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Preparation of large size reduced graphene oxide wrapped ammonium polyphosphate and its enhancement on the mechanical and flame retardant properties of thermoplastic polyurethane Yan Zhang, Bibo Wang, Bihe Yuan, Yao Yuan, Kim Meow Liew, Lei Song, and Yuan Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 16, 2017

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Industrial & Engineering Chemistry Research

Preparation of large size reduced graphene oxide wrapped ammonium polyphosphate and its enhancement on the mechanical and flame retardant properties of thermoplastic polyurethane Yan Zhanga,b,c, Bibo Wanga, *, Bihe Yuand, Yao Yuana, Kim Meow Liewc, Lei Songa, Yuan Hua,b,* a

State Key Laboratory of Fire Science, University of Science and Technology of China,

96 Jinzhai Road, Hefei, Anhui 230026, People's Republic of China. b

Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study,

University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, People's Republic of China. c

Department of Architecture and Civil Engineering, City University of Hong Kong,

Tat Chee Avenue, Kowloon, Hong Kong. d

School of Resources and Environmental Engineering, Wuhan University of

Technology, Wuhan, Hubei 430070, People's Republic of China.

*Corresponding author. Fax/Tel: +86-551-63601664. E-mail address: [email protected] (Yuan Hu) *Corresponding author. Fax/Tel: +86-551-63601664. E-mail address: [email protected] (Bibo Wang)

ACS Paragon Plus Environment


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ABSTRACT A facile and novel approach is provided to improve the dispersion of large size reduced graphene oxide (LRGO) in polymer matrix through attaching on the surface of

ammonium

polyphosphate

(APP)

in

the

medium

of

(3-aminopropyl)

triethoxysilane (APTS). Here, a series of LRGO wrapped APP (named LRAPP) were firstly prepared and successfully characterized, and then blended into thermoplastic polyurethane (TPU). The introduction of LRGO increased the interfacial adhesion between the APP and polymer thus getting remarkable enhancement in mechanical and flame retardant properties. Especially, TPU/LRAPP1, where APP is encapsulated by 1% LRGO, can obtain huge increase on tensile strength and elongation at break by 189.7% and 24.6 % than those of TPU/APP. In addition, LRAPP0.5 encapsulated by only 0.5% of LRGO, could effectively restrain the melt-dripping phenomenon in TPU composites and acquired the lowest pHRR value of 170.6 kW/m2. This novel strategy aims to broaden extensive application of large size graphene. KEYWORDS: large size reduced graphene oxide; thermoplastic polyurethane; microencapsulation; well-dispersion; anti-dripping

1. Introduction

As a commercial engineering elastomer materials, thermoplastic polyurethanes (TPUs) are extensively used in many fields, such as coating materials, wires and cables, and adhesives, due to its excellent processability, good abrasive resistance, high mechanical performances and sound chemical stability.1-4 However, the high


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flammability (LOI about 16-19%) is accompanied by melt-dripping phenomenon, which might induce the terrible spread of fire.5 Another extreme challenge is the large production of visible smoke and toxic gases in the process of burning, such as CO, HCN, NOx, and so on.6 Therefore, much attention has been paid to search for efficient and environmental friendly halogen-free flame retardants to decrease the fire hazard of TPU. Nanocomposites have triggered wide interest of researchers in recent years for the combination effects between nanoadditives and polymer matrix.7-9 A small amount of nanoparticles (nanospheres, nanorods, nanotubes, nanoplatelets, etc.) can acquire remarkable enhancements in mechanical properties, thermal stability and flame retardancy.10-12 Recently, graphene has been developed as a promising nanomaterial for its excellent thermal conductivity, intensive mechanical properties and outstanding electrical conductivity and electromagnetic interference shielding function.13-16 The graphene is mostly prepared by the reduction of graphene oxide (GO) through chemical or thermal treatment, and the obtained reduced graphene oxide (RGO) is more stable for the removal of labile oxygen groups on the surface of GO.17 More investigations demonstrate that graphene-based flame retardant shows promising flame retardant performance in TPU, mainly because of its physical barrier effect.18 Jaber et al. incorporated RGO into IFR-PU composites exhibited excellent anti-dripping properties as well as UL-94 V0 rating.19 Cai et al. reported that TPU contained 2% of functionalized graphene which was noncovalently modified by ligninsulfonate and iron ion (Fe-lignin) can get almost half of the decrease of pHRR



