History

An entrepreneur/engineer set up a company to design, assemble, and market electronically controlled motorised wheelchairs for handicapped children. Materials of construction included steel, glass-reinforced plastics (GRP), rubber and all the thermoplastics normally associated with electrical circuitry. The product was well designed and was the recipient of numerous awards.

The business however did not prosper and the company was in a poor financial state when a large consignment was ordered by a customer in the USA. Champagne was consumed, overdrafts were increased, and the consignment was shipped to the States.

The wheelchairs were shipped in wooden crates. On opening the crates the customer found that all of the polymeric components were either swollen, shrunk, sticky, or fractured. The consignment (valued at £250,000) was rejected.

Inspection and analysis

It was reasonable to suppose that the materials had been subject to attack during the time of crating and shipping. The uniformity of the attack suggested that an aggressive and all pervasive vapour was responsible.

A rubber tube with the most severe swelling and therefore most likely to harbour residual vapour was selected for gas chromatography/mass spectrometry (GC/MS) analysis using a dynamic headspace (purge and trap) injection technique. This revealed the dominant presence of a complex amine.

A sachet of ‘antirust’ agent was belatedly discovered amongst the debris. An unused sachet was acquired and subjected to the same analysis. The same complex amine was identified.

Failure diagnosis

The shipping agent had (gratuitously) added a sachet of a proprietary amine-based antirust formulation to each crate to protect the steel parts during transit. The amine vapour caused rubber parts to swell, extracted plasticiser from PVC, and stress cracked polycarbonate switches, PMMA panel lights, ABS consoles etc.

Lessons and consequences

1. Fluids that are benign or beneficial to metals may be aggressive to polymers. Antirust agents or corrosion inhibitors have been the direct cause of many failures (e.g. Case 6.5.3). The high cost of amine attack on nitrile seals in the offshore oil and gas sector [53] has raised awareness but only within the sector.
2. Most ESC failures involve contact with small amounts of proprietary fluids that are secondary to part function or purpose.
3. The company folded and a well designed product was lost.