Application of a novel plasma-based method of introducing maleic acid and pentaerythritol phosphate urea salt onto polyester fabrics for durable flame-retardant treatment

Abstract

In this work, a novel anti-flame poly(ethylene terephthalate) (PET) fabric was successfully fabricated by plasma-based technology combined with pad-dry-cure treatment finishing. Low-temperature plasma can graft maleic acid (MA) onto a PET polymer backbone, which can activate the PET fiber and provide the opportunity of reaction with pentaerythritol phosphate urea salt (PEPAS) to achieve an excellent and durable anti-flame ability. Furthermore, the chemical reaction between PET-MA and PEPAS can form a complex and compact net structure that can effectively improve the anti-dripping property of PET. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy analysis showed that MA and PEPAS were successfully introduced onto the surface of the PET fabric, and a compact protective carbon layer was formed during the combustion process due to the synergistic effect of phosphorus and nitrogen. The flame-retardant and anti-dripping properties were evaluated by thermogravimetric analysis, the limiting oxygen index (LOI) and a vertical burning test. The treated samples showed the maximum LOI value of 29.3, possessing excellent thermal stability and self-extinguishing ability and inhibiting melt-dripping ability. Moreover, tensile strength at break of the treated PET fabric was slightly greater than that of the original PET fabric, indicating that this type of treatment had little negative impact on the bulk of the PET fabric. After 20 laundering cycles, the LOI value of the treated sample still remained at 26.8, which indicated durability in the flame-retarding effect.

Keywords

anti-flame plasma maleic acid polyester fabric anti-dripping

Poly(ethylene terephthalate) (PET) is the most widely used synthetic raw material for textile industries and possesses many excellent benefits, such as high chemical stability, good mechanical performance, spinnability, dyeability and low cost. However, there is still the critical problem of easy flammability and severe melt dripping, which greatly limit its application in the field of textiles.^1,2 To endow PET with anti-flame and anti-dripping properties, the many methods exploited by scientists and engineers to impart PET anti-flame and anti-dripping properties can be mainly divided into three categories: introducing flame-retardant monomers into PET polymer chains by copolymerization strategies;^3–5 adding flame-retardant additives, such as phosphorus-containing agents, nanoparticles and nitrogenous heterocyclic compounds, into PET during the mixing process;^1,6,7 using flame-retardant agents to finish PET fabrics by both physical and chemical technologies.^8–10 Although the first two methods have the advantages of easy preparation, durability, dynamic tenability and so on, there is the main problem of the number of flame-retardant compositions. Increasing the proportion of active ingredients in modified PET materials may result in negative effects on application performances and mechanical properties, while a smaller fraction of active ingredients cannot achieve the expected anti-flame goal. In particular, using the second method, active ingredients may apparently leak out from modified PET fabrics due to great differences in chemical composition and poor compatibility.¹¹ Therefore, more and more scientists and engineers are concerned with developing more convenient and inexpensive flame-retardant treatments for PET fabrics.

With a home-made flame-retardant poly (2-hydroxy propylene spirocyclic pentaerythritol bisphosphonate) (PPPBP) used to finish PET fabrics, Chen et al.¹² found that the chars generated by the decomposition of PPPBP during combustion could effectively enhance the flame retardancy and anti-dripping performance of the finished fabrics. With the help of the photo-induced method, Yu et al.⁸ used 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) and sulfamic acid combined with the pad-cure process to improve the anti-flame properties of the PET fabric, for which the limiting oxygen index (LOI) value could reach up to 25.9 when the grafting percent was 22.5%. With the action of polyethyleneimine (PEI), negatively charged oxidized sodium alginate (OSA) and hypophosphorous acid (HA), Pan et al.¹³ improved the fireproof function of polyester–cotton blend fabrics, as well as good laundering durability, by a layer-by-layer assembled coating technology and a cross-linking technology. To achieve the activation effect of the outer surface, Younis¹⁴ exposed the PET fabrics to ultraviolet (UV)/O₃ treatment for a certain irradiation time and subsequently immersed the fabrics into the coating solution to obtain the durable anti-flame performance. According to their findings, durability plays a vital role in flame-retardant treatments for PET fabrics. However, so far the durability has not been satisfactory for practical production and further studies are still needed.

Low-temperature plasma can generate charged particles that are accelerated by the electric field to collide on the material surface. Subsequently, these charged particles can etch the surface of the treated polymer material, improve the adhesion effect due to the increased roughness, separate hydrogen from the main chain of the polymer and create free radicals and new active functional groups on the polymer surface.^15,16 Meanwhile, plasma technology can effectively modify the material surface without hurting its bulk properties and has characteristics such as good energy conservation and environment protection, low cost and being easily prepared, resulting in it being widely applied in the textile manufacturing industry to achieve the goals of improving the dyeing ability and fastness, hydrophobic and hydrophilic performance, pill resistance, removal of wicks, etc.^17–19 The original PET fabric (O-PET) is difficult to subject to functional finishing due to its characteristics of densification and chemical stability, but free radicals and new active functional groups can be effectively introduced onto the PET fabric by plasma technology; thus, a durable chemical finishing effect on the PET fabric can be successfully obtained because of the formation of covalent bonding, hydrogen bonding and ionic bonding.^20,21 Lv et al.²² used low-temperature oxygen plasma to etch polyester fabrics and achieved durable antistatic and antibacterial properties. Gotoh et al.²³ modified a PET film to possess an extremely stable wet ability by the plasma-coating-oxidation technique. Haji et al.^24,25 used plasma treatment to assist in depositing amino functionalized carbon nanotubes onto the surface of PET fabric to improve the electrical conductivity and microwave shielding behavior. Belhaj et al.²⁶ employed plasma to realize the cross-linking of sericin on polyester fabric, and the treated samples yielded a more durable hydrophilic finish with a high capillarity. Tarek et al.²⁷ introduced two different azo pH-indicator dyes onto the surface of PET textile to achieve halochromic properties, using plasma-assisted sol–gel coating technology. Jaššo et al.²⁸ treated PET fibers with low-temperature nitrogen plasma to perform surface activation, then the activated PET fibers were soaked in a maleic acid (MA) solution to graft with MA in order to enhance their adhesion to the styrene–butadiene rubber blend.

