Superelastic, Anticorrosive, and Flame-Resistant Nitrogen-Containing

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Superelastic, Anticorrosive and Flame Resistant Nitrogen-containing Resorcinol Formaldehyde/Graphene Oxide Composite Aerogels Lei Wang, Jianlei Wang, Longhui Zheng, Zhenming Li, Lixin Wu, and Xin Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01735 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Sustainable Chemistry & Engineering

Superelastic, Anticorrosive and Flame Resistant Nitrogen-containing Resorcinol Formaldehyde/Graphene Oxide Composite Aerogels Lei Wanga,b, Jianlei Wangb, Longhui Zhengb, Zhenming Lia, Lixin Wub,*, Xin Wangc,** a Institute of Safety Engineering, Zhejiang University of Technology, 288 Liuhe Road, Xihu District, Hangzhou, Zhejiang, 310023, PR China b CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Gulou District, Fuzhou, Fujian, 350002, PR China c Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui, 230026, PR China

AUTHOR INFORMATION Corresponding Author *Lixin Wu, Email address: [email protected] **Xin Wang, Email address: [email protected] ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Energy conservation requires next-generation thermal-insulating materials feature with multiple functions (e.g. ultralight, anticorrosion, mechanically resilient, and highly flame retardant); however, creating such thermal-insulating materials has still proven challenging. Herein, we employed a self-assembly copolymerization strategy combined with ambient-pressure drying technique to synthesize a new series of nitrogenous resorcinol formaldehyde/graphene oxide composite aerogels. Rational integration of metals ions, urea-formaldehyde (UF), resorcinol-formaldehyde (RF) and graphene oxide (GO) in a system led to the interpenetrating quaternary network, rendering the as-prepared RUFG aerogel extraordinary compressibility (the maximum strain of 80 % and robust stability), high corrosion and combustion resistance, superior to many commercial thermal-insulating materials. Further considering its ultralight weight and low thermal conductivity, this RUFG aerogel can be quite adequate to serve as a new building material with high energy efficiency. KEYWORDS: Graphene oxide, Compressibility, Corrosion resistance, Combustion resistance, Thermal conductivity INTRODUCTION The energy used in the building occupies a significant part of the world’s total energy consumption.1-3 Currently, an effective approach to control the energy efficiency of buildings is to utilize thermal-insulation materials with low thermal conductivity values.3,4 Recent serious fires occurred in high rise buildings emphasized the importance of non-flammability and structural integrity for thermal-insulating materials during burning.4 Commercial insulation materials, such as expanded polystyrene (EPS) and expanded polyurethane (EPU), are typically lightweight, anticorrosive, flexible and cheap, yet their high flammability can promote the spread of fire in a fire disaster, thus restricting their future application prospects.5,6 To solve this problem, some fire retardants were added to ameliorate the fire retardancy of these materials. However, most commonly used flame retardants have been identified as persistent pollutants, owing to their toxicity and slow biodegradability in natural environment.7,8 Herein, it is imperative, but also challenging, to develop new high-performance insulation materials that simultaneously own excellent anticorrosive, flame-retardant and mechanical properties. Phenolic resin aerogels, a well-known polymeric aerogels, featured with low thermal conductivity and corrosion resistance, were regarded as ideal thermal-insulating materials.9,10 Specially, phenolics has been used in ablative thermal protection applications as thermal resistant thermosets, and addition of melamine or ACS Paragon Plus Environment

