Brightness reversion of mechanical pulps. Part XVI: The effect of oxygen on photostabilization of high-yield mechanical pulps treated with UV absorbers and a fluorescent whitening agent

Photoyellowing of softwood (SW) and hardwood (HW) bleached chemithermomecha nicaI pulps (BCTMP) irradiated under argon, oxygen, and air was studied in the absence and presence of UV absorbers and a fluorescent whitening agent. Brightness measurements indicated that BCTMP readily photoyellowed under a n inert atmosphere of argon. Indeed, in the early phase of brightness rever sion the rates of photoyellowing under an argon or air atmosphere were virt ually the same. Photoyellowing was more significant for SW than for HW BCTM P sheets under all atmospheric conditions studied. In the presence of 2,4-d ihydroxy-benzophenone (DHB) or 2-[2-hydroxy-3-t-butyl-5-(2-octyloxycarbonyl )-ethyl-phenyl]2H-benzotriazole (HPB), the photoaging process was found to be very similar for HW BCTMP handsheets under an argon or air atmosphere. B rightness reversion of SW and HW BCTMP handsheets treated with a fluorescen t whitening agent was detrimentally impacted significantly when they were p hotolyzed in an oxygen atmosphere. These results ave supportive of a multi- reaction pathway lending to the photodiscoloration of high-brightness mecha nical pulps.