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Plastic storage tanks for heating oil have been on the market since the early 1970s in Germany, mainly made from polyethylene grades. Tank manufacturers and experts examining the tanks recommend the replacement of the tanks to ensure the safety after a period of 30 years. The tank manufacturer is legally obliged to specify the service life of the tanks.
Polyethylene is subject to ageing by alteration of the properties during its life cycle. A distinction is made between internal and external ageing processes. The internal ageing, such as the breakdown of internal stresses, post-crystallization, phase sepa-ration of multicomponent systems and plasticizer migration is attributed to thermodynamically instable conditions of the plastic material. The external ageing in form of stress cracking, fatigue cracking, thermooxidative degradation or swelling is based on physical or chemical effects of the environment of the polyethylene grade. The degree of ageing and the nature of the degradation process mainly depend on the chemical degradation of the polyethylene, the wall thickness of the tank and the environmental conditions. There are no results available on the long-term behaviour of the polyethylene grades used as materials for heating oil storage tanks, especially after a service life of more than 25 years.
The aim of this investigation was the determination of the factual degree of damage in comparison to the uncontaminated polyethylene grades. Data on the melt flow rate (MFR) and density of the used polyethylene grades are available because the BAM was the competent authority for the approval of the plastic heating oil storage tanks until the end of the 1980s.
The technical service of specialist companies for fuel tank disposal supplied tank sections from the bottom, the shell and the roof of 8 segregated heating oil storage tanks. Some of the tanks had been in service for more than 30 years. Two polyethylene grades mainly were used as materials for these tanks over this time. The determination of the MFR according to EN ISO 1133 and the FTIR spectroscopy followed standard test methods.
The highest average percentage increase in the MFR was determined as 31- 43 % for the bottom section of the tanks, for both the inner shell and the outer shell. The lowest increase in MFR was measured for the top of the tank, with 6 - 12 % for the inner shell and 0 - 8 % for the outer shell. The tank shell sections had an average percentage increase in MFR of 10 - 33 % for the inner shell and of 14 - 40 % for the outer shell.
The oxidative damage of the polyethylene grades was highest in the bottom area due to the permanent contact with the heating oil and degradation products.
The microtome cuttings were analyzed with image-guided infrared microscopy in the transmission mode and confirmed the results obtained with the MFR measurements.
In summary it can be concluded that the oxidative damage of the polyethylene grades after long-term contact with heating oil is relative low.