Mechanical pulps are known to yellow on exposure to light, much of it due to the photochemical reactions oflignin. Advances in bleaching and pulping technology have enabled the production of high brightness mechanicalpulps that provide paper with attractive mechanical and printing properties. Nevertheless, the high value printingand writing grades of paper are almost exclusively composed of chemical or Kraft pulp. There is however, agrowing interest in using bleached mechanical pulps in high value-added paper grades. A hindered hydroxylaminebased radical scavenger (RS) that imparts effective light stability to lignin containing papers, has been recentlydeveloped. As its affinity for pulp fibers is very low, it must be applied to the paper surface as surface size or in thecoating color. This work examines the influence of the application mode of RS, as well as coatweight and coatingcomposition, on the efficacy of brightness reversion inhibition of coated groundwood sheets. A statistically designedstudy was used to examine the effect of the aforementioned parameters on (ⅰ) initial brightness (ISO), (ⅱ) Δbrightness (accelerated aging, 6 days), (ⅲ) Δ brightness (ambient aging, 75 days), and, (ⅳ) Δ brightness (darkaging, 75 days). The effectiveness of RS in inhibiting brightness reversion, under both ambient and acceleratedaging, is immediately apparent by comparing the treated and untreated (control) samples. At low coatweights, 6 and8 gsm, the average initial brightness as well as the drop in brightness on aging, for all the coatings studied weresimilar. This implies that the activity of RS is independent of its mode of application. Intuitively, the best way toapply the RS to a sheet would appear to be surface sizing prior to coating since the RS functions by scavengingradicals generated by photolysis of lignin. However, this work shows that the photostability of coated sheets can beimproved by simply adding the RS to the coating color, without modifying the regular coating process.
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