Case Study - Hot Water Bottle

Our Consultant team was contacted following the degradation of a rubber hot water bottle in service after just 3 months leading to minor injuries. The manufacturer showed diligence by implementing a full analysis of the failure.

History

Over the years an appreciable number of cases have been reported of hot water bottles failing in service. In this instance a bottle failed catastrophically after about 3 months use, resulting in hot water coming into contact with the user. It was understood that the bottle had been used according to instructions, being filled with hot rather than boiling water.

Inspection & Analysis

The bottle had been moulded with one side externally smooth & the other externally ribbed. It was the smooth side that had split. The bottle was cut open & the inside surfaces examined. As can be seen in the photograph, the whole of the split side was visibly degraded, whilst the opposite side was in good condition.

Samples were taken from the undamaged area & tensile stress strain properties measured in accordance with BS 903 Part A2. The undamaged rubber was found to comply with the requirements of BS 1970, the specification for rubber hot water bottles.

Samples were taken from the damaged side & energy dispersive x-ray analysis (EDAX) carried out to determine metal content on both the inner & outer surfaces. The results showed that there was copper & nickel present in the inner degraded surface but not in the outer surface.

Failure Diagnosis

There have been cases where the sort of damage visible on the outside had been caused by the bottle being stored against a hot water pipe, but it was clear in this case that the whole of one side was badly degraded on the inside. The undamaged side of the bottle conformed to BS 1970 in respect of strength & elongation, so it had been manufactured from a satisfactory compound.

It is well known that copper causes a catalytic action in natural rubber (which is used for hot water bottles) in the presence of oxygen, resulting in rapid degradation (depolymerisation) at elevated temperatures. There is a recognised rough correlation between the frequency of premature failure & the concentration of copper in the water supply. As copper was detected on the inside degraded area but not on the outside it could be concluded that the failure was caused by copper accelerated ageing.

It was, however, curious that only one side of the bottle had degraded & that the degradation was particularly severe in a short space of time. Two factors emerged about the details of how the bottle was used. The owner filled it from the hot water cylinder, which was made of copper, so there was likely to be a high copper content in the water. She was also a person of habit & always placed the bottle in the bed with the smooth side uppermost. Hence, for much of the time the degraded side was in contact with a pocket of hot air, which is an ideal circumstance for rapid oxidation.

Lessons & Consequences

1. The frequency of hot water bottle failures combined with the risks of serious injury would suggest that the testing & quality standards for this product are not sufficiently rigorous or demanding. For example BS 1970 demands a level of property retention after immersion in water, but the quality of the water is not defined in the standard. Therefore the durability of the product in contact with hot water containing higher than normal levels of copper has been ignored or overlooked.
2. This case clearly demonstrates the general rule that air is a more powerful medium for thermo-oxidation than water at the same temperature. BS 1970 includes an air ageing test. This case reveals clearly that the most aggressive ageing condition is first exposure to hot water (absorption of copper ions) followed by exposure to hot air.
3. For similar reasons, the thermo-oxidative degradation in a wide range of fluid storage products is most severe at the fluid/air interface (i.e. the splash zone).