Novel Fully Biobased Benzoxazines from Rosin - American Chemical

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10682-10692

Novel Fully Biobased Benzoxazines from Rosin: Synthesis and Properties Xiaoyun Liu,*,† Ruhong Zhang,† Tianquan Li,† Pengfei Zhu,† and Qixin Zhuang*,‡ The Key Laboratory of Advanced Polymer Materials of Shanghai, School of Materials Science and Engineering and ‡Key Laboratory of Specially Functional Polymeric Materials and Related Technology, Ministry of Education, East China University of Science and Technology, No. 130, Meilong Road, Shanghai 200237, China

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ABSTRACT: In the present study, two novel fully biobased benzoxazines were synthesized using modified products of rosin, dehydroabietylamine, guaiacol, 4-methylumbelliferone, and paraformaldehyde. The chemical structures of DM (dehydroabietylamine, 4methylumbelliferone) and DG (dehydroabietylamine, guaiacol) were characterized by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR) and 13C NMR spectra, dimensional nuclear magnetic and high-resolution mass spectrometry. The curing process of DM and DG was monitored by differential scanning calorimetry and in situ FTIR. The results demonstrated that the corresponding polymers PDM and PDG had high thermal stability. In addition, they had low dielectric constants below 3.30 at 25 °C and 1 kHz condition. Water contact angle measurements, OPC-time curves, and Tafel plots of PDG and PDM were also studied. The results showed that anticorrosion performances of PDG and PDM were stable during the immersion process and had strong abilities as shield corrosive media. Therefore, these two benzoxazine resins based on dehydrobietylamine may have potential applications in many fields. KEYWORDS: Benzoxazine, Rosin, Biobased resin, Dielectric properties, Anticorrosion

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oil, and the resulting benzoxazines with a long flexible alkyl chain have been synthesized.24,25 Even though these types of monomers suffer from a very low glass transition temperature (Tg) and the side alkyl chains hinder the reaction of difunctional monomers, they can be used as diluents for epoxy.26 Eugenol can be extracted from clove and the fabricated polybenzoxazines with allyl,27−29 and it can participate in ringopening polymerization to enhance the cross-linking density of the system.28 Guaiacol is a lignin derivative that is abundant in nature,30−32 and furfurylamine and stearylamine can be derived from furfural and vegetable oil, respectively. Guaiacol condensed with paraformaldehyde and furfurylamine or stearylamine can make two monomers with the copolymer showing better thermal stability than those of the pure polymers.30 Vanillin is a flavor that can be manufactured from lignin.33−35 Vanillin-based benzoxazine monomers contain an aldehyde group. With the help of the aldehyde group, the curing temperature of monomers can be decreased; however, it can generate carbon dioxide.34 Castor oil derivatives, coumarin derivatives, jute fiber, card bisphenol, arbutin, and other green materials have also contributed to the manufacturing of benzoxazines.31,36−41

INTRODUCTION Benzoxazine, as a type of phenol derivative, can be synthesized by amine and paraformaldehyde via the Mannich condensation reaction.1,2 The benzoxazine ring can cross-link via thermal cationic ring-opening without any initiator or catalyst.3,4 The corresponding polymer, polybenzoxazine, has attracted significant interest in the scientific and industrial communities for its excellent properties, such as high thermal stability, mild curing conditions, near-zero curing shrinkage, good mechanical properties, low water absorption, low dielectric, flame retardancy, and high molecular design flexibility.4−12 Therefore, benzoxazine resins can be used in electronics, composites, aerospace, and other fields.13 Taking advantage of their molecular design flexibility, a series of polybenzoxazines with well-defined properties have been synthesized over the past few decades.10,14,15 Blending polybenzoxazines with other high performance polymers or inorganic particles is also a good method for improving the performance of resins.16−23 However, the majority of benzoxazines are based on petroleum resources. The global energy crisis and current environmental problems require materials scientists to focus on green chemistry. Over the past years, a great number of petroleum-based polymers have been replaced by biobased renewable polymers. Using the molecular design flexibility of benzoxazines, renewable phenolic and amino-based compounds have also been used to synthesize biobenzoxazines. For instance, cardanol has been isolated from cashew nut shell © 2017 American Chemical Society

Received: August 2, 2017 Revised: September 22, 2017 Published: October 12, 2017 10682

