Introduction

Lyocell fibres are made from cellulose solution in an N-methylmorpholine-N-oxide (NMMO) system via a dry-jet wet spinning technique (Jiang et al. 2020; Zhang et al. 2018a, b). These fibres are highly flammable, which limits their application range. There has been an urgent demand for flame-retardant cellulose materials in recent years. Flame-retardant cellulose materials, such as combustion droplets of typical flame-retardant thermoplastic materials of nylon and polyester, can avoid secondary injury risks. Therefore, improving the flame retardancy of lyocell fibres has become an important focus in the textile industry (Alongi et al. 2015; Lazar et al. 2020; Qi et al. 2023). State-of-the-art flame-retardant technology is used to create a flame-retardant coating on the surface of lyocell fabric (Alongi et al. 2015; Lazar et al. 2020; Qi et al. 2023). Nevertheless, only a few studies have been performed on the direct preparation of flame-retardant lyocell fibre materials.

The flame-retardant coating treatment of lyocell fabric is the primary direction of flame-retardant cellulose materials. The most mature technology uses tetrakis(hydroxymethyl)phosphonium chloride (THPC) (Basch et al. 1979; Dong et al. 2015; Mengal et al. 2016; Roy et al. 2018). This technology requires the use of a flame-retardant coating by polymerization involving base-promoted condensation of THPC with polyamines. The dipping-nipping method can be applied to flame-retardant technologies for fabrics, such as using dicyandiamide and amine compounds (Chen et al. 2021; Guo et al. 2021a, b; Liu et al. 2018a, b; Liu et al. 2020; Ren et al. 2020; Su et al. 2021; Tan et al. 2021; Xiao et al. 2021; Zhang et al. 2020; Zhao et al. 2023); ammonium polyphosphate; and tyramine (Li et al. 2023). In addition, in recent years, researchers have developed plasma-aided surface modification, layer-by-layer assembly technology and sol–gel treatment technology to prepare flame-retardant lyocell fabrics (Jenny et al. 2011; Lazar et al. 2020; Warunee et al. 2017). However, the use of THPC produces formaldehyde and degrades fabric performance (Lazar et al. 2020; Mengal et al. 2016; Qi et al. 2023; Roy et al. 2018). The dipping-nipping method is applied to lyocell fabrics in the form of coatings but not to cellulose fibres. The preparation process of layer-by-layer assembly technology is complex and time-consuming, and sol–gel treatment requires a complicated preparation process involving a flame-retardant gel solution.

Flame-retardant lyocell fibres are usually generated by chemical reactions or physical blending.

The chemical reaction method involves the formation of covalent bonds between cellulose and the flame retardants. For example, Guo et al. (Guo et al. 2021a, b) prepared flame-retardant materials based on carboxymethylation and aluminium ion chelation. Several groups (Nguyen et al. 2012; Megumi et al. 2012) produced flame-retardant lyocell fibres by covalent bonding between phosphorus-containing flame retardants and cellulose hydroxy groups via the addition of acrylamide or the nucleophilic substitution reaction of cyanuric chloride. Liu et al. (Liu et al. 2018a, b) fabricated durable and flame-retardant lyocell fibres by a one-pot chemical treatment between ethanolamine diphosphoric acid and cellulose. However, the fibres prepared in this research have a limiting oxygen index (LOI) of less than 28% and cannot protect firefighters when exposed to fire. The effect of chemical reactions on the mechanical strength has not been determined. On the other hand, the phosphorylation of cellulose is also a new approach for flame retardancy (Bai et al. 2014; Ghanadpour et al. 2015; Rol et al. 2020). Due to the limitation of the tensile strength of phosphorylated cellulose, it is used to prepare flame-retardant gel/sol and ion-exchange materials (Ghanadpour et al. 2018; Illy et al. 2015; Rol et al. 2020).

