Effect of polydimethylsiloxane viscosity on silica fume-based geopolymer hybrid coating for flame-retarding plywood

Construction and Building Materials 239 (2020) 117814

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Effect of polydimethylsiloxane viscosity on silica fume-based geopolymer hybrid coating for flame-retarding plywood YaChao Wang a,b,⇑, JiangPing Zhao b, Jinglong Chen b a b

Post-Doctoral Station of Architecture, Xi’an University of Architecture & Technology, PR China School of Resources Engineering, Xi’an University of Architecture & Technology, Xi’an 710055, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 PDMS doped silica fume-based

geopolymer hybrid coating is prepared.  PDMS modified geopolymer coating holds enhanced fireproof property.  PDMS viscosity of 350 cps exerts the highest flame resistance.  Appropriate PDMS triggers the crosslinking Q41 and Q42 groups.

a r t i c l e

i n f o

Article history: Received 20 June 2019 Received in revised form 4 December 2019 Accepted 5 December 2019

Keywords: Geopolymer Flame retardancy Heat release rate 29 Si MAS NMR Organic–inorganic hybrid coating

a b s t r a c t Hydroxyl-terminated polydimethylsiloxane (PDMS) modified silica fume-based geopolymer explores a novel challenging and interesting flame-retardant hybrid coating, and the effect of PDMS viscosity on the flame resistance and microstructure is investigated firstly. The optimum kinetic viscosity of PDMS is estimated as 350 cps according to the results of cone calorimeter (CC), which imparts a 50% decreased fire growth index (FGI) with the fire performance index (FPI) of 3.80 s
m2
kW 1 to the hybrid coating. The incorporation of PDMS renders the decreases in the weight loss percent (52.51%) and the average combustion velocity Vac (6.74  10 2%
s 1), as well as excellent smoke suppression. The doped PDMS (350 cps) facilitates the transformation from Q40 (no aluminum ion in „Si-O-Si„ chains) to Q41 or Q42 groups (one or two aluminum ions Al3+ replace the pristine Si4+) involved in geopolymer coating, evidenced by the results of 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR) spectra, leading to a continuous, compact, and intact barrier layer from microstructure analysis. Moreover, the retarding mechanism of PDMS in the hybrid coating is elucidated including physical filling, heat sink effect of „Si-Ochains, hydrogen bonding, and crosslinking. However, excessive PDMS viscosity exerts adverse flame resistance to the silica fume-based geopolymer hybrid coating. Ó 2019 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Post-Doctoral Station of Architecture, Xi’an University of Architecture & Technology, PR China. E-mail address: [email protected] (Y. Wang). https://doi.org/10.1016/j.conbuildmat.2019.117814 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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Y. Wang et al. / Construction and Building Materials 239 (2020) 117814

