Reproducible Step Evaluation for TGA Curves

Purpose

The basic principles for the evaluation of TGA curves have been described in Section 3.2.1. Unfortunately, the steps of interest are not always completely separated. The use of a slower heating rate, isothermal steps, reduced pressure or the MaxRes software normally improves the separation. But even these measures do not always result in an ideal separation of different effects in elastomer analysis. Various evaluation possibilities are described here that nevertheless allow reproducible results to be obtained.

Sample

Elastomer based on an NR (33.8%) / EPDM (20.2%) blend

Conditions

Measuring cell: TGA/SDTA851e with gas controller 

Pan: Alumina 30 µl

Sample preparation: Elastomer sample of approx. of 20 mg

TGA measurement: Heating from 50°C to 625°C 

                                    Heating rate: 30 K/min

Atmosphere: Nitrogen, 50 ml/min

The upper part of the diagram displays the normalized TGA curve and its first derivative, the DTG curve. The TGA curve shows that the sample first loses mass due to the vaporization of volatile compounds before the actual pyrolysis reaction itself begins. This then takes place in two steps. The determination of the limits marked 1, 2, 3, and 4 in the diagram is important so that the step heights can be accurately evaluated.

Evaluation

Determining the limits:

Limit 1: Is a temperature at the beginning of the measurement at which no loss of mass has occurred (not critical).

Limit 2: Defines the beginning of the pyrolysis. Neither the TGA nor the DTG curve shows a definite point to which this limit can be assigned. The second derivative of the TGA curve (d2TG) allows the beginning of the pyrolysis reaction to be localized at about 300 °C, which is the starting point for the corresponding peak. The second derivative can be used to define the starting point of a peak. In the case considered, the fourth derivative of the TGA curve has been used (the d4TG curve was expanded 1000 times). In this curve, the point of intersection of the tangent to the corresponding peak with the zero crossover is reproducible and can be regarded as the correct limit. This point is determined as “Peak Onset”. Here it is important that the first tangent is a horizontal line at the zero crossover. 

Limit 3: This is the limit between the overlapping TGA steps in the pyrolysis reaction. This limit corresponds to the maximum between the two negative peaks in the DTG curve. For better reproducibility, the corresponding zero crossover in the d2TG curve can be used. 

Limit 4: A value after the pyrolysis reaction in which the TGA curve no longer changes and the DTG or its first derivative is zero.

After establishing the limits, the mass losses for the individual steps can be determined both from the step height of the TGA curve or from the peak area of the DTG curve. The zero line in the DTG curve serves as the baseline for the peak area determination. The following table compares the results of the evaluation with the corresponding details of the elastomer formulation: 

Interpretation

The first pyrolysis step can be assigned to NR. The second is due mainly to EPDM. 

While the first step is about 4.5% less than the actual NR content, the step height of the second pyrolysis step is about 3.5% more than the EPDM content. This difference is due to the fact that the pyrolysis of NR takes place in two steps (see Section 4.1.1). One of the main steps is just below 400 °C. A smaller step at about 450 °C is 7% of the first step. If this is taken into account, the step heights should be corrected by about 2%. The first pyrolysis step then gives a content of 31.5% NR and the second step 21.5% EPDM.

Conclusions

Reproducible evaluations based on one measured curve can be performed in elastomer analysis by using the DTG curve and higher derivatives of the TGA curve. These evaluations require the Mathematics software option. 

Comments

If the steps are so small that only a shoulder can be seen in the DTG curve, then the second derivative of the TGA curve shows a minimum that can be used as the limit for the separation of the steps. An example of this is shown in the following diagram (limit 3)

^{Reproducible Step Evaluation for TGA Curves | Thermal Analysis Handbook No.HB415 | Application published in METTLER TOLEDO TA Application Elastomers, Volume 1.}