Comprehensive Study of Thermal and Fire Behavior of para-Aramid

Chapter 6

Comprehensive Study of Thermal and Fire Behavior of para-Aramid and Polybenzazole Fibers 1

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Xavier Flambard , Serge Bourbigot *, Sophie Duquesne , and Franck Poutch 3

Downloaded by EMORY UNIV on August 13, 2015 | http://pubs.acs.org Publication Date: September 15, 2001 | doi: 10.1021/bk-2001-0797.ch006

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Laboratoire de Génie et Matériaux Textiles (GEMTEX), UPRES EA2161, Ecole Nationale Supérieure des Arts et Industries Textiles (ENSAIT), BP 30329, 59056 Roubaix Cedex 01, France Laboratoire de Génie des Procédés d'Interactions Fluides Réactifs-Matériaux (GEPIFREM), UPRES EA2698, Ecole Nationale Supérieure de Chimie de Lille (ENSCL), BP 108, 59652 Villeneuve d'Ascq Cedex, France Centre de Recherche et d'Etude sur les Procédés d'Ignifugation des Matériaux (CREPIM), Parc de la Porte Nord, Rue Christophe Colomb, 62700 Bruay-la-Buissière, France 2

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In this work, we study the thermal and fire degradation of poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers in comparison with traditional p-aramide (PPT) fibers. The superior performance of P B O in comparison with PPT fibers in terms of heat resistance and flame retardancy is demonstrated. During the degradation of the fibers in a well-ventilated room, only C O , C O and H O are observed in the gas phase. In the condensed phase, PPT and P B O fibers decompose to polyaromatic compounds which are able to trap free radicals. 2

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© 2001 American Chemical Society

In Fire and Polymers; Nelson, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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Introduction


The field of high performance fibers has witnessed considerable growth in the last three decades (1). A large number of high performance polymeric fibers are on the market today and they exhibit many enhanced properties in comparison with the traditional polymeric materials, such as high mechanical properties, heat resistance or good flame retardancy. Their applications are numerous (fragmentation barrier, protective clothing, heat barrier, ...). Poly(pphenylene-2,6-benzobisoxazole) (PBO) registered under the trademark Zylon® is a new high performance fiber (1). It is a polybenzazole containing an aromatic hetero-cyclic ring (Figure 1). It is a rigid rod isotropic crystal polymer.

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> Figure 1. Repeat unit of PBO

The polybenzazoles have been developed by U S A i r Force researchers as super heat resistant polymer which could surpass the traditional aramide fibers. P B O has superior tensile strength and modulus compared to the classical paramide fibers (poly-p-phenylenediamine-terephtalamide fibers (PPT)). In our laboratory, we have recently shown that this fiber, as a knitted fabric, has excellent properties in cutting and stab resistance (3). The combination of various knitted layers gives exceptional results with P B O fibers (stab-resistance of 25 Joules for a textile structure of 3 kg/m with an English blade). P B O has good flame resistance and thermal stability among organic fibers and, in particular, in comparison with p-aramide fibers. A s an example, the limiting oxygen index (LOI measured according to N F G 07-128) of P B O is 68 vol.-% (1) whereas that of p-aramide fibers is only 30 vol.-% (4). The L O I test is not very representative of a fire, even i f it enables one to quantitatively rate the materials. The approach that is developed to evaluate fibers as knitted fabric is to use the cone calorimeter as fire model. In this paper, we will compare and will discuss the thermal and fire behavior of P B O fibers in comparison with PPT. The gas phase will be studied 2


65 during the fire degradation using FTIR and the composition of the materials in the condensed phase will be investigated using solid state C N M R . 13


Experimental P B O fibers were supplied by Toyobo (Japan) and is registered under the trademark Zylon®. PPT fibers are classical Kevlar®. The yarns used in this study have the following characteristics: PBO : Ne 20/2 (Nm 2/34 - 60 Tex) spun yarn PPT : Ne 16/2 (Nm 2/28 - 70 Tex) spun yarn P B O and PPT fibers have been knitted on an automatic flat machine gauge E7. The texture used is a double woven rib. The two samples have a surface weight equaling 1.08 kg/m (4 yarns knitted together in the case of PPT and 5 yarns in the case of PBO). T G analyses were performed using a Setaram M T B 10-8 thermobalance at 10 °C / m i n from20°C to 1200°C under air flow (Air Liquide grade, 5 x l 0 m /s measured in standard conditions) and under nitrogen flow (N45 A i r Liquide grade, 5xl0" m /s measured in standard conditions) . Samples (about 10 mg) were placed in vitreous silica pans; precision on temperature measurements is ±1.5°C. The Stanton Redcroft Cone Calorimeter was used to carry out measurements on samples following the procedure defined in A S T M 1354-90. The standard procedure used involves exposing specimens measuring 100 mm χ 100 mm χ 2.5 mm thick in the horizontal orientation. External heat fluxes of 50, 75 and 100 k W / m have been used; these fluxes have been chosen because 50 k W / m is common heat flux in mild fire scenario and 75 & 100 k W / m represents postflashover conditions (5). When measured, R H R (rate of heat release) and VSP (volume of smoke production (6)) values are reproducible to within ±10%. The results presented in the following are averages; the cone data reported in this paper is the average of three replicated experiments. The weight loss of the samples during combustion was not recorded, because of the low mass of the textile. The curves were too noisy to be used and yields of C O and C 0 and the heats of combustion could not be computed. Sample of decomposition gases were then taken from an exhaust pipe of the cone calorimeter continuously pumped with a capacity of 8 liters/minutes and analyzed by FTIR. A smoke filter removes soot particles from the gas sample before it reaches the FTIR analyzer. The sampling line, approximately 4 meter long, is maintained at about 183°C in order to avoid condensation. The FTIR spectrophotometer (Nicolet 710C) was placed on-line and it has enabled the continuous detection of gas components. The gas cell had a volume of 2

