coatings

Article Ecological -Modified Geopolymeric Coating for Flame-Retarding Plywood

Yachao Wang * and Jiangping Zhao

School of Resource Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China * Correspondence: [email protected]; Tel.: +86-29-82205869

 Received: 28 June 2019; Accepted: 26 July 2019; Published: 29 July 2019 

Abstract: An ecological ammonium thiocyanate (NH4SCN)-modified geopolymeric coating was facilely prepared for flame-retarding plywood. The effect of NH4SCN on the flame resistance was preliminarily investigated using cone calorimeter (CC), scanning electron microscope (SEM), X-ray diffraction (XRD), and thermal gravimetry (TG). The results show that 1 wt.% NH4SCN as dopant is of paramount importance to generate a compact and continuous coating. The formation of a smooth, intact, and uniform-swelling siliceous layer during combustion facilitates enhanced fire resistance, evidenced by the increased fire performance index (FPI), reduced fire growth index (FGI), and 39.7% decreased value of peak heat release rate (pHRR), in comparison to those of the sample without NH4SCN. Because of the reducibility of O2-consuming NH4SCN, the compact shielding-layer containing carbonate and sulfate, as well as the release of NH3, the NH4SCN-modified geopolymeric coating exerts an enhancement on the flame-retardant efficiency.

Keywords: flame-retardancy; ammonium thiocyanate; cone calorimeter; thermogravimetry

1. Introduction The organic-inorganic halogen-free hybrid flame-retardant coating has been a hotspot, due to its excellent fireproofing property and ecology, which has become the main developing trend of the coating industry currently [1–4]. This increasing recognition identifies that silica particles offer some advantages including well-defined ordered structures, facile surface modification, and cost-effective preparation [2]. Moreover, inorganic silicate is employed as a physical filler to provide an enhanced barrier effect, and also it serves as a functional filler to enhance the structural strength and adhesion forces [3]. It is also reported to provide an improved anti-aging coating by doping TiO2 and clay nanoparticles [4]. Inspired by a report of inorganic silicate-doped coatings, an exploration of the novel flame-retardant coatings using amorphous Si(OH)4-enriched sol as the binder has been undertaken; these coatings have a self-adhering and cross-linking structure through a facile geopolymerization between the Si(OH)4 and Al(OH)4 [5]. Furthermore, previous investigations have explored the promising application to geopolymeric coating through a facile sol–gel method [6,7]. A novel inorganic coating, the geopolymer serves as a flame-retardant wood primer with a cost-effective edge and has not been widely reported on. Therefore, a multiplication of modification efforts for geopolymeric coatings is necessary to boost a sustainable, ecological, and recyclable economy on a global scale, which holds promising potential and is attracting increasing attention for research and applications.

Coatings 2019, 9, 479; doi:10.3390/coatings9080479 www.mdpi.com/journal/coatings Coatings 2019, 9, 479 2 of 11

In addition, the most prominent bottleneck for inorganic coatings has been identified as the inferior flame retardancy compared to organic coatings, ascribed to lower expansibility and heat absorptivity. To address this problem, phase change materials (PCMs) have been designed in the building envelope as a passive system to reduce heat loss owing to their latent heat storage [8], which also could be used to prepare halogen-free flame retardants [9,10]. Consequently, paraffin and expandable graphite (EG) have been used to fabricate hybrid flame retardants [11,12]. The enhancement of the flame resistance of novel geopolymeric coatings modified by a decanoic/palmitic eutectic mixture has been investigated [13]. Moreover, ammonium thiocyanate (NH4SCN), as a kind of solid-solid PCM, can be exploited as solid polymer electrolyte due to its high conductivity and amorphous phase [14]. However, the effect of NH4SCN on the flame-retarding property of coatings is a virgin area for hybrid flame retardants. Consequently, this investigation scrutinized the effect of NH4SCN on the fire resistance of hybrid geopolymeric coatings using a cone calorimeter (CC). Its microstructure was examined using scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetry (TG) to preliminarily illuminate its flame-retarding mode of action.

2. Materials and Methods

2.1. Preparation of NH4SCN-Modified Geopolymeric Coatings

2.1.1. Starting Materials The starting material for the geopolymer was gray fumed silica collected by the Linyuan company in Xi’an of China, solid powder derived from silicon-smelting, which had a Blaine specific surface area of 25 m2 g 1 and a density of 1.62 g cm 3. Analysis using X-ray fluorescence (XRF, bruker · − · − AXS Co., Karlsruhe, Germany, LTDS4 PIONEER) determined a high silica content (87.46 wt.%) as shown in Table1. Analytically pure (AP) Na SiO 9H O and KOH, which served as chemical 2 3· 2 activators, were purchased from the HongYan reagent factory in Tianjin of China. The modifier NH4SCN (AP) was obtained from the FuChen reagent factory of Tianjin in China. Hydrophobic polydimethylsiloxane (PDMS) was prepared by the Tianjin YaoHua reagent factory. The thickener and film-forming agent, polyacrylamide (PAM), was synthesized by the Shanghai reagent factory in China. EG served as the charring fortifier [15] and was fabricated by the Qingdao chemical group in China with a volume-expansion rate of 300 mL g 1 and an average particle size of 48 µm. The second-class · − flame-retarding plywood was produced in the Xi’an timber processing plant.

