- Email: [email protected]
An eco-friendly and effective flame retardant coating for cotton fabric based on phytic acid doped silica sol approach
Progress in Organic Coatings 141 (2020) 105539
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
An eco-friendly and effective flame retardant coating for cotton fabric based on phytic acid doped silica sol approach
T
Xian-Wei Chenga,*, Ren-Cheng Tanga, Jin-Ping Guana, Shao-Qiang Zhoub a b
National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, 199 Renai Road, Suzhou 215123, China Nanjing Customs Industrial Products Testing Center, 39 Chuangzhi Road, Nanjing 210000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coating Sol-gel Phytic acid Silica Cotton Flame retardancy
The high flammability of cellulosic materials somewhat limits their practical application. The introduction of an environmentally benign flame retardant coating for cotton fabric was achieved by hydrolyzing tetraethoxysilane and doping the silica sol with phytic acid in the presence of sodium alginate. The hybrid silica sol had high condensation degree and the modified silica particles had a spherical structure. The coated cotton fabrics with the hybrid PA/silica sol systems showed enhanced thermal stability at high temperatures, and displayed significantly suppressed heat and smoke generation ability, compared with the uncoated one. The hybrid silica sol coatings influenced the combustion performance of cotton via a charring action, and endowed the cotton fabrics with self-extinguishing ability during the vertical flammability test, demonstrating the improvement in fire resistance. The coated cotton fabrics displayed enhanced char yields, whilst maintained their texture structures after burning. Besides, the silicon- and phosphorus-containing components were also found to have a synergistic flame retardant action on cotton.
1. Introduction Flame retardant (FR) functional treatment of textile materials has long been a major issue because of their massive application in everyday life and high flammability which make them a notable contributor to fire disasters. Cotton is a highly appreciated natural fiber and its consumption also takes a significant percentage of the total fiber consumption. The researchers all over the world keep trying to reduce the overall fire risk of cotton textiles by preventing their combustion or at least suppressing the fire spread [1,2]. Among the effective achievements, the halogen-based and formaldehyde-containing (eg. pyrovatex CP and proban) FR agents were the most favored commercial approaches for producing FR cotton textiles over the decades. Now, people are pursuing a healthy life and caring more about environmental protection and sustainable development. Thus, the application of such kinds of chemicals, which are recognized to be detrimental to human health and environment, has been gradually restricted or prohibited [1–4]. The development of eco-friendly, sustainable and cost-effective FR systems for textiles becomes one of the urgent and great challenges. As of late, the study of natural products from plants and animals as potential FR agents has gained a broad consideration because of their easily accessible and renewable characteristics. It has been recognized that these FR approaches, contributing to the generation of insulating
⁎
char layer on the surface of target substrates, have the ability to provide efficient FR functionality [5], and thus may serve as promising candidates to replace halogen-based FR chemicals. Indeed, almost all of the recently reported FR approaches based on the renewable resources follow this mechanism, and they could improve the FR performance of polymeric materials including textiles through the condensed phase action [5–8]. Phytic acid (PA) is one of the attractive bio-based compounds in the field of FR modification owing to the biological and renewable features, along with the high phosphorus content (28 wt%). Like most of the phosphorus-containing FR agents, PA will decompose at first during combustion and the released phosphorus/polyphosphoric acid can promote the generation of phosphorus-rich and thermally stable char layer, protecting the substrates below and thus improving the fire behavior of the target materials. As individual FR agent, PA was found to be able to enhance the flame retardancy of wool [9], whereas it was not effective enough to heighten the FR property of cotton according to our preliminary experiment. It is supposed that the difference between the FR performance of the PA modified wool and cotton fabrics lies in the nitrogen-containing groups of wool fiber; a synergistic FR effect may form between the phosphorus- and nitrogen-containing groups on wool fiber. As a result, the combinations of PA with other FR agents are proposed to further enhance its functional efficiency on other textile
Corresponding author. E-mail address: [email protected] (X.-W. Cheng).
https://doi.org/10.1016/j.porgcoat.2020.105539 Received 3 September 2019; Received in revised form 23 November 2019; Accepted 2 January 2020 0300-9440/ © 2020 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
discussion section, Cotton-1, Cotton-2 and Cotton-3 samples denote the coated fabrics with the hybrid sol solutions containing 0, 0.2 and 0.6 mol/L TEOS respectively, together with 0.2 mol/L PA.
materials like cotton. By means of the electrostatic attraction, PA has great potential to react with cationic chemicals to yield complexes [6]. Therefore, PA was applied together with nitrogen- or silicon-containing compounds to develop intumescent FR coatings for preparing FR cotton [6,10,11], polyamide 66 [12], polyacrylonitrile [13] and polyester [14] fabrics through the layer by layer assembly route, which involves multiple cycles of impregnation to deposit sufficient chemicals on textiles surface to impart desired functionality. Over the past decade, inorganic nanoparticles and nanotechnology have been applied in the field of the functional modification of polymeric materials for enhanced UV protection, antibacterial, hydrophobic, self-cleaning and FR properties. By acting as a thermal insulator, inorganic nanoparticles are capable to absorb the released heat and restrain the generation of volatiles, thus enhancing the heat and fire resistance of polymeric substrates [15,16], and they seem to generate much more effective flame retardancy when operated in synergistic or joint effects with phosphorus or nitrogen-based chemicals [17,18]. It was also reported that PA can catalyze the hydrolysis of tetraethoxysilane and tetraethoxysilane (TEOS) to prepare phosphosilicate gels with high phosphorus content through the sol-gel process [19,20]. The aforementioned studies enlightened us to develop the PA/silica organic-inorganic hybrid sol (hybrid silica sol) system for enhancing the FR property of cotton fabrics by using TEOS as a silicon precursor. The applications of alginate as biodegradable polymeric carriers for the drug delivery systems in controlling the release of solid dosage forms and improving the drug dissolution have gained a wide interest [21]. Alginate grafted on particle surface also showed the ability to control the aggregation and dispersion of colloids via steric interactions [22]. In this study, sodium alginate was thus used as an anti-agglomeration agent and stabilizer for the hybrid silica sol in the presence of citric acid as cross-linking agent between sodium alginate and cotton fiber. The surface morphology, particle size distribution, functional groups and condensation degree of the developed hybrid silica sol product were investigated. The surface morphology, thermal stability, heat and smoke generation ability as well as FR performance of the coated fabrics were studied. The FR mode of action of the fabrics was studied via the morphological and elemental analyses of the residues.
2.4. Characterizations The surface images of the sol particles were captured by the Hitachi HT7700 transmission electron microscopy (TEM) and Hitachi S-4800 field-emission scanning electron microscope (SEM). The X-ray diffraction (XRD) measurement of sample was conducted on an X'Pert-Pro MPD X-ray diffractometer (PANalytical B.V., Netherlands) equipped using Cu-Kα radiation of wavelength 0.15418 nm. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were recorded by the Thermo Scientific Nicolet iS50 FT-IR spectrometer. The solidstate 29Si nuclear magnetic resonance (NMR) spectrum of the freezedried hybrid silica sol products was recorded by the Bruker Advance III 400 MHz spectrometer with tetramethylsilane as an internal reference; the condensation degree (Dc) of Q species in the products was calculated based on the deconvoluted peak areas using the following equation:
Q1+2Q2+3Q3+4Q4 ⎤ Dc (%) = ⎡ × 100 ⎢ ⎥ 4 ⎣ ⎦
(1)
For the cotton samples, the morphological performance was studied by the Hitachi S-4800 field-emission and TM3030 tabletop SEM, and energy disperse spectroscopy (EDS) spectrometer fitted to TM3030 SEM was applied for elemental analysis; the flammability tests concerning the limiting oxygen index (LOI) and vertical burning were evaluated based on GB/T 5454-1997 and GB/T 5455-2014, respectively; the burning behavior was rated on the basis of GB/T 17591-2006. The heat and smoke release performance were measured on the basis of ASTM D7309 (Method A) and ISO 5659.2 using the FTT0001 microscale combustion calorimetry and FTT0064 NBS smoke density test chamber, respectively. The thermogravimetric (TG) analysis was conducted on the Perkin-Elmer Diamond TG/DTA SII thermal analyzer. The tensile strength test was conducted on the Illinois Instron 3365 Universal Testing Machine on the basis of ISO 13934-1-2013. The Hunter whiteness index (WI) was calculated based on the lightness [L], rednessgreenness value [a] and yellowness-blueness value [b], which were determined by the HunterLab UltraScan PRO reflectance spectrophotometer, using the following equation:
2. Experimental 2.1. Materials
WI = 100 − [(100 − L)2 + a2 + b2]1/2 The scoured cotton fabric (100 g/m2) was supplied by Suzhou Printing and Dyeing Factory Co. Ltd., China. PA was supplied by Chengdu AiKeda Chemical Technology Co. Ltd., China. TEOS, sodium alginate, citric acid, ethanol, and triethanolamine were supplied by Sinopharm Chemical Reagent Co. Ltd., China.