1. Introduction Polydimethylsiloxane (PDMS) with „Si-O- backbones and –CH3 side-chains, as a novel multifunctional precursor for protective coating materials, exhibits excellent heat resistance, antioxidation and anti-ozone properties [1]. Due to its inherently strong chemical bonding within their siloxane (-Si-O-) networks, thermal stability, and inertness against chemical attack, PDMS has attracted the increasing interest. Moreover, PDMS possesses the following established properties, such as the anticorrosion, anti-biofouling, anti-icing, self-cleaning, anti-reflection [2–3], biocompatibility, and excellent hydrophobicity [4–6]. Its long flexible polymeric chain-modifies impart the enhanced durability [7–8], repelling potential contaminants to coatings effectively [9–10], as well as the high elasticity and low expansion coefficient after solidification [11]. The research and development of flame retardant in preventing or reducing the damage during the increasing city conflagration has globally aroused common awareness [12], due to the inflammability of decorative materials including plywood, particleboard, and externally thermal insulation materials, further constituting the fireproof materials and control system is imminent. It is a common consensus that a good flame retardant coating endows the excellent flame resistance to the underlying substrate, lengthens evacuation time for victims during firing, and releases nontoxic or less toxic smoke, as well as low carbon footprint and cost. Meanwhile, inorganic silicate-based composite coatings have become the competitive alternative to halogen-containing or phosphorous flame retardants, ascribed to their low or non-toxicity and cost-effectiveness, although the flame resistance and weak durability need to be ameliorated further [12]. Nevertheless, silicatebased inorganic coatings modified by organics [13], as organically modified nano-clay [14], hold great potential in the benign design of inorganic–organic hybrid flame-retarding coatings, due to the blocking and suppressing effect. Furthermore, the recycling of industrial solid wastes has been the hotspot of academic study for reducing environmental pollutions. Highly value-added reusing of the misplaced resources is urgent to prompt a cleaner production and ecological circular economy. Regarding the geopolymer mainly fabricated by industrial solid wastes, it belongs to a kind of amorphous to semicrystalline silicate binders comprised of „Si-O-Si„ or „Si-OAl„ chains [15]. It holds inherent incombustibility and thermal stability, has the potential to protect the underlying substrate from burning or firing effectively. Therefore, the initial exploring of alkali-activated silica fume based geopolymer coating for flameretarding plywood is investigated, using the PDMS as a modifier to improve its hydrophobicity [15], due to the high elasticity and hydrophobicity of PDMS [16]. Additively, it is well known that the viscosity of PDMS is a crucial parameter for optimizing the performance of composites with PDMS [17], which is highly determining the performance of composites [18], including the processing, glass transition temperature, and curing reaction [19]. However, the effect of hydroxylterminated PDMS viscosity on coatings has been seldom investigated. Our previous studies establish sustainable flame-retarding geopolymer coatings modified by the PDMS, which is mainly served as the hydrophobic film-forming agent [15,20–21]. Consequently, the effect of PDMS viscosity on the fireproof property of hybrid coatings is preliminarily examined. The Na2SiO3/KOH activated silica fume-based geopolymer is exploited as the coating binder, using plywood as the carrier. The study’s novelty relates to the effect of PDMS viscosity on the flame resistance of geopolymeric hybrid coating. It is firstly characterized by 29Si magic angle spinning nuclear magnetic resonance (29Si MAS NMR), x-ray diffraction

(XRD), and scanning electron microscopy equipped with energy dispersive spectrometer (SEM-EDS). Finally, the retardant mechanism of PDMS in geopolymer coatings is elaborated, boosting the development of more sustainable building materials. 2. Experiment and methods 2.1. Raw materials Silica fume was collected from the Linyuan company of Xi’an, held a density of 1.62 g
cm 3 and Blaine specific surface area of 25 m2
g 1, its chemical composition tested by X-ray fluorescence was 87.18 wt% SiO2, 6.89 wt% Al2O3, 1.18 wt% Fe2O3, 0.66 wt% K2O, 0.60 wt% CaO, 0.45 wt% MgO, 0.28 wt% Na2O, 0.32 wt% SO3, and 2.44 wt% loss on ignition. The activators including analytically pure Na2SiO3 and KOH were all purchased from the HongYan chemical reagent factory in Tianjin of China, the molar ratio of Na2O to SiO2 was 1.0. Hydroxyl-terminated PDMS was served as the organic modifier to improve water resistance with various kinetic viscosities m of 50, 100, 350, 500, and 1000 cps, respectively, which was purchased from Xiameter (R) PMX-200. Analytically pure polyacrylamide was purchased from the Hongyan chemical reagent factory in Tianjin. The plywood was purchased from the timber processing plant of Xi’an in China. 2.2. Preparation of samples The geopolymeric binder was prepared by dispersing 50 g silica fume into the chemical activator mixture under magnetic stirring at 60 rpm and 60 , which consisted of 1 mol
L 1 Na2SiO3 and 2 mol
L 1 KOH, the addition process lasted about 0.5 h. And then the 1.5 wt% PDMS and 1 wt% polyacrylamide were sequentially added into the aforesaid sol during stirring of 200 rpm within 5 min [15], followed by another vigorous stirring at 1000 rpm for about 30 min. Finally, the geopolymer hybrid coating was manually brushed on the plywood surface with a thickness of about 0.8 mm tested by the film thickness gauge (PosiTector 200, America) with an error of 5 lm, which was obtained from the average value of five results collected in the four sides and the center of the sample, according to the ASTM D 7091–05, using the average value of 3 determinations as the final thickness of coating. The surface drying time and drying time were about 42 min and 2.4 h respectively, according to the Chinese standard of GB 1728 [22]. The sample incorporated PDMS with the kinematic viscosity m of 50, 100, 350, 500, and 1000 cps was defined as S1 ~ S5, respectively, using the sample without PDMS as the control denoted S0. The brushed content of coatings was approximately 600 ± 10 g
m 2. According to the standard of ASTM D870 [23–24], all edge-sealing samples S0 ~ S5 (after 7d of drying) were immersed in distilled water at 25 °C for 24 h. The mass change was calculated by DM = (M2 M1)/M1  100, where DM was mass change (%), M1 was sample mass (g) before water immersion, and M2 was sample mass (g) after water immersion. All the samples received a mass gain after water immersion that was 19.36% for S0, 5.69% for S1, 4.92% for S2, 4.35% for S3, 4.45% for S4, and 4.39% for S5. The remarkable water resistance was examined for the samples with PDMS, but the mass gain presented a slight fluctuation with the increasing viscosity of PDMS. It indicated that the viscosity of PDMS played a negligible role in the water-resistance of composite coatings. 2.3. Characterizations The cone calorimeter (CC, ZY6243, Zhongnuo instrument company, China) test was carried out on the samples of

