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 retardant: use of FR fighter pilots, tank crews, and astronauts.

52 Lenzinger Berichte 89 (2011) 50 – 59

The meta–aramid nonwovens are also used study the energy and mass changes of for hot gas filtration and thermal various FR fibers during temperature insulation. Para–aramid fibers are used for increase. The use of these FR fibers in ballistic and flame protection. Generally, clothing requires sufficient physiological meta–aramides are used in heat protective comfort. Therefore, FR fibers have to clothing. However, para–aramid fibers are provide good sorption properties. added to improve the mechanical stability Additionally to the thermal study, moisture [1]. sorption isotherms and WRV (water Meta–aramid fibers are blended with retention value) measurements were done Lenzing FR® and FR–treated wool. on FR samples to have a notion about the physiological behavior of FR fibers in use. Polyimide By SEM, the visualization of fiber residue Another aromatic copolyamide fiber is after burning was conducted. polyimide (brand name P84®; produced by Evonik Industries). The fiber does not melt Experimental but becomes carbonized at 500 °C with LOI value 36–38%. The application of Materials high–performance P84® fibers include protective clothing, as a sealing or packing In this work, thermal behavior of Lenzing material, for hot gas filtration and in FR® fibers was compared with flame– aviation and space including aircraft seats retardant polymers. The characteristics of [1]. used samples are given in Table 2.

Phenolics Table 2. Fibres used for thermal and sorption Melamine polymer is obtained by study. condensation reaction between melamine and formaldehyde. The cross–linked Fibre Type Characteristics polymer is then spun to melamine fibers. Titer [dtex] / They can be used in continuous service at 200 °C. Above 370 °C, thermal Length [mm] degradation result in char formation rather Lenzing FR® 2.2 / 51 then molten drip [1]. Lenzing Modal® 1.3 / 38 Modacrylic TENCEL® 1.3 / 38 FR modacrylic fiber is a copolymer of acrylonitrile, vinyl–chloride or vinyliden– Meta–aramid 1.7 / 50 chloride in ratio of 60:40 wt% along with a Para–aramid Fabric sulphonated vinyl monomer [1]. Modacrylic 2.8 / 38 Aim of our work Polyimide 2.2 / 53 DSC and TGA are important means to Melamine 2.2 / 90 study thermal properties of textile Flame–retardant 7.0 / 60 materials, fabrics and fibers. However, DSC spectra provide changes of polyester temperature of the material in pyrolysis and the changes of heat release, but no other thermal properties. TGA can show Methods weight losses of fibers and fabrics under different temperature, but not the concrete Thermal analysis pyrolysis processes and products. In this Thermogravimetric Analysis (TGA) study, DSC and TGA were employed to Approximately 20 mg of compressed fibers rolled in a small ball were placed in 53 Lenzinger Berichte 89 (2011) 50 – 59 the sample pan. The temperature TIIL–PRÜFANWEISUNG 092/00. The programme was set from 23 °C until 600 fiber sample (0.35 g) was weighed in the °C with a heating rate 10 Kmin-1. All centrifuge glass and wetted for 10 minutes measurements were done under air in deionized water (15 mL). After that, the atmosphere to get more realistic results sample was centrifuged for 15 minutes at since oxygen is needed to burn the fibers. 3000 rpm and weighed immediately (mw). Before measurements all fibers were Then the samples were dried for 24 h at carded to ensure maximum homogeneity 105 °C and weighed again (md). The WRV of the fibers and better reproducibility of was calculated according to Equation (1). the results. A TA instruments TGA 2050 The value expresses the water amount held was used. in the pores inside the fibers. m  m Differential Scanning Calorimetry (DSC) WRV % w d 100 10–12 mg of fibers were compressed and md placed in the sample pan. Four holes were (Eq. 1) made in the sample lid to allow the off– gases to escape. Heating rate was set at 10 Visualization of burned fibers Kmin-1. For these experiments, standard Lenzing FR®, modacrylic, FR polyester pan and standard lids were used (pan and meta–aramid fibers were burned and 900786.901; lid 900779.901). Preparation viewed under SEM. The samples were of samples for DSC is more sensitive than Au–coated for 60 s. A Hitachi S–400 SEM for TGA and more attention had to be paid was used with a relatively low accelerating to the sample preparation. Two parallel voltage of 5 kV to prevent beam damage measurements were done and the average of the samples. value was calculated. An untreated sample was also measured in parallel in order to Results and discussion investigate the effect of the incorporated polymers on the temperature stability. Thermal analysis Mass change by TGA Sorption study TGA is a basic method for characterization Dynamic Sorption Analysis of flame–retardant materials under high– Moisture sorption measurements were temperature environments. By the first done on dynamic sorption analyzer (SPS derivation of the mass change curve it is 11 Analyser, Projekt–Messtechnik, D– possible to determine the temperature at Ulm). The measurements of the fiber which the degradation rate is the highest samples by dynamic sorption analyzer (the peak of the first derivation). In this were described in detail by Okubayashi et work this temperature is defined as al. [6]. In brief, the atmosphere in the degradation point [7]. analyzer was conditioned at 25 °C and 0 % The TGA plots of examinated fibers are relative humidity (RH) until equilibrium shown in Figure 1, Figure 2 and Figure 3. was achieved. Then, the moisture cycle Plots of standard and modified cellulosics was started increasing the RH in 10 % RH are summarized in Figure 1. There are no steps (interval method). The maximal differences in degradation point between achieved RH was 90 % for measurements TENCEL® and Lenzing Modal® (305 °C). at 25 °C. At 450 °C the whole cellulose is burned. Very good results in heat protection show Water Retention Value Lenzing FR® fibers. The thermal The WRV value denotes the amount of decomposition of Lenzing FR® fibers water held inside the fiber after wetting occurs in two steps: the first point is at 250 and centrifuging. The measurements were °C (most of the FR–agent reacts) and the performed following a Lenzing protocol second degradation point is 458 °C. At the

