Flame Retardant Epoxy Derived from Tannic Acid as Biobased Hardener

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Flame Retardant Epoxy Derived from Tannic Acid as Bio-based Hardener Young-O Kim, Jaehyun Cho, Hyeonuk Yeo, Byoung Wan Lee, Byung Joon Moon, Yu-Mi Ha, Ye Rin Jo, and Yong Chae Jung ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04851 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 30, 2019

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ACS Sustainable Chemistry & Engineering

Flame Retardant Epoxy Derived from Tannic Acid as Bio-based Hardener Young-O Kim,† Jaehyun Cho,† Hyeonuk Yeo,‡ Byoung Wan Lee,† Byung Joon Moon,† Yumi Ha,† Ye Rin Jo,† Yong Chae Jung *† †Institute

of Advanced Composite Materials, Korea Institute of Science and Technology

(KIST), 92, Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea ‡Department

of Chemistry Education, Kyungpook National University, 80, Daehak-ro, Buk-

gu, Daegu 41566, Republic of Korea

Keywords: bio-based materials, flame retardants, epoxy resins, tannic acids Author’s e-mail: [email protected] (Y.-O.Kim), [email protected] (J.Cho), [email protected] (H. Yeo), [email protected] (B.W.Lee), [email protected] (B.J.Moon), [email protected] (Y.Ha), [email protected] (Y.R.Jo), [email protected] (Y.C.Jung) * To whom correspondence should be addressed: Yong Chae Jung, Ph.D. Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), Korea (Republic of)

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Tel: 82-63-219-8153 E-mail: [email protected]


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[Abstract] Flame retardant epoxy is closely related to safety of human’s life against the surrounding fire threat. Flame retardant properties can be obtained by supplementing with additives, such as phosphorous compounds and nanomaterials, or synthesizing flame retardant monomers. The principle of improving flame retardancy are based on the capture of oxygen radicals and the formation of a char layer, which blocks flammable gases. This paper focuses on a flame retardant epoxy resin using naturally occurring tannic acid (TA) as a hardener, which is both an oxygen-radical quencher and a charring agent. TA is reacted with commercially-available diglycidyl ether of bisphenol A (DGEBA). The reaction between the epoxy ring of the DGEBA and multiple functional groups in TA is empirically demonstrated using dynamic scanning calorimetry (DSC) and Brillouin spectra. The most effective flame-retardant TA-DGEBA (TD) thermoset had an limiting oxygen index (LOI) value 46 % higher than the control sample. This result suggests that TA-based epoxy resins could be promising flame-retardant polymers. KEYWORDS. bio-based materials, flame retardants, epoxy resins, tannic acids



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INTRODUCTION Epoxy resin is one of thermosetting polymers, which has been widely used in a multitude of fields such as coatings,1-5 adhesives,6-9 and advanced composites10-15 because of their excellent adhesion, chemical resistance, and mechanical properties. However, its flammable property has limited the applications related to fire resistance. Therefore, many researches have been conducted to suppress their flammability. At the initial stage, halogenated compounds were usually used as a flame retarding additive.16 Nevertheless, because they can expel toxic halogencontaining gases during burning process and threaten human’s health, various alternatives such as phosphorous containing compounds17-18 and nano-additives such as clays,19-23 carbon nanotubes,20,

23-24

and graphenes25-27 have been suggested. Phosphorous containing

compounds, when heated, produce a polymeric phosphoric acid, which sequentially triggers a flame retarding barrier. Nano-additives have an advantage improving mechanical property of matrix as a filler as well as a flame retardancy. In common with phosphorous containing compounds, they play a role as a layer blocking flammable gases. High contents of these additives rather deteriorate the original mechanical and thermal properties of polymer matrix. Only changing a kind of additives reaches the limitation. Ultimately, modifying epoxy monomers as a fundamental solution is needed. Generally, epoxy or hardener containing phosphorous element has been synthesized for flame retardant epoxy resin.28-32 Recently, bio-resource derivatives instead of petroleum-based chemicals has been used and synthesized for fabricating the bio-based epoxy due to the increasing environmental interest. For example, lignin,33-34 starch,35 daidzein,36 cardanol,37-38 and natural acids39 such as gallic acid, ferulic acid, citric acid and malonic acid have been used as ACS Paragon Plus Environment

