An infiltration method to synthesize thermoplastic polyurethane composites based on size-controlled graphene foams

Accepted Manuscript An infiltration method to synthesize thermoplastic polyurethane composites based on size-controlled graphene foams Yanbei Hou, Lijin Duan, Zhou Gui, Yuan Hu PII: DOI: Reference:

S1359-835X(17)30085-4 http://dx.doi.org/10.1016/j.compositesa.2017.02.023 JCOMA 4584

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

18 October 2016 19 February 2017 20 February 2017

Please cite this article as: Hou, Y., Duan, L., Gui, Z., Hu, Y., An infiltration method to synthesize thermoplastic polyurethane composites based on size-controlled graphene foams, Composites: Part A (2017), doi: http:// dx.doi.org/10.1016/j.compositesa.2017.02.023

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An infiltration method to synthesize thermoplastic polyurethane composites based on size-controlled graphene foams Yanbei Hou, Lijin Duan, Zhou Gui , Yuan Hu State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, PR China Abstract Graphene foams (GFs) with different sizes were prepared and applied to design thermoplastic polyurethane (TPU)/GF nanocomposites by infiltration method. Size-controlled GFs were successfully synthesized with variable concentration of graphene oxide (GO). Stable framework of GFs contributed to uniformity of composites and endowed them preferable thermal and mechanics performance. Results of TGA and MCC manifested that thermostability and flame retardancy of composites were superior to pure polymer, which was contributed to laminar barrier effect of GFs. Compression modulus of composite reached up to 2041.29 KPa, which was much higher than GFs. Due to porous structure, both GFs and TPU/GF composites exhibited quite low value of thermal conductivity. Char residue of TPU/GF composites not only remained original shape, but withstood certain pressure, which decreased potential fire risk. Polymeric materials design, based on GFs, is a feasible scheme to obtain composite with good integrated performance. Key words: graphene foam; thermoplastic polyurethane composites; polymeric material design; thermal and mechanical properties.



Corresponding author. Tel/Fax: +86 551 63601664 (Y. Hu) +86 551 63601669 (Z. Gui). E-mail address: [email protected] (Y. Hu) [email protected](Z. Gui).

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1. Introduction Three-dimensional (3D) carbon materials are composed of carbonic framework and interconnected holes. Recently, 3D carbon materials have attracted more and more attention [1-3]. They have already applied in a variety of fields including physics [4, 5], chemistry [6], and environmental applications [7, 8]. It has reported that chemical reduction of graphene oxide (GO) is a feasible method to prepared 3D architectures of graphene [9]. 3D nitrogen-doped graphene aerogels was synthesized and applied as supercapacitors [10]. Micropores and mesoporous provide space for gas and oil absorption. Qu et al. [11] prepared graphene frameworks and studied their capacitive behaviors and reversible adsorption for organic solvents. The ultrahigh and versatile 3D material offers great technological promise for a variety of sustainable applications [12-14]. Shi et al. [15] synthesized a nanocomposite based on graphene aerogel and used as enzymatic catalyst. It is believed that developing the 3D structures of graphene will further expand its significance in applications. It is important to note that the integration of GFs and polymers is a facile method of polymeric materials design [16, 17]. Consecutive pores endow foamed materials with many superior characteristics, high strength-to-density ratio, high modulus-to-density ratio, low thermal conductivity and so on. It has reported that graphene aerogels material can be prepared with an extremely low thermal conductivity[18]. However, the mechanical properties of GFs were unsatisfactory[19]. On the other hand, small size also restricts its applications. Thermoplastic polyurethane (TPU) is extensively used due to its excellent physical properties, flexibility at low temperature, abrasion resistance, variable hardness, etc. TPU, as other polymeric materials, presents some drawbacks including low thermal stability and flammability. It has confirmed that graphene can enhance flame retardancy of polymers [20, 21]. It is evident that TPU and GFs can complement each other's advantages to produce new composites with good mechanical and other properties. Since frameworks of GFs are stably formed, dispersibility of nanocomposites in matrix will not be involved, which contributes to uniformity of 2

