Breakthrough in Yellowing Inhibition of Mechanical Pulp, 1999 International Mechanical Pulping Conference Proceedings

Peter McGarry, Zhirun Yuan, Cyril Heitner & John Schmidt

Paprican

Raymond Seltzer, Glen Cunkle & Jean-Pierre Wolf

Ciba Specialty Chemicals

An inhibitor system consisting of (2-(2¢-hydroxy-3¢,5¢-di-tert-amylphenyl)benzotriazole, an ultraviolet (UV) absorber, and 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl, a nitroxide radical, imparts unprecedented colour stability to peroxide-bleached softwood TMP (BTMP). During 600 days of continuous exposure to ambient office lighting (fluorescent lighting + window-filtered daylight), BTMP treated with 1% of the UV absorber and 1% of the nitroxide lost only 10 points ISO brightness, the same brightness loss as bleached kraft pulp. Untreated BTMP lost over 40 points brightness during the same exposure period. When the amount of nitroxide was reduced to 0.1%, identical stability was observed for 300 days exposure, with a modestly larger brightness loss at longer exposure times. The nitroxide radical also inhibits thermal yellowing of lignin-containing pulps, at very low charges. This system is the basis for the first commercially viable technology to stabilise the brightness of mechanical pulps.