Synthesis of Biobased Benzoxazines Suitable for Vacuum-Assisted

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Synthesis of Bio-based Benzoxazines Suitable for Vacuum Assisted Resin Transfer Molding (RTM) Process via Introduction of Soft Silicon Segment Jinyue Dai, Na Teng, Xiaobin Shen, Yuan Liu, lijun cao, Jin Zhu, and Xiaoqing Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04716 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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Industrial & Engineering Chemistry Research

Synthesis of Bio-based Benzoxazines Suitable for Vacuum Assisted Resin Transfer Molding (RTM) Process via Introduction of Soft Silicon Segment Jinyue Dai 1,2, Na Teng 1,3, Xiaobin Shen 1, 2, Yuan Liu 1,2, Lijun Cao1,2, Jin Zhu 1,3, Xiaoqing Liu 1,3*

1

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, (P. R. China) 2

3

University of Chinese Academy of Sciences, Beijing 100049 (P. R. China)

Key Laboratory of Bio-based Polymeric Materials Technology and Application of Zhejiang Province Corresponding Author: Dr. Xiaoqing Liu, E-mail: [email protected];

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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

ABSTRACT: A serial of bio-based benzoxazine oligomers suitable for vacuum assisted resin transfer molding (RTM) were prepared from eugenol derivatives, furfurylamine, diamine and paraformaldehyde. After the structure of these oligomers were identified by fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (1H NMR and

13

C NMR), their curing behaviors

and processability were evaluated by differential scanning calorimetry (DSC), FT-IR and rheological test. Results showed that the oligomers had stable viscosity lower than 1 Pa·s in the temperature range from 60 to 190 °C and their processing window was found to be wider than 160 °C. The cured resins exhibited excellent thermal properties (10 % thermal weight loss at 406 °C and glass transition temperature higher than 110 °C), good hydrophobic performance and low moisture absorption. The benzoxazine oligomers reported here demonstrated great potential in the fabrication of composites, especially the green composites via more efficient process, such as RTM.


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1. INTRODUCTION Vacuum assisted resin transfer molding (RTM), as an efficient process for composites manufacture, has received extensive attention in the past two decades owing to the characteristics of high efficiency, favorable working environment and high accuracy of controlling. As a general rule, the resins suitable for RTM should have low and stable viscosity, low shrinkage during the curing process without release of small molecules.

1-3

Up to now, a large quantity of thermosetting resins, including epoxies,

polyesters, vinyl esters, unsaturated, bismaleimide and urethanes, have been taken to fabricate composites via RTM process. However, in many cases, their intrinsic thermal or mechanical properties have to be compromised in order to meet the requirements for RTM process after the addition of reactive diluents. In addition, the shrinkage is usually observed during the curing process, which directly influences the dimension stability of the final products. Therefore, how to develop the potential resins suitable for RTM is always an essential task facing us. 4-6 Due to the outstanding thermal properties, excellent chemical resistance, good dimensional stability, low water absorption and flammability, polybenzoxazines have attracted much more attention and widely used in the fields of coatings, adhesives, microelectronics and composites.

7-9

In recent years, with the rapid development of

polybenzoxazines and RTM technology, more and more benzoxazines suitable for RTM have already been reported.

10-15

For example, Liu and co-workers

10

synthesized a series of benzoxazine oligomers from bisphenol-F, aniline and 4, 4′-diamino-diphenylmethane, and they exhibited excellent processability. Yin and his co-workers

12

studied the rheological properties of benzoxazine resins synthesized

from menthane diamine and phenol. Results showed that this resin had stable viscosity lower than 0.8 Pa·s from 85 to120 °C. Currently, the raw materials for polybenzoxazine resins are almost derived from fossil resources. With the increasing exhaustion of petrochemical resources and serious environmental pollution, the conversion of renewable biomass into useful materials has received more and more attention both in academia and industry. 16-18 Up to now, varieties of benzoxazines have been already synthesized from the bio-based feedstocks such as cardanol, ACS Paragon Plus Environment

19-21


diphenolic acid,

22

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vanillin,23-25 guaiacol,26 eugenol,27-35 furfurylamine and

stearylamine.26, 29 However, the research on bio-based benzoxazine suitable for RTM process was rarely reported. Considering the diversity and specialization of bio-based chemicals, it should be possible for us to design and synthesize the RTM benzoxazine resins from renewable resources. And if the combination of bio-based resins together with advanced RTM technology was realized, the green composites can be manufactured via a greener strategy, namely using green resins and more efficient manufacturing process. That will be an evident progress towards the sustainable development of polymer composites industry. Eugenol is a relatively inexpensive (ca. $5 kg-1) bio-based aromatic compound with multi-functional groups, which can be obtained from lignin.