DOI: 10.1021/acssuschemeng.7b02650 ACS Sustainable Chem. Eng. 2017, 5, 10682−10692

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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of DG and DM

Revisiting the history of biobased benzoxazines, a large number of renewable phenol derivatives have been used to synthesize benzoxazine monomers, although the number of bioamines is deficient. Renewable amines, which are suitable for benzoxazines, are relatively rare. From the history of biobased benzoxazines, renewable multiamines and chitosan have been reported only by a few studies.42,43 Furfurylamine and stearylamine are popular with researchers, and they have been synthesized from many different renewable phenolic compounds. In addition, it has been reported that the furan ring of furfurylamine can react and improve the cross-linking density of the system.28,30,44,45 Therefore, the study of biobased benzoxazine is important. Rosin is a nonvolatile natural resin originating from pine resin. It is an abundantly renewable resource and is often utilized in industries for preparing varnishes, paper sizing agents, adhesives, odorants, alkyd resins, binders for printing inks, and so on.46 In addition, rosin has been utilized to synthesize a number of high-performance thermosetting resins, such as epoxy resin, phenolic resin, vinyl ester resin, and polyurethane.47−49 Rosin was first used in benzoxazine by Min Tao et al.50 The authors utilized rosin acid to synthesize maleopimaric acid (MPAIP), and MPAIP played the role of phenol compounds.50 However, this benzoxazine is not a real biobased benzoxazine because MPAIP is not renewable. Dehydroabietylamine is one of the important modified products of rosin and the main component of disproportionated rosin amine. It has an optical active base on its tricyclic phenanthrene structure. In recent years, with the improvement of extraction and purification technology, it is expected that dehydroabietylamine will be widely utilized in the field of photochemical resolution, and its application in the field of fine processing of metal processing, beneficiation, surfactant, dye, coating, medicine, and pesticides will be further strengthened. To the best of our knowledge, it has never previously been used in benzoxazines. In the present study, we describe two novel fully biobenzoxazines, DM (dehydroabietylamine, 4-methylumbelliferone) and DG (dehydroabietylamine, guaiacol), based on dehydroabietylamine. They are characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FTIR), and high-resolution mass

spectrometry (HRMS). The curing process and thermal properties were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). In addition, the properties of corresponding polybenzoxazines were also studied, including the dielectric constant, water contact angle, and anticorrosion performance.

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EXPERIMENTAL SECTION

Materials. Guaiacol (99%), 4-methylumbelliferone (99%), dehydroabietylamine (90%), dioxane (AR grade), ethanol (AR grade), sodium bicarbonate (99%), and paraformaldehyde (95%) were purchased from Aladdin Reagent (Shanghai) Co., Ltd., China. CClD3 and acetone-d6 with 0.03 vol % of tetramethylsilane (TMS) as the reference were obtained from Meryer (Shanghai) Chemical Technology Co., Ltd. All chemicals were used as received without any purification. Synthesis of DG. Under condensed reflux conditions, guaiacol (10 mmol, 1.24 g), paraformaldehyde (20 mmol, 0.6 g), dehydroabietylamine (10 mmol, 3.1667 g), and dioxane (50 mL) were placed into a 100 mL round-bottomed flask (Scheme 1). The mixture was stirred at 70 °C until it was completely dissolved, and then the mixture was heated to 85 °C and maintained at this temperature for 20 h. After cooling to room temperature, the crude product was concentrated under reduced pressure to remove dioxane, and the residual solid was washed with 1% NaHCO3 and deionized water several times to remove mainly the unreacted guaiacol. The product was purified by recrystallizing in ethanol to remove dehydroabietylamine and byproducts. The resulting powdery brown solid was dried at 60 °C in a vacuum. The yield was 64%, and the melting point was 104 °C. FTIR (KBr, cm−1): 2930, 2865, 1678, 1584, 1463, 1386, 1263, 1223, 1100, 1018, 932. 1H NMR (acetone-d6, ppm): 7.19−6.58, 4.77, 3.97, 3.80, 2.90−2.81, 2.30, 1.94−1.36, 1.20, 0.89. 13C NMR (acetone-d6, ppm): 148.47−110.15, 84.94, 62.43, 55.17, 53.95, 44.02, 38.76−33.40, 25.17, 23.52, 19.34−18.2. Synthesis of DM. Under condensed reflux conditions, 4methylumbelliferone (10 mmol, 1.76 g), paraformaldehyde (20 mmol, 0.6 g), dehydroabietylamine (10 mmol, 3.1667 g), and dioxane (50 mL) were placed into a 100 mL round-bottomed flask (Scheme 1). The mixture was stirred at 70 °C until it was completely dissolved, and then the mixture was heated to 90 °C and maintained at this temperature for 20 h. After cooling to room temperature, the crude product was concentrated under reduced pressure to remove dioxane, and the residual solid was washed with 1% NaHCO3 and deionized water several times to remove unreacted guaiacol. The product was purified by recrystallizing in ethanol to remove dehydroabietylamine and byproduct. The resulting yellow spherical crystals were dried at 60 10683


