British Polymer Journul20 (1988) 61-67

Use of DTA with Infrared Analysis of Evolved Gas to Investigate the Effect of

Flame Retardants on Gas Evolution from Pyrolysed Cellulose (Cotton)*

Dennis Price,a A. R. (Dick) Horrocks’ and Mehmet Akalin”

Department of Chemistry and Applied Chemistry, Univcrsity of Salford, Salford M5 4WT, UK ’ Department of Textile Studics, Bolton Institute of Higher Education, Bolton BL3 5AB, U K

(Received 9 April 1987; revised version received 18 June 1987; accepted 16 July 1987)

Abstract: DTA combined with infrared analysis of the evolved gas (EGA) has been used to study the temperature behaviour, both in air and nitrogen, of commercial phosphorus- and nitrogen- and/or bromine-containing flame retardants applied to cotton. By reference to the DTA traces and also the maxima occurring in the CO, CO, and H,O evolution rates, seven significant peak temperatures were assigned. Two new relatively low temperature peaks have been observed: the higher of these is DTA-sensitive and occurs in all unretarded and retarded samples and is ascribed to the formation of an ‘activated cellulose’ state previously proposed by Bradbury, Sakai and Shafizadeh;” the lower peak is an exotherm (1 88-220°C) associated with H,O and CO, evolution rrom flame-retarded samples only. These observations are interpreted in terms of the mechanism of the cellulose pyrolysis/combustion and the influence of the flame retardants.

Key words: Cellulose (cotton), DTA, evolved gas analysis, flame retardants, mechanism, pyrolysis.

1 INTRODUCTION

Although textiles are the initial material ignited in only 25% of fires reported annually in the United Kingdom, such fires result in over 50% of the consequent deaths.’ This disproportionate fatality rate emphasises the importance of the development of successful flame-retardant systems for textile materials. Unfortunately, flame retardants, whilst inhibiting flame ignition, evolve highly toxic gases (e.g. CO, HBr) when consumed in a conflagration. It is important, therefore, to obtain as much inform- ation as possible concerning the chemical behaviour of flame-retardant textiles.

* Presented at the joint meeting of the RSC Thermal Methods Group and the RSC/SCI Macro Group on ‘Applications of Thermal Methods to Polymers’, Loughborough, UK, January 1987.

Flame-retardant systems commonly applied to cotton fabrics function in the condensed and/or vapour phase depending on their chemical struc- ture.* Phosphorus- and nitrogen-containing retard- ants such as ammonium phosphate derivatives and phosphonium salt-urea-ammonia polycondensates (e.g. Proban 210, Albright and Wilson Ltd) function in the condensed phase by reducing the temperature at which pyrolysis occurs and enhancing char formation at the expense of production of flam- mable volatiles. In the case of cotton fabrics above 300“C, the vola tiles consist mainly of levoglucosan. Halogen-containing retardants rarely influence pyrolysis but function in the gas phase by interfering with the flame chemistry. Both types of retardant system may fundamentally change the character of the combustion gases which in real fires are the major factor in determining loss of life.

The authors have recently initiated a programme 61

British Polymer Journal 0007-1641/88/$03.50 0 Society of Chemical Industry, 1988. Printed in Great Britain

62 Dennis Price, A . R. (Dick) Horrocks, Mehmet Akalin

of investigations into the combustion behaviour of cotton and flame-retardant cotton fabrics. This paper reports novel preliminary information ob- tained using differential thermal analysis (DTA) coupled with infrared analysis of the evolved gases (EGA). The major gaseous products generated from cotton fabrics treated with selected phosphorus- and nitrogen- and/or bromine-containing flame retard- ants are CO, CO, and H,O. All three gases are evolved during combustion whilst CO and H,O are the anticipated major evolved pyrolysis products. By observing the behaviour of all three gases in inert and in oxidising atmospheres the effect of each flame retardant on both pyrolysis and combustion may be better understood. This initial study provides the basis for an ongoing detailed kinetic and mechan- istic investigation of combustion product evolution from flame-retardant cellulosic materials.

The application of thermal analysis techniques for the assessment of polymer flammability has been discussed e l ~ e w h e r e . ~ ' ~ Use of DTA,5-s DSC.9- l 1

and TG5 - l4 techniques is well established for investigation of mechanisms by which flame retard- ants function on cellulose.

