Rodolphe Sonnier
email: rodolphe.sonnier@mines-ales.fr

Loïc Dumazert

Ebastien Livi

Thi Khanh

Ly Nguyen

Jannick Duchet-Rumeau

Henri Vahabi

Pascal Laheurte

Flame retardancy of phosphorus-containing ionic liquid based epoxy networks

Keywords:

Various amounts of a phosphonium ionic liquid (IL169) were used as curing agents as well as flame retardants of epoxy prepolymer in order to prepare epoxy networks with improved fire properties. The flame retardancy of these resins was investigated using thermogravimetric analysis (TGA), pyrolysiscombustion flow calorimetry (PCFC) and cone calorimetry. Phosphonium ionic liquid significantly reduced flammability without further addition of flame retardants due to the high amount of phosphorus (up to 3.69 wt%). The Peak of heat release rate decreases from 1099 kW/m 2 to 300 kW/m 2 in cone calorimeter at 35 kW/m 2 when incorporating 30 phr of IL169. Phosphorus modifies pyrolysis pathway, promotes charring and may act as flame inhibitor. The char layer protects the underlying polymer, leading to a high unburnt polymeric fraction. Char properties were studied using PCFC, Raman spectroscopy and X-ray tomography. Phosphorus improved the graphitization of the char and its thermooxidative stability.

Introduction

Epoxy resins are widely used in many applications, including composite and coating industries. They are among the highest flammable thermosets together with unsaturated polyesters. Thus, many efforts have been made to improve the flame retardancy of these resins in order to extend their applications [START_REF] Levchik | Thermal decomposition, combustion and flame-retardancy of epoxy resins -a review of the recent literature[END_REF]. Phosphorusbased additives have been used as alternatives to halogen-based flame retardants (FRs) while some of the latter were banned due to environmental concerns [1e4]. Phosphorus-based epoxy monomers or curing agents have also been investigated as flame retardants [5e10]. Indeed, the reactive approach seems to be slightly more efficient, allows maintaining transparency, and is believed to suppress possible migration of flame retardants over time [11e12]. More recently, biobased additives or reactive hardeners have been developed [START_REF] Menard | Synthesis of biobased phosphorus-containing flame retardants for epoxy thermosets: comparison of additive and reactive approaches[END_REF][START_REF] Menard | Synthesis of biobased phosphate flame retardants[END_REF] and some fully biobased and flame retarded epoxy resins have been synthetized [START_REF] Menard | From a bio-based phosphorus-containing epoxy monomer to fully bio-based flame retardant thermosets[END_REF].

According to the literature, phosphate and phosphonate-based FRs act mainly in condensed phase by promoting charring of the epoxy resin [5e6]. Intumescent mono-component solutions led to a significant reduction of flammability. Phosphorus compounds decompose into phosphoric acids resulting in the dehydration of the epoxy resin which is necessary for promoting charring. However, the presence of phosphorus compounds could also reduce the thermal stability of the resin. Phosphinate and phosphine oxidebased FRs may be more active in gas phase, decreasing the combustion efficiency. The extent of flame retardancy depends primarily on the phosphorus content (typically higher than 1 wt%). Ionic liquid (IL)-based epoxy resins are a new class of epoxy thermosets [START_REF] Silva | New epoxy systems based on ionic liquid[END_REF]. According to the literature, Livi et al. have demonstrated that the basicity of the ionic liquids (phosphinate, phosphate or dicyanamide) initiates the epoxy polymerization and binds covalently to the network through an anionic polymerization. These new curing agents of epoxy prepolymer lead to networks with enhanced final properties such as an excellent thermal stability, higher hydrophobic behavior and good thermo-mechanical properties [16e20]. Thanks to the great diversity of IL, properties can be finely tuned [START_REF] Fraser | Phosphonium-based ionic liquids: an overview[END_REF]. The thermal stability and flammability of ILs have already been studied, and vary to a great extent depending on their nature [22e23]. While some ionic liquids contain phosphorus groups, low-flammability epoxy resins can be expected without further addition of flame retardants.

The objective of this article is to assess the flame retardancy of epoxy resins crosslinked with various amounts of a phosphorusbased ionic liquid (namely IL169). The modes-of-action of phosphorus were identified and the properties of the char formed during burning were investigated in detail.

Experimental

Materials

Diglycidyl ether of bisphenol A (DGEBA) based epoxy prepolymer (DER 332) with an epoxide equivalent weight (EEW) of 175 g mol À1 was purchased from DOW Chemical Company. Jeffamine D230 was supplied by Huntsman and was used as conventional curing agent. Phosphonium ionic liquid: Tributyl(ethyl) phosphonium diethyl-phosphate named IL169 was kindly provided by Cytec, Inc. All the chemical structures are shown in Fig. 1.

To prepare samples, DGEBA and curing agents (i.e. Jeffamine D230 and IL169) were mixed with a suitable ratio under stirring at room temperature for 30 min. The mixture was then degassed in an ultrasonic bath for 15 min, and was poured into silicone molds prior to curing. The ratio between DGEBA and curing agents as well as the curing and post-curing conditions are presented in Table 1.

Differential scanning calorimetry

Differential Scanning Calorimetry thermograms (DSC) of epoxy/ amine and epoxy/IL networks were recorded using a DSC Q20 TA instruments from 20 to 250 C during heating at a rate of 10 K min À1 under nitrogen flow of 50 mL min À1 .

Thermogravimetric analysis

Thermogravimetric analyses (TGA) were performed using a Setsys Evolution apparatus (Setaram). 10 (±2) mg-samples were heated under nitrogen flow (100 mL/min) at a heating rate equal to 10 C/min from room temperature to 900 C.

Pyrolysis-combustion flow calorimeter

Flammability was investigated using a pyrolysis combustion flow calorimeter (PCFC) which was developed by Lyon and Walters [START_REF] Lyon | Pyrolysis combustion flow calorimetry[END_REF]. The sample (3 ± 1 mg) was first heated from 80 to 750 C at 1 C/s in a pyrolyzer under nitrogen flow and the degradation products were sent to a combustor where they were mixed with oxygen in excess at 900 C. In such conditions, these products were fully oxidized. The heat release rate (HRR) was then calculated by oxygen depletion according to Huggett's relation (1 kg of consumed oxygen corresponds to 13.1 MJ of released energy) [START_REF] Huggett | Estimation of rate of heat release by means of oxygenconsumption measurements[END_REF]. All materials were tested twice at least. The variation in main data (peak of heat release rate, total heat release and heat of complete combustion) was lower than 10% and the variation in the temperature of pHRR was around 10 C.

The thermo-oxidative stability of residues from cone calorimeter was also studied in PCFC according to method B, i.e. in aerobic pyrolysis, for various oxygen fractions in pyrolysis chamber (from 0.01 to 0.4) as proposed elsewhere [START_REF] Sonnier | Studying the thermooxidative stability of chars using pyrolysis-combustion flow calorimetry[END_REF]. The temperature of char degradation is the temperature at which the HRR reaches 50 W/g.

Cone calorimeter

Some flame retardant effects (as barrier effect or flame inhibition) are not effective in PCFC. Therefore fire behavior was also studied using a cone calorimeter (Fire Testing Technology) which is a powerful tool to investigate the fire behavior of polymers. A horizontal sample sheet of 100 Â 100 Â 3 mm 3 was placed at 2.5 cm below a conic heater and isolated by rock wool. The samples were exposed to 35 kW/m 2 in well-ventilated conditions (air rate 24 L/s) in the presence of a spark igniter to force the ignition. HRR was determined according to oxygen depletion (Huggett's relation) as in PCFC. This test was performed according to the ISO 5660 standard. All materials were tested twice with a good repeatability. The variation on main data (time-to-ignition, peak of heat release rate, total heat release, smoke extinction area, residue content and effective heat of combustion) was lower than 10% except for pHRR of 10IL and the char content was 20IL (15e20% of variation).