Y. Wang et al. / Construction and Building Materials 239 (2020) 117814

100  100  3 mm3 according to BS ISO 5660–1:2015. The specimens were conditioned to equilibrium at 70% relative humidity and 28 °C before testing. Each specimen enwrapped in a single layer of aluminum foil was exposed horizontally to an external radiation intensity of 35 kW
m 2. The following parameters were real-timely recorded including heat release rate (HRR, kW
m 2), time to ignite (TTI, s), peak heat release rate (p-HRR, kW
m 2), time to p-HRR (Tp, s), total heat release (THR, MW
m 2), average effective heat of combustion (av-EHC, MJ
kg 1), and weight loss (WL, g). Meanwhile, the fire growth index (FGI, kW
m 2
s 1) was defined as the ratio of p-HRR to Tp (FGI = p-HRR/Tp), a higher FGI corresponds to accelerated flame propagation. The fire performance index (FPI, s
m2
kW 1) was defined as the ratio of TTI to p-HRR (FPI = TTI/p-HRR), a higher FPI means longer evacuation time for victims under the same externally thermal flux. The avEHC was calculated by formula av-EHC = THR/WL [15,20–21]. Additively, the total smoke production (TSP) and smoke production rate (SPR) were real-timely recorded by CC. A swollen barrier layer derived from the burning of the sample coating was observed by SEM-EDS (Quanta 200) with an area of 0.8 mm2 approximately, which was preferentially covered by a conductive golden layer. The XRD patterns of samples after burning in CC were recorded by a D/MAX-2400 X-ray diffractometer equipped with a rotation anode using Cu Ka radiation. The 29Si MAS-NMR spectrum of samples after burning in CC was collected by the Avance-400 Bruker apparatus with operating at 79.46 MHz for 29Si under the rotation frequency of 10 kHz, which was loaded in 4 mm PSZ rotors with an accessible volume of 0.115 cm3. All measurements were taken at room temperature with TMS (tetramethylsilane) for 29Si as the external standard. The error in chemical shift values was estimated to be lower than 0.5 ppm.

3. Results 3.1. Flame retardant property tested by CC Fig. 1a presents the shrunk HRR for the specimens with PDMS than that of the control S0. Although the Tp fluctuates nearly at the time of 340 s, the reduced p-HRR is examined. The S3 with a viscosity of 350 cps exhibits the lowest value of p-HRR, which decreases from 115 (S0) to 60 kW
m 2. However, it is worth pointing that the samples with PDMS viscosity >350 cps exert the higher value of p-HRR in comparison to that of S3, which climbs with the increasing PDMS viscosity. The sample S5 with PDMS viscosity of 1000 cps presents the p-HRR of 95 kW
m 2, corresponding to weak fire resistance. A distinct ‘‘right shift” and shrinkage of the smoke temperature peak is observed with the increasing viscosity of PDMS in Fig. 1b. The S3 presents the lowest value of 68 °C at 494 s, indirectly indicates the highest flame resistance, because the smoke temperature is negatively correlated to flame resistance of samples. However, the reverse increase presents with the increasing viscosity of PDMS > 350 cps, which is completely consistent with the results of HRR. Moreover, compared with the slight fluctuation of Tp, the peak of smoke temperature continuously shifts towards the right, indicates that an obviously retarding phenomenon pronounces between the HRR and smoke temperature. Because the diffusion of heat highly depends on the temperature gradient, a higher HRR favors a faster heat diffusion, leading to an earlier emergence of smoke temperature peak with a higher value. The CC indirectly provides effective and efficient evidence for assessing the fireproof property of the PDMS modified geopolymer coating, rather than the direct flame-retarding parameters as the flame propagation rate, flame intensity, or the movement of flame