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values.20 In this case, large size graphene sheets are more desirable to prevent the heat transport and diffusion of the volatile pyrolysis products for its large specific surface area.21 However, graphene sheets especially the lager one, are easily to aggregate and restack due to the intense van der Waals forces and π-π interactions between graphene layers, which is one of the challenging factors affecting the dispersion and reinforcing effect of graphene.22-25 To obtain the homogeneous dispersion and synergistic effect of graphene in polymer matrix, surface organic functionalization, nanohybrids and microencapsulation methods are the common ways to get refined.26-28 For instance, Yu et al. reported the preparation of flame-retardant-wrapped graphene/EP by in situ polymerization with POCl3 and DDM, and only 2 wt% FRGO can lead to a 43% reduction in the pHRR values for the FRGO/EP composites.29 However, there still remain some drawbacks by means of nanocomposites. In situ polymerization and solution blending are common methods to obtain homogeneous dispersed graphene/polymer nanocomposites (GPNs), which are based on a lot of organic solvents and complicated procedures.30 Besides, few reports are carried out that nanocomposites can reach up to the flame-retardant-rating via the test of limited oxygen index and vertical burning method. To meet the requirement, intumescent flame retardant (IFR) is considered to be a desirable alternative to flame retardant polymers and its relatively simple processing method as melt blending contributes to its wide application in academic and commercial area. Generally, a typical IFR is mainly composed by three basic components, namely acid source, char-forming agent and blowing agent. APP as an efficient acid source and blowing agent is also widely


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applied in TPU composites for the rich P element content (31 wt%) and the formation of fluffy protective char layer.31-32 By now, there still remain some problems with the large addition of APP. It can not only worsen the mechanical properties of the TPU matrix, but also produce much more toxic smoke during the combustion.6 In order to reduce the addition of flame retardant but maintain enough flame retardancy at the same time, it is essential to further explore the modification of APP particles. Shi et al. prepared g-C3N4 wrapped APP (CNAPP) incorporated in PS and the THRR value was reduced by 39% with 20% of CNAPP, which not only improves the homogeneous dispersion of g-C3N4, but also enhances the flame retardancy of APP.33 Based on this concept, we try to attach RGO sheets on the surface of APP to explore whether it can obtain the combined effect. This work aimed to provide a simple encapsulation method to improve the dispersion of LRGO in TPU resin, and employed APP as the main flame retardant agent to obtain the synergistic effect with barrier effect. Here, APP was encapsulated by a series content of LGO through the medium of APTS by two steps, and then incorporated into the TPU resin by the melt blending procedure. APTS functioned as surface modifier can not only reduce the polarity of APP, but also attach the LRGO sheets onto its surface. The structure of LRAPP was successfully characterized by X-ray

diffraction,

Raman

spectroscopy,

X-ray

photoelectron

spectroscopy,

transmission electron microscopy and so on. In addition, the effect on mechanical properties, thermal degradation, and flame retardancy of graphene-based APP (LRAPP) in TPU composites are also investigated systemically.



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2. Experimental section 2.1 Raw materials Chopped carbon fiber (0.3 mm length) was supplied by Complex new material technology (Shanghai) Co., Ltd.Potassium permanganate (KMnO4), hydrochloric acid (HCl, 37% aq.), sodium nitrate (NaNO3), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2, 30% aq.), (3-aminopropyl) triethoxysilane (APTS), ethanol, ammonia (25-28% aq.) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). APP (phase II, the degree of polymerization > 1000) was supplied by Shandong

Hongchuang

Flame

Retardant

Reagent

Co.,

Ltd.

(China)

Thermoplastic polyester polyurethane (TPU85E85) was provided by Bangtai Material Co., Ltd. (China). All reagents were used as received without further purification. 2.2 Sample preparation 2.2.1 Preparation of LGO LGO was prepared from chopped carbon fiber according to Hummers’ method reported in prior study.18 In brief, 1.0 g of chopped carbon fiber and 0.5 g of NaNO3 were stirred vigorously in 23 mL of H2SO4 (98 wt%) under an ice bath. Then 3.0 g of KMnO4 was slowly added to the mixture under strong agitation and the temperature of the system was controlled below 10 °C. Afterwards, remove the ice bath and heat the mixture at 35 °C for 1 h. Maintain this temperature, 46 mL of deionized water was carefully added dropwise to the suspension. After that, another 70 mL of lukewarm deionized water was slowly poured into the mixture. Furthermore, 2.5 mL of 30% hydrogen peroxide was added to obtain the resultant bright-yellow suspension. The


