Paper Properties – ISO brightness

Posted: October 13, 2012 in cie lab, Color, ISO, ISO 12647-2, ISO Press standardisation, Paper
Tags: , ,

The natural fibrous raw material from which newsprint is manufactured has a creamish shade due to a
content of yellow chromophores. This natural shade is more or less neutralized during the papermaking
process by the introduction of complementary blue or violet dyes to make the sheet look brighter or
actually less yellow.

ISO Brightness

The absorption of light in the untreated raw material has a maximum for blue light since blue is the
complementary colour to yellow. Consequently the light reflectance is the lowest for blue light. If the
purpose of a reflectance measurement on a newsprint sheet is to obtain a measure of the chromophore
content of the pulp, the method will be most sensitive if the measurement is performed with blue light.
This, then, is the purpose of the ISO-Brightness method in which a blue filter is used.
In newsprint production, the quantity of wood chromophores in the paper depends upon the raw material
and the process conditions. The chromophore content of the raw material, i.e. the wood, is increased and
hence the brightness is decreased with increased time in storage. In the manufacture of mechanical pulp of
good brightness it is therefore essential to use as much fresh wood as possible.
The process conditions which most greatly influence the chromophore content of mechanical wood pulp
are primarily refining temperature and bleaching. If the wood is old and dry, the refining temperature will
be higher and more chromophores will be formed. To achieve high brightness, therefore, it is essential to
avoid the use of too old and too dry wood.
The need to use increasingly larger quantities of waste paper as a supplementary raw material and the need
to close the process water system in the paper mill make it difficult and costly to maintain brightness levels
above about 55% for grammages less than 48.8 g/m
2
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