This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem., 2019, 43, 10183--10189 | 10183

Cite this: New J. Chem., 2019,

43, 10183

A facile coating with water-repellent and flame-retardant properties on cotton fabric†

Jing Fu,ab Fuchao Yang,b Guopeng Chen,b Guofeng Zhang,b Can Huangb andZhiguang Guo *bc

A facile coating consisting of chitosan (CS), phytic acid (PA) and silicon dioxide (SiO2) modified with

hexamethyldisilylamine (HMDS) was fabricated with a simple repeated coating technique and applied

onto cotton fabric (CF) to achieve both superhydrophobicity and flame retardancy. SEM observation

revealed that CS/PA/HMDS–SiO2 CF has a hierarchical rough surface. ATR-FTIR characterization results

and XPS spectra verified the structure of the coating on the CF surface. The as-prepared coating on CF

showed excellent waterproofing ability, mechanical stability and thermal stability in air. A flame test indi-

cated the coating can protect cotton sample burning off. In particular, the coated CF still retained

water-repellent and flame-retardant properties to some extent even after 50 abrasion cycles with sand-

paper (400 cW). Hence, the coating on CF possesses potential application in outdoor clothing, equipment

and the military.

1. Introduction

Recently, water-repellent properties applied on cotton textilesurfaces have been broadly explored and applied in self-cleaning, oil–water separation, anti-condensation and otherfields.1,2 Superhydrophobic surfaces with a water contact anglegreater than 1501 and a sliding angle (SA) below 101 resulted inthe prevention of water absorption and a self-cleaning effect.3,4

Nevertheless, most textiles are highly flammable, and this hasled to lots of fire disasters, which brings not only massivedamage to human properties and lives worldwide but alsosevere destruction to our surrounding environment.5 Thefabrication of a flame-retardant and superhydrophobic fabricis necessary.

Fabrics with waterproof and flame-retardant coatings arewidely utilized in civilian and military applications,6 such asautomotive interiors, outdoor clothing and tents, firefighterapparel and military garments. Among the many flame-retardant treatments of fabrics, surface modification is the

most effective and convenient strategy.7–9 The treatment ofthe surface of fabrics does not modify the intrinsic properties(e.g., mechanical properties) of the textiles and can be purpo-sely integrated with functional components to achieve multiplefunctions.10

Various waterproof and flame-retardant coatings on fabricsurfaces have been fabricated by researchers in recent years.The Wei group synthesized linear a,o-di-[(4-butoxypiperazin-1-yl)phosphinic acid methyl ether]-terminated polysiloxane andcyclic-shaped poly[tetra(tetramethylcyclosiloxylpiperazin)phosphinicacid methyl ether] macromolecules as flame retardants.11,12 Thesemultifunctional copolymers have both flame retardant and hydro-phobic properties for cotton fabrics, and showed excellentdurability. Chen et al. reported flame-retardant and super-hydrophobic cotton textiles, which were fabricated with intro-duction of fluorine–silicon-containing polymer networks and9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide onto thesurface of cotton fabric.13 The Chen team have demonstratedthe fabrication of flame-retardant and self-healing superhydro-phobic cotton fabric by using a solution-dipping method thatinvolves the sequential deposition of a trilayer of branchedpoly(ethylenimine), ammonium polyphosphate, and fluorinateddecyl polyhedral oligomeric silsesquioxane.14 These cases involvethe participation of organosilicon, which is a smoke suppressorand low surface energy material. The Si–O–Si structure could beincorporated into the flame-retardant isolation layer and enhancestability.15,16 Si–R is a hydrophobic group, which can endow cottonfabric with water repellency.17 However, these approaches still havesome disadvantages, such as complicated and time-consumingoperations and the use of expensive and toxic compounds.

a School of Chemistry and Environment Engineering, Wuhan Institute of Technology,

Wuhan 430205, People’s Republic of Chinab Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials

and Ministry of Education Key Laboratory for the Green Preparation and

Application of Functional Materials, Hubei University, Wuhan 430062, People’s

Republic of Chinac State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China.

