Color Measurement of Flourescent Paper Grades, 1998 Engineering Conference Proceedings

The paper industry’s demand for high white paper products is increasing. This requires usage of well bleached fibers and high dosage of fluorescent brightening agents (FBA) often applied both in the wet end and at the size press. The dosing level of FBAs at both processes is quite often near the saturation point. Specialty paper makers may also use fluorescent colorants for shaded grades, in combination with non-fluorescent colorants and sometimes FBAs. Thus there is an immediate need for reliable, repeatable, and reproducible measurement of colour in fluorescenct grades.

The paper industry uses a variety of spectrophotometers and calorimeters as their laboratory quality reference instruments. Unfortunately, even on non-fluorescent grades the agreement between different color instruments is not at the level provided by measurements of properties such as weight or caliper. Furthermore, laboratory instruments measure a stack of sheets, while on-machine instruments measure a single layer, causing additional sources of instrument disagreement and quality deviations.

Color target specifications and grade limits are usually supplied as calorimetric quantities. This is barely adequatz for non-fluorescent grades and problematic for fluorescent grades. Moreover, the standard vocabulary for color terms is ambiguous concerning certain phenomena relevant to measuring fluorescent samples. Thus the paper maker may have to learn the theory and pitfalls of fluorescent color measurement in the hard way.

This paper describes the principal factors affecting color measurement for fluorescent grades. Process and instrument issues are discussed. Ambiguities in standard terminology and practices are identified, and improvements are recommended. Examples are given with measurements made at several paper mills.