Flammability and thermal properties of cotton fabrics modified with a novel flame retardant containing triazine and phosphorus components

Abstract

A novel formaldehyde-free flame retardant containing phosphorus and dichlorotriazine components (CTAP) for cotton fabrics was synthesized. As an active group, the dichlorotriazine could react with cotton fabric via covalent reaction. The addition of 20.7 wt% CTAP into the cotton fabric obtained a high limiting oxygen index value of 31.5%, which was 13.5% higher than the pure cotton fabric. The results of heat release rate, total heat release and effective heat combustion indicated that CTAP effectively imparted flame retardancy to cotton fabric by the cone calorimetry test. With respect to the untreated cotton fabrics, the treated cotton fabrics degraded at lower decomposition temperature and form a consistent and compact char layer, which could be observed by thermogravimetric analysis, Fourier transform infrared spectroscopy and scanning electron microscopy. Compared to the untreated cotton fabrics, CTAP performed an effective role in flame retardancy for treated cotton fabrics. Meanwhile, it stimulated the formation of char and promoted the thermal stability of treated cotton fabrics during combustion.

Keywords

cotton fabrics flame retardant thermal degradation properties combustion behavior

Cotton fabrics have been widely used in garments, curtains, towels, decorations and other areas ascribed to their outstanding properties.¹ Although cotton fabrics have advantages of eco-friendliness, biodegradability, softness, hygroscopicity and breathability, their inherent flammability limits their applications.^2,3 What is more, the smoke and heat emitted during burning can injure or even kill people and animals and may cause personal properties damage, as well as create a threat to the environment. Hence, flame retardants appear to meet the requirement, which could substantially enhance the flame retardancy of the materials. For instance, in the previous literature,⁴ many halogen flame retardants, such as chlorine-containing and bromine-containing retardants and their derivatives, have been extensively applied on cotton fabrics. They can efficiently suppress oxygen and heat transfer, as a consequence of retarding the fire propagation. However, they are forbidden due to toxic smoke and corrosive gas generated in a real fire scenario. In a continuous seeking way, some organic and inorganic composites,^5–7 metal oxides⁸ and metal hydroxides^9–11 have been proved to have good flame retardancy when applied on cotton fabrics with a favorable compatibility. In recent years, halogen-free, non-toxic and environment friendly flame retardants^9,12,13 have exhibited a worthy substitution of the conventional flame retardant additives, which are attributed to their superior flame retardancy and eco-friendly properties. Flame retardants containing P, N and Si elements attract plenty of attention, due to for their excellent flame resistance and slight influence on the natural properties of materials.^11,12,14

Phosphorus-containing flame retardants, such as phosphate, phosphite, phosphorous oxynitride, red phosphorous and phosphorous derivatives, have shown promising fire inhibition.^3,14–16 Confirmed by some previous literatures, the mechanism is defined as both condensed phase and gas phase.^17,18 The phosphorous-containing flame retardants always degrade at a relative lower temperature to form phosphoric acid and polyphosphoric acid, which act as a dehydration agent and promote char formation. On the other hand, it releases a PO· radical, which by capturing the hydrogenous radical results in self-extinguishing behavior.¹⁴ In addition, triazine-containing compounds have been proved to have a significant influence on thermal stability, char forming and flame retardancy of flammable materials.^19–21

In this study, a novel formaldehyde-free flame retardant containing phosphorus and dichlorotriazine components (CTAP) was synthesized and applied on cotton fabrics. CTAP can react with cotton fabric via a covalent reaction, which exhibits excellent flame retardancy with non-formaldehyde and good washing durability. As commonly seen, the limiting oxygen index (LOI) test and vertical burning test are employed to characterize the simple flame retardant properties. The details of the combustion behavior and thermal degradation of the cotton samples during combustion have been evaluated through the cone calorimetry test and thermogravimetric analysis (TGA), respectively. In addition, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) have also been used to investigate the structure and morphology of char residues, respectively.

