Novel Fully Biobased Benzoxazines from Rosin: Synthesis and

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Novel fully bio-based benzoxazines from rosin: synthesis and properties Xiaoyun Liu, Ruhong Zhang, Tianquan Li, Pengfei Zhu, and Qixin Zhuang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02650 • Publication Date (Web): 12 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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ACS Sustainable Chemistry & Engineering

Novel fully bio-based benzoxazines from rosin: synthesis and properties Xiaoyun Liu1*, Ruhong Zhang1, Tianquan Li1, Pengfei Zhu1, Qixin Zhuang2* 1.The Key Laboratory of Advanced Polymer Materials of Shanghai, School of Materials Science and Engineering, East China University of Science and Technology, No. 130, Meilong Road, Shanghai 200237, China Xiaoyun Liu. Email: [email protected] 2. 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 Qixin Zhuang. Email: [email protected]

KEYWORDS. Benzoxazine, rosin, bio-based resin, dielectric properties, anticorrosion In the present study, two novel fully bio-based benzoxazines were synthesized using modified products of rosin, dehydroabietylamine, guaiacol, 4-methylumbelliferone, and paraformaldehyde. The chemical structures of DM (Dehydroabietylamine, 4-methylumbelliferone) and DG (Dehydroabietylamine, guaiacol) were characterized by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR) and 13C NMR spectra, dimensional nuclear magnetic (HSQC) and high-resolution mass spectrometry (HRMS). 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

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had high thermal stability. In addition, they had low dielectric constants below 3.30 at 25 oC 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.

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 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 to improve 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 bio-based 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 oil and the resulting


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benzoxazines with long flexible alkyl chain 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 di-functional 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 the ring-opening polymerization to enhance the cross-linking density of the system.28 Guaiacol is a lignin derivative that is abundant in nature,30-32 while 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 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 Revisiting the history of bio-based benzoxazines, a large number of renewable phenol derivatives have been used to synthesize benzoxazine monomers, while the number of bio-amines is deficient. Renewable amines, which are suitable for benzoxazines, are relatively rare. From the history of bio-based benzoxazines, renewable multi-amines 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 a lot of 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 bio-based benzoxazine is important. Rosin is a non-volatile 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



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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 bio-based benzoxazine since 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 pesticide will be further strengthened. To the best of our knowledge, it has never been used in benzoxazines before. In the present study, we describe two novel fully bio-benzoxazines 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. EXPERIMENTAL SECTION Materials Guaiacol (99%), 4-methylumbelliferone (99%), dehydroabietylamine (90%), dioxane (AR grade), ethanol (AR grade), sodium bicarbonate (99%), and paraformaldehyde (95%) were purchased


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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 oC until it was completely dissolved and then the mixture was heated to 85 oC 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 by-products. The resulting powdery brown solid was dried at 60 oC in a vacuum. The yield was 64% and the melting point was 104 oC. FTIR (KBr, cm1

): 2930, 2865, 1678, 1584, 1463, 1386, 1263, 1223, 1100, 1018, 932.

1

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.

13

H NMR (acetone-d6, C 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.

Scheme 1 Synthesis of DG and DM



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Synthesis of DM Under condensed reflux conditions, 4-methylumbelliferone (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 oC until it was completely dissolved and then the mixture was heated to 90 oC 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 by-product. The resulting yellow spherical crystals were dried at 60 oC in a vacuum. The yield was 70% and the mp. was 131 oC. 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.

13

C NMR

(acetone-d6, 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 poured into ceramic models. Then, the ceramic models were placed into a vacuum drying oven at 80 oC for 24 h to remove the solvent. After that, they were cured in a tube furnace and the steps were as follows: 100 oC (1 h), 150 oC (1 h), 180 oC (1 h), 200 oC (1 h), and 230 oC (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 ଵଶ ) and washed with acetone to remove rust and degrease. DG and DM were


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dissolved in N, N-Dimethylformamide with concentration of 250 g/L. The clean Q2345A was dip into the solutions with a withdrawing speed of 300 mm/min for 1min. The coated samples were placed overnight and cured according to the curing process. The sickness of the coatings was estimated to be around 5 µm. Characterization FTIR spectra were measured with a Bruker Vector 22 FTIR analyser. All samples were finely ground with KBr powder and pressed into a thin disk. In situ FTIR were taken in the absorbance mode to monitor the curing process. The 1H NMR and

13

C NMR spectra were recorded with a

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 a LTQ/Orbitrap Elite equipped with an atmosphericpressure matrix-assisted 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 oC/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 oC. TGA was carried out on a TA Instrument Model 2050 in the range between 20 oC and up to 800 oC with a heating rate of 10

o

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 oC and 200 oC. 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.