Raman spectroscopy

Raman spectra were carried out to study the graphitization of the chars. Spectra were measured using a Horiba Jobin Yvon Lab-RAM Aramis spectrophotometer fitted with a 532 nm laser and a 200 mm confocal pinhole. The laser was focused using a  10 objective. All spectra were obtained with an integration time of 30 s accumulation of 4 spectra recorded in the spectral range 800e1900 cm À1 .

X-ray tomography analyses

X-ray tomography is a powerful tool to assess the morphology of the char [27e29]. To obtain 3D visualization of the inner residues, a fraction of 30 Â 30 Â 30 mm 3 of the residue collected from cone calorimeter test was analyzed using non-destructive Analysis X-ray micro-tomography EasyTom with a LaB6 filament and a flat panel with a resolution of 2320 Â 2336. The volume of material is 30 Â 30 Â 30 mm 3 with a resolution of 34 microns. The volume reconstruction was obtained from a set of 1900 slices obtained after 1800 rotations. The diagram of the analysis is shown in Fig. 2.

Scanning electron microscopy coupled with Energy Dispersive X-ray spectroscopy (SEM-EDX)

Chars were observed using a FEI Quanta 200 scanning electron microscope and char composition (C, O, P atoms) was determined by Energy Dispersive X-ray spectroscopy (EDX) using Oxford INCA Energy 300 system and a detector of 133 eV (at Mn). For each char, the composition of at least three zones was analyzed. Note that hydrogen content cannot be measured using SEM-EDX.

Results and discussion

Micro-scale behavior

Thermogravimetric (TG) and differential thermogravimetric (DTG) curves are shown in Fig. 3 for all samples. The main data are summarized in Table 2. The phosphorus-free resin is decomposed in one step starting at 300 C and leaving a small residue. The incorporation of phosphorus reduces only slightly the thermal stability (measured using the temperature for 5% of mass loss T 5% ). The peak of mass loss rate decreases and its temperature increases by a few degrees. A shoulder is observed at higher temperatures (close to 400 C). For 30IL, a second shoulder is also noted above 450 C. The residue content is higher (around 20 wt%) than for 0IL (10.4 wt%). Its value is stable whichever the phosphorus content. Around 400 C, the discrepancy between the residue contents for 0IL and for phosphorus-based epoxy resins is higher: 21 wt% and 40e42 wt% respectively.

Heat release rate curves from PCFC tests are shown in Fig. 4. The main data from these tests can be found in Table 2. Even if the heating rate is higher than in TGA (60 C/min versus 10 C/min), the curve profiles are similar in both tests. The decomposition occurs through one main step but shoulders can be observed at high temperature, particularly for 30IL. Phosphorus slightly reduces the thermal stability (the heat release starts at lower temperature than for 0IL) but the temperature of the peak T pHRR is slightly higher (382e391 versus 374 C for 0IL). The peak of heat release rate, related to the peak of mass loss rate in TGA, decreases linearly with the phosphorus content (see Fig. 5). Such a relation was already observed for epoxy resins flame retarded with phosphoruscontaining additives [START_REF] Menard | Synthesis of biobased phosphorus-containing flame retardants for epoxy thermosets: comparison of additive and reactive approaches[END_REF][START_REF] Menard | Synthesis of biobased phosphate flame retardants[END_REF]. Total heat release (THR) is lower for phosphorus-containing resins (22.6e23.9 kJ/g versus 26.5 kJ/g for 0IL). Char contents are slightly lower in PCFC than in TGA. As noted by Müller et al. [START_REF] Müller | Melamine poly(zinc phosphate) as flame retardant in epoxy resin: decomposition pathways, molecular mechanisms and morphology of fire residues[END_REF], this difference is due to the higher heating rate in PCFC. Nevertheless in both cases, the char content for ILcontaining resins is more or less constant and slightly higher than for 0IL. The heat of complete combustion of the volatiles (h c,gas ) is calculated as the THR in PCFC divided by the mass loss. The heat of complete combustion of volatiles is similar for all epoxy resins (around 27e28 kJ/g).

On the whole, the incorporation of phosphorus from ionic liquid IL169 allows to reduce the peak of heat release rate and to moderately increase the residue content without lowering the thermal stability, which is quite often observed when phosphorus flame retardants are added. The reduction in pHRR with increasing phosphorus content can be attributed to a change in the thermal decomposition mechanism that results in less heat being released over a broader temperature range. While some flame retardant effects are not available at microscale, cone calorimeter tests were also performed.

Cone calorimeter results (bench scale)

Fig. 6 shows the heat release rate curves for all materials studied in cone calorimeter at 35 kW/m 2 . Table 3 summarizes the main data. The phosphorus-free epoxy resin (0IL) presents a very high peak of heat release rate (pHRR) (higher than 1000 kW/m 2 ). The residue content is very small and the combustion is almost complete. The increase in phosphorus content allows for a significant reduction of the pHRR to 300 kW/m 2 (-73% for 30IL). The residue content increases up to 39 wt% for 30IL. The effective heat of combustion is lowered (19.1e21.6 kJ/g versus 24.3 kJ/g). Consequently the total heat release is divided by almost 2 (from 23.7 kJ/g for 0IL to 12.7 kJ/g for 30IL). Interestingly, the time-to-ignition remains stable around 100s for all resins. This is not common, as many phosphorus flame retardants promote fast ignition due to a loss in thermal stability. This is in agreement with TG and PCFC analyses: thermal stability is similar whatever the IL169 content. Fig. 7 shows the residues sampled from cone calorimeter tests. It can be observed that the residues from epoxy resins containing ionic liquid IL169 are expanded. The higher the IL169 content, the higher the expansion is during the test. Movie M1 (see supporting information) shows the development of the expanded residue for 30IL. The intumescence starts after ignition and continues throughout the test. Nevertheless, the expansion is quite slow, in comparison to other flame retarded epoxy resins [START_REF] Menard | Synthesis of biobased phosphorus-containing flame retardants for epoxy thermosets: comparison of additive and reactive approaches[END_REF]. The rate of char formation and expansion needs further investigation.

Modes-of-action of phosphorus

Phosphorus can act by modifying pyrolysis mechanisms but also by flame inhibition or barrier effect. These various modes-of-action need to be assessed.