However, flame-retardant cellulose materials are difficult to prepare by physical blending. There are several problems in preparing special lyocell fibres by physical blending. First, additives or flame retardants must be thermally stable due to the dissolution of cellulose at high temperatures (Edgar et al. 2020; Jiang et al. 2020; Li et al. 2014). Second, the fibres must be filled with additives by fine dispersion; otherwise, the spinnability of the lyocell fibres may be reduced (Edgar et al. 2020). Third, the flame retardant must be alkali resistant because lyocell fibres need to be dyed under alkaline conditions. The durability and permanence of flame-retardant fibres are highly problematic is because regular crystals lead to the expansion of lyocell fibres in water, causing difficulty to maintain durability inside fibres (Okugawa et al. 2020, 2021; Sawada et al. 2021; Zuckerstatter et al. 2013). For instance, Gao et al. (Gao et al. 2012) blended resins containing a microencapsulated flame retardant into epoxy, but this technique could not be applied to lyocell fibres because the microcapsule was dissolvable in NMMO solution. Zhang et al. (Zhang et al. 2018a, b) suppressed the flammability of cellulose fibres by incorporating alginate fibres. These fibres could not be utilized within the conventional application range because the seaweed was dissolved in salt. Yang et al. (Yang et al. 2022) blended melamine cyanurate into fibres; however, this component lacked flame-retardant permanency or alkali resistance, causing difficulty in its applications.

Due to the lack of research on the direct production of the permanent flame-retardant lyocell fibres without coating and the large demand for fibres in many applications, we proposed a novel method for fabricating permanent flame-retardant lyocell fibres with desirable performance. Therefore, in this study, a preparation technology for durable flame-retardant lyocell fibres was provided. We mixed MTT into lyocell fibres to improve the condensed-phase flame retardancy, and we further phosphorylated these fibres to improve the gas-phase flame retardancy (referred to as P-MTT/lyocell). Furthermore, the synergistic role and mechanism of these two flame retardant treatment methods were examined. This fibre (P-MTT/lyocell) could simultaneously meet the needs of high strength and excellent durable flame-retardant functionality. This is the first report on the preparation of permanent flame-retardant lyocell fibres, with high mechanical strength.

Materials and methods

Materials

Wood pulp (6.0 wt% water content, DP = 640, 95% α-cellulose content) was obtained from COSMO Specialty Fibers, Inc., and the NMMO solution was obtained from the China Textile Academy (China). In addition, laboratory phosphoric acid, ammonium dihydrogen phosphate, sodium phosphate, sodium hydrogen phosphate, potassium hydrogen phosphate, urea, the fatty alcohol polyoxyethylene ether (AEO) and sodium dodecyl sulfate (SDS) were used throughout the study.

Phosphorylation of lyocell fibres

Phosphorous compounds and urea were dissolved in water at a temperature of 50℃ to prepare solutions with concentrations of 1 mol/L and 2 mol/L, respectively. The fibres (1 g) were soaked in this solution (100 mL) for 10 min, after which the excess solution was removed until the total weight was 2.6 g. The fibres were reacted for 30 min at 160 °C in an oven. Subsequently, the fibres were cleaned, and the experiment was repeated twice. Finally, the LOI values of the fibres were measured to investigate the effect of different phosphorous compounds on the LOI values. In addition, phosphoric acid-treated fibres were prepared according to previous methods (Ablouh et al. 2021).

Diammonium hydrogen phosphate and urea were dissolved in water at a temperature of 50℃ to prepare solutions with concentrations of 2 mol/L and 4 mol/L, respectively. The fibres (10 g) were soaked in this solution (1000 mL) for 10 min, after which the excess solution was removed until the total weight reached 26 g. The fibres were reacted for different durations at different temperatures in an oven. The fibres were cleaned, and the experiment was repeated three times. Finally, the LOI values of the fibres were measured to investigate the effect of the reaction time and reaction temperature on the flame retardancy.

A diammonium hydrogen phosphate solution with a concentration of 1 mol/L was prepared, and urea was added at 0, 1, 1.5, 2, 2.5, and 3 mol/L. The fibres (1 g) were soaked in this solution (100 mL) for 10 min, after which the excess solution was removed until the total weight was 2.6 g. The mixture was reacted for 30 min at 160 °C in an oven. The fibres were cleaned, and the above operation was repeated twice. Finally, the LOI values of the fibres were measured to determine the effect of the urea content on the flame retardancy.

Preparation of the MTT/lyocell blended fibres

First, the fatty alcohol polyoxyethylene ether (AEO, CAS: 111–09-3), sodium dodecyl sulfate (SDS, CAS: 151–21-3) and flame retardants (Table 1) were added to water, and the concentrations of these were 10%, 3% and 10%, respectively. Then, the mixture was stirred for 30 min in a high-speed shearing machine (JRH200-S, China) to prepare a stable solid suspension.

