High-efficiency and durable glycerol-based reactive flame retardants for cotton fabric

Abstract

A novel efficient and durable glycerol-based reactive flame retardant, ammonium triphosphoric acid glycerol ester, was successfully synthesized. Ammonium triphosphoric acid glycerol ester was grafted onto cotton fabrics through dip-pad-dry-cure technology, and cotton fabrics with excellent and durable flame retardancy were prepared. The limiting oxygen index value of the cotton fabric treated with 20% ammonium triphosphoric acid glycerol ester reached 37.3%, which was maintained at 26.5% after 50 laundry cycles. The treated cotton fabrics were not ignited in the vertical burning tests, much char formed, and still kept their fiber shapes after combustion. The results of X-ray diffraction patterns and Fourier transform infrared spectroscopy showed that the treatment had little detrimental effect on the original structure of the cotton fabrics. Ammonium triphosphoric acid glycerol ester can effectively promote cellulose dehydration into char, and play a flame retardant role in both condensed phase and gas phase. Mechanical property testing shows that the mechanical properties of cotton fabrics treated with 20% ammonium triphosphoric acid glycerol ester are maintained well, and can serve as clothing fabrics and decorative supplies.

Keywords

Cotton fabric flame retardant glycerol-based durable

Cotton fabrics have characteristics of good hygroscopicity, anti-static electricity, and comfort for personal wear.^1,2 However, cotton fabric is flammable. Cotton fabric burns easily and fiercely once it is ignited, and can cause significant casualties and property damage.^3,4 Therefore, many flame retardants (FRs) have been synthesized to improve flame retardancy of cotton fabrics.

FRs typically contain halogen, phosphorus, nitrogen, boron, silicon elements and so on. Other specialty FR materials include graphite and alkaline earth metallic compounds.⁵ Halogen-containing FRs are highly effective in capturing high-energy free radicals in the thermal decomposition process. Nevertheless, halogen-containing FRs have been banned in many countries and regions due to carcinogenic and toxic compounds possibly released into the environment during combustion and directly affecting human health.⁶ Boron-containing flame retardants for cotton fiber often lack durability and are inefficient.^7–9 Alkaline earth metallic compounds can achieve satisfactory flame resistance. However, many metallic FRs must be deposited in the substrate, and a high weight gain rate affects the properties of the substrate.¹⁰ Nitrogen-containing additives are often combined with other flame-retardant elements to attain better flame resistance.¹¹ Recently, many researchers have investigated nanoparticles (e.g. clay,¹² carbon nanotubes,¹³ and polyhedral oligomeric silsesquioxane)¹⁴ as new FRs. Nanoparticles can be deposited on the surface of substrates as a physical barrier to reduce flames spreading. Although the heat release rate of the treated sample was decreased, other important fire parameters (e.g. time to ignition and total heat release) were not effectively improved.^15,16 Phosphorous-containing FRs have no dust and relatively eco-friendly advantageous characteristics and have been the most studied type of FRs.^17,18 Proban (Rhodia) and Pyrovatex CP (BASF) are two typical organophosphorus FRs used in cotton fabrics. However, cotton fabric treated with these two FRs will release formaldehyde. In recent years, researchers have focused more on green FRs, so a wide range of renewable biomass materials have been investigated as FRs. Some phosphorus-containing bio-based substances such as deoxyribonucleic acid, casein, hydrophobin, and phytic acid have potential applications as environmentally friendly FRs.^19–22 However, the extractions of DNA, casein, and hydrophobin were inefficient. Previously, the authors have used phytic acid to synthesize a biomass-based reactive ammonium phytate (APA) FR for cotton fabric, where cotton fabric treated with APA showed poor flame retardancy after 40 laundry cycles (LCs).²³

Glycerol is a natural compound with low molecular weight, which is produced industrially in mass quantities. Some flame retardants based on glycerol have been synthesized to improve the flame retardancy of silk and jute fabrics.^24,25 Glycerol can also react with phosphoric acid to produce a small flame-retardant molecule with triphosphoric acid glycerol ester (TPGE), which is chemically similar to phytic acid. In this research, a new type of FR, ammonium triphosphoric acid glycerol ester (ATPAGE) based on glycerol, was synthesized for cotton fabric. The ATPAGE containing three reactive −P=O(NH₄⁺)₂ groups can react with cellulose through –P(=O)-O-C- covalent bonds using dicyandiamide as catalyst. Cotton fabric treated with ATPAGE showed excellent flame retardancy and persistence. In addition, the synthesis process of ATPAGE did not use formaldehyde, which is highly beneficial for using in apparel and other cotton fabric applications (Figure 1).

