# Comparison of two flame-retardant treatment methods on bulk hemp fibers: Towards a large-scale viability as a raw material for building insulation

Thomas Schatzmayr Welp Sà, Sandrine Marceau, Clément Piégay, Philippe

Glé, Fouad Laoutid, César Segovia, Etienne Gourlay

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Thomas Schatzmayr Welp Sà, Sandrine Marceau, Clément Piégay, Philippe Glé, Fouad Laoutid, et al. Comparison of two flame-retardant treatment methods on bulk hemp fibers: Towards a large-scale viability as a raw material for building insulation. Industrial Crops and Products, 2025, 228, pp.120903. 10.1016/j.indcrop.2025.120903. hal-05005171

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# Comparison of two flame-retardant treatment methods on bulk hemp fibers: Towards a large-scale viability as a raw material for building insulation

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Thomas Schatzmayr Welp Sá<sup>a,b</sup>, Sandrine Marceau<sup>a,*</sup>, Clément Piégay<sup>b,D</sup>, Philippe Glé<sup>b,D</sup>, Fouad Laoutid<sup>c</sup>, César Segovia<sup>d</sup>, Etienne Gourlay<sup>e,D</sup>

## ARTICLE INFO

Keywords:

Hemp fibers

Bio-based material

Flammability

Fireproof treatment

## ABSTRACT

The high flammability of bio-based insulation materials derived from plant fibers is a major barrier to their widespread use, necessitating the development of effective flame-retardant treatments. This study aims to optimize the treatment of raw hemp fibers by comparing two environmentally-friendly application methods: immersion and spraying. The fibers are treated with a solution containing phytic acid and urea, as biobased sources of phosphorus and nitrogen. The efficiency of the treatments is assessed by characterizing the quantity of phosphorus grafted, the thermal degradation of the fibers by thermogravimetric analysis and their flammability using a mass loss calorimeter. Both application methods are effective, but the spraying method demonstrated superior performance, resulting in a higher level of phosphorus grafted to the fibers and a reduced heat release rate. Additionally, the formation of a protective char layer on the fiber surface helps to limit flame propagation. Notably, this method requires ten times less treatment product than immersion, making it a highly scalable and efficient solution for the industrial production of flame-retardant insulating materials.

## 1. Introduction

The benefits of using bio-based materials in construction are numerous. They range from environmental aspects, linked to carbon capture and low-processing and low-energy intensive solutions (Zieger et al., 2020; Arehart et al., 2021; Pittau et al., 2021; Galimshina et al., 2022), to technical aspects, due to high-level performance in terms of acoustics (Berardi and Iannace, 2015; Chabriac et al., 2016; Piégay et al., 2021) and hygrothermics (Collet et al., 2011; Korjenic et al., 2016; Moudood et al., 2016; Amziane and Collet, 2017). For building insulation, for example, lignocellulosic fibers provide multi-functional features that make them a strong alternative to conventional solutions.

However, the main drawback of natural lignocellulosic fibers is their high flammability (Freivalde et al., 2014; Kozłowski and Muzyczek, 2020). During the thermal decomposition of bio-based materials, there is formation of levoglucosan, that decomposes into reactive species, which reacts with the oxidizing agent (usually the oxygen in the air) (Hajj et al., 2020). Therefore, it fuels the combustion reaction

maintaining and propagating the flame (see Fig. 1). To overcome this problem, the use of flame-retardants (FRs) is needed. Different types of FRs are available and widely investigated for this purpose such as minerals, halogenated, phosphorus-based, nitrogen-based, silicon-based, and nanometric compounds (Morgan and Gilman, 2013; Costes et al., 2017; Taib et al., 2022). Among these options, the phosphorus-based flame retardants seem to be one of the most promising, as they are considered to be non-toxic, unlike the halogenated ones, and can act by forming a carbonaceous layer (char) during thermal degradation and combustion of bio-based materials, reducing their flammability (Costes et al., 2017; Kozłowski and Muzyczek, 2020; Hajj et al., 2020). This carbonaceous structure acts as a physical barrier, reducing the heat diffusion through the material, the oxygen penetration, the volatilization of combustible compounds and the flame spread (Costes et al., 2017; Taib et al., 2022). This is illustrated in Fig. 1. Furthermore, there are bio-based sources of phosphorus, such as the phytic acid, which can be found in seeds and grains, which aligns with the purpose of working with bio-based materials (Daneluti and Matos,

<footnote><sup>a</sup> Université Gustave Eiffel, MAST-CPDM, CEDEX 2, Marne-la-Vallée 77454, France</footnote>

<footnote><sup>b</sup> Cerema, Université Gustave Eiffel, UMRAE, 11 Rue Jean Mentelin, Strasbourg F-67035, France</footnote>

<footnote><sup>c</sup> Laboratory of Polymeric and Composite Materials, Materia Nova Research Center - University of Mons (UMONS), Av. Nicolas Copernic 3, Mons 7000, Belgium</footnote>

<footnote><sup>d</sup> CETELOR - Université de Lorraine, 27 Rue Philippe Seguin, Epinal 88051, France</footnote>

<footnote><sup>e</sup> Cerema, Equipe-recherche BPE, 11 Rue Jean Mentelin, Strasbourg 67035, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: sandrine.marceau@univ-eiffel.fr (S. Marceau).</footnote>
2013).

