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Abstract

In this study, a novel flame-retardant diethyl methacryloylphosphoramidate containing phosphorus and nitrogen was synthesized and characterized by Fourier transform infrared and nuclear magnetic resonance. The synthesized compound was grafted onto cotton fabrics using electron beam irradiation and pad dry cure processes. Scanning electron microscope and X-ray photoelectron spectroscopy were used to characterize the surfaces of the modified cotton fabrics to confirm that diethyl methacryloylphosphoramidate was grafted on cotton fabrics successfully. Both electron beam–cotton and pad dry cure–cotton exhibited efficient flame retardancy which was proved by limiting oxygen index and vertical flammability test. Thermogravimetric analysis results showed that both electron beam-cotton and pad dry cure–cotton degraded at lower temperature and produced higher yields at 600℃. The tensile loss of electron beam–cotton was lower than that of pad dry cure–cotton, and within the acceptable range in flame retardant finishing.

Keywords

Cellulose phosphoramidate flame retardancy electron beam

Introduction

Cotton is widely used in clothing, home furnishings, and various industrial and military products due to its excellent breathability and water absorbancy [1 –3]. However, a major drawback of cotton is its high flammability, which may lead to fire disaster and threat the lives of human beings, and then greatly limits the application of cotton in some areas [4]. Therefore, the flame-retardant finishing of cotton fabrics becomes an essential and challengeable issue in textile industry in order to satisfy the strict safety requirements [5,6].

Application of a flame retardant is the most common method to produce flame-retardant textile fabrics. Among various kinds of flame retardants, organophosphorus compounds have been established as excellent flame retardants due to their positive effect on flame-retardant property and producing less toxic gas [7 –11]. These compounds are capable of reacting with cellulose fibers or forming cross-linking structures on fibers [12]. During combustion, phosphorus-containing compounds can phosphorylate C(6) of the glucose monomer, prevent the formation of levoglucosan and promote the dehydration process, which can accelerate the formation of char residue [13]. Moreover, the phosphorus–nitrogen systems have attracted much attention because of the synergistic effect between P and N. The presence of N additives improves the char content and P retention on the substrates during the combustion process [14]. Sabyasachi Gaan et al. investigated the effects of three nitrogen additives (urea, guanidine carbonate and melamine formaldehyde) on the flame-retardant action of cotton fabrics, and the results showed that adding nitrogen additives increased the activation energy at higher degree of degradation, thus indicating better thermal stability at higher temperatures [15]. However, the flame-retardant cotton fabrics were usually finished by traditional pad dry cure (PDC) process with the aid of cross-linking agent or an initiator. Normally, the conventional chemical grafting requires higher temperature and initiators which could seriously decrease the breaking strength of cotton fibers.

Recently, electron beam (EB) irradiation process has attracted much attention in modifying different surfaces due to the uniform treatment, energy-saving, and less environmental problems [16 –18]. It is the process of generating electrons off a cathode in a vacuum environment from commercial electricity. EB has been widely used in degrading organic pollutants because it is thought to offer higher dose rate capability and no nuclear waste [19]. Moreover, it can also be applied to many other fields such as food manufacturing, agricultural phytosanitary treatments, and semiconductor industries [20,21]. The formulation to be cured or crosslinked by EB irradiation usually contains unsaturated monomers, oligomers, and other additives according to the desired properties. In this study, a novel flame-retardant diethyl methacryloylphosphoramidate (DMPP) was synthesized and grafted to cotton fabrics via both EB and conventional chemical grafting technique. This paper made a comparative study of cotton fabrics treated with two finishing processes. The grafted cotton fabrics were characterized by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The flame retardancy of the grafted cotton fabrics was determined by the vertical flammability test and limiting oxygen index (LOI). Thermogravimetric analysis was conducted to analyze the thermal property of the grafted cotton fabrics. The breaking strength of the grafted cotton fabrics was also measured.

Experimental

Materials

The scoured and bleached cotton fabric ((40 s × 40 s)/146 × 63, 132 g/m²) was purchased from Zhejiang Guandong Textile Dyeing Garment Co., Ltd. Diethyl chlorophosphate and methacrylamide were obtained from Suzhou Haofan Biological Technology Co., Ltd and the J&K Chemical Co., Ltd, respectively. Other chemicals were from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were reagent grade and used without further purification.

