《高分子物理》课程教学资源（文献资料）Vanillin-Derived High-Performance Flame Retardant Epoxy Resins

Vanillin-Derived High-Performance Flame Retardant Epoxy Resins: Facile Synthesis and Properties Sheng Wang,†,‡ Songqi Ma,*,† Chenxiang Xu,†,‡ Yuan Liu,†,‡ Jinyue Dai,†,‡ Zongbao Wang,§ Xiaoqing Liu,† Jing Chen,† Xiaobin Shen,†,‡ Jingjing Wei,† and Jin Zhu*,† † Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Zhenhai District, Ningbo 315201, P. R. China ‡ University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing 100049, P. R. China § Ningbo University, Ningbo 315201, P. R. China ABSTRACT: Lignin derivative vanillin when coupled with diamines and diethyl phosphite followed by reaction with echichlorohydrin yields high-performance flame retardant epoxy resins. Biorenewable and environment-friendly flame retardant alternatives to bisphenol A epoxy resins (having plenty of applications such as coatings, adhesives, composites, etc.) have captured great attention due to their ecological and economic necessity. Vanillin, an industrial scale monoaromatic compound from lignin, is a promising sustainable candidate for highperformance polymers, while synthesis of diepoxies is challenging. Meanwhile, bio-based epoxy resins combining high performance and excellent fire resistance are more difficult to be achieved. In this paper, two novel bio-based epoxy monomers EP1 and EP2 were synthesized by one-pot reaction containing Schiff base formation and phosphorus−hydrogen addition between vanillin, diamines, and diethyl phosphite, followed by reacting with epichlorohydrin. Their reactivities are similar to bisphenol A epoxy resin DGEBA. After curing they showed excellent flame retardancy with UL-94 V0 rating and high LOI of ∼32.8%, which was due to the outstanding intumescent and dense char formation ability. Meanwhile, it was found that the cured vanillin-based epoxies had exceedingly high Tgs of ∼214 °C, tensile strength of ∼80.3 MPa, and tensile modulus of ∼2709 MPa, much higher than the cured DGEBA with Tg of 166 °C, tensile strength of 76.4 MPa, and tensile modulus of 1893 MPa; the properties of vanillin-based epoxies are easy to be regulated by using different “coupling” agentsdiaminesduring the synthesis process. ■ INTRODUCTION Epoxy resins, as one of the three most important thermosetting polymers, have been widely employed in a multitude of fields such as coatings, adhesives, laminated circuit board, electronic component encapsulations, and advanced composites because of their excellent adhesion, chemical resistance, mechanical properties, and dielectric properties.1−5 Nowadays, almost all of the epoxy resins are produced from fossil resources, and 90% of the commercially available epoxy resins are diglycidyl ether of bisphenol A (DGEBA) via the reaction of bisphenol A with epichlorohydrin.6 Bisphenol A, fully dependent on fossil resources, accounts for greater than 67% of the molar mass of DGEBA.7 In addition, bisphenol A is a reprotoxic compound;8 and even if chemically incorporated into polymers, a small amount of bisphenol A can still release from the polymers with time due to the not completely stable chemical bonds linking bisphenol A.9 As a result, it is under close monitoring, and its application has been restricted in many countries. Recently, the finite and rising price of fossil resources, climate change from CO2 emission, and other environmental problems have raised interests in polymers from biorenewable raw materials which have a wide variety of biomass resources with low price and enhanced environment benefits. There are numerous bionewable resources including vegetable oils,10−14 cardanol,15 isosorbide,9,16 rosin,17,18 gallic acid,19−21 ferulic acid,22 lignin, and its derivatives23−26 that have been investigated as feedstocks for epoxy resins. Because of the long flexible aliphatic chain and low reactivity of internal epoxy groups, epoxidized vegetable oil and cardanol often exhibit poor thermal and mechanical properties.6,27 Although isosorbide, rosin, gallic acid, and ferulic acid-based epoxy resins exhibited satisfied