54 Lenzinger Berichte 89 (2011) 50 – 59 temperature 250 °C to 700 °C, the material the garment fabric helping to eliminate air degrades slowly with a residue of 8% movement and heat transfer to the interior (w/w) at 700 °C. skin area. As both the fiber and the fabric thicken together this increases the

® 100 Lenzing FR insulating barrier and therefore reduces TENCEL® heat transfer to the wearer. ® 80 Lenzing Modal Polyimide (brand name P84®) is more ]

% Degradation

[ stable then meta–aramid and FR polyester. ® s 60

s Lenzing FR = 250 °C a ® It degrades at 510 °C.

M TENCEL = 305 °C

e ®

v 40

i Lenzing Modal = 305 °C t a l e

R 20 100 Para-aramid Meta-aramid AIR Polyimide

0 ] 80 FR polyester 100 200 300 400 500 600 700 % [

s s Temperature [°C] a 60 M

Figure 1. TGA graph of cellulosics. e Degradation v i t Para-aramid=521 °C a

l 40 Phosphorous containing FR agents e Meta-aramid=476 °C ® R Polyimide=510 °C incorporated in Lenzing FR fibers work 20 FR polyester=431 °C/517 °C according to following principle: during AIR burning, a protective layer is formed on the 0 100 200 300 400 500 600 700 surface of fabric by the production of Temperature [°C] polyphosphoric acid and carbonization, e.g. by release of water. These P– Figure 2. TGA plots of synthetic FR fibers. containing FR agents have to decompose TGA plots of other high–performance before the cellulose decomposes. If the synthetic fibers melamine and modacrylic decomposition of cellulose occurs at 305 are shown in Fig. 3. Both, melamine and °C (derived from TGA curves), then the P– modacrylic fibers degrade in two major agents have to decompose below this steps. Modacrylic is less stable then temperature to form a protective layer [8]. melamine. Modacrylic degrades at 236 °C In Figure 2 the traces of synthetic (first point) and 503 °C (second point). polymers show significantly higher The two–step degradation of this polymer degradation points then measured for is explained by the formation of acrylic cellulosics. The role of high–performance and modacrylic (halogen containing) polymers is not to react before the basic components in a ratio approx. 60:40 wt%. material (like in the case of cellulose), then Halogens contained in modacrylic work on to postpone the melting point to higher the principle of radical reaction. The temperatures. The heat energy is spent on acrylonitrile component works on the melting of polymer or on radical reaction principle of char formation. Melamine also like in FR polyester. The tested samples degrades in two steps: 363 °C and 617 °C. degrade at two major points: 431 °C and The thermal degradation in melamine 517 °C which indicate that various results in char formation rather then modifications are present in the polymer. molten drip. Para–aramid degrades at 521 °C. Meta–aramid, due to the meta–position of aromatic rings is less stable then para– aramid. Meta–aramid degrades at 476 °C. The physical action of the meta–aramid fiber when exposed to a heat source is that the fiber itself absorbs heat energy during the carbonization process. The fiber swells and thickens in size and seals openings in