Experimental details

Materials

Fabric of 100% scoured and bleached plain-woven cotton (14.75 tex × 14.75 tex, 122 g/m²) was purchased from Weifang Qirong Textiles Co., Ltd. Ethanolamine (C₂H₇NO) was supplied by Tianjin Guangcheng Chemical Reagent Co., Ltd. Phosphorous acid (H₃PO₃) was provided by Tianjin Hongyan Chemical Reagent Works. Cyanuric chloride (C₃H₂Cl₃N₃) was obtained from Chengdu Aikeda Chemical Reagent Co., Ltd. Acetone (CH₃COCH₃) and concentrated sulfuric acid (H₂SO₄) were purchased from Laiyang Jingxi Chemical Works. Sodium carbonate anhydrous (Na₂CO₃) was supplied by Tianjin Bodi Co., Ltd.

Synthesis of 2-aminoethyl phosphite

To a 250 mL three-neck round bottom flask equipped with a reflux condenser, a dropping funnel and a thermometer, phosphorous acid (41 g, 0.5 mol) was added firstly. A mixture solution of ethanolamine (30.5 g, 0.5 mol) and a catalytic amount of concentrated sulfuric acid was added into the dropping funnel and mixed into the flask dropwise with a magnetic stirrer. The resulting solution was gradually heated to 160℃ under nitrogen protection for 6 h. Thereafter, a resulting yellowish viscous liquid was obtained, which was the intermediate of CTAP, named DAP (Scheme 1).
Scheme 1.
The synthesis of the intermediate and CTAP.

Synthesis of 2-(2-aminoethyl hydrogen phosphite)-4,6-dichloro-1, 3,5-triazine

To a 500 mL three-neck round bottom flask equipped with a thermometer and a mechanical stirrer, a mixture of cyanuric chloride (18.45 g, 0.1 mol) dissolved in appropriate acetone was added. When the flask was cooled to 0–5℃ in an ice bath, 10% Na₂CO₃ aqueous solution (13.75 g, 0.11 mol) was dropped in slowly, and the mixture was stirred for 7 h. During this reaction period, the pH value of system was controlled at 7–8 using 10% Na₂CO₃ aqueous solution. Thus, the resulting mixture was filtered and purified under the reduced process to evaporate the volatile substances, including water, acetone, etc. The obtained product was dried in a vacuum oven at 60℃ for 2 h to give CTAP (yield) as white powder (Scheme 1).

FTIR (KBr) (cm^–1): 3440 (versus N–H), 3350 (versus –OH), 2800–2900 (versus –CH₂), 2460 (versus P-H), 1722, 1614, 1404, 725 (versus C=N, C–N and C–Cl), 1150 (versus P=O), 1067 (versus P–O–C).

Sample preparation

The cotton fabric samples were immersed into a finishing bath containing distilled water (the bath ratio was 1:30), CTAP (300 g l^–1) and Na₂CO₃ (15 g l^–1). Then the beaker containing samples treated with CTAP was kept at 20℃ in a thermostatic bath for 100 min. After finishing, the samples were picked up and then dried. The treated cotton fabrics were washed with water before they were dried to a constant mass finally (Scheme 2).
Scheme 2.
Substitution reaction between CTAP and cotton fiber.

The add-on percentage mass of CTAP was calculated from the following equation
$add - on % = [(W_{a} - W_{b}) / W_{b}] \times 100 %$
where W_a and W_b represent the mass of the samples after and before CTAP treatment, respectively.

Measurements

The LOI test was examined on an Oxygen Index Apparatus (Fire Testing Technology Ltd), according to standard ASTM D2863-08.

According to standard ASTM D6413, a CZF-3 type instrument (Nanjing Jiangning Analytical Instrument Factory, China) was employed to investigated the vertical burning test.

TGA was performed on a HTG-1 thermal analyzer (Beijing Hengjiu Instrument Factory, China), with a heating rate of 10℃ min^–1 from 25℃ to 700℃ in a nitrogen atmosphere, with a gas flow rate of 50 ml min^–1.

The cone calorimeter test was carried out on a FFT0007 type instrument (Fire Testing Technology Ltd) according to standard ISO 5660. The specimens (100 × 100 × 3 mm³) were exposed to a 30 kW m^–2 incident heat flux and examined horizontally on a specimen holder. It is considered that the parameters such as time to ignition (TTI, s), total heat release (THR, MJ m^–2), heat release rate (HRR, kW m^–2) and effective heat combustion (EHC, MJ kg^–1) were very useful to evaluate the combustion behavior, which revealed the thermal properties of burning materials in this way.