Figure 1.

Graphic abstract of study.

Experimental

Materials

Bleached white cotton fabrics (133.56 g/m²) were bought from Chaotianmen Market (Chongqing, China). Ethyl alcohol was supplied by Chengdu Kelong Chemical Reagent Co., Ltd (Sichuan, China). Glycerol, phosphoric acid, and urea were gained from Chongqing Chuandong Chemical Co., Ltd (Chongqing, China). Dicyandiamide was supplied by Aladdin Regent Co. Ltd (Shanghai, China).

Synthesis of ATPAGE

Glycerol (9.23 g, 0.1 mol) and phosphoric acid (24.61 g, 0.3 mol) were added in a beaker (250 ml) and heated at 120°C with magnetic stirring. After 4 h, a viscous and faint yellow liquid was obtained. Then, the solution was mixed with urea (18.02 g, 0.3 mol) and some distilled water, which reacted at 140°C until the solution reached a pH of 6. Then the crude product, ATPAGE, was subsequently obtained and purified by precipitation using ethanol, filtration, and oven-drying. A white solid ATPAGE was obtained with 71.2% yield. The synthesis route to ATPAGE is presented in Figure 2.

Figure 2.

Synthesis route of ammonium triphosphoric acid glycerol ester (ATPAGE).

Treatment by ATPAGE

ATPAGE was diluted in distilled water to obtain FR solutions with various mass concentrations. As a catalyst 5% of dicyandiamidet was added to the solutions. Then, cotton fabrics were added (liquid ratio of 1:20) into these mixed solutions for 10 min at 60°C. After two dips and two nips, the cotton fabrics were cured in an automatic thermostatic setting machine at 170°C for 5 min. Then the samples were washed with tap water and dried at 60°C. The possible reaction between ATPAGE and cotton fabric is shown in Figure 3. The −P=O(NH₄⁺)₂ groups of ATPAGE were gradually decomposed to form −P=O(OH)₂ groups, and the phosphoric anhydride was formed under the catalysis of dicyandiamide, which reacted with the reactive hydroxyl groups on the cellulose glucose ring to form a –P(=O)-O-C- covalent bonds. The weight gain (WG) of the treated cotton samples was computed using the follow formula:

WG = (W_{1} - W_{0}) / W_{0} \times 100 %

Figure 3.

The possible reaction between ammonium triphosphoric acid glycerol ester (ATPAGE) and cotton fabric.

W₁ is the final weight of the treated cotton fabric, W₀ is the weight of cotton fabric before the treatment.

Characterization

The ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and ³¹P NMR spectra of ATPAGE were tested using by a Bruker AV III 600 MHz nuclear magnetic resonance spectrometer (USA) and using deuterium oxide (D₂O) as the reactive solvent.

Vertical burning test results were conducted used the YG815B vertical fabric flame retardant tester, in accordance with the American Society of Testing Materials (ASTM) D6413˗99 standard.

The fourier transform infrared (FT-IR) spectra of ATPAGE and treated cotton fabrics were recorded by spectrum GX apparatus (PE Co., USA), using KBr tableting technique at a scanning range of 400–4000 cm^˗¹ and a resolution of 2.0 cm⁻¹.

The thermal performance of the cotton fabric was evaluated by using the Pyris 1 thermogravimetric (TG) analyzer (Perkin-Elmer, USA). The temperature was from 40–600°C with a heating rate of 20°C/min under nitrogen and air atmosphere.