Plasma technology also has been applied in the field of flame retardants for synthetic fiber textiles and researchers always focus on the following three aspects: (a) surface etching to improve the specific surface area, introduce oxygen-containing polar functional groups and generate free radicals; (b) polymerization induced by plasma and the film formed on the surface of fiber; (c) the direct grafting reaction.^29,30 Carosio et al.²⁹ provided a combination method of plasma surface activation and montmorillonite nanoparticle adsorption to prepare anti-flame PET fabrics, and the treated fabrics made remarkable progress in terms of thermal stability, time to ignition and heat release rate by comparison with O-PETs. Tsafack and Joelle³⁰ investigated the argon plasma-induced graft-polymerization of four acrylate monomers containing phosphorus, diethyl(acryloyloxyethyl)phosphate, diethyl-2-(methacryloyloxyethyl) phosphate, diethyl(acryloyloxymethyl) phosphonate and dimethyl(acryloyloxymethyl) phosphonate, for the fire-proofing of polymeric substrates. Wafaa et al.³¹ used dielectric barrier discharge (DBD) air plasma treatment for fiber surface activation to facilitate the deposition of aluminum oxide, nano-silver and nano-titanium dioxide onto polyester fabric and the results showed that air plasma-Al₂O₃ treatment obviously improved the flame retarding effect. Plasma technology has brought great advances in the anti-flame effect for synthetic polymer fabrics, but there are still some shortcomings, such as weak absorption, limited functional groups, growing less powerful with time, being easily washed away and being difficult to synthesize.

Herein, we designed a novel anti-flame method, in which MA was chosen as a bridge to combine PET with pentaerythritol phosphate urea salt (PEPAS). The plasma-induced PET-MA can exist for a long period and esterification occurs with polyhydroxy compound PEPAS, which has been used in cotton fabrics³² and plastic products,³³ under the condition of catalysis; thus, the anti-flame element can be firmly fixed onto PET and complex and compact net structure can be effectively constructed. The whole process could be divided into three stages. Firstly, O-PETs were immersed in the solution of MA after alkali deweighting treatment, and then they were air-dried. Secondly, as described in Figure 1, low-temperature plasma was used to graft MA onto the macromolecular chain of PET. Finally, PEPAS was fixed onto the modified PET fabrics by the dip-padding method and high-temperature chemical finishing. The fabricated PET fabrics were investigated by infrared (IR) light, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) for chemical structure and microstructure analysis. Thermogravimetry (TG) and differential thermal analysis (DTA) were used to analyze thermal stabilities of the PET fabrics. The characteristics, including the LOI, durable wash fastness and mechanical stabilities, were addressed to evaluate the application performance.

Figure 1.

The proposed mechanism of the flame-retardant finishing for poly(ethylene terephthalate) fabrics. MA: maleic acid; PEPAS: pentaerythritol phosphate urea salt.

Experimental details

Materials

Commercial knitted PET fabrics (weight 184 g/m²) were kindly provided by Zhangjiagang Helian Textile Co., Ltd (Jiangsu, China). Before treatments, the PET fabrics were cleaned in an acetone solution under ultrasonic condition, and were subsequently washed with deionized water several times and the dried fabrics were stored in plastic sealing bags. The alkali deweighting promoter and ECE nonphosphate reference detergent were provided by Yancheng Dyeing and Finishing Co., Ltd (Jiangsu, China). Absolute alcohol, urea, MA and sodium hypophosphite were obtained from Shanghai Shengxiang Chemical Reagent Co., Ltd (Shanghai, China). Pentaerythritol, phosphoric acid, acetic acid, acetone, paraxylene and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).

Alkali deweighting treatment

With the liquor ratio of 1:30, the O-PET was soaked in a solution of 30 g/L NaOH and 5 g/L alkali deweighting promoter at 80℃ for 60 min. After being taken out of the bath, the fabric was washed with hot deionized water and soaked in an acetic acid solution to be neutralized, then washed with hot deionized water and cold deionized water several times until pH neutral. Under the alkali treatment, PET fiber undergoes hydrolysis and tapers, resulting in inter-fiber cohesive force being diminished and gaps, hydrogen bonds, specific surface areas and the capillary effect being effectively increased.³⁴ Therefore, the adsorption characteristic of the PET fabric can be improved. After the alkali treatment, the weight losses of the PET fabrics were 11 ± 1%. The alkali deweighting promoter was a combination of a variety of agents in which 2-pentanol,1,1′,1″,1″′-(1,2-ethandiyldinitrilo) tetrakis was its major component. This promoter can adsorb on the PET surface to reduce surface tension and cationic groups in the promoter can absorb hydroxyl ions, resulting in the abundance on the PET surface, which can favor completion of the hydrolysis reaction.

Plasma-grafting treatment

A certain amount of MA was dissolved in deionized water and diluted with deionized water to 5–25% (mass%). With the liquor ratio of 1:50, the PET fabric after alkali treatment was immersed in the MA solution to absorb the MA by the dip-padding technique at room temperature. Before the plasma treatment, the PET fabric was dried in vacuum drier at 80℃ for 30 min. The activation process for the PET fabric was carried out using a Model HD-1B plasma instrument (Changzhou Zhongke Changtai Plasma Technology Co., Ltd, China) with cold oxygen atmosphere and at the pressure of 10 Pa. After being treated at a certain power (50–250 W) for 5 min at an oxygen gas flow rate of 5 LPM, the plasma-treated fabric (P-PET) was taken out from the metallic chamber for the next treatment.

