ABSTRACT

In the present study we have analyzed the behavior of plastic materials against thermal degradation.
Thermoplastics are subjected to transformation processes that imply the material heating. This can be the source of their degradation and it can change its properties consequently.
Some materials have a wide operational margin between the transformation temperature and degradation, but in other cases this margin becomes narrow. In such cases it is necessary to use heat stabilizers and the knowledge of their degradation mechanism allow us to design suitable stabilizers. To know these degradation mechanisms, it a non-isothermal kinetics analysis has been employed, which presents advantages over the isothermal analysis. We used a non-isothermal methods based on linear temperature programs. This allows the determination of kinetic parameters to be used in a wide temperatures processing window. Thus it has been possible to obtain useful information of several parameters, such as high decomposition temperature.
Some methods, known as classical methods, have been applied to several materials (elastomers, poly(propylene), poly(carbonate), poly(vinyl chloride) and acrylonitrile-butadiene-styrene) to study their degradation. These methods have certain disadvantages:
- These methods do not fit correctly the material degradation process so they are not suitable models to be applied.
- It was obtained good results using low heating rates, which imply excessively long experiments.
To solve these problems it has been proposed a new thermal analysis method that improves the results as a result of the increasing in the equipment capacity while the time required for experiments is reduced.
As a result of the good results obtained with the studied materials, this new model was applied in the analysis of other material to confirm its effectiveness: ethylene vinyl acetate (EVA).
First the influence of the sample weight was studied. Secondly, kinetic parameters were obtained applying the mathematical model to isothermal and dynamic data. Finally, a degradation onset temperature of the material was found.
From the results obtained, it can be concluded that the weight of the sample does not have influence in the degradation process, except when low initial weights it is used that implies a strong dependence on the kinetic parameters.
The kinetic parameters obtained applying the new analysis method proposed shows good linearity, and it was observed a good fit in the whole process range.
To conclude, it is important to remark that when this new method it is applied, it is not used any value of the curve to predict its own behavior, unlike conventional methods.