Table 1. Compositions of fumed silica tested using X-ray fluorescence (XRF).

Weight (wt.%) Material CaO SiO2 Al2O3 Fe2O3 MgO Na2OK2O SO3 Loss Fumed silica 0.61 87.46 6.90 1.17 0.46 0.29 0.67 0.31 2.12

2.1.2. Preparation of NH4SCN-Modified Geopolymeric Coatings The geopolymeric coatings were prepared by slowly dispersing 50 g of fumed silica into 200 mL alkali-activated solution (comprising 1 mol L 1 Na SiO and 2 mol L 1 KOH) under 60 C, which was · − 2 3 · − ◦ placed on a magnetic stirrer at 60 r min 1 [6]. After stirring for about 0.5 h, the modifiers including · − PAM (1 wt.%), PDMS (1.5 wt.%), and EG (5 wt.%) were sequentially added into the aforesaid solution during constant stirring at 60 ◦C, followed by another stirring for about 0.5 h [13]. Finally, the 0.5, 1, 1.5, 2, and 3 wt.% NH4SCN was doped into the abovementioned mixture, respectively, the hybrid coating was achieved by another vigorous stirring (1000 r min 1 for about 15 min). · − Coatings 2019, 9, 479 3 of 11

2.1.3. Preparation of Samples The samples were the plywood (100 100 4 mm3) covered by geopolymeric coatings after × × brushing the surfaces three times with an interval time of 40 min. The samples with the NH4SCN dosage of 0.5, 1, 1.5, 2, and 3 wt.% were denoted as S1, S2, S3, S4, and S5, respectively. Coatings without NH4SCN and pristine raw plywood were used as the controls, which were defined as S0 and Sr. The coating thicknesses were the same, 0.4 mm approximately.

2.2. Characterizations

2.2.1. Flame Resistance The horizontal external heat flux was 30 kW m 2 (about 625 C for complete decomposition of · − ◦ plywood) to examine the combustion parameters of samples in real time, which was conducted in a CC (ZY6243, Zhongnuo instrument company, Dongguan, China) according to BS ISO 5660-1:2015 [16], the average value was obtained from the three samples with a deviation of <5%. The heat release rate (HRR), time to ignite (TTI), the peak heat release rate (pHRR), time to pHRR (Tp), O2 concentration and smoke temperature in exhaust smoke, and time to the cessation of flaming (Tb) were recorded in real time. The fire performance index (FPI, FPI = TTI/pHRR) and fire growth index (FGI, FGI = pHRR/Tp), were the most important indexes to distinguish the flame-retardant efficiency [7,13,15], the sample with a higher FPI and lower FGI corresponded to a stronger flame resistance.

2.2.2. Microstructure Analysis The Quanta 200 SEM (FEI Co., Hillsboro, OR, USA) was employed to observe the micro-morphologies of coating residues after combustion in the CC, the operating voltage was 20 kV with a working distance of 10 mm. The sample with a superficial area of 25~30 mm2 was placed under a vacuum degree of <1.33 10 3 Pa prior to gold sputtering for observation. The D/MAX-2400 × − X-ray diffractometer with Cu Kα was used to detect mineral phases of samples, the coating residue was ground into powder for preparing the testing sample with a working voltage of 30 kV and a working current of 40 mA, the scanning speed was 5 min 1 with a range of 10 ~50 . The Mettler thermal ◦· − ◦ ◦ balance was exploited to record the TG/DSC values in real time of bare pre-dried sample coatings of about 10~20 mg, which were tested at 50~200 ◦C (the complete decomposition of NH4SCN occurred below 180 C) with a heating rate of 5 C min 1 under nitrogen (flow of 50 cm3/min) in an alumina pan. ◦ ◦ · − 3. Results

3.1. Flame-Retardant Properties

The decreased pHRR value for the specimens with NH4SCN is presented in Figure1a, the occurrence of a gradual right-shift indicates a delayed flame propagation, with the increasing content of NH SCN. S2 appears to have the lowest pHRR value of 248 kW m 2 with a 39.7% decrease 4 · − in comparison to that of S0, which determines that NH4SCN imparts an improved flame resistance to geopolymeric coatings. The combustion process of wood can be divided into four stages, namely, drying, pre-carbonization, carbonization, and combustion, and the HRR property depends on the structural stability of the wood component. The decomposition of wood occurs sequentially as light volatile and partial hemicellulose, hemicellulose, cellulose, and partial lignin [17]. The flame propagation retards or temporarily stops after 80 s, ascribed to surficial charring of plywood [18], with the increasing accumulated heat, the sharply increased HRR indicates a flashover for Sr. However, the curve of pHRR rises when the dosage of NH4SCN exceeds 1 wt.%, this indicates that excess NH4SCN is detrimental for obtaining an excellent flame retardancy, implying that an appropriate dosage of NH4SCN is beneficial to constitute an advantageous fire resistance, the reason is illustrated in Section4. Coatings 2019, 9, 479 4 of 11 Coatings 2019, 9, x FOR PEER REVIEW 4 of 11

