What effect does material aging have on the performance of HDPE thread fittings

Photo-oxidative degradation of materials is one of the main causes of performance degradation. In the molecular chain of high-density polyethylene (HDPE), methylene groups are prone to chain scission reactions under the action of ultraviolet radiation. When ultraviolet light with a wavelength of 290 to 400 nanometers is continuously irradiated, a large number of oxidation products such as carbonyl and hydroxyl groups will be generated on the surface of the material. This photo-oxidative effect is particularly significant in outdoor overhead laying scenarios. Experimental data show that after six months of exposure to an environment with an ultraviolet intensity of 300 microwatts per square centimeter, the impact strength of the outer wall of the joint may drop by more than 40%. The accumulation of oxidation products will not only change the surface morphology of the material, but also form a microcrack network, thereby providing a channel for the penetration of the medium, and ultimately leading to the failure of the joint seal.

Under high temperature conditions, the thermal oxidation aging effect is particularly obvious. When the system operating temperature exceeds 60 degrees Celsius, the free radical reaction rate in the HDPE molecular chain increases exponentially, which in turn leads to a widening of the molecular weight distribution of the material and a decrease in crystallinity. This thermodynamic damage is particularly prominent in chemical pipeline systems. 

The erosion of chemical media is also an important factor in accelerating material aging. In an industrial environment containing chloride ions (Cl⁻), the chlorination reaction of the HDPE molecular chain will increase the brittleness of the material. When the concentration of Cl⁻ in the medium exceeds 50ppm, the stress cracking resistance (ESCR) of the joint can decrease at a rate of 3 times that at normal temperature and pressure. A coastal sewage treatment plant used ordinary HDPE threaded joints to treat saline wastewater. After only 18 months of operation, batch leakage occurred. Inspection found that pitting pits with a depth of 0.2 mm had formed on the inner wall of the joint.

The environmental stress cracking (ESC) phenomenon is a typical manifestation of the coupling of material aging and stress. When the pipeline system transports a medium containing surfactants, the HDPE molecular chain is prone to crack propagation under the action of continuous stress. Experiments show that in a 0.5% sodium dodecyl sulfate solution, the crack growth rate of a threaded joint subjected to an internal pressure of 0.8 MPa is two orders of magnitude higher than that of a pure water medium. This type of environmental stress cracking is particularly dangerous in buried pipelines. For example, a gas pipeline in a certain city suffered a catastrophic rupture of its joint after five years of operation due to an imbalance in soil-side pressure.