Brightness Reversion of Mechanical Pulps. XIX. Photostabilization of Mechanical Pulps by UV Absorbers: Surface Photochemical Studies Using Diffuse Reflectance Technique

Abstract

The photostabilization of softwood (SW) and hardwood (HW) bleached chemithermomechanical pulp (BCTMP) by UV absorbers has been studied using diffuse UV/VIS reflectance technique. Irradiation of SW and HW BCTMP testsheets resulted in the formation of two absorption bands in the near UV with λmax at 360 and 420 nm. Along with these absorption bands there was a near constant absorption in the visible range from 550 to 800 nm. Irradiation of SW and HW BCTMP testsheets treated with 2,4‐dihydroxybenzophenone (DHB) or and 2‐[2‐hydroxy‐3‐t‐butyl‐5‐(2‐octyloxycarbonyl)ethyl‐phenyl]2H‐benzotriazole (HPB) resulted in a significant differences in the UV/VIS spectra of the treated testsheets in comparison to the untreated pulps. For the treated pulps, no absorption was observed at λmax∼360 nm and absorption at λmax∼425 nm was significantly decreased with respect to the results acquired with the untreated testsheets. The rate of formation of chromophores that absorbed light in 550–800 nm for the treated testsheets was also smaller than the untreated testsheets. The results showed that DHB was a better light stabilizer than HPB, possible pathways for photostabilization were discussed in terms of the mechanism of photoyellowing of mechanical pulps.