# Transmission Reduction for UV and IR Radiation with Dyed Lyocell Knitted Textiles
## 1 Introduction
Textiles for reducing the transmission of IR radiation are primarily found in the field of PPE (personal protective equipment) or military technology [50]. Both inorganic (e.g., lanthanum hexaboride LaB\({}_{6}\), titanium dioxide TiO\({}_{2}\) or zinc oxide ZnO) and organic substances (e.g., benzotriazole, coumarin or curcumin) are used [34; 51; 52]. They can either be applied subsequently as additives or incorporated directly into the fiber by spin doping. The effect of TiO\({}_{2}\) in particular has been proven several times [23; 53]. Dyes usually serve aesthetic aspects and a wide range of dyes allows a multitude of design possibilities. Here, the color depends on the chemical structure of the chromophore and its ability to absorb radiation and transfer electrons into the excited state. In principle, compounds with conjugated double bonds are suitable for this [54]. So-called \"functional dyes\", such as IR dyes used to protect barcodes against forgery [55]. These functional dyes exhibit absorption of radiation in the wavelength range from 700 nm and are preferably not colored. The basic problem in the application for dyes that have transmission-reducing properties in both the UV and IR spectral range is that the absorption and remission properties of the dye components used are dedicated to visible light in the range of 400 nm to 700 nm. For the dyeing of regenerated cellulosic fibers, reactive dyes, and substantive dyes are commonly used [56]. Reactive dyes consist of a coloring chromophore, a solubilizing group, and a reactive group characteristic for this dye class, with which the dye can be covalently bound to the fiber. With the hydroxide groups of cellulose, either a nucleophilic addition, in the case of vinyl sulfones, or a nucleophilic substitution, in the case of aromatic reactive groups, takes place [57; 58]. Monoazo dyes, which have narrow and steep absorption bands due to their small molecular structure, are particularly popular, resulting in very brilliant color shades. Other chromophore systems contain anthraquinone, phthalocyanine, and formazan structural units. Diazo dyes are only rarely used because their greater substantivity has a negative influence on washing-out behavior [59]. Substantive dyes are also named as direct dyes. For these dyes, large-molecular, planar diazo or polyazo chromophores are preferred [60]. The bond between dye and cellulose fibers is based on hydrogen-like bridges between the chromophore of the dye and the hydroxyl groups of the cellulose. As a prerequisite for this, the chromophore must have a planar structure and a high number of conjugated double bonds in order to have a sufficient affinity to the cellulose [61; 62]. In the current investigation, dyestuffs from different dye classes were applied on Lyocell knitted fabrics with the purpose to reduce the transmission of UV light and near-infrared light. The main objective of this research is to realize a textile material offering protection against both UV and near IR light while keeping mainly textile properties such as flexibility or hand feel. To guarantee the properties in terms of use the washing and abrasion stability are investigated. A dependence on the protective effect is determined for the chemical structure of the dye and on the presence of TiO\({}_{2}\) in the fiber material. Finally, the proof-of-concept was made, that the transmission for both UV light and near-infrared light can be decreased by developed Lyocell materials. By this, a first step is completed for the development of clothing materials offering additional UV protection also a certain capability for protection against infrared radiation.