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#12 (46), 2006
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PEXb: frequently asked questions  #4 (38), 2006

PEX


 

1. What’s PEX?

PEX means cross-linked polyethylene. In other words, it’s polyethylene that is chemically and mechanically modified to organize three-dimensional links. After these modifications thermoplastic polyethylene (creeping when heated) turns into elastomer.

 

2. Cross-linking

There are three variants that can be resorted to for cross-linking of polyethylene: peroxide, radiation and silane technologies. According to ISO standard minimal cross-linking degree (gel fraction content) should be 75% for peroxide cross-linking, 65% for silane and 60% for radiation cross-linking technology.

Peroxide cross-linking (PEXa)

Here PE is mixed with peroxide during the extrusion process. When the temperature is raised, peroxide decomposes producing radicals. When the radicals interact, they form cross-linking groups. PE melt is partly cross-linked when leaving the mixing zone and it’s then molded to a pipe. Then the pipe is heated to finish peroxide cross-linking. After cooling the product has considerably lower density.

Property Substituting HDPE for PEX
Coefficient of melt creep Lower
Density No changes or slightly lower
Tensile strength No changes or slightly higher
Elongation at rupture Lower
Resistance to rapid crack Superior
Resistance to slow crack Superior, PEX is designed to resist hydrostatic pressure
Chemical strength Superior
Fatigue strength Superior
Wear strength Superior

Silane cross-linking (PEXb)

Silane cross-linking process includes grafting an active silane molecule (usually vinyl trimethoxisilane to PE initiated by peroxide. There exist two variants of this technology – Sioplas and Monosil.

Following Monosil technology, vinylsilane is incorporated into PE by extruding a PE pipe. Sioplas technology implies PE is compounded with peroxide and vinylsilane at the first stage and then the compound is mixed with a catalyst superconcentrate and passed on to the extruder to manufacture a pipe. In both cases the essence is confined to incorporating an active silane molecule onto the PE backbone. The pipe produced is then cross-linked during several hours in hot water or under vapor exposure.

Radiation cross-linking (PEXc)

PE-pipes produced by extrusion are irradiated for cross-linking. The cross-linking happens under exposure to intense radiation: like gamma irradiation or electron injector treatment. Due radiation radical groups are formed on polymer backbones. Two radicals that appear as a result of this process are combined making the cross-links.

Normally a PE-pipe is cross-linked being many times exposed to electron injection until the estimated radiation dose needed for cross-linking and cross-linking degree are achieved.

3. Advantages of PEXb

Silane cross-linked PE-pipes production does not require vast investments: PEXb pellets are processed at a standard HDPE extruder with standard feeder.Main advantages:

  • Considerable heat resistance improvement
  • Low temperature characteristics
  • Impact strength and tensile strength
  • Chemical strength and resistance to solvents (does not dissolve)
  • Resistance to slow crack
  • Notch sensitivity
  • Low creep and corrosion susceptibility under stress
  • Higher fatigue and wear resistance
  • Lower coefficient of linear thermal expansion
  • Scratch resistance and strength
  • Strain and ability to restore initial shape (memory effect)
  • Acoustic isolation (do not hum)
  • Non-toxicity
  • Easy processing and transportation

4. Organoleptic and toxicity characteristics

Padanaplast composites are certified by NSF (USA). The certification implies examining the materials the product is made of, extraction products, toxicity research and manufacture audit. NSF certificate means that the certified formula can be controlled once a year without notification, in this way stable quality and formula are guaranteed. Several national certifying organizations have admitted Padanaplast PEXb pipes for using in hot and cold water supplies composites’ formula components are admitted for this kind of application.

5. Polidan PEX by Solvay Padanaplast

Polidan ® PEX is a silane cross-linked HDPE, made by Sioplas technology.

Polidan® PEX is a two-component system containing silane-grafted composite and composite with catalyst and antioxidant superconcentrate. The components are blended in 95:5 proportion and extruded similarly to standard HDPE.

Polidan ®T/A perfectly suits for making thin-walled structures.

Polidan® T/A-HF is designed for making very flexible and shiny pipes so normally it is used for outer layers of pipes that are laid where they are visible.

Besides other brands Padanaplast makes super-light brands with various rheology for easier processing at non-standard equipment. To select a special brand you should address a regional office of Solvay Padanaplast or directly the producing plant.

6. Application spheres and market segments Automobile construction

Polidan T/A is used for extrusion of small-diameter pipes used for liquid media transportation inside the automobile. Choosing this material you bring down expenses and material consumption compared to the traditionally used polyamides, preserving the necessary temperature and pressure resistance.

Drinking water

Polidan is used for pipes that contact with drinking water and are certified by the supervising organizations in the countries where it’s necessary.

Metal-plastic pipes

The most considerable sphere of application for PEX is metal-plastic pipes, that is pipes of PEX/Al/PEX construction.

Using metal-plastic pipes aims at providing all the benefits of metal pipes, among which are mechanical strength, stable properties in exploitation without their drawbacks such as corrosion exposure, huge mass, stiffness and electrical conductivity.

Heating

PEX pipes are becoming more and more popular for heating, and this market is expanding, as PEX pipes are replacing PB, PP and PVC in the newly built accommodation.

The decisive factor to be considered choosing an all-plastic pipe is oxygen permeability. Here the best solution may be PEX coextrusion with EVOH barrier layer.

Polidan is used for metal-plastic pipes with barrier covering. Our customers produce pipes with EVOH covering as the middle layer (5-layer pipes) or as the outer layer (3-layer pipes) ensuring protection from oxygen penetration. This minimizes boilers’ steel parts corrosion caused by solute oxygen.

Introduction of a heating system

Polidan PEX is used for making pipes for introduction of a heating system together with a special superconcentrate raising pipes’ mechanical properties. Polidan PEX pipes have been run for many years showing perfect strength at high temperatures and hydrostatic pressure heating hundreds of houses in the Northern Europe. There is a unique method for old pipe lines repair that implies broaching PEX pipes: a small-diameter pipes is broached and then ballooned under vapor or water pressure to fix the shape due to cross-linking.

Application in industry

Resistance to hydrolysis, acids and bases and low swelling in oil allow using PEX pipes for industrial purposes.

Liquid chemicals transportation oil pipe-lines fettling, thermal waters transportation – these are only some of the spheres Polidan is resorted to due to its chemical strength.

Gas transportation

Polidan PEX is characterized by extremely low liquid and natural gas and odorants permeability, which, together with exquisite mechanical strength makes them preferable for being applied in this sphere.

 

 


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