In general, 100% cotton or cotton blends will be perceived as whiter by making them less yellow. And to make them less yellow we automatically make the colour of the product appear bluer. Visually blue and yellow are opposites and an increase in blue results in a decrease in yellow. This is the basic theory behind the processes of achieving a whiter linen product.
Visually we associate yellowness with soiling or the lack of cleanliness and an attribute that should be removed during the laundering process.
The traditional method for removing the yellow colouration is through bleaching. Chemical bleaching (oxygen and/or chlorine compounds) removes some degree of yellowing by chemically destroying the colourant that caused the discolouration.
Bleach alone may not achieve the level of whiteness that industry and consumers expect from a white linen product. Even with extensive bleaching most cotton textiles are, at best, an off-white colour with a light yellow tint. Historically, to correct for the yellowness that bleaching has not removed, bluing was added to the washing chemicals. Bluing is a blue dye that masks the natural yellowing of cottons and increases the visual whiteness because the product appears bluer. Bluing may still be used, but today most institutions add fluorescent whitening agents or optical brighteners to white textiles.
The perception of whiteness of a test piece or any other ‘white’ product consists of three components of the colour ‘white’.
Fluorescent white: is the addition of optical brighteners on white, resulting in an increase in blue reflectance.
Shaded white: is the amount of whiteness increase due to the compensation of yellowness by the addition of a product such as bluing.
Base white: is the contribution to the whiteness by the fabric itself.
It is a pure reflectance of a cotton fabric that absorbs some of the blue light and thus produces a yellow tint. The whiteness of the base fabric is a very important component of whiteness because it determines the extent by which the yellowness can be compensated by physical means, i.e. bleaching and/or optical brighteners.
What are Fluorescent Whitening Agents?
Fluorescent Whitening Agents (FWAs) are additives to laundry processing chemicals that are specifically designed to increase the perceived whiteness of linens. The fibres, yarns and fabric absorb FWAs thus gaining fluorescence and a whiter appearance. FWAs are not stable compounds and are affected by heat, pH and light. The main reason for adding FWAs to detergents is to restore the original optical brightness of linens that may have been lost due to abrasion, light exposure, bleaching or just wash-out.
FWAs make a fabric appear ‘whiter’ by absorbing light in the near-ultraviolet portion of the spectrum (typically 340-360nm) and re-emitting that energy in the blue region of the spectrum, from 420 to 460nm.
Fluorescence is blue light that is added to a (normally yellow) substrate thus increasing the perceived whiteness by making the object appear brighter and less ‘yellow’. Fluorescent whitening agents are extremely effective in compensating for the yellowness occurring in cottons. They are water-soluble compounds that have an affinity for cotton materials. In simplistic terms FWAs are similar to dyes applied in the textile industry and they are applied using the same techniques during processing.
Instrumental Measurement of White
The colour ‘white’ is defined in colourimetric terms as a colour with the highest luminosity, no hue and no saturation. Instruments measure the colour of ‘white’ as a property of reflecting light at all wavelengths, that is, the reflectance of the whole visible spectrum of 100%. This defines the ‘ideal white’; a condition rarely found in nature.
‘Whiteness’ is the attribute of colour perception by which an object is compared to approach the preferred white. The ‘Whiteness Index’ is an umber, computed by an instrument that indicates the level of whiteness. In the testpiece report, the higher the Whiteness Index value, the greater the whiteness of the measured testpiece. If the preferred white fabric has a high reflectance, then the ideal Whiteness Index for textile materials should approach 100.
The use of FWAs can make the sample measurements difficult. If the Whiteness Index of fluorescent samples is measured without a filter, the resulting Whiteness Index value will consist of both reflected and fluorescing light. The resulting value would provide an indication of the effect of FWAs, but is also influenced by the amount of UV in the light source of the instrument itself. The resulting Whiteness Index may exceed 100 as illustrated by Whiteness Index values of 120-130.
The resulting Whiteness Index is not a true measure because the human eye cannot detect the UV value of this measurement.
Ignoring the effect FWAs have on the visual evaluation of the Whiteness Index increases the possibility that the instrumental evaluation of the testpiece will not agree with the visual determination that the subscriber sees. An alternative to including all of the fluorescence is to calibrate the instrument to include a portion of the fluorescence.
Compiled from various sources