Table 1 The flame retardants used in this research
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Second, the solid suspension was ground for 120 min, and the solid particle size was controlled to 2 µm by a sand mill (RTSM-10AD, China). Then, the wood pulp, the 80% NMMO solution and the suspension were added to a custom-made reaction kettle. The ratios of NMMO, cellulose, water and flame retardant were 8:1:n:m, where n and m were determined according to the needed flame retardant content. Then, the cellulose solution was prepared by negative pressure at 100 °C from the above mixtures.

Third, the lyocell fibres were spun from cellulose solution via dry-jet wet spinning, and the linear density of the fibres was controlled within 1—3 dtex.

Finally, the residual NMMO, AEO and SDS on the fibres were removed by washing, and the fibres were obtained by drying.

Characterization

The limiting oxygen index (LOI) of each fibre sample was measured by a limiting oxygen index tester (K-R2406S, China) according to the FZ/T 50016–2011 standard. A 0.3 g sample was twisted to produce a fibre bundle of 25 cm. We injected a mixture of nitrogen and oxygen into the tester and ignited the fibre bundle. The maximum oxygen concentration at which the fibres could extinguish was the LOI. To ensure reproducibility, the mean test value from the results of 15 samples were taken. The durability of the flame retardant was evaluated by washing for 30 min during the standard washing procedure, which was repeated 12 times.

The mechanical properties of the fibres were evaluated by an XQ-1 tensile tester (Donghua University, China). The test length was 20 mm, and the tensile speed was 5 mm/min. To ensure reproducibility, the mean test value from the results of 50 samples were calculated.

The surface and cross-sectional morphology of the fibres were verified via scanning electron microscopy (SEM, S-4700, Japan). The fibre samples were coated with approximately 20 nm of copper to make the samples more conductive and suitable for SEM analysis. The SEM was operated using 10 kV.

A Nicolet-10 Fourier transform infrared (FT-IR) spectrometer was used to identify the infrared spectrum of the fibres. The IR spectra were scanned over the wavenumber range of 4000–500 cm−1.

X-ray photoelectron spectroscopy (XPS) spectra were measured by ESCALAB 250 (USA). The samples were irradiated with monochromatic Al K Alpha (100 eV) using a spot size of 500 µm × 500 µm. In addition, high-resolution scan XPS spectra of P 1 s, Si 2p and Al 2p were recorded with a pass energy of 30 eV, and the energy step size was 0.100 eV, from which the surface chemical compositions were obtained. To ensure reproducibility, the samples were analysed in duplicate or triplicate, and data analysis was performed using the instrumentation software.

X-ray diffraction (XRD) patterns were measured by a Siemens-Bruker (D5000, Germany) using Cu Kα radiation. Scattered radiation was detected in the range of 2θ = 5–60° at a scan step size of 0.05°. Curve fitting was performed using PeakFit software (www.systat.com) to identify individual peaks (Isable et al. 2019).

Thermogravimetric (TG) and TG-infrared (TG-IR) analyses of samples (10 mg) heated from 35 to 800 °C were performed using a PerkinElmer TGA4000 (USA). The heating rate was set at 20 °C/min under a continuous flow of N2 (30 mL/min).

Raman spectroscopy was used to evaluate the char residues of the fibres with an XploRA PLUS (Horiba, Japan) in the range of 500–2500 cm−1. The laser wavelength was 532 nm. Curve fitting was performed using PeakFit software (www.systat.com) to identify individual peaks. The peaks were resolved into two reflections.

Inductive coupling plasma (ICP, ICP‒OES725, USA) was used to determine the phosphorus content. The phosphorus content was measured after microwave mineralization in the presence of nitric acid, followed by ICP. At least two measurements were performed for each sample.

Results and discussion

Influence of the phosphorylation on the lyocell fibres

The reaction between phosphorus compounds and cellulose was esterification. This reaction could occur in the crystal region of the lyocell fibres. In this reaction, urea was used as a catalyst, and enhanced fibre expansion prevented cellulose degradation. It is usually difficult to phosphorylate cellulose (Wang et al. 2018; Ablouh et al. 2021).

The effect of different phosphorylation reagents on the LOI of lyocell fibres is shown in Fig. 1a. The method for preparing phosphoric acid-treated fibres was based on previous methods (Ablouh et al. 2021). The results indicated that diammonium hydrogen phosphate was the ideal phosphorylation reagent. Although phosphoric acid-treated fibres exhibited better flame retardancy, this method hydrolysed cellulose and caused a significant decrease in fibre tensile strength.

