Combined TGA and DSC Analysis of an EPDM/SBR Blend
Purpose
If the pyrolysis temperatures of the polymer constituents of an elastomer are close together, it is not possible to distinguish between the two polymers by TGA. It will be shown how DSC measurements can be used to obtain additional information on the composition, using an SBR/EPDM blend as an example.
Sample
Technical carbon black filled elastomer. The polymer content is 12.6% SBR and 25.5% EPDM.
Conditions
Measuring cell: TGA/SDTA851e and DSC822e with liquid nitrogen cooling option
Pan: TGA measurements: alumina 30 μl
DSC measurements: aluminum 40 μl, pierced lid
Sample preparation: Cubes of approx. 10 mg cut from the starting material
TGA measurement: Heating from 30 °C to 550 °C at 2 K/min.
Atmosphere: Nitrogen, 50 ml/min
The diagram displays the TGA and DTG curves.
Interpretation and Evaluation
Two processes can be seen in the TGA curve. The first step, in the temperature range 150 °C to 300 °C, involves the loss of volatile compounds, namely the oil used as plasticizer. The sample was found to contain 14.2% oil.
The second step between 300 °C and 450 °C is due to polymer pyrolysis. Since the two polymers used pyrolyze in the same temperature range, it is not possible to distinguish between them by TGA. The total polymer content (39%) is determined from the step height. This value agrees well with the formulation (38.1%).
DSC measurement: One possibility to distinguish between the two polymers is to measure a sample with DSC and compare the glass transitions.
In Section 4.5.3. Glass transition of incompatible polymer blends it was shown that the relative polymer content could be estimated by evaluating the step heights at the glass transitions. DSC measurements were therefore performed in the temperature range –130 °C to 50 °C at a heating rate of 10 K/min.
The diagram shows the DSC curve with and without evaluation. The glass transition of EPDM occurs at about –60 °C. This is followed by the melting peak of the EPDM, which is overlapped by the glass transition of SBR. While the glass transition of the EPDM is easy to evaluate (T is 57.1 °C and 'c is 0.135 J/gK), it is very difficult to resolve the glass transition of SBR. First of all, a tangential baseline was used to determine the melting peak (see Section 4.1.3. Comparison of different types of EPDM by DSC). The glass transition of the SBR is then evaluated using a tangent that corresponds to the baseline for the peak evaluation. This gives a glass transition temperature of –40.4 °C and a step height, Δcp, of 0.058 J/gK. The Δcp value is particularly uncertain. The polymer content can then be estimated from the measurement values by two different methods:
Method 1: The step height of the EPDM glass transition can be determined relatively accurately. The EPDM content, αEPDM, is therefore calculated using the equation
where Δcp,EPDM is the measured change of the specific heat capacity at the EPDM glass transition. A Δcp,1 value of 0.47 J/gK is used for the step height of the glass transition of pure EPDM (see Section 4.1.3. Comparison of different types of EPDM by DSC). The EPDM content estimated in this way is 28.7%. Since the TGA measurement gives a total polymer content, αp, of 39.0%, it follows that the SBR content must be 10.3%. This method does not use the (uncertaintain) step height of the SBR glass transition. The information from pure EPDM is however required
Method 2:
where αSBR is the SBR content and 'Δcp,SBR is the corresponding measured step height. Furthermore, the total polymer content, αp, equals αEPDM+αSBR. F
rom these two equations, the following equation for the EPDM content can be derived:
This calculation gives a value of 27.3% for EPDM, and consequently 11.7% for SBR.
The main problem in the evaluation of the DSC curves is that the EPDM melting peak overlaps the SBR glass transition. Since the melting peak of the EPDM is relatively small, a better resolution of the glass transition can be obtained by using ADSC. This method takes advantage of the fact that in the heat capacity curve (complex cp) measured by ADSC, a melting peak occurs but its area decreases when shorter periods are used. If the period is sufficiently short, the glass transition of the SBR can in fact be measured without it being overlapped to any appreciable extent by the melting peak. The evaluation is then performed using method 2. This requires only relative changes of the heat capacity. A calibration of the heat capacity signal is therefore not necessary. p The ADSC measurement was performed at an underlying heating rate of 2 K/min, a temperature amplitude of 0.5 K and a period of 24 s.
The values determined for the step height are summarized in the table. The polymer contents determined according to method 2 gave values of 26.2% EPDM and 12.8% SBR
Summary of results:
The evaluation of the ADSC curve is more reliable and reproducible than with conventional DSC measurements because the separation of the glass transition and melting is greatly improved.
Conclusions
The combination of TGA and DSC measurements improve the accuracy of elastomer analysis. It is possible to determine whether an elastomer consists of one or more polymer constituents. The evaluation of the glass transition step allows information to be obtained relating to the composition of the sample. ADSC measurements can be used to separate various thermal processes that overlap in conventional DSC measurements. This improves the interpretation and evaluation of DSC measurements.
Combined TGA and DSC Analysis of EPDM/SBR Blend | Thermal Analysis Application No. HB 475 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2