At the time of writing, there is no clear consensus as to how to quantify lightfastness for the paperboard industry. However, it can be stated that as a rule of thumb the lightfastness of FWA-containing grades is largely determined by the lightfastness of the effect obtained from the FWAs used. In FWA-free grades the yellowing of the wood fibres used in the paperboard is usually the dominant factor.
Mechanically processed pulps retain most of their lignin, this being the substance in wood that binds the wood fibres together and makes it possible for the tree to stand erect. When these types of pulps are exposed to light, the ultraviolet radiation in the light starts to break down the lignin, and the result is a yellow shade. Mechanically processed and unbleached chemically processed pulps also contain wood resin. Oxygen in the air initiates a resin oxidation process which can also yellow the fibres.
Chemically processed and fully bleached pulps are free from both the lignin and the resin and hence have a higher degree of lightfastness.
The paperboard choice
When choosing a paperboard, decisions must be made as to the colour properties required. Packages for luxury products usually have very high whiteness levels. The exact colour properties required are strongly influenced by the end use environment because perceived whiteness varies across cultures and markets. It is always worthwhile to examine the paperboard under realistic lighting conditions relevant to the intended end use.
The whiteness of the fibres has a strong influence on the whiteness of the paperboard. Bleached chemical pulp has a high whiteness level and is often used in the outer plies instead of other types of pulp. Any coatings used also have a large impact on the colour properties. The whiteness of the paperboard is often increased by the inclusion of fluorescent whitening agents (FWAs) and dyes.
Visualisation of the shade change pulp undergoes when exposed to light during 24h, commonly known as the yellowing effect.
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