An investigation of smoke generation upholstery"

from textile

John A Smith Dept of Technology, Scottish College of Textiles, Netherdale, Galashiels TD1 3HE UK

Much is known about the application of flame retardants to textile fabrics. However, there is a gap in present knowledge on the effects that these flame retardants have on smoke generation, and in the use of smoke suppressants on fabrics and other substrates. The aim of this investigation was to investigate the application and effects produced in the treatment of upholstery foam/fabric systems, with a view to laying down basic criteria for improving the treatments used. It was found that the surrounding material used, the Zirpro flame-retarding treatment, the use of combustion- modified foam and the use of smoke suppressants all increased smoke generation. Solvents such as water applied to the foam also greatly increased the amount of smoke generated.

INTRODUCTION Flammability has long been recognised as a danger to life and property. Legislation in this area has been directed at reducing the flammability of materials and controlling the use of those materials that cannot be eliminated. Even though the flammability of items is now tightly control- led, deaths are still increasing. The key to surviving a fire is escape, but recent research has shown that the pres- ence of heavy smoke prevents fire victims from escaping. Prolonged exposure to smoke results in the inhalation of toxic gases, so that the person trapped is overcome, and several deaths have been attributed to this cause.

The aim of this research was to investigate the applica- tion and effects produced in the treatment of foadfabric systems, as most fires occur with bedding and uphol- stery. Once the effects produced are known, some at- tempt to reduce the smoke generation can then be made.

EXPERIMENTAL

Foams Non-flame-retarded (NFT) and combustion-modified (CM) foams were used. The use of CM foam became widespread after the Furniture and Furnishings (Fire) (Safety) Regulations 1988 specified that all polyurethane foams used in furnishings should be of this type.

Limiting oxygen index (LOI) The potential of a fabric sample to retard flame was assessed by measurement of its limiting oxygen index (LOI), which is the minimum concentration of oxygen in an oxygednitrogen atmosphere that is necessary to ini- tiate and support a flame. The higher the LO1 value of the sample, the greater the flame-retardant effect it exerts.

The LO1 values of wool (untreated and Zirpro-treated), cotton and acrylic fabric samples and of foam samples

This paper was one of three that reached the final stages of the Society's 1992 Student Research Project competition.

were determined. The fabric samples posed no problems in testing, but the CM foam sample gave difficulties as a light brown liquid was produced when the foam was exposed to flame.

NBS smoke chamber The flammability can be assessed by measurement of the amount of smoke generated over a given time. The test uses a completely enclosed cabinet in which a square specimen is supported in a frame, in such a manner that a specific area is exposed to radiant heat. The smoke generated interrupts a vertical light beam directed onto a photoelectric cell. The light source and photoelectric cell are so arranged that the electrical output of the cell is used as a measure of the attenuation of light by smoke. As the density of the smoke increases, less light reaches the photocell and so the electrical signal is also reduced.

In this research, the values are given in terms of D, (density of smoke), derived using Eqn 1:

where T is the light transmission value measured in the NBS smoke chamber.

A number of samples were tested and four results that did not vary by more than 20% were chosen for study. Four values were quoted in the report:

The D,value measured after 5 min would relate to the amount of smoke a person would experience in es- caping quickly from a fire (D , (5 min)) The D, value measured after 10 min would relate to the amount of smoke a person would experience when exposed to a fire and was unable to escape (D, (10 min)) The time, in seconds, required for the smoke to re- duce the visibility by one quarter (D, = 16) An initial smoke production rate ( R ) was defined to

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provide some idea of the amount of smoke generated over the first five minutes; R = ((D, (5 min)/5),

In short, the time at D, = 16 should be as long as possible, and the initial smoke generation rate R should be as low as possible, i.e. little or no smoke produced. Together, these two values give a quick indication of the smoke- generating abilities of the test sample.