The FTIR analysis was performed using a Nicolet 5700 FTIR model (Thermo Nicolet Corporation, USA) to observe the residues after the cone calorimeter test, with the KBr pellet technique.

The surface morphology of the residues after the cone test of the treated cotton samples could be seen from the SEM, which was demonstrated by a JSM-6010LA SEM instrument (Japan Electron Optics Laboratory Co., Ltd). Before the experiment, all specimens were coated with platinum at room temperature.

Results and discussion

Flame retardancy

As is commonly known, the LOI test and vertical burning test have been widely utilized for evaluating the flammability of materials. The data of the LOI test are shown in Table 1. The LOI value is defined as the minimum concentration of oxygen in a mixture of oxygen and nitrogen, which is just sufficient to sustain a constant burning of the treated cotton fabrics.²² The LOI value is conducive to describe the flame retardancy of the treated cotton samples. It can be revealed from Table 1 that the LOI values of the treated cotton fabrics are higher than those of the untreated cotton fabric. The untreated cotton fabric has the lowest LOI value of 18.0%. The LOI value can be enhanced from 18.0% to 31.5% when 20.7 wt% CTAP is added. In addition, the LOI value increases from 22.4% to 27.2% when the addition of CTAP increases from 7.5 wt% to 12.2 wt%. Compared to the untreated cotton fabric, it is clearly seen that CTAP has a significant influence on flame inhibition.
Table 1.
Flammability of the treated/untreated cotton fabric samples

Add-on/% LOI/% Vertical burning test

After-flame time/s After-glow time/s Char length/mm

0 18.0 – – –

7.5 22.4 3.3 1.2 122

12.2 27.2 0 0 75

20.7 31.5 0 0 50

LOI: limiting oxygen index.

Furthermore, the vertical burning test also indicates a similar result, as concluded from the parameters such as the after-flame time, after-glow time and char length. The measured results, which showed a pronounced decrease in char length as the added amount of CTAP increased, are collected in Table 1. It can be noted that when the add-on is 7.5, 12.2 and 20.7 wt%, the corresponding char length is 122, 75 and 50 mm, respectively. The after-flame time and after-glow time are 0 seconds with the loading of 12.2 and 20.7 wt% CTAP. This indicates that the fire will self-extinguish after the ignition apparatus is removed. The char of the treated cotton fabric is much thicker and denser than that of the untreated cotton fabric. This phenomenon demonstrates that CTAP has a remarkable effect on suppressing the oxygen and heat transfer and promotes char formation. As far as the collected data are concerned, the greater the mass gain of the treated cotton fabrics, the better flame retardant performance can be obtained.²³

Combustion behavior

In recent years, the cone calorimetry test has been an effective method for investigating the combustion behavior and flame retardant properties of materials.^14,24,25 Many parameters can be examined, such as TTI, THR, HRR, CO yield, CO₂ yield and so on, which are extensively applied in fire safeness and engineering, the flame retardant performance of materials and other evaluating areas. In this study, the CTAP treated cotton fabric was compared with the untreated cotton fabric via the cone calorimetry test at a heat flux of 30 kW m^–2. The HRR, THR, EHC and mass curves are depicted in Figure 1. Meanwhile, all the corresponding experimental data are shown in Table 2.
Figure 1.
(a) Heat release rate. (b) Total heat release. (c) Effective heat combustion. (d) Mass curves of the untreated and treated cotton fabrics.

Table 2.
Combustion data of the untreated/treated cotton fabric from the cone calorimetry test

Sample TTI/s P-HRR/kW m^–2 A-HRR/kW m^–2 THR/MJ m^–2 A-EHCMJ kg^–1 A-COYKg kg^–1 A-CO₂YKg kg^–1

Untreated cotton 2 107.99 46.50 5.65 8.70 0.08 1.82

Treated cotton 5 19.81 10.80 1.23 2.29 0.14 1.08

TTI: time to ignition; HRR: heat release rate; THR: total heat release; EHC: effective heat combustion.