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LGO solution was purified by centrifugation and thorough washing with a 10% HCl solution and deionized water to reach a pH near 7. 2.2.2 Preparation of LRAPP A series of LRAPP were prepared by the medium of APTS.34 During the procedure, a 500 mL round bottom flask was equipped with a mechanical stirrer, thermometer and reflux condenser under nitrogen atmosphere. 50 g of APP and 5 g of APTS were added into 200 mL of absolute ethanol in the flask and stirred for 30 min under room temperature. Then it was heated to reflux temperature and kept reaction until there was no ammonia release. Next, the LGO solution was dispersed by sonication in 100 mL of ethanol and 100 mL of deionized water for 30 min. Then the modified APP mixture was immediately poured into LGO solution at 50 °C and stirred under sonication for 30 min to get LGAPP (means APP wrapped by LGO). After that, the temperature was increased to 80 °C and 1g of hydrazine hydrate was added to make LGO reduced. The reaction was further proceeded for 5 h under the same temperature and the color changed gradually from brown to black to get LRAPP. Finally, the mixture was filtered to remove solvent, and the crude product was washed with ethanol for 3 times, and then vacuum-dried at 100 °C for 8 h to remove residual solvents. To facilitate the description, LRAPP0.25, LRAPP0.5 and LRAPP1 were designed as the APP particles wrapped with 0.25%, 0.5% and 1% of LRGO, respectively. The schematic diagram of LRAPP is presented in Scheme 1. 2.2.3 Preparation of flame retarded TPU composites The commercial TPU, APP and the prepared LRAPP were dried in a vacuum oven



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at 80 °C overnight in order to remove absorbed water. And all of the samples were prepared using a two-roll mixing mill (Rheomixer XSS-300, Shanghai Ke Chuang China) at 170 °C, and the roll speed was maintained at 80 rpm. TPU was first added into the mill at the beginning of the blending procedure. After the melting of TPU, the flame retardant with desired amount were added, and the mixture was processed for about 15 min. The resulting samples were hot-pressed at about 180 °C under 10 MPa for 10 min into sheets with a thickness of 3.0 ± 0.1 mm for UL-94 classification and limiting oxygen index. Other samples were fabricated in the same procedure. The formulations of prepared samples are listed in Table 1. 2.3 Characterization X-ray diffraction (XRD) patterns were obtained by a Japan Rigaku Dmax X-ray diffractometer equipped with graphite monochromatized high-intensity Cu Kα radiation (λ= 1.54178 Å). Transmission electron microscopy (TEM) was probed by a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) images were recorded on a PHILIPS XL30E scanning electron microscope at an acceleration voltage of 5 kV. The composite sample was cryogenically broken after immersion in liquid nitrogen, and the fractured surfaces were observed by SEM. Mechanical properties of tensile strength (TS) and elongation at break (EB) were measured with a universal testing machine (Instron 1185) at temperature of 25 + 2 °C according to ASTM D412. The cross-head speed was 200 mm/min and the gauge


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length of the samples is 20 mm. Dynamic mechanical analysis was measured with DMA Q800 (TA, USA). The dynamic storage modulus were determined at a frequency of 10 Hz and a heating rate of 5 °C/min over the range of -100 °C to 100 °C in the tensile conﬁguration on film. The dimensions of the samples were approximately 1 mm thickness, 20 mm length and 4 mm width. The storage modulus was plotted. Thermogravimetric analysis (TGA) was carried out using a Q5000 thermoanalyzer instrument under a nitrogen flow of 50 mL/min. The samples (about 5 mg) were heated from room temperature to 800°C at a linear heating rate of 20 °C/min. The thermal conductivity was carried out with a hot-disk thermal analyzer (TC 3000E, Xia xi technology, China) at room temperature, adopting the transient plane source technique. Limiting oxygen index (LOI) was measured by an HC-2 oxygen index meter 207 (Jiangning Analysis Instrument Co., China) according to ASTMD 2863. The dimensions of each specimen were 100 × 6.5 × 3 mm3. Vertical burning test (UL-94) was conducted by a CZF-II horizontal and vertical burning tester (Jiangning Analysis Instrument Co., China) according to ASTM D3801. The dimensions of the samples were 127 × 12.7 × 3 mm3. The combustion test was performed on the cone calorimeter (FTT, UK) test according to ISO 5660 standard procedures, with 100 × 100 × 3 mm3 specimens. Each specimen was wrapped in an aluminum foil and exposed horizontally to 35 kW/m2 external heat flux.