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urea can further give rise to various advantages, such as the reduced cost and improved flame retardancy of the resin, which are very important for their practical utilization as thermal insulating building material.11-15 Although significant improvement can be achieved by incorporation of these nitrogenous compounds, the obtained copolymer is limited by its poor mechanical properties, as these aerogels are usually brittle and fragile.16 Previous study has revealed nanomaterials can serve as promoters to improve the physical and chemical properties of polymer matrix. For example, the emplyment of carbon nanotubes, clays, or graphene as templates was demonstrated to strengthen the polymeric gel skeletons.17-19 Thermoelectric function was also reported to be enhanced by in-situ polymerization or chemical reaction between specific carbon materials (carbon nanotubes and graphene nanoplatelets) and polymer molecules.20-22 Besides, some bivalent metal ions (e.g. Co2+, Ni2+, etc.) were reported to play a positive role in promotion of the mechanical properties for threedimensional (3D) polymer/nanomaterials composites by serving as synergic ion-bonding cross-linkers.16,23-27 These cases provide us a promising way to select suitable foreign materials to form more robust and economical phenolic resin-based 3D structure via a synergistic enhancing effect. Herein, in this work, we took resorcinol-formaldehyde (RF), a conventional phenolic resin, as an example, and chose GO, Co2+ and low-cost urea as property-enhancing components. Through a low-temperature template-copolymerization followed by an ambient-pressure drying process, the as-made optimal resorcinolurea-formaldehyde/graphene oxide (RUFG) aerogel exhibited the integrated properties of low density, high corrosion-resistance, super recyclable compressibility, low thermal conductivity and high fire-resistance. In the interpenetrating quaternary networks, urea, GO and bivalent metal ions played different important roles: 1) The employment of urea reduced the cost and improved the flame retardancy. 2) GO acted as a template skeleton and anti-shrinkage additive. 3) Co2+ ions served as inducers and catalysts for assembling GO into 3D porous gels and catalyzing the polymerization of phenolic resin according to the previous reports.16,28-31 Owing to those outstanding properties, the optimum RUFG aerogel has great potential as highperformance insulation materials in the building field. EXPERIMENTAL SECTION Materials. All the chemicals were analytical grade and commercially available from Shanghai Chemical Reagent Co. Ltd, and were used without further purification. Synthesis of Resorcinol-Urea-Formaldehyde (RUF) Copolymer Resin and Resorcinol-UreaFormaldehyde/Graphene Oxide (RUFG) Aerogels. GO was prepared from natural graphite powder via ACS Paragon Plus Environment


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acid-oxidation according to the modified Hummers’ method.16 Resorcinol, urea and formaldehyde were added at a molar ratio of 1:1:4 into the solution containing 0.1g/L CoCl2·6H2O and different contents of GO (Table S1). After further stirring to form a homogenous solution, the mixture was transferred to the autoclave, then maintained at 85 oC in an oven for 12 h. When the reactor was cooled down naturally to room temperature, the wet gel was washed with distilled water for removing impurities and completely dried at 40 oC under ambient pressure to yield the final RUFG aerogels. RUF copolymer resin was synthesized under similar conditions without GO. Characterization. The apparent densities of aerogels were calculated by weighing them and measuring their volumes. The morphology of as-made RUFG samples were investigated by scanning electron microscope (SEM, JEOL-6700F) and transmission electron microscope (TEM, HitachiH-7650). The chemical structure in the RUFG aerogels was analyzed by Fourier transform infrared spectroscopy (FTIR, MAGNA-IR 750) and X-ray diffraction (XRD, Philips X’Pert PRO SUPER X-ray diffractmeter). The local coordination environment and valence state of Co2+ ion in the final RUFG aerogels were measured by X-ray photoelectron spectroscopy (XPS, ESCALAB MK II X-ray photoelectron spectrometer). The compressive stress-strain tests for the as-prepared RUFG aerogels were measured by using Instron 5565A equipped with two flat-surface compression stages and 500 N load cells, and the strain ramp rate was confined to be 5 mm/min for the tests. Dynamic viscoelastic measurements of RUFG aerogels were performed using a DMA Q800 instrument. The corrosion resistance for RUFG-1 aerogels was examined by the compressive performances after three samples with a size of 1.5 cm×1.5 cm×1.5 cm were put in pure water and the other three samples were immersed in diluted sulfuric acid (pH 3) for 1 month. The thermal conductivity of as-prepared RUFG aerogels was determined by the Hot Disk TPS 2500 thermal constant analyzer at room temperature. Thermal property was performed by thermogravimetric analyses (TGA) from room temperature to 700 oC with a heating rate of 10 oC/min under air and nitrogen atmosphere (PerkinElmer TGA 7). The combustion behavior of samples was investigated by limiting oxygen index (LOI) (Fire Testing Technology, UK) according to ASTM D 2863-97 and microscale combustion calorimeter (MCC) (GOVMARK MCC-2). The char from composite aerogel after burning was analyzed at room temperature with laser raman spectrometer (SPEX-1403, USA), provided in back-scattering geometry by a 532 nm argon laser line, and Xray photoelectron spectroscopy (XPS). RESULTS AND DISCUSSION ACS Paragon Plus Environment

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Fabrication and Characterization of RUFG Aerogels. For the synthesis of RUFG aerogels (Figure 1a), urea, resorcinol, formaldehyde and CoCl2·6H2O were first homogeneously dispersed in a certain concentration of GO dispersion. Then, through sequential steps including self-assembly, copolymerization and ambient pressure drying, the final RUFG aerogels were obtained. As shown in Table S1, the apparent density varied via altering the synthetic parameters, indicating the controllability for our synthesis. All RUFG aerogels were very light and their density could be adjusted from 24.8 to 38.2 mg·cm-3. The scanning electron microscopy (SEM) images (Figure 1b) revealed a highly porous 3D configuration for RUFG aerogel that was consisted of interconnected and distorted belt-like structures with a length of several micrometers. Transmission electron microscopy (TEM) images in Figure 1c displayed such distorted sheets had a larger thickness than primitive graphene flakes indicating that RUF might be successfully wrapped into graphene layers and served as spacers to prevent graphene from restacking.16,32