PEPAS synthesis

A total of 13.6 g (0.1 mol) of pentaerythritol and 80 mL of paraxylene were added into a 250 mL four-necked flask. After the flask was equipped with a reflux condenser tube, a water separator and a mechanical stirrer, 17.2 g (0.176 mol) of phosphoric acid was dropwise added into the flask, and subsequently the solution was well mixed. After heating up to 140℃, the solution was kept at that temperature until the amount of water in the water separator stopped rising. Then the reaction mixture was treated with two different kinds of processes: in the first process, the mixture was filtered while it was hot to remove the solvent and get the crude product. Then the filter cake was washed thoroughly with absolute alcohol and vacuum dried, and a white powered solid product, pentaerythritol phosphate (PEPA), could be obtained. In the second process, 22 g urea was added directly into the reaction mixture and it was kept at 140℃ for 3 h. The mixture was filtered while hot to get the final product PEPAS and vacuum dried at 170℃. The reaction mechanism is described in Figure 2.

Figure 2.

Synthesis processes for pentaerythritol phosphate urea salt.

Anti-flame finishing treatment

An aqueous solution consisting of PEPAS (100–250 g/L) and sodium hypophosphite (60 g/L) was prepared and its pH was adjusted to 3. The P-PET was treated with the prepared solution by two dipping and two padding methods at ambient temperature with a pick-up of 100%. The fabric was subsequently dried at 80℃ for 3 min and cured at 160℃ for 3 min in a Model M-6 heat setting machine (Nantong Baolai Textile Instruments Factory Co., Ltd, China). After this, the finished PET fabric (F-PET) was washed thoroughly with water containing 2 g/L Na₂CO₃ and 2 g/L soap flakes and cold tap water before being air-dried.

Characterization

The chemical structure of the fabrics (before and after treatment) and the synthetic anti-flame product were investigated by Fourier transform infrared spectroscopy (FT-IR) (Tensor27, Bucker, Germany) with the attenuated total reflectance (ATR) method at a resolution of 2 cm⁻¹ in the range of 650–4000 cm⁻¹ and the KBr squashed method at a resolution of 2cm⁻¹ in the range of 400–4000 cm⁻¹, respectively.

The surface morphologies of the fabrics and residual char from the burning experiments were observed by a Quanta 200 scanning electron microscope analyzer (FEI, USA). Elemental analysis was also performed with a plane scan of energy-dispersive spectroscopy (EDS). Prior to observation, the PET samples were sputtered with gold under vacuum.

Surface chemical characterizations of the PET fabrics were analyzed with XPS analyses. The XPS experiments were performed on a PHI-5000C ESCA system (Perkin Elmer, USA) with a mono Al Kα X-ray anode source (h = 1486.6 eV) at 14.0 kV and 250 W under ultrahigh vacuum.

Thermogravimetric analysis (TGA) and DTA were carried out by employing a TG 209 analyzer (Netzsch, Germany). A total of 5 mg of the PET sample was loaded into an alumina oxide pan and heated from 25℃ to 730℃ under air atmosphere at a heating rate of 10℃ min⁻¹.

Mechanical properties, such as tensile strength (TS), elongation at break (Eb) and elasticity modulus (EM), were examined using a H5K-S universal material testing machine (Hounsfield, England) under the conditions of ambient temperature and 65% relative humidity at a speed of 12 mm min⁻¹ with a 5000 N sensor loaded.

Anti-flame test

LOI tests were measured on a HC-2C oxygen index meter (Jiangning Analysis Instrument Company, China) according to GB/T 5454-1997 (China) with fabric dimensions of 150 mm × 58 mm. The LOI is the minimum oxygen concentration of an oxygen–nitrogen mixture that will barely support flaming combustion under specified practical conditions, and is described by the oxygen volume percentage.

The vertical burning tests (Underwriter Laboratory 94) were conducted by using a JF-3-type vertical burning flame-retardant tester (Jiangning Analysis Instrument Company, China) with a dimensional size of 300 mm × 80 mm according to GB/T 5455-1997 (China).

The durability performance of F-PET samples was subjected to an accelerated domestic laundering method that was proposed in our previous work.³⁵ Twenty laundry cycles were carried out with the ECE nonphosphate reference detergent at 40℃. The anti-flame performance indicators of the fabrics were investigated after the laundering process.

Results and discussion

FT-IR studies

Figure 3(a) shows the FT-IR spectrum of pentaerythritol and PEPA. As shown in Figure 3(a), it is clear to observe that the absorption peaks at 3417.56 cm⁻¹ of pentaerythritol and 3334.05 cm⁻¹ of PEPA are ascribed to the stretching vibration of -OH groups. Since the P=O bond is successfully introduced into the chemical structure, the inter-and intra-molecular hydrogen bonds are effectively enhanced, which leads to shifting to the low wave number direction and the broadening of the peaks. The peaks between 2860 and 2960 cm⁻¹ arise from the stretching of saturated C-H groups. The peaks at 1717.06, 1718.78 and 1717.24 cm⁻¹ may be caused by the carbonyl groups of the PET polymer backbone. Compared to pentaerythritol, the characteristic peaks centered at 2365.87 and 1234.19 cm⁻¹ of the PEPA correspond to the vibration of P-OH···O and P=O bonds. These results show that the phosphate esterification is successfully carried out. Figure 3(b) shows the FT-IR spectrum of O-PET, P-PET (150 W and 20% MA) and F-PET (150 W, 250 g/L PEPAS and 20% MA). After plasma treatment, the characteristic peak centered at 3435.64 cm⁻¹ is due to the stretching vibration of O-H, while there is no corresponding peak of the pristine PET, which may be caused by the grafting MA and the formation of hydroxyl groups on the PET polymer backbone. The peaks at 1661.45 and 1548.19 cm⁻¹ of F-PET may arise from the stretching vibration of C=O and the bending vibration of N-H of urea. The peak at 1193.74 cm⁻¹ may be related to the P=O bond, the peak at 971.96 cm⁻¹ may be due to the P-O bond and the stronger peak at 1021.48 cm⁻¹ compared with the other two spectra curves may be caused by the P-O-C bond, indicating that the synthetic anti-flame agent is successfully incorporated into the polymer matrix.