E-mail: zguo@licp.cas.cn; Fax: +86-931-8277088; Tel: +86-931-4968105

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj02240f

Received 1st May 2019,Accepted 28th May 2019

DOI: 10.1039/c9nj02240f

rsc.li/njc

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Considering green chemistry and environmental issues, thecoating treatments of textiles that come in close contact withthe body have created a desire to use ‘‘green’’ materials.Intumescent coating is a subset of flame-retardant technologywith a simple and environmentally friendly preparation com-monly used to protect cotton textiles.18 We chose the chitosan(CS)–phytic acid (PA) system as an intumescent flame-retardantcoating to apply in our work. CS is an abundant, natural andcationic biopolymer with nontoxicity, biodegradability andantibacterial activity properties.19 The natural, non-noxious,renewable and phosphorus-rich characteristics make PA anattractive green product which can be coupled with CS to forman eco-friendly flame-retardant system.20,21 In this study, wefabricated a flame-retardant and water-repellent coating oncotton fabric (CF) using pollution-free compounds and by thesimple repeated coating technique. The flame-retardant coat-ing consists of a bilayer of CS/phosphoric acid (PA) fabricatedwith the formation of a polyelectrolyte complex (PEC). Subse-quently, a hydrophobic layer of –Si(CH3)3 functionalized SiO2

nanoparticles was deposited on the PEC surfaces. The water-repellent properties of the textile depended on the non-fluorinated hydrophobic SiO2 nanoparticles.

2. Experimental2.1 Materials

Tetraethyl orthosilicate (TEOS, 98%), hexamethyldisilylamine(HMDS, 98%) and PA (50 wt% solution in H2O) were purchasedfrom Shanghai Macklin Biochemical Co. Ltd (Shanghai, China).CS (deacetylation degree Z95%; the viscosity-average molecularweight (Mv) of CS was 4.7 � 105 g mol�1) was acquired fromAladdin Industrial Co. Ltd (Shanghai, China). Acetic acid, ammoniasolution (25–28%) and ethanol were obtained from SinopharmChemical Reagent Co. Ltd (Shanghai, China). Deionized water wasused for all the experiments. All reagents were used as received andwithout further purification. The CF was a commercial product andcut into 3 cm � 3 cm pieces before being used.

2.2 Preparation of CS solution and PA solution

0.5 g CS powder was magnetically stirred in 20 mL of 2% (v/v)acetic acid solution for 2 h at ambient temperature until thepowder was completely dissolved to form the CS solution.PA solution was prepared at 5.0 wt% concentration usingdeionized water.

2.3 Preparation of HMDS-modified SiO2 homogeneoussolution

The SiO2 sol was synthesized by using TEOS as the precursorcatalyzed with dilute ammonia in ethanol at room temperature.22

Dilute ammonia solution was prepared by adding 1 mL ammoniasolution into 9 mL deionized water. Briefly, TEOS (3.5 mL) wasadded into 25 mL of ethanol under magnetic stirring with heatingto 35 1C in a water bath, and the pH value was adjusted to 8–9 with5% dilute ammonia solution. After continuously stirring for 3.5 h,HMDS (4.5 mL) and ethanol (4.5 mL) were then added into the

above SiO2 sol dispersion under stirring and the reaction waskept for additional 1.5 h to obtain a dispersion of HMDS-modified SiO2 (labeled as HMDS–SiO2) homogeneous solution(Scheme 1). The resulting white dispersion was cooled to roomtemperature and aged for 24 h.

2.4 Fabrication process of flame-retardant and water-repellent coating on the cotton fabric

The CF was hyperacoustically rinsed with deionized water andethanol, and dried in an oven at 60 1C for 20 min before usage.The pristine CF was immersed in CS solution for 10 min toobtain a layer of polycations and dried in an oven at 60 1C(coded as CS CF). The weight per unit area of sample increasedabout 1.5 � 10�3 g cm�2 after this step. Then, the CS CF wassoaked in PA solution for 10 min which can pair with CS. Thequantity of PA absorbed was about 2.0 � 10�3 g cm�2 aftermeasuring. The dried textile was called CS/PA CF after this step.The low-surface-energy material HMDS–SiO2 was finally depos-ited on the textile surface to generate superhydrophobicity. TheCS/PA/HMDS–SiO2 CF was prepared by soaking CS/PA CF inHMDS–SiO2 homogeneous solution for 10 min. The massiveHMDS–SiO2 particles were deposited on the surfaces and theweight per unit area increased about 0.8 � 10�3 g cm�2. It isworth noting that the CF became slightly more rigid after thecoating treatment. The procedure for fabricating the water-repellent and flame-retardant CS/PA/HMDS–SiO2 CF is shownin Scheme 1.