55 Lenzinger Berichte 89 (2011) 50 – 59

100 Melamine 0 Modacrylic -2 -4 80 ] ] 1 - g

% -6 [

W s [ Para-aramid

s 60 -8 a w Meta-aramid o -10 l M

F

e Polyimid

t v -12 i a

t 40 e

a Degradation

l -14 H e Melamine=363 °C / 617 °C

R -16 20 Modacrylic=236 °C / 503 °C -18 Endo Up AIR -20 0 100 200 300 400 500 600 100 200 300 400 500 600 700 Temperature [°C] Temperature [OC] Figure 3. TGA plots of melamine and modacrylic Figure 5. DSC Plots of synthetic FR fibers. fibers. The DSC traces of melamine and Energy changes by DSC modacrylic show first smaller peaks The heat released or absorbed during the (where char is formed) and the reaction is thermal conversion is measured by DSC. finished (Figure 6). The second part of the thermal degradation is melting. Like in 2 0 other high–performance fibers, the melting -2 of melamine occurs at the beginning of the -4 ]

1 reaction. -

g -6 W [

-8 w

o -10 ® l Lenzing FR 0 F

t -12 ®

a TENCEL e -14 ® -2 H Lenzing Modal

-16 ]

1 -4 -

-18 Endo Up g

W -6 [

-20 Melamin 100 200 300 400 500 600 w -8 o l Modacrylic F Temperature [°C] t -10 a e

Figure 4. DSC Plots of cellulosics. H -12 In Figure 4, DSC traces of cellulosics are -14 Endo Up -16 shown. At the beginning of the 100 200 300 400 500 600 measurement, there is a small endothermic Temperature [OC] peak due to the water evaporation. Both standard and Lenzing FR® show Figure 6. DSC plots of melamine and exothermic peaks while those for Lenzing modacrylic fibers. FR® are significantly smaller and narrower If we compare TGA and DSC data, the due to the phosphorous/sulfur based correlation of the degradation points of the agents. temperatures show exact fitting. The DSC plots of high–temperature polymers combination of these two analytical are shown in Figure 5. Exothermic peaks methods provides a very useful tool in are not measured at these samples, which fiber characterization and product indicate, that the exothermic reaction development. which started at 350 °C is still ongoing at 600 °C. Sorption study Water vapor sorption Dynamic sorption analysis is a very precise method for material characterization. It covers the whole range

56 Lenzinger Berichte 89 (2011) 50 – 59 of relative humidities, where the 50–70 % the materials (Table 3 and Table 4). As RH range is relevant for physiological TENCEL® is more porous then Lenzing comfort. So, the comparison of the Modal®, the WRV is higher. The values materials in this paper focuses on this decrease slightly with incorporation of FR humidity range. All isotherms show particles. Like in vapor sorption, WRV sigmoid–shaped curves, which are typical values for synthetics are significantly for hygroscopic materials. In Figure 7, lower. The sorption is in this case higher TENCEL® and Lenzing Modal® show the for melamine and meta–aramid fibers. highest sorption (9 – 13 %) where the sorption capacity of Lenzing FR® is Table 3. WRV of standard and modified slightly lower due to the incorporated cellulosics. additives (7 – 11%). The isotherms of Fiber WRV [%] synthetic fibers are rather linear (Figure 8). Lenzing FR® 48.6 ± 0.16 In the humidity range 50 – 70 %, meta– TENCEL® 67.1 ± 0.51 aramid and melamine absorb 5 – 6% H2O. Lenzing Modal® 57.6 ± 0.95 Para–aramid and polyimide absorb 2 – 3 % and the lowest sorption is measured for modacrylic – only 0.5 to 1%. Table 4. WRV of synthetic FR fibers.