The untreated cotton fabric exhibits a vigorous burning with a high HRR of 107.99 kW m^–2 according to Figure 1(a) and Table 2. This is agreement with the LOI value, in that untreated cotton fabric has the lowest LOI value (18.0%), burning fast after ignition. In contrast, the incorporation of CTAP into the cotton fabric shows a dramatic decline of the HRR. The peak of the HRR reduces from 107.99 to 19.81 kW m^–2, which is a decrease of almost 80% of the HRR. The figures show that CTAP has a remarkable influence on the flammability of cotton fabric. When CTAP is added into cotton fabrics, it can degrade at a relative lower temperature, which can reduce the production of volatile species and form carbonaceous char. As far as the heat release is concerned, the reduction of volatile species becomes a key factor. Meanwhile, the char layer generated during combustion can act as a physical barrier, which prevents oxygen and heat diffusing into the underlying cotton substrate. With respect to A-HRR in Table 2, the treated cotton sample reduces by 35.70 kW m^–2 compared to the untreated cotton fabric. As far as the HRR curves are concerned, the lower data means better flame resistance. Moreover, the detention of the TTI also demonstrates the phenomenon as stated above.

As shown in Figure 1(b), the THR of the treated cotton sample is much lower than that of the untreated cotton sample, where THR is the summation of the released heat from ignition to self-extinguishing. The low THR value of the treated cotton sample (1.23 MJ m^–2) arises from the formation of the carbonaceous layer. The untreated cotton fabric has an intense combustion with a higher THR value, whereas by the incorporation of phosphorus- and triazine-containing flame retardant CTAP, a much lower THR value can be seen. It can be observed that CTAP efficiently suppresses the fire spread and heat release and inhibits the underlying material’s further decomposition. With regard to the EHC curve (Figure 1(c)), it is considered to reflect the burning degree of the volatile gas in the gas phase. The EHC value declines significantly, as depicted in Figure 1(c) and Table 2. It is thought that some of the cotton fabrics burn insufficiently with the incorporation of CTAP. The decrease in EHC value means that CTAP has a good performance in the gas phase, which is attributed to phosphorus and triazine groups.¹² The phosphorus components generate PO·during combustion, which can entrap free radicals (H·and·OH), leading to incomplete burning of the cotton sample. The volatile species (such as levoglucose, furan and furan derivatives) can be obviously reduced, which are the primary reasons for lower values of HRR, THR and EHC. At the same time, the nonflammable gases emitted from triazine groups of CTAP also have an influence on flame retarding due to their dilution effect on volatile species. With reference to the data of the HRR and EHC, we can find that the decreasing trend of the HRR value is much higher than that of the EHC value, which reveals that CTAP is also effective in the condensed phase. Furthermore, the flame inhibition can be confirmed by the mass profile in Figure 1(d). The mass of the treated cotton sample decreases from 7.14 to 2.07 g; meanwhile, that of the untreated sample declines from 6.25 to 0.31 g. The untreated cotton fabric has a fast and intense combustion with nearly no residues. In contrast, the treated cotton fabric has denser and thicker residues, which refer to the char layer. This indicates that the addition of CTAP into cotton fabrics can distinctly suppress the fire and heat release, while promoting the formation of the charring layer, which can prevent further burning in turn. It is worth noting that CTAP is analogous to the flame retardant mechanism of other phosphorous-containing flame retardants, both in the condensed and gaseous phases.^12–15

CO yield and CO₂ yield as a function of heating time are equally important in evaluating combustion performance. Thereby, the analyses of the CO yield and CO₂ yield curves are helpful to explain the combustion mechanism of the cotton samples.²⁶ Table 2 shows that the average CO yield of the treated cotton sample is 0.14 kg kg^–1 which is higher than 0.08 kg kg^–1 of the untreated cotton sample. Otherwise, the average CO₂ yield decreases from 1.82 kg kg^–1 of untreated cotton fabric to 1.08 kg kg^–1 in the presence of CTAP. CO₂ comes from a reaction of CO and oxygen; thus, a lower CO₂ yield can be ascribed to the incomplete combustion of the fabric and the absence of oxygen. On the other hand, it is reported that triazine components are good for fire retarding due to their rich nitrogen elements.¹⁹ In other words, the triazine components can produce inert gases during combustion, leading to the flame being extinguished. CO and smoke are produced by incomplete burning, which is attributed to the formation of the carbonaceous layer, dilution of the volatile gas and the capture of the hydrogenous radical.