The thermogravimetric- infrared (TG-IR) was assessed by a thermogravimeter (Pyris 1, Perkin Elmer, USA) connected with an infrared spectrometer (Nicolet6700). Cotton fabric samples were conducted from 40–700°C under a nitrogen atmosphere. The heating rate was 20°C/min.

Cone calorimetry was conducted to evaluate the combustion behavior of control and treated cotton fabrics (100 mm × 100 mm) according to the ASTM E1354 standard under an irradiative heat flux of 35 kW/m² during the tests.

The surface morphologies of cotton fabrics and the residue of treated cotton fabric after combustion were observed using a Phenom Prox Holland scanning electron microscope (SEM). The test voltage was 10 kV, and the magnifications were 800 and 2000, respectively. Chemical compositions of treated cotton fabrics before and after burning were evaluated by energy˗dispersive X˗ray spectroscopy (EDX) (JEOL˗6300F).

The crystalline structure was analyzed by a Rigaku XD˗3 X˗ray diffraction (XRD). Control cotton fabric and treated cotton fabrics were tested with CuK radiation, and the measured voltage and current were 36 kV and 20 mA, separately. The diffraction angle range was from 5°–50°, and the step size was 0.02° (λ = 0.154 nm).

Limiting oxygen index (LOI) test was evaluated by the M606B digital oxygen index apparatus in accordance with the ASTM D2863-2000 standard.

The durability of treated cotton fabrics was tested with reference to AATCC 61-2013 1A standard method. The washing temperature was 45°C, the wash time was 45 min for five LCs.

The tensile strengths of cotton fabrics (25 mm ×180 mm) were determined in accordance with the ASTM 5035˗2006 standard by using an electronic fabric tension tester (HD026N, Nantong Hongda Experiment Instruments Co. Ltd, China). The crease recovery angles were evaluated on a TSE-A002 tester in accordance with the AATCC 66-2014 standard.

Results and discussion

Characterization of ATPAGE

The chemical structure of ATPAGE was characterized by FT-IR and NMR, and the TG of ATPAGE was also studied. The FT-IR spectra, TG and derivative weight loss (DTG) curves are shown in Figure 4. From Figure 4(a), peaks are shown at 3139, 1403, 1288, 1075 and 969 cm⁻¹. Peaks at 3139 and 1403 cm⁻¹ were assigned to the stretching and scissoring vibration absorption of NH₄⁺ groups, the peak at 1288 cm⁻¹ corresponded to the vibration absorption of P=O, the peak at 1075 cm⁻¹ corresponded to the absorption of P-O-C, and the peak at 969 cm⁻¹ belonged to the absorption of P-O.

Figure 4.

Fourier transform infrared spectroscopy (FT-IR) spectra of ammonium triphosphoric acid glycerol ester (ATPAGE) (a); thermogravimetric (TG) and derivative weight loss (DTG) curves of ATPAGE under N₂ atmospheres.

The results of ¹³C NMR, ¹HNMR, and ³¹P NMR were summarized as follows: ¹³C NMR (D₂O, 150 MHz) δ (ppm): 77.87 (CH) and 85.88 (3CH₂); ¹HNMR (D₂O, 600 MHz) δ (ppm): 3.38 (l s, CH), 3.66 (s, 2CH₂) and 4.80 (s, deuterium oxide); ³¹P NMR (D₂O, 243 MHz) δ (ppm): –10.66 (s, 3P). The results of FT-IR and NMR were consistent with the structure of ATPAGE.

Figure 4(b) shows the TG and DTG curves of ATPAGE under a nitrogen atmosphere. In the range of 100–300°C, ATPAGE decomposed to release NH₃ and phosphonic acid. When the temperature reached above 500°C, the further dehydration and cross-linking of phosphonic acid occurred.

