Answer Happy

Background A problem was discovered in a large number of office canteens equipped with hot water urns Some of the plastic taps of the urns had suffered attack and were severely eroded (Figure 3) Attack varied from place to place with no obvious rationale for the seemingly random behaviour I Figure 3 Eroded plastic taps (the upper sections of the taps are the acetal components) The taps were constructed from acetal resin, a type of material used widely in plumbing systems (for joints and connections, for example). Figure 4 is a close-up of the damage at one location on the knurled (the surface with grooves to aid in gripping) edge of the tap

Figure 4 Close-up of the damaged area Investigation It was known that four different cleaning fluids were used on the urns, all of which may have come into contact with the taps at some time. These cleaning Nuids were all water based, but each one had a different pH as shown in Table 1 (recall that low pH is acidic and high pH is alkaline; pH 7 is neutral) Table 1 Cleaning fluids and their measured pH pH Product pH 0.85 Kemico descaler 0.97 7.42 Ecolab phosphoric descaler Brillo brass and steel cleaner Johnson Endblac sanitizer 12.30 It was also known that the temperature at which the hot water was maintained in the urns was in the range 75-80'C.

New samples of the acetal used to make the taps were therefore obtained and exposed under controlled conditions to the four different cleaning fluids at varying temperatures. The results of these experiments are shown in Table 2 Table 2 Results of experiments on the tap material Result Experiment No effect 1 Acetal resin exposed to each of the fluids at 25°C for 24 hours No effect 2 Acetal resin heated to 80 °C and then exposed to fluids at 25 C for 24 hours No effect 3 Acetal resin exposed to fluids at 80 °C for 10 minutes 4 Acetal resin exposed to fluids at 80 "C for 24 hours Attack by the Kemico and Ecolab descalers a Outline any similarities and differences between the urn failure described here and the failure of an acetal fitting at Loughborough University as described in Block 2 Part 4 (Section 5,3) (5 marks) D. Suggest why erosive attack on the plastic resin used to make the taps occurred with only two of the fulds tested What mechanism could account for this attack? (6 marks) c Suggest why erosive attack was only discovered in the fourth senes of experiments, and indicate how such conditions could have occurred in practice (7 marks) a Descalers are often used to remove the calcium carbonate deposits that occur in hard water areas Knowing that not water urns have to be maintained at a high temperature for long periods of time, what advice would you give to the maintenance staff to prevent a future recurrence of the problem of erosive attack on the taps? How could the structural integnty of ums be affected if the problem of erosion is not tackled? 17 marks)