Products of Rigid PVC Burning under Various Fire Conditions

Chapter 26

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Products of Rigid PVC Burning under Various Fire Conditions Krzysztof Lebek, T. Richard H u l l , and Dennis Price Centre for Materials Research and Innovation, University of Bolton, Deane Road, Bolton BL3 5AB, United Kingdom

The low flammability of P V C as a bulk polymer is counterbalanced by its higher toxicity in a fire. Rigid P V C has been subjected to decomposition, early well-ventilated and developed, low-ventilation flaming combustion in a tube furnace (the Purser furnace) following the methodology described in IEC 60695-7-50. Analysis of the temperature profiles within the tube furnace showed stratification at low air flow rates and uneven mixing within the tube. Conversely, within the effluent dilution chamber, it was shown that complete mixing had taken place. In comparision with hydrocarbon polymers, yields of carbon monoxide are shown to increase in the presence of H C l , giving high levels of fire toxicity for both well ventilated and low ventilation conditions. Under smouldering conditions, large quantities of HCl are evolved but most of the carbon remains as residue.

334

© 2006 American Chemical Society

Wilkie and Nelson; Fire and Polymers IV ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

335


Introduction and Background PVC is widely used for wire and cable jacketing and insulation (replacing natural rubber), for pipes and conduits (replacing metallic tubing) and for window frames (replacing wood). As a consequence of the widespread use of PVC-based materials, they are often present at the scene of a fire and it is, therefore, of interest to investigate their behaviour. Toxic gases are the cause of most fire fatalities, and carbon monoxide poisoning is responsible for the majority of these deaths (1) . Its contribution to the toxicity and irritancy of fire gases has been the subject of much recent debate. It is acknowledged that smallscale tests do not generally accurately represent the burning conditions occurring in a fully developed fire. Since most fire deaths result from breathing fire gases, and real-scale fires are very expensive to set up, there is a need for a robust means of replicating real-scale fires on a small scale. UK fires cost 600 lives and £4.9 billion per year. The main killer in fire is carbon monoxide. CO is colourless and odourless, so it is inhaled unnoticed. CO combines with haemoglobin in blood, forming carboxyhaemoglobin which is 200 times more stable than oxyhaemoglobin, dramatically reducing the oxygen supply to the cells and the brain. The toxicity of CO may be expressed as an LC . This is the concentration of CO at which 50% of the population would be expected to die in a fixed time, normally within 30 minutes. This has been quoted (2) as 5700 ppm. Different strategies have been employed in order to replicate these fire types on a small scale. Closed box tests, such as NBS smoke chamber, attempt to estimate combustion toxicity in an atmosphere that starts off well-ventilated but which becomes increasingly oxygen depleted during the test. These suffer from a lack of definition of fire conditions, which change during the test, and so are not suitable for use in fire safety engineering. More sophisticated are the flowing air tests of which four established methods exist for small-scale measurement of fire gas toxicity: the DIN 53436 (3), the FM Fire Propagation Apparatus [ASTM E2058] (4), the French test [NFX70-100] (5) and the Purser furnace [BS 7990 (6) and IEC 60695-7-50 (7)]. Only the Purser furnace is capable of forcing a steady state under the most toxic oxygen depleted conditions. It does so by feeding the sample and air into a tube furnace at fixed rates, so that the flame front is held stationary relative to the furnace. This makes it the only small-scale apparatus capable of giving reliable data on the product yields over the full range of fire conditions. Figure 1 presents a simplified view of the first step of the decomposition of PVC, showing the theoretical maximum yield of HC1. Above 190°C, HC1 evolution is rapid and autocatalytic. The resultant conjugated polyene then undergoes scission, resulting in the formation of benzene, toluene, polyaromatic species and carbonaceous char, depending on the conditions. Decomposing 50



