How FILAB helped one of its customers understand the yellowing of a silicone compound.
Silicones are now widely used in various fields thanks to their remarkable physical and chemical properties, such as excellent thermal stability, high resistance to oxidation and hydrolysis, and excellent gas permeability. Finally, their non-toxicity and chemical inertness allow many types of silicones to be used in cosmetics, dentistry, medicine, surgery, pharmacy, and the food industry.
As part of its component molding process for medical applications, our client encountered yellowing problems in a batch of material used on several pieces of equipment, a phenomenon that worsened during oven recoking. After initial internal investigations, FILAB was called in to understand the cause of the phenomenon.
Comparative studies: an essential methodology
When investigating failures, it is very helpful, where possible, to be able to compare the non-compliant sample with a compliant sample, i.e., one on which no degradation has been observed. This makes it possible to identify the source of a defect more quickly. In this case, our client was able to provide us with a sample from a compliant batch and then one from a non-compliant batch.
The first analytical hypothesis was that external contamination had impacted the formulation of the non-compliant batch. Here, we proposed to study organic contamination by GC-MS and inorganic contamination by ICP-AES.
The second hypothesis was to determine the thermal properties of the two materials to find out whether the non-compliant batch could have been affected by the process temperature. The data was obtained by thermogravimetric analysis (TGA).
These first two analytical approaches concluded that the yellowing was not caused by inorganic or organic contamination, nor by different behavior of the materials with respect to temperature. However, GC-MS revealed differences in formulation that had to be validated using additional analysis.
A combination of several techniques is often necessary to understand a phenomenon
ATG spectra of the different samples
Fourier transform infrared spectrometry
Fourier transform infrared spectroscopy (FTIR) is an analytical method commonly used to study polymers. It works by identifying the chemical bonds between the atoms that make up the polymer. It can also be used to detect abnormal degradation in a material by comparing degraded and new samples, or to characterize cross-linking mechanisms.
After studying the spectra, the FTIR analysis did not reveal any significant differences in composition.
In addition to the results obtained previously, it was ultimately NMR spectroscopy, a technique that exploits the magnetic properties of certain atomic nuclei, that showed that the formulations of the two silicones were not equivalent. Here, NMR revealed, for example, that:
- The yellowing gum shows the presence of vinyl bonds, which are absent in the gum that remains white.
- The phenomenon of water absorption on the silica surface is also visible for the non-compliant sample, whereas it is not visible for the compliant silicone.
- The presence of higher quantities of carbon also probably contributes to the yellowing.
- Finally, silicon is more present in the non-compliant batch.
Why FILAB?
FILAB has extensive expertise and silicone characterization techniques tailored to a variety of needs, such as:
- Deformulation of unknown silicones
- Study of silicone thermal behavior using DSC or TGA
- Searching for impurities in silicone oils or fluids
- Expertise in silicone failures: odor, color, contamination, degradation issues, etc.
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Frequently asked questions
How can the cause of silicone yellowing after post-curing be investigated?
The approach consists of comparing a yellowed batch with a compliant batch, then testing several hypotheses: contamination, thermal behaviour and formulation differences. In this study, FILAB combined GC-MS, ICP-AES, TGA, FTIR and NMR. Their complementary nature made it possible to identify differences between the formulations of the two silicones.
Why provide compliant silicone in addition to the yellowed sample?
A compliant sample provides a reference for isolating differences related to the defect. In this study, comparing the two batches helped examine the hypotheses of contamination and thermal sensitivity, then directed the analysis toward the formulation. The reference should be selected under conditions as comparable as possible to those of the non-compliant batch.
What is the role of TGA in an investigation into silicone yellowing?
Thermogravimetric analysis measures changes in the sample’s mass during a thermal program. It makes it possible to compare certain behaviors of the two materials when heated. In this study, the results did not reveal any difference that could explain the yellowing solely through the thermogravimetric behavior measured.
Why continue with NMR when FTIR shows no significant difference?
FTIR and NMR investigate different types of structural information and can therefore complement each other. In this case, FTIR had not revealed any significant discrepancy, whereas NMR highlighted differences between the formulations, including the presence of vinyl groups in the yellowing rubber. Therefore, the absence of a difference with one technique does not necessarily mean that the investigation is complete.
Do formulation differences alone prove the exact yellowing mechanism?
Formulation differences provide explanatory leads that must be considered alongside the process conditions. In this study, NMR showed that the two silicones were not equivalent and helped guide the interpretation of the defect. Targeted testing can then help verify the role of the identified constituents and assess the proposed modifications.