(2)
3. Results and discussion 3.1. Characterization of the hybrid silica sol 3.1.1. TEM analysis Fig. 1 displays the TEM micrograph and the particle size distribution of the hybrid silica sol. The spherical silica particles were uniformly dispersed, and they were speculated to be surrounded by sodium alginate, as appeared in lighter color under the microscope, leading to the formation of fruit/tree-like three-dimensional structures. The average diameter of the hybrid silica particles was about 119 nm. Besides, according to the SEM observation (Fig. S1), the hybrid silica particles had an average diameter of about 95 nm, which is accordance with the result of TEM observation.
2.2. Preparation of the hybrid silica sol Firstly, triethanolamine was used to adjust pH of PA solution to 4, and then sodium alginate (0.5 %) and citric acid (6 %) were dissolved in PA solution. The mixture consisting of TEOS and ethanol, with a molar ratio of 2:1, was added dropwise into PA solution under high-speed constant stirring at 70 °C within 1 h. Then, the reaction was allowed to continue for 3 h. The sol solutions containing 0.2 mol/L PA and various dosages of TEOS (0, 0.2, 0.4, 0.6 and 0.9 mol/L) with a total volume of 100 mL were prepared.
3.1.2. FT-IR analysis As shown in Fig. 2, for the spectrum of PA, the peak at around 1630 cm−1 is attributed to the hydration of water molecules; the peaks between 1180 and 995 cm−1 are associated with the stretching vibration of P]O and P-O-C structures [23]. For spectra of the hybrid silica sols, the new peak at 1710 cm−1 corresponds to the stretching vibration of
2.3. Preparation of the coated cotton fabrics After being treated with sol solutions, the cotton samples were rollsqueezed to a 100 ± 5 % wet pickup. The samples were dried at 80 °C and then cured at 160 °C for 3 min. Afterwards, the samples were rinsed thoroughly with tap water, and then air-dried. In the results and 2
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Fig. 3. Solid-state
29
Si NMR spectrum of the hybrid silica sol product.
the antisymmetric stretching vibration of Si-O-Si structure appeared at 1081 cm−1 was overlapped with the absorption of CeOCe structure. The new peak at around 791 cm−1, which was not observed in the spectrum of sample b, is attributed to the symmetric stretching vibration of SieOSei structure [26,27]. Besides, the EDS mapping also showed a high Si content (5.4 %) for the hybrid silica sol product. These results above confirm the formation of hybrid silica sol network.
3.1.3. NMR analysis In order to further confirm the formation of silica network, 29Si NMR spectroscopy was conducted and the typical 29Si NMR spectrum of the hybrid silica sol is shown in Fig. 3. The signal fell in the range of -95 to -120 ppm, which can be attributed to the presence of a 29Si nucleus in a tetrahedral oxygen environment, conventionally indicated by the Qn notation. Different types of Qn sites are distinguished by the secondnearest neighbor ligand, n ranging from 0 to 4 depending on the number of oxygens bonding with another Qn unit. The chemical shifts at around -92, -101 and -110 ppm are assigned to the Q2 (double threerings), Q3 (double four-rings) and Q4 (3D six-rings) species, respectively [28]. The condensation degree of the hybrid silica sol reached 76.2 %, indicating the high cross-linking extent of TEOS. Besides, the XRD spectrum (Fig. S2) suggests that the developed hybrid silica sol product is amorphous, which is accordance with the reported literatures [19,20].
Fig. 1. TEM micrograph (a) and particle size distribution (b) of the hybrid silica sol.
3.2. Characterization of the coated cotton fabrics 3.2.1. FT-IR analysis As shown in Fig. 4, the signal at around 3300 cm−1 in spectrum of the uncoated cotton is ascribed to OH stretching vibration of cellulose fibers; the characteristic signals at 1161, 1060 and 995 cm−1 are accordance with C-O-C and C-O stretching vibration of glucosidic units [29], overlapping with the main peaks of phosphorus- and siliconcontaining structures of the hybrid silica sols [23,26]. Thus, it might be difficult to detect the signals of the coatings on the fabrics. Even so, the spectra differences between the cotton samples before and after the treatment at 1081−995 cm−1, which is ascribed to the overlapped OePCe and Si-O-Si stretching vibration, were found; the new peak observed in the spectra of Cotton-2 and Cotton-3 samples at around 791 cm-1 is due to the symmetric stretching vibration of Si-O-Si structure. Besides, in the case of the coated samples, the new signal at 1591 cm−1 should be assigned to NeH bending vibration of triethanolamine; the signal at 1737 cm−1 should be ascribed to C]O stretching [24], confirming the ester cross-linking of sodium alginate and cotton fiber by citric acid.
Fig. 2. FT-IR spectra of PA (a), and the hybrid sols containing 0 (b), 0.2 (c) and 0.6 (d) mol/L TEOS.
carbonyl group of citric acid [24]. The peak at 1587 cm−1 is due to the NHe bending vibration of triethanolamine; the peak at 1386 cm−1 is ascribed to the stretching vibration CeO structure of alginate; the strong peaks between 1081 and 1027 cm−1 relate to the antisymmetric stretching vibration of CeOCe structure of alginate [25]. In addition, 3
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
elements were homogeneously distributed on the cross-section of the fibers; but for P and Si elements, most of them were found to distribute on the outler layer of the fibers, and only few dispersed on the core of the fibers. This phenomenon was more obvious for Si element. Taking Cotton-3 as a sample, as shown in Fig. 6, P and Si content of the fibers were 1.874 and 0.893 %, respectively; but for the cross-section, the content of P and Si elements decreased to 1.083 and 0.415 %, respectively. These results above indicate that the hybrid silica sol product mainly adheres on the surface of cotton fiber. 3.2.3. Thermal stability For cotton fabric, three thermal degradation steps were clearly detected in an air atmosphere (Fig. 7). The first process was observed below 150 °C and the negligible weight loss is attributed to the loss of water attached to cotton fiber. But it is difficult to distinguish this step for the coated samples. For all of the fabrics, the highest weight loss took place in the second region during 150–400 °C, corresponding to the depolymerization of the glycosyl units to produce volatiles species (such as levoglucosan, furan and furan derivatives) and the dehydration of the main chain to form thermal stable char [30]; afterward, the further oxidation of the formed char took place at higher temperature [31]. Compared with the cotton samples, the hybrid silica sol showed a rapid weight loss and low thermal stability below 300 °C, and had a high thermal stability at following degradation stage. As shown in Fig. 7 and Table 1, the introduction of the hybrid silica sol coating advanced the decomposition of cotton and decreased the Tmax2 value. It is supposed that the phosphorus-containing compounds decompose at first, inducing the degradation of cotton and favoring the generation of thermal stable char layer at a lower temperature. The beforehand degradation was advantageous for improving the thermal stability of cotton as the thermal stable carbonaceous structures formed at the beginning could retard the following cotton degradation. Thus, although the thermal stability of the coated cotton at low temperature decreased, it was significantly improved at the end of the decomposition process, as indicated by the heightened Tmax3 value and increased char residue at 400 and 600 °C.
Fig. 4. FT-IR spectra of the coated cotton fabrics.
3.2.2. SEM-EDS analysis The surface images of the coated cotton fibers were firstly captured by TM3030 SEM at low magnification. All cotton fibers exhibited an inhomogeneous structure because of natural growth (Fig. S3). The surface images captured by S-4800 field-emission SEM at high magnification are shown in Fig. 5. Compared with the clean surface of the untreated one, the deposition of FR compounds on the fiber surface was found for the coated cotton samples. A homogeneous and continuous coating appeared on the fiber surface. Although the hybrid silica particles were embedded inside the polymer matrix and their surface may be covered, some of them were observed by S-4800 field-emission SEM at higher magnification, demonstrating the deposition of the hybrid silica sol coating on the fiber surface. Besides, EDS spectrum was used to determine the elements of the fibers and the corresponding cross-section. All the elements (C、O、P and Si) were finely dispersed on the coated cotton fibers. According to the elemental mapping of the cross-section of the fibers, both C and O
Fig. 5. SEM images of the coated cotton fibers (captured by S-4800). 4
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Fig. 6. EDS spectra of the fiber (a) and corresponding cross-section (b) of Cotton-3.
Fig. 7. TG (a) and DTG (b) curves of the cotton fabrics in air.
