Yellowing Mechanism and Kinetics of Thick Handsheets of Softwood Thermomechanical Pulp

Two major UV absorption peaks were observed when thermomechanical papers (black spruce, balsam fir) were exposed to a UV light in the range 300–400 nm for up to 65 hours. One of the peaks around 425 nm was the result of the formation of three different chromophores. The other one at 360 nm was a composite band resulted from the disappearance of one chromophore, and the appearance of a different chromophore, which seemed to be the intermediate molecule of two steps photoinduced reaction. The kinetic observed always follows a first order reaction rate. We believe that the two chromophores observed at 360 nm were related to the formation of the chromophores appearing at 425 nm. This resulted in a system of four chromophores which occurred in yellowing involving three reaction pathways. One of the reaction involved a chromophore which was affected by peroxide and borohydride bleaching. The elimination of this chromophore by either oxidation or reduction minimized its effect on color reversion. The other reaction pathways could also be modulated either by borohydride or peroxide. One of the major problem in using thick handsheets is highlighted. The photo and thermal stabilizing effects of 2,2′-thiodiethanol (1), 2,2′-dithiodiethanol (2), and 2,2′-oxydiethanethiol (3) were investigated for high brightness bleached chemithermomechanical pulps (BCTMP). The dithiol additive 3 was shown to be the most effective photostabilizing reagent for freshly impregnated handsheets. Mechanistic studies suggest that dithiol additives photostabilize BCTMP pulps no more effectively than monothiol additives. Long-term thermal reversion studies indicated that 2,2′-oxydiethanethiol also stabilized BCTMP handsheets against thermal yellowing. Photolysis of the aged handsheets indicated that the photostabilizing efficiency of additive 3 was detrimentally impacted upon by long term storage prior to photolysis.