36

It has demonstrated

great potential to promote the development of “Green Chemistry” due to its abundant availability, unique structure and low cost. Up to now, several kinds of polybenzoxazine resins have been synthesized from eugenol or its derivatives, and they showed comparable to or better properties when compared with their petroleum-based analogues.

27-35

For example, Thirukumaran et al

27

prepared a new

kind of eugenol-based benzoxazine and the cured resin exhibited excellent dynamic mechanical properties, good thermal stability and flame retardancy. Dumas et al

28

also synthesized the polybenzoxazines from eugenol, 1, 4-phenylenediamine and phenol. Results showed that the eugenol-based polybenzoxazines possessed the glass transition temperature of 150 to 220 °C and the 5 % thermal degradation temperature in the range of 300 to 340 °C. In order to modify the eugenol-based benzoxazines, furfurylamine has also been employed as the co-monomer. After the incorporation of reactive furan groups, the thermal stability and crosslink density of the cured resin could be increased further. 31 Besides that, other chemicals including bismaleimide, 30, 35

octa(aminophenyl) silsesquioxane

32

and stearylamine, 33, 34 were also taken as the

co-monomers to functionalize or modify the eugenol-based benzoxazines. Based on the reported results, it could conclude that the eugenol-based polybenzoxazines could demonstrate a very broad spectrum of properties due to its unique structure and the molecular design flexibility of benzoxazine. ACS Paragon Plus Environment

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In this work, the soft eugenol derivatives containing silicon unit, 4, 4’-(1, 3-dipropyl-tetramethyldisiloxane) bis-2-methoxyphenol (SIE), were synthesized at first. And then a one-pot reaction was taken to prepare the main-chain type benzoxazine oligomers from SIE, furfurylamine, paraformaldehyde and different diamines (1, 2-ethylenediamine, 1, 4-butanediamine, 1, 6-hexanediamine). The structure features, processability and curing behaviors of these oligomers as well as the properties of cured resins, in terms of thermodynamic properties, thermal stability and moisture resistance, were investigated. As we know, the silicon segments usually demonstrates high molecular flexibility, low water absorption and high thermal stability. All these advantages might improve the comprehensive properties of polybenzoxazines if the silicon units were incorporated. The objective of this work is to provide us a new strategy for the synthesis of bio-based benzoxazine suitable for composites fabrication, especially via the RTM process.

2. EXPERIMENTAL SECTION 2.1. Materials. 1, 2-ethylenediamine (99 %), 1, 4-butanediamine (99 %), 1, 6-hexanediamine (99 %), isopropyl alcohol (99.5 %) and paraformaldehyde (95 %) were purchased from Zhejiang Guoguang Biochemistry Co., Ltd. Petroleum ether, 1, 4-dioxane (99.5 %), eugenol (98 %), 1, 1, 3, 3-tetramethyldisiloxane (99 %), toluene (99 %), chloroplatinic acid (99.5 %) and furfurylamine (99 %) were all obtained from Aladdin Reagent, China. All chemicals were used as received without any purification.

2.2. Measurements. Nuclear magnetic resonance spectroscopy (1H NMR and

13

C

NMR) were collected with a 400 MHz AVANCE III Bruker NMR spectrometer (Bruker, Switzerland). Deuterated chloroform and tetramethysilane (TMS) were used as the solvent and internal standard, respectively. The concentration was about 10 mM and all the samples were scanned 16 times for 1H NMR measurement and 1024 times for