Fig. 1
figure 1

Effect of phosphorus compound type (a), reaction temperature and time (b) and reagent part (c) on the flame retardancy of fibres

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The effects of phosphorylation temperature and time on the LOI of the lyocell fibres were investigated by using diammonium hydrogen phosphate (Fig. 1b). The effect of the ratio of diammonium hydrogen phosphate and urea on the LOI of the fibres was optimized (Fig. 1c).

As shown in Fig. 1, the phosphorylation reaction did not occur below 110 °C, and the optimal molar ratio of phosphate to urea was 1:2.

As shown in Fig. 2a, the effects of phosphorylation degree on the LOI, phosphorus content and fibre structure were investigated. The results showed that the phosphorus content increased linearly with increasing reaction time. In general, when the phosphorus content was 1.5–4%, the fibres exhibited excellent self-extinguishing performance (LOI > 25%).

Fig. 2
figure 2

Effect of phosphorylated cellulose time on the fibre LOI and phosphorus contents (a) and the effect of the phosphorylated cellulose degree on the lyocell fibre crystal (b)

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The crystallinity index (CrI) of each sample at different reaction times was calculated according to previous methods (Isable et al. 2019) (The specific method see Supproting materials). When the reaction time was shorter than 30 min, no significant change was observed in the crystallinity of the fibres. The phosphorus content continued to linearly increase when the reaction time exceeded 30 min. In this case, phosphorylation occurred in the crystallization zone, resulting in a decrease in fibre crystallinity (Fig. 2b) and a decrease in strength (Fig. 3).

Fig. 3
figure 3

Durable flame retardancy and alkaline conditions of different blended fibres

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Therefore, the use of diammonium hydrogen phosphate for phosphorylation produced excellent flame retardancy performance of the lyocell fibres. However, the reaction occurred in the crystalline region, destroyed the fibres and had no commercial value.

Properties of the retardant fibres prepared by physical blending of different flame retardants

In this study, conventional flame retardants were added to lyocell fibres by blending.

MPP is a flame retardant containing phosphorus and nitrogen. The flame retardancy of the MPP/lyocell blended fibres decreased after washing because the lyocell fibres expanded under wet conditions, causing the loss of flame retardants inside the fibres.

Melamine cyanurate (MCA) is an important nitrogen flame retardant that is suitable for polyamide materials. However, MCA dissolves in alkaline solution, preventing MCA/lyocell fibres from resisting alkaline conditions.

Although the DBDPE/lyocell blended fibres exhibited good flame retardancy, they produced toxic gases during combustion and were inherently biohazardous. Therefore, this approach was not appropriate for lyocell fibres.

Nanoz-zinc borate (ZB) is an environmentally friendly nonhalogen flame retardant. The greater expansion of fibres under alkaline conditions caused a loss of the flame retardancy in the ZB/lyocell fibres.

NLD is a flame retardant for viscose fibres containing phosphorus nitrogen compounds. NLD was dissolved in NMMO and diffused into water during spinning, leading to ineffective flame retardancy of the fibres.

MTT is used as a flame retardant or flame retardant synergist (Liu et al. 2011; Sanusi et al. 2020). Although the LOI of the MTT/lyocell fibres was low, the fibres possessed superior permanent flame retardancy and alkaline resistance. The above discussion is the basis for the subsequent studies.

Phosphorylated MTT/lyocell blended fibres

In this section, the excellent permanent flame retardancy of MTT/lyocell fibres was evaluated. The MTT/lyocell fibres were phosphorylated to prepare high-strength and permanent flame-retardant fibre materials. The synergistic flame retardant effects of these two methods were investigated.

Based on the linear change trend of phosphorylation degree with time, as shown in Fig. 2, this section provided a discussion on the use of the phosphorylation time to represent the phosphorylation degree.

Figure 3a shows the synergistic flame-retardant curve between the phosphorylation degree and the MTT content on the flame retardancy. The LOI of the phosphorylated control lyocell fibres showed less improvement, while the LOI of the MTT/lyocell fibres showed improvement after phosphorylating. MTT and phosphorylated cellulose had synergistic flame retardancy effects.

The LOI increased from 18 to 21% as the MTT content increased from 0 to 33%. Nevertheless, the fires on the fibres were not extinguished in the air. However, the LOI of the fibres containing 33% and 50% MTT reached 28% after phosphorylation; this value was significantly greater than that of the control fibre without MTT. In addition, the fibres filled with MTT lost strength after 30 min of phosphorylation because the addition of MTT changed the crystallinity of the cellulose. In addition, the 50% MTT/lyocell fibres had low strength and could not be used commercially.

