Bleaching study of kenaf mechanical pulps.

Application: Kenaf bark pulp may be a suitable raw material for newsprint and specialty papers in countries where wood resources for fiber are in short supply.

The researchers carried out bleaching experiments refiner mechanical pulps composed of (a) whole kenaf with 65% core fiber ("kenaf") and (b) kenaf bark with 22 % core fibers ("bark-22"). Both pulps were treated with different dosages of hydrogen peroxide ([H.sub.2][O.sub.2]) and sodium hydroxide (NaOH). A bleach liquor containing NaOH, sodium silicate, magnesium sulfate, and ([H.sub.2][O.sub.2]) was added to preheated pulp samples, which were treated for two hours at 60 [degrees]C. Later, pulp pads were formed after adequate washing and were analyzed for ISO brightness.

The results showed that the peroxide consumption is higher for kenaf than for bark-22, which can be attributed to higher extractives and lignin content of the core fibers. The initial pulp brightness had a substantial effect on the bleaching efficiency. For each combination of the [H.sub.2][O.sub.2] and NaOH dosages tested, the bleaching efficiency was better for the bark-22 RMP than for the kenaf RMP. With only 1.0% [H.sub.2][O.sub.2] and 1.0% NaOH, the kenaf and bark-22 pulps reached brightnesses of 62.0% and 64.0%, respectively. With higher NaOH and [H.sub.2][O.sub.2] dosages, both types of pulp can be bleached to more than 75.0% ISO brightness in a single hydrogen peroxide stage.

At the time of this project, Mohta and Roy were with the Faculty of Forestry, University of Toronto, Toronto, ON M5S 3B3, Canada. Mohta is now with Buckman Laboratories (Asia) Pte Ltd., 33 Tuas South St. 1, Singapore 638038. Whiting is with Trojan Technologies Inc., 3020 Gore Road, London, ON N5V 4T7, Canada. Email Roy at [e-mail]