The flame retardancy of the phosphorus compounds can be improved by the combination with nitrogen-based products to form an intumescent flame-retardant system (Laoutid et al., 2009; Costes et al., 2017). The nitrogen flame retardants release gases when thermally degrading (Costes et al., 2017; Moussa et al., 2020), which can lead to an expansion of the char layer, which will improve the protection of the lignocellulosic fibers against burning by reducing heat transfer between the heat source and the material's surface, and limiting the fuel transfer and the oxygen diffusion (Laoutid et al., 2009). The synergism between phosphorus and nitrogen compounds have been previously explored and shown to be an interesting approach to reduce the flammability of bio-based materials (Zhang et al., 2019; Moussa et al., 2020; Skyam et al., 2021; Antoun et al., 2022; Taib et al., 2022).

Different techniques can be used for binding the phosphorus to natural fibers (Hajj et al., 2018; Teixeira et al., 2018; Kozlowski and Muzyczek, 2020). One of them consists in chemically grafting phosphorus through fiber immersion in a solution, followed by heating to activate covalent reaction and finishing by washing and drying steps (Dorez et al., 2014a; Hajj et al., 2018; Moussa et al., 2020; Antoun et al., 2022). Research has shown that reduced flammability of lignocellulosic fibers is directly associated with the phosphorus content achieved by the treatment (Hajj et al., 2018; Moussa et al., 2020; Antoun et al., 2022; Taibi et al., 2022). Therefore, high phosphorus levels lead to improved char formation and lower flammability.

This immersion technique has proved effective in phosphorous grafting and flammability reduction (Dorez et al., 2014a; Hajj et al., 2020; Moussa et al., 2020; Antoun et al., 2022). However, given its multiple steps and demands for time, energy, and materials, it is important to assess its scalability for large-scale applications. The Table 1 presents the fireproof treatment details on natural fibers, by immersion, performed by studies found in the literature. Most of the studies treated low amount of fibers, and required some type of pre-treatment, increasing the number of steps, time and materials used. Furthermore, among the flame retardants used, only the phytic acid is considered to be a bio-based source of phosphorus, which corresponds well with the nature of vegetal fibers (Sykam et al., 2021).

An alternative method, less frequently discussed in the literature (Deborde et al., 2023), involves applying flame-retardants via pulverization. This technique appears to require fewer resources and less time, but its efficiency and practical feasibility needs further investigation due to limited application reports.

The studies carried out to date have demonstrated the efficiency of the treatments used, but only on a few grams of fibers. There is a need to develop treatments that can be applied on an industrial scale to hemp fiber insulation materials, and to evaluate the impact of this treatment on their hygrothermal and acoustic performance. This is the final objective of this research. Therefore, the first stage of this work is to

compare two flame-retardant treatment techniques for fireproofing: one considered as the traditional method by immersion, and an alternative by pulverization. The objective is to optimize the treatment process for bulk hemp (Cannabis sativa) fibers, without any binder, with a focus on scalability for large-scale applications.

## 2. Materials and methods

### 2.1. Materials description

Bulk hemp fibers that were used for the evaluation of the two fireproof treatment methods have been obtained from AgroChanvre (France). These fibers have been previously retted and a portion of them can be seen in Fig. 2a. The untreated hemp fibers used as reference are named as REF. For the treatment, a phytic acid (CAS: 83–86–3) in solution with a concentration of 50 % in mass, and urea (CAS: 57–13–6, purity > 99.5 %) were used. Nitric acid (CAS: 7697–37–2) 65 %, was also used for to perform the spectroscopy analysis (ICP-AES).

### 2.2. Treatment

Before the treatment, the solution used was prepared by manually mixing 10 wt% of phytic acid and 30 wt% of urea, for 5 minutes, with distilled water. Two methods of application have been tested: by immersion and pulverization on 300 g of fibers. In the end of the treatment and before doing the following tests, the fibers had their water content and temperature stabilized at room temperature of 21 ± 2°C and relative humidity of 35 ± 5 %.

