Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings

Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings

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Journal Pre-proof Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings Long Yan, Zhisheng Xu, Nan Deng PII:

S0141-3910(19)30392-1

DOI:

https://doi.org/10.1016/j.polymdegradstab.2019.109064

Reference:

PDST 109064

To appear in:

Polymer Degradation and Stability

Received Date: 11 August 2019 Revised Date:

21 November 2019

Accepted Date: 30 December 2019

Please cite this article as: Yan L, Xu Z, Deng N, Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings, Polymer Degradation and Stability (2020), doi: https://doi.org/10.1016/ j.polymdegradstab.2019.109064. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings Long Yan*, Zhisheng Xu, Nan Deng Institute of Disaster Prevention Science and Safety Technology, School of Civil Engineering, Central South University, Changsha 410075, China Corresponding author: Long Yan (ORCID:0000-0001-5641-6150), Tel: +86 18163650767, Fax: +86 731 85119593, Email: [email protected], Address: Institute of Disaster Prevention Science and Safety Technology, Railway Campus, Central South University, 22 South Shaoshan Road, Changsha 410075, China

Abstract: A series of organophosphate-functionalized graphene oxide flame retardants (GPPBs) were successfully synthesized by grafting flexible phosphate ester (PPB) on the surface of graphene oxide (GO), and well characterized by Fourier transform infrared (FTIR) spectroscopy and 1H nuclear magnetic resonance (1H NMR) spectroscopy. The obtained GPPBs were then incorporated into amino resin to produce transparent fire-retardant coatings for reducing the fire hazard of wood. The transparency analysis shows that the GPPBs endow the resulting coatings with a high degree of transparency even at a relatively high GO contents due to the uniform dispersion and completely exfoliated states of GO in amino matrix. The evaluation of combustion behavior reveals that the introduction of GO greatly reduces the weight loss, char index, flame spread rating (FSR), smoke production and heat release of the coatings concomitant with an increase in the insulation property, for example, 60.7% reduction in smoke density rating (SDR) and 33.3% reduction in total heat release (THR) are observed in the case of MGPPB3 coating obtained from GPPB3 with 0.07wt% GO in comparison to MGPPB0 coating obtained from PPB. Thermo-gravimetric analysis shows that the thermal stability and residual weight of the coatings are improved after introduction of GO, and MGPPB3 exhibits the highest residual weight of 34.5 % at 700 °C. The main mechanism of the GPPBs for enhancing the fire safety of transparent fire-retardant coatings is ascribed to the formation of high-quality intumescent char with excellent compactness, anti-oxidation ability and barrier effect against fire.

Keywords: Functionalized graphene oxide; flexible phosphate ester; flame retardancy;

smoke suppression; transparent fire-retardant coating

1 Introduction Wood is widely applied as building and decoration materials due to its excellent mechanical properties, low cost, and unique aesthetic characteristics [1]. However, the inherent flammability of wood restricts its practical application in many fields with high fire safety requirements especially in densely populated areas. To eliminate this potential fire hazard, the deposition or impregnation of flame-retarded additives as coatings has been applied to enhance the fire safety of wood and wood-based materials [2, 3]. Inorganic salts such as borates, phosphates, silicates and potassium carbonates, are widely utilized to reduce the flammability of wood by a simple impregnation process due to their low cost and high flame-retardant efficiency [4]. However, the cost and energy-intensive drying process brings a potential problem for large-scale industrial application of the impregnated wood. The application of inorganic or organic coatings is an effective and simple approach to protect the wood from the threat of fire [5, 6]. Compared to opaque coatings, transparent coatings are appreciated widely in some fields such as cultural relics, heritage conservations and high-quality furniture [7, 8]. Especially, transparent intumescent fire-retardant coating is one of attractive transparent coatings and possesses satisfactory applications in wood and wood-based products due to its excellent hydrophobicity, decorative property and fire protection performance [9, 10]. It is well known that transparent intumescent coatings are usually produced by physical blending phosphate esters with amino resin [11, 12]. When exposed to heat or flame, phosphate esters can capture OH· and H· radicals in the gas phase by releasing P–O· radicals and catalyze the carbonation of carbon-rich components in the condensed phase by releasing inorganic acid, thus effectively reducing the combustion intensity [13]. Meanwhile, amino resin produces incombustible gases (NH3 and H2O) to dilute the oxygen in the combustion zone and promote the intumescence of the molten char. However, most of those kinds of amino transparent coatings have a relatively low flame-retardant efficiency. To improve the flame-retardant and smoke

suppression efficiencies of transparent intumescent coatings, some synergistic elements containing boron, silicon, aluminum and magnesium are introduced into phosphate esters [14-16]. In addition, the incorporation of nano-fillers as synergists exhibits enormous potential to enhance the flame retardancy and smoke suppression properties of transparent fire-retardant coatings, and these nano-fillers include nano-silica [17], organically modified montmorillonite [18], polyhedral oligomeric silsesquioxane (POSS)[19]. Graphene oxide (GO) as the precursor of graphene with a two-dimensional layered structure and rich functional groups that could be easily functionalized by flame-retardant compounds or polymers [20]. GO has superior thermal property and barrier effect that make itself promising application in the field of flame-retarded materials [21]. However, the van der Waals forces between the graphene sheets result severe agglomeration inclination of GO in polymer matrix, which induce significant light scattering and thus restrict its application in transparent materials [22, 23]. To solve this problem, many efforts have focused on the functionalization of GO with flame retardants containing phosphorus, nitrogen and/or silicon elements to obtain uniform dispersion of GO sheets in polymer matrix. Shi [24] and Ran [25] et al. reported on the decorating GO with P, N-containing flame retardants for acquiring well dispersion of GO and superior flame retardancy and smoke suppression properties of polymer materials. Bao et al. found that the introduction of N and P elements on the surface of GO results in compact and continuous char to protect the underlying materials against the fire, and the enhanced char can greatly improve the fire safety of polymeric materials [26]. Wang et al. grafted silicon-phosphorus flame retardant (DOPO-VTES) on the surface of GO to synthesis functionalized graphene oxide (DV-GO), and found that the DV-GO can be applied to develop high performance resins with excellent flame retardancy [27]. Thus, it is anticipated that functionalized GO with phosphate esters could provide superior flame retardancy and smoke suppression properties to amino transparent fire-retardant coatings concomitant with a high degree of transparency. However, the influence of functionalized GO on the optical transparency, flame retardancy and smoke suppression properties of