thermal and mechanical properties, isosorbide-based one suffered from relatively high hydrophilicity,28 and rosin, gallic acid, and ferulic acid have seldom spare capacity due to their variety applications and limited resources. Lignin is the second most abundant natural organic material accounting for approximately 30% of organic carbon in the biosphere; it is also the only scalable renewable feedstock consisted of aromatic monomers,29 and it is highly underutilized.30 Epoxy resins directly from lignin exhibit several drawbacks such as slow curing rate, unstable properties, and issues with processability Received: January 16, 2017 Revised: February 20, 2017 Published: February 24, 2017 Article pubs.acs.org/Macromolecules © 2017 American Chemical Society 1892 DOI: 10.1021/acs.macromol.7b00097 Macromolecules 2017, 50, 1892−1901 Downloaded via GUANGDONG UNIV OF TECHNOLOGY on November 15, 2022 at 01:40:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles

due to their high molecular weights, insolubility, and composition variability depending on the species or the time of year,30−32 which greatly limited their utilization. Despite a great deal of efforts, decomposition of lignin into small molecules remains a challenge. Fortunately, the lignin-tovanillin process has been commercially utilized, and vanillin, as the only monoaromatic compounds produced on an industrial scale from lignin, has already exhibited tremendous potential to be used as a renewable building block for polymers.33,34 However, vanillin has only one group suitable to be introduce epoxy group; consequently, producing diepoxies is challenge. Oxidation or reduction of vanillin’s formyl group was conducted to achieve two suitable groups to introduce two epoxy groups,25,26 and using pentaerythritol reacting with formyl group and “coupling” two vanillin to achieve a difunctional phenol followed by reacting with epichlorohydrin is also a reported way to produce diepoxies.23 These methods are relatively complex, and toxic compounds were utilized; the fundamental mechanical properties of the vanillin-based epoxies were not reported. The second drawback of epoxy resins is their flammability, which blocks their application in the fire resistance required fields.35 Phosphorus-containing compounds have been regarded as effective and environment-friendly flame retardants for polymers including epoxy resins, and phosphoruscontaining flame retardants have attracted intensive interests36 after some halogenated fire retardants were banned by the European Union due to their toxicity.37 Recent years have witnessed rapid development of phosphorus-containing epoxy resins, while most of the phosphorus-containing epoxy systems manifested a decrease in glass transition temperature (Tg) than the phosphorus-free systems due to the limited chemical structure design and relatively low cross-link density of the cured epoxy resin.38 Sustainability, reduction of environmental impacts, and green chemistry are increasingly guiding the development of phosphorus-containing materials from renewable resources,39 while there is limited report on biobased phosphorus-containing epoxy resins. Thus, in this paper, two novel high-performance flame retardant bio-based epoxy resins containing phosphorus were synthesized from vanillin. Diamines were used as the coupling agents in this work, and the Schiff base intermediates (produced from the coupling reaction between vanillin and diamines), without being extracted from the systems, further reacted with diethyl phosphite to obtain phosphorus-containing difunctional phenols; and then the flame retardant epoxy resins EP1 and EP2 were obtained by reacting these difunctianal phenols with epichlorohydrin. The chemical structures of the monomers were characterized by FTIR, 1 H NMR, and 13C NMR. DDM was used to cure EP1, EP2, and DGEBA (the control) to obtain three thermosetting resins EP1-DDM, EP2- DDM, and DGEBA-DDM. The curing behaviors of the systems were examined by differential scanning calorimetry (DSC). Flame retardancy and flame retarding