Inhibition of Brightness Reversion in Coated Groundwood Sheets: Effect of Coatweight, Application Method and Coating Ingredients, 2002 Coating Conference Proceedings

T. Ghosh, D. Cogswell, A. Cunningham and D. Raue

Mechanical pulps are known to yellow on exposure to light, often due to the photochemical reactions of lignin. Advances in bleaching and pulping technology have enabled the production of high brightness mechanical pulps that provide paper with attractive mechanical and printing properties. Nevertheless, the high-value printing and writing grades of paper are almost exclusively composed of chemical or Kraft pulp. There is, however, a growing interest in using bleached mechanical pulps in high value-added paper grades. A hindered hydroxylamine-based radical scavenger (RS) that imparts effective light stability to lignin-containing papers, has been recently developed. As its affinity for pulp fibers is very low, it must be applied to the paper surface as surface size or in the coating color. This work examines the influence of the application mode of RS, as well as coatweight and coating composition, on the efficacy of brightness reversion inhibition of coated groundwood sheets. A statistically-designed study was used to examine the effect of the aforementioned parameters on (i) initial brightness (ISO), (ii) brightness (accelerated aging, 6 days), (iii) brightness (ambient aging, 75 days), and (iv) brightness (dark aging, 75 days). The effectiveness of RS in inhibiting brightness reversion, under both ambient and accelerated aging, is immediately apparent by comparing the treated and untreated (control) samples. At low coatweights, 6 and 8 gsm, the average initial brightness as well as the drop in brightness on aging for all the coatings studied were similar. This implies that the activity of RS is independent of its mode of application. Intuitively, the best way to apply the RS to a sheet would appear to be surface sizing prior to coating since the RS functions by scavenging radicals generated by photolysis of lignin. However, this work shows that the photostability of coated sheets can be improved by simply adding the RS to the coating color without modifying the regular coating process.