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precursors for epoxy resins. Among various bio resources, tannins, which are polyphenolic compounds extracted from plants, would be a multifunctional epoxy hardener. Tannins can be classified into condensed tannins and hydrolysable tannins with chemical structures.40-42 Condensed tannins are made up of repetitive flavonoid units, while hydrolysable tannins are saccharides partly or fully substituted by gallic or ellagic acid. Due to their molecular structure consisting of a number of aromatic rings and hydroxyl groups, they enable covalent and noncovalent interactions, which can directly affect the mechanical property of the epoxy resins. In addition, they possess reducing activity and quenching effect of singlet oxygen.43-44 During the degradation process of materials containing phenol groups, phenoxy radicals could quench the oxygen free radicals without phosphorous elements in polymer structure. Moreover, they have high char forming efficiency when burned by fire,45-46 leading high yield of char layer, or protective layer, which blocks heat, oxygen, and flammable gases.47 Harmonious effect of properties of tannins would be expected to increase not only flame retardancy but mechanical properties of polymeric materials. Among tannins, we focused on hydrolysable tannin or tannic acid (TA). Because it has less conjugated and simple structure than condensed tannins, it has some advantages that it is soluble in organic solvents and facilitates the fabrication of plastics.48 TA and its derivatives have been previously used as a monomer and an additive for epoxy resins which were made by functionalizing the TA with oxirane group.49-53 Lang et. al. also used the TA as a coating material for a low flammable epoxy foam.54 In this study, TA as a multifunctional and flame retardant epoxy hardener was reacted with diglycidyl ether of bisphenol A (DGEBA) for flame retardant epoxy resin. Figure 1a illustrates the representative structure of a biologically extracted TA and ACS Paragon Plus Environment


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the epoxy specimen synthesized from it. It is difficult for all of the 25 hydroxyl groups of TA to react with epoxy groups of DGEBA, due to changes of pKa of phenolic –OH or steric hindrance with epoxidation proceeding. Therefore, controlling temperature and hydroxyl/epoxy molar ratio must be needed for optimization of curing condition. Herein, we examine the mechanical and flame retardant properties of TA-based epoxy resins depending on the TA content in epoxy resins.

RESULTS AND DISCUSSION As far as we know, there is no clear evidence for which of the three –OH groups of gallic acid moieties in TA molecules is more reactive. Therefore, ab-initio calcuations to theoretically determine the preferential reaction mechanism as well as reaction selectivity were carried out. For efficient quantum mechanical calculations, local structures of TA and DGEBA structures with 1,2,3-benzenetriol (so called pyrogallol) and ethyloxirane were modelled, respectively. The interaction energies of the stable complexes of 1,2,3-benzenetriol with ethyloxirane molecule were acquired by applying the conformation optimization to two different reaction systems. Since more negative value of interaction energy coincides with the higher reaction selectivity to organic materials, as shown in Figure S1, we are able to conclude that 2 position –OH of the terminal phenyl group of 1,2,3-benzenetriol is more readily reacted with the epoxy groups of ethyloxirane compared with its 1 or 3 position due to the electron donating effect of – OH groups. Analgously, Route 2, leading to the formation of

2-(2-hydroxybutoxy)benzene-1,3-diol, is

a more energetically stable among two different possible pathways (Figure S1b). Typically, epoxy resins are directly mixed with a hardener, poured into a mold, and cured. However, due to high melting point of TA powder, using this method, subsidence of TA powder in DGEBA during a curing process resulted in a non-ideal epoxy thermoset. This problem was solved by dissolving DGEBA