Figure 1. a) The schematic diagram, b) SEM and c) TEM images of RUFG aerogel. In order to study the chemical structure between GO and RUF in the RUFG aerogels, Fourier transform

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infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were performed (Figure S1 and Figure S2). FTIR spectrum of RUF and RUFG aerogels displayed characteristic peaks at 1660, 1232, 1027, 1480, 1618 and 1445 cm-1, attributed to the stretching vibration of amide carbonyl groups, phenolic hydroxyl groups, hydroxyl groups, methylene groups and benzene rings, demonstrating polymerization of RUF did occur during the fabrication process.9,33,34 Simultaneously, we could find that after the reaction the significant C=O stretching vibration peak at 1730 cm-1 for GO disappeared, indicating that GO was partly reduced.16 Moreover, XRD patterns of RUFG aerogels and pure GO flakes showed that the characteristic diffraction peak of GO at 11˚ vanished after the reaction, suggesting GO sheets in composite aerogels were possibly coated by polymer.35 To investigate the chemical composition and state of Co2+ ions in the final RUFG aerogels, X-ray photoelectron spectroscopy (XPS) (Figure S3) was carried out. The result from Figure S3a indicated the existence of C, N, O and Co elements in our samples. Furthermore, as revealed in Figure S3b, the peaks of 2p3/2 and 2p1/2 spin-orbit components at 781.6 and 797.0 eV meant that Co in the final aerogels kept their original oxidation valance of +2, and Co2+ was tetrahedrally coordinated by O atoms.16,36-38 Based on the above analysis, we have proposed a schematic diagram for such RUFG composite aerogels, showed in Figure 1a. During the synthesis process, there simultaneously existed three types of interactions: 1) Hydroxymethylated resorcinol (RF) and hydroxymethylated urea (UF) polymerized under the acidic conditions regulated by Co2+ and joined with each other either by methylene or ether bridges to give RUF;9 2) Co2+ linked RUF with GO sheets, GO sheets with GO sheets, RUF with RUF by coordination between hydroxyl groups and oxygen-containing/amine groups;16,39 3) Apart from the interactions induced by Co2+, hydrogen bonds between RUF and GO also contributed to the strong linkage.16,39 Mechanical and Anticorrosive Properties of RUFG Aerogels. Practical thermal-insulating applications required aerogels to be sufficiently strong and resilient to sustain compression loading and recover their original

shape

from

deformation.

From

Table S1 and Figure S4,

we

found

that with a low amount of GO templates, the as-made 2RUFG aerogel cannot afford the compression, which has been confirmed previously.11 Similarly, if the cont ent of resin precursor decreased substantially, the sheet layer in resulted 3RUFG aerogel becomes much thinner so that it cannot bear a high resistance to the severe outer bending forc e.9 Noteworthy, 1-RUFG aerogel with the reasonable ratio of composition displayed the best compressibility among the synthesized aerogels. As revealed in the compressive stress-strain curves of Figure 2a, 1-RUFG aerogel could tolerate a high compression strain (ε) up to 80%. Three regimes of deformation were observed ACS Paragon Plus Environment

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in the loading stress-strain curve: nearly line, which was lastic regime at low strain, corresponding to bending of the cell walls; relatively flat stress plateau, resulted from the elastic buckling of cell walls; and abrupt stress increasing regime, owing to densification of cells.16 The fatigue strain-stress curves at ε=50% with 50 loading/unloading cycles for 1-RUFG aerogel were demonstrated in Figure 2b. The plastic deformation only arrived at about 12.5% after fatigue testing, and the maximum degradation under compressive stress was smaller than 10% after 50 cycles, which corroborated the excellent structural robustness of the compressible 1-RUFG aerogel. Moreover, no obvious morphology changes were observed after the fatigue test, further confirmed the highly compressive stability for 1-RUFG aerogel (Figure 5S). Dynamic compressive viscoelastic measurements in Figure 2c and 2d revealed that the storage modulus and loss modulus of 1-RUFG aerogel were nearly stable. From Figure 2c, storage modulus was one order of magnitude higher than loss modulus during the angular frequency range, demonstrating that the elastic response was predominant and this aerogel contained strong, viscoelastic networks.40,41 Moreover, as indicated in Figure 2d, the good thermal stability of such aerogel was demonstrated by the independent values of storage modulus and loss modulus for the entire temperature range (28-95 oC). Combined with the above data analysis, the high compressibility for 1-RUFG aerogel might be ascribed to the following factors: 1) Co2+ linked RUF and GO networks, which formed complex synergic ionic bonding networks to enhance the load transfer capability of the material;42 2) Hydrogen bonding network between RUF and GO preserved the memory of initial state of this aerogel;43 3) Appropriate amount of RUF wrapped on graphene layers brought about the relatively high ratio of thickness to length for the layers, gave a high resistance to a severe outer bending force.44 Moreover, corrosion resistance was also important to thermal insulating materials in special environments. 1-RUFG aerogel displayed hydrophobic surfaces with contact angles of 150° (Figure S6a). After immersing the 1-RUFG aerogel into the pure water (pH 7) and sulfuric acid solution (pH 3) for 1 month, the compressive stress did not greatly decrease (Figure S6b). Here, corrosion resistance of 1-RUFG aerogel was due to the natural inertness of phenolic resin and the anticorrosion barrier of graphene.45-48



