Synergistic effects of a highly effective intumescent flame retardant based on tannic acid functionalized graphene on the flame retardancy and smoke suppression properties of natural rubber

Synergistic effects of a highly effective intumescent flame retardant based on tannic acid functionalized graphene on the flame retardancy and smoke suppression properties of natural rubber

Journal Pre-proofs Synergistic effects of a highly effective intumescent flame retardant based on tannic acid functionalized graphene on the flame ret...

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Journal Pre-proofs Synergistic effects of a highly effective intumescent flame retardant based on tannic acid functionalized graphene on the flame retardancy and smoke suppression properties of natural rubber Lin Li, Xiaolin Liu, Xiaoming Shao, Licong Jiang, Kai Huang, Shuai Zhao PII: DOI: Reference:

S1359-835X(19)30464-6 https://doi.org/10.1016/j.compositesa.2019.105715 JCOMA 105715

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

6 June 2019 25 October 2019 24 November 2019

Please cite this article as: Li, L., Liu, X., Shao, X., Jiang, L., Huang, K., Zhao, S., Synergistic effects of a highly effective intumescent flame retardant based on tannic acid functionalized graphene on the flame retardancy and smoke suppression properties of natural rubber, Composites: Part A (2019), doi: https://doi.org/10.1016/ j.compositesa.2019.105715

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© 2019 Published by Elsevier Ltd.

Synergistic effects of a highly effective intumescent flame retardant based on tannic acid functionalized graphene on the flame retardancy and smoke suppression properties of natural rubber Lin Li*, Xiaolin Liu, Xiaoming Shao, Licong Jiang, Kai Huang, Shuai Zhao* Key Lab of Rubber-plastics, Ministry of Education/Shandong Provincial Key Lab of Rubber-plastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China ‫כ‬Corresponding author E-mail address: [email protected](L. Li) and [email protected] (S. Zhao). Abstract Tannic acid (TA), natural phenolic compounds abundant in many plants, exhibit low flammability and good absorbility because of multidentate properties. Based on it, a novel intumescent flame retardant (IFR) system (AGT) that consists of ammonium polyphosphate (APP)/TA functionalized graphene (TGE) presents synergistic flame retardant and smoke suppression for natural rubber (NR) due to the dual flame retardant functions of each component. The novel IFR (AGT) system is of interest to a variety of fields because of its distinct flame retardant and relatively good mechanical properties in comparison to traditional IFR system. Cone calorimeter results reveal that the AGT system could clearly change the decomposition behavior of NR and form a strong a highly graphitized and phosphorous-containing carbonaceous structure char layer on the surface of the composites, consequently resulting in synergistic effects on the flame retardancy and smoke suppression properties. Moreover, TGE endows NR composite excellent mechanical properties within an effective additive mass of AGT system. 1

Keywords: A. Graphene; A. Nanocomposites; B. Thermal properties; E. Cure 1 Introduction For environmental concerns, intumescent flame retardant (IFR) is considered as a promising halogen-free flame retardant with the advantages of anti-drop, low fire toxicity and low smoke

[1-2]

. Typically, IFR consists of three components, including

an acid source, a carbonization agent charring agent and a gas source

[3]

. During

combustion, the IFR produces an expanded multicellular char layer, which acts as a physical barrier against heat transmission and oxygen diffusion to protect the underlying material from the action of fire, thus prevents materials pyrolysis into volatile combustible compounds

[4]

. In traditional IFR, polyammonium phosphate

(APP) is superior to other flame retardants due to its small load, no additional smoke, low cost and good processing performance [5]. However, in comparison with halogenbased retardant, the traditional APP-based IFR suffers from the drawbacks of weak moisture resistance and low flame retardant efficiency

[6-9]

.