transparent fire-retardant coatings has been rarely investigated. In this work, a flexible phosphate ester (PPB) was grafted on the surface of GO to obtain a series of organophosphate-functionalized GO flame retardants (GPPBs), and well characterized by FTIR and 1H NMR spectra. Then, the obtained GPPBs were added into amino resin to produce transparent fire-retardant coatings. The influence of the GPPBs on the optical transparency, mechanical properties, fire protection performance, smoke suppression properties of the coatings was investigated by various analytical methods, and the potential flame-retardant and smoke suppression mechanism of the GPPBs in the coatings was also proposed.

2 Experimental 2.1 Materials Graphite oxide (mass fraction of C element: 47±5%, molar ratio of C/O: 0.6±0.1) was provided by the Sixth Element (Changzhou) Materials Technology Co., Ltd (Changzhou, China). Polyethylene glycol with molecular weight of 200 (PEG200) and n-butyl alcohol (n-BA) were acquired from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Boric acid (BA, purity: 99.5%) and phosphoric acid (PA, 85.0% in water) were acquired from Hunan Huizhong Chemical Reagent Co., Ltd. (Changsha, China). Pentaerythritol (PER, purity: 99.5%) was acquired from Hangzhou JLS Flame Retardant Co., Ltd., China. Melamine formaldehyde resin (MF, model 303-80, 58-62 % in n-BA) was provided by Jiyang Sanqiang Chemical Reagent Co., Ltd. (Shandong, China). All the above reagents were used as received without further purification. 2.2 Synthesis of PPB and the GPPBs Flexible phosphate ester (PPB) was synthesized by the esterification of cyclic phosphate ester (PEA) and polyethylene glycol borate (PEG-BA) with a mass ratio of 85:15 according to the procedure as reported [16]. In the first step, the mixture of PA (138.4 g,1.2 mol), PER (46.3g, 0.34 mol), and n-BA (14.8 g, 0.2 mol) was added into a 500-mL three-neck flask, and then magnetically stirred under refluxing for 4h at 105 °C in an oil bath. Afterwards, the transparent liquid named PEA was obtained by

cooling and removing water under reduced pressure distillation. In the second step, PEG200 (0.3 mol, 60g) and BA (6.2 mol) were added into a 500 ml three-necked flask with magnetic stirring at 130 °C for 3h. After the reaction finished, the yellow transparent liquid named polyethylene glycol borate (PEG-BA) was obtained by cooling and removing the water under reduced pressure distillation. In the third step, 85g PEA and 15g PEG-BA were added to a 500-mL three-necked flask with magnetic stirring under 50 °C for 1h, then the temperature raised to 115°C for refluxing 4h. Finally, a transparent liquid named PPB was obtained by distillation under reduced pressure for removing the water. The above PPB was grafted on the surface of GO to obtain a series of organophosphate-functionalized graphene oxide flame retardants (GPPBs) via a two-step process, as shown in Scheme 1. In the first step, 0.5g graphene oxide was dispersed in 500 mL deionized water by ultrasonication under 50 °C for 1h, and then the above homogenous suspension was filtrated and dried at 60 °C for 24h to obtain graphene oxide (GO). In the second step, an excessive content of PPB was reacted with GO to remain a relatively high acid number that can promote the ambient-temperature curing of the amino resin. Briefly, the mixtures of GO and PPB at different mass ratios (i.e., 99.97:0.03, 99.95:0.05, 99.93:0.07, and 99.9:0.1) were added in a 500-mL three-necked flask with magnetic stirring at 50 °C for 1 h, then stirred under 90 °C for 6 h to obtain liquid product. The liquid product was distilled under reduced pressure to remove water, then a series of homogenous liquid was obtained and designated as GPPBs. The theoretical loading of GO in the GPPBs and the acid number values of the GPPBs are listed in Table 1, and the acid number values of the GPPBs were tested according to ASTM D664-11a (2017).

Scheme1. Synthesis route of the GPPBs flame retardants

Table 1 The composition and acid number values of PPB and the GPPBs Samples Acid number (mg KOH/g) w(GO)/wt%

PPB

GPPB1

GPPB2

GPPB3

GPPB4

298.9±5

265.8±4

247.5±4

226.8±5

202.2±5

0

0.03

0.05

0.07

0.1

2.3 Preparation of transparent fire-retardant coatings The preparation procedure of transparent fire-retardant coatings is shown in Fig.1. The coating liquid was obtained by mixing 50g PPB or GPPBs ethanol solution (60 wt%) and 60 g MF n-butyl alcohol solution (58-62 wt%). The coating liquid was then coated on the plywood boards (75 mm×75 mm×4 mm, 100 mm×100 mm×4 mm, 150 mm×150 mm×4 mm and 600 mm×90 mm×4 mm) in an amount of 500 g/m2, and the thickness of dry film reached to 0.4 mm±0.02 mm. The coating liquid coated on the plywood boards with dimension of 300 mm×150 mm×4 mm in an amount of 250 g/m2, and the coatings were cured to 0.2 mm±0.02 mm thick film. The coatings obtained from PPB and GPPB1-GPPB4 were designated as MGPPB0 and MGPPB1-MGPPB4, respectively.