composites. In addition, GF based polymeric materials with functional composites have been studied [22-25], which further confirm the feasibility of the incorporation of TPU and GF to prepare functional composite. This work provided an infiltration method to prepare composites with favorable properties. GO was prepared by a modified Hummer’s method [26]. With variable concentration of GO and different volume of autoclaves, size-controlled GFs were obtained through reduction reaction of GO and glucose. Modified GFs, infiltrated with TPU/DMF solution, dried in a vacuum, and then TPU/GF composites were obtained without excess treatments. Thermostability, Flame retardancy, thermal conductivity and mechanical properties of TPU/GF composites were satisfactory from the experiments data. Certain stress tolerance of combustion residue and low heat conductivity coefficiently enhanced fire safety of composites. 2. Experiment section 2.1 Raw materials Carbon source was generously provided by Complex new material technology (Shanghai) Co., Ltd. (Shanghai, China). Sulfuric acid (H2SO4, 98%), sodium nitrate (NaNO3, AP), potassium permanganate (KMnO4, AP), hydrazine hydrate (85% aq.), hydrochloric acid (HCl, 37% aq.), N, N-dimethylformamide (DMF, AP), glucose (AP) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Thermoplastic polyurethane (TPU, 85E85) was obtained from Baoding Bangtai Chemical Industry Co., Ltd. (Baoding, China). 2.2 Synthesis and modification of 3D GFs Scheme 1a shows schematic synthesis route of GFs. Sonication-assisted stirrer was employed to obtain homogeneous GO suspension with concentration of 1~4 mg/mL. Appropriate amount of glucose was dispersed in GO suspension (2.5 mg/mL) with continuing ultrasonic treatment for 0.5 h. Then, the mixed suspension was transferred into 80 or 400 mL autoclave, and held temperature at 180 oC for 18 h. Suspended graphene hydrogel with clarified water were obtained. For GFs preparation, the as-prepared reduced graphene hydrogel was freeze-dried to remove 3

the absorbed water. Surface modification of GFs was conducted to smoothly introduce TPU/DMF solution into inner space. GFs were coated with a conformal layer of simethicone (SM) to create low-energy surfaces by employing a vapor deposition technique [27] that allows the entire surface of the porous material to be coated. Briefly, as shown in Scheme 1b, sealed GFs and SM stamp together in autoclave, raised temperature to 235 oC and maintained for 4 h. Natural cooling to room temperature, SM-coated GFs were obtained without shape change. 2.3 Preparation of TPU/GF composites TPU was introduced into GFs layer by layer with increased concentration gradient (0. 1 g/mL, 0. 2 g/mL and 0. 5 g/mL) of TPU/DMF solution. As shown in scheme 1c, modified GFs were immersed in the mixed solution at 80 °C and would not be removed until liquid level was no longer falling. The composites were dried in a vacuum oven at 60 oC to remove DMF. The TPU composites obtained were named TPU/GF-1, TPU/GF-1, and TPU/GF-3, denoting as the first, second and third cycle of infiltration process. Curing shrinkage of polymer led to slight volume decrease of composites when DMF was removed. Furthermore, the neat TPU sample was prepared by a melting method. Briefly, a given mass of TPU was filled in a model and heated to 185 oC. Force the TPU at 10 MPa for 1 min to prepare TPU plate for later use. 2.4 Characterization The structure of GO and GFs were characterized via X-ray Diffraction (XRD) patterns of samples were performed with a Japan Rigaku D/Max-Ra using Cu-Kα radiation (λ= 0.1542 nm) at 40 kV and 200 mA. Raman spectroscopy measurements were conducted at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co.). X-ray photoelectron spectroscopy (XPS) was recorded using a Kratos Axis Ultra DLD spectrometer. Transmission electron microscopy (TEM) micrographs 4

were obtained by JEOL 2010 with an acceleration voltage of 200 kV. Dilute GO solution drops and placed onto a lacy carbon film supported by an ultra-thin Cu grid. Atomic force microscope (AFM) was applied to characterize the GO sheets by Multimode V scanning probe microscope (Digital Instrument) with tapping mode at a scanning rate of 1 Hz. Nitrogen adsorption–desorption isotherms at -196 °C were recorded on a Micromeritics ASAP2010C nitrogen adsorption instrument. The morphology of GFs and TPU/ GF composites were characterized by a JSM-6800F scanning electron microscopy (SEM) produced by JEOL. Thermogravimetric analysis (TGA) was conducted on a Q5000 IR thermogravimetric analyzer (TA Instruments/Waters) under N2 or air atmosphere, using a heating rate of 20 °C/min from room temperature to 800 °C in a 60 cm3·min-1 stream. The samples of MCC tests were obtained from a Govmak MCC-2. Mixed the pyrolysis products of samples with oxygen (20 mL·min-1) prior to entering a 900°C combustion furnace. The heats of combustion of the pyrolysis products were measured by the oxygen consumption principle. Laser Raman spectroscopy (LRS) measurements were carried out at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA) with excitation provided in backscattering geometry by a 514.5 nm argon laser line. Mechanical testing was conducted at a temperature of 23 ± 2 °C using an Instron universal testing machine with a crosshead speed of 5 mm/min. 3. Results and discussion 3.1 Characterization of GO and GFs In the XRD curves (Fig. 1a), a typical (001) diffraction of GO was located at 2θ=10.4°, implying that the d-spacing was 0.83 nm (GO). After chemical reduction, a 5

