Thermal and Sorption Study of Flame–Resistant Fibers
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Lenzinger Berichte 89 (2011) 50 – 59 THERMAL AND SORPTION STUDY OF FLAME–RESISTANT FIBERS Ksenija Varga1,2, Michael F. Noisternig3, Ulrich J. Griesser3, Linda Aljaž4 & Thomas Koch5 1Christian–Doppler Laboratory for Chemistry of Cellulosic Fibers & Textiles, University of Innsbruck 2Lenzing AG, Fiber Science Group, Innovation & Business Development Textiles, A–4860 Lenzing, Phone:+43 7672 701 2757; Fax:+43 7672 918 2757; E–Mail: [email protected] 3Institute of Pharmacy, Pharmaceutical Technology, University of Innsbruck 4Faculty of Natural Sciences & Engineering, University of Ljubljana, Slovenia 5Institute of Materials Science and Technology, Vienna University of Technology The characterization of FR fibers and release during decomposition while fabrics is of crucial importance giving TGA show weight loss of the tested the main accent to the thermal behavior materials, but no specific pyrolysis of of materials in practical use. Since FR the products. Dynamic sorption analysis fabrics are often made from a very in broad humidity range show high limited number of materials, it is sorption of cellulosic Lenzing FR® and important to use fibers which provide lower for synthetic materials. good physiological comfort. Lenzing Visualization of the residues of FR® fibers are characterized by thermal combusted materials by SEM confirms and sorption methods in comparison the theory of different decomposition with other flame–resistant materials. mechanisms during heat exposure. DSC and TGA are important means to study thermal properties of textiles. Keywords: Lenzing FR®, FR fibers, DSC spectra provide changes of heat protective wear __________________________________________ Introduction Safety of human beings has been an In the process of accomplishing flame important issue all the time. A growing protection, however, the garment may be segment of the industrial textiles industry so thermally insulative and water vapor has therefore been involved in a number of impermeable that the wearer may begin to new developments in fibers, fabrics and suffer discomfort and heat stress. Body protective clothing. Major innovations in temperature may rise and the wearer may the development of heat–resistant fibers become wet by sweat. Attempts have and flame–protective clothing for fire- therefore been made to develop thermal fighters, foundry workers, military and and flame protective clothing which can be space personnel, and for other industrial worn without any discomfort [1, 2]. workers who are exposed to hazardous This paper provides an overview of mostly conditions have been carried out. For heat used fibers for thermal and flame and flame protection, requirements range protection. The thermal behavior of such from clothing for situations in which the fibers is studied by using following wearer may be subjected to occasional thermal methods: thermogravimetry exposure to a moderate level of radiant (TGA) and differential scanning heat as part of normal working day, to calorimetry (DSC). Scanning electron clothing for prolonged protection, where microscopy (SEM) images showed the the wearer is subjected to severe radiant residues of the fibers after combustion. and convective heat to direct flame. Sorption study highlights the importance 50 Lenzinger Berichte 89 (2011) 50 – 59 of using cellulosic flame–retardant fibers expected to support the wearer’s work; in the protective fiber blends. they should not shrink or melt, and if then decompose, from char. These requirements Thermal behavior of fibers cannot be met by thermoplastic fibers and The effect of heat on textile materials can so resource must be made from aramid produce physical and chemical changes. fibers, flame–resistant cellulosics or wool. The physical changes occur at the glass It may also be noted that the aramid fibers transition (Tg ) , and melting temperature with their high limited oxygen index (LOI) and high thermal stability, have not been (T ) in thermoplastic fibers. The chemical m found suitable for preventing skin burns in changes take place at pyrolysis molten metal splashes because of their temperature (Tp ) at where thermal high thermal conductivity. The mode of degradation occurs. The combustion is a decomposition and the nature of the complex process that involves heating, decomposition products (solid, liquid and decomposition leading to gasification (fuel gaseous) depend on the chemical nature of generation), ignition and flame the fiber, and also on the type of applied propagation. finishes or coatings. If the decomposition A self–sustaining flame requires a fuel products are flammable, the atmospheric source and a means of gasifying the fuel, oxygen raises ignition, with or without after which