Comparative Study of Thermal Aging Effects on Old Newspaper Pulps Bleached with Different Chemicals

This study investigates the influence of different bleaching systems on the thermal aging behavior of old newspaper pulps (ONP) under controlled laboratory conditions. Hydrogen peroxide, sodium dithionite, and formamidine sulfinic acid (FAS) were applied at their respective optimum dosages, and the resulting pulps were subjected to accelerated aging at (103 ± 2) °C for up to 240 h. Aging was evaluated as a multidimensional process rather than being limited to brightness changes. Optical properties were assessed together with mechanical strength retention and structural modifications associated with hornification and pore evolution. Brightness, whiteness, yellowness, and color difference (ΔE) were measured alongside breaking length, burst index, Cobb water absorption, and air permeability to characterize the overall response of the fiber network during thermal exposure. Statistical analysis was performed using one-way ANOVA followed by Duncan’s multiple range test. Distinct differences were observed among the bleaching systems. Peroxide-treated pulps maintained relatively stable optical and mechanical performance during aging. In contrast, dithionite-bleached samples showed pronounced color reversion and a reduction in strength. FAS-treated pulps exhibited intermediate behavior. The results indicate that initial brightness gain alone is insufficient to predict long-term performance, emphasizing the need to consider durability under thermal conditions when selecting bleaching strategies for recycled paper.