Study of the Features of Climatic Aging of Stabilized High-Density Polyethylene in Severe Natural and Climatic Conditions

Many regions of Siberia and the Far East are characterized by extreme natural and climatic conditions including significant seasonal (from –60 to 40°C) and daily (up to 30°C) temperature fluctuations with repeated transitions through 0°C, high humidity, and intensive solar radiation. The results of a study of the climatic resistance of high-density polyethylene (HDPE) in complex natural and climatic conditions are presented. The HDPE of 273-83 grade containing the stabilizer of SO-4 grade is studied. Field exposure at the test site is carried out over a period of 15 months. When studying the relative elongation at break, it is found that materials containing 1.0 wt % of SO-4 have the highest climatic resistance. In addition, the stabilizer allows one to significantly increase the resistance of HDPE to oxidative processes without participating in the Norrish II reaction accompanied by the formation of terminal vinyl groups. Analysis of the melt flow index shows that during aging at the initial stages (up to 3 months), predominantly the reactions of macromolecular chain cross-linking proceed in the materials. With increasing exposure time, reactions accompanied by the breaking of macromolecular chains begin to prevail over cross-linking processes leading to a decrease in melt viscosity. The use of a stabilizing additive makes it possible to reduce the rate of both competing reactions. It is found that the effectiveness of the stabilizer is determined by its optimal concentration, which depends on the solubility of the additive in the polymer matrix. Insufficient amount of stabilizer leads to its premature consumption during inhibition of radical chain reactions occurring under the influence of UV radiation and atmospheric oxygen. If there is an excess, part of the additive appears on the surface of the material causing the formation of internal defects that lead to embrittlement of the HDPE. The obtained results can be used to assess the reliability and durability of the HDPE products and stabilized materials on HDPE basis when exposed to specified natural and climatic conditions to prevent equipment failures during operation.