Brightness (Low, Variable,High)
LOW BRIGHTNESS may be due to inadequate bleaching, the presence of certain metal ions such as iron or manganese, problems with the size-press application, and various other factors. The recommended strategies for increasing brightness depend on the paper grade, type of furnish, and circumstances under which the condition was discovered. If the low brightness represents a change, then it makes sense to fix whatever went wrong to cause the lower brightness.
One of the first things to check is the brightness of the main fiber constituent in the furnish. There may be deficiencies in bleaching. Also, test the brightness of the broke; it is possible that the level of broke in the paper happens to be high and that the broke has a lower brightness than the supplied pulp. Though it is possible to use chelating agents or polyphosphates to counteract the effects of transition metal ions such as iron and manganese, a better approach usually is to try to remove such ions by treating the water before it is brought into the paper mill. Transition metals also can interfere with peroxide bleaching; in that case it is usual to treat the furnish with silicate to tie up the metal ions. In the case of recycled office waste paper, low brightness sometimes can be associated with inadequate removal of inks. In the short term such problems may be attacked with higher levels of deinking chemicals - though such chemicals are then likely to be transferred to a greater extent to the paper machine, where they may interfere with sizing. In principle it is possible to increase brightness by using bright filler that scatters light effectively. Precipitated calcium carbonate is often a good choice. For papers that must to manufactured under acidic conditions, one can use calcined clay, other high-brightness clay products, or even aluminum trihydrate (ATH). Titanium dioxide is very effective, but usually it is not cost-justified if the only objective is to increase brightness. Some other possible sources of low brightness include (a) humic acids or transition metal compounds in the incoming water, (b) a failure of the filler addition system, (c) excessive addition of dyes, and (d) biological decay. In the case of high-yield and unbleached kraft pulps the brightness can be expected to decrease significantly with increasing pH, especially above the neutral point. The effect of pH has been attributed to changes in the light-absorbing characteristics of lignin and its byproducts in the furnish. This effect may be compensated by bleaching with peroxide or hydrosulfite (dithionite) where this makes sense.
VARIABLE BRIGHTNESS is likely to be a much more serious problem than a persistent condition of either high or low brightness. Some likely causes of variability in brightness include variations in first-pass retention, variations in broke content, variation in contaminate levels, variations in pulp yield, or variations in bleaching. It is recommended to look for any periodicity in the brightness variations. Sometimes brightness variations can follow the batch-wise preparation cycles of a starch (possibly subject to biological decay), retention aid (possibly related to swings in first-pass retention), or biocide (often added intermittently and also affecting retention efficiency in certain cases). If the problem is new, then one needs to ask about recent changes in the process or the types and qualities of the materials or fresh water. Efforts to control retention to a steady value, adjustment in biocide type or addition cycles, and attention to dye additives often will resolve problems with brightness variations.
BRIGHTNESS TOO HIGH is seldom considered a serious problem by papermakers, especially if the condition is steady. High brightness easily can be "solved" by addition of a small amount of inexpensive black dye. Alternatively, papermakers can reduce bleaching chemical concentrations or select less expensive fiber or filler types. Lower-brightness clays are typically less expensive that high-brightness clays.
Bristow, J. A., "ISO Brightness - A More Complete Definition," TAPPI J. 82 (10): 54 (1999).
Scott, W. E., Abbott, J. C., and Trosset, S., Properties of Paper. Introduction, TAPPI Press, Atlanta, 1995.
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (email@example.com). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.