13

C NMR spectra. Fourier Transform Infrared spectra (FT-IR) were recorded in

absorbance mode using a NICOLET 6700 (NICOLET, America) spectrometer. The ACS Paragon Plus Environment


samples were prepared by casting the oligomers onto a KBr plate. The spectra were recorded from 400 to 4000 cm-1 with a wavenumber resolution of 4 cm-1. 32 scans were collected for each sample. Molecular weight and molecular weight distribution were determined by a HLC-8320 (TOSOH ECOSEC, Japan) Gel Permeation Chromatograph (GPC). The standard for molecular weight calibration was mono-disperse polystyrene ranging from 2500 to 1110,000 g/mol and the eluent was THF with a flow rate of 1 ml min-1 at 40 °C. The concentration of the test sample was 1 mg/ml. Viscosity and gelation behaviors were investigated by a dynamic rheological analyzer (Anton Paar Physica MCR-301) using a parallel plate. The diameter of plate was 25.0 mm and the gap between two plates was set to be 0.30 mm. The test was conducted under steady shear mode with the heating rate of 3 °C min-1 and frequency of 1Hz. Differential Scanning Calorimetric (DSC) thermograms were recorded with a Mettler-Toledo MET DSC (METTLER TOLEDO, Switzerland). The samples were placed into an aluminum pan and heated from 25 to 300 °C at a heating rate of 20 °C min-1. The pure nitrogen was used as the protection atmosphere with the flow of 20 ml min-1. Dynamic Mechanical Analysis (DMA) measurements were carried out with a TA Instrument (TA Q800, USA) in a tension mode with an amplitude of 10 mm. All the cured resins with the dimension of 20 mm ×5 mm × 0.5 mm were tested with a heating rate of 3 °C min-1 from 0 to 200 °C at a frequency of 1 Hz. The thermal stability of cured resins was evaluated by a Thermogravimetric analysis (TGA) (Mettler-Toledo, Switzerland). Approximate 10 mg of cured resins was placed inside an alumina crucible and it was scanned from 50 to 800 °C at a heating rate of 20 °C min-1 under nitrogen or air. As for the isothermal stability measurement, the cured resins were maintained at different temperatures (250 °C, 280 °C, 300 °C) and the weight loss as a function of time was recorded for analysis. The X-ray photoelectron spectroscopy (XPS) spectra of the char residue (surface) of the cured resins were recorded with an AXIS ULTRA apparatus (Kratos, England). Water absorption was calculated based on the weight differences of swollen and dry specimens before and after they were soaked in water for a certain period. Before immersing in water, the samples were dried at 80 °C in vacuum oven for 24 h to get the constant weight. ACS Paragon Plus Environment

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Water contact angles were measured with a contact angle goniometer system (Data Physics OCA20, Germany) at room temperature. 2 µL distilled water droplet was formed using a syringe and deposited onto the sample surface. Then the syringe was moved away and the shape of water droplets was recorded by a charge coupled device (CCD) camera. The contact angle was obtained by fitting water droplet shape according to Young’s equation. In order to ensure accuracy, at least five tests were conducted for each sample and the average was reported.

2.3. Synthesis of 4, 4’-(1, 3-dipropyl-tetramethyldisiloxane) bis-2-methoxyphenol (SIE). Into a 500 ml round flask equipped with magnetic stirrer, condenser, thermometer and dropping funnel, 32.8 g (0.2 mol) of eugenol and 2.4 g of chloroplatinic acid in isopropyl alcohol (the molar ratio of double bond to Pt was 1: 6×10-5) together with 100 ml 1, 4-dioxane were mixed together. After the mixture was stirred at room temperature for 10 min., 13.4 g (0.1 mol) 1, 1, 3, 3-tetramethyldisiloxane was added dropwise into the reaction system within 2 h. Then the temperature was increased to 60 °C and maintained at this temperature for 12 h. Finally, the solvent was stilled out of the system and the product was washed by petroleum ether several times. After the residual solvent and petroleum ether were removed in the vacuum oven (-0.1 MPa) at 80 °C, the target product 4, 4’-(1, 3-dipropyl-tetramethyldisiloxane) bis-2-methoxyphenol (SIE) weighting 44.5 g was obtained (yield: 96 %). The synthetic route was illustrated in Scheme 1.

