Regenerated cellulose fibers with reduced flammability can be obtained by incorporation of silica into viscose fibers. Existing technologies can be applied for the textile mechanical and textile chemical processing. Besides the special functionality textiles made from such fibers exhibit the good wear comfort which is typical for viscose fibers.
Safety concerns have led to specifications for many textile applications with regard to reduced flammability during use. Frequently fiber modifications are achieved with flame-retardant finishes. The global production of about 3 million tons/year of such chemicals leads to a substantial environmental burden during production, application and disposal of the treated products [1, 2]. Flame-retardant high-performance fibers represent another concept for textiles with reduced flammability, however the moderate wear comfort restricts their use to products for special purposes e.g. fire fighters, blast furnace workers and technical products.
Cellulosic fibers can be imparted with flame-retardant properties by special textile finishing operations (e.g. cotton) or by incorporation of flame-retardant chemicals during fiber production (e.g. viscose FR). The impact of heat leads to formation of an intumescent structure as a consequence of reactions between the finishing chemicals and the cellulose polymer. These reduce access of oxygen to the site of combustion and lower the heat flux from the site of combustion to the surface of the material. Timely products have to match with requirements of sustainability and minimal environmental impact during production, use and disposal in addition to the requests of reduced flammability. Additionally, products which are worn next to the skin should provide a pleasant wear comfort and must not release potentially irritating chemicals to the skin.
Cellulose-silica hybrid fibers
A new product concept to impart cellulosic fibers with flame-retardant properties utilizes the incorporation of silica during the viscose fiber process into the fiber matrix . Silica easily dissolves in alkaline solutions, thus a homogenous solution can be achieved in the alkaline xanthogenate spin dope. During the process of cellulosic fiber regeneration in the acidic precipitation bath also silica becomes insoluble, precipitates and remains entrapped in the viscose fiber matrix. A general scheme of the silica incorporation during cellulosic fiber production is shown in Fig. 1. As a result, a hybrid fiber consisting of cellulose and silica is obtained, which can contain up to 20 % of the inorganic polymer.
In case of fire, the cellulose matrix decomposes. However, the thermally stable silica structure remains, and this retards and interrupts the combustion process. The porous inorganic structure reduces the access of oxygen to site of combustion and also reduces heat transfer through infrared irradiation to the textile material.
Microphotograph of a reactive dyed knitted fabric from 100 % Danufil BF after burn testing, showing the remaining silica structure, the fire limit and the intact fabric (Source: Kelheim Fibres)
In Fig. 2 a photomicrograph of a knitted fabric after a burn test is shown. The orange-brown stripe indicates the fire limit, where the blue reactive dye has been thermally decomposed. The remaining fabric structure consists of silica and carbon residues.
An incorporation of silica up to 20 wt.% of the total fiber mass can be achieved without unacceptably high losses in fiber strength. The LOI (Limiting Oxygen Index) then increases up to 27-30 % O2, with the exact value being dependent on the respective material construction and density. Representative values for fiber tenacity have been determined with 14-18 cN/tex and for the elongation with 20-24 %. The fiber properties permit good spinnability also for yarn which consists to 100 % of the new hybrid fiber.
As expected for regenerated cellulose fibers moisture sorption is at a high level (9-13 wt.%) which forms the basis for good wear comfort and the ability to regulate body climate conditions. The soft hand makes the fibers interesting for products which are worn next to the skin. As no special functional chemicals are released from the material, the fibers also are of interest for bedding textiles.
For an assessment of the ignitability of bedding items and the classification of the burning behavior of such products ignition, experiments are simulated in a model-setup (EN ISO 12952-1 /EN 14533). A smoldering cigarette serves as ignition source and the burning and afterglowing are observed under standardized experimental conditions (Fig. 3a). Neither burning nor afterglowing were observed with samples manufactured from the new hybrid fiber, while pronounced afterglowing and sample destruction over longer distances occurred with normal regenerated cellulosic fibers.
Products with direct contact to skin
The introduction of bedding textiles with reduced flammability is of interest for care homes and hospitals. As the reduced flammability has not been achieved through chemical finishing, there will be no risks of a release of chemicals from the bedding to the skin, which then could cause allergic reactions. Cellulose fibers are not thermoplastic, thus no melting or formation of molten polymer residues will occur upon impact of heat.
Very low flammability of products can be achieved when fiber blends with modacrylic fibers are used. A fabric containing 50:50 modified cellulosic fiber and modacrylic fibers is able to pass the vertical flame test for flammability. Technical development activities for reduction of the modacrylic content in the fiber blend are part of the ongoing research.
The incorporation of silica into the cellulose matrix does not lead to changes in the coloristic behavior of the material, which can be dyed with reactive dyes. A textile chemical treatment of the fibers with aluminate leads to formation of insoluble aluminosilicate inside the fibers, which contributes to the wash permanency of the incorporated silica. During multiple wash operations in alkaline wash baths a reduction of the silica content in the fibers can occur, thus washing at reduced alkalinity is recommended. The durability of the ashes content in the fiber during multiple washing operations is shown in Fig. 3b.
Left: Fabric sample manufactured from 100 % hybrid fiber after ignition tests with smoldering cigarette Right: Ashes content over 25 washing operations in a commercial household washing machine (Source: Kelheim Fibres) Dyeing properties
In a favorable process variant, the continuous cold pad batch dyeing process is combined with the aluminate treatment. In this case no additional processing steps have to be introduced for the reactive dyeing of fabric made from the hybrid fiber. The technology already has successfully reached production scale level. The dyeing results with the modified fibers demonstrate that the high level of wash and rub-fastness of the dyeing remain preserved. The incorporation of silica into regenerated cellulosic fibers thus does not reduce the fastness level of the reactive dyes tested.
Regenerated cellulosic fibers are sustainable products and will take an important role for a future circularity of all cellulose-based textile products, including cotton. Textile products will have to consider the requirements of the current EU legislation for a circular textile economy. An important aspect of any new product development then will be the appropriate design for recycling, which has to include the fate of a product after use either in material recycling or composting. Problems can arise from the presence of high amounts of finishing chemicals both during cellulose recycling and during biodegradation. In case of the silica-cellulose hybrid fibers fiber recycling and biodegradation could be possible at the end of the product life. For the production of the inorganic silica, no petrol-based raw materials are required as feedstock.
The modified viscose fibers represent a new generation of functional fibers for garment, home textiles and technical applications which are entering the market and are able to combine reduced flammability with aspects of sustainability and circularity.
Danufil = registered trademarkAcknowledgements
This research was financially supported by the Austrian research promotion agency K-Project Textile Competence Center Vorarlberg tccv2 (882502).References
 Yasin, S.; Curti, M.; Rovero, G.; Behary, N.; Perwuelz, A.; Giraud, S.; Migliavacca, G.; Chen, G.; Guan, J.: An alternative for the end-of-life phase of flame-retardant textile products: Degradation of flame retardant and preliminary settings of energy valorization by gasification, BioResources 12 (2017) 5196-5211
 Samani, P.; van der Meer, Y.: Life cycle assessment (LCA) studies on flame retardants: A systematic review, J. Clean. Prod. 274 (2020) 123259
 Paul, B.; Mahmud-Ali, A.; Lenninger, M.; Eberle, S.; Bernt, I.; Mayer, D.; Bechtold, T.: Silica incorporated cellulose fibres as green concept for textiles with reduced flammability, Polym. Degrad. Stab. 195 (2022) 109808
Kelheim Fibres GmbH, Kelheim/Germany
Research Institute of Textile Chemistry and Textile Physics, University of Innsbruck/Austria
This Article was published in Technical Textiles 5/2022