Thermal Analysis UserCom 22

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 isothermal 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.

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Introduction

Polymer engineering and manufacturing materials consist of organic macromolecules to which a number of functional additives have been added according to the requirements specified for the particular material. Since polymer materials contain carbon and hydrogen, they are usually easily combustible. For safety reasons, high demands are therefore put on fire prevention, depending on the field of application (e.g. building industry, electrical engineering, transport, etc.). These demands cannot be met by the polymer base material itself.
The addition of suitable fire retardants [1], however, allows adequate fire protection properties to be achieved even with easily flammable large-volume plastics such as ABS or polyolefines (PE, PP).

To generate and sustain a fire, three main requirements have to be fulfilled: a source of combustible material, a source of oxygen and a sufficiently high activation energy. Once a fire has started, complex, usually free radical, decomposition processes (pyrolysis and oxidation reactions) occur. As a rule, these are exothermic in nature. From this point of view, fire retardants can exert their effect by

• Producing competing endothermic chemical reactions to consume the exothermic combustion energy. This helps to cool the system
  
• Interfering with the free radical and oxidative decomposition processes
  
• Forming a non-flammable, often foam-like crust as a protective layer, e.g. through charring or carbonization, or the formation of inorganic, glassy materials
  
• Displacing or eliminating (through chemical reaction) surrounding oxygen, or the dilution of mixtures of flammable gases and oxygen
  

Nowadays, combinations of different chemicals are also used as fire retardants in order to achieve synergetic effects.
A number of standardized methods are used to investigate fire behavior [1]. These include procedures for determining certain characteristics of materials (e.g. the Limited Oxygen Index (LOI) according to ISO 4598 or Cone Calorimetry according to ISO 5660) as well as industry-specific methods (e.g. Bunsen burner test according to UL 94 for electrical engineering). These measurement and test procedures usually give a practical insight into the fire behavior of the materials or components under investigation and are therefore frequently specified in their requirement profiles.

The characterization of a material with regard to fire behavior according to the current standards is more difficult if only a small amount of material is available or if the geometry of the material available is unsuitable. This is usually this is case with more detailed investigations of materials and damage. In such situations, thermal analysis is then often very helpful when used together with conventional chemical analytical methods.

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Literature

[1] P.F. Ranken, 12. Flame Retardants, in H. Zweifel (Editor), Plastics Additives Handbook, Carl Hanser Verlag, München, 5th Edition (2001), p. 681-698

TOPEM^®is a new temperature-modulated DSC technique. Latent and sensible heat flows can be separated and the frequency- dependent heat capacity determined in one single measurement.

Introduction

In TOPEM^®, a temperature program is used in which an underlying temperature ramp or an isothermal segment is modulated with a series of small temperature pulses of random pulse width. Using this technique, it is possible to separate sensible and latent heat flows from one another. In addition, the frequency-dependent heat capacity can be determined, which allows conclusions to be drawn about the dynamics of processes. In this article, we will present several applications that demonstrate the power of this new technique.

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