Biobased Composites Prepared Using an Environmentally Friendly

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Materials and Interfaces

Bio-based Composites Prepared Using an Environmentally-friendly Water-slurry Methodology Zhongxiang Zhao, Yongjian Lou, Zenghui Dai, Feiya Fu, and Xiangdong Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018

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Industrial & Engineering Chemistry Research

Bio-based Composites Prepared Using an Environmentally-friendly Water-slurry Methodology Zhongxiang Zhao, Yongjian Lou, Zenghui Dai, Feiya Fu*, Xiangdong Liu* Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, College of Materials and Textile, Zhejiang Sci-Tech University, Xiasha Higher Education Zone, Hangzhou 310018, P.R. China. E-mail: [email protected]; [email protected].

KEYWORDS: Bio-composites, water-slurry molding, benzoxazine, cotton fabrics, environmentally-friendly method ABSTRACT To improve processability of benzoxazine monomer in preparation of composites, a water-slurry strategy was examined using several laboratory-scale instances. The water-slurries

were

fabricated

by

mixing

solid

resin

powder

of

3-furfury-8-methoxy-3,4-dihydro-2H-1,3- benzoxazine (Bzf) with water and one type of filler particles i.e., calcium carbonate (CC), montmorillonite (MMT), or hollow glass beads (HGMS). Experimental data show that approving liquidity can be achieved when more than 180 phr of water was mixed in the solid mixtures containing the resin powder and 100 phr of solid filler. The bio-based composite prepared using the optimized condition exhibits outstanding mechanical properties and antifatigue performance as the composites prepared via solvent method. This water-slurry approach provides an environmentally-friendly strategy to manuscript benzoxazine composites, offering benzoxazine with more promising applications in a lot of industries such as building, wind energy, aircraft, and automobile. 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 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

INTRODUCTION Polybenzoxazines have excellent electrical insulativity, good thermal stability, high glass transition temperature, approvable flame retardancy, outstanding mechanical property, promising molecular design flexibility, and approximately zero shrinkage.1-9 These advantages were particularly attractive in fabrication of high-performance materials.10-15 Most of benzoxazine monomers melt at above 120 oC, and they are of no fluidity at room temperature.16-21 To satisfy the fluidity requirements in forming process such as preparation of composites, benzoxazine monomers are often treated by three methodologies: (i) melting by heat,22-24 (ii) dissolving in organic solvents,25-28 and (iii) crushing into powder using machines.29, 30 Melting solid benzoxazine monomer has been frequently reported to obtain fluidity in forming processing due to its simplicity.22-24 However, the high temperature and high viscosity are always problematic in operations like mixing, transporting, and coating. By dissolving benzoxazine monomer in organic solvents like acetone,25 xylene,28 and DMF,26 various polybenzoxazine composites were successfully prepared. Although acceptable fluidity can be obtained, the dissolving methods generally require additional process to remove the organic solvent before curing, and more serious is the environmental issues due to the exhaust of organic solvents.31-34 The powder method has been used to prepare graphene oxide/benzoxazine nanocomposites and wood-plastic materials using polybenzoxazine as a binding 2 ACS Paragon Plus Environment

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material.29, 30 The problems of organic solvents were successfully avoided in those works, but the poor flow-ability of benzoxazine powders does not meet a large part of processing requirements.35 To obtain good fluidity and to avoid organic solvent pollution, a slurry method by mixing benzoxazine powder with water is proposed in the present work. Solid benzoxazine monomer is first crushed into micrometers and then dispersed into water to make paste like mixtures. A series of the water-slurries are prepared by mixing water with the powder of 3-furfury-8-methoxy-3,4-dihydro-2H-1,3-benzoxazine (Bzf) and a variety of fillers of calcium carbonate (CC), montmorillonite (MMT), or hollow glass beads (HGMS). These fillers have been applied in bio-based composites due to their safety for the environment protection.36-38 The composites fabricated using the water-slurry method are characterized and compared with previous studies of benzoxazine composites.39-47 The curing behavior and rheological property of these water-slurries are studied using Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and rotational rheometer analyses, respectively. After composted with cotton fabric, mechanical and thermomechanical properties of the resulted composites are evaluated by elongation tests and dynamic mechanical analyses (DMA). EXPERIMENTAL Materials Bzf monomer was synthesized as previous literature,48 and the detailed method was described in Supporting Information. Bisphenol F benzoxazine (BF, containing 75 wt% 3 ACS Paragon Plus Environment


butanone solvent) was purchased from Suzhou shellfish US Electronic Materials Co., Ltd. (Suzhou, China). Methyl p-toluenesulfonate (PTSM), furfurylamine, guaiacol, paraformaldehyde, CC (particle size = 400-900 nm), MMT (particle size = 800-2000 nm), and other materials were purchased from Aladdin Reagent Co. (Shanghai, China). HGMS (particle size = 8-20 µm) was purchased from Qinhuangdao Aoge Group Co. Ltd. (Hebei, China). All the chemicals were used without further purification. Cotton fabric (CO, warp density 60 stick/cm, weft density 30 stick/cm, thickness 0.42 mm, square meter quality 120 g/m2, specific area 35.2 m2/g) was provided by Huizhou Xinhui instrument and Equipment Co., Ltd. (Anhui, China). Before the composite processing, cotton fabrics were activated as previous literature.49 Briefly, cotton fabric was soaked in a solution of NaOH (100 ml, 2 wt%) at 25 oC for 2 h, dried at 150 oC for 4 h, washed with distilled water (100 ml × 3 times), and dried in a vacuum at 80 oC overnight. Preparation of benzoxazine/water-slurries To a beaker under mechanical stirring, Bzf (12.15 g, 50.00 mmol) and PTSM (0.47 g, 2.50 mmol) were added. The mixture was heated at 130 oC and vigorously stirred (2000 rpm) for 30 min, cooled to room temperature, crushed using a grinder, and finally screened to obtain Bzf resin powder having particle size ranged in 3.5-8.5 µm. Bisphenol F benzoxazine (BF) resin powder was prepared in the same way. The Bzf/CC/water-slurry (180 phr water) was prepared by mixing the Bzf resin powder (5.0 g), CC powder (5.0 g), and water (9.0 ml). The mixture was further 4 ACS Paragon Plus Environment

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mixed using a conditioning mixer (Thinky Co., AR-100, Tokyo, Japan) at 2000 rpm for 30 min to obtain the slurry sample Bzf-CC+180. Other slurries were prepared with the similar way but a varying mass ratio (Table 1). Preparation of Bzf composites. A Bzf/CC/water-slurry (10 g, Bzf-CC+180) was uniformly coated on each cotton fabric (8 cm × 8 cm, 4 pieces), dried at 80 oC for 4 h, heated in a hot pressing device (10 MPa, 180 oC) for 2 h to obtain the composite Bzf-CC (Figure 1). Other composites were prepared by using similar process listed in Table 1. As a control, the composite simple was fabricated via a dissolving method. In short, a solution of bisphenol F benzoxazine in butanone (40 ml, 75 wt% butanone, BF 46.70 mmol) and PTSM (0.025 g, 2.34 mmol) were mixed using the AR-100 conditioning mixer at 2000 rpm for 30 min, coated on each cotton fabric (8 cm × 8 cm, 4 pieces).

Table 1. Mechanical properties of the benzoxazine composites prepared by water-slurry method. Sample

a

Bzf-CC+100 Bzf-CC+140 Bzf-CC+180 Bzf-CC+220 Bzf-CC+260 Bzf-HGMS Bzf-MMT BF-CC BFS-CC a b

Filler CC CC CC CC CC HGMS MMT CC CC

Water

Stress

Strain

(phr)

(MPa)

(%)

18.1±1.1 21.3±0.9 24.6±1.0 23.2±1.3 22.1±1.1 11.7±1.1 25.3±2.1 29.5±1.6 27.2±1.2

14.6±0.7 14.7±0.5 14.7±0.3 14.3±0.5 14.5±0.4 14.1±0.7 9.6±0.6 11.2±0.6 12.9±0.9

100 140 180 220 260 180 180 180 —b

The mass ratios of both fabric and filler were 100 phr. As a reference sample, BFS-CC was prepared by a dissolving method using butanone solvent.

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Figure 1. Preparation of the cotton fabric reinforced benzoxazine composites by water-slurry method. The precursor was heated at 80 oC under reducing pressure to remove butanone solvent, heated in a hot pressing device (10 MPa, 180 oC) for 2 h to obtain BFS-CC composite. Characterization. 1

H NMR spectra were recorded on a Bruker Ascend 400 spectrometer (Bruker Corp.,

Switzerland). FTIR spectra were obtained on a Nicolet 5700 FTIR plus spectrometer (Nicolet Corp., USA) with 4 cm-1 resolution in the wavelength range from 400 cm−1 to 4000 cm−1. Viscosity was measured by Anton Paar Physical MCR 301 rotational rheometer (Anton Paar Corp., Germany). Dynamic light Scattering (DLS) analyses were carried out using a Zetasizer Nano ZS (Malvern Corp., UK), equipped with a 5.0 Mw laser (λ = 650 nm) at room temperature. Surface morphology of the powders was investigated using a scanning electron microscopy (SEM) set (SU8010, Hitachi Corp., Japan) and an Olympus IX51 optical microscope (Olympus Corp., Japan). DSC tests were performed on a DSC-Q2000 (TA Instruments Corp., USA) at a heating rate of 10 o

C/min in a nitrogen flow (35 ml/min). The stress-stain data of the composite samples

were obtained from a universal material testing machine (HESON HS300C, Shanghai 6 ACS Paragon Plus Environment

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Hesheng Instrument Technology Co., Ltd., China). The sample dimension was 50 × 10 × 1 mm and the stretching rate was 40 mm/min. Fatigue properties were examined by a fatigue test machine (DSW-3200, Sichuan Dexiang Kechuang Instrument Co., Ltd., China). Dynamic mechanical analyses (DMA) were performed with DMA-1 analyser (Mettler-Toledo Corp., Switzer-land) used controlled single cantilever bending mode (1 Hz, 15 mm amplitude) at a heating rate of 3 oC/min from 25 oC to 270 oC. The samples were of 10.0 mm width and approximate 1.0 mm thickness. Thermogravimetric analysis (TGA) was performed on a TGA/DSC1 (Merrler-Toledo Corp., Switzer-land) at a heating rate of 10 oC/min from 30 oC to 800 oC under a nitrogen flow of 120 ml/min. Water absorption property was characterized using the ASTM D70-98 standard method with a few modifications. Five composite specimens (100 mm × 10 mm × 2 mm) were dried in an oven at 50 oC for 24 h (weight: W1), immersed in distilled water at 23 ± 1 oC, weighed after certain periods of time (weight:

W2). The water absorption rates of specimens were calculated by eq 1:

water absorption rate =

× 100%

(1)

RESULTS AND DISCUSSIONS Curing behavior of the Bzf mixtures The bio-based benzoxazine monomer, Bzf, was synthesized using a reported non-solvent method.48 As shown in Figure S1, guaiacol has FTIR peaks at 1010 and 1218 cm−1, which are assignable to the symmetrical and asymmetric C-O-C stretching 7 ACS Paragon Plus Environment


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of the methoxy group on the benzene ring. However, these peaks shift to 1020 and 1220 cm-1 for the Bzf monomer. In addition, the FTIR spectrum of Bzf shows characteristic peaks at 1178 cm-1 (symmetric C-N-C stretching), 1314 cm-1 (CH2 oxazine ring wagging), 1482 cm-1 (substituting of substituted benzene ring), and 945 cm-1 (C-H out-of-plane deformation mode), suggesting that the oxazine ring fused to the benzene ring was formed. Moreover, the furan ring structure in Bzf was confirmed by the characteristic peaks observed at 1588, 970, and 765 cm−1. The 1H NMR spectrum of Bzf is presented in Figure S2. The peaks appeared at around 4.02 and 4.99 ppm can be assigned to -Ph-CH2-N- and -OCH2-N- of the oxazine ring, respectively. While the characteristic peaks found in the range of 6.26-6.84 ppm and 7.41 ppm are contributable to the protons of the furan ring, and the peak at 3.96 ppm is to the -CH2-N- structure of the furan ring. These spectra of Bzf are consistent with the results published recently.50 Moreover, elemental analysis result is consistent with the theoretical calculation of Bzf. Generally, the ring opening polymerization of benzoxazine monomers often happens at a high temperature more than 200 oC. By addition of PTSM, the ring opening polymerization temperature can be reduced to 180 oC.48,

51

The curing

behavior of the resins of Bzf cured at isothermal conditions was studied by DSC analyses. As shown in Figure 2a, there are no exothermic peaks on the curves of 180 o

C and 190 oC, meaning that Bzf can be fully cured at 180 oC for 2 h. Based on the

result, the water-slurries containing Bzf and CC, MMT, or HGMS filler were cured at 180 oC for 2 h, and the cured samples were subjected to DSC analyses again. 8 ACS Paragon Plus Environment

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Figure 2. DSC thermograms of Bzf after different curing temperature. 2 h/25 oC, 80 o

C, 120 oC, 150 oC, 180 oC and 190 oC. (b). DSC thermograms of the mixtures of Bzf

and solid fillers at 180 oC for 2 hours. As shown in Figure 2b, there are no obvious exothermic peaks appearing up to 260 oC, indicating that the benzoxazine monomer (Bzf) in these composites has been completely cured. The FTIR spectra of the mixtures and their cured samples are shown in Figure 3. Before heating, all the four mixtures have a characteristic peak at 930 cm−1, which is contributable to the benzoxazine structure.52 However, this peak is almost completely disappeared upon heating at 180 oC for 120 min. This suggests that the curing process of the mixtures progressed well as pure Bzf monomer. Thus, the curing condition of the composite procession is determined to be 180 oC/2 h. Rheological properties of the water-slurry As shown in Figure 4 and 5, the DLS results and the optical images indicates that the Bzf powders have a particle size range from 3.5 µm to 6.5 µm, whereas the particle size of CC, MMT, and HGMS are 400-900 nm, 800-2000 nm, and 8-20 µm, respectively. 9 ACS Paragon Plus Environment


Figure 3. FTIR spectra of the cured resins of (a) Bzf, (b) Bzf-CC, (c) Bzf-MMT and (d) Bzf-HGMS. Curing conditions: 180 oC for 2 hours.

Figure 4. SEM (a) and optical microscope (b) images of HGMS. The flowing properties of the Bzf/CC/water-slurries are displayed in Figure 6. As intuitively imaged in Figure 6a, the water quantity in the slurries has a strong effect on rheological properties of the slurries containing CC particles. With increasing water mass ratio, the rheological properties of the Bzf/CC/water-slurries are improved. 10 ACS Paragon Plus Environment

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Figure 5. Particle sizes of the benzoxazine powder and solid fillers. (a) Bzf, (b) CC, (c) MMT, and (d) HGMS. (a, b and c were obtained from dynamic light scattering measurements, and the graph d was calculated from SEM images) When the water mass ratio was less than 180 phr, the water-slurries cannot be extruded due to their poor flow-ability. The extrusion amounts of the water-slurries were measured and shown in Figure 6b. They are 1.76 ml and 3.95 ml for the slurries containing 180 phr and 260 phr of water, respectively. The flow-ability of the water-slurries gradually increases with increasing water ratio, leading to the improved processability in preparation of the benzoxazine composite material. Constant viscosity is often required in a fabrication process of composites because thermoforming process need a suitable flowing property.53, 54 As shown in Figure 6c, the viscosity values of these slurries become to constant when the testing time was over 400 s, and the apparent viscosity values of the slurries containing 100 phr, 180 phr, and 260 phr of water are 0.945 Pa·s, 0.443 Pa·s and 0.202 Pa·s, respectively. 11 ACS Paragon Plus Environment


Figure 6. The optical images reflecting the fluidity of the Bzf/CC/water-slurries (a), the extrudate volume data of the Bzf/CC/water-slurries (b), and the effect of water weight ratios on viscosity (c). Expectably, they decrease with increasing of water content, suggesting that the processability of the slurries become better with increasing the water quantity. Mechanical and dynamic mechanical properties The water-slurries were used to prepare benzoxazine composites with cotton fabric, and their fracture strength and elongation were tested by a universal material testing machine, because the mechanical properties are the most important indicator for evaluating composite performances.22 As shown in Figure 7a and Table 1, the Bzf-MMT benzoxazine composites exhibit a break strength of 25.3 MPa, which is similar to that of Bzf-CC (24.6 MPa) and much higher than that of the Bzf-HGMS composites (11.7 MPa). The weakness of the 12 ACS Paragon Plus Environment

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Figure 7. Stress-strain behavior of the benzoxazine composites. Bzf-HGMS composites may be contributed to the large size of the hollow glass beads and the poor adhesion capability with the polybenzoxazine resin.37, 38 These results suggest that the benzoxazine composites prepared by the water-slurry method have good mechanical properties and good applicability for filler particles. As shown in Figure 7b, the break strength of benzoxazine composite BF-CC, which was prepared using the bisphenol F benzoxazine (BF) monomer via the water-slurry method, is 29.5 MPa. This value is higher than that of the composite Bzf-CC only by 4.9 MPa, reflecting that the bio-based benzoxazine composites have high mechanical properties as the commercially available monomer, BF. As a contrast, BF monomer was used to prepare composites with cotton fabric and CC filler via a traditional processing method using organic solvent, butanone. The 13 ACS Paragon Plus Environment


break strength of the resulting BFS-CC composite is 27.2 MPa, which is in good agreement with previous report.22 By using melting method, Dayo et al. have successfully prepared reinforced benzoxazine composites with natural hemp fiber.22 Their fracture strength was maintained at 20-45 MPa, and the elongation at break was maintained between 1.2-1.7 %. These comparison data indicate again that, the water-slurry method can provide good mechanical properties like the traditional methods. Fatigue resistance is an important index to determine the service life of composite materials.55-57 As shown in Figure 8, after 10000 times of fatigue test, the Bzf-CC, Bzf-MMT, Bzf-HGMS and BF-CC benzoxazine composites still exhibit strong material properties. Their break strength values are 18.9, 20.3, 8.2 and 23.2 MPa, respectively, which all keep at about 80 % by comparing with the original samples before the fatigue test. These results indicate that the water-slurry method can give the composites with good fatigue resistance and longer service life. Furthermore, the fatigue strength value of the BFS-CC is 21.1 MPa, meaning that at least more than 20 % break strength was lost during the fatigue test. This value is lower than that of the BF-CC composite, suggesting the water-slurry preparation provides the benzoxazine composites with better antifatigue ability than the dissolving method does. As shown in Figure 9a, the storage modulus values (50 oC) of Bzf-CC, Bzf-MMT and Bzf-HGMS composites are 2.07, 2.29 and 1.63 GPa, respectively. The Bzf-HGMS exhibits the lowest storage modulus among the three composites. This result agrees with the breaking strength, and can be contributed to the larger size of 14 ACS Paragon Plus Environment

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Figure 8. Antifatigue properties of the benzoxazine composites. the hollow glass beads and poor adhesion capability to polybenzoxazine resins. The tan δ curves reveal Tg values of the composites.58 As shown in Figure 9b, the Tg values of the composites of Bzf-CC, Bzf-MMT, and Bzf-HGMS are all centered at about 145 oC. These Tg values are in good agreement with the Tg value of the neat resin Bzf, which measured by a DSC method.48, 51 In contrast, in other work reported by Ribeiro et al., the polybenzoxazine composites fabricated have a storage modulus between 500-900 MPa, and a Tg between 90-125 oC.59 These results suggest that the bio-based benzoxazine composites prepared by the water-slurry method have approving thermo-mechanical properties for general applications like such as building, wind energy, aircraft, and automobile. As shown in Figure 9c and 9d, the thermo-mechanical properties of the BF-CC and BFS-CC composites are very similar, indicating that the water-slurry method leads to a complete curing process of benzoxazine monomer, and no significant difference is found when compared with the traditional method using organic solvent.



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Figure 9. DMA curves. (a) and (c) storage modulus versus temperature, (b) and (d) tan δ versus temperature. The TGA and DTG curves of the composites of Bzf-CC, BF-CC, BFS-CC, Bzf-MMT, and Bzf-HGMS are depicted in Figure 10, and the T5%, T10%, and Yc values are summarized in Table 2. The weight loss prior to 100 oC is contributed to the moisture present in the samples. The volume of the void spaces in the fabric-matrix may adsorb moisture, but the weight loss for all composites are very diminutive.22

Table 2. TGA summary for the benzoxazine composites. Sample

T5% (oC)

T10% (oC)

Yc (%)

Bzf-CC Bzf-HGMS Bzf-MMT BF-CC BFS-CC

270 229 265 271 270

309 274 304 311 310

39 55 40 37 37


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Figure 10. TGA (a) and DTG (b) profiles of the composites heated under nitrogen environment. As shown in Figure 10b, the DTG curves for all composites show weight loss peaks at the temperature range from 330 to 350 oC, indicating that they have similar thermal stability. Moreover, both BF-CC and BFS-CC composites display similar thermal stability under nitrogen atmosphere, suggesting that the water-slurry method brings not negligible impact on thermal stability, even on char yield and Yc values. The water absorption behavior of benzoxazine composites was studied for 100 hours and the results are illustrated in Figure 11. All samples show a continuous rise in water uptake till 14 hours. The saturated water content for Bzf-CC, Bzf-MMT, Bzf-HGMS, BF-CC and BFS-CC were recorded as 11.06, 11.18, 14.74, 10.92 and 17 ACS Paragon Plus Environment


Figure 11. Water absorption rate of composites. 10.89 %, respectively. It seems that the water absorbability of the composites is higher than the results reported by Dayo and co-workers, who claimed that only 4.4 % water is absorbed by the polybenzoxazine composites containing 25 vol % natural hemp fiber.22 The reason can be assigned to the high content of the inorganic fillers. On the other hand, as shown in Figure 11, the similarity of the composites of Bzf-CC, Bzf-MMT, BF-CC, and BFS-CC suggests that both the water-slurry method and the bio-based benzoxazine will not raise new problems in water absorption when compared with the BF composites prepared by the dissolving method. In addition, the highest water absorbability of Bzf-HGMS composite may be due to the cracked glass microbeads. CONCLUSION In summary, the water-slurry method is environmentally-friendly and can be used to improve processability of benzoxazine monomers in composite processes. The rheological tests revealed that approving viscosity about 0.443 Pa·s can be achieved by mixing suitable water in the slurry. In addition, the water-slurry method can give benzoxazine composites with strong mechanical properties, high storage modulus and 18 ACS Paragon Plus Environment

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glass transition temperature, similar to that of the composites prepared via the traditional method using organic solvent. Furthermore, the water-slurry method does not affect the thermal stability of the composites, as the decomposition temperatures (T5% and T10%) and the char yields of the composites prepared by water-slurry method and the dissolving method have no significant difference. When this methodology was used to fabricate composites of bio-based benzoxazine monomers and natural fiber like cotton, fully green and sustainable way towards wide environmentally-friendly applications can be paved. CORRESPONDING ANTHOR Xiangdong Liu*, Professor, E-mail: [email protected]. Tel: (0571) 86843785 Feiya Fu*, E-mail: [email protected]. ACKNOWLEDGEMENTS This work was financially supported by the Natural Science Foundation of China (51573167), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, and the State Education Ministry (1101603-C). SUPPORTING INFORMATION The Supporting Information for this paper illustrates the detailed synthesis process of the Bzf monomer, the good repeatability of the FTIR and 1H-NMR spectra. REFERENCES (1) Xu, Q.; Zeng. M.; Chen, J.; Zeng, S.; Huang, Y.; Feng, Z.; Xu, Q.; Yan, C.; Gu, Y. Synthesis, polymerization kinetics, and high-frequency dielectric properties of novel main-chain benzoxazine copolymers. React. Funct. Polym. 2018, 122, 158. 19 ACS Paragon Plus Environment


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Graphical Abstract

A water-slurry method using benzoxazine monomer powder provides an environmentally-friendly way to fabricate excelent composites.


Biobased Composites Prepared Using an Environmentally Friendly

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