It is concluded that the incorporation of the phosphorous- and triazine-containing flame retardant has endowed cotton fabrics with excellent flame retardancy. The flammability of the untreated cotton fabric can be significantly inhibited when CTAP is added, owing to the function of both the solid phase and gas phase of the phosphorus-containing components during combustion. The char layers generated can act as a physical barrier, which can insulate oxygen and heat transmission and effectively decrease the formation of volatile species and flammability of cotton fabrics.²⁷

Thermal degradation properties

The char residues after the cone calorimetry test have been comprehensively investigated by TGA, with a heating rate of 10℃ min^–1 from 25℃ to 700℃ in a nitrogen atmosphere, in order to assess the thermal properties of the untreated and treated cotton fabrics. The TG curves of the samples are depicted in Figure 2. Table 3 clearly shows the parameters of the experiment in nitrogen.
Figure 2.
Thermogravimetric curves of the untreated and treated cotton fabrics in nitrogen.

Table 3.
The data of thermogravimetric curves of cotton samples after the cone calorimetry test

Sample T_onset/^oC Residue/%
T_max/^oC

550℃ 600℃ 650℃

Untreated cotton 327.4 13.60 12.51 11.60 377.3

Treated cotton 226.8 49.38 48.52 47.73 278.6

As far as the untreated cotton fabric is concerned, the T_onset value is 327.4℃ and T_max value is 377.3℃, respectively, shown in Table 3 and Figure 2. As reported in some literatures,^7,15,22,28 the untreated cotton fabric degradation occurs by one step in nitrogen atmosphere, with little char residue. This can be attributed to its high flammability and burning mechanism. The mass of the untreated cotton sample is lost quickly and releases a large heat. As is known, cotton fabrics dehydrate and form carbonaceous char at a relative lower temperature during combustion. At a higher temperature it will produce levoglucose and further pyrolyze other flammable volatile species. During these processes, a lot of heat will be released, which is in agreement with HRR, THR and EHC curves of the cone calorimetry test. On the contrary, the treated cotton fabric has a lower T_onset value with respect to the untreated cotton fabric (226.8℃, Table 3 and Figure 2), due to the phosphorous groups of CTAP. Compared to the untreated cotton sample, the T_max value of the treated cotton sample is 278.6℃. When cotton fabric is treated with CTAP, a coating adheres to the surface of the treated cotton fibers. The thermal stability of P-O-C is weaker than that of C-C, which causes decomposition of the flame retardant before that of cotton fabric. The obtained results have revealed that the phosphorous component can accelerate dehydration and char forming of the treated cotton fabric. This is advantageous for reducing volatile species during combustion and preventing underlying materials from further burning. In Figure 2, the curve of the treated cotton sample at 500–650℃ has a slight decomposition peak caused by thermal degradation of some residues, which is in accordance with the trend of the untreated cotton sample. Furthermore, it also can be noted that the char residue of the treated sample is significantly higher than the untreated sample at 550℃, 600℃ and 650℃, respectively (Table 3).

As can be seen from the curves and parameters, we can conclude that CTAP imparts flame resistance and thermal stability to cotton fabrics. The TG curves in Figure 2 show that the treated cotton sample has an earlier decomposition temperature with respect to the untreated cotton sample, which is ascribed to the lower decomposition temperature of the phosphorous groups. The phosphorous-containing component can be degraded to phosphate and polyphosphoric acid, which can act as a dehydration agent to accelerate dehydration and promote the formation of the carbonaceous char. In addition, it is believed that the triazine groups can form nonflammable gases (such as N_2, NH_3, CO₂ and NO₂), which will dilute the volatile species, thereby suppressing the propagation of the fire.²⁸

Combined with the information from the LOI test, vertical burning test and cone calorimetry test, it can be seen that the phosphorous- and triazine-containing flame retardant effectively enhance the flame resistance of the treated cotton fabrics and stimulate the formation of the char. In the presence of CTAP, it can decrease the decomposition temperature and increase the thermal stability of cotton fabrics. As a consequence, the collected results have demonstrated that CTAP containing phosphorus and triazine groups has a visible flame inhibiting performance.