According to the deductions from Fig. 3a and b, the ideal condition was phosphorylation of the 33% MTT/lyocell fibres for 15 min. The LOI was 28%, and the tensile strength was 2.0 cN/dtex. The tensile strength of the viscose fibre was usually between 1.5 cN/dtex and 2.7 cN/dtex. Although the tensile strength of the fibres decreased by approximately 50%, this value was still close to the strength of viscose fibres and showed definite commercial value.

In addition, the strength of the untreated fibres was 3.9 cN/dtex. Materials such as ramie fibres could be applied to increase the strength of original lyocell fibres to increase the strength of flame-retardant lyocell fibres.

The results from XPS and high-resolution scan XPS showed that P was successfully grafted on the fibres, and the MTT material was successfully blended into the lyocell fibres (Fig. 4a). The new peaks at 75 eV, 103 eV and 134 eV for the treated samples were attributed to Al 2p, Si 2p and P 2p, respectively. The FTIR results (Fig. 4b) showed that the characteristic peak at 3614 cm−1 of the montmorillonite material was interlaminar water, which disappeared after mixing with cellulose because of the high-temperature preparation of the cellulose solution. Instead, a new peak was generated at 1450 cm−1 and was the bending vibration absorption of C-H; this result indicated that there was a strong intermolecular force between MTT and cellulose. Due to this result, the MTT/lyocell fibres had excellent permanent flame retardancy (Fig. 5). The characteristic peak at 1230 cm−1 was attributed to the P = O band, and the peak at 900 cm−1 was attributed to the P-O-C band; thus, the MTT/lyocell fibres were successfully grafted with phosphorus. The characteristic peak at 1730 cm−1 was attributed to the C = O stretching mode because urea and cellulose reacted to produce carbamate.

Fig. 4
figure 4

Effect of different MTT contents and phosphorylated cellulose degree on the flame retardancy (a) and tensile strength (b) of the fibres

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Fig. 5
figure 5

XPS (a) and FTIR (b) spectra of the fibres

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Table 2 shows the washability and durability of the different flame-retardant fibres. The results showed that the phosphorylated MTT/lyocell fibres had excellent permanent flame retardancy, and the flame retardancy did not change after 12 separate washing. In addition, the material could maintain excellent flame retardancy under alkaline washing conditions. Other flame-retardant fibres did not consider the strength, washing resistance and LOI values

Table 2 Comparison of the flame retardancy, strength, and washing resistance of different lyocell fibres
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Figure 6 shows SEM images of several flame-retardant lyocell fibres. Phosphorylation had no significant effect on the morphology of the lyocell fibres. The MTT/lyocell and P-MTT/lyocell fibres had uneven surfaces due to the addition of MTT.

Fig. 6
figure 6

SEM images of the control lyocell (a), P-lyocell (b), MTT/lyocell (c) and P-MTT/lyocell (d) fibres

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Flame retardancy mechanism of P-MTT/lyocell

In this section, the flame retardancy mechanism of P-lyocell, MTT/lyocell and P-MTT/lyocell was analysed. The LOIs of MTT/lyocell and P-lyocell were only 23–24%. However, the LOI of P-MTT/lyocell reached 28% when the two flame-retardant methods were combined.

TG-IR analysis and TG analysis

TG-IR analysis can better explain the thermal degradation behaviour of cellulose materials and reveal the composition and structure of the volatile components. Figure 7 shows the TG-IR results for four different lyocell fibres. The peak at 3600–3900 cm−1 was attributed to the stretching vibration of -OH, which was derived from water vapour produced during the thermal degradation. The peak at 2900 cm−1 was attributed to the vibration absorption of the C-H bond derived from the flammable alkane compounds. The peak at 2360 cm−1 was attributed to the stretching vibration peak of CO2. The peak at 1745 cm−1 was assigned to the C = O of carbonyl compounds, which are considered pyrolysis products of cellulose.