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ACS Sustainable Chemistry & Engineering °C in a vacuum. The yield was 70%, and the mp was 131 °C. FTIR (KBr, cm−1): 2930, 2865, 1730, 1610, 1506, 1447, 1386, 1263, 1223, 1146, 1026, 1016, 900. 1H NMR (acetone-d6, ppm): 7.54−6.75,612, 4.89, 4.14, 2.90−2.81, 2.43, 1.94−1.36, 1.20, 0.89. 13C NMR (acetoned6, ppm): 159.94, 158.15, 153.37, 151.86, 147.38−108.38, 112.86, 85.64, 62.89, 48.15, 44.2, 38.50−33.52, 25.10, 23.50, 19.08−18.00. Preparation of Corresponding Polybenzoxazines. The two benzoxazine monomers, DG and DM, were dissolved in dimethylformamide, and the solution was poured into ceramic models. Then, the ceramic models were placed into a vacuum drying oven at 80 °C for 24 h to remove the solvent. After that, they were cured in a tube furnace, and the steps were as follows: 100 °C (1 h), 150 °C (1 h), 180 °C (1 h), 200 °C (1 h), and 230 °C (1 h). Thereafter, samples were cooled to room temperature, and the resulting polymers were named PDG and PDM, respectively. Preparation of PDG and PDM Coatings. Prior to coating, Q2345A low-carbon steel (1 cm × 1 cm × 0.2 cm) was blasted by a milling machine (Sa 2 1 ) and washed with acetone to remove rust and 2 degrease. DG and DM were dissolved in N,N-dimethylformamide with a concentration of 250 g/L. Clean Q2345A was dipped into the solutions with a withdrawing speed of 300 mm/min for 1 min. The coated samples were placed overnight and cured according to the curing process. The thickness of the coatings was estimated to be around 5 μm. Characterization. FTIR spectra were measured with a Bruker Vector 22 FTIR analyzer. All samples were finely ground with KBr powder and pressed into a thin disk. In situ FTIR were taken in absorbance mode to monitor the curing process. 1H NMR and 13C NMR spectra were recorded with an NMR spectrometer (Bruker, 400 MHz) using CClD3 and acetone-d6 as the solvent and TMS as the internal standard. Dimensional nuclear magnetic (HSQC) of DM and DG were provided to attributed peaks more clearly. HRMS uses an LTQ/Orbitrap Elite equipped with an atmospheric pressure matrixassisted laser desorption/ionization source (AP-MALDI PDF+, Masstech). The MALDI-produced ions can be investigated by accurate mass measurement (subppm relative error) and structural confirmation by MSn. DSC was carried out on an Instruments DSC Model 2920. Both samples were heated at a rate of 10 °C/min under a nitrogen flow of 50 mL/min, and an indium standard was used for calibration. The Tg of the corresponding polybenzoxazine was studied by DSC in the temperature range of 25−300 °C. TGA was carried out on a TA Instrument Model 2050 in the range between 20 °C and up to 800 °C with a heating rate of 10 °C/min under nitrogen flow of 40 mL/min. The dielectric properties of the corresponding polybenzoxazines were analyzed with a Concept 40 Broadband dielectric analyzer at 25 and 200 °C. Coatings of polybenzoxazines on slides and aluminum sheets were prepared to study their anticorrosion properties. Water contact angles were measured with a Contact Angle Measuring Instrument JC2000D2. OPC-time curves and Tafel plots were studied with an Electrochemical Workstation CHI 660D.

