# Flammability of flax plant fractions: a whole-plant study from micro- to bench-scale

Solène Galinier, Clément Brendlé, Johnny Beaugrand, Antoine Barbulée, Nicolas Le Moigne, Rodolphe Sonnier

## ► To cite this version:

Solène Galinier, Clément Brendlé, Johnny Beaugrand, Antoine Barbulée, Nicolas Le Moigne, et al. "Flammability of flax plant fractions: a whole-plant study from micro- to bench-scale. Polymer Degradation and Stability, 2025, 241, pp.111532. 10.1016/j.polymdegradstab.2025.111532. hal-05165025

HAL Id: hal-05165025

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# Flammability of flax plant fractions: a whole-plant study from micro- to bench-scale

Solène Galinier<sup>a</sup>, Clément Brendlé<sup>a</sup>, Johnny Beaugrand<sup>b</sup>, Antoine Barbulée<sup>c</sup>, Nicolas Le Moigne<sup>a, *, #</sup>, Rodolphe Sonnier<sup>a, *</sup>

## ARTICLE INFO

### Keywords:

Fire behaviour

Flammability

PCFC

Cone calorimetry, Lignocellulose, Biopolymers,

Minerals

## ABSTRACT

The flammability of a large set of 7 flax plant fractions, namely roots, shives, scutched fibres, tows, fines, seeds and capsules, has been studied using pyrolysis-combustion flow calorimetry and cone calorimetry in order to characterize the most performant fractions in terms of fire reaction. In addition, scutched fibres from twelve flax varieties with different growth locations and retting conditions have been considered to assess the variability of flammability properties and understand the scattering of results observed in the literature. The fines, produced in small amounts and rich in minerals, are the least flammable fraction, with a low heat release (6.1 kJ/g after anaerobic pyrolysis at microscale) and a high content of mineral-enriched residue (45 wt %). On the contrary, seeds release a large amount of energy (19.6 kJ/g) during combustion due to their oil-rich composition (> 30 wt %). In addition, variables such as flax variety, growth location and retting have a significant influence on the flammability properties of scutched fibres although the respective contributions of these factors cannot be easily discriminated.

## 1. Introduction

Due to sustainability concerns, bioresources are more and more considered as valuable alternatives to synthetic materials. Biopolymers like chitosan, alginate can be used as matrix or binders. They can also be synthesized from monomers derived from bioresources, as is the case for PA11 (using castor oil) or PLA (from fermented starch and sugars) and thermosets such as epoxy (from triglycerides, lignin...). Plant fibres as cotton, flax or hemp are used in textiles but agro and forestry residues have shown clear potential as alternative raw materials for the manufacture of composite materials, i.e. particleboard, polymer composites and bio-based concretes. Miscanthus, hemp shives and cereal straws can be cited as examples [1-5]. Straws are also directly used as bales [6].

France is the world's leading flax producer, with 850,000 tonnes in 2021, i.e. over 50 % of global production, ahead of Belgium and Belarus, with <100,000 tonnes each [7]. Flax is multi-purpose plant, which provides different value-added fractions: its bast fibres, in the form of scutched fibres or tow, are used in textile and composite industries while oil can be extracted from their seeds. Additionally, shives are used for

mulching, bedding [8], energy applications [9], and can also find applications in particle board [10] or as fillers in cement [11,12] or polymer composites [13,14].

Many of these applications require flame-retardant properties. As lignocellulosic materials, flax and its components are carbon rich and obviously flammable. The energy released during combustion is of first importance in assessing the fire hazard of a material. For example, in building industry, materials are classified according to several Euro-classes from A1, for the most performing materials in terms of non-flammability, to F, for the most hazardous materials [15]. For bio-concretes, A1/A2 ratings depend on the heat stored in the material, i.e. the quantity of bioresource and its heat release capacity, which varies from one bioresource to another [16].

The heat released also depends on the lignocellulosic composition, which in turn depends on a couple of parameters such as the species, growth location, climate conditions, as well as specific treatments (example for a jute fabric [17]). But of course, the first influencing parameter is the fraction considered within the bioresource. The anatomy of plants is complex, consisting of several structural and conductive

<footnote><sup>a</sup> Polymers Composites and Hybrids (PCH), IMT Mines Ales, Ales, France</footnote>

<footnote><sup>b</sup> Biopolymères Interactions Assemblages (BIA), INRAE, 44 316 Nantes, France</footnote>

<footnote><sup>c</sup> DEPESTELE, Teillage Vandecandelaere, 5 rue de l'Eglise, 14540 Bourguébus, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: nicolas.le-moigne@mines-ales.fr (N. Le Moigne), rodolphe.sonnier@mines-ales.fr (R. Sonnier).</footnote>
<table><caption>**Table 1**
The three series (s1, s2, s3) of tested flax samples and their main characteristics (variety, fraction, season, retting, location).</caption><thead><tr><th>Series</th><th>Variety</th><th>Plant fraction</th><th>Season</th><th>Retting</th><th>Location (Department)</th></tr></thead><tbody><tr><td rowspan="10">1</td><td rowspan="10">Bolchoï</td><td>scutched fibres</td><td rowspan="10">Spring</td><td rowspan="10">Standard</td><td rowspan="10">Bertrimont (76)</td></tr><tr><td>Tow</td></tr><tr><td>Top-stem shives</td></tr><tr><td>Whole-stem shives</td></tr><tr><td>Capsules</td></tr><tr><td>Straw</td></tr><tr><td>Roots</td></tr><tr><td>Seeds</td></tr><tr><td>Fines</td></tr><tr><td rowspan="4">2</td><td>Lisette</td><td>Scutched fibres</td><td rowspan="4">Spring</td><td>Low</td><td>Etouteville (76)</td></tr><tr><td rowspan="3">Mélina</td><td>Whole-stem shives</td><td rowspan="3">High</td><td rowspan="3">Tôtes (76)</td></tr><tr><td>Scutched fibres</td></tr><tr><td>Whole-stem shives</td></tr><tr><td rowspan="14">3</td><td>Aramis</td><td></td><td rowspan="14">Spring</td><td rowspan="14">Standard</td><td>Douvres-la-Délivrande (14)</td></tr><tr><td>Aretha 1</td><td></td><td>Bretteville-l'Orgueilleuse (14)</td></tr><tr><td>Aretha 2</td><td></td><td>Vacognes-Neuilly (14)</td></tr><tr><td>Bolchoï</td><td></td><td>Bretteville-l'Orgueilleuse (14)</td></tr><tr><td>Damara</td><td></td><td>Potigny (14)</td></tr><tr><td>Daurea</td><td></td><td>Tailleville (14)</td></tr><tr><td>Elixir 1</td><td></td><td>Grainville-Langannerie (14)</td></tr><tr><td>Elixir 2</td><td></td><td>Bretteville-l'Orgueilleuse (14)</td></tr><tr><td>Idéo</td><td></td><td>Occagnes (61)</td></tr><tr><td>Lisette</td><td></td><td>Tailleville (14)</td></tr><tr><td>Malika</td><td></td><td>Cagny (14)</td></tr><tr><td>Olga 1</td><td rowspan="3">Scutched fibres</td><td></td><td rowspan="3">Standard</td><td>Maltot (14)</td></tr><tr><td>Olga 2</td><td></td><td>Avenay (14)</td></tr><tr><td>Olga 3</td><td></td><td>Creances (50)</td></tr><tr><td>Toundra 1</td><td></td><td>Standard</td><td>lfs (14)</td></tr><tr><td>Toundra 2</td><td></td><td></td><td>Reviers (14)</td></tr></tbody></table>