Surface morphology of the char residues

The char residues of the treated and untreated cotton fabric after the cone calorimetry test have been investigated by SEM. From Figure 3, it can be seen that the surface morphology of the untreated cotton sample is different from the treated cotton sample. As shown in Figures 3(a) and (c), the structure of the untreated cotton sample is almost destroyed during combustion. The char residue exhibits less continuous and compactness than the treated cotton sample (see Figures 3(b) and (d)). However, in the presence of CTAP, the cotton fabric exhibits a uniformly continuous and integrated char structure, which reveals that CTAP has an important role in flame inhibiting. The morphology of the fiber is reserved obviously and the damage of the fiber structure is slight. It is worthy of note that the surface of the residues depicted in Figure 3(d) has an irregular carbonaceous layer with some bubbles and holes, which is caused by the nitrogen element in triazine groups and some gaseous molecules during combustion. This porous and intumescent char layer is probably ascribed to the decomposition of the phosphorous- and triazine-based components.
Figure 3.
Scanning electron micrographs of the char residues of (a) ×100, (c) ×1000 the untreated cotton fabric and (b) ×100, (d)×1000 the treated cotton fabric.

As far as SEM analysis is concerned, the incorporation of CTAP into the cotton fabric can remarkably increase the quantity, stability and compactness of the char residues. The char layer working as a thermal barrier inhibits the generation of volatile species and energy transmission, which leads to the self-extinguishing of cotton samples. It is also demonstrated that the phosphorous-containing and triazine-based flame retardant displays an effective flame retardancy and suppression of oxygen and heat transfer.

FTIR analysis of the char residues

The structures of the char residues after the cone calorimetry test have been characterized by FTIR, as shown in Figure 4. The images directly show the comparison between the untreated cotton sample and the treated cotton sample. Although there are many similarities between the two cotton samples, some characteristic peaks can be observed in the treated cotton sample. Moreover, the FTIR spectra shows absorption at 1140, 964 and 867 cm^–1 assigned to the asymmetric stretching vibration of P=O and P–O, respectively, which exhibits the generation of phosphoric and polyphosphoric acid.^17,22,29,30 Absorption peaks at 964 and 867 cm^–1 are attributed to the formation of P–O–P, which is caused by the cleavage of P–O–C.³¹ The peak at 2370 cm^–1 belongs to the P-H stretching vibration, which can further confirm the inference as stated above.²³ Furthermore, the characteristic peak seen at 1592 cm^–1 is attributed to C=N stretching vibration, suggesting the presence of triazine groups.³² As the combustion proceeds, the triazine groups are decomposed into nitrogenous compounds, which also take part in the flame retarding of cotton fabrics.³³ As expected, the data in Figure 4 support the conclusion that the treated cotton fabric can produce phosphorous- and nitrogen-containing compounds during the combustion, which can suppress oxygen and heat transmission. The obtained results are consistent with the cone calorimetry test, TGA and SEM analysis. According to the analysis above, we conclude that the phosphorus- and triazine-containing flame retardant CTAP evidently reduces the flammability of the untreated cotton fabric.
Figure 4.
Fourier transform infrared spectra of the char residues after the cone calorimetry test.

Washing durability

The data of the flame retardancy of the treated cotton samples after different washing times are shown in Table 4. The LOI values have an obvious decrease as the washing times increases, which indicates decline of the flame retardancy of the cotton fabric sample. However, the cotton sample still has a good flame resistance, with a LOI value of 26.0% after 30 washing times. The results demonstrate that cotton samples treated with CTAP have excellent washing durability because the covalent bond between the cotton fiber and the active group of CTAP.
Table 4.
Washing durability of the treated cotton samples

Washing times 0 1 10 20 30

LOI/% 31.5 30.4 29.4 27.9 26.0

LOI: limiting oxygen index.