600 110 (214, 545) (b) (181, 107) S0 (a) Sr 500 S4 105 S2 S2 (366, 97) S1

-2 400 (278, 356) S5 100 S3 S3 S4 (80, 299) 300 S1 95 S5 S0 200 Sr

HRR / kW*m HRR/ 90

100 (238, 248) oC / Smoketemperature 85

0 80 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time / s Time / s

21.0 (c)

20.8

20.6 Sr S0 / % 2

O S2 20.4 (378, 20.52) S1 S5 S4 20.2 S3

(196, 20.08) 20.0 0 100 200 300 400 500 600 Time / s FigureFigure 1. Real-time 1. Real-time combustion combustion parameters parameters of samplesof samples including including (a )(a heat) heat release release rate rate (HRR), (HRR), (b ()b smoke) smoke temperature, and (c) O2 concentration. temperature, and (c)O2 concentration.

FigureFigure1b presents 1b presents the the smoke smoke temperature temperature of of sa samplesmples during during the whole the whole process process of burning, of burning, the the obviousobvious “right “right shift” shift” of the of thesmoke smoke temperature temperature peak was peak examined, was examined, and the lowest and peak the of lowest 97 °C was peak of located on S2 at 366 s, while that of Sr was 107 °C at 181 s. The delayed and shrunk peak indirectly 97 ◦C was located on S2 at 366 s, while that of Sr was 107 ◦C at 181 s. The delayed and shrunk peak demonstrates that S2 with 1 wt.% NH4SCN exerts the highest flame resistance. However, a reverse indirectly demonstrates that S2 with 1 wt.% NH SCN exerts the highest flame resistance. However, increase and “left shift” in the smoke temperature4 peak were observed when the content of NH4SCN a reverseexceeded increase 1 wt.%, and which “left shift”is consistent in the smokewith the temperature results of HRR peak in were Figure observed 1a. Moreover, when thethe contentO2 of NHconcentration4SCN exceeded in exhaust 1 wt.%, gas of which smoke is tube consistent takes on witha similar the tendency results ofto that HRR of inthe Figure HRR, the1a. highest Moreover, the OO22 concentrationconcentration peak in exhaust of 20.52% gas appeared of smoke in S2tube at 378 takes s, while on that a similar of Sr was tendency 20.08% at to 196 that s as of shown the HRR, the highestin Figure O2 concentration1c. Because the peak O2 is of an 20.52% essential appeared element in for S2 atthe 378 burning s, while of thatsamples, of Sr wasthe higher 20.08% O at2- 196 s as shownconsumption in Figure implies1c. Because fiercer thecombustion, O 2 is an S2 essential with the element lowest forO2-consumption the burning corresponds of samples, to the the higher slowest burning, which also confirms that an appropriate dosage of NH4SCN favors the enhancement O2-consumption implies fiercer combustion, S2 with the lowest O2-consumption corresponds to the of geopolymeric coatings effectively. slowest burning, which also confirms that an appropriate dosage of NH4SCN favors the enhancement Table 2 shows an increase of Tb for the samples with NH4SCN, but an enhancement of flame of geopolymeric coatings effectively. resistance was clarified. For S2, the FPI increased from 0.14 (S0) to 0.41 s·m2·kW−1, and FGI dropped T Tablefrom 1.882 shows (S0) to an 1.04 increase kW·m−2·s of−1, indicatingb for the an samples enhanced with flame NH resistance4SCN, but [19]. an It enhancementalso implies that of S2 flame 2 1 resistanceholds wasa superior clarified. flame For resistance S2, the than FPI the increased others, therefore, from 0.14 an (S0) appropriate to 0.41 s dosagem kW of− NH, and4SCN FGI serves dropped 2 1 · · fromto 1.88 improve (S0) to the 1.04 flame-retardant kW m− s− ,efficiency indicating of angeopolymeric enhanced flamecoatings. resistance [19]. It also implies that S2 · · holds a superior flame resistance than the others, therefore, an appropriate dosage of NH4SCN serves to improve the flame-retardantTable 2. Combustion efficiency parameters of geopolymeric of samples tested coatings. by cone calorimeter (CC).