1

H-NMR (400 MHz, CDCl3, d, ppm): 6.97 (s, 1H), 6.86 (s, 1H), 6.67-6.69 (m, 4H),

3.85 (s, 6H), 2.58 (m, 4H), 1.64 (m, 4H), 0.59 (m, 4H), 0.08 (s, 12H). 13

C-NMR (400 MHz, CDCl3, d, ppm): 146.91 (s), 143.70 (s), 134.59 (s), 121.00 (s),

114.40 (s), 111.21 (s), 55.79 (s), 39.36 (s), 25.75 (s), 18.18 (s), 1.07 (s). FT-IR (KBr): 3456 cm-1 (-OH), 2955 cm-1 (-CH3), 2924, 2857cm-1 (-CH2-), 1612, 1514 cm-1 (Benzene:-C=C-), 1055 cm-1 (-Si-O-Si-)



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Scheme 1 Synthesis and chemical structures of SIE 2.4. Synthesis of main-chain type benzoxazine oligomers. The main-chain type benzoxazine oligomers were synthesized via the Mannich condensation reaction of SIE, furfurylamine and different diamines (1, 2-ethylenediamine, 1, 4-butanediamine, 1, 6-hexanediamine) in the presence of paraformaldehyde. The synthesis of different oligomers was followed the same procedure: into a 500 ml round flask equipped with a magnetic stirrer, condenser and thermometer, 15.02 g of 1, 2-ethylenediamine (0.25 mol), 48.56 g of furfurylamine (0.5 mol) and 60.06 g of paraformaldehyde (2 mol) together with 200 ml toluene were mixed together. After the mixture was stirred at room temperature for 30 min, 231.4 g SIE (0.5 mol) and 50 ml toluene were added and then it was stirred vigorously at 90 °C for 12 h. Then the toluene was stilled out of the system and the product was washed by petroleum ether several times. After the residual solvent was removed by the vacuum oven (-0.1 MPa) and the final product named as poly (SIE-ef) was obtained. When 1, 4-butanediamine or 1, 6-hexanediamine was used as the starting material, the target product was abbreviated as poly (SIE-bf) and poly (SIE-df), respectively. The synthetic route was illustrated in Scheme 2 and the formulations for different oligomers were shown in Table 1. In Table 1, the overall yields of poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were also listed.

Table 1. Feed composition, overall yield, molecular weight and bio-based content of synthesized benzoxazine oligomers Sample

Diamine

Yield

Mn

Mw

Ð

Bio-based Content

Poly(SIE-ef)

1, 2 -ethylenediamine

71%

1000

1600

1.6

64%

Poly(SIE-bf)

1, 4-butanediamine

69%

1000

1400

1.4

65%

Poly(SIE-df)

1, 6-hexanediamine

80%

1400

2400

1.7

67%


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Scheme 2 Synthesis and chemical structures of different benzoxazine oligomers

2.5. Curing process for benzoxazine oligomers. After the oligomers were degassed under vacuum oven (-0.1 MPa) at 50 °C for 20 min, they were casted onto glass plates to prepare the films with the dimension of about 20 mm ×5 mm × 0.5 mm. Then they were cured at 180 °C for 2 h, 200 °C for 2 h and 220 °C for 2 h. 10 After the curing reaction, the cured dark brown films were obtained by soaking the coated glass plates in water and the films were dried in the vacuum oven before properties investigation.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Structure characterization. The main-chain type bio-based benzoxazine oligomers were synthesized through a one-pot reaction starting from eugenol derivatives (SIE), furfurylamine, paraformaldehyde and different diamines (1, 2-ethylenediamine,1, 4-butanediamine, 1, 6-hexanediamine). As for the eugenol derivatives, the occupied ortho and para position with respect to the phenolic hydroxyl group will lead to a relatively lower cross-link density of the resulted polybenzoxazines resins.

35, 37

That was why the bio-based furfuryl amine, which

could provide extra crosslinking point during the curing reaction, was introduced. ACS Paragon Plus Environment