336 organochlorine compounds can also form a mixture of toxic compounds such as phosgene, chlorobenzene, chlorotoluene, dichlorobenzene and chlorinated dibenzodioxins and dibenzofurans (8,9) at lower concentrations, which can be detected by FTIR or GC-MS. PVC generates very dense smoke in the initial phase of the fire. The fire effluent gases are corrosive and contain approximately five times the amount of carbon monoxide, compared to polyolefins such as PE, PP. Tests performed according to ASTM 1354-90 using the cone calorimeter (10) showed the main corrosive and irritant gas is hydrogen chloride which can be evolved from unplasticised PVC with a yield of up to the maximum theoretical yield of 0.58 g/g of polymer, based on the atomic weights on the elements.

r r ?' r r r r r r r r HL

H-CI

H-CI

H

9

H-CI

H

5

H-CI

H«

H-CI

H«

H-CI

H~

H-CI

H

9

H~CI

H«

H-CI

H-

H-

H-CI

H~CI

Figure L Simplifiedfirst step of decomposition of PVC

Experimental Materials. Rigid PVC pellets obtained from Northern Industrial Plastics, Manchester, U.K. were used for this study. The Purser furnace. The Purser furnace (11,12) (figure 2) consists of a tube furnace (13) through which a polymer sample is driven at a fixed rate, while being supplied with a fixed feed rate of primary air. This tube furnace has two major advantages over other small-scale physical fire models for studying fire toxicity. By fixing the fuel and primary air feed rates, the equivalence ratio (fuel to air ratio over stoichiometric fuel to air ratio) can be carefully controlled. By driving the sample into a rising temperature gradient within the furnace, increasing the applied heat flux, combustion is forced even under reduced ventilation. In this way, steady state burning is set up before the hottest part of the furnace is reached, and provided steady flaming is obtained, the results are largely independent of the furnace temperature. Furnace temperature profiles have been recorded and reported elsewhere (14). The effluent from the tube is


337 made up to 50 litres per minute with secondary air by dilution within the chamber. The secondary oxidiser is supplied with gaseous sample taken from the effluent dilution chamber. The temperature is set at 900 °C so that partially burnt molecules, benzene, CO, and soot particles are oxidised to C 0 . Carbon dioxide concentrations in effluent dilution chamber and secondary oxidiser are detected using non-dispersive infrared analysers ( Edinburgh Sensors Ltd.). The oxygen concentration in the effluent dilution chamber is measured using a paramagnetic analyzer (Servomex Ltd.). The carbon monoxide concentration is measured using a electrochemical cell (City Technology Ltd.). HC1 was trapped by a series of three bubblers into deionised water and titrated against 0.1M sodium hydroxide, using congo red as an indicator. Toxic product yields were determined using the IEC 60695-7-50 tube furnace method which controls the rate of burning through the sample feed rate. The ratio of primary to secondary air is altered, and once steady state conditions have been established, the fire toxicity at different fire conditions can be determined. The PVC was studied using the experimental conditions relating to the 3 fire scenarios described in IEC 60695-7-50 (Table I). The feed rate was set to 1 g/min by loading the sample boat which travelled at a speed of 35.6 mm/s. Flaming combustion occurred within the first half of the tube furnace before the temperature maxima was reached.


2

Table I Fire types according IEC 60695-7-50. fire type smouldering well-ventilated flaming developed fire-low ventilation

furnace temperature / C 350

primary air flow /l/min 1.1

secondary air flow /l/min 48.9

650

22.6

27.4

825

2.7

47.3

Results Oxygen Concentration Figure 3 shows data for three different fire types. For a developed fire, the steady state occurred from 9 to 26 minutes. For other conditions a shorter steady state was observed. Under smouldering conditions ignition does not take place, but smoke appears. During well-ventilated flaming, ignition occurs, and there is a significant drop in the oxygen concentration. Flaming reaches a peak, and then subsides in the latter part of experiment.



