Polyethylene raised-temperature materials

Recently, polyethylene raised-temperature materials (PE-RT), which are well-established hot water piping materials in Europe, have been introduced into the North American market. Also, multilayer specialty piping products using PE-RT with various barrier and protective layers have been introduced. 

The temperature dependence of ESR spectra of fractured polyethylene labelled by nitroxide radicals is shown in Fig. 10.2., which shows the case of high density polyethylene. W indicated in Fig. 10.2. means extrema separation (outermost separation) and is the same quantity as S in Eq. (9,5). W is a measure of the mobility of the materials and the value of W is shown to decrease with raising temperature reflecting a much more mobile character of the observed molecules at elevated temperatures. As mentioned in the previous section, the varying extrema separation with temperature has been widely discussed in connection with molecular motion. 

Thermal properties are interesting from both theoretical and practical viewpoints. An amorphous material begins to show rubberiness as its temperature is raised above the Tg. In the case of crystallizable polymers rubbery properties may be affected by crystallization up to temperatures close to the Thus at normal ambient temperatures both polyethylene and polypropylene are not rubbery even though they are above the Tg. 

Thermoplastics soften when heated (and eventually liquefy) and harden when cooled — processes that are totally reversible and may be repeated. On a molecular level, as the temperature is raised, secondary bonding forces are diminished (by increased molecular motion) so that the relative movement of adjacent chains is facilitated when a stress is applied. Irreversible degradation results when a molten thermoplastic polymer is raised to too high a temperature. In addition, thermoplastics are relatively soft. Most linear polymers and those having some branched structures with flexible chains are thermoplastic. These materials are normally fabricated by the simultaneous application of heat and pressure (see Section 15.22). Examples of common thermoplastic polymers include polyethylene, polystyrene, poly(ethylene terephthalate), and poly(vinyl chloride). 

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