To make paperboard with a high whiteness value the major components of the paperboard must also have a high whiteness value. A pulp with a high whiteness level is obtained through bleaching. Bleaching reduces light absorption, particularly in the blue region of the electromagnetic spectrum.
Brightness has traditionally been an important quality criterion and brightness values are often quoted in specifications for paperboard. It has, however, been increasingly accepted that brightness as it has been defined in current test methods is inadequate for specifying the optical properties of a paperboard.
The bleaching stages in this illustration are shown in the photos on the previous page. The unbleached pulp is represented by the blue line, step 2 by the yellow line and step 5 by the brown line.
Brightness (ISO 2470)
Brightness is a good property to consider when controlling a bleaching process but it is not a useful property to consider when discussing a paperboard that may contain both dyes and FWAs. Brightness is not a property related to human perception studies. Brightness is the reflectance of an object measured through a blue filter with a peak pass wavelength of 457 nm. Brightness is reported as a percentage of the anticipated result from a perfect diffuse reflector.
Illustration of the ISO brightness measurement area when measuring a typical paperboard.
An illustration of the difference in spectral reflecting power of an OBA-containing board and an OBA-free board given that there is UV radiation in the incident light (D65).
Click to enlarge images.
Fluorescent whitening agents and dyes
Fluorescent whitening agents (FWAs), sometimes referred to as optical brightening agents (OBAs), absorb ultraviolet light and emit visible light. In very rough terms the light from these FWAs increases the stimulation of the short wavelength-receptive cones while not stimulating the medium and long wavelength-receptive cones. Considering the tri-stimulus values mentioned earlier, Z will increase while X and Y are largely unaffected, therefore x and y will both be reduced and so according to the CIE whiteness equation the perceived whiteness will increase.
Dyes modify the colour of an object. The violet dyes used in the paperboard industry impart the violet shade that the whiteness equation indicates will result in an increase in perceived whiteness. The use of these dyes is limited by their tendency to cause the paperboard to take on a grey appearance.
Fluorescence (ISO 11475)
Fluorescence is the difference in the CIE whiteness values of the same sample measured under D65 lighting and D65 with the ultraviolet portion of the illuminant filtered out. This measurement corresponds to the effect obtained from FWAs under D65 illumination.