Estimation of the Long-Term Stability of Materials using Advanced Model Free Kinetics

The article describes how advanced model free kinetics can be used to make predictions about the long term-stability of materials, using the decomposition of polystyrene (PS) as an example. A combination of heating and isothermal measurements as well as an iterative comparison between the predictions of the kinetics and measurement results proved successful.

Introduction

An important application of model free kinetics is to predict the course of a chemical reaction for conditions under which the reaction can only be measured with difficulty or not at all. To do this the reaction is measured in a readily accessible temperature range and the results used to predict the behavior in another temperature range. The extrapolation of kinetics data can however only be performed with good accuracy if the reaction mechanism at a conversion, α, does not change significantly with temperature. This is generally the case for predictions close to the temperature range in which the measurements were performed. However, in the estimation of the long-term stability of materials, the measurements are performed in a relatively high temperature range (e.g. polymer decomposition at about 300 °C) while the predictions are for room temperature. Care must be taken when making predictions about the kinetics of a reaction over such a large temperature range because reaction mechanisms can change considerably and in particular the changed influence of diffusion processes must be taken into account. A prediction should therefore whenever possible be compared with experimental data, for example from iso-thermal measurements. Here of course a compromise has to be made between the measurement time available and the observed temperature range. Predictions from kinetics data are more accurate at low measurement temperatures, but the measurement time required and the demands put on the measurement technique are however higher. The advanced model free kinetics software allows data from heating measurements, isothermal measurements as well as from measurements with any temperature segments to be simultaneously included in the calculation. This is why this kinetics software is especially suitable for predicting the long-term behavior of materials.

This article describes a procedure for the prediction of reactions at low temperatures. Thermogravimetric data was used for the evaluation; polystyrene was chosen as model substance.

Experimental Details

The decomposition of polystyrene was measured using a TGA/SDTA851e. The samples weighed about 10 mg and were contained in 30-μL crucibles; the purge gas was nitrogen. The change in mass of the samples was measured at different heating rates in the range 50 °C to 500 °C or at a constant reaction temperatures. Relatively low heating rates of 1, 2, 5 and 10 K/min were used because predictions for the course of the reaction at low temperatures were required

The isothermal measurements were performed by inserting the samples into the furnace at 50 °C and heating them to the reaction temperature at different heating rates between 10 and 40 K/min. This temperature program was chosen to ensure that no reaction occurred when the sample was inserted, and also to allow a reaction that occurred on heating to be taken into account in the kinetics evaluation. If the heating segment employed to reach the isothermal reaction temperature always used the same heating rate, the reaction occurring in this segment would not be adequately taken into account in the kinetics evaluation.

Conclusions

The possibilities offered by advanced MFK are ideal for the iterative improvement of predictions on the course of reactions if one wants to extrapolate over a large temperature range. To do this, the predictions of the kinetics evaluation are first compared with the isothermal measurements. In a further step, the measured data is then used for the kinetics calculations. This approach allows the predictions of long-term behavior of materials to be significantly improved. The example discussed in this article is based on thermogravimetric data in which the reaction is measured to completion. For practical applications it is often not always necessary to measure the full course of the reaction. The measurement can be terminated at a conversion greater than 10% in order to reduce measurement time, especially with long measurement times. This must however be taken into account in the calculation of the conversion curves (see box). Furthermore the predictions of the kinetics evaluation are valid only up to the smallest final value of the measured conversion curves.

The approach discussed here can of course also be applied to DSC measurements.

The study of reactions with low reaction rates (such as occur at relatively low temperatures) requires measured data with good resolution for kinetics evaluation and necessitates the use of sensitive measuring instruments. An ultramicro balance is recommended for thermogravimetric measurements. The sensitivity of DSC measurements can be improved by using the HSS7 sensor.

Estimation of the Long-Term Stability of Materials using Advanced Model Free Kinetics | Thermal Analysis Application No. UC223 | Application published in METTLER TOLEDO Thermal Analysis UserCom 22