Application of smoke suppressants to foams Solutions of ferrocene (dicyclopentadienyl iron) (1 and 0.5% solutions) in ethanol were prepared. A known volume of each of these solutions was added to a foam sample. The foam was then compacted to ensure that the solution had penetrated the bulk, transferred to a watch glass and placed in an oven for 180 min at 70°C. A final air dry at room temperature overnight was permitted.

Equal volumes of 1% aqueous borax and boric acid were mixed and a known volume applied in a similar manner as described above, using an oven temperature of 100°C.

Application of Zirpro flame retardant to wool fabric The Zirpro treatment was developed by the Interna- tional Wool Secretariat during the early 1970s to increase the natural flame resistance of wool fabrics. It is based on the use of potassium hexafluorozirconate (K2ZrF6) and potassium hexafluorotitanate (K2TiF6) metal complexes. Botany serge wool fabric of the required dimensions was treated to 5 and 10% o.w.f., while ensuring that satisfac- tory levelling was obtained, within a suitable time pe- riod.

Scanning electron microscopy of foam samples Scanning electron microscopy (SEM) was carried out in order to investigate differences between normal and burnt CM foam, in order to gain some understanding of the effect of fire on the structure.

Thermal analysis Differential scanning calorimetry (DSC) is a useful tech- nique in examining physical and chemical changes that occur when a substance is heated, providing data on melting point(s) and deformation temperature regions. NFT and CM foams, both with and without the presence of ethanol, were analysed.

RESULTS AND DISCUSSION

LO1 values It was found that the LO1 values of untreated fabrics increased from 17.8 (cotton), through 18.2 (acrylic) to 23.9 (wool). Although different from values quoted in the literature, measurements were considered to be correct as they were consistent over five samples.

The Zirpro-treated wool gave increased LO1 values, and hence flame resistance, compared with untreated wool (5% K2TiF6 25.4, 10% K2TiF6 27.2, 5% KzZrF6 26.6,

10% K2ZrF6 27.0). It was interesting to note that the hexafluorozirconate appeared to give a more effective result at the 5% application level, but at the 10% applica- tion level gave a lower change than that of the hexafluorotitanate.

It was noted that, as the flames passed down the wool surface, the fibres became liquid-like and amounts of blacldgrey smoke were also generated as the front trav- elled down the sample.

The NFT foam gave a value of 15.3, but the CM foam produced a light brown liquid that inhibited any attempt to determine its LOI; hence it was initially concluded that the CM foam appeared to be safer. The fact that an LO1 value could not be determined for pure CM foam, to- gether with the value obtained for the NFT foam, sug- gests that the combustion modification was effective. However, what is of more concern is that: (a) The ethanol treatment modified the CM foam so that

an LO1 value could be obtained. (b) The LO1 values obtained from the ethanol-treated

foam was further increased by the application of a ferrocene solution.

Smoke generation Table 1 shows the results for the Zirpro-treated samples, which were found to have increased smoke-generating abilities, suggesting that the treatment increased the amount of volatiles. The hexafluorozirconate gave better results than the hexafluorotitanate in terms of D, = 16 and R. In fact the 10% hexafluorozirconate values were less than the 10% hexafluorotitanate values.

Table 1 Smoke generation from Zirpro-treated wool samples

0 s 0 s Time (s) at Sample (5min) (10 min) /Is= 16 R

Wool (untreated) 10.4 15.0 2.1 0

+ 5% K,ZrF, 19.6 29.9 257.3 3.93 + 5% K,TiF, 29.6 34.8 120.8 5.91

+ 10% K,TiF, 33.2 38.1 104.3 6.64 + 10% K,ZrF, 29.9 36.8 124.5 5.98

This could be related to the ionic radius of the of the hexafluorotitanate anion, which is smaller than that of the hexafluorozirconate, and enables it to penetrate the wool better. Then, during exposure to heat, this could permit the complex retardation reactions to take greater effect, inhibiting the burning of the fabric but increasing the smoke generated. The hexafluorozirconate is more likely to remain on the surface of the wool fabric and so be vaporised by the exposure to heat, thus reducing retardation reactions and smoke generated. So if a flame retardant treatment is required, the hexafluorozirconate would be the preferred choice, but this would have to be balanced against the efficiency of the flame-retardant effect and the toxic nature of zirconium salts.

