PROCESSING | STANDARDS & TESTING
constant pressure tests were performed at 65 and 75°C and the cyclic pressure tests were performed at 75°C. “The failure curves from the constant pressure testing clearly showed two distinguished regions controlled by strain accumulation and stress crack growth (SCG) failure modes,” said Vahidi.
Above:
Scientists at Drexel
University have developed a test method to evaluate the small, thin- walled PE tubes under internal pressure – such as those used in catheters
caused by the uncertainty of the notch quality,” the researchers concluded.
Pressure’s on In a similar vein, scientists at Drexel University in Philadelphia, USA have developed a test system to evaluate the failure behaviour of a small-diameter, thin-walled PE tube under internal pressure. These types of pipe are widely used in flow lines for fluids and gases in a variety of applications, including pneumatics and hydraulics applications – and, increasingly, in new designs of heat exchang- er as a replacement for metal tubing. In the biomedical field, PE tubes have been used as catheters for accessing different parts of human body to obtain data or deliver fluids. “Although stress cracking of HDPE has been well
studied, there is no test standard for thin-wall and small diameter HDPE tube,” said Siavash Vahidi, co-author of a paper on the research, presented at last year’s Antec conference. “This is a new test design for both constant and cyclic pressure patterns.” The test setup delivered constant (static) and cyclic (dynamic) pressure patterns, while maintain- ing an elevated testing temperature in order to accelerate the failure. While it is based on the test set-up presented in ASTM D2143-15 standard, for reinforced composite thermoset pipes, it had to be modified for small-diameter, thin-walled PE pipe. Water was used as the internal pressurising fluid due to its incompressibility, which ensured a uniform pressure distribution in the tube. Pressure patterns were obtained by controlling the opening and closing duration of a series of solenoid valves. A water- sensing system was used to detect the failure time, particularly for small brittle failure. A data acquisition system controlled and recorded the applied pressures and failure times. The
16 PIPE & PROFILE EXTRUSION | April 2018
Chlorinated effect Researchers in Japan have studied the effect of residual chlorine on the performance of PEX and polybutylene (PB) pipes that are used in polymer electrolyte fuel cells (PEFCs) – a growing market in Japan. The pre-bent pipes are used to supply hot water to the PEFCs. Because the pipes are difficult to replace, it is important to evaluate their durability and predict their lifetime, said the researchers. Takehiro Fujii of Shinwa Sangyo, told Antec
delegates that a residual chlorine solution immer- sion test for the bent PB and PEX pipes was carried at 80, 90 and 98°C. In the test, the concentration of the residual chlorine solution is 5ppm and 10ppm for 80°C, 5ppm at 90°C and 5ppm at 98°C. The tests revealed that the bent PB pipes developed cracks at 36,000 hours at 5ppm and 90°C; 40,000 hours at 10ppm and 80°C; and 44,000 hours at 5ppm and 80°C. On the other hand, the bent PEX pipe did not crack at all even at exposures of 47,000 hours. The Polymer Competence Centre Leoben
(PCCL) in Austria has also studied the effect of chlo- rinated water on PE pipe – but in this case for drinking water. Researchers tested three PE pipe grades to determine the effect of 10ppm of chlorine dioxide and 100ppm of sodium hypochlo- rite at 60°C. “The presence of disinfectants creates an
oxidative environment, which may lead to acceler- ated consumption of stabilisers and even to the degradation of the materials,” said Márton Bredács, in an Antec presentation. The team has developed a new exposure device
that can offers more precise control and monitor- ing of the test condition, he said. Among other factors, it can hold the temperature at 60°C with a variation of only 1°C, as well as close control of flow rate and ClO2
concentration, and measurement of
pH, free chlorine and oxidation-reduction potential (ORP). “To obtain a more thorough understanding of
the relevant ageing processes, and determine the chlorine species responsible for material degrada- tion and anti-oxidant consumption, further acceler- ated ageing studies will be performed,” he said.
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