Thermal Yellowing of Hornbeam Chemi-mechanical Pulps Bleached with Hydrogen Peroxide and Sodium Dithionite

The thermal yellowing of hornbeam chemi-mechanical pulps (CMP) after bleaching with hydrogen peroxide and sodium dithionite was investigated. The hornbeam chips were randomly chosen from Mazandaran wood and paper industries. The CMP pulps prepared with 85% yield were separately bleached with diethylenetriamine pentaacetic acid (DTPA), without DTPA, and hydrogen peroxide. Some pulps were bleached with sodium dithionite. The optical properties of prepared hand-sheets of 60 g/m(2) after spraying with 0.5% DTPA were measured using TAPPI standard methods. All prepared papers were thermally aged separately in an oven at 105 degrees C for 0, 10, 20, 30, and 40 h. The optical properties, such as brightness, yellowness, coefficient of absorption, k/s ratio, post color (PC) number, and a* factor, before and after thermal aging were measured. The results showed that from 0 to 40 h, the optical properties of paper increased except brightness and greenness. This increase was more extensive up to 15 h. Additionally, among the various treatments, DTPA treatment in longterm thermal aging and the use of sodium dithionite and hydrogen peroxide in the short-term aging had noticeable effects on brightness durability and decrease in the color reversion. Thus, there was an increase in the durability of the paper against thermal deterioration.