association has conducted a survey and identified the following alternative possibilities:-

• Mechanical recovery
  • Grinding of PE-X to a fine powder enables the product to be used as a filler in other plastics or in other compositions. These powders have been in hand cleaner formulations also.
  • Other techniques using high shear compounding techniques to break down the cross-linked network have also been patented by Alcatel.
    
• Chemical recovery
  • The chemical Si-O-Si bond in silane crosslinked polyethylene can be broken under conditions of temperature and pressure using super critical alcohols as described in Japanese Hitachi patents.
• This concept has been further developed by Hitachi using compounding techniques.
• The commercial viability of such techniques has not been demonstrated.
  
• Thermal recovery
  • Polyethylene has a similar calorific value as oil at 45MJ/kg making thermal recovery an attractive option.
  • PE-X pipe is relatively easy to separate as waste and should represent an attractive viable low cost energy recoverable route.