Hindered nitroxide: A new yellowing inhibitor for mechanical pulps

An inhibitor consisting of 2-(2'-hydroxy-3',5'-di-tert-amylphenyl)benzotria zole absorber (UVA), and 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl fre e radical (NO .), a nitroxide radical trap, imparted unprecedented colour s tability to peroxide-bleached softwood TMP (BTMP). During 600 days of conti nuous exposure to ambient office lighting fluorescent lighting + window-fil tered daylight), BTMP treated with 1% of the UVA and 1% of the radical trap lost only 10 points ISO brightness, the same brightness loss as bleached k raft pulp. Untreated BTMP lost over 40 points brightness during the same ex posure period. When the amount of nitroxide was reduced to 0.1%, identical stability was observed for 250 days exposure, with a modestly larger bright ness loss at longer exposure times. The nitroxide also inhibits thermal yel lowing of lignin-containing pulps, at very low charges. The hydroxylamine c orresponding to the hindered nitroxide is equally efficient as an anti-yell owing agent.