To solve these issues, great efforts have been made to develop high performance APP-based IFR system

[10-13]

. While these achievements are quite impressive, the

efficiency and environmental protection of APP-based IFR can be further improved. Some synergists including carbon nanomaterials have been used as adjuvants for IFR to enhance its overall performances

[14-15]

. Graphene (GE) are a commonly used

carbonbased nanofillers for improving the flammability flame retardance of polymer due to its excellent fire retardant effect

[16]

. However, strong interactions between

molecules limits limit the manipulation and processing of graphene, resulting in their uneven dispersion in the polymer matrix. In recent years, efforts have been made to functionalize graphene with the aim of improving its solubility and compatibility with polymers by through changing its surface properties 2

[17-20]

. In most cases, their

dispersion is improved, though however, it simultaneously deteriorates their flame retardancy. In our previous work, we have proposed the direct liquid-phase exfoliation of graphite into graphene using TA as stabilizer and regulator for aqueous exfoliation of graphite [21]

. TA, phenolic-based natural products found mostly in the bark of pine, the wattle

of mimosa and hemlock and in the wood of certain trees such as quebracho and sumach, have many special properties, such as antioxidant effects antibacterial effects capability

[26]

[24]

, the ability to precipitate proteins

[25]

[22]

, inhibition

[23]

,

, and a reducing

. Moreover, mainly concentrated in the bark of trees, TA are an a

naturally efficient intumescent flame retardant because during a fire, TA efficiently reduce oxidants and radicals to maximize the unburned solid and charred remain, aiding tree survival [27]. Inspired by such a natural fire-protecting strategy, an efficient natural TA-based intumescent flame-retardant system (AGT) has been designed in this study by applying TA functionalized graphene combined with APP. For AGT system, each component has a dual identity identities, such APP both acting as acid source and gas source, TA both acting as acid source and carbonization agent and graphene both acting as carbonization agent and carbon layer reinforcing agent. In this work, AGT intumescent flame-retardant system is mainly focused on the flame-retardany effects of rubber, as shown in Scheme 1. Rubber, especially for NR, is an important commercial material widely being used not only for automobile and industrial applications but also for space components

[28]

. However, it suffers from some

weaknesses especially high fire hazard for such applications. Unfortunately, the addition of fire retardant can remarkably change the mechanical behavior of rubber materials. Their fire retardant action is accompanied by negative effects, such as 3

contamination of mechanical properties. The developed AGT intumescent flameretardant can simultaneously endow NR with the functions of flame retardancy and reinforcing effect within an appropriate loading. play both flame retardancy and reinforcing effect for rubber materials within an appropriate loading. The thermal degradation and combustion properties of AGT/NR composites are demonstrated using thermogravimetric and microscale combustion calorimetric analyses. The formation microstructure of intumescent char is examined using SEM-EDX, FTIR and Raman. Flame retardant mechanism is proposed based on the analysis of thermal decomposition behavior of polymer composites and char features. 2 Experimental and materials 2.1 Materials Ammonium polyphosphate (APP, type is TF-201, the degree of polymerization is 1500) was generously supplied by Shandong Taifeng New Flame Retardant Co., Ltd, China. Graphene (GE) was supplied by the sixth element (Changzhou) material technology Co. Ltd. (China). Tannic acid (TA) was purchased from Aladdin Technology Co., LTD (China). NR latex (solid content: 60 wt %) was provided by Qingdao Double Butterfly Group Co., Ltd. (China). Other reagents, including sulfur, N-cyclohexyl-2-benzothiazolesulfenamide (CZ), 2,2-dibenzothiazoledisulfide (DM), zinc oxide (ZnO), and N98 (98% SiO2, average diameter: 200 nm) were all industrial grade and kindly supplied by Qingdao Topsen Chemical co. LTD (China). 2.2 Characterization Raman spectra were recorded using a high-resolution FRS-100S (Bruker) machine with a CCD detector. The spectral purity is quoted to be < 700 at wavelength > 2,100 cm-1 from a set wavelength. The infrared spectra were obtained with a Bruker Vertex 4