tissues, with very different biochemical compositions and microstructures. Basically, a flax plant is made up of roots (» 10 wt %), stem (» 75 wt %) and ramifications bearing seeds (» 4–7 wt %) inside capsules (» 3 wt %) [18]. Within the stem, shives (up to 50 wt %) come from the conductive tissues, i.e. the xylem, at the woody core of the stem, while bast fibres (25 wt %) are the supporting tissue at the periphery of the stem just below the epidermis and embedded in the cortical parenchyma [19]. These different tissues of the stem are primarily composed of cellulose, hemicelluloses, lignin and pectins, bast fibres being rich in cellulose (up to 80 wt %), shives containing high amount of lignin (up to 20–30 wt %), like roots, and the cortical parenchyma being rich in pectins. The seeds are rich in oil (up to 50 wt %) and proteins (up to 20 wt %) [20].

A first objective of the present work is to assess the thermal stability and the energy released by the different fractions that can be refined from the flax plant. Another motivation for this work is to understand why flax flammability data in the literature are scattered. Indeed, Dorez et al. have studied different bioresources including flax fibres using Pyrolysis-Combustion Flow Calorimeter (PCFC). The authors found that the peak of Heat Release Rate (pHRR) reaches 115 W/g at 371 °C and the Total Heat Release (THR) is 5.8 kJ/g [21]. Hajj et al. investigated the fire behaviour of flax fabric. pHRR was 208 W/g at 364 °C and THR was 9.3 kJ/g [22]. Galaska et al. found similar results for flax fibres, i.e. pHRR of 213 (±1) W/g at 379 (±1) °C and THR of 8.2 (±0.2) kJ/g [23]. Brendlé et al. have analysed short flax fibres, containing shives, and measured a pHRR of 100 W/g at 344 °C and THR of 7.3 kJ/g [24]. The origin, growth conditions and treatments (if any) of flax in these different works

are usually not specified in detail, therefore the cause of the differences in fire behaviour cannot be identified precisely.

To address these two issues, the fire behaviour of several batches of flax materials including different fractions of the flax plant but also various flax varieties, with different retting and growth location, was investigated at micro- and bench-scales using PCFC and cone calorimetry. The results are studied in relation to the biochemical composition of the different fractions, i.e. monosaccharide composition, lignin and extractives contents, as well as flax variety, growth location and retting conditions.

## 2. Materials and methods

### 2.1. Materials

Three series of flax samples, supplied by Depestele (France), were harvested in 2023 in four French departments (Calvados 14, Manche 50, Orne 61, Seine Maritime 76) of the Normandy region (see Table 1):

* Series 1 (s1) gathers different flax plant fractions, i.e. straw, shives from the whole stem and from only the top of the stem, scutched fibres, tows, roots, fines, capsules and seeds, obtained from the mechanical fractionation of the whole plant and belonging to the Bolchoï variety, sown in spring and having undergone standard retting. It should be pointed out that the « fines » fraction can contain significant quantity of soil residues still present on the stems when scutching.
<table><caption>**Table 2**
Biochemical composition of the different flax samples (series 1 and 2).</caption><thead><tr><th>Flax samples (series)</th><th>Flax variety</th><th>Retting</th><th>Monosaccharides (wt%)</th><th>Lignin (wt%)</th><th>Glucose (wt%)</th><th>Extractives (wt%)</th><th>Moisture (wt%)</th></tr></thead><tbody><tr><td>Scutched fibres (s1)</td><td>Bolchoï</td><td>Standard</td><td>87.1 ± 5.2</td><td>2.2 ± 0.1</td><td>71.3 ± 3.5</td><td>5.9</td><td>9.9 ± 1.5</td></tr><tr><td>Scutched fibres (s2)</td><td>Mélina</td><td>High</td><td>88.8 ± 3.4</td><td>2.8 ± 0.6</td><td>71.8 ± 3.5</td><td>7.1</td><td>8.5 ± 1.1</td></tr><tr><td>Scutched fibres (s2)</td><td>Lisette</td><td>Low</td><td>95.1 ± 1.0</td><td>3.0 ± 0.9</td><td>65.8 ± 0.7</td><td>8.6</td><td>9.8 ± 1.0</td></tr><tr><td>Whole-stem shives (s1)</td><td>Bolchoï</td><td>Standard</td><td>67.3 ± 2.7</td><td>25.9 ± 0.5</td><td>33.5 ± 3.4</td><td>6.4</td><td>10.1 ± 0.4</td></tr><tr><td>Whole-stem shives (s2)</td><td>Mélina</td><td>High</td><td>74.5 ± 4.1</td><td>29.0 ± 0.1</td><td>39.9 ± 1.6</td><td>6.6</td><td>9.9 ± 0.3</td></tr><tr><td>Whole-stem shives (s2)</td><td>Lisette</td><td>Low</td><td>64.3 ± 3.4</td><td>28.0 ± 0.9</td><td>36.1 ± 1.7</td><td>6.8</td><td>9.5 ± 1.1</td></tr><tr><td>Tow (s1)</td><td></td><td></td><td>85.2 ± 5.9</td><td>3.6 ± 0.2</td><td>67.1 ± 5.0</td><td>5.7</td><td>11.8 ± 3.0</td></tr><tr><td>Top-stem shives (s1)</td><td></td><td></td><td>64.7 ± 5.1</td><td>28.8 ± 0.4</td><td>30.4 ± 2.5</td><td>6.0</td><td>10.2 ± 0.5</td></tr><tr><td>Straw (s1)</td><td></td><td></td><td>85.2 ± 5.6</td><td>11.9 ± 0.1</td><td>62.4 ± 3.3</td><td>5.1</td><td>10.2 ± 1.4</td></tr><tr><td>Roots (s1)</td><td>Bolchoï</td><td>Standard</td><td>72.3 ± 1.1</td><td>22.4 ± 0.2</td><td>39.6 ± 0.7</td><td>3.7</td><td>9.8 ± 0.4</td></tr><tr><td>Capsules (s1)</td><td></td><td></td><td>55.2 ± 0.8</td><td>22.4 ± 0.3</td><td>26.8 ± 0.6</td><td>12.1</td><td>10.9 ± 0.4</td></tr><tr><td>Seeds (s1)</td><td></td><td></td><td>25.2 ± 1.3</td><td>4.3 ± 0.5</td><td>6.8 ± 0.3</td><td>40.6</td><td>4.5 ± 0.2</td></tr><tr><td>Fines (s1)</td><td></td><td></td><td>25.2 ± 4.4</td><td>6.5 ± 0.9</td><td>11.6 ± 2.0</td><td>12.0</td><td>5.2 ± 1.0</td></tr></tbody></table>

* Series 2 (s2) gathers scutched fibres and whole-stem shives from two spring flax varieties, namely Mélina and Lisette with two retting levels, i.e. high and low respectively.