Figure 3.

Fourier transform infrared spectroscopy spectra of (a) pentaerythritol phosphate (PEPA), pentaerythritol and (b) the original poly(ethylene terephthalate) (PET) fabric (P-PET), the plasma-treated fabric (P-PET) and the completely finished PET fabric (F-PET).

Microstructure

Figures 4(a) and (b) present the surface morphology of O-PET and P-PET, respectively, by SEM. We can see clearly that the surface morphology of O-PET is smooth, while that of P-PET becomes obviously rough, with the appearance of many grooves, fragments and particles. This can be explained by the PET fibers being bombarded to increase active groups and the specific surface area due to the plasma etching effect, which is beneficial for grafting and adsorbing MA.³⁶ After treating with PEPAS, we can see that it is successfully deposited on the surface of PET fibers with heavy dense embossment and granulated structures, as presented in Figure 4(c), indicating that MA can firmly fix PEPAS onto the surface of PET fibers, which results in the improvement of anti-flame performance.

Figure 4.

Surface scanning electron microscopy pictures of (a) original PET fabric (O-PET), (b) plasma-treated fabric, (c) finished PET fabric (F-PET), (d) char residues of O-PET and (e) and (f) char residues of F-PET. PET: poly(ethylene terephthalate).

For studying the solid phase flame retardancy, the inner surface morphologies of char residues of O-PET and F-PET are shown in Figures 4(d)–(f). From these SEM images, we can observe clearly that lots of bubbles and large holes spread all over the inner surface (Figure 4(d)), which can transfer heat and diffuse fuel gas to aid the flame. However, Figures 4(e) and (f) show that the compact and intact char microstructures and the surface of char residues of F-PET are showing roughness and are covered with a uniformly distributed high density of substances. These specific compact micro substances form a barrier that can effectively provide the heat insulation and protect the PET matrix from contacting fire, as well as inhibit smoke, to achieve good anti-flame performance and non-dripping effects. Since the element composition of char residues play a vital role in the flame-retardant performance, the energy-dispersive X-ray spectroscopy (EDXS) map analyses of char residues are shown in Figure 5. It reveals that the introduced phosphorus and nitrogen retardant elements are successfully deposited on the surface of the char residues.

Figure 5.

The energy-dispersive X-ray spectroscopy map analyses of the char residues of finished poly(ethylene terephthalate) fabric.

Thermal stability

The thermal behaviors under air atmosphere of O-PET and F-PET samples were investigated by TGA, as shown in Figure 6. From Figure 6(a), both samples exhibit two major thermal degradation stages and F-PET begins to degrade at a lower temperature than O-PET because of the decomposition of PEPAS, which can release active ingredients to inhibit combustion. The first stage corresponds to the degradation of the polymer main chain, while the second stage arises from a further oxidative process. It can also be easily visualized that the weight loss curve of O-PET drops more dramatically than that of F-PET, indicating that the latter has a relatively small heat generation rate. The percentage of remaining residues of O-PET is 0.164%, while that of F-PET is 6.004% at 730℃.

Figure 6.

(a) The thermogravimetry analysis. (b) The derivative thermogravimetric analysis (DTG). (c) The differential thermal analysis (DTA) of the original PET fabric (O-PET) and finished PET fabric (F-PET). PET: poly(ethylene terephthalate).

From Figure 6(b), for the maximum decomposition rate temperature (T_max) of the first stage, there is little difference between O-PET (455.33℃) and F-PET (453.48℃). However, the peak at the second stage can be improved from 515.68℃ to 555.40℃ after plasma and anti-flame treatment, which is attributed to the formation of dense char layers and the stable cross-linking networks. Cross-linking can be formed between two polymer main chains, since the synthetic anti-flame agent includes at least two active groups. Furthermore, the height of the peak in O-PET is higher than that of F-PET, indicating that grafting PEPAS can effectively retard drastic decomposition. DTA is also adopted to evaluate the burning behaviors of O-PET and F-PET, and the data is displayed in Figure 6(c). The two curves show similar glass transition and melting behaviors, while the two decomposition exothermic peaks of the two curves changed significantly. Compared with O-PET, F-PET not only shows smaller intensity but also has a later exothermal temperature range, which indicates that the anti-flame treatment could reduce the total heat release, promote the formation of char and improve the anti-dripping property.

