Effect of molecular structure on the durability of high-density polyethylene☆☆☆

A STUDY of the durability of polyethylene showed that as a crystalline polymer it is subject to two types of breakdown, i.e. plastic rupture under big stresses, preceded by the development of a "neck", and brittle fracture, marked by cracking under moderate stresses. This is reflected in the break that appears o15 the durability isotherms o f PE. Two durability parameters were therefore introduced for PE [2]: the long-term plastic strength (LPS), and the long-term brittle strength (LBS), by which is meant the static resistance of the material to plastic and brittle breakdown respectively. I t was shown in references [2, 3] that the existence of these two types of breakdown of PE may be related to the dual nature of its structure, the elements of the latter being on the one hand the PE macromolecules, and on the other hand the supermoleeular crystalline formations. In view of this there will be two competing thermo-aetivated kinetic processes of breakdown developing simultaneously in the polymer from the moment that the tensile stress is applied, namely reerystallization (breakdown at the supermolecular level) and static fatigue (breakdown at the molecular level); Depending of which of these two competing processes predominates there will be either the development of a neck, or brittle fracture. When the recrystallization process predominates, the supermolecular crystalline formations are obviously the main factor involved in the resistance of the material to plastic rupture, and in this case the effect of a the individual molecules will be of quite minor importance. On the other hand, where the process of static fatigue predominates, there will be the effect of the supermolecular formations, and that of the individual molecules ~dso. The effect of the supermolecular crystalline formations is due to the fact that the aggregation of macromolecules ensures more mfiform acceptance of the stress. Therefore, while the LPS is sensitive only to the supermolecular structure, the LBS is sensitive to the molecular structure as well. Experience of the actual behaviour of PE structures :m.der service conditions, together with the results of temperaturet ime extrapolation of the durabil-