Optical brighteners

A topic that is gradually coming into the focus of our industry concerns the optical brighteners that are used in papers.

Optical brighteners are added in the pulp and their effect is that they absorb the light from the ultra violet end of the spectrum (370-400 nm) and in turn give it off in the visible range (400-700nm), usually toward the blue range (430nm), resulting in a whiter and brighter appearance. A white paper treated with an optical brightener emits more visible light than shines on it, making it appear brighter by hiding the yellow and brown tones. Different requirements are placed depending on the paper grade.

However the amount of optical brighteners used is not easy to be measured since the light sources of spectrophotometers usually starts at the 400nm. The question is how can the pulp manufacturing process be controlled, and how can the effects of the optical brighteners be measured over time and under different lightning conditions.

Paper manufacturers are required to produce paper at the lowest cost and sell it at the lowest possible price. This means that they have to use more optical brighteners instead of bleaching which is more costly. In addition, bleaching might effect the other physical properties of the paper in an adverse manner.

A very interesting demonstration to see the variation in the use of optical brighteners would be to take a number of sheets from a given paper batch that has optical brighteners and inspect it under a black light. What you will see will be most likely astonishing.

How consistent is the use of optical brighteners in a single batch of paper I do not know. It would be useful if there was a way to measure the effects of optical brighteners. As I see it, the problem lies in being able to measure and monitor the manufacturing process in order to produce pulp that has a known amount of optical brighteners and to communicate it to the printer.

A good white paper on the issue, especially in relation to proofing, is found at:
http://www.cgs.de/news/whitepapers/WP0511_OpticalBrighteners.pdf.