The fluorescence spectra of untreated, chemically treated and photoyellowed sheets of various mechanical pulps were studied. Untreated pulps gave broad structureless emission bands between 375 and 600 nm with maxima around 450 nm when excited at 350 nm. On irradiation, with either different light sources or monochromatic light at selected wavelengths, a broadening of the emission band, a shift of the emission maximum to longer wavelengths and a slight increase in the total emission intensity were observed. Sulphonation caused a strong increase in the fluorescence intensity and shifted the emission maximum to shorter wavelengths compared with the unsulpbonated pulps. Photoyellowing of the sulphonated pulps caused a marked decrease in the fluorescence intensity and a similar shift in the emission maximum to longer wavelengths, as found for unsulphonated pulps. In addition, sulphonated samples were found to be much more vulnerable to photoyellowing than unsulphonated samples, as shown by the changes in their fluorescence properties. Bleaching of both unsulphonated and sulphonated pulps with hydrogen peroxide strongly increased the fluorescence intensity, but the position of the emission maximum remained unchanged. Bleached pulps were found to be much more sensitive to irradiation than unbleached pulps. This was manifested as a rapid decrease in the emission intensity and a shift in the emission maximum to longer wavelengths when irradiated with either monochromatic light at selected wavelengths or different light sources.