* Series 3 (s3) gathers scutched fibres from different varieties, i.e. 9 spring flax (Aramis, Aretha, Bolchoï, Damara, Daurea, Elixir, Idéo, Lisette, Malika) and 2 winter flax (Olga, Toundra) varieties. These scutched fibres also differ in their growth location and retting conditions.

Before testing, all the samples were stored at 50 % relative humidity and 23 °C to ensure identical conditioning. Their moisture content was measured on 0.5 g of samples using an infrared moisture balance (MA160-1, Sartorius) heated at 80 °C. 5 replicates were performed on each sample.

## 2.2. Methods

### 2.2.1. Biochemical composition analysis

**Monosaccharides.** Both the identification and the quantification of monosaccharides were performed according to the protocols reported in [25]. In brief, neutral carbohydrates were quantified by gas chromatography (Clarus 580; Perkin Elmer) whereas acidic monosaccharides (uronic acids) were quantified thanks to a colorimetric method. All tests were done in triplicate independent samples and values given are mean values and expressed as a % of the dry matter mass.

**Lignin.** The analyses were performed on three independent spectrophotometric measurements, following the acetyl bromide method as reported earlier [25]. The lignin content is expressed as a percentage of the dry matter.

**Extractives.** Extractives were removed from the Alcohol-insoluble material (AIM) according to a protocol from [26] slightly modified. They were obtained after boiling samples fractions for 20 min in 70 % ethanol (laboratory grade 15 g/250 mL), and were collected after separation of the AIM thanks to filtration on a G2 sintered glass filter. The extractives were then dried from the alcohol by aspiration under vacuum and finally weighed to determine the yield.

### 2.2.2. Oil extraction from flax seeds

Flax seeds were first ground using a Moulinex® AR110830 grinder. Subsequently, 10 g of the dry sample (moisture content: 5.5 %, determined using a Sartorius MA-160 infrared moisture analyzer; sample mass = 1 g, at 23 °C and 57 % RH) were subjected to Soxhlet extraction

with n-hexane for 45 cycles. Finally, the extracted flax oil was recovered using a rotary evaporator.

### 2.2.3. Fourier transform infrared spectroscopy

FTIR spectra were obtained using a Bruker VERTEX 70 spectrometer in attenuated total reflectance mode, by performing 32 scans between 400 and 4000 cm⁻¹ with a resolution of 4 cm⁻¹.

### 2.2.4. Flammability at microscale

A pyrolysis-combustion flow calorimeter (PCFC - Fire Testing Technology Ltd., UK) was used according to the test method ASTM D7309. Pyrolysis was conducted under nitrogen flow (method A - anaerobic pyrolysis) or under air (method B - aerobic pyrolysis). Around 3 mg of sample are placed in a crucible and inserted into a pyrolysis chamber. The chamber is heated up to 750 °C at 1 °C/s. The pyrolytic gases are sent to a second chamber, called combustor, at 900 °C in the presence of N₂/O₂ (80/20) flow. These conditions ensure complete combustion. The energy released is calculated according to the Huggett's relation, which states that 1 kg of oxygen consumed corresponds to an energy release of 13.1 MJ [27]. Each fraction was tested as received. For most of them, particle size is small, and a 2-5 mg sample is representative of the whole fraction. Nevertheless, for some other ones (like capsules or roots), the homogeneity of the fraction at microscale is more questionable. It is the reason why 5 samples for each fraction of series 1 and 2 were tested according to method A in order to assess the mean value and the standard deviation which was considered satisfactory. For series 3, 2 to 3 samples, and for method B, 2 to 4 samples were tested.

### 2.2.5. Fire behaviour at bench scale

A FTT cone calorimeter was used according to ISO 5660-1 standard at an external heat flux of 35 kW/m². Around 20 g of samples are placed in the sample holder of 88.4 cm² with a ceramic plate at the bottom to ensure thermal insulation. Metallic frame was used but no aluminium foil. Sample thickness of the particles' bed depends on the sample density and ranges from 5 to 30 mm. Two samples were tested for each fraction. Additional tests were performed on some fractions at various heat fluxes. Heat release is also measured according to the Huggett's relation. Other recorded data include time-to-ignition, mass loss and smoke opacity.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Composition in Lignin, monosaccharides and extractives of flax samples from series 1 and 2 (circle diameter is proportional to the extractives content), i.e. scutched fibres, shives and various flax plant fractions (tow, roots, capsules, seeds, fines).</figcaption></figure>

<caption>**Table 3**</caption>

Main flammability data measured in pyrolysis-combustion flow calorimetry (method A) for flax samples from series 1 and 2.

<table><thead><tr><th>Flax samples</th><th>pHRR (W/g)</th><th>TpHRR (°C)</th><th>THR (kJ/g)</th><th>Residue fraction (-)</th><th>Δh (kJ/g)</th></tr></thead><tbody><tr><td>Scutched fibres (s1)</td><td>147 ± 9</td><td>372 ± 8</td><td>8.2 ± 0.2</td><td>0.17 ± 0.01</td><td>9.8 ± 0.2</td></tr><tr><td>Scutched fibres (s2)</td><td>120 ± 7</td><td>363 ± 6</td><td>6.2 ± 0.6</td><td>0.20 ± 0.02</td><td>7.8 ± 0.8</td></tr><tr><td>Melina</td><td>107 ± 15</td><td>357 ± 6</td><td>6.6 ± 0.5</td><td>0.22 ± 0.01</td><td>8.4 ± 0.6</td></tr><tr><td>Lisette</td><td>143 ± 7</td><td>370 ± 7</td><td>8.3 ± 0.6</td><td>0.16 ± 0.01</td><td>10.0 ± 0.7</td></tr><tr><td>Tow (s1)</td><td>107 ± 9</td><td>377 ± 9</td><td>9.2 ± 0.6</td><td>0.16 ± 0.02</td><td>11.0 ± 0.6</td></tr><tr><td>Whole-stem shives (s1)</td><td>114 ± 4</td><td>360 ± 7</td><td>8.7 ± 0.8</td><td>0.20 ± 0.01</td><td>10.9 ± 0.8</td></tr><tr><td>Whole-stem shives (s2) Melina</td><td>107 ± 13</td><td>377 ± 6</td><td>8.6 ± 1.0</td><td>0.18 ± 0.02</td><td>10.5 ± 1.1</td></tr><tr><td>Whole-stem shives (s2) Lisette</td><td>113 ± 17</td><td>382 ± 5</td><td>9.9 ± 0.8</td><td>0.17 ± 0.01</td><td>11.9 ± 0.8</td></tr><tr><td>Top-stem shives (s1)</td><td>150 ± 10</td><td>374 ± 5</td><td>9.1 ± 0.8</td><td>0.15 ± 0.01</td><td>10.8 ± 0.9</td></tr><tr><td>Straw (s1)</td><td>121 ± 8</td><td>388 ± 5</td><td>9.6 ± 1.1</td><td>0.16 ± 0.01</td><td>11.4 ± 1.3</td></tr><tr><td>Roots (s1)</td><td>78 ± 10</td><td>369 ± 4</td><td>7.6 ± 1.0</td><td>0.21 ± 0.04</td><td>9.7 ± 1.5</td></tr><tr><td>Capsules (s1)</td><td>178 ± 31</td><td>423 ± 4</td><td>19.6 ± 1.8</td><td>0.14 ± 0.02</td><td>22.7 ± 1.7</td></tr><tr><td>Fines (s1)</td><td>41 ± 8</td><td>367 ± 3</td><td>6.1 ± 1.5</td><td>0.55 ± 0.05</td><td>13.3 ± 2.0</td></tr></tbody></table>