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RESULTS AND DISCUSSION Characterization of DG and DM Fig. 1 shows the FTIR of DG and DM. DG showed characteristic absorption peaks at 2930 cm-1 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 cm-1 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 FTIR of DG, DM also showed absorption peaks at 2930 cm-1, 2865 cm-1, 1263 cm-1, and 1026 cm1

. The absorption peak of the oxazine ring was also found at 932 cm-1. In addition, the stretching

vibration of C=O of coumarin moiety appeared at 1730 cm-1.3,37

Fig. 1 FTIR spectra of DG and DM Fig. 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 at


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the range of 7.19–6.58 ppm belonged to the aromatic ring. The chemical shift at 3.80ppm corresponded to the methoxy group. Similar results were found in the 1H NMR of DM. The oxazine ring could be proved by the chemical shifts at 4.89 ppm and 4.14 ppm. The singlet at 6.12 ppm was produced by the CH=C of the coumarin unit.

Fig. 2 1H NMR spectra of DG and DM Fig. 3 shows the

13

C NMR spectra of DG and DM. The chemical shifts of the aromatic ring of

DG appeared between 148.47 ppm and 110.15 ppm. The 55.17 ppm was generated by the methoxy of guaiacol of DG. However, the 158.15 ppm, 153.37 ppm, and 147.38–108.38 ppm in



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the

13

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C NMR spectra of DM belonged to the chemical shifts of the aromatic protons. The 159.94

ppm, 151.86 ppm, 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 (ArCH2-N-) for DG and 85.64 ppm (-O-CH2-N) and 48.15 ppm(Ar-CH2-N-) for DM.

Fig. 3 13C NMR spectra of DG and DM


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Fig. 4 shows the HSQC of DG and DM. It proves that the previous analysis was correct and two new bio-based benzoxazine were successfully synthesized.

Fig. 4 HSQC spectra of DG and DM The HRMS of DG and DM are shown in Fig. 5. The theoretical molecular weights of DG and DM were 433.63 and 485.66, respectively. Fig. 5 shows that the biggest molecular mass peak of DG and DM were 433.29 and 485.56, respectively, which are consistent with theoretical values. Since the highest m/z peak appears in the middle of picture, the compound may consist of two



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stable structure 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 to the main part of dehydroabietylamine. Therefore, the C-CH2-N may be the weak pond and broken off during the measuring process. The molecular weight of guaiacol and 4-methylumbelliferone are 124 g/mol and 176 g/mol respectively. This shows no residual reagents in benzoxazine monomers. The results of NMR, FTIR, and HRMS analysis prove that dehydroabietylamine-based benzoxazine monomers were successfully synthesized.

Fig. 5 HRMS of DG and DM Curing behaviour of the bio-based benzoxazine monomers


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The curing behaviour of DG and DM were studied by DSC (Fig. 6) and in situ FTIR (Fig. 7). The bulky amine moiety will not degrade during the cure. 47,50 As shown in Fig. 6 and Table 1, the endothermic peaks at 104 oC and 133 oC were assigned to the melting points of DG and DM, respectively. The exothermic peaks demonstrate the ring-opening polymerization of oxazine rings. The onset and maximum ring-opening temperatures of DG were 180 oC and 225 oC, respectively. The values for DM were 198 oC and 228 oC, 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. 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 bio-based benzoxazine monomers, but had no effect on the ring-opening polymerization.51,52 The abovementioned results are consistent with these studies. In addition, the peak curing temperatures of DG and DM were slightly lower than most other petrol-based benzoxazines, which occur between 220 and 260 oC.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-catalytic the curing of DM.3 The processing windows of DG and DM were 76 oC and 65 oC, respectively, which mean they have good processing performance. Compared to other bio-based benzoxazines synthesized from the same phenol compound but different amine compounds, DG and DM had lower curing temperatures. The onset curing temperature of bio-based benzoxazine Bzs, synthesized from guaiacol and stearylamine, was 232 o

C.30 The peak curing temperature of bio-based benzoxazine Bzf, synthesized from guaiacol and

furfurylamine, was 240 oC.30 The curing of benzoxazine Mu-a, synthesized from 4-methylumbelliferone and aniline, began at 229 oC and showed the endothermic peak at 232 oC.3,38 The possible reason for these



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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 phenanthrene ring is an electron donating group, which is stronger than the benzene ring and furan ring.

Fig. 6 DSC curves of DG and DM Table 1 DSC data of DG and DM monomers at a heating rate of 10 oC /min Monomer Tm(oC) Tonset(oC) Tmax(oC) ∆H (J/g) DG

104

180

225

173.5

DM

133

198

228

152.7

In situ FTIR was utilized to further clarify the curing process of DG and DM. As shown in Fig. 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.


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Fig. 7 In situ FTIR of 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 (Fig. 8). From the DSC curves, the Tg values of PDG and PDM were 124 oC and 207 oC, respectively. As is well-known, both the rigidity of the polymer chain and the cross-linking density can influence the Tg value.44 Since there is only one benzoxazine ring in the structure, these resins should be mainly linearly polymerized without formation of a massive crosslinked network. The possible reason for 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



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and PDM with other polybenzoxazines that have similar chemical structures. Comparing Tg values of PDG to PBzs (Tg = 82 oC) and PBzf (Tg = 148 oC), 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, while 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 oC),3,38 the rigidity of PDM was higher, which might be attributed to the huge phenanthrene ring structure of PDM.