Conclusions

A novel formaldehyde-free flame retardant containing phosphorus and dichlorotriazinyl components has been successfully synthesized and applied on cotton fabrics. It can obviously reduce their flammability. At the same time, the combustion behavior and thermal properties have been comprehensively investigated by the cone calorimetry test and TGA, respectively. It has been confirmed that CTAP has a significant effect on the flame retardancy and thermal stability of cotton fabrics. The obtained results showed that the treated cotton fabric has a high LOI value of 31.5% with a 20.7 wt% loading, which is somewhat higher than the untreated cotton fabric with a lower LOI value of 18.0%. Moreover, the cone calorimetry test also reveals a superior flame resistant performance, according to the lower HRR and THR values. Furthermore, the TG curves have shown lower decomposition temperature and higher char residues of 46.4% at 700℃, due to the mechanism of phosphorous- and triazine-containing components and the synergistic effect of P and N elements. In the meantime, the residue structure and surface morphology have been assessed by FTIR and SEM, which exhibit a continuous, compact and integral carbonaceous char. As a consequence, CTAP has endowed the cotton fabric with flame resistance and favored the formation of char acting as a physical barrier, which effectively suppresses the transfer of oxygen, heat and energy.

Add-on/%	LOI/%	Vertical burning test
0	18.0	–	–	–
7.5	22.4	3.3	1.2	122
12.2	27.2	0	0	75
20.7	31.5	0	0	50

Sample	TTI/s	P-HRR/kW m^–2	A-HRR/kW m^–2	THR/MJ m^–2	A-EHCMJ kg^–1	A-COYKg kg^–1	A-CO₂YKg kg^–1
Untreated cotton	2	107.99	46.50	5.65	8.70	0.08	1.82
Treated cotton	5	19.81	10.80	1.23	2.29	0.14	1.08

Sample	T_onset/^oC	Residue/%	T_max/^oC
Untreated cotton	327.4	13.60	12.51	11.60	377.3
Treated cotton	226.8	49.38	48.52	47.73	278.6

Footnotes

Declaration of conflicting interests

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

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work was supported by The Project Sponsored by the 48th Scientific Research Foundation for the Returned Overseas Chinese Scholars.

References

Laufer

Carosio

Martinez

. Growth and fire resistance of colloidal silica-polyelectrolyte thin film assemblies. J Colloid Interface Sci 2011; 356: 69–77.

Horrocks

. Flame retardant challenges for textiles and fibres: new chemistry versus innovatory solutions. Polym Degrad Stab 2011; 96: 377–392.

Bosco

Carletto

Alongi

. Thermal stability and flame resistance of cotton fabrics treated with whey proteins. Carbohydr Polym 2013; 94: 372–377.

De Wit

. An overview of brominated flame retardants in the environment. Chemosphere 2002; 46: 583–624.

Mostashari

. Combustion pathway of cotton fabrics treated by ammonium sulfate as a flame-retardant studied by TG. J Therm Anal Calorim 2008; 91: 437–341.

Wang

. A novel phosphorus-containing copolyester/montmorillonite nanocomposites with improved flame retardancy. Eur Polym J 2007; 43: 2882–2890.

Song

. The potential of ferric pyrophosphate for influencing the thermal degradation of cotton fabrics. J Therm Anal Calorim 2012; 109: 27–32.

Lam

Kan

Yuen

CWM

. Flame-retardant finishing in cotton fabrics using zinc oxide co-catalyst. J Appl Polym Sci 2011; 121: 612–621.

Zhou

Chen

. Synthesis and application of a formaldehyde-free flame retardant for bamboo viscose fabric. Text Res J 2014; 84: 1515–1527.

10.

Liu

Wang

. Effects of magnesium hydroxide and its synergistic systems on the flame retardance of polyformaldehyde. J Appl Polym Sci 2012; 125: 968–974.

11.

Wang

. The influence of silicone shell on double-layered microcapsules in intumescent flame-retardant natural rubber composites. J Therm Ana Calorim 2014; 118: 349–357.

12.

Zhou

Guo

. Synthesis of a novel flame retardant phosphorus/nitrogen/siloxane and its application on cotton fabrics. Text Res J 2015; 85: 701–708.

13.

Yuan

Yin

Cai

. Effect of a novel flame retardant containing silicon and nitrogen on the thermal stability and flame retardancy of polycarbonate. J Therm Ana Calorim 2013; 111: 1531–1537.

14.

Liao

Zhou

. Synthesis of a novel phosphorus-nitrogen-silicon polymeric flame retardant and its application in poly(lactic acid). Ind Eng Chem Res 2014; 53: 10015–10023.

15.