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Table 2 Smoke generation from untreated composites

0 s 4 Time (s) at Sample (5 min) (10 min) D s = 16 R

NFTfoam 41.2 67.5 107.5 8.23 +wool 32.0 45.2 109.5 6.43 +cotton 40.0 54.5 99.3 8.00 + acrylic 72.2 106.6 95.3 14.40

CM foam 41.2 70.3 102.0 8.23 +wool 36.4 65.6 100.0 7.33 +cotton 35.2 63.1 136.5 7.04 + acrylic 77.5 131.1 89.0 15.50

Since upholstery is a complex composite, the next stage was to consider the smoke generated by the fabric and foam together. Table 2 shows the variations in smoke generated by NFT and CM foams surrounded by un- treated wool, cotton or acrylic fabrics. D, (5 min) and D, (10 min) figures for wool and cotton composites show that smoke generation was reduced, which is supported by the figures for the initial smoke generation. However, the acrylic unit had far greater values of D, (5 min), D, (10 min), D, = 16 and R, with the CWacrylic composite having larger values than its NFT/acrylic counterpart. These results were related back to the separate behaviour of CM foam and of acrylic fibre. The acrylic, when ex- posed to heat, formed a hard structure; this could be likened to the first steps in the production of carbon fibre from acrylic. The foam when exposed to the same amount of heat produced a liquid. This is thought to have soaked into the hard structure left by the acrylic, which acted as a wick, permitting the total pyrolysis of the liquid, thus increasing the total smoke generation.

Since the foam was identified as the major source of smoke generated, consideration was given to the appli- cation of smoke suppressants to the foam. Two different treatments were used, the first of these was ferrocene, the results for which are shown in Table 3. This shows that application of ferrocene actually increased the smoke generated from both NFT and CM foams, the CM foam being affected to a greater degree. One possibility that was not explored was to examine the effect of applying the powdered ferrocene to the liquid foam before it had the gas blown through it, to see if the ferrocene did actually have a positive effect.

Table 3 Smoke generation from ferrocene-treated foams

0 s 0 s Time (s) at Sample (5 min) (10min) D,= 16 R

NFT foam 41.2 67.5 107.5 8.23 + 0.5% ferrocene 57.9 76.2 48.0 11.58 + 1% ferrocene 59.5 78.4 55.5 11.90

CM foam 41.2 70.3 102.0 8.23 + 0.5% ferrocene 53.5 97.6 82.8 10.70 + 1% ferrocene 67.6 127.1 61.5 13.52

It was assumed that the use of ethanol as a solvent could serve to promote smoke generation from the foam, so it was decided to use a water-based boraxboric acid treatment. Table 4 shows that the use of this treatment again increased smoke generation with both foam types. The increased smoke generation here and with ethanol suggested two possible mechanisms, as follows. (a) Even though the foam had been 'totally' dried, some

solvent must still have been left in the internal struc- ture of the foam, and this increased the amount of smoke generated (this would be valid only in the case of ethanol).

(b) The application of the solvent in some way affected the internal structure of the foam, making it more combustible. This could be achieved by breaking down the main structure noted in the SEMs (dis- cussed later) into smaller sections or by the breaking of the films over the main structure, which are then more likely to pyrolyse during testing.

