Using OBA for brightening High Yield Pulps

The interactions of Optical Brightening Agents (OBA) with High Yield Pulp (HYP) fibres under two methods of OBA addition were evaluated on a softwood TMP and hardwood CTMP. The methods for OBA addition were: 1) Method A: OBA was added with the peroxide bleach liquor and 2) Method B: OBA was added following the conventional procedure at the wet-end. The treated pulps were fractionated and OBA retention and efficiency in each fraction were evaluated with respect to process conditions such as temperature and contact time, as well as fibre morphology and chemical composition of the pulp fractions. Results show that pulp morphology, lignin content, temperature, and contact time played a key role in the adsorption of OBA onto the pulp fibres. Adding OBA to the bleach liquor gave a higher OBA retention than adding OBA in the wet-end to bleached pulp. Hardwood CTMP had a higher OBA brightening efficiency than softwood TMP.;The OBA penetration and diffusion into the fibre wall were studied by a fluorescent microscopy technique. The image analysis of OBA processed fibres revealed that kraft pulp fibres being essentially lignin free, had a higher affinity for OBA and higher brightening efficiency than HYP; in contrast, HYP, absorbed less OBA due to its high lignin content. Fibres processed by Method A gave a deeper penetration than Method B. A "two-shoulder" pattern of OBA penetration and diffusion was observed for HYP fibres, suggesting that OBA diffuses into fibre wall from both the fibre surface inward and from the lumen outward. For kraft pulp fibres, in which most of the fibre walls are collapsed, OBA penetration and diffusion occurred mainly from the outer wall inwards.