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and TA in ethanol: the subsequent evaporation of ethanol resulted in a homogeneous mixture. The homogeneous incorporation of TA into DGEBA induces the increase of the viscosity of the mixture (Figure S2). To examine optimal curing conditions, dynamic differential scanning calorimetry (DSC) scans were conducted between 50 and 290 °C; TA decomposes at temperatures above 300 °C. As shown in Figure 1b, the first broad endothermic band starting at 95 °C corresponds to the evaporation of hydrated water molecules, which can be confirmed with the DSC curve of neat TA (Figure S3). Previous research showed that reaction between phenolic –OH groups and epoxy rings occurs at approximately 100 °C, thus, we expected an additional peak around 100 °C to be observed. However, the reaction in our study starts at 195 °C and peak at 226 °C; the total curing energy is approximately 230 J/g. This information was used to perform multiple isothermal DSC scans, within an optimal temperature range, and determine the curing time at each temperature (Figure 1c). Curing times were faster at higher temperatures. However, exact curing times could not be determined, as heat flow did not converge to zero. Therefore, microBrillouin spectroscopy, which can monitor change in elastic property in an in-situ, non-destructive manner, was used.55-57 Figure 1d-e shows dependence of the Brillouin shift of the TA/DGEBA mixture on temperature and time. The Brillouin shift was obtained from fitted Brillouin spectra (Figure S4), which consisted of a longitudinal acoustic mode and center peak, with changing curing temperature and time. The Brillouin spectra were fitted using a Lorentzian function convoluted with the instrumental Gaussian function. The shift of the peak position toward the high-frequency range represents the solidification of the resin. Figure 1d shows the dependence of Brillouin shift on temperature. The DGEBA has a high viscosity at room temperature, and its viscosity slightly decreases with increasing temperature up to 140 °C. However, hardening starts at approximately 150 °C, and more changes are observed at approximately 180 °C, 200 °C, and 260 °C, which is in close accordance with DSC results. This can be interpreted as the temperatures at which the -OH groups in TA molecules are activated. Compared with the three broad peaks observed in DSC data, curing temperatures were observed by changes in phonon energy were more identifiable. ACS Paragon Plus Environment


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Figure 1e shows measurements taken at 210 °C in 5 min intervals. The longer the curing time, the larger the peak shift. It cures rapidly for the first 20 min, and then slows.

Figure 1. Synthesis of the TD epoxy thermoset. a) Schematic illustration of TD epoxy thermoset from the curing reaction between TA and DGEBA. b) DSC curve (heating rate: 10 °C/min under N2 atmosphere). Red arrows indicate the curing temperatures. c) Isothermal DSC curves. d) Dependence of Brillouin shift on curing temperature. Red arrows indicate the curing points. e) Brillouin shift of a 1:1 TA:DGEBA mixture measured at 210 °C up to 120 min. To investigate the optimal TA:DGEBA ratio, Fourier transformed infrared (FT-IR) analysis was conducted (Figure 2). The TD0.5 thermoset was the only thermoset sample to exhibit a peak at 913 cm-1: this peak corresponds to epoxy ring groups and is therefore weaker than that of DGEBA.58 Samples with ACS Paragon Plus Environment

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a 0.8 TA molar ratio and above do not have epoxy ring groups. Theoretically, a peak corresponding to the epoxy ring group should disappear in samples above a 1.0 TA molar ratio. However, it should be noted that DGEBA can be homopolymerized, thereby reducing the concentration of TA needed for the complete polymerization of the epoxy ring. In addition, a broad peak in the 3000-3700 cm-1 region, indicating the –OH group of TA, shifted to lower frequencies with increasing concentration of TA. This implies an increase in hydrogen bonds between –OH groups not participating in a hardening reaction, which can affect the mechanical properties of TD thermosets.

Figure 2. a) FT-IR spectra of TA, DGEBA, and TD thermosets synthesized with various TA-DGEBA molar ratios. b) FT-IR spectra enlarged within the 2900-3750 cm-1 region of a). Glass transition temperature (Tg) is a critical factor for thermosetting epoxy resins, and is affected by crosslinking density and interactions between crosslinked polymeric chains. To investigate the effect of TA concentration on the Tg of the cured TD thermosets, DSC and dynamic mechanical analysis (DMA) tests were performed. The DSC and DMA results are shown in Figure 3, Tg is determined from the points of inflection and the peak temperatures of tan δ, respectively. The DSC and DMA data show a similar propensity for change in Tg of the TD epoxy resins; they present an early increasing and later decreasing trend with increasing concentration of TA. However, the DMA data of TD1.0 and TD1.2 shows an additional broad Tg shoulder approximately 180 °C indicating that domains of polymeric chains are separated depending on crosslinking degree. Data obtained from DSC and DMA analyses are summarized,



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in detail, in Table S1. Based on these results, the crosslinking densities of the cured TD epoxy thermosets were calculated using modulus values at the rubbery region according to the following equation39, 59:

