Photoyellowing of mechanical pulp part 1: Examining the wavelength sensitivity to light-induced-yellowing by monochromatic radiation
Paper made from mechanical pulp was exposed to monochromatic radiation to determine the wavelength sensitivity of light-induced yellowing. Both photoyellowing and photobleaching were observed as wavelength-dependent phenomena in the wavelength range of 260-600 nm. Transition from photoyellowing to photobleaching was observed between 340 nm and 400 nm. The increase in yellowness per available photon decreased logarithmically with the wavelength of exposure. Results also suggest that the change in yellowness index is a linear function of light intensity at a given wavelength.