Flame inhibition may be assessed using different parameters, including smoke production rate, combustion efficiency and CO/ CO 2 ratio. Usually, combustion efficiency is calculated as the effective heat of combustion in cone calorimeter divided by the heat of complete combustion of the volatiles (h c,gas ). Nevertheless, in our case, the calculation is questionable, particularly for 30IL because the mass loss is not the same at microscale and at bench scale. However for 0IL and 10IL the char content is similar in both tests. It can be noted that the combustion efficiency is much lower for 10IL: 0.71 versus 0.87 for 0IL. CO/CO 2 ratio curves during cone calorimeter test are shown in Fig. 8. It can be noted that this ratio increases with the IL169 content during burning (100e400 s). The mean value for 30IL is six times the value for 0IL (0.12 versus 0.02). A fast increase in the CO/CO 2 ratio is observed at the end of the test when the flame starts to vanish (the temperature in the combustion zone decreases). Table 3 shows the Smoke Extinction Area (SEA), which represents the effective optical obscuring area generated by 1 kg of mass loss. This parameter depends on the combustion efficiency but also on the degradation rate. SEA is the lowest for the phosphorus-free epoxy resin despite a much higher heat release rate. These observations are not final evidence of a flame inhibition effect provided by IL169, but are in agreement with such an assumption. Fig. 9 presents the residue content in cone calorimeter and the difference between the residue contents in cone calorimeter and in PCFC as a function of phosphorous content in the resin. The residue content in PCFC is lower for 0IL sample (5.5 wt%) than for ILcontaining resins but stable for phosphorus-containing epoxy resins (14.5e16 wt%). On the contrary, the residue content in cone calorimeter linearly increases when IL169 content increases. Müller et al. have shown that the char content from epoxy resins is dependent on the heating rate [START_REF] Müller | Melamine poly(zinc phosphate) as flame retardant in epoxy resin: decomposition pathways, molecular mechanisms and morphology of fire residues[END_REF]. This may explain the slight difference observed between the char contents in cone calorimeter and in PCFC for 0IL and 10IL. On the contrary, a significant gap is observed for 20IL and 30IL. The barrier effect provided by the expanded char in cone calorimeter test is very efficient for 20IL and 30IL. Then a fraction of the underlying polymer is not fully decomposed leading to an increase in the residue content. In this case, the residue is not composed of char only, but also of nonpyrolyzed polymer. The very high amount of residue for 30IL is a convincing indication evidencing the role of the expanded char as barrier layer.

Study of chars

Table 4 shows the elemental composition of the chars sampled from cone calorimeter tests. The residues obtained at the end of cone calorimeter test were picked up in the core of char under a protective top layer for 10, 20 and 30IL. Therefore, they do not correspond to the fraction of non-pyrolyzed resin located at the bottom of the residue. On the contrary, any cohesive layer is formed on the whole surface during burning for 0IL. This sample collected is hardly comparable to that of phosphorus-containing resins. Its composition is very different from the other ones, with a very high content of carbon even if this sample was not protected from heat and oxygen by a top layer at the end of the test (when thermooxidation may occur). The composition of chars from phosphorus-containing resins is quite similar but the content in carbon tends to decrease from 71.6 wt% for 10IL to 65.4 wt% for 30IL. On the contrary, the oxygen and phosphorus contents tend to increase.

Phosphorus contents in residues from PCFC and cone calorimeter tests are plotted versus the initial phosphorus contents in epoxy resins in Fig. 10. The phosphorus content in residues, from PCFC or cone calorimeter, increases linearly with the initial phosphorus content in resin. Moreover PCFC and cone calorimeter tests lead to similar phosphorus contents in residues considering standard deviations.

Considering the phosphorus contents in PCFC residue and in epoxy resin and the mass loss in PCFC analysis, it is possible to calculate the amount of phosphorus remaining in condensed phase. Approximately 30e35% of phosphorus is remaining in condensed phase for the three resins containing IL169. These calculations show that more than 50 wt% of phosphorus is released in vapor phase. This is consistent with a possible flame inhibition effect of phosphorus.

Intumescence is a powerful strategy to reduce fire hazard. For example, intumescent coatings (including epoxy-based coating) are used to protect metallic structures. For such applications, the expanded char integrity must be maintained for a long time. If pyrolysis is believed to be anaerobic during flaming combustion, the char may undergo thermo-oxidation (i.e. aerobic pyrolysis) after flame out. The thermo-oxidative stability of chars sampled from cone calorimeter was assessed using PCFC by changing the oxygen fraction in pyrolyzer as recently proposed by Sonnier et al. [START_REF] Sonnier | Studying the thermooxidative stability of chars using pyrolysis-combustion flow calorimetry[END_REF].

As already mentioned, the residue from 0IL is not sampled from the core of the char and its composition is significantly different. Therefore the method was carried out only on the chars from IL-containing resins (Fig. 11). It appears clearly that the increase in phosphorus content leads to a higher thermo-oxidative stability. The temperature of char degradation increases from 10IL to 30IL whichever the oxygen fraction. The increase is about 60 C at an oxygen fraction of 0.2.

It has already been proved that thermal stability is not only related to elemental composition but also to graphitization. The The integration of bands (using a Lorentzian fitted curve method) showed that I D /I G ratio is decreasing for the samples containing phosphorous (2.33, 2.17, 2.56, respectively for 10IL, 20IL and 30IL) compared to the sample without phosphorous (2.94). Thus, it can be assumed that the presence of phosphorous increases the graphitization of char. Nevertheless, the change is not monotonous when increasing phosphorous content. This point is still under investigation.

The efficiency of an expanded char as a barrier layer to heat and gases largely depends on its structure (porosity, density, cell size …). Therefore the expanded char layers from cone calorimeter residues were studied using X-ray microtomography. The thickness of chars increased from 10IL to 30IL, as explained earlier (Fig. 7 e Residues in cone calorimeter), and a closed surface is observed in all cases, but the inner structure also changes to a large extent. Fig. 13 displays the pictures obtained from this measurement for 10IL, 20IL and 30IL samples. Open pores are observed particularly for 20IL and 30IL. The size of cells increases with increasing phosphorous percentage and is particularly high for the chars from 20IL and 30IL (several cm 3 ). There are more cells in the char from 10IL and the size of cells is smaller but the expansion is much more limited. Moreover, there is a hollow space on the back side of this char (Fig. 14).

Conclusions

Phosphorus-based ionic liquid is able to impart flame retardancy of epoxy networks wherein it is used initially as hardener. The peak of heat release rate and the total heat release in cone calorimeter decrease by 73% and 48% respectively in the presence of 30 phr of IL169, without reducing the time-to-ignition. Increasing incorporation of IL169 leads to increasing phosphorus content in char. Phosphorus improves the graphitization as well as the thermo-oxidative stability of the char. Expanded char allows protecting the underlying polymer even if its structure is not fully optimized.

Phosphorus modifies the decomposition pathway and promotes charring but may also act as flame inhibitor. It is interesting to note that both the cation and the anion of IL169 contain phosphorus. Different phosphorus moieties may act differently in condensed and gaseous phases in the studied materials.

Various ionic liquids containing phosphorus in different oxidation states or other flame retardant elements (as bromine or

Fig. 1 .

 1 Fig. 1. Chemical structures used in this study.

Fig. 3 .

 3 Fig. 3. TG (top) and DTG (bottom) curves for studied epoxy resins (under nitrogen flow).

Fig. 4 .

 4 Fig. 4. Heat release rate curves for epoxy resins in test.

Fig. 5 .

 5 Fig. 5. Peak of heat release rate in PCFC test versus phosphorus content in epoxy networks.

Fig. 6 .

 6 Fig. 6. Heat release rate curves for epoxy cured with amine (D230) and ionic liquid (IL169) in cone calorimeter test (heat flux 35 kW/m 2 ).

Fig. 7 .

 7 Fig. 7. Residues after cone calorimeter tests (from left to right: 0IL, 10IL, 20IL, 30IL).

Fig. 8 .Fig. 9 .

 89 Fig. 8. CO/CO 2 ratio curves for the epoxy resins during cone calorimeter tests.

Fig. 10 .Fig. 11 .Fig. 12 .

 101112 Fig. 10. Phosphorus content in char for residues sampled from cone calorimeter and PCFC tests.

Fig. 13 .

 13 Fig. 13. X-ray microtomography images of chars from phosphorus-based epoxy resins.

Fig. 14 .