3

front, which has been an effective and widely used assessment method to flame resistance currently. Table 1 summarizes the heat release properties of samples, the S3 presents the highest flame retardancy, evidenced by the highest FPI, the lowest FGI and avEHC, which are important indicators to evaluate the fireproof properties [25]. The higher of FPI and the lower of FGI, the weaker of the flashover propensity is. The FPI increases from 0.83 to 3.80 s
m2
kW 1 while the FGI drops from 0.34 to 0.17 kW
m 2
s 1, indicates the higher fire resistance. The THR decreases 69.7% approximately with a 62.6% decreased av-EHC in comparison to that of S0. It identifies that the PDMS viscosity is crucial to exert excellent flameretardant efficiency, and the optimum kinetic viscosity is 350 cps. Meanwhile, the samples are subjected to the continuous weight loss due to the decomposition of plywood as shown in Fig. 1c, under the externally constant heat flux of 35 kW
m 2. However, the weight loss percent of samples varies clearly as shown in Table 2, which firstly decreases and then increased with the increasing viscosity of PDMS. The S3 exhibits the lowest weight loss percent of 52.5% with an average combustion velocity Vac of 6.74  10 2%
s 1, the retarded tmax is examined at 521 s, corresponding to the highest flame resistance. The steep curve appears for the sample S4 with Vac of 12.91  10 2%
s 1, which further increases for S5 with a Vac of 17.37  10 2%
s 1, indicates the faster combustion occurs for samples with the PDMS viscosity > 500 cps. It demonstrates that the appropriate hydroxyl-terminated PDMS with the viscosity of 350 cps retards the formation and propagation of flame effectively, leading to the slow decomposition and combustion of plywood. Apart from the abovementioned combustion parameters, the TSP and SPR of samples were presented in Fig. 1d-e. The S3 exerts the lowest TSP (41 m2) and SPR among the samples (that of S0 is 194 m2), due to the barrier effect of hybrid coatings. There are two obvious peaks on the SPR curve. The former is mainly attributed to the burning of volatiles and hemicellulose, the latter is caused by the pyrolysis and combustion of cellulose [26]. The coating as a physical barrier retards the transfer of heat and mass, leading to a decrease in plywood decomposition, evidenced by the reduction of TSP and THR. However, the excessive PDMS viscosity makes the reverse increases in the TSP and SPR when the PDMS viscosity >350 cps. It indicates the higher viscosity is detrimental to optimize the fire resistance of the hybrid coating. Generally, it demonstrates that an appropriate PDMS viscosity facilitates enhanced flame resistance and smoke suppression simultaneously. While excessive viscosity plays an adverse influence on fire resistance and smoke suppression, the reason is analyzed in the section of discussion. 3.2.

29

Si MAS-NMR analysis

The obtained spectra, Gaussian fitting peaks, and deconvolution results of raw silica fume and samples after burning are shown in Fig. 2. The peak of spectrum produces obvious ‘‘left shift” for samples in Fig. 2b, but the broader and poorly-ordered peaks appear than that of raw silica fume in Fig. 2a, indicates a higher degree of amorphousness in the siliceous layer [27]. Because the increased amorphous silicate derives from the alkali-activation, which transforms the discrete silica powder into the amorphous binder for adhering to plywood effectively [15]. The peaks at 109 ~ 119 ppm are assigned to the Q40 groups (no aluminum ion Al3+ in „Si-O-Si„ chains). Peaks at 99 ~ 107 ppm are assigned to the Q41 groups (one Al3+ replaces the pristine Si4+ and transforms into „Si-O-Al„). Peaks at 93 ~ 95 ppm are assigned to the Q42 groups (two Al3+ replace the pristine Si4+ and transform into „Al-O-Si(OH)2-O-Al„). Peaks at 91 ~ -88 ppm are assigned to the Q43 groups (three Al3+ replace the pristine Si4+ as („AlO-)3„Si-O-), and peaks at 79 ~ 82 ppm are assigned to the Q44 groups (the Si4+ is completely surrounded by aluminum-oxygen