mechanism of the epoxy resins were investigated by vertical burning and limit oxygen index test and char analysis using inductively coupled plasma optical emission spectrometer (ICP-OES), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The thermal stability, glass transition temperature, and mechanical properties were studied by thermogravimetric analysis (TGA), DSC, and tensile test, respectively. ■ EXPERIMENTAL SECTION Materials. Vanillin, 4,4-diaminodiphenylmethane (DDM), tetrabutylammonium bromide, p-phenylenediamine (PDA), and diethyl phosphite were purchased from Aladdin-reagent Co., China. Zinc chloride, epichlorohydrin (ECH), ethanol, petroleum ether, dichloromethane, and sodium hydroxide were obtained from Sinopharm Chemical Reagent Co., Ltd., China. Epoxy resin (DGEBA, trade name DER331, epoxy value 0.53) was supplied by DOW Chemical Company. Synthesis of Tetraethyl(4,4′-methylenebis(4,1-phenylene)- bis(azanediyl))bis((4-hydroxy-3-methoxyphenyl)methylene)- diphosphonate (DP1). DP1 was synthesized by a quintessential condensation reaction between vanillin and 4,4-diaminodiphenylmethane, followed by an addition reaction with diethyl phosphite. 20 g (0.13 mol) of vanillin was dissolved in 150 mL of ethanol in a 500 mL three-neck flask equipped with a magnetic stirrer and a reflux condenser. 11.83 g (0.06 mol) of 4,4-diaminodiphenylmethane dissolved in 50 mL of ethanol was added dropwise into the flask over a period of 0.5 h at 25 °C, and they were reacted at 40 °C for 1 h. Then diethyl phosphite (24.86 g, 0.18 mol) and zinc chloride (0.5 g) as the catalyst for the addition reaction were added into the system, and the mixture was heated from 40 to 80 °C under a nitrogen atmosphere and kept at 80 °C for 8 h. Finally, the mixture was poured into 1000 mL of petroleum ether and stirred for 0.5 h after being cooled to room temperature. A white solid was collected by filtration; then the white crystalline product DP1 (29.7 g) with the yield of around 93.3% was obtained after being washed with ethanol twice followed by drying at 70 °C for 24 h in a vacuum oven. The synthetic route is illustrated in Scheme 1. FTIR (KBr), cm−1 : 3364 (N−H); 3225 (O−H); 1278 (PO); 1026 (P−O). 1 H NMR (DMSO-d6, ppm): δ = 1.02 (t, 6H), 1.14 (t, 6H), 3.51 (d, 2H), 3.65 (m, 6H), 3.83 (m, 2H), 3.99 (m, 4H), 4.76 (dd, 2H), 5.96 (dd, 2H), 6.64 (dd, 6H), 6.75 (d, 4H), 6.84 (d, 2H), 7.07 (s, 4H), 8.85 (s, 2H). 13C NMR (DMSO-d6, ppm): δ = 16.68 (dd, 4C), 39.94 (s, 1C), 53.64 (s, 1C), 55.17 (s, 1C), 56.12 (s, 4C), 62.65 (dd, 2C), 113.09 (d, 2C), 114.07 (s, 4C), 115.40 (s, 2C), 121.38 (d, 2C), 128.00 (s, 2C), 129.18 (s, 4C), 130.78 (s, 2C), 145.76 (d, 2C), 146.34 (d, 2C), 147.69 (d, 2C). Scheme 1. Synthetic Routes of DP1 and DP2 Macromolecules Article DOI: 10.1021/acs.macromol.7b00097 Macromolecules 2017, 50, 1892−1901 1893

Synthesis of Tetraethyl(1,4-phenylenebis(azanediyl))bis((4- hydroxy-3-methoxyphenyl)methylene)diphosphonate (DP2). DP2 was also synthesized by a quintessential condensation reaction between vanillin and p-phenylenediamine followed by an addition reaction with diethyl phosphite. 20 g (0.13 mol) of vanillin was dissolved in 150 mL of ethanol in a 500 mL three-neck flask equipped with a magnetic stirrer and a reflux condenser. 6.4884 g (0.06 mol) of p-phenylenediamine dissolved in 50 mL of ethanol was added dropwise into the flask over a period of 0.5 h at 25 °C, and they were reacted at 40 °C for 1 h. Then diethyl phosphite (24.86 g, 0.18 mol) and zinc chloride (0.5 g) as the catalyst for the addition reaction were added into the system, and the mixture was heated from 40 to 80 °C under a nitrogen atmosphere and kept at 80 °C for 8 h. Finally, the mixture was poured into 1000 mL of H2O and stirred for 0.5 h after being cooled to room temperature. A light yellow solid was collected by filtration; then the light yellow crystalline product DP2 (23.4 g) with the yield of around 88.3% was obtained after being washed with acetone twice followed by drying at 70 °C for 24 h in a vacuum oven. The synthetic route is illustrated in Scheme 1. FTIR (KBr), cm−1 : 3390 (O−H, N−H); 1285 (PO); 1040 (P− O).1 H NMR (DMSO-d6, ppm): δ = 1.02 (t, 6H), 1.16 (t, 6H), 3.62 (m, 2H), 3.71 (s, 6H), 3.82 (m, 2H), 4.00 (p, 4H), 4.65 (dd, 2H), 5.39 (dd, 2H), 6.51 (d, 4H), 6.64 (d, 2H), 6.81 (d, 2H), 7.05 (s, 2H), 8.84 (s, 2H). 