2.5 Characterization

The surface morphologies of the fabrics were characterized byusing a JSM-5601LV scanning electron microscope (SEM) oper-ating at 5 kV. Before analysis, the samples were sputtered withgold under high vacuum to improve their conductivity. Atte-nuated total reflectance (ATR) Fourier transform infrared(FTIR) spectra were collected at room temperature in the rangeof 4000–400 cm�1 with a Thermo Scientific Nicolet iS10 FTIRspectrophotometer. X-ray photoelectron spectroscopy (XPS) wasconducted with a Thermo Scientific ESCALAB 250 Xi. Contactangle (CA) and SA were assessed by a contact angle meter(POWEREACH JC2000D1), and the corresponding dropletvolumes were 5 mL and 10 mL at 25 1C, respectively. Thermalstability tests were conducted with a thermogravimetric analyzer(Mettler 1100LF) under an air atmosphere from 30 to 800 1C at aheating rate of 10 1C min�1. Combustion behavior was determinedby a vertical flammability test using an automatic vertical

Scheme 1 Schematic illustration of the preparation process for CS/PA/HMDS–SiO2 CF.

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flammability cabinet (Qingdao SHANFANG ZR-02). Optical photo-graphs and videos were obtained with a Huawei Mate 10.

3. Results and discussion

We aimed to design an environmentally friendly coating systemcontaining CS and PA which can impart flame retardancy, andHMDS–SiO2 acts as a hydrophobic component. CS is a linearnatural polysaccharide backbone with abundant functionalhydroxyl and amino groups (Scheme 2a).19 In dilute acidsolution, CS displayed a cationic nature and paired with manyanion polyelectrolytes to prepare functional PECs with theprotonation of NH2 groups. PA contains 6 phosphate groupsand 12 hydroxyl groups in very close proximity, enabling it tointeract with almost all kinds of positively charged compounds(Scheme 2b).20 For this reason, PA can interact with CS to formPEC through ionic bonds on cotton surface by a simpledeposition method (Scheme 2c).23

Under alkaline conditions, hydrophilic SiO2 nanoparticleswere obtained from the hydrolysis and the self-condensationreaction of TEOS.24 These nanoparticles contained abundant–OH groups on their surface. These groups reacted with HMDSto convert some of the hydrophilic silanol groups (Si–OH) tohydrophobic groups through the formation of �Si–O–Si(CH3)3

linkages (Scheme 2d).25 TEOS and HMDS hydrolytic reactioncould lead to the formation of semitransparent hydrophobicSiO2 sols. Based on the large amount of methyl groups coveredon the surfaces, the hydrophilic SiO2 nanoparticles were con-verted into hydrophobic nanoparticles.

3.1 Surface morphology

The surface morphologies of CF were observed by using SEM asshown in Fig. 1. From Fig. 1a, the pristine CF fibers wereusually smooth and round, on which was present naturalstructure of grain and distortion. Due to the excellent film-forming ability of CS, there was an integral film on the surface

of CS CF, as shown in Fig. 1b (left). However, this does notcause great changes of the surface morphologies compared tothe pristine CF, which is shown in Fig. 1b (right). The SEMimage shown in Fig. 1c (left) illustrates that the CS/PA PECcoating deposited and grew on the CS/PA CF surface. Thiscoating changed the surface morphology (Fig. 1c, right). TheSEM images of the pristine CF and CS/PA/HMDS–SiO2 CF(Fig. 1d, left) showed no significant differences. However,Fig. 1d (right) illustrates the surface of coated fibers becamerough because the fibers were fully covered with sphericalparticles after treatment with HMDS–SiO2. On the other hand,it is clearly revealed that the last sample has a typical nano- andmicroscale hierarchical structure. The EDS results indicatedthat the CS/PA/HMDS–SiO2 CF mainly contains C, O, N, P andSi elements (Fig. 1e). Moreover, the EDS elemental mappingof the CS/PA/HMDS–SiO2 CF is shown in Fig. 1f, in whichthe abovementioned elements were well detected. The resultsuggested that CS, PA and HMDS–SiO2 were successfullydeposited on the CF surface.