The immersion method is based on previous works (Dorez et al., 2014a; Hajj et al., 2020; Moussa et al., 2020; Antoun et al., 2022), but has been adapted, for potential industrial-scale applications, by reducing the number of steps and simplifying the whole procedure. In this method, the fibers were fully immersed in the solution (Fig. 2b), at a solution-to-fiber mass ratio of 10 for 1 hour at room temperature. This method ensured a homogeneous application of the flame retardants in all fibers. Following immersion, the fibers were oven-dried at either 80°C or 120°C for 2 hours. They were then washed and dried in an oven at 60°C. Fibers treated by this immersion method are labeled IMM 80°C or IMM 120°C.

The pulverization method consisted on applying the solution with a spray, shown in Fig. 2d. This approach required significantly less solution than the immersion method, needing only enough to evenly coat the fibers. A solution-to-fiber mass ratio of 1 was determined to be sufficient for effective treatment without soaking. The fibers were spread out on a surface (Fig. 2c) with half of the solution sprayed on one side. They were then flipped and the remaining solution was sprayed on the other side. After the application, the fibers were heated in an oven at either 80°C or 120°C for 2 hours, followed by drying at room temperature. Fibers

<figure><img src="image_4.png" /><figcaption>Fig. 1. Scheme of the flame retardancy mechanism.</figcaption></figure>
<table><caption>**Table 1**

Characteristics of the fireproof treatment by immersion.</caption><thead><tr><th>Type of fiber</th><th>Quantity treated</th><th>Phosphorus FR</th><th>Mass ratio FR/fiber (%)</th><th>Pre-treatment</th><th>Source</th></tr></thead><tbody><tr><td>Flax</td><td>50 g</td><td>DAP<sup>1</sup>, PA<sup>2</sup>, APP<sup>3</sup></td><td>2.5–10</td><td>Removal of waxes and pectin with ethanol</td><td>Dorez et al., (2014a)</td></tr><tr><td>Flax</td><td>0.25 g</td><td>VPA<sup>4</sup></td><td>400</td><td>-</td><td>Hajj et al., (2018)</td></tr><tr><td>Miscanthus and flax</td><td>0.25 g</td><td>C6, C8, C10 bisphosphonic acids and VPA<sup>4</sup></td><td>400–800</td><td>Removal of extractives with toluene/ ethanol</td><td>Hajj et al., (2020)</td></tr><tr><td>Hemp</td><td>10 g</td><td>HEDP<sup>5</sup></td><td>130</td><td>Steam explosion and bleaching</td><td>Moussa et al., (2020)</td></tr><tr><td>Hemp</td><td>10 g</td><td>Phytic acid</td><td>3–7</td><td>Steam explosion and bleaching</td><td>Antoun et al., (2022)</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Materials and treatment methods (a) Bulk hemp fibers; (b) Immersion method; (c) Pulverization method; (d) Spray used for pulverizing the solution.</figcaption></figure>

treated by the pulverization method are labeled PUL 80°C or PUL 120°C.

## 2.3. Homogeneity test of the pulverization method

The homogeneity of the pulverization method was evaluated by the application of a blue-colored solution over the hemp fibers, simulating the same procedure as the application of the flame retardants. The fiber to solution mass ratio was of 1. 10 g of fibers were spread in a surface, and half of the solution was applied in one side of the fibers, and half of the solution on the other side. The homogeneity of the treatment was evaluated by a visual inspection.

## 2.4. Weight, real density and fiber diameter assessment

To determine the weight gain or loss after the treatment, the fibers were oven-dried and weighted before and after the treatment. A Mettler Toledo scale with a precision of 0.01 g was used to weight the material. The procedure consisted in drying the fibers at 60°C until constant mass, which was obtained until their mass variation from 3 measurements, obtained from 3 consecutive days, were lower than 0.5 %. The same procedure was repeated after the treatment, and the results are reported as a percentage of the initial mass. The true density of the hemp fibers was determined using a Micrometrics Accupyc II helium gas pycnometer, with fibers pre-dried under the same conditions as described above.

Additionally, the diameter of five fiber bundles from each treatment condition was measured every centimeter along its length, both before and after the treatment. Measurements were taken on fiber bundles rather than individual fibers. To do so, fiber segments of approximately 5 cm in length were positioned straight, without tension, and images

were captured every 0.5 cm using an optical microscope and a ruler, to identify the position. The evaluations of the fiber diameter were made using ImageJ software.

## 2.5. Fourier transform infrared spectroscopy (FTIR)

The Fourier-transform infrared spectroscopy spectra of all samples were obtained by a Thermo Scientific Nicolet iS20, using an Attenuated Total Reflection (ATR) sampling technique. Before analysis, the hemp fibers were dried in an oven for 3 days at 60°C. A small sample of hemp fibers was manually compacted to reduce the empty spaces between fibers, and positioned in the equipment for analysis. Before each analysis, the background spectra were obtained and subtracted from the fibers' spectra. A total of 32 scans were made for each sample in the range between 625 and 4000 cm-1, with a spectral resolution of 0.25 cm-1. The spectra are presented only up to 2000 cm-1, in order to focus the analysis on the wavenumber range of interest.