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Y. Wang et al. / Construction and Building Materials 239 (2020) 117814

120 100

HRR / kW*m-2

90 80 70

(349, 60)

60

180

50 40

S0 S5 S1 S4 S2 S3

30 20 10 0

150

200

300

194 171

115 90 60

46 41

30 0 0

100

217

120

-10 0

(d)

S0 S1 S2 S3 S4 S5

210

S0 S1 S2 S3 S4 S5

TSP / m2

110

240

(338, 115)

(a)

400

500

600

700

800

900

100

200

300

400

1000

500

600

700

800

900 1000

Time / s

Time / s 0 80

(309, 78)

S0

75

70

S0 S1 S2 S3 S4 S5

S5 (494, 68)

S4 S1

65

SPR / *10-3 m2*s-1

Smoke Temperature / oC

(b)

S2 S3

60

55 0

100

200

300

400

500

600

700

800

900

1000

Time / s 100

S3 S2 S1 S4 S0 S5

Residual char / wt %

90 80 70

(c) S0 S1 S2 S3 S4 S5

60

100 200 300 400 500 600 700 800 900 1000

3 2 1 0 2.61 1.74 0.87 0.00 1.38 0.92 0.46 0.00 1.77 1.18 0.59 0.00 2.7 1.8 0.9 0.0

(e) S5

S4

S3

S2

S1

3.6 2.4 1.2 0.0

S0 0

100 200 300 400 500 600 700 800 900 1000

Time / s

50 40 30 0

100

200

300

400

500

600

700

800

900 1000

Time / s Fig. 1. HRR (a), smoke temperature (b), residual weight (c), TSP(d), and SPR(e) of samples are real-timely recorded by CC.

Table 1 HRR parameters of samples are recorded by CC under 35 kW
m Samples

TTI/s

Tp/s

p-HRR/kW
m

S0 S1 S2 S3 S4 S5

96 205 168 228 132 104

338 330 347 349 336 338

115 87 70 60 81 95

2

2

. FPI/s
m2
kW 0.83 2.36 2.40 3.80 1.63 1.09

1

FGI(kW
m 0.34 0.26 0.20 0.17 0.24 0.28

2


s

1

)

WL/g

THR/MW
m

28.1 22.4 20.7 22.5 24.2 24.1

34.18 14.73 14.32 10.34 17.55 17.78

2

av-EHC/kJ
g 10.70 5.79 6.09 4.04 6.38 6.49

1

5

Y. Wang et al. / Construction and Building Materials 239 (2020) 117814 Table 2 Weight loss of samples under the externally constant heat flux of 35 kW
m

2

.

Samples

S0

S1

S2

S3

S4

S5

Residual weight/% Weight loss percent/% tmax t 0.05/s t 0.90/s t 0.50/s Vac  102/%
s 1

32.8 67.2 357 108 613 349 13.20

34.4 65.6 382 87 791 366 9.30

44.4 55.6 416 37 645 383 9.15

47.5 52.5 521 34 813 442 6.74

38.2 61.8 378 33 512 307 12.91

37.3 62.7 293 38 403 253 17.37

The tmax is the time of the maximum weight loss, t-0.05, t-0.90, and t-0.50 are the times corresponding to the total weight loss of 5%, 90%, and 50%, respectively. The Vac is the average combustion velocity defined as the rate of weight loss percent to the time interval from t-0.05 to t-0.90, that is Vac = weight loss percent/(t 0.90- t 0.05), %
s 1.

3.00E+008

(a)

2.50E+008

Intensity (a.u.)

octahedra as Si(-O-Al„)4) [28–30]. The content of Q40, Q41, Q42, Q43, and Q44 groups in S3 after burning is 19.13%, 38.06%, 26.34%, 12.78%, and 3.69%, respectively. While that of the virgin S0 is 27.41%, 34.41%, 18.49%, 14.27%, and 5.43%, respectively. The increased content of the Q41, Q42 groups reveals that the doped PDMS alters the transformation of „Si-O- microstructure to some extent, the Al3+ participates or joins in the condensation involved in the „Si-O- chains and Si(OH)4. Because the trivalent Al3+ is hardly to obtain a charge balance as [Al(OH)4] [31], which could generate the electrostatic interactions within the coating matrix, favors the condensation and homogenization of coating. Generally, the increased Al3+ in silica-oxygen tetrahedra chains determines that the new crosslinking between „Si-O- chains and activated Al3+ occurs after doping the PDMS in geopolymer, leading to an increase in flame resistance.

