1

COMPLEX CHAR FORMATION IN FLAME-RETARDED FIBRE-INTUMESCENT

COMBINATIONS - IV. MASS LOSS AND THERMAL BARRIER PROPERTIES

B.K.Kandola and A.R.Horrocks

Faculty of Technology, Bolton Institute, Deane Road, Bolton BL3 5AB

ABSTRACT

Previously reported [1-3] fibre/intumescent composite textile systems are leading to the development

of a range of high performance thermal barrier fabrics. These systems comprise nonwoven cores of

flame retardant cellulose/regenerated cellulose throughout which fibre-interactive intumescent

chemicals (melamine and phosphate based) are dispersed. When heated, both components decompose

by compatible physical and chemical mechanisms leading to a complex carbonaceous 'char-bonded

structure'. Mass loss studies of these samples indicate that decomposition takes place in three stages, an

initial volatilisation, char formation (both occurring during the first 3.5 minutes exposure period) and a

char oxidative stage continuing over a much longer period. Kinetic analysis of mass loss curves gives

insight into their thermal behaviour. Thermal insulative properties of these samples have been observed

by embedding thermocouples within layers of fabrics and measuring their temperatures as functions of

time. Using this technique, a measure of the time dependence of the thermal barrier properties has been

gained at heat fluxes in the range 25 - 75 kWm-2. Derived thermal resistivity values illustrate the

superior barrier properties afforded by the complex chars formed.

2

1. INTRODUCTION

Most non-thermoplastic polymers and textile fibres when ignited, burn through a non-linear

feedback process. The polymer decomposes releasing flammable gases that fuel the flames.

Radiation from the flames heats the neighbouring polymer and completes the feedback cycle.

Simultaneously, polymers having functional side-groups and especially, hydroxyl groups have a

tendency to form char. This occurs through dehydration and cross-linking reactions at the expense

of chain scission decomposition reactions, thus reducing volatile flammable fuel gases. Once

formed, char acts as an insulator protecting the underlying polymer and sub-surfaces from the heat

of the flame [4]. The flammability of any polymer decreases as its char-forming tendency increases

[5]. This char-forming property can be further enhanced by adding appropriate flame retardant

additives to the polymers. In order to compare the char-forming effectiveness of different flame

retardant additives, some analytical techniques like thermogavimetry (TG) and more recently, cone

calorimetry (or mass loss measurements in this case) are required. To mathematically model the

solid-phase decomposition reactions of the polymer, kinetic analysis of the mass loss vs time or

temperature curves is required from which the Arrhenius parameters, activation energies and

frequency factors, can be calculated [6]. These values are good indicators for distinguishing

between different reaction mechanism types occurring during polymer pyrolysis.

A recently developed [1-3, 7-9] fibre/intumescent composite textile has been observed to be an

effective heat barrier. This system contains an intumescent interspersed within a flame retardant

fibrous assembly and both components char by an intermediate semi-fluid phase and by similar

chemistries from which a so-called char-bonded structure arises. This integrated fibrous-

intumescent char structure has a physical integrity superior to those of charred fibre or intumescent

alone [3]. In addition and partly because of reduced oxygen accessibility, the structure demonstrates

an unusually high resistance to oxidation when exposed to temperatures above 4000C [2,3,7,8] and

3

even up to 12000C [9]. Thermal stabilities of these systems and the physical and chemical nature of

these char-bonded structures have been previously studied by thermal analysis (DSC, TGA,

TMA), SEM and EDAX studies [2,3,7,8]. In the present work mass loss studies of char formation

have been undertaken and used to undertake kinetic studies of char formation and oxidation at heat

fluxes of 25, 50 and 75 kWm-2. These three heat fluxes were chosen to observe barrier effects of

these fabrics under heat fluxes that were less than, equal to and greater than the generally accepted

standard value of 50 kW/m2 for fully developed fires.

In addition, an attempt to determine barrier performance as a time-dependent thermal gradient across

each fabric exposed to a high heat flux has been undertaken by embedding thermocouples between

adjacent fabric layers.

2. EXPERIMENTAL

2.1. Fabric composite samples

Preparation of fabric composites was undertaken as previously described [3,9]. Essentially, 200 gm-

2 nonwoven needle-punched webs of the flame retardant viscose fibres Visil (Sateri (formerly Kemira)

Fibres, 40mm, 3.5 dtex), Viscose FR (Lenzing, 51mm, 3.5 dtex) and cotton fibres (30 mm, 1.5 dtex)

were prepared (see Table 1). Nonwoven cotton webs of similar properties were scoured and bleached

to remove impurities and flame retardant - finished using a range of commercially available products

as shown in Table 1 and more fully described elsewhere [8]. For each fibre, one web was taken and

each intumescent system (50% intumescent with respect to fibre weight) [3] with Vinamul 3303 resin

(Vinyl acetate – ethylene copolymer) (Vinamul Ltd., UK) was applied by a brushing technique. The

resin emulsion was used at 15% (w/w) solids level with respect to intumescent. After drying, a second

200 gm-2 fibre web was attached to it and the whole assembly was then needle-punched into a coherent

single web of about 515 gm-2 nominal area density. Needle punching helps in dispersing the

4

intumescent layer throughout the composite structure as well as creating a coherent structure which has

textile fabric properties and is suitable for characterisation of derived chars.