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The 1H NMR and 13C NMR spectra of SIE were presented in Figure 1. As shown in Figure 1(a), the characteristic peaks corresponding to the formation of SIE (C-CH2-Si and to C-CH2-C) were clearly shown at 0.59 ppm (Peak f) and 1.64 ppm (Peak e). And other signals were all assigned accordingly. In Figure 1(b), the peaks showing at 146.9, 143.7, 134.6, 121.0, 114.4 and 111.2 ppm stood for the carbons in benzene ring and the signals observed at 25.7 ppm and 18.2 ppm were attributed to the carbon atoms in -CH2-Si (Peak i) and C-CH2-C (Peak j), respectively. Figure 2 represents the FT-IR spectra of poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) in the range of 500-1800 cm-1. It was easy to notice that the different oligomers showed quite similar absorption peaks. The absorption bonds appeared at 730 and 1513 cm-1 indicated the successful incorporation of furan moieties. 38 The broad bond showing at 1059 cm-1 was assigned to the Si-O-Si group in SIE units and the absorption at 1146 cm-1 indicated the presence of C-N-C. The sharp peak observed at 1251 cm-1 was corresponded to the anti-symmetric stretching of C-O-C in oxazine ring and the characteristic absorption of oxazine ring attached on benzene was shown at 925 cm-1. In order to obtain more detailed structural information, 1H-NMR spectra for the synthesized oligomers were also shown in Figure 3. The characteristic peaks of benzoxazine corresponding to the methylene units in oxazine rings (O-CH2-N and Ph-CH2-N) were observed at 3.96-4.02 ppm and 4.91-4.96 ppm, respectively. Resonance peaks for protons on furan ring were observed at 6.26, 6.33 and 7.42 ppm. In addition, the characteristic peaks for aromatic protons were shown in the range of 6.25-6.84 ppm. The 1H NMR results were in good agreement with the FT-IR spectra, which confirmed the successful synthesis of poly (SIE-ef), poly (SIE-bf) and poly (SIE-df). The molecular weight and dispersity (Ð) of poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were determined by GPC and the results were listed in Table1. Based on the number-average molecular weight (Mn) of 1028 to 1396 g/mol, the degree of polymerization (DP) for different oligomers were calculated to be about 3-5. And in addition, the Ð was in the range of 1.4-1.7. These all indicated that the benzoxazine oligomers with defined structures were obtained successfully.


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Figure 1. 1H-NMR and 13C-NMR spectra of 4, 4’-(1, 3-dipropyl-tetramethyldisiloxane) bis-2-methoxyphenol (SIE)

Figure 2. FT-IR spectra of different synthesized benzoxazine oligomers



Figure 3. H-NMR spectra of synthesized benzoxazine oligomers

For bio-based materials, besides their thermal or mechanical properties, the bio-based content is also very important for them. In order to regulate the development of bio-based materials, several standards for the determination of bio-based content have been built. For example, United States Department of Agriculture (USDA) defined the bio-based content of a product as the amount of bio-based carbon in the product as a percentage of the weight of the total organic carbon in the product”. 17 According to this definition, the bio-based content of poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) was calculated to be 64 %, 65 % and 67 %, respectively (In table 1). Those were high enough and in line with the principle of green chemistry.

3.2. Processability Evaluation. Usually, the process temperature window of thermosetting resins, defined as the temperature difference between the melting


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temperature and gelation temperature, is taken as the key indicator for their processability evaluation. 39 In this work, the viscosity of synthesized benzoxazine oligomers was measured by the rheological analyzer and the viscosity as a function of temperature was shown in Figure 4(a). Different from the conventional benzoxazine monomers, poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were liquid at room temperature. Their viscosities were very low ( poly (SIE-bf) >poly (SIE-df). This result could be attributed to the dilution effect of different diamines. When the length of diamine was increased (from 1, 2-ethylenediamine to 1, 6-hexanediamine), the curing reactivity of resulted oligomers was decreased.


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Figure 4. Viscosity as a function of temperature with the heating rate of 3 °C min-1 (a) and viscosity as function of time at different temperatures: (b) at 100 °C; (c) at 120 °C; (d) at 140 °C and (e) at 180 °C for the synthesized benzoxazine oligomers

3.3. Polymerization behaviors. The curing behaviors of poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were monitored by DSC and FT-IR. As shown in Figure 5a, an obvious exothermic peak, associated with the oxazine ring opening polymerization, 42 was observed at 236.1-249.3 °C for all the oligomers. In the order of poly (SIE-ef) < poly (SIE-bf) < poly (SIE-df), the peak curing temperatures were increased slightly and the curing enthalpy was decreased from 39.4 J/g for poly (SIE-ef) to 29.0 J/g for poly (SIE-df). The reason might be that, from poly (SIE-ef) to poly (SIE-df), the concentration of oxazine rings per unit weight was decreased accordingly, which influenced the reactivity and curing enthalpy. This result was in a good agreement with the above rheological analysis. In order to reduce the possible degradation of benzoxazine group during curing reaction, the maximal curing temperature of 220 °C and a little longer curing period were applied, which was an usual curing condition for common benzoxazines.

