Burning behaviour of foam/cotton fabric combinations inthe cone calorimeter

Dennis Pricea,*, Yan Liua, T. Richard Hulla, G. John Milnesa,Baljinder K. Kandolab, A. Richard Horrocksb

aInstitute for Material Research, Cockcroft Building, University of Salford, Salford M5 4WT, UKbFaculty of Technology, Bolton Institute, Deane Road, Bolton BL3 5AB, UK

Received 12 August 2001; received in revised form 19 November 2001; accepted 25 November 2001

Abstract

The burning behaviours of polyurethane foam/cotton fabric combinations were investigated using cone calorimetry. One non-flame retarded and two flame retarded polyurethane foams containing melamine and melamine plus a chlorinated phosphaterespectively, were combined with four cotton fabrics, i.e. two types of commercial cotton, one without flame retardant (CN) andanother flame retarded with Proban (CPR); and another two flame retardant cottons which were treated with Pyrovatex (CPY) and

(NH4)2HPO4 (CDA) respectively in the laboratory. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Cotton fabric; Polyurethane foam; Foam/fabric combination; Flame retardancy; Cone calorimeter

1. Introduction

Upholstered furniture and furnishings play an impor-tant part in our daily life to make life more comfortable.However, statistics [1] show that more than 94% of thefire fatalities occurred in dwellings and that a major causeof fire fatalities in dwellings was associated with burningof upholstered furniture. From the point of fire hazardassessment, information is needed concerning rates offire development and toxic gas and smoke release fromfoam/fabric combinations used in such furniture.

Basic materials for upholstery composites are cover-ing and filling materials. Polyurethane flexible foams arewidely used as filling material and have frequently beenclaimed to produce large amount of heat when burning[2]. However, the effective heat of combustion of poly-urethane foam is relatively low. It is in the same rangeas wool (20 to 26 MJ/kg), slightly higher than cellulose(13.6 MJ/kg) and only about 53% of that of poly-ethylene [3]. According to our previous study [4], thepolyurethane foam was the easier to ignite and gave ahigher heat release rate compared to wool fabric. Such

flammability characteristics of flexible polyurethanefoam are mainly due to a very large surface for per unitmass, the cellular structure which allows ready access tothe oxygen (air) required for the combustion, and thelow thermal inertia (k�c where k is thermal con-ductivity, � the volumetric mass and cp the heat capacityat constant pressure), rather than to the chemical con-stitution of the foam. It is well known that tests carriedout on component materials in isolation could not givea true indication of their behaviour when combinedwith other component materials in a piece of furniture.Therefore regulatory and advisory agencies have turnedto composite testing rather than flammability testing ofindividual components [5]. In this paper, the burningbehaviour of the foam/cotton fabric combinations in acone calorimeter will be reported.

2. Experimental

2.1. Materials

The materials investigated included two types of flameretarded polyurethane foams, containing melamine andmelamine plus a chlorinated phosphate respectively,three types of flame retarded cotton fabrics, i.e. one

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00036-8

Polymer Degradation and Stability 77 (2002) 213–220

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +44-161-295-4262; fax: +44-161-

295-5111.

E-mail address: d.price@salford.ac.uk (D. Price).

commercial Proban-treated cotton and another twowhich were treated with Pyrovatex and diammoniumorthophosphate respectively in the laboratory [6]. Forcomparison purposes, non-flame retarded foam andcotton fabric were also tested under the same condi-tions. General details of these samples are provided inTables 1 and 2.

2.2. Assembling foam/fabric combination samples

The foam samples used in this work were preparedand cut into 100 � 2 mm � 100 � 2mm � 50 � 2 mmblocks by British Vita PLC. The covering fabric was cutaccording to a template as previously described [4]. Bostik1782 adhesive was used to stick the fabric to form a shell.After using the adhesive, the shell was left to dry in acupboard overnight. To assemble the foam and fabricinto a combination sample, the foam was compressedwith the fingers and inserted into the fabric shell. It wasimportant to make sure that the foam block was lined upexactly at the corners of the fabric shell and there were nogaps between the foam and fabric. After weighing thecomposite, each of the four sides was stapled in the man-ner recommended by Babrauskas and Wetterlund [7].

The base and sides of the sample were wrapped in asingle layer of aluminium foil, shiny side toward the

sample. The bottom of the sample holder was filled with alayer of low-density refractory fibre blanket with a thick-ness of about 13 mm. The retainer frame was placedaround this assembly so that only 88 cm2 of sample topsurface was exposed to the radiating conical heater.

