INVESTIGATION ON BRIGHTNESS STABILITY OF ACETYLATED AND BLEACHED HORN BEAM CMP PULP FOLLOWING THERMAL AND OPTICAL AGING

摘要

The mechanical and high-yield pulps are susceptible to photo-oxidative reactions which cause the pulps to become discolored and brightness reversion. In this research, Horn Beam chips were chosen randomly from chips pile at Mazandaran Pulp and Paper Mill in north of Iran and pulped under CMP conditions at the yield of 85%. The CMP pulps were bleached separately with DTPA and without DTPA and with hydrogen peroxide. Some of the pulps were bleached with sodium dithionite. Then 60 gr/m2 hand sheets were made from bleached and unbleached pulp. Then DTPA 0.5% were sprayed on the hand sheets and hand sheet optical properties were measured by standard TAPPI. A number of unbleached hand sheets were acetylated using acetic anhydride at 80°C for 30 and 60 minutes. The hand sheets were irradiated for zero, 10, 20, 30 and 40 minutes for optical aging and at 105 degrees in the autoclave for thermal aging. The optical characteristics of the hand sheets were measured before and after optical and thermal aging. The results of this study showed that following bleaching and acetylating, absorption coefficient, K/S ratio, opacity, yellowness, greenness and post color (PC) number were decreased, and brightness was increased. Following aging up to 40 hours, all optical properties (except brightness) were increased. The post color (PC) number is considered as a scale for paper aging, which is zero for zero hour of accelerated aging. These changes are more tangible up to 20 hours irradiation. The DTPA treatment in long-term irradiation and the use of sodium dithionite and hydrogen peroxide in short-long aging have better brightness stability. Papers brightness durability in thermal aging is more than in irradiation aging. Among the hand sheets, acetylated hand sheets, especially those acetylated for 60 minutes, have better brightness stability and less brightness reversion and therefore better resistance towards optical deterioration.