Figure 1. FTIR spectra of DG and DM.

FTIR of DG, DM also showed absorption peaks at 2930, 2865, 1263, and 1026 cm−1. The absorption peak of the oxazine ring was also found at 932 cm−1. In addition, the stretching vibration of CO of the coumarin moiety appeared at 1730 cm−1.3,37 Figure 2 shows the 1H NMR spectra of DG and DM. The characteristic protons of the oxazine ring of DG appeared at 4.77 ppm (O−CH2−N) and 3.97 ppm (Ar−CH2−N). The chemical shifts in the range of 7.19−6.58 ppm belonged to the aromatic ring. The chemical shift at 3.80 ppm corresponded to the methoxy group. Similar results were found in the 1H NMR of DM. The oxazine ring could be proven by the chemical shifts at 4.89 and 4.14 ppm. The singlet at 6.12 ppm was produced by the CHC of the coumarin unit. Figure 3 shows the 13C NMR spectra of DG and DM. The chemical shifts of the aromatic ring of DG appeared between 148.47 and 110.15 ppm. The 55.17 ppm was generated by the methoxy of guaiacol of DG. However, the 158.15, 153.37, and 147.38−108.38 ppm in the 13C NMR spectra of DM belonged to the chemical shifts of the aromatic protons. The 159.94, 151.86, and 112.86 ppm were given by α-pyrone of the coumarin unit.3 In addition, the characteristic protons of the oxazine ring appeared at 84.94 ppm (-O−CH2−N-) and 53.95 ppm (Ar−CH2−N-) for DG and 85.64 ppm (-O−CH2−N) and 48.15 ppm (Ar−CH2−N-) for DM. Figure 4 shows the HSQC of DG and DM. It proves that the previous analysis was correct and that two new biobased benzoxazines were successfully synthesized. The HRMS of DG and DM are shown in Figure 5. The theoretical molecular weights of DG and DM were 433.63 and 485.66, respectively. Figure 5 shows that the biggest molecular mass peaks of DG and DM were 433.29 and 485.56, respectively, which are consistent with theoretical values. Because the highest m/z peak appears in the middle of the picture, the compound may consist of two stable structures linked by weak bonds. The difference between 433 (DG) and highest m/z 178 is 255, which is the same as the difference between 485 (DM) and highest m/z 230. This shows DG and DM lost the same fragments. The difference of 255 just equals the main part of dehydroabietylamine. Therefore, the C−CH2− N may be the weak bond and broken off during the measuring process. The molecular weight of guaiacol and 4-methylumbelliferone are 124 and 176 g/mol, respectively. This shows no residual reagents in benzoxazine monomers. The results of

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RESULTS AND DISCUSSION Characterization of DG and DM. Figure 1 shows the FTIR of DG and DM. DG showed characteristic absorption peaks at 2930 and 2865 cm−1, which were assigned to symmetric stretching and asymmetric stretching of methyl groups, respectively.28 The stretching vibration of the C−O−C of the methoxy group appeared at 1463 cm−1. In addition, the characteristic absorption peaks of -NH2 (3421 cm−1) of dehydroabietylamine disappeared, and some new absorption peaks appeared. The symmetric and asymmetric stretching of the C−O−C of the oxazine ring was observed at 1263 and 1018 cm−1, respectively.35,36 The characteristic peaks at 1100 cm−1 corresponded to symmetric C−N−C stretching. The peak of C−H of benzene with an attached oxazine ring appeared at 932 cm−1 for its out-of-plane deformation mode.33 These proved that the oxazine rings formed. Similar to the 10684


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ACS Sustainable Chemistry & Engineering

Figure 2. 1H NMR spectra of DG and DM.

