TGA of Fluoroelastomers (FPM)
Purpose
Comparative measurements to determine the composition of fluoroelastomers
Sample
Two elastomers based on fluororubber
Conditions
Measuring cell: TGA/SDTA851e
Pan: Alumina 30 µl, without lid
Sample preparation: Pieces of elastomer of approx. 5 mg
TGA measurement: Program 1 - Heating from 50 °C to 600 °C under nitrogen (80 ml/min), then from 600 °C to 800 °C under air (80 ml/min) Heating rate 30 K/min
TGA measurement: Program 2 - Heating from 50 °C to 800 °C under nitrogen (80 ml/min) Heating rate 30 K/min
TGA measurement: Program 3 - Heating from 50 °C to 600 °C under nitrogen (80 ml/min) at 10 K/min, followed by cooling to 200 °C at 30 K/min and an isothermal segment of 5 min. Afterward heating to 750 °C at 5 K/min in air.
Atmosphere: Nitrogen and air, automatically switched
The upper part of the diagram shows the TGA curves of measurement programs 1 (continuous line) and 2 (dashed line). The lower part shows only the DTG curve from measurement program 1.
Interpretation
Pyrolysis of the fluororubber occurs between 400 °C and 550 °C. The peak maxima of the DTG peaks are at about 510 °C. Combustion of the carbon black takes place on switching to air. Sample 2 exhibits two distinct peaks in this step. The separation indicates that two different types of carbon black burn. To check that this was in fact due to combustion, the measurement was repeated without switching over to air at 600 °C (measurement program 2). No combustion was then measured. This confirms that both two steps are due to reaction with oxygen.
Analogous to the chlorinated elastomers, one expects that fluorinated materials also form carbon black during the pyrolysis. Measurements were therefore performed using measurement program 3 in order to distinguish between this carbon black and the carbon black filler.
The diagram displays the TGA curves from measurement program 3. For comparison, the curves obtained with measurement program 1 are also included. It can be seen that pyrolysis at 30 K/min is shifted to higher temperatures compared with the measurements at 10 K/min. The most significant differences, however, are in the measurement segment in air. While sample 1 shows only one combustion step when measured at 30 K/min, several steps are observed at the lower heating rate. First of all, the carbon black formed during the pyrolysis burns, and then the carbon black filler. The sharp step at 540 °C is due to the strongly exothermic effect of carbon black combustion. To avoid this self-ignition, the sample can be cut into small pieces and mixed with about 20 mg α-aluminum oxide as diluent.
Combustion of the pyrolysis carbon black also occurs first in sample 2. Afterward the carbon black filler burns at relatively high temperatures. This sample contained carbon black filler with a relatively low surface area (see Section 4.3.3. Analysis of elastomers with different types of carbon black.)
To make the evaluation procedure clearer, the diagram also displays the parts of the TGA curves measured under air according to program 3 as well as their DTG curves.
Evaluation
The DTG curves were used to evaluate the different carbon black combustion steps. After the first peak, the rate of combustion drops to almost zero. The corresponding minimums in the DTG curve at about 475 °C (sample 1) and 515 °C (sample 2) are used as the end values for the first combustion step in the TGA curves. The results from measurement program 2 are summarized in the following table:
The polymer content of the samples corresponds to the sum of the step heights from the pyrolysis step and the first combustion step. It follows that the fluororubber contents of samples 1 and 2 are 71.7% and 74.0% respectively. From this data, the amount of carbon black formed in the pyrolysis of pure fluoroelastomers can be calculated using the following equation:
where wc is the proportion of pyrolysis carbon black for a pure elastomer, w1 is the height of the first carbon black step and wPolymer is the polymer content. Values of 7.9% and 7.7% pyrolysis carbon black for the combustion of the pure polymer were obtained from the two samples. The second carbon black step corresponds to the amount of carbon black filler. The residue consists of inorganic additives and ash.
Conclusions
Fluoroelastomers undergo pyrolysis at about 500 °C and produce carbon black. This type of carbon black can be distinguished from the carbon black filler by slowly heating the pyrolysis products under air after they have been cooled to 200 °C. The results showed that about 8% carbon black was formed during the pyrolysis of the fluoroelastomers measured in this study.
TGA of Fluoroelastomers (FPM) | Thermal Analysis Application No. HB460 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2