ρ=

𝐸𝑟

(1)

3𝑅𝑇

Where Er is the storage modulus at Tg2 + 60 °C, R is the gas constant, and T is the absolute temperature. The major Tg2 peak, which mainly affects the mechanical properties of epoxy thermosets, was used to calculate crosslinking density. The Er and crosslinking density obtained from this equation shows a similar propensity to Tg. A TA molar ratio above 0.8 induces interactions such as hydrogen bonding, π- π stacking, and van der Waals interaction, between TA molecules in the polymer matrix, which makes further covalent crosslinking challenging. The effect of TA concentration on the mechanical properties of TD thermosets is shown in Figure 3c and Figure S5, S6. The tensile mechanical properties of the TD thermosets were tested using universal testing machine (UTM); at least four specimens were tested for reproducibility. Typically, the stiffness and strength of epoxy resins are related to crosslinking density and molecular structure, which in our case can be varied by the concentration of TA. As shown in Figure 3c, the Young’s modulus of the TD thermosets increases with increasing TA concentration until a TA molar ratio of 0.8; this can be explained by the introduction of the rigid aromatic structure of TA. However, the Young’s modulus of TD thermosets above a 0.8 TA molar ratio decreased. DSC and DMA data reveals that above a certain TA concentration phase separation occurs; the partially reacted TA molecules may act as a plasticizer, thereby reducing the overall stiffness of the epoxy resins.60-61 However, inverse to the trend of the Young’s modulus change in TD thermosets, tensile strength initially increases with increasing TA concentration. As previously noted, the strength reinforcement by TA crosslinking is no longer possible above a certain TA concentration due to steric hindrance and phase separation. Although crosslinking density is limited by the physical structure of TA, the multiple polar groups (-OH) of the TA provides H-bonding sites between TA and neighboring epoxy chains, which results in a further increase in tensile strength.62 HACS Paragon Plus Environment

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bonding effect from the partially reacted TA could be confirmed from the water uptake measurement. As shown in Figure S7, the water uptake of TD thermosets above a 0.8 TA molar ratio was increased with increase of TA content, which is attributed to the increasing amount of H-bonding between the partially reacted TA and water.

Figure 3. Thermomechanical properties of the cured TD thermosets. a) DSC curves showing Tg. b) Tan δ obtained from DMA data. c) Tensile properties obtained from UTM analysis. The TA-based bio epoxy was designed to be a flame retardant by utilizing the charring effect of TA. For this reason, the degradation behaviors of the TD thermoset, when exposed to thermal energy, were checked through thermogravimetric analysis (TGA) in nitrogen atmosphere as shown in Figure S8 and Table S1. The initial decomposition temperature (T5%), and the remaining weights after heating up to 700 °C (char yield), varied between samples. In nitrogen atmosphere, the T5% shows the decreasing propensity with increasing concentration of TA, which is attributed to the volatiles attached to TA46. In contrast, char yield increases with increasing concentration of TA. Based on these results, the TA in the epoxy matrix plays an important role in enhancing the flame retardancy by inducing the formation of char. The flame-retardant behaviors of TD thermosets were evaluated by limiting oxygen index (LOI) tests and micro combustion calorimetry (MCC) (Figure 4; Figure S9). All TD thermosets have a higher LOI value than the control sample, and the LOI values increase with increasing concentration of TA. This might be ascribed to the quenching effect of the phenoxy radicals formed during combustion43-44 and the formation of a char layer which acts as a barrier obstructing heat flux and gas transport45-46. In addition, the peak heat release rate (PHRR) and total heat release (THR) of the TD thermosets were estimated, ACS Paragon Plus Environment


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using MCC, as a measure of flame retardant efficiency. The PHRR of the TD0.5 thermoset is higher than the control sample, but the PHRRs of the TD thermosets decrease with increasing TA concentration. Comparatively, THR decreases with increasing the concentration of TA, as in accordance with LOI results. The higher PHRR of the TD0.5 thermoset compared to the control sample could be explained by the fact that the TD0.5 thermoset contained unreacted DGEBA as shown in Figure 2. The PHRRs of samples with TA molar ratio higher than 0.8 might depend on both thermal conductivity of the TD thermosets and the gas diffusion barrier effect caused by char layer formed upon combustion. Thermal conductivities of the TD thermosets are listed in Table S2. As the amount of the TA increased, thermal conductivity was increased, which should increase PHRR. As evidenced by decreasing PHRR of the TD thermosets with increasing the TA amount, however, the effect of char layer might be far larger than that of the thermal conductivity,.