2.2. Mass loss measurements

A mass loss calorimeter (Fire Testing Technology Ltd., UK) was used at heat fluxes of 25, 50 and 75

kW m -2 in an air atmosphere under free convective air flow conditions to expose 100 x 100 mm fabric

samples. It has been observed from our previous cone calorimeter studies [9] that single layers, when

mounted on ceramic fibres as described in ISO 5660, tended to crack and shrink excessively during

exposure. Preliminary mass loss experiments on some Visil composites showed that a three-layered

sample of each composite withstood exposure times for up to 10 minutes without disintegration of the

top layer [9]. Therefore, for the present work, three layers of fabric (total thickness equalling about 10

mm) were used. The top layer was slightly larger than the others (110 x 110 mm) to enable it to be

tucked down the sides of the others. Using this sample presentation method, reproducibility of mass

loss curves was high. Two repeats of each experiment were conducted and the results averaged.

In order to compare their behaviour with other barrier fabrics, the following commercially available

heat barrier fabrics were also examined :

(i) Panotex Felt Black, P39 (Universal Carbon Fibres) (400 gm-2) comprising oxidised acrylic fibre

(Panox, RK Textiles, UK) in the main;

(ii) Dufelt -cotton calico face (Proban treated) comprising 50% Panox/50% para - aramid back (Duflot)

(400-500 gm-2).

2.3. Measurement of thermal barrier properties

To study thermal gradients across the layer of fabric samples under specified heat fluxes, four layers of

fabric (total thickness equalling about 14 mm) were constructed with a thermocouple embedded

between each adjacent layer, and two repeats of each experiment were conducted at varying heat fluxes

(25-75 kWm-2). All samples were studied at 50 kWm-2 whereas at 25 and 75 kWm-2 only Visil,

5

Proban-treated cotton composites and commercial fabrics were investigated. Typical results for

Visil, Visil/MPC1000, Proban-treated cotton and its composite with Antiblaze NW are shown in

Fig.1. “Therm 1-4” responses in Figs. 1 (a) - (d) represent respective thermocouples embedded on

top, between and under respective layers. The chimney thermocouple is the thermocouple

embedded in the chimney above the cone heater. The cone temperature represents the temperature

of the cone heater (measured by the thermocouples on the cone heater coil and recorded on the

control unit), i.e., 538, 726 and 8510C, respectively for each heat flux value. Surface temperatures on

the sample holder at different heat fluxes were observed by embedding a thermocouple in very thin

layer of ceramic wool and values at 25, 50 and 75 kWm-2 heat flux were 460, 580 and 7100C,

respectively.

From the temperature versus time curves (see Fig.1), time - dependent thermal conductivities and

thermal resistivities of samples were also calculated. During the conduction of heat in solids, if the

thermal environment of a solid has been constant for a sufficient time (i.e., the boundary conditions

have not changed in value), it achieves a steady temperature distribution [10]. According to Rockett

and Mike [10] in a solid plate of thickness h, if the two surface temperatures, T0 and Tb are known,

the heat flux per unit area through the plate will be

Q" = (T0 - Tb) k / h (1)

where k is the thermal conductivity. In the present case, assuming that all heat supplied by the cone

heater is passing through the fabric, conductivities of different layers can be calculated. However,

since all the experiments were conducted under similar conditions, and in order to obtain

significantly comparable data, free of the problems associated with excessive damage of the top

layer, the difference in temperatures of the first and fourth thermocouples was used to define the

combined thermal conducting behaviour of three layers of fabric.

6

3. RESULTS AND DISCUSSION

3.1. Mass Loss Measurements

3.1.1. General observations

Mass loss studies were undertaken for all samples listed in Table1 plus the Dufelt and Panotex

fabrics at all three heat fluxes. Most composite fabric samples did not ignite at all or if ignited, the

flame went out within a few seconds (5 - 30 seconds, depending upon the sample and heat flux

used). The Viscose FR samples at 50 and 75 kWm-2 ignited immediately and burned until the end of

each experiment. All FR viscose composites, FR cotton samples and their composites did not ignite,

but after 180-240 s, the respective upper layers started cracking and curling, exposing the lower

ones.

The Dufelt fabric at 25 kWm-2 ignited at 60 s but the flame went out at about 86 s, at 50 kWm-2 it

ignited at 14 sec and extinguished at 93 s, but at 75 kWm-2 after immediate ignition, it burned

throughout with a small flame. Panotex at 25 kWm-2 did not ignite, but at 50 and 75 kWm-2, it

ignited immediately and burned with a small flame for about 180 and 420 s, respectively.

3.1.2. Mass loss studies

The mass loss versus time curves for all the samples at 50 kWm-2 are shown in Fig.2 and expressed

as (m/m0) x 100 where m and m0 are the masses at time t and zero, respectively. In each case

composite structures containing intumescent are more stable than the control ones. Plotting mass

loss curves as ln vs. t (see Fig.3), (where m/m0 = ) show that three regions of mass loss exist, an

initial volatilisation, char formation stage (both occurring during the first 210 s exposure period) and a

char oxidative stage continuing over a much longer period. Weight losses occurring during these three

stages are shown in Table 2. The weight losses for first stage (0-60 s) at 50 and 75 kWm-2 for nearly

7

all samples are more than double the respective values at 25 kWm-2, whereas for the subsequent

stages, differences are not so great. This is because at higher temperatures (under higher heat flux),

the volatilization and decomposition reactions are very fast and active. For all composite samples

the weight loss after each stage is significantly less (sometimes half) than for control fabrics. This

shows the relatively greater stability of these composite char systems. For Panotex and Dufelt the

initial weight loss is quite low. Both fabrics comprise aromatised fibres and so do not decompose

quickly whereas all the FR fibres used in the present work are cellulosics where the initial

decomposition to char is rapid.