-116: 17.73 % -112: 36.13 % -107: 27.95 % -100: 13.34 % -95: 4.75 % -87: 3.09 %

2.00E+008 1.50E+008

-112 -116 -107 -100

1.00E+008

-95

-87 5.00E+007 0.00E+000 -70

-80

-90

-100

-110

-120

Chemical shift / ppm

3.3. XRD analysis

Intensity (a.u.)

4.00E+008

3.00E+008

2.00E+008

-119: 3.75 % -114: 7.71 % -109: 15.95 % -95 -105 -105: 17.64 % -100 -109 -100: 16.77 % -91 -114 -95: 18.49 % -91: 14.27 % -86: 5.43 %

1.00E+008

(b)

Although the XRD patterns of the different samples are quietly comparable as shown in Fig. 3, there is a huge hump from 2h = 15° 10

25

S0

-110

-116: 4.23 % -110: 14.90 % -104: 18.56 % -99: 19.50 % -93: 26.34 % -88: 12.78 % -79: 3.69 %

(c)

5.00E+008

4.00E+008

-93 -88 -99

3.00E+008

-104

-110 2.00E+008

q

270 0

S2

870

290 0

s

Area=15784

S3

s q

660 330

-80

q - quartz s - silicon

580

0.00E+000 -70

50

Area=16752 s q

990

-116

-79

1.00E+008

45

540

-120

Chemical shift / ppm

Intensity / cps

-100

40

Area=17050

810 -90

35

q

290 10800

-80

30

580

0.00E+000

Intensity (a.u.)

20

qs

870

-119

-86

15

-90

-100

-110

-120

Chemical shift / ppm Fig. 2. 29Si MAS NMR spectra of specimen coating after burning contain raw silica fume (a), S0(b), and S3(c).

0 10

S5 15

Area=17080 20

25

30

35

40

45

50

2θ / o Fig. 3. XRD spectra of specimen after burning in CC are detected by x-ray diffractometer.

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Y. Wang et al. / Construction and Building Materials 239 (2020) 117814

to 2h = 40° corresponding to the amorphous silicate, which is conducive to prevent flame propagation as a physical barrier. The area of the amorphous silicate shrinks slightly when the PDMS is incorporated into the coating. The S3 presents the smallest area (Area = 15784) assigned to the lowest content of amorphous silicate. Because the doped PDMS triggers the crosslinking among the silica-oxygen Si(OH)4 and aluminum-oxygen tetrahedron [Al (OH)4] according to the result of 29Si MAS-NMR, the amorphousness of coatings is optimized by incorporation of PDMS slightly. However, the excessive viscosity of PDMS hinders the crosslinking involved in the coating, which blocks the condensation involved in the „Si-O- chains and Si(OH)4, leading to an increase in the hump area of 17080. It reveals that an appropriate viscosity of PDMS is crucial to adjust the amorphous silicate involved in the silica fume-based geopolymer coating. The competition between the crosslinking and blocking effect of PDMS dominates the interactions involved in the PDMS doped geopolymer hybrid coating essentially. On the other hand, there are many superposed peaks on the hump assigned to quartz (SiO2, JCPDS 46–1045) and silicon (Si, JCPDS 27–1402). The quartz mainly corresponds to the 2h of 20.8° and 26.6°, and the silicon is mainly assigned to the 2h of 28.4° [32,33], respectively. The quartz mainly derives from the thermal decomposition of amorphous „Si-O- chains and Si(OH)4. The dehydration involved in the „Si-O- chains and Si(OH)4 occurs during burning. It transforms into the hexagonal SiO2 and vapor, leading to a heat sink effect. Generally, the doped PDMS affects the crosslinking of the amorphous silicate structure slightly. The coexistence of quartz and silicon is detected in the residual coating. 3.4. Macrograph of specimens after burning The broken pieces of the siliceous layer present in Fig. 4a, which become gradually intact with the increasing viscosity of PDMS in Fig. 4b ~ d. Especially for the S3 in Fig. 4d, an intact and continuous residual layer covers the surface of plywood completely. However, cracks are observed for the specimen with the PDMS viscosity