70 variable temperature Fourier Transform Infrared Spectrometer. The morphologies of the residual carbon layer were observed using JSM-6700F SEM-EDX (Japan Electronics Corp.). The SEM samples were coated with gold for 60 s. TGA measurement was carried out using a TGA-7 type thermo-analysis instrument (Perkine Elmer Company, USA) from room temperature to 800 °C at a heating rate of 10 °C/min under N2 atmosphere. Dynamic mechanical analysis (DMA) was performed on a DMA242 machine (NETZSCH) in tensile mode with a temperature increment of 3°C·min−1. The frequency and stain were fixed at 1 Hz and 0.5%, respectively. Limited Oxygen Index (LOI) was measured using a HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China) according to the standard oxygen index test ASTM D2863. The specimen dimension were is 130*6.5*3.2 mm3. UL-94 Vertical Burning Test was performed using a vertical burning instrument (CFZ-1 type, Jiangning Analysis Instrument Co., China) according to UL-94 test ASTM D3801-2010. The dimensions dimension of samples were is 130*13*3.2 mm3. The Cone Calorimeter Test (CCT) was performed on the cone calorimeter (Fire Testing Technology, U.K.) according to ASTM E1354/ISO 5660. The dimensions dimension of samples were is 100*100*3.2 mm3. Each specimen was wrapped in an aluminum foil and exposed horizontally to 35 kW/m2 external heat flux. The thermal conductivity was measured by a laser flash system (LFA 447 NanoFlash). Tensile and tear testing were carried out using an AI-7000S Universal Material Tester, with a dumbbell specimen at a tensile speed of 500 mm·min-1 according to ISO 528:2009. The abrasion test was carried out with the DIN abrader according to ISO-4649. The abrasion was designed as a volumetric loss of a strip sample. The optimum curing time of NR and its composites was determined at 150 °C by an ALPHA MDR2000 UCAN rheometer. 5

2.3 Preparation of TGE Variable GE was added into a vial containing 30 ml TA aqueous solution which the concentration of TA is variable according to the batch compositions shown in Scheme 1 Table S1. The mixture was then sonicated for 30 min in a 30-35°C water bath using a bath sonicator (Elmasonic E30H, 40 W, 37 kHz) to prepare TGE [21]. 2.4 Preparation of the batches A certain amount of TGE aqueous dispersion (condensed into 5 mg·mL-1) was slowly added into NR latex under vigorous mechanical stirring. After coagulation upon the addition of sodium chloride aqueous solution (1 wt %), the TGE/NR compounds were cut into pieces and washed with distilled water to thoroughly remove the sodium chloride. Then, TGE/NR compounds were dried in a vacuum oven at 60 °C overnight. Mixing was performed in a 200 ml Banbury mixer at a rotor speed of 60 rpm for the mixing stage at a temperature of 120 °C. A certain amount of TGE/NR compounds was fed into the mixer and premixed for 2 min. This was followed by the sequential addition of N98 (5 phr), ZnO (5phr), APP (variable), N330 (30 phr) and mixed for 8 min. And then the mixture was discharged onto a two roll mill at 40 °C, where the sulfur (1 phr) , CZ (1.2 phr) and DM (0.6 phr) was were added. The prepared composite is labeled [email protected], as shown in Scheme 1, and the batch compositions for [email protected] and AGT system are respectively shown in Table 1 and Table S1. For comparison purpose, reference sample NR/APP composites (designed as [email protected]) was prepared from APP and NR subjecting to the same procedures that were used to prepare [email protected]

6

Scheme 1 The preparation process for the flame retarded [email protected] composites. Table 1 Batch compositions. Samples

NR Latex

CB

ZnO

CZ

DM

S

N98

Amount (g)

166.7

30

5

1.2

0.6

1

5

APP

variable variable

3 Results and discussion 3.1 Flame retardant properties of [email protected] and [email protected] composites

7

AGT

Table 2 LOI and UL-94 tests experimental data for various samples.