2.3. Cone calorimeter

The burning behaviours of flexible polyurethanefoam/cotton fabric combinations were studied using aCone Calorimeter, made by Fire Testing TechnologyLimited, under fully ventilated conditions. A pre-setheat flux of 35 kW/m2 was used for this study. This isthe level required by the FAA and CAA regulations fortesting aircraft seating materials [8].

3. Results and discussion

3.1. Ignition time

It has been reported in previous study [4] that flexiblepolyurethane foams were very easily ignited in the conecalorimeter at the heat flux level of 35kW/m2, even ifthey were flame-retarded. The values of ignition time(tig) of the foam/cotton fabric combinations are given in

Table 1

Details of polyether polyurethane foams

Foam Flame retardant Density (kg/m3) Resistance to BS 5852 (ignition source No. 5)

PU No 24–26 Noa

PUM Melamine 24–27 Passa

PUMC Melamine and chlorinated phosphate 23–25 Passa

a Dr. Hillier working in British Vita PLC supplied this information.

Table 2

Details of cotton fabrics

Cotton fabric Flame retardant P% (W/W) Area density (g/m2) Thicknessa(mm) Resistance to BS 5438

CN None 0 375 0.67 No

CDA (NH4)2HPO4 2.8 420 0.71 Pass

CPY Pyrovatex CP 1.3 445 0.73 Pass

CPR Proban 3 335 0.61 Pass

a The thickness was measured with a micrometer with measurable area 506 mm2.

Table 3

Ignition time (tig) and re-ignition time (tre-ig) of the foam/fabric combinations over 1 and 6 min after ignition

Foam

PU PUM PUMC

Cotton tig (s) tig (s) tre-ig (s) tig (s) tre-ig (s)

CN 16 17 a 15 –

CDA 16 20 b 19 375

CPY 15 20 300 21 242

CPR 19 16 b 17 300

a No extinction occurred.b Re-ignition did not occur up to a burn time of 600 s.

214 D. Price et al. / Polymer Degradation and Stability 77 (2002) 213–220

Table 3. It can be seen that these combinations gave tigvalues in the range 15–21 s, which were much higherthan foam alone (tig=3 s). However, the presence of theflame-retardants in the fabric and/or in the foam did notsignificantly delay the time to ignition.

Only the combination containing non-FR foam and/or non-FR cotton sustained combustion. The extinctionof the FR foam/FR cotton composites occurred within

1 min after ignition. The PUM/FR cotton combinationsamples appeared the higher resistance to re-ignition ascompared with the PUMC/FR cotton samples. Their re-ignition times (tre-ig) are also given in Table 3. For thesame FR foam, the fabrics CDA and CPR containingthe higher phosphorus concentration, showed the higherresistance to re-ignition compared to the CPY fabricwhich had the lower phosphorus content.

Fig. 1. Heat release rates of (a) the foam/CN; (b) the foam/CDA; (c) the foam/CPY and (d) the foam/CPR combinations.

D. Price et al. / Polymer Degradation and Stability 77 (2002) 213–220 215

3.2. Heat release

3.2.1. Peak heat releaseFigs. 1a–d show the heat release rates of the foam/

cotton fabric combinations at pre-set heat flux of 35kW/m2. Any FR cotton as covering for the three typesof polyurethane foams significantly reduced the peakheat release rate compared to using non-FR cotton. Thepeak heat release rate (PHRR) is believed by many firescientists to be the major determinant of the onset offlashover for furniture in the real fire situation [9]. In

order to compare the peak heat release rate of the com-posites and to identify and study possible synergistic,additive, and antagonistic effects between the individualPU foams and cotton fabrics based on the PHRR, thecalculation given in Eq. (1) was used. This equationtests whether reduction in PHRR of the FR cover andFR foam individually is reflected in the FR cover/FRfoam composite.

q_cal ¼ q_PU=CN ��q_FR=foam ��q_FR cotton ð1Þ

Fig. 1 (continued).

216 D. Price et al. / Polymer Degradation and Stability 77 (2002) 213–220

where q_cal symbolises the calculated PHRR value of FRfoam/FR cotton composite. q_PU=CN represents thePHRR value of PU/CN composite, in which there is noFR at all, �q

_FRfoam represents the decreased value of

PHRR of FR foam/CN composite due to the presenceof FR in foam, and �q

_FRf cotton represents the decreased

value of PHRR of PU/FR cotton composite due to thepresence of FR in cotton. �q

_FRfoam and �q

_FRf cotton can

be obtained from the following Eqs. (2) and (3).