25 Lenzing FR® Fiber WRV [%] Lenzing Modal® Para–aramid 5.9 ± 0.19 20 TENCEL®

] Meta–aramid 12.1 ± 0.80 %

[ Polyimide 5.6 ± 0.26

n 15

o Melamine 11.8 ± 0.23 i t p

r Modacrylic 7.3 ± 0.17 o

S 10

e r u t From this results it can be concluded, that s i 5 o the application of hygroscopic FR fibers in M protective clothing is crucial to provide 0 0 10 20 30 40 50 60 70 80 90 100 good wear comfort of protective textiles. RH [%] Visualization of burned fibers Figure 7. Sorption isotherms of cellulosics.

25 Para-aramid SEM images of burned fibers support the Meta-aramid theory about the decomposition ] 20 Polyimide % mechanism of various FR fibers. In Figure [ Melamine n

o 9 images of five mainly used protective i Modacrylic t 15 ® p

r fibers are shown. Lenzing FR keeps o S

e 10 shape after burning. A carbon–layer is r u

t formed on the surface and prevents the s i o 5 fibers from further burning. Modacrylic M fibers shrink in the contact with flame and 0 tear afterwards. FR polyester melts during 0 10 20 30 40 50 60 70 80 90 100 heat exposure, which is not an ideal option RH [%] since molten fibers can stick to the skin Figure 8. Sorption isotherms of synthetic FR fibres. and skin burns are possible. Meta–aramid fibers expand during the thermal treatment, Water retention values (WRV) (volume increase) and form a protective Parallel to the vapor sorption, sorption of barrier which prevents from further liquid water is measured for textile burning of the fiber. materials which refers to the porosity of

57 Lenzinger Berichte 89 (2011) 50 – 59

Figure 9. SEM images of FR fibers before and after burning.

Lenzing FR® before burning after burning

10 µm 10 µm

Modacrylic before burning after burning

20 µm 70 µm

FR polyester before burning after burning

300 µm 20 µm

Meta–aramid before burning after burning

30 µm 20 µm

58 Lenzinger Berichte 89 (2011) 50 – 59

Conclusions and outlook [3] Horrocks A. R., Hall M. E. & Roberts D: Environmental consequences of using In this work, Lenzing FR® fibers were flame–retardant textiles – a simple life characterized by different analytical cycle analytical model, Fire and Materials methods and their behavior in practical use 21 (1997), pp. 229–234. has been investigated in comparison to other protective fibers. [4] Thermal methods – DSC and TGA are http://www.lenzing.com/fasern/lenzing- precise tools for observing thermal fr.html degradation – mass / energy changes with increasing temperature. The results of both [5] Rüf H., Firgo H. & Kroner G.: thermal methods showed good correlation. Cellulosic Molded Body, Method for Improved physiological performance of Manufacturing it and Use Thereof, US protective clothing is important and the 2008/0233821 A1, 2008. standards for protective clothing require materials with improved comfort properties. [6] Okubyashi S., Griesser U.J. & Therefore, new developments in this Bechtold T.: A kinetic study of moisture segment are crucial to be at the state of the sorption and desorption on lyocell fibres, art with the market trends. Carbohydrate Polymers 58 (2004), pp. The disadvantage of using synthetic fibers is 293–299. associated with poor wear comfort. The [7] Aljaz L., Varga K. & Koch T.: DSC & emphasis is placed on the use of Lenzing ® th FR® in workwear as best solution between TGA Analysis of Lenzing FR Fibres, 41 functionality and comfort. This statement is International Symposium on Novelties in supported by the sorption isotherms showing Textiles, 27–29 May 2010, Ljubljana, high absorbency for cellulosics and low Slovenia. absorbency for synthetics. [8] Einsele U.: Über Wirkungsweise und In the field of cellulosic materials, newest synergistische Effekte bei developments have been focused on spun– ® Flammschutzmittel für Chemiefasern, dyed Lenzing FR , especially in high– Lenzinger Berichte 40 (1976), pp. 102– visibility colors for protective wear. 116. Acknowledgements

Authors gratefully acknowledge to Ms. Sandra Schlader for SEM images and to Ms. Agnes Uwaleke for WRV data.

References

[1] Bajaj P.: Heat and Flame Protection, in Handbook of Technical Textiles (Ed. Horrocks R. A. & Anand, S.C.), Woodhead Publishing Limited (2000), ISBN 1 85573 385 4.

[2] Hull T. R. & Kandola B. K. (Ed.): Fire Retardancy of Polymers – New Strategies & Mechanisms, © Royal Society of Chemistry 2009, ISBN 978–0–85404–149–7.

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