BRIGHTNESS REVERSION OF MECHANICAL PULPS .4. A STUDY ON THE ACTION OFTHIOLS AND DISULFIDES ON HARDWOOD BCTMP

Stabilization against thermal and photo-induced brightness reversion o f high-brightness BCTMP (bleached chemithermomechanical pulp) by vario us sulphur compounds was investigated. Ethylene glycol bisthioglycolat e and 3-mercapto-1,2-propanediol were the most effective additives tha t provided bleaching action and prevented thermal and light-induced br ightness reversion. The stabilizing activity of thiols is detrimentall y influenced by long-term storage, particularly for the mercapto-carbo xylic acids. However, the photostabilization effect of disulphides was moderate and can be retained during storage. In addition, the results of sulphur analysis for treated BCTMP handsheets support the occurren ce of the Michael-type addition of thiols for photostabilizing mechani cal pulps.