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The present work deals with the valorization of the lignin. The lignin is a by-product of the black liquor of the paper industry, which has a very complex composition structure. In this study, this lignin is obtained from the Alfa grass (Stipa Tenacissima L, also named Esparto grass). A composite material with polymer matrix (unsaturated polyester) reinforced with the lignin at various proportions has been elaborated and a comparison of its mechanical and physico-chemical characteristics to another type of composite material constituted of polymer matrix (unsaturated polyester) reinforced with Alfa fibers has been achieved. The characterization of these composites materials is based on tensile as well as thermal degradation tests under isothermal conditions. In order to explain the deviation from the linear profile (weight losses), a mathematical model has been used to show that the degradation energy is the same for all temperature ranges. This model allowed us to calculate the activation energy (48 kJ/mole.K), which corresponds to the process of off-gassing, break of macromolecular chains and weight loss. [ABSTRACT FROM PUBLISHER]