3.1.3. Comparison with TGA studies

If the mass loss calorimeter is considered to be a “macro TGA”, the results from this and former

studies should be comparable. In our previous work [7,8] we have reported TGA results of

individual pulverised FR fibres, intumescent components and their mixtures in 1:1 ratio. Direct

comparison between the two studies is not straightforward since in TGA the sample sizes were very

small and the conditions were dynamic, i.e., temperature rises from ambient to 9000C at 10 deg min-

1, whereas in the mass loss calorimeter, the temperatures of the conical heater at 25, 50 and 75

kWm-2 in this work are 538, 726 and 851 0C respectively and the samples are also ignited.

Nevertheless, there is some similarity in the nature of the curves. TGA analyses of these samples

[7,8] also showed three stages of weight loss (labelled I, II and III in Table 2) and DTG maxima for

these are listed in Table 2. For all flame retardant fibres the first stage below 2000C represents

dehydration reactions due to sorbed moisture and acid catalysed dehydration - decomposition

reactions of the cellulose moiety. Sometimes if the weight loss is very gradual, a DTG peak is not

observed. The second stage with a maximum of weight loss in the temperature range of 200 - 410

0C is the main decomposition stage to char and volatiles. Above 410 0C, oxidation of char occurs.

8

In the case of fibre/intumescent mixtures, ammonium polyphosphate and melamine phosphate

decompose around 215 and 270 0C, respectively releasing phosphoric acid [7] which catalyses

dehydration - decomposition reactions of cellulose and pentaerythritol (carbon source) [11-13]. In

the case of Visil and Viscose FR/intumescent mixtures, this same range covers the first stage of

combined component decomposition, whereas for FR cotton/intumescent combinations this forms

an initial part of the main decomposition stage, which comprises two or three apparent steps

depending upon the sample. For FR cottons the first stage is below 200 0C. During the second stage

various reactions are taking place simultaneously over the temperature range 270 - 400 0C. These

are release of polyphosphoric acid, acid - catalysed dehydration of the polyols and cross-linking

reactions of polyol phosphate and cellulose phosphate [7, 11]. Also during this period the reaction

mixtures of intumescent and cellulosic components being in semi - liquid form (as observed by

SEM studies of chars [2,3]) , physically mix together and chemically interact enabling the final

formation of a char-bonded structure to occur. Close to 400 0C, melamine sublimes and probably

releases large amounts of ammonia, which helps in foaming the char formed. Above 400 0C

oxidation of this char takes place, represented by weight losses during the third stage. All these

results, their interpretations and various reactions taking place during these stages have been

reported in detail in our previous publications [3,7,8].

In the mass loss curves, the reactions discussed above are occurring isothermally in the same

sequence, and because the cone radiant temperatures are very high, initial volatilization and

decomposition occurs in the first 60 s, with subsequent char-forming reactions continuing upto 210

s, followed by oxidation of char reactions above this time.

3.1.4. Char yield values

From our earlier TGA studies on FR fibre, intumescent systems and their mixtures (1:1 ratio) it was

observed that fibre/intumescent systems behaved as expected from calculated averages of individual

9

fibre and intumescent sample behaviours upto 400 0C, but above 400 0C, the systems were more

stable than expected with much higher char yields. The previously published observed and expected

(as calculated average) char yield values at 7000C are also given in Table 3 [7,8]. This

comparatively high mass value is due to the formation of the char-bonded structure whose

mechanism of formation has been discussed earlier [3,7,8].

Similarly high char yield results are also observed in the mass loss curves when intumescents are

present. Char residual yields after 300 and 600 s exposure at three heat fluxes are given in Table 3

and for 50 kWm-2 only are shown in Figs. 4(a) and 4(b) (previously reported in ref 14), respectively.

It is clear that composite fabrics containing intumescent produce more char than control fabrics and,

more significantly, that the results from these cellulosic-based structures are comparable to

commercial fabrics comprising highly flame and heat resistant aromatic-structurred fibres.

Furthermore, it is seen that at 50 and 75 kWm-2 and after 600 s exposure, control flame retarded

viscose and cotton fabrics have completely oxidised, whereas, the fabrics with intumescents have

high amounts of residual char left and hence may retain a thermal barrier function.

3.1.5. Kinetic studies of mass loss curves

It is well known that the decomposition of cellulose is a complex process [13]. Presence of flame

retardants, and intumescent chemicals as well, makes it even more complicated and it is very

difficult to mathematically model such reactions as Nelson et al. [15] have shown. Nevertheless, the

basic decomposition process can perhaps be simply described as a series of combined stages :

10

Volatiles

FR cellulose +

Intumescent [O2]

Reactive Char Oxidised

Intermediates char

I II III

Scheme I

Assuming that this set of simplified multi-steps is a series of first order reactions, then the following

equation (1) for first order kinetics can be applied to each stage at a defined temperature T:

ln = - kt (2)

where = m / m0 , m is mass at time t, m0 is initial mass and k is the rate constant. Typical ln vs t

plots for Proban-treated cotton and its composites at 25, 50 and 75 kWm-2 are shown in Fig. 3 (a) -

(c). Each linear stage of these plots represents one of the overall mechanistic steps at a given cone

radiance (as external heat flux) and hence black body temperature T. Determination of T is not

straight forward since the black body temperature at the fabric surface as measured by Therm 1 (see

Fig.1) will be influenced by the chemical reactions taking place and their associated enthalpic

contributions. As expected the surface temperatures of the various samples (Therm1 in Fig.1) show

significant variations over the initial 0-100 s time interval depending on respective sample

exothermic reactions and also whether ignition occurs or not. However, the cone temperature

thermocouple output as shown by straight line in Fig.1, is close in value to the later more stable

zones of respective Therm 1 traces at each particular heat flux and hence, for simplicity has been

considered here as the surface temperature, T. For each stage and cone heat flux, plots of ln k

against 1/T enable the Arrhenius parameters Ea and A to be calculated. Ea is the apparent or average

activation energy, and A is the pre-exponential or frequency factor.