The calculated TG curves of the coated cotton were obtained according to the simple additive contribution of cotton and the hybrid silica sol coating. The difference between the experimental and calculated TG curves was used to evaluate the interactions between cotton and the hybrid silica sol during heating. Taking Cotton-3 as a sample, the experimental curve had higher thermal stability and char residues than the calculated one (Fig. 7 and Table 1). These results indicate that certain interactions rather than the simple additive interaction between cotton and the hybrid silica sol occur during heating, confirming the above hypothesis. Besides, the char residues at 600 °C exhibited growing trend with increasing TEOS concentration due to the growing Si content on fabrics (discussed later), indicating that the P/Si synergistic FR effect contributes to the generation of more complicated and thermal stable residues.
Table 1 TG data of the cotton fabrics in air. Sample
Tmax1 (oC)
Residue at 150 °C (%)
Tmax2 (oC)
Residue at 400 °C (%)
Tmax3 (oC)
Residue at 600 °C (%)
Hybrid sol Control Cotton-1 Cotton-2 Cotton-3a Cotton-3b
– 50.6 – – – –
96.2 93.9 96.6 97.0 96.8 94.3
– 350.2 293.4 292.2 293.8 –
48.6 23.4 39.9 42.4 43.9 27.8
– 489.5 505.6 505.7 504.9 –
37.0 0.0 8.1 12.3 13.8 6.2
Note:
a
Experimental one;
b
Calculated one.
5
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Fig. 8. HRR (a) and Ds (b) curves of the cotton fabrics.
3.2.4. Heat and smoke release capacity In this section, the potential fire risk of the coated samples was assessed using the PCFC and smoke density tests. The heat release in the PCFC test is related to the oxidation of volatile matters. As shown in Fig. 8, the temperature (Tmax) at peak heat release rate (pHRR) declined in the presence of the coatings, which was also observed for another PAbased system [6]. The phenomenon, which is considered to be a harmful action by instinct, is beneficial to the formation of the shielding layer at low temperature, and thus impedes the further generation of combustion matters and smoke particles at high temperature. Consequently, the coated samples exhibited a remarkable decline in pHRR, total heat release (THR) and maximum Ds (Dsmax) value (Table 2), demonstrating less volatile and smoke generation. Furthermore, the coatings promoted the generation of protective char and raised char residue of cotton (Table 2). The hybrid sol coating could accelerate the generation of char, and thus more organic matters were catalytically carbonized during charring procedure, instead of being transferred into combustible gas and smoke particles.
Fig. 9. Weight gain and FR performance of the cotton fabrics coated with the hybrid silica sols at various TEOS concentrations.
hybrid sol containing 0.6 mol/L TEOS, had an increased LOI of 29.8 % and a decreased char length of 13.2 cm. These results indicate that the hybrid silica sol coatings are effective to heighten the FR property of cotton. In order to quantify the synergistic FR effect between phosphorusand silicon-containing components on cotton fabric, the synergism effectiveness (SE) parameter based on the LOI results was calculated according to the previously reported literatures [32,33]. As shown in Table 3, the calculated SE for Cotton-3 sample was 1.05, which is higher than 1.0, indicating that the silicon- and phosphorus-containing components have synergistic FR ability together. Unfortunately, the hybrid silica sol system is not durable on cotton fabric due to the water solubility of PA and the lack of chemical interaction between the FR agents and cotton fiber. The treated cotton fabric may find its potential application in sofa fabric, mattress, electric blanket, wall covering and curtains for upholstery, movie screen and proscenium curtain in public, the disposable protective clothing for work wear and so on. In future work, some measures should be taken to improve the washing resistance of the coated cotton fabrics.
3.2.5. Flammability According to GB/T 17591-2006, there are two different ratings of the FR furnishing textiles based on the vertical burning test: (1) B1 rating: char length ≤ 15 cm, after flame time ≤ 5 s, after glow time ≤ 5 s; (2) B2 rating: char length ≤ 20 cm, after flame time ≤ 15 s, after glow time ≤ 15 s. The uncoated cotton fabric was lit easily and then consumed completely with very little gossamery residue left, and it also displayed after flame and afterglow phenomenon. As a result, the uncoated one had the highest char length of 30 cm and no rating was reached; it also had the lowest LOI of 18.0 %, displaying low FR performance. Evidently, the hybrid silica sol coatings affected the burning behavior of cotton in a positive way. It exhibited effective FR ability, resulting in no burning of the coated fabrics once removal the flame source. As shown in Fig. 9, higher weight gain was obtained by increasing TEOS concentration, leading to the increased Si content and almost unchanged P content of the coated cotton fabrics (discussed later); the coated samples also showed elevated FR performance as demonstrated by the increased LOI and reduced char length. All of the coated cotton fabrics reached B1 rating (char length ≤ 15 cm) in the vertical burning test. For example, Cotton-3 sample, for which the Table 2 PCFC and smoke density parameters of the cotton fabrics. Sample
Control Cotton-1 Cotton-2 Cotton-3
PCFC
Smoke density o
pHRR (W/g)
THR (kJ/g)
Tmax ( C)
Char residue (%)
Dsmax
229.1 ± 6.8 91.8 ± 3.5 78.0 ± 2.6 60.3 ± 1.3
11.7 ± 0.1 3.2 ± 0.2 2.8 ± 0.1 2.3 ± 0.1
357.0 ± 5.3 274.3 ± 1.4 272.8 ± 1.5 269.5 ± 1.1
8.8 ± 0.6 31.9 ± 0.2 33.1 ± 0.3 34.7 ± 0.3
72.2 ± 3.7 21.7 ± 1.4 20.7 ± 1.1 19.6 ± 1.3
6
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Table 3 LOI of the untreated and treated cotton fabrics. Sample
LOI (%)
Control Cotton-PA Cotton-Sol Cotton-3 SEa (Cotton-3)
18.0 28.5 18.7 29.8 1.05
a Synergistic effectiveness parameter from LOI data.
3.3. Char residue analyses As described above, the presence of the coatings altered the combustion mode of cotton via charring action and enhanced the amount of char residue after burning. A small piece of gossamery residue was left, and they exhibited sloppy and fragile features (Fig. 10). As for the coated samples, the residues were integral, and the original texture was also well maintained after burning, responding to the high thermal resistance of the coated fibers. Additionally, for Cotton-3 sample, intumescent bubbles were also found in the char residue. As shown in Fig. 10, Si and P elements were homogeneously distributed and finely dispersed on and within the char residues. The content of these two elements showed a similar changing trend before and after burning (Fig. 11). The Si content experienced a significant increase by increasing TEOS dosage, whereas P content had little change. It is worth to note that the P and Si content of the samples increased significantly after burning, suggesting that these elements acted in the solid phase and participated in forming the char layer. As a conclusion, the coated samples released fewer pyrolysis products than the uncoated one because of the thermally insulating action of
Fig. 11. Si and P content of the cotton fabrics and corresponding char residues determined by ICP-OES.
