PROCESSING | STANDARDS & TESTING Right:
Chlorine-based disinfectants keep drinking water clean, but can have a detrimental effect on
plastic pipes
Checking with CRB PCCL researchers have also assessed the relatively new cyclic cracked round bar (CRB) test as the basis to characterise PE pipe grades. The CRB test was recently standardised by ISO18489 as an alternative – and faster – way of assessing the slow crack growth (SCG) performance of PE pipe grades. The researchers used the test to characterise 35
grades PE80, PE100 and PE100-RC material. For the six materials with highest SCG resistance, PENT test data was only available in terms of breakup times – as the materials showed no failure after long test times. However, the CRB test used failure cycle numbers to differentiate between the six grades. Overall, the researchers categorised the main advantages of the CRB test as: n Much faster testing times: characterising PE pipe grades with very high SCG resistance can be done within two weeks. The speed is achieved by the specimen geometry – which leads to a quick crack initiation – and the cyclic loading. n Material is tested at ambient temperatures: while test methods such as NPT, FNCT or PENT are done at 80°C or higher, the CRB test characterises SCG at ambient temperatures – which is much closer to the material performance of real pipes in the field. n Results correlate with those of traditional test methods: the SCG ranking based on CRB provides the same order as those of the FNCT test, said the researchers – and with a much shorter test time. n Differentiation of PE pipe grades with high SCG resistance: in contrast to other test methods – which are stopped after a defined time in order to pass a ‘non-failure criterion’ – the CRB test provides quantitative results based on real crack initiation and SCG resistance. Because of the technical and economic advan-
tages of the test, the researchers recommended that the US market adopt it as a complementary – or alternative – method to the existing PENT test. In a variant on this, PCCL carried out CRB tests in
order to correlate it with strain hardening (SH) – an- other method for measuring SCG. Strain hardening is a quick, reliable test to determine the disentan- glement capacity of the tie molecules in PE patterns. The researchers tested 12 commercially avail- able PE pipe grades using CRB. Results were compared with those from SH testing, and showed a linear correlation.
“Earlier studies confirm the correlation between these two test methods but point out that the SH modulus is [more] sensitive to co-monomer type of
18 PIPE & PROFILE EXTRUSION | April 2018
material than the failure time of Cyclic CRB Test – which could be a reason for the mismatch in correlation relating to some materials,” said the researchers.
Chinese success Chinese researchers have also reported success in using CRB testing – and say they have used it to show that a polypropylene-based pipe had similar long-term performance to one made of PE100. “In this work, the SCG resistance of a new
PP-based compounded material – with flame retardant and antistatic properties, which can be used in coal mines – was investigated using the CRB test,” said Wenbin Liang of the National Institute of Clean-and-Low-Carbon Energy (Nice Energy), in a presentation at Antec. “The results show that the new compound has excellent long-term performance.” The weight fraction of the fillers in the pipe
grade was 28%. Bars of material – both the new grade, and the comparative PE100 – were tested on servo-hydraulic machines from MTS. Samples were also tested using dynamic mechanical analysis (DMA), in order to investigate creep. Scanning electron microscopy (SEM) was used to confirm the failure mode of the specimens. “The cycles to failure and the rate of crack opening displacement of the new grade were comparable to PE100 pipe material – which indi- cated that it may find applications in industrial pipes with excellent long- term performance,” said Liao.
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