Lightweight and Ultrastrong Polymer Foams with Unusually Superior

Jul 14, 2017 - High-performance flame-retardant materials are urgently needed to address outstanding issues that pertain to safety. Traditional flame ...
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Lightweight and Ultrastrong Polymer Foams with Unusually Superior Flame Retardancy Linli Xu,† Linhong Xiao,† Pan Jia,† Karel Goossens,‡ Peng Liu,∥ Hui Li,† Chungui Cheng,† Yong Huang,† Christopher W. Bielawski,‡,§ and Jianxin Geng*,† †

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, P. R. China ‡ Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea § Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ∥ Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: High-performance flame-retardant materials are urgently needed to address outstanding issues that pertain to safety. Traditional flame retardants are toxic to the environment and/or lack the physical properties required for use in many contemporary applications. Here, we show that isocyanate-based polyimide (PI) foam, a flammable material, can exhibit unusually superior flame retardancy as well as other excellent properties, such as being lightweight and displaying high mechanical strength, by incorporating red phosphorus (RP)-hybridized graphene. The covalent bonds formed between the graphene platelets and the PI matrix provide the resultant PI foam with a specific Young’s modulus (83 kNm kg−1) that is comparable to or even higher than those displayed by state-of-the-art foams, including silica aerogels, polystyrene foams, and polyurethane foams. In addition, even a low content of the RP-hybridized graphene (2.2 wt %) results in an exceptionally higher limiting oxygen index (39.4) than those of traditional flame-retardant polymer-based materials (typically 20−30). The resultant PI foam also exhibits thermal insulation properties that are similar to that of air. Moreover, the RP-hybridized graphene is prepared using a one-step ball milling process in 100% yield, and does not require solvent or produce waste. The preparation of the flame-retardant PI foams can be scaled as the starting materials are commercially available and the techniques employed are industrially compatible. KEYWORDS: graphene, red phosphorus, polyimide foam, fire retardant, compressive property, thermal conductivity

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INTRODUCTION Flame-retardant materials play a major role in controlling fire accidents. Traditional inorganic and brominated flame retardants often are inefficient, are toxic to the environment, and/or lack the required mechanical strength needed for use in contemporary applications.1−3 Therefore, there is a growing need to develop high-performance, halogen-free flame retardants to overcome the intrinsic drawbacks of traditional flame retardants. For many industrial and consumer applications, the density as well as the material’s mechanical strength play decisive roles in determining practicality and utility. As such, the development of materials that combine an excellent flame retardancy with other outstanding characteristics, such as a low density as well as superior mechanical properties, is warranted. Recently, flame-retardant graphene foams that are ultralight and also exhibit compressible characteristics were reported.4 While such an advance is attractive, the handling large-scale fabrication and overcoming the environmental issues associated © XXXX American Chemical Society

with the chemical production of graphene remain challenging. Polymer foams are also lightweight and, due to their variable chemical structures, can be rendered processable and used in many applications.5,6 Unfortunately, polymer foams are prone to catching fire. Over the past several decades, various types of flame retardants, including halogen- and phosphorus-based agents,7,8 intumescent compounds,9−11 inorganic layered materials,12−14 and other flame-resistant materials15−18 have been successfully incorporated into a broad range of polymer foams to minimize flammability. In addition, hybrid flame retardants composed of inorganic layered compounds or carbon nanomaterials, such as nanotubes and graphene, have also been shown to be efficient flame retardants for polymeric materials.19−23 Nevertheless, a critical challenge thus far is to develop a scalable and low-cost approach for fabricating Received: May 4, 2017 Accepted: July 14, 2017 Published: July 14, 2017 A

