# Transmission Reduction for UV and IR Radiation with Dyed Lyocell Knitted Textiles
## 4 Discussion
Thus, the brightness of a shade cannot necessarily be related to the UV protective effect of color. Washing and abrasion increase the optical transmission of investigated fabrics (see Figures 6 and 9). Nevertheless, very good to excellent UPF values are still achieved, which proves the good usability of the textiles. A loss of dye, as confirmed by the color measurement, seems to have a stronger effect on the transmission of Sologhenyl Bordeaux 3 BLE than on Drimaren Yellow HF-CD, especially as the loss of dye in the latter leads to a decrease in brightness, whereas the Sologhenyl Bordeaux 3 BLE knitted fabrics became significantly brighter. If one disregards the clear brightening effect of TiO\({}_{2}\), the use of TiO\({}_{2}\) as a component of the fiber can be assessed as positive. The transmission could be significantly reduced with Sologhenyl Bordeaux 3 BLE as well as with Drimaren Yellow HF, whereby the absorbing effect of TiO\({}_{2}\) in the UVA and the reduction in the IR range is particularly striking. Furthermore, the knitted fabrics with TiO\({}_{2}\) are much more resistant to mechanical stress, which was shown by a lower increase in transmission and a lower fiber abrasion quantity in the abrasion test. With regard to the choice of dye to reduce transmission in the various spectral ranges, the factors of dye concentration, color depth, and color shade have an influence, although, with regard to color shade and color depth, no sweeping statements may be made about possible behavior in the optical spectra. Furthermore, the structural composition of the dye is essentially responsible for the transmission. Sologhenyl Bordeaux 3 BLE has an elongated, two-dimensional character and has two azo groups (disazo) as part of the chromophore. In addition to the aromatic character, this dye has the feature of a copper complex. Possibly this is responsible for a shift of the red spectrum into the NIR range, which may explain the decreased transmission in the spectral range of 700 nm to 900 nm. Drimaren Yellow HF-CD is a monoazo dye with two reactive anchor groups. The structural formula is not published by the manufacturer and is therefore unknown. In other investigations, the use of dyes with monochlorotriazine anchor groups was described, which positively influenced the transmission in the UV range [72], perhaps similar structures are present here. Although the structure of the dyes shown is quite different, in the case of Sologhenyl Bordeaux 3 BLE it can be stated that the presence of several aromatic structures and a thus more delocalized \(\pi\)-electron system is possibly a criterion for their good protective effect in the UV and IR spectral range. Furthermore, an elongated structure as in Sologhenyl Bordeaux 3 BLE also seems to have a positive effect. In the literature, different approaches are reported to realize textile materials with reduced transmission for UV light and near-infrared light. Prominent examples are based on coatings with pearlescent effect pigments, short basalt fiber, or pigments from indium tin oxide [51; 67; 73]. The application of pearlescent pigments offers good UV protection due to their content of titanium dioxide. Their capability to reduce transmission for near infrared light depends on interference effects and can yield to reduced transmission values of around 25% at 750 nm [51]. The developed Lyocell materials exhibit lower transmission in this spectral range and are therefore advantageous. The application of short basalt fibers with a high concentration of more than 40 wt-% can lead to textile materials with reduced transmission for near-infrared light with transmission values of around 10% [67]. With a view on this low transmission values, these basalt fiber-containing coatings are advantageous compared to the currently developed Lyocell knitted fabrics. However, due to the high content of inorganic basalt fiber in the coating, the textile properties of these coated materials are lost.