phosphorus- and silicon-based char layer, and thus obtained good flame retardancy. 3.4. Tensile strength and whiteness of the fabrics Tensile strength and whiteness of the coated cotton fabrics were evaluated. As shown in Table 4, the whiteness of the cotton fabrics had little decrease after the coating treatment because the citric acid treatment contributes to the yellowing of cotton fabric. The treatment also decreased the tensile strength of cotton fabric as PA can hydrolyze the ether bond of cellulose unites at weak acid and high-temperature curing condition. Besides, the crosslinking reaction between cellulose
Fig. 10. SEM micrographs of the char residues after vertical flammability tests. 7
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Table 4 Tensile strength, elongation and whiteness of the cotton fabrics. Sample
Whiteness
Tensile strength (N)
Elongation (%)
Control Cotton-1 Cotton-2 Cotton-3
80.0 77.4 75.7 75.2
643.5 556.4 535.0 530.8
6.15 8.72 8.42 8.32
[2] G. Malucelli, F. Carosio, J. Alongi, A. Fina, A. Frache, G. Camino, Materials engineering for surface-confined flame retardancy, Mat. Sci. Eng. R 84 (2014) 1–20. [3] L.S. Birnbaum, D.F. Staskal, Brominated flame retardants: cause for concern? Environ. Health Persp. 112 (2004) 9–17. [4] I. Van Der Veen, J. De Boer, Phosphorus flame retardants: properties, production, environmental occurrence, toxicity and analysis, Chemosphere 88 (2012) 1119–1153. [5] L. Costes, F. Laoutid, S. Brohez, P. Dubois, Bio-based flame retardants: when nature meets fire protection, Mat. Sci. Eng. R 117 (2017) 1–25. [6] G. Laufer, C. Kirkland, A.B. Morgan, J.C. Grunlan, Intumescent multilayer nanocoating, made with renewable polyelectrolytes, for flame-retardant cotton, Biomacromolecules 13 (2012) 2843–2848. [7] J. Alongi, R.A. Carletto, A. Di Blasio, F. Carosio, F. Bosco, G. Malucelli, DNA: a novel, green, natural flame retardant and suppressant for cotton, J. Mater. Chem. A 1 (2013) 4779–4785. [8] F. Carosio, G. Fontaine, J. Alongi, S. Bourbigot, Starch-based layer by layer assembly: efficient and sustainable approach to cotton fire protection, ACS Appl. Mater. Inter. 7 (2015) 12158–12167. [9] X.W. Cheng, J.P. Guan, G. Chen, X.H. Yang, R.C. Tang, Adsorption and flame retardant properties of bio-based phytic acid on wool fabric, Polymers 8 (2016) 122. [10] X. Wang, M.Q. Romero, X.Q. Zhang, R. Wang, D.Y. Wang, Intumescent multilayer hybrid coating for flame retardant cotton fabrics based on layer-by-layer assembly and sol-gel process, RSC Adv. 5 (2015) 10647–10655. [11] Z.F. Li, C.J. Zhang, L. Cui, P. Zhu, C. Yan, Y. Liu, Fire retardant and thermal degradation properties of cotton fabrics based on APTES and sodium phytate through layer-by-layer assembly, J. Anal. Appl. Pyrolysis 123 (2017) 216–223. [12] C.K. Kundu, W. Wang, S. Zhou, X. Wang, H. Sheng, Y. Pan, L. Song, Y. Hu, A green approach to constructing multilayered nanocoating for flame retardant treatment of polyamide 66 fabric from chitosan and sodium alginate, Carbohydr. Polym. 166 (2017) 131–138. [13] Y. Ren, T. Huo, Y. Qin, X. Liu, Preparation of flame retardant polyacrylonitrile fabric based on sol-gel and layer-by-layer assembly, Materials 11 (2018) 483. [14] Z. Jiang, C. Wang, S. Fang, P. Ji, H. Wang, C. Ji, Durable flame-retardant and antidroplet finishing of polyester fabrics with flexible polysiloxane and phytic acid through layer-by-layer assembly and sol-gel process, J. Appl. Polym. Sci. 135 (2018) 46414. [15] P. Jamshidi, D. Ghanbari, M. Salavati-Niasari, Sonochemical synthesis of La(OH)3 nanoparticle and its influence on the flame retardancy of cellulose acetate nanocomposite, J. Ind. Eng. Chem. 20 (2014) 3507–3512. [16] D. Ghanbari, M. Salavati-Niasari, S. Khaghani, F. Beshkar, Preparation of polyvinyl acetate (PVAc) and PVAc-Ag-Fe3O4 composite nanofibers by electro-spinning method, J. Clust. Sci. 27 (2016) 1317–1333. [17] J. Alongi, F. Carosio, G. Malucelli, Current emerging techniques to impart flame retardancy to fabrics: an overview, Polym. Degrad. Stabil. 106 (2014) 138–149. [18] A.M. Grancaric, L. Botteri, J. Alongi, A. Tarbuk, Silica precursor as synergist for cotton flame retardancy, Int. J. Cloth. Sci. Tech. 28 (2016) 378–386. [19] C. Samba-Fouala, J.C. Mossoyan, M. Mossoyan-Déneux, D. Benlian, C. Chanéac, F. Babonneau, Preparation and properties of silica hybrid gels containing phytic acid, J. Mater. Chem. 10 (2000) 387–393. [20] D. Qiu, P. Guerry, J.C. Knowles, M.E. Smith, R.J. Newport, Formation of functional phosphosilicate gels from phytic acid and tetraethyl orthosilicate, J. Sol-Gel Sci. Techn. 48 (2008) 378–383. [21] M.L. González-Rodrıguez, M.A. Holgado, C. Sanchez-Lafuente, A.M. Rabasco, A. Fini, Alginate/chitosan particulate systems for sodium diclofenac release, Int. J. Pharmaceut. 232 (2002) 225–234. [22] F. Sabri, K. Berthomier, A. Marion, L. Fradette, J.R. Tavares, N. Virgilio, Sodium alginate-grafted submicrometer particles display enhanced reversible aggregation/ disaggregation properties, Carbohydr. Polym. 194 (2018) 61–68. [23] G. Jiang, J. Qiao, F. Hong, Application of phosphoric acid and phytic acid-doped bacterial cellulose as novel proton-conducting membranes to PEMFC, Int. J. Hydrogen Energ. 37 (2012) 9182–9192. [24] C.Q. Yang, FT-IR spectroscopy study of the ester crosslinking mechanism of cotton cellulose, Text. Res. J. 61 (1991) 433–440. [25] G. Lawrie, I. Keen, B. Drew, A. Chandler-Temple, L. Rintoul, P. Fredericks, L. Grøndahl, Interactions between alginate and chitosan biopolymers characterized using FTIR and XPS, Biomacromolecules 8 (2007) 2533–2541. [26] R. Al-Oweini, H. El-Rassy, Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)4 and R′′Si(OR′)3 precursors, J. Mol. Struct. 919 (2009) 140–145. [27] A. Fidalgo, L.M. Ilharco, The defect structure of sol-gel-derived silica/polytetrahydrofuran hybrid films by FTIR, J. Non-Cryst. Solids 283 (2001) 144–154. [28] B. Lebeau, J. Maquet, C. Sanchez, F. Beaume, F. Lauprêtre, Structural and dynamical studies of hybrid siloxane–silica materials, J. Mater. Chem. 7 (1997) (1997) 989–995. [29] C. Chung, M. Lee, E.K. Choe, Characterization of cotton fabric scouring by FT-IR ATR spectroscopy, Carbohydr. Polym. 58 (2004) 417–420. [30] A.A. Faroq, D. Price, G.J. Milnes, A.R. Horrocks, Thermogravimetric analysis study of the mechanism of pyrolysis of untreated and flame retardant treated cotton fabrics under a continuous flow of nitrogen, Polym. Degrad. Stabil. 44 (1994) 323–333. [31] D. Price, A.R. Horrocks, M. Akalin, A.A. Faroq, Influence of flame retardants on the mechanism of pyrolysis of cotton (cellulose) fabrics in air, J. Anal. Appl. Pyrol. 40 (1997) 511–524. [32] A.R. Horrocks, G. Smart, S. Nazaré, B. Kandola, D. Price, Quantification of zinc hydroxystannate** and stannate** synergies in halogen-containing flame-retardant polymeric formulations, J. Fire Sci. 28 (2010) 217–248. [33] A.M. Grancaric, L. Botteri, J. Alongi, G. Malucelli, Synergistic effects occurring between water glasses and urea/ammonium dihydrogen phosphate pair for enhancing the flame retardancy of cotton, Cellulose 22 (2015) 2825–2835.
fibers by using citric acid may also reduce the tensile strength of cotton fabric. However, a decrease of 17.0 % for tensile strength can be acceptable in the wet processing of cotton fabric. 4. Conclusions The prepared organic-inorganic hybrid silica sol system based on naturally occurring materials effectively improved the FR performance of cotton. In vertical burning tests, the coated cotton fabrics with the hybrid silica sols could extinguish the flame, and the residue chars still preserved the texture structure, demonstrating the good efficiency of the FR coatings on cotton. The high phosphorus and silicon content of the char residues and intumescent bubbles formed on the surface of char residues suggested an evident intumescent charring mechanism for the coated cotton. The samples showed anticipated degradation at a lower temperature but higher thermal stability at higher temperatures. In addition, the heat and smoke release ability of the coated cotton were greatly reduced, compared to the uncoated one, as a result of the barrier and insulating behavior of phosphorus-containing char residues. The present hybrid silica sol system is considered as an effective and environmentally sustainable FR approach for textile materials. However, further investigations are needed to enhance the washing resistance of the hybrid sol system with the aim of broadening its application fields. Data availability statement The authors confirm that the data supporting the findings of this study are available within the article. Author statement Xian-Wei Cheng performed the experiments, analyzed the data and wrote the manuscript. Ren-Cheng Tang proposed and supervised the work, and revised the manuscript. Jin-Ping Guan and Shao-Qiang Zhou provided lots of constructive suggestions about the work. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements This study was funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions [No. 2018-87]. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2020. 105539. References [1] A.R. Horrocks, Flame retardant challenges for textiles and fibers: new chemistry versus innovatory solutions, Polym. Degrad. Stabil. 96 (2011) 377–392.