13C NMR (DMSO-d6, ppm): δ = 16.68 (dd, 4C), 54.54 (s, 2C), 56.08 (s, 4C), 62.5 (m, 2C), 112.99 (s, 2C), 115.37 (s, 4C), 121.32 (s, 2C), 128.23 (s, 2C), 139.51 (d, 4C), 146.2 (s, 2C), 147.66 (s, 2C). Synthesis of Tetraethyl(4,4′-methylenebis(4,1-phenylene)- bis(azanediyl))bis((3-methoxy-4-(oxiran-2-ylmethoxy)phenyl)- methylene)diphosphonate (EP1). 5 g of DP1 (0.0058 mol), 53.7 g of epichlorohydrin (0.58 mol), and 0.5 g of tetrabutylammonium bromide (10 wt % of DP1) as the catalyst were placed in a threenecked flask equipped with a magnetic stirrer and a reflux condenser, and the mixture was reacted at 80 °C for 3 h. After being cooled to 5 °C, 2.32 g of 40 wt % aqueous sodium hydroxide solution (0.058 mol NaOH) was added dropwise into the flask and reacted for 5 h. The solution was washed with distilled water three times after diluted with dichloromethane; then a light yellow solid was collected after removing dichloromethane and epichlorohydrin. Finally, the light yellow solid product EP1 (5.3 g) with the yield of around 93.5% was obtained after being washed with petroleum ether twice followed by drying at 50 °C for 24 h in a vacuum oven. The synthetic route is illustrated in Scheme 2. FTIR (KBr), cm−1 : 3300(N−H); 1265 (PO); 1027 (P−O); 910 (epoxy). 1 H NMR (CDCl3, ppm): δ = 1.07 (t, 6H), 1.27 (t, 6H), 2.72 (dd, 2H), 2.88 (t, 2H), 3.37 (s, 2H), 3.70 (m, 4H), 3.84 (s, 6H), 4.01 (m, 8H), 4.19 (dt, 2H), 4.62 (d, 2H), 6.47 (t, 4H), 6.91 (dd, 10H). 13C NMR (CDCl3, ppm): δ = 16.38 (dd, 4C), 40.06 (s, 1C), 44.92 (s, 2C), 50.13 (s, 2C), 55.17 (s, 1C), 55.96 (s, 4C), 56.68 (s, 1C), 63.23 (d, 2C), 70.12 (s, 2C), 111.37 (s, 2C), 113.71 (s, 2C), 113.93 (s, 4C), 120.19 (d, 4C), 129.41 (s, 4C), 131.72 (s, 2C), 144.48 (s, 2C), 147.60 (s, 2C), 149.59 (s, 2C). Synthesis of Tetraethyl(1,4-phenylenebis(azanediyl))bis((3- methoxy-4-(oxiran-2-ylmethoxy)phenyl)methylene)- diphosphonate (EP2). 5 g of DP2 (0.0065 mol), 60.1 g of epichlorohydrin (0.65 mol), and 0.5 g of tetrabutylammonium bromide (10 wt % of DP2) as the catalyst were placed in a threenecked flask equipped with a magnetic stirrer and a reflux condenser, and the mixture was reacted at 80 °C for 3 h. After being cooled to 5 °C, 2.6 g of 40 wt % aqueous sodium hydroxide solution (0.065 mol of NaOH) was added dropwise into the flask and reacted for 5 h. The solution was washed with distilled water three times after diluted with dichloromethane; then a light yellow solid was collected after removing dichloromethane and epichlorohydrin. Finally, the white solid product EP2 (4.3 g) with the yield of around 75.2% was obtained after being washed with petroleum ether twice followed by drying at 50 °C for 24 h in a vacuum oven. The synthetic route is illustrated in Scheme 2. FTIR (KBr), cm−1 : 3383 (N−H); 1264 (PO); 1033 (P−O); 910 (epoxy). 1 H NMR (CDCl3, ppm): δ = 1.14 (t, 6H), 1.27 (t, 6H), 2.74 (dd, 2H), 2.90 (t, 2H), 3.39 (s, 2H), 3.71 (m, 2H), 3.85 (s, 6H), 4.03 (m, 8H), 4.19 (dt, 2H), 4.55 (d, 2H), 6.43 (s, 4H), 6.92 (m, 6H). 13C NMR (CDCl3, ppm): δ = 16.37 (dd, 4C), 44.95 (s, 2C), 50.11 (s, 2C), 55.98 (d, 4C), 57.52 (s, 2C), 63.16 (m, 2C), 70.10 (s, 2C), 111.30 (s, 2C), 113.61 (s, 2C), 115.45 (s, 4C), 120.18 (d, 2C), 129.62 (s, 2C), 139.24 (d, 2C), 147.55 (s, 2C), 149.54 (s, 2C). Preparation of the Cured Epoxy Resins. Both EP1 and EP2 were cured with a commonly used curing agent DDM. Epoxy monomers were mixed with DDM in a 1:1 equiv ratio. The mixtures were cured by a plate vulcanizer at 200 °C for 3 min (1 min for removing bubbles, 2 min for heat pressing) to get films with thickness of approximately 100 μm for thermal and mechanical properties examination. The mixtures were also melted and poured into stainless steel molds and cured at 160 °C for 2 h to obtain samples with dimensions of 80 mm × 6.5 mm × 3 mm for flame retardancy investigation. Both films and rectangular samples were postcured at 200 °C for 2 h and 230 °C for 2 h in a vacuum oven. For comparison, commonly used bisphenol A epoxy resin (DGEBA) was also cured with the same curing agent. Characterization. 