 14 Fig. 14. Pictures of residues at the end of cone calorimeter test for 10IL sample, a) front side b) back view-Red ring corresponds to the hollow at back side of sample. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1

 1 Formulations studied in this article.

	Formulation Glass transition temperature ( C) Curing agent Curing agent part (phr) Curing conditions	Post-curing conditions Phosphorus content (wt%)
	0IL	90	D230	34	2 h at 80 C, 3 h at 125 C 1 h at 200 C	0
	10IL	114	IL169	10	2 h at 80 C, 3 h at 160 C 3 h at 200 C	1.45
	20IL	108	IL169	20	2 h at 80 C, 3 h at 160 C 3 h at 200 C	2.67
	30IL	93	IL169	30	2 h at 80 C, 3 h at 160 C 3 h at 200 C	3.69

Fig. 2. Diagram of X-ray tomography analysis.

Table 2

 2 Main data from thermogravimetric analyses and PCFC tests.

	Formulation	P content (wt%)	TGA			PCFC
			T 5% ( C)	T peak ( C)	Residue at 700 C (wt%)	pHRR (W/g)	T pHRR ( C)	THR (kJ/g)	Residue at 750 C (wt%)	h c,gas (kJ/g)
	0IL	0	344	363	10.4	555	374	26.5	5.5	27.9
	10IL	1.45	343	377	21.5	506	382	22.6	15.5	26.8
	20IL	2.67	334	374	21.1	470	391	22.9	16	27.3
	30IL	3.69	331	369	18.6	388	383	23.9	14.5	27.9

Table 3

 3 Main data from cone calorimeter tests (heat flux 35 kW/m 2 ).

	Formulation	P content (wt%)	TTI (s)	pHRR (kW/m 2 )	THR (kJ/g)	Residue (wt%)	EHC (kJ/g)	SEA (m 2 /kg)
	0IL	0	105	1099	23.7	2.5	24.3	810
	10IL	1.45	96	629	16.2	15.1	19.1	1138
	20IL	2.67	104	345	16.3	23.8	21.6	949
	30IL	3.69	102	300	12.7	39.0	20.9	946

Table 4

 4 Elemental composition of chars.

	Formulation	From cone calorimeter
		C (wt%)	O (wt%)	P (wt%)
	0IL	85.4	14.5	0
	10IL	71.6	23.5	4.7
	20IL	69	25.6	5.5
	30IL	65.4	26.5	8.1

chlorine) are available. They offer the possibility to finely tune the fire behavior of the epoxy networks through a reactive approach. Some of them are currently under investigation.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.polymdegradstab.2016.10.009.


Nicolas Hernandez

Rodolphe Sonnier
email: rodolphe.sonnier@mines-ales.fr

Ste ´phane Giraud

Influence of grammage on heat release rate of polypropylene fabrics

Keywords: Polypropylene, fabric, cone calorimeter, thermally thin materials

The flammability of nine polypropylene fabrics or sheets has been tested using cone calorimeter at various heat fluxes (25, 35, 50, and 75 kW/m 2 ) in order to assess the relevance of this fire test for thermally thin materials. The chosen procedure uses a grid and allows maintaining a constant exposed surface during the test, except for the lightest fabric. The structure of the knitted fabrics has a relatively small influence on the main flammability parameters. On the contrary, the area density of the sample (from 218 to 5729 g/m 2 ) impacts strongly the time to ignition, the peak of heat release rate, and the increase in heat release rate after ignition (fire growth rate). At a fixed heat flux, thicker is the sample, higher are the time to ignition and the peak of heat release rate and lower is the fire growth rate. Moreover, thick samples exhibit the highest sensitivity of peak of heat release rate and the lowest sensitivity of fire growth rate to heat flux. This study emphasizes the fact that a moderate change in weight may have a significant influence on cone calorimeter results, without any significance on real flammability.

Introduction

Heatproof and flame-retardant (FR) textiles have applications not only for protective clothing but also in areas where fire protection is essential, that is, public transport or places open to the public. These materials fall into the composition of multiple products for furniture, bedding, seat covers, housing (wall coverings), structural composite materials, filters, acoustic and thermal insulation panels, and so on. The textile fire behavior [START_REF] Lewin | Flame retardance of fabrics[END_REF][START_REF] Horrocks | Textiles[END_REF] and the associated fireproofing principles [START_REF] Weil | Flame retardants in commercial use or development for textiles[END_REF][START_REF] Horrocks | Flame retardant challenges for textiles and fibres: new chemistry versus innovatory solutions[END_REF] have already been the subject of numerous bibliographic summaries. Industrial requirements for FR textiles are linked to the legislation and standards that define the required levels of fire resistance. These standards, whether national or international, mandatory or recommended, are specific to the sectors of application (transport, building, furnishing, etc.). Most of fire tests only allow rating textiles without providing relevant data to study in detail their fire behavior.

Methods based on oxygen depletion measurements are powerful tools available in many laboratories. [START_REF] Huggett | Estimation of rate of heat release by means of oxygen consumption measurements[END_REF] Pyrolysis-combustion flow calorimetry has been proven to provide reliable measurements on flame-retarded fibres [START_REF] Yang | Investigation of the flammability of different textile fabrics using micro-scale combustion calorimetry[END_REF] but some phenomena cannot be observed at microscale. [START_REF] Sonnier | Combining cone calorimeter and PCFC to determine the mode of action of flame-retardant additives[END_REF] Cone calorimeter is probably the device most used in research laboratories to study the flame retardancy because it provides many data such as time to ignition (TTI), mass loss, and heat and smoke release. Moreover, the geometry of the test is relatively simple: a radiant cone delivers a controlled and homogeneous heat flux on the sample surface. A comprehensive article has reviewed this test in great details. [START_REF] Schartel | Development of fire-retarded materials-Interpretation of cone calorimeter data[END_REF] Unfortunately, cone calorimeter is not optimized for thin specimens, [START_REF] Schartel | Development of fire-retarded materials-Interpretation of cone calorimeter data[END_REF] as films or textiles.

Thin specimens are difficult to study for several reasons. First, the heat flux may not be fully absorbed. [START_REF] Fina | Ignition of polypropylene/montmorillonite nanocomposites[END_REF][START_REF] Jiang | Absorption of thermal energy in PMMA by in-depth radiation[END_REF][START_REF] Sonnier | Influence of multiwall carbon nanotube (MWCNT) dispersion on ignition of poly(dimethylsiloxane)-MWCNT composites[END_REF][START_REF] Alongi | How much the fabric grammage may affect cotton combustion?[END_REF] Second, samples are prone to distort or to blow during heating. Therefore, the exposed surface is not constant and the measurements are not reliable anymore. Third, the response time of the oxygen consumption system is too long for accurate measurements in the case of fast burning. [START_REF] Filipczak | Heat release rate measurements of thin samples in the OSU apparatus and the cone calorimeter[END_REF] Despite these issues, cone calorimeter has been used to study thin materials and textiles. Especially, ignition mechanisms of thermally thin materials have been addressed by several researchers. [START_REF] Babrauskas | Ignitability measurements with the cone calorimeter[END_REF][START_REF] Mikkola | On the thermal ignition of combustible materials[END_REF][START_REF] Nelson | Ignition mechanisms of thermally thin thermoplastics in the cone calorimeter[END_REF][START_REF] Tsai | Experimental and numerical study of autoignition and pilot ignition of PMMA plates in a cone calorimeter[END_REF] It is well known that the dependence of TTI over heat flux is different for thermally thin and thick materials. TTI is proportional to the reverse of heat flux for thermally thin materials (vs the squared reverse of heat flux for thermally thick materials). [START_REF] Mikkola | On the thermal ignition of combustible materials[END_REF][START_REF] Hopkins | Material fire properties and predictions for thermoplastics[END_REF] For a same material, TTI is lower for thinner specimen, especially at low heat flux. Critical heat flux (CHF), that is, the minimum heat flux needed for ignition to occur, has also been studied as an important criterion to assess the fire hazard from a material. [START_REF] Mikkola | On the thermal ignition of combustible materials[END_REF][START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF] Most often, the research works have investigated a series of various materials. There are few works studying and comparing a large series of specimen of same nature but various thicknesses. Some articles have addressed the influence of fabrics parameters such as area density and thickness on their fire behavior measured using cone calorimeter. Alongi et al. [START_REF] Alongi | How much the fabric grammage may affect cotton combustion?[END_REF] have studied three cotton fabrics with an area density (or grammage) ranging from 100 to 400 g/m 2 . Some tests are not reproducible, in particular for the thinnest fabric at low heat flux. TTI at 25 kW/m 2 is higher for an area density of 200 g/m 2 than for an area density of 400 g/m 2 , but it is reverse at 35 kW/m 2 . Total heat release per gram of cotton greatly increases (from 8.3 to 13.2 kJ/g) when increasing the sample weight (i.e. also the area density) despite the fact that the amount of char is higher for the weightiest sample. The authors assume that the normalized peak of heat release rate (pHRR; i.e., pHRR divided by the mass) is the only intrinsic characteristic of cotton. Nevertheless, only three cotton fabrics were tested.