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approximately 750 cm , a 3.77 meter path length and was operated at 650 torr. The FTIR was set to generate one spectrum every 10 sec. C N M R measurements were performed on a Brucker ASX100 at 25.2 M H z (2.35j T ^ w i t h magic angle spinning (MAS), high power H decoupling (DD) and H - C cross polarization (CP) using a 7 mm probe. The HartmannHahn matching condition was obtained by adjusting the power on *H channel for a maximum C FID signal of adamantane. A l l spectra were acquired with contact times of 1 ms. A repetition time of 10 s was used for all samples. The reference used was tetramethylsilane and the spinning speed was 5000 H z . The values of chemical shift are verified using adamantane and adamantanone before starting a new experiment. For these products, the chemical shifts were within ± 0.2 ppm. Downloaded by EMORY UNIV on August 13, 2015 | http://pubs.acs.org Publication Date: September 15, 2001 | doi: 10.1021/bk-2001-0797.ch006

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Results and Discussion T G curves (Figure 2) show the high heat resistance of P B O fibers in comparison with PPT fibers. Whatever the atmosphere, the heat resistance of P B O fibers is always higher. Under air, PPT fibers begin to degrade at about 450°C and form a 3 wt-% residue at 1200°C whereas P B O fibers degrade at about 600°C and form a 3 wt.-% residue at 1200°C. Under nitrogen, the degradation of fibers begins at higher temperature (about 550°C for PPT and about 700°C for PBO). Larger amounts of residue are observed at 1200°C (38 wt.-% for PPT and 65 wt.-% for PBO). This shows the strong influence of oxygen on the thermal degradation of the fibers. This last consideration is important when it is recalled that the burning process depend on the thermal degradation reactions in the condensed phase, which generate volatile products and, that the polymeric substrate heated by an external source is pyrolyzed with the generation of combustible fuel in a zone where there is depletion of oxygen (7). Rate of Heat Release (RHR) curves (Figure 3) of knitted PPT and P B O fibers under three external heat fluxes (50, 75 and 100 kW/m ) show that P B O fibers show very good fire behavior in comparison with PPT fibers. R H R peaks of P B O under 50 kW/m , 75 k W / m and 100 k W / m are respectively only 60 kW/m , 150 kW/m and 250 k W / m respectively compared with 400 kW/m , 430 k W / m and 650 k W / m for PPT fibers. Moreover, the time to ignition of P B O is longer than that of PPT (i.e, 27 s vs. 54 s at 75 kW/m ). This demonstrates the high fire resistance of PBO. Smoke obscuration is significantly lowered for P B O in comparison with paramide fibers (Figure 4). PPT fibers evolve smoke with a peak at 0.012 m /s and this peak does not depend on the external heat flux, except for the time when it occurs (72s at 50 kW/m , 42s at 75 k W / m and 25s at 100 kW/m ). P B O fibers do not contribute to the smoke obscuration during fire whereas the smoke production of burning p-aramide material is comparatively high. If fibers are to be used as flexible fire barriers, this becomes important in term of safety of people because the obscuration of a room or a corridor generally leads to panic; indeed panic gives rise to more deaths than the fire itself (8). 2

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Figure 2. TG curves of PPT and PBO fibers under nitrogen (dashed lines) and air (plain lines) (heating rate = 10°C/min)



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PPT 50 k W / m

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Figure 3. RHR curves of knitted PPT and PBO fibers


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PBO 75 k W / m Downloaded by EMORY UNIV on August 13, 2015 | http://pubs.acs.org Publication Date: September 15, 2001 | doi: 10.1021/bk-2001-0797.ch006

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Figure 4. VSP curves of knitted PPT and PBO fibers