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Figure 2. a) Compressive stress-strain curves of the 1-RUFG aerogel at different set strains of 30, 50 and 80%. b) Cyclic stress-strain curves of the 1-RUFG aerogel at a maximum strain of 50%. c) The frequency dependence of the storage and loss modulus for 1-RUFG aerogel. d) Storage and loss modulus as a function of temperature at a fixed frequency (4 Hz) for 1-RUFG aerogel. Thermal Conductivity and Flame Resistant Properties of RUFG Aerogels. Low thermal conductivities, high thermal stabilities and good flame retardancy were also essential for the thermal insulating materials.49 Thermal transport in materials, depending on gaseous conduction, solid conduction and infrared radiative transfer, has been confirmed to alter as a function of density.50 The thermal conductivity of RUFG aerogels varied from 29 to 37 mW·m-1·K-1 (Table S1 and Figure 3a), comparable with those reported for common insulation materials such as mineral wool (30-40 mW·m-1·K-1), cellulose (40-50 mW·m-1·K-1), silica aerogel (17-41 mW·m-1·K-1), expanded polyurethane (20-30 mW·m-1·K-1) and expanded polystyrene (30-40 mW·m1·K-1).1,49,51,52

The ability of RUFG aerogels to yield less λ values could be related to the thermal properties of components and microstructure in the cell walls.49 Firstly, high porosity of RUFG aerogels limited the crosssectional area for heat conduction.52 Secondly, the use of nanosized component (GO) imparted a significant


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interfacial thermal resistance, which reduced the solid conduction of walls.53 Thirdly, phonon scattering effects, induced by defects and RUF wrapped on graphene layers, could reduce λ values for the composite aerogel wall.54 Lastly, graphene, as an efficient infrared adsorbent, could lessen the radiation.49

Figure 3. a) The thermal conductivity values of RUFG aerogels and other thermal insulation materials. b) The peak of heat release rates (PHRR) of RUF copolymer resin, 1-RUFG aerogel, commercial EPU and commercial EPS during microscale combustion calorimeter (MCC) test. The thermal stability of RUF copolymer resin and 1-RUFG aerogel under air and nitrogen atmosphere was studied by TGA analysis (Figure S7a, Figure S7b and Table S2). From Figure S7a, the temperature at 10% mass loss (T-10) and the temperature at 50% weight loss (T-50) of RUF copolymer resin and 1-RUFG aerogel under air atmosphere were recorded. T-10 and T-50 of RUF were 59.2 oC and 105.2 oC, while these of 1-RUFG composite aerogel got to 95.2 oC and 406.1 oC, respectively. As shown in Figure S7b, T-10 of 1RUFG composite aerogel reached at 258.9 oC, compared to that of RUF with 53.3 oC. Moreover, 1-RUFG composite aerogel yielded a carbon-rich final residue of 73 wt% at a temperature up to 800 oC, much more than that of RUF with 34 wt% under nitrogen atmosphere. The results indicated our fabricated 1-RUFG aerogel had higher thermal stability. Recent work has shown that the fire retardancy of copolymer resin can be further significantly improved by the addition of well-distributed inorganic fillers and some metal ions.11,17,55-59 The limiting oxygen index (LOI), gave the oxygen concentration to keep a material burning, was examined. Their values obtained from the average of three determinations were collected in Table S2, and errors were ±0.5%. The results showed that 1-RUFG composite aerogel could reach at 27±0.5% (Table S2), demonstrated very good fire retardancy, which was higher than O2 level in air (21%) and commercial flame retardant-containing polymer-based foams, having typical LOI values between 22% and 25%.60 In addition, their flame retardancy was assessed by ACS Paragon Plus Environment