Preparation of DMPP

DMPP was synthesized using diethyl chlorophosphate and methacrylamide as starting materials. The reaction equation is shown in Scheme 1. Diethyl chlorophosphate (0.1 mol) was dissolved in tetrahydrofuran (THF, 100 mL) and cooled to 0℃. Methacrylamide (0.1 mol) was dissolved in THF (50 mL) and added dropwise into the solution of diethyl chlorophosphate. Triethylamine (0.1 mol) was added as acid-binding agent. The mixture was stirred at 35℃ for 10 h. At the end of the reaction, the solution was filtered and THF was removed under reduced pressure. The yellow oil products were obtained with yields near 85%. The ¹H NMR spectra were also recorded and the chemical shift values (ppm) were found as follows: ¹H NMR (400 MHz, DMSO): δ (ppm): 5.49 (2 H, CH₂=), 4.01 − 3.74 (4 H, 2CH₃–CH₂–), 1.78 (3 H, CH₃–C), 1.26 − 1.06 (6 H, 2CH₃–CH₂–).

Scheme 1.

The synthesis scheme of DMPP.

Fabric treatment

EB irradiation and traditional PDC methods were applied in this study. In EB finishing process, DMPP was dissolved in ethanol at concentrations ranging from 10 to 35 wt%. Cotton swatches were soaked in the treatment solution for 30 min, and padded through a laboratory wringer to give a 100% wet pickup. Then the swatches were irradiated at room temperature by using an EB accelerator. Irradiation dose was set up in 32.5 KGy, the energy of the EB accelerator was 130 KW, and the average beam current was maintained at 1 mA. After irradiation, the swatches were dried at 45℃. Then the grafted cotton fabrics were immersed in 0.5% detergent solution for 15 min, washed with distilled water and dried at ambient temperature. The high-energy irradiation of EB directly induces the grafting of fabrics with flame-retardant monomer, without the presence of initiator and other reagents. In PDC finishing process, DMPP was dissolved in ethanol at concentrations ranging from 10% to 35%, containing 3 wt% potassium persulfate as catalyst and 1.5 wt% polyoxyethylene octyl phenol ether-10 (OP-10) as penetrating agent. Cotton swatches were impregnated in the treatment solution for 30 min at 70℃, and padded with wet pick-up of 100%. Then the swatches were dried at 95℃ for 5 min, and cured at 130℃ for 2 min. The grafted cotton fabrics were immersed in 0.5% detergent solution for 15 min, washed with distilled water and dried at ambient temperature.

Characterization

The structure of DMPP was verified by an AVANCE III 400 MHz Digital NMR spectrometer (Bruker AXS GmbH, Germany) using CDCl₃ as the solvent. FT-IR spectra were obtained using a NICOLET 10 FT-IR spectrometer (Nicolet Instrument Corporation, Madison, WI) with the scanning number of 500–4000 cm⁻¹. Surface morphology of cotton fibers was investigated by TM3030 SEM (Hitachi, Tokyo, Japan) with a setting of 15 KV. In order to determine the chemical state of elements on the surface of the grafted cotton fabrics, XPS were gained with an ESCALAB 20Xi (Thermo Scientific, USA). Thermogravimetric analysis (TGA) was performed using DTC-60 H series thermal analysis (Shimadzu, Japan) at a heating rate of 10℃/min with a continuous nitrogen atmosphere and a flow rate of 100 mL/min from 25 to 600℃. The vertical flammability test was carried out on YG815 instrument according to GB/T 5455-1997. LOI test was conducted using a digital display oxygen index instrument LFY-605 according to GB/T 5454-1997. Tensile strength was tested with YG (B) 026D-250 Electronic Fabric Strength Tester according to GB/T 3923-1997.

Results and discussion

FT-IR spectra characterization of DMPP

Figure 1 shows the FT-IR spectra of DMPP, diethyl chlorophosphate and methacrylamide. The characteristic absorption bands of the C=C is shown near 1634 cm⁻¹ [22]. The absorption bands of 1231 and 1027 cm⁻¹ are assigned to P=O and P–O–C [23,24]. The C=O stretching band appeared at 1676 cm⁻¹. The characteristic absorption peaks of C=C, P=O, P-O-C, and C=O appeared on the curve of A, indicate that flame-retardant DMPP was synthesized.

Figure 1.

FT-IR spectra of the DMPP (a), diethyl chlorophosphate (b), and methacrylamide (c).

Characterization of the grafted fabrics

DMPP (35 wt%) was applied to cotton fabric by PDC and EB finishing processes. SEM was used to investigate the morphologies of the ungrafted and grafted fabrics. The surface of ungrafted fabric was smooth, on which presented the natural structure of distortion and ravines, as shown in Figure 2(a). The cotton fabrics were uneven and rough after treatment, as shown in Figure 2((b) and (c)).

Figure 2.

SEM images of (a) cotton; (b) PDC–cotton; and (c) EB–cotton.