Flame resistance and durability

LOI and vertical burning tests are widely used quantitative methods to evaluate the flammability of materials. Both LOI and vertical burning tests correlate well with the flame resistance of textiles.²⁶ The LOI values of cotton fabrics treated with different concentrations of ATPAGE and after different LCs are shown in Tables 1 and 2. From Table 1, the LOI value of the control cotton fabric was 17.7%, which was much lower than the FR standard 26%.²⁷ During the vertical burning tests (Table 2), the control cotton fabric burned vigorously and completely, with an after-flame time of 6 s and an after-glow time of 12 s. After the treatments with different concentrations ATPAGE, the treated cotton fabrics self-extinguished as soon as the flame source was removed. The LOI value of 5% ATPAGE-treated cotton fabric was 25.4% (<26%), and the treated cotton fabric was hard to be ignited in the air with an after-flame time of 8 s and an after-glow time of 0 s, suggesting that 5% ATPAGE improved the flame retardancy of cotton fabrics. The LOI values of 10% ATPAGE-treated cotton fabric reached 30.3%, which retained at 26.2% after 10 LCs. The LOI value of 15% ATPAGE-treated cotton fabrics increased to 34.5%, which maintained at 26.3% after 30 LCs. From the results, the 15% ATPAGE-treated cotton fabric could be considered to be a semi-durable flame-retardant textile. When the concentration of ATPAGE increased to 20%, 25%, and 30%, the LOI value of treated cotton fabric increased to 37.3%, 40.4%, and 41.9%, which was still retained at 26.5%, 28.5%, and 29.1% after 50 LCs, respectively. It is suggested that ATPAGE was firmly grafted on the cotton fabric via covalent bonds and could endow cotton fabric with splendid flame resistance performance and durability, 20%, 25%, and 30% ATPAGE treated cotton fabric could be used for durable functional materials. From these results, the durability of cotton fabrics treated with ATPAGE was better than those treated with APA. Compared with APA, ATPAGE has a molecular weight of only half of the APA and more easily penetrated into the interior of the cotton fabric.²³

Table 1.

Limiting oxygen index (LOI) values of treated cotton fabrics after different laundering cycles

Concentration of ATPAGE	LOI (%)
Concentration of ATPAGE	0 LCs	10 LCs	30 LCs	50 LCs
Control cotton	17.7	–	–	–
5% ATPAGE treated cotton	25.4	21.3	–	–
10% ATPAGE treated cotton	30.3	26.2	24.1	22.3
15% ATPAGE treated cotton	34.1	29.2	26.3	24.2
20% ATPAGE treated cotton	37.3	32.1	28.6	26.5
25% ATPAGE treated cotton	40.4	35.3	30.2	28.5
30% ATPAGE treated cotton	41.9	36.6	31.4	29.1
20% APA treated cotton	43.2	38.8	30.5	24.7

ATPAGE: ammonium triphosphoric acid glycerol ester; LC: laundry cycle.

Table 2.

The results of vertical burning tests for treated cotton fabrics

Samples	WG (%)	After-flame time (s)	After-glow time (s)	Char length (mm)
Control cotton	–	6 ± 2	12 ± 2	0
5% ATPAGE treated cotton	5.3	8 ± 1	0	350 ± 5
10% ATPAGE treated cotton	9.3	2 ± 1	0	176 ± 6
15% ATPAGE treated cotton	14.8	0	0	64 ± 2
20% ATPAGE treated cotton	18.1	0	0	54 ± 3
25% ATPAGE treated cotton	23.2	0	0	52 ± 1
30% ATPAGE treated cotton	28.5	0	0	48 ± 1
10% ATPAGE treated cotton after 50LCs	7.4	10 ± 1	0	350 ± 2
15% ATPAGE treated cotton after 50LCs	12.1	3 ± 1	0	164 ± 4
20% ATPAGE treated cotton after 50LCs	15.7	0	0	58 ± 3
25% ATPAGE treated cotton after 50LCs	20.8	0	0	54 ± 2
30% ATPAGE treated cotton after 50LCs	26.1	0	0	50 ± 1

ATPAGE: ammonium triphosphoric acid glycerol ester; WG; weight gain.