>350 cps in Fig. 4e, which worsens further for S5 with the viscosity of 1000 cps in Fig. 4f, leading to inferior flame resistance. It infers that the higher viscosity of PDMS disturbs the homogeneous dispersion of „Si-O-Si„ or „Si-O-Al„, evidenced by the brittle barrier layer. The homogeneous barrier layer generates an effective and efficient thermal barrier with the continuous „Si-O-Si„ or „Si-O-Al„ cross-linking chains as the prerequisites. While the stacked or inhomogeneous dispersed silica tetrahedron is prone to transform into discontinuous barrier layers, leading to poor thermal protection. 3.5. Morphology observation by SEM The porous and fluffy surface of S0 presents in Fig. 5a, the irregular and porous barriers (Fig. 5a-1) mainly consist of C, Na, Si, and K from the result of EDS in Fig. 5a-2. The incorporation of PDMS achieves the increased Al yield in Fig. 5b-2, leading to the appearance of lamellar with a thickness of 2 lm approximately in Fig. 5b1. And the scaffolds originated from the aluminosilicate geopolymer composites appear in Fig. 5b [31], which gradually become obvious with the increasing viscosity of PDMS in Fig. 5c. Especially for S3, a continuous, swollen, and intact residual layer appears in Fig. 5d. Meanwhile, a smooth, uniform and compact barrier layer presents in Fig. 5d-1. The increased content of Al and C in Fig. 5d-2 indicates that the incorporation of PDMS is beneficial for cross-linking and compact barrier layers. Because the inert PDMS could physically insert and fill the clearances originated from the amorphous porous geopolymer due to its low surface tension. Meanwhile, Con et al. [34] also suggested that PDMS held the filling effect, presenting a compact morphology. However, when the PDMS with the viscosity of 500 cps is doped into the hybrid coating, the rugged surface is observed in Fig. 5e, due to the inhomogeneous geopolymer binder. The increasing large pores or holes appear in Fig. 5f, which might be attributed to the non-uniform dispersion of high-viscous PDMS, hindering or disturbing the condensation of „Si-O-Si„ or „Si-O-Al„ chains. Because the large pore prompts the air convection and accelerates the heat transfer [35], leading to a decrease in flame resistance.

Fig. 4. Macrographs of specimens after burning in CC contains (a)S0, (b)S1, (c)S2, (d)S3, (e)S4, and (f)S5.

Y. Wang et al. / Construction and Building Materials 239 (2020) 117814

(a)

(a-1)

7

(a-2)

20 μm

50 μm

(b)

(b-2)

(b-1)

20 μm

50 μm

(c)

50 μm

Fig. 5. SEM-EDSs of samples after burning contain (a)S0, (b)S1, (c)S2, (d)S3, (e)S4, and (f)S5, and the a-1, b-1, and d-1 are corresponding to the images with 5000, the a-2, b2, and d-2 are corresponding to the EDS.

8

Y. Wang et al. / Construction and Building Materials 239 (2020) 117814

(d)

(d-2)

(d-1)

20 μm

50 μm

(e)

(f)

50 μm

50 μm

Fig. 5 (continued)

4. Discussion The combustion of plywood begins with the decomposition of the non-charring lighter compounds including hemicellulose and partial lignin, leading to the formation of flammable volatiles. And then the highly cross-linked material transforms into barrier char including cellulose (300 ~ 350 °C) and lignin left (370 ~ 400 °C). Finally, the oxidation of the charred wood residue occurs when the temperature > 450 °C [26,36]. Combining with the results of flame resistance, 29Si MAS-NMR, XRD, and SEMEDS, the flame-retarding mechanism of the PDMS modified silica fume-based geopolymer coating could be analyzed from the following reasons. Because the abundant hydroxyls exist in the amorphous geopolymer binder including (HO)3„Si-O-, (HO)3„Al-O-, Si(OH)4, as well as the „Si-O-Si„ chains, which could release vapor through an endothermic decomposition during firing, dilutes the flammable volatiles and favors the swelling of geopolymer coating during firing, leading to a heat sink action with the formation of SiO2 (Fig. 3) at high decomposition temperature (>500 °C) [37–38]. The FGI drops to 0.17 kW
m 2
s 1 with a 50% decrease, and the beginning time of weight loss (t 0.05) emerges earlier for S3 in Table 2.