XPS analysis

To further investigate the relationship between the chemical structure and flame retardance, XPS spectra were studied on the fire residues of the LOI test. As shown in Figure 7(a), it is clearly seen that the fire residues of F-PET mainly contain C (C_1s, 284.4 eV), O (O_1s, 532.7 eV), N (N_1s, 400.1 eV) and P (P_2p, 134.1 eV) elements, while those of O-PET mainly contain C (C_1s 284.6 eV) and O (C_1s 533.1 eV) elements. These data are in accordance with others reported in the literature.^22,37,38 After treatment, the relative content of C_1s decreased from 76.17% (that of O-PET) to 71.29%, while O_1s increased from 21.26% (that of O-PET) to 22.05%, and the relative contents of P_2p and N_1s were 2.42% and 4.24%, respectively. The XPS deconvolution scans for the P_2p spectra of F-PET are shown in Figure 7(b), and the binding energy peaks centered at 133.40 and 134.25 eV correspond to P-O and P=O,³⁹ indicating that phosphorous compounds, such as P₂O₅ and phosphate ester, are formed due to the oxidation and pyrolysis process.⁴⁰ According to the above analysis, it can be confidently concluded that the existence of phosphorous and nitrogen, by the synergistic effect, can effectively promote the formation of a stable carbon layer, extinguish flames and inhibit dripping for PET fabrics.

Figure 7.

X-ray photoelectron spectroscopy survey scan spectra of fire char for original PET fabric (O-PET) and finished PET fabric (F-PET). PET: poly(ethylene terephthalate).

Mechanical properties

The typical stress–strain curves of O-PET, PET after the alkali bath and F-PET are presented in Figure 8. The average tensile stress at break of O-PET, PET after the alkali bath and F-PET are 14.40, 11.54 and 15.23 MPa, respectively; the values for Eb are 119.40%, 102.53% and 77.61%, respectively; and the modulus elasticity values are 12.73, 15.17 and 30.99 MPa, respectively. All three types of samples display soft and flexibility features and have excellent mechanical properties, which indicates the treatment has had little adverse impact on the bulk of the PET fabrics. After alkali deweighting treatment, tensile stress at break and Eb show a trend of decline, which may be mainly caused by the degradation effects of PET under alkali conditions. However, we are interested in finding whether the TS has a certain improvement instead of decreasing after the anti-flame treatment. Although the plasma and alkaline treatments cause a negative effect on the mechanical properties, the cross-linking effects, hydrogen bonds and inter-molecular forces between the modified PET macromolecules can effectively enhance the tensile stress at break. Yet, it is this enhancement that inhibits the slip ability of PET macromolecules and the Eb decreases significantly. In addition, the baking steps of the finishing process may result in recrystallization behaviors, which may be partly responsible for the improvement of tensile stress at break.

Figure 8.

Stress–strain curves of original PET fabric (O-PET), poly(ethylene terephthalate) (PET) after the alkali bath and finished PET fabric (F-PET).

Flame retardancy

The flame retardancy of the PET fabrics was firstly evaluated by the LOI, and a single factor analysis of the flame-retardant finishing process was carried out. The corresponding experimental data are listed in Table 1.

Table 1.

Single factor analysis of the effect on the limiting oxygen index (LOI) under the condition of adjusting plasma power, the amount of maleic acid (MA) and the amount of pentaerythritol phosphate urea salt (PEPAS)

Sample number	Plasma power (W)	The amount of MA (%)	The amount of PEPAS (g/L)	LOI
0	–	–	–	20.2 ± 0.16
1	50	20%	250	25.7 ± 0.32
2	100	20%	250	28.0 ± 0.64
3	150	20%	250	29.1 ± 0.31
4	200	20%	250	28.4 ± 0.23
5	250	20%	250	27.7 ± 0.45
6	150	20%	100	23.4 ± 0.50
7	150	20%	150	25.6 ± 0.35
8	150	20%	200	27.1 ± 0.35
9	150	20%	300	29.3 ± 0.29
10	150	5%	250	24.6 ± 0.45
11	150	10%	250	25.5 ± 0.29
12	150	15%	250	27.9 ± 0.40
13	150	25%	250	28.6 ± 0.40

Note: plasma 5 min, curing temperature 160℃, curing time 3 min, sodium hypophosphite 60 g/L, penetrating agent 2 g/L, pH = 3.

It is shown that the LOI value increases with the increase in plasma power density at first, but if the power continues to increase, the LOI values show a descending tendency. A proper increase of plasma power can generate more free radicals, which results in an improved grafting degree and thus more MA monomer molecules are successfully introduced onto the surface of PET fabrics and frequently react with PEPAS to form strong covalent bonds. After the power density exceeds 150 W, the polymerization of MA itself is enhanced, while the grafting between MA and PET macromolecule decreases, which leads to the decline in the covalent bonds between PEPAS molecules and PET macromolecules. On the other hand, if the power density is too high, the surfaces of the plasma-treated PET fabrics will be seriously etched and their weave structures will be loosened and damaged to a certain extent, and then the contacting part with oxygen will be enhanced during the combustion, which has a negative impact on the anti-flame effect. With the consideration of each performance of various plasma power densities, the optimum value is 150 W.

As shown in Table 1, PEPAS was added in the amount of 100–300 g/L to the anti-flame finishing solution, and we found that the LOI value increased to 29.1 when the concentration of PEPAS reached 250 g/L, then even if more PEPAS was added into the solution, there were no significant changes in the LOI. PEPAS could improve the anti-flame properties of PET in two ways. On the one hand, it can generate POċ to capture Hċ, which effectively interrupts the exothermic process and inhibits combustion, but this is just a minor aspect. On the other hand, it can change the thermal cracking process of PET and is beneficial to the formation of the char layer and water; thus, the amount of combustible gas is greatly decreased. The latter is the major aspect. Furthermore, PEPAS is an integrated organophosphorus intumescent fire-proofing agent. The phosphorus element removes water and displays condensed phase flame retardation to promote the formation of the carbon layer. N₂ and NH₃, which are derived from the nitrogen element in PEPAS, can dilute the flammability of the gas and result in foaming and intumescing phenomena to form a highly dense and micro-porous fire-proofing char layer. Integrating phosphorus and nitrogen into an anti-flame-retardant agent, they can create synergistic effect with each other and greatly improve the efficiency of the flame retardant.²⁶ The pentaerythritol component acts as a carbon source and is the foundation of the foam char layer. Moreover, a PEPAS molecule can simultaneously form more than two covalent bonds with different modified PET macromolecules due to having several hydroxyl groups in the structure, so it can be firmly combined with the PET matrix and the TS of the treated PET fabric can be effectively improved, which has been proved in the mechanical test. As the grafting degree of MA is determined by the previous process, too much PEPAS cannot be well bonded with the modified PET macromolecules due to the limited active sites. Therefore, no obvious enhancement of the LOI was found by further increasing the amount of PEPAS; thus, 250 g/L was chosen as the optimum amount of PEPAS.

