To achieve the recommendations of the European Union [1], bottle producers must add 25% of recycled PET by 2025 and 30% by 2030. Industrials have already started to use such recycled PET and met different problems such as bottles explosion and the need of empiric change of blowing machine parameters. The origin of these problems requires to be understood to improve the use of recycled PET in the bottles. A hypothesis which may explain these problems is the impact of contaminants.

In the literature, scientists showed the impact of contaminants on the physical-mechanical properties on PET [2-4]: acceleration of the crystallization and increase of the crystallinity, rigidification of the material, and opacification. However, the stretch ability is not studied, which creates a scientific lack of information about the blowing of bottles. Indeed, during the blowing step, PET is both uni and bi-axially stretched. Understanding how physical phenomena such as strain-induced crystallization may occur during this process is then crucial.

For this purpose, the first studies are led on the nature, numbers of contaminants and intrinsic properties. Crystallization in quiescent conditions is favored, especially cold crystallization. Such crystallization must be avoided to keep transparency and stretch ability. Contaminants are also more numerous in recycled PET.

Mechanical testings have been performed in temperature to demonstrate the influence of recycled PET. Moreover, the behavior observed over first blowing tests have showed a higher ability in being largely stretched in the case of recycled PET, compared to virgin one. On Figure 1, three bottles blown according to the same processing parameters (same time in the oven, pressure and heating power applied) are compared: the volume obviously differs. A part of the explication is due to the difference of thermal behavior, as the infra-red ray absorption is not equivalent from a material to another. Another part of the discrepancy can be explained by intrinsic mechanical behavior and by the presence of contaminants.