11

Typical Arrhenius plots for Proban-treated cotton and its composites with two intumescents and for

each of three stages at 50 kWm-2 are shown in Fig.5 and values of apparent, average activation

energies for all samples are given in Table 4. In general, the activation energies for char formation

(60-210 s) and char oxidation (>210 s) stages for composite fabrics are higher than for control ones,

which indicates that interaction between flame retardant fibre and intumescent has occurred to give

chars with increased resistance to oxidation. The activation energies for these two stages are also

higher for composite fabric samples than those respective stages for Panotex and Dufelt barrier fabrics

corroborating the evidence of their improved oxidation resistance in Fig.4.

Results from Table 4 indicate that the initial interaction between flame-retardant fibre and

intumescent depends upon the flame - retardant present in the fibre. The Visil/MPC 1000

composite has a lower activation energy than Visil alone, but Visil/Antiblaze NW has a higher

value, indicating that presence of Antiblaze NW increases resistance to volatilization. In the second

(char-forming) stage values are higher for Visil/intumescent composites showing the effect of the

intumescent. In the third stage, however, there is very little effect of intumescent because in all

samples, including Visil alone, the residual silica present (see Table 1) is apparently effective in

reducing ease of oxidation of the residual char.

Viscose FR samples show almost constant activation energy values for the first stage whereas there

is significant increase of activation energy values for the later two stages. Here the flame - retardant

component is the phosphorus and sulphur - containing Sandoflam 5060, which is quite different

from components of intumesecnt systems and the intumescent effect is clearly seen.

Proban-treated cotton also shows a considerable increase in activation energy values in the presence

of intumescent for all stages. Proban releases polyphosphoric acid on heating [16], which also

12

helps the intumescent reactions in reducing volatilization and increasing char formation. During the

char oxidation stage of fibres alone, carbonaceous residues, free of phosphorus following

volatilisation as P2O5, are not very resistant to oxidation. This contrasts with char promotion in the

presence of intumescent where there is an enhanced oxidative resistance of a more complex char

possibly via formation of P-N, P-O-C and similarly unreactive bonds.

For Amgard TFR1, and hence N-methylol dimethyl phosphonopropionamide - containing samples,

intumescents again increase the activation energy for the volatilzation stage. For the second and

third stages the activation energies for FR2-cotton/intumescent systems are higher and similar for

both intumescents present. The phosphonamide present behaves differently than Proban and it does

not release polyphosphoric acid on heating [16] but changes the cellulose decomposition

mechanism by reducing levoglucosan formation [16]. The results in Table 4 suggest that this char is

more difficult to oxidise than that from Proban - cotton alone. This is possibly because the

phosphorus is already bonded into the char structure in the former and not released as acid as in the

latter. Hence, intumescents, when added to this system have little effect on increasing char

formation and reducing the ease of subsequent oxidation.

Amgard LR2 is ammonium polyphosphate, some of which phosphorylates cellulose during

application, thus introducing phosphorus into the char. The presence of additional ammonium

polyphosphate in MPC 1000 will increase this effect further in the complex char and so, increase

the activation energy. A similar interaction is proposed to occur in the Amgard LR2/Antiblaze NW

formulation.

3.2. Thermal Insulative (or Gradient) Studies

3.2.1. General behaviour

13

As mentioned earlier, thermal insulative properties of selective fabrics have been studied by

embedding four thermocouples one each under each of four layers of fabric and temperatures were

measured as a function of time at 25, 50 and 75 kWm-2 and typical results for Visil,

Visil/MPC1000, Proban-treated cotton and its composite with Antiblaze NW are shown in Fig.1.

For hybrid viscose, Visil samples, the temperature of each layer rises gradually in the beginning

although at rates dependent on their locations in the layered sample (see Fig. 1(a)). After about two

minutes the temperatures of second and third layers approach that of the first layer and become

constant after about 210 s. After 100 s a drop in temperature of the upper layer is observed because

the combustion of volatiles has ceased and the fabric has been converted to nearly stable char. Once

the char is fully formed, the temperature becomes constant. In Fig. 1(b) for Visil/MPC 1000

composites, this temperature fall of the first layer is delayed until 285 s possibly because of the

exothermic character of the larger amount of char. In addition, the temperature difference between

each of the first three layers is much higher than for Visil alone, showing the higher thermal

stability of the composite system and its higher thermal insulative character. Similar behaviour is

observed for Visil/Antiblaze NW, and all respective samples at 25 and 75 kWm-2. At 25 kWm-2 the

temperatures of different layers rise more slowly whereas, at 75 kWm-2 they rise quickly relative to

the 50 kWm-2 behavoiur as can be seen from Fig.6.