## 2.6. Phosphorus content

The phosphorus content of treated fibers was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) model ULTIMA 2000 from Jobin Yvon Horiba Company. Prior to ICP-AES analysis, 0.3 g of hemp fibers were digested using a nitric acid/ distilled water (50/50) solution for 4 hours at 100°C, under magnetic stirring. The remaining solution was filtered, and the phosphorus concentration was determined using calibration curves obtained from ICP-AES analysis of standard solutions at different concentrations. The test was repeated 10 times.

<footnote n="1">¹ Dihydrogen ammonium phosphate;</footnote>

<footnote n="2">² Phosphoric acid;</footnote>

<footnote n="3">³ Ammonium polyphosphate;</footnote>

<footnote n="4">⁴ Vinyl phosphonic acid;</footnote>

<footnote n="5">⁵ 1-Hydroxyethylidene-1,1-diphosphonic acid</footnote>
## 2.7. Thermal degradation

The thermal degradation of the treated and untreated fibers was assessed by thermogravimetric analysis (TGA) from TA Instruments model SDT650. Samples of 15 ± 3 mg were heated from 30°C to 800°C, at 10°C.min⁻¹, under air. Three samples were tested for each condition, to ensure repeatability.

## 2.8. Fire reaction

Three different tests were used to characterize the fiber's reaction to fire. In all of them the samples weighted 24 g, which indicates an apparent density of 60 kg.m⁻³. The single flame-source test was used to assess the ignitability of the hemp fibers, following the standard EN ISO 11925-2 (2020) (https://www.iso.org/standard/71204.html). The sample was burned for 15 seconds and is observed during 60 seconds after the test begins. The height of the burned area was measured and three samples were tested for each condition.

The mass loss calorimeter test ISO 5660-1 (2015) (https://www.iso.org/standard/57957.html) was used to quantify the flammability of the hemp fibers. The samples used had dimensions of 100x100x40 mm³, and they were exposed to a heat flux of 35 kW.m⁻². This test provides numerous comparable parameters regarding the flammability of a materials: time to ignition (TTI), heat release rate (HRR), peak HRR (pHRR), total heat release (THR) and mass loss rate were measured. This test was repeated four times for each condition.

A smoke density apparatus was also used to assess the effects of the treatment on smoke opacity, following the ASTM D2843 (2022) (American Society for Testing and Materials, 2022). Samples of 4 g of hemp fiber were burned during 4 minutes and the opacity of the air in the chamber was measured. The results are reported as a curve of the absorptivity versus time. This test was repeated 2 times.

# 3. Results and discussion

## 3.1. Homogeneity test

Images of the hemp fibers before and after the homogeneity test are shown in Fig. 3. Fig. 3a shows the raw hemp fibers spread in a surface before the application of the colorant. Then, Fig. 3b shows the aspect of the fibers after full application of the colored solution on both sides. Even though the small amount of solution used (fiber to solution mass ratio of 1), it can be seen that an evenly spread of the colored solution is achieved.

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Homogeneity test (a) Bulk hemp fibers before spraying; (b) Fibers after full application of the colorant.</figcaption></figure>

## 3.2. Physical properties

The results for the mass variations and their standard deviation after flame retardant treatments are reported on Table 2. All treated samples showed an increase in mass after the treatment. The weight increase of the immersed fibers was 3.2 % for IMM 80°C and 5.1 % for IMM 120°C. In contrast, the pulverized samples exhibited a much higher weight increase, with gains of 14.3 % for PUL 80°C and 12.8 % for PUL 120°C. These differences are due to the different finishing methods applied during treatment. In fact, since immersed fibers are washed, non-grafted phosphorylated molecules are removed and only covalently bonded molecules remain. On the contrary, pulverized fibers are not washed, and thus the weight increase is higher and correspond to the initial reagents amount used, grafted and non-grafted.

Another reason behind the difference in mass increase observed between the immersed and pulverized fibers, was identified by measuring the mass change on a sample (REF W 80°C) that underwent the same treatment steps as the immersed fibers, but without the use of the flame retardants. In other words, the fibers from this sample were immersed in water for 1 hour, then were heated at 80°C and were finally washed and dried. This sample allowed the assessment of the effects of water immersion, heating and rinsing alone. A 5 % mass decrease was observed, attributed to the partial removal of waxes, pectin, and ashes. Therefore, the actual weight increase due to phosphorus grafting in the IMM 80°C and IMM 120°C samples is higher than the measured values.