NMR, FTIR, and HRMS analysis prove that dehydroabietylamine-based benzoxazine monomers were successfully synthesized. Curing Behavior of the Biobased Benzoxazine Monomers. The curing behaviors of DG and DM were studied by DSC (Figure 6) and in situ FTIR (Figure 7). The bulky amine moiety will not degrade during the cure.47,50 As shown in Figure 6 and Table 1, the endothermic peaks at 104 and 133 °C were assigned to the melting points of DG and DM, respectively. The exothermic peaks demonstrate the ringopening polymerization of oxazine rings. The onset and maximum ring-opening temperatures of DG were 180 and 225 °C, respectively. The values for DM were 198 and 228 °C, respectively. Comparing DG and DM, the onset curing temperature of DM was higher. The possible reason for this is that the α-pyrone of the coumarin unit has bigger steric

hindrance. However, the peak curing temperatures of DG and DM are almost equal. A similar phenomenon has been reported by Allen et al.52 These authors found that the phenol substitution had a large influence on the melting temperatures of biobased benzoxazine monomers but no effect on the ringopening polymerization.51,52 The results presented above are consistent with these studies. In addition, the peak curing temperatures of DG and DM were slightly lower than those of most other petrol-based benzoxazines, which occur between 220 and 260 °C.4−7 The possible reason for this is that phenyl substitution stimulated by the positioning effect of the methoxy group may take place when DG is cured.46 Meanwhile, the coumarin moiety of DM may self-catalytically cure DM.3 The processing windows of DG and DM were 76 and 65 °C, respectively, which means they have good processing performance. 10685


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Figure 3. 13C NMR spectra of DG and DM.

phenanthrene ring is an electron donating group, which is stronger than the benzene ring and furan ring. In situ FTIR was utilized to further clarify the curing process of DG and DM. As shown in Figure 7, the characteristic peaks of the benzoxazine ring of DG, which were at 1263 cm−1, 1018 cm−1 (asymmetric and symmetric stretching of C−O−C), 1100 cm−1 (symmetric stretching vibrations of C−N−C), and 932 cm−1 (out-of-plane bending of C−H) decreased. The in situ FTIR of DM showed similar changes, which indicates the occurrence of ring-opening reactions in DG and DM. Thermal Properties of Polybenzoxazines. After curing DG and DM, the corresponding polybenzoxazines named PDG and PDM, respectively, were developed. First, the Tg of the polybenzoxazines were studied by DSC (Figure 8). From the DSC curves, the Tg values of PDG and PDM were 124 and 207 °C, respectively. As is well-known, both the rigidity of the

Compared to other biobased benzoxazines synthesized from the same phenol compound but different amine compounds, DG and DM had lower curing temperatures. The onset curing temperature of biobased benzoxazine Bzs, synthesized from guaiacol and stearylamine, was 232 °C.30 The peak curing temperature of biobased benzoxazine Bzf, synthesized from guaiacol and furfurylamine, was 240 °C.30 The curing of benzoxazine Mu-a, synthesized from 4methylumbelliferone and aniline, began at 229 °C and showed the endothermic peak at 232 °C.3,38 The possible reason for these differences is due to the structure of dehydroabietylamine. Electronic effects can influence curing temperatures and it has been observed that amine components with electron withdrawing groups result in higher curing temperatures.39 Dehydroabietylamine has a large phenanthrene ring and the 10686


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Figure 4. HSQC spectra of DG and DM.

The thermal stabilities of PDG and PDM were studied by TGA. The results are shown in Figure 9 and summarized in Table 2. From Table 2, T5, T10, and char yields at 800 °C of PDM were higher than those of PDG, which means PDM was more thermally stable. The possible reason for this is that the αpyrone of PDM is more thermally stable. In addition, as shown in Table 2, limiting oxygen index (LOI) values of both PDG and PDM were over 30. Meanwhile, from Figure 9, the weight of PDG and PDM displayed no changes below 300 °C, which indicates they were more thermally stable than other biobased polybenzoxazines. This means that they have good potential application in materials used below 300 °C. It is well-known that most biological amines are defective for the use of synthesizing benzoxazines due to their poor thermal stability. However, in the present study, we discovered that the thermal stability of dehydroabietylamine-based polybenzoxazines were better than most other biopolybenzoxazine, such as stearylamine-based polybenzoxazines. Dehydroabietylamine is

polymer chain and the cross-linking density can influence the Tg value.44 Because there is only one benzoxazine ring in the structure, these resins should be mainly linearly polymerized without formation of a massive cross-linked network. The possible reason that the Tg of PDM was higher than that of PDG is that the α-pyrone of coumarin causes higher rigidity. To discuss the effect of structure on Tg, we compared Tg values of PDG and PDM with other polybenzoxazines that have similar chemical structures. Comparing Tg values of PDG to PBzs (Tg = 82 °C) and PBzf (Tg = 148 °C), it was found that Tg (PBzf) > Tg (PDG) > Tg (PBzs).30 The possible reason for this is that the furfurylamine group improves the cross-linking density of polybenzoxazines, whereas the dehydroabietylamine group has no effect on the cross-linking density. Therefore, Tg of PDG is higher than PBzs and lower than PBzf. Comparing Tg of PDM and PMu-a (Tg = 170.2 °C),3,38 the rigidity of PDM was higher, which might be attributed to the huge phenanthrene ring structure of PDM. 10687