Tata et al. [START_REF] Tata | Optimization of the procedure to burn textile fabrics by cone calorimeter: part I. Combustion behavior of polyester[END_REF] have studied the influence of sample geometry and test conditions on the flammability of four polyester fabrics, one of which is flame retarded. The area density ranges from 148 to 574 g/m 2 . They found that the use of a grid to maintain the specimen improves the repeatability of the results. They observed that the fabric structure, the area density, and the fiber diameter influence the TTI and the pHRR. When several layers of fabrics are superimposed, the pHRR increases, but the influence on TTI is unclear.

Nazare´et al. [START_REF] Nazare´s | Use of cone calorimetry to quantify the burning hazard of apparel fabrics[END_REF] have studied the flammability of fabrics (area density from 71 to 180 g/m 2 ) using a cone calorimeter at different heat fluxes. They also observed that a grid improves the reproducibility. On the contrary, the stacking of several layers is not satisfying, increases the standard deviation, and leads to some inconsistent results. They also were further in the interpretation of data, calculating CHF and pHRR at 0 kW/m 2 . They suggest that the area density is an important parameter, but the studied fabrics were very different (cotton, silk, wool, acrylic, and polyester/cotton) preventing a more detailed comparison.

In comparison to the previous studies, this study deals with more samples (all based on polypropylene (PP)) and a larger range of area density from 255 to 5729 g/m 2 . Moreover, all fabrics have been prepared using the same yarn (70 Tex) to avoid a possible effect of the fiber diameter on the flammability data.

The objectives of this work are as follows:

To identify the most relevant parameters to study the flammability of thermally thin samples in cone calorimeter tests;

To assess if fabrics have specific behavior during cone calorimeter tests (in comparison to dense sheets); To evaluate the role of knitted structure (if any) on the flammability.

We have chosen to consider the results as measured without any further corrections on heat release rate (HRR) [START_REF] Filipczak | Heat release rate measurements of thin samples in the OSU apparatus and the cone calorimeter[END_REF] or external heat flux (which is only partially absorbed by thin samples).

Materials and methods

Materials

A commercial grade of homopolymer PP, PPH 9069 from Total, with a Melt Flow Index of 25 g/10 min was used to prepare fabrics and sheets. The PP pellets were dried at 60°C for 24 h before use (injection-molding or spinning).

Processing

Injection-molding. PP sheets with a surface area of 100 cm 2 and a thickness ranging from 1 to 6 mm were prepared by molding-injection at 210°C (mold temperature 40°C) using an injection-molding machine KraussMaffei 50T-KM50/180CX. Spinning. PP pellets were fed in hopper of the melt spinning machine (driver SPINBOY I Busschaert Engineering, Belgium), before drying during 12 h at 60°C. After passing thought the single-screw extruder, a volumetric pump ensures injection with a rotation speed of 13 r/min, thereby ensuring a constant flow through holes with a diameter of 1.2 mm. In total, two bundles of 40 monofilaments were obtained which are cooled in air and combined into a multifilament. The final melt-spun multifilament consists of 80 monofilaments which were coated by spin finish oil and rolled up on two heated rolls with varying speeds (S1 and S2) to ensure a draw. The theoretical drawing is given by the draw ratio (DR) = S2/S1. Spinning parameters of melt spinning device are shown in Table 1. The obtained PP yarn has a count of 70 Tex (g/km). Knitting. Fabrics were knitted with a computerized flat knitting machine SES122ff Shima Seiki (Japan) with E7 gauge (seven needles by inch). Different structures were prepared with various thickness and area density. Several machine parameters such as stitch density or number of yarns used, allowed having a similar density for each structure. Some properties of fabrics are given in Table 2 for information. Note that cone calorimeter results appeared to be particularly related to area density of fabrics and sheets. Therefore, the discussion in the following will consider only this physical parameter.

Cone calorimeter

The flammability was investigated using a cone calorimeter (Fire Testing Technology) according to the ISO 5660 standard. A horizontal sample sheet of 100 3 100 mm 2 was placed at 2.5 cm below a conic heater (Figure 2). The influence of various test conditions has already been discussed, for example, by Babrauskas and Parker. [START_REF] Babrauskas | Ignitability measurements with the cone calorimeter[END_REF] The role of substrate on ignition of materials, especially for thermally thin materials, has been highlighted. [START_REF] Babrauskas | Ignitability measurements with the cone calorimeter[END_REF] These authors proposed to use a special sample holder to test fabrics as air-supported materials. Nevertheless, in our study, the fabrics were isolated by rock wool as in standard tests. The edge effects were also pointed out, especially for wood specimen. [START_REF] Babrauskas | Ignitability measurements with the cone calorimeter[END_REF] In this study, the edges as well as the bottom of the fabrics were covered by aluminum foil as usual. The samples were exposed to various heat fluxes (25, 35, 50 and 75 kW/m 2 ) in well-ventilated conditions (air rate 24 L/s) in the presence of a spark igniter to force the ignition. To avoid the distortion of samples during the test, a grid was used as suggested by Tata et al. [START_REF] Tata | Optimization of the procedure to burn textile fabrics by cone calorimeter: part I. Combustion behavior of polyester[END_REF] The influence of the grid is discussed in Supporting Information 1. HRR was determined according to oxygen depletion (Huggett's 5 relation). In total, 8-12 samples were tested for each formulation. Mean values are calculated for each sample and each heat flux, and the coefficients of determination of the curves 1/TTI (or pHRR or fire growth rate (FGR)) = f(heat flux) are calculated from these mean values.

PP is a noncharring polymer which is fully degraded during cone calorimeter tests even at the lowest heat flux (25 kW/m 2 ). Moreover, combustion is almost complete in all cases. Therefore, effective heat of combustion (and total heat release) is similar for all tests (40 6 2 kJ/g). Moreover, mass loss rate can be easily calculated by dividing HRR by effective heat of combustion. Therefore, this work is focused on properties as TTI and pHRR. Moreover, the increase in HRR just after ignition (called FGR) is especially considered. FGR is calculated from the time when HRR exceeds 50 kW/m 2 until HRR reaches 90% of pHRR (see Figure 3). The 6-mm-thick sample exhibits an intermediate thick noncharring behavior, that is, the ignition is followed by a first increase in HRR. Then, a plateau occurs and a second increase in HRR is observed up to pHRR. In this case, the calculated FGR corresponds to the first increase in HRR.