In order to confirm the surface composition and chemical states of cotton fibers, XPS was performed in ungrafted cotton, EB grafted cotton (EB-cotton) and PDC grafted cotton (PDC–cotton). The four elements of C, O, P, N were tested in each samples. The results are shown in Figure 3. For ungrafted cotton, two elements were detected which corresponded to C1s (283 eV) and O1s (531 eV), respectively. The XPS survey of EB–cotton and PDC–cotton exhibited new peak at 143 and 409 eV, which were assigned to P2p and N1s, respectively. The new peak of P2p and N1s indicated that the surface of fibers has been successfully modified by DMPP.

Figure 3.

XPS spectra of cotton, EB–cotton and PDC–cotton.

Flame-retardant performance

LOI and vertical flammability test are two important parameters to evaluate the flame retardancy [25]. At the end of the vertical flammability test, the after-flame time and after-glow time were recorded, and the char length was measured. Figure 4 shows the images taken after the vertical flammability test, and Table 1 summarizes the tests results of the cotton fabrics treated by EB and PDC finishing processes. Table 1 shows that the add-ons of the grafted cotton increased with the increase of DMPP concentration, which might be due to that there are more reaction sites on the cotton fabrics than the amount of DMPP.

Figure 4.

Vertical flammability test results of the ungrafted and grafted cotton fabrics.

Table 1.

Vertical flammability and limiting oxygen index (LOI) tests of the grafted cotton with different concentrations of diethyl methacryloylphosphoramidate (DMPP) via different processes.

Finishing process	Concentrations (%)	Add-ons (%)	After-flame time (s)	After-glow time (s)	Char length (cm)	LOI (%)
EB	35	16	0	0	12.5	30.2
	30	12	0	0	21.5	28.2
	20	9	5.5	0	>30	26.4
	10	5	8.0	2.5	>30	23.8
PDC	35	14	0	0	17.1	28.7
	30	11	0	0	22.3	27.2
	20	8	6.3	1.0	>30	25.3
	10	4	12.4	3.1	>30	23.4

EB: electron beam; PDC: pad dry cure.

During the experiment, there was no dripping or melting occurred in the burning for all grafted samples. From Figure 4 and Table 1, the vertical flammability test showed the effectiveness of DMPP as flame retardant by EB and PDC finishing process. The grafted cotton fabrics showed the phenomenon of self-extinguishing when the concentration of DMPP reached 30% in both EB and PDC finishing process. In addition, the char length and after-flame time decreased with the increasing concentration of DMPP. Moreover, when the concentration of DMPP increased to 35%, the char length of grafted cotton fabrics by EB process was 12.5 cm, which was lower than the required maximum char length of 15.0 cm to pass a vertical flammability test [24,25].

The flame retardancy of DMPP is further supported by LOI result. Normally, the LOI value of the ungrafted cotton fabric was 18%, and could not pass a vertical flammability test [24]. As shown in Table 1, the LOI value of the grafted cotton by EB process increased significantly from 23.8% to 30.2%, while the LOI value of grafted cotton by PDC process increased from 23.4% to 28.7% with increasing concentration of DMPP. The increased LOI value resulted in lower combustibility and better flame-retardant property, which was proved by vertical flammability test shown in Figure 4.

From Table 1 and Figure 4, it is obvious that the flame retardancy of the grafted cotton fabrics by EB process is better than that of grafted cotton fabrics by PDC process. In the process of PDC finishing, radicals originated from hydroxymethyl of position C(6). But in the case of EB irradiated cotton fabric, radicals originated from hydrogen abstraction from positions 2, 3, or 4 of a glucose ring [26,27]. Therefore, the add-ons of grafted cotton fabrics by EB were improved compared that by PDC, which led to the improvement of the flame retardancy of the grafted cotton fabrics by EB.

Thermal property

The thermal properties of cotton, EB–cotton, and PDC–cotton were investigated by TGA and the results are shown in Figure 5. EB–cotton and PDC–cotton were both treated with DMPP at the concentration of 35%. It is reported that the pyrolysis of cellulose fiber includes three stages: initial stage, main stage, and decomposition of residue char [28,29]. The main pyrolysis stage occurs in the temperature range of 300–380℃ with maximum weight loss rate temperature of 383℃, 319℃, and 319℃ for cotton, EB–cotton and PDC–cotton, respectively [30]. And in this stage, l-glucose and many combustible gases are produced. From the TGA and Derivative thermogravimetry analysis (DTG) curves in Figure 5, it could be found that the pyrolysis of the grafted cotton fabrics showed the similar stages, but with lower decomposition temperatures and weight loss than those of the ungrafted cotton fabrics.

Figure 5.

TGA and DTG curves of cotton, EB–cotton and PDC–cotton.

