Structure-Property Relationships in Bimodal Polyethylene from Indentation Measurements

Polyethylene is an advantageous material for the construction of buried pipelines. It is corrosion resistant, seismic tolerant, and utilizes low cost installation methods. The exceptional long term performance of polyethylene pipe has been fostered by an understanding of the structure-property relationships for the polyethylene chains, particularly the impact of molecular weight and short chain branching on tie chain formation. An area that remains a challenge for predicting performance is the thermal fusion bond created when two pipe sections are joined. The strength of the joint is predicated on the ability of the polyethylene chains to inter-diffuse and form inter-crystalline tie-chains across the two polyethylene surfaces. Testing the strength of the fusion bond is difficult because failure occurs under a biaxial stress state. A number of different destructive and non-destructive tests have been developed, but these allow ranking of joints. Instrumented indentation has the capability to measure elastic, plastic, and visco-elastic/ plastic properties under a multi-axial stress state. In this work, instrumented indentation is used to develop structure-property relationships as a function of the local microstructure. Five different polyethylene resins used for pipe manufacturing are investigated. The impact of thermal processing is investigated by imposing three different thermal cooling histories (0.4, 9, and 100 degrees C/min) on the polyethylenes. The goal is to determine the impact of chain architecture, molecular weight, and crystallinity on modulus, hardness, yield stress, and viscoelastic properties. The results show that instrumented indentation using is capable of measuring many aspects of polyethylene behavior related to performance.