FT-IR and XRD analysis of cotton fabrics

The FT-IR spectra of the control and 20% ATPAGE treated cotton fabric are shown in Figure 5(a). For both the control and treated cotton fabrics, the absorption peak at 3332 cm⁻¹ was assigned to the stretching vibration of -OH groups, the peak at 2921 cm⁻¹ corresponded to the stretching vibration absorption of C-H, peaks at 1154, 1108, and 1027 cm⁻¹ belonged to the absorption of C-O-C.²⁸ Comparing with the absorption of control cotton fabric, the treated cotton fabric showed some new absorption peaks. The absorption peak at 1234 cm⁻¹ was attributed to P=O stretching vibration, and the absorption peak at 816 cm⁻¹ was assigned to the P-O-H peak.²⁹ The absorption peak at 1703 cm⁻¹ was assigned to the C=O vibration, the C=O group may generate from oxidation of the glucosidic bond of cellulose molecule under high temperature acid condition.³⁰ The absorption peak at 1050 cm⁻¹ was enhanced, which was caused by –P(=O)-O-C- covalent bonds between cellulose and ATPAGE, suggesting that ATPAGE was successfully grafted into the cellulose.³¹

Figure 5.

Fourier transform infrared spectroscopy (FT-IR) spectra (a) and X-ray diffraction patterns (b) of the control and treated cotton fabrics. ATPAGE: ammonium triphosphoric acid glycerol ester.

The XRD analyses of 20% ATPAGE-treated fabric and control cotton fabric are shown in Figure 5(b). The peak shapes and positions of these two samples were similar. The diffraction peaks at 15.09°, 16.49°, and 22.94° were assigned to the crystal planes (1–10), (110), and (200) of cellulose I, respectively.³² The diffraction peaks of ATPAGE treated cotton fabric were a little weaker than those of the control cotton fabric. The main reason presumably was that ATPAGE permeated into the cotton fibers during the treatment to react with cellulose, leading to a decrease in the crystalline regions. From the results, the treatments by ATPAGE had little effect on the crystal structure of cotton fabrics.

TG analysis of cotton fabrics

The decomposition and charring capacity of control cotton fabric and ATPAGE treated cotton fabrics were analyzed by TG. The TG analysis results of the control fabric and 20% ATPAGE treated cotton fabric under N₂ and air atmospheres are presented in Figure 6. The related data, such as the 10% mass loss temperature (T_10%), the maximum mass loss rate (V_max), the temperature at V_max (T_max), and the residual weight of cotton fabrics at 600°C (R_w600), are also listed in Figure 6.

Figure 6.

Thermogravimetric (TG) curves of the cotton fabrics under N₂ (a) and air (c) atmospheres, derivative weight loss (DTG) curves of the cotton fabrics under N₂ (b) and air (d) atmospheres. ATPAGE: ammonium triphosphoric acid glycerol ester.

As shown in Figure 6(a) and (b), the heat-degradation of the control cotton fabric occurred at 335–401°C.³³ Before the initial degradation temperature, the weight loss of the control cotton fabric was primarily due to the releasing of water vapor.³⁴ In the beginning of the thermal decomposition process, cellulose decomposed to aliphatic char and then further depolymerized into volatile gases. The T_max was 361.87°C, and the V_max was 2.51%/°C. When the temperature was above 401°C, the cotton fabric continuously decomposed to release volatile gases, and only 5.01% of residue retained at 600°C. By contrast, the thermal degradation processes of treated cotton fabrics were different from that of control cotton fabric. For the ATPAGE treated cotton, the first degradation occurred in the range of 276–313°C, where the T_max and V_max were 305.46°C and 2.26%/°C. Compared with the control sample, the T_10%, T_max, and V_max of treated cotton fabrics were markedly reduced, and the ATPAGE treated sample formed more residue at 600°C. The results suggested that ATPAGE changed the thermal decomposition pathway of the cotton fabric, and was highly effective in promoting char formation of cellulose. Because of the low cracking temperature, ATPAGE could generate phosphoric acid and polyphosphoric acid at low temperature before the pyrolysis of cotton fabric to promote the char formation of cellulose, causing the decrease of T_10% and T_max of treated cotton fabrics.