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Raman spectroscopy was performed on a SPEX-1403 laser Raman spectrometer (SPEX Co., U.S.) with excitation provided in backscattering geometry by a 514.5 nm argon laser line.

3. Results and discussion

3.1 Microstructure and morphology of LGO The XRD patterns shown in Figure S1 were used to investigate the crystal structure of graphite and LGO. There remains a remarkable difference between graphite and LGO. A sharp peak of graphite at 26.6° indicates the interlayer spacing of about 3.43 Å. The XRD trace of GO powders exhibits a diffraction peak at 9.6° (corresponding to interlayer spacing of about 0.92 nm), suggesting a significant enlargement in interlayer spacing. To provide a direct observation for the morphology of LGO, TEM image of LGO was performed in Figure 1. As can be seen, the morphology of LGO is wrinkled and almost transparent, and the surface of LGO sheets is fairly smooth and large. The ultrathin nature of GO sheets makes them nearly invisible, whereas the multilayered stacks of the GO make them relatively clear.35 All above represent that GO sheet with large size was obtained successfully in our experiment. 3.2 Microstructure and morphology of LRAPP. The phase of samples investigated by XRD technique was shown in Figure S2. It is obvious to find that the characteristic peaks of APP at 2θ=14.81°, 15.61°, 26.28°, 27.58°, 29.20°, 30.69° corresponding to (200), (110), (310), (111), (211) and (301) are exactly assigned to commercial APP.36 As for LRGO, a broad diffraction halo at about 24.5° can be observed after chemical reduction of LGO, which is according to the


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previous reports.18 However, it is just obvious to find strong crystal peaks assigned to APP instead of LRGO. To reveal intuitively the existence of LRGO on APP, the digital photos are used to verify it. Scheme 1 also shows the appearance of APP, LGAPP0.5 and LRAPP0.5. The difference among their color can be more obvious when dispersed by sonication in ethanol. With the chemical grafting of LGO, the color of APP powder changes from white to brown. After chemical reduction, it is expected to change into black due to the further reduction of LGO. Furthermore, the XPS spectrum in the C1s of LRAPP0.5 is shown in Figure 2. It is clearly demonstrated that four peaks appearing at 284.8, 285.8, 286.8, and 288.8, which are belong to the to the nonoxygenated ring C atoms, the C atoms bonded to C atoms in defective structures, the C atoms in hydroxyl and epoxy/ether groups, and the carbonyl C structure, respectively.37 Compared with previous reports, the peak intensities of oxygenated C in epoxy/ether and the carbonyl groups were much lower than those in GO.38 It can also indicate that the LGO on the surface of APP was successfully reduced by hydrazine hydrate. To have a microscopic view of the samples, the SEM images are present in Figure 3. From the SEM images, APP particles without the microencapsulation of LRGO show a smooth and neat surface. Particular, the LRAPP0.25 containing only 0.25% of LRGO, is not clearly to find the LRGO sheets but the surface is obvious rougher than APP, which is due to the hydrolysis reaction of APTS. With the increasing loading of LRGO, it is obvious to find that the rough surface has increasing LRGO layers. The aforementioned images demonstrate that APP particles are successfully wrapped by



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LRGO sheets by the medium of APTS. Besides, Raman spectrum can also be adopted as a qualitative approach to evaluate the loading of LRGO on APP particles. As can be seen from Figure S3, pure APP particles have strong absorption and the characteristic peak appears at 300-1200 cm-1. As the loading of LRGO increases, the characteristic peak of APP are still remain but the whole intensity decreases, and the D band (1360 cm-1) and G band (1600 cm-1) corresponding to the absorption of LRGO are becoming more clearly. Thus, the Raman spectrum can be taken as an assistant method to charify that LRGO was successfully attached on the surface of APP . 3.3 Thermal stability of LRAPP. The influence of the chemical grafting of LRGO on the thermal stability of APP was investigated by TGA in nitrogen atmosphere. Figure 4 shows the TGA (a) and DTG (b) curves of APP and LRAPP. The detailed data are listed in Table 2. All of the samples are composed of two main degradation steps based on the DTG profile. The first-step decomposition of APP occurs in the range of 200-400 °C, resulting from the decomposition of thermally unstable groups such as the terminal chain groups [-OP(O)(ONH4)2] and midchain groups [-OP(O)(ONH4)-] and the elimination of NH3 and H2O during the thermal decomposition of polyphosphate, while the second-step decomposition is initiated by the release of phosphoric acid, polyphosphoric acid, and metaphosphoric acid beyond 500 °C.33 Compared with APP, the T