2 EXPERIMENTAL

2.1 Equipment

DTA-EGA experiments were carried out using a Stanton-Redcroft DTA Instrument No. 673-4 coupled to a Wilks MIRAN infrared analyser. Fabric samples (20mg) were placed in a platinum crucible and subjected to a 5" min- temperature rise over the range 20-600°C. The influences of oxidising (air) and non-oxidising (nitrogen) atmos- pheres were investigated, the flow rate over the sample being 500 ml min - '. Because of the slow scan speed of the MIRAN, separate experiments were conducted to investigate the temperature de- pendence of the evolution rates of the three products of interest: water monitored at 6.6 pm, carbon monoxide at 4.7 pm and carbon dioxide at 4.25 pm.

2.2 Materials

Experiments were conducted on samples of a heavyweight cotton fabric treated with the durable flame retardant Proban 210 or a non-durable flame retardant, which was either ammonium poly- phosphate or ammonium phosphate-ammonium bromide (Amgard TR and Amgard CD, respectively from Albright and Wilson Ltd). The 193grn-' cotton fabric was commercially prepared and bleached; flame retardants were applied by standard pad-dry-ammonia cure (Proban 210) or pad-dry

(Amgard TR and Amgard CD) methods.15 Results from these fabrics are contrasted with those obtained from the untreated fabric. Details of the area density and the phosphorus, nitrogen and bromine contents of the various fabrics are given in Table 1 together with an indication of their modes of flame-retardant action.

3 RESULTS

The DTA and EGA-temperature curves for the four fabric types investigated are presented in Figs. 1-4. More accurate estimates of the various peak temperatures are provided in Table 2. It should be noted that the DTA responses were generally small and not easy to interpret. This could be an inherent problem with the current experiments, particularly those carried out in nitrogen.

The lowest peak temperature T, was observed as low intensity DTA, CO, and H,O responses for all three flame-retarded fabrics. The second peak T2 was seen only by DTA. This occurred around 300°C and was exothermic except for the cotton/N, case. Dehydration always occurred in a single step

Fig. 1. DTA records for (a) pure cotton (b) Proban-treated cotton, (c) Amgard TR-treated cotton and (d) Amgard CD-

treated cotton, in nitrogen (full line) and air (broken line).

BRITISH POLYMER JOURNAL VOL.20, NO. l . 1988

Effect of flame retardants on gas evolution from pyrolysed cellulose 63

TABLE 1. Area density, phosphorus, nitrogen and bromine content, limiting oxygen index (LOI) and summary of mode of action of durable (Proban 210) and non-durable (Amgard)

flame-retarded cotton fabrics investigated

Cotton Proban 210 Amgard TR Amgard CD

Area density (gm-’) Phosphorus (%) Nitrogen (%) Bromine (%) LO1 (single layer, 20°C)

Flame-retardant action Acid catalysed dehydration + Char Reduces volatile (levoglucosan) formation Char consolidation Vapour phase flame inhibition

193.0 __ - - 0.1 85

192.0 2.91 2.83

0.320 -

Yes Yes Yes No

21 3.5 21 6.0 2.1 9 1.19 2.92 1.48 - 4.42 0.369 0.47 1

Very significant Yes Yes No No No Yes

TABLE 2. Peak temperatures ( - C ) observed in DTA-EGA study of various fabrics (shoulders and minor peaks not included)

Fabric

Cotton in air

Cotton in N2

Proban in air

Proban in N,

Amgard TR in air

Amgard TR in N,

Amgard CD in air

Arngard CD in N,

DTA H2O co CO, DTA HZO co COZ DTA H2O 188 co

DTA

co

DTA HZO 189 co CO, 189 DTA

co CO, 192 DTA 220 H J J co co2 DTA 198 HZO co COZ 21 8

co2

H2O

CO,

H,O 191

331 350

346

365

297

300-350

295

31 2

284

31 0

263

266

276

303

250 292

262

350 354

369

333

325

292

281

276

336 26 1

471 471

466 373 478

372

466 31 0 503

320

504 339 526

41 4 274

462 343 51 2

427 272

BRITISH POLYMER JOURNAL VOL.20. N O . l , 1988

64 Dennis Price, A. R. (Dick) Horrocks, Mehmet Akalin

I I I I

.-.

i I , I .

