PREVENTION OF PAPERS PHOTO-INDUCED YELLOWING BY FLUORESCENT WHITENING AGENTS - MOLECULAR MODELING OF THEORETICAL SPECTRA VS. EXPERIMENTAL RESULTS

Our previous studies have shown that Fluorescent Whitening Agents (FWA) and Optical Brighteners (OB) can be efficiently used to reduce photo-induced yellowing of papers with high lignin content like those from thermomechanical pulps (TMP). However, the complete understanding of the efficiency of these FWAS and OB, as well as their exact mechanism, is by far incomplete. Our previous works lead-us to believe that there is a relationship between the energy of the HOMO-LUMO, and there variants, transition involved in the FWA spectra and the energy levels of the "components" of the lignin molecule. In this study, we try to establish a relationship between the FWA and OB spectra obtained in solution, those obtained when the FWA and OB are deposited over high lignin (TMP) and low lignin (Kraft) papers, and those obtained by molecular simulation in ab initio, DFT and semiempirical methods. The experimental spectra were taken according to the Kubelka-Munk theory. The molecular simulations were done using Gaussian 09W. This study aims at the understanding of the exact mechanism involved, paving the way to the synthesis of even more efficient FWA and OB.