Determining Fluoroelastomer Content of Rubbers Using Thermal Degradation and Spectroscopic Methods of Analysis

Thermogravimetric Analysis (TGA) is commonly used to determine the volatile, polymeric and filler components of rubber materials. Tests conducted with various copolymers, terpolymers and base resistant polymers under nitrogen purge showed that polymer degradation was complete by 600°C. However, when such polymers were incorporated into a typical rubber matrix a thermally resistant residue was seen at 600°C. This resulted in under determination of the elastomer content as well as inaccuracies in filler determination.

 Given the multivariate complexity of the rubber compound it was decided to first investigate the thermal degradation phenomenon using a (simple) mixture of poly Vinylidene fluoride/Hexafluoropropylene, N990 and  wt % carnauba wax. The error associated with polymer determination was 3.0 wt. %. Using a design of experiment methodology the effects of metal oxides /hydroxides that are commonly used in FKM rubbers were investigated. The errors increased to 4 wt.% in the individual and combined presence of MgO and Ca(OH)₂. Fe₂O₃ and ZnO were seen to have marginal effects (1.0 wt.%). The effect of TiO₂ was most dramatic. At 4.0 wt. % TiO₂ the fluoroelastomer content was underestimated by 17.0 wt.%. TiO₂ caused a lowering of the degradation onset temperature and this effect increased with increases in concentration.

 The Investigations established that non isothermal oxidizing conditions (air purge) can be used to eliminate or significantly minimize the formation of residual fluoro organics.  For the materials tested the temperature requirements did not signifcantly overlap with the thermal oxidation of the carbon black  used.  Hence it was possible to determine polymer and filler components from a single test. This technique did not work for materials containing Fe₂O_3. In this case the better condition was to increase test temperatures to over 900°C in nitrogen. Generally an initial analysis of the rubber via FTIR and WDXRF were useful for determining test conditions for thermal analysis.

A combination of X-ray photoelectron spectroscopy (XPS), FTIR and XRF tools were used to investigate the mechanism involved.