Aging behavior and lifetime assessment of polyolefin liner materials for seasonal heat storage using micro-specimen

Highlights

• Global aging experiments of a commercial PE-RT and a novel PP-R grade.

• Critical environmental condition for PE-RT is hot water, for PP-R hot air.

• Lifetime assessment was done by accumulation of damages at service temperatures.

• Lifetimes in most critical environment: 1 year for PE-RT and 16 years for PP-R.

Abstract

The paper deals with the lifetime assessment of polyethylene (PE) and polypropylene (PP) liner materials for seasonal heat storages. Temperature loading profile was estimated for a storage type with maximum temperatures of 85 °C in September and minimum temperatures of 50 °C in March. Aging experiments in hot air or water at elevated temperatures were carried out using micro-sized specimen. The aging indicator strain-at-break was monitored. From experimental data at elevated temperatures endurance times at service-relevant temperatures were extrapolated using an Arrhenius approach. By weighting the endurance times with the loading profiles and assuming cumulative damages, lifetime values were deduced. The investigated PP random copolymer liner material exhibited a significant better long-term performance than the well-established PE liner material.

Introduction

Common polymeric liner materials for hot water seasonal heat storages are based on polyethylene raised temperature resistance grades (PE-RT) or polypropylene random copolymer grades (PP-R) (Novo et al., 2010, Pfeil and Koch, 2000). A ductile material behavior in the service temperature range between 40 and 95 °C (Novo et al., 2010, Wallner et al., 2013) as well as weldability, waterproofness and resistance to hydrolysis are required. The long term stability of polyolefin-based liner materials against thermo-oxidation is essential to ensure functionality and durability of hot water heat storages at service temperatures up to 95 °C for at least 15–20 years (Schramm and Jeruzal, 2006, Paranowska and Pedersen, 2016). Compared to applications of such materials for hot water piping, the maximum service temperatures and the exposure times are significantly higher for seasonal storages. Hence, special attention has to be given to the investigation of polyolefines accounting for the more demanding temperature loading profile.

The main objective of this paper is to investigate the aging behavior and to assess the lifetime of two different polyolefin grades using micro-sized specimen. A well-established PE grade should be compared to a novel PP random copolymer liner material. Temperature loading profiles were estimated for a medium temperature heat storage type with a maximum temperature of 85 °C in September and a minimum temperature of 50 °C in March. The technological parameter strain-at-break was determined for micro-sized specimen after exposure to hot air or water at elevated temperatures of at least 95 °C. Using an Arrhenius approach, endurance times at service-relevant temperatures were calculated. For lifetime assessment the theoretical loading profile and the extrapolated endurance times were combined using the Miner’s rule (Wallner et al., 2016).