Study of aging of polypropylene


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Polypropylene has many applications, particularly in equipment for protection against falls from height.

However, this polymer is highly sensitive to physical and chemical aging, which due to its semi-crystalline structure, is a very complex process. Physical aging at room temperature was shown to produce a strong increase in the fragile-ductile transition temperature, making the material more and more fragile. This increase was attributed to a change in conformation of the molecules in the amorphous zone at the amorphous-crystalline interface, leading to a drop in entropy. However, physical aging at room temperature does not affect plastic yielding, which is controlled by the crystalline part. For its part, low temperature physical aging corresponds to a relaxation of molecular chains in the amorphous part, and its effect on mechanical properties is seen primarily in the Young modulus.

Finally, at high temperatures, physical aging leads instead to chain scission and to an increase in crystallinity, and thus to an increase in yield stress and deformation at break. For its part, chemical aging affects the structure of polyethylene, but does not influence the kinetics of its physical aging. At room temperature, it acts both on the Young modulus and on the yield stress. Significant differences were observed in terms of the effect on mechanical properties, depending on the temperature at which thermo-oxidative processes are carried out.

These results have been published in an international scientific journal and in five conference proceedings. They are also the subject of Dr. M. El Majdoubi's thesis and of G. Gbeuli's Master's dissertation.