TGA of Elastomers Containing SBR as One Constituent
Purpose
Further examples of the content analysis of elastomer blends are described using elastomers based on SBR as examples.
Samples
Three samples with the following compositions were analyzed. (Sample E was already analyzed in Section 4.5.1. TGA of blends of NR with other polymers):
Conditions
Measuring cell: TGA/SDTA851e
Pan: Alumina 30 µl
Sample preparation: Piece of elastomer of approx. 20 mg
TGA measurement: Heating from 50°C to 625°C under nitrogen (50 ml/min), then from 625°C to 1000°C under air (50 ml/min) Heating rate 30 K/min
Atmosphere: Nitrogen then air, automatically switched
The diagram shows the measurement curves and their first derivatives (DTG). The letters E to G are used to identify the samples. The numbers 1 to 4 indicate the individual weight loss steps.
Evaluation
The results of the evaluation are summarized in the following table:
Interpretation
Step 1: Volatile compounds are evolved from the samples. As with the oil-free samples analyzed in Section 4.5.1. TGA of blends of NR with other polymers, the step is 3.5% (Sample F). Water can also be evolved in this step. This could be present for example as residual moisture in carbon black filler introduced on mixing or be adsorbed from atmosphere during storage of the material. The step is somewhat larger for the samples that contain oil. Sample G loses its oil content completely in this step. The midpoint of about 284 °C is relatively high.
Step 2: In sample F, this step occurs at lower temperature (midpoint: 385 °C). It is below the decomposition temperature of NR and is clearly visible in the DTG curve as a shoulder. This is probably caused by the decomposition of less stable constituents such as resins and polyethylene glycol that were added to this material in about 8% concentration. With the other samples, this step has to do with the pyrolysis of the polymer constituents. In sample E, the midpoint of 388 °C can be assigned to the pyrolysis of NR (see discussion in Section 4.5.1. TGA of blends of NR with other polymers.) The midpoint of about 394 °C in sample G corresponds to the pyrolysis of EPDM. The step height correlates well with the content of this polymer.
Step 3: This step is primarily due to the pyrolysis of polymers. With sample E, this step corresponds to the SBR content. Sample F is a blend of SBR and BR. These two polymers cannot be distinguished from each other under these conditions. In this case, online gas analysis with MS or FTIR would be useful. With sample G, the step should correspond to the pyrolysis of SBR. It is however apparent that the step is shifted to higher temperatures. Besides this, a small shoulder can be seen on the high temperature side of the DTG peak (see arrow in the zoomed curve). This indicates that a further process occurs in this temperature range. This process is the reason why the step height is about 7% greater than the SBR content. This effect is caused by the thermal decomposition of additives.
Step 4: During this step, the carbon black burns in air. A carbon black content of about 1% is measured for samples F and G. This is due to pyrolysis residues. With sample E, the step height agrees very well with the carbon black content.
Residue: All the samples analyzed in this section are elastomers that contain a high proportion of inorganic fillers. The contents are determined with reasonable accuracy from the residue:
Conclusions
The composition of the above elastomer samples can be analyzed by TGA under the conditions described. To obtain more definite results, especially with regard to the oil content, it would be advisable to vary the experimental conditions. This is described in Section 4.6.1. In particular with materials of high inorganic filler content, water or CO is evolved in the same temperature range as the pyrolysis of the polymer. This affects the TGA results to such a degree that the polymer content appears to be higher. This is probably the case with sample G. 2 With some polymers, the pyrolysis steps occur in practically the same temperature range. In this case, only the total polymer content can be determined by TGA. Section 4.5.4 describes how the total polymer content can be separated into individual polymer constituents.
TGA of Elastomers Containing SBR as one Constituent | Thermal Analysis Application No. HB468 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2