-5wt%

is decreased

after being capsulated with LRGO, which is because LRGO possesses high thermal conductivity. Nevertheless, the major difference between APP and LRAPP is that


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LRAPP possesses higher Tmax2 and more residue at 800 °C. Furthermore, with the increasement of LRGO, the Tmax2 and residue of LRAPP rise remarkably and regularly, which might due to the protective effect of large RGO sheets. For instance, LRAPP1 with the loading of 1% LRGO, has a 19.1% increase of Tmax2 and 141.4% increase of residue than that of APP. 3.4 Mechanical properties The mechanical properties of TPU composites are investigated by tensile testing and dynamic mechanical analysis. In addition, SEM analyses are also carried out on the fracture surfaces of the FR TPU composites to obtain more information. The tensile strength (TS) and elongation at break (EB) of flame retardant TPU composites are shown in Figure 5. It is reported that the tensile strength values of micro-scale particulate filled composite mainly rely on three factors, the addition amount of the filler, the shape and structure of the filler, and the adhension between filler and matrix.5 There shows a dramatically decrease in TS with the addition of 5% APP in TPU, which is due to the weak interfacial adhension and poor compatibility between the APP and polymer matrix.31 The marked polarity discrepancy between APP and the polymer matrix makes them thermodynamically immiscible and thus causing weak interfacial adhesion, which can be hazardous to the tensile strength values of TPU composites. However, The TS shows a huge increase with the same addition of LRAPP, and the higher content of LRGO wrap APP particals, the higher TS values are reached. Here RGO layer is able to improve the negative impact of APP particles in TPU matrix, which is because its layer like structure raises loading bearing capcity



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than only particulate filler structure as APP particles.39 Besides, it is also clearly to find the great increase in EB followed by the similar regular. The improvement on mechanical properities can attribute to the better strong interfacial adhesion between LGO and TPU matrix. To further verify the interaction between the boundry, the freeze fracture surface is shown in Figure S4. As can be seen, the surface of TPU/APP is fairly flat. However, there present more obvious fold and bulge with the same addition of LRAPP, which reflects the strong interaction between the filler and matrix. The mechanical reinforcement is also reflected from the dynamic mechanical property. The storage modulus (E’) of the material is correlation with its load bearing capacity and the values at 10Hz are plotted in Figure 6 for TPU and its composites. There shows obvious improvement on storage modules with the addition of a series of LRAPP and APP. As shown, at -100 °C which is below glass transition temperature, the storage module of pure TPU cannot reach 3000 MPa. However, the TPU/LRAPP can all reach up to 4000 MPa. It can be attributed to the fact that the storage modules can be increased if the filler has higher stiffness than the matrix.5 In addition, TPU/5% LRAPP composites exhibit higher storage modules than that of TPU/5% APP composites. Besides, TPU/LRAPP1 containing 1% of RGO presents highest storage modules owing to the strong interfacial adhension with TPU matrix. All aforementioned results are noted that RGO layers play a role as surface modifier to improve polymer-filler interactionand hence improve the mechanical properities.40 3.5 Thermal stability of TPU composites. The thermal degradation behaviors of flame retardant TPU composites are


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investigated by TGA under nitrogen atmosphere. The TGA and DTG curves and the related data are shown in Figure S5 and Table S1. It is obvious to find that the degradation process of TPU and flame retardant TPU composites are all containing two steps. Compared with neat TPU, the flame retardant TPU composites have lower initial decomposition temperature and higher char residue. This is because APP accelerates the decomposition of TPU, resulting in lower decomposition temperature of these composites and the polyphosphoric acid decomposed from APP can catalyze the formation of more char residue. As shown, there is no big difference in initial and the maxium degradation temperature between the APP and LRAPP flame retardant TPU composites. The LRAPP0.25 flame retardant TPU composites reveal the highest char residue. Therefore, LRAPP0.25 achieves better joint effect. The physical barrier effect of RGO is another major reason to improve the char residue and thermal stability of TPU composites. On the other hand, the addition of LRGO improves the thermal conductivity of TPU composites, which is one of the mainly reason to decrease the decomposition temperature and the time to ignition. Here, the thermal conductivity data of TPU composites are present in Figure 7. Generally speaking, the thermal conductivity of polymer containing LRGO is associated with two parts, which is the dispersion degree and intrinsic property of LRGO.20 Thus, the great enhancement on thermal conductivity of TPU/LRAPP0.5 attributes to its well-dispersion and the formed thermal conductivity network. However, the higher loading of LRGO leads to a decrease on thermal conductivity, which is because of the aggregation and restack of LRGO on the surface of APP.