DOI: 10.1021/acsami.7b06282 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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was dried in a vacuum oven set at 120 °C for 5 h before use. Polyaryl polymethylene isocyanate (PAPI) (PM-200) was purchased from Yantai Wanhua Polyurethanes Co., Ltd. (China). Polyether modified polysiloxane (DC-193) was supplied by Foshan Daoning Chemical Co., Ltd. (China). All other chemicals were purchased from Sinopharm Chemical Co., Ltd. (China) and were used as obtained. Preparation of the RP-Hybridized Graphene. The RPhybridized graphene was prepared by ball milling mixtures of RP and graphite at various mass ratios (i.e., 2:1, 4:1, and 6:1) (Scheme 1a). The corresponding composites were designated as GPx, with x representing the mass ratio of RP to graphite: i.e., GP2, GP4, and GP6 when the RP/graphite ratio was 2:1, 4:1, and 6:1, respectively. Briefly, RP and graphite (10 g in total weight but with different ratios) were added to a 100 mL stainless steel ball-mill capsule. The capsule was filled with Ar, sealed, and then agitated on a planetary ball-milling machine at 480 rpm for 48 h. As a control, RP and graphite were independently ball-milled. Preparation of Flame-Retardant PI Foams. The PI foams were prepared via a two-step process (Scheme S1): synthesis of PI precursor foams by chemical foaming during the reaction between benzophenone-4,4′-dimethoxycarbonyl-3,3′-dicarboxylic acid (BDMDA) and PAPI, followed by imidation of the PI precursor foams to form the PI foams. In a typical process, BTDA (8.137 g) was dissolved in N,N-dimethylformamide (DMF) (8.20 g, 8.65 mL) by stirring at 80 °C. Next, methanol (1.221 g, 1.55 mL) was added to the BTDA solution to form BDMDA through alcoholysis at 70 °C. Any unreacted methanol was kept in the solution to act as a foaming agent. Deionized water (0.814 g, 0.82 mL) as an additional foaming agent was added to the solution. Polyether modified polysiloxane (1.221 g) was also added as a surfactant to facilitate foaming. After the addition of the reagents, the aforementioned BDMDA solution was cooled to room temperature, followed by the addition of graphite, RP, or a GPx composite at various loadings with respect to the masses of BTDA and PAPI. The mixture was then agitated at 6000 rpm for 2 min using a high-speed mixer to obtain a homogeneous suspension. In some cases (e.g., to reduce the brittleness of the PI foam; see below), polyethylene glycol-600 (PEG-600) (0.814 g) was also added to the BDMDA suspension. PAPI (16.274 g) was dissolved in DMF (9.48 g, 10.00 mL), and the obtained solution was added to the aforementioned BDMDA suspension that was precooled using an ice/water bath. The resultant mixture was agitated at 6000 rpm for 5−15 s using a high-speed mixer to obtain a homogeneous suspension and then immediately transferred to a mold for foaming at room temperature. In this process, the reactions between BDMDA and PAPI led to the formation of the framework of the PI precursor foams (Scheme S1). The CO2 released from the reactions between the isocyanate groups of PAPI and methanol/water acted as a foaming gas to form the cellular structures. Once the PI precursor foams were verified to be no longer sticky by touch, they were thermally treated at 180 °C for 2 h and then at 250 °C for 1 h to achieve the PI foams. General Characterization Methods. Raman spectra were collected on a Renishaw inVia-Reflex confocal Raman microscope with an excitation wavelength of 633 nm. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Focus diffractometer with an incident wavelength of 0.154 nm (Cu Kα radiation) and a Lynx Eye detector. The step size and counting time per step were set to be 0.02° and 0.5 s, respectively. X-ray photoelectron spectroscopy (XPS) data were recorded on a PHI Quantera Scanning X-ray Microprobe using monochromated Al Kα radiation (1486.7 eV). Thermal measurements were performed using a TA Instruments Q50 thermogravimetric analysis (TGA). Samples of ca. 3 mg were loaded and scanned from room temperature to 1000 °C at a heating rate of 10 °C min−1 under an atmosphere of air or N2. Scanning electron microscopy (SEM) observations were performed on a Hitachi S-4800 field emission microscope operated at an acceleration voltage of 10 kV. SEM samples were prepared by cutting the PI foams or their residual chars obtained after burning into small pieces; the cut surfaces were analyzed. Energydispersive X-ray spectroscopy (EDS) analysis was performed on the same microscope. Transmission electron microscopy (TEM) was

materials that have superior flame-retardancy but also display desirable properties, such as low densities, high mechanical strength, and favorable thermal characteristics. Herein, we present the fabrication of superior flame-retardant polyimide (PI) foams that are also of high strength and are lightweight through the incorporation of red phosphorus (RP)hybridized graphene (Scheme 1). The RP-hybridized graphene Scheme 1. Preparation and Testing of a Flame-Retardant PI Foama

a

(A) Schematic illustration of preparing the GPx composites by simply ball milling graphite and RP. (B) Schematic illustration of the molecular structure of a graphene platelet with RP nanoparticles attached. Photographs taken of (C) a GP2-containing PI foam and (D) a pristine PI foam being torched by the flame of an alcohol lamp.