Samples TTI (s) Tb (s) Tp (s) pHRR (kW·m−2) FPI (s·m2·kW−1) FGI (kW·m−2·s−1) Table 2. Combustion parameters of samples tested by cone calorimeter (CC). Sr 18 289 214 545 0.03 2.55 2 2 1 2 1 SamplesS0 TTI58 (s) T298b (s) 219Tp (s) pHRR 411 (kW m ) FPI (s 0.14m kW ) FGI (kW 1.88m s ) · − · · − · − · − S1Sr 78 18 312 289 234 214 301 545 0.33 0.03 2.55 1.29 S2S0 102 58 356 298 238 219 248 411 0.41 0.14 1.88 1.04 S3S1 81 78 373 312 243 234 261 301 0.33 0.33 1.29 1.07 S4S2 10259 342 356 252 238 343 248 0.17 0.41 1.04 1.36 S5S3 47 81 320 373 278 243 356 261 0.13 0.33 1.07 1.28 S4 59 342 252 343 0.17 1.36 S5 47 320 278 356 0.13 1.28

TTI—time to ignite; Tb—time to the cessation of flaming; pHRR—peak heat release rate; Tp—time to pHRR; FPI—fire perfomance index, FPI = TTI/Phrr; FGI—fire growth index, FGI = pHRR/Tp. Coatings 2019, 9, x FOR PEER REVIEW 5 of 11

CoatingsTTI—time2019, 9, 479 to ignite; Tb—time to the cessation of flaming; pHRR—peak heat release rate; Tp—time to5 of 11 pHRR; FPI—fire perfomance index, FPI = TTI/Phrr; FGI—fire growth index, FGI = pHRR/Tp.

3.2. Morphologies

3.2.1. Macro-Morphology of Samples The cinereous geopolymeric coatings covered on the plywood are displayed in Figure 22a,a, nono bubbling oror peeling peeling is observed,is observed, but ebutfflorescence efflorescence is obviously is obviously seen, due seen, tothe due strong to the alkaline strong conditions alkaline conditionsderived from derived the alkali from activators. the alkali It activators. reveals the It geopolymeric reveals the geopolymeric coatings could coatings serve as could primers serve rather as primersthan finishing rather coatings than finishing due to thecoatings whitening due andto the non-transparency. whitening and With non-transparency. the increasing contentWith the of NH SCN, the phenomenon of efflorescence weakens as shown in Figure2b, and the samples take on increasing4 content of NH4SCN, the phenomenon of efflorescence weakens as shown in Figure 2b, and theuniformly samples smooth take on surfaces uniformly in Figure smoot2hc,d. surfaces However, in Figure a rough 2c,d. coating Howe surfacever, a rough was observed coating surface for S4 with was 2 wt.% NH SCN as shown in Figure2e, the strands of coating appear for S5 in Figure2f, due to the observed for4 S4 with 2 wt.% NH4SCN as shown in Figure 2e, the strands of coating appear for S5 in Figurenon-uniform 2f, due dispersion to the non-uniform and filming. dispersion and filming.

Figure 2. AppearanceAppearance of of specimens specimens before before combustion combustion including including (a) ( aS0,) S0, (b) ( bS1,) S1, (c) (S2,c) S2, (d) (S3,d) S3,(e) (S4,e) S4,(f) (S5.f) S5.

After combustion in the CC, a swollen siliceous la layeryer was generated as shown in Figure 3 3,, whichwhich protected the underlying plywood from burning effect effectively,ively, Figure3 3aapresents presentsa a thinnerthinner andand brokenbroken shielding layer. However, However, a gradually swelling intact layer presents with the increasing dosage of NH4SCNSCN in in Figure Figure 33b,b, andand FigureFigure3 3cc takes takes on on a a continuous continuous and and uniform uniform layer, layer, corresponding corresponding to to the the improved flameflame resistance. But some pores are presen presentt in Figure 3 3d,d, whichwhich mightmight bebe attributedattributed toto thethe escape of excess NH 3 derivedderived from from the the transformation transformation of NH 4SCN,SCN, evidenced evidenced by by the the fish-scale fish-scale feature as shown inin FigureFigure3 e.3e. Moreover, Moreover, non-uniform non-uniform swelling-induced swelling-induced cracks cracks spread spread rapidly rapidly for S5,for withS5, with the theincreasing increasing dosage dosage of NH of4SCN, NH4 leadingSCN, leading to a chipped to a andchipped rough and surface rough in Figuresurface3f. in This Figure demonstrates 3f. This demonstratesthat the flame resistancethat the flame mainly resistance depends onmainly the formation depends of on an the intact formation siliceous of shielding an intact layer, siliceous due to shieldingits incombustibility layer, due andto its excellently incombustibility thermal and stability. excellently thermal stability.

(a) (b) (c)

Figure 3. Cont.