3.2 ATR-FTIR

To confirm the functional groups, the ATR-FTIR spectra of thepristine CF, CS CF, CS/PA CF and CS/PA/HMDS–SiO2 CF wereobtained and are presented in Fig. 2a. By comparison of theFTIR spectra of the CF before and after modification, thecharacteristic peak at 3329 cm�1 corresponded to the stretchingpeak of –OH in cellulose, and the bands at around 1242 cm�1

and 1712 cm�1 were ascribed to the stretching vibration of C–Oand flexural vibration of CQO in cellulose, respectively. Thesignals of CS CF were difficult to be precisely distinguishedfrom pristine CF due to some similar chemical structures of CSand cellulose.26 For the spectrum of CS/PA CF, the absorptionpeaks at 1164 cm�1 and 981 cm�1 were associated with thestretching vibrations of PQO, O–P–O and P–O groups.27,28

Scheme 2 Schematic illustrations of (a) CS and (b) PA. (c) Schematicillustration for the formation of PEC between CS and PA. (d) Schematicillustration for the preparation of HMDS–SiO2.

Fig. 1 SEM images of (a) pristine CF, (b) CS CF, (c) CS/PA CF, and (d) CS/PA/HMDS–SiO2 CF. (e) EDS spectrum of the CS/PA/HMDS–SiO2 CF.(f) SEM-EDS elemental mapping of the CS/PA/HMDS–SiO2 CF.

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In comparison, a new absorption band at 1540 cm�1 corre-sponded to the NH3

+ vibrational mode of the PEC, which isformed between CS and PA.29,30 As shown in the spectrum ofthe CS/PA/HMDS–SiO2 CF, the strong characteristic bandsappearing at 840 cm�1 and around 1053 cm�1 were attributedto the Si–C and Si–O–Si in HMDS–SiO2 nanoparticles,31 respec-tively. Overall, the ATR-FTIR spectra revealed PEC formationbetween CS and PA, and the presence of hydrophobic SiO2

nanoparticles which were produced by the hydrolysis-condensation reaction of TEOS and HMDS.

3.3 XPS

The surface composition of the fabric samples was obtainedfrom XPS spectra, and element content (Fig. 2c) was calculatedfrom areas of the bands in the spectra. Fig. 2b indicates that Cand O were detected on the pristine CF belonging to thecellulose fibers, whereas N, P and Si originating from thecoating were detected on the sample. In the case of CS CFand CS/PA CF, the CS coating was confirmed by the presence ofthe N 1s peak, which was detected at a binding energy of 402 eV(Fig. 2d). However, the N 1s peak was not apparent in the XPSspectrum of the CS/PA/HMDS–SiO2 CF. The reason might berelated to the following two aspects. The first is the relativedecrease of the N element content with the increase in coatingon the CF surface, and the other point is the CS coating wascompletely covered after treatment with the HMDS–SiO2 layer.In addition to C and O, the P signal located at 133 eV belongingto the PA coating on the CS/PA CF (Fig. 2e) and CS/PA/HMDS–SiO2 CF surfaces (Fig. 2b) was observed. Si was detected on theCS/PA/HMDS–SiO2 CF (Fig. 2f), which confirmed the presenceof the hydrophobic SiO2 nanoparticles on the fabric surface.

3.4 Wettability

The wettability was determined by measuring the CAs and SAsof samples directly. Fig. 3a displays the state of a water dropdeposited on the cotton samples. There was no obvious differ-ence between the uncoated and coated cotton in terms ofappearance. The left-hand image is the pristine CF, and thedrop was absorbed quickly. However, the drop maintained a