Fig.1. Preparation process of the transparent fire-retardant coatings on wood substrates

2.4 Measurements and characterization Fourier transform infrared spectra (FTIR) were recorded on a Nicolet FTIR IS5 spectrometer (Nicolet Instrument Co., USA) using a KBr pellet in the range of 4000-500 cm−1. 1

H nuclear magnetic resonance (1H NMR) spectra were obtained on a Bruker

Ascend 500 MHz NMR spectrometer (Bruker, Switzerland), using D2O as solvent. Optical transparency of the fire-retardant coatings was measured using an LS116-type light transmittance meter (Shenzhen Linshang Technology Co. Ltd, China). All the films were coated on a transparent glass slide with a thickness of 0.2 ± 0.02 mm. Five runs were repeated for each sample and the average value was recorded. X-ray diffraction (XRD) patterns were collected on a D/MAS-YA X-ray diffractometer (Rigaku, Japan), using a Cu-Kα radiation (α=0.1542 nm) range from 5 ° to 40 ° at a scanning rate of 2 °/min. Optical digital images of the coatings applied on wood substrates were observed by using a VHX-6000 3D optical digital microscope (Keyence, Japan). The hardness of the coatings was measured by pencil test according to ISO 15184-2012. The adhesion property of the coatings was analyzed by tape test according to ASTM D3359-09. In the pencil and tape tests, the coatings with the thickness of 0.4 mm±0.02 mm were applied on plywood boards under test, and finial values of the test were the average of five determinations.

Scanning electron microscope (SEM) images were acquired on a MIRA 3 LMU scanning electron microscopy (Tescan, Czech Republic) under a voltage of 20 kV. Energy dispersive X-ray spectroscopy (EDS) maps were conducted on an X-Max20 X-ray probe (Oxford instruments, UK) for elemental analysis. Cabinet method test was conducted using an XSF-1-type fire-resistant paint tester (small room mode) (Jiangning Analysis Instrument Company, China) according to GB12441-2018 and ASTM D1360-2011, and the dimensions of each specimen was 300 × 150 × 4 mm3. The final values of the test were the average of five determinations. Tunnel method test was carried on a SDF-2-type 2-foot flame tunnel instrument (Jiangning Analysis Instrument Company, China) according to ASTM D3806-2011, and the dimensions of specimens were 600 ×90 ×4 mm3. The final flame spread rating (FSR) values of the test were the average of five determinations. The heat insulation properties of the coatings were examined by a small scale “big panel method” test, and the coated side of specimens was exposed to a Bunsen burner that offered an approximately 900 °C high-temperature flame. The dimension of specimens was 150 mm × 150 mm × 4 mm. Smoke density test was carried on a smoke density test machine (PX-07-008, Suzhou Phinix Analysis Instrument Co. Ltd, China) according to ASTM D 2843-99. The coated side of specimen was exposed to the propane burner, and the dimensions of each specimen were 75mm × 75 mm × 4 mm. Cone calorimeter test was conducted to record the heat release of the coatings according to ISO5660-2002 (FTT, UK) under an external heat flux of 50 kW/m2. The size of the specimen was 100 mm × 100 mm × 4 mm, and the side of the specimen with coatings was horizontally exposed to the heat radiator. Thermogravimetric

(TG)

analysis

was

performed

on

a

SOTA-851

(Mettler-Toledo, Switzerland) instrument with a heating rate of 10 °C/min under nitrogen flow of 40 mL/min. X-ray photoelectron spectroscopy (XPS) was measured on an ESCALAB250Xi electron spectrometer (ThermoFisher-VG Scientific, USA). The energy step sizes for

XPS survey spectra and sole element XPS spectra were selected as 1.0 eV and 0.05 eV, respectively.

3 Results and discussion 3.1 Characterization of the GPPBs flame retardants The FTIR spectra of GO, PPB and GPPB3 are shown in Fig.2. In the spectrum of GO, the peaks at 3448, 2921, 1638, 1404, and 1110 cm-1 are assigned to the stretching vibration of –OH, C–H, C–O in COOH, C–OH and C–O–C, respectively [28, 29], indicating GO has C–OH groups that provide the ability to synthesize GPPBs flame retardants. In the spectrum of PPB, the peaks at 2361, 1638, 1466, 1352, 1119, 991 and 885 cm-1 are assigned to C–H stretch vibration, –OH bending vibration, –CH2 deformation vibration, B–O–C stretch vibration, C–O–C stretch vibration, exocyclic P–O–C stretch vibration, and cyclic P–O–C stretch vibration, respectively [16], revealing the successful synthesis of PPB in Scheme 1. After the functionalization of GO by PPB, the majority of characteristic peaks appeared in the spectra of both GO and PPB are found in the spectrum of GPPB3, confirming that PPB was successfully grafted on the surface of GO.

Fig.2. FTIR spectra of GO, PPB and GPPB3

The chemical structures of PPB and GPPB3 are further confirmed by 1H NMR spectra. In the spectrum of PPB, two major signals in 3.16-3.27 ppm and 4.81 ppm are

assigned to the H atoms adjacent to exocyclic P–O–C groups (labeled 3) and the H atoms adjacent to cyclic P–O–C groups (labeled 4 and 5), respectively. The peaks at 0.46 ppm and 0.90-1.22 ppm are assigned to the H atoms in the structure of n-BA (labeled 1) and PEG-BA (labeled 2), respectively. When GO is introduced, the major peaks of exocyclic H atoms (labeled 3) and the H atoms in n-BA (labeled 1) and PEG-BA (labeled 2) in the spectrum of GPPB3 exhibit lower frequency than those of PPB due to the anisotropy of protons in olefin structures of GO. In addition, the cyclic H atoms (labeled 4 and 5) move to high frequency of 4.93 ppm in the spectrum of GPPB3 from 4.81 ppm in the spectrum of PPB due to the increase of electronegativity after introduction of P–O–C groups adjacent to GO. The above results confirm that PPB was successfully grafted on the surface of GO to prepare GPPBs flame retardants as shown in Scheme1.