it must be mixed with oxygen flame. When the heat evolved is higher and heat. When a fiber is subjected to heat, then that required for thermal decomposition, it can spread the ignition to it pyrolyses at Tp and volatile liquids and gases, which are combustible, act as fuels the total material destruction. for further combustion. After pyrolysis, if In addition to the fiber characteristics and the temperature is equal or greater than fabric finish, several garment characteristics also influence the thermal combustion temperature T , flammable c protection. For a given fabric thickness, volatile liquids burn in the presence of the lower the density, the greater is the oxygen to give products such as carbon thermal resistance. This applies on char– dioxide and water. When a textile is forming fibers such cellulosics and wool. ignited heat from an external source raises Hence, thicker fabric made from its temperature until it degrades. The rate cellulosics or wool or other non–melting of this initial rise in temperature depends fibers give good thermal protection, on: the specific heat of the fiber, its whereas the thicker thermoplastic fibers– thermal conductivity and also the latent fabric produce burns. Thermal properties heat of fusion (for melting fibers) and the of some fibers are given in Table 1 [1]. heat of pyrolysis. Concerning environmental point of view, In protective clothing, it is desirable to Horrocks et al. developed an analytical have a low propensity for ignition from a model for understanding the environmental flaming source or, if the item ignites, a consequences of using flame–retardant slow fire spread with low heat output textiles. An environmental rank value is would be ideal. In general, thermoplastic given at each stage in the manufacturing fibers such as polyamide or polyester process and product life of each flame– fulfill these requirements because they retardant fibers and fabrics. The results shrink from the flame. If they burn, they show that each of the eleven generic fibers burn with a small and slowly spreading analyzed showed an environmental index flame and ablate. value within the range of 32–51% where For protective clothing, however, there are 100 denotes the worst environmental additional requirements, such as protection position possible [3]. against heat by providing an insulation, as well as high dimensional stability of fabrics. Upon heat exposure, they are 51 Lenzinger Berichte 89 (2011) 50 – 59 Table 1. Thermal and flame–retardant properties of some fibres [1]. Fibre Tg [ C ] Tm [ C ] Tp [ C ] Tc [ C ] LOI Glass Melt Pyrolysis Combustion [ % ] transition Viscose (CV) - - 350 420 18.9 Cotton (CO) - - 350 350 18.4 Wool (WO) - - 245 600 25 Polyester (PES) 80-90 255 420 –477 480 20–21.5 Acrylic (PAN) 100 >320 290 >250 18.2 Modacrylic (MAC) <80 >240 273 690 29–30 Meta–aramid (AR) 275 375 310 500 28.5–30 Para–aramid (AR) 340 560 590 >550 29 Materials for flame protection co-monomers during co–polymerization, introduction of an FR additive during Flame Resistance Cellulosics extrusion and application of flame Inherently flame–resistant viscose fibers retardant finishes or coating. Additives and are produced by incorporating FR co–monomers are halogen or phosphorus– additives (fillers) in the spinning dope based [1]. before extrusion. For example: phosphorous additives, polysilicic acid or Flame-retardant Acrylic polysilicic acid and aluminum [1]. Like other synthetic fibers, acrylic fibers shrink when heated, which can decrease Lenzing AG currently produces Lenzing the possibility of accidental ignition. FR® which contains phosphorus / sulfur– However, once ignited, they burn containing additives [4]. vigorously accompanied by black smoke. Halogen–based and particularly bromine Rüf et al. patented a new procedure for derivatives or halogen– or phosphorus– producing TENCEL® fibers with reduced containing co–monomers, are the most flammability. The invention relates to a effective flame retardants used in acrylic cellulosic molded body containing a fibers. A number of spinning dope cellulose / clay nanocomposite. The clay additives are also known to render acrylic component of said nanocomposite fibers flame retardant, for example esters comprises a material selected from the or antimony, tin and their oxides, SiO2, group consisting of unmodified hectorite halogenated paraffins, halogenated clays and hydrophilically modified aromatic compounds and phosphorus hectorite clays [5]. compounds [1]. There are also viscose fibers with incorporated polysilicic acid available on Aramids the market [1]. Aromatic polyamides char above 400 °C and can be exposed to temperatures up to Flame-retardant Polyester 700 °C. Meta–aramid fibers have been There are three methods of rendering developed for protective clothing for synthetic fibers flame