near sphere on the right-hand CS/PA/HMDS–SiO2 CF surface.The CA reached 150 � 21 (Fig. 3a inset), and the SA of thecoated cotton sample was about 51 (Fig. 3b and Movie S1, ESI†).Therefore, the CS/PA/HMDS–SiO2 CF can be called an almostsuperhydrophobic surface. Based on classical interface wett-ability theory,32,33 the superhydrophobic properties are relatedto the hierarchical micro-/nanostructures and the ordinary low-surface-energy compound of the material surface. Judging fromthe SEM image (Fig. 1d), secondary structures of the coatedcotton can be observed. Due to these secondary structures, thesurface possesses a certain roughness. In addition, the surfacewas covered with plenty of hydrophobic groups of RSi–O–Si(CH3)3 which were produced from the hydrolysis of TEOS andHMDS (Scheme 2d). According to the above two aspects, themodification of the cotton surface with HMDS–SiO2 led tosuperhydrophobic properties. The chemical durability of acoating is an important index for superhydrophobic surfaces.In order to evaluate how the wettability of the coating variedwith the pH value, the CA was measured after treatment withcorrosive droplets. As shown in Fig. 3c, the CA of the waterdroplets of pH from 1 to 14 on the surface of superhydrophobicsample was in the vicinity of 1501. The result indicated that thewettability of CS/PA/HMDS–SiO2 CF had chemical durabilitytoward corrosive droplets. It is well known that mechanicalstability is another significant indicator for the functionof a coating because it is very easy to damage in practicalapplications.34 In general, the hierarchical micro-/nanostruc-tures of superhydrophobic surface are relatively weak and couldbe easily destroyed. Therefore, an abrasion test was conductedon the CS/PA/HMDS–SiO2 CF for validating the mechanicalstability. The sample was placed on sandpaper (400 cW) andloaded with 200 g weight, which was dragged by an iron folderfor a length of 10 cm (Fig. 3d). This operation was defined asone abrasion cycle. And then, this process was repeated and theCA of the coating measured every five abrasion cycles. As shownin Fig. 3e, the CA was recorded to show the variation ofwettability. It is obvious that the CA maintained a relativesteady value (150 � 41) before 40 abrasion cycles and quicklydecreased below 1401 after 40 cycles. In order to confirm the

Fig. 2 (a) ATR-FTIR spectra, (b) XPS survey spectra, and (c) elementcontent of pristine CF, CS CF, CS/PA CF and CS/PA/HMDS–SiO2 CF. XPSsurvey spectra of (d) N 1s of CS/PA CF, (e) P 2p of CS/PA CF, and (f) Si 2p ofCS/PA/HMDS–SiO2 CF.

Fig. 3 (a) A water drop on the pristine CF and CS/PA/HMDS–SiO2 CFsurface (inset is a CA image of CS/PA/HMDS–SiO2 CF). (b) SA images ofCS/PA/HMDS–SiO2 CF. (c) CA changes with pH value. (d) Schematic imageof the abrasion test. (e) CA changes during friction test. (f) SEM images ofCS/PA/HMDS–SiO2 CF after 50 abrasion cycles.

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surface morphologies of the coated cotton after friction tests,SEM was conducted after 50 cycles. Fig. 3f shows that thecoated sample fibre was severely damaged, and the coatingwas inevitably destroyed. This means that the secondary struc-tures were destroyed and the superhydrophobic property dis-appeared. But the surface of the damaged sample still kept thehydrophobic property (CA 4 1201), which showed that therewas a portion of coating left on the sample such that the waterdrop cannot completely wet the sample. Furthermore, the fire-resistant property of the damaged CS/PA/HMDS–SiO2 CF wasstudied and is discussed later (Fig. 6c). The test indicated thecoating can resist friction to some extent.

3.5 Thermal stability in air

Thermal stability of the uncoated and coated cotton fabrics wasinvestigated by thermogravimetric analysis in air, as shown inFig. 4. This process was done in air atmosphere at a heatingrate of 10 1C min�1 from 30 to 800 1C. For the pristine CF, itexhibited weight loss of 5.9% at 30–336 1C and 80.2% at336–448 1C (Fig. 4a), which was mostly due to the stage of cellulosedegradation.35 The thermal degradation of CS/PA/HMDS–SiO2 CFstarted at about 200 1C, which is the temperature at which thesample weight loss began (Fig. 4b). Onset weight loss of 7%occurred in the temperature range of 200 1C to 250 1C due todecomposition and oxidation of the CS main chain.36,37 Thesample exhibited weight loss of 21.3% at 250–400 1C, and the rateof weight loss showed a distinct decline compared with theuncoated fabric. This might be due to the phosphoric acidproduced by the PA layer with increasing temperature, whichcan accelerate the dehydration of cellulose and CS to form a charlayer and inhibit volatile production.38 In addition, the largeamount of water formed in this process will tend to quench theflame. The remaining residues at 600 1C were 10.72% and 29.66%for the pristine CF and CS/PA/HMDS–SiO2 CF samples, respec-tively. It is evident that the residues of the coated samples wereabout 3 times higher than that of the uncoated sample. Thisphenomenon is attributable to the synergistic effect39 between theorganosilicon and the PA coating on the CS-covered CF. Moreover,the sample coating affected by temperature was tested. The CS/PA/HMDS–SiO2 CF was placed into an oven with different temperature(20 1C to 120 1C for 1 h). The wettability of the sample surface wasmeasured soon after heating. Fig. 4c shows that, at under 120 1C,temperature cannot generate any huge effect on the wettability ofthe coating which means that this CS/PA/HMDS–SiO2 coating canmaintain its surface characteristic at under 120 1C. Observed fromthe TG curve (Fig. 4b), the coating can maintain its surfacecharacteristics at 200 1C. And this is enough for everyday needs.

