Color (Off-shade, Metamerism, Variable)
Color may be the first thing that a buyer or user of paper products notices when looking at a sample of your product. The human eye is very sensitive to small differences in color, especially when comparing two pieces of paper placed side by side. As a consequence, complaints often result when printers attempt to compile books or magazines from paper that comes from different batches if the color is not precisely controlled.
In modern papermaking operations it is most common to continuously add three dyes and to control the product color by adjusting the flows. The amounts of blue, yellow, and red dyes are adjusted periodically to keep the paper within specified ranges for color. Though color control can be greatly enhanced by online monitoring, it is always important to calibrate the online equipment with offline tests. Online color test results can be affected by changes in opacity and by the fact that the hues of certain dyes are changed by the hot temperatures of the paper at the dry end where such measurements are made.
Though many color problems require little more than an adjustment in the dosages of the dyes, often it is better to address the root causes of color variation. For example, certain biocide treatments can gradually destroy dye molecules; this can be minimized by making sure that the residual chlorine dioxide or hypochlorite is below about 1 ppm at the point where dye is added. Color tends to be more stable in systems where first-pass retention also has a stable value. Variations in the BRIGHTNESS or color of the incoming furnish are likely causes of color variations in the product.
METAMERISM is a word describing the situation in which a sample of paper may exactly match a standard sample under one condition of illumination, but it fails to match the standard under a different type of illumination. In the case of white paper, the most common cause of such a problem is a difference in fluorescent whitening effects of the two paper samples being compared. A simple way to judge the fluorescent whitening effects of paper is to observe them under ultraviolet light. The comparison can be made quantitative by using a brightness meter that is equipped with a filter to either block or permit passage of ultraviolet light in the incident beam. The difference of the "unfiltered" minus the "filtered" brightness gives a rough measure that can be used to control this parameter. If the fluorescent effect in the current product is lower than the standard, then the recommended solution is to add fluorescent whitening agents (FWA) or increase its dosage. If the fluorescent effect is too high, then the problem is likely to be more difficult or costly to resolve. Highly charged cationic polymers such as poly-diallyldimethylammonium chloride (poly-DADMAC) tend to "quench" the FWA. However, such an approach is likely to affect retention, sizing, and the hues of other dyes, etc. About the only other way to get rid of unwanted fluorescent whitening (especially in waste paper) is to bleach the pulp.
In the case of colored papers, metamerism often results when the hues of the dyes used in manufacture are different from those that were used when making the standard for that paper grade. A plot of diffuse reflectance versus wavelength ought to agree with that of the standard paper over the visible range of the light spectrum. Fortunately it is not necessary to use the exact same dye products. Dye suppliers usually have software that allows a quick selection of suitable dyes to match a given standard.
VARIABLE COLOR is likely to be a much more serious issue than a steady error in color relative to a standard. Some likely causes of variability in color include variations in first-pass retention, variations in broke content, or variations in pulp quality or bleaching. It is recommended to look for any periodicity in the color variations. Sometimes color 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 (also affecting retention efficiency in certain cases). Also, if the problem is new, one needs to ask about recent changes in the process or the types and qualities of the materials. Efforts to control retention to a steady value, appropriate biocide use, and attention to dye additives often will resolve problems with color variations.
Jay, S. L., "Color Control for the Paper Producer in the 90's," Proc. 1991 Papermakers Conf., 93.
Lips, H. A., "Dyeing," in Casey, J. P., Ed., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. 3, Ch. 19, 1627 (1981).
Newton, R. J., "Continuous Dyeing: Theory and Practice," Paper Technol. Ind. 24 (4): 140 (1983).
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 (firstname.lastname@example.org). 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.