1 H NMR and 13C NMR spectra were recorded by an AVANCE III Bruker NMR spectrometer (Bruker, Switzerland) with DMSO-d6 and CDCl3 as solvents, operating at 400 and 75.5 MHz, respectively. The infrared spectra (FTIR) were measured with a NICOLET 6700 FTIR (NICOLET, America) using the KBr pellet method. Differential scanning calorimetry (DSC) measurements were performed under a nitrogen atmosphere on a Mettler Toledo Star 1 apparatus and a PerkinElmer DSC8000 apparatus for nonisothermal curing kinetics and glass transition temperature (Tg), respectively. The mixtures (around 3−5 mg) of epoxy monomers and DDM were taken for the study of nonisothermal curing kinetics. A heat scan ranging from 50 to 250 °C was performed at varying heating rates of 5, 10, 15, and 20 °C min−1 , respectively. Cured samples (around 8−10 mg) were heated from 50 to 250 °C at a heating rate of 20 °C min−1 and held at 250 °C for 3 min to eliminate thermal history, and then they were cooled to 50 °C at a cooling rate of 20 °C min−1 followed by being heated again to 250 °C at a rate of 20 °C min−1 . The Tg was obtained from the second heating curve of the cured epoxy resins. Cured samples with dimensions of 30 mm × 0.5 mm × 100 μm were used to examine tensile properties by a Universal Mechanical Testing Machine (Instron 5569A) with a cross-head speed of 0.5 mm min−1 . The tensile properties of each sample were reported as the average of five Scheme 2. Synthetic Routes of EP-1 and EP-2 Macromolecules Article DOI: 10.1021/acs.macromol.7b00097 Macromolecules 2017, 50, 1892−1901 1894

measurements. Thermogravimetric analysis (TGA) was carried using a Mettler-Toledo TGA/DSC1 thermogravimetric analyzer (METTLER TOLEDO, Switzerland). Approximately 3−5 mg samples were heated from 50 to 700 °C at a heating rate of 10 °C min−1 under nitrogen and air atmospheres. The limit oxygen index (LOI) was obtained on a JF-3 oxygen index instrument (Jiangning Analysis Instrument Company, China) with sample dimensions of 80 mm × 6.5 mm × 3 mm according to ASTM D2863-10. UL-94 vertical burning tests were conducted according to ASTMD2863-97 on an AG5100B vertical burning tester (Zhuhai Angui Testing Equipment Company, China) with sample dimensions of 80 mm × 6.5 mm × 3 mm. The morphologies of residues collected after LOI tests were observed by scanning electron microscopy (SEM, EVO18) with an accelerating potential of 20 kV. DGEBA-DDM could not self-extinguish, so it was blown out during the test in order to investigate the morphology of its surface during the burning. The residues of phosphorus content after LOI tests were performed by inductively coupled plasma optical emission spectrometer (ICP-OES) on a PerkinElmer Optima 2100DV apparatus. 20 mL of aqua regia (10 mL of H2O, 7.5 mL of HCl, and 2.5 mL of HNO3) was added into a 120 mL volumetric flask with approximately 0.02 g of carbon residue; then the systems were kept at 180 °C for 1 h; finally, water was added to make the systems reach 100 mL. 5 mL of the obtained solution was taken for test. Each sample was examined twice under the same conditions, and the data were averaged. The X-ray photoelectron spectroscopy (XPS) spectra of the char residues (surface and inside (cut off the surface layer with thickness of 2 mm)) were recorded with an AXIS ULTRA apparatus (Kratos, England). ■ RESULTS AND DISCUSSION Synthesis and Characterization of DP1, DP2, EP1, and EP2. In this paper, vanillin was coupled by diamines through Schiff base condensation, and the generated Schiff base further reacted with diethyl phosphite by phosphorus−hydrogen addition reaction to yield phosphorus-containing vanillinbased diphenols DP1 and DP2. Significantly, the Schiff base formation and phosphorus−hydrogen addition reaction were finished in one pot, which means that the Schiff