Coatings 2019, 9, 479 6 of 11 Coatings 2019, 9, x FOR PEER REVIEW 6 of 11 Coatings 2019, 9, x FOR PEER REVIEW 6 of 11

(d) (e) (f) (d) (e) (f) Figure 3. Appearance of specimens after combustion including (a) S0, (b) S1, (c) S2, (d) S3, (e) S4, (f) S5. FigureFigure 3. 3. AppearanceAppearance of of specimens specimens after after combustion combustion including (a)) S0,S0, ((bb)) S1,S1, ((cc)) S2,S2, ( d(d)) S3, S3, ( e(e)) S4, S4, ( f()f) S5. S5. 3.2.2. Micro-Morphology after Combustion 3.2.2. Micro-Morphology after Combustion The amorphous spicules were dispersed on the matrix with many pores or holes as shown in The amorphous spicules were dispersed on the matrix with many pores or holes as shown in Figure 44a.a. AfterAfter dopingdoping NHNH44SCNSCN into into the the coatings, coatings, a a decrease decrease in in the the volume of of pores or holes was Figure 4a. After doping NH4SCN into the coatings, a decrease in the volume of pores or holes was observed, presenting a continuous and uniform barrier layer as shown in Figure4b. S2 exhibits observed, presenting a continuous and uniform barrier layer as shown in Figure 4b. S2 exhibits a a smoother, more homogeneous and continuous structure as shown in Figure4c, compared with smoother, more homogeneous and continuous structure as shown in Figure 4c, compared with that that of others. However, a lot of crystalline filaments and pores with an average pore diameter of of others. However, a lot of crystalline filaments and pores with an average pore diameter of approximately 5 μµm were observed as seen in Figure4 4d,d, whenwhen thethe contentcontent ofof NHNH44SCN increased to approximately 5 μm were observed as seen in Figure 4d, when the content of NH4SCN increased to 3 wt.%, they might be the channels left by NH3− escaping.escaping. 3 wt.%, they might be the channels left by NH3− escaping.

(a’) (b’)

(a) (b) (c’) (d’)

(c) (d) Figure 4. SEM images of specimens after combustion including including ( (aa)) S0, S0, ( (bb)) S1, S1, ( (cc)) S2, and ( d) S5 with Figure 4. SEM images of specimens after combustion including (a) S0, (b) S1, (c) S2, and (d) S5 with 2500×,2500 , and and the the same same images images (5000×) (5000 ) contain contain ( (a’a’)) S0, S0, ( (b’b’)) S1, S1, ( (c’c’)) S2, S2, and and ( (d’d’)) S5. S5. 2500×,× and the same images (5000×)× contain (a’) S0, (b’) S1, (c’) S2, and (d’) S5.

Coatings 2019, 9, x FOR PEER REVIEW 7 of 11 Coatings 2019, 9, 479 7 of 11 Coatings 2019, 9, x FOR PEER REVIEW 7 of 11 3.3. TG/DSC 3.3.3.3. TGTG/DSC/DSC The weight loss was essentially equal to approximately 15.5% for both S2 and S5 as shown in FigureTheThe 5, weightweightthe slight lossloss waschangewas essentiallyessentially in the weightequalequal toto loss approximatelyapproximately indirectly determines 15.5%15.5% forfor bothboth that S2 S2the andand doped S5S5 asas shownNHshown4SCN inin Figure 5, the slight change in the weight loss indirectly determines that the doped NH4SCN completelyFigure5, the dissolves slight change or decomposes, in the weight rather loss indirectly than transforms determines into that a thecomplicated doped NH structure4SCN completely through insertiondissolvescompletely or or linkagedissolves decomposes, [20]. or However,decomposes, rather than the transformsweightrather thanloss into temperaturetransforms a complicated into (WLT) a complicated structure increased through from structure 78.5 insertion to through82.2 °C or insertion or linkage [20]. However, the weight loss temperature (WLT) increased from 78.5 to 82.2 °C withlinkage the [20 increasing]. However, content the weight of NH loss4SCN. temperature According (WLT) to increasedthe TG-DTG from method 78.5 to 82.2[21],◦ Cthe with onset the with the increasing content of NH4SCN. According to the TG-DTG method [21], the onset temperatureincreasing content changed of NH slightly4SCN. while According the end-set to the temp TG-DTGerature method climbed [21], from the onset 96.5 temperatureto 102.9 °C as changed shown temperature changed slightly while the end-set temperature climbed from 96.5 to 102.9 °C as shown inslightly Figure while 5a,b. theBecause end-set the temperature evaporation climbed of free wa fromter 96.5occurs to 102.9at <105◦C °C, as the shown increases in Figure in WLT5a,b. and Because end- in Figure 5a,b. Because the evaporation of free water occurs at <105 °C, the increases in WLT and end- setthe temperature evaporation ofindicate free water that occurs the doped at <105 NH◦C,4SCN the increasesslightly restrains in WLT and the end-setvaporization temperature heat of indicate water set temperature indicate that the doped NH4SCN slightly restrains+ the vaporization heat of water involvedthat the doped in the samples, NH4SCN due slightly to strong restrains interactions the vaporization between NH heat4 and of water H2O. involved in the samples, involved in the samples, due to strong interactions+ between NH4+ and H2O. due to strong interactions between NH4 and H2O.