10

Figure 5b represented the DSC curves for poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) after curing reaction. It was seen that no significant residual exothermic peaks were observed for all the systems, which confirmed the fully curing reaction. However, a deviation of base line was observed for all the systems when the


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temperature increased to higher than 280 °C, which might be caused by the post-curing or the thermal degradation of cured resins at a higher temperature. 33, 35

Figure 5. (a) DSC heating curves for poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) before curing reaction; (b) DSC heating curves for the cured poly (SIE-ef), poly (SIE-bf) and poly (SIE-df)

In order to further analyze the curing behaviors of poly (SIE-ef), poly (SIE-bf) and poly (SIE-df), FT-IR technology was employed to verify the ring-opening reaction and the polymerization of oxazine rings. In this work, poly (SIE-ef) was taken as the example for analysis and Figure 6 represents the FT-IR spectra of poly (SIE-ef) after accumulative curing at each stage for 2 h. Along with the curing process, the characteristic absorption for oxazine rings shown at 925 cm-1 and the C-N-C absorption appeared at 1146 cm-1 were gradually decreased. 43 After it was cured at 220 °C for 2 h, the curing reaction was finished, which was evidenced by the



unchanged FT-IR spectrum when the curing time was prolonged to 4 h. Based on the above FT-IR and DSC analysis, the curing procedures was proved to be suitable for the curing reaction. According to previously reported curing mechanism, the proposed chemical structures of cross-linked network was shown in Scheme 3. 26

Figure 6. FT-IR spectra of poly(SIE-ef) at different curing stages

Scheme 3 The possible chemical structures of final polybenzoxazine.

3.4. Thermal properties. The dynamic mechanical properties of cured poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were investigated by DMA. The curves of storage modulus and Tan δ as a function of temperature are shown in Figure 7, and the related values including the glass transition temperature (Tg) determined by the peak temperature in the Tan δ vs temperature graph, storage modulus and cross-link density are all listed in Table 2. All the cured samples demonstrated similar DMA behaviors and the storage modulus of the cured poly (SIE-ef), cured poly (SIE-bf) and cured


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poly (SIE-df) at 25 °C were found to be 1.7 GPa, 1.5 GPa and 1.0 GPa, respectively. In Figure 7b, their Tg were ranged from 73 °C to 110 °C. It was noted that both storage modulus and Tg were decreased with the length of diamine units in the resins. Compared with the polybenzoxazine synthesized from bisphenol A/F,

44, 45

demonstrating the Tg (determined by DMA Tan δ curve) of 148 °C for poly (BF-a) and 171 °C for poly (BPA-a), the modulus and Tg of cured poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were a little lower. These results were reasonable because the soft silicone units were contained in their architectures, which would lead to decreased modulus

and

Tg.

However,

when

compared

with

the

main-chain

type

polybenzoxazines reported in previous literatures, the Tg of 110 °C and modulus of 1.5 GPa were comparable to or higher. 40, 46 For example, Zhang and his coworkers reported a bio-based main-chain type polybenzoxazine with the Tg (DMA Tan δ) of 113 °C. 40 Lligadas et al synthesized three kinds of main-chain type polybenzoxazines and the highest modulus and Tg (obtained from DMA Tan δ curve) was 1706 MPa and 87 °C, respectively. 46



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Figure 7. DMA curves of cured poly(SIE-ef), poly(SIE-bf) and poly(SIE-df) The cross-link density is defined as the number of crosslink points per unit volume in the crosslinked systems. With the theory of rubber elasticity, the cross-link density (νe) could be calculated from the plateau of the elastic modulus in the rubbery state based on the following equation (1) 47, 48:

= ⁄3 1 Strictly, it was used only to make qualitative comparison of crosslinking level for the thermosetting resins. Where E’ is the storage modulus in the rubbery plateau region (E’ at the temperature of 180 oC was chosen in this work), T is the temperature in K and R is the gas constant. The calculated crosslink density data for cured poly (SIE-ef), cured poly (SIE-bf) and cured poly (SIE-df) was collected in Table 2. It could be seen that the cross-link density was decreased in the order of: cured poly (SIE-df) < cured poly (SIE-bf) < cured poly (SIE-ef). This result was in line with the above modulus and Tg values, and it was reasonable because their molecular rigid was in the order of: poly (SIE-ef) > poly (SIE-bf) > poly (SIE-df).

Table 2. DMA and TGA data for the cured benzoxazine oligomers Tga o

Cured samples

( C)

cured poly (SIE-ef) cured poly (SIE-bf) cured poly (SIE-df) a

E’b (GPa)

νec/103 (mol m−3)

T maxd (oC)

Td5%e (oC)

N2

Air

N2

Td10%f (oC) Air

N2

R800g (%)

Air

N2

110

1.7±0.2

47.0

416

416

384

384

406

405

55±2

19±2

85

1.5±0.1

43.9

414

413

384

379

401

397

49±1

17±1

73

1.0±0.1

30.6

412

411

367

370

390

392

42±1

15±1

Glass transition temperature determined by the peak temperature in the Tan δ vs temperature

graph;

Air

b

Storage modulus at 25 °C;

c

Caculated crosslink density; Temperature at which the

maximum degradation rate was observed;

e

Temperature at which the 5 % weight loss was

occurred; f Temperature at which the 10 % weight loss was occurred; g The residual char yield at 800 °C.


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The thermal stability and char yield of cured poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were investigated by thermogravimetric analysis (TGA) under nitrogen and air atmosphere. The TGA curves are shown in Figure 8 and related data is collected in Table 2. As shown in Figure 8(a), all the cured oligomers demonstrated excellent thermal stability. The values of 5 % weight loss temperature, Td5%, for the cured poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) were 384, 384 and 367 °C, respectively, and their 10 % weight loss temperature, Td10%, were 406, 401 and 390 °C. In addition, the char yields at 800 °C (R800) were ranged from 42.3 to 55.1 wt%, which reflected the intrinsic properties of polybenzoxazine resins. As for the samples heated under air atmosphere (Figure 8(b)), similar results for Td5% and Td10% were obtained except the much lower char yields, which was in the range of 15.4 % to 18.8 %. It was noted that both the key parameters for thermal stability evaluation, such as Td5% and Td10%, and the residual char yield at 800 °C (R800) all followed the order of cured poly (SIE-ef) > cured poly (SIE-bf) > cured poly (SIE-df), regardless of the protection atmosphere, nitrogen or air. This result indicated that the cured poly (SIE-ef) possessed the best thermal stability, followed by cured poly (SIE-bf), and then cured poly (SIE-df). The reason might be related to their different crosslink density and silicon content, and it was easy to understand that the char yield was increased with the content of silicon. 49, 50



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Figure 8. TGA curves for the cured benzoxazine oligomers: (a) under nitrogen; (b) under air atmosphere

Usually, the char yield is related to the flame retardancy of polymeric materials. 51 In order to preliminarily analyze the flame retardancy of cured benzoxazine oligomers, especially the relationship between the silicon content and the residual char formed after TGA treatment under nitrogen atmosphere, the X-ray photoelectron spectroscopy (XPS) was employed here to study the surface element content of the char residue. The XPS spectra were shown in Figure 9 and the element content was collected in Table 3. The content of silicon element in the char surface was as high as 40.91, 38.98 and 36.34 wt% for poly (SIE-ef), poly (SIE-ef) and poly (SIE-ef), respectively. Similar to the results reported in previous literatures,

52, 53

the silicon

atom might exist in the form of SiO2 or other oxides, which could serve as the protection layer or coatings to prevent further thermal degradation or combustion of matrix.

Table 3. Element content of the char residue determined by XPS. Elements (at. %)

O

N

C

Si

char residue of cured poly (SIE-ef)

54.37

0

4.72

40.91

char residue of cured poly (SIE-bf)

57.43

0

3.59

38.98

char residue of cured poly (SIE-df)

60.58

0

3.08

36.34


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Figure 9. XPS spectra for the formed char residues after TGA treatment

The thermal stability of cured benzoxazine oligomers were further examined by the isothermal TGA at different temperatures under nitrogen and the thermograms were shown in Figure 10. As shown in Figure 10a, when isothermally heated at 250 °C for 120 min, all the cured resins showed the similar degradation behaviors and less than 4% weight losses were observed. As reported in the literature, 54 the initial slight weight loss in polybenzoxazine networks was attributed to the Mannich base cleavage. When the temperature was increased to 280 °C, a little higher weight loss was noticed in 120 min. And a more obvious thermal degradation occurred after they were heated at 300 °C, indicated by the weight loss percentage was increased to the range of 5.3 to 7.5 wt%. Based on these results, it could conclude that all the cured resins exhibited good thermal stability under high temperature, although the slight thermal degradation was observed during the long time heating treatment. In addition, the isothermal stability of cured poly (SIE-ef) was better than that of cured poly (SIE-df), indicated by the lower weight loss percentage under the same condition, which was in good line with the above crosslink density results.



Figure 10. Isothermal TGA curves for the cured benzoxazine oligomers at different temperatures: (a) at 250°C; (b) at 280°C and (c) at 300°C

3.5. Water sorption and water contact angle. Besides the excellent mechanical and thermal properties, the low water absorption is also one of the most important features of polybenzoxazine resins. In Table 4, the water absorptions of cured poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) after being soaked in water for different periods were


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listed. As could be seen, all of the cured resins exhibited a very low water absorption in the range of 0.3 % to 0.6 % after being soaked in water for 24 h. When the soaking time was increased to 30 days, only about 0.5 % increment was noticed. That was similar to the traditional petroleum-based polybenzoxazines. Generally speaking, water absorption is mainly dependent on the free volume and intrinsic property of the polymeric resins. In this work, the hydrophobicity of siloxane moieties in the molecular architectures might make a positive contribution to the low water sorption. The higher content of the siloxane moiety in the polybenzoxazines, the lower water absorption they might have. The hydrophilic-hydrophobic property of the cured poly (SIE-ef), poly (SIE-bf) and poly (SIE-df) was further characterized by the contact angle measurements. The picture for water contact angle was shown in Figure 11 and the average values were collected in Table 4. The results showed that the water contact angles of all the cured resins were above 90o, indicating the hydrophobic surface of cured resins. Compared with the commercialized polybenzoxazines (BA-a), 55 which showed the water contact angle of less than 90°, the bio-based polybenzoxazines in this work showed a much better hydrophobic property (from 99.7° to 107.3°). That was due to the presence of siloxane moieties. Especially, due to the longest fatty chain of poly (SIE-df) among these three benzoxazine oligomers, the cured poly (SIE-df) shown the highest water contact angle.

Table 4. Water absorption and water contact angles of the cured oligomers Water sbsorption Samples

Contact angle 24h

7*24 h

30*24 h

cured poly (SIE-ef)

99.7° ± 0.4°

0.4% ± 0.1%

0.7% ± 0.1%

0.9% ± 0.1%

cured poly (SIE-bf)

105.5° ± 1.0°

0.3% ± 0.1%

0.7% ± 0.1%

0.8% ± 0.1%

cured poly (SIE-df)

107.3° ± 0.7°

0.6% ± 0.1%

1.0% ± 0.1%

1.1% ± 0.1%



Figure 11. The water contact angle of cured benzoxazines

4. CONCLUSIONS A serial of bio-based main-chain-type benzoxazine oligomers, poly (SIE-ef), poly (SIE-bf) and poly (SIE-df), were synthesized from eugenol derivatives, furfurylamine, diamine and paraformaldehyde via a one-pot reaction. The bio-based content of synthesized oligomers was determined to be higher than 60 % according to the standard built by USDA. In the temperature range from 60 to 190 °C, the viscosity of all the oligomers was lower than 1 Pa.s and remained very stable. Even isothermally heated at 120 °C for 2 h, their viscosity was still remained as low as about 1 Pa.s, which indicated their excellent processing performances and the suitability for RTM technology. The cured resins showed good thermal and mechanical properties, as well as extremely low moisture absorption. Considering the comprehensive properties of these oligomers, they could be used as diluent or toughening agents for the traditional RTM resins. Alternatively, in the field that do not require high Tg, they also could be used as the green matrix resins for the fabrication of fiber reinforced composites via a greener strategy, namely using bio-based resins and more efficient RTM process.

NOTES The authors declare no competing financial interest. *Corresponding authors: [email protected] (Xiaoqing Liu)


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ACKNOWLEDGEMENTS The authors are grateful for the financial support from National Natural Science Foundation of China (Grant No.51373194), National Key Technology Support Program (2015BAD15B08) and the project co-funded by Chinese MIIT Special Research Plan on Civil Aircraft with the Grant No. MJ-2015-H-G-103.

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TOC


Synthesis of Biobased Benzoxazines Suitable for Vacuum-Assisted

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