FGR is a measurement of the fire growth rate as the well-known FIGRA (which is also an acronym for fire growth rate) defined as the maximum value of the function (HRR)/ (elapsed test time). Both parameters are not calculated in the same way but have the same dimension (in kW/m 2 s). From our point of view, FIGRA is more difficult to use because it does not really represent how the heat is released contrary to the defined FGR parameter which represents the true acceleration of heat release. Supporting Information 2 summarizes some additional results about the measurement of the upper surface temperature during cone calorimeter tests. These measurements were carried out to check whether some relevant parameters such as temperature at ignition change for fabrics. No significant difference in temperature at ignition, temperature at plateau during burning, or heating rate before ignition was observed.

Results and discussion

As examples, Figures 4 and5 show the HRR curves for all materials for heat flux of 35 kW/m 2 . Not only TTI, pHRR and FGR change in a large extent, but the whole HRR curve is dependent on the sample aspect. Nevertheless, at the end of the test, a fraction of PP tends to accumulate under the edges of sample holder. Therefore, the final part of the curve is less reproducible and difficult to analyze.

Thermally thin behavior

The thermal behavior of a sample depends on its thermophysical properties, its thickness L, and the external heat flux. Lyon et al. [START_REF] Lyon | Combustibility of cyanate ester resins[END_REF] proposed a criterion to assess whether the sample is thermally thin (equation ( 1)). This criterion needs to know the thermal diffusivity of the material. In our case, considering thermophysical properties of PP from literature, it is possible to assess the behavior of dense PP sheets. Specific heat of PP is 1.88 kJ/kg K, thermal conductivity is 0.15 W/m K, and density is 902 kg/m 3 . Therefore, thermal diffusivity a is 0.88.10 27 m 2 /s. [START_REF] Lyon | Polymer flammability[END_REF] A 1-mm-thick sheet exhibits a thermally thin behavior for all heat fluxes while the criterion is not respected at all heat fluxes for thicker sheets (criterion fails at 75 kW/m 2 for 2mm-thick sheets and at 35, 50, and 75 kW/m 2 for 6-mm-thick sheets). This criterion must be considered as an indication and not as a strict limit between two extreme behaviors. In all cases, the ratio L= ffiffiffiffiffiffiffiffi ffi TTI p is still lower than 4 ffiffiffi a p

L ffiffiffiffiffiffiffiffi ffi TTI p \2 ffiffiffi a p ' 5:93:10 À4 ð1Þ
Thermal diffusivity of the fabrics is not known. But while the weight of knitted structures is always equal or lower than the weight of 1-mm-thick sheet, it is reasonable to consider their behaviors as thermally thin. Moreover, according to the typology proposed by Schartel and Hull, [START_REF] Schartel | Development of fire-retarded materials-Interpretation of cone calorimeter data[END_REF] the HRR curves are typical of a thermally thin behavior, except 6-mm-thick sheet for which the behavior can be described as intermediate thick noncharring.

Ignition

Piloted ignition occurs when pyrolysis gases mix with air and reach the lower flammability limit in the presence of an ignition source. Several criteria were proposed to predict the TTI. Ignition occurs when a critical surface temperature, mass flux, or HRR is reached. [START_REF] Nelson | Ignition mechanisms of thermally thin thermoplastics in the cone calorimeter[END_REF][START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF] The dependence of these criteria on heat flux is usually considered negligible or limited [START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF] even if Rich et al. [START_REF] Rich | Mass flux of combustible solids at piloted ignition[END_REF] have shown that mass flux at ignition changes according to the test conditions, especially heat flux. In this work, we have checked that the surface temperature at ignition does not depend on the heat flux, in good agreement with the above remark.

TTI of thermally thin samples can be calculated using equation (2). [START_REF] Schartel | Development of fire-retarded materials-Interpretation of cone calorimeter data[END_REF] Nevertheless, this equation fails to fit the experimental data in most cases (see in Figure 6 for 1-mm-thick sheet), even by choosing a CHF equal to the value obtained from our experimental data (i.e. 6 kW/m 2 rather than 15 kW/m 2 in Lyon and Quintiere [START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF] ). Several reasons can be proposed to explain this discrepancy. The first one is that thermophysical properties used in equation ( 2) are measured at room temperature but change during heating. The second one is the heat distortion of the sample during the heating. The third one is the partial absorption of heat by thin samples. Fina et al. [START_REF] Fina | In-depth radiative heat transmittance through polypropylene/nanoclay composites[END_REF] have shown that such PP thin samples transmit directly a significant fraction of heat. Alongi et al. [START_REF] Alongi | How much the fabric grammage may affect cotton combustion?[END_REF] have also observed that the transmitted heat flux through cotton fabrics increases during the test. However, equation ( 2) is based on the assumption of total heat absorption by the surface

TTI = Lrc T ig À T 0 q 00 ext À CHF ð2Þ
where L is the thickness of the sample, r the density, c the specific heat, T ig the temperature of the upper surface at ignition, T 0 the room temperature, q 00 ext the external heat flux, and CHF the critical heat flux (15 kW/m 2 according to Lyon and Quintiere [START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF] ). Note that the product Lr corresponds to the area density used in the following.

TTI values are given in Table 3 for all samples and for each heat flux. The slope of the curve 1/TTI = f(heat flux) is also shown with a high coefficient of determination in all cases. Obviously, the TTI decreases when the heat flux increases. Fina et al. [START_REF] Fina | Ignition of polypropylene/montmorillonite nanocomposites[END_REF] have also studied the flammability of a PP (thickness 6 mm) at various heat fluxes. The TTI measured by the authors is consistent with our results in all cases: 117, 70, and 31 s, respectively, at 25, 35, and 50 kW/m 2 .

The difference between the samples is more significant at low heat flux. At 75 kW/m 2 , the difference between the lowest and the highest TTI is only 10 s. For a fixed heat flux, the TTI decreases with the area density of the sample (i.e. with the weight of the sample since the surface area is the same for all samples). Nazare´et al. [START_REF] Nazare´s | Use of cone calorimetry to quantify the burning hazard of apparel fabrics[END_REF] have also observed that TTI of multilayered fabrics increases when more layers are used. Nevertheless, it seems that the TTI is higher for fabrics than for dense sheets at similar area density (compare, for example, Ottoman and 1-mm-thick sheet-see Figure S6 in Supporting Information 3). Double face and Interlock 2 yarns fabrics exhibit close area density and TTI values. One specific behavior is observed for Locknit. Its area density is the lowest one, but its TTI is relatively high at low heat flux. Moreover, its CHF is the highest one among all the samples (19 kW/m 2 ). This behavior is probably due to its dimensional instability (leading to an inconstant exposed surface) but may also be related to the behavior of some very thin samples studied elsewhere. [START_REF] Alongi | How much the fabric grammage may affect cotton combustion?[END_REF][START_REF] Tata | Optimization of the procedure to burn textile fabrics by cone calorimeter: part I. Combustion behavior of polyester[END_REF] Indeed, in these studies, the thinnest samples do not ignite in cone calorimeter test or TTI is longer.