0.0 0.0 (a) 60.1 (b) 62.9 100 0.0 100 0.0 (a) 60.1 (b) 62.9 100 100 -0.1 -0.1

95 -0.1 95 -0.1 DTG

95 95 DTG -0.2

DTG -0.2 DTG -0.2 90 90 -0.2 Weight / % / Weight Weight / % Weight 90 90 Weight / % / Weight -0.3 Weight / % Weight -0.3

-0.3 85 -0.3 85 78.5 96.5 85 85 -0.4 82.2 102.9 -0.4 40 60 80 100 120 140 160 180 200 78.5 96.5 40 60 80 100 120 140 160 180 200 -0.4 82.2 102.9 o -0.4 40 60 80 100T / C 120 140 160 180 200 40 60 80 100T / o 120C 140 160 180 200 o T / C o T / C Figure 5. TG/DTG curves of samples including (a) S2 and (b) S5. FigureFigure 5.5. TGTG/DTG/DTG curvescurves ofof samplessamples includingincluding ((aa)) S2S2 andand ((bb)) S5.S5. Moreover, thethe heat heat absorption absorption of S5of decreased S5 decreased slightly slightly and a tinyand left-shift a tiny ofle theft-shift peak of temperature the peak Moreover, the heat absorption of S5 decreased slightly and a tiny left-shift of the peak temperatureoccurred compared occurred with compared that of S2 with as shown that of in S2 Figure as shown6. It hasin Figure been reported 6. It has that been the reported transformation that the transformationtemperature occurred of NH4 SCNcompared into thioureawith that occurs of S2 asat shownapproximately in Figure 140 6. It°C has [22,23], been reportedno endothermic that the of NH4SCN into occurs at approximately 140 ◦C[22,23], no endothermic peak appeared at peaktransformation appeared at of 140 NH °C,4SCN implying into thiourea that the occursNH4SCN at approximatelyreacted under strong140 °C alkaline [22,23], conditions.no endothermic Due 140 ◦C, implying that the NH4SCN reacted under strong alkaline conditions. Due to its high topeak its highappeared solubility at+ 140 and °C, ionized implying NH that4+, the the priorNH4 SCNreaction+ reacted between under NH strong4+ and alkaline OH− occurs conditions. with heat Due and ionized NH4 , the prior reaction between NH4 and OH− occurs with heat released, leading to a to its high solubility and ionized NH4+, the prior reaction between NH4+ and OH− occurs with− 1heat released,slight decrease leading in to absorption a slight decrease heat, evidenced in absorption by theheat, shrunken evidenced area by h the(224.1 shrunken J g 1area) of h theS5 (224.1 endothermic J·g ) of S5 − −1 thereleased, endothermic leading peakto a slight in comparison decrease in1 to absorption hS2 (297.6 heat, J·g−1). evidenced by the shrunken· area hS5 (224.1 J·g ) of peak in comparison to hS2 (297.6 J g− ). the endothermic peak in comparison· to hS2 (297.6 J·g−1).

0.0

0.0 S2

-1 -0.5 S5 S2 -1

-1 h =224.1 J g -0.5 h S5 S5 S2 h =224.1 J g-1 -1.0 h S5 S2 -1.0 83.1 83.1 -1.5

-1.5

Heat flow / mW*mg 88.2 h =297.6 J g-1 -2.0 S2 Heat flow / mW*mg 88.2 h =297.6 J g-1 -2.0 S2

-2.5 20 40 60 80 100 120 140 160 180 200 -2.5 o 20 40 60 80 100T / C 120 140 160 180 200 o T / C Figure 6. HeatHeat flow flow of samples including S2 and S5. Figure 6. Heat flow of samples including S2 and S5. 3.4. XRD 3.4. XRD 3.4. XRDFigure 7 mainly presents huge humps assigned to amorphous silicate at approximately Figure 7 mainly presents huge humps assigned to amorphous silicate at approximately 2θ = 2θ = 15 ~30 . Small diffraction peaks superimposed on the hump corresponded to the poorly 15°~30°.Figure◦ Small◦ 7 mainlydiffraction presents peaks huge superimposed humps assigned on the humpto amorphous corresponded silicate to atthe approximately poorly crystalline 2θ = crystalline phases including tridymite (SiO2, PDF No.: 14-0260), quartz (SiO2, PDF No.: 46-1045), phases15°~30°. including Small diffraction tridymite peaks(SiO2, PDFsuperimposed No.: 14-0260), on the quartz hump (SiO corresponded2, PDF No.: 46-1045), to the poorly natrite crystalline (Na2CO3, PDFphases No.: including 37-0451), tridymite sodium sulfite (SiO2, (NaPDF2SO No.:3, PDF14-0260), No.: 37-1488),quartz (SiO an2d, PDF sodium No.: sulfate 46-1045), (Na natrite2SO4, PDF (Na 2No.:CO3 , PDF No.: 37-0451), sodium sulfite (Na2SO3, PDF No.: 37-1488), and sodium sulfate (Na2SO4, PDF No.:

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Coatings 2019, 9, x FOR PEER REVIEW 8 of 11 natrite (Na2CO3, PDF No.: 37-0451), sodium sulfite (Na2SO3, PDF No.: 37-1488), and sodium sulfate 01-0990).(Na2SO4, PDFHowever, No.: 01-0990). the absence However, of peaks the absence corresponding of peaks corresponding to thiocyanate to thiocyanatedetermines determinescomplete dissolutioncomplete dissolution or decomposition, or decomposition, which is whichin agreement is in agreement with the with finding the finding of literature of literature [24,25]. [24 The,25]. obviousThe obvious characteristic characteristic diffraction diffraction peaks peaks of tridymit of tridymitee and and quartz quartz derive derive from from the the transformation transformation of of fumedfumed silica. silica. The The natrite natrite forms forms through through the the reactions reactions shown shown in in Equations Equations (1)–(3), firstly firstly the NH 4SCNSCN − 2− 2−2 reactsreacts with the OHOH− and produces NH33,,S S −, ,and and CO CO33 ,− following, following by by the the formation formation of of sodium sodium sulfite sulfite 2 2− via oxidizingoxidizing reactionreaction between between the the S − Sand and O2 derivedO2 derived from from air, finally air, finally partial partial sodium sodium sulfite is sulfite oxidized is − 2 2− + + oxidizedfurther and further transfers and transfer into sodiums into sulfate.sodium SCNsulfate.− can SCN transform can transform into S −into, CO S2,, andCO2 NH, and4 NHunder4 under acid acidenvironment environment via a via reduction a reduction reaction reaction [24]. However,[24]. However, the hump the hump of amorphous of amorphous silicate silicate covers thecovers spectra the spectraof Na2CO of 3Na, Na2CO2SO3, 3Na, and2SO Na3, and2SO Na4, because2SO4, because of their of lowtheir contents low contents in XRD in patterns.XRD patterns.

− 2− 2− NH4SCN + 4OH = 2NH3↑+ CO2 3 + S2 + H2O (1) NH SCN + 4OH− = 2NH + CO − + S − + H O (1) 4 3↑ 3 2 22S2− + 3O2 = 2SO322− (2) 2S − + 3O2 = 2SO3 − (2)

2 2− 22− 2SO2SO3 −3+ +O O2 2= =2SO 2SO44 − (3)(3)

3000 n-natrite t-tridymite q t s-sodium sulfite q-quartz t t s'-sodium sulfate 2500 q t n t n 2000 t q t s t S0 s's s' s 1500 nn n t s' t s's q n 1000 n s' S2 Intensity / a.u. t t ss' n n s t n n s' 500 t S5 0 10 20 30 40 50 θ O 2 / Figure 7. XRDXRD of of samples samples after after combustion. combustion. 4. Discussion 4. Discussion It has been reported that mercerization increases the interfacial bonding strength between It has been reported that mercerization increases the interfacial bonding strength between lignocellulosic fibers and organics [26–28], the alkaline condition facilitates removal of soluble lignocellulosic fibers and organics [26–28], the alkaline condition facilitates removal of soluble components [29,30], and, therefore, chemical bonding between the siliceous gel and plywood forms. components [29,30], and, therefore, chemical bonding between the siliceous gel and plywood forms. Furthermore, SCN− is prone to associate with nonpolar surfaces due to its low charge density and Furthermore, SCN− is prone to associate with nonpolar surfaces due to its low charge density and weak-tightly bonding hydration shell [31], and the plywood surface mainly consists of nonpolar weak-tightly bonding hydration shell [31], and the plywood surface mainly consists of nonpolar cellulose and hemicellulose, which prompts a strong adhesive property of geopolymeric coatings, cellulose and hemicellulose, which prompts a strong adhesive property of geopolymeric coatings, favoring the more pronounced fireproof property. favoring the more pronounced fireproof property. On the other hand, it can be inferred that the improved flame resistance of geopolymeric coatings is On the other hand, it can be inferred that the improved flame resistance of geopolymeric attributed to the following factors according to the results of CC and XRD. Firstly, NH4SCN postpones coatings is attributed to the following factors according to the results of CC and XRD. Firstly, the rising pHRR as a suitable O2− consuming reductant. NH4SCN with a melting temperature of NH4SCN postpones the rising pHRR as a suitable O2− consuming reductant. NH4SCN with a melting 149.6 ◦C can transform into thiourea at 140~180 ◦C[22], the strong alkaline condition triggers the prior temperature of 149.6 °C can transform into thiourea at 140~180 °C [22], the strong alkaline condition oxidizing reaction and the firing provides the conditions for the formation of sulfate, this implies that triggers the prior oxidizing reaction and the firing provides the conditions for the formation of sulfate, the phase change performance of NH4SCN is limited or inhibited, but the reducibility is beneficial for this implies that the phase change performance of NH4SCN is limited or inhibited, but the reducibility retarding flame-propagation by consuming O2. Secondly, NH4SCN mainly favors the formation of a is beneficial for retarding flame-propagation by consuming O2. Secondly, NH4SCN mainly favors the continuous layer as a dopant for reinforcing the siliceous layer, due to its natural amorphousness [32], formation of a continuous layer as a dopant for reinforcing the siliceous layer, due to its natural which facilitates its diffusion and migration in the amorphous geopolymeric coatings, and the formation amorphousness [32], which facilitates its diffusion and migration in the amorphous geopolymeric of carbonate and sulfate prompts a continuous and compact siliceous layer as a barrier filler. Finally, coatings, and the formation of carbonate and sulfate prompts a continuous and compact siliceous layer as a barrier filler. Finally, NH4SCN suppresses the diffusion of heat and flame by releasing NH3−the NH3 derives from two routes, one is the combination of NH4+ and OH−, the other is the