Figure 4. Flame retardant properties of the TD epoxy thermosets measured with MCC: a) LOI, b) PHRR and THR. The TD thermosets were characterized by SEM to further explore evidence of improved flame retardant efficiency (Figure 5a-d). SEM images show the morphologies of the char residues after the control sample and TD thermosets were calcined, in a furnace, in air conditions. Since combustion is controlled by the diffusion of combustible gases, from the polymer surface to the flame and from the air to the polymer surface, forming physical barriers is important to obstruct the mass transport pathways of gases. The char residue of the control is a porous film consisting of small, creased flakes (Figure S10b). ACS Paragon Plus Environment

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In all TD thermosets, the char layer consists of larger, smoother flakes but there are no apparent differences between them. The significant difference in LOI value and THR between the control and the TD thermosets could stem from the difference in char morphology. We thought that differences of flame retardancy could be related to the inner structure of the char residues, therefore, a 3D micro-CT scan was used to observe the non-destructed inner structure of the char residue of the TD thermosets (Figure 5e-h). Carbonaceous structures of samples were reconstructed using CTvox software and shown as 1.5 × 1.5 × 1.5 mm3 cubic regions of interest (ROIs). The surfaces of all the char residues exhibited a flake-like morphology, similar to the SEM images. The inner structure of the TD0.5 thermoset has a larger portion of void space than the other TD thermosets. Superficially, the micro-CT images of the TD thermoset samples were similar, with the exception of the TD0.5 thermoset. However, using CTAn software, it was possible to confirm a difference in percent-volume of char residue at the inner space (Vc). Between TA molar ratio of 0.8 and 1.2, Vc increases with decreasing TA concentration, which is likely to be related to crosslinking density. These results show that, when undergoing combustion, a char layer is formed on the surface of and inside of the TD thermoset (Figure 5i). On the basis of SEM and micro-CT analysis, we suppose that the mass transport barrier effect is not displayed at the inner space so much as at the surface of char residue. Therefore, the surfaces of char residues were analyzed using Raman spectroscopy (Figure 5j; Figure S11), to investigate the structural integrity of the char.63-64 The Raman spectra present a disordered graphite peak at approximately 1370 cm-1 (D peak) and an ordered graphite peak at approximately 1600 cm-1 (G peak). A lower relative intensity ratio of the two peaks (ID/IG) indicates a more graphitized structure.63-64 We confirmed that increased concentration of TA stabilized the char layer and decreased defects. Accordingly, the higher the concentration of TA in the epoxy matrix, the more stable the graphitized structure produced after burned, which improves flame retardant performance.



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Figure 5. a-d) SEM images and e-h) micro-CT results of the char residues of the TD thermosets (a,e) TD0.5, b,f) TD0.8, c,g) TD1.0 and d,h) TD1.2) after being calcined in a furnace in air conditions. The surface of char residue is red. Cubic size of micro-CT images is 1.5 × 1.5 × 1.5 mm3. The percent volume of the char residue is shown below each micro-CT image. i) Illustration of TD thermoset after ignition. The flame retarded TD thermoset contains a char layer formed during ignition. j) Comparison of ID/IG extracted from Figure S9.

CONCLUSION Although the use of TA as a bio-based epoxy hardener had been previously suggested, a systematic analysis of the reaction between multiple phenolic –OH groups in TA and epoxy ring containing monomer, had not been reported. This study provided that the hardening process of DGEBA and TA, with the variation of temperature, was observed using DSC and Brillouin spectroscopic analysis. High TA ACS Paragon Plus Environment

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concentration in TD thermosets induced phase separation and incomplete hardening. Nevertheless, increased tensile strength was observed in TD thermosets with a high TA concentration; it was deduced from multiple hydrogen bonding sites in multifunctional cluster TA. Additionally, the increased flame retardant efficiency observed with increasing TA concentration in the TD thermosets was confirmed to be due to charring effect of TA and might also be ascribed to the radical quenching of phenolic derivatives. Hence, this analysis of the direct reaction between TA and an epoxy monomer, could be considered as a promising approach toward the synthesis of flame retardant epoxy thermosets applicable in aerospace, construction, and automobile fields.