# 3. Results and discussion

## 3.1. Composition of flax samples from series 1 and 2

Table 2 presents the biochemical composition and moisture content (at 50 % RH, 23 °C) of the different flax samples from series 1 and 2. These samples can be gathered in several categories (Fig. 1). The first one gathers scutched fibres and tow having a high monosaccharide content (> 85 wt %) and low lignin (< 4 wt %) and extractives (< 9 wt %) contents. The second one gathers shives having high lignin content (> 25 wt %), a moderate monosaccharide content (64–75 wt %) and a low extractive content (~ 6.5 wt %). Capsules, straw and roots have

intermediate composition. Capsules have slightly lower lignin (~ 22 wt %) and monosaccharide (~ 55 wt %) contents than shives and a higher extractives content (12 wt %). Straw composition is close to that of scutched fibres but lignin content reaches almost 12 wt %. Roots have only slightly lower lignin content than shives (22.4 wt %). Finally, two fractions greatly differ from the above-mentioned lignocellulosic materials. Seeds contain low monosaccharide and lignin contents (25.2 wt % and 4.3 wt % respectively) but a very high extractives content (> 40 wt %) as expected from literature. Literature reports on the use of some valuable extractives as flame retardant components [28] but in that case, it corresponds probably to highly flammable oils. Fines have similar contents in monosaccharide and lignin as seeds, and a moderate extractive content (12 wt %). The sum of these three components barely exceeds 50 wt %. Indeed, as will be discussed below, fines contain a large quantity of minerals, mainly silica. As expected, moisture content ranges between 8.5 and 12 wt % for lignocellulosic samples. Lower moisture content (close to 5 wt %) was measured for seeds and fines.

## 3.2. Micro-scale flammability of flax samples from series 1 and 2

Flammability was firstly studied at microscale using pyrolysis-combustion flow calorimetry according to anaerobic (method A) and aerobic (method B) pyrolysis. Table 3 shows the flammability data obtained with method A. Selected HRR curves can be found in Fig. 2.

For all materials, only one apparent peak of heat release rate is observed, centred at 360–380 °C and corresponding to cellulose decomposition. A shoulder or even a minor peak can be observed for some fractions, especially shives, roots and capsules when the ratio between glucose and other saccharides is low. Indeed, the thermal decomposition of cellulose (formed from glucose monomers) is responsible for the main peak while other saccharides can decompose at lower temperatures, contributing to the shoulder mentioned. The peak temperature is slightly higher for roots (~ 390 °C) and greatly higher for seeds (~ 420 °C). For the latter fraction, the heat release is mainly driven by a component other than cellulose, i.e. by lipids present in extractives (see below). Furthermore, the pHRR is the lowest for fines (41 W/g) and capsules (78 W/g) and the highest for seeds (178 W/g). For the lignocellulosic fractions, i.e. fibres, shives, roots, the pHRR varies in a narrow range between 107 and 150 W/g.

As expected for lignocellulosic materials, the residue fraction ranges from 14 to 22 wt %, except for fines (55 wt %). This high residue content is mainly due to the presence of mineral, as will be evidenced in the
<figure><img src="image_4.png" /><figcaption>Fig. 2. HRR curves versus pyrolysis temperature in pyrolysis-combustion flow calorimeter for flax samples from series 1 (method A).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. pHRR in PCFC versus glucose content (A) and monosaccharide content (B) for flax samples from series 1 and 2.</figcaption></figure>
<caption>**Table 4**</caption>

Main flammability data measured in pyrolysis-combustion flow calorimetry (method B) for flax samples from series 1 and 2 – due to high variability, standard deviations are not done for pHRR.

<table><thead><tr><th>Flax samples</th><th>pHRR1 (W/g)</th><th>TpHRR1 (°C)</th><th>pHRR2 (W/g)</th><th>TpHRR2 (°C)</th><th>pHRR3 (W/g)</th><th>TpHRR3 (°C)</th><th>THR (kJ/g)</th><th>Residue fraction (-)</th><th>Δh (kJ/g)</th></tr></thead><tbody><tr><td>Scutched fibres (s1)</td><td>175</td><td>344</td><td>322</td><td>409</td><td></td><td></td><td>15.8 ± 1.4</td><td>0.01 ± 0.00</td><td>16.0 ± 1.4</td></tr><tr><td>Scutched fibres (s2) Mélina</td><td>253</td><td>335</td><td>326*</td><td>393*</td><td>77*</td><td>422*</td><td>15.7 ± 0.5</td><td>0.00 ± 0.01</td><td>15.7 ± 0.5</td></tr><tr><td>Scutched fibres (s2) Lisette</td><td>518</td><td>321</td><td>76</td><td>432</td><td></td><td></td><td>15.2 ± 0.3</td><td>0.02 ± 0.01</td><td>15.5 ± 0.2</td></tr><tr><td>Tow (s1)</td><td>407</td><td>338</td><td>306*</td><td>388*</td><td></td><td></td><td>15.5 ± 0.7</td><td>0.01 ± 0.02</td><td>15.7 ± 0.6</td></tr><tr><td>Whole-stem shives (s1)</td><td>159</td><td>331</td><td>241</td><td>401</td><td>74</td><td>458</td><td>17.2 ± 0.4</td><td>0.01 ± 0.00</td><td>17.4 ± 0.4</td></tr><tr><td>Whole-stem shives (s2) Mélina</td><td>253</td><td>340</td><td>133*</td><td>432*</td><td>122*</td><td>460*</td><td>17.4 ± 0.6</td><td>0.01 ± 0.01</td><td>17.5 ± 0.6</td></tr><tr><td>Whole-stem shives (s2) Lisette</td><td>237</td><td>336</td><td>204</td><td>428</td><td></td><td></td><td>17.7 ± 0.5</td><td>0.01 ± 0.01</td><td>18.0 ± 0.4</td></tr><tr><td>Top-stem shives (s1)</td><td>210</td><td>336</td><td>173</td><td>437</td><td>102*</td><td>447*</td><td>17.8 ± 0.4</td><td>0.00 ± 0.00</td><td>17.8 ± 0.4</td></tr><tr><td>Straw (s1)</td><td>322</td><td>335</td><td>122</td><td>425</td><td>143*</td><td>443*</td><td>16.9 ± 0.7</td><td>0.01 ± 0.02</td><td>17.1 ± 0.4</td></tr><tr><td>Roots (s1)</td><td>227</td><td>334</td><td>194</td><td>403</td><td></td><td></td><td>17.4 ± 0.8</td><td>0.02 ± 0.02</td><td>17.7 ± 0.6</td></tr><tr><td>Capsules (s1)</td><td>79</td><td>329</td><td>286</td><td>446</td><td></td><td></td><td>18.0 ± 0.7</td><td>0.03 ± 0.02</td><td>18.5 ± 0.3</td></tr><tr><td>Seeds (s1)</td><td>172</td><td>370</td><td>117</td><td>507</td><td></td><td></td><td>23.9 ± 1.6</td><td>0.04 ± 0.01</td><td>24.8 ± 1.5</td></tr><tr><td>Fines (s1)</td><td>61</td><td>336</td><td>36</td><td>487</td><td></td><td></td><td>10.9 ± 1.2</td><td>0.45 ± 0.03</td><td>19.7 ± 1.2</td></tr></tbody></table>

following.