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NH4SCN suppresses the diffusion of heat and flame by releasing NH3—the NH3 derives from two + 3 routes, one is the combination of NH4 and OH−, the other is the transformation of N − in SCN−. The former mainly takes place during the process of preparation, leading to negligible flame resistance, but the latter possesses an obviously inhibiting effect during combustion. However, flame resistance declines when the dosage of NH4SCN exceeds 1 wt.%, this can be ascribed to the rapid increase in viscosity of coatings, evidenced by the non-uniform dispersion and filming, as well as the increase in WLT from TG. The coating is prone to fattening for S5, due to strong water-consuming hydrolysis of NH4SCN, especially for the alkaline environment that accelerates and 2 2 2 boosts the formation of NH3. The superfluous SCN− could transform into CO3 −, SO3 −, and SO4 − according to the result of XRD, which consumes O2 to delay the propagation of flames, evidenced by the gradual right-shift of pHRR with increasing NH4SCN. However, the O2-consuming effect is weak (low content) because the formation of an intact and uniform shielding layer is of vital importance for flame retardancy. The thermal conductivity is remarkably decreased because of the discontinuous coating or film [33], which blocks heat conduction and charring of the hybrid coating during combustion, as well as the formation of swelling siliceous layers, leading to a chipped and rough surface. Although this paper presents a cost effective and ecological geopolymeric coatings for flame-retarding plywood, an important issue to highlight is the interactions between NH4SCN and PDMS, EG, and other components. As seen from the charring mechanism on the carbon/siliceous layer, SEM only provides a final morphology, but a pyrolysis kinetic is essential to investigate the novel coatings. This work might be the first step toward preparing novel NH4SCN-doped geopolymeric coatings for flame-retarding plywood as a primer, proposing a new pathway for meeting the sustainable demand of “end-of-waste”. Furthermore, some deep problems remain in durability and chemical structure, which need to be analyzed and evaluated in detail.

5. Conclusions To obtain an ecological, cost-effective, and high flame-retarding coating, a facilely prepared halogen-free flame-retardant coating was explored by mixing the alkali-activated fumed silica-based geopolymer with PDMS, EG, and NH4SCN. Its flame resistance and microstructure were preliminarily investigated and the following conclusions were drawn.

A novel NH SCN-modified geopolymeric coatings for flame-retarding plywood was prepared via • 4 a facile sol–gel method, and the optimal dosage of NH4SCN was determined as 1 wt.%, evidenced by the highest FPI, the lowest FGI, and the pHRR dropping to 248 kW m 2 with a 39.7% decrease. · − Microstructure analysis determined that moderate use of NH SCN as a dopant is of importance • 4 for a uniform and continuous coating before combustion, which is beneficial for the formation of a smooth and intact swollen siliceous shielding layer during combustion. The enhancement of flame resistance mainly depends on the formation of a compact and continuous siliceous layer.

Author Contributions: Conceptualization, Y.W. and J.Z.; methodology, Y.W.; software, Y.W.; validation, Y.W.; formal analysis, Y.W.; investigation, Y.W.; resources, J.Z.; data curation, J.Z.; writing—original draft preparation, Y.W.; writing—review and editing, Y.W.; visualization, Y.W.; supervision, J.Z.; project administration, Y.W.; funding acquisition, Y.W. Funding: This research was funded by the Natural Science Foundation of Shanxi Province, grant number 2018JQ5131. Conflicts of Interest: The authors declare no conflict of interest.

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