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Figure 5. HRMS of DG and DM.

abundant in the natural world; therefore, dehydroabietylamine is a good choice as a biological source amine to use in biobased benzoxazines. Dielectric Properties of PDG and PDM. There are methods for reducing the dielectric constant of the polymer, e.g., increase the free volume of polymer material, introduction of fluorine atoms, and becoming nanoporous. Short side chains, flexible bridge structures, and large groups that can limit the mutual attraction between chains can increase the free volume of the polymer. Increasing the free volume of the polymer can reduce the number of polar groups per unit volume and reduce the dielectric constant.53 Figure 10 shows the dielectric constant of PDG and PDM at 25 and 200 °C. At 25 °C and 1 kHz, the dielectric constants of PDG and PDM were 3.30 and 3.15, respectively. The values are comparable with some modified polybenzoxazines and other resins. The dielectric

constant of these materials is usually in the range of 2.5 to 4.54−56 In addition, at 200 °C, PDG and PDM still maintained low dielectric constants. We presume that the giant phenanthrene ring structure increases the free volume of polybenzoxazines and that the phenanthrene ring is an electronics supply group that can reduce the polarity of the C−N bond. In addition, the structure of 4-methylumbelliferone is larger than guaiacol, which may result in the lower dielectric constant of PDM. Lower dielectric constants mean PDG and PDM have potential applications in the field of integrated circuits. Anticorrosion Performance of PDG and PDM Coatings. Slides and aluminum sheets were chosen to obtain coatings on different substrates. Figure 11 provides pictures of benzoxazines coatings and corresponding polybenzoxazines coatings on slides and aluminum sheets. DG and DM were 10688


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Figure 6. DSC curves of DG and DM.

Figure 8. DSC curves of PDG and PDM for Tg determination.

Figure 9. TGA curves of PDM and PDG.

Table 2. TGA Results of PDG and PDM

a

sample

T5a (°C)

T10b (°C)

char yield (%)

LOI values

PDG PDM

321 362

358 405

24 32

30.1 33.3

T5: 5% weight loss temperature. bT10: 10% weight loss temperature.

Figure 7. In situ FTIR of DG and DM.

Table 1. DSC Data of DG and DM Monomers at a Heating Rate of 10 °C/min monomer

Tm (°C)

Tonset (°C)

Tmax (°C)

ΔH (J/g)

DG DM

104 133

180 198

225 228

173.5 152.7

Figure 10. Relationship between the dielectric constant and frequency of PDG and PDM at different temperature conditions.

dissolved in N,N-dimethylformamide. Solvent was removed under 110 °C to easily obtain uncured benzoxazine coatings DG-S, DG-A, DM-S, and DM-A, and the coatings were cured

as in the previously mentioned curing step. The resulting polybenzoxazines coatings were named PDG-S, PDG-A, PDMS, and PDM-A. 10689


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Figure 11. Pictures of DG and DM monomer coatings obtained on slides (DG-S and DM-S) and aluminum sheets (DG-A and DM-A); pictures with corresponding capital letters represent the cured polymer.

The water contact angles of polybenzoxazines coatings were measured at 25 °C. From Figure 12, water contact angles of

Figure 13. Anticorrosion performance for origin low-carbon steel and coated low-carbon steel in 3.5 wt % sodium chloride solution: (a) OPC-time curves and (b) Tafel plots.