The slope of the curve 1/TTI = f(heat flux) is plotted versus the area density in Figure 7. Interestingly, the slope is almost constant for all PP sheets (between 0.0006 and 0.0008 m 2 / kW s). This is not in agreement with equation ( 2). This may be due to the fact that the behavior of the weightiest samples goes from thermally thin to intermediate or thick. Indeed, at high heat flux, the TTI is the same for 2-, 4-, and 6-mm-thick sheets as predicted by equation ( 3) corresponding to a thermally thick behavior 8

TTI = p 4 krc T ig À T 0 q 00 ext À CHF 2 ð3Þ
where k is the thermal conductivity. The slope may be slightly higher for fabrics, which means that the ignition of these samples is more sensitive to heat flux (compare 1-mm-thick sheet and Ottoman). Nevertheless, the differences with dense sheets are not very obvious except for Locknit for which the dimensional instability makes the measurements more suspicious. Data from the work of Nazare´et al. [START_REF] Nazare´s | Use of cone calorimetry to quantify the burning hazard of apparel fabrics[END_REF] were used to calculate the same slope for six different fabrics having a low area density (\180 g/m 2 ). The slopes ranged from 0.0021 to 0.006 m 2 /kW s, that is, much higher than the values found in our work for denser fabrics. This confirms that the slope increases when area density decreases. pHRR pHRR for all samples and each heat flux is given in Table 4. Obviously, pHRR increases when the heat flux increases. At low heat flux, the pHRR seems to be linearly related to the area density. But at higher heat flux, the curve pHRR = f(area density) is slightly more curved (see Figure S7 in Supporting Information 4). Maybe, it is only due to the results obtained for Locknit which are less reliable. The relation between the area density and the pHRR can be explained easily. In thermally thin samples, the development of the pyrolysis zone and then its greatest thickness, are not limited by the heat conduction (which transfers the heat from the surface to the deeper layers) but by the thickness of the sample. In other words, the pyrolysis zone extends continuously until all the material is pyrolyzed. Therefore, the pHRR is lower for lighter fabrics than for heavier dense sheets. Ottoman fabric and 1- mm-thick sheet exhibit similar pHRR (except at 50 kW/m 2 ). Same conclusion can be drawn for Interlock 2 yarns and double face. Note that Nazare´et al. [START_REF] Nazare´s | Use of cone calorimetry to quantify the burning hazard of apparel fabrics[END_REF] have also observed an increase in pHRR for multilayered fabrics when the number of layers increases. Nevertheless, they also noted that pHRR of some fabrics becomes constant for more than two or three layers. This result observed in our study is very important for the interpretation of the flame retardancy. Indeed, many treatments for fabrics like coatings lead to increase significantly the weight. It can be expected that the pHRR of such treated fabrics lowers just or partly due to the increase in area density (and not due to the efficiency of the FR treatment).

Alongi et al. [START_REF] Alongi | How much the fabric grammage may affect cotton combustion?[END_REF] claimed from cone calorimeter results on three cotton fabrics that the normalized pHRR (i.e. pHRR divided by the mass) is the only intrinsic characteristic of cotton. Nevertheless, it is clearly not the case in this study. The normalized pHRR is not constant nor over the range of tested samples, neither over the heat flux.

The slope of the curve pHRR = f(heat flux) is also given in Table 4, and the coefficients of determination R 2 are still very good (see an example in Figure 8) except for Locknit. As observed by Nazare´et al. [START_REF] Nazare´s | Use of cone calorimetry to quantify the burning hazard of apparel fabrics[END_REF] for different fabrics, pHRR is directly proportional to heat flux. Once again, the results show a clear influence of the area density (Figure 9). The slope increases when the area density increases, but for the weightiest samples (4-and 6-mm-thick sheets), the slope becomes constant. It means that the pHRR is less sensitive to heat flux for the lightest samples. When heat flux increases, the pyrolysis zone can further extend. But in thinner samples, this extension is limited by the shortage of fuels. Then, the increase in pHRR is rapidly limited. This limitation is less obvious for thicker samples. Nazare´et al. [START_REF] Nazare´s | Use of cone calorimetry to quantify the burning hazard of apparel fabrics[END_REF] have also measured the same slope for a series of fabrics. While the area density of these fabrics was very low (\200 g/m 2 ), the slope was also lower than those measured for PP fabrics. Their values were added in Figure 9 and are in good agreement with the tendency pointed out in this article.

FGR

The FGR after ignition is not a common parameter considered in cone calorimeter studies. Nevertheless, it is interesting to consider it for thermally thin materials, since we have seen that pHRR is dependent on the fuel shortage rather on the intrinsic properties of the material. Table 5 summarizes the data for all samples at each heat flux. It appears that the FGR increases when the heat flux increases. Moreover, once again, there is a clear relation between the FGR at a fixed heat flux and the area density (see, for example, Figure S8 in Supporting Information 5): higher is the area density, lower is the FGR. This may explain by the fact that in thick samples, the heat absorbed on surface is partially transferred by conduction to the deeper layer. Therefore, the pyrolysis zone extends more slowly.

Figure 10 shows that the FGR increases linearly with the heat flux. The slopes of the curve FGR = f(heat flux) for all samples are given in Table 5. In all cases, the coefficient of determination is good (or acceptable for Locknit). Figure 11 shows the change in slope values versus the area density. Except once again Locknit, it can be observed that the slope decreases when the area density increases. It means that the sensitivity of FGR to the heat flux is lower for thicker samples. The reason is probably related to the heat conduction which may transfer the heat from the surface to the bulk. This phenomenon is proportional to the heat gradient into the sample. When the heat absorbed on surface increases (due to higher heat flux), the heat transfer by conduction also increases. Then, the extension of the pyrolysis zone when increasing heat flux is slower than for thermally thin samples (where heat gradient is small). An interesting observation is that the influence of area density on the slopes of the curves pHRR = f(heat flux) and FGR = f(heat flux) is reverse (compare Figures 9 and11). Surprisingly, the product of the two slopes is roughly constant if Locknit is excluded: 4.8 (60.9) s 21 . We have explained that the slope of pHRR = f(heat flux) increases for thicker samples because there is no limitation of the extension of the pyrolysis zone due to fuel shortage. But for thicker samples, the slope of FGR = f(heat flux) is rather low because the heat conduction transfers the heat absorbed on the surface to the deeper ''cold'' layers. It appears that both phenomena compensate each other. The meaning of this observation, as well as the value of the product of both slopes, needs further investigations.

CHF

CHF corresponds to the lowest heat flux allowing the ignition of the sample. It is usually calculated from the curve TTI 21 = f(heat flux). CHF does not seem to depend on the area density and ranges from 5 to 19 kW/m 2 according to the sample. The mean value is 12 (65) kW/m 2 in good agreement with the value given in Lyon and Quintiere: [START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF] 11-15 kW/m 2 . Nazare´et al. [START_REF] Nazare´s | Use of cone calorimetry to quantify the burning hazard of apparel fabrics[END_REF] have also calculated CHF for a series of fabrics. CHF ranged from 9 to 20 kW/m 2 as in our study. Only two of these fabrics were of same nature (cotton). CHF of heavy cotton (180 g/m 2 ) was 10 versus 17 kW/m 2 for light cotton (87 g/m 2 ). The difference is in the same order as for PP fabrics studied in our article and may not be significant. Babrauskas and Parker 14 have noted that the CHF for various commercial fabrics is in the same narrow range (10 6 5 kW/m 2 ) despite the great variability of the fabrics (based on cotton, wool, polyolefin, and polyurethane). Mikkola and Wichman [START_REF] Mikkola | On the thermal ignition of combustible materials[END_REF] have measured the CHF of various materials. Interestingly, they found the same value (12 kW/m 2 ) for thermally thin and thick spruce. All these results allow concluding that CHF is not greatly influenced by the area density. If the CHF is in the same range for all the materials tested in the articles cited above, it is probably because these materials are not intrinsically fire resistant. Indeed, according to Lyon and Quintiere, [START_REF] Lyon | Criteria for piloted ignition of combustible solids[END_REF] CHF is much higher (above 20-25 kW/m 2 and up to 50 kW/m 2 ) only for halogen-containing polymers and fire-resistant polymers such as polyimides.