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From this point, LRAPP0.5 achieves better joint effect. 3.6 Flame retardancy and mechanism LOI, vertical burning test and cone calorimeter are widely used to evaluate the flammability and elucidate the combustion behaviors of FR materials. Table 3 lists the detailed results of the aforementioned tests. LOI as an important evaluation parameter for flame retardance, is based on the minimum oxygen concentration by volume for maintaining the burning of a material. Here, the LOI values and UL-94 testing results of the TPU composites are listed in Table 3. The neat TPU possesses a lower LOI values as 22%, with the addition of 5% APP or LRAPP, the LOI values can all reach up to about 25%. There is no obvious differernce between the APP and LRAPP flame retardant TPU composites. However, the LOI test cannot detect the difference among the flame retardant TPU composites. That is because nanoparticles themselves are often not efficient in reducing the overall flammability of polymers. The UL 94 classification and oxygen index usually remain similar or are even worsened by the addition of nanoparticles.41 While the vertical burning test highlights the differences. As shown in Figure 8, the neat TPU can not reach any ratings with melt dripping during the vertical burning test. With the addition 5% APP into TPU composites, the LOI value can improve obviously, while the sample still accompoing serious dripping during the vertical burning test, and only reach V-2 rating. However, the APP particles encapsulated by only 0.25% or 0.5% of LRGO can depress the droppings in TPU/LRAPP composites, and reach V-0 rating. When the content of LRGO increases to 1%, the LRAPP1 can not achieve such association effect. As reported, the


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synergistic effect of LRGO increases the melt viscosity of TPU composites which hinders the polymer material from dripping and improves the char residues.39 However, too much excess LRGO can cause the opposite effect which might because more LRGO on the surface of APP can not only get excessive accumulation but also restrain the reaction between APP and TPU resin during burning. As an easy and useful test to rank the flame retardance of flame retardant polymers, LOI, however, is only a small-scale test but not a reliable instruction of the real performance in a fire. Cone calorimeter that is one of the most effective bench-scale methods were used and the results are carried out in Figure 9 and Table 3. Many key parameters can be obtained from cone calorimeter, such as time to ignition (TTI), heat release rate (HRR), total heat release (THR), and peak heat release rate (pHRR). TTI is used to determine the influence of the FR on ignitablity of materials. It can be observed that the neat TPU burns vigorously and its HRR shapes a single peak with a highest pHRR of 1013.7 kW/m2. The HRR curves of FR TPU becomes squat, which is corresponded to the literatures. However, the addition of APP and LRAPP lead to a a eariler ignition observed from the great decreased TTI values, which might because APP decomposes earilier than neat TPU resin, and many small volatile molecules are generated from the decomposition of APP.33 Besides, the high thermal conductivity of LRGO also accelerates the degradation.42 In addition, with increasing LRGO loadings, the TTI presents a regular decline. The LRGO leads to the rapid decoposition of APP to form more protective layers to put off the combustion of underlaying composites. Particularly, TPU/LRAPP0.5 possesses the lowest pHRR values and THR values as



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170.6 kW/m2 and 44.1 MJ/m2, respectively. To get a further judgement of fire hazard, the fire performance index (FPI) is calculated based on the cone calorimeter data, and calculated as FPI =TTI/pHRR.43 Generally, the high FPI value means higher safety rank. It is noted that TPU/LRAPP0.5 owns the highest FPI value, indicating that it possess the most effective flame retardancy. What’s more, TPU composite containing 5% of LRAPP0.5 possess the lowest production of CO and CO2, we suspected that more carbon-containing compounds are converted into char residue. To demonstrate it, the char residues for TPU and its composites after cone calorimer test are present by digital photos and SEM images. As can be seen from Figure S6, an unexpanded char layer is achieved after combustion. However, the char residues become more expandable after the addtion of 5% flame retardant. In addition, the TPU/LRAPP composites show more intumescent char residue than that of TPU/APP composites. Thus we can conclude that the existence of LRGO on the surface of APP contribute to generate more char residue during the combustion of TPU composites. Furthermore, It is not obvious to find too much intrinsic difference through the SEM images of the char residue. However, TPU/APP shows plenty of bubbles and some of them are broken, which might due to the production of gas during the combustion. The char layers of pure TPU is similar to that of TPU/LRAPP, but the former is more filmy and broken. Generally, the formation of a protective layer is not only controlled by its amount, but also determined by its properties. Therefore, the compact, intumecent and more thermally stable char layer can reduce the heat and mass transfer and protect the inner