Viscose FR and flame retardant - treated cotton fabric composite samples generally behaved in a

similar manner to each other. After about 120 s, the first layer degraded and cracked, exposing the

underlying layer and then gradually the second and third layers behaved in a similar way. This

effect is represented by a reduction in the temperature of the first thermocouple to a value below

those of the underlying thermocouples after about 105 s as can be seen in Fig.1(c). The gradual

decrease of this upper thermocouple response reflects its tendency to reach the blackbody

temperature above the composite which is less than the former fabric temperature where exothermic

oxidation was occurring. The underlying fabrics now oxidise and raise adjacent and underlying

14

thermocouple temperatures above the fixed cone temperature. This effect is either not observed at

all or happens at much higher times for viscose FR and FR cotton composites with MPC 1000 and

Antiblaze NW where oxidation is much less rapid and char integrity is greater (see Fig.1(d)).

3.2.2. Time - dependent conductivity curves

Using equation (1) and the method discussed in section 2.3., apparent thermal conductivities and

thermal resistivities of samples were calculated. The calculated apparent thermal conductivities as a

function of time for selected samples at 50 kWm-2 are shown in Fig.7 and values after 300 s are

presented in Table 5. As thermal resistivity, r, is the reciprocal of the thermal conductivity, k, the

samples with lower thermal conductivity will have higher thermal resistivity. As can be seen from

Fig.7 for control fabrics, conductivity rapidly increases with time indicating that thermal resistivity

decreases. In contrast intumescent impregnated composite fabrics offer resistance for considerably

longer periods of time. As all samples are of the same original thickness and all other experimental

conditions are the same, the thermal resistivity values can be directly compared. Thermal resistivity

values for all samples calculated after 300 s at 50 kWm-2 are plotted in Fig.8 (b) and selected

samples at 25 and 75 kWm-2 in Figs. 8 (a) and (c), respectively. The results show that the thermal

resistivities for the control fabrics after 300 s are much less than for the commercial thermal barrier

fabrics whereas, for the FR-fibre/intumescent combinations, the thermal resistivity is much higher

and more comparable with the commercial fabric. At 25 kWm-2 thermal resistivities for the

composite materials are low compared to commercial fabrics, but at 75 kWm-2, values are very

similar. This indicates that these commercial fabrics, although having relatively more resistance at

low heat fluxes, become relatively less resistant at high heat fluxes probably because the similarly

resistant FR-fibre/intumescent combinations form complex, oxidation - resistant chars more

efficiently under these conditions.

15

The oxidation resistance of the char has been previously observed by thermogravimetric analysis

where the behaviour of the composite samples was quite similar above 5000C in both nitrogen and

air atmospheres [7]. The reason for this apparent oxidation-resistance of the chars is not very well

understood, although it seems to be result of both physical and chemical effects. Earlier SEM

evidence [2,3] has suggested that increased physical thermal resistance might arise from the

encapsulation of fibrous chars by expanded intumescent char voids within it. Furthermore, this

structure will reduce the ease of diffusion of atmospheric oxygen through the char and so reduce the

rate of its oxidation. Chemical effects may arise from the complex nature of the char formed (as

evidenced previously by thermal analytical techniques [7,8] and SEM - EDS study of the chars [3] )

that might contain cross-linked structures having conjugated double bonds and P-N, P-O or C-O-P

bonds which are difficult to oxidise [7].

4. CONCLUSIONS

The interaction between flame retarded cellulosic fibres and selected intumescents previously observed

by thermal analytical techniques [7,8] has also been confirmed by mass loss measurements at various

heat fluxes. The residual char formed from this interaction, is much higher in quantity above 5000C, is

resistant to oxidation and has higher thermal insulative properties compared to control fabrics

comprising inherently fire resistant synthetic fibres. In addition, mass loss measurements at various

heat flux intensities has enabled pseudo kinetic analyses to be undertaken of three assumed first

order regions of mass loss. These relate to volatilisation and initial char formation, full char-

development and subsequent char oxidation. Derived rate constants subjected to Arrhenius plot

analysis have yielded apparent activation energies for each of these three reaction zones for each of

the composite structure investigated. Except for LR2 cotton composites, addition of intumescent

increases the activation energy of the char oxidation stage thereby demonstrating that these complex

structures have increased resistance to oxidation. The presence of intumescent increases Ea values

16

for the full char development stage for all flame retardant cellulosic fibre composites. This

demonstrates the proposed interaction between the different flame retardants and fibre substrates

present.

Thermal conductivity values across four layers of charring composite are significantly reduced for

each system studied after the introduction of an interactive intumescent. This clearly demonstrates

the improvement in thermal barrier properties that the introduction of intumescents may confer.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support from the Engineering and Physical Sciences

Research Council during this work.

17

REFERENCES

1. A.R.Horrocks S.C.Anand.and B.J.Hill, UK Patent GB 2279084 B, 20 June 1995.

2. A.R.Horrocks, S.C.Anand and D.Sanderson, Polymer, 37(15), 3197 (1996).

3. B.K.Kandola and A.R.Horrocks, Text.Res.J., 69 (5), 374 (1999).

4. S.Bourbigot and J.-M Leroy in ‘Fire Retardancy of Polymers. The use of Intumescence’, M. Le

Bras, G Camino, S Bourbigot and R Delobel (eds), RSC, Cambridge, 1998, p129.

5. D.W.Van Krevelen, Polymer, 16, 615 (1975).

6. J.D.Cooney, M.Day and D.M.Wiles, J.Appl.Polym.Sci., 28, 2887 (1983).

7. B.K.Kandola and A.R.Horrocks, Polym. Degdn and Stab., 54, 289 (1996) .

8. B.K.Kandola, S.Horrocks and A.R.Horrocks, Thermochim. Acta, 294, 113 (1997).

9. A.R.Horrocks and B.K.Kandola, proc. Flame Retardants '96 Queen Elizabeth II Conference

Centre, Westminster, London, Interscience Communications Ltd., 1996, p145.