The Heat of complete combustion (Δh) is in the range 8–12 kJ/g for most flax fractions. Once again, this is typical of lignocellulosic materials. Δh seems to be slightly higher for high-lignin fractions (shives, roots, straw) than for low-lignin fractions (fibres, tow, capsules). Δh is significantly higher for fines and even higher for seeds which have the highest extractives content. Indeed, extractives content for fines is 12 wt % of the total sample mass, but 21.8 wt % of its organic fraction (~55 wt %); and the mineral fraction (close to 45 wt % - see below) does not contribute to gas release and hence to heat of complete combustion.

THR is the product of mass loss and heat of combustion. Except seeds (THR = 19.6 kJ/g), THR values range from 6.1 to 9.9 kJ/g. THR is the lowest for fines and some fibre samples, and the highest for shives, straw and roots (THR > 8.5 kJ/g). Nevertheless, it should be pointed out that these differences are rather limited.

Data were compared to those measured for a large set of synthetic, animal and plant-based materials (mostly recorded in our laboratory database and measured in the same experimental conditions) (see Figure S1). As most of plant-based materials (apart from some extractives as oleic oil), flax components exhibit rather lower pHRR, THR and Δh than synthetic polymers. Nevertheless, their relatively low thermal stability as well as their thermally thin behaviour (due to their use as bulk particles – see below for bench scale trials) are the main characteristics that determine their fire hazard.

Based on these results, we can assume that the main component

driving the heat release of the different flax samples tested is cellulose, except for seeds due their lipid-rich composition. Therefore, pHRR should be mainly correlated to the glucose content. At first glance, Fig. 3A shows such a significant correlation. Indeed, pHRR increases from low (fines) to high (fibres, tow and straw) glucose contents. Capsules, shives and roots exhibit intermediate values. Nevertheless, the correlation is not perfect, and some fibres show a relatively low pHRR. A better fit can be obtained by considering all monosaccharides (Fig. 3B), but the correlation remains not fully satisfying, as the other saccharides, from hemicelluloses or pectins, are less thermally stable and decompose at lower temperature. Therefore, their decomposition step only partially overlaps with the cellulose decomposition step and do not contribute as much as glucose to the pHRR. Other microstructural and supramolecular features of the cell walls as intermolecular linkages between bio-polymers may also have an influence, making it difficult to predict pHRR accurately on the basis of biochemical composition alone.

Flammability was also assessed in the case of aerobic pyrolysis (method B). Table 4 presents the main data. HRR curves were more complex to analyse in aerobic conditions. A first peak corresponding to the decomposition of the lignocellulosic materials is generally observed at a lower temperature than in anaerobic conditions. A second pHRR was observed at higher temperatures, due to char oxidation, with the two peaks possibly merging. Moreover, a third pHRR was observed in some cases. An example of this different behaviour for the same material is shown in Figure S2. This variability, which was already observed for

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Δh (kJ/g) in PCFC (method B) versus extractives content in organic fraction (wt %).</figcaption></figure>

<footnote>* not systematically observed.</footnote>
<table><caption>**Table 5**
Main flammability data measured in cone calorimeter for flax samples from series 1 and 2 (heat flux 35 kW/m²).</caption><thead><tr><th>Flax samples</th><th>Density (kg/m³)</th><th>TTI (s)</th><th>pHRR (kW/m²)</th><th>tpHRR (s)</th><th>THR (kJ/g) at 250 s</th><th>Mass loss (g) at 250 s</th><th>EHC (kJ/g) at 250 s</th><th>TSR (m²/g) at 250 s</th><th>EHC (kJ/g) at 50 s</th><th>χ</th></tr></thead><tbody><tr><td>Scutched fibres (s1)</td><td>77</td><td>3 / 6</td><td>164 / 128</td><td>11 / 13</td><td>7.2 / 8.1</td><td>14.3 / 14.7</td><td>10.4 / 11.1</td><td>1.7 / 3.1</td><td>10.7</td><td>1.1</td></tr><tr><td>Scutched fibres (s2) Mélina</td><td>80</td><td>3 / 5</td><td>135 / 128</td><td>11 / 14</td><td>6.3 / 7.2</td><td>12.9 / 12.7</td><td>10.6 / 11.7</td><td>2.0 / 3.8</td><td>10.7</td><td>1.4</td></tr><tr><td>Scutched fibres (s2) Lisette</td><td>77</td><td>9 / 7</td><td>141 / 131</td><td>14 / 15</td><td>6.6 / 6.5</td><td>13.2 / 13.2</td><td>10.1 / 9.9</td><td>10.3 / 6.0</td><td>9.9</td><td>1.2</td></tr><tr><td>Tow (s1)</td><td>83</td><td>2 / 3</td><td>127 / 120</td><td>10 / 13</td><td>6.6 / 6.8</td><td>14.8 / 12.6</td><td>9.5 / 11.2</td><td>4.3 / 3.6</td><td>10.4</td><td>1.0</td></tr><tr><td>Whole-stem shives (s1)</td><td>113</td><td>5 / 7</td><td>150 / 154</td><td>15 / 14</td><td>7.3 / 7.2</td><td>12.2 / 12.3</td><td>11.8 / 11.9</td><td>4.8 / 8.1</td><td>12.2</td><td>1.1</td></tr><tr><td>Whole-stem shives (s2) Mélina</td><td>107</td><td>12 / 11</td><td>153 / 156</td><td>19 / 20</td><td>6.5 / 7.8</td><td>10.1 / 13.4</td><td>9.4 / 12.1</td><td>15.8 / 10.4</td><td>11.7</td><td>1.1</td></tr><tr><td>Whole-stem shives (s2) Lisette</td><td>101</td><td>12 / 7</td><td>145 / 153</td><td>15 / 16</td><td>8.6 / 6.1</td><td>8.5 / 10.9</td><td>11.6 / 11.5</td><td>25.5 / 12.6</td><td>12.9</td><td>1.2</td></tr><tr><td>Top-stem shives (s1)</td><td>156</td><td>10 / 13</td><td>136 / 141</td><td>17 / 19</td><td>6.1 / 5.5</td><td>10.4 / 9.8</td><td>12.2 / 11.7</td><td>10.7 / 13.0</td><td>12.2</td><td>1.0</td></tr><tr><td>Straw (s1)</td><td>74</td><td>4 / 8</td><td>165 / 160</td><td>13 / 16</td><td>8.6 / 8.3</td><td>14.3 / 13.6</td><td>11.6 / 12.3</td><td>5.8 / 5.9</td><td>11.5</td><td>1.1</td></tr><tr><td>Roots (s1)</td><td>118</td><td>9 / 11</td><td>172 / 193</td><td>20 / 27</td><td>9.1 / 9.6</td><td>14.7 / 15.8</td><td>12.4 / 12.3</td><td>7.2 / 2.3</td><td>12.1</td><td>1.1</td></tr><tr><td>Capsules (s1)</td><td>78</td><td>3 / 5</td><td>142 / 196</td><td>9 / 10</td><td>6.4 / 6.1</td><td>10.6 / 10.5</td><td>12.8 / 11.8</td><td>11.9 / 23.1</td><td>13.5</td><td>1.4</td></tr><tr><td>Wet seeds (s1)*</td><td>460</td><td>85 / 63</td><td>324 / 271</td><td>95 / 84</td><td>6.9 / 6.5</td><td>11.4 / 11.9</td><td>12.1 / 11.3</td><td>109.6 / 93.1</td><td>-</td><td>-</td></tr><tr><td>Dry seeds (s1)*</td><td>-</td><td>45 / 49</td><td>391 / 379</td><td>54 / 59</td><td>11.7 / 11.2</td><td>11.7 / 11.5</td><td>20.1 / 19.9</td><td>171.2 / 185.8</td><td>-</td><td>-</td></tr><tr><td>Fines (s1)</td><td>165</td><td>17 / 18</td><td>76 / 72</td><td>25 / 24</td><td>2.7 / 2.0</td><td>5.4 / 5.1</td><td>10.0 / 8.2</td><td>127.5 / 120.6</td><td>9.3</td><td>0.7</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** HRR curves versus time in cone calorimeter for flax samples from series 1 (heat flux 35 kW/m²).</figcaption></figure>

