Analysis of a CR/NBR blend by TGA

Purpose

If one of the polymer constituents of an elastomer contains chlorine, hydrochloric acid (HCl) is formed during pyrolysis. The HCl can react with other constituents. Besides this, carbon black is formed as a major pyrolysis product. As an example, the TGA analysis of an elastomer based on an NBR/CR blend is described. The results obtained using two different measurement programs are discussed. 

Sample

Vulcanized elastomer based on an NBR/CR blend

Conditions

Measuring cell: TGA/SDTA851e 

Pan: Alumina 30 µl

Sample preparation: Piece of elastomer of approx. 15 mg  

TGA measurement: Program 1  

                                    Heating from 50°C to 625°C under nitrogen (50 ml/min),

                                    then from 625°C to 950°C under air (50 ml/min)

                                    Heating rate 30 K/min

The diagram shows the TGA curve and its first derivative (DTG) and also the SDTA curve. 

TGA measurement: Program 2

                                    Heating from 50°C to 625°C under nitrogen (50 ml/min) a 10 K/min, cooling from 625°C to 250°C under nitrogen (50 ml/min) at 25 K/min and then heating from 250°C to 800°C under air (50 ml/min) at 10 K/min

Atmosphere: Nitrogen then air, automatically switched

Evaluation

The following table summarizes the step heights evaluated for both measurement

Interpretation

Step 1: The samples lose volatile compounds. This step corresponds well with the plasticizer content of 9%. In the second measurement it is however too small because the decomposition of a polymer component is shifted to lower temperature at a heating rate of 10 K/min. 

Step 2: This has to do with the decomposition of CR. The SDTA signal shows that an exothermic reaction takes place. HCl is eliminated. The curve measured with program 2 exhibits two steps because the lower heating rate results in better resolution. The step heights are 15.5% and 14.4% respectively.

Step 3: This step is due to the decomposition reaction of NBR. The SDTA signal shows that it is an endothermic reaction. The step height correlates well with the NBR content of the blend. 

Step 4: This step is due mainly to the combustion of the carbon black formed during the decomposition of CR. Separation of the step is possible with program 2 due to the double peak in the DTG curve. In program 1, the DTG curve initially shows a more rapid combustion process. This information is used to separate the combustion step into two partial steps. It is assumed that the carbon formed during the pyrolysis burns more quickly than the carbon black filler. The CR content corresponds to the sum of the step heights of step 2 and step 4. With program 1, a value of 25.5% is obtained; program 2 yields 24.8%. This agrees very well with the formulation (24.4%)

Step 5: This step is die to the combustion of carbon black. If one assumes that this carbon corresponds to the carbon black filler, then a value that is about 8% too high is obtained. Apparently, the HCl formed in the CR decomposition leads to other reactions in which carbon is formed. Further information on the effects caused by the HCl formed can be obtained by evolved gas analysis.

Residue: The inorganic residue is determined as 2.6% and 2.9% respectively. On the basis of the formulation, at least 4.4% is expected. The reason for the low values could be due to a reaction of the inorganic fillers with the HCl.

Conclusions

The determination of the composition of elastomers by TGA is difficult if the elastomer contains chlorinated polymers because HCl is formed. It is nevertheless possible to determine the polymer constituents with good accuracy. Errors in the determination of the carbon black content are to be expected however. Measurements with other methods such as DSC are recommended in order to confirm the TGA results. The DSC curve of this elastomer is discussed in Section 4.5.3. Glass transition of incompatible polymer blends.

_{Analysis of a CR/NBR Blend by TGA | Thermal Analysis Application No. HB 469 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2}