Samples Neat NR [email protected] [email protected] [email protected] [email protected] [email protected] [email protected]

NR (phr) 100 100 100 100 100 100 100

APP (phr) -20 14.5 40 29.1 60 43.6

GE (phr) --3.67 -7.27 -10.93

TA (phr) --1.83 -3.63 -5.47

LOI (%) 19.5 20.5 21.5 24.5 25.1 30.5 32.1

UL-94

Dripping

NC NC NC V-2 V-1 V-0 V-0

Yes Yes Yes Yes NO NO NO

The LOI values and UL-94 rating are experimentally and theoretically investigated to figure out the thermal combustion properties of [email protected] and [email protected] The neat NR burns fiercely after ignited and accompanies with dripping. As shown in Table 2, the LOI data show that the neat NR is flammable for its LOI value is only 19.5%, and it cannot pass UL-94 test. The digital photos of char residue for the neat NR (Fig.1) directly show a sparse char residue with less charring amount. By Incorporation of 20 phr AGT results in a little bit increase of LOI value of the [email protected] compared with the [email protected], but no enhancement on UL-94 rating. With the further addition of flame retardant, all the LOI values and UL-94 ratings of composites are improved on the increasing trend as a whole. The UL-94 test shows that [email protected] burns to the dripping point during the second phase of ignition and reaches only V-2 rating. In comparison with a LOI value of 24.5% and V-2 rating of the [email protected], it is notable that adding 40 phr AGT the [email protected] significantly improves the flame retardancy of NR that [email protected] with a LOI value of 25.1% passed V-1 rating. This demonstrates that a critical amount of the flame retardant is required to provide a significant increase in the LOI value significantly improve the fire resistance of NR due to intrinsic flammability of the neat NR. NR composites incorporated with 60 phr 8

APP or AGT both pass V-0 rating, however, the LOI value of [email protected] is highest and can reach to 32.1%. the LOI value of [email protected] is much higher and with a 5.2% increment. The digital photos of char residues for [email protected] composites (Fig.1) after cone calorimeter tests show that the char residue of the APP system is compact and strong but not integrated and not swelled. However, the char residue is integrated and compact and strong for the [email protected] composites. Compared with those of APP system, charring amount of AGT system, at equivalent loading, are much higher, which representing that AGT system in the composite effectively work to accelerate the strong char formation. The results can also certificated by thermal analysis. The TGA and DTG curves of APP and AGT under N2 atmosphere are shown in Fig. 2, and the related TGA data are recorded in Table S2. It can be found that the thermal degradation of AGT are only composed of two main steps compared with three degradation steps of APP due to the synergistic interaction of three components on the thermal degradation process. Compared with APP, both the initial degradation temperature (T-5%, where 5 wt % mass loss takes place in our laboratory) and the maximum weight loss temperature (T1max) of AGT decrease owing to the thermal degradation of TA. However, AGT has higher thermal stability (T2max ) after first degradation step and residues than those of APP. This may be due to the fact that the synergistic catalytic charring effect of TA results in the generation of more phosphor carbonaceous char residue in compare to APP. Moreover, the high-aspect-ratio graphene can hold the char particles together to generate a high-strength adiathermic char shield on the inner materials. The results indicate that flame retarded efficiency of AGT is obviously higher than APP due to synergistic flame retardant mechanism.

9

Fig.1 The digital photos of char residues for neat NR, [email protected] and [email protected] composites after CCT test.

10

Fig.2 TGA and DTA thermograms of APP and AGT in N2 atmosphere. In order to further investigate the flame retardancy of the [email protected] and [email protected] composites, more scientific cone calorimeter test is also employed to evaluate the combustion performance, parameters such as time to ignition (TTI), peak heat release rate (PHRR), total heat release (THR), smoke release rate (RSR), total smoke rate (TSR), total smoke production (TSP) and mass residue after combustion as shown in Fig. 3, and the corresponding data are displayed in Table 3. Peak value of HRR (PHRR), and total heat release (THR) values have been proved to be the critical parameters in characterizing fire safety of polymers. Compared with the neat NR, introducing APP system or AGT system into neat NR brings about an improvement of fire retardancy. Moreover, AGT system is more efficient in fire retardancy of NR compared to APP system. As shown in Fig.3(a) and Fig.3(b), the neat NR is an inherently flammable polymeric material, which burns rapidly with high PHRR value of 731.6 kWm-2 and THR of 151.1 MJm-2. For The [email protected] composite give 11

