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High yield and chemi-Mechanical pulps are susceptible to oxidation-optical and thermal reactions. These reactions result discoloration and brightness reversion in pulps. In this research, chemi-mechanical pulps were selected randomly from Mazandaran pulp and paper mill and were bleached separately by P, H, PH and HP sequences and sheets with 60 gr/m2 gramage were prepared from these bleached pulps. All the prepared papers went through thermal aging, separately at 0, 12, 24, 36 and 48 hours at 1050C in oven, and their optical properties, including, their brightness, their a, b and L factor, α exponent, yellowness, opacity, K/S ratio, and PC number before and after thermal aging were measured and compared based on TAPPI standard. The results of this study showed that, during thermal aging from 0 to 48 h, paper optical properties such as brightness, L and a factor were decreased and b factor, yellowness, α, K/S ration and PC number were increased. As a result, bleaching by P and HP sequences had the highest amount of brightness and L factor and lowest amount of a and b factor, α exponent, yellowness, K/S ratio and PC number. Therefore, these bleaching sequences had important influences on brightness stability and decrease in paper brightness reversion and increasing their durability against thermal degradation.

The objective of this study was the evaluation of optical, mechanical properties of chemi-mechanical pulp which were bleached by one and two stage bleaching sequence of hydrogen peroxide and sodium hypochlorite. Also the environmental effects of effluents from bleached paper liquor were studied. For this purpose, unbleached CMP pulp mixed of hardwoods was prepared from Mazandaran Wood and Paper mill. Then, these pulps were bleached using P (hydrogen peroxide), H (Sodium Hypochlorite), PH (Hydrogen Peroxide and Sodium Hypochlorite) and HP (Sodium Hypochlorite and hydrogen peroxide) stages. 60 gr/m2 hand sheets were prepared and the optical, mechanical properties of hand sheets and also their effluent bleaching liquor characteristics were measured and analyzed based on TAPPI and APHA Standards. The results showed that when peroxide was applied at the last stage of bleaching, then the properties such as tear, tensile and brightness have increased and yellowness has decreased. Generally, the role of single sequence of bleaching by P and two sequence of bleaching with HP were more effective than single sequence of H or unbleached pulp. Based on results, the highest and lowest COD load is related to bleaching liquor of H and PH respectively. Moreover, bleaching liquor of hydrogen peroxide had the highest BOD load and the lowest amount of BOD is related to PH sequence of bleaching.