8
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
An eco-friendly and effective flame retardant coating for cotton fabric based on phytic acid doped silica sol approach
T
Xian-Wei Chenga,*, Ren-Cheng Tanga, Jin-Ping Guana, Shao-Qiang Zhoub a b
National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, 199 Renai Road, Suzhou 215123, China Nanjing Customs Industrial Products Testing Center, 39 Chuangzhi Road, Nanjing 210000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Coating Sol-gel Phytic acid Silica Cotton Flame retardancy
The high flammability of cellulosic materials somewhat limits their practical application. The introduction of an environmentally benign flame retardant coating for cotton fabric was achieved by hydrolyzing tetraethoxysilane and doping the silica sol with phytic acid in the presence of sodium alginate. The hybrid silica sol had high condensation degree and the modified silica particles had a spherical structure. The coated cotton fabrics with the hybrid PA/silica sol systems showed enhanced thermal stability at high temperatures, and displayed significantly suppressed heat and smoke generation ability, compared with the uncoated one. The hybrid silica sol coatings influenced the combustion performance of cotton via a charring action, and endowed the cotton fabrics with self-extinguishing ability during the vertical flammability test, demonstrating the improvement in fire resistance. The coated cotton fabrics displayed enhanced char yields, whilst maintained their texture structures after burning. Besides, the silicon- and phosphorus-containing components were also found to have a synergistic flame retardant action on cotton.
1. Introduction Flame retardant (FR) functional treatment of textile materials has long been a major issue because of their massive application in everyday life and high flammability which make them a notable contributor to fire disasters. Cotton is a highly appreciated natural fiber and its consumption also takes a significant percentage of the total fiber consumption. The researchers all over the world keep trying to reduce the overall fire risk of cotton textiles by preventing their combustion or at least suppressing the fire spread [1,2]. Among the effective achievements, the halogen-based and formaldehyde-containing (eg. pyrovatex CP and proban) FR agents were the most favored commercial approaches for producing FR cotton textiles over the decades. Now, people are pursuing a healthy life and caring more about environmental protection and sustainable development. Thus, the application of such kinds of chemicals, which are recognized to be detrimental to human health and environment, has been gradually restricted or prohibited [1–4]. The development of eco-friendly, sustainable and cost-effective FR systems for textiles becomes one of the urgent and great challenges. As of late, the study of natural products from plants and animals as potential FR agents has gained a broad consideration because of their easily accessible and renewable characteristics. It has been recognized that these FR approaches, contributing to the generation of insulating
⁎
char layer on the surface of target substrates, have the ability to provide efficient FR functionality [5], and thus may serve as promising candidates to replace halogen-based FR chemicals. Indeed, almost all of the recently reported FR approaches based on the renewable resources follow this mechanism, and they could improve the FR performance of polymeric materials including textiles through the condensed phase action [5–8]. Phytic acid (PA) is one of the attractive bio-based compounds in the field of FR modification owing to the biological and renewable features, along with the high phosphorus content (28 wt%). Like most of the phosphorus-containing FR agents, PA will decompose at first during combustion and the released phosphorus/polyphosphoric acid can promote the generation of phosphorus-rich and thermally stable char layer, protecting the substrates below and thus improving the fire behavior of the target materials. As individual FR agent, PA was found to be able to enhance the flame retardancy of wool [9], whereas it was not effective enough to heighten the FR property of cotton according to our preliminary experiment. It is supposed that the difference between the FR performance of the PA modified wool and cotton fabrics lies in the nitrogen-containing groups of wool fiber; a synergistic FR effect may form between the phosphorus- and nitrogen-containing groups on wool fiber. As a result, the combinations of PA with other FR agents are proposed to further enhance its functional efficiency on other textile
Corresponding author. E-mail address: [email protected] (X.-W. Cheng).
https://doi.org/10.1016/j.porgcoat.2020.105539 Received 3 September 2019; Received in revised form 23 November 2019; Accepted 2 January 2020 0300-9440/ © 2020 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
discussion section, Cotton-1, Cotton-2 and Cotton-3 samples denote the coated fabrics with the hybrid sol solutions containing 0, 0.2 and 0.6 mol/L TEOS respectively, together with 0.2 mol/L PA.
materials like cotton. By means of the electrostatic attraction, PA has great potential to react with cationic chemicals to yield complexes [6]. Therefore, PA was applied together with nitrogen- or silicon-containing compounds to develop intumescent FR coatings for preparing FR cotton [6,10,11], polyamide 66 [12], polyacrylonitrile [13] and polyester [14] fabrics through the layer by layer assembly route, which involves multiple cycles of impregnation to deposit sufficient chemicals on textiles surface to impart desired functionality. Over the past decade, inorganic nanoparticles and nanotechnology have been applied in the field of the functional modification of polymeric materials for enhanced UV protection, antibacterial, hydrophobic, self-cleaning and FR properties. By acting as a thermal insulator, inorganic nanoparticles are capable to absorb the released heat and restrain the generation of volatiles, thus enhancing the heat and fire resistance of polymeric substrates [15,16], and they seem to generate much more effective flame retardancy when operated in synergistic or joint effects with phosphorus or nitrogen-based chemicals [17,18]. It was also reported that PA can catalyze the hydrolysis of tetraethoxysilane and tetraethoxysilane (TEOS) to prepare phosphosilicate gels with high phosphorus content through the sol-gel process [19,20]. The aforementioned studies enlightened us to develop the PA/silica organic-inorganic hybrid sol (hybrid silica sol) system for enhancing the FR property of cotton fabrics by using TEOS as a silicon precursor. The applications of alginate as biodegradable polymeric carriers for the drug delivery systems in controlling the release of solid dosage forms and improving the drug dissolution have gained a wide interest [21]. Alginate grafted on particle surface also showed the ability to control the aggregation and dispersion of colloids via steric interactions [22]. In this study, sodium alginate was thus used as an anti-agglomeration agent and stabilizer for the hybrid silica sol in the presence of citric acid as cross-linking agent between sodium alginate and cotton fiber. The surface morphology, particle size distribution, functional groups and condensation degree of the developed hybrid silica sol product were investigated. The surface morphology, thermal stability, heat and smoke generation ability as well as FR performance of the coated fabrics were studied. The FR mode of action of the fabrics was studied via the morphological and elemental analyses of the residues.
2.4. Characterizations The surface images of the sol particles were captured by the Hitachi HT7700 transmission electron microscopy (TEM) and Hitachi S-4800 field-emission scanning electron microscope (SEM). The X-ray diffraction (XRD) measurement of sample was conducted on an X'Pert-Pro MPD X-ray diffractometer (PANalytical B.V., Netherlands) equipped using Cu-Kα radiation of wavelength 0.15418 nm. The attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra were recorded by the Thermo Scientific Nicolet iS50 FT-IR spectrometer. The solidstate 29Si nuclear magnetic resonance (NMR) spectrum of the freezedried hybrid silica sol products was recorded by the Bruker Advance III 400 MHz spectrometer with tetramethylsilane as an internal reference; the condensation degree (Dc) of Q species in the products was calculated based on the deconvoluted peak areas using the following equation:
Q1+2Q2+3Q3+4Q4 ⎤ Dc (%) = ⎡ × 100 ⎢ ⎥ 4 ⎣ ⎦
(1)
For the cotton samples, the morphological performance was studied by the Hitachi S-4800 field-emission and TM3030 tabletop SEM, and energy disperse spectroscopy (EDS) spectrometer fitted to TM3030 SEM was applied for elemental analysis; the flammability tests concerning the limiting oxygen index (LOI) and vertical burning were evaluated based on GB/T 5454-1997 and GB/T 5455-2014, respectively; the burning behavior was rated on the basis of GB/T 17591-2006. The heat and smoke release performance were measured on the basis of ASTM D7309 (Method A) and ISO 5659.2 using the FTT0001 microscale combustion calorimetry and FTT0064 NBS smoke density test chamber, respectively. The thermogravimetric (TG) analysis was conducted on the Perkin-Elmer Diamond TG/DTA SII thermal analyzer. The tensile strength test was conducted on the Illinois Instron 3365 Universal Testing Machine on the basis of ISO 13934-1-2013. The Hunter whiteness index (WI) was calculated based on the lightness [L], rednessgreenness value [a] and yellowness-blueness value [b], which were determined by the HunterLab UltraScan PRO reflectance spectrophotometer, using the following equation:
2. Experimental 2.1. Materials
WI = 100 − [(100 − L)2 + a2 + b2]1/2 The scoured cotton fabric (100 g/m2) was supplied by Suzhou Printing and Dyeing Factory Co. Ltd., China. PA was supplied by Chengdu AiKeda Chemical Technology Co. Ltd., China. TEOS, sodium alginate, citric acid, ethanol, and triethanolamine were supplied by Sinopharm Chemical Reagent Co. Ltd., China.