�q_FRfoam ¼ q_PU=CN � q_FRfoam=CN ð2Þ

�q_FR cotton ¼ q_PU=CN � q_PU=FR cotton ð3Þ

It can be noticed from the data in Table 4 that calcu-lated values and tested values of PUM/CDA andPUMC/CDA composites correspond well. This indicatesthat there is probably only an additive effect between thetwo FR polyurethane foams and the non-durable FRcotton (CDA). For PUMC/CPY and PUMC/CPRcomposites, their calculated values were much lowerthan their tested values. Based on the peak heat releasevalues this indicates an antagonistic effect between thePUMC foam and the two durable FR-cottons.

However, for PUM/CPY and PUM/CPR, their cal-culated values were comparable with their measuredvalues. The percentage relative standard deviation forthe HRR as calculated from cone calorimeter experi-ments is about �5%. The results obtained for PUM/CPY and PUM/CPR therefore do not conclusivelyindicate an additive or an antagonistic effect.

3.2.2. Average heat release rate (AvHRR)Any FR cotton as covering for the three types of

polyurethane foams significantly reduced the averageheat release rate during ignition to ignition plus 180s(q: 00bs), compared to using non-FR cotton. According to

Babrauskas [10], the average heat release rate duringignition to ignition plus 180s at an external heat flux of35 kW/m2 might be used to predict the full-scale peakheat release rate (q

:fs) of chairs involved in a propagating

fire.A similar calculation method as described by Eqs.

(1)–(3) was used to study possible synergistic, additiveand antagonistic effects between the individual PUfoams and cotton fabrics based on the AvHRR (180 s).Both the calculated values and the measured results,given in Table 5 show that the calculated values werealmost as twice as high as their measured values. Thisindicates that there is a synergistic effect between the FRfoam and the FR cotton of the composite according toaverage heat release rate during ignition to ignition plus180s. It can be seen that synergism between the FRfoam and the CPY cotton was weaker than that of theothers.

3.2.3. Total heat releaseSimilar calculations were also used to study possible

synergistic, additive and antagonistic effects between theindividual PU foams and cotton fabrics based on thetotal heat release up to a burn time of 500 s. Compar-ison of the calculated and the measured values, given inTable 6, shows that a synergistic probably existed only

Table 4

Both the calculated PHRR values and test PHRR values (kW/m2) for the FR foam/FR cotton composites

Composite Calculated PHRR Tested PHRR Composite Calculated PHRR Tested PHRR

PUM/CDA 127 128 PUMC/CDA 100 102

PUM/CPY 109 121 PUMC/CPY 82 120

PUM/CPR 90 100 PUMC/CPR 63 117

Table 5

Both the calculated AvHRR values and test AvHRR values (kW/m2) for these FR foam/FR cotton composites

Composite Calculated AvHRR Tested AvHRR Composite Calculated AvHRR Tested AvHRR

PUM/CDA 55 22 PUMC/CDA 45 22

PUM/CPY 60 31 PUMC/CPY 50 28

PUM/CPR 59 17 PUMC/CPR 49 24

Table 6

Both the calculated THR values and test THR values (MJ/m2) for these FR foam/FR cotton composites until test time of 500 s

Composite Calculated THR Tested THR Composite Calculated THR Tested THR

PUM/CDA 27.3 9.9 PUMC/CDA 21.4 15.7

PUM/CPY 15.7 17.9 PUMC/CPY 19.8 21.4

PUM/CPR 15.8 5.6 PUMC/CPR 19.9 16.4

D. Price et al. / Polymer Degradation and Stability 77 (2002) 213–220 217

between the FR foam and the CDA and CPR cottonfabrics. In the case of the FR foam/CPY fabric combi-nation, the calculated and measured values are close.

Overall, phosphorus-nitrogen synergism seems toexist between the FR in the PUM foam and the FR inCDA or CPR, based on AvHRR (180 s) and THR (500s). Both CDA (P: 2.8%) and CPR (P: 3.0%) fabricshave a higher phosphorous content than does CPY (P:1.3%). Melamine as the FR in the PUM foam can offer

nitrogen compounds. However, the AvHRR (180 s) andthe THR (500 s) values indicate that the synergisticeffect between the FR in the PUMC foam and the FR inthe CDA and CPR fabrics was not as strong as thatbetween the FR in PUM foam and the FRs in the CDAor CPR fabrics. This is probably due to the vapour phaseflame retardant in PUMC being partly disabled by the FRcovering fabric, or even the vapour phase flame retardantreducing the synergism of phosphorus-nitrogen.

Fig. 2. Rates of smoke release of (a) the foam/CN; (b) the foam/CDA; (c) the foam/CRY and (d) the foam/CPR combinations.

