Analysis of Carbon Black in SBR Elastomers with Different Degrees of Cross-Linking
Purpose
As has already been shown in Section 4.1.1, SBR is one of the types of rubber that produces very little carbon black on pyrolysis. This means that the carbon black filler content can be determined relatively easily and accurately. This is shown using samples of L-SBR with different degrees of cross-linking.
Sample
L-SBR with different degrees of cross-linking prepared from different formulations
Conditions
Measuring cell: TGA/SDTA851e
Pan: Alumina 70 µl
Sample preparation: Pieces of elastomer of approx. 10 mg
TGA measurement: Heating from 35 °C to 600 °C under nitrogen (50 ml/min), then from 600 °C to 850 °C under air (50 ml/min) Heating rate 10 K/min
Atmosphere: Nitrogen then air, automatically switched
Interpretation
As has already been shown in Section 4.2.2, the rate of the pyrolysis reaction of SBR cross-linked with sulfur depends on the sulfur content. The carbon black burns on switching the atmosphere from nitrogen to air at 600 °C. With SBR, only about 1% carbon black is formed during pyrolysis. The carbon black filler content can therefore be determined with good accuracy by TGA.
Evaluation
The volatile compounds are evaluated together with the pyrolysis step. The results are summarized in the table:
The value determined for the carbon black filler content is about 31% and agrees very well with the value given in the formulation. It is apparent that the measured value of carbon black is slightly larger than the formulation value. This difference increases with the sulfur content as shown in the diagram below:
where wm is the carbon content as determined by TGA, and wc is the carbon black filler content of the sample according to the formulation. The difference, wm-wc , corresponds to the carbon black formed during pyrolysis
Conclusions
The amount of sample degraded during pyrolysis is about 66.5%. The carbon black content determined is 31%. This is somewhat larger than the value given for the carbon black filler content in the formulation. The error in the carbon black determination is of the order of 1% and depends on the sulfur content. The larger the sulfur content, the larger the proportion of pyrolysis carbon black.
Analysis of Carbon Black in SBR Elastomers with Different Degrees of Crosslinking | Thermal Analysis Application No. HB454 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2.