The increased use of fast growth tropical hardwoods has led to a significant increase in issues relating to vessel picking. Hardwood pulps contain vessel elements, large diameter cells which function to transport water throughout the tree. Different HW species will have different quantities (and sizes) of vessel elements ranging from less than 10% to greater than 50% by volume. These large, tubular-shaped vessels are easily picked from the sheet by tacky inks and deteriorate offset print quality. Issues related to vessel elements include dusting on the paper machine, coating picks or streaks, linting or picking in the printing operations, and print spots or voids (ink refusal).
Traditional methods of coping with vessel picking issues include the use of high intensity refining, fractionation by hydrocyclone, dry strength additives, and surface sizes. Refining to the point of eliminating vessel picking is not practical for most mills as it is expensive and generally has a negative impact on drainage and optical properties. Concentration of the vessel elements by hydrocyclone followed by subsequent high consistency refining of the vessel rich stream is probably the most effective option, but is very capitol intensive for mills that don’t already have the required equipment. Dry strength additives and surface sizes can help but high dosages are usually required to have much impact.
A leading chemical company has developed several products that are effective for reducing vessel picking and has secured an ongoing application at customer site. The acacia pulp was being treated and a 20% reduction in refining energy was achieved while more importantly increasing IGT (A picking test in which a paper strip is printed with a tacky ink at ever increasing speeds. The speed just prior to the sample beginning to pick determines the pick resistance. The same instrument can also give a pick count.)values.
A laboratory test method has been developed for evaluating vessel picking and determining the effectiveness of the treatments to reduce vessel picking. The difference in vessel element structure of treated Vs untreated vessels picked from laboratory handsheets made from refined pulp. In the handsheets made from the treated pulp, the picks are significantly fewer and tend to be only fragments vessel elements rather then the intact vessel elements seen from the untreated pulp handsheets. We have seen very positive results in reducing vessel picking for a number of different pulp species (Table 1) and are working to secure additional applications. As there is wide variety of HW pulps being used by the pulp and paper industry, a number of products for vessel pick reduction have been developed. It is recommended that pulp samples be evaluated in the laboratory in order to determine the best product option for any given application.