If the brightening agent is distributed throughout the paper thickness, should that make it more or less likely to fade?
There are more factors having an influence on the shift of paper white in time. In the end Aardenburg will have the verdict on how the paper behaves. Mark gave a good summary on the Digital B&W list of the factors that have an influence. I assume he wouldn't mind a quote here:
> I believe there are several factors that influence OBA burnout and
> media white point stability. Here are some of the key variables.
> 1) concentration of OBAs, ie. how much does the initial media color
> depend on the incorporated OBAs. If very little, then total burnout
> produces only slight effect. If a lot, then total burnout produces
> large effect.
> 2). Location of OBAs. When located in top microporous coatings, the
> OBAs are extremely sensitive to oxidation (just like other dyes). The
> oxidation can be photochemically induced, but even more so due to
> ozone induced oxidation. The OBA's are more protected when located in
> subbing layers and paper core. Many RC papers, for example, have
> subbing layers below the top ink receptor layer and above the PE/TiO2
> layer where the manufacturer can include some OBAs.
> 3). Molecular structure of the OBA and interaction with the layer(s)
> in which they are embedded. For example, swellable polymers will
> indeed protect them more from oxidation, which in part explains why
> OBAs in traditional darkroom type photo papers have not gotten as bad
> a reputation (although some problems have been experienced in the
> field with traditional photo papers as well).
> 4) As a corrollary to item 3), the pore size of the micro/nano porous
> silcates used in the inkjet paper probably plays a role as it can
> also affect the oxygen penetration rates to the OBAs as well as the
> final physical shape of the OBA molecular chain structure due to
> electronic charge influences from the silicates distorting the bonds
> in the dye molecule.
> 5) Inclusion of additional tinting pigments in the paper
> size/coatings to achieve cooler media white point thus lessening the
> need for higher OBA concentrations. Typically you will see lower L*
> values for papers that add some cool-white hue with tinting
> additives, and UV-cut spectral data should still show blue wavelength
> region effects due to the added colorant.
> There's undoubtedly other variables as well, but these are some key
> variables that affect OBA fading impact on media white point
> The manufacturers can also add anti-oxidants.
end of quote
If you check the Aardenburg paper white shifts of papers and my plots you will see some correlation between the paper construction (FBA placing) and white paper shifts but there isn't a strict 1:1 rule.
I prefer to use shift instead of fading. it is quite usual that the overall white reflectance actually (initially) increases a bit in the Aardenburg test but the brightness goes down in time due to FBA breakdown. I guess that first effect is like bleaching linen on the meadow. Due to the FBA loss a shift to a warmer paper happens.
On the brightness versus white reflectance. In the graphic industry the brightness number was used as a general indication of whiteness for papers that had very similar spectral plots. By measuring only the 457 NM reflectance is was an easy and fast method. We see however that it isn't an accurate description for the papers we use. And I think that simple rule may have had an influence on the use of FBAs to give that paper a boost to get in a higher category. Not a problem for magazine paper though.
met vriendelijke groeten, Ernst Dinkla
New: Spectral plots of +230 inkjet papers:http://www.pigment-print.com/spectralplots/spectrumviz_1.htm