CELLULOSE CHEMISTRY AND TECHNOLOGY Cellulose Chem. Technol., 47 (1-2), 93-109 (2013) STUDYING THE EFFECT OF PHOTO-YELLOWING ON THE BRIGHTNESS PROPERTY OF CHEMI-MECHANICAL PULPING PAPER MOHAMMAD NEMATI, AMIR HOOMAN HEMMASI, MOHAMMAD TALAEIPOUR and AHMAD SAMARIHA Department of Wood and Paper Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran Received November 11, 2011 The main objective of this study was to evaluate and determine the variation in brightness characteristics of chemi- mechanical papers, which were produced by Mazandaran Wood and Paper Company through treatment with some transitional metal ions in addition to relevant techniques to decrease it. The existing metal ions in chemi-mechanical pulps lose their brightness upon reaction of the paper fiber with lignin. Although some of their properties will be increased, including yellowness, the brightness of the pulps produced through mechanical or chemi-mechanical procedures are preferred according to investigations on the aging degree and other relevant brightness parameters. Some unbleached samples of paper were prepared at the factory and then, they were treated using following two chemical materials. Firstly, they were wetted using EDTA (Ethylene Diamine Tetraacetic Acid) as chelating agent, at various levels of concentration. The process neutralized available ions in the samples of CMP (chemi-mechanical papers) and also the existing metal ions, which can trigger aging and yellowness regimes. Secondly, after EDTA-wetted samples were dried, they were wetted again by solutions containing transitional element ions. The obtained results revealed that the most prevalent effect on brightness improvement was associated with Fe 2+ ions, while the least considerable one was related to ions of Al 3+ . Similar to a neutral ion, Al 3+ did not show a significant effect on decreasing the brightness. Besides, when the samples were wetted using different concentrations of EDTA, a significant decrease occurred in detrimental effects, reducing the brightness and increasing the yellowness degrees. Keywords: accelerated optical aging, brightness, EDTA, hardwood CMP, transitional metal ions INTRODUCTION Several methods have been considered to decrease the negative effect of ions, one of them being the use of chelating agents. 1 Since chelating materials employed in previous experiments, such as DTPA, are rather expensive, emphasis here will be put on inexpensive substitutes, like EDTA. Improvement in durability without causing negative effects on brightness, physical properties, and strength of CMPs is one of the main targets for all manufacturers. Since the chemi-mechanical process produces part of the paper consumed in Iran, it is important to study optical properties, as well as solutions to mitigate yellowness and aging effects. Ions of transitional metals, such as Fe 2+ , Cu 2+ , and Mn 2+ , cause paper to be colored. 2 The affinity of these ions to react with lignin and form colorful complexes in samples produced from species containing high lignin contents is greater than in those with lower lignin contents. During a typical pulping process, water, equipments, and even wood chips can act as sources of transitional metal ions. Using distilled water is not economically feasible to mitigate the negative effects of ions. Thus, some alternative methods have been suggested, namely using chelating elements like EDTA, decreasing pH value of the pulp, 1 and acetylation. 3 Scientifically speaking, paper produced from chemi-mechanical pulp is composed of cellulose, hemicelluloses, lignin, extractive materials, additives, starch and starching materials, fillers, and pigments. Mechanically produced and high- yield pulps, such as CMP with a lot of lignin, will intrinsically experience decline in whiteness and brightness, or the well-known “yellowness” phenomenon in the absence of light, or in the presence of light and UV waves in the 300-400 nm range with higher amplitudes. Researchers