lignocellulosic materials tested according to method B (unpublished data) strongly limits the comparison between the various fractions. Nevertheless, some data, as THR and heat of combustion, remain fully reliable.

All flax samples leave no residue, evidencing their fully organic composition. The only exception is fines which leave a mineral residue of 45 wt %. It means that the residue measured according to method A (anaerobic pyrolysis – Table 3) is mainly mineral with a small amount of char (55 – 45 = 10 wt % of char, i.e. 18 wt % of the organic fraction as the other lignocellulosic fractions). In overall, the heat of complete combustion reaches 15.5–18 kJ/g as it is commonly measured for lignocellulosic bioresources (for example by PCFC in method B or by oxygen bomb calorimeter [16]) High-lignin fractions (i.e. shives and, to a lesser extent, straw and roots) show a slightly higher Δh than fibres and tow (mean values 17.5 kJ/g vs 15.7 kJ/g). Indeed, lignin stores a large amount of heat (24.9–29.3 kJ/g according to the monomer, namely 4-hydroxyphenyl, guaiacyl and syringyl – values calculated from Huggett’s relation) which is fully released in aerobic pyrolysis. Higher values were obtained for fractions with high extractives content, i.e. capsules, fines and seeds. In particular, seeds present a Δh close to 25 kJ/g, evidencing the presence of other fuels in their structure, such as lipids. Fig. 4 highlights these observations.

### 3.3. Bench scale flammability of flax samples from series 1 and 2

Table 5 lists the main flammability data for the flax plant fractions from series 1 and 2 measured in cone calorimeter (at external heat flux 35 kW/m²). Fig. 5 shows HRR curves for some of these fractions (other curves are shown in Figures S3 and S4). Pictures of samples before and after cone calorimeter tests are shown in Fig. 6. All HRR curves correspond to a so-called thermally thin behaviour according to Schartel and Hull [29]. It means that heat conductivity is so reduced that heat cannot be transferred from the top volume to the material’s bulk.

The density of the particle bed was calculated as the ratio between its initial mass (around 20 g) and its occupied volume in the sample holder. Except for seeds, the density is very low (< 160 kg/m³) for most fractions. It is even lower than 90 kg/m³ for fibres and tow. This explains why the heat conductivity is very low. Bossu et al. have measured the heat conductivity of panels made with different fractions of Guyanese wood fibres and barks. They found that heat conductivity is around 0.05 W/(m.K) in many cases for densities ranging between 100 and 250 kg/m³ [30].

Time-to-ignition is very short (< 20 s, most often < 10 s) except for seeds. In fact, seeds undergo a specific phenomenon that is discussed below and makes the data inaccurate. While heat diffusion is negligible, heat cannot be evacuated from the top volume (absorbing directly the

<footnote>* Inaccurate data due to seed skipping phenomenon; For each fraction, values from both tests are indicated.</footnote>
<figure><img src="image_8.png" /><figcaption>Fig. 6. Flax samples from series 1 and 2 before and after cone calorimeter tests.</figcaption></figure>

heat flux). Then, the temperature increases very quickly up to the ignition temperature. The peak of heat release rate is reached few seconds after ignition. Then, HRR quickly decreases up to flame out, but stabilizes at a non-null value (around 20–30 kW/m²). This is due to the thermo-oxidation of char and/or unpyrolyzed particles.

For most flax samples, pHRR is close to 150 (± 17) kW/m². Roots have a slightly higher pHRR (183 kW/m²). In contrast, pHRR is much higher for seeds (298 kW/m²) and much lower for fines (74 kW/m²).

In order to compare properly the different curves, THR, mass loss, Effective heat of combustion (EHC) and TSR were calculated at 250 s, which corresponds to the time when HRR reaches the plateau at 20–30 kW/m². Mass loss ranges from 10 to 14 g at 250 s, except for fines. Indeed, the mass loss for this fraction is almost half (5.2 g) which can be related to its mineral fraction of about 45 wt %. EHC is also similar for all fractions from 10 to 12.4 kJ/g which is characteristic of lignocellulosic materials [21,23]. Also note that this value is close to the heat of complete combustion measured in PCFC (method A), confirming that the combustion is almost complete. Besides, the combustion efficiency $\chi$ is slightly higher than 1, which is theoretically not possible. This is

probably due to the thermo-oxidation of char which may occur when the flame starts vanishing (even before the complete flame out): while char stores a huge amount of energy, the combustion of gases released by its thermo-oxidation increases EHC which becomes higher than $\Delta h$ measured in PCFC (where thermo-oxidation does not occur).

Fines exhibit a slightly lower effective heat of combustion (9.1 kJ/g). This value is significantly lower than the heat of complete combustion, leading to an apparent very low combustion efficiency ($\chi < 0.7$). The reason for such low combustion efficiency is probably a very incomplete burning. Flame out occurs after 40 s and even during the flaming period, flame is rather small. In other words, a large pyrolyzed fraction is unburnt. The lower heat of combustion of gases, the limited organic fraction and the possible additional barrier effect from the mineral fraction which accumulates at the surface and may slow down the pyrolysis rate are responsible for this phenomenon.