Table 4 Smoke generation from borax/boric acid-treated foams

0 s 0 s Time (s) at Sample (5 min) (10 min) Ds= 16 R

NFT foam Untreated 41.2 67.5 107.5 8.23 Treated 51.2 74.3 96.9 9.71

CM foam Untreated 41.2 70.3 102.0 8.23 Treated 55.3 87.8 87.6 10.46

Together these ideas can be categorised as 'foam modifi- cation', i.e. the application of a solvent that can modify foam to increase smoke generation. This effect could have serious consequences. For example, an alcoholic drinkspilt on furniture and allowed to'dry' might modify the nature of the foam, causing it to be a serious hazard in the future. The everyday spillage of a cup of tea or coffee, or even a glass of water, could also affect the smoke-generating properties of foam-filled furniture.

Thus it was decided to investigate the effect of drying over five weeks at room temperature (25 f lT) on ethanol and water applied to NFT and CM foam. On ethanol- treated samples it was found that the smoke generation was increased in all cases when compared with that of the untreated foam, with the CM foam being affected to a greater degree. The results for the water-treated foams were more dramatic, showing that water had a greater effect than ethanol in increasing smoke generation. These results are summarised in Table 5, in which a positive value indicates an increase and a negative value a de- crease in the values obtained.

The effects on the LO1 of the ethanol-treated foam were also investigated; these were somewhat difficult to explain. With NFT foam an increase in LO1 was observed (change in LO1 = LO), making the foam less ready to

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Table 5 Effect of prolonged drying on NFT and CM foams treated with ethanol and water

0 s 4 Time (s) at Sample (5min) (10 min) D,= 16 R

Ethanol treated NFT foam 3.2 3.3 -14.5 0.64 CM foam 7.6 13.7 -19.5 1.12

Water treated N FT foam 27.8 21.2 -36.5 5.55 CM foam 11.6 24.8 -25.0 2.32

support burning. The CM foam, for which an LO1 value was initially unobtainable, produced an LO1 value of 24.5. These results suggest that the relationship between LO1 and smoke generation is a highly complex one, requiring further detailed research. Together the smoke generation results and the LO1 information strongly suggest that the foam modification concept is a valid one.

After finding that treatment of the foam was not vi- able, attention was focused on the surrounding fabric. It was decided to assess the effect of the Zirpro flame- retarding treatment on the composites. Table 6 shows that with NFT foam composites treatment increased the smoke generation in all cases, as shown by D, = 16 and R values. Also the concentration level had a great effect. The hexafluorotitanate concentration when increased from 5% 0.w.f. to 10% 0.w.f. increased the D, (5 min) value from 69.1 to 102.2 and the D, (10 min) value from 93.2 to 132.6. This effect was not as pronounced with the hexafluoro-zirconate, the D, (5 min) value only increas- ing from 59.2 to 62.2 and the D, (10 min) value from 93.7 to 107.9.

Table 6 Smoke generation from wool/foam composites treated with Zirpro solutions

DS 0 s Time (s) at Sample (5min) (10min) Ds= 16 R

N f T foam + untreated wool + 5% K,TiF, + 5% K,ZrF, + 10% K,TiF, + 10% K,ZrF,

CM foam + untreated wool + 5% K,TiF, + 5% K,ZrF, + 10% K,TiF, + 10% K,ZrF,

32.0 45.2 109.5 69.1 93.2 54.0 59.2 93.7 57.0

102.2 132.6 53.3 62.2 107.9 57.8

36.4 65.6 100.0 54.6 89.4 75.8 57.4 96.5 66.0 59.3 89.3 70.5 58.5 99.8 68.3

6.43 14.03 11.85 20.43 12.15

7.33 10.91 11.48 11.87 11.70

With the CM foam composites treatment increased the smoke generation but not to the same degree as with the NFT units. Also changing the concentration level from 5 to 10% 0.w.f. on gave marginal increases in the D, (5 min) and D, (10 min) values.

These effects have relatively minor influence on smoke generation, which is increased no matter what the con- centration of Zirpro treatment used.

