Source: http://www.techexchange.com

The application of FWAs (Fluorescence Whitening Agents) is a common practice in some textile applications where the need to achieve high whiteness is a requirement. Other applications include the use of fluorescent colorants to dye textile substrates in order to accomplish high chromatic colors for style or high visibility apparel. The visual and instrumental evaluation of such materials requires the simulation of the ultra-violet component found in daylight.

 

In order to achieve this simulation it is critical to define the specific qualities of the illumination. The sample illumination should be the same for which colormetric values have been calculated. The common use in todays visual and instrumental measurement is simulation of illuminant D65.

Additional considerations in instrumental measurement relate to geometry, measurement area, UV content of the light source, and UV calibration of the light source.

The objective of this paper is to discuss the practical uses of instrumentation for those applications that use FWAs in their process. We need to realize that there are many complex issues regarding instrument measurement and our purpose here is to outline those that are most important, from a practical viewpoint of the average instrument user.

 

Light and Color:

 

The human eye has the ability to see colors that fall between 400nm to 700nm (nanometers) on the visible spectrum. At the lower end of the spectrum (400nm) the color we see is violet; the hue we recognize as blue is around 480nm; green is approx. between 480nm and 560nm; yellow between 560-590nm; and orange between 590-630nm. We see the color red at those wavelengths longer than 630nm. The energy below 400nm is categorized as ultra violet light and is invisible to the eye just as light energy above 700nm is near infra-red light and also invisible to the eye.

When light strikes an object there are two things that can happen relative to color. Either the light transmits through the object or reflects from the object. It is possible that some of the light may pass through, some may reflect, some may scatter, and some do all of the above but for the purpose of our discussion the general categories of transmittance or reflectance will suffice. With the above basics in mind it is now that we approach the issue of fluorescence, what it is and its applications within the textile industry.

 

Fluorescence:

 

The use of FWAs (Fluorescent Whitening Agents) or optical brightners has been used in the textile industry for those applications where a high degree of whiteness or chromatic color was required. In the case of white fabrics, the nature of these chemicals is to absorb light in the invisible, or near ultra-violet, region of the spectrum and then re-emit this light as fluorescence in the visible region of the spectrum. This re-emitted light generally occurs between 420-500nm. The effect is a greater degree of reflectance in the blue region of the spectrum, therefore a bluer white. In effect, this process produced much whiter whites! Shades that are white have a high degree of lightness and a low chroma. More important, the perception of white is generally caused by a high lightness with a very low amount of yellowness. Yellowness causes a white fabric to look faded or degraded over time. Therefore, a fabric that has high lightness with a lack of yellow direction (or toward the blue region of the spectrum) will appear whiter to the eye.

Prior to the use of FWAs, the bleaching process was used to achieve a higher degree of white. As an example, cotton is gray in its unbleached state and the bleaching process is used to increase its level of whiteness. Bleaching does increase the level of lightness over the entire visible spectrum (400nm-700nm) with a somewhat higher level in the blue region. Therefore, after the bleaching process the cotton is perceived to be whiter. In order to further increase this perception of a whiter effect, FWAs were added to the process. As described above, these chemicals absorb light in the near UV and re-emit by fluorescence as visible blue light. This effect gives a white fabric a higher degree of blueness therefore a greater perceived whiteness. (See Figure 1)