Abstract
This work deals with two aspects of slow pyrolysis of lignin in an inert atmosphere from room temperature to 800 °C, the main examinated product is the solid residue of the degradation. We first deal with the physico-chemical aspects, in particular the TGA, DTG, DTA and DSC analyses and then the thermomechanical aspects relating to the dimensional variations (DV) and the SEM characterizations. The mass loss measurements show that lignin decomposes slowly in the studied temperature range and loses 50% of its initial mass at 800 °C. This loss is accompanied by a 50% increase in the calorific value of the char. The lignin degradation occurs by breaks in the β-O-4, C-C, β-β bonds, by the condensation and polymerization reactions, breaks and reactions suggested by the different peaks of the DTG plot. The DTA and DSC analyses indicate that the thermal treatment of the lignin is exothermic. The DV measurements show that the material undergoes shrinkage at 180 °C, a temperature corresponding to the glass transition of the material. Beyond this temperature, the material increases in volume and this one is quadrupled to 260 °C. The characterization by SEM shows the development of a plastic phase by crazing mainly in volume and the formation of cracks at the surface. The genesis of this phase is at the origin of the low mass loss of the lignin, the high yield of carbon and the stability of the char. The ultimate stability of the plastic phase is reached after the second warming of the material by DSC.
A phase shift (φ) is observed between the maximum of DTG peak of the lignin and the maximum of its DTA peak. The phase shift between the cause and the effect is related to the viscosity which results from the internal frictions in the structure. Internal frictions are representative of the thermo-mechanical behavior of the thermally stressed material. Figure 3: DTG and DTA curves of lignin. [Display omitted]
•DTA and DSC analyses indicate that the thermal treatment of lignin is exothermic.•The material increases in volume after glass transition temperature.•Development of a plastic phase by crazing mainly in volume and cracks formation at surface.•The plastic phase is the origin of the low mass loss of lignin, the high yield of carbon and the char stability.•Ultimate stability of the plastic phase is reached after material second warming by DSC.