3.6 Water repellence, self-cleaning, and delaying-icingproperties

As a superhydrophobic surface, a spherical water droplet couldslide from the CS/PA/HMDS–SiO2 CF surface if the rolling anglewas less than 101. Besides the water droplet, the surface alsopresents a great resistance for a water column. Fig. 5a (Movie S2,ESI†) shows that, when the water column ejects to the surface(sample was put on glass with an inclination angle of about 301),it was bounced out without any spread and did not wetthe surface of the sample. This phenomenon indicated theCS/PA/HMDS–SiO2 CF surface has excellent water repellence. Inpractical applications, the CF is often contaminated with sand,clay, and muddy water. As a superhydrophobic surface, airpockets get trapped in the interface between solid and liquid,which lead to the water droplets easily carrying away contami-nants, resulting in self-cleaning property. As shown in Fig. 5b–dand Movies S3, S4 (ESI†), we dropped muddy water onto thesurface of the pristine CF and the CS/PA/HMDS–SiO2 CF whichwere tilted at 301, to confirm that the CS/PA/HMDS–SiO2 CF wasable to remain clean. Muddy water was added dropwise ontothe surface by a dropping pipette. The pristine CF was con-taminated by the polluted water rapidly and the clay stuck onthe surface (Fig. 5c). As shown in Fig. 5d, the muddy watercould effortlessly roll off along the CS/PA/HMDS–SiO2 CFsurface which took away the pollutant. Comparing with thepristine CF, the superhydrophobic surface was not wetted andwas clean in the whole process (Fig. 5b). These antifoulingexperiments tested the self-cleaning and antifouling propertiesof the CS/PA/HMDS–SiO2 CF. Moreover, the delaying-icingperformance of the superhydrophobic sample was tested underthe overcooled condition in a fridge freezer (below 10 1C). Inorder to ensure the sample temperature was consistent with theenvironment, the samples were stuck on glass and placed in thefreezer for 5 minutes before depositing the water droplet(10 mL) on the surface. For the pristine CF, water slowly wettedthe cotton and completely iced after 133 s, whereas the droplet

Fig. 4 TG and DSC curves of (a) pristine CF and (b) CS/PA/HMDS–SiO2

CF. (c) CA changes after thermal treatment.

Fig. 5 (a) Water repellency of the CS/PA/HMDS–SiO2 CF surface. (b) Thepristine CF and the CS/PA/HMDS–SiO2 CF after antifouling experiments.(c) Muddy water was dripped on the pristine CF surface. (d) Muddy waterwas dripped on the CS/PA/HMDS–SiO2 CF surface. (e) The process ofdelaying icing on the CS/PA/HMDS–SiO2 CF surface (inset is a magnifiedicing image).

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on the coated cotton took about 520 s to become a spheroidalice, as shown in Fig. 5e. For the CS/PA/HMDS–SiO2 CF, morethan double the time was required to freeze the water dropletcompared to the uncoated cotton. Due to the extremely smallcontact area between the water droplet and the superhydro-phobic substrates, the air layer can greatly impede heat trans-fer, which delayed the freezing process.

