The effect of gamma irradiation on physical, thermal and antioxidant properties of kraft lignin

Physico-chemical, thermal and antioxidant properties of lignin irradiated with gamma source of doses 30, 60 and 90 kGy have been investigated by Electron spin resonance (ESR), Fourier transform infrared, Differential scanning calorimetry, X-ray diffraction, Scanning electron microscope (SEM) and spectrophotometer techniques. ESR studies confirm the presence of poly-conjugated radicals in unirradiated lignin, whereas irradiated lignin possess as both poly-conjugated and peroxy radicals. The peroxy radicals decay near the glass transition point on thermal heating while poly-conjugated radicals are stable even at a high temperature of 450 K. The absorption band at 1615 cm−1 reveals the presence of conjugated structures whose concentration increases with the dose of irradiation. Upon irradiation, an increase in glass transition temperature as well as amorphous nature of lignin was observed. SEM micrographs depicts that particle size was considerably decreased on irradiation. Antioxidant studies show that the irradiated lignin with decreased particle size and high Π-conjugation exhibit high antioxidant activity compared to the unirradiated lignin.