To show how the kinetics of a vulcanization reaction can be investigated by DSC. Experiments were performed at vulcanization temperatures of 70 °C and 130 °C.
Unvulcanized NBR with 50% polymer content
Measuring cell: DSC822e
Pan: Aluminum 40 Pl, with pierced lid
Sample preparation: Cubes of approx. 25 mg were cut from the starting material immediately before the measurement. For the vulcanization reaction at 70 °C, a number of samples were prepared and stored in a separate oven at 70 °C. Individual samples were removed from the oven after different reaction times. For the vulcanization reaction at 130 °C, a fresh sample was placed in a DSC that had been preheated to the reaction temperature and then removed after a reaction time, tr. This procedure was repeated for a number of samples for different values of tr using the automatic sample robot.
DSC measurement: Heating from 50 °C to 220 °C at 10 K/min
Atmosphere: Nitrogen, 50 ml/min
The enthalpy of reaction of the post-vulcanization reaction can be determined from the heat flow curves.
The conversion is calculated from the equation:
where Δh0 is the specific enthalpy of reaction of the unreacted sample. The conversion can be displayed as a function of the reaction time from the calculated conversion values.
The samples that had been allowed to react at 70 °C were also measured in the same way as the samples that had been preheated at 130 °C in the DSC. The following diagram shows the conversion as a function of time at vulcanization temperatures of 130 °C and 70 °C. The logarithmic time axis was used so that the conversion curves of both reactions could be displayed in one diagram.
At a vulcanization temperature of 70 °C, the sample is initially stable; the reaction only begins at a noticeable rate after about 30 minutes.
Since the reaction peaks are relatively broad and flat, the evaluation of an individual measurement is somewhat unreliable. Reliability is improved when a series of samples is evaluated. The reaction rate at 70 °C is so slow that it cannot be measured in a direct isothermal measurement in the DSC. The measurement of the post-vulcanization reaction thereby improves the accuracy and the detection limits of a DSC analysis. In particular, it saves measurement time when slow reactions are involved
Measurement of Isothermal Vulcanization Kinetics | Thermal Analysis Handbook No.HB410 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 1