The TG analysis results of the control and 20% ATPAGE treated cotton fabric under air atmospheres are displayed in Figure 6(c) and (d). The control and modified cotton fabrics explicitly displayed two different phase of degrade. The first step of thermal degradation occurred at 310–362°C for control cotton. In this stage, the initial degradation of cotton fabric started at 310.51°C, and the T_max was 352.98°C, and the V_max was 3.27%/°C. The second degradation stage appeared in the range of 362–600°C, which was the further oxidation of the residue in the presence of oxygen. When the temperature came up to 600°C, there was almost no residue left. For the ATPAGE treated cotton, the T_10%, V_max, and T_max of treated cotton fabrics were lower than those of control cotton fabric. From the results, ATPAGE reduced the further oxidation of cellulose, there was 27.86% residue remaining at 600°C, suggesting that ATPAGE could promote cellulose to form a char layer to protect the internal cotton fiber.

TG-IR analysis of cotton fabrics

TG-IR analysis was used to investigate the composition of the gases produced by thermal cracking of the 20% ATPAGE treated cotton fabric and control cotton fabric, and the FT-IR spectra of the gaseous volatiles during the pyrolysis process are shown in Figure 7. From Figure 7, the maximum amount of gaseous volatiles of treated cotton fabrics were released at 348°C, which was earlier than that of control cotton fabric (396°C). Compared with control cotton fabric, there were no new peaks appearing in the FT-IR spectra of treated cotton fabric. The absorption peaks at 3592, 2967, 2355, 1757, and 1060 cm⁻¹ were attributed to stretching vibrations of O-H from water vapor,³⁵ stretching vibrations of C-H from diverse alkanes,³⁶ CO₂,³⁷ stretching vibrations of C=O in carbohydrates,³⁸ and stretching vibrations of C-O-C from ethers,³⁹ respectively. Among them, water vapor and CO₂ were incombustible gases, but the diverse alkanes, carbohydrates, and ethers were flammable. For treated cotton fabric, the absorption peak intensities of the stretching vibrations of C–H and C=O were much weaker than those of control cotton fabric, suggesting that ATPAGE could reduce the flammable gases released by cotton fabrics. Combined with the results of TG analysis, APTAGE altered the thermal cracking process of cotton fabrics, promoting the formation of cellulose to char during pyrolysis rather than releasing flammable gases.

Figure 7.

Fourier transform infrared spectroscopy (FT-IR) spectra of gas products from control and ammonium triphosphoric acid glycerol ester (ATPAGE) treated cotton samples during the pyrolysis process.

Cone calorimetry

Cone calorimetry tests were used to analyze the combustion performance of the control and 20% ATPAGE treated cotton fabrics. The heat release rate (HRR) and the total heat release (THR) curves are shown in Figure 8, and the corresponding data is presented in Table 3.

Figure 8.

Heat release rate (HRR) (a) and total heat release (THR) (b) curves of cotton fabrics. ATPAGE: ammonium triphosphoric acid glycerol ester.

Table 3.

Cone calorimetry tests data of cotton fabrics

Samples	TTI (s)	T_p (s)	PHRR (kW/m²)	FGR (kW/m²/s)	THR (MJ/m²)	EHC (MJ/kg)	CO₂/CO	Residue (wt %)
Control cotton	6.1	22	195.1	8.9	2.8	19.0	86.18	0.02
Treated cotton	–	40	11.5	0.3	1.5	14.4	2.76	29.7

Samples

TTI (s)

T_p (s)

PHRR

(kW/m²)

FGR (kW/m²/s)

THR (MJ/m²)

EHC (MJ/kg)

CO₂/CO

Residue (wt %)

Control cotton

6.1

195.1

8.9

2.8

19.0

86.18

0.02

Treated cotton

–

11.5

0.3

1.5

14.4

2.76

29.7

EHC: effective heat of combustion; FGR: flame growth rate; PHRR: peak heat release rate; THR: total heat release; TTI: time to ignition.