(2)
3. Results and discussion 3.1. Characterization of the hybrid silica sol 3.1.1. TEM analysis Fig. 1 displays the TEM micrograph and the particle size distribution of the hybrid silica sol. The spherical silica particles were uniformly dispersed, and they were speculated to be surrounded by sodium alginate, as appeared in lighter color under the microscope, leading to the formation of fruit/tree-like three-dimensional structures. The average diameter of the hybrid silica particles was about 119 nm. Besides, according to the SEM observation (Fig. S1), the hybrid silica particles had an average diameter of about 95 nm, which is accordance with the result of TEM observation.
2.2. Preparation of the hybrid silica sol Firstly, triethanolamine was used to adjust pH of PA solution to 4, and then sodium alginate (0.5 %) and citric acid (6 %) were dissolved in PA solution. The mixture consisting of TEOS and ethanol, with a molar ratio of 2:1, was added dropwise into PA solution under high-speed constant stirring at 70 °C within 1 h. Then, the reaction was allowed to continue for 3 h. The sol solutions containing 0.2 mol/L PA and various dosages of TEOS (0, 0.2, 0.4, 0.6 and 0.9 mol/L) with a total volume of 100 mL were prepared.
3.1.2. FT-IR analysis As shown in Fig. 2, for the spectrum of PA, the peak at around 1630 cm−1 is attributed to the hydration of water molecules; the peaks between 1180 and 995 cm−1 are associated with the stretching vibration of P]O and P-O-C structures [23]. For spectra of the hybrid silica sols, the new peak at 1710 cm−1 corresponds to the stretching vibration of
2.3. Preparation of the coated cotton fabrics After being treated with sol solutions, the cotton samples were rollsqueezed to a 100 ± 5 % wet pickup. The samples were dried at 80 °C and then cured at 160 °C for 3 min. Afterwards, the samples were rinsed thoroughly with tap water, and then air-dried. In the results and 2
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Fig. 3. Solid-state
29
Si NMR spectrum of the hybrid silica sol product.
the antisymmetric stretching vibration of Si-O-Si structure appeared at 1081 cm−1 was overlapped with the absorption of CeOCe structure. The new peak at around 791 cm−1, which was not observed in the spectrum of sample b, is attributed to the symmetric stretching vibration of SieOSei structure [26,27]. Besides, the EDS mapping also showed a high Si content (5.4 %) for the hybrid silica sol product. These results above confirm the formation of hybrid silica sol network.
3.1.3. NMR analysis In order to further confirm the formation of silica network, 29Si NMR spectroscopy was conducted and the typical 29Si NMR spectrum of the hybrid silica sol is shown in Fig. 3. The signal fell in the range of -95 to -120 ppm, which can be attributed to the presence of a 29Si nucleus in a tetrahedral oxygen environment, conventionally indicated by the Qn notation. Different types of Qn sites are distinguished by the secondnearest neighbor ligand, n ranging from 0 to 4 depending on the number of oxygens bonding with another Qn unit. The chemical shifts at around -92, -101 and -110 ppm are assigned to the Q2 (double threerings), Q3 (double four-rings) and Q4 (3D six-rings) species, respectively [28]. The condensation degree of the hybrid silica sol reached 76.2 %, indicating the high cross-linking extent of TEOS. Besides, the XRD spectrum (Fig. S2) suggests that the developed hybrid silica sol product is amorphous, which is accordance with the reported literatures [19,20].
Fig. 1. TEM micrograph (a) and particle size distribution (b) of the hybrid silica sol.
3.2. Characterization of the coated cotton fabrics 3.2.1. FT-IR analysis As shown in Fig. 4, the signal at around 3300 cm−1 in spectrum of the uncoated cotton is ascribed to OH stretching vibration of cellulose fibers; the characteristic signals at 1161, 1060 and 995 cm−1 are accordance with C-O-C and C-O stretching vibration of glucosidic units [29], overlapping with the main peaks of phosphorus- and siliconcontaining structures of the hybrid silica sols [23,26]. Thus, it might be difficult to detect the signals of the coatings on the fabrics. Even so, the spectra differences between the cotton samples before and after the treatment at 1081−995 cm−1, which is ascribed to the overlapped OePCe and Si-O-Si stretching vibration, were found; the new peak observed in the spectra of Cotton-2 and Cotton-3 samples at around 791 cm-1 is due to the symmetric stretching vibration of Si-O-Si structure. Besides, in the case of the coated samples, the new signal at 1591 cm−1 should be assigned to NeH bending vibration of triethanolamine; the signal at 1737 cm−1 should be ascribed to C]O stretching [24], confirming the ester cross-linking of sodium alginate and cotton fiber by citric acid.
Fig. 2. FT-IR spectra of PA (a), and the hybrid sols containing 0 (b), 0.2 (c) and 0.6 (d) mol/L TEOS.
carbonyl group of citric acid [24]. The peak at 1587 cm−1 is due to the NHe bending vibration of triethanolamine; the peak at 1386 cm−1 is ascribed to the stretching vibration CeO structure of alginate; the strong peaks between 1081 and 1027 cm−1 relate to the antisymmetric stretching vibration of CeOCe structure of alginate [25]. In addition, 3
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
elements were homogeneously distributed on the cross-section of the fibers; but for P and Si elements, most of them were found to distribute on the outler layer of the fibers, and only few dispersed on the core of the fibers. This phenomenon was more obvious for Si element. Taking Cotton-3 as a sample, as shown in Fig. 6, P and Si content of the fibers were 1.874 and 0.893 %, respectively; but for the cross-section, the content of P and Si elements decreased to 1.083 and 0.415 %, respectively. These results above indicate that the hybrid silica sol product mainly adheres on the surface of cotton fiber. 3.2.3. Thermal stability For cotton fabric, three thermal degradation steps were clearly detected in an air atmosphere (Fig. 7). The first process was observed below 150 °C and the negligible weight loss is attributed to the loss of water attached to cotton fiber. But it is difficult to distinguish this step for the coated samples. For all of the fabrics, the highest weight loss took place in the second region during 150–400 °C, corresponding to the depolymerization of the glycosyl units to produce volatiles species (such as levoglucosan, furan and furan derivatives) and the dehydration of the main chain to form thermal stable char [30]; afterward, the further oxidation of the formed char took place at higher temperature [31]. Compared with the cotton samples, the hybrid silica sol showed a rapid weight loss and low thermal stability below 300 °C, and had a high thermal stability at following degradation stage. As shown in Fig. 7 and Table 1, the introduction of the hybrid silica sol coating advanced the decomposition of cotton and decreased the Tmax2 value. It is supposed that the phosphorus-containing compounds decompose at first, inducing the degradation of cotton and favoring the generation of thermal stable char layer at a lower temperature. The beforehand degradation was advantageous for improving the thermal stability of cotton as the thermal stable carbonaceous structures formed at the beginning could retard the following cotton degradation. Thus, although the thermal stability of the coated cotton at low temperature decreased, it was significantly improved at the end of the decomposition process, as indicated by the heightened Tmax3 value and increased char residue at 400 and 600 °C.
Fig. 4. FT-IR spectra of the coated cotton fabrics.
3.2.2. SEM-EDS analysis The surface images of the coated cotton fibers were firstly captured by TM3030 SEM at low magnification. All cotton fibers exhibited an inhomogeneous structure because of natural growth (Fig. S3). The surface images captured by S-4800 field-emission SEM at high magnification are shown in Fig. 5. Compared with the clean surface of the untreated one, the deposition of FR compounds on the fiber surface was found for the coated cotton samples. A homogeneous and continuous coating appeared on the fiber surface. Although the hybrid silica particles were embedded inside the polymer matrix and their surface may be covered, some of them were observed by S-4800 field-emission SEM at higher magnification, demonstrating the deposition of the hybrid silica sol coating on the fiber surface. Besides, EDS spectrum was used to determine the elements of the fibers and the corresponding cross-section. All the elements (C、O、P and Si) were finely dispersed on the coated cotton fibers. According to the elemental mapping of the cross-section of the fibers, both C and O
Fig. 5. SEM images of the coated cotton fibers (captured by S-4800). 4
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Fig. 6. EDS spectra of the fiber (a) and corresponding cross-section (b) of Cotton-3.
Fig. 7. TG (a) and DTG (b) curves of the cotton fabrics in air.
