Typical TGA of an SBR Elastomer

Purpose

The basic principles of compositional analysis by TGA is demonstrated with an example.

Sample

Technical SBR elastomer 

Conditions

Measuring cell: TGA/SDTA851^e with gas controller

Pan: Alumina 30 µl

Sample preparation: Cube of approx. 15 mg cut from the starting material

TGA measurement: Heating from 50 °C to 600 °C at 10 K/min under nitrogen (50 ml/min), then from 600 °C to 850 °C at 10 K/min under air (50 ml/min). 

Atmosphere: Nitrogen then air, automatically switched 

Interpretation

The TGA curve shows three steps. The first (below about 300 °C) is not clearly defined. It corresponds to the loss of a small quantity of relatively volatile compounds.

The pyrolysis of the elastomer takes place between 300 °C and 550 °C. The step height corresponds to the polymer content

Afterward, the carbon present burns on switching from nitrogen to air. With many elastomers, the amount of carbon black formed during pyrolysis can be neglected. The height of the third step can therefore usually be taken as a measure of the carbon black filler content of the sample. Exceptions to this rule are elastomers that contain chlorine such as chloroprene (CR). A discussion on carbon black determination in such materials follows in Section 4.3.3. 

The residue corresponds to the ash content and can often be assigned to inorganic additives. 

Evaluation

The first derivative of the TGA curve, the DTG curve, often improves the evaluation of the step in a TGA curve. In a DTG curve, a TGA step due to a change in mass is displayed as a peak. This makes it easier to determine the limits of the TGA steps (shown as minima between peaks in the DTG curve). The first poorly defined step in the TGA curve is not visible as a distinct peak in the DTG curve either. The beginning of the pyrolysis reaction can be clearly identified in the DTG curve. This is marked in the figure by the dotted line at about 330 °C in the figure.

The end of the pyrolysis reaction and the beginning of the combustion of carbon black are well separated from each other. In this case, the choice of the beginning and end of the evaluation is not at all critical. 

The evaluation of the step height gave the following results:

volatile content: 3.1%

polymer content: 62.9%

carbon black content: 31.5%

inorganic residues: 2.3%. 

In the example discussed, the results agree well with the known constituents of the elastomer formulation: 62% SBR, 31% carbon black and 1.8% zinc oxide. 

Conclusions

The main constituents of elastomers can often be analyzed with good accuracy by TGA. The technique can distinguish between volatile compounds, the polymer, carbon black and inorganic residues.

Comments

For accurate compositional analyses, a blank curve should be subtracted from the measured curve and the instrument should be operate under stable thermal conditions. Under such conditions the baseline can then be used for a reasonably long period.

^{Typical TGA of an SBR Elastomer | Thermal Analysis Handbook No. HB412 | Application published in METTLER TOLEDO TA Application Elastomers, Volume 1.}