As shown in Table 3 and Figure 8, the control sample showed a steep HRR initially, where the time to ignition (TTI) was 6.1 s, the peak heat release rate (PHRR) was 195.1 kW/m², the time to reach the peak heat release rate (t_p) was 22 s, and THR was 2.8 MJ/m², suggesting that the control cotton fabric was highly flammable. Once combustion began, the cotton fabric burned rapidly and the flame spread quickly to release heat and flammable gases. For the treated cotton fabric, there was no TTI, which indicated that the sample failed to ignite during the cone calorimetry tests. Besides, the HRR curve was stable and without a peak on the curve, the PHRR was 11.5 kW/m², t_p was 40 s (much lower than that of control cotton fabric), and the THR of treated cotton fabric was only half of the control cotton fabric. These results indicate that the ATPAGE treated cotton fabrics showed excellent flame resistance.

Flame growth rate (FGR) is a parameter to assess potential for fire hazards.⁴⁰ The FGR of treated cotton fabric was 0.3 kW/m²/s, which was far lower than that of control cotton fabric (8.9 kW/m²/s), suggesting that the ATPAGE treated cotton fabric showed distinguished flame resistance. The effective heat of combustion (EHC) was the ratio of the rate of heat release to the rate of mass loss at the same time, which reflected the degree of combustion of the volatile components produced by the material during combustion. The EHC of ATPAGE treated cotton fabric was much lower than that of control cotton fabric. This result suggested that the combustible gas volatiles from cotton fabrics were significantly decreased after the treatment by ATPAGE, which was consistent with the results of TG-IR. The CO₂/CO value is used to evaluate whether material burns sufficiently during combustion.⁴¹ The CO₂/CO of treated cotton fabric decreased significantly compared to the control cotton fabric, which showed that the combustion of ATPAGE treated cotton fabric was hindered. After the tests, there was almost no residue left for control cotton fabric, while the residue of treated cotton fabric was 29.7%.

Surface morphologies

The surface morphologies of the control fabric, 20% ATPAGE treated fabric and 20% ATPAGE treated fabric after burning are shown in Figure 9. The control cotton fibers (Figure 9(a) and (b)) displayed natural convolution and flat structure, where the fringe of the fiber was slightly curled inward. The morphologies of 20% ATPAGE treated cotton fibers (Figure 9(c) and (d)) were nearly identical to the control cotton fibers. There was no damage was caused on fibers, ATPAGE permeated the interior space of the cotton fabrics to induce a little swelling of the flat structure during the treatment. After burning, the char of treated cotton fabric showed an intact char skeleton with a complete surface (Figure 9(e) and (f)), because ATPAGE could effectively promote the char formation of cellulose during burning. In addition, some bubbles were observed on the surface of the char. These results suggested that ATPAGE mainly exhibited condensation flame-retardant mechanism, accompanied by the releasing of nonflammable gases.

Figure 9.

Scanning electron microscope (SEM) images of cotton fabrics: control cotton fabrics (a) and (b), 20% ammonium triphosphoric acid glycerol ester (ATPAGE) treated cotton fabrics (c) and (d), 20% ATPAGE treated cotton fabrics after burning (e) and (f).

The elemental compositions of the control cotton fabric, 20% ATPAGE treated cotton fabric and 20% ATPAGE treated cotton fabric after burning are shown in Table 4. The control cotton fabric only detected C and O elements, and the mass concentrations of C and O were 50.31% and 49.69%, respectively. And 3.93% P and 5.18% N elements were detected in 20% ATPAGE treated cotton fabric, suggesting that ATPAGE was introduced into the cotton fabrics. After burning, 4.69% P and 8.72% N elements retained in the residue, because ATPAGE played a role in the condensed phase to promote the formation of char.

Table 4.

Energy-dispersive X-ray spectroscopy (EDX) data of cotton fabrics

Sample	C (%)	O (%)	N (%)	P (%)
Control cotton fabric	50.31	49.69	–	–
20% ATPAGE treated cotton fabric	44.54	46.35	5.18	3.93
20% ATPAGE treated cotton fabric after burning	62.24	24.35	8.72	4.69

ATPAGE: ammonium triphosphoric acid glycerol ester.