The calculated TG curves of the coated cotton were obtained according to the simple additive contribution of cotton and the hybrid silica sol coating. The difference between the experimental and calculated TG curves was used to evaluate the interactions between cotton and the hybrid silica sol during heating. Taking Cotton-3 as a sample, the experimental curve had higher thermal stability and char residues than the calculated one (Fig. 7 and Table 1). These results indicate that certain interactions rather than the simple additive interaction between cotton and the hybrid silica sol occur during heating, confirming the above hypothesis. Besides, the char residues at 600 °C exhibited growing trend with increasing TEOS concentration due to the growing Si content on fabrics (discussed later), indicating that the P/Si synergistic FR effect contributes to the generation of more complicated and thermal stable residues.
Table 1 TG data of the cotton fabrics in air. Sample
Tmax1 (oC)
Residue at 150 °C (%)
Tmax2 (oC)
Residue at 400 °C (%)
Tmax3 (oC)
Residue at 600 °C (%)
Hybrid sol Control Cotton-1 Cotton-2 Cotton-3a Cotton-3b
– 50.6 – – – –
96.2 93.9 96.6 97.0 96.8 94.3
– 350.2 293.4 292.2 293.8 –
48.6 23.4 39.9 42.4 43.9 27.8
– 489.5 505.6 505.7 504.9 –
37.0 0.0 8.1 12.3 13.8 6.2
Note:
a
Experimental one;
b
Calculated one.
5
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Fig. 8. HRR (a) and Ds (b) curves of the cotton fabrics.
3.2.4. Heat and smoke release capacity In this section, the potential fire risk of the coated samples was assessed using the PCFC and smoke density tests. The heat release in the PCFC test is related to the oxidation of volatile matters. As shown in Fig. 8, the temperature (Tmax) at peak heat release rate (pHRR) declined in the presence of the coatings, which was also observed for another PAbased system [6]. The phenomenon, which is considered to be a harmful action by instinct, is beneficial to the formation of the shielding layer at low temperature, and thus impedes the further generation of combustion matters and smoke particles at high temperature. Consequently, the coated samples exhibited a remarkable decline in pHRR, total heat release (THR) and maximum Ds (Dsmax) value (Table 2), demonstrating less volatile and smoke generation. Furthermore, the coatings promoted the generation of protective char and raised char residue of cotton (Table 2). The hybrid sol coating could accelerate the generation of char, and thus more organic matters were catalytically carbonized during charring procedure, instead of being transferred into combustible gas and smoke particles.
Fig. 9. Weight gain and FR performance of the cotton fabrics coated with the hybrid silica sols at various TEOS concentrations.
hybrid sol containing 0.6 mol/L TEOS, had an increased LOI of 29.8 % and a decreased char length of 13.2 cm. These results indicate that the hybrid silica sol coatings are effective to heighten the FR property of cotton. In order to quantify the synergistic FR effect between phosphorusand silicon-containing components on cotton fabric, the synergism effectiveness (SE) parameter based on the LOI results was calculated according to the previously reported literatures [32,33]. As shown in Table 3, the calculated SE for Cotton-3 sample was 1.05, which is higher than 1.0, indicating that the silicon- and phosphorus-containing components have synergistic FR ability together. Unfortunately, the hybrid silica sol system is not durable on cotton fabric due to the water solubility of PA and the lack of chemical interaction between the FR agents and cotton fiber. The treated cotton fabric may find its potential application in sofa fabric, mattress, electric blanket, wall covering and curtains for upholstery, movie screen and proscenium curtain in public, the disposable protective clothing for work wear and so on. In future work, some measures should be taken to improve the washing resistance of the coated cotton fabrics.
3.2.5. Flammability According to GB/T 17591-2006, there are two different ratings of the FR furnishing textiles based on the vertical burning test: (1) B1 rating: char length ≤ 15 cm, after flame time ≤ 5 s, after glow time ≤ 5 s; (2) B2 rating: char length ≤ 20 cm, after flame time ≤ 15 s, after glow time ≤ 15 s. The uncoated cotton fabric was lit easily and then consumed completely with very little gossamery residue left, and it also displayed after flame and afterglow phenomenon. As a result, the uncoated one had the highest char length of 30 cm and no rating was reached; it also had the lowest LOI of 18.0 %, displaying low FR performance. Evidently, the hybrid silica sol coatings affected the burning behavior of cotton in a positive way. It exhibited effective FR ability, resulting in no burning of the coated fabrics once removal the flame source. As shown in Fig. 9, higher weight gain was obtained by increasing TEOS concentration, leading to the increased Si content and almost unchanged P content of the coated cotton fabrics (discussed later); the coated samples also showed elevated FR performance as demonstrated by the increased LOI and reduced char length. All of the coated cotton fabrics reached B1 rating (char length ≤ 15 cm) in the vertical burning test. For example, Cotton-3 sample, for which the Table 2 PCFC and smoke density parameters of the cotton fabrics. Sample
Control Cotton-1 Cotton-2 Cotton-3
PCFC
Smoke density o
pHRR (W/g)
THR (kJ/g)
Tmax ( C)
Char residue (%)
Dsmax
229.1 ± 6.8 91.8 ± 3.5 78.0 ± 2.6 60.3 ± 1.3
11.7 ± 0.1 3.2 ± 0.2 2.8 ± 0.1 2.3 ± 0.1
357.0 ± 5.3 274.3 ± 1.4 272.8 ± 1.5 269.5 ± 1.1
8.8 ± 0.6 31.9 ± 0.2 33.1 ± 0.3 34.7 ± 0.3
72.2 ± 3.7 21.7 ± 1.4 20.7 ± 1.1 19.6 ± 1.3
6
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Table 3 LOI of the untreated and treated cotton fabrics. Sample
LOI (%)
Control Cotton-PA Cotton-Sol Cotton-3 SEa (Cotton-3)
18.0 28.5 18.7 29.8 1.05
a Synergistic effectiveness parameter from LOI data.
3.3. Char residue analyses As described above, the presence of the coatings altered the combustion mode of cotton via charring action and enhanced the amount of char residue after burning. A small piece of gossamery residue was left, and they exhibited sloppy and fragile features (Fig. 10). As for the coated samples, the residues were integral, and the original texture was also well maintained after burning, responding to the high thermal resistance of the coated fibers. Additionally, for Cotton-3 sample, intumescent bubbles were also found in the char residue. As shown in Fig. 10, Si and P elements were homogeneously distributed and finely dispersed on and within the char residues. The content of these two elements showed a similar changing trend before and after burning (Fig. 11). The Si content experienced a significant increase by increasing TEOS dosage, whereas P content had little change. It is worth to note that the P and Si content of the samples increased significantly after burning, suggesting that these elements acted in the solid phase and participated in forming the char layer. As a conclusion, the coated samples released fewer pyrolysis products than the uncoated one because of the thermally insulating action of
Fig. 11. Si and P content of the cotton fabrics and corresponding char residues determined by ICP-OES.
phosphorus- and silicon-based char layer, and thus obtained good flame retardancy. 3.4. Tensile strength and whiteness of the fabrics Tensile strength and whiteness of the coated cotton fabrics were evaluated. As shown in Table 4, the whiteness of the cotton fabrics had little decrease after the coating treatment because the citric acid treatment contributes to the yellowing of cotton fabric. The treatment also decreased the tensile strength of cotton fabric as PA can hydrolyze the ether bond of cellulose unites at weak acid and high-temperature curing condition. Besides, the crosslinking reaction between cellulose
Fig. 10. SEM micrographs of the char residues after vertical flammability tests. 7
Progress in Organic Coatings 141 (2020) 105539
X.-W. Cheng, et al.
