BRIGHTNESS IMPROVEMENT OF DOUGLAS-FIR THERMOMECHANICAL PULP BY EDTA AND ASCORBIC-ACID TREATMENTS ON CHIPS

This investigation studies the use of Douglas fir wood as feedstock fo r thermomechanical pulp (TMP). Douglas fir wood extractives include fl avonoids and other polyphenolic compounds, which make the pulp suscept ible to discolouration. Pulp darkening is promoted by the formation of metal-chelates and phenolic oxidation products. The effectiveness of various wood pretreatments to prevent the discoloration of polyphenoli c extractives in Douglas fir wood was investigated in lab-scale experi ments. Iron-mediated brightness losses (up to 5.1% ISO) could be preve nted by wood pretreatment with 0.2% EDTA. Treatment of wood meal slurr ies at temperature levels comparable to those applied in thermomechani cal pulping caused wood discoloration due to polyphenol oxidation. The brightness losses could be reduced by the addition of 0.5% ascorbic a cid. The anti-oxidizing agent was more effective with sapwood compared to heartwood. EDTA pretreatment allowed an increase in the proportion of heartwood from 12% to 39% (w/w) that could be tolerated as feedsto ck for the production of dithionite bleached TMP with a brightness of 60% ISO. Wood chip impregnation with both EDTA and ascorbic acid only proved advantageous over EDTA alone in Douglas fir samples consisting almost exclusively of sapwood.