The evolved gases collected from the cone experiments (external heat flux = 75 kW/m ) were analyzed by FTIR (Figure 5 and Figure 6). In these conditions (well ventilated room), only C O (2000-2250 cm" ), C 0 (2250-2400 c m and 635-720 c m ) and H 0 (3500-4000 c m and 1350-1850 cm- ) are observed. The amount of the gases is always lower in the case of the combustion of P B O (typical example in Figure 7). A s the cone combustion is highly overventilated, it is not a surprise that only major products of combustion are detected. This also means that the hazard of P B O during a fire is lower than that of PPT especially, i f it is used as a fire blocker. The R H R curves of PPT and P B O fibers enable the determination of several characteristic times at an external heat flux of 75 kW/m (Figure 3). The first event to notice is the heating of the fibers (t=20s for PPT and 30s for PBO). The ignition of the materials occurs at 27s for PPT and 54s for PBO. After the ignition, a sharp peak is observed corresponding to the combustion of the fibers (t = 33s for PPT and t = 90s for PBO). It is noteworthy that between the ignition and the end of the R H R peak, the two behaviors are very different: PPT burns with a large flame whereas P B O burns with a small flame. After the R H R peak, glowing of the fibers occurs which leads to their degradation (t > 75s for PPT and t > 90s for PBO). 2

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2000 Wavenumbers Îcm-1)

Figure 5. Infrared spectra of evolved gaseous products of PPTfibersversus time (conditions of the cone calorimeter, heatflux= 75 kW/m ) 2

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Time (minutes)

Figure 7. Evolved CO of PBO and PPTfibersversus time measured using FTIR connected to the cone calorimeter ( heat flux = 75 kW/m ) 2

In order to understand what species are formed as a function of time during combustion in the condensed phase, the external heat flux (75 kW/m ) is shut down at the different characteristic times defined above. Then the sample is removed from the cone calorimeter and quenched in the air before C N M R analyses. The assignments of the bands of the two fibers before degradation are shown in Figure 8 and Figure 9. The spectra (Figure 10 and Figure 11) reveal that the fibers decompose after the R H R peaks when glowing phenomenon occurs (t > 75s PPT and t > 90s PBO). Indeed for the two fibers, the bands in the aromatic region (100-160 ppm) broaden and only one band is then observed. It is centered about 130 ppm and it can be assigned to the formation of aromatic/polyaromatic species(9),(10). The width of the band suggests the presence of several nonmagnetically equivalent carbons (11). A s discussed before in previous work(12), the band can be assigned to several types of partially oxidized aromatic and polyaromatic species. A t long times, the loss of the signal can be assigned to the paramagnetic behavior of the sample. The fibers are transformed into polyaromatic compounds which can trap free radicals (12). 2

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Figure 10. C NMR spectra of PPT fibers versus characteristic time (conditions of the cone calorimeter, heat flux = 75 kW/m ) 2



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(conditions of the cone calorimeter, heatflux= 75 kW/m )

Conclusion This work has demonstrated the superior performance of P B O in comparison with traditional p-aramide (PPT) fibers in terms of thermal and fire behavior. During the degradation of the fibers in the conditions of a well ventilated room, only CO, C 0 and H 0 are observed in the gas phase. In the condensed phase, PPT and P B O fibers decompose to polyaromatic compounds which are able to trap free radicals. 2

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Acknowledgment The authors are indebted to Mister Dubusse and Mister Noyon from C R E P I M for their skilful experimental assistance in cone calorimeter experiments. N M R experiments were made in the common research center of the University of Lille and Mister Bertrand Revel is acknowledged for helpful discussion and experimental assistance.


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References (1) Hongu, T.; Philips, G.O. In“NewFibers”, Woodhead Publishing Limited, Cambridge; England, 1997. (2) Kitagawa, T.; Murase, H . ; Yabuki, K. J. Polym. Sci Part Β : Polym. Phys., 1998, 36, 39-48. (3) Flambard, X.; Polo, J., In Fiber Society Spring Conference “Sustainability and recycling of textile materials”,Ferreira, F. N. Ed., Fiber Society; Guimares, Portugal, 2000, 147-148. (4) Yang, H.H., In “Kevlar aramid fiber”, Yang, H . H . Ed., John Wiley & Sons; Chichester, 1993, 191-192. (5) Babrauskas, V . Fire and Mat., 1984, 8, 81-95. (6) Babrauskas, V.; Grayson, S.J. In "Heat Release in Fires", Elsevier Science Publishers Ltd; London, 1992. (7) Lewin, M. In Fire Retardancy of Polymers : The Use of Intumescence, Le Bras, M.; Camino, G.; Bourbigot, S.; Delobel, R., Eds., Royal Chem. Soc.; Cambridge, 1998, 3-32. (8) Akalin M.; Horrocks A.R.; Price D . J. Fire Sci, 1988, 6, 333-334. (9) Earl, W.L.; Vanderhart, D . L . J. Magn. Reson., 1982, 48, 35-54. (10) Supaluknari, S.; Burgar, I.; Larkins, F.P. Org. Geochim., 1990, 15, 509519. (11) Maciel, G.E.; Bartuska, V.J.; Miknis, F.P. Fuel, 1979, 58, 391-394. (12) Bourbigot, S.; Le Bras, M.; Delobel, R ; Descressain, R.; Amoureux, J.P. J. Chem. Soc. - Faraday Trans., 1996, 92, 149-158.


Comprehensive Study of Thermal and Fire Behavior of para-Aramid

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