Fig. 8 DSC curves of PDG and PDM for Tg determination The thermal stability of PDG and PDM was studied by TGA. The results are shown in Fig. 9 and summarized in Table 2. From Table 2, T5, T10, and char yields at 800 oC of PDM were higher than that of PDG, which means PDM was more thermally stable. The possible reason for this is that the α-pyrone 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 Fig. 9, the weight of PDG and PDM displayed no changes below 300 oC, which indicates they were more thermally stable than other bio-based polybenzoxazines. This means that they have good potential application in materials used below 300 oC.


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Fig. 9 TGA curves of PDM and PDG Table 2 TGA results of PDG and PDM Sample T5* (oC)

T10 ** (oC)

Char yield (%)

LOI values

PDG

321

358

24

30.1

PDM

362

405

32

33.3

*T5: 5% weight loss temperature; **T10: 10% weight loss temperature 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 bio-polybenzoxazine, such as stearylamine-based polybenzoxazines. Dehydroabietylamine is abundant in the natural world; therefore, dehydroabietylamine is a good choice as a biological source amine to use in bio-based 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, the introduction of fluorine atoms, and the formation of nano-porous.



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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 Fig. 10 shows the dielectric constant of PDG and PDM at 25 oC and 200 oC. At 25 oC, 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 oC, PDG and PDM still maintained low dielectric constants. We presume that the giant phenanthrene ring structure increases the free volume of polybenzoxazines and the phenanthrene ring is an electronics supply group that can reduce the polarity of the C-N bond. In addition, the structure of 4methylumbelliferone 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.

Fig. 10 The relationship between the dielectric constant and frequency of PDG and PDM at different temperature conditions Anticorrosion performance of PDG and PDM coatings


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Slides and aluminum sheets were chosen to obtain coatings on different substrates. Fig. 11 provides pictures of benzoxazines coatings and corresponding polybenzoxazines coatings on slides and aluminum sheets. DG and DM were dissolved in N, N-dimethylformamide. Solvent was removed under 110 oC 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, PDM-S, and PDM-A.

Fig. 11 Pictures of DG and DM monomers coating 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 oC. From Fig. 12, water contact angles of 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.



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

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


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The anticorrosion performances of PDG and PDM were studied by measuring OPC-time curves and Tafel plots of low-carbon 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 Fig. 13a, the initial EOCP of Q2345A, PDG-Q, and PDM-Q were –0.590, –0.415, and – 0.323V vs Ag/AgCl, respectively. In contrast to Q2345A, the EOCP of PDG-A and PDM-A increased by 175 mV and 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 origin low-carbon steel (Q2345A) were still over 180 mV vs Ag/AgCl. The value is comparable to 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 Fig. 13b, Tafel plots also provided corrosion potential (Ecorr) and corrosion current (Icorr) 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 micro pores prepared from higher density polymer could inhibit the essential step in metal corrosion. Polybenzoxazine has the advantages of nearzero curing shrinkage and low water absorption.58 It also has been reported that some polybenzoxazines have similar anticorrosion performances.57,58,59 In this study, the possible reason that PDG and PDM behave good anticorrosion performances might owe to the lower water resistance and good adhesion to steel for the PDG and PDM, which are beneficial for the longterm use of coatings.

CONCLUSION



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In the present study, we explored two novel fully bio-based 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

13

C 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 oC and 225 oC, respectively. The properties of the corresponding polybenzoxazines (PDM and PDG) were also investigated. The PDG revealed Tg at 124 oC, while Tg of PDM was 207 oC. The 5% weight loss temperature of PDG was 321 oC, which is lower than that of PDM (T5 = 362 o

C), which means both PDM and PDG have high thermal stability. In addition, PDG and PDM

have low dielectric constants at 25 oC (1 kHz, dielectric constants of PDG and PDM were 3.30 and 3.15, respectively). The initial EOCP of Q2345A, PDG-Q, and PDM-Q were –0.590, –0.415, and –0.323V vs Ag/AgCl, respectively. After immersing in sodium chloride solution for 6000 s, the differences of EOCP between PDM-Q, PDG-Q, and Q2345A were still over 180 mV vs Ag/AgCl. Tafel plots test 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.

AUTHOR INFORMATION Corresponding Author

* Xiaoyun Liu. Email: [email protected] * Qixin Zhuang. Email: [email protected] ORCID


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* Xiaoyun Liu: 0000-0002-9838-1109 * Qixin Zhuang: 0000-0001-8883-3805

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

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For Table of Contents Use Only

Synopsis Two kinds of fully bio-based benzoxazines were synthesized from sustainable materials and their polymers have good anticorrosion performance.


Novel Fully Biobased Benzoxazines from Rosin: Synthesis and

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