Fig.3. 1H NMR spectra of PPB and GPPB3 flame retardants

3.2 Optical transparency analysis The digital photos of the flame retardants and their resulting transparent coatings applied on wood substrates are shown in Fig.4. It can be found that the transparency of the GPPBs gradually decreases with increasing GO content, and GPPB4 forms a black and uniform liquid without phase separation, as viewed by naked eyes. The uniform distribution of GO in the GPPBs helps to endow the resulting coatings with a high degree of transparency. From the appearance of MGPPBs coatings, it can be seen

that the transparency value of MGPPBs is gradually decreased with increasing GO loading, which is ascribed to the light scattering of GO hampers the transmission of visible light and then decreases the level of transparency. In detail, MGPPB4 exhibits the lowest transparency value of 80.1 % among the coatings, which is consistent with the observation of GPPB4 flame retardant. The above results show that a moderate content of GO is essential to maintain the high transparency of fire-retardant coatings.

Fig.4. Digital photos of the flame retardants and their resulting transparent coatings applied on wood substrates

The fractured surfaces of MGPPB3 are observed by optical digital microscope (as seen in Fig.1) and SEM (as seen in Fig.5). It can be seen from Fig.1 that the natural appearance of wood surfaces is clearly visible after application of MGPPB3 coating. As shown in Fig.5, the GO sheets are uniformly distributed and completely exfoliated in the coating matrix. In general, the loss of transparency for transparent polymer nanocomposites is ascribed to the scattering of light by nanoparticles, and the well dispersion of GO with dimensions less than the wavelength of visible light can effectively minimizes scattering phenomenon and achieves a high level of transparency [22]. This is the reason why the incorporation of GO in the transparent coatings retains a high level of transparency (as seen in Fig.4).

Fig.5. SEM images of the fractured surfaces of MGPPB3 coating magnified at 500 and 2000 times

XRD is employed to analyze the structure of GO and the MGPPBs (Fig.6). The sharp diffraction peak of GO at 2θ=11.2 ° features an interlayer spacing of 0.79 nm, corresponding to the diffraction of (002) plane of GO. In addition, a weak and broad peak of GO appears at 2θ=21.1 ° corresponding to an interlayer spacing of 0.42 nm, which is caused by unexfoliated graphite [30]. After introduction of GO into the coatings, the two major diffraction peaks of GO disappear in the MGPPB3 and MGPPB4, indicating that the amino resin and PPB are intercalated into the layers of GO that achieve well dispersion and completely exfoliated state of GO nanosheets. This is consistent with the observation from SEM images.

Fig.6. XRD patterns of GO and the resulting transparent coatings

3.3 Hardness and adhesion analyses The pencil hardness and adhesion classification of the coatings are listed in Table 2. It can be seen that the introduction of GO slightly increases the pencil hardness of the coatings, and the pencil hardness of MGPPB4 is increased to 2B. In addition, the adhesion classification of the coatings is slightly increased to 2B and then the decreased to 3B with increasing GO content, indicating that a moderate content of GO is beneficial to impart the optimum adhesion property to the coatings. And, an excessive content of GO will greatly decrease the acid number of the organophosphate concomitant with the decrease of cross-linking activity, resulting in the decrease of crosslinking density and adhesion property of the amino coatings. Table 2 Hardness and adhesion properties of the transparent coatings Samples Pencil hardness Adhesion classification

MGPPB0

MGPPB1

MGPPB2

MGPPB3

MGPPB4

3B

3B

3B

2B

2B

4B

4B

3B

2B

3B

3.4 Fire protection tests The results from the cabinet method and tunnel method tests are listed in Table 3. It can be found that the application of transparent fire-retardant coatings greatly decreases the weight loss, char index and FSR values of the samples, indicating the improvement of fire protection performance. In addition, the introduction of GO can further improve the fire protection performance of the samples, and MGPPB3 exhibits the lowest weight loss (2.3 g), char index (3.3 cm3) and FSR (6.9) concomitant with the highest intumescent factor of 90.5 among the samples. By combining Fig. 7, it can be found that the coatings containing GO generate a more compact and intumescent char that provides an effective physical barrier against the heat and mass transfer, and MGPPB3 exhibits the densest and highest char among the samples in accordance with the best fire protection performance. In addition, it is noted that an excessive content of GO will suppress the swelling process and diminish its positive effect on improving the barrier effect and fire protection performance of the coatings. Table 3 Fire protection performance of the samples assessed by the cabinet method and tunnel

method tests Samples

Uncoated

MGPPB0

MGPPB1

MGPPB2

MGPPB3

MGPPB4

Weight loss (g)

15.5±0.6

3.2±0.5

3.0±0.3

2.8±0.3

2.3±0.1

2.5±0.1

3

Char index (cm )

35.6±3.2

11.3±0.6

9.1±2.1

8.4±0.2

3.3±0.5

6.4±1.2

FSR

66.2±4

17.5±3

12.7±2

10.7±1

6.9±1

8.7±1

0

35.5±5

48.0±4

63.5±5

90.5±4

71.3±5

Intumescent factor

Fig.7. Digital photographs of the char residues after the cabinet method test

The heat insulation property of the coatings was assessed by the big panel method test, and the results are presented in Fig.8. It can be seen that the backside temperature of uncoated sample increases rapidly and reaches 247.3 °C at 200 s, and the application of the coatings gradually decreases the backside temperature of the samples. From MGPPB0 to MGPPB4, the equilibrium backside temperature at 900s is 229.3, 205.4, 189.4, 156.9 and 168.8 °C, respectively. In particular, MGPPB3 exhibits the best heat insulation property as well as fire protection performance among the samples, coinciding with that obtained by the cabinet method and tunnel method tests. Based on the above results, it can be found that the introduction of GO exerts excellent synergistic effect on enhancing the fire protection performance of the transparent coatings, and this synergistic effect depends on the content of GO.

Fig.8. Backside time-temperature curves of the samples assessed by the big panel method test

3.5 Smoke density test Light absorption curves of the transparent fire-retardant coatings are presented in Fig.9. It can be seen that the light absorption value of the coatings continuously increases and reaches the maximum value at 240 s. When GO is introduced, the GPPBs impart lower light absorption values to their resulting coatings than those of PPB, indicating the reduction of smoke release. In particular, MGPPB3 containing GPPB3 exhibits the lowest light absorption value of 31.8% at 240 s, and this value is 42.6% lower than that of MGPPB0 containing PPB.