Secondly, the enhanced fireproof property of the PDMS modified geopolymer coating could be analyzed by the following aspects. (i) The physical insertion and filling between geopolymer and PDMS. The geopolymer reduces the flammability of PDMS [39], due to the thermal stability and non-flammability of geopolymer. (ii) The hydrogen bonding of silanol Si-OH involved in Si(OH)4 and hydroxyl-terminated PDMS [31,40–41], facilitates the crosslinking of „Si-O-Si„ and tends to form network structure during the film formation. (iii) The strong chemical stable, elastic PDMS is beneficial to reinforce and toughen the inherently fragile geopolymer as flexible nano-chains. It could absorb more strain energy derives from thermal stress during burning, evidenced by the continuous residual layer. (iv) Apart from the coupling and lubricity, PDMS could also improve the dispersion of inorganic additives as a surfactant [42]. The increased content of the amorphous Q41 and Q42 groups in silicate (Fig. 2) is contributed to the substitution of Si(OH)4 with [Al(OH)4] . The negative charge prompts a uniform-dispersion and crosslinking due to the electrostatic repulsion, evidenced by the formation of an intact, compact, and smooth siliceous layer on the surface of the substrate (in Figs. 4-5). However, an excessive viscosity corresponds to more flammable –CH3 within PDMS, which favors combustion and

Y. Wang et al. / Construction and Building Materials 239 (2020) 117814

accelerates the flame propagation, leading to an increase in FGI and an early Tp. Meanwhile, the excessive viscosity results in a slower diffusion rate of PDMS due to its high molecular weight. It inhibits or disturbs the movement of „Si-O-Si„ chains and the connections among „Si-OH derived from the alkali-activated silica fume, leading to a discontinuous morphology (Fig. 4). Courtat et al. [43] suggested that an increase of viscosity could prevent the gases transfer between the composite and the flame. Besides, the excessive viscosity of PDMS is difficult to be uniformly dispersed into the geopolymer coating. It wraps siliceous sol and hinders the network forming through the excessive hydrogen bonding in silanol, leading to a decrease in the fireproof efficiency. Nevertheless, the flame-retarding property of coating on plywood is inappropriate or negligible when plywood is placed in high-temperature furnace, due to its quick decomposition and burning, indicates that choosing an appropriate substrate of the geopolymer coating is the primary consideration before applying this coating in flame-retarding design. Besides, the interactions between the PDMS and the geopolymer coating matrix needs to be investigated further. Such as the bonding, transformation of the doped PDMS during burning, and the combustion kinetics, it holds high research value and broad potential prospects in our next research.

5. Conclusions Hydroxyl-terminated PDMS modified geopolymer composites are novel challenging and interesting organic–inorganic hybrids for the development of flame-retarding plywood coatings, due to the tailored possibilities concerning the fireproof property and competitive cost. The influence of PDMS viscosity (m = 50, 100, 350, 500, and 1000 cps) on the flame-retarding efficiency and microstructure of the coating is investigated firstly, and the following conclusions are drawn. (1) Hydroxyl-terminated PDMS with the kinematic viscosity of 350 cps imparts the highest flame resistance and smoke suppression to the geopolymer hybrid coating for flameretarding plywood. The FPI increases from 0.83 to 3.80 s
m2
kW 1, while the FGI decreases from 0.34 to 0.17 kW
m 2
s 1 with a weight loss percent of 52.51% and an average combustion velocity Vac of 6.74  10 2%
s 1. The TSP decreases from 194 m2 to 41 m2, due to the continuous and compact barrier. (2) PDMS facilitates the increase in the content of Q41 and Q42 groups detected by 29Si MAS NMR spectra and SEM-EDS, leading to a continuous and intact barrier layer. But an excessive viscosity is detrimental for improving the fireproof property.

CRediT authorship contribution statement YaChao Wang: Conceptualization, Writing - review & editing, Formal analysis, Methodology, Project administration, Visualization, Funding acquisition. JiangPing Zhao: Funding acquisition, Resources. Jinglong Chen: Data curation, Investigation, Software, Supervision, Validation, Visualization.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgment This work was supported by the China Scholarship Council (CSC No. 201808610034) and Shaanxi Natural Science Fund (No. 2018JQ5131).

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