BLEACHING AND BRIGHTNESS STABILITY OF ASPEN BIOMECHANICAL PULPS

Fungal treatment of wood chips prior to mechanical pulping saves energ y and improves paper strength but reduces pulp brightness by as much a s 15 points. If biomechanical pulping (BRMP) is to become commercially viable, BRMP must be bleached to the same brightness as other high-yi eld mechanical pulps. In this study the bleachability and brightness s tability of BRMP from aspen treated with fungus (Phanerochaete chrysos porium or Ceriporiopsis subvermispora) were established and compared w ith those of traditional high-yield aspen pulps. Hydrogen peroxide and sodium hydrosulfite were used to bleach BRMP, refiner mechanical pulp (RMP), groundwood pulp, thermomechanical pulp, and chemithermomechani cal pulp (CTMP) to nominal brightness levels of 60%, 70%, and 80%. Asp en BRMP could be bleached to 80% brightness with alkaline hydrogen per oxide and to 60% brightness with sodium hydrosulfite. Brightness stabi lity was evaluated by subjecting bleached-pulp handsheets to accelerat ed photo- and thermal-aging tests. Stability of BRMP was slightly lowe r than that of RMP but slightly higher than that of CTMP