EXPERIMENTAL SECTION Synthesis of epoxy resin TA (Sigma Aldrich, molecular weight of completely substituted TA: 1702.20 g/mol), dissolved in 50 mL of ethanol (Daejung chemical), was poured into 40 g of DGEBA (Kukdo chemical, trade name YD128) in a 250 mL round bottom flask. The weight of TA for TD0.5, 0.8, 1.0, and 1.2 was 8, 12.85, 16, and 19.2 g, respectively. The mixture was homogenized using a bath-type sonicator (JAC-3010, KODO Technical Research) for 30 min, and ethanol was removed by reduced pressure at 80 °C. The epoxy-TA paste was poured into a mold and cured at 210 °C for 2 h. To produce the control sample, 40 g of DGEBA and 20 g of polyetheramine hardener (Kukdo chemical, trade name D-230) were poured into PTFE container and mixed using a centrifugal mixer (Thinky Corp., ARE-310). The mixture was poured into a mold and cured at 80 °C for 2 h. Analysis for determining hardening condition of TD thermoset DSC thermograms were recorded with a DSC Q20 (TA Instruments, USA) at a scan rate of 10 °C min-1. Brillouin spectroscopy was performed using a conventional tandem six-pass Fabry-Perot interferometer (TFPI, JRS Co.) to monitor changes in hardness under temperature variation. A diodeACS Paragon Plus Environment


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pumped solid-state laser (Excelsior 532-300, Spectra Physics) at a wavelength of 532 nm was used as the excitation source. A micro-Brillouin spectrometer was used for the backscattering experiment, where a modified microscope (BX-41, Olympus) was adopted. A compact cryostat (THMS600, Linkam) was placed on the stage of the microscope to control temperature variation. Two Brillouin analyses were performed. In one analysis measurements were taken as temperature increased from 30 °C to 270 °C in 10 °C increments, in the other measurements were taken at a contstant temperature of 210 °C in 5 min intervals. Scans with a resolution of 4 cm-1 were taken with an FT-IR (FT/IR-6600, JASCO, USA) equipped with an ATR accessory (ATR Pro One). Computational method All ab initio electronic structure calculations were performed using the Hyperchem 8.5 and Gaussian 09 package. The geometrical structures of the complexes were optimized at the MP2/6-31G** level. Basis set superposition error (BSSE) was applied by employing Boys and Bernardi's standard counterpoise method. The supermolecular approach was utilized to calculate the interaction energy of the complexes. The interaction energy between two different molecules at a given level of theory was calculated by the following equation65-66: (2)

𝐸𝑖𝑛𝑡 = 𝐸𝐴𝐵 ― 𝐸𝐴 ― 𝐸𝐵 + ∆𝐸𝐵𝑆𝑆𝐸

where Eint is the corrected intermolecular potential energy, EAB is the energy of the complex, EA (or EB) is the energy of the monomer A (or B), and ΔEBSSE is the difference between the corrected and uncorrected energies. In addition, each reaction energy (with ZPVE corrections) was calculated via the difference between the energies of product and reactant species. Thermal characterization TGA was performed on a Discovery TGA (TA Instruments, USA). Samples were heated to 800 °C, at a rate of 20 °C min-1, under nitrogen or air atmospheres. For DMA analysis, samples were cut to a


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rectangular shape (60 × 13 × 4 mm3) and analyzed with a DMA Q800 (TA Instruments, USA) with a load frequency of 1 Hz and a heating rate of 2 °C min-1, in air, using a dual cantilever clamp. Mechanical characterization UTM measurements were performed with a UTM 5567A instrument (Instron, USA). Thermosets were prepared as a suitable size to the ASTM D638 standard. The used load cell and crosshead speed for UTM was 3 kN and 1 mm min-1, respectively. Water uptake test TD thermosets were cut to 38 × 25.4 × 1 mm3 using diamond-blade saw. Before testing, all of the TD thermosets were dried in vacuum chamber at 90 °C for 24 h and were weighed (Wd). And then, the samples were immsersed into distilled water at 20 °C. After 24 h, water drops on the surface of them were wiped using WypAll® and weighed (W) using a digital balance with 0.01 mg resolution. The water uptake percentage (M%) was calculated from the following equation:

𝑀% =

― 𝑊d [𝑊 𝑊d ] × 100%

(3)

Flame retardant characterization LOI values were measured by an ISO 4589-2 and ASTM D2863 instrument (Fire Testing Technology, UK). Themosets were cut to 100 × 10 × 4 mm3 using Superjet-T500-3015-Hybrid (TOPS, KR). MCC was performed with the Federal Aviation Administration (FAA) micro calorimeter (IMDEA Materials, UK). About 5 mg of each samples was triplicatly tested with heating to 700 °C at a heating rate of 1 C° s-1. Morphological analysis of char residues



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The micro-morphology of the char residues was characterized using a FEI-NOVA Nano SEM50. The samples were coated with a gold layer, of 15 nm thickness, to improve the conductivity of the surface of the samples and reduce the accumulation of electrostatic charge. Micro-CT images of the char samples were obtained using a Skyscan 1172 (Bruker, Belgium). Due to the fragility of the char samples, they were contained inside Eppendorf tubes and fixed to the sample holder using wax. Images were scanned with a size of 4000 × 1332 pixels, an operating voltage of 30 kV, a 0.2° step size, with frame averaging of 2° and 180° rotation. The resulting projections were reconstructed with NRecon software (version 1.6.9.8, Bruker, Kontich, Belgium). Three cubic regions of interest (ROIs) were defined through each scaffold (1.5 × 1.5 × 1.5 mm3) to investigate the intact structure of the char residues. Vc values were determined from binary images using CTAn software (version 1.16.4.1, Bruker, Belgium). Raman measurements were carried out with a inVia Renishaw Raman (diode laser 532 nm, power = 1 mW) in an ambient atmosphere at room temperature.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Computational results, viscosity measurement, DSC curve, representative Brillouin spectra, DMA data, Representative stress-strain curves, water uptake propensity, TGA graph, Thermal properties, HRR curves of TD thermosets, TGA and SEM data of the char residue of the control sample, Representative Raman spectra of TD thermosets after ignition ACKNOWLEDGEMENTS This work was supported by the KIST Institutional Program.

AUTHOR INFORMATION Notes ACS Paragon Plus Environment

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The authors declare no competing financial interest.



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[Table of Contents]

Epoxy thermosets using tannic acid as a bio-based hardener have been successfully synthesized and show efficient flame-retardant properties due to radical quenching and charring effect of tannic acid.



Figure 1. Synthesis of the TD epoxy thermoset. a) Schematic illustration of TD epoxy thermoset from the curing reaction between TA and DGEBA. b) DSC curve (heating rate: 10 °C/min under N2 atmosphere). c) Isothermal DSC curves. d) Dependence of Brillouin shift on curing temperature. e) Brillouin shift of a 1:1 TA:DGEBA mixture measured at 210 °C up to 120 min. 133x149mm (300 x 300 DPI)


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Figure 2. a) FT-IR spectra of TA, DGEBA, and TD thermosets synthesized with various TA-DGEBA molar ratios. b) FT-IR spectra enlarged within the 2900-3750 cm-1 region of a). 83x51mm (300 x 300 DPI)



Figure 3. Thermomechanical properties of the cured TD thermosets. a) DSC curves showing Tg. b) Tan δ obtained from DMA data. c) Tensile properties obtained from UTM analysis. 38x9mm (300 x 300 DPI)


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Figure 4. Flame retardant properties of the TD epoxy thermosets measured with MCC: a) LOI, b) PHRR and THR. 48x16mm (300 x 300 DPI)



Figure 5. a-d) SEM images and e-h) micro-CT results of the char residues of the TD thermosets after being calcined in a furnace in air conditions. The surface of char residue is red. Cubic size of micro-CT images is 1.5 × 1.5 × 1.5 mm3. The percent volume of the char residue is shown below each micro-CT image. i) Illustration of TD thermoset after ignition. The flame retarded TD thermoset contains a char layer formed during ignition. j) Comparison of ID/IG extracted from Figure S9. 119x93mm (300 x 300 DPI)


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Flame Retardant Epoxy Derived from Tannic Acid as Biobased Hardener

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