DEPTH PROFILING OF A PHOTOCHEMICALLY YELLOWED PAPER .1. UV-VISIBLE REFLECTANCE AND FLUORESCENCE SPECTROSCOPY

A strongly photochemically yellowed paper having a reflectance at 457 nm of 53% (initial reflectance 73%) containing high-yield pulp from softwood was ground off to varying depths. The resulting papers were studied with the use of UV-visible diffuse reflectance and fluorescence spectroscopy. Yellowing penetration depth was determined and the chromophore distribution in the z direction was characterized with the aid of difference spectra. Expressed as the amount of chromophores based on the specific adsorption coefficient, 25% of the yellowing had occurred at a depth of 10-12 mu m, 50% at 15-20 mu m, and 75% at 25-30 mu m. The fluorescence spectroscopy results showed that 50% of the changes induced by light had occurred at a depth of about 20 mu m.