TGA of Epichlorohydrin and Halogenated Butyl Elastomers
Purpose
To determine the composition of elastomers made from one polymer component with different compositions. Besides the polymer component itself, the materials differ in their cross-linking systems, fillers and plasticizers.
Samples
Elastomers based on epichlorohydrin rubber (ECO), bromobutyl rubber (BIIR) and chlorobutyl rubber (CIIR). CIIR Polymer 68.4% 51.3%
The samples analyzed had the following composition:
Conditions
Measuring cell: TGA/SDTA851e
Pan: Alumina 30 µl, no lid
Sample preparation: Pieces of elastomer of about 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
Evaluation
The TGA and DTG curves were evaluated according to the algorithms described in Chapter 3. The curves were separated into the maximum number of individual steps in the evaluation:
.
Interpretation
Steps one and two have to do with the elimination of volatile substances. These steps are usually not separated. However, with these particular elastomers a reproducible separation can be achieved using the first and second derivatives of the DTG curves. All the samples evolved moisture and plasticizers in the first two steps.
Step 1: The samples lose volatile compounds. The height of the step in all three elastomers is 1.5% to 2.5%.
Step 2: As in step one, this is due to the loss of volatile compounds, but at a temperature that is about 70 K higher. With BIIR, the step is somewhat larger than with the other elastomers (4.5%). This sample contains 7.2% plasticizer (of which 4.8% is oil). The fact that the step height is lower indicates that part of the oil is evolved in the temperature range in which pyrolysis of the elastomer occurs. The two effects overlap. Section 4.6.1 describes how the oil content of elastomer formulations can be determined with better accuracy.
Step 3: This step is due primarily to the pyrolysis of the polymer component. The step height often corresponds directly to the polymer content. However, errors can arise in some cases due to the following:
• Oil additive is still evolved during the pyrolysis. In this case, the step height is somewhat too large. But then the step before the pyrolysis is correspondingly too small. An example of this is the BIIR sample with a polymer content of 47.6% and a step height of 48.9%. To find out how to achieve better results, see Section 4.6.1.
• Carbon black is formed during the pyrolysis of the polymer. The carbon black remains as a solid residue and burns together with the carbon black filler only after the atmosphere has been switched to air or oxygen. This carbon black residue can be neglected with many polymers. In other cases, however, e.g. especially halogen-containing polymers, the effect can be quite significant. With these elastomers, the pyrolysis step is too small and the combustion step higher than that expected from the carbon black filler content. Examples of this are ECO and CIIR. With ECO, the step height of the pyrolysis step is 64.1%, and the polymer content 68.4%. The combustion step (step 5) is however about 4% higher than the carbon black filler content. With CIIR, the combustion step of the carbon black is about 5% too large. Possibilities for distinguishing between carbon black formed during pyrolysis and carbon black filler are described in Section 4.3.3.
• If the elastomer contains larger amounts of inorganic fillers, the pyrolysis step can be larger than expected. In these cases, it is possible that the inorganic substances decompose in this temperature range, e.g. with the loss of water. The pyrolysis step is then too high, but the ash content after carbon black combustion is less than the filler content. An example of this is the CIIR sample. In this sample, the effect of carbon black formation during the pyrolysis must also be taken into account, through which the carbon black step increases by about 5%. The expected decrease of the pyrolysis step height due to this effect is compensated by the loss of mass from the inorganic compounds, which are present to the extent of 44.2% in the elastomer. As a result, approx. 6% too little inorganic content is measured. The inorganic residue is 36.9%.
Step 4: This is the temperature range between the end of the pyrolysis of the polymer and the switchover to air. In this range, there is a small loss of mass (about 1%). The step height with the two chlorinated polymers (ECO and CIIR) is somewhat larger than with BIIR.
Step 5: During this step, carbon black burns in air. The step height with BIIR corresponds very well with the carbon black filler content of the sample. With the two chlorinated samples, the step is larger than the carbon black content because carbon black was formed during pyrolysis of the polymer. Possibilities for separating the two types of carbon black have already been discussed in Section 4.3.3
Residue: The residue consists of ash and the inorganic fillers and additives (ECO and BIIR). Elastomers containing relatively large amounts of inorganic fillers give values lower than those expected because some fillers eliminate water or carbon dioxide. This effect is observed with CIIR.
Conclusions
The temperature program described above allows approximate compositional analyses to be performed with TGA. Problems can occur with elastomers that contain large amounts of oil or inorganic fillers or that are based on chlorinated polymers. In such cases, the accuracy can be improved by varying the conditions (Sections 4.3.3 and 4.6.1).
TGA of Epichlorohydrin and Halogenated Butyl Elastomers | Thermal Analysis Handbook No.HB459 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2