Even with the weight increase from the treated hemp fibers, the real density of them barely changed, and no significant differences were found. All fibers presented a density near 1.50 g.cm⁻³, as shown in Table 2, which are in accordance with the literature (Pereira et al., 2015). In theory, considering that the fibers are shaped as a perfect cylinder, with a height *h*, a diameter *d*, a mass *m* and a density *ρ*, the fiber's diameter can be expressed as the Eq. 1.

$$d = 2 \cdot \sqrt{\frac{m}{\pi \cdot h \cdot \rho}} \qquad (1)$$

As the treated hemp fiber mass increased and their real density remained the same or slightly decreased, a diameter increase is expected. By considering the mass and the density variations of the treated samples, and that the fiber's height is big enough so it does not change when the fibers are treated, the Eq. 1 gives the expected percentage increment of the diameter from the treated samples, when compared with the reference fibers. The samples IMM 80°C, IMM 120°C, PUL 80°C and PUL 120°C presented an expected diameter increase of 1.5 %, 3.3 %, 7.2 % and 6.7 %, respectively.

In order to verify this phenomenon, images of the same hemp fibers were made before and after the treatment. Fig. 4 illustrates two examples of diameter measurement, with images taken before (4a and 4c), and after (4b and 4d) treatment. In both cases, the marks on the fibers show that they did not twist or shift during treatment, allowing precise location of the measured positions. That was, however, not always the case. Frequently, the fibers shifted, twisted, or separated from their bundles after being treated, making the comparison between them difficult. Two examples are shown on Fig. 5, where Fig. 5a and Fig. 5c shows two images made before the treatment, and Fig. 5b and Fig. 5d

<table><caption>**Table 2**
Mass and fiber diameter change.</caption><thead><tr><th>Sample</th><th>Mass increase (%)</th><th>Diameter increase (%)</th><th>Real density (g.cm<sup>-3</sup>)</th></tr></thead><tbody><tr><td>REF</td><td>-</td><td>-</td><td>1.503 ± 0.004</td></tr><tr><td>REF W 80°C</td><td>-5.0 ± 0.7</td><td>-1 ± 5</td><td>-</td></tr><tr><td>IMM 80°C</td><td>3.0 ± 0.8</td><td>0 ± 5</td><td>1.504 ± 0.002</td></tr><tr><td>IMM 120°C</td><td>5.1 ± 1.7</td><td>6 ± 6</td><td>1.479 ± 0.003</td></tr><tr><td>PUL 80°C</td><td>14.3 ± 4.6</td><td>8 ± 4</td><td>1.496 ± 0.003</td></tr><tr><td>PUL 120°C</td><td>12.8 ± 2.4</td><td>6 ± 3</td><td>1.489 ± 0.002</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Example of measurement of fiber diameter. (a) and (c) Before the treatment; and (d) After the treatment.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. Example of a discarded measurement of fiber diameter. (a) and (c) Before the treatment; (b) and (d) After the treatment.</figcaption></figure>

shows two images made after the treatment. In both cases, it is possible to see that the fibers rotated their position when compared with their initial position, which caused a large difference in their diameter when observed from the microscope. Like those cases, measurements were excluded if the fiber's position was unclear before and after treatment, or if fiber bundles had attached or detached in certain areas.

Additionally, fibers that were visibly twisted were also excluded, as they are not perfectly round and this affected measurement accuracy. For each condition, 45 measurements were taken, with about half discarded for these reasons.

Morphological analysis of the fiber bundles evidences that the treated samples generally showed a diameter increase of 6 % and 8 %, in
respect to their initial size, except for sample IMM 80°C, which showed no change. These results are shown in Table 2, alongside their standard error. However, significant variation between measurements led to a high standard deviation, limiting the relevance of comparisons among treated samples despite careful measurement protocols. Despite this, the overall trend of diameter increase suggests an effect of the treatment on fiber morphology.

### 3.3. Fourier transform infrared spectroscopy analysis

The FTIR-ATR spectra from all hemp fiber samples are presented in Fig. 6. The treated samples were compared to the reference to identify the changes in the spectra after the treatment. The peaks linked to the bonds between plant fibers and phytic acid are indicated with a dashed line in the Figure. At 790 cm⁻¹, a bond between phosphorus, oxygen and carbon was identified (Taibi et al., 2022). The peaks at 1240 cm⁻¹ and 1450 cm⁻¹, correspond to the P = O bonds absorption (Li et al., 2015, Marcilla et al., 2015, Taibi et al., 2022). The presence of these peaks confirms the formation of covalent bonds between the phosphorus molecules from the phytic acid and the cellulose and hemicellulose from the hemp fibers. The peak 1630 cm⁻¹ originates from the N-H bonds, which comes from the use of urea as a flame retardant (Marcilla et al., 2015).