new wide diffraction peak appeared at 23.9°corresponding to an interlayer spacing of 0.38 nm. Changes of peaks indicated occurrence of reduction reaction, which resulted in the formation of graphene hydrogel [9]. Raman spectroscopy is widely employed to investigate the structure of carbonaceous materials. The corresponding Raman spectra of GO and GFs (Fig. 1b) showed the existence of the D, G bands. The G band, corresponding to ordered carbonic structure [28], anchored at 1598 cm-1 for GO and moved to 1589 cm-1 for GFs. The G band of GFs was close to the value of the pristine graphite which confirmed the reduction of GO during the chemical treatment. However, the existence of the D band at 1355 and 1350 cm-1, corresponding to GO and GFs, predicted the defect of the sample [29]. A higher ID/IG ratio indicates a higher degree of disorder [30]. Due to thermal reductive reaction, the ID/IG value of GO (1.07) is smaller than GF (around 0.96) C1s (280～300 eV) and O1s (525～545 eV) in XPS spectra (Fig. 1c) showed ratio change of carbon and oxygen. Peak value of O1s from GO was relatively higher than C1s, but the case for GFs was just the opposite. The C 1s curves of GO (Fig. 1d) indicated the presence of three kinds of carbon: C=O (carbonyl, ~288.3 eV), C–O (hydroxyl and epoxy, ~286.5 eV) and C=C in the graphitic domain. C1s peak of GFs (Fig. 1e) was consisted of a main components arising from C=C (~284.8 eV) and four minor components from C-C (285.6 eV), C-O /C-O-C groups, C=O (287.8 eV) and O-C=O groups (289.3 eV) [31-33]. These two curves revealed that carbonyl groups were nearly removed, and the C-O bonds decreased as well. Decreased oxygen content manifested reduction reaction mainly occurred between glucose and oxygen-containing functional groups. AFM height image of the GO with different magnification were illustrated in Fig. 2a. TEM image of GO (Fig. 2b) directly showed lamellar GO with large area was successfully prepared. AFM images indicated average thickness of the GO sheets were around 0.75 nm, which was consistent with previous reports [34]. A pristine graphene sheet is atomically flat with thickness of ~0.34 nm. GO sheets are expected to be thicker due to the presence of covalently bound oxygen and the displacement of 6

the sp3-hybridized carbon atoms slightly above and below the original graphene plane [35]. Larger thickness in magnified AFM image, caused by fluctuant GO sheets, indirectly prove the existence of oxygen-containing functional groups, which provide reduction sites for reaction [36]. Columned graphene hydrogels and GFs were shown in Fig. 2c and d. Concentration of GO in aqueous solution and volume of autoclave codetermined the size of GFs. The size of GFs increased with increasing concentration of GO at fixed volume of autoclave. However, cracks appeared on the surface of as-prepared GFs when concentration of GO was higher than 5 mg/mL. The SM-modified GFs became superhydrophobic, as indicated by its high water-contact angle (Fig. 2e). The contact angle increases from 95o to 143o confirmed successful modification has been conducted. 3.2 Characterization of GFs and TPU/GF composites With hydrophobic surface and extra high porosity, GFs could easily absorb TPU/DMF solution. The volume of cylindrical GFs and their composites can be roughly calculated. As shown in Fig. 3, the shape of composites changes little, comparing with GFs. For cylinder structure of GFs and TPU/GF composites, the basal diameter (D) and height (H) can be measured by vernier caliper. Therefore, the approximate density of the GFs and TPU/GF composites can be calculated by equation:

, where M means the weight of GFs and TPU composites. The

calculated densities of GFs and its composites are ~30, ~300, 375 and 750 mg/cm3, respectively, which mean that the mass fraction of GFs in composite is ~10, 8 and 4 wt% for TPU/GF-1, TPU/GF-1, and TPU/GF-3. It is worth pointing out that variable content of GF will be facilely prepared by changing concentration of TPU/DMF solution. The microstructures of GFs and TPU/GF composites were characterized by SEM (Fig. 3). Enlarged photograph of GFs indicated that graphite sheets interlocked each other at reduction reaction sites and constituted three-dimensional framework with micron-sized pores. Surface of TPU/GFs exhibited obviously different morphology. 7

Most pores were blocked and the size distribution of them became nonuniform. Partial enlarged view of the composite demonstrated that graphitic sheets maintained their original structure and were covered by TPU on both side. These microstructures directly contributed to uniformity of composite. With the increasing concentration of TPU, the pores of GF were generally filled up. However, the skeleton of GF could be observed. To further verify the integrity of GFs skeleton in composites, the electrical conductivity tests of GFs and TPU/GF-1 were conducted by a simple voltammetry method and the results presented in Fig. 4a. The samples were cylindrical with a 15 mm height and 10 mm diameter. The resistance of TPU/GFs (64 Ω) was higher than that of GFs (28.2 Ω), indicating that the addition of TPU hinders the circulation of electronic. Compared with insulating TPU matrix, the low-resistance TPU/GF composite directly proved that integrated conductive pathways indeed existed in composite. It also confirmed that the integrity of GFs skeleton well-preserved during the preparation process of TPU/GF composites. The nitrogen adsorption isotherms for GFs and TPU/GF-1 showed in Fig. 4b and c. Isotherm of GF exhibited typical H4 (IUPAC classification) type hysteresis, which confirmed that internal space structure was consisted by layered graphene. While TPU composite displayed a H3 type, characteristic of slit shape pores [37]. The differences between the isotherms were essentially caused by the changes of open-framework structure. The BET surface areas of GFs and their composites were 168.9 m²/g and 0.93 m²/g respectively. The decreased area directly proved the successful introduction of TPU and remained voids existed in composite, which is in agreement with SEM results. Porous TPU/GFs are constituted by continuous phase (TPU and GFs) and dispersed phase (air). The thermal conductivity of air is much lower than that of solid skeleton, leading to a lower thermal conductivity of the whole material [38]. Thermal conductivities of TPU and TPU/GF-1 were obtained by a DRE-2c thermal analyzer (Instrument and meter Co., Xiangtan, China) at room temperature. The average thermal conductivity values of GFs and TPU/GF composites were 0.0365 and 0.0333 8

W m-1 K-1, which are much lower than that of TPU (0.2044 W m-1 K-1). Decreased porosity contributed to higher thermal conductivity, but thermal conductivity of graphene was much higher than TPU [18, 39], these mutual effect endowed composite with a lower thermal conductivity. 3.3 Thermostability, flame retardancy and mechanical properties The influence of GFs on the thermal stability of TPU was investigated by TGA. Fig. 5 showed the TGA curves of TPU and TPU/GFs under N2 (a) and air (b) atmosphere. The mass loss of GFs occurred at the beginning of thermal treatment, which caused by the decomposition of oxygen-containing functional groups from GO [40]. Therefore, the addition of GFs led to the initial decomposition temperature of composites. Due to thermostability of GFs at higher temperature, they can protect the composite from oxygen and suppress pyrolysis of TPU. Fig. 5a shows that TPU rarely generate char residue, while carbon residue value of TPU/GF-1 is as high as 11.5 %, confirmed that the addition of GFs highly improve the char yield of TPU. It can be observed that with the increasing cycle number, the char yield of TPU composites decreased obviously, which is attributed to the increasing content of TPU in TPU/GF composites. The similar tendency of pyrolyzation can be observed under air conditions (Fig. 5b). Both under air and N2 conditions, the range of decomposition temperature for the composites became broader than that of pure TPU, implying the pyrolytic processes of TPU composites were prolonged. Due to barrier effect of layered reduced GO layers, the degradation process of TPU was suppressed and the carbon-forming effect was promoted. Microscale combustion calorimetry (MCC) is one of the most effective bench-scale methods for investigating the combustion properties of polymeric materials. The combustion properties of the pure TPU and TPU/GF composites were characterized by MCC. Heat release rate (HRR) is one of the most important material parameters to evaluate the fire risk. The HRR curves of the TPU and TPU/GF composites were presented in Fig. 5c. It is obvious that with the concentration of GF decreasing, the flame retardancy of TPU composites is apparently weakened. TPU 9