10. J.A.Rockett and J.A.Mike, 'Conduction of Heat in Solids' Chapter 2, in the SFPE Handbook of

'Fire Protection Engineering', NFPA, Quincy, MA, USA, 1995, p 1-25.

11. G.Camino and L.Costa, Rev. Inorg. Chem., 8(1/2), 69 (1986).

12. F.J.Kilzer and A.Broido, Pyrodynamics, 2, 151 (1965).

13. B.K.Kandola, A.R.Horrocks, D.Price and G.V.Coleman, J.M.S.- Rev. Macromol. Chem. Phys.,

C36(4), 721-794 (1996).

14. A.R.Horrocks proc. Flame Retardants 2000 Queen Elizabeth II Conference Centre, Westminster,

London, Interscience Communications Ltd., 2000, p156.

15. M.I.Nelson and J.Brindley, Thermochim. Acta, 258, 175-188 (1995).

16. P.Nousiainen, 'Chemical Flame Retardation Mechanisms of Viscose-polyester Fabrics', Technical

research Centre of Finland, Publictions, 45, Espoo, 1988.

18

Fig.1. Time-dependent temperature values of thermocouples embedded in different layers for Visil,

Visil/MPC 1000, FR cotton (Proban-treated) and FR cotton/Antiblaze NW at 50kWm-2.

Visil

0

100

200

300

400

500

600

700

800

900

100 200 300 400 500 600

Time, s

Te

mp

, 0C

(a)

Visil/MPC 1000

0

100

200

300

400

500

600

700

800

900

100 200 300 400 500 600

Time, s

Te

mp

, 0C

(b)

FR cotton (Proban-treated)

0

100

200

300

400

500

600

700

800

900

100 200 300

Time, s

Te

mp

, 0C

Therm 4

Therm 3

Therm 2

Therm 1

Chimney

Cone Temp

(c)

FR cotton/Antiblaze NW

0

100

200

300

400

500

600

700

800

900

100 200 300 400

Time, s

Te

mp

, 0C

(d)

19

Fig.2. Mass loss curves for respective FR fibres ( ), FR fibre/MPC 1000 ( ), FR

fibre/Antiblaze NW ( ), and Dufelt and Panotex at 50kWm-2 heat flux.

0

20

40

60

80

100

0 100 200 300 400 500 600

Time, s

Ma

ss, %

Visil

Visil/MPC 1000

Visil/Antiblaze NW

(a)

0

20

40

60

80

100

0 100 200 300 400 500 600

Time, s

Ma

ss, %

Viscose FR

FRV/MPC 1000

FRV/Antiblaze NW

(b)

0

20

40

60

80

100

0 100 200 300 400 500 600

Time, s

Mass

, %

FR1 cotton (Proban)

FRC/MPC 1000

FRC/Antiblaze NW

(c)

0

20

40

60

80

100

0 100 200 300 400 500 600

Time, s

Ma

ss, %

FR2 cotton (TFR1)

FRC/MPC 1000

FRC/Antiblaze NW

(d)

0

20

40

60

80

100

0 100 200 300 400 500 600

Time, s

Ma

ss, %

FR3 cotton (LR2)

FRC/MPC 1000

FRC/Antiblaze NW

(e)

0

20

40

60

80

100

0 100 200 300 400 500 600

Time, s

Mass

, %

Panotex

Dufelt

(f)

20

Fig.3. Plots of ln vs t for FR1 cotton (Proban - treated), FRC/MPC 1000 and FRC/Antiblaze NW

at 25 ( ), 50 ( ) and 75 kWm-2 ( ) heat fluxes.

(a)FR1 cotton (Proban)

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0 100 200 300 400 500 600

Time, s

ln

25kW

50kW

75kW

(b) FRC/MPC 1000

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0 100 200 300 400 500 600

Time, s

ln

25kW

50kW

75kW

(c) FRC/Antiblaze NW

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0 100 200 300 400 500 600

Time, s

ln

25kW

50kW

75kW

21

Fig.4. Percentage composite char residues for different fibres following exposures to heat

fluxes of 50kWm-2 for 300 and 600 s [14].

(a) 50kW m -2

, 300 s

0

10

20

30

40

50

60

70

80

Visil FRV LR2 Proban TFR1 Dufelt Panotex

Ma

ss

, %

FR fibre

FR fibre/MPC 1000

FR fibre/Antiblaze NW

(b) 50kW m -2

, 600 s

0

10

20

30

40

50

60

70

80

Visil FRV LR2 Proban TFR1 Dufelt Pnotex

Ma

ss

, %

FR fibre

FR fibre/MPC 1000

FR fibre/Antiblaze NW

22

Fig.5. Plots of ln k vs 1/T for FR cotton (Proban - treated), FRC/MPC 1000 and

FRC/Antiblaze NW for three different stages of decomposition.