Figure 2. The Purser Furnace



339

Carbon Dioxide Figures 4 and 5 show the variations of C 0 for the effluent dilution chamber and secondary oxidiser during runs. In figure 4, the steady state for the developed fire is observed between 10 and 25 minutes, showing a deviation only at the start and at the end. The deviation at the start shows a slow ignition process, and the peak at the end may be due to an increase in flaming once all the HCl has been pyrolysed. As dehydrochlorination occurs at 190°C, the supply of HCl would finish before the residual char further along the sample boat had been oxidised. The drop in C 0 concentration between 20 and 25 minutes suggests temporary extinction of flaming for the well ventilated flaming scenario. During smouldering, only very low concentrations of C 0 were observed. Greater concentrations of C 0 are observed for the secondary oxidiser because inside the secondary oxidiser the temperature is always 900°C and all carbonaceous material is converted into C 0 . The concentration of C 0 during smouldering at 350°C showed a slow increase, corresponding to the longer distance the boat needed to travel into the furnace in order to achieve a steady state. 2

2

2

2

2

2



340

" d e v e l o p e d fire-low ventiaJtion - well-ventilated flaming - smouldering

Figure 5. Variation of C0 concentration in secondary oxidiser during experimental runs 2


341


Carbon Monoxide Figure 6 shows the variation of carbon monoxide with time. Again the longest steady state was obtained under fully developed conditions, since the higher furnace temperature means that the sample is exposed to temperatures above ignition for a greater period of travel of the sample boat. The sharp rise in the CO concentration, between 17 and 20 minutes during well ventilated flaming, suggests some inhibition mechanism (possibly by HCl) which resulted in temporary extinction of flaming by reducing heat feedback to the sample.

It is very important to assess CO concentration because this is the most significant toxicant found in a real fire. The highest CO concentration occurred for a developed fire at low ventilation and the lowest CO concentration occurred for smouldering. There is a very strong correlation between fire conditions and CO yield. For a developed fire at low ventilation, the CO yield is nearly 6 times greater than for smouldering and 3.5 times greater than for well-ventilated. As CO is the main toxic element from fires, this shows the importance of fire scenario on the fire gas toxicity.


342 Optical Density


Figure 7 shows hazard from smoke (presented as optical density). For wellventilated flaming much more smoke is produced than for other conditions. The lowest level of smoke is produced during smouldering fire types.

time /min

Figure 7. Variation of optical density during experimental runs

HC1 Detection The assessment of fire gas toxicity of halogen-containing materials, such as PVC, requires measurement of HC1 evolution, which makes a major contribution to the fire gets irritancy, (as an irritant gas which suppresses breathing and so hinders escape), most notably under smouldering conditions (where the smouldering may be driven by heat or fuel from other sources). A methodology for determination of HC1, which is extremely soluble in condensable vapours, and easily adsorbed onto many other surfaces, was developed. Starting with pellets of rigid PVC, the determination was optimised in order to get consistent and quantitative yields of HC1 from PVC. Initially it was believed that the best approach would be to sample the gas directly from the furnace tube, where it would not have had a chance to condense. In some preliminary work using slightly different ventilation conditions to IEC 60695-7-50, comparison was made of the HC1 concentrations at the top and



343 bottom of tube (Figure 8), and at the top and bottom of the effluent dilution chamber (Figure 9). For low ventilation conditions inside the tube, six times more HCl was detected at the top of the tube than at the bottom (Table II). Based on this information, subsequent samples were only taken from effluent dilution chamber. The variations in the data led to temperature measurements within the tube, and the unexpected discovery of stratification within the furnace tube, with the hottest layers at the top at low air flow rates, and in the middle at higher air flow rates. Further, the HCl concentration was found to vary significantly according to the position of the probe in the furnace tube. Conversely, grab samples taken from the effluent dilution chamber were found to give nearly quantitative yields of HCl from PVC under certain conditions, and consistent yields under all conditions. Subsequent analysis from different positions within the effluent dilution chamber verified that adequate mixing had taken place within the chamber. Table II Comparison HCl yield for top and bottom the tube furnace material Pure PVC sample taken from top of tube Pure PVC sample taken from bottom of tube Pure PVC sample taken from top of tube Pure PVC sample taken from bottom of tube

temp. /°C

HCl yield in tube/%

HCl yield in chamber /%

350

19.9

36.5

350

4.5

35.0

825

30.8

48.1

825

4.7

47

Figure 8. Measurements from top and bottom of tube furnace


344 16

16

20]

