The most important values characterizing the properties of polyethylene are density and molecular weight. A whole range of mechanical properties, including yield stress and flexural and torsional rigidity, depend particularly on density, while toughness (notched impact strength), especially at low temperatures, wear resistance, heat deflection temperature and resistance to stress cracking all depend on molecular weight.
The density of the UHMWPE is in the range of 0.925 to 0.95 g/cm3 for virgin material. As a comparison the HDPE – High Density Polyethylene has a density in the range of 0.94 – 0.97 g/cm3.
The extremely high molecular weight imparts properties to this plastic that limit the processing of the virgin material powder to ram extrusion and compression molding processes. As the molecular weight of the polyethylene increases, higher values are obtained for a number of technically important properties:
• Notched impact strength
• Wear resistance
• Energy absorption capacity at high stress rates
• Heat deflection temperature
• Stress crack resistance
The molecular weight for regenerated material that can be produced is in general lower than virgin material and in practice the regenerated material in the market available depends on the mixing of different types of raw material.
The yield strength is the stress the material can withstand without permanent deformation. For UHMWPE the characteristic value for the tensile strength at yield is ≥ 17 Mpa.
UHMWPE has an extremely high impact strength, which is retained even at very low temperatures. The impact strength is so high, in fact, that under the standard notched impact test (ISO 179) UHMWPE does not break. Therefore, a special, “double notched” impact test is used.
UHMWPE has a good wear resistance. A suitable test method for the purpose of comparison is the sand-slurry test. The lower the value the better the wear resistance.
UHMWPE is an excellent material for sliding applications. The material possesses self-lubricating properties, particular in dry sliding movement against metal surfaces.
UHMWPE is water-repellent and does not swell. It is resistance to sea water.
Through the addition of light stabilizers the outdoor service life can be extended. The most economic and simple protection means is to incorporate carbon black than no oxidative degradation is evident. Despite being efficient, the Carbon Black has a color issue – all parts must be black. When the final product cannot be black, one must use chemical protection.
UHMWPE is a very effective insulator, although it can be reduced to the antistatic region by the addition of conductive carbon black.
