TGA of Blends of NR with Other Polymers - METTLER TOLEDO

TGA of Blends of NR with Other Polymers

Purpose

To illustrate the analysis of elastomer blends containing different polymers by measuring several samples based on NR. 

 

Samples

Five samples with the following compositions were measured: 

Conditions

Measuring cell: TGA/SDTA851

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 TGA measurement curves and their first derivatives (DTG). The letters A to E are used to identify the samples. The numbers 1 to 4 indicate the individual weight loss steps.

 

Evaluation

The TGA and DTG curves were evaluated as previously described in Section 3.2.3. The results obtained from the measurement curves are summarized in the following table. 

Interpretation

Step 1: Volatile compounds are evolved from the sample. For the samples that do not contain oil, the midpoint temperatures are about 240 °C and the step height is between 3.5% and 4.0%. Water present in the sample is also evolved in this step. For the samples containing oil, the step is somewhat larger and shifted to higher temperature. The difference in step height between samples that contain oil and the oil-free samples is much less than that expected from the formulation. It is probable that part of the oil is not evolved until the pyrolysis step. The separation of oil and the pyrolysis step is described in Section 4.6.1.

Step 2: This has to do mainly with the pyrolysis of less stable polymers. The peak maximum of the DTG curve is at about. 390 °C. It is due to the pyrolysis of NR. This step also shows significant differences between the samples that contain oil (D and E) and the oil-free samples (A, B and C). The steps of the oil-free samples are in principle somewhat smaller than the NR content. With samples A and C, the measured step height corresponds to 96.4% and 97.3% of the NR content. With sample B the step is only about 88% of the NR. This behavior is typical for NR because NR pyrolyzes in two steps - a large step at about 400 °C and a small step at about 450 °C (see Section 4.1.1). With the samples that contain oil, the step is greater than that expected from the NR concentration. This effect is due to the fact that it includes a contribution from the oil. If one assumes that the first step without the presence of oil is about 4% and one subtracts the remainder of the first step (0.7% in sample D and 0.4% in sample E) from the plasticizer content of the elastomer, one obtains the expected contribution of the oil to this step (6.95% and 9.2%). The subtraction of this part from the step height yields 37.35% for sample D and 25.3% for sample E. This corrected step height then corresponds to the NR content. For a better separation of polymer pyrolysis and oil vaporization see Section 4.6.1.

Step 3: This step is due mainly by the pyrolysis of the second polymer component. With samples A to C, this is EPDM. The maxima of the DTG curves lie between 481 °C and 485 °C. From the evaluation of the step height, one obtains values that are about 10% to 15% greater that the EPDM content. (With reference to the total mass of the sample, this corresponds to an error of about 3.5% to 1% and can therefore be neglected.) Samples D and E contained BR or SBR. The step heights for these samples agree with the contents of the corresponding polymers. 

Step 4: This step is due to the combustion of carbon black in air. In each case, the step height agrees very well with the carbon black content of the sample. The midpoint temperature increases with increasing carbon black content because the carbon black burns at an almost constant rate. 

Residue: Inorganic residues remain behind after the combustion of carbon black is completed. With samples A to D, the residue corresponds roughly to the ZnO content. Sample E contains inorganic fillers. The sum of these fillers and the ZnO give 50.6%. The residue at the end of the measurement is 48.7%. The difference is due to the elimination of water and CO2 from the fillers during the measurement. 

 

Conclusions

The results show that it is possible to analyze the elastomer samples under the measurement conditions used. The polymer content, carbon black content and inorganic residue can be determined quite accurately. The different polymer components can also be separated. With samples A to C (NR/EPDM blends without oil), the total polymer content corresponds to the sum of step heights 2 and 3. With the other two samples, the results are influenced by the oil. In this case, measurements at reduced pressure are often useful (see Section 4.6.1). 

TGA of Blends of NR with Other Polymers | Thermal Analysis Application No.HB467 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2