Influence of Sample Preparation on TGA Curves
Purpose
To investigate the influence of sample mass on TGA measurements.
Sample
A technical elastomer made from a blend of NR (approx. 30%), SBR (approx. 45%) and carbon black (approx. 23%).
Conditions
Measuring cell: TGA/SDTA85e with gas controller
Pan: Alumina 30 µl
Sample preparation: Elastomers pieces of different sizes:
Sample 1: a cube with a mass of approx. 5 mg
Sample 2: a cube with a mass of approx. 16 mg
TGA measurement: Heating from 50°C to 600°C under nitrogen (80 ml/min), then switching to air (80 ml/min) and heating to 900°C
Heating rate 30 K/min
Atmosphere: Nitrogen then air, automatically switched
Evaluation
The figure shows the TGA and DTG curves of the two different sample masses. It can be seen that two processes actually occur in the pyrolysis step between 350 °C and 500 °C. The first process can be assigned to the pyrolysis of NR and the second to that of SBR. The corresponding peaks in the DTG curve are only slightly different. The deviation is about 5 K. This is of the same order as the reproducibility of the pyrolysis step. This uncertainty is caused by inhomogeneity of the sample composition, diffusion of the gaseous pyrolysis products and temperature gradients in the sample. Larger differences, however, occur during the combustion of the carbon black after switching to air atmosphere. The carbon black burns much more rapidly in the small sample because the diffusion of oxygen is faster than in the large sample.
Conclusions
Geometry and sample mass have an important influence on the behavior of the sample in reactions involving reactive gases. If the combustion behavior of samples containing carbon black is analyzed or compared, then the sample geometry must be the same. Sample geometry influences the pyrolysis of the polymer constituents of elastomers to a lesser extent.
Influence of Sample Preparation on TGA Curves | Thermal Analysis Handbook No.HB413 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 1