Secondary air supply

Effluent dilution chamber


230

80

Figure 9. Comparison concentrations of HCl on top and bottom of effluent dilution chamber

This demonstrates that the concentrations in different parts of the effluent dilution chamber are the same because the results obtained (for both sampling lines) were the same. HCl chain strips from PVC at low temperature (~200°C), so at 350°C fairly high HCl concentrations are observed. However, because only the total HCl is measured during the run, it is not possible to quantify the amount of chlorine remaining in the undecomposed material, particularly during smouldering. This is because the maximum furnace temperature is 350°C, and many parts will be below 200°C. At 825°C nearly all the furnace will be above 200°C, so HCl loss form the polymer will be complete. HCl was measured from the secondary oxidiser in order to determine the organochlorine content of the fire effluent, by difference with the effluent dilution chamber HCl yield. The deionised water in the bubbler from the secondary oxidiser showed evidence of chlorine formation, a yellow colour and bleaching of the congo red indicator. This arises from oxidation of HCl, which is favourable at 900°C.

0

H0

HCl + 2 -> 2 + Cl The Cl then undergoes partial hydrolysis to hydrochloric and hypochlorous acid. 2

2

Cl + 2

H0

HOCI + HCl

2


345 There also seems to be evidence for some loss of HCl in the tubing running to the secondary oxidiser, most obviously at 825°C when the organochlorine compound concentration is lowest.


Summary of Results for Different Fire Types Table III shows the data for the decomposition of PVC under the three fire conditions. The mass loss for smouldering could correspond to loss of all HCl (58.5%) plus loss of a smaller portion of the aromatic residue. More complete decomposition occurs for the two flaming fire conditions with around 90 and 99% mass loss. The oxygen concentration (average concentration for a steady state), taken from the effluent dilution chamber shows only a small level of oxygen uptake by the sample, which concurs with the low levels of C 0 produced. Under smouldering conditions there is only a small yield of C 0 in the effluent dilution chamber, but a larger concentration of products of incomplete combustion. Surprisingly, under well-ventilated flaming, there is a similar low yield of C 0 in the fire effluent, but a much higher concentration of products of incomplete combustion. This suggests that if flaming occurred it was only partial or sporadic. This brings into question the lack of guidance in the IEC standard over the continuity of the flaming condition. By contrast, BS 7990 stipulates that if flaming is not continuous the furnace temperature should be increased in 25°C steps and the experiment repeated until steady flaming occurs. At the fully developed fire stage, despite the lower concentration, there is a higher yield of C 0 but still a larger yield of products of incomplete combustion. The carbon monoxide yield is consistently high in proportion to the amount of mass lost. Under smouldering conditions, there is little release of carbonaceous material, but a significant CO yield. Under well-ventilated and developed fire/low-ventilation conditions, potentially toxic levels of CO are clearly evident. The C0 /CO ratio, which is normally around 100 for wellventilated flaming for hydrocarbon polymers, is very low, due to the inhibiting effect of HCl on the conversion of CO to C 0 . The HCl yield for the PVC under different fire conditions shows that significant chain stripping yielding HCl, aromatics and char precursors is occurring, even under smouldering conditions. The theoretical maximum yield of HCl from the PVC is 0.585 g/g, suggesting that considerable HCl is lost (or converted to Cl and not then hydrolysed), particularly at 825°C for the fully developed fire condition. The slightly lower yields of HCl from the secondary oxidiser for the developed fire condition, giving an apparent negative organochlorine yield, may arise from deposition of HCl between the effluent dilution chamber and the secondary oxidiser, or may be within the limits of experimental error. 2

