We used a combination of materials testing and characterisation techniques, including environmental exposure testing and Fourier transform infrared (FTIR) spectroscopy, to help Siemens Mobility ensure train door buttons remain fit for purpose.
Cracked and discoloured train door buttons
The challenge: understanding the cause of polycarbonate material degradation
Siemens Mobility asked for our help to understand why train door buttons had become discoloured and cracked while in service, and ensure that this cracking would not result in operational failure going forward.
Applying nC2’s failure investigation expertise
Rachel Triggs, Senior Consultant, examining the sample door buttons
Work package 1: Ruling out a mechanical cause and identifying next steps
Siemens provided 8 samples for this phase: 1 new button, 1 used interior button and 6 used exterior buttons.
Macroscopic inspection of the samples revealed discolouration and cracking across the entire surface of the exterior buttons, including on the thicker raised ‘arrows’. This suggested the damage was not caused by mechanical force.
Destructive examination showed that the cracking typically appeared only on the outside of the button lids and did not extend to the inside of the button.
FTIR spectroscopy was undertaken which identified no significant degradation of the polycarbonate.
Work package 1 conclusions: We suspected that the degradation could be due to environmental stress cracking. Potential causes could be UV exposure or chemical attack; these can both result in cracking but with different underlying molecular mechanisms.
Work package 2: Environmental exposure and comparative FTIR
For this phase we tested 4 buttons that had not previously been exposed to UV: 2 used interior buttons, and 2 new buttons with protective coating.
Simulated environmental exposure: The samples were loaded into an accelerated weathering cabinet and exposed to UV light for 4 weeks, and to a cyclical UV/condensation cycle to simulate outdoor conditions for a further 3 weeks.
After testing, the used interior buttons had become discoloured but there was no visible cracking, suggesting that UV exposure was not the cause. The new coated buttons appeared neither yellowed nor cracked; this was confirmed under magnification.
Comparative FTIR testing showed that UV damage did not cause the surface cracking
FTIR spectroscopy: The next step was to compare the FTIR ‘fingerprints’ of the UV-exposed buttons with that of an exterior cracked button from work package 1.
- A comparison of the spectra showed changes to the buttons that had undergone accelerated UV weathering, indicating that long polymer chains within the polycarbonate had become damaged, or ‘chopped up’ into smaller molecules, by the UV light.
- In contrast, the spectrum from the cracked exterior button did not exhibit this FTIR signature. In fact, there was very little detectable difference between unexposed polycarbonate and the cracked button polycarbonate spectra.
- The FTIR results also showed the coating on the new buttons provided an excellent degree of UV protection, with no observable degradation. The coating was also observed during the condensation stage of testing to be hygrophobic.
Exposure to thermal extremes: To further confirm these results, one of the UV-exposed buttons was exposed to thermal extremes to initiate rapid expansion and contraction within the material; this had no visible effect on the polycarbonate, with no cracking or crazing seen under magnification, confirming that the cracking observed in service was not consistent with UV degradation of these buttons.
Work package 2 conclusions: Our testing and analysis showed that while the buttons were susceptible to UV damage, comparative FTIR showed this was not the cause of the cracking of the polycarbonate surface, even after exposure to rapid extremes of temperature.
Our analysis
We advised that exposure to plasticising chemicals (such as ethanol or isopropyl alcohol), was likely the primary factor. This causes the long polymer chains to mostly pull apart without damaging their chemical make-up, causing cracking with little visible change to the FTIR spectrum.
We also concluded the coating of the new buttons is UV resistant and may provide an increased resistance to chemical attack due to their hygrophobicity.
nC2’s added value
- Internationally recognised expertise in failure investigation.
- The ability to harness the right combination of knowledge, techniques and instrumentation to find solutions.
- Independent, evidence-based opinion.
- Flexible packages, allowing Siemens to consider initial findings before commissioning further work.
The outcome: new insights and quality assurance for Siemens
Siemens gained a better understanding of the cause of the material degradation, and reassurance that the visual changes were purely cosmetic and not of structural concern. Further reassurance was gained with the new-style coated buttons being more robust, enabling them to move ahead with confidence.