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microscale combustion calorimeter (MCC), which was one of the most effective bench scale methods for investigating the combustion properties of polymer materials. The heat release rate (HRR) curves of RUF, 1RUFG and commercial polymeric insulation materials, such as EPU and EPS, were shown in Figure 3b and the corresponding peak heat release rate (PHRR) values were summarized in Table S2. After introducing GO into RUF, 1-RUFG aerogel displayed the lower PHRR value. For example, PHRR value of 1-RUFG reduced to 123.4 W/g compared with 203.6 W/g of RUF, while EPS and EPU reached up to 322.8 W/g and 184.3 W/g, respectively. Besides, the total heat release (THR) and heat release capacity (HRC) were also important parameters for evaluating flame retardancy of a material. Compared with pure RUF, the incorporation of GO made THR value reduced from 15.7 to 10.9 KJ/g, and HRC value decreased from 184.1 to 112.3 J/g·K. These data revealed that the fire hazard of 1-RUFG was smaller. Flame Resistant Mechanism. To obtain deeper insight into the flame retardant mechanism, the properties and components of char residues from 1-RUFG aerogel subjected to flame were analyzed using raman and XPS. The raman spectrum for RUF and 1-RUFG aerogel (Figure 4a) depicted two bands at around 1350 and 1590 cm-1, which were defined as D and G peak, respectively, and the area ratio of D to G and (ID/IG) was adopted to evaluate the graphitization degree of the char residue. The value of ID/IG for RUF was 0.3387, whereas 1-RUFG aerogel exhibited higher value (0.4856), revealing the highly-perfect graphitized carbon, which was known to be compact and efficient in terms of thermal insulation, provided a protective shield that isolated the resin from oxygen and heat.49 In addition, the XPS survey (Figure 4b) data collected in Table S3, demonstrated that the surface of samples were composed of C, O, N and Co elements. The relative content of element C in the char of 1-RUFG was higher than that of RUF, and the O content in production of 1-RUFG was less than that in RUF, due to the thermal stability of rGO in high temperature coupled with char catalyzed by Co2+.61 Hence, the protective char layer from 1-RUFG aerogel during burning presented better thermal oxidative resistance, that acted as a barrier to inhibit mass and heat transfer between condensed and gas phase.62


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Figure 4. a) Raman spectra and b) XPS survey spectra of RUF copolymer resin and 1-RUFG aerogel after burning. c) The illustration for the flame-retardant mechanism for RUFG aerogel. On the basis of the aforementioned analysis, a possible flame retardant mechanism of RUFG aerogel was postulated in Figure 4c. On one hand, carbon layer derived from carbonization of polymer and thermal stable rGO nanosheets acted as mass barriers.

63-65

Meanwhile, the catalytic charring effect of Co2+ leaded to

form higher graphitized char residue.55-57 Then the formed char particles combined with the high aspect ratio graphene sheets together to generate a compact char shield on the surface of inner materials.61 On the other hand, aerogels could also release incombustible gases (water, ammonia, etc.) during the combustion, thus diluting the concentration of combustible gas and oxygen, which further improved the flame retardancy of composite.10 CONCLUSION In conclusion, we develop a simple sequential low-temperature template-copolymerization and ambientpressure drying strategy to fabricate a new type of RUFG composite aerogels. Unlike traditional, fragile and



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brittle phenolic resin aerogel, as-made optimal RUFG composite aerogel combined with superelasticity, effective anticorrosion, ultralow thermal conductivity, high thermal stability, good flame retardancy and ease of synthesis all together. We anticipate that such exceptional multifunctional thermal-insulation material will open up numerous opportunities for the building areas. ASSOCIATED CONTENT Supporting Information Synthetic parameters and physical properties of samples, FTIR, XRD, XPS SEM, TGA, LOI and MCC for samples, contact angle and compressive stress-strain curves of 1-RUFG aerogel before and after immersing in water or acid solution for 1 month, elemental compositions of the residual char ACKNOWLEDGMENTS The work was financially supported by the funding from Natural Science Foundation of Education Department of Zhejiang Province (GZ18661080015), Natural Science Foundation of Zhejiang Province (Q16E040005 and LY14E030005).


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For Table of Contents Use Only

Through low-temperature template-copolymerization and ambient-pressure drying process, the optimal RUFG aerogel exhibited integrated properties of recyclable compressibility, corrosion-resistance and fireresistance.


Superelastic, Anticorrosive, and Flame-Resistant Nitrogen-Containing

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