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The role of oxygen in the ignition of polystyrene by a small flame
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THE ROLE OF OXYGEN I N THE IGNITION OF POLYSTYRENE BY A SMALL FLAME
by Ferrers R.S. Clark
ANALYZED
Reprinted from
Journal of Polymer Science
Polymer Chemistry Edition, Vol. 22, 1984 p. 263
-
268DBR Paper No. 1186
Division of Building Research
Price $1 .OO OTTAWA
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L I B R A R YC e t t e e t u d e p o r t e s u r l ' i n f l a m m a t i o n p a r une p e t i t e flamme d ' h y d r o g s n e - o x y g h e d e p l a q u e s d e p o l y s t y r h e t r h r b i s t a n t a u x c h o c s . On a r e l e v ' e l e s d ' e l a i s d ' i n f l a m m a t i o n e t l a v i t e s s e i n i t i a l e d e d6veloppement d e l a flamme a p r b l ' i n f l a m m a t i o n e n f o n c t i o n d e l a t e m p ' e r a t u r e d e s g a z e t d e l a s g p a r a t i o n e n t r e l a flamme e t l a s u r f a c e du polymsre. Ides d 6 l a i s d ' i n f l a m m a t i o n s u i v e n t une l o i du t y p e A r r h e n i u s a v e c une 6 n e r g i e d ' a c t i v a t i o n d e 98
+
18 kJ mole-l. Lesv i t e s s e s d e d6veloppement d e l a flamme d i m i n u e n t B m e s u r e que
l a t e m p e r a t u r e d e s g a z augmente. Pour l e s d 6 l a i s d ' i n f l a m m a t i o n d e l o n g u e d u r & , l e c o e f f i c i e n t d e t r a n s f e r t d e c h a l e u r a p p a r e n t B l a s u r f a c e d e s C c h a n t i l l o n s a chut'e d ' e n v i r o n 100 W m-2 K-l ?t d e s v a l e u r s p r o c h e s d e c e l l e s a u x q u e l l e s o n p e u t s ' a t t e n d r e p o u r u n e flamme d e g a z chaud r e n c o n t r a n t 5 a n g l e d r o i t une s u r f a c e f r o i d e . Pour l e s d ' e l a i s d e c o u r t e dur'ee, c e c o e f f i c i e n t e t a i t p l u s 6lev'e e t p l u s c o n s t a n t a v e c une v a l e u r d ' e n v i r o n 100 W m-2 K-l.
The Role of Oxygen in the Ignition of Polystyrene by a
Small Flame
FERRERS R. S. CLARK, National Research Council, Division of Building Research, Ottawa, Canada KIAOR6
Synopsis
The ignition of slabs of high-impact polystyrene by a lean hydrogen-oxygen flat flame was studied. The ignition delays and initial rates of flame development after ignition are reported as functions of gas temperature and the separation between flame and polymer surface. The delays follow an Arrhenius-type expression with an activation energy of 98 f 18 kJ mol-'. The rates of flame de- velopment drop as the gas temperature increases. During long ignition delays the apparent heat transfer coefficient at the sample Hurface dropped from about 100 W m-2 K-1 to values close to that expected for a hot gas impinging a t right angles on a cold surface. For short delays it was higher and more constant at about 100 W m-2 K-l. Although the surface temperature reached before ignition exceeded that required for nonoxidative pyrolysis, the polymer surface charred only when oxygen was present. It is concluded that both oxidative and nonoxidative pyrolysis contribute to the ignition of polystyrene.
INTRODUCTION
Previous articles in this ~ e r i e s l . ~ described studies of the ignition of poly(methy1 methacrylate) (PMMA) and low density polystyrene (LDPE) by a small, lean hydrogen-oxygen flat flame. In these conditions nonoxidative pyrolysis was evident during the ignition delay of PMMA, in agreement with existing litera- ture? but the length of the ignition delay was probably determined by the con- centration of key chain-propagating radicals in the gas phase.l Conversely, and again in agreement with the literature? the ignition of LDPE by the hydrogen- oxygen flame clearly involved oxidative pyrolysis, but the delay and the rate of flame development after ignition was determined by the concentration of chain propagating radicah2
Brauman4 studied the combustion of polystyrene and found that when the level of radiative heating varied the rate of pyrolysis was the same, whether in an atmosphere of air or nitrogen. This result has been taken as evidence that oxygen is not needed for the pyrolysis of polystyrene during combustion. The pyrolysis rate was insensitive to oxygen concentration during steady-state py- rolysis at mass loss rates comparable to those of the burning of polystyrene in the same conditions. A study of the actual burning rate as a function of oxygen concentration in this type of experiment would not distinguish between the ef- fects of solid-phase pyrolysis and gas-phase reactions.
Because the PMMA and LDPE studies had provided some insight into the role of oxygen in the ignition of these polymers, a similar study of high-impact polystyrene (HIPS) was undertaken to extend the work by B r a ~ m a n . ~
In the following sections, the ignition delays and initial rates of flame devel- opment after ignition for HIPS exposed to a hydrogen-oxygen flame are reported as functions of the gas temperature and the separation between the igniting flame
Journal of Polymer Science: Polymer Chemistry Edition, Vol. 22,263-268 (1984)
CLARK TABLE I Ignition Delays for HIPSa
Separation Equivalence (mm) ratio 5 10 15 20 25 30 0.10 8.9 17.8 31.4 31.0 47.6 44.7 0.11 4.8 7.6 11.9 14.9 17.7 19.3 0.13 3.0 5.4 8.4 9.8 11.9 12.2 0.15 2.5 4.3 6.9 8.0 9.3 9.6 0.17 2.1 3.6 5.7 6.4 7.6 8.0 0.18 1.8 3.3 5.0 5.6 5.8 6.5 0.19 1.8 3.1 4.6 5.0 5.2 5.9 0.21 1.6 2.9 4.1 4.6 5.0 5.1 a Means of four delays, in seconds.
and the polymer surface. These results are compared with those for PMMA and LDPE. Further, the temperature of the surface during the delay and the degree of char formation with and without oxygen are discussed. These experiments show that the mechanism of HIPS ignition in these conditions must include oxidative and nonoxidative pyrolysis.
EXPERIMENTAL
Samples of white, opaque, high-impact polystyrene, 50 X 50 X 5 mm, were assumed to have the following values of thermal properties, all determined a t room temperature: thermal conductivity K, 0.084 W m-1 K-l; specific heat C p , 1400 J kg-l K-l; density p, 1100 kg m-3; thermal diffusivity h , 5 X m2 s-l.5
Slabs of HIPS were placed on the sample plate of the apparatus previously described.l The sample could be raised to a predetermined distance or sepa- ration below the burner sinter to allow exposure. The temperature was con- trolled by changing the equivalence ratio of the burner gases, defined as the volume of oxygen required to convert all the hydrogen to water divided by the volume of oxygen supplied. The range of gas temperatures, gas velocity, Reynolds number, and residual radiant flux at the sample position were the same as in the earlier work.l,2
When exposed to the flame, the surface of the polymer gradually charred. The blue glow observed above PMMA before ignition1 was not observed with HIPS. During the delay the pyrolysis gases were almost white and of low absorbance. Ignition occurred with a bright flash of light and the evolution of large volumes of dark, sooty smoke.
The ignition delay was recorded four times for each of eight equivalence ratios and six separations (Table I); like the deviations found for PMMA and LDPE, the deviation among replicate delays did not exceed 10%. The initial rate of flame development after ignition was also recorded five times each at four sep- arations and eight equivalence ratios. The rates varied more than those reported for PMMA and LDPE1,2 (Table 11).
Variation in surface temperature during the delay was measured by a method previously described1 in which chromel-alumel thermocouples of 0.12-mm
IGNITION OF POLYSTYRENE 265 TABLE I1
Initial Rate of Flame Development, HIPSa
-
Separation
Equivalence imm)
ratio 5 10 15 20
a Mean, standard deviation of five rates of flame development in v s-l.
nominal bead diameter were used. The maximum temperature reached before ignition did not correlate with delay, gas temperature, or separation. Averaged over all the separations and equivalence ratios, the maximum temperature was 293 f 42OC.
Because the surface temperature, as a function of the exposure time, was known, the apparent heat transfer coefficient could be estimated at any time during the exposure by use of the following equations, provided that heat dissi- pation into the solid was as for one-dimensional heat flow into an inert semiin- finite slab:
( T , - Tr)/(Tg - Tr) = 1 - exp X 2 erfc X (1)
X = (hlK)(kt)lI2 (2)
where T,, T,, and Tr are the temperatures of the gas, specimen surface, and room, respectively, t is the time of exposure, and h is the apparent heat transfer coef- f i ~ i e n t . ~
Plots of h vs. t were prepared for a range of separations and equivalence ratios; an example is given in Figure 1. Although the results varied, a clear trend was apparent. For short delays h soon reached a relatively constant value, about 100 W m-2 K-l, in a manner similar to LDPE,2 and ignition occurred soon after. When delays were longer, behavior akin to that of PMMA1 was observed; from high initial values h dropped with time to about 50 W m-2 K-I or below, com- parable with the value expected for a hot gas impinging on an inert cool plate.] Possible explanations for this phenomenon have been described1; in this case an energy demand appears to develop gradually on the surface during the delay. The build-up of pyrolysis gases above the polymer during the delay presumably displaces the oxygen. Thus exothermic, oxidative pyrolysis is gradually replaced by endothermic, nonoxidative pyrolysis as exposure proceeds.
Finally, slabs of HIPS were heated with a radiant source in the presence and absence of oxygen. The heating element from a NBS Smoke Chamber Test (ASTM E662-79)7 was mounted directly above the sample at a distance of 80 mm. The radiant flux at the upper surface of the slab was approximately 40 W m-l. In air, surface charring and bubbling occurred and deepened with time; throughout the exposure, an acrid smoke was observed. In nitrogen almost no charring occurred in the same exposure time (6 min), no bubbling was seen, and
CLARK
. .
0 1. Z 2 . 4 3.6 4 . 8 6 . 0 TIME, s
Fig. 1. Variation of heat transfer coefficient during ignition delay; equivalence ratio, 0.13,20 mm
separation.
the fumes were sweeter smelling. If the sample exposed in nitrogen was further exposed in air, char formed and the smell became acrid once again.
DISCUSSION
Brauman4 found that in pyrolytic conditions the mass flux from small-dim- eter polystyrene samples was the same in air and nitrogen, even when the py- rolysis rate was similar to that observed when the samples were burning. Thus it was concluded that oxygen did not affect the combustion of polystyrene in these conditions. However, the char formation in the present study clearly shows that the polymer behaves differently in the presence of excess oxygen.
This study has considered two aspects of HIPS combustion for which oxygen involvement may be critical: the ignition delay and the initial rate of flame development, both of which are potentially valuable descriptors of the fire hazard of the polymer.
By examining the dependence of the delay on the gas temperature (Fig. 2) it was found that an Arrhenius-like relationship existed. Plots of In (delay) versus the inverse of the absolute gas temperture were linear; in all cases the correlation coefficients were greater than 0.987. From the slope of these lines the activation energy was estimated to be 98 f 18 k mol-l. According to Madorsky: the ac- tivation energy of this polymer decomposing in vacuo is 242 kJ mol-l, but these numbers are difficult to compare because the temperature of the solid decom- posing in the present experiment is only approximately the same as that in Madorsky's experiment (350°C). Use of the gas temperature in the Arrhenius analysis leads to overestimation of the activation energy. Thus, nonoxidative pyrolysis does not govern the ignition delay. On the other hand, Dickens found the activation energy of oxidative pyrolysis of polystyrene to be 90.2 f 0.8 kJ
mol-I in conditions in which the oxygen supply is not limiting.g
The relationship between ignition delay and initial rate of flame development for HIPS was similar to that for PMMA in that the rate of flame development increased as delay increased; as in PMMA and LDPE, the rate of flame devel- opment for HIPS increased as separation decreased.
IGNITION OF POLYSTYRENE
T E M P E R A T U R E . 'C
Fig. 2. Ignition delay as a function of gas temperature: (0) 5 mm, ( A ) 10 mm, (B) 15 mm, (@) 20 mm, (0) 25 mm, ( A ) 30 mm.
MECHANISM
The activation energy of the delay process was less than that for nonoxidative pyrolysis, although the surface temperatures reached were sufficient to allow pyrolysis to occur.$ Surface charring during the delay was identical to that which
I I
I occurred during radiant heating of HIPS only in the presence of oxygen; thereby
evidence of oxidative pyrolysis was provided.
Conversely, the variation in the apparent heat transfer coefficient during the long delays suggests that endothermic pyrolysis became important as exposure was prolonged. Finally, PMMA and HIPS exhibited the same form of rela- tionship between the delay and rate of flame development.
I t is tentatively proposed that, as in PMMA,l the delay is determined by the accumulation of species critical to the chain propagation steps of the combustion process. For ignition to occur the concentration of these species must exceed the minimum values for chain propagation. The concentration of pyrolysis gases will also increase with exposure time and on ignition will determine the initial rate of flame development.
This mechanism is identical to that proposed for PMMA, in which only non- oxidative pyrolysis was seen in the condensed phase.l Yet for HIPS oxidative processes were evident on the polymer surface. The important events that de- termined the delay and initial rate of flame development occur in the gas phase and are to some degree decoupled from condensed phase reactions.
The author acknowledges the assistance of Raymond Flaviani, who conducted many of the ex- periments. This article is a contribution from the Division of Building Research, National Research Council of Canada, and is published with the permission of the Director of the Division.
I
References1. F. R. S. Clark, J. Polym. Sci. Polym. Chem. Ed., accepted for publication. 2. F. R. S. Clark, J. Polym. Sci. Polym. Chem. Ed., 21,3225 (1983). 3. S. J. Burge and C. F. H. Tipper, Combust. Flame, 13(5), 495 (1969).
268 CLARK
5. F. Rodriguez, Principles of Polymer Systems, McGraw-Hill, New York, 1970, pp. 520-525. 6. A. M. Kanury, Fire Res. Abstr. Rev., 14,24 (1972).
7 . Annual Book of ASTM Standards, Part 18,1266 (1980). 8. S. L. Madorsky, J . Polym. Sci., 9(2), 133 (1952). 9. B. Dickens, Polym. Degradation Stability, 2,249 (1980).
Received April 29,1983 Accepted June 9,1983
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