0-60 s

-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4

0.85 0.9 0.95 1 1.05 1.1 1.15 1.2

103.T

-1 (K)

ln K

560610660710760810860

T (0C)

FR1 cotton (Proban)

FRC/MPC 1000

FRC/Antiblaze NW

(a)

60-210 s

-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4

0.85 0.9 0.95 1 1.05 1.1 1.15 1.2

103.T

-1 (K)

ln K

560610660710760810860

T (0C)

FR1 cotton (Proban)

FRC/MPC 1000

FRC/Antiblaze NW(b)

210-600 s

-8

-7.5

-7

-6.5

-6

-5.5

-5

-4.5

-4

0.85 0.9 0.95 1 1.05 1.1 1.15 1.2

103.T

-1 (K)

ln K

560610660710760810860

T (0C)

FR1 cotton (Proban)

FRC/MPC 1000

FRC/Antiblaze NW

(c)

23

Fig.6. Time-dependent temperature values of thermocouples embedded in different layers for

Visil/MPC 1000 at 25 and 75 kWm-2.

Visil/MPC1000, 25kwWm-2

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600

Time, s

Te

mp

, 0C

(a)

Visil/MPC 1000, 75 kWm-2

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400 500 600

Time, s

Te

mp

, 0C

Therm 4

Therm 3

Therm 2

Therm 1

Chimney

Cone Temp

(b)

24

Fig.7. Thermal conductivity curves for respective FR fibres ( ), FR fibre/MPC 1000 ( ),

FR fibre/Antiblaze NW ( ), and Dufelt and Panotex at 50kWm-2 heat flux.

0.000

0.001

0.002

0.003

0.004

0.005

0.006

60 160 260 360 460 560

Time, s

Therm

. conductivity, k

W.m

-1 K

-1 Visil

Visil/MPC 1000

Visil/Antiblaze NW

(a)

0.000

0.001

0.002

0.003

0.004

0.005

0.006

60 160 260 360

Time, s

Th

erm

. co

nd

uctivit

y, kW

.m-1

K-1

FR viscose

FRV/MPC 1000

FRV/Antiblaze NW

(b)

0.000

0.001

0.002

0.003

0.004

0.005

0.006

60 160 260 360 460 560

T ime, s

Th

erm

. c

on

du

cti

vit

y,

kW

.m-1

K-1

P anotex

D ufelt

(d)

0.000

0.001

0.002

0.003

0.004

0.005

0.006

60 160 260 360

Time, s

Th

erm

. c

on

du

cti

vit

y,

kW

.m-1

K-1 FR1 cotton (Proban)

FRC/MPC 1000

FRC/Antiblaze NW

(c)

25

Fig.8. Thermal resistivities of respective FR fibre, FR fibre/MPC 1000 and FR

fibre/Antiblaze NW after 300 s under various external heat fluxes.

(a) 25kW m-2

, 300 s

0

500

1000

1500

2000

Visil Proban Panotex Dufelt

Th

erm

. re

sis

tiv

ity

, K

m/k

W

(b) 50 kW m-2

, 300 s

0

500

1000

1500

2000

V isil FRV Proban LR2 Panotex Dufelt

Th

erm

. re

sis

tiv

ity

, K

m/k

W

(c ) 75kW m-2

, 300 s

0

500

1000

1500

2000

V is il P roban P anotex

Th

erm

. re

sis

tiv

ity

, K

m/k

W

C on tro l

F ib re /M P C 1000

F ibre /A ntib laze N W

26

TABLE 1. Flame retardant fibre-intumescent combinations producing char-bonded structures [3,7,8]

Fibre components

Fibre Flame retardant

Visil (Sateri, formerly Kemira, Finland) Polysilicic acid, as 30% by silica

Viscose FR (Lenzing, Austria) Sandoflam 5060 (2,2-oxybis(5,5-dimethyl-1,2,3-dioxaphosphorinane-2,2-disulphide))

FR1 cotton Tetrakis(hydroxymethyl)phosphonium chloride (THPC) + urea (durable flame retardant)

(Proban CC, Albright and Wilson) at 3.15% P

FR2 cotton N-methylol dimethyl phosphonopropionamide (durable flame retardant)

(Amgard TFR1, Albright and Wilson) at 2.77% P

FR3 cotton Ammonium polyphosphate + urea (semi-durable flame retardant)

(Amgard LR2, Albright and Wilson) at 3.0% P

Intumescent components

Intumescent Chemicals Molar ratio

MPC 1000 (Albright and Wilson) Ammonium polyphosphate, melamine, pentaerythritol 3 : 1 : 1

Antiblaze NW (formerly MPC 2000) Melamine phosphate and dipentaerythritol. Between 1 and 2 : 1

(Albright and Wilson)

Note : Since this work was undertaken, the use of the name Amgard has been replaced by Antiblaze and Amgard TFR1 has been discontinued.

27

Table 2. Mass losses (%) from mass loss calorimetric curves and related DTG transitions

Sample 25 kWm-2 50 kWm-2 75 kWm-2 DTG maxima (0C) from TGA

Curves in static air in stages

[7,8]