From the data listed in Table 1, the LOI value increases from 24.6 to 29.1 with the concentration of MA adjusted from 5% to 20%. Subsequently, the LOI value tends to slightly decline when the concentration of MA reaches 25%. By increasing the concentration of MA after alkali deweighting treatment, the amount of MA on the PET fabric increases by adsorption. Therefore, more active sites can be generated correspondingly, which effectively improve the reaction with PEPAS. However, the growth reduces gradually due to the saturation of the adsorption capacity. Furthermore, an excessive amount of MA also enhances the chances of self polymerization and weakens the combination of MA and PET, which results in the slightly declined LOI.

According to the above analysis, the technical parameters of anti-flame finishing were determined: MA 20%, PEPAS 250 g/L, pH = 3, sodium hypophosphite 60 g/L, penetrating agent 2 g/L, power density 150 W, plasma time 5 min, curing temperature 160℃, curing time 3 min. Then the optimal treated fabrics were tested by the LOI and vertical burning test, and the results are listed in Table 2. After ignition, the O-PETs could not self-extinguish and almost burnt out, with obvious melts dripping away from the flaming fabrics. The treated PET samples could reach UL-94 V-0 grade and the flame almost instantly went out after removing the igniter, showing an excellent anti-flame performance and self-extinguishing property. Neither type of sample showed a smoldering phenomenon. During the UL-94 flame test for the treated samples, the absorbent cotton could not be ignited, which demonstrated that the melt dripping property was suppressed successfully. The excellent anti-dripping effect may be attributed to the cross-linking behavior, which can increase the melt viscosity and reduce the flowability.⁴¹ Moreover, when the treated PET fabrics burnt, the three-dimensional cross-linking system acted as a net in which the molten droplets could be encased, and thus prevent dripping.⁴² Furthermore, esterification, dehydration, expanding and cross-linking occur during the heating process for the treated samples, which results in the formation of foaming and an intumescing char layer on the surface of the PET matrix. The char layer can effectively inhibit the thermal decomposition of the interior PET matrix and decrease the amount of combustible gas, subsequently decreasing the heat of combustion and the heat release rate. Therefore, the formation of molten droplets becomes relatively difficult. In order to evaluate the durability of the anti-flame agent to washing, the treated samples were investigated by the accelerated laundering method (20 times of domestic laundering). From the data listed in Table 2, the LOI value declines from 29.3 to 26.8. This may be caused by the ablation of PEPAS in the washing process. Compared with O-PET fabric, which has LOI = 20.2, F-PET shows a durability in the flame-retarding effect, and this suggests that PEPAS can be firmly combined with the PET fabric. The LOI values of the treated fabrics are comparable to those of P(ET-co-BP)_x (25.9–29.2),⁴³ BAnPETs (22.0–31.0),³ BD_xPET (28.4)⁴⁴ and sPMCMSs/PET (25.5–27.2)⁴⁵ and higher than those of A_xPET and F_xPET (23.3–26.0),⁴⁶ GMA-g-PET/FR (25.9)⁸ and PET-g-PGMA/DVB (21.5),⁴⁷ but still much lower than those of PET/AF (35.3),⁴⁸ PET/HNCP/POE-g-MA (26.8–35.1)¹ and PAN/PET (42.0),³⁶ so many aspects, such as the catalyst composition, plasma atmosphere and gas flow rate, need to be researched to get further improvements in the anti-flame performance of PET fabrics.

Table 2.

The limiting oxygen index (LOI) and vertical flame test of original PET fabric (O-PET), finished PET fabric (F-PET) and F-PET after laundering 20 times

Sample	LOI (%)			Vertical burning test
Sample	LOI (%)	UL-94	Melt dripping	Molten droplets within 10 s	Ignition cotton
O-PET	20.2 ± 0.16	V-2	heavy	8	Yes
F-PET	29.3 ± 0.29	V-0	none	0 (self-extinguish)	No
F-PET after laundering	26.8 ± 0.42	V-0	little	3 (self-extinguish)	No

PET: poly(ethylene terephthalate).

