Creep behaviour of rotomouldable grade materials: A comparative study

Abstract

Rotational moulding industry is looking to increase its utility spectrum from simple hollow products for storage purposes to complex engineered products used in structural and automobile applications. As such applications require satisfactory performance of the products for several decades, consideration of their creep performance in design is extremely important. Off late, Polypropylene (PP) is looked upon as a preferred choice of material for such products apart from the conventional high density and linear low-density polyethylene (LLDPE) grade materials, mainly due to its excellent static mechanical properties. This study presents a comparative analysis of the creep behaviour of PP with two commercially used hexene (C6) and butene (C4) grades of LLDPE to understand the long-term performance of products made out of these materials. For this purpose, accelerated creep testing using stepped isothermal method was carried out at three commonly used stress levels of 3.5, 4, 4.5 MPa and the creep behaviour was predicted at reference temperatures of 25 °C, 40 °C, and 60 °C. To compare and validate these findings, conventional creep experiments were performed up to one year, and the creep behaviour was extrapolated using Findley's equation. Rheological studies and microstructural characterisation involving wide-angle X-ray scattering, Raman mapping, solid-state NMR, and FT-IR spectroscopy were done to correlate the above findings. The study was further extended to develop a creep model using Norton Bailey law to predict the primary and secondary creep stages. This model can be efficiently used to predict the creep behaviour in a shorter time and thus can be used in the design of engineered products where creep failure may be a concern.