0-60 60-210 210-600 0-60 60-210 210-600 0-60 60-210 210-600

s s s s s s s s s I II III

Visil 9.8 21.3 16.1 26.8 19.7 22.1 28.4 21.4 29.6 - 332 474

Visil/MPC 1000 5.3 9.2 10.3 12.1 14 18.8 13.1 17.6 24.5 197 327 -

Visil/Antiblaze NW 1.1 7.9 11.4 13.9 19.5 19.9 17.2 21.0 24.4 242 348, 408 -

Viscose FR 13.2 32.8 43.3 33.2 33.7 33.1 42.1 32.8 25.1 197 265 582

FRV/MPC 1000 5.9 12.2 16.5 20.4 18.1 32.7 24.4 24.4 36.9 259 332 -

FRV/Antiblaze NW 6.4 12.9 16.4 20.2 23.2 27.8 21.1 21.4 30.9 266 402 -

FR1 cotton (Proban) 18.8 25.6 38.6 30.1 25.0 44.9 37.8 23.3 38.9 - 317 559

FRC/MPC 1000 6.6 12.0 16.9 18.3 18.8 25.4 19.9 21.4 32.9 - 220,318 616

FRC/Antiblaze NW 6.4 12.8 15.9 15.7 19.3 29.3 24.7 24.5 30.4 - 255,328,409 663

FR2 cotton (TFR1) 13.4 24.4 29.4 28.2 30.6 41.2 38.5 32.6 28.9 - 220, 326 563

FRC/MPC 1000 10.0 16.9 24.6 20.1 21.3 38.5 18.1 21.1 36.2 - 209,277,326 653

FRC/Antiblaze NW 7.2 9.6 13.5 13.7 23.0 29.5 23.8 23.4 30.9 - 265,334,400 679

FR3 cotton (LR2) 22.3 26.3 26.4 34.1 30.6 35.3 42.2 31.3 26.5 173 288 568

FRC/MPC 1000 6.5 7.0 17.4 14.7 21.5 34.5 18.9 24.7 39.6 183 262,307,408 672

FRC/Antiblaze NW 5.8 12.8 17.3 15.1 22.3 30.2 24.3 24.5 34.0 178 261,305408 681

Panotex 3.1 10.3 19.0 9.7 15.7 25.7 16.0 16.5 30.3

Dufelt 3.3 19.4 26.7 13.6 20.2 29.0 18.2 21.8 32.6

FRV = Viscose FR , FRC = Flame retardant cotton

28

Table 3. Char yields from mass loss calorimetric and thermogravimetric curves [7,8]

Sample Char yields (%) from mass loss calorimetric studies Char yields (%) from TGA

curves at 700 0C [7,8]

25 kWm-2 50 kWm-2 75 kWm-2

Observed Expected

300 s 600 s 300 s 600 s 300 s 600 s (calculated average)

Visil 62.3 52.7 46.8 31.4 41.8 20.6 30.0

Visil/MPC 1000 82.1 75.2 68.8 55.1 62.3 44.7 41.5 18.5

Visil/MPC 2000 86.9 79.6 60.4 46.7 53.7 37.4 37.5 17.5

Viscose FR 40.4 10.7 21.4 - 16.6 - 4.8

FRV/MPC 1000 78.0 65.4 54.5 28.7 41.9 14.1 24.2 5.9

FRV/Antiblaze NW 76.5 64.3 49.5 28.8 49.5 26.6 25.5 4.9

FR1 cotton (Proban) 45.5 17.1 33.2 - 28.7 - 6.3

FRC/MPC 1000 77.2 64.5 56.8 37.4 50.7 25.8 23.9 11.4

FRC/Antiblaze NW 76.7 64.9 57.0 35.7 43.4 20.4 21.7 8.1

FR2 cotton (TFR1) 51.9 32.8 28.8 - 14.2 - 9.8

FRC/MPC 1000 66.8 48.5 49.3 20.1 51.7 24.6 27.3 13.2

FRC/Antiblaze NW 79.4 69.1 56.2 33.8 44.4 21.9 24.3 9.9

FR3 cotton (LR2) 43.1 25.0 23.5 - 12.2 - 5.2

FRC/MPC 1000 82.3 69.1 56.5 29.3 46.2 16.8 21.5 10.8

FRC/Antiblaze NW 76.6 64.1 54.3 32.4 41.7 17.2 21.1 7.6

Panotex 81.0 67.6 68.2 48.9 59.3 37.2 -

Dufelt 69.9 50.6 57.7 37.2 50.4 27.4 -

FRV = Viscose FR , FRC = Flame retardant cotton

29

Table 4. Apparent Activation Energies (kJmol-1) in the Hypothesis of First Order Decomposition

Stages of Composite Structures

Sample 0-60 s 60-210 s 210-600 s

Visil 139 74 353

Visil / MPC 1000 102 207 365

Visil/Antiblaze NW 357 290 413

Viscose FR 267 140 102

FRV/MPC 1000 288 245 450

FRV/Antiblaze NW 271 214 348

FR1 cotton (Proban) 172 77 59

Proban/MPC 1000 288 224 348

Proban/Antiblaze NW 326 276 428

FR2 cotton (TFR1) 192 219 449

TFR1/MPC 1000 296 285 533

TFR1/Antiblaze NW 357 302 487

FR3 cotton (LR2) 162 174 435

LR2/MPC 1000 308 398 437

LR2/Antiblaze NW 390 299 427

Panotex 274 157 253

Dufelt 470 72 160

FRV = Viscose FR , FRC = Flame retardant cotton

30

Table 5. Thermal conductivity (kW/m.K x 10 - 4) Values for Fabric Samples after 300 s at Different Cone Irradiances

Material 25 kWm-2 50 kWm-2 75kWm-2

Visil 15.1 26.7 34.6

Visil/MPC 1000 9.2 11.0 19.4

Visil/Antiblaze NW 9.8 13.2 18.6

Viscose FR -

FRV/MPC 1000 11.4

FRV/Antiblaze NW 14.1

FR1 cotton (Proban) 19.9 45.3 31.8

FR1 cotton/MPC 1000 8.2 17.4 37.0

FR1 cotton/Antiblaze NW 7.2 11.8 20.4

FR3 cotton (LR2 ) -

FR3 cotton/MPC 1000 22.2

FR3 cotton/Antiblaze NW 12.1

Panotex 4.9 10.6

Dufelt 6.4 13.1 17.9