Scanning electron microscopy The scanning electron micrographs made the differences in the two foam structures quite clear. Figure l(a) is a general view of the structure of the foam, which is quite bulky and consisting mainly of thin, film-like elements. Figure l(b) shows a close up of a film with a tear in it, highlighting the fragility of the structure. This suggests that foam modification could quite easily break up the films, making them more available for combustion.

Figure 1 Scanning electronmicrograph of CM foam; (a) 33x magni- fication, (b) 83x magnification

Figure 2(a) is a view of burnt CM foam; the basic shape of the main structure remains but its bulk is dramatically reduced. This is explained, in part, by a loss of the film structure, and also by the break-up mentioned above. This effect is even better illustrated in Figure 2(b), also of burnt CM foam; this shows a close up of one of the cut edges of the remaining structure. The loss in bulk is attributable to the retraction of the edges, a 'swiss roll' effect in which the edges retract, curling in on themselves when heat is applied.

Figure 2 Scanning electronmicrograph of burnt CM foam; left 83x magnification, right 440x magnification

Thermal analysis The thermal analysis trace of NFT foam in Figure 3(a) shows that heat was absorbed up to 300°C at which point there was a sudden large heat-flow peak, termed an

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I I I I

100 200 300 400 Temp., O C

1 r ~~ ~~ I

100 200 300 4oa Temp., O C

Figure 3 Thermal analysis traces of (a) NFTfoam and (b) NFTfoam + ethanol

'exotherm'. Above 310°C this fell off equally rapidly as the sample began to absorb heat again. This pattern is char- acteristic of a substance absorbing heat, reaching its igni- tion temperature, then burning up to leave ash which in turn begins to absorb heat. Adding ethanol to the system exerted a dramatic effect, as shown in Figure 3(b). The foam, rather than absorbing heat up to ignition, in fact released small but increasing amounts of heat until it reached its ignition temperature. After the ignition maxi- mum, the trace begins to fall but then rises again. This may be because the ethanol had modified the foam, as outlined above.

I

I I I I

100 200 300 400 Temp., OC

100 200 300 400 Temp., 'C

Figure 4 Thermal analysis traces of (a) CM foam and (b) CM foam + ethanol

With CM foam the trace shows a radically different pattern to that of NFT foam (Figure 4(a)). In this case the heat was released all the time, from the moment the test started at 50°C. The exotherm at 270-305°C is indicative of foam ignition. Above 305°C the ash did not begin to absorb heat, but continued to emit it. Adding ethanol to the system had the effect of preventing the samples from releasing heat (Figure 4(b)). Heat was absorbed up to 250"C, at which point there was a release of heat. As the temperature was raised there were further exotherms in the regions 300-340°C and above 360°C. This three-peak effect, when compared with the behaviour of untreated foam, strongly suggests major modification had occurred.

CONCLUSIONS The following conclusions were drawn from the present work. (a) The use of combustion-modified foam, as required

by law, increases the amount of smoke generated in a fire involving upholstered furniture, when com- pared with fires involving non-flame-retarded foam. When the time of escape is all important in surviving a fire, this is a factor that should be of major concern.

(b) The material surrounding the foam plays a major part in the smoke-generating abilities of the final composite. For example, wool is a naturally flame- resistant fibre that imparts some resistance to smoke generation, whereas acrylic acts to increase smoke generation.

(c) Treating the wool surrounding the foam with Zirpro was found to increase smoke generation, as com- pared with that from the untreated fabric. This sug- gests that a great deal of work remains to be done on how to achieve effective flame retardation without increasing the smoke-generating ability of the mat- erials. The use of solvents such as ethanol and water has very serious effects on the smoke-generating abilities of both types of foam. This finding is supported by results from smoke generation, limiting oxygen in- dex values and thermal analysis experiments.

(e) Ferrocene and boraxboric acid smoke suppressants are only two examples of reagents that can be used. Foam modification may have a greater effect on smoke generation than does the action of the suppressants. This could mean that suppressants may have to be introduced during foam manufacture.

458 JSDC VOLUME 108 OCTOBER 1992