For a clear representation of the degradation information, the onset temperature (10% weight loss) and the char yield at 600℃ of cotton, EB–cotton, and PDC–cotton are shown in Table 2. As seen in Table 2, ungrafted cotton fabrics showed onset temperature (10% weight loss) at 326℃, and gains 11% char residue at 600℃. In the meantime, the temperature of onset degradation of EB–cotton and PDC–cotton was 245℃ and 261℃, and they provided char yield of 30% and 29% at 600℃, respectively. It is clear that EB–cotton and PDC–cotton degrade at a lower temperature but provide higher char content at 600℃ compared to ungrafted cotton fabrics due to the addition of DMPP. The char residue resulting from the grafted cotton fabrics is significantly higher, showing that more charred products were produced after the flame-retardant treatment. Moreover, EB–cotton showed lower onset temperature than PDC–cotton. The reason might be that EB–cotton obtained higher add-ons compared to PDC–cotton and the onset temperature of the grafted cotton fabrics decreased with the increase of flame retardant.

Table 2.

The degradation information of cotton and treated cotton.

Samples	Onset temperature (10% weight loss, ℃)	Char yield at 600℃
Cotton	326	11
EB–cotton	245	30
PDC–cotton	261	29

Generally, the reduction of onset degradation temperature may be attributed to the lower stability of the P–O–C bond compared to the C–C bond [31]. But both EB–cotton and PDC–cotton were thermally stable than ungrafted cotton because phosphorus-containing compounds may degraded first and formed phosphoric acid at 260–300℃. Then the phosphoric acid acted as an acid catalyst to accelerate the dehydration and promotes the char formation [32]. The char residue can protect the cotton fabrics from heat and oxygen, resulting in extinguishing of fire. Thus, the combination of P and N offers char formation from phosphorus containing compound and incombustible gas from nitrogen containing compound when heated and ignited, which indicated DMPP can notably enhance the thermal stability of cotton fabrics at high temperature.

Mechanical property

The tensile strength of cotton and cotton treated with 35 wt% DMPP via EB and PDC are presented in Table 3. Compared to ungrafted cotton, PDC–cotton and EB–cotton showed some degree of decrease in tensile strength in both warp and weft directions. For PDC–cotton, the strength loss can be attributed to the acidity of the treatment solution, which causes depolymerization of the cellulose macromolecules [33] and scission of cellulosic chains because of the heat treatment [34]. For EB–cotton, the loss of strength is related to high energy of EB irradiation which caused some breakage of bonds in the structure of cotton fabrics and deformation or breakage of glucosidic bond [26]. From Table 3, it can be seen that the tensile strength of EB–cotton could maintain 78% and 75% of the original tensile strength in warp and weft directions, which is higher than that of PDC–cotton, which is one of the advantages of the EB process to produce flame-retardant cotton fabrics over the traditional PDC process.

Table 3.

The tensile strength of cotton and treated cotton.

Samples	Warp (N)	Weft (N)
Cotton	731 (12)	402 (13)
PDC–cotton	537 (15)	281 (13)
EB–cotton	572 (17)	301 (4)

Conclusion

A novel flame-retardant DMPP containing phosphorus and nitrogen was successfully synthesized and characterized by FT-IR and ¹H-NMR. DMPP was grafted on cotton fabrics through EB irradiation and PDC processes. SEM and XPS showed that DMPP was successfully grafted onto the surfaces of cotton. According to the LOI and vertical flammability tests, it was found that the char length of EB-cotton treated at the concentration of 35% was 12.5 cm while the char length of PDC-cotton was 17.1 cm. The LOI of EB-cotton was 30.2% and the LOI of PDC–cotton was 28.7%. Both EB–cotton and PDC–cotton degraded at lower temperature and produced higher yields at 600℃. It is obvious that the flame retardancy of EB-cotton is better than that of PDC–cotton. In addition, the tensile strength of EB–cotton could maintain 78% and 75% of the original tensile strength in warp and weft directions, which is higher than that of PDC–cotton. The grafted cotton fabrics can be applied in multiple industries to improve public safety protections, including but not limited to packaging, decorations, and firefighter uniform.

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 Project of Jiangsu Science and Technological Innovation Team, the Fundamental Research Funds for the Central Universities (No. JUSRP51722B, No. JUSRP11806), the national first-class discipline program of Light Industry Technology and Engineering (LITE2018-2), and 111 Projects (B17021).

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Flame-retardant cotton fabrics modified with phosphoramidate derivative via electron beam irradiation process

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of DMPP

Fabric treatment

Characterization

Results and discussion

FT-IR spectra characterization of DMPP

Characterization of the grafted fabrics

Flame-retardant performance

Thermal property

Mechanical property

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References