It is interesting to check whether CHF may be calculated from the curves pHRR = f(heat flux) and FGR = f(heat flux). Considering the former curve, CHF is found too low and close to 0 kW/m 2 : 24 (66) kW/m 2 (without considering Locknit). On the contrary, CHF is correctly estimated from the curve FGR = f(heat flux). The mean value is 12 (64) kW/m 2 .

Conclusion

Despite the melting of PP before ignition and the very small amount of fuels for some samples, relevant conclusions can be drawn about the influence of physical properties on flammability. Only the lightest fabric (Locknit-sample weight 2.3 g) exhibits a dimensional instability despite the use of a grid, which makes the data more suspicious.

The cone calorimeter results show that for thermally thin samples, the area density impacts significantly not only the main flammability parameters (such as TTI, pHRR, or FGR after ignition) at a fixed heat flux but also the sensitivity of these parameters to the heat flux. In other words, it is difficult to compare two thermally thin materials using cone calorimeter test if they do not have a very similar area density. The treatments used to impart flame retardancy to fabrics can increase the area density. Therefore, the change in flammability may be partially assigned to the change in weight rather than to the FR effect.

Interestingly, the sensitivities of pHRR and FGR change adversely with area density. Higher is the area density, lower is the sensitivity of FGR to heat flux and higher is the sensitivity of pHRR. This can be explained, respectively, by the conduction of the heat from the surface to the bulk and by shortage of fuels.

Finally, it can be observed that the precise structure of the knitted fabrics has only a negligible influence on the flammability except for TTI (moderate influence). The TTI is higher for fabrics than for sheets at similar area density.

Note also that this study focuses on PP due to the numerous available knitted structures. PP melts at 160°C, and therefore, the knitted structure is fully distorted during heating before ignition. The results of this study may be extended to other noncharring thermoplastics, but it is expected that charring or heat-resistant materials (flax and other natural fibers, Kevlar, etc.) may exhibit a completely different behavior.

Figure 1 .

 1 Figure 1. Pictures of the knitted fabrics studied in this work.

Figure 2 .

 2 Figure 2. Scheme of sample presentation during cone calorimeter tests.

Figure 3 .

 3 Figure 3. Typical heat release rate curve versus time and main data studied in this work (curve for 1-mmthick PP at 35 kW/m 2 ).

Figure 4 .

 4 Figure 4. Heat release rate curve versus time for PP sheets at 35 kW/m 2 .

Figure 5 .

 5 Figure 5. Heat release rate curve versus time for PP fabrics at 35 kW/m 2 .

Figure 6 .

 6 Figure 6. 1/TTI versus heat flux for 1-mm-thick sheets: experimental values and calculated values using equation (2).

Figure 7 .

 7 Figure 7. Slope of 1/TTI = f(heat flux) curve versus area density of all samples.

Figure 8 .

 8 Figure 8. pHRR versus heat flux for 2-mm-thick sheets.

Figure 9 .

 9 Figure 9. Slope of pHRR = f(heat flux) curve versus area density of all samples and fabrics tested by Nazare ´et al. 21 (dotted line is only done for eye guideline).

Figure 10 .

 10 Figure 10. FGR versus heat flux for 2-mm-thick sheets.

Figure 11 .

 11 Figure 11. Slope of FGR = f(heat flux) curve versus area density of all samples (dotted line is only done for eye guideline).

Table 2 .

 2 Main properties of fabrics.

	Name of knitting fabric	Yarn path diagram	Thickness (mm)	Area density (g/m 2 )	Density (g/cm 3 ) Air permeability a (L/m 2 s)
	Ottoman		5.4 (60.3) 1195 (636) 0.221 (60.009)	2273 (-)
	Interlock 2 yarns		3.0 (60.1) 698 (621)	0.234 (60.006)	3790 (6130)
	Double face		3.3 (60.1) 690 (627)	0.208 (60.004)	1079 (673)
	Interlock		2.6 (60.1) 442 (614)	0.171 (60.008)	5816 (6162)
	Locknit		1.3 (60.1) 218 (611)	0.162 (60.013)	6560 (-)

a Measured using a FX 3300 instrument from Textest.

Table 1 .

 1 Spinning parameter of PP.

	Extruder temperature profile (°C)			Roll 1		Roll 2		Draw
												ratio
	T1	T2	T3	T4	T5	T6	T7	T (°C)	Speed	T (°C)	Speed
									(m/min)		(m/min)
	190	195	200	205	210	210	210	70	150	80	600	4

PP: polypropylene.

Table 3 .

 3 Mean TTI versus heat flux for all samples studied in this work.

	Structure	Area density (g/m 2 )	TTI (s) 25 kW/m 2 35 kW/m 2 50 kW/m 2 75 kW/m 2	Slope 1/TTI (m 2 /kW s)	R 2
	6-mm-thick sheet 5729	131	68	39	23	0.0007	0.99
	4-mm-thick sheet 3831	101	60	40	24	0.0006	0.99
	2-mm-thick sheet 2048	86	55	38	21	0.0007	0.99
	1-mm-thick sheet 1145	66	46	30	19	0.0008	0.99
	Ottoman	1195	100	59	37	16	0.0011	0.97
	Interlock 2 yarns	698	80	42	26	16	0.0010	0.99
	Double face	690	72	45	29	19	0.0008	0.99
	Interlock	442	71	37	28	16	0.0009	0.99
	Locknit	218	109	54	26	13	0.0014	0.99

TTI: time to ignition.

Table 4 .

 4 Mean pHRR versus heat flux for all samples studied in this work.

	Structure	Area	pHRR (kW/m 2 )			Slope	R 2
		density (g/m 2 )	25 kW/m 2	35 kW/m 2	50 kW/m 2	75 kW/m 2	pHRR
	6-mm-thick sheet	5729	913	1252	1551	2100	23.0	0.99
	4-mm-thick sheet	3831	607	746	1235	1764	24.0	0.99
	2-mm-thick sheet	2048	399	598	743	1093	13.4	0.99
	1-mm-thick sheet	1145	306	386	725	877	12.0	0.93
	Ottoman	1195	286	364	509	870	11.8	0.98
	Interlock 2 yarns	698	276	316	422	743	9.5	0.96
	Double face	690	320	348	480	710	8.1	0.98
	Interlock	442	215	281	369	576	7.2	0.99
	Locknit	218	119	214	238	278	2.8	0.80
	pHRR: peak of heat release rate.

Table 5 .

 5 Mean FGR versus heat flux for all samples studied in this work.

	Structure	Area density (g/m 2 )	FGR (kW/m 2 s) 25 kW/m 2 35 kW/m 2	50 kW/m 2	75 kW/m 2	Slope (s 21 )	R 2
	6-mm-thick sheet	5729	2.3	4.5	6.4	10.7	0.16	0.99
	4-mm-thick sheet	3831	2.5	4.2	7.6	13.5	0.22	0.99
	2-mm-thick sheet	2048	5.0	9.4	13.0	22.7	0.34	0.99
	1-mm-thick sheet	1145	6.5	10.3	22.2	32.7	0.54	0.98
	Ottoman	1195	5.0	7.5	13.7	24.1	0.39	0.99
	Interlock 2 yarns	698	6.6	10.5	16.9	35.4	0.58	0.98
	Double face	690	5.6	10.5	15.9	32.9	0.54	0.98
	Interlock	442	7.6	12.7	18.9	33.8	0.52	0.99
	Locknit	218	5.3	9.8	17.5	21.9	0.33	0.94

FGR: fire growth rate.

Acknowledgments

The authors would like to thank Benjamin Gallard and Loı¨c Dumazert for their help in preparing the sheets and carrying out the cone calorimeter tests.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.