Tensile-Creep Behavior of Polypropylene Block Copolymer Resin for Non-pressure, Sanitary Corrugated Pipe for Drainage

Session 5B
11:20 AM

Adel Haddad, Equistar Chemicals, LP

A high modulus polypropylene copolymer, typically used in 3-layer corrugated pipe for sanitary applications, was investigated to determine its creep behavior. Two test methods were employed in this study. The first was a short-term test method per ASTM D 6992, Accelerated Tensile Creep and Creep-Rupture of Geosynthetic Materials Based on Time-Temperature Superposition Using the Stepped Isothermal Method. The test was conducted at 500 psi and 1000 psi, in 7 ºC temperature ramp between 23 ºC and 83 ºC over 10,000 seconds period. The second was a long-term ISO creep test which was conducted at 500 psi, 750 psi and 1000 psi and 23 ºC, 40 ºC and 60 ºC for duration of 1000 hours at each stress and temperature combination. The 50-year tensile creep at 500 psi and 23 ºC was predicted using time temperature superposition (TTS) and obeyed the Arrhenius equation. The SIM method and ISO method predicted tensile creep values of 1.24 and 1.17% strain respectively. Based on test data, it was concluded that the SIM method is equally adequate as the ISO method in predicting long-term tensile creep of high modulus polypropylene copolymer. The experiments also showed that the creep rate of high modulus polypropylene copolymer decreased with time, as the greater part of deformation occurred earlier in the test procedure.

As PP is making inroads into waste water piping systems, a study was undertaken to compare the slow crack growth (SCG) performance of PP and HDPE evaluating both the effect of molecular weight, additive package and comonomer incorporation between the two resin families to determine which parameter is significant in SCG. Standard SCG tests such as bent strip, notched constant tensile load and notched constant ligament stress were utilized along with long-term stability testing via DSC IT and OIT. Flow rate and gel permeation chromatography were used to look at the effect of molecular weight and molecular weight distribution while triple detector Temperature Rising Elution Fractionation was used to study comonomer incorporation. Stiffness and tensile stress and elongation properties were also evaluated for the resins in each polymer family.

Adel Haddad Consulting Researcher
Douglas Keller Senior Principal Engineer