and TPU/GF composites displayed two-step combustion process from the HRR curves. The first combustion region occurred around from 400 to 450

o

C,

corresponding to the heat release of hard segments. The second region happened at 500-600 oC and the maximum combustion temperatures appeared around 550-570 oC, which was associated with the thermal degradation of TPU chains. The above results were in agreement with the TGA analysis. It was evident from Fig. 5c that the initial combustion temperature of pure TPU and TPU/GF composites had no distinct difference. However, PHRR of the TPU/GF-1 (253.18W/g) was remarkably lower than that of pure TPU (390.02 W/g), which was attributed to the heat resistance effect and barrier property of GO sheets. GF created physical protective layers on the surface of the composites during combustion, which postponed the time to PHRR (from 364 to 503.5 s), thus heat release of TPU was suppressed. Longer time to PHRR benefits less potential fire hazards. In summary, incorporation of GF and TPU endowed composites better flame retardancy performance than pure TPU. Compared with granular and two-dimensional fillers, including metal hydroxide, nano carbon, and layered double hydroxide, the application of 3D GFs as flame retardant propose a novel pathway to prepare composites [41, 42]. The stable framework of GFs guarantees the homogeneity of composites, confirmed by Fig. 3. The char formed during combustion is important for fire safety applications. Morphology of residue, treated at 700 °C for 10 min, was tested by SEM (Fig. 6a). There was almost no char yield for pure TPU. Compared with shapeless residue of TPU, TPU/GF composite remains its original shape after high-temperature processing. Local enlargement of TPU/GF residue revealed that most GF survived and functioned as skeleton for carbon residue (Fig. 6b). The original morphology of TPU/GF composites was disappeared, but the layered sheets and porous structure remained. To further understand the formation of the carbonaceous char, LRS was performed and the specific components in the residue of samples were calculated. The two peaks, located at 1399 and 1600 cm-1, are corresponding to the vibration of carbon atoms 10

from disordered graphite and glassy carbons (D band) and vibration of carbon atoms in graphite layers (G band) respectively [43]. The graphitization degree of char could be calculated by a ration of the integrated intensity of D and G bands (ID/IG). Fig. 6 c-d illustrated that the ID/IG ratio of TPU/GF was higher than that of GF, indicating a lower graphitization degree of composites, which is attributed to the barrier effect of graphene sheets during thermal decomposition process. GF sheets prolonged the combustion of TPU and promote higher char residue with disordered formation. The chemical composition of residues was shown in Fig. 6e. The ratio of oxygen and carbon could explain the char formation of GF and its composites. The ratio follows the sequence of GF (12.5:87.5)
< 10%, and a plateau

with gradually increasing slope until very high strains up to 65%. The composites exhibited similar tendency with much higher compression strength. Introducing TPU into GFs markedly improved mechanical property of GFs, which is mainly contributed by the excellent mechanical performance of TPU. Stress and compression strength at

=10%, modulus of compression of GFs and TPU/GF-1are listed in table

1. What to be noted is that combustion residue of composite has a satisfactory mechanics performance (Fig. 7b). The maximum value of compression strength reached 35.5 KPa. Brittle fracture of residue occurred when the compressive strain was more than 25%. All results above confirm that the mechanical property and fire safety of composite were highly enhanced. The cyclic strain–stress curves at strain up to 50% are shown in Figure 7c (GFs) and d (TPU/GF-1). The curves of GFs shows similar tendency of TPU/GF-1. Take TPU composite for example, during the first test, the stress increases rapidly and 11

linearly when the strain is less than 10% after which stress increases slowly. The stress increases sharply again when the strain is more than 40%. The first compression cycle is different from the subsequent ones showing higher Young’s modulus, maximum stress and large energy loss coefficient. The hysteresis loop for the second cycle shrinks compared to the first one, but the maximum stress appearing at

