How do these designations relate to MDPE and HDPE?

Prior to the adoption of international standards (CEN, ISO) PE pipe materials were more commonly designated by their density within the range associated with PE i.e. Low Density PE (LDPE), Medium Density PE (MDPE) and High Density PE (HDPE). The reference to density gave an indication of the material strength. For example HDPE pipe had a higher long term strength or minimum required strength (MRS) than LDPE. But this gave no indication as to other long term properties, such as slow crack growth or resistance to rapid crack propagation.

MDPE was developed in the 1970's specifically for pipes for the gas distribution market. National standards were subsequently developed to incorporate not only strength requirements but also toughness requirements in relation to slow crack growth. These standards were used as the model for the introduction of international standards and the designation of PE 80.

Further developments in the 1990's saw the introduction of a higher strength, higher toughness PE. As the density of the polymer was in the same range as the traditional HDPE confusion arose when describing or specifying the product. A short term solution was to describe the new PE as High Performance Polyethylene (HPPE). However with the introduction of the CEN and further development of the ISO standards the new improved PE was designated PE 100.

References

ISO 9080:2003 Plastics piping and ducting systems - determination of long term hydrostatic strength of thermoplastics materials in pipe form by extrapolation.
EN ISO 12162:2009 Thermoplastics materials for pipes and fittings for pressure applications - Classification and designation - overall Service (Design) coefficient.

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