Conclusions

Herein, a plasma-based method is reported to achieve a durable anti-flame effect for polyester fabrics. Alkali deweighting treatment can effectively destroy the surface compactness of PET and enlarge the specific surface area to absorb more MA, which can form a bridge between PET and PEPAS. The plasma-induced PET-MA can exist for a long period and esterification with polyhydroxy compounds and PEPAS occurs under the condition of catalysis; thus, the anti-flame element can be firmly fixed onto PET and the complex and compact net structure can be effectively constructed. The optimal anti-flame finishing process conditions were MA 20%, PEPAS 250 g/L, pH = 3, sodium hypophosphite 60 g/L, penetrating agent 2 g/L, power density 150 W, plasma time 5 min, curing temperature 160℃, curing time 3 min. Under optimum formulation, the LOI value of the treated sample reached 29.3 and showed better anti-dripping performance than that of the untreated sample. The results of characterization with FT-IR, EDS and XPS showed that MA and PEPAS were successfully introduced onto the surface of the PET fibers. From the SEM, it could be seen that the dense char layer formed at high temperature was enhanced obviously due to the firm coverage of the anti-flame agent. Compared with the O-PETs, the TSs and thermal stabilities of the treated fabrics exhibited significant improvement. In the vertically burning test, the treated fabrics could achieve a self-extinguishing effect, while the untreated samples almost burnt out. Moreover, the cross-linking structure formed between PEPAS and PET could impart good anti-dripping performance. After 20 laundering cycles, the LOI value of the treated sample remained at 26.8 and the treated sample still reached the UL-94 V-0 grade, which meant a good laundering durability. This modification method might provide a novel, environmental way to improve the flame retardancy of polyester fabrics as well as good laundering durability.

Footnotes

Acknowledgments

Many thanks to Yancheng Dyeing and Printing Co., Ltd, for generously providing some of the chemicals.

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: This work was supported by the Science Project of the China Textile Industry Federation (Project Number: 2015021), the Open Foundation of the Provincial Research Platform of Yancheng Vocational Institute of Industry Technology (Project Number: YGKF-201707) and the National Natural Youth Science Foundation of China (Project Number: 51803175).

References

Zhang

L-P

, et al. The influence of synergistic effects of hexakis (4-nitrophenoxy) cyclotriphosphazene and POE-g-MA on anti-dripping and flame retardancy of PET. J Ind Eng Chem 2013; 19: 993–999.

Eslami

Müller-Plathe

. Structure and mobility of poly(ethylene terephthalate): a molecular dynamics simulation study. Macromolecules 2009; 42: 8241–8250.

J-N

Chen

Lin

Teng

, et al. New application for aromatic Schiff base: high efficient flame-retardant and anti-dripping action for polyesters. Chem Eng J 2018; 336: 622–632.

Zhang

Y-P

M-X

, et al. Phosphorus-containing copolyesters: the effect of ionic group and its analogous phosphorus heterocycles on their flame-retardant and anti-dripping performances. Polymer 2015; 60: 50–61.

Zhao

H-B

Wang

Y-Z

. Design and synthesis of PET-based copolyesters with flameretardant and antidripping performance. Macromol Rapid Commun 2017; 170045: 1–14.

Guo

W-Y

Chuan

K-Y

. Crystallization, dispersion behavior and the induced properties through forming a controllable network in the crosslinked polyester-SiO₂ nanocomposites. J Appl Polym Sci 2012; 123: 1773–1783.

Liu

H-M

Wang

. Static and dynamic mechanical properties of flame-retardant copolyester/nano-ZnCO₃ composites. J Appl Polym Sci 2011; 121: 3131–3136.

L-H

Zhang

Liu

, et al. Improving the flame retardancy of PET fabric by photo-induced grafting. Polym Degrad Stabil 2010; 95: 1934–1942.

Carosio

Laufer

Alongi

, et al. Layer-by-layer assembly of silica-based flame retardant thin film on PET fabric. Polym Degrad Stabil 2011; 96: 745–750.

10.

Zhu

S-F

Shi

M-W

Tian

M-W

, et al. Burning behavior of irradiated PET flame-retardant fabrics impregnated with sensitizer. Mater Lett 2015; 160: 58–60.

11.

Jenny

Federico

Giulio

. Current emerging techniques to impart flame retardancy to fabrics: an overview. Polym Degrad Stabil 2014; 106: 138–149.

12.

Chen

D-Q

Wang

Y-Z

X-P

, et al. Flame-retardant and anti-dripping effects of a novel char-forming flame retardant for the treatment of poly(ethylene terephthalate) fabrics. Polym Degrad Stabil 2005; 88: 349–356.

13.

Pan

Liu

L-X

Wang

, et al. Hypophosphorous acid cross-linked layer-by- layer assembly of green polyelectrolytes on polyester-cotton blend fabrics for durable flame retardant treatment. Carbohyd Polym 2018; 201: 1–8.

14.

Younis

A-A

. Evaluation of the flammability and thermal properties of a new flame retardant coating applied on polyester fabric. Egypt J Petrol 2016; 25: 161–169.

15.

Waicek

A-E

Jurak

Gozdecka

, et al. Interfacial properties of PET and PET/starch polymers developed by air plasma processing. Coll Surf A 2017; 532: 323–331.

16.

Štular

Primc

Mozetič

, et al. Influence of non-thermal plasma treatment on the adsorption of a stimuli responsive nanogel onto polyethylene terephthalate fabric. Prog Org Coat 2018; 120: 198–207.

17.

Mehmood

Kaynak

Dai

X-J

, et al. Study of oxygen plasma pre-treatment of polyester fabric for improved polypyrrole adhesion. Mater Chem Phys 2014; 143: 668–675.

18.

Vinisha

R-K

Sarma

. Plasma treatment on cotton fabrics to enhance the adhesion of reduced graphene oxide for electro-conductive properties. Diam Relat Mater 2018; 84: 77–85.

19.

Canbolat

Kilinc

Kut

. The investigation of the effects of plasma treatment on the dyeing properties of polyester/viscose nonwoven fabrics. Procedia Soc Behav Sci 2015; 195: 2143–2151.

20.

Cioffia

M-O-H

Voorwalda

H-J-C

Mota

R-P

. Surface energy increase of oxygen-plasma treated PET. Mater Charact 2003; 50: 209–215.

21.

Vesel

Mozetic

Zalar

. XPS study of oxygen plasma activated PET. Vacuum 2008; 82: 248–251.

