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/m2 after spraying with 0.5% DTPA were measured using TAPPI standard methods. All prepared papers were thermally aged ‎separately in an oven at 105 °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 long-term 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.