A dramatic solvent effect on high-yield pulp yellowing inhibition for a benzophenone-based ultraviolet absorber

Abstract UV screens used to photostabilise high-yield pulp do not work as well when deposited from water as they do when deposited from organic solvents. For a water-soluble ultraviolet absorber (UVA) based on 2-hydroxybenzophenone, the water-effect is dramatic. For example, during light exposure a 78% ISO brightness paper sheet made from lignin-containing peroxide bleached softwood thermomechanical pulp (BTMP) lost 27 brightness points. A BTMP sheet treated with 0.5% by weight of the UVA 5-benzoyl-4-hydroxy-2-methoxy-benzenesulfonic acid, 1 (Uvinul MS40™) delivered from ethanol lost only 19 brightness points. However, a sheet treated with 0.5% of the same UVA from water lost 25 brightness points. For a benzotriazole UVA, 5-benzotriazolyl-4-hydroxy-3- sec -butyl-benzenesulfonic acid, 2 (Cibafast W™) the adverse water-effect is smaller. Our experiments suggest several reasons for the poor performance of aqueous-delivered 1 : attenuation and broadening of the absorption spectra on paper when the additive is delivered from water, disruption of the internal hydrogen bond, partial formation of phenolate ion, and changes in the distribution of the additive through the thickness of the paper sheet. This effect has been found to be general across several water-soluble benzophenone- and benzotriazole-type UVAs. One exception to the rule is found for a benzotriazole that has a PEO side chain on the hydroxyphenyl ring. Thus, choice of solvent used in testing new paper stabilisers is of central importance to stabiliser performance.