Fig. 2. Temperature dependence of the rate of CO evolution from (a) pure cotton, (b) Proban-treated cotton, (c) Amgard TR- treated cotton and (d) Amgard CD-treated cotton, in nitrogen (full line) and air (broken line). (Ordinate scale same as that for

Fig. 3.)

corresponding to T4. For cotton, T4 was slightly higher in the inert atmosphere than in air; in all other cases the reverse was true. The stronger influence of the Amgard flame retardants on the dehydration process is shown by T4 being some 40°C lower for the Amgard-treated fabrics compared to that for Proban. Under nitrogen, initial evolution of CO, corresponding to T3, usually occurred just before that of water whilst in air this order was shown only for pure and Proban-treated cotton. Again, for cotton the peak temperature was higher in nitrogen than in air. The flame-retarded fabrics showed the reverse trend. The first CO, peak (T,) followed soon after the water peak. In the case of the cotton and Proban samples T5 was similar in air and in nitrogen. Both Amgard samples had significantly higher T,

I Temperaturel'C I 100 200 3bO l'00

Fig. 3. Temperature dependence of the rate of water evolution from (a) pure cotton, (b) Proban-treated cotton, (c) Amgard TR- treated cotton and (d) Amgard CD-treated cotton, in nitrogen (full line) and air (broken line). (Ordinate scale same as that for

Fig. 2.)

values in air compared with nitrogen. In air, all samples gave evidence for second CO(T,) and CO,( T7) evolution processes occurring above 400°C. The Amgard-treated fabrics also showed some CO evolution above 400°C in nitrogen.

4 DISCUSSION

4. I Cellulose pyrolysis and oxidation

The simplest mechanism for the pyrolysis of cellulose is that proposed by Kilzer and Broido,I6 shown in Fig. 5(a). This was later modified by Bradbury et who postulated a three-step mechanism in which an 'initial reaction' leads to the

BRITISH POLYMER JOURNAL VOL.20, NO. 1,1988

EfJect of flame retardants on gas evolution from pyrolysed cellulose 65

(b)

I + Temperaturefi loo 200 300 4011

Fig. 4. Temperature dependence of the rate of CO, evolution from (a) pure cotton, (b) Proban-treated cotton, (c) Amgard TR- treated cotton and (d) Amgard CD-treated cotton, in nitrogen (full line) and air (broken line). (Ordinate scale twice that of Figs.

2 and 3.)

formation of an ‘active cellulose’ which subsequently decomposes by two competitive first-order reac- tions, one yielding volatiles and the other char and a gaseous fraction; see Fig. 5(b). The current DTA results support the latter theory if it is supposed that the peak at T, corresponds to the formation of the activated species. The peaks associated with the first evolution of CO( T3) and CO,( T,) and also that for water (T4) occur fairly close together. They are in a similar temperature region for decomposition in both air and nitrogen. Water would be evolved during the dehydration of the Cell* to form char, with concurrent evolution of CO and CO,. Carbon monoxide and carbon dioxide would also be formed by decomposition of volatiles such as levoglucosan evolved during this dehydration stage. As these

Char + CQ+ H20(+C01 200- 2800Cf

Cell ___* Cell */ Volati‘es

( b) \Char + gaseous fraction

Fig. 5. Mechanism proposed for cellulose pyrolysis by (a) Kilzer and Broido16 and (b) Bradbury et aL” Lev0 = levo-

glucosan; Ccll* = ‘activated cellulose’.

processes were also observed for decomposition under nitrogen, the required oxygen must derive from the parent cellulose molecule. The second CO(T,) and CO,(T,) evolution peaks observed at higher temperatures in air must be due to char combustion.’’ These observations allow us to propose the more detailed scheme for cellulose combustion, based on the original work of Kilzer and Broido16 and Bradbury et aZ.,” presented in Fig. 6.

Oxygen plays three roles in this scheme, namely possible catalysis of pyrolysis and char formation, oxidation of volatiles generated and the above- mentioned char oxidation. In addition to the last, which occurs above 400”C, Figs. 2(a) and 4(a) for untreated cotton show enhancements of both CO and CO, production within the lower temperature range 346-373°C. The considerable increase in CO evolution in the presence of air is accompanied by a shift in its peak temperature (T3) value to 346°C from the value of 365°C in nitrogen (Table 2). Thus not only is volatile pyrolysis product oxidation occurring but catalysis of levoglucosan evolution by oxygen is also possible.