While mass loss and heat of combustion are rather close, THR is also similar for most fractions: 5.8–7.8 kJ/g, with a slightly higher value for straw and roots. Once again fines exhibit a much lower value (2.4 kJ/g) due to low mass loss and effective heat of combustion.
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Total smoke release (in m² per g of gas released) in cone calorimeter (heat flux 35 kW/m²) versus extractive content in organic fraction (wt %).</figcaption></figure>

Overall, no significant differences are observed between the various scutched fibres or shives (Figures S3 and S4). It means that the influence of retting is almost negligible when studying bench scale flammability. Nevertheless, it should be kept in mind that retting is not the only parameter that differentiates the flax samples: the flax variety is also different (Bolchoï, Lisette and Mélina). The influence of retting can therefore be concealed by other factors as flax variety and growth conditions, which will be further studied in Section 3.5.

Total smoke release was calculated in m² per gram of gas released in order to compare the different materials from series 1 and 2. According to values given in Table 5, smoke production is very limited for most flax fractions but increases when extractives content is high (i.e. for capsules, fines and seeds). Fig. 7 plots the total smoke release versus extractives content. Note that for fines, extractives content is calculated considering only the organic fraction, since the mineral fraction does not contribute to the gas release.

Smoke production depends on the nature of materials under burning but also on the pyrolysis rate. The pyrolysis rate is not higher for either the capsules or the fines (according to the peak of mass loss rate $pMLR = $

$pHRR/EHC$). It has been shown that the smoke production depends on the fraction of oxygen atoms in the material [31]. When this fraction is high, smoke production is reduced. This is particularly the case for lignocellulosic materials (oxygen fraction is 49.3 wt % in cellulose). On the contrary, even if the exact composition of extractives is not known, they include lipids (at least for seeds). Lipids are poor in oxygen but rich in carbon and hydrogen. Therefore, smoke production is significantly enhanced in presence of such extractives. For fines, decomposition without burning also contributes to the smoke production. Indeed, smoke production continues to some extent even after flame out. Nevertheless, more samples are needed to confirm the trend observed here.

As mentioned above, seeds exhibit a peculiar behaviour that results inaccurate HRR data. Indeed, seeds explode and jump out of the sample holder in the first seconds (before ignition), and this phenomenon continues during a long time with the jumping of ignited particles. All these particles contribute to the mass loss but not to the oxygen consumption measured in cone calorimeter, reducing the heat release. Additionally, this phenomenon is of course very hazardous because it

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** HRR and mass curves versus time in cone calorimeter for dry and wet seeds (heat flux 35 kW/m²) and pictures of wet seeds before and after the “popcorn jumping” phenomenon.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 9. FTIR spectrum obtained for extractives from flax seeds.</figcaption></figure>

can easily propagate a fire (see Movie S5 in supporting information). Although seeds contain only 4.5 wt % moisture (Table 2), the increase in pressure in the seeds when moisture is heated and vaporized leads to their explosion as popcorn. When ignition occurs, the mass loss has already reached >20 wt % for wet seeds. If seeds are pre-dried at 105 °C during 24 h, the phenomenon is greatly limited, but not completely prevented. Fig. 8 shows the HRR and mass curves versus time for wet and dry seeds. Ignition occurs much earlier for dried seeds. Mass loss at ignition is <2wt %. Peak of heat release rate is also increased from 298 to 385 kW/m².

In addition to this “popcorn jumping” effect, seeds are clearly different from other flax fractions. This can be assigned to the presence of a large amount of extractives. In this study, in order to assess the influence of these extractives, additional analyses were performed on this fraction.

Soxhlet extraction carried out on seeds led to a mass loss of approximately 32 wt %, which is in agreement with the expected oil content of flax seeds [32,33]. Flax seeds are a rich source of oils, typically comprising 35–45 wt % of their total composition. These oils are predominantly made up of α-linolenic acid (approximately 70 %), along with monounsaturated fatty acids, primarily oleic acid (20 %), and a smaller fraction of saturated fatty acids, including palmitic and stearic acids (9–10 %) [34]. The FTIR spectrum (Fig. 9) of the recovered extractive is identical to those obtained by other authors for flaxseed oil and confirms the presence of unsaturated esters [35]. Indeed, the band at 1740 cm⁻¹ corresponds to the stretching vibration of the ester carbonyl (C=O), while the bands at 1240, 1100, and 1160 cm⁻¹ correspond to the stretching vibration of the C—O bond. In addition, the band at 3010 cm⁻¹ is attributed to the vibration of the C=C double bonds present in the hydrocarbon chains. The bands at 2920 and 2860 cm⁻¹ are

<figure><img src="image_12.png" /><figcaption>Fig. 10. Experimental pHRR vs calculated pHRR (using Eq. (1)) for the materials (textiles, foams, woods, fibers in bulk, bioconcretes, panels) tested in [36] (white circles) and for the flax samples tested in this work (black circles).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 11. Reciprocal of TTI versus heat flux for scutched fibres, roots and fines from series 1.</figcaption></figure>

attributed to the asymmetric and symmetric C—H stretching vibrations. Finally, the bands at 720 and 1460 cm⁻¹ correspond, respectively, to the in-plane bending and scissoring of CH₂.

The flammability of extractives and the extractives-free seeds was tested using PCFC (method A – Table S1). Extractives exhibit a very high THR of 34.8 kJ/g, a pHRR of 518 W/g, and no residue. The high heat of complete combustion (34.8 kJ/g) is typical for oils and very close to the theoretical value for α-linolenic acid: C₁₈H₃₀O₂ calculated according to Huggett’s relation. It is also noticeable that the THR, pHRR, and TpHRR values obtained for extractives-free seeds are significantly lower than those obtained for unmodified seeds (compare data in Tables 3 and S1) confirming that oil-rich extractives are responsible for the high flammability of seeds.

### 3.4. Modelling of flammability data at bench scale

In a previous work, a model has been proposed to estimate the pHRR of thermally thin materials in cone calorimeter [36]. This model assumes that pHRR depends only on the decomposition of the material which absorbs directly the heat flux from the radiant cone. The mass absorbing the heat flux is called $m_a$ and then only the mass $f_0 \times m_a$ contributes to the pHRR, with $f_0$ the organic fraction having an effective heat of combustion $EHC_o$. The Eq. (1) was checked on around 400 materials tested at different heat fluxes: textiles, foams, woods, fibers in bulk, bioconcretes, panels.

$$pHRR = EHC_o \times \alpha \times (f_0 \times m_a)^\beta \times C_{Grid} \quad (1)$$

$C_{Grid}$ is a factor depending on the presence of a grid (dimensionless). $C_{Grid} = 1$ in presence of a grid (mesh 2 × 2 cm²) and 1.4 without a grid (the grid is sometimes used to avoid the distortion of thin materials as textiles; this is not the case here). α and β are two parameters depending on the heat flux HF and established from data obtained by El Gazi et al. [37] (around 120 tests at various HF) according to Eqs. (2) and (3), respectively.

$$\alpha = 0.0818 \times HF + 3.2427 \quad (2)$$

$$\beta = 0.0025 \times HF + 0.2197 \quad (3)$$

The absorbing mass $m_a$ was assessed to be 4.14 g. $f_0$ is equal to 1 for all the materials tested here, except for fines ($f_0$ = 0.55). $EHC_o$ was calculated over the first fifty seconds of cone tests (during which pHRR is reached – see Table 5). For seeds, due to the “popcorn jumping” phenomenon, $EHC_o$ was calculated from $\Delta h$ measured in PCFC ($0.9 \times \Delta h$ as

suggested in [36]). For dry seeds, this value was multiplied by $\frac{1}{(1-0.045)}$ to consider the moisture content (PCFC tests were performed on seeds stored at 50 wt % RH, thus having a moisture content of 4.5 wt %).