rather low peaks of HRR and THR (555.2 kWm-2 and 105.8 MJm-2) under heat flux of 35 kWm-2 compared to [email protected] composites with values of 698.0 kWm-2 and 127.9 MJm-2. For the [email protected] composite, its PHRR value sharply decreases with a reduction of 17.6% compared with the result that of [email protected] composite. With increasing the loading of fire retardant, the gap in THR and PHRR between APP and AGT systems is gradually narrowing. In order to understand the fire hazard of several two studied materials systems more clearly, the fire performance index (FPI) the fire growth index (FIGRA)

[30]

[29]

and

are selected. FPI is defined as the proportion of

TTI and PHRR. It has been reported that there is a certain correlation between the FPI value of material and the time to flashover. When the value of FPI reduces, the time to flashover will be advanced

[29]

. Therefore, it can be accepted that when the FPI value

of a material is smaller, the fire risk is higher. FIGRA is defined as the proportion of PHRR and the time to peak HRR. According to the previous report, the larger the FIGRA value, the shorter time it takes to arrive at a high peak HRR, and the more fire hazard the materials have. The comparison between the The values of FPI and FIGRA of these composites has have been shown in Table 3. Compared with those of APP system, the FPI and FIGRA of AGT system, at equivalent loading, are respectively much higher and much lower which indicates more effective at reducing flammability of NR. The FPI value of the [email protected] is highest and the FIGRA value of that is the lowest among seven samples., accompanied by a minimum FIGRA among these composites. It means that fire risk of the [email protected] is the smallest. This might be the synergistic effect between APP and TGE, leading to the generation of more compact and more phosphor carbonaceous char residue for [email protected] (Fig.3f). composites which is more compact than the char in [email protected] composites. It is of significant importance to reduce smoke release during combustion since the 12

smoke toxicity of organic materials causes the vast majority of fire deaths and injuries. The dynamic smoke production behaviors of [email protected] and [email protected] composites are characterized by total smoke production (TSP), total smoke rate (TSR) and smoke release rate (RSR), as shown in Fig. 3(c, d and e), and the data are summarized in Table 3. As shown in Fig. 3c, RSR of [email protected] composites remarkably decrease compared with those of [email protected], which P-RSR of the [email protected] is reduced from 22.5 m2·s-1·m-2 of the neat NR to 11.1 m2·s-1·m-2 with a decrement of 51.2%, and reduced from 14.1 m2·s-1·m-2 of the [email protected] to 11.1 m2·s-1·m-2 with a decrement of 21.3%. The reduction of smoke indicates that AGT flame retardant system can act as the smoke suppression agent, and increase the chances of survival. TSR (Fig. 3d) and TSP (Fig. 3e) decrease with increasing the amount of AGT fire retardant system. Moreover, compared with those of APP system [email protected] composites, TSR (Fig. 3d) and TSP (Fig. 3e) of AGT system [email protected] composites, at equivalent loading, are respectively much lower. The main reasons maybe that the graphene can also act as a physical barrier to limit soot transfer, and maybe the increase in residue mass is also larger which representing that TA and graphene in the composite effectively work to accelerate the strong char formation The main reasons maybe that the graphene can act as a physical barrier to limit soot transfer, and the catalytic carbonization of TA and char layer reinforcement of GE can synergistically function. The surface of formed intumescent char layer is further oxidized and the CO2 and CO is generated and released out, as a result, the carbon content decreased. The reduction reductions in the release of CO and CO2 production rate (Fig. 4) for [email protected] composites also directly prove the higher ability of char formation the higher char formation capacity compared with [email protected] composites. The measured average effective heat comb. (EHC) further provide a better understanding of the reduced flammability resulting 13

from char formation. So far, the synergistic improvements of the thermal stability and the flame retardancy are proven by the thermal combustion properties of [email protected] composites. The synergism interaction between APP and TGE as a flame retardant treatment for NR is demonstrated and quantified, following the method proposed by Lewin

[31]

and Horrocks et al.