Previous studies’ show, however, that whilst cellulose pyrolysis is influenced by oxygen below

Char + H20 + C02 4

Cell

effect I-----

Fig. 6. Mechanism for pyrolysis-combustion of cellulose (cotton) indicating the influence of flame-retardant treatment, proposed on the basis of current study. [O] represents oxygen

available from the initially charred cellulose.

BRITISH POLYMER JOURNAL VOL.20, N O . l , 1988

64 Dennis Price, A . R. (Dick) Horrocks, Mehmet Akalin

300”C, above this temperature both the rate” and products” are little affected. Within the range 275-34OoC, Fairbridge et al.” have provided evidence that oxygen catalyses the formation of volatiles as well as interacting with the char- promoting reactions. Very recently Horrocks et ~ 1 . ~ ~ have added support to this argument and shown that volatile formation may in fact be favoured in preference to char formation when oxygen is present.

Thus the current understanding of cellulose pyrolysis is confused and requires further study.

4.2 The effect of flame retardants

The above peaks are present in the flame-retarded samples with T,, the ‘activated cellulose’ formation DTA peak, significantly lowered, especially under nitrogen, for Amgard CD and Amgard TR treat- ments. The closely related T3 (CO), T4 (H,O) and T , (CO,) temperatures are reduced and the respec- tive values of the ammonium-salt-treated fabrics are lower under nitrogen than under air conditions. This latter effect is especially noticeable for the T , (CO,) peak. Whilst for Proban-treated fabrics T, increases from 310°C to 320°C on replacing air by nitrogen, such a change in atmosphere causes reductions of 65°C and 71°C for Amgard TR- and Amgard CD- retarded cotton respectively.

This effect is puzzling and, in the absence of further evidence, suggests that in the presence of flame retardants oxygen may in fact negatively influence the reactions associated with the T, to T, peaks. This effect is opposite to that which oxygen has on the T3 peak in untreated cotton as discussed above. Thus whilst the presence of flame retardants shifts T2 to temperatures below 300”C, oxygen appears to reduce the magnitude of this shift. Interpretation of T, and hence Cell,* however, is not simple and is dependent on the flame retardant; further work is necessary.

The char oxidative temperature T6 (CO) for all fabrics studied, except Amgard TR in air, shows little variation from 466°C seen for pure cotton. Dollimore and Hoath report a DTA maximum of 460°C for this peak.’ The higher T6 value of 504°C observed for ammonium polyphosphate finished cotton suggests that this retardant is red.ucing the oxidative tendency of the char. That simple am- monium phosphates are afterglow retardants as well as flame retardants is well known.23 They function by favouring oxidation of carbon via the less exothermic route to CO rather than to C0,.24 It is noteworthy that the gas evolution maximum rate ratios T3(C0)/T6(C0) are lower for all the flame- retarded samples heated in air than the respective values for pure cotton. Thus the flame retardants, by

reducing formation of volatiles, decrease the as- sociated CO generation; furthermore by enhancing char formation, resultant oxidation gives rise to increased CO formation above 460°C. The relative T5 (CO,)/T, (CO,) ratios are remarkably similar for all fabrics although absolute CO, evolutions at T, decrease for flame-retarded samples reflecting the reduced volatile formation. The apparent increased T6 (CO)/T, (CO,) ratios for the retarded samples with respect to cotton, in spite of different respective char levels, perhaps indicate that CO generation is favoured via the reduced afterglow mechanism mentioned above.

In the absence of oxygen, Proban-treated cotton shows negligible tendency of the char to self-oxidise above 400°C whilst this is not true of either of the phosphate-treated samples. Amgard TR under nitrogen promotes some CO formation at 414°C whilst Amgard CD shows greater CO generation at 427°C than at 262°C (T3). Relative to pure cotton pyrolysed under nitrogen, both these retardants, and especially Amgard CD, suppress both CO ( T3) and CO, ( T5) generations below 400°C. This suggests that the chars above 400°C may be more highly oxygenated than either the pure cotton or Proban- cotton chars thereby enabling greater self-oxidation with concomitant higher CO formation. That the Proban-cotton char does not self-oxidise is probably a consequence of the known char-consolidating property of this type of organophosphorus-nitrogen- containing retardant. The opposite may be true of the Amgard CD-derived char where the reduced CO formation at 262°C in nitrogen may be a conse- quence of inhibition of its formation by the ammonium bromide present. However, at higher temperatures, the enhanced oxygen content of this char ensures that CO formation is considerable at 427°C. Surprisingly CO, generation under nitrogen is also quite significant in this temperature region.