Table 4 Tensile strength, elongation and whiteness of the cotton fabrics. Sample
Whiteness
Tensile strength (N)
Elongation (%)
Control Cotton-1 Cotton-2 Cotton-3
80.0 77.4 75.7 75.2
643.5 556.4 535.0 530.8
6.15 8.72 8.42 8.32
[2] G. Malucelli, F. Carosio, J. Alongi, A. Fina, A. Frache, G. Camino, Materials engineering for surface-confined flame retardancy, Mat. Sci. Eng. R 84 (2014) 1–20. [3] L.S. Birnbaum, D.F. Staskal, Brominated flame retardants: cause for concern? Environ. Health Persp. 112 (2004) 9–17. [4] I. Van Der Veen, J. De Boer, Phosphorus flame retardants: properties, production, environmental occurrence, toxicity and analysis, Chemosphere 88 (2012) 1119–1153. [5] L. Costes, F. Laoutid, S. Brohez, P. Dubois, Bio-based flame retardants: when nature meets fire protection, Mat. Sci. Eng. R 117 (2017) 1–25. [6] G. Laufer, C. Kirkland, A.B. Morgan, J.C. Grunlan, Intumescent multilayer nanocoating, made with renewable polyelectrolytes, for flame-retardant cotton, Biomacromolecules 13 (2012) 2843–2848. [7] J. Alongi, R.A. Carletto, A. Di Blasio, F. Carosio, F. Bosco, G. Malucelli, DNA: a novel, green, natural flame retardant and suppressant for cotton, J. Mater. Chem. A 1 (2013) 4779–4785. [8] F. Carosio, G. Fontaine, J. Alongi, S. Bourbigot, Starch-based layer by layer assembly: efficient and sustainable approach to cotton fire protection, ACS Appl. Mater. Inter. 7 (2015) 12158–12167. [9] X.W. Cheng, J.P. Guan, G. Chen, X.H. Yang, R.C. Tang, Adsorption and flame retardant properties of bio-based phytic acid on wool fabric, Polymers 8 (2016) 122. [10] X. Wang, M.Q. Romero, X.Q. Zhang, R. Wang, D.Y. Wang, Intumescent multilayer hybrid coating for flame retardant cotton fabrics based on layer-by-layer assembly and sol-gel process, RSC Adv. 5 (2015) 10647–10655. [11] Z.F. Li, C.J. Zhang, L. Cui, P. Zhu, C. Yan, Y. Liu, Fire retardant and thermal degradation properties of cotton fabrics based on APTES and sodium phytate through layer-by-layer assembly, J. Anal. Appl. Pyrolysis 123 (2017) 216–223. [12] C.K. Kundu, W. Wang, S. Zhou, X. Wang, H. Sheng, Y. Pan, L. Song, Y. Hu, A green approach to constructing multilayered nanocoating for flame retardant treatment of polyamide 66 fabric from chitosan and sodium alginate, Carbohydr. Polym. 166 (2017) 131–138. [13] Y. Ren, T. Huo, Y. Qin, X. Liu, Preparation of flame retardant polyacrylonitrile fabric based on sol-gel and layer-by-layer assembly, Materials 11 (2018) 483. [14] Z. Jiang, C. Wang, S. Fang, P. Ji, H. Wang, C. Ji, Durable flame-retardant and antidroplet finishing of polyester fabrics with flexible polysiloxane and phytic acid through layer-by-layer assembly and sol-gel process, J. Appl. Polym. Sci. 135 (2018) 46414. [15] P. Jamshidi, D. Ghanbari, M. Salavati-Niasari, Sonochemical synthesis of La(OH)3 nanoparticle and its influence on the flame retardancy of cellulose acetate nanocomposite, J. Ind. Eng. Chem. 20 (2014) 3507–3512. [16] D. Ghanbari, M. Salavati-Niasari, S. Khaghani, F. Beshkar, Preparation of polyvinyl acetate (PVAc) and PVAc-Ag-Fe3O4 composite nanofibers by electro-spinning method, J. Clust. Sci. 27 (2016) 1317–1333. [17] J. Alongi, F. Carosio, G. Malucelli, Current emerging techniques to impart flame retardancy to fabrics: an overview, Polym. Degrad. Stabil. 106 (2014) 138–149. [18] A.M. Grancaric, L. Botteri, J. Alongi, A. Tarbuk, Silica precursor as synergist for cotton flame retardancy, Int. J. Cloth. Sci. Tech. 28 (2016) 378–386. [19] C. Samba-Fouala, J.C. Mossoyan, M. Mossoyan-Déneux, D. Benlian, C. Chanéac, F. Babonneau, Preparation and properties of silica hybrid gels containing phytic acid, J. Mater. Chem. 10 (2000) 387–393. [20] D. Qiu, P. Guerry, J.C. Knowles, M.E. Smith, R.J. Newport, Formation of functional phosphosilicate gels from phytic acid and tetraethyl orthosilicate, J. Sol-Gel Sci. Techn. 48 (2008) 378–383. [21] M.L. González-Rodrıguez, M.A. Holgado, C. Sanchez-Lafuente, A.M. Rabasco, A. Fini, Alginate/chitosan particulate systems for sodium diclofenac release, Int. J. Pharmaceut. 232 (2002) 225–234. [22] F. Sabri, K. Berthomier, A. Marion, L. Fradette, J.R. Tavares, N. Virgilio, Sodium alginate-grafted submicrometer particles display enhanced reversible aggregation/ disaggregation properties, Carbohydr. Polym. 194 (2018) 61–68. [23] G. Jiang, J. Qiao, F. Hong, Application of phosphoric acid and phytic acid-doped bacterial cellulose as novel proton-conducting membranes to PEMFC, Int. J. Hydrogen Energ. 37 (2012) 9182–9192. [24] C.Q. Yang, FT-IR spectroscopy study of the ester crosslinking mechanism of cotton cellulose, Text. Res. J. 61 (1991) 433–440. [25] G. Lawrie, I. Keen, B. Drew, A. Chandler-Temple, L. Rintoul, P. Fredericks, L. Grøndahl, Interactions between alginate and chitosan biopolymers characterized using FTIR and XPS, Biomacromolecules 8 (2007) 2533–2541. [26] R. Al-Oweini, H. El-Rassy, Synthesis and characterization by FTIR spectroscopy of silica aerogels prepared using several Si(OR)4 and R′′Si(OR′)3 precursors, J. Mol. Struct. 919 (2009) 140–145. [27] A. Fidalgo, L.M. Ilharco, The defect structure of sol-gel-derived silica/polytetrahydrofuran hybrid films by FTIR, J. Non-Cryst. Solids 283 (2001) 144–154. [28] B. Lebeau, J. Maquet, C. Sanchez, F. Beaume, F. Lauprêtre, Structural and dynamical studies of hybrid siloxane–silica materials, J. Mater. Chem. 7 (1997) (1997) 989–995. [29] C. Chung, M. Lee, E.K. Choe, Characterization of cotton fabric scouring by FT-IR ATR spectroscopy, Carbohydr. Polym. 58 (2004) 417–420. [30] A.A. Faroq, D. Price, G.J. Milnes, A.R. Horrocks, Thermogravimetric analysis study of the mechanism of pyrolysis of untreated and flame retardant treated cotton fabrics under a continuous flow of nitrogen, Polym. Degrad. Stabil. 44 (1994) 323–333. [31] D. Price, A.R. Horrocks, M. Akalin, A.A. Faroq, Influence of flame retardants on the mechanism of pyrolysis of cotton (cellulose) fabrics in air, J. Anal. Appl. Pyrol. 40 (1997) 511–524. [32] A.R. Horrocks, G. Smart, S. Nazaré, B. Kandola, D. Price, Quantification of zinc hydroxystannate** and stannate** synergies in halogen-containing flame-retardant polymeric formulations, J. Fire Sci. 28 (2010) 217–248. [33] A.M. Grancaric, L. Botteri, J. Alongi, G. Malucelli, Synergistic effects occurring between water glasses and urea/ammonium dihydrogen phosphate pair for enhancing the flame retardancy of cotton, Cellulose 22 (2015) 2825–2835.
fibers by using citric acid may also reduce the tensile strength of cotton fabric. However, a decrease of 17.0 % for tensile strength can be acceptable in the wet processing of cotton fabric. 4. Conclusions The prepared organic-inorganic hybrid silica sol system based on naturally occurring materials effectively improved the FR performance of cotton. In vertical burning tests, the coated cotton fabrics with the hybrid silica sols could extinguish the flame, and the residue chars still preserved the texture structure, demonstrating the good efficiency of the FR coatings on cotton. The high phosphorus and silicon content of the char residues and intumescent bubbles formed on the surface of char residues suggested an evident intumescent charring mechanism for the coated cotton. The samples showed anticipated degradation at a lower temperature but higher thermal stability at higher temperatures. In addition, the heat and smoke release ability of the coated cotton were greatly reduced, compared to the uncoated one, as a result of the barrier and insulating behavior of phosphorus-containing char residues. The present hybrid silica sol system is considered as an effective and environmentally sustainable FR approach for textile materials. However, further investigations are needed to enhance the washing resistance of the hybrid sol system with the aim of broadening its application fields. Data availability statement The authors confirm that the data supporting the findings of this study are available within the article. Author statement Xian-Wei Cheng performed the experiments, analyzed the data and wrote the manuscript. Ren-Cheng Tang proposed and supervised the work, and revised the manuscript. Jin-Ping Guan and Shao-Qiang Zhou provided lots of constructive suggestions about the work. Declaration of Competing Interest The authors declare no conflict of interest. Acknowledgements This study was funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions [No. 2018-87]. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2020. 105539. References [1] A.R. Horrocks, Flame retardant challenges for textiles and fibers: new chemistry versus innovatory solutions, Polym. Degrad. Stabil. 96 (2011) 377–392.
8