### 3.4. Phosphorus content

A significant decrease in the hemp fibers flammability is expected with the grafting of phosphorus, as presented by Antoun et al. (2022). Therefore, the fiber's phosphorus content was measured and the results and their standard deviation are reported on Table 3. The fibers treated by immersion, at 80°C and 120°C, presented a phosphorus content of 0.61 % and 0.66 %, respectively. A paired t-student test showed that both phosphorus contents are statistically similar between themselves. The fibers treated by pulverization achieved a higher phosphorus concentration, reaching 1.85 % and 1.56 %, for the fibers treated at 80°C and 120°C, respectively. The higher phosphorus content presented by the pulverization method, in comparison with the immersion method, was explained by the absence of washing at the end of the treatment.

### 3.5. Thermal degradation

The thermal degradation curves of the hemp fibers and their derivatives are presented on Figs. 7 and 8, revealing two main stages of mass loss. For the reference fibers, degradation starts at 230°C, corresponding to the breakdown of the hemicellulose fraction, due to its low crystallinity and reduced thermal stability (Yang et al., 2007; Dorez et al., 2014b). This is followed by the cellulose degradation, which continues up to 380°C. Then, a second mass loss takes place between 380°C and 500°C, corresponding to a further degradation of products formed in the initial stage, known as humins (Bausch et al., 2021). The lignin does not have an evident peak as it degrades gradually in multiple stages between 150°C and 700°C (Zhao et al., 2014; Costes et al., 2017).

<table><caption>**Table 3**
Peak DTG, residues at 500°C and 750°C and phosphorus content.</caption><thead><tr><th>Sample</th><th>Peak DTG (°C)</th><th>Residue 500°C (%)</th><th>Residue 750°C (%)</th><th>P content (%)</th></tr></thead><tbody><tr><td>REF</td><td>333 ± 1</td><td>4.2 ± 0.6</td><td>2.4 ± 1.0</td><td>0.08 ± 0.01</td></tr><tr><td>IMM 80°C</td><td>304 ± 2</td><td>23.5 ± 0.2</td><td>2.1 ± 0.8</td><td>0.61 ± 0.16</td></tr><tr><td>IMM 120°C</td><td>294 ± 5</td><td>26.4 ± 2.1</td><td>2.2 ± 0.3</td><td>0.66 ± 0.24</td></tr><tr><td>PUL 80°C</td><td>277 ± 4</td><td>30.4 ± 0.8</td><td>10.0 ± 0.6</td><td>1.85 ± 0.40</td></tr><tr><td>PUL 120°C</td><td>277 ± 1</td><td>29.5 ± 3.7</td><td>8.7 ± 2.8</td><td>1.56 ± 0.26</td></tr></tbody></table>

In the case of treated fibers, premature thermal degradation is observed. In fact, the peak of the first degradation step occurs at 333°C for untreated fibers and between 277 and 304°C for modified fibers. This shift is related to changes in the degradation pathways of cellulose and hemicellulose due to dehydration catalyzed by the phosphorus flame retardant (Hajj et al., 2020). The accelerated dehydration of cellulose promotes char formation, which protects the inner parts of the fibers and increases the residue formed during thermal degradation (Hajj et al., 2020; Moussa et al., 2020; Taibi et al., 2022). This is confirmed by TGA analysis that highlight that the amount of the residue formed at 500 °C is higher for treated fibers (between 23 % and 31 %) compared with only about 4.2 % for the untreated fibers. At 750°C, the immersed fibers show no significant increase in residue compared with the reference sample, whereas pulverized fibers maintain high residue levels, 10.0 % and 8.7 %, for the fibers treated 80°C and 120°C respectively. These results, and their respective standard deviation, are summarized in the Table 3 along with phosphorus content measured by ICP-AES.

The TGA results also demonstrated a good correlation with the phosphorus content of the fibers. It is evident that higher phosphorus content corresponds to a greater amount of residue remaining during thermal degradation, particularly at temperatures above 500°C. This indicates that the quality of the char formed is enhanced, providing derivatives are presented on Figs. 7 and 8, revealing two main stages of mass loss. For the reference fibers, degradation starts at 230°C, corresponding to the breakdown of the hemicellulose fraction, due to its low crystallinity and reduced thermal stability (Yang et al., 2007; Dorez et al., 2014b). This is followed by the cellulose degradation, which continues up to 380°C. Then, a second mass loss takes place between 380°C and 500°C, corresponding to a further degradation of products formed in the initial stage, known as humins (Bausch et al., 2021). The lignin does not have an evident peak as it degrades gradually in multiple stages between 150°C and 700°C (Zhao et al., 2014; Costes et al., 2017).

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** FTIR spectra of treated and untreated hemp fibers.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. TGA curves from treated and untreated hemp fibers.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. DTG curves from treated and untreated hemp fibers.</figcaption></figure>

better protection for the fibers and increasing the mass retained during the test.