With regard to water evolution, whilst all three flame-retarded cottons enhance water formation at the T4 peak with respect to cotton, this is not the case in air where little apparent change exists except for Amgard CD. The presence of ammonium bromide must be suppressing the dehydration activity of the ammonium phosphate present. In nitrogen, how- ever, at the levels applied, both Amgard TR and Amgard CD generate the greatest concentrations of water thereby demonstrating their prime function as dehydrating, condensed-phase flame retardants. This is further demonstrated by the reduction of T4 to 281°C and 261°C respectively. The lower tem- perature shifts of peaks T, to T, for Proban-finished cotton with respect to pure cotton gives added support to its multirole function in terms of dehydration, phosphorylation and volatile reduc- tion and char con~o l ida t ion .~~

BRITISH POLYMER JOURNAL VOL.20, NO. l , 1988

Effect ofjarne retardants on gas evolution from pyrolysed cellulose 67

4.3 The low temperature peak

A most interesting observation arising from our experiments is that of a low intensity exothermic peak ( T , ) at around 200°C for all flame-retarded samples. This peak coincides with water and carbon dioxide evolution. This behaviour must be due to the enhanced dehydration and char formation proper- ties of the flame-retardant treatments. This process is included in the cellulose combustion mechanism proposed in Fig. 6. It is considered that this transition is especially sensitive to condensed-phase flame retardants and so is only resolved as a separate peak in their presence. The peak is particularly noticeable in the ammonium-phosphate-derived finishes and seems to be independent of the presence of oxygen, unlike T2 to T,. It may therefore be considered as a separate reaction. Several other workers6-8.11.14 have observed peaks in the 200°C region although no analysis of evolved products has been reported. Hendrix et ~ 1 . ~ suggest that an endotherm starting at 175°C for a cellulose-phos- phoric acid system is associated with phosphory- lation of the C(6) anhydroglucopyranose hydroxyl group; furthermore, for P 2 2.4% on diammonium hydrogen phosphate (DAP)-treated cotton an endo- therm at 210°C is associated with DAP decompo- sition. Neither proposal would give rise to both CO, and H,O generation observed at T , from Amgard TR-treated fabric in this work. Shafizadeh et al.,I4 using t.g.a., showed that DAP-treated cellulose starts to generate carbon-containing species above 210°C leading to a maximum rate at 305°C. Langley et al.' ' demonstrated that selected cotton-organo- phosphorus-compound combinations show no DSC-sensitive peaks below 200°C. However, iso- thermal TGA shows that two phosphoramide derivatives applied to cotton promote well-defined mass-loss reactions within the 132-199°C tempera- ture rCgime. These are associated with undefined decompositions of the flame retardants prior to their promotion of charring above 200°C. More recent studies by Jain et u1.' demonstrate that low temperature, weak DTA endotherms are shown by cellulose phosphate in air at 140°C and 159°C. DTG maxima also occur at 143°C (= 10% estimated mass loss) and at 179°C (=25% estimated mass loss). From infrared analyses of chars, these peaks were associated with dephosphorylation-decomposition reactions and their intensities were reduced in nitrogen.

Clearly the elucidation of the TI peak is not simple and further work is necessary to ascertain its cause. Significantly it is the only transition observed in this work at which H,O and CO, evolutions (Amgard TR) occur at identical temperatures.

5 CONCLUSIONS

Use of DTA has identified a peak T2 which could be due to the formation of the 'activated cellulose' species Cell* first postulated by Bradbury et al.17 We have extended the simple cellulose pyrolysis model and elaborated the role of oxygen in this model. The influence of different flame retardants on the relative rates of reaction within the extended model have been discussed.

A new observation is that of an exothermic process at around 200°C for all flame-retarded samples. This is an oxygen-independent transition occurring under inert as well as oxidising atmos- pheres. This coincides with H20-CO, evolution but not CO. Thus in addition to the expected and generally observed lowering of the pyrolysis temper- ature, phosphorus- and nitrogen-containing flame retardants also may cause an initial decomposition reaction at an even lower temperature than pre- viously observed.

ACKNOWLEDGEMENTS

The authors wish to thank Albright and Wilson Ltd for provision of the samples used in this work and one of us (M.A.) for financial support.

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