A client experienced cracking failure of PTFE (Polytetrafluoroethylene) diaphragm during commissioning a sulphuric acid concentration plant. The diaphragm was purposely designed and purchased as a standard component from a recognised supplier. The operating conditions in the plant were believed to be well within the allowable operating limits for the diaphragm. The liquid in the plant had been water (for testing) or sulphuric acid at ambient temperature of about 30°C. Diaphragm which had operated successfully during water test failed when commissioning with acid. It was suspected that the quality of the PTFE raw material may not meet specification.
1. Infrared spectrometry (FT-IR) analysis on the as-received raw material to identify the generic polymer type.
2. Differential scanning calorimetry (DSC) analysis on both the raw material and failed diaphragm samples and comparison of the results should indicate whether regrind material (recycled PTFE) was used.
Results & Analysis:
The raw material was identified by FT-IR as being based on PTFE. The DSC traces indicated there were differences in thermal performance between the raw material and failed diaphragm samples, with the raw material showing a main melting peak at about 341°C and another small peak at 327°C, while the diaphragm sample showing only one melting peak at about 326°C. It is well documented that using differential scanning calorimetry (DSC) at a common heating rate, e.g. 10°C/minute, the virgin (as-polymerised) PTFE exhibits a peak melting temperature of about 342 + 2°C; the corresponding melting temperature of the sintered PTFE (melt- crystallised) being 328 + 2°C. It was clear from the DSC comparison that the raw material sample was not pure virgin PTFE and contained sintered PTFE, which most likely came from the recycled/reprocessed polymers. One of the main issues of using reprocessed PTFE is that it introduces contaminants into the material, leading to deterioration of the inherent chemical resistance of PTFE and causing failure when in contact with aggressive chemicals, e.g. concentrated sulphuric acid.
Conclusions & Recommendation:
The cracking failure of the PTFE diaphragm was suggested to be caused by the use of raw material blended with reprocessed polymers. It was recommended to take action to ensure virgin PTFE is used for manufacturing the component and quality screening approach is taken to monitor batch to batch variation of raw material.