### 3.6. Fire reaction

The mass loss cone calorimeter test provided several valuable parameters for characterizing the fire behavior of the treated hemp fibers. The evolution of the heat release rate (HRR) as a function of temperature for the reference and treated fibers is shown in Fig. 9, with a summary of all test results and their standard deviation on Table 4.

First of all, the reference fibers presented a quick ignition, with a TTI of about 3.5 seconds. The samples IMM 80°C and IMM 120°C, delayed the ignition to 5.3 s and 6.0 s, respectively, and the samples PUL 80°C and PUL 120°C, delayed it even more, having a TTI at about 11.0 s and 9.5 s, respectively, as shown in Table 4. This indicates the tendency to increase the fibers resistance to ignition after the treatment.

Significant improvements were also observed in the pHRR, and THR. The average of the pHRR from the reference fibers was of 109 kW.m⁻². The samples IMM 80°C, IMM 120°C, PUL 80°C and PUL 120°C had an

average of 92 kW.m⁻², 70 kW.m⁻², 49 kW.m⁻² and 57 kW.m⁻², respectively, representing a reduction of 16%, 36%, 55% and 48%. The THR, measured from the beginning of the test, until the flame extinction, showed a substantial decrease for the treated fibers, reaching nearly 90% for the pulverized fibers. The overall reduction in fiber's flammability aligns with their phosphorus content, shown in Table 3. All treated samples exhibited a significant enhancement of fire-retardant behavior, particularly the pulverized fibers, which contain a higher phosphorus level. These results are in accordance with the residue increase observed in the TGA.

The mass loss curves over the duration of the test are shown in Fig. 10. During the tests, flames extinguished much faster for most treated fibers. The reference fibers burned intensively, with an average burning duration of about 4.5 minutes, leaving an average final mass of 17% of the initial mass. The IMM 80°C sample showed a slight enhancement, with an average burning time of just under 4 minutes and a final mass was of 34%. Significant improvement was observed for the other treated fibers. The IMM 120°C sample had an average burning duration of 1 minute and 9 seconds, preserving 70% of the initial mass.
<figure><img src="image_12.png" /><figcaption>Fig. 9. HRR curves of treated and untreated hemp fibers as a function of time.</figcaption></figure>

<table><caption>**Table 4**
Cone calorimeter results.</caption><thead><tr><th>Sample</th><th>TTI (s)</th><th>pHRR (kW.m<sup>-2</sup>)</th><th>THR (MJ.m<sup>-2</sup>)</th></tr></thead><tbody><tr><td>REF</td><td>3.5 ± 1.9</td><td>109 ± 7</td><td>13.2 ± 0.6</td></tr><tr><td>IMM 80°C</td><td>5.3 ± 4.2</td><td>92 ± 15</td><td>7.5 ± 0.9</td></tr><tr><td>IMM 120°C</td><td>6.0 ± 4.0</td><td>70 ± 6</td><td>2.9 ± 0.3</td></tr><tr><td>PUL 80°C</td><td>11.0 ± 1.2</td><td>49 ± 9</td><td>1.5 ± 0.6</td></tr><tr><td>PUL 120°C</td><td>9.5 ± 3.0</td><td>57 ± 13</td><td>1.8 ± 0.4</td></tr></tbody></table>

Both pulverized samples burned for only 50 seconds, preserving 77 % of their initial mass.

Fig. 11 illustrates the appearance of the fibers after testing. A char layer is observed on all treated samples, evidencing the effect of the phosphorus-based treatment. This char helped the treated samples retain their shape, allowing them to be removed from the mold intact. In contrary, the reference sample was highly deteriorated, fragile and unable to maintain its form. The reduction of burning time and the increase of the remaining masses is linked to the formation of these char layer

barrier on top of the fibers, creating a protection to the unexposed fibers.

Notably, samples IMM 120°C, PUL 80°C and PUL 120°C retained, at some extent, their fiber morphology of the fibers, which can be correlated to their improved performance. In contrast, portions of the IMM 80°C sample developed a smooth surface, losing the original fiber structure.

### 3.7. Ignitability and smoke production

The ignitability of the fibers was assessed using the single flame source test. The results are presented in the Fig. 12. All the fibers presented a burned area below 15 cm, which meets the requirements for the European classification (Euroclass) classes E and F. The immersed fibers, despite not showing a statistically significant reduction in the burned area height when compared with the reference fibers, they cease burning as soon as the flame was removed. For the pulverized fibers, a statistically significant reduction in burned area was observed, and they also stopped burning immediately after the flame was removed.