was prepared by simply ball milling mixtures of graphite and RP, which resulted in efficient exfoliation of the graphite followed by hybridization of the resultant graphene platelets with the nanosized RP particles. We show that the contributions of the nanosized RP particles and the 2D graphene platelets were synergistic and significantly enhanced the flame-retardant properties of the resultant RP-hybridized graphene, particularly when compared to pristine RP. The RPhybridized graphene was incorporated into a PI foam by mixing with an appropriate foaming precursor. Chemical reactions between the graphene platelets and the PI matrix resulted in a significantly enhanced compressive strength of the resultant PI foam as well as a uniform distribution and good retention of the flame-retardant material in the polymer matrix. Only a low incorporation of the RP-hybridized graphene was required (2.2 wt %) to achieve a PI foam with a superior resistance to flame. For example, the limiting oxygen index (LOI) of the resultant PI foam was measured to be 39.4, a value that is 1.5 times higher than those reported for traditional flame-retardant polymer-based materials (typically 20−30). Furthermore, the starting materials used for preparing the flame-retardant PI foams are commercially available, and the employed techniques are industrially compatible. These features should make the scale-up of the synthesis of the foams described herein feasible.

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EXPERIMENTAL SECTION

Materials. Graphite powder ( GP4 > GP6 > RP. This order is the reverse of the RP content in the composites. This trend was attributed to the sizes of the RP particles in the corresponding composites (Figure S8). The LOI, which represents the minimum oxygen concentration (%) required to keep a material burning, was determined for a series of PI foams (Table S4). The LOI measured for the foam containing GP2 (39.4) was nearly twice the O2 level in air (21%). This value is significantly higher than E

DOI: 10.1021/acsami.7b06282 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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phase during combustion by scavenging reactive free radicals,1−3,42,43 phosphorus compounds can be vapor phase or condensed phase flame retardants depending on their chemical structures and their interactions with the polymer matrices.42,43 In this research, GPx composites were composed of graphene platelets, with phosphonic acid groups covalently bonded to the edges and RP nanoparticles attached to the surfaces. The flame inhibition (i.e., reduced pHRR and THR) upon the addition of GPx into PI foams (shown in Table 1) indicated the vapor phase mechanism of flame retardancy. Meanwhile, compared to pristine PI foam, the enhanced char yield demonstrated the condensed phase mechanism of flame retardancy (shown in Table 1), due to the two-dimensional feature of the graphene platelets as well as the covalent bonding between the graphene platelets and the PI matrix. As a result, GPx composites inhibited flame propagation through a combination of vapor phase and condensed phase mechanisms. In order to obtain deeper insight into the flame-retardant mechanism, the thermal properties of GP2 were analyzed using TGA under an atmosphere of air (Figure 5a). At ca. 200 °C, GP2 exhibited a gradual increase in mass due to oxidation of phosphorus to pyrophosphate and polyphosphate by O2, which is a requisite step for the RP nanoparticles to show flameretardant properties.1,37 The maximum increase in mass was found to be ca. 35.8 and 53.6 wt % with respect to the composite and the RP in the composite, respectively, with the peak temperature (Tpeak) observed at 406 °C. Such increase in mass was significantly higher than that recorded for pristine RP (ca. 15.4%) and was attributed to the decreased size of the RP particles in GP2. In particular, among the three composites prepared, GP2 exhibited the greatest increase in mass as well as the highest Tpeak (Figure S15 and Table S6). The enhanced increase reflected improved reaction extent of the RP in GP2 with O2. The elevated Tpeak may be ascribed to the interactions between the RP nanoparticles and the graphene platelets. The assignment of the increased mass was supported by a series of complementary TGA experiments that were performed under an atmosphere of N2 (Figure S16); in these experiments, increases in mass were not detected. The pyrophosphate and polyphosphate species catalyze the formation of char, which protects the underlying material from heat and shields the fuel gases released from the surface.1,37 In the gas phase, active radicals (e.g., PO2•, PO•, and HPO•) scavenge H• and OH• radicals to inhibit flame formation. Compared with the pristine RP, the superior flame retardancy of GP2 can be attributed to the following aspects: First, since the conversion of RP to pyrophosphate and polyphosphate is a requisite step in the flame retardation mechanism, the

that measured for the pristine PI foam (25.9) as well as other common, flame retardant-containing polymer-based foams such as PU and PS foams (typically 20−30).36−40 Furthermore, the GP2 content required to achieve such good flame resistance is lower than that of other flame retardants used in polymer-based foams (usually 3−57 wt %).36−40 The flame-retardant properties of the PI foams were further assessed using cone calorimetry (Figure S13 and Table 1). The time to ignition Table 1. Parameters of the PI Foams Obtained from Cone Calorimetrya pHRR THR [kW m−2] [MJ m−2]

char yield at 300 s [%]

sample

tign [s]

tpHRR [s]

pristine PI foam PI foam_2.2 wt % graphite PI foam_2.2 wt % RP PI foam_2.2 wt % GP2 PI foam_2.2 wt % GP4 PI foam_2.2 wt % GP6

10 12

22 108

139.8 74.6

12.0 9.5

4.30 12.9

8

18

94.4

11.0

23.1

7

14

48.4

4.1

56.8

7

14

62.9

5.0

56.7

7

14

64.6

8.7

31.5

a

The data were derived from cone calorimetry data shown in Figure S13.