267 mV, respectively. After immersing in sodium chloride solution for 6000 s, the EOCP of Q2345A, PDG-A, and PDM-A samples decreased to −0.625, −0.435, and −0.420 V vs Ag/ AgCl, respectively. The differences of EOCP between the coated low-carbon steel (PDM-Q and PDG-Q) and original lowcarbon steel (Q2345A) were still over 180 mV vs Ag/AgCl. The value is comparable to those of some modified polybenzoxazines.57 These results show that the anticorrosion performance of PDG and PDM was stable during the immersion process and had strong ability as a shield corrosive media. From Figure 13b, Tafel plots also provided corrosion potential (E corr ) and corrosion current (I corr ) by the extrapolation method. The Ecorr of Q2345A, PDG-Q, and PDM-Q samples were −0.675, −0.49, and −0.478 V vs Ag/ AgCl. The results show that PDG and PDM have good anticorrosion performance. Furthermore, Icorr of Q2345A, PDG-Q, and PDM-Q were 4.04, 0.88, and 0.65 μAcm−2, respectively. This result also demonstrates that the coating can effectively reduce the electrochemical corrosion of low-carbon steel. Organic coatings are not impenetrable, and coatings with fewer micropores prepared from higher density polymer could inhibit the essential step in metal corrosion. Polybenzoxazine has the advantages of near-zero curing shrinkage and low water absorption.58 It also has been reported that some polybenzoxazines have similar anticorrosion performances.57−59 In this study, the possible reason that PDG and PDM behave with good anticorrosion performances might be because of the lower water resistance and good adhesion to steel for PDG and PDM, which are beneficial for the long-term use of coatings.

Figure 12. Contact angle measurements (water) of samples, slides, and aluminum sheets.

PDG-S, PDG-A, PDM-S, and PDM-A were around 100°. They are larger than that of slides and aluminum sheets because the phenanthrene ring of polybenzoxazine acts as an aliphatic group. Furthermore, methoxy of guaiacol and CO groups of coumarin units can introduce additional intramolecular hydrogen bonding with phenolic OH groups. The anticorrosion performances of PDG and PDM were studied by measuring OPC-time curves and Tafel plots of lowcarbon steel (Q2345A), PDG-coated Q2345A (PDG-Q), and PDM-coated Q2345A (PDM-Q) low-carbon steel in 3.5 wt % sodium chloride solution over different times. From Figure 13a, the initial EOCP of Q2345A, PDG-Q, and PDM-Q were −0.590, −0.415, and −0.323 V vs Ag/AgCl, respectively. In contrast to Q2345A, the EOCP of PDG-A and PDM-A increased by 175 and 10690


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CONCLUSION In the present study, we explored two novel fully biobased benzoxazines, DM and DG, using dehydrobietylamine (an important modified product of rosin), guaiacol, 4-methylumbelliferone, and paraformaldehyde. The chemical structures of DM and DG were characterized by FTIR, 1H NMR and 13C NMR spectra, and HRMS. The curing process of DM and DG was monitored by DSC and in situ FTIR. The results showed that the peak curing temperatures of DG and DM were 227 and 225 °C, respectively. The properties of the corresponding polybenzoxazines (PDM and PDG) were also investigated. The PDG revealed Tg at 124 °C, and the Tg of PDM was 207 °C. The 5% weight loss temperature of PDG was 321 °C, which is lower than that of PDM (T5 = 362 °C), which means both PDM and PDG have high thermal stability. In addition, PDG and PDM have low dielectric constants at 25 °C (1 kHz, dielectric constants of PDG and PDM were 3.30 and 3.15, respectively). The initial EOCP of Q2345A, PDG-Q, and PDMQ were −0.590, −0.415, and −0.323 V vs Ag/AgCl, respectively. After immersing in sodium chloride solution for 6000 s, the differences of EOCP among PDM-Q, PDG-Q, and Q2345A were still over 180 mV vs Ag/AgCl. Tafel plot tests showed the Ecorr of Q2345A, PDG-Q, and PDM-Q were −0.675, −0.49, and −0.478 V, respectively. These results demonstrate that the anticorrosion performances of PDG and PDM are stable during the immersion process and have strong abilities as shield corrosive media. Therefore, these two benzoxazine resins based on dehydrobietylamine may have applications in many fields.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiaoyun Liu: 0000-0002-9838-1109 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Shanghai Natural Science Foundation (16ZR1407700), National Natural Science Foundation of China (51573045 and 51773060), and Shanghi Scient ific and Technological Innovation Project (16520722000).

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Novel Fully Biobased Benzoxazines from Rosin - American Chemical

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