Polymer Degradation- FTIR Analysis of Products Formed from Overheating a Silicone Material
This is an illustrative example of a polymer degradation investigation, primarily by FTIR analysis, but assisted by XPS analysis and residual gas mass spectroscopy analysis.
A composite aerospace structure was subjected to a flame resistance test with longer burning after the flame was removed than expected. The metal hardware in the structure was found afterward to have a melted red material at the base of the metal anchored in the composite material known to be a red siloxane based material. Further out on the metal hardware, the surface was covered with a black material, and still further out there was a thin layer of a slightly milky to clear material. FTIR was used to determine the chemical nature of these materials. XPS analysis was also used to characterize them.
The FTIR spectra taken with the ATR attachment of the three potential polymer degradation products included the melted red (spectrum in black), charred black (spectrum in green), and the nearly clear material thin layer (spectrum in blue), are shown in the overlaid spectra below:
We searched the databases of FTIR spectra below and found a match with a high spectral correlation between the melted and bubbled red material on the metal hardware and a red RTV gasket forming material made by Permatex with product number 28BR with a good resistance to thermal degradation. Our unknown red material spectrum is shown in black and the red spectrum is the Permatex RTV 28BR material. The small differences in these spectra are explained by a small loss of methyl groups and increased oxidation. XPS analysis measured the decrease in carbon relative to a good dimethyl siloxane to be 2.5% and the increase in oxygen to be 21%, with much of the increase in the form of hydroxyl groups.
We found that the unknown nearly clear thin layer on the hardware was a good match with a silicon gel material found in the FTIR spectral databases:
The milky white to clear material from the hardware is shown in blue in the overlaid spectrum above and the reference silica gel material is shown in blue. XPS showed this material to be a highly disordered silica, but not really a silica gel. Silica gel is often similar and has a high degree of disorder, but this material was quite devoid of hydroxyl groups.
It appeared that the melted red material, the black charred material, and the milky to clear materials were the degradation products simply of the overheated siloxane material from the composite material. We had a red gasket forming Permatex RTV 26BHTR material available in the laboratory. We overheated it and obtained similar bubbled and melted red material, black charred material, and a nearly clear material of silica, depending on the temperature of heating and the time of heating. No other material was found, so the highly combustible material appears to have been completely vaporized. We concluded that the unexpected burn in the flame test may have been caused by under-cured siloxane resin or by a flammable volatile contaminant.
In time, we were able to prevail upon our customer to supply the silicone materials in an unheated, but supposedly cured condition. We were able to show that surfaces were covered with short-chain dimethyl siloxanes and that heating the materials, especially a silicone sealant used in the structure, caused an outpouring of very short-chain siloxanes which we measured using mass spectroscopy. Such short-chain siloxanes are highly flammable. The long-chain siloxanes are resistant to flame, while the short-chain siloxanes readily combust.
Inquiries about FTIR analysis and polymer degradation concerns may be made at our “Contact Us” page or directly to Dr. Vaidheeshwar Ramasubramanian at Vaidheeshwar.R@andersonmaterials.com