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materials to slow decomposition. The properties of the char residues, in a sense, are more important than their amount. The Raman spectra were employed to clarify the graphitic structure of exterior char residues for TPU and its FR composites, and the detailed data are shown in Figure 10. Generally, two remarkable peaks at 1360 and 1590 cm-1 are belong to D and G bands. The ratio of the integrated intensities of D to G band (ID/IG) is usually employed to evaluate the degree of graphic structure. The lower ratio of ID/IG, the higher graphitization degree of char. And the higher graphitization degree of the char structure lead to better protection of materials from thermal oxidation. It can be clearly observed that the ID/IG values follows the sequence of TPU (2.96) > TPU/APP (2.88) > TPU/LRAPP0.5 (2.72), indicating that the char residue of TPU/LRAPP0.5 possess the highest graphitization degree and the most thermally stable char structure. It is noted that a higher graphitization degree in the char structure tends to form compact and efficient thermal insulated residual char layers. Therefore, the highest graphitization degree of TPU/LRAPP0.5 might be another reason to improve the thermal stability of TPU, which matches well with the LOI, vertical burning test and cone results.

4. Conclusion

In summary, a series of LRAPP was synthesized successfully via the reaction between APP and LGO in the medium of APTS. This method can not only contribute to the interfacial compatibility of APP in TPU matrix, but also improve the dispersibility of large scale graphene. Compared to APP, LRAPP shows attractive mechanical properties and flame retardancy when functions in TPU matrix. The



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fracture morphology analysis proves that LRAPP possesses higher interfacial action, in which LRGO plays a role as linking agent. Especially, LRAPP0.5 containing only 0.5% of LRGO could effectively restrain the melt-dripping phenomenon in TPU composites and acquired the lowest pHRR value of 170.6 kW/m2. That might attribute to the fact that a balanced amount of LRGO can function well with APP particles for the shielding effect. Moreover, after cone calorimeter test, TPU/LRAPP composites possess most of the char residue and thermally stable graphitic structure char, which is significant to improve the flame retardance. In conclusion, LRAPP shows a remarkable effect in enhancing the flame retardance and facilitates the extensive application of large size graphene.

Acknowledgments

The work was financially supported by the National High-tech R&D program (No. 2016YFB0302104), the National Natural Science Foundation of China (No. 51303167), the Fundamental Research Funds for the Central Universities (No. WK2320000032), the Research Grants Council of the Hong Kong Special Administrative Region, China (No.9042354, CityU 11261216) Associated Content Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI. The related TGA data of TPU and FR TPU composites; XRD patterns of graphite and LGO; XRD patterns of APP, LGAPP0.5 and LRAPP0.5; Raman spectra of APP


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and LRAPP; SEM images of the fracture surface; TGA and DTG curves of TPU composites; Digital photos and SEM images of the char residue for TPU and its composites after cone calorimer test. References (1) Zhong, W.; Cao, Z.; Qiu, P.; Wu, D.; Liu, C.; Li, H.; Zhu, H., Laser-Marking Mechanism of Thermoplastic Polyurethane/Bi2O3 Composites. ACS Appl. Mater. Inter. 2015, 7, 24142. (2) Valentini M, Piana F, Pionteck J. Electromagnetic properties and performance of exfoliated graphite (EG)-Thermoplastic polyurethane (TPU) nanocomposites at microwaves. Compos. Sci. Technol. 2015, 114, 26-33. (3) Sun X C, Xi B J. Study on preparation and mechanical performance of TPU/nonwoven

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Table captions Table 1. Formulations of FR TPU composites. Table 2. The related data of APP and LRAPP in nitrogen atmosphere. Table 3. LOI, UL-94 rating and cone calorimeter data for TPU composites


