MATERIALS | ENGINEERING THERMOPLASTICS


Right: Metal district heating pipes require frequent replacement due to corrosion


molecular structure, which also allows it to respond to additional requirements such as the ability to be shaped (pipes and fittings) and fuse-welded, as well as suppleness (allowing the pipe to be coiled onto a spool).


Non metallics In addition, the typical conditions that non-metallic substitutes will have to deal with in district heating networks include: n Nominal pipe diameter: 40mm to 600 mm; n Internal pressure: 3 to 6 bar; n Sustained operating temperature: 110°C (and up to 130°C);


n Operational lifetime: 10 years or more; n Sliplining using coiled pipes (environmental constraint);


n Use of electrowelding or butt-welding for joints; and,


n A high-performance stabilising system (anti- oxidants) to slow chemical degradation of the polymer matrix, especially through oxidation;


n A polymer matrix with minimal sensitivity to oxidative attack, independent of the stabilising system in the matrix; and,


n A macromolecular and crystalline structure that significantly impedes slow creep-crack growth at high temperatures. The first requirement requires modification of conventional polymer solutions, such as polybutyl- ene (PB), polyamide (PA) and polyethylene (PE). Cross-linked polyethylene (PEX) and polyethylene of raised temperature resistance (PE-RT), or even polybutylene (PB), would meet these demands, but only for a permanent operating temperature of 110°C or less – and, for limited operational life- times. Regardless of how effective the stabilising system in the polymer matrix might be, its effective- ness over time will be limited, due to the low amount that can be incorporated into the matrix – which is susceptible to photo-thermo-oxidative attack due to its molecular structure. The second requirement calls for a polymer


matrix that is almost impervious to any kind of oxidation, due to the limited number of sites liable to oxidative attack on the carbon chain, through partial or complete substitution of the labile hydrogen atoms. The third requirement calls for a semi-crystalline


14 PIPE & PROFILE EXTRUSION | January/February 2019


n Resistance to the pH of the water transported: 9.1. Taken together, these technical requirements point to partially-fluorinated polymers such as polyvinylidenedifluoride (PVDF), or ethylene chlorotrifluoroethylene (E-CTFE). Polytetrafluoro- ethylene (PTFE) is omitted from the list of solutions due to technical limitations linked to its shaping, and its unsuitability for welding or gluing – a consequence of its high thermal stability due to the saturation of the repeat unit of the carbon chain by four fluorine atoms. In fact, the increased number of substituted fluorine atoms increases the thermal stability of the material to the detriment of its ability to be shaped and welded.


PVDF promise PVDF is a fluorinated thermoplastic that is white to translucent in colour. It was originally exploited for its electrical and piezoelectrical characteristics, and was later used to make coatings – and, eventually, for pipes and accessories in the chemical and pharmaceutical industries due to its high chemical purity, low permeability and resistance to most chemical compounds. The high chemical and thermal resistance of PVDF arises from the partial saturation of the carbon chain by two fluorine atoms in the repeat unit of the polymer. PVDF is a semi-crystalline polymer whose continuous operating temperature interval lies between −30°C and +150°C. Its glass transition temperature (where it passes from the glass state to a ‘rubbery’ semi-crystalline state) is at about -40°C. It is easily shaped into pipes, fittings and moulded pieces using conventional extrusion and injection techniques. The available pipe diameters generally range from 16mm to 315mm, but the


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