Alongi

Carletto

Cuttica

. Intrinsic intumescent-like flame retardant properties of DNA-treated cotton fabrics. Carbohydr Polym 2013; 96: 296–304.

16.

Van der Veen

de Boer

. Phosphorus flame retardants: properties, production, environmental occurrence, toxicity and analysis. Chemosphere 2012; 88: 1119–1153.

17.

Velencoso

Ramos

Klein

. Thermal degradation and fire behaviour of novel polyurethanes based on phosphate polyols. Polym Degrad Stab 2014; 101: 40–51.

18.

Liang

Shi

. Thermal behaviour and degradation mechanism of phosphate di/triacrylate used for UV curable flame-retardant coatings. Polym Degrad Stab 2004; 84: 525–532.

19.

Chen

Qian

. Component ratio effects of hyperbranched triazine compound and ammonium polyphosphate in flame-retardant polypropylene composites. J Appl Polym Sci 2014; 131: 41006–41013.

20.

Qian

Qiu

Sun

. Pyrolysis route of a novel, flame retardant constructed by phosphaphenanthrene and triazine-trione groups and its flame-retardant effect on epoxy resin. Polym Degrad Stab 2014; 107: 98–105.

21.

Song

. The potential of ferric pyrophosphate for influencing the thermal degradation of cotton fabrics. J Therm Anal Calorim 2012; 109: 27–32.

22.

Liu

Chen

Wang

. A novel phosphorus-containing flame retardant for the formaldehyde-free treatment of cotton fabrics. Polym Degrad Stab 2012; 97: 2487–2491.

23.

Yang

Wang

Lei

. A durable flame retardant for cellulosic fabrics. Polym Degrad Stab 2012; 97: 2467–2472.

24.

Carosio

Di Blasio

Alongi

. Green DNA-based flame retardant coatings assembled through layer by layer. Polymer 2013; 54: 5148–5153.

25.

Alongi

Di Blasio

Cuttica

. Bulk or surface treatments of ethylene vinyl acetate copolymers with DNA: Investigation on the flame retardant properties. Eur Polym J 2014; 51: 112–119.

26.

Alongi

Carletto

Bosso

. Caseins and hydrophobins as novel green flame retardants for cotton fabrics. Polym Degrad Stab 2014; 99: 111–117.

27.

Alongi

Milnes

Malucelli

. Thermal degradation of DNA-treated cotton fabrics under different heating conditions. J Anal Appl Pyrol 2014; 108: 212–221.

28.

Dong

Zhu

. Combustion behaviors of cotton fabrics treated by a novel guanidyl- and phosphorus-containing polysiloxane flame retardant. J Therm Anal Calorim 2015; 119: 349–357.

29.

Zhao

Chen

. A phosphorus-containing inorganic compound as an effective flame retardant for glass-fiber-reinforced polyamide 6. J Appl Polym Sci 2011; 119: 2379–2385.

30.

Jiang

Zhang

. Synthesis, characterization, and utilization of a novel phosphorus/nitrogen-containing flame retardant. Ind Eng Chem Res 2015; 54: 2974–2982.

31.

Pan

Wang

Pan

. Formation of self-extinguishing flame retardant biobased coating on cotton fabrics via Layer-by-Layer assembly of chitin derivatives. Carbohydr Polym 2015; 115: 516–524.

32.

Xie

Gao

Zhang

. Flame retardant finishing of cotton fabric based on synergistic compounds containing boron and nitrogen. Carbohydr Polym 2013; 98: 706–710.

33.

Chen

Peng

. A novel flame retardant containing phosphorus, nitrogen, and sulfur: Synthesis and application in thermoplastic polyurethane. J Therm Anal Calorim 2014; 115: 1639–1649.

Add-on/%	LOI/%	Vertical burning test
Add-on/%	LOI/%	After-flame time/s	After-glow time/s	Char length/mm
0	18.0	–	–	–
7.5	22.4	3.3	1.2	122
12.2	27.2	0	0	75
20.7	31.5	0	0	50

Sample	T_onset/^oC	Residue/%			T_max/^oC
		550℃	600℃	650℃
Untreated cotton	327.4	13.60	12.51	11.60	377.3
Treated cotton	226.8	49.38	48.52	47.73	278.6