218 D. Price et al. / Polymer Degradation and Stability 77 (2002) 213–220

Based on both total heat releases, there is no syner-gistic effect between the FR in FR foam and the FR inCPY. The reason for no synergistic effect in PUM/CPYcomposite is possible due to an effective N/P atomicratio in the CPY cotton fabric, furthermore the phos-phorus concentration contained in CPY was only 1.3%.

Based on the measured and calculated peak heatrelease rate values, there was no synergy between theFR foams and the FR cotton fabrics. The reason isprobably because the peak heat release rate was mainlydue to the covering cotton fabric, and the time to peak

heat release rate was too short for the FR in the poly-urethane foam to act effectively.

3.3. Smoke release

Fig. 2a–d presents the smoke release rates measuredfor the foam/fabric combinations in the cone calori-meter at heat flux of 35 kW/m2. Fig. 2a shows that thePUM/CN combination gave a lower peak RSR value,but a higher rate of smoke production after the peaktime compared to the PU/CN and the PUMC/CN. The

Fig. 2 (continued).

D. Price et al. / Polymer Degradation and Stability 77 (2002) 213–220 219

PU/FR cotton combinations gave the much lower peakRSR values compared to the FR foam/FR cotton com-posites, which also gave a much higher value of totalsmoke release due to smouldering. There were no sig-nificant differences in rates of smoke release betweenPUM and PUMC covered by FR cotton during theirsmouldering.

3.4. CO

The average CO yield values over 1 and 6 min afterignition for the foam/fabric combinations are given inTable 7. These results show that using the FR cottonfabrics as cover significantly increased the averagevalues of the CO yield, compared to using non-FR cot-ton, and using FR foam resulted in the higher averagevalues of the CO yield, compared to using the PU foamwhen covered by the same fabric.

4. Conclusions

� The peak heat release rates and ignition times of thecomposites mainly depended on the cotton fabrics.

� Both the FR cotton fabrics and the FR foamsaffected the re-ignition times of the FR foam/FRcotton composites.

� Unfortunately, the FR foam/FR cotton combi-nations significantly increased the smoke and COrelease, especially during the smouldering period.

� Based on the total heat release and average heatrelease rates, synergy was observed between theFR’s in in the foams and the FR’s in the CDA orCPR fabrics, both of which had relatively highphosphorus contents (2.8% and 3.0% respec-

tively). The vapour phase active FR [6], appearedto show less synergism compared to that usingusing melamine only as FR.

Acknowledgements

The authors wish to thank Dr. K. Hillier of BritishVita PLC for providing the polyurethane foams andAlbright & Wilson for providing the Proban treatedcotton fabric and the smoke measurement equipment.One of us (Y.L.) wishes to thank the University of Salfordfor the award of a studentship.

References

[1] Collier P, Watson L. Fire Statistics United Kingdom 1997. The

Home Office, The Government Statistical Office, UK, ISSN 0143

6384, 1998.

[2] Damant GH. Flammability of furnishings: someone had to be

first. Flame Retardants ’90 (Proceedings). London: Elsevier, 1990.

p. 242–64 [and references therein].

[3] Babrauskas V. Heat release in fires. Chapman & Hall, 1996.

p. 215 [Chapter 4].

[4] Price D, Liu Y, et al. Polym Int 2000;49:1153–7.

[5] BS 5852: Methods of Tests for the Ignitability of Upholstered

Composites for Seating by Flaming Sources. BSI London, 1982

[part 2].

[6] Price D, Horrocks AR, Tunc M. Chem Br 1987:235.

[7] Babrauskas V, Wetterlund I. Fire Safety J 1998;30(2):179–94.

[8] Peterson JM. In: Babrauskas V, Grayson SJ, editors. Heat

release in fire. London: Elsevier Applied Science, 1992. p. 568.

[9] Ames AA, Rogers SP. Large and small scale fire calorimetry

assessment of upholstered furniture. Proceedings of Interflam.

London: Interscience Communications Ltd, 1990. p. 221 [and

references therein].

[10] Babrauskas V. Heat release in fires. Chapman & Hall, 1996.

p. 519–44.

Table 7

Average CO yield values (kg/kg) of the foam/fabric combinations over 1 and 6 min after ignition

Foam

Foam PU PUM PUMC

Cotton 1 Min 6 Min 1 Min 6 Min 1 Min 6 Min

CN 0.0035 0.0016 0.0056 0.0109 0.0069 0.0391

CDA 0.0119 0.0130 0.0322 0.0439 0.0310 0.0382

CPY 0.0123 0.0207 0.0264 0.0513 0.0278 0.0392

CPR 0.0188 0.0125 0.0337 0.0460 0.0303 0.0385

220 D. Price et al. / Polymer Degradation and Stability 77 (2002) 213–220