According to Fig. 10, a reasonable agreement is found between the experimental and calculated values. The mean error is 14 % (vs 18.1 % in [36]). This result confirms that the pHRR can be correctly predicted from flammability data at microscale. Indeed, $EHC_o$ is close to the heat of complete combustion measured using PCFC.

Additional data were collected for some flax samples, i.e., roots and scutched fibres. Fines and roots were selected because they have the lowest and the highest pHRR (except seeds) in cone calorimetry, respectively. Scutched fibres were chosen because they are usually the most interesting fraction for material applications.

Tests were performed at different heat fluxes in order to calculate the critical heat flux (CHF), i.e. the minimum heat flux below which the ignition does not occur. CHF can be calculated according to Eq. (4), which is valuable for thermally thin materials and assuming emissivity equal to 1 [29].

$$TTI = \frac{l\rho c (T_{ig} - T_0)}{\varepsilon \times HF - CHF} \quad (4)$$

$l, \rho, c$ and $\varepsilon$ are respectively the thickness, density, specific heat and emissivity of the sample. $T_{ig}$ and $T_0$ are respectively the surface temperature at ignition and the room temperature.

Fig. 11 plots TTI⁻¹ versus external heat flux. Despite a moderate R² (especially for scutched fibres), CHF can be estimated. Calculated CHF were found to be 20.1, 16.1 and 13.7 kW/m² respectively for fines, roots and scutched fibres. These values slightly overestimate the experimental values (maybe because emissivity is a little lower than 1). Indeed, no ignition was observed at 15, 10 and 5 kW/m² while ignition is observed at 18, 15 and 10 kW/m² respectively for the three materials.

### 3.5. Investigating flammability variability at microscale on scutched flax fibres from series 1, 2 and 3

As shown in Table 1, series 3 gathers batches of scutched fibres from numerous flax varieties with different growth (season, location) and retting conditions. These samples were tested using PCFC according to method A (anaerobic pyrolysis). Figs. 12A and B show the pHRR and THR values for all these samples, together with scutched fibres from series 1 and 2. It is noticeable that minimum and maximum pHRR ranges from 90 to almost 160 kW/m². The lowest values are obtained for the
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** pHRR (A) and THR (B) measured in PCFC (method A) for 19 different batches of scutched fibres (series 1, 2 and 3) with different growth location and retting conditions.</figcaption></figure>

low-retted spring Lisette variety (series 2) and the standard-retted winter Toundra variety (series 3, Reviers). The highest pHRR are observed for the standard-retted spring Bolchoï variety (series 1) and the low-retted winter Olga variety (series 3, Creances). Most of the samples exhibit values close to 130–150 kW/m². Considering THR, values range from approximately 6 to 11 kJ/g. The lowest values are obtained for the two scutched fibres from series 2, i.e. Mélina and Lisette varieties which were grown in Seine Maritime, with high and low retting respectively. The highest THR value corresponds to the standard-retted spring Damara variety (series 3) and standard-retted winter Toundra variety (series 3, Reviers). For both pHRR and THR, the ratio between the highest and the lowest values is close to 1.9 evidencing the large variability of the values. As discussed in the introduction, this variability is comparable to

that observed in literature, ranging from 100 to 213 W/g and 5.8 to 9.3 kJ/g for pHRR and THR respectively [21–24].

Considering the three batches from winter Olga variety, which were grown in three different locations (Maltot, Avenay and Creances) with standard and high retting, minimum and maximum pHRR and THR values vary from 110 to 150 kW/m² and from 7 to 10 kJ/g, respectively. This variation is of the same order of magnitude as the maximum differences observed between two flax varieties. In other words, we can assume that the growth and retting conditions, i.e. the location, season and related temperature, sunlight, soil nature, humidity, have a major impact on the composition and cell wall structure of scutched fibres and therefore on their flammability. At least, these conditions are as influent as the flax variety. One-way ANOVA statistics tests were carried out on
these data and revealed that, when considered independently, the effect of flax variety, retting and growth location on pHRR and THR values is significant at the 0.05 level, whereas the season has no significant effect. However, when conducting three-way ANOVA on these three influential factors (flax variety, retting and growth location), the interactions in between are not significant at the 0.05 level, highlighting the difficulty to discriminate their effect on flammability.

## Conclusions

In this work, the flammability of different flax plant fractions and various flax variety has been studied at micro and bench scales. The influence of some factors on the fire behaviour of flax has been highlighted:

- The main peak of heat release rate at microscale (anaerobic pyrolysis) is centered at around 360-370 °C at microscale, and corresponds to the cellulose decomposition. Therefore, this pHRR is roughly proportional to the glucose content. Nevertheless, this peak is partially overlapped with the earlier decomposition peak of other polysaccharides (from hemicelluloses or pectins). The heat release from the pyrolysis of these saccharides also contributes a little to the main pHRR.

- Lignin content has a little influence on the heat of complete combustion after aerobic pyrolysis because lignin stores higher energy than cellulose.

- The presence of extractives, found in capsules, seeds and fines, increases the heat of complete combustion (especially for aerobic pyrolysis) and the smoke production.

- At bench scale, almost all the fractions exhibit very similar HRR curves and a thermally thin behaviour. Only fines (with a high mineral fraction, a low pHRR and heat release and a fast flame out) and seeds (with a high pHRR and a late ignition due to a “popcorn jumping” phenomenon) exhibit a peculiar behaviour.

- From an extended set of scutched fibres (with different variety, season, retting and growth location), a large discrepancy was found in terms of heat release (pHRR and THR). Growth location and retting appear to be as influent as the flax variety. However, the respective contributions of these factors cannot be easily discriminated, and further work is needed to state on their role.

## CRediT authorship contribution statement

**Solène Galinier:** Writing – original draft, Visualization, Investigation, Formal analysis, Data curation. **Clément Brendlé:** Writing – review & editing, Visualization, Investigation, Formal analysis, Data curation. **Johnny Beaugrand:** Writing – review & editing, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation. **Antoine Barbulée:** Writing – review & editing, Validation, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. **Nicolas Le Moigne:** Writing – review & editing, Visualization, Validation, Supervision, Software, Project administration, Methodology, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Rodolphe Sonnier:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Nicolas Le Moigne reports financial support was provided by The French Agency for Ecological Transition. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in

this paper.

## Acknowledgments

The authors would like to thank the French Agency for Ecological Transition (ADEME) for funding the ProACTIF project (GRAINE 2019 - n°2103D0029). SG, NLM and RS thank Monica Pucci and Anne-Sophie Caro for scientific discussions and Loïc Dumazert for his help in setting up the micro- and bench-scale flammability experiments. Authors thank Lèna Brionne and Lucie Le Bot for their help and skills in biochemical analysis.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.polymdegradstab.2025.111532.

## Data availability

Data will be made available on request.

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