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The temperature rise (between 250 and 300 ° C.) of linear polyesters such as PET has the consequence of favoring the degradation and condensation reactions responsible respectively for the breaking of the macromolecular chains and for the structural rearrangement. While the physical aging of the material can take place in the solid state, the chemical degradation of the material takes place essentially in temperature (thermal degradation), in particular during the melt processing of the polymer: in the presence of water in the material or oxygen in the extruder, the molecules degrade very rapidly by hydrolytic and thermo-oxidative degradation respectively. Overall, the greater the recycling of PET, the more we promote the breakdown of molecular chains (by hydrolysis, thermal degradation, oxidative breakdowns ...) which reveal an increasing number of COOH acid functions at the end of the molecular chain. This results in changes in mechanical properties such as the decrease in viscosity, the average molar mass Mw and the resistance to stretching (...), and an increase in the degree of crystallinity and coloring. Carboxylic acid chains are therefore the consequence of the thermal and hydrolytic degradation of the material and the quantification of these chain ends is therefore a good indicator of the progress of the degradation of the material. The ultimate goal is to find a correlation between the COOH function rate and the PET recycling rate (or the PETV content of RPET). The determination of the carboxylic acid functions of the PET is carried out according to the analytical method described by POHL1, by simple and reproducible acid-base assay. About 0.1 g of PET is dissolved in 5 ml of hot benzyl alcohol. The mixture is then cooled and poured into 10 ml of chloroform. The operation is repeated once with 5 ml of benzyl alcohol to ensure the complete dissolution of the PET. The solution thus obtained is assayed with concentrated sodium hydroxide at about 0.05N prepared with crystals of NaOH dissolved in benzyl acid. The phenol red determines the balance of the acid-base dosage.