3.7 Flame-retardant property

In general, cotton fabric can be burned easily by fire andthe process is irreversible. Hence, flame-retardant property ofthe uncoated and coated cotton fabrics was assessed using thevertical flame testing method. The test was carried out onsamples with 25 mm � 130 mm size. A 40 mm-high fire froma gas burner was applied to ignite the tested fabrics for 3 s whilerecording the afterflame and afterglow time and then removed.It was clear that the pristine CF was consumed by the brightflame quickly and only the edges of the holder retained a fewashes with afterflame for 17 s (Fig. 6a, f and Movie S5, ESI†).Upon exposure to direct flame, the CS CF ignited immediatelyand the flame quickly spread as for the uncoated CF. The flameon the CS CF was vigorous and bright after ignition, as shownin Fig. 6b. The coated CS fabric was completely burned within21 s and remained with black ash (Fig. 6f and Movie S6, ESI†).When the CF was coated with CS and PA, no afterflameappeared during the test, which indicates that the coatinghas a certain flame-retardant effect, but was not sufficient tostop the spread of the fire (Fig. 6c, f and Movie S7, ESI†). For theCS/PA/HMDS–SiO2 CF, it exhibited a less vigorous flame thanthe CS/PA CF. Afterflame time was also reduced and afterglowwas eliminated (Fig. 6d and Movie S8, ESI†). After burning, thechar length and black area of the CS/PA/HMDS–SiO2 CF werethe smallest among the coated CF samples, whereas most of thetop of the fabric remained intact (Fig. 6f).

The flame retardancy of this cotton was attributed to highefficiency of the CS/PA layer in hindering the transportation of

heat and oxygen during the whole process and to the non-flammable property of SiO2 particles. Since pristine CF does notpossess such properties, it was easily burned. In this work, CSand PA were employed to fabricate intumescent flame-retardant coatings on CF. Typical intumescent systems containthree components: carbon donor, acid source, and gas source.These components react upon heating to generate a swollenmulticellular insulating layer that protects the underlyingmaterial from heat and flame.18 In the combustion process,CS can be easily carbonized and release inert gases, whichenable it to act as a carbon source and a blowing agent forthe intumescent flame-retardant system.20,40 From a flame-retardant perspective, PA with 28 wt% P can serve as an acidsource and deliver more active flame-retardant atoms permolecule.41,42 During combustion, combination of nitrogenand phosphorus has synergistic inhibitory effects to protectcotton sample from burning off.43,44

The CS/PA/HMDS–SiO2 CF after 50 abrasion cycles wasfurther confirmed to have flame retardancy. The CF withabrasive coating also completely stopped the flame almostimmediately after ignition leaving most of the fabric preserved,as for the CS/PA/HMDS–SiO2 CF (Fig. 6e and Movie S9, ESI†). Itwas remarkable that the damaged sample still maintained aflame-retardant character. This phenomenon demonstrates theresidual coating after abrasion still protected the cotton fromburn out. The results manifested that the CS/PA layer andHMDS–SiO2 particles of the coating can endow the cotton withflame retardancy.

4. Conclusions

In summary, we have successfully fabricated functional CF withsuperhydrophobic surface and excellent fire-resistance propertyby coating with CA/PA layer and hydrophobic silicon dioxidenanoparticles. The fabrication process can be accomplished ina beaker and without any usage of toxic reagents. The CA ofcoated CF reached 150 � 21, and the SA was about 51. For thecoated sample, the fire spread more slowly and the burningprocess only lasted 3 s. In addition to this, the coated CFexhibited considerable self-cleaning and anti-fouling character-istics. In particular, the coated CF still retained water-repellentand flame-retardant properties to a certain extent even after50 abrasion cycles with sandpaper (400 cW). The coating on thecotton satisfies the requirements of real-life applications anddisplayed relatively good survivability under mechanical fric-tion, cold and hot natural environments. Hence, the materialpossesses application potential in outdoor clothing, equipmentand the military. This work provided an eco-friendly wayto prepare water-repellent and flame-retardant CF. Furtherimprovement in mechanical properties is required for prac-tical applications.

Conflicts of interest

There are no conflicts to declare.

Fig. 6 Vertical flammability test of (a) pristine CF, (b) CS CF, (c) CS/PA CF,(d) CS/PA/HMDS–SiO2 CF, and (e) damaged CS/PA/HMDS–SiO2 CF. (f) Thecombustion residue of the uncoated CF, coated CF and damaged CF aftervertical testing.

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Acknowledgements

This work is supported by the National Nature Science Founda-tion of China (51675513, 51735013).

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