Based on the above results, the mechanism of ATPAGE for cotton fabrics was that ATPAGE released phosphoric acid and polyphosphoric acid under high temperature conditions, which promoted dehydration of cellulose to form char layer. The char layer prevented spread of oxygen and heat into the inner portion of the fabric. The poor thermal conductivity of the char layer hindered the external heat from entering the inner material, retarding the thermal decomposition of inner materials and leaving cotton fabric to a large amount of residue after the combustion. In addition, ATPAGE reduced the release of combustible gases by changing the thermal cracking process of cotton fabrics. In short, the flame resistance mechanism of ATPAGE was a typical condensation phase flame resistance mechanism.

Mechanical property and whiteness

The whiteness, breaking strength, elongation at break, and crease recovery angle of control cotton fabric and 20% ATPAGE treated cotton fabric were evaluated, and the results are shown in Table 5.

Table 5.

The whiteness, breaking strength, elongation at break, and crease recovery angle of cotton fabrics

Sample	Whiteness (%)	Breaking strength (N)		Elongation at break (%)		Crease recovery angle (°)
Sample	Whiteness (%)	Warp	Weft	Warp	Weft	Warp	Weft
Control cotton	93.98	997	633	6.47	15.45	94.5	105.2
treated cotton	78.64	732	468	6.23	13.27	95.5	106.4

The whiteness of the control cotton fabric was 93.98%, compared to 78.64% of the treated cotton. This signified a 15.34% decrease in whiteness of the 20% ATPAGE treated cotton fabric dropped. During the treatment, ATPAGE gradually decomposed the −P-O⁻H⁺ groups at high temperatures. The presence of catalyst enabled the grafting reaction to proceed rapidly, but some unreacted −P-O⁻H⁺ groups degraded a portion of the cotton fabric, which made the cotton fabric yellow and reduced the whiteness. For the control cotton, the warp break strength was 997 N and weft break strength was 633 N. After treating with 20% ATPAGE, the warp and weft break strengths of the cotton fabric were 732 N and 468 N, signifying the 25.07% and 23.52% decrease, respectively. This was likely on account of the degradation of the glucosidic bond of cellulose under high temperature acidic conditions.⁴² The warp elongation at break of control cotton fabric and ATPAGE treated cotton fabric was almost the same, although the weft elongation at break of the treated cotton fabric slightly declined, suggesting that ATPAGE reacted with multiple cellulose chains. The strong cross-linking reaction between the chain of cellulose caused by ATPAGE reduced the cotton fabric deformation in response to external forces. In addition, the changes of crease recovery angles for treated cotton fabrics were insignificant, which demonstrated that ATPAGE had little impact on the original properties of the cotton fabric.

Conclusions

A sustainable, natural, and durable phosphorous-containing FR, ATPAGE, was synthesized based on glycerol for cotton fabric. ATPAGE contains three reactive −P=O(NH₄⁺)₂ groups, which can react with cellulose to form −P(=O)-O-C- covalent bonds using dicyandiamide as catalyst, and ATPAGE can be firmly grafted onto cotton fabric. After the treatment with ATPAGE, the flame retardancy of cotton fabric improved observably. ATPAGE treated cotton fabric had lower HRR, THR, and PHRR values than those of the control cotton fabric. The LOI value of cotton fabric treated with 20% ATPAGE reached 37.3%, which maintained at 26.5% after 50 LCs, implied that ATPAGE could be regarded as a durable flame retardant for cotton fabric. During combustion, ATPAGE showed a typical condensation phase flame resistance mechanism. In addition, the structure of the treated cotton fabric was not obviously changed, while the whiteness and mechanical properties slightly decreased. In short, this formaldehyde-free, natural, durable, and bio-based FR has good potential for use in cellulosic materials.

Footnotes

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Research Foundation for Youth Scholars of Beijing Technology and Business University (Grant No. QNJJ2021-20), the Beijing Science and Technology Plan Project (Grant No. Z211100004321003 and Z211100004321004) and the Natural Science Foundation of Chongqing (Grant No. cstc2019jcyj-msxmX0412).

ORCID iDs

Guangxian Zhang

Fang Xu

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