2

2

2

2

2

2


346 Table III Summary of results for different fire types Wellventilated

Mass loss %

63.81

89.65

0 concentration %

20.75

19.95

19.5

2

C 0 yield as g/g polymer

0.0947

0.1038

0.6612

Secondary C 0 yield as g/g polymer

0.3659

1.5298

0.9516

0.2712

1.426

0.2904

0.0268

0.0437

0.1550

0.37

0.48

0.55

0.39

0.51

0.51

0.02

0.03

-0.04

0.0688

0.0769

0.3662

2


Developed fire-low ventilation 98.93

smouldering

2

Products of incomplete combustion as g C0 /g polymer CO yield as g/g polymer 2

HCl yield as g/g polymer Secondary HCl as g /g polymer Organochlorine as g HCl/g polymer Smoke as optical density

Conclusions The Purser Furnace is capable of forcing a steady state under the most toxic oxygen depleted conditions, even for intrinsically low flammability materials, such as rigid PVC. HCl yields must be undertaken carefully to minimise loss of analyte. The measurements of fire gas concentrations taken from the effluent dilution chamber are more reliable than those taken directly form the tube, where stratification appears to occur, particularly at low primary air flow rates. At higher furnace temperatures, greater yields of HCl, CO, C 0 are observed, but in contrast to CO or smoke, the HCl yield is less dependent on fire conditions. It should be noted that toxic yield assessment may also be pendent on concentration and identity of the organochlorine species detected through secondary oxidation. 2


347

References 1. 2.


3.

4.

5. 6. 7. 8. 9. 10.

11.

12. 13. 14.

Fire statistics United Kingdom 1997, Home Office Statistical Bulletin, pl3 Issue 25/98 (1998) ISO/TR9122-5:1993 Toxicity testing of fire effluents - Part 5: Prediction of toxic effects of fire effluents Purser D. A.; Fardell P. J.; Rowley J.; Vollam J.; Bridgeman B. An improved Tube Furnace method for the generation and measurement of toxic combustion products under a wide range of fire conditions. Flame Retardant '94 Conference, London (1994) Standard Test Methods for Measurement of Synthetic Polymer Material Flammability Using a Fire Propagation Apparatus (FPA) ASTM E2058: 2002 NFX 70-100 (06/86) Essais de comportement au feu, analyse de gaz de pyrolyse et de combustion, methode au four tubulaire, 1986 BS 7990 : 2003. Tube furnace method for the determination of toxic product yields in fire effluents, 2003 Toxicity of fire effluent - Estimation of toxic potency: Apparatus and test method; IEC 60695-7-50:2002, 2002 Paul K. T. Fire Mater, 1989, 14, 43-58 Hirschler M. M. Eur. Polym. J. 1986, 22, 153-160 Sultan B. .; Ericsson K.; Hirvensalo M.; Hjertberg T.; Hanninen M.; Novel halogen free flame retardant polyolefins intended for internal wiringProperties and flame retardant mechanism. 47th International Wire & cable symposium, 1998, Philadelphia, USA Price D.; Ebdon J. R.; Hull T. R.; Milnes G. J.; Hunt B. J. in Fire and Polymers, Materials and Solutions for Hazard Prevention, Ed. G. L. Nelson and C.A.Wilkie, ACS Symposium series 797, 2001, pp.307-320. Hull T. R.; Carman J. M.; Purser D. A. Polym. Int. 2000, 49, 1259-1265 BS 7990:2003 Tube furnace method for the determination of toxic product yield in fire effluents Carman J. M.; Purser D. A.; Hull T. R.; Price D.; Milnes G. J. Polym. Int. 2000, 49, 1256-1258


Products of Rigid PVC Burning under Various Fire Conditions

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