[32]

and based on the calculation of synergism

effectiveness parameter (SE) as shown in Table S3. The SE value which is greater than 1 is sufficient to indicate the synergism interaction between APP and TGE. Our research also shows that a profound effect on the synergistic activity is the addition amount of flame retardant. Although SE value decreases with the increase of flame retardant additive due to complex rubber formulation systems, the synergistic effect is still obvious under the maximum flame retardant additive.

14

Fig.3 CCT results of [email protected] and [email protected] composites under an external heat flux of 35 kWm−2: (a) HRR, (b) THR, (c) RSR, (d) TSR, (e) TSP and (f) MASS.

15

Fig.4 CCT results of [email protected] and [email protected] composites under an external heat flux of 35 kW·m−2: the release of (a) CO production rate and (b) CO2 production rate. Table 3 Cone calorimetry experimental data for various samples. FPI (10-2m2s/ kW))

Samples

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

TSP (m2)

MASS (g)

Neat NR

45

731.6

151.1

27.44

28.4

6.1

5.83

[email protected]

48

698.0

127.9

11.96

33.8

6.9

4.65

[email protected]

52

555.2

105.8

10.79

37.8

9.4

3.83

14.28

[email protected]

51

508.1

109.2

9.91

41.4

10.0

3.74

15.25

[email protected]

55

418.8

110.9

7.83

44.5

13.0

4.52

13.11

[email protected]

56

436.8

101.4

8.63

45.7

12.8

4.22

15.18

[email protected]

58

443.4

94.8

6.15

46.3

13.1

2.76

8.58

16

FIGRA (kW/(m2/s))

EHC (MJ/kg) 21.56 18.97

3.2 Morphology of intumescent char layer

Fig.5 SEM images of the char residue after CCT test: (a) neat NR, (b) [email protected], (c) [email protected] and (d) [email protected]

17

In order to elucidate how the formation of chars affects the combustion of the NR composites, the morphology and structure of the chars after combustion are investigated with SEM. Fig. 5 and Fig. 6 show the SEM images of the residual chars for neat NR, [email protected] and [email protected] composites after combustion.

Fig.6 SEM images of the char residue after CCT test: (a) [email protected], (b) [email protected], (c) [email protected] For the chars char residue of the neat NR, which it is easy to burn and only some char residue can be observed at the end of the test, moreover, loose char layers emerged with a number of big holes are seen (Fig. 5(a-1)). With high magnification (Fig. 5(a18

3)), molten drops obviously display for the neat NR. For the chars char residue of [email protected] composites, the continuous and compact char layers without expanding, with slight melting and dropping, are observed clearly (Fig. 5b-d), which can act as protective shields on the surface of the residues inner materials, thus inhibiting the transmission of heat and heat fuel diffusion when exposed to flame or heat source. The char residue of [email protected] composites with expanding (Fig.6) are much compacter and denser than those of [email protected], which indicates that the combination synergistic effect of APP and TGE can promote the formation of strong and intumescent char layers enhances the char strength. Under high magnification (Fig.6 (a-2, b-2 and c-2)), in compare to [email protected] composites, the intumescent protective char layer for [email protected] composites is much finer and more integrated that mostly effectively protects the underlying material from the action of the heat flux or the flame due to the synergistic effect of AGT system., thus greatly improves the flame retardant properties of NR composites. [email protected] composites can expand during the combustion process, and they carbonize and extinguish right after being deviated from fire. Specially for the char residue of the [email protected] composite under high magnification (Fig.6(c-3)), no flaws can be seen on the surface of the char residue due to sufficient and tight char formation or condensed char during the burning process. The distribution of flame retardant elements in char residue is investigated by SEMEDX analysis. In Fig. 7, compared with the elemental mappings of [email protected] composites (Fig. 7a, c, e), those of [email protected] composites (Fig. 7b, d, f) reveal the better uniform distribution of C, N, O and P atoms on the char surface. The increase of P atom for [email protected] composites further verify the high phosphorus residue after combustion in compare to [email protected] composites as shown in Table S4. Thus it just provides a higher resistance during pyrolysis and combustion. 19