(tign) and time to peak heat release (tpHRR) of the PI foams with GPx decreased to ca. 7 and 14 s, respectively, probably due to the higher conductivity of the graphene platelets and the reaction of the RP with oxygen.41 Compared with the pristine PI foam, addition of the various flame retardants led to reduced peak heat release (pHRR) and total heat release (THR) as well as increased char yield (Table 1). The changing tendencies of pHRR, THR, and char yield recorded for RP- and GPxcontaining PI foams indicated that GP2 exhibited the best flame-retardant performance, consistent with the results obtained from burning tests (Figure S12). Significantly, the pHRR and THR values recorded for GP2-containing PI foam were measured to be 34.6% and 34.2% of those recorded for pristine PI foam, and the char yield recorded for the former was 13.2 times higher than that recorded for the latter. Microscale combustion calorimetry data collected for the PI foams (Figure S14) were consistent with the results obtained from cone calorimetry and supported the notion that the GP2-containing PI foam exhibits the highest flame-retardant performance among the various flame retardants (Table S5). Flame Retardation Mechanism. While RP is a vapor phase flame retardant that reduces the heat release from the gas

Figure 5. Flame retardation mechanism. (A) TGA data recorded under an atmosphere of air for graphite, RP, and GP2 (indicated). (B) An SEM image of a GP2-containing PI foam (2.2 wt % of GP2) after being subjected to flame. F

DOI: 10.1021/acsami.7b06282 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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nanoscale RP particles in GP2 are preferentially oxidized. Second, the graphene platelets, which are a well-known 2D material with high thermal and chemical stabilities,27,38 contribute to the formation of a shielding layer to protect the underlying material.42 Indeed, SEM images of the GP2containing foam subjected to flame revealed the presence of a uniform, thin protecting layer formed on the cell walls of the cellular structures (Figure 5b).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Hundred Talents Program” of the Chinese Academy of Sciences, the Natural Science Foundation of China (21274158, 91333114), and CAS President’s International Fellowship for Visiting Scientists (2013T1G0019). C.W.B. is grateful to the IBS (IBS-R019D1) and the BK21 Plus Program as funded by the Ministry of Education and the National Research Foundation of Korea.

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CONCLUSIONS In summary, a common isocyanate-based PI foam was shown to exhibit exceptionally superior flame retardancy, light weight, and a high compressive strength due to incorporation of RPhybridized graphene. The excellent integrated properties of the resultant PI foam are ascribed to the hybridization of graphene platelets with the RP nanoparticles as well as the covalent crosslinking between the graphene platelets and the PI matrix. A PI foam containing only 2.2 wt % of GP2 shows unusually superior resistance to flame and exhibits an LOI value of up to 39.4, which is ca. 1.5 times higher than those of common, flame-retardant-containing polymer-based foams (typically 20− 30). In addition, the GP2-containing PI foam also exhibits a very low density (0.016 g cm−3), a high Young’s modulus (1.33 MPa), and excellent thermal insulation (30 mW m−1 K−1). The RP-hybridized graphene is prepared in one step and in 100% yield using a process that does not require solvent or produce solvent waste. The starting materials for preparing the flameretardant PI foams are commercially available, and the employed techniques are industrially compatible, making the scale-up of this approach feasible. Our results provide important insights into the development of superior flameretardant materials that also display outstanding mechanical strength and thermal insulation by utilizing environmentally friendly resources. It is expected that such materials will find utility in thermal insulation or vibration damping required in civil engineering applications as well as in modern modes of aerospace, maritime, and railway-based transportation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06282. Morphological and compositional data of the GPx composites, preparation and additional characterization data of the isocyanate-based PI foams, and additional flame-retardant data of the various PI foams (PDF) Movie showing horizontal burning test of a pristine PI foam (AVI) Movie showing horizontal burning test of a PI foam containing 2.2 wt % of GP2 (AVI) Movie showing combustion test of a pristine PI foam that is placed on top of a flame (AVI) Movie showing combustion test of a PI foam containing 2.2 wt % of GP2 that is placed on top of a flame (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianxin Geng: 0000-0003-0428-4621 G

DOI: 10.1021/acsami.7b06282 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b06282 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX