The Thermal Decomposition of PA 6.6 Compounds Using Model Free Kinetics (MFK)

Kinetic calculations based on TGA measurements of PA 6.6 compounds were performed to assess the influence of additives on the course of thermal decomposition.

Introduction

The flow behavior and flame protection properties of flame resistant, polyamide 6.6 compounds containing fillers was investigated as part of a research project at the Kunststoff Zentrum in Leipzig GmbH (www.kuz-leipzig.de).

The thermal properties of the materials were characterized by performing TGA measurements at different heating rates. The decomposition kinetics were evaluated from the measurement curves using model free kinetics (MFK).

The flame retardant used was melamine cyanurate. This compound contains nitrogen and acts mainly in the gas phase. The cooling effect produced during the combustion process is due to the strongly endothermic decomposition reaction of the additive. In addition, the gaseous non-combustible decomposition products reduce the oxygen concentration at the surface of the polymer.

Classification in the flammability rating UL-94 V-0 can be achieved for polyamide 6.6 (PA 6.6) by adding 10 mass % of this compound. The addition of this amount of melamine cyanurate reduces the mechanical and electrical properties of PA 6.6 only to a relatively small extent. Fillers are often added to composites of PA 6.6 and flame retardants in order to improve their mechanical properties. The research project referred to above examines the question of how the addition of an inert, silica filler influences combustion behavior.

The analysis of reaction kinetics is used to describe the course of the decomposition reaction and to simulate TGA measurement curves. In principle, two different approaches are possible, namely

• model-based kinetics, and
  
• model free kinetics based on the iso-conversion method.

In model-based methods, suitable reaction models are first chosen for the type of reaction involved; the activation energy for each reaction step is constant [1]. This type of approach is however not suitable for calculating the kinetics of complex reactions, to which polymer reactions belong. The reason for this is the large number of secondary reactions in which intermediate products participate. This causes the activation energy of the total reaction to change as the reaction proceeds.

For practical kinetic analysis, model free kinetics (MFK) is therefore advantageous, in which the activation energy is taken into account as a function of conversion [2]. The STARe software model free kinetics option also allows curves to be simulated. This enables TGA curves to be obtained that cannot be directly measured, for example due to technical reasons (heating rate is too high) or because of time limitations (heating rate is too low) [2].

Furthermore, isothermal data can be calculated from non-isothermal measurement data. The course of a reaction can then be estimated as a function of time at different temperatures. The calculations are based on measurement curves of the reaction recorded at three or more different heating rates.

This article shows examples that explain how the evaluation of the TGA data by MFK can be used to investigate the questions raised above and determine the limitations that arise.

Conclusions

To apply model free kinetics (MFK), the TGA curves must be measured at three or more different heating rates. Furthermore, the conversion curves derived from the TGA curves must not cross in the evaluation range.

MFK offers a large number of possibilities for evaluating thermal data. It allows complex reactions such as the thermal decomposition of a polymer to be described in detail based on the conversion-dependent calculation of the activation energy.

The possibility of determining the course of the activation energy for overlapping steps of a reaction is a great advantage compared with simpler kinetic calculation methods in which only one value of the activation energy is determined for the whole course of the reaction.

A further advantage lies in the simple and rapid simulation of the course of the reaction under different conditions, for example at temperatures or heating rates at which practical measurements cannot be performed. This enables complex reactions to be comprehensively described over a wide temperature-time window.

Evaluation of the decomposition kinetics of the above-mentioned PA 6.6 compounds under nitrogen yielded plausible results.

_{The Thermal Decomposition of PA 6.6 Compounds Using Model Free Kinetics (MFK) | Thermal Analysis Application No. UC 434 | Application published in METTLER TOLEDO Thermal Analysis UserCom 43}