22.

J-C

Zhou

Q-Q

Zhi

, et al. Environmentally friendly surface modification of polyethylene terephthalate (PET) fabric by low-temperature oxygen plasma and carboxymethyl chitosan. J Clean Prod 2016; 118: 187–196.

23.

Gotoh

Shohbuke

Kobayashi

, et al. Wettability control of PET surface by plasma-induced polymer film deposition and plasma/UV oxidation in ambient air. Colloid Surface A 2018; 556: 1–10.

24.

Haji

Ruhollah

S-R

Ahmad

M-S

. Plasma assisted attachment of functionalized carbon nanotubes on poly(ethylene terephthalate) fabric to improve the electrical conductivity. Polimery 2015; 60: 337–342.

25.

Haji

Ruhollah

S-R

Ahmad

M-S

. Improved microwave shielding behavior of carbon nanotube-coated PET fabric using plasma technology. Appl Surf Sci 2014; 311: 593–601.

26.

Belhaj

Imene

N-L

Behary

, et al. Crosslinking of Sericin on air atmospheric plasma treated polyester fabric. J Text I 2017; 108: 840–845.

27.

Tarek

Simon

El-Sayed

A-A

, et al. Plasma-assisted surface modification of polyester fabric for developing halochromic properties. Fiber Polym 2017; 18: 731–740.

28.

Jaššo

Hudec

Alexy

, et al. Grafting of maleic acid on the polyester fibres initiated by plasma at atmospheric pressure. Int J Adhes Adhes 2006; 26: 274–284.

29.

Carosio

Alongi

Frache

. Influence of surface activation by plasma and nanoparticle adsorption on the morphology, thermal stability and combustion behavior of PET fabrics. Eur Polym J 2011; 47: 893–902.

30.

Tsafack

M-J

Joelle

L-Gr

. Plasma-induced graft-polymerization of flame retardant monomers onto PAN fabrics. Surf Coat Tech 2006; 200: 3503–3510.

31.

Wafaa

M-R

Usama

S-R

Hanan

E-S

, et al. Ultraviolet protection, flame retardancy and antibacterial properties of treated polyester fabric using plasma-nano technology. Mater Sci Appl 2011; 2: 1432–1442.

32.

Deng

J-Y

Liu

Dong

X-L

, et al. Preparation of novel N-P flame retardant and its flame retardant properties in cotton fabrics. J Text Res 2017; 38: 97–101.

33.

Zhong

H-X

Liu

, et al. Synthesis, characterization and thermal stability of flame retardant containing pentaerythrityl phosphate. Chem Res Appl 2012; 24: 127–132.

34.

J-D

Cai

G-Q

Liu

J-Q

, et al. Eco-friendly surface modification on polyester fabrics by esterase treatment. Appl Surf Sci 2014; 295: 150–157.

35.

Zhou

T-C

X-M

Guo

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

36.

Zhou

T-C

Pang

Z-Y

, et al. Flame retardancy and conductive properties of polyester fabrics coated with polyaniline. Text Res J 2016; 86: 1171–1179.

37.

Wang

C-X

J-C

Gao

D-W

, et al. Surface modification and aging effect of polysulfonamide yarns treated by atmospheric pressure plasma. Fibers Polym 2013; 14: 1478–1484.

38.

Shen

Wang

Long

J-J

. Biodegumming of ramie fiber with pectinases enhanced by oxygen plasma. J Clean Prod 2015; 101: 395–403.

39.

Malmgren

Ciosek

Hahlin

, et al. Comparing anode and cathode electrode/electrolyte interface composition and morphology using soft and hard X-ray photoelectron spectroscopy. Electrochim Acta 2013; 97: 23–32.

40.

Chu

F-K

X-J

Hou

Y-B

, et al. A facile strategy to simultaneously improve the mechanical and fire safety properties of ramie fabric-reinforced unsaturated polyester resin composites. Compos Part A 2018; 115: 264–273.

41.

J-N

Qin

Z-H

Chen

, et al. Tailoring Schiff base cross-linking by cyano group toward excellent flame retardancy, anti-dripping and smoke suppression of PET. Polymer 2018; 153: 78–85.

42.

Tang

H-Y

Chen

J-Y

Guo

Y-H

. A novel process for preparing anti-dripping polyethylene terephthalate fibers. Mater Design 2010; 31: 3525–3530.

43.

Z-Z

Y-P

, et al. Effect of biphenyl biimide structure on the thermal stability, flame retardancy and pyrolysis behavior of PET. Polym Degrad Stabil 2018; 155: 162–172.

44.

Guo

D-M

Ruan

, et al. A new approach to improving flame retardancy, smoke suppression and anti-dripping of PET: via arylene-ether units rearrangement reactions at high temperature. Polymer 2015; 77: 21–31.

45.

Xue

B-X

Niu

Yang

Y-Z

, et al. Multi-functional carbon microspheres with double shell layers for flame retardant poly (ethylene terephthalate). Appl Surf Sci 2018; 435: 656–665.

46.

Guo

D-M

Chen

X-Q

Tang

, et al. PET-based copolyesters with bisphenol A or bisphenol F structural units: their distinct differences in pyrolysis behaviours and flame-retardant performances. Polym Degrad Stabil 2015; 120: 158–168.

47.

Chen

Wang

Dai

G-L

, et al. Radiation grafting of glycidyl methacrylate and divinylbenzene onto polyethylene terephthalate fabrics for improving anti-dripping performance. Radiat Phys Chem 2016; 127: 256–263.

48.

Khalifah

A-S

Ali

Pietro

, et al. Comprehensive study on flame retardant polyesters from phosphorus additives. Polym Degrad Stabil 2018; 155: 22–34.