Analysis of Carbon Black in Elastomers Based on Chloroprene
Purpose
Only small amounts of carbon black are formed during the pyrolysis of most types of rubber. Some elastomers, however, especially those containing chlorine, produce relatively large amounts of carbon black during pyrolysis. The TGA determination of the carbon black filler is then more difficult with these types of polymers. This experiment describes a TGA method to determine the carbon black filler content using chloroprene elastomer as an example.
Sample
Elastomer from an NBR/CR blend and carbon black
Conditions
Measuring cell: TGA/SDTA851e
Pan: Alumina 30 µl
Sample preparation: Pieces of elastomer of about 15 mg
TGA measurement: Program 1 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
TGA measurement: Program 2 Heating from 50 °C to 625 °C under nitrogen (50 ml/min), cooling to 250 °C at 20 K/min under nitrogen (50 ml/min), then switching to air (50 ml/min) and heating to 800 °C. Heating rate 10 K/min
Atmosphere: Nitrogen then air, automatically switched
The diagram shows the TGA curves of both measurements. This section discusses the quantitative determination of the carbon black added as filler. We will therefore restrict ourselves to the part of the curve measured on switching the atmosphere from nitrogen to air, i.e. the part in which the combustion of carbon black takes place. The evaluation of the measurement curve during the pyrolysis, which is completed by about 500 °C, has already been discussed in detail in Section 4.1.1. TGA of chloroprene rubber (CR). DTG curves of the region in which carbon black combustion takes place are also displayed.
Evaluation
In the curve measured at 30 K/min, combustion appears to take place in one step. The carbon black formed during pyrolysis cannot therefore be distinguished from the carbon black added as filler. In contrast, the second curve (measured at 10 K/min and with cooling) shows a distinct difference in the combustion of the two types of carbon black. The carbon black filler burns somewhat later than the carbon black formed during pyrolysis. This difference is particularly evident in the DTG curve. If the temperature of the minimum in the DTG curve at 515 °C is used as a basis for the separation of the two steps, a value of 43.0% for the carbon black filler content is obtained. The carbon black formed during the pyrolysis of CR amounts to 10.4%.
Conclusions
Measurement program 2 allows the carbon black formed during the pyrolysis of chloroprene to be distinguished from the carbon black added as filler. To achieve this separation, the sample is cooled under inert conditions immediately after the polymer pyrolysis is completed. The heating rate of 10 K/min results in a better resolution of the step in which the two different types of carbon black burn. The separation can be improved even more by a further reduction of the heating rate under air.
Analysis of Carbon Black in Elastomers based on Chloroprene | Thermal Analysis Application No. HB455 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2