Whiter than white
Why is it that two white paperboards can look so different? And as a consumer of paperboard, what should you look for? If you’ve ever selected paperboard, you know how difficult it can sometimes be to choose one white paperboard over another.
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“We have no visual memory for colour at all, so I recommend that all customers take their time when comparing samples,” advises Daniel Hawkrigg, a senior research scientist at Iggesund. “View samples in pairs, select one, then close your eyes for a while or move the sample out of view for a few seconds – and then look at it again. If you have a light box, use it.”
It sounds a little like doing the Coca-Cola versus Pepsi blind taste test, but these small efforts can help make the difference when selecting paperboard. It’s also helpful to be aware of a number of underlying factors that affect a paperboard’s whiteness. For example, whiteness can be subjective, which is why context is so important.
“Views on whiteness change with geography and market,” Hawkrigg says. “As you go south, people tend to prefer a greener white. Here in the North they like a blue or pure non-tinted white. A reddish tint is preferred in China, perhaps because red is considered lucky there.”
Geography, market and personal taste all affect people’s perceptions and preferences when it comes to whiteness. At the same time, some consumers rely strictly on CIE whiteness figures when it comes to selecting both paper and paperboard.
“View samples in pairs, select one, then close your eyes for a while or move the sample out of view for a few seconds – and then look at it again.”
CIE whiteness is a standard that is recognized worldwide in the pulp and paper industries. While the CIE figures are important, judging a paperboard on the numbers alone can be misleading, Hawkrigg says. A high whiteness value is no guarantee that a customer has selected the best paperboard for the occasion.
“A CIE whiteness value does not specify a particular colour,” he says. “It indicates a plane in the three-dimensional colour space used to define colour, and this plane encompasses a range of visibly different colours.” The CIE whiteness value does not correspond to how white a sample is, but rather how white it is perceived to be. “Perhaps counterintuitively, there is not a direct correlation between proximity to the physical property that is white and the human experience of whiteness,” Hawkrigg says.
A high CIE whiteness level is often achieved by using large amounts of dyes in the paperboard. So while the sample has a high value, the paperboard can in fact look dirty. Adding a lot of dye to paperboard can also affect the print quality. This is why achieving a high lightness level can often result in a more desirable whiteness.
L*A*B* IS A three-dimensional system for describing colour that is designed to correspond with the perception of the human eye. The L* corresponds to lightness and A* and B* to the colour-opponent dimensions. A sample with high whiteness obtained through an aggressive use of dye will have a low L* value. A sample with a similar whiteness value obtained without the aggressive use of dye will have a higher L* level.
“Iggesund is very good at manipulating OBAS (Optical Brightening Agents) or FWAS (Fluorescent Whitening Agents) to give us high levels of fluorescence, which in turn lead to the perception of higher whiteness without having to add dye,” Hawkrigg says.
OBAS have been used in paperboards for many years, but there has been a trend to increased levels in recent years. Paperboard producers have been adding increasing amounts of OBA, in particular in the surface-size and baseboard, in their efforts to increase fluorescence levels while still maintaining the stability of their products.
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Judging colour can be a complicated business, so Hawkrigg keeps it simple for those considering whiteness when selecting paperboard. “Don’t worry too much about the figures,” he says. “Look at the samples and select what looks good to you.”
TEXT: CARI SIMMONS
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