= 50%

is only 3% lower than for the first one, for TPU/GF composites. Since the fourth cycle, the stress-strain curves almost remain unchanged. TPU overcovered both sides of GO sheets and immobilized the interlocking structure, which endowed the composite better performance. With partial broken during test, GFs and TPU/GF composites cannot fully recover to their original volume at the first cycle, However, the volume shows almost no change in the followed cycles, resulting in the larger hysteresis loop observed at first cycle in fig. 6c and d, which is similar with the phenomenon has reported [44]. 4. Conclusion In summary, TPU/GF composites, which were prepared by infiltration method, possessed advantages of foamed materials and thermoplastic elastomeric materials. It exhibited better thermostability and flame retardancy than pure TPU. Specially, char residue of TPU/GF composites not only remained its pristine shape, but also performed certain mechanical properties, which means better fire safety was obtained. Extremely low thermal conductivity of TPU/GF composites indicated that it is an excellent thermal insulation material. Modulus of compression and cycle mechanical performance of TPU/GF composites were much higher than that of GFs. Assembly of GFs and TPU achieves win-win results by complementing each other with their own advantages. Notably, with appropriate solvents, most polymers, such as polystyrene, polypropylene, polyethylene and so on, can prepare GF-based composites by this 12

method. Acknowledgments The authors are grateful to the National Basic Research Program of China (973 Program) (No. 2012CB922002), National Key Technology R&D Program (2013BAJ01B05) and National Natural Science Foundation of China (21374111).

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thermal and flame retardant properties by combining graphene nanosheets and metal hydroxide nanorods for Acrylonitrile-Butadiene-Styrene copolymer composite. Compos Pt A-Appl Sci Manuf. 2014;64:203-10. [43] Yuan BH, Wang BB, Hu YX, Mu XW, Hong NN, Liew KM, et al. Electrical conductive and graphitizable polymer nanofibers grafted on graphene nanosheets: Improving electrical conductivity and flame retardancy of polypropylene. Compos Pt A-Appl Sci Manuf. 2016;84:76-86. [44] Hu H, Zhao ZB, Wan WB, Gogotsi Y, Qiu JS. Ultralight and Highly Compressible Graphene Aerogels. Adv Mater. 2013;25(15):2219-23.

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Figure Captions Scheme 1 Schematically synthetic routes of GF (a), SM-coated GF (b) and TPU/GF composite (c) Fig. 1 XRD (a) and Raman (b) curves of GO and GF; XPS spectra (c) and C 1s peaks of GO (d) and GF (e) Fig. 2 AFM and TEM photograph of GO (a, b) and digital pictures of graphene hydrogel of different sizes (c), GF (d), and results of contact angle measurement (e). Fig. 3 Digital and SEM photographs of GF , TPU/GF composite Fig. 4 Volt-ampere characteristics curves (a) and nitrogen adsorption isotherms for the GF (b) and TPU/GF composite (c) Fig. 5 TGA results of GF and TPU/GF composites under N2 (a) and air (b) atmosphere; HRR curve of TPU and TPU/GF composites (c) Fig. 6 SEM images (a, b), Raman spectra (c, d), and EDS results (e) of the char residues of GF and TPU/GF composites Fig. 7 Stress–strain curves of GF and TPU/GF-1 (a, c-d) and stress–strain curves of TPU/GF-1 after heat treatment (b)

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Scheme 1 Schematically synthetic routes of GF (a), SM-coated GF (b) and TPU/GF composite (c)

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Fig. 1 XRD (a) and Raman (b) curves of GO and GF; XPS spectra (c) and C 1s peaks of GO (d) and GF (e)

22

Fig. 2 AFM and TEM photograph of GO (a, b) and digital pictures of graphene hydrogel of different sizes (c), GF (d), and results of contact angle measurement (e)

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Fig. 3 Digital and SEM photographs of GF , TPU/GF composites

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Fig. 4 Volt-ampere characteristics curves (a) and nitrogen adsorption isotherms for the GF (b) and TPU/GF composite (c)

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Fig. 5 TGA results of GF and TPU/GF composites under N2 (a) and air (b) atmosphere; HRR curve of TPU and TPU/GF composites (c)

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Fig. 6 SEM images (a, b), Raman spectra (c, d), and EDS results (e) of the char residues of GF and TPU/GF composites

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Fig. 7 Stress–strain curves of GF and TPU/GF-1 (a, c-d) and stress–strain curves of TPU/GF-1 after heat treatment (b)

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Table 1 Stress, compression strength and modulus of compression of GF and TPU/GF composite Sample

F   10% (N)

P   10% (MPa)

Ec(KPa)

GF

6.47

0.0228

198.53

TPU

475.83

1.0523

8130.41

TPU/GF

68.24

0.8693

2041.29

29