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Table 1. Formulations of FR TPU composites. Sample

TPU

APP

LRAPP0.25

LRAPP0.5

LRAPP1

LRGO content

FR content

(g)

(g)

(g)

(g)

(g)

(wt %)

(wt %)

0

0

0

5%

0.0125%

5%

0.025%

5%

0.05%

5%

TPU

100

TPU/APP

95

TPU/LRAPP0.25

95

TPU/LRAPP0.5

95

TPU/LRAPP1

95

5 5 5 5



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Table 2. The related data of APP and LRAPP in nitrogen atmosphere. Sample

T -5wt%(°C)＊

T max1 (°C)

T max2 (°C)

Residue at 800 °C (wt %)

APP

303.5

290.3

579.8

15.2

LRAPP0.25

278.8

285.8

621.5

29.9

LRAPP0.5

281.7

289.8

638.3

33.8

LRAPP1

285.2

289.8

690.5

36.7

＊

T

-5wt%

the initial degradation temperature (the initial degradation temperature is

deﬁned as T-5 wt%, where 5 wt% mass loss takes place in our laboratory)


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Table 3. LOI, UL-94 rating and cone calorimeter data for TPU composites. Sample

LOI

UL-94

(vol%)

Melt

av-t1 av-t2 TTI

dripping

(s)

(s)

(s)

pHRR

THR

FPI

(kW/m2) (MJ/m2) (m2/s/kW)

TPU

22.0±0.3

V-2

Y/Y

9.14

2.28

62

1013.7

71.8

0.06

TPU/APP

25.5±0.3

V-2

N/Y

7.02

2.65

45

247.8

49.9

0.18

TPU/LRAPP0.25 25.0±0.3

V-0

N/N

1.15

2.82

37

236.4

45.5

0.16

TPU/LRAPP0.5

25.5±0.3

V-0

N/N

1.48

6.03

35

170.6

44.1

0.21

TPU/LRAPP1

25.0±0.3

V-2

N/Y

6.32

8.78

35

215.6

49.8

0.16



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Figure captions Scheme 1. Schematic diagram of LRAPP Figure 1. TEM image of LGO. Figure 2. C1s XPS spectrum of LRAPP0.5 Figure 3. SEM images of (a) APP, (b) LRAPP0.25, (c) LRAPP0.5 and (d) LRAPP1 (scale bar size:2 µm). Figure 4. (a) TGA and (b) DTG curves of APP and LRAPP in nitrogen atmosphere. Figure 5. (a) Tensile strength and (b) elongation at break of TPU composites Figure 6. Storage modulus values (E’) of TPU composites. Figure 7. Thermal conductivity plots of TPU composites. Figure 8. Digital photos of the (a) TPU, (b) TPU/APP, (c) TPU/LRAPP0.25, (d) TPU/LRAPP0.5 and (e) TPU/LRAPP1 during the UL-94 test process (10s after the first and the second ignition) Figure 9. (a) HRR, (b)THR, (c) COPR and (d) CO2PR curves of TPU composites. Figure 10. Raman spectra of the external chars from (a) TPU, (b) TPU/APP and (c) TPU/LRAPP0.5


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Scheme 1. Schematic diagram of LRAPP



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Figure 1. TEM image of LGO.


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Figure 2. C1s XPS spectrum of LRAPP0.5



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Figure 3. SEM images of (a) APP, (b) LRAPP0.25, (c) LRAPP0.5 and (d) LRAPP1 (scale bar size:2 µm).


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Figure 4. (a) TGA and (b) DTG curves of APP and LRAPP in nitrogen atmosphere.



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Figure 5. (a) Tensile strength and (b) elongation at break of TPU composites


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Figure 6. Storage modulus values (E’) of TPU composites.



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Figure 7. Thermal conductivity plots of TPU composites.


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Figure 8. Digital photos of the (a1, a2) TPU, (b1, b2) TPU/APP, (c1, c2) TPU/LRAPP0.25, (d1, d2) TPU/LRAPP0.5 and (e1, e2) TPU/LRAPP1 during the UL-94 test process (10s after the first and the second ignition)



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Figure 9. (a) HRR, (b)THR, (c) COPR and (d) CO2PR curves of TPU composites.


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Figure 10. Raman spectra of the external chars from (a) TPU, (b) TPU/APP and (c) TPU/LRAPP0.5



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Preparation of Large-Size Reduced Graphene ... - ACS Publications

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