bases produced during the reaction did not need to be extracted from the system. The two phosphorus-containing bio-based epoxy resins EP1 and EP2 were synthesized by reacting DP1 and DP2 with epichlorohydrin by a typical method. The chemical structures of the synthesized monomers were confirmed by FTIR, 1 H NMR, and 13C NMR. Figure 1 presents the FTIR spectra of DP1, DP2, EP1, and EP2. In the FTIR spectra of DP1 and DP2, there are characteristic peaks at 3200−3400 cm−1 belonging to N−H and O−H, peaks at around 1280 cm−1 ascribed to PO, and peaks at around 1030 cm−1 representing P−O. For the FTIR spectra of EP1 and EP2, the peaks for O−H disappeared and the peaks for epoxy group at around 910 cm−1 appeared. As seen from the 1 H NMR and 13C NMR spectra of DP1, DP2, EP1, and EP2 (Figures 2 and 3), the chemical shift and integral area of all the peaks match well with the protons and carbons of chemical structures for DP1, DP2, EP1, and EP2. These results indicate that the target compounds were synthesized successfully. Curing Process. The nonisothermal curing kinetics of EP1- DDM, EP2-DDM, and DGEBA-DDM were studied by DSC. The Kissinger’s method (eq 1) 40−42 and Ozawa’s method (eq 2) 40−43 were used to obtain the apparent activation energy during the curing process. − =− ln( / ) / ln( / ) q T E RT AR E p 2 ap a (1) ln 1.052 / ln( / ) ln ( ) 5.331 q E RT AE R F x =− + − − ap a (2) where q is the heating rate, Tp is the exothermic peak temperature, Ea is the activation energy, R is the gas constant, A is the pre-exponential factor, and F(x) is a conversiondependent term. Figure 4 reveals the plots of −ln(q/Tp 2 ) versus 1/Tp based on Kissinger’s equation and ln q versus 1/Tp based on Ozawa’s theory for the EP1-DDM, EP2-DDM, and DER331-DDM systems, and the Eas calculated from the slope of the linear fitting plots are concluded in Table 1. It can be seen that EP1-DDM and EP2-DDM presented similar Ea to DGEBA-DDM, which is indicative of the comparable reactivity of EP1 and EP2 to that of DGEBA toward the curing agent DDM. In order to investigate the curing degree of the systems, FITR spectra of EP1-DDM and EP2-DDM after curing are exhibited in Figure 5. Compared with the FTIR spectra of EP1 and EP2, the peaks for epoxy groups at around 910 cm−1 disappeared and broad peaks ranged from 3200 to 3600 cm−1 representing O−H from the ring-opening reaction between epoxy groups and amines appeared in the FTIR spectra of EP1- DDM and EP2-DDM. This means that the epoxy systems were cured well under the former mentioned conditions. Flame Retardancy and Flame Retarding Mechanism of the Cured Epoxy Resins. The flame retardancy of the cured epoxy resins was investigated by vertical burning test and limit oxygen index (LOI) examination. The UL-94 ratings and LOI values of the cured epoxy resins are listed in Figure 6. Obviously, the UL-94 V0 rating which was the highest flame retarding rating by vertical burning test was achieved for the EP1-DDM and EP2-DDM systems. Meanwhile, the LOI values of EP1-DDM and EP2-DDM were 31.4% and 32.8%, respectively. Nevertheless, the DGEBA-DDM showed no UL- 94 rating and low LOI value of 24.6%. These results illustrate that EP1-DDM and EP2-DDM presented excellent flame retardancy, while DGEBA-DDM was flammable. Generally, the char residue structure after combustion can reflect the flammability characteristics of polymer materials. Digital photos and SEM photographs of the char residues after LOI test are illustrated in Figure 7, and the phosphorus content in char residues examined by inductively coupled plasma optical emission spectrometer (ICP-OES) is concluded in Table 2. It can be clearly seen in Figure 7a that EP1-DDM and EP2-DDM formed great intumescent char residues, accounting for 35.8% Figure 1. FTIR spectra of DP1, DP2, EP1, and EP2. and 41.2% of the original weight of EP1-DDM and EP2-DDM, Macromolecules Article DOI: 10.1021/acs.macromol.7b00097 Macromolecules 2017, 50, 1892−1901 1895

《高分子物理》课程教学资源（文献资料）Vanillin-Derived High-Performance Flame Retardant Epoxy Resins - Facile Synthesis and Properties