Case Studies

Case Study 1 – Cracking Failure of PTFE Component

30/11/16

Background:

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.

Approaches:

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.

Case Study 2 – Fractured Epoxy Resin Coating

21/11/16

Background:

A client manufactures electrical components for the aerospace industry and experienced fracturing of an epoxy resin coating surrounding an electrical component in one of their products; as a result, they decided to cease producing the components until the issue was resolved. The epoxy resin was produced by mixing a catalyst with a low molecular weight (Mw) resin and this mixture then being cured. After evaluation of the data supplied by the client it seemed likely that the fracture was caused by something wrong in the curing process.

Approaches:

1. Optical microscopy and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) to try to determine the mode of the fracture and provide information on the structural and elemental composition of the epoxy resin coating.
2. Differential scanning calorimetry (DSC) on the fractured coating sample to determine if residual catalyst from the mixing process was present, if so during DSC analysis any residual catalyst will react with the insufficiently cured resin at elevated temperature to generate an exothermic peak.
3. Thermogravimetric analysis (TGA) on the fractured and a reference (good quality) coating samples toexamine the bulk composition and decomposition temperature of the cured resins.

Results & Analysis:

By examination of the fracture surface under microscope, it was confirmed that the epoxy resin coating failed in a brittle manner. The general structure of the polymer showed some air voids/bubbles along the fracture face. Additionally, the polymer resin appeared to be mixed with calcium-based white material, likely an inorganic filler, which was not evenly distributed in the resin matrix. Uneven distribution of fillers in polymer material tends to generate stress concentration and trigger cracking when subjected to mechanical loading. There was no cure exotherm in the first heating DSC traces of the fractured epoxy resin samples, indicating that no further curing reaction occurred in the sample during DSC testing. It was shown from TGA that in comparison with the reference sample, the onset of main-chain polymer decomposition occurred earlier for the fractured coating. This implied lower level of curing in the fractured sample, since the polymer with lower level of curing has lower density of cross-linking and tends to decompose at lower temperature during a TGA testing. Meanwhile, the much higher content of low molecular weight species detected in the fractured sample suggested the presence of more uncured epoxy resin in the fractured coating.

Conclusions & Recommendation:

Based on the testing results, the fracturing problem was very likely caused by uneven/inappropriate mixing of the ingredients (e.g. the inorganic filler) and insufficient curing of the resin material. It was therefore recommended to choose an epoxy resin system with longer pot life to enable complete mixing of all ingredients and achieve more effective curing.