Fig.9. Light absorption curves of the transparent fire-retarded coatings

The smoke density rating (SDR) values of the coatings are presented in Fig.10. It is obviously seen that the introduction of GO greatly decreases the SDR value of the coatings, and MGPPB3 possesses the lowest SDR value of 14.2% concomitant with the best smoke suppression effect among the samples. From the above results, it can be concluded that the introduction of GO performs well synergistic smoke suppression effect in the transparent coatings, and an excessive content of GO will diminish the smoke suppression efficiency.

Fig.10. Smoke density rating (SDR) values of the transparent fire-retarded coatings

3.6 Cone calorimeter test The heat release rate (HRR) and total heat release (THR) curves of the transparent fire-retardant coatings are presented in Fig.11a and Fig.11b, respectively. It can be observed that MGPPB0 burns rapidly after ignition with the highest peak HRR (PHRR) value and THR value of 120.4 kW/m2 and 2.4 MJ/m2, respectively. As expected, the introduction of GO decreases the PHRR and THR values of the coatings, indicating that the presence of GO is effective in reducing the heat release of the coatings. In detail, the PHRR values from MGPPB1 to MGPPB4 are 113.6, 106.8, 90.8 and 96.6 kW/m2, respectively, and the THR values from MGPPB1 to MGPPB4 are 2.1, 2.0, 1.6 and 1.8 MJ/m2, respectively. Among the above coatings, MGPPB3 exhibits the lowest PHRR and THR values, and these values are reduced by 24.6 % and 33.3 %, respectively, compared to the values of MGPPB0. By combining the char residues of Fig.10c, it can be found that the introduction of GO is beneficial to produce a more compact and intumescent char that effectively prevents the underlying

materials from further combustion, thus exhibiting less heat release during burning. In particular, MGPPB3 shows the highest intumescent char concomitant with the lowest heat release among the samples, further verifying that a moderate content of GO is crucial to exhibit the optimum synergistic flame-retarded effect in the coatings. This is consistent with the results obtained from the fire protection tests.

(c) MGPPB0

MGPPB1

MGPPB2

MGPPB3

MGPPB4

Fig.11 Heat release rate curves (a), total heat release curves (b) and digital photos of char residues (c) of the transparent fire-retardant coatings after the cone calorimeter test

3.7 TG analysis The TG and DTG curves of the transparent fire-retardant coatings are presented in Fig.12, and the related data are listed in Table 4. And, Ton is defined as the onset decomposition temperature at 5% weight loss, Tmax means the temperature of peak mass loss rate, and PMLR is defined as the mass loss rate at Tmax. It can be observed from Fig.12 that the coatings exhibit five decomposition processes in the temperature ranges of 60-200 °C, 200-280 °C, 280-450 °C, 450-550 °C and 550-700 °C. The first stage at 60-200 °C is ascribed to the release of small molecules accompanying with a little weight loss (<7.5 wt%). The second stage at 200-280°C corresponding to a strong DTG peaks is ascribed to the breakage of P– O–C groups in the structure of PPB and the GPPBs, and the GPPBs impart higher PMLR value and weight loss to the resulting coatings than those of PPB at this stage. The third stage at 280-450 °C is the dominated one and is assigned to the decomposition of flame retardants and amino resin. In this stage, the phosphate

derivatives and ethylene glycol ester released from phosphate interact with triazine compounds and incombustible gases released from amino resin to form multicellular char layer. The fourth stage at 450-550 °C is ascribed to the aromatization and polymerization of unstable double bonds or structures, corresponding to a weak DTG peak. The fifth stage at 550-700 °C is assigned to the decomposition of unstable carbonized backbones in the char residues. As shown in Table 4, it can be seen that the introduction of GO greatly improves the Ton and residual weight at 700 °C of the coatings concomitant with the decrease of mass loss rate and PMLR at the main decomposition stage, indicating the enhancement of thermal stability and char formation. And, the increased residual weight is beneficial for reducing the amounts of combustible gases and smoke precursors during combustion, thus decreasing the heat release and smoke production. In particular, MGPPB3 exhibits highest residual weight of 34.5 % at 700 °C among the samples, corresponding to the lowest smoke production and heat release. According to above results, it can be concluded that the introduction of GO plays a positive effect on improving the thermal stability and char formation of the coatings.

Fig.12. TG (a) and DTG (b) curves of the transparent fire-retardant coating under nitrogen atmosphere at a heating rate of 10 °C/min Table 4 Thermal parameters of the transparent fire-retardant coatings under nitrogen atmosphere at a heating rate of 10 °C/min Samples

Ton/°C

Tmax/°C

PMLR/(%/min)

Residue at 700 °C /%

MGPPB0 MGPPB1 MGPPB2 MGPPB3 MGPPB4

171.2 176.4 179.5 183.3 186.1

270.2b, 329.2c,486.3d 253.6b,330.8c,492.7d 253.6b,333.9c,495.9d 255.3b,332.7c,492.4d 253.6b,332.3c,486.3d

2.1b,3.0c,1.6d 3.4b,2.6c,1.6d 3.2b,2.5c,1.5d 3.0b,2.2c,1.4d 2.9b,2.4c,1.4d

31.1 31.9 32.9 34.5 33.7

Note: superscript b represents the second degradation stage, c denotes the third degradation stage, and d illustrates the fourth degradation stage.

3.8 SEM-EDS analysis The SEM images and EDS maps of the char residues after the cabinet method test are shown in Fig.13. As can be observed from Fig. 13(a) that the char residue of MGPPB0 has a lot of voids and holes with different sizes that provide channels for the transport of oxygen and combustible volatiles between the inner matrix and the flame. In Fig. 13(b), the char residue of MGPPB3 is very compact and continuous with few holes and voids, thus providing an effective physical barrier against the heat transfer and mass transportation. The results of EDS maps show that MGPPB3 remains more P and B elements in the char residue than that of MGPPB0. Generally,

the char residue rich in P and B elements is beneficial to generate more crosslinking structures in the condensed phase that can enhance the barrier effect and thermal stability of char. In addition, the high C/O ratio in the char residue of MGPPB3 further illustrates that the introduction of GO contributes to enhance the cross-linking density and anti-oxidation ability of the char residue, thus exhibiting better compactness and barrier effect.