Fig. 7 EDX spectrum of (a) [email protected], (b) [email protected], (c) [email protected], (d) [email protected], (e) [email protected] and (f) [email protected] 3.3 Thermal and mechanical properties of [email protected] and [email protected] composites

20

Fig. 8 (a) abrasion loss, (b) tensile strength, (c) tear strength and (d) thermal conductivity of [email protected] and [email protected] composites. The mechanical property is a non-ignorable performance index for flexible rubber materials in various applications. Unfortunately, the addition of fire retardant can remarkably change the mechanical behavior of NR. Their fire retardant action is accompanied by negative effects, such as contamination of mechanical properties. In this regard, poor particle dispersion and interface interaction are presumably responsible for the negative effects due to polarity differences between polar fire retardant and nonpolar NR. With increasing incorporation of APP, the tensile strength and tear strength of [email protected] composites decrease monotonically (Fig. 8b). In comparison with [email protected] composites, the conjugation of AGT system into the NR demonstrates a noticeable increase in tensile strength and tear strength (Fig. 8b and c), 21

within the loading of 20 phr AGT, in comparison with neat NR and [email protected] especially with the loading of 20 phr AGT, the tensile strength of the [email protected] composite is actually higher than that of the neat NR due to the reinforcement contribution of graphene. Moreover, abrasion performance of [email protected] composites is always better than that of [email protected] composites and the neat NR. It may be relative to the increase of crosslinking density due to TA involved in the crosslinking reaction and the reinforcing effect of graphene for NR

[21]

. The significant improvement

further benefits from both the uniform dispersion of TGE and the strong interfacial adhesion between TGE and NR

[28,33]

through the dual roles of dispersant and

interfacial regulator of TA. Nevertheless, the phenomenon is incompatible with the change trend of the glass transition temperature (Tg) for [email protected] and [email protected] composites, as shown in Fig.S1. Due to the confinement and free volume occupation effects of polar flame retardant, the Tg of composites all shift to higher temperatures compared with the neat NR

[34]

. Generally, the limitation of rubber chain mobility

resulting from the strong interfacial adhesion between TGE and NR should impart increase to Tg for [email protected] composites in contrast to [email protected] composites. However, the Tg of [email protected] composites shifts toward lower temperature compared with [email protected] composites. This because the graphene acts as a plasticizer increasing the flexibility of chain segments of the NR matrix[35], and the π-π conjugate interaction between graphene and TA induces the NR chains crosslinked to TA slip along the graphene surface [36]. The thermal conductivities of NR/APP [email protected] and [email protected] composites as a function of fire retardant loadings are further investigated as shown in Fig. 8d. Thermal conductivities of [email protected] and [email protected] composites improve remarkably with APP or AGT loadings. Nevertheless, at equivalent loading, abrasion 22

performance and the thermal conductivities of [email protected] composites are respectively much larger than NR/APP [email protected] composites, which representing that thermal conductive path formed in [email protected] composites is better due to the existence of the graphene. 3.4 Flame retardant mechanism

Fig. 9 Illustrative scheme of the flame retarded mechanism for [email protected] composites.

23

Fig.10 FTIR (a) and Raman spectra (b) of char residue of composites. On the basis of the aforementioned analysis, a possible flame retardant mechanism of AGT system in NR matrix is postulated in Fig. 9. First, for condensed phase, during the earlier degradation, thermally stable graphene sheets act as a mass barrier to restrain the permeation of flammable gases

[37-38]

