Gas Analysis During Vulcanization using a TGA-MS Combination

Purpose

A TGA-MS combination allows very small quantities of evolved gases to be analyzed. This is demonstrated using the vulcanization reaction of NBR rubber as an example.

Sample

Unvulcanized NBR rubber with 50% polymer content 

Conditions

Same conditions as in Section 4.2.1. TGA measurements of the vulcanization of NBR

Measuring cell: TGA/SDTA841e (large oven) coupled to a Balzers ThermoStar mass spectrometer

Pan: Alumina 70 µl

Sample preparation: Five pieces, each weighing approx. 10 mg, were cut from the starting material. 

TGA measurement: Heating from 35 °C to 300 °C at 10K/min

Atmosphere: Argon

Evaluation

The DTG curve shows a shoulder between 135 °C and 185 °C that correlates with the vulcanization reaction (see Section 4.2.1. TGA measurements of the vulcanization of NBR). This region is marked with vertical lines. The change in mass amounted to about 0.1%. The extremely small quantities of gas produced were analyzed by mass spectrometry

To illustrate this, the MS signals of several ions are shown as a function of temperature. The ordinate scale of some of the curves was expanded by a factor, F, for display purposes

Interpretation

It can be seen that the m/z 2 curve is similar to that of the TGA curve. Hydrogen accompanies every loss of mass. The m/z ions 17 and 18 are related to the loss of water. The signal intensities decrease with increasing temperature, but show a distinct peak in the region of the vulcanization. There are two reasons for the gradual loss of water. First of all, atmospheric water enters the TGA when the sample is introduced. This effect can be reduced by lengthy purging before beginning the measurement. Secondly, the sample itself loses moisture in this temperature range. The peak in the temperature range marked by the vertical lines, however, shows that water is produced in the vulcanization reaction. The m/z ion 12 is characteristic for carbon and the release of hydrocarbons. Before vulcanization, this signal could hardly be detected. A define step can however be seen that coincides with vulcanization reaction. At higher temperatures, other carbon-containing compounds are evolved. The m/z 64 and 45 ions indicate the release of sulfur-containing compounds during the vulcanization reaction. 

Conclusions

TGA-MS measurements allow information to be obtained on chemical processes that occur during the vulcanization reaction. Very low gas concentrations can be analyzed in this way.

A detailed analysis of the chemical reaction would require the evaluation of many more MS signals than we have measured here. This would, however, go beyond the scope of this booklet. 

^{Gas Analysis During Vulcanization using a TGA-MS Combination | Thermal Analysis Application No. HB439 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2}