The smoke density test provides additional insight into the reaction

<figure><img src="image_13.png" /><figcaption>Fig. 10. Mass loss during the cone calorimeter test.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 11. Hemp fibers aspect after the cone calorimeter test.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 12. Burned area height from the single flame source test.</figcaption></figure>

to fire analysis of both treated and untreated hemp fibers, by examining smoke production during combustion. Results are expressed as the light absorption percentage, or absorptivity, as a function of time, as seen in Fig. 13. Higher absorptivity indicates less light passing through the smoke to reach the detector. Initially, smoke density rises quickly to a peak before starting to decrease, which happens due to chamber's bottom opening, that allow airflow and smoke exit. At a certain point, the rate of smoke production falls below the rate at which smoke exits, causing the curves to decline.

The IMM 80°C fibers showed no improvements regarding the smoke production with a smoke production rate similar to that of the reference fibers. Both achieved comparable maximum absorptivity, 93.9% for reference fibers and 92.8% for IMM 80°C fibers. In contrast, the IMM 120°C, PUL 80°C and PUL 120°C samples presented a reduced smoke production rate and lower maximum absorptivity values, reaching peaks of 89.8%, 84.5% and 81.7%, respectively.

<figure><img src="image_16.png" /><figcaption>Fig. 13. Absorptivity as a function of the test duration.</figcaption></figure>

### 3.8. Comparison between immersion and pulverization treatments

Based on the results presented, both treatment methods effectively reduce the flammability of the hemp fibers, although key differences exist. Notably, the pulverization method resulted in higher phosphorus content in the fibers due to the absence of a final washing step, leading to reduced flammability and smoke production, which makes this method advantageous in flame-retardant performance.

From a technical and industrial scaling perspective, each treatment method has positive and negative aspects. The pulverization method uses a FR/Fiber mass ratio of 1, instead of 10 for the immersed fibers, requires fewer washing and drying steps, and yields fibers that remain loose, making it a less resource-, time-, and energy-intensive approach.

Conversely, it was noted that the fireproof treatment imparts a viscous texture to the fibers when handled, a sensation not experienced with the immersed fibers. This characteristic is attributed to the absence of a washing step in the pulverization treatment, which leaves non-grafted flame-retardant on the fiber surface. Therefore, this method may not be suitable for applications involving frequent user contact, such as textiles. In such cases, the immersion method is preferable, as it effectively removes any residual substances from the fiber surface.

Additionally, some side effects of the treatment on the fibers were noted, including an increase in weight and a tendency for the fiber diameter to increase as well. However, the real density of the fibers remained unchanged. These changes at the fiber level could also influence the acoustic and thermal performance of the fibers in applications such as building insulation.

## 4. Conclusion

Hemp fibers stacks were treated with a bio-based flame-retardant system, combining phytic acid and urea, using two application methods: immersion and pulverization. As opposed to the techniques previously seen in the literature (Table 1), the methods presented in this study aim to be adapted to an industrial production level, treating a large quantity of fibers while being efficient in the amount of flame retardant used and the number of steps required for the whole treatment. The results showed that the thermal and flame-retardant properties of the fibers were affected by both the treatment method and drying temperature.

For the immersion method, although both fiber samples had similar phosphorus content, the fibers treated at 120°C exhibited greater residue at 500°C during TGA, lower pHRR and THR during mass loss calorimeter test, and a lower smoke production, in respect to untreated fibers and those treated by immersion at 80°C.

With the pulverization method, flammability results were similar across both temperatures, despite a higher phosphorus content in fibers treated at 80°C. Therefore, treating fibers at lower temperatures is preferable, as it reduces energy use, costs, and avoid potential fiber degradation.

The compounds used for the flame-retardant treatment of hemp fibers and the pulverization application method can be used to treat fibers on a large scale and manufacture insulating materials based on plant fibers. This work has demonstrated the effectiveness of this treatment on raw fibers. One of the prospects of this work is to manufacture insulating
panels using industrial processes to validate these results on a material scale.

In addition, as the treatment of the fibers can have an impact on their diameter, the characterization of their hygrothermal and acoustics performance will also be carried out in order to study its influence on the functional properties of the fibers.

## Funding

This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie COFUND grant agreement No 101034248 and from the French Agence Nationale de la Recherche (ANR) under grant ANR-23-CE22-0012 (BIOMETA).

## CRediT authorship contribution statement

**Segovia César:** Writing – review & editing, Supervision, Resources, Methodology, Conceptualization. **Glé Philippe:** Writing – review & editing, Supervision, Conceptualization. **Gourlay Etienne:** Writing – review & editing, Supervision, Conceptualization. **marceau sandrine:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. **Schatzmayr Welp Sá Thomas:** Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis. **Laoutid Fouad:** Writing – review & editing, Validation, Supervision, Resources, Methodology, Conceptualization. **Piégay Clément:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

Data will be made available on request.

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