. Meanwhile, the catalytic charring

effect of metaphosphoric acid (HPO4) decomposed from APP and the absorbed TA absorbed on the surface of graphene structure results in the generation of much phosphor carbonaceous char residue, which is confirmed by the obviously depressed reducing of gaseous products (such as CO2 and CO shown in Fig. 4 and EDX results shown in Fig. 7). Then the formed char layer is inflated by ammonia gas released from APP. Moreover, the high-aspect-ratio graphene can hold the intumescent char particles together to generate a high-strength adiathermic char shield on the inner materials. Secondly, for gas phase, ammonia gas released from APP can effectively dilute volatile products during the combustion process. It can be concluded that the tripartite cooperative mechanism of AGT system is the main source of the excellent flame retardancy of NR composites. To confirm the conjecture, the chemical structure of char residue is analyzed by FTIR and Raman spectrometer. The FTIR spectra 24

(Fig.10a) of char residue for all composites are nearly flat, which indicates that the raw sample can nearly graphitize after complete combustion. Meanwhile, the formation of phosphorous-containing carbonaceous structure is also detected due to the characteristic absorption peaks of P-O-C at 1105 cm−1 [39]. This result indicates an effective thermal insulator (phosphorous-containing carbonaceous layer) to slow down the heat and mass transfer between gas and condensed phases [40-41]. The flameretardant mechanism of APP and AGT are also revealed by Raman spectra. As shown in Fig. 10b, each spectrum is subjected to peak fitting using the curve fitting software Originpro 2017/Peak Fitting Module to resolve the curve into 2 Gian bands (Fig. S2). Typically, the integrated intensities of D to G bands (ID/IG) embody the graphitization degree of the char residue, and a lower ID/IG value suggests better char residue structures

[42]

. Notably, the ID/IG ratio always follows the sequence of [email protected] <

[email protected] with the same loading (Table S5), demonstrating that AGT system can facilitate the formation of highly graphitized and thermally insulated char layers, which is primarily responsible for the suppressed flammability. Conclusion In this work, the novel AGT flame retardant presents synergistic flame retardancy and smoke suppression for NR due to intumescent flame retardant mechanism. Raman, FTIR results and SEM-EDX of char residue show that the AGT system could clearly facilitate the formation of highly graphitized and phosphorous-containing carbonaceous structure char layers, consequently resulting in efficient reduction of the flammability parameters and the fire risk. Moreover, the AGT system is beneficial for mechanical performances of NR within an appropriate amount of addition, even though, in the case of high loading, AGT take much smaller damage in mechanical 25

performances than APP system with the same loading. Acknowledgements The authors gratefully acknowledge support from National Natural Science Foundation of China [grant numbers 51703111 and 51603111] and Shandong Provincial Natural Science Foundation, China [grant numbers 2018GGX102015 and ZR2017BEM011] and China Postdoctoral Science Foundation [grant number 2018M642626 ]. References [1] Laoutid F, Bonnaud L, Alexandre M, Lopez-Cuesta JM, Dubois P. New prospects in flame retardant polymer materials: from fundamentals to nanocomposites. Mater Sci Eng R Rep 2009; 63(3):100-125. [2] Bourbigot S, Bras ML, Duquesne S, Rochery M. Recent advances for intumescent polymers. Macromol. Mater. Eng. 2004; 289: 499-511. [3] Bai G, Guo C, Li L. Synergistic effect of intumescent flame retardant and expandable graphite on mechanical and flame-retardant properties of wood flourpolypropylene composites. Construct. Build. Mater. 2014; 50: 148-153. [4] Lee SY, Kim KM, Seong DG, Lee DJ. Synergistic improvement of flame retardant properties of expandable graphite and multi-walled carbon nanotube reinforced intumescent polyketone nanocomposites. Carbon 2019; 143: 650-659. [5] Lim KS, Bee ST, Sin LT, Tee TT, Ratnam CT, Hui D, Rahmat AR. A review of application of ammonium polyphosphate as intumescent flame retardant in thermoplastic composites. Composites Part B 2016; 84: 155-174. [6] Chen XF, Huang CY, Shi YQ, Yuan BH, Suna YR, Bai ZM. MoO3-ZrO2 solid acid for enhancement in the efficiency of intumescent flame retardant. Powder Technology 2019; 344: 581-589. 26

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Authors Declaration We confirm that the manuscript has been read and approved by all named authors. We declare that we have no conflict of interest. Sincerely,

Lin Li

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