Strain hardening test on the limits of Slow Crack Growth evaluation in high resistance polyethylene resins: Effect of comonomer type

Long term performance assessment of polyethylene pipes is an issue that has greatly increased in importance in recent years due to the incorporation in the market of high resistance to crack polyethylene grades (PE100RC), where established Slow Crack Growth (SCG) evaluation using traditional tests such as Full Notch Creep Test (FNCT) or Pennsylvania Notch Tensile (PENT) Test is insufficient. The development in recent years of fast evaluation techniques such as Strain Hardening (SH) modulus has opened an important alternative for quick SCG evaluation since it correlates well with other conventional tests such as FNCT and PENT. In this work, a large number of commercial and experimental polyethylene pipe resins with different comonomer types were evaluated in order to define their SH values to rank the resins as PE100 or PE100RC. A relationship is proposed that utilizes SH test results to estimate the SCG resistance of PE pipes. 1-Butene copolymer resins display threshold SH values of 38 and 53 MPa that have been assigned to PE100 and 100RC grades, respectively. Moreover, dependence of the SH values on comonomer type used has been demonstrated. The experimental results show that 1-hexene copolymer resins exhibit higher SH values than 1-butene comonomer based resins.