Fig. 7
figure 7

TG-IR curves of control lyocell (a), MTT/lyocell (b), P-lyocell (c) and P-MTT/lyocell (d) fibres

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The intensity of the characteristic peaks of lyocell and MTT/lyocell reached a maximum at 420 °C, whereas that of the P-lyocell and P-MTT/lyocell fibres reached a maximum at 360 °C. Thus, the maximum degree of thermal degradation was reached at the matching temperature. This result was attributed to the thermal degradation by phosphorylated cellulose, which also enhanced the quenching of pyrolysis. This process protected the material surface from heat and air earlier during combustion. In addition, the two phosphorylation materials no longer produced flammable alkanes, indicating that they inhibited the diffusion of combustion gases to the pyrolysis zone. P-lyocell and P-MTT/lyocell showed gas-phase flame retardancy mechanisms (Fig. 8).

Fig. 8
figure 8

Weight and derivative weight loss curves of the control lyocell, MTT/lyocell, P-lyocell and P-MTT/lyocell fibres with increasing temperature

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TG test results showed that the thermal degradation temperature of MTT/lyocell was consistent with those of the control lyocell fibres. The thermal degradation temperature of the two fibres decreased after phosphorylation, which was consistent with the TG-IR curves. The three kinds of fibres had higher char yields than the control fibres. The P-MTT/lyocell fibres had the highest char yield.

Raman analysis

The carbon residue of the different fibres after combustion was measured via Raman spectroscopy to analyse the flame retardancy mechanism. The Raman spectra of the fibres after combustion consists of overlapping peaks at 1360 cm−1 and 1580 cm−1. These two peaks corresponded to amorphous carbon (D band) and graphitic carbon (G band), respectively. The ratio of the intensity of the D band to that of the G band (ID/IG) is used to characterize the degree of graphitization of the carbon residue. In general, a lower ID/IG indicates a higher degree of graphitization, a denser carbon layer and greater thermal stability. Our results indicated that the fibre had a better role in isolating the exchange of oxygen and heat.

As shown in Fig. 9, the ID/IG of the control, MTT/lyocell, P-lyocell and P-MTT/lyocell groups were 4.5, 3.8, 3.2 and 2.8, respectively. MTT strengthened the amorphous carbon conversion to graphitized carbon in the phosphorylated cellulose, resulting in a better ability to prevent the exchange of oxygen and heat. In summary, the lyocell fibres exhibited high flame retardancy due to the synergistic effect of the condensed phase and vapour phase flame retardancy.

Fig. 9
figure 9

Raman spectra of the control lyocell (a), MTT/lyocell (b), P-lyocell (c) and P-MTT/lyocell (d) fibres

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Mechanism

The flame retardancy mechanism is shown in Fig. 10. Montmorillonite produced carbonaceous substances after heating, and the phosphorylated cellulose promoted this action, which isolated the exchange of oxygen and heat. The catalytic effect of the phosphorylated cellulose produced from phosphorus-containing acids could facilitate cellulose dehydration to generate additional carbonaceous substances. On the other hand, the phosphorylated cellulose decomposed to form numerous PO• free radicals, which could capture the gas phase free radicals produced by the decomposition of cellulose molecules. Moreover, non-combustible gases (such as CO2 and H2O) produced by fibres diluted the O2 concentration on the fibre surface and reduced the rate of heat release during combustion, and the fibres no longer produced flammable alkanes. In summary, these fibre materials exhibited flame retardancy in both the condensed-phase and gas-phase.

Fig. 10
figure 10

Proposed flame retardancy mechanism for the P-MTT/lyocell fibres during burning

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Conclusion

In this study, new flame-retardant fibre materials were designed with excellent and durable flame retardancy and high mechanical strength.

Flame-retardant fibres with phosphorylated cellulose have no commercial value because excessive phosphorylation damages the crystallinity of cellulose and results in a decrease in the mechanical strength of the fibres. The MTT/lyocell fibres exhibited minor flame retardancy with permanent flame retardancy. In contrast, the combination of MTT and phosphorylated cellulose produced flame retardant fibre materials with LOIs higher than 28% and tensile strengths higher than 2.0 cN/dtex.

FT-IR and XPS analyses showed that the phosphorus compound was grafted onto the lyocell fibres by P-O-C covalent bonding, and the MTT was blended into lyocell by fine spreading. TG-IR and TG results showed that phosphorylated cellulose played a role in condensation-phase and gas-phase flame retardancy mechanisms, and MTT played a role in condensation-phase flame retardancy mechanisms. Raman spectroscopy revealed that the synergistic effect of the two flame retardant methods improved the residual char of the fibres. In conclusion, phosphorylated cellulose with inorganic flame-retardant filling is a new flame-retardant cellulose material. These fibres have durable flame retardancy and great tensile strength and have shown promise for industrial-scale preparation.