Fig.13. SEM images and EDS maps of the char residues after the cabinet method test: (a) MGPPB0 and (b) MGPPB3

3.9 XPS analysis The elemental information and chemical structures of the char residues after the cabinet method test are analyzed by XPS, and the detailed composition is listed in Table 5. As shown in Table 5, the O/C, N/C and P/C ratios of MGPPB3 char are higher than those of MGPPB0, indicating more P, N and O-containing cross-linking structures remained in the MGPPB3 char. This result can be ascribed the fact that the barrier effect of GO sheets can effectively reduce the release of NH3, volatile phosphorus and boron, thus leaving more P, N and B for char formation. Table 5 XPS data of the char residues obtained after the cabinet method test Samples

C, at%

O, at%

N, at%

P, at%

B, at%

O/C

N/C

P/C

MGPPB0 MGPPB3

50.5 31.2

34.0 47.3

6.1 7.7

7.1 10.3

2.3 3.5

0.67 1.52

0.12 0.25

0.14 0.33

The full-scan XPS spectra and high-resolutions of C1s, N1s, O1s, P2p and B1s XPS spectra of the char residues after the cabinet method test are presented in Fig.14, and

the corresponding results are listed in Table 6. As shown in Fig. 14a, the char residues of MGPPB0 and MGPPB3 mainly contain carbon, nitrogen, oxygen, phosphorus and boron elements. The C1s spectra present three bands at 284.5, 285.0 and 286.3 eV as shown in Fig. 14b, and the peak at 284.5 eV is assigned to the C–H and C–C groups of aliphatic and aromatic species [31]. As for N1s spectra, three bands are found at 399.5, 400.2 and 402.1 eV that can be assigned to the C=N groups in triazine rings, C–N groups and N–H groups, respectively [32, 33]. As shown in Fig. 14d, the O1s spectra show two bands at 531.6 and 533.1 eV that is assigned to the =O– groups (C=O and P=O groups) and –O– groups (C−O−C, P−O−P and/or C−OH groups), respectively [34]. The P2p spectra present two bands at 134.0 and 134.6 eV as shown in Fig.14e that can be assigned to the P−O−C, P−O−P and/or PO3− groups in phosphorus-rich cross-links [35]. As shown in Fig. 14f, two peaks at 190.2 and 192.2 eV in the B1s spectra can be attributed to B−O groups (B−OH and B−O−B) and B−O−C groups, respectively [16]. It can be observed from Table 6 that MGPPB3 produces more C−N, P−O−C, P−O−P and B−O−C groups in the char compared to those of MGPPB0, revealing that the introduction of GO is beneficial for generating more crosslinking structures that enhance thermal stability and barrier effect of char against the mass transportation and heat transfer during burning.

Fig.14. XPS spectra of the char residues obtained from the cabinet method test: (a) a full-scan, (b) C1s XPS spectra, (c) N1s XPS spectra, (d) O1s XPS spectra, (e) P2p XPS spectra, and (f) B1s XPS spectra Table 6 Fitting results of C1s, N1s, O1s, P2p and B1s spectra of the char residues obtained after the cabinet method test Elements

C1s

Binding energy/eV

Groups

MGPPB0/at%

MGPPB3/at%

284.5

C–H and C–C C–O, C–N and C–O–P C=O, C=C and C=N

8.7

6.9

26.0

13.9

15.8

10.4

0.7

1.1

285.0 286.3

N1s

399.5

C=N

400.2 402.1

C–N N–H

2.8 2.6

3.4 3.2

O1s

531.6 533.1

=O– group –O– group

12.4 21.6

23.0 24.3

P2p

134.0 134.6

P–O and/or PO3 P–O–C

1.7 5.4

4.8 5.5

B1s

190.2 192.2

B–O B–O–C

0.3 2.0

0.8 2.7

3.10 Flame-retardant and smoke suppression mechanism An illustration of the potential flame-retardant and smoke suppression mechanism of the GPPBs in the transparent fire-retardant coatings is presented in Fig.15. The mechanism of the GPPBs in the transparent coatings is ascribed to both gas phase and condensed phase mechanisms. When exposed to heat or flame, the coatings begin to decompose and release incombustible gases (H2O and NH3) that dilute the combustibles gases and reduce the combustion intensity. With the temperature increasing, the phosphorus-based species (mainly PO· and P· radicals) released from PPB and GPPBs react with free radicals (mainly HO· and H· radicals) produced by amino resin, which can effectively reduce the degradation products of the coatings escaping into the gas phase. As a result, the decrease of degradation products leads to a reduction in smoke production and heat release during burning. More importantly, the interaction between GO sheets and phosphate ester or its derivatives leads to generate more crosslinking structures in the condensed phase that strengthen the compactness, anti-oxidation ability and insulation property of the char layer, whose “barrier effect” could effectively isolate the heat, oxygen and combustible gas transfer, thus preventing the underlying materials from further combustion. Based on the above discussion, the flame retardancy and smoke suppression properties of the coatings containing GPPBs are greatly improved.

Fig.15. Potential flame-retardant and smoke suppression mechanism of the GPPBs in the transparent fire-retardant coatings

4 Conclusions In this work, a series of efficient organophosphate-functionalized GO flame retardants (GPPBs) were synthesized by functional modification of graphite oxide with PPB, and FTIR and 1H NMR spectra confirm the successful grafting of PPB on the surface of GO. Then, the obtained GPPBs were incorporated into amino resin to produce transparent fire-retardant coatings. The results show that the introduction of GO can improve the hardness and adhesion property of the coatings, and MGPPB3 shows the best adhesion property among the samples. Transparency analysis reveals that the uniform dispersion and completed exfoliated states of GO sheets in the amino matrix can retain the high transparency in the coatings even at high GO contents, as determined by XRD pattern and SEM images. Fire protection tests show that the uniform dispersion of GO sheets can greatly decrease the weight loss, char index and FSR values of the coatings concomitant with the increase of insulation property and intumescent factor, and MGPPB3 possesses the lowest SDR value of 14.2% and highest intumescent factor of 90.5 among the samples. Cone calorimeter and smoke density tests indicate that the presence of GO sheets greatly decreases the heat release and smoke production of the coatings, exhibiting excellent synergistic flame retardant

and smoke suppression effects. Especially, MGPPB3 shows the best synergistic effect among the samples, for example, 60.7 % reduction in SDR value and 33.3 % reduction in THR value are observed for MGPPB3 compared to those of MGPPB0. TG results show that the introduction of GO plays an positive effect on improving the thermal stability and char formation of the coatings, and MGPPB3 exhibits the highest residual weight of 34.5 % at 700 °C. Char residue analysis shows that the remarkable enhancements in the flame retardancy and smoke suppression properties are largely ascribed to the high-quality char with outstanding compactness, anti-oxidation ability and insulating property, which are able to isolate the transport of volatile degradation products and heat between gas phase and condensed phase during burning. And, the improvement in the yield, anti-oxidative ability and barrier effect of intumescent char is mainly attributed to the fact that the introduction of GO results in the formation of more crosslinking structures containing C−N, P−O−C, P−O−P and B−O−C groups in the condensed phase. In summary, the results provide a new insight into the application of functionalized GO in the design and preparation of super transparent fire-retardant coatings.

Notes The authors declare no conflicts of interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51676210 and 51906261) and the Natural Science Foundation of Hunan Province (No. 2018JJ3668).

References [1] B. Guo, Y. Liu, Q. Zhang, F. Wang, Q. Wang, Y. Liu, J. Li, H. Yu, Efficient flame-retardant and smoke-suppression properties of Mg-Al layered double hydroxide nanosheets on wood substrate, ACS Appl. Mater. Inter. 9 (2017) 23039-23047. [2] Q. Fu, L. Medina, Y. Li, F. Carosio, A. Hajian, L.A. Berglund, Nanostructured wood hybrids for fire retardancy prepared by clay impregnation into the cell wall, ACS Appl. Mater. Inter. 41 (2017) 36154-36163. [3] M. Nikolic, J.M. Lawther, A.R. Sanadi. Use of nanofillers in wood coatings: a scientific review, J. Coat. Technol. Res. 12 (2015) 445-461.

[4] S. He, W. Wu, M. Zhang, H. Qu, J. Xu, Synergistic effect of silica sol and K2CO3 on flame-retardant and thermal properties of wood, J. Therm. Anal. Calorim. 128 (2017) 825-832. [5] F. Carosio, F. Cuttica, L. Medina, L.A. Berglund. Clay nanopaper as multifunctional brick and mortar fire protection coating-wood case study, Mater. Design 93 (2016) 357-363. [6] Z. Xiao, S. Liu, Z. Zhang, C. Mai, Y. Xie, Q. Wang, Fire retardancy of an aqueous, intumescent, and translucent wood varnish based on guanylurea phosphate and melamine-urea-formaldehyde resin, Prog. Org. Coat. 121 (2018) 64-72. [7] W.N. Nkeuwa, B. Riedl, V. Landry, Transparent UV-cured clay/UV-based nanocomposite coatings on wood substrates: surface roughness and effect of relative humidity on optical properties, J. Coat. Technol. Res. 14 (2017) 555-569. [8] L. Yan, Z. Xu, D. Liu, Synthesis and application of novel magnesium phosphate ester flame retardants for transparent intumescent fire-retardant coatings applied on wood substrates, Prog. Org. Coat. 129 (2019) 327-337. [9] W. Xing, P. Zhang, L. Song, X. Wang, Y. Hu, Effects of alpha-zirconium phosphate on thermal degradation and flame retardancy of transparent intumescent fire protective coating, Mater. Res. Bull. 49 (2014) 1-6. [10] X. Liu, J. Guo, J. Sun, X. Gu, W. Feng, W. Liu, H. Li, S. Zhang, The preparation of a bisphenol A epoxy resin based ammonium polyphosphate ester and its effect on the char formation of fire resistant transparent coating, Prog. Org. Coat. 129 (2019) 349-356. [11] G. Wang, Y. Huang, X. Hu, Synthesis of a novel phosphorus-containing polymer and its application in amino intumescent fire resistant coating, Prog. Org. Coat. 76 (2013) 188-193. [12] C. Tian, C. Wang, X. Ren, L. Hong, Synthesis of silane-modified polyphosphate esters and its application in transparent flame-retardant coatings, J. Appl. Polym. Sci. 136 (2019) 47199. [13] J. Alongi, Z. Han, S. Bourbigot, Intumescence: Tradition versus novelty. A comprehensive review, Prog. Polym. Sci. 51 (2015) 28-73. [14] Z. Xu, Z. Chu, L. Yan, Enhancing the flame-retardant and smoke suppression properties of transparent intumescent fire-retardant coatings by introducing boric acid as synergistic agent, J. Therm. Anal. Calorim. 133 (2018) 1241-1252. [15] Y. Shi, G. Wang, The novel silicon-containing epoxy/PEPA phosphate flame retardant for transparent intumescent fire resistant coating, Appl. Surf. Sci. 385 (2016) 453-463. [16] L. Yan, Z. Xu, N. Deng, Effects of polyethylene glycol borate on the flame retardancy and smoke suppression properties of transparent fire-retardant coatings applied on wood substrates, Prog. Org. Coat. 135 (2019) 123-134. [17] L. Yan, Z. Xu, X. Wang, Influence of nano-silica on the flame retardancy and smoke suppression properties of transparent intumescent fire-retardant coatings, Prog. Org. Coat. 112 (2017) 319-329. [18] L. Yan, Z. Xu, X. Wang, Synergistic effects of organically modified montmorillonite on the flame-retardant and smoke suppression properties of transparent intumescent fire-retardant coatings, Prog. Org. Coat. 122 (2018) 107-118. [19] B. Yu, Y. Tao, L. Liu, Y. Shi, H. Yang, G. Jie, S. Lo, Q. Tai, L. Song, Y. Hu, Thermal and flame retardant properties of transparent UV-curing epoxy acrylate coatings with POSS-based phosphonate acrylate, RSC Adv. 5 (2015) 75254-75262. [20] X. Shi, X. Peng, J. Zhu, G. Lin, T. Kuang, Synthesis of DOPO-HQ-functionalized graphene oxide as a novel and efficient flame retardant and its application on polylactic acid: Thermal property,

flame retardancy, and mechanical performance, J. Colloid. Interf. Sci. 524 (2018) 267-278. [21] X. Wang, E.N. Kalali, J. Wan, D. Wang, Carbon-family materials for flame retardant polymeric materials, Prog. Polym. Sci. 69 (2017) 22-46. [22] J. Loste, J. Lopez-Cuesta, L. Billon, H. Garay, M. Save, Transparent polymer nanocomposites: An overview on their synthesis and advanced properties, Prog. Polym. Sci. 89 (2019) 133-158. [23] W. Xu, X. Wang, X. Wu, W. Li, C. Cheng, Organic-Inorganic dual modified graphene: Improving the dispersibility of graphene in epoxy resin and the fire safety of epoxy resin, Polym. Degrad. Stabil. 165 (2019) 80-91. [24] Y. Shi, B. Yu, Y. Zheng, J. Yang, Z. Duan, Y. Hu, Design of reduced graphene oxide decorated with DOPO-phosphanomidate for enhanced fire safety of epoxy resin, J. Colloid. Interf. Sci. 521 (2018) 160-171. [25] S. Ran, F. Fang, Z. Guo, P. Song, Y. Cai, Z. Fang, H. Wang, Synthesis of decorated graphene with P, N-containing compounds and its flame retardancy and smoke suppression effects on polylactic acid, Composites Part B: Engineering 170 (2019) 41-50. [26] C. Bao, Y. Guo, B. Yuan, Y. Hu, L. Song, Functionalized graphene oxide for fire safety applications of polymers: a combination of condensed phase flame retardant strategies, J. Mater. Chem. 22 (2012) 23057-23063. [27] Z. Wang, W. Wu, M.H. Wagner, L. Zhang, S. Bard, Synthesis of DV-GO and its effect on the fire safety and thermal stability of bismaleimide, Polym. Degrad. Stabil. 128 (2016) 209-216. [28] G.Z. Papageorgiou, Z. Terzopoulou, V. Tsanaktsis, D.S. Achilias, K. Triantafyllidis, E.K. Diamanti, D. Gournis, D.N. Bikiaris, Effect of graphene oxide and its modification on the microstructure, thermal properties and enzymatic hydrolysis of poly(ethylene succinate) nanocomposites, Thermochim. Acta 614 (2015) 116-128. [29] W. Guo, B. Yu, Y. Yuan, L. Song, Y. Hu, In situ preparation of reduced graphene oxide/DOPO-based phosphonamidate hybrids towards high-performance epoxy nanocomposites, Composites Part B: Engineering. 123 (2017) 154-164. [30] K. Zhou, Z. Gui, Y. Hu, S. Jiang, G. Tang, The influence of cobalt oxide-graphene hybrids on thermal degradation, fire hazards and mechanical properties of thermoplastic polyurethane composites, Composites Part A: Applied Science and Manufacturing 88 (2016) 10-18. [31] L. Yan, Z. Xu, X. Wang, N. Deng, Z. Chu, Preparation of a novel mono-component intumescent flame retardant for enhancing the flame retardancy and smoke suppression properties of epoxy resin, J. Therm. Anal. Calorim. 134 (2018) 1505-1519. [32] B. Yuan, A. Fan, M. Yang, X. Chen, Y. Hu, C. Bao, S. Jiang, Y. Niu, Y. Zhang, S. He, H. Dai, The effects of graphene on the flammability and fire behavior of intumescent flame retardant polypropylene composites at different flame scenarios, Polym. Degrad. Stabil. 143 (2017) 42-56. [33] Z. Xu, Z. Chu, L. Yan, H. Chen, H. Jia, W. Tang, Effect of chicken eggshell on the flame-retardant and smoke suppression properties of an epoxy-based traditional APP-PER-MEL system, Polym. Composite. 40 (2019), 2712-2723. [34] Y. Guan, J. Huang, J. Yang, Z. Shao, Y. Wang, An effective way to flame-retard biocomposite with ethanolamine modified ammonium polyphosphate and its flame retardant mechanisms, Ind. Eng. Chem. Res. 54 (2015) 3524-3531. [35] P. Wang, L. Xia, R. Jian, Y. Ai, X. Zheng, G. Chen, J. Wang, Flame-retarding epoxy resin with an efficient P/N/S-containing flame retardant: Preparation, thermal stability, and flame retardance. Polym. Degrad. Stabil. 149 (2018) 69-77.

Research Highlights of Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings

1. 2. 3. 4.

Organophosphate-functionalized graphene oxide (GPPBs) was synthesized and studied The uniform dispersion of GO sheets maintains the high transparency of the coatings The GPPBs impart superior flame retardancy and smoke suppression properties to the coatings Remarkable enhancement on barrier effect of char was achieved after introducing GO.

Authors’ contributions Dear editor, The paper entitled “Synthesis of organophosphate-functionalized graphene oxide for enhancing the flame retardancy and smoke suppression properties of transparent fire-retardant coatings” was contributed to Long Yan, Zhisheng Xu and Nan Deng. The detail of author’s contributions in this paper are as following: Dr. Yan and Prof. Xu conceived and designed the study, Mr. Deng performed the